Harald O. Jeschke - Publications

 Refereed publications 

 2025 

197. 	K. Ma, S. López-Paz, K. Gornicka, H. O. Jeschke, T. Klimczuk, F. O. von Rohr
     	Discovery of the type-II superconductor Ta4Rh2C1-δ with a high upper critical field
     	Phys. Rev. Res., in press (2025), (abstract).(hide abstract). We report on the discovery of superconductivity in the previously unknown compound Ta4Rh2C1-δ. Ta4Rh2C1-δ crystallizes in the η-carbide structure type, in the cubic space group Fd-3m (No. 227) with a unit cell parameter of a = 11.7947 . Temperature-dependent magnetic susceptibility, resistivity, and specific heat capacity measurements reveal that Ta4Rh2C1-δ is a type-II bulk superconductor with a critical temperature of Tc = 6.4 K, and a normalized specific heat jump ΔC/γTc = 1.56. Notably, we find Ta4Rh2C1-δ has a high upper critical field of μ0Hc2(0) = 17.4 T, which is exceeding the BCS weak coupling Pauli limit of μ0HPauli = 11.9 T.

196. 	P. Schmoll, H. O. Jeschke, Y. Iqbal
     	Tensor network analysis of the maple-leaf antiferromagnet spangolite
     	submitted (2025) (preprint,pdf), (abstract).(hide abstract). Spangolite (Cu6Al(SO4)(OH)12Cl⋅3H2O) is a hydrated layered copper sulfate mineral whose crystal structure is well described by a depleted triangular lattice of Cu2+ ions in each layer. Experimental measurements reveal a non-magnetic ground state at T∼8K with magnetization properties dominated by dimerisation. We propose a concrete model of Cu2+ spin-1/2 degrees of freedom on this geometrically frustrated and effectively two-dimensional maple-leaf lattice. The distorted geometry of the depleted triangular lattice layers results in a spatially anisotropic Heisenberg model featuring five symmetry inequivalent couplings with ferromagnetic bonds on hexagons and antiferromagnetic triangular bonds. The validity of the proposed Hamiltonian is demonstrated by state-of-the-art tensor network calculations which can assess both the nature of the ground state as well as low-temperature thermodynamics, including effects of a magnetic field. We provide theoretical support for a dimerized ground state by calculating the static spin structure factor as well as the magnetic susceptibility, the latter is shown to be in good agreement with experiment. We further predict the emergence of magnetisation plateaus at high values of an external magnetic field and study their melting with increasing temperature.

195. 	M. G. Gonzalez, Y. Iqbal, J. Reuther, H. O. Jeschke
     	Field-induced spin liquid in the decorated square-kagome antiferromagnet nabokoite KCu7TeO4(SO4)5Cl
     	Commun. Mater., in press (2025) (preprint,pdf), (abstract).(hide abstract). Quantum antiferromagnets realizing the square-kagome lattice are proving to be a promising platform for nontrivial phenomena in frustrated magnetism. We carry out a theoretical analysis of the recently synthesized compound nabokoite KCu7TeO4(SO4)5Cl based on a density functional theory energy mapping which establishes an unusual Heisenberg Hamiltonian with a hierarchy of couplings which suggests a description in terms of effective spin liquid models. Owing to the decoration of the square-kagome lattice with a seventh Cu site, nabokoite is characterized by highly frustrated square pyramids that are antiferromagnetically coupled in a chiral arrangement. Employing classical Monte Carlo simulations, we are able to explain the two transitions observed in the low-temperature magnetization curve from the experimental measurements. Remarkably, we show that the low-field phase is only realized when the inter-layer coupling is included. On the other hand, the intermediate-field phase is also found in a purely two-dimensional model and is described by a spin liquid featuring a subextensive degeneracy with a ferrimagnetic component. We show that this phase can be approximated by a checkerboard lattice in a magnetic field. We finally assess the effects of quantum fluctuations in zero fields using the pseudo-Majorana functional renormalization group method and predict the spin structure factor for comparison with future neutron scattering measurements.

194. 	J. Ebad-Allah, D. Guterding, M. Varma, M. Diware, S. Ganorkar, H. O. Jeschke, C. A. Kuntscher
     	Near room-temperature ferromagnetism from double-exchange in van der Waals material CrGeTe3: evidence from optical conductivity under pressure
     	Phys. Rev. B 111, L140402 (2025) (Editors' Suggestion) (doi,pdf,supplement), (abstract).(hide abstract). The unexpected discovery of intrinsic ferromagnetism in layered van der Waals materials has sparked interest in both their fundamental properties and their potential for novel applications. Re- cent studies suggest near room-temperature ferromagnetism in the pressurized van der Waals crystal CrGeTe3. We perform a comprehensive experimental and theoretical investigation of magnetism and electronic correlations in CrGeTe3, combining broad-frequency reflectivity measurements with density functional theory and dynamical mean-field theory calculations. Our experimental optical conductivity spectra trace the signatures of developing ferromagnetic order and of the insulator- to-metal transition (IMT) as a function of temperature and hydrostatic pressure. With increasing pressure, we observe the emergence of a mid-infrared feature in the optical conductivity, indicating the development of strong orbital-selective correlations in the high-pressure ferromagnetic phase. We find a peculiar relationship between the plasma frequency and Curie temperature of CrGeTe3, which strongly suggests that a double-exchange mechanism is responsible for the observed near room-temperature ferromagnetism. In contrast to previous studies, our results clearly demonstrate the existence of an optical gap in the metallic phase, ruling out the collapse of the charge transfer gap under pressure.

193. 	L. D. C. Jaubert, Y. Iqbal, H. O. Jeschke
     	Spin-Peierls transition in the frustrated spinels ZnCr2O4 and MgCr2O4
     	Phys. Rev. Lett. 134, 086702 (2025) (Editors' Suggestion) (doi,pdf,supplement), (abstract).(hide abstract). The chromium spinels MgCr2O4 and ZnCr2O4 are prime examples of the highly frustrated pyrochlore lattice antiferromagnet. Experiment has carefully established that both materials, upon cooling, distort to lower symmetry and order magnetically. We study the nature of this process by a combination of density-functional-theory based energy mapping and classical Monte Carlo simulations. We first computationally establish precise Heisenberg Hamiltonian parameters for the high temperature cubic and the low temperature tetragonal and orthorhombic structures of both spinels. We then investigate the respective ordering temperatures of high symmetry and low symmetry structures. We carefully compare our results with experimental facts and find that our simulations are remarkably consistent with a type of spin-Peierls mechanism, adapted to three dimensions, where the structural distortion is mediated by a magnetic energy gain due to a lower degree of frustration.

192. 	G. Scharf, D. Guterding, B. Hen, P. M. Sarte, B. R. Ortiz, G. Kh. Rozenberg, T. Holder, S. D. Wilson, H. O. Jeschke, A. Ron
     	Pressure tuning of intrinsic and extrinsic sources to the anomalous Hall effect in CrGeTe3
     	Phys. Rev. Res. 7, 013127 (2025) (doi,pdf,supplement), (abstract).(hide abstract). The integrated Berry curvature is a geometric property that has dramatic implications for material properties. This study investigates the integrated Berry curvature and other contributions to the anomalous Hall effect in CrGeTe3 as a function of pressure. The anomalous Hall effect is absent in the insulating phase of CrGeTe3 and evolves with pressure in a dome-like fashion as pressure is applied. The dome’s edges are characterized by Fermi surface deformations, manifested as mixed electron and hole transport. We corroborate the presence of bipolar transport by ab-initio calculations which also predict a nonmonotonic behavior of the Berry curvature as a function of pressure. Quantitative discrepancies between our calculations and experimental results indicate that additional scattering mechanisms, which are also strongly tuned by pressure, contribute to the anomalous Hall effect in CrGeTe3.

191. 	K. D. Röseler, C. Witteveen, C. Besnard, V. Pomjakushin, H. O. Jeschke, F. von Rohr
     	Efficient Soft-Chemical Synthesis of Large van-der-Waals Crystals of the Room-Temperature Ferromagnet 1T-CrTe2
     	J. Mater. Chem. A, advance publication (2025) (doi,pdf,supplement,cif), (abstract).(hide abstract). We herein report on a fast and convenient soft-chemical synthesis approach towards large 1T-CrTe2 van-der-Waals crystals. This compound is formed X-ray diffraction pure, with a complete conversion within just over 2 h from flux-grown LiCrTe2 crystals using diluted acids. Due to the availability of high-quality single crystals, we have confirmed the crystal structure for the first time by single-crystal X-ray diffraction experiments. For the acid deintercalated 1T-CrTe2 crystals, we find long-range ferromagnetic order with a Curie temperature of TC = 318 K. We further revealed the magnetic structure of 1T-CrTe2 using low-temperature neutron powder diffraction experiments and determine the magnetic Hamiltonian using density functional theory. X-ray diffraction experiments of post-annealed crystals suggest a thermal stability of 1T-CrTe2 up to at least 100 °C. Our findings expand the synthesis methods for 1T-CrTe2 crystals, which hold promise for integrated room-temperature spintronics applications.

      2024 

190. 	P. Ghosh, T. Müller, Y. Iqbal, R. Thomale, H. O. Jeschke
     	Effective spin-1 breathing kagome Hamiltonian induced by the exchange hierarchy in the maple leaf mineral bluebellite
     	Phys. Rev. B 110, 094406 (2024) (doi,pdf), (abstract).(hide abstract). As a highly frustrated model Hamiltonian with an exact dimer ground state, the Heisenberg antiferromagnet on the maple leaf lattice is of high theoretical interest, and a material realization is intensely sought after. We determine the magnetic Hamiltonian of the copper mineral bluebellite using density functional theory based energy mapping. As a consequence of the significant distortion of the spin S = 1/2 maple leaf lattice, we find two of the five distinct nearest neighbor couplings to be ferromagnetic. Solution of this Hamiltonian with density matrix renormalization group calculations points us to the surprising insight that this particular imperfect maple leaf lattice, due to the strongly ferromagnetic Cu2+ dimer, realizes an effective S = 1 breathing kagome Hamiltonian. In fact, this is another highly interesting Hamiltonian which has rarely been realized in materials. Analysis of the effective model within a bond-operator formalism allows us to identify a valence bond solid ground state and to extract thermodynamic quantities using a low-energy bosonic mean-field theory. We resolve the puzzle of the apparent one-dimensional character of bluebellite as our calculated specific heat has a Bonner-Fisher-like shape, in good agreement with experiment.

189. 	M. G. Gonzalez, V. Noculak, A. Sharma, V. Favre, J-R. Soh, A. Magrez, R. Bewley, H. O. Jeschke, J. Reuther, H. M. Rønnow, Y. Iqbal, I. Živković
     	Dynamics of K2Ni2(SO4)3 governed by proximity to a 3D spin liquid model
     	Nature Comm. 15, 7191 (2024) (doi,pdf,supplement), (abstract).(hide abstract). Quantum spin liquids (QSLs) have become a key area of research in magnetism due to their remarkable properties, such as long-range entanglement, fractional excitations, pinch-point singularities, and topologically protected phenomena. In recent years, the search for QSLs has expanded into the three-dimensional world, where promising features have been found in materials that form pyrochlore and hyper-kagome lattices, despite the suppression of quantum fluctuations due to high dimensionality. One such material is the S=1 K2Ni2(SO4)3 compound, which belongs to the langbeinite family consisting of two interconnected trillium lattices. Although magnetically ordered, K2Ni2(SO4)3 has been found to exhibit a highly dynamical and correlated state which can be driven into a pure quantum spin liquid under magnetic fields of only B≃4 T. In this article, we combine inelastic neutron scattering measurements with pseudo-fermion functional renormalization group (PFFRG) and classical Monte Carlo (cMC) calculations to study the magnetic properties of K2Ni2(SO4)3, revealing a high level of agreement between the experiment and theory. We further reveal the origin of the dynamical state in K2Ni2(SO4)3 by studying a larger set of exchange parameters, uncovering an 'island of liquidity' around a focal point given by a magnetic network composed of tetrahedra on a trillium lattice.

188. 	X. Guo, W. Liu, J. Schwartz, S. H. Sung, D. Zhang, M. Shimizu, A. L. N. Kondusamy, L. Li, K. Sun, H. Deng, H. O. Jeschke, I. I. Mazin, R. Hovden, B. Lv, L. Zhao
     	Extraordinary Phase Transition Revealed in a van der Waals Antiferromagnet
     	Nature Comm. 15, 6472 (2024) (doi,pdf,supplement), (abstract).(hide abstract). While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. We leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface is responsible for this enhanced surface magnetism. Our results not only show counterintuitive, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.

187. 	J. Otsuki, K. Yoshimi, H. Shinaoka, H. O. Jeschke
     	Multipolar ordering from dynamical mean field theory with application to CeB6
     	Phys. Rev. B 110, 035104 (2024) (doi,pdf), (abstract).(hide abstract). Magnetic and multipolar ordering in f electron systems takes place at low temperatures of order 1-10 Kelvin. Combinations of first-principles with many-body calculations for such low-energy properties of correlated materials are challenging problems. We address multipolar ordering in f electron systems based on the dynamical mean-field theory (DMFT) combined with density functional theory. We derive the momentum-dependent multipolar susceptibilities and interactions in two ways: by solving the Bethe-Salpeter (BS) equation of the two-particle Green's function and by using a recently developed approximate strong-coupling formula. We apply the formalism to the prototypical example of multipolar ordering in CeB6 using the Hubbard-I solver, and demonstrate that the experimental quadrupole transition is correctly reproduced. The results by the approximate formula show good agreement with those by the BS equation. This first-principles formalism based on DMFT and BS equation has applications which are beyond the reach of the traditional RKKY formula. In particular, more itinerant electron systems including 5f, 4d and 5d electrons can be addressed.

186. 	H. O. Jeschke, M. Shimizu, I. I. Mazin
     	CuAg(SO4)2: A doubly strongly correlated altermagnetic three-dimensional analog of the parent compounds of high-Tc cuprates
     	Phys. Rev. B 109, L220412 (2024) (doi,pdf,supplement), (abstract).(hide abstract). The discovery of high-temperature superconductivity (HTSC) in strongly correlated cuprates opened a new chapter in condensed matter physics, breaking existing stereotypes of what is a material base for a good superconductor (“Matthias rules”), at the same time emphasizing richness and challenge of strongly correlated physics, personified by the most strongly correlated 3d ion, Cu2+. A recently reported new compound, CuAg(SO4)2, combines in a fascinating way the same ion with the most strongly correlated 4d one, Ag2+. In this Letter, we present a detailed analysis of electronic and magnetic properties of this material, and show that it is very different from the HTSC cuprates in several different ways, and opens a door into further research of superconductivity and magnetism, in particular altermagnetism, in strongly correlated materials.

185. 	K. Shibata, M. Naka, H. O. Jeschke, J. Otsuki
     	Spin-Orbital Ordering in Alkali Superoxides
     	Phys. Rev. B 109, 235115 (2024) (doi,pdf) (Editors' Suggestion), (abstract).(hide abstract). Akali superoxides AO2 (A=Na, K, Rb, Cs), due to an open p shell of the oxygen ion O2- with degenerate π orbitals, have spin and orbital degrees of freedom. The complex magnetic, orbital, and structural phase transitions observed experimentally in this family of materials are only partially understood. Based on density functional theory, we derive a strong-coupling effective model for the isostructural compounds AO2 (A=K, Rb, Cs) from a two-orbital Hubbard model. We find that CsO2 has highly frustrated exchange interactions in the a-b plane, while the frustration is weaker for smaller alkali ions. We solve the resulting Kugel-Khomskii model in the mean-field approximation. We show that CsO2 exhibits an antiferro-orbital (AFO) order with the ordering vector q=(1,0,0) and a stripe antiferromagnetic order with q=(1/2,0,0), which is consistent with recent neutron scattering experiments. We discuss the role of the pi-orbital degrees of freedom for the experimentally observed magnetic transitions and interpret the as-yet-unidentified Ts2=70K transition in CsO2 as an orbital ordering transition.

      2023 

184. 	N. Niggemann, N. Astrakhantsev, A. Ralko, F. Ferrari, A. Maity, T. Müller, J. Richter, R. Thomale, T. Neupert, J. Reuther, Y. Iqbal, H. O. Jeschke
     	Quantum paramagnetism in the decorated square-kagome antiferromagnet Na6Cu7BiO4(PO4)4Cl3
     	Phys. Rev. B 108, L241117 (2023) (doi,pdf,supplement), (abstract).(hide abstract). The square-kagome lattice Heisenberg antiferromagnet is a highly frustrated Hamiltonian whose material realizations have been scarce. We theoretically investigate the recently synthesized Na6Cu7BiO4(PO4)4Cl3 where a Cu2+ spin-1/2 square-kagome lattice (with six site unit cell) is decorated by a seventh magnetic site alternatingly above and below the layers. The material does not show any sign of long-range magnetic order down to 50 mK despite a Curie-Weiss temperature of −212 K indicating a quantum paramagnetic phase. Our DFT energy mapping elicits a purely antiferromagnetic Hamiltonian that features longer range exchange interactions beyond the pure square-kagome model and, importantly, we find the seventh site to be strongly coupled to the plane. We combine two variational Monte Carlo approaches, pseudo-fermion/Majorana functional renormalization group and Schwinger-Boson mean field calculations to show that the complex Hamiltonian of Na6Cu7BiO4(PO4)4Cl3 still features a nonmagnetic ground state. We explain how the seventh Cu2+ site actually aids the stabilization of the disordered state. We predict static and dynamic spin structure factors to guide future neutron scattering experiments.

183. 	M. Shimizu, J. Otsuki, H. O. Jeschke
     	Magnetic fluctuations in Pb9Cu(PO4)6O
     	Phys. Rev. B 108, L201105 (2023) (doi,pdf), (abstract).(hide abstract). The hope that copper doped lead apatite Pb9Cu(PO4)6O is a room-temperature superconductor has largely been dashed by global research efforts. Nevertheless, according to the current state of knowledge, the material has interesting magnetic properties, and research groups around the world have prepared high quality samples. We use a fluctuation exchange approximation (FLEX) approach to study the magnetic tendencies in Pb9Cu(PO4)6O. We find ferromagnetic fluctuations very close to the filling of the stoichiometric compound which can be understood from Fermi surface nesting at the M point. This is similar to the one-band triangular lattice Hamiltonian at three-quarter filling. Interestingly, the special kz dependence of the Pb9Cu(PO4)6O band structure makes it very sensitive to doping. Only slight charge doping switches between antiferromagnetic and ferromagnetic fluctuations. If the material could become superconducting, it might be easily switchable between singlet and triplet superconductivity.

182. 	H.-X. Xu, M. Shimizu, D. Guterding, J. Otsuki, H. O. Jeschke
     	Pressure evolution of electronic structure and magnetism in the layered van der Waals ferromagnet CrGeTe3
     	Phys. Rev. B 108, 125142 (2023) (doi,pdf), (abstract).(hide abstract). Layered van der Waals ferromagnets, which preserve their magnetic properties down to exfoliated monolayers, are fueling a bonanza of fundamental research and nanoscale device demonstration. CrGeTe3 is a prime example for this class of materials. Its temperature-pressure phase diagram features an insulator-to-metal transition and a signicant increase of ferromagnetic Curie-Weiss temperatures upon entering the metallic state. We use density functional theory to understand the magnetic exchange interactions in CrGeTe3 at ambient and elevated pressure. We calculate Heisenberg exchange couplings, which provide the correct ferromagnetic ground state and explain the experimentally observed pressure dependence of magnetism in CrGeTe3. Furthermore, we combine density functional theory with dynamical mean eld theory to investigate the eects of electronic correlations and the nature of the high pressure metallic state in CrGeTe3.

181. 	H. Yamaoka, H. O. Jeschke, H. Li, T. He, N. Tsujii, N. Hiraoka, H. Ishii, H. Goto, Y. Kubozono
     	Correlation between electronic structure and emergence of superconductivity in Bi2-xSbxTe3-ySey (y ∼ 1.2) studied by x-ray emission spectroscopy and density functional theory
     	Phys. Rev. B 108, 035146 (2023) (doi,pdf,supplement), (abstract).(hide abstract). Chemical composition and pressure dependencies of the electronic structures of Bi2-xSbxTe3-ySey (y ∼ 1.2) have been studied by the high-resolution x-ray absorption spectroscopy. We find a shift of the Bi-L3 absorption edge due to Sb substitution, suggesting a change in the Bi charge state. In the pressure dependence, the electronic structures of Bi2Te2Se and Bi1.5Sb0.5Te2Se start to change below the pressure of the first structural phase transition where the emergence of the superconductivity was observed and then show a large change just around that pressure. This is in contrast to the behavior observed in Bi2Se3-based compounds where the structural phase transition was necessary for the onset of superconductivity.We performed density functional theory calculations using the experimentally determined structures for Bi2Te2Se and Bi1.5Sb0.5Te2Se.We show that both compounds become metallic within the rhombohedral phase below the pressure of the first structural transition and we thus corroborate the experimental observation. The experimental and calculated results show that closing the gap and increasing the density of states in the rhombohedral phase are the triggers to induce superconductivity.

180. 	M. Gen, H. Ishikawa, A. Miyake, T. Yajima, H. O. Jeschke, H. Sagayama, A. Ikeda, Y. H. Matsuda, K. Kindo, M. Tokunaga, Y. Kohama, T. Kurumaji, Y. Tokunaga, T. Arima
     	Breathing pyrochlore magnet CuGaCr4S8: Magnetic, thermodynamic, and dielectric properties
     	Phys. Rev. Mater. 7, 104404 (2023) (doi,pdf), (abstract).(hide abstract). We investigate the crystallographic and magnetic properties of a chromium-based thiospinel CuGaCr4S8. From a synchrotron x-ray diffraction experiment and structural refinement, Cu and Ga atoms are found to occupy the tetrahedral A-sites in an alternate way, yielding breathing pyrochlore Cr network. CuGaCr4S8 undergoes a magnetic transition associated with a structural distortion at 31 K in zero magnetic field, indicating that the spin-lattice coupling is responsible for relieving the geometrical frustration. When applying a pulsed high magnetic field, a sharp metamagnetic transition takes place at 40 T, followed by a 1/2-magnetization plateau up to 103 T. These phase transitions accompany dielectric anomalies, suggesting the presence of helical spin correlations in low-field phases. The density-functional-theory calculation reveals that CuGaCr4S8 is dominated by antiferromagnetic and ferromagnetic exchange couplings within small and large tetrahedra, respectively, in analogy with CuInCr4S8. We argue that A-site-ordered Cr thiospinels serve as an excellent platform to explore diverse magnetic phases along with pronounced magnetoelastic and magnetodielectric responses.

179. 	M. Gen, A. Ikeda, K. Aoyama, H. O. Jeschke, Y. Ishii, H. Ishikawa, T. Yajima, Y. Okamoto, D. Nakamura, S. Takeyama, K. Kindo, Y. H. Matsuda, Y. Kohama
     	Signatures of a magnetic superstructure phase induced by ultrahigh magnetic fields in a breathing pyrochlore antiferromagnet
     	Proc. Natl. Acad. Sci. USA 120, e2302756120 (2023) (doi,pdf,supplement), (abstract).(hide abstract). The mutual coupling of spin and lattice degrees of freedom is ubiquitous in magnetic materials and potentially creates exotic magnetic states in response to the external magnetic field. Particularly, geometrically frustrated magnets serve as a fertile playground for realizing magnetic superstructure phases. Here, we observe an unconventional two-step magnetostructural transition prior to a half-magnetization plateau in a breathing pyrochlore chromium spinel by means of state-of-the-art magnetization and magnetostriction measurements in ultrahigh magnetic fields available up to 600 T. Considering a microscopic magnetoelastic theory, the intermediate-field phase can be assigned to a magnetic superstructure with a three-dimensional periodic array of 3-up-1-down and canted 2-up-2-down spin molecules. We attribute the emergence of the magnetic superstructure to a unique combination of the strong spin–lattice coupling and large breathing anisotropy.

178. 	A. Samartzis, S. Chillal, H. O. Jeschke, D. J. Voneshen, Z. Lu, A. T. M. N. Islam, B. Lake
     	Magnetic excitation spectrum and Hamiltonian of the quantum spin chain compound BaCuTe2O6
     	Phys. Rev. B 107, 184435 (2023) (doi,pdf), (abstract).(hide abstract). The magnetic excitation spectrum and Hamiltonian of the quantum magnet is studied by inelastic neutron scattering (INS) and density functional theory (DFT). INS on powder and single crystal samples reveals overlapping spinon continuua - the spectrum of an antiferromagnetic spin-1/2 spin chain - due to equivalent chains running along the a, b, and c directions. Long-range magnetic order onsets below TN=6.3 K due to interchain interactions, and is accompanied by the emergence of sharp spin-wave excitations which replace the continuua at low energies. The spin-wave spectrum is highly complex and was successfully modelled achieving excellent agreement with the data. The extracted interactions reveal an intrachain interaction, J3=2.9 meV, while the antiferromagnetic hyperkagome interaction J2, is the sub-leading interaction responsible for coupling the chains together in a frustrated way. DFT calculations reveal a similar picture for of dominant J3 and sub-leading J2 antiferromagnetic interactions and also indicate a high sensitivity of the interactions to small changes of structure which could explain the very different Hamiltonians observed in the sister compounds SrCuTe2O6 and PbCuTe2O6.

177. 	Y. N. Huang, H. O. Jeschke, I. I. Mazin
     	CrRhAs: a member of a large family of metallic kagome antiferromagnets
     	npj Quant. Mater. 8, 32 (2023) (doi,pdf,supplement), (abstract).(hide abstract). Kagome lattice materials are an important platform for highly frustrated magnetism as well as for a plethora of phenomena resulting from flat bands, Dirac cones and van Hove singularities in their electronic structures. We study the little known metallic magnet CrRhAs, which belongs to a vast family of materials that include 3d, 4f and 5f magnetic elements, as well as numerous nonmagnetic metals and insulators. Using noncollinear spin density functional calculations (mostly spin spirals), we extract a model magnetic Hamiltonian for CrRhAs. While it is dominated by an antiferromagnetic second nearest neighbor coupling in the kagome plane, the metallic nature of the compound leads to numerous nonzero longer range couplings and to important ring exchange terms. We analyze this Hamiltonian and find unusual ground states which are dominated by nearly isolated antiferromagnetic triangles that adopt 120° order either with positive or with negative vector chirality. We discuss the connection to the few known experimental facts about CrRhAs. Finally, we give a brief survey of other interesting magnetic members of this family of kagome compounds.

176. 	C. Witteveen, E. Nocerino, S. A. López-Paz, H. O. Jeschke, V. Y. Pomjakushin, M. Månsson, F. O. von Rohr
     	Synthesis and Anisotropic Magnetic Properties of LiCrTe2 Single Crystals with a Triangular-Lattice Antiferromagnetic Structure
     	J. Phys.: Materials 6, 035001 (2023) (doi,pdf,supplement), (abstract).(hide abstract). We report on the synthesis of LiCrTe2 single crystals and on their anisotropic magnetic properties. We have obtained these single crystals by employing a Te/Li-flux synthesis method. We find LiCrTe2 to crystallize in a TlCdS2-type structure with cell parameters of a = 3.9512(5) Å and c = 6.6196(7) Å at T = 175 K. The content of lithium in these crystals was determined to be near stoichiometric by means of neutron diffraction. We find a pronounced magnetic transition at TNab = 144 K and TNc = 148 K, respectively. These transition temperatures are substantially higher than earlier reports on polycrystalline samples. We have performed neutron powder diffraction measurements that reveal that the long-range low-temperature magnetic structure of single crystalline LiCrTe2 is an A-type antiferromagnetic (AFM) structure. Our DFT calculations are in good agreement with these experimental observations. We find the system to be easy axis with moments oriented along the c-direction experimentally as well as in our calculations. Thereby, the magnetic Hamiltonian can be written as H = HHeisenberg + Σi Kc (Siz)2 with Kc = −0.34K (where |Sz| = 3/2). We find LiCrTe2 to be highly anisotropic, with a pronounced metamagnetic transition for H ⊥ ab with a critical field of μHMM(5 K) ≈ 2.5 T. Using detailed orientationdependent magnetization measurements, we have determined the magnetic phase diagram of this material. Our findings suggest that LiCrTe2 is a promising material for exploring the interplay between crystal structure and magnetism, and could have potential applications in spin-based 2D devices.

175. 	D. Crawford, E. Mascot, M. Shimizu, R. Wiesendanger, D. K. Morr, H. O. Jeschke, S. Rachel
     	Increased localization of Majorana modes in antiferromagnetic chains on superconductors
     	Phys. Rev. B 107, 075410 (2023) (doi,pdf), (abstract).(hide abstract). Magnet-superconductor hybrid (MSH) systems are a key platform for custom-designed topological superconductors. Ideally, the ends of a one-dimensional MSH structure will host Majorana zero-modes (MZMs), the fundamental unit of topological quantum computing. However, some of the experiments with ferromagnetic chains show a more complicated picture. Due to tiny gap sizes and hence long coherence lengths MZMs might hybridize and lose their topological protection. Recent experiments on a niobium surface have shown that both ferromagnetic and antiferromagnetic chains may be engineered, with the magnetic order depending on the crystallographic direction of the chain. While ferromagnetic chains are well understood, antiferromagnetic chains are less so. Here we study two models inspired by the niobium surface: a minimal model to elucidate the general topological properties of antiferromagnetic chains, and an extended model to more closely simulate a real system by mimicking the proximity effect. We find that in general for antiferromagnetic chains the topological gap is larger than for ferromagnetic ones and thus coherence lengths are shorter for antiferromagnetic chains, yielding more pronounced localization of MZMs in these chains. While topological phases for both ferromagnetic and antiferromagnetic chains both depend on the magnetic moment of the adatoms and the chemical potential, we find that antiferromagnetic chains also have a strong dependence on the magnitude of Rashba spin-orbit coupling at the surface.

      2022 

174. 	D. Crawford, E. Mascot, M. Shimizu, L. Schneider, P. Beck, J. Wiebe, R. Wiesendanger, H. O. Jeschke, D. K. Morr, S. Rachel
     	Majorana modes with side features in magnet-superconductor hybrid systems
     	npj Quant. Mater. 7, 117 (2022) (doi,pdf,supplement), (abstract).(hide abstract). Magnet-superconductor hybrid (MSH) systems represent promising platforms to host Majorana zero modes (MZMs), the elemental building blocks for fault-tolerant quantum computers. Theoret- ical description of such MSH structures is mostly based on simplified models, not accounting for the complexity of real materials. Here, based on density functional theory, we derive an effective superconducting 80-band model to study an MSH system consisting of a magnetic manganese chain on the s wave superconductor niobium. For a wide range of values of the superconducting order parameter, the system is a topological superconductor, with MZMs exhibiting non-universal spatial patterns and a drastic accumulation of spectral weight on both sides along the magnetic chain. Performing scanning tunneling spectroscopy experiments on the same system, we observe a spatial structure in the low-energy local density of states that is consistent with the theoretical results. Our results open a first-principle approach to the discovery of topological superconductors.

173. 	M. Fujihala, H. O. Jeschke, K. Morita, T. Kuwai, A. Koda, H. Okabe, A. Matsuo, K. Kindo, S. Mitsuda
     	Birchite Cd2Cu2(PO4)2SO4 · 5H2O as a model antiferromagnetic spin 1/2 Heisenberg J1-J2 chain
     	Phys. Rev. Mater. 6, 114408 (2022) (doi,pdf), (abstract).(hide abstract). S = 1/2 Heisenberg J1-J2 chain antiferromagnets have been investigated extensively due to their exotic magnetic states. Here, we report the magnetic behavior of birchite Cd2Cu2(PO4)2SO4 · 5H2O and its effective spin model. Experimental studies by magnetic susceptibility, magnetization, heat capacity, and μSR measurements indicate the absence of long-range order down to 0.4 K. Theoretical studies reveal that birchite is a model compound for the J1-J2 antiferromagnetic chain: the intrachain interactions J1 and J2 are antiferromagnetic and their magnitude is about 100 times larger than the interchain interactions. The magnitude of J2 is two to three times larger than that of J1, thus the spin gap is expected to be only a few percent of that of J1. The temperature dependence of the specific heat shows a broad peak at about 1 K (≃ 0.036J1), which suggests the presence of a spin gap.

172. 	H. Fujiwara, K. Terashima, J. Otsuki, N. Takemori, H. O. Jeschke, T. Wakita, Y. Yano, W. Hosoda, N. Kataoka, A. Teruya, M. Kakihana, M. Hedo, T. Nakama, Y. Onuki, K. Yaji, A. Harasawa, K. Kuroda, S. Shin, K. Horiba, H. Kumigashira, Y. Muraoka, T. Yokoya
     	Anomalously large spin-dependent electron correlation in the nearly half-metallic ferromagnet CoS2
     	Phys. Ref. B 106, 085114 (2022) (doi,pdf), (abstract).(hide abstract). The spin-dependent band structure of CoS2, which is a candidate for a half-metallic ferromagnet, was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS2 is not a half-metal but very close. Band calculations using density functional theory with generalized gradient approximation showed good agreement with the observed majority spin eg bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin eg bands. Our results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

171. 	I. Olejniczak, B. Barszcz, P. Auban-Senzier, H. O. Jeschke, R. Wojciechowski, J. A. Schlueter
     	Charge-Ordering and Structural Transition in the New Organic Conductor δ′-(BEDT-TTF)2CF3CF2SO3
     	J. Phys. Chem. C 126, 1890 (2022) (doi), (abstract).(hide abstract). We report structural, transport, optical properties and electronic structure calcula-tions of the δ′-(BEDT-TTF)2CF3CF2SO3 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) organic conductor that has been synthesized by electrocrystallization. Electronic structure calculations demonstrate the quasi-one-dimensional Fermi surfaces of the compound while the optical spectra are characteristic for a dimer-Mott insulator. The single-crystal X-ray diffraction measurements reveal the structural phase transition at 200 K from the ambient-temperature monoclinic P21/m phase to the low-temperature orthorhombic Pca21 phase, while the resistivity measurements clearly show the first order semiconductor-semiconductor transition at the same temperature. This transition is accompanied with charge-ordering as it is confirmed by splitting of charge-sensitive vibrational modes observed in the Raman and infrared spectra. The horizontal stripe charge-order pattern is suggested based on the crystal structure, band structure calculations and optical spectra.

170. 	M. Hering, F. Ferrari, A. Razpopov, I. I. Mazin, R. Valenti, H. O. Jeschke, J. Reuther
     	Phase diagram of a distorted kagome antiferromagnet and application to Y-kapellasite
     	npj Comput. Mater. 8, 10 (2022) (doi,pdf,supplement), (abstract).(hide abstract). We investigate the magnetism of a previously unexplored distorted spin-1/2 kagome model consisting of three symmetry-inequivalent nearest-neighbor antiferromagnetic Heisenberg couplings and uncover a rich ground state phase diagram even at the classical level. Using analytical arguments and numerical techniques we identify a collinear Q=0 magnetic phase, two unusual non-collinear coplanar Q=(1/3,1/3) phases and a classical spin liquid phase with a degenerate manifold of non-coplanar ground states, resembling the jammed spin liquid phase found in the context of a bond-disordered kagome antiferromagnet. We further show with density functional theory calculations that the recently synthesized Y-kapellasite Y3Cu9(OH)19Cl8 is a realization of this model and predict its ground state to lie in the region of Q=(1/3,1/3) order, which remains stable even after inclusion of quantum fluctuation effects within variational Monte Carlo and pseudofermion functional renormalization group. Interestingly, the excitation spectrum of Y-kapellasite lies between that of an underlying triangular lattice of hexagons and a kagome lattice of trimers. The presented model opens a new direction in the study of kagome antiferromagnets.

      2021 

169. 	H. Yamamoto, T. Sakakura, H. O. Jeschke, N. Kabeya, K. Hayashi, Y. Ishikawa, Y. Fujii, S. Kishimoto, H. Sagayama, K. Shigematsu, M. Azuma, A. Ochiai, Y. Noda, H. Kimura
     	Quantum spin fluctuations and hydrogen bond network in the antiferromagnetic natural mineral henmilite
     	Phys. Rev. Mater. 5, 104405 (2021) (doi,pdf,supplement,cif,press release Tohokudai,press release Okadai), (abstract).(hide abstract). Henmilite (Ca2Cu(OH)4[B(OH)4]2) is a blue calcium copper borate mineral found only in the Fuka mine, Okayama Prefecture, Japan. Crystal structure refinement, magnetic and specific heat measurements, as well as density functional theory (DFT) calculations are performed to clarify its magnetic properties. The synchrotron x-ray diffraction experiments reveal that the hydrogen bonded chains are arranged in an antiferroelectric manner, doubling the unit cell along the a-axis. An antiferromagnetic transition is observed at 0.2 K at zero magnetic field. Furthermore, a dome-shaped antiferromagnetic ordering region exists in the temperature-magnetic field phase diagram, indicating the presence of quantum spin fluctuations. The obtained crystal structure in combination with DFT calculations suggests that the system has a coupled two-leg ladder magnetic lattice, explaining the very low ordering temperature.

168. 	I. Živković, V. Favre, C. Salazar Mejía, H. O. Jeschke, A. Magrez, B. Dabholkar, V. Noculak, R. S. Freitas, M. Jeong, N. G. Hegde, L. Testa, P. Babkevich, Y. Su, P. Manuel, H. Luetkens, C. Baines, P. J. Baker, J. Wosnitza, O. Zaharko, Y. Iqbal, J. Reuther, H. M. Rønnow
     	Magnetic field induced quantum spin liquid in the two coupled trillium lattices of K2Ni2(SO4)3
     	Phys. Rev. Lett. 127, 157204 (2021) (doi,pdf,supplement), (abstract).(hide abstract). Quantum spin liquids are exotic states of matter which form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of themagnetic interactions. Three-dimensional cases are especially challenging due to the significant re-duction of the influence of quantum fluctuations. Here, we report the magnetic characterization of K2Ni2(SO4)3 forming a three dimensional network of Ni2+ spins. Using density functional theory calculations we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. Magnetic field B≤4 T diminishes the ordered component and drives the system in a pure quantum spin liquid state. This shows that a system of interconnected S=1 trillium lattices exhibits a significantly elevated level of geometrical frustration.

167. 	K. Matano, R. Ogura, M. Fountainea, H. O. Jeschke, S. Kawasaki, G.-q. Zheng
     	Antiferromagnetic spin fluctuations and superconductivity in NbRh2B2 and TaRh2B2 with a chiral crystal structure
     	Phys. Rev. B 104, 224508 (2021) (doi,pdf), (abstract).(hide abstract). We report the 11B nuclear magnetic resonance (NMR) measurements on non-centrosymmetric superconductors NbRh2B2 (superconducting transition temperature Tc = 7.8 K) and TaRh2B2 (Tc = 5.9 K) with a chiral crystal structure. The nuclear spin-lattice relaxation rate 1/T1 shows no coherence peak below Tc, which suggests unconventional nature of the superconductivity. In the normal state, 1/T1T increases with decreasing temperature T at low temperatures below T = 200 K for TaRh2B2 and T = 15 K for NbRh2B2, while the Knight shift remains constant. These results suggest the presence of antiferromagnetic spin fluctuations in both compounds. The stronger spin fluctuations in TaRh2B2 compared to NbRh2B2 is discussed in the context of spin-orbit coupling.

166. 	H.-X. Xu, D. Guterding, H. O. Jeschke
     	Theory for doping trends in titanium oxypnictide superconductors
     	Phys. Rev. B 104, 184519 (2021) (doi,pdf), (abstract).(hide abstract). A family of titanium oxypnictide materials BaTi2Pn2O (Pn = pnictogen) becomes superconducting when a charge and/or spin density wave is suppressed. With hole doping, isovalent doping and pressure, a whole range of tuning parameters is available. We investigate how charge doping controls the superconducting transition temperature Tc. To this end, we use experimental crystal structure data to determine the electronic structure and Fermi surface evolution along the doping path. We show that a naive approach to calculating Tc via the density of states at the Fermi level and the McMillan formula systematically fails to yield the observed Tc variation. On the other hand, spin fluctuation theory pairing calculations allow us to consistently explain the Tc increase with doping. All alkali doped materials Ba1-xAxTi2Sb2O (A = Na, K, Rb) are described by a sign-changing s-wave order parameter. Susceptibilities also reveal that the physics of the materials is controlled by a single Ti 3d orbital.

165. 	L. Ding, X. Xu, H. O. Jeschke, X. Bai, E. Feng, A. S. Alemayehu, J. Kim, F. Huang, Q. Zhang, X. Ding, N. Harrison, V. Zapf, D. Khomskii, I. I. Mazin, S.-W. Cheong, H. Cao
     	Field-tunable toroidal moment in a chiral-lattice magnet
     	Nature Commun. 12, 5339 (2021) (doi,pdf,supplement), (abstract).(hide abstract). A toroidal dipole moment appears independent of the electric and magnetic dipole moment in the multipole expansion of electrodynamics. It arises naturally from vortex-like arrangements of spins. Observing and controlling spontaneous long-range orders of toroidal moments are highly promising for spintronics but remain challenging. Here we demonstrate that a vortex-like spin configuration with a staggered arrangement of toroidal moments, a ferritoroidal state, is realized in a chiral triangular-lattice magnet BaCoSiO4. Upon applying a magnetic field, we observe multi-stair toroidal transitions correlating directly with metamagnetic transitions. We establish a first-principles microscopic Hamiltonian that explains both the formation of toroidal states and the metamagnetic toroidal transition as a combined effect of the magnetic frustration and the Dzyaloshinskii-Moriya interactions allowed by the crystallographic chirality in BaCoSiO4.

164. 	K. Ma, R. Lefèvre, K. Gornicka, H. O. Jeschke, X. Zhang, Z. Guguchia, T. Klimczuk, F. O. von Rohr
     	Group-9 Transition-Metal Suboxides Adopting the Filled-Ti2Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields
     	Chem. Mater. 33, 8722 (2021) (doi), (abstract).(hide abstract). Ti2Ni and the related η-carbide structure are known to exhibit various intriguing physical properties. The Ti2Ni structure with the cubic space group Fd3̅m is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related η-carbide compounds correspond to a filled version of the Ti2Ni structure. Here, we report on the structure and superconductivity in the η-carbide-type suboxides Ti4M2O with M = Co, Rh, and Ir. We have successfully synthesized all three compounds in the single-phase form. We found all three compounds to be type-II bulk superconductors with transition temperatures of Tc = 2.7, 2.8, and 5.4 K and with normalized specific heat jumps of ΔC/γTc = 1.65, 1.28, and 1.80 for Ti4Co2O, Ti4Rh2O, and Ti4Ir2O, respectively. We found that all three superconductors exhibit high upper critical fields. Particularly noteworthy in this regard is Ti4Ir2O with an upper critical field of μ0Hc2(0) = 16.06 T, which exceeds by far the weak-coupling Pauli limit─widely considered as the maximal upper critical field─of μ0HPauli = 9.86 T. The role of the void-filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti2Co. In contrast to the metallic η-carbide-type suboxides Ti4M2O, we found that Ti2Co displays a semimetallic behavior down to 0.75 K. Below 0.75 K, we observe a broad decrease in the resistivity, which can most likely be attributed to an onset of a superconducting transition at lower temperatures. Hence, the octahedral void-filling oxygen plays a crucial role in the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron–acceptor atoms may in the Ti2Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.

163. 	K. Riedl, E. Gati, D. Zielke, S. Hartmann, O. M. Vyaselev, N. D. Kushch, H. O. Jeschke, M. Lang, R. Valenti, M. V. Kartsovnik, S. M. Winter
     	Spin vortex crystal order in organic triangular lattice compound
     	Phys. Rev. Lett. 127, 147204 (2021) (doi,pdf,supplement), (abstract).(hide abstract). Organic salts represent an ideal experimental playground for studying the interplay between magnetic and charge degrees of freedom, which has culminated in the discovery of several spin-liquid candidates, such as κ-(ET)2Cu2(CN)3 (κ-Cu). Recent theoretical studies indicate the possibility of chiral spin liquids stabilized by ring-exchange, but the parent states with chiral magnetic order have not been observed in this material family. In this work, we discuss the properties of the recently synthesized κ-(BETS)2Mn[N(CN)2]3 (κ-Mn). Based on analysis of specific heat, magnetic torque, and NMR measurements combined with ab initio calculations, we identify a spin-vortex crystal order. These observations definitively confirm the importance of ring-exchange in these materials, and support the proposed chiral spin-liquid scenario for triangular lattice organics.

162. 	L. Heinze, H. O. Jeschke, I. I. Mazin, A. Metavitsiadis, M. Reehuis, R. Feyerherm, J.-U. Hoffmann, M. Bartkowiak, O. Prokhnenko, A. U. B. Wolter, X. Ding, V. Zapf, C. C. Moya, F. Weickert, M. Jaime, K. C. Rule, D. Menzel, R. Valentí, W. Brenig, S. Süllow
     	Magnetization process of atacamite: a case of weakly coupled S=1/2 sawtooth chains
     	Phys. Rev. Lett. 126, 207201 (2021) (doi,pdf,supplement), (abstract).(hide abstract). We present a combined experimental and theoretical study of the mineral atacamite Cu2Cl(OH)3. Density functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateau-like magnetization at about Msat/2. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

161. 	K.-Y. Ma, K. Gornicka, R. Lefevre, Y. Yang, H. M. Ronnow, H. O. Jeschke, T. Klimczuk, F. O. von Rohr
     	Superconductivity with high upper critical field in the cubic centrosymmetric η-carbide Nb4Rh2C1-η
     	ACS Materials Au 1, 55 (2021) (doi,pdf,supplement), (abstract).(hide abstract). The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as μ0HPauli ≈ 1.86[T/K] · Tc for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy-fermionic and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbide-type compound Nb4Rh2C1-η, with a normalized specific heat jump of ΔC=γTc = 1.64. We find that this material has a remarkably high upper critical field of μ0Hc2(0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of μ0HPauli = 18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.

160. 	L. Chen, A. Ide, H. O. Jeschke, K. Kobayashi
     	Hole doping and chemical pressure effects on the strongcoupling superconductor PdTe
     	Phys. Chem. Chem. Phys. 23, 13331 (2021) (doi), (abstract).(hide abstract). Chemical doping of known superconductors is a probate strategy to test and enhance our understanding which parameters control the critical temperature Tc and the critical magnetic fields. The transition metal chalcogenide PdTe is considered a conventional type II superconductor but its resilience to magnetic Fe doping is noteworthy. Isoelectronic Ni doping has been done, but the effect of doping charges into PdTe is so far unexplored. We follow two strategies to introduce holes into PdTe and to exert chemical pressure on it: By pnictogen doping on the chalcogen site PdTe1−xSbx and by systematically introducing a Pd deficiency Pd1−yTe. We find that the superconducting Tc is very sensitive to both kinds of doping. We employ density functional theory to rationalize the observations. We conclude that in PdTe, the effects of charge doping take the lead but we can also identify a structural parameter that correlates with Tc.

      2020 

159. 	S. Chillal, Y. Iqbal, H. O. Jeschke, J. A. Rodriguez-Rivera, R. Bewley, P. Manuel, D. Khalyavin, P. Steffens, R. Thomale, A. T. M. N. Islam, J. Reuther, B. Lake
     	Evidence for a three-dimensional quantum spin liquid in PbCuTe2O6
     	Nature Commun. 11, 2348 (2020) (doi,pdf,supplement), (abstract).(hide abstract). The quantum spin liquid is a highly entangled magnetic state characterized by the absence of static magnetism in its ground state. Instead, the spins fluctuate in a highly correlated way down to the lowest temperatures. Quantum spin liquids are very rare and are confined to a few specific cases where the interactions between the magnetic ions cannot be simultaneously satisfied (known as frustration). Lattices with magnetic ions in triangular or tetrahedral arrangements, which interact via isotropic antiferromagnetic interactions, can generate such a frustration. Three-dimensional isotropic spin liquids have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we present a three-dimensional lattice called the hyper-hyperkagome that enables spin liquid behaviour and manifests in the compound PbCuTe2O6. Using a combination of experiment and theory, we show that this system exhibits signs of being a quantum spin liquid with no detectable static magnetism together with the presence of diffuse continua in the magnetic spectrum suggestive of fractional spinon excitations.

158. 	H. Zhang, Z. Zhao, D. Gautreau, M. Raczkowski, A. Saha, V. O. Garlea, H. Cao, T. Hong, H. O. Jeschke, Subhendra D. Mahanti, T. Birol, F. F. Assaad, X. Ke
     	Coexistence and interaction of spinons and magnons in an antiferromagnet with alternating antiferromagnetic and ferromagnetic quantum spin chains
     	Phys. Rev. Lett. 125, 037204 (2020) (doi,pdf,supplement), (abstract).(hide abstract). In conventional magnets with magnetic long range order (LRO), low-energy excitations are carried by spin waves, represented by massless bosons called magnons with S = 1. However, in one-dimensional (1D) antiferromagnetic quantum spin systems, quantum fluctuations destroy LRO. Their low-energy excitations are spinons, a fractionalized fermion with S = 1/2, instead of magnons. In quasi-1D antiferromagnets with quantum spins, magnetic excitations are carried by either magnons or spinons in different energy regimes: they do not coexist independently, nor could they interact with each other. Here, by combining inelastic neutron scattering, quantum Monte Carlo simulations and Random Phase Approximation calculations, we report the discovery and discuss the physics of the coexistence of magnons and spinons and their interactions in Botallackite-Cu2(OH)3Br. This is a unique quantum antiferromagnet consisting of alternating ferromagnetic and antiferromagnetic Spin-1/2 chains with weak inter-chain couplings. Our study presents a new paradigm where one can study the interaction between two different types of magnetic quasiparticles, magnons and spinons.

157. 	N. D. Kushch, V. A. Kopotkov, G. V. Shilov, A. V. Akimov, S. V.Tokarev, E. B. Yagubskii, V. N. Zverev, S. S. Khasanov, S. M. Winter, H. O. Jeschke
     	Radical cation salts of BETS and ET with dicyanamidocuprate anions demonstrating metal-insulator and semiconductor – semiconductor transitions
     	Polyhedron, 189, 114705 (2020) (doi), (abstract).(hide abstract). Electrocrystallization of bis(ethylenedithio)tetraselenafulvalene (BETS) and bis(ethylenedithio)tetrathiafulvalene (ET) in the presence of an electrolyte (Ph4P)[Cu(dca)]3·H2O [(dca) = N(CN)2] has been studied using different solvents. A new organic metal (BETS)2Cu(dca)3 (1), and the first radical cation salt with 3D dicyanamidocuprate anion, incorporating both diamagnetic Cu1+ and paramagnetic Cu2+ atoms, (ET)2Cu1.8(dca)4 (2) have been obtained. Crystal structure, conducting properties of both salts, as well as the electronic structure of 2 and its structural analog (ET)2CuMn(dca)4 (3) were analysed. The salts 1 and 2 have layered structures. In contrast to BETS radical cation salt 1, complex 2 is characterized by the presence of a 3D polymeric anion built into the ET radical cation layers. At ambient pressure, the resistance of the crystals 1 shows semimetal behaviour down to 30 K. Below 30 K, the resistance sharply increases and the sample becomes insulating. The application of a pressure of about 3 kbar suppresses the metal-insulator transition. The salt 2 undergoes a semiconductor I - semiconductor II phase transition, which is manifested by a jump in the resistance and a hysteresis in the R(T) curves in a very large temperature range (185 – 250 K). The phase transition is accompanied by changes in the charge state of the ET molecules, and notable structure changes in the anion layer, especially in the environment of Cu2+. For the salt 3 the calculated selfconsistent charges show upon cooling a sharp increase of charge order between 291 K and 285 K.

156. 	K. Iida, H. K. Yoshida, A. Nakao, H. O. Jeschke, Y. Iqbal, K. Nakajima, S. Ohira-Kawamura, K. Munakata, Y. Inamura, N. Murai, M. Ishikado, R. Kumai, T. Okada, M. Oda, K. Kakurai, M. Matsuda
     	q=0 long-range magnetic order in centennialite CaCu3(OD)6Cl2⋅0.6D2O: A spin-1/2 perfect kagome antiferromagnet with J1-J2-Jd
     	Phys. Rev. B 101, 220408(R) (2020) (doi,pdf,supplement), (abstract).(hide abstract). Crystal and magnetic structures of the mineral centennialite CaCu3(OH)6Cl2⋅0.6H2O are investigated by means of synchrotron X-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu3(OH)6Cl2⋅0.6H2O crystallizes in the P-3m1 space group and Cu2+ ions form a geometrically perfect kagome network with antiferromagnetic J1. No intersite disorder between Cu2+ and Ca2+ is detected. CaCu3(OH)6Cl2⋅0.6H2O enters the magnetic long-range ordered state below TN=7.2 K, and the q=0 magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3 K is suppressed to be 0.58(2)μB. Our DFT calculations indicate the presence of antiferromagnetic J2 and ferromagnetic Jd superexchange couplings of a strength which place the system at the crossroads of three magnetic orders (at the classical level) and a spin-1/2 PFFRG analysis shows a dominance of q=0 type magnetic correlations, consistent with and indicating proximity to the observed q=0 spin structure. The results suggest that this material is located close to a quantum critical point and the first compound of J1-J2-Jd kagome antiferromagnet.

155. 	M. Shimizu, N. Takemori, D. Guterding, H. O. Jeschke
     	Importance of the Fermi surface and magnetic interactions for the superconducting dome in electron doped FeSe intercalates
     	Phys. Rev. B 101, 180511(R) (2020) (doi,pdf,supplement), (abstract).(hide abstract). The van-der-Waals gap of iron chalcogenide superconductors can be intercalated with a variety of inorganic and organic compounds that modify the electron doping level of the iron layers. In Lix(C3N2H10)0.37FeSe, a dome in the superconducting transition temperature Tc has been reported to occur in the doping range of x=0.06 to x=0.68. We use a combination of density functional theory and spin fluctuation theory to capture the evolution of superconducting transition temperatures theoretically. We clearly demonstrate how the changing electronic structure supports an increasing superconducting Tc. The suppression of Tc at high doping levels can, however, only be understood by analyzing the magnetic tendencies, which evolve from stripe-type at low doping to bicollinear at high doping.

154. 	H. Yamaoka, H. O. Jeschke, X. Yang, T. He, H. Goto, N. Hiraoka, H. Ishii, J. Mizuki, Y. Kubozono
     	Electronic structures of Bi2Se3 and AgxBi2Se3 under pressure studied by high-resolution x-ray absorption spectroscopy and density functional theory calculations
     	Phys. Rev. B 102, 155118 (2020) (doi,pdf,supplement), (abstract).(hide abstract). The pressure-induced change in the electronic structures of the superconductors Bi2Se3 and AgxBi2Se3 has been measured with high-resolution x-ray absorption spectroscopy. As common feature for these compounds, we find that pressure causes the broadening of the Se 4p band and an energy shift of the Bi 6s band above the Fermi level up to the pressure of the first structural transition. These results, corroborated by density functional theory calculations, correlate with an increase of the carrier density, the disappearance of the band gap, and the emergence of superconductivity. The electronic structure changes significantly at the pressure of the first structural transition which may be a trigger of the emergence of superconductivity, while above the pressure of the first phase transition it does not change much even around the second phase transition pressure, corresponding to the nearly constant Tc above the pressure of the second structural transition.

153. 	Y. Naijo, K. Hada, T. Furukawa, T. Itou, T. Ueno, K. Kobayashi, I. I. Mazin, H. O. Jeschke, and J. Akimitsu
     	Unusual non-valence-skipping electronic state of Sn in AgSnSe2
     	Phys. Rev. B 101, 075134 (2020) (doi,pdf), (abstract).(hide abstract). AgSnSe2, by formal electron count, should have Sn in a highly unusual 3+ valence state, and was therefore suggested to be a valence-skipping compound with potential for negative-U centers and local electron pairing. It has been proposed that the latter may be the mechanism beyond seemingly conventional superconductivity in this compound. We report NMR measurements and first principles calculation that agree with each other perfectly, and both indicate that valence skipping does not take place and the highly unusual Sn3+ state is realized instead, likely because of geometrical constraint prohibiting a breathing distortion that could screen the on-site Coulomb repulsion.

152. 	I. Olejniczak, R. Wesolowski, H. O. Jeschke, R. Valentí, B. Barszcz, J. A. Schlueter
     	Charge ordering and low temperature lattice distortion in the β′-(BEDT-TTF)2CF3CF2SO3 dimer Mott insulator
     	Phys. Rev. B 101, 035150 (2020) (doi,pdf,supplement), (abstract).(hide abstract). We present single-crystal X-ray diffraction measurements, optical investigations and electronic structure calculations for the organic charge-transfer salt β′-(BEDT-TTF)2CF3CF2SO3 synthesized by electrocrystallization. Electronic structure calculations confirm the quasi-one-dimensional behavior of the compound and optical conductivity measurements reveal the dimer-Mott insulating nature of the system. The splitting of the charge-sensitive ν2 mode in Raman spectra demonstrates the onset of an interlayer charge-ordered phase below 25 K, also suggested by the crystal structure considerations. This transition is accompanied by clear signatures of a lattice distortion in the BEDT-TTF donor layer, as shown by a splitting of the vibrational ν3 mode in infrared spectra. At the same time, the sharp redshift of the ν1 mode involving the BEDT-TTF ethylene groups strongly suggests a significant modification of the hydrogen-type bonding present between the BEDT-TTF donor layer and the CF3CF2SO3 anion layer. These observations point to a subtle interplay of charge and lattice degrees of freedom at the phase transition.

      2019 

151. 	I. I. Mazin, M. Shimizu, N. Takemori, H. O. Jeschke
     	Novel Fe-based superconductor LaFe2As2 in comparison with traditional pnictides
     	Phys. Rev. Lett. 123, 267001 (2019) (doi,pdf), (abstract).(hide abstract). The recently discovered Fe-based superconductor (FeBS) LaFe2As2 seems to break away froman established pattern that doping FeBS beyond 0.2e/Fe destroys superconductivity. LaFe2As2 has an apparent doping of 0.5e, yet superconducts at 12.1 K. Its Fermi surface bears no visual resemblance with the canonical FeBS Fermiology. It also exhibits two phases, none magnetic and only one superconducting. We show that the difference between them has nonetheless magnetic origin, the one featuring disordered moments, and the other locally nonmagnetic. We find that La there assumes an unusual valence of +2.6 to +2.7, so that the effective doping is reduced to 0.30-0.35e. A closer look reveals the same key elements: hole Fermi surfaces near Γ-Z and electron ones near the X-P lines, with the corresponding peak in susceptibility, and a strong tendency to stripe magnetism. The physics of LaFe2As2 is thus more similar to the FeBS paradigm than hitherto appreciated.

150. 	P. Ghosh, Y. Iqbal, T. Müller, R. T. Ponnaganti, R. Thomale, R. Narayanan, J. Reuther, M. J. P. Gingras, H. O. Jeschke
     	Breathing chromium spinels: a showcase for a variety of pyrochlore Heisenberg Hamiltonians
     	npj Quantum Materials 4, 63 (2019) (doi,pdf,supplement), (abstract).(hide abstract). We address the long-standing problem of the microscopic origin of the richly diverse phenomena in the chromium breathing pyrochlore material family. Combining electronic structure and renormalization group techniques we resolve the magnetic interactions and analyze their reciprocal-space susceptibility. We show that the physics of these materials is principally governed by long-range Heisenberg Hamiltonian interactions, a hitherto unappreciated fact. Our calculations uncover that in these isostructural compounds, the choice of chalcogen triggers a proximity of the materials to classical spin liquids featuring degenerate manifolds of wave-vectors of different dimensions: A Coulomb phase with three-dimensional degeneracy for LiInCr4O8 and LiGaCr4O8, a spiral spin liquid with two-dimensional degeneracy for CuInCr4Se8 and one-dimensional line degeneracies characteristic of the face-centered cubic antiferromagnet for LiInCr4S8, LiGaCr4S8 and CuInCr4S8. The surprisingly complex array of prototypical pyrochlore behaviors we discovered in chromium spinels may inspire studies of transition paths between different semi-classical spin liquids by doping or pressure.

149. 	Y. Feng, Q. Jiang, B. Feng, M. Yang, T. Xu, W. Liu, X. Yang, M. Arita, E. F. Schwier, K. Shimada, H. O. Jeschke, R. Thomale, Y. Shi, X. Wu, S. Xiao, S. Qiao, S. He
     	Rashba-like spin splitting along three momentum directions in trigonal layered PtBi2
     	Nature Commun. 10, 4765 (2019) (doi,pdf,supplement), (abstract).(hide abstract). Spin-orbit coupling (SOC) has gained much attention for its rich physical phenomena and highly promising applications in spintronic devices. The Rashba-type SOC in systems with inversion symmetry breaking is particularly attractive for spintronics applications since it allows for flexible manipulation of spin current by external electric fields. Here, we report the discovery of a giant anisotropic Rashba-like spin splitting along three momentum directions (3D Rashba-like spin splitting) with a helical spin polarization around the M points in the Brillouin zone of trigonal layered PtBi2. Due to its inversion asymmetry and reduced symmetry at the M point, Rashba-type as well as Dresselhaus-type SOC cooperatively yield a 3D spin splitting with αR ≈ 4.36 eV Å in PtBi2. The experimental realization of 3D Rashba-like spin splitting not only has fundamental interests but also paves the way to the future exploration of a new class of material with unprecedented functionalities for spintronics applications.

148. 	R. Shirakami, H. Ueda, H. O. Jeschke, H. Nakano, S. Kobayashi, A. Matsuo, T. Sakai, N. Katayama, H. Sawa, K. Kindo, C. Michioka, K. Yoshimura
     	Two magnetization plateaus in the kagome fluoride Cs2LiTi3F12
     	Phys. Rev. B 100, 174401 (2019) (doi,pdf), (abstract).(hide abstract). We synthesized a kagome fluoride Cs2LiTi3F12 with S = 1/2 spins, and studied magnetic properties of the compound. The temperature dependence of the magnetic susceptibility indicates that it has dominant antiferromagnetic interactions and that it has no magnetic order down to 2K. We found two magnetization plateaus in its magnetization process approximately at (1/3) μB and 0.8 μB per Ti. The monoclinic crystal structure gives four inequivalent nearest-neighbor exchange interactions. Our density functional theory calculations suggest that three of them are antiferromagnetic and one of them is weakly ferromagnetic, resulting in a magnetic system composed of antiferromagnetically coupled linear chains and Δ chains. This explains the observed suppression of magnetic order. Numerical diagonalization gives magnetization curve in good agreement with the experimental results.

147. 	T. He, X. Yang, T. Taguchi, T. Ueno, K. Kobayashi, J. Akimitsu, H. Yamaoka, H. Ishii, Y.-F. Liao, H. Ota, H. Goto, R. Eguchi, K. Terashima, T. Yokoya, H. O. Jeschke, X. Wu, Y. Kubozono
     	Pressure-induced superconductivity in Bi2-xSbxTe3-ySey
     	Phys. Rev. B 100, 094525 (2019) (doi,pdf,supplement), (abstract).(hide abstract). We systematically investigated the pressure-dependence of electric transport and the crystal structure of topological insulator, Bi2-xSbxTe3-ySey, which did not show superconductivity down to 2.0 K at ambient pressure; the crystal took a rhombohedral structure (space group No. 166, R-3m, termed phase-I). The Bi2-xSbxTe3-ySey crystal showed two structural phase transitions under pressure, from rhombohedral structure (phase-I) to monoclinic structure (space group No. 12, C2/m, termed phase-II), and from phase-II to another monoclinic structure (space group No. 12, C2/m, termed phase-III). Furthermore, an additional monoclinic phase (space group No. 15, C2/c, termed phase-II’) was present with phase-II and phase-III, in Bi2-xSbxTe3-ySey at x ≠ 0. Interestingly, the pressure causing the first and second structural phase transitions gradually increased with increasing x (amount of Sb). Superconductivity appeared when applying pressure; actually the superconductivity of all Bi2-xSbxTe3-ySey samples emerged in phase-I, and the onset pressure for emergence of superconductivity seems to be a little high, at x = 0.5. The superconducting transition temperature, Tc, increased against pressure in a pressure range of 0 – 15 GPa for all Bi2-xSbxTe3-ySey samples, and the maximum Tc was 5.45 K, recorded at 13.5 GPa in Bi2-xSbxTe3-ySey at x = 0 and y = 1.0. The magnetic field (H) dependence of R - T plot for Bi2-xSbxTe3-ySey was measured at some pressures，suggesting a possible p-wave superconducting pairing.

146. 	M. Saito, M. Watanabe, N. Kurita, A. Matsuo, K. Kindo, M. Avdeev, H. O. Jeschke, H. Tanaka
     	Successive phase transitions and quantum magnetization plateau in the spin-1 triangular-lattice antiferromagnet Ba2La2NiTe2O12 with small easy-axis anisotropy
     	Phys. Rev. B 100, 064417 (2019) (doi,pdf), (abstract).(hide abstract). The crystal structure and quantum magnetic properties of the spin-1 triangular-lattice antiferromagnet Ba2La2NiTe2O12 are reported. Its crystal structure is trigonal R-3, which is the same as that of Ba2La2NiW2O12 [Y. Doi et al., J. Phys.: Condens. Matter 29, 365802 (2017)]. However, the exchange interaction J/kB≃19 K is much greater than that observed in the tungsten system. At zero magnetic field, Ba2La2NiTe2O12 undergoes successive magnetic phase transitions at TN1=9.8 K and TN2=8.8 K. The ground state is accompanied by a weak ferromagnetic moment. These results indicate that the ground-state spin structure is a triangular structure in a plane perpendicular to the triangular lattice owing to the small easy-axis-type anisotropy. The magnetization curve exhibits the one-third plateau characteristic of a triangular-lattice quantum antiferromagnet. Exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate the large difference in the exchange constants between tellurium and tungsten systems and the good two-dimensionality of the tellurium system.

145. 	P. Naumov, S. Huangfu, X. Wu, A. Schilling, R. Thomale, C. Felser, S. Medvedev, H. O. Jeschke, F. O. von Rohr
     	Large Resistivity Reduction in Mixed-Valent CsAuBr3 Under Pressure
     	Phys. Rev. B 100, 155113 (2019) (doi,pdf,supplement), (abstract).(hide abstract). We report on high-pressure p≤45 GPa resistivity measurements on the perovskite-related mixed-valent compound CsAuBr3. The compounds high-pressure resistivity can be classified into three regions: For low pressures (p <10 GPa) an insulator to metal transition is observed; between p= 10 GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; above p= 14 GPa a semiconducting state is observed. From this pressure up to the highest pressure of p= 45 GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than 6 orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show - by an analysis of the electronic band structure evolution of this material - that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving towards one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.

144. 	H. O. Jeschke, H. Nakano, T. Sakai
     	From kagome strip to kagome lattice: Realizations of frustrated S=1/2 antiferromagnets in Ti(III) fluorides
     	Phys. Rev. B 99, 140410(R) (2019) (doi,pdf), (abstract).(hide abstract). We investigate the connection between highly frustrated kagome based Hamiltonians and a recently synthesized family of materials containing Ti3+ S=1/2 ions. Employing a combination of all electron density functional theory and numerical diagonalization techniques, we establish the Heisenberg Hamiltonians for the distorted kagome antiferromagnets Rb2NaTi3F12, Cs2NaTi3F12 and Cs2KTi3F12. We determine magnetization curves in excellent agreement with experimental observations. Our calculations successfully clarify the relationship between the experimental observations and the magnetization-plateau behavior at 1/3 height of the saturation and predict characteristic behaviors under fields that are higher than the experimentally measured region. We demonstrate that the studied Ti(III) family of materials interpolates between kagome strip and kagome lattice.

143. 	Y. Iqbal, T. Müller, P. Ghosh, M. J. P. Gingras, H. O. Jeschke, S. Rachel, J. Reuther, R. Thomale
     	Quantum and classical phases of the pyrochlore Heisenberg model with competing interactions
     	Phys. Rev. X 9, 011005 (2019) (doi,pdf), (abstract).(hide abstract). We investigate the quantum Heisenberg model on the pyrochlore lattice for a generic spin-S in the presence of nearest-neighbor J1 and second-nearest-neighbor J2 exchange interactions. By employing the pseudofermion functional renormalization group (PFFRG) method, we find, for S = 1/2 and S = 1, an extended quantum spin liquid phase centered around J2 = 0, which is shown to be robust against the introduction of breathing anisotropy. The effects of temperature, quantum fluctuations, breathing anisotropies, and a J2 coupling on the nature of the scattering profile, and the pinch points, in particular, are studied. For the magnetic phases of the J1-J2 model, quantum fluctuations are shown to strongly renormalize phase boundaries compared to the classical model and shift the ordering wave vectors of spiral magnetic states, however, no new magnetic orders are found to be stabilized.

142. 	J. C. Leiner, H. O. Jeschke, R. Valentí, S. Zhang, A. T. Savici, J. Lin, M. B. Stone, M. D. Lumsden, J. Hong, O. Delaire, W. Bao, C. L. Broholm
     	Frustrated magnetism in Mott insulating (V1−xCrx)2O3
     	Phys. Rev. X 9, 011035 (2019) (doi,pdf), (abstract).(hide abstract). V2O3 famously features all four combinations of paramagnetic versus antiferromagnetic and metallic versus insulating states of matter in response to percent-level doping, pressure in the GPa range, and temperature below 300 K. Using time-of-flight neutron spectroscopy combined with density functional theory calculations of magnetic interactions, we have mapped and analyzed the inelastic magnetic neutron scattering cross section over a wide range of energy and momentum transfer in the chromium-stabilized antiferromagnetic and paramagnetic insulating phases. Our results reveal an important magnetic frustration and degeneracy of the paramagnetic insulating phase which is relieved by the rhombohedral-to-monoclinic transition at TN=185 K. This leads to the recognition that magnetic frustration is an inherent property of the paramagnetic phase in (V1−xCrx)2O3 and plays a key role in suppressing the magnetic long-range-ordering temperature and exposing a large phase space for the paramagnetic Mott metal-insulator transition to occur.

      2018 

141. 	M. Shimizu, N. Takemori, D. Guterding, H. O. Jeschke
     	Two-dome superconductivity in FeS induced by a Lifshitz transition
     	Phys. Rev. Lett. 121, 137001 (2018) (doi,pdf,supplement), (abstract).(hide abstract). Among iron chalcogenide superconductors, FeS can be viewed as a simple, highly compressed relative of FeSe without nematic phase and with weaker electronic correlations. Under pressure, however, the superconductivity of stoichiometric FeS disappears and reappears, forming two domes. We perform electronic structure and spin fluctuation theory calculations for tetragonal FeS in order to analyze the nature of the superconducting order parameter. In random phase approximation we find a gap function with d-wave symmetry at ambient pressure, in agreement with several reports of a nodal superconducting order parameter in FeS. Our calculations show that as function of pressure, the superconducting pairing strength decreases until a Lifshitz transition takes place at 4.6 GPa. As a hole pocket with large density of states appears at the Lifshitz transition, the gap symmetry is altered to sign-changing s-wave. At the same time the pairing strength is severely enhanced and increases up to a new maximum at 5.5 GPa. Therefore, our calculations naturally explain the occurrence of two superconducting domes in FeS.

140. 	E. Gati, J. K. H. Fischer, P. Lunkenheimer, D. Zielke, S. Köhler, F. Kolb, H.-A. Krug von Nidda, S. M. Winter, H. Schubert, J. A. Schlueter, H. O. Jeschke, R. Valentí, M. Lang
     	Evidence for electronically-driven ferroelectricity in the family of strongly correlated dimerized BEDT-TTF molecular conductors
     	Phys. Rev. Lett. 120, 247601 (2018) (doi,pdf,supplement), (abstract).(hide abstract). By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ-(BEDT-TTF)2Hg(SCN)2Cl, we provide evidence for order-disorder type electronic ferroelectricity which is driven by charge order within the (BEDT-TTF)2 dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and non- or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intra-dimer charge degrees of freedom are of particular importance in correlated κ-(BEDT-TTF)2X salts and can create novel states, such as electronically-driven multiferroicity or charge-order-induced quasi-1D spin liquids.

139. 	Y. Iqbal, T. Müller, H. O. Jeschke, R. Thomale, J. Reuther
     	Stability of the spiral spin liquid in MnSc2S4
     	Phys. Rev. B 98, 064427 (2018) (doi,pdf), (abstract).(hide abstract). We investigate the stability of the spiral spin-liquid phase in MnSc2S4 against thermal and quantum fluctuations as well as against perturbing effects of longer-range interactions. Employing ab initio DFT calculations we propose a realistic Hamiltonian for MnSc2S4, featuring second (J2) and third (J3) neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong J2 and J3 couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spin-structure factor within the pseudofermion functional-renormalization group technique we find that close to the magnetic phase transition the sizeable J3 couplings induce a strong spiral selection effect, in agreement with experiments. With increasing temperature the spiral selection becomes weaker such that around three times the ordering temperature an approximate spiral spin-liquid is realized in MnSc2S4.

138. 	K. Kobayashi, Y. Ai, H. O. Jeschke, J. Akimitsu
     	Enhanced superconducting transition temperatures in the rocksalt-type superconductors In1-xSnxTe (x≤0.5)
     	Phys. Rev. B 97, 104511 (2018) (doi,pdf), (abstract).(hide abstract). We investigate superconductivity in In1-xSnxTe (x≤0.5) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature Tc for increasing tin concentration x. These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with x, Tc saturates around x=0.4. Electronic structure calculations indicate that the Tc modulation is brought on by the change of the density of states in the vicinity of the Fermi energy [N(EF)]. However, differences between the calculated N(EF) and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

137. 	X. Wu, H. O. Jeschke, D. Di Sante, F. O. von Rohr, R. J. Cava, R. Thomale
     	Origin of the pressure-dependent Tc valley in superconducting simple cubic phosphorus
     	Phys. Rev. Mater. 2, 034802 (2018) (doi,pdf), (abstract).(hide abstract). Motivated by recent experiments, we investigate the pressure-dependent electronic structure and electron-phonon (e-ph) coupling for simple cubic phosphorus by performing first-principle calculations within the full potential linearized augmented plane wave method. As a function of increasing pressure, our calculations show a valley feature in Tc, followed by an eventual decrease for higher pressures. We demonstrate that this Tc valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the band-resolved contributions to the e-ph coupling. Below the first Lifshitz transition, the phonon hardening and shrinking of the γ Fermi surface with s orbital character results in a decreased Tc with increasing pressure. After the second Lifshitz transition, the appearance of δ Fermi surfaces with 3d orbital character generate strong e-ph inter-band couplings in αδ and βδ channels, and hence lead to an increase of Tc. For higher pressures, the phonon hardening finally dominates, and Tc decreases again. Our study reveals that the intriguing Tc valley discovered in experiment can be attributed to Lifshitz transitions, while the plateau of Tc detected at intermediate pressures appears to be beyond the scope of our analysis. This strongly suggests that besides e-ph coupling, electronic correlations along with plasmonic contributions may be relevant for simple cubic phosphorous. Our findings hint at the notion that increasing pressure can shift the low-energy orbital weight towards d character, and as such even trigger an enhanced importance of orbital-selective electronic correlations despite an increase of the overall bandwidth.

136. 	D. Guterding, H. O. Jeschke
     	An efficient GPU algorithm for tetrahedron-based Brillouin-zone integration
     	Comp. Phys. Commun. 231, 114 (2018) (doi,preprint), (abstract).(hide abstract). We report an efficient algorithm for calculating momentum-space integrals in solid state systems on modern graphics processing units (GPUs). We extend the tetrahedron method by Blöchl et al. to the more general case of the integration of a momentum as well as energy dependent quantity and implement the algorithm based on the CUDA programming framework. We test this method by applying it to a simple example, the calculation of the orbital-resolved density of states. We benchmark our code on the problem of calculating the orbital-resolved density of states in an iron-based superconductor and discuss the design choices made in the implementation. Our algorithm delivers large speedups of up to a factor ∼165 also for moderately sized workloads compared to standard algorithms executed on central processing units (CPUs).

135. 	M. Souto, M. C. Gullo, H.-B. Cui, N. Casati, F. Montisci, H. O. Jeschke, R. Valentí, I. Ratera, C. Rovira, J. Veciana
     	Role of the open-shell character in the pressure-induced conductivity in an organic radical D-A dyad
     	Chem. Eur. J. 24, 5500 (2018) (doi) (abstract).(hide abstract). Single-component conductors based on neutral organic radicals have received a lot of attention due to the possibility that the unpaired electron can serve as a charge carrier without the need of a previous doping process. Although most of these systems are based on delocalised planar radicals, we reported here a nonplanar and spin localised radical based on a tetrathiafulvalene (TTF) moiety, linked to a perchlorotriphenylmethyl (PTM) radical by a conjugated brige, which exhibits a semiconducting behavior upon application of high pressure. The synthesis, electronic properties and crystal structure of this neutral radical TTF-PTM derivative (1) is reported and implications of its crystalline structure on its electrical properties dicussed. On the other hand, the non-radical derivative 2, which is isostructural with the radical 1, shows an insulating behavior at all measured pressures. The different electronic structures of these two isostructural systems have a direct influence on the conducting properties as demonstrated by band structure DFT calculations.

134. 	M. Kaliappan, H. O. Jeschke, R. Valentí
     	Dynamics and fragmentation mechanism of (CH3-C5H5)Pt(CH3)3 on SiO2 Surfaces
     	Beilstein J. Nanotechnol. 9, 711 (2018) (doi,pdf), (abstract).(hide abstract). The interaction of CH3-C5H5)Pt(CH3)3 (trimethyl methylcyclopentadienyl platinum) molecules on the fully and partially hydroxylated SiO2 surfaces and their dynamics were investigated using density functional theory (DFT) calculations to explain the initial reactions occurring during the electron beam induced deposition process of Pt. Partially hydroxylated surfaces that contain exposed surface Si atoms are found to activate the precursor. Our DFT results illustrate that the fragmentation of CH3-C5H5)Pt(CH3)3 are orientation dependent and might occur either with the release of a methyl group or the cyclopentadienyl ring. To favor the assistance of the methyl group, the presence of another active site on the SiO2 surface is necessary and the fragmented methyl groups are bound to the surface active site. Our molecular dynamics simulations furthermore illustrate, that the separation of the methyl-cyclopentadienyl ring from Pt might be the source for organic contamination in initial layers of the deposits. A detailed mechanistic pathway has been computed for the dissociation of the CH3-C5H5)Pt(CH3)3 precursor on SiO2 surfaces, which illustrates that oxidized Pt is formed on the surface, prior to metal deposition.

      2017 

133. 	P. Schütz, D. V. Christensen, V. Borisov, F. Pfaff, P. Scheiderer, L. Dudy, M. Zapf, J. Gabel, Y. Z. Chen, N. Pryds, V. A. Rogalev, V. N. Strocov, C. Schlueter, T.-L. Lee, H. O. Jeschke, R. Valentí, M. Sing, R. Claessen
     	Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy
     	Phys. Rev. B 96, 161409(RC) (2017) (doi,pdf,supplement), (abstract).(hide abstract). The spinel/perovskite heterointerface γ-Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

132. 	D. Guterding, H. O. Jeschke, R. Valentí
     	Basic electronic properties of iron selenide under variation of structural parameters
     	Phys. Rev. B 96, 125107 (2017) (doi,pdf), (abstract).(hide abstract). Since the discovery of high-temperature superconductivity in the thin-film FeSe/SrTiO3 system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron selenide if the structural parameters could be tuned at liberty. We calculate the momentum dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a nontrivial way. These results are consistent with the known experimental behavior of binary iron chalcogenides and, at the same time, reveal two promising ways of tuning superconducting transition temperatures in these materials: on one hand by expanding the iron lattice of FeSe at constant iron-selenium distance and, on the other hand, by increasing the iron-selenium distance with unchanged iron lattice.

131. 	Y. Iqbal, T. Müller, K. Riedl, J. Reuther, S. Rachel, R. Valentí, M. J. P. Gingras, R. Thomale, H. O. Jeschke
     	Signatures of a gearwheel quantum spin liquid in a spin-½ pyrochlore molybdate Heisenberg antiferromagnet
     	Phys. Rev. Mater. 1, 071201(R) (2017) (doi,pdf,supplement), (abstract).(hide abstract). We theoretically investigate the low-temperature phase of the recently synthesized Lu2Mo2O5N2 material, an extraordinarily rare realization of a S=1/2 three-dimensional pyrochlore Heisenberg antiferromagnet in which Mo5+ are the S=1/2 magnetic species. Despite a Curie-Weiss temperature (ΘCW) of −121(1) K, experiments have found no signature of magnetic ordering or spin-freezing down to T*≈0.5 K. Using density functional theory, we find that the compound is well described by a Heisenberg model with exchange parameters up to third nearest-neighbors. The analysis of this model via the pseudofermion functional renormalization group method reveals paramagnetic behavior down to a temperature of at least T=|ΘCW|/100, in agreement with the experimental findings hinting at a possible three-dimensional quantum spin liquid. The spin susceptibility profile in reciprocal space shows momentum-dependent features forming a "gearwheel" pattern, characterizing what may be viewed as a molten version of a chiral non-coplanar incommensurate spiral order under the action of quantum fluctuations. Our calculated reciprocal space susceptibility maps provide benchmarks for future neutron scattering experiments on single crystals of Lu2Mo2O5N2.

130. 	J. Tapp, C. R. dela Cruz, M. Bratsch, N. E. Amuneke, L. Postulka, B. Wolf, M. Lang, H. O. Jeschke, R. Valentí, P. Lemmens, and A. Möller
     	From magnetic order to spin-liquid ground states on the S=3/2 triangular lattice
     	Phys. Rev. B 96, 064404 (2017) (doi,pdf,supplement), (abstract).(hide abstract). The series of compounds AAg2Cr[VO4]2, with A=Ag, K, or Rb, is layered S = 3/2 triangular-lattice (TL) systems in which the magnetic exchange interactions between Cr3+(3d3) ions are mediated by nonmagnetic [VO4]3- entities. Here, the relative orientation of the vanadate is altered with respect to the TL as a function of the A site, which corresponds to an induced symmetry change of the [CrO6] complex. All members of this series of compounds belong to the class of frustrated TL antiferromagnets. We find that the distorted TL (A=Ag) exhibits collinear antiferromagnetic long-range order (LRO) at TN≈10 K, whereas the high-symmetry cases (A=K, Rb) evade LRO in zero field down to 0.03 K, the lowest temperature of our experiments. The latter members of the series belong to the undistorted TL and are candidates for spin-liquid ground states presumably not related to Ising anisotropy or dimerization.

129. 	M. Sing, H. O. Jeschke, F. Lechermann, R. Valentí, R. Claessen
     	Influence of oxygen vacancies on two-dimensional electron systems at SrTiO3-based interfaces and surfaces
     	Eur. Phys. J. Special Topics 226, 2457 (2017) (doi), (abstract).(hide abstract). The insulator SrTiO3 can host high-mobility two-dimensional electron systems on its surfaces and at interfaces with other oxides. While for the bare surface a two-dimensional electron system can only be induced by oxygen vacancies, it is believed that the metallicity of heterostructure interfaces as in LaAlO3/SrTiO3 is caused by other mechanisms related to the polar discontinuity at the interface. Based on calculations using density functional and dynamical mean-field theory as well as on experimental results using photoemission spectroscopy we elucidate the role of oxygen vacancies, thereby highlighting their importance for the electronic and magnetic properties of the systems under study.

128. 	D. Guterding, H. O. Jeschke, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valentí
     	Non-trivial role of interlayer cation states in iron-based superconductors
     	Phys. Rev. Lett. 118, 017204 (2017) (doi,pdf,supplement), (abstract).(hide abstract). Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond-angles or -distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or dp model Hamiltonians only.

127. 	A. J. Kim, H. O. Jeschke, P. Werner, R. Valentí
     	J-freezing and Hund′s rules in spin-orbit-coupled multiorbital Hubbard models
     	Phys. Rev. Lett. 118, 086401 (2017) (doi,pdf,supplement), (abstract).(hide abstract). We investigate the phase diagram of the spin-orbit-coupled three orbital Hubbard model at arbitrary filling by means of dynamical mean-field theory combined with continuous-time quantum Monte Carlo. We find that the spin-freezing crossover occurring in the metallic phase of the non-relativistic multiorbital Hubbard model can be generalized to a J-freezing crossover, with J=L+S, in the spin-orbit-coupled case. In the J-frozen regime the correlated electrons exhibit a non-trivial flavor selectivity and energy dependence. Furthermore, in the regions near n=2 and n=4 the metallic states are qualitatively different from each other, which reflects the atomic Hund′s third rule. Finally, we explore the appearance of magnetic order from exciton condensation at n=4 and discuss the relevance of our results for real materials.

126. 	Y. Li, S. M. Winter, H. O. Jeschke, R. Valentí
     	Electronic excitations in γ-Li2IrO3
     	Phys. Rev. B 95, 045129 (2017) (doi,pdf), (abstract).(hide abstract). We investigate the electronic properties of the three-dimensional stripyhoneycomb γ-Li2IrO3 via relativistic density functional theory calculations as well as exact diagonalization of finite clusters and explore the details of the optical conductivity. Our analysis of this quantity reveals the microscopic origin of the experimentally observed (i) optical transitions and (ii) strong anisotropic behavior along the various polarization directions. We furthermore reanalyze within this approach the electronic excitations in the known two-dimensional honeycomb systems α-Li2IrO3 and Na2IrO3 and discuss the results in comparison to γ-Li2IrO3.

125. 	D. Guterding, S. Backes, M. Tomic, H. O. Jeschke, R. Valentí
     	Ab-initio perspective on structural and electronic properties of iron-based superconductors
     	Phys. Stat. Sol. B 254, 1600164 (2017) (doi,preprint,pdf), (abstract).(hide abstract). The discovery of iron pnictides and iron chalcogenides as a new class of unconventional superconductors in 2008 has generated an enourmous amount of experimental and theoretical work that identifies these materials as correlated metals with multiorbital physics, where magnetism, nematicity and superconductivity are competing phases that appear as a function of pressure and doping. A microscopic understanding of the appearance of these phases is crucial in order to determine the nature of superconductivity in these systems. Here we review our recent theoretical efforts to describe and understand from first principles the properties of iron pnictides and chalcogenides with special focus on (i) pressure dependence, (ii) effects of electronic correlation and (iii) origin of magnetism and superconductivity.

124. 	M. Knöner, E. Gati, B. Wolf, M. de Souza, A. Haghighirad, M. Kuhnt, W. Assmus, H. O. Jeschke, M. Tomic, R. Valentí, M. Lang
     	Combined experimental and theoretical studies of pressure effects in La2Sb
     	Phys. Stat. Sol. B 254, 1600168 (2017) (doi), (abstract).(hide abstract). The 5 K superconductor La2Sb shares some structural similarities with the iron-pnictides and -chalgogenides, especially the so-called 122-compounds, despite lacking a 3d-metal component. In order to look for similarities and dissimilarities of this layered metal pnictide with the higher-Tc Fe-based superconductors, we have performed a combined experimental and theoretical investigation of the effects of hydrostatic pressure. Magnetic measurements reveal a linear increase of Tc with pressure at a rate dTc/dP=(0.71±0.01)KGPa-1. Band  structure  calculations show that (i) the states at the Fermi level have predominantly La 5d character and (ii) that the density of states increases linearly with increasing pressure. By using these theoretical results and a BCS formalism, a quantitative description of the experimentally revealed pressure dependence of Tc is possible within a reasonable range of parameters. Our results indicate that La2Sb, despite its structural similarities to the 122 Fe-based superconductors, exhibits a BCS-type of superconductivity.

      2016 

123. 	S. Backes, T. C. Rödel, F. Fortuna, E. Frantzeskakis, Le Fèbre, F. Bertran, M. Kobayashi, R. Yukawa, T. Mitsuhashi, M. Kitamura, K. Horiba, H. Kumigashira, R. Saint-Martin, A. Fouchet, B. Berini, Y. Dumont, A. J. Kim, F. Lechermann, H. O. Jeschke, M. J. Rozenberg, R. Valentí, A. Santander-Syro
     	Hubbard band or oxygen vacancy states in the correlated electron metal SrVO3?
     	Phys. Rev. B 94, 241110(R) (2016) (doi,pdf,supplement), (abstract).(hide abstract). We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating d0 transition metal oxides.

122. 	A. Chernenkaya, A. Morherr, S. Backes, W. Popp, S. Witt, X. Kozina, S. A. Nepijko, M. Bolte, K. Medjanik, G. Öhrwall, C. Krellner, M. Baumgarten, H. J. Elmers, G. Schönhense, H. O. Jeschke, R. Valentí
     	Microscopic origin of the charge transfer in single crystals based on thiophene derivatives: A combined NEXAFS and density functional theory approach
     	J. Chem. Phys. 145, 034702 (2016) (doi,preprint), (abstract).(hide abstract). We have investigated the charge transfer mechanism in single crystals of DTBDT-TCNQ and DTBDT-F4TCNQ (where DTBDT is dithieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]dithiophene) using a combination of near-edge X-ray absorption spectroscopy (NEXAFS) and density functional theory calculations (DFT) including final state effects beyond the sudden state approximation. In particular, we find that a description that considers the partial screening of the electron-hole Coulomb correlation on a static level as well as the rearrangement of electronic density shows excellent agreement with experiment and allows to uncover the details of the charge transfer mechanism in DTBDT-TCNQ and DTBDT-F4TCNQ, as well as a reinterpretation of previous NEXAFS data on pure TCNQ. Finally, we further show that almost the same quality of agreement between theoretical results and experiment is obtained by the much faster Z+1/2 approximation, where the core hole effects are simulated by replacing N or F with atomic number Z with the neighboring atom with atomic number Z+1/2.

121. 	M. Souto, H.-B. Cui, M. Peña-Alvarez, V. García Baonza, H. O. Jeschke, M. Tomic, R. Valentí, D. Blasi, I. Ratera, C. Rovira, J. Veciana
     	Pressure-Induced Conductivity in a Neutral Non-planar Spin-localized Radical
     	J. Am. Chem. Soc. 138, 11517 (2016) (doi), (abstract).(hide abstract). There is a growing interest in the development of single-component molecular conductors based on neutral organic radicals that are mainly formed by delocalized planar radicals, such as phenalenyl or thiazolyl radicals. However, there are no examples of systems based on non-planar and spin-localized C-centered radicals exhibiting electrical conductivity due to their Mott insulator behavior with large Coulomb energy (U) repulsion and narrow electronic bandwidth (W). Here we present a new type of neutral radical conductor by linking a tetrathiafulvalene (TTF) unit to a neutral polychlorotriphenylmethyl radical (PTM) with the important feature that the TTF unit enhances the overlap between the radical units as a consequence of short intermolecular S···S interactions. This system becomes semiconducting upon the application of high pressure thanks to increased electronic bandwidth and charge reorganization opening the way to develop a new family of neutral radical conductors.

120. 	J. Zabaleta, V. S. Borisov, R. Wanke, H. O. Jeschke, S. C. Parks, B. Baum, A. Teker, T. Harada, K. Syassen, T. Kopp, N. Pavlenko, R. Valentí, J. Mannhart
     	Hydrostatic pressure response of an oxide two-dimensional electron system
     	Phys. Rev. B 93, 235117 (2016) (doi,pdf,supplement), (abstract).(hide abstract). Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO3-SrTiO3 interfaces and measure a pronounced, unexpected response. Pressure of ∼2 GPa reversibly doubles the 2D carrier density ns at 4 K. Along with the increase of ns, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure.

119. 	P. Puphal, D. Sheptyakov, N. van Well, L. Postulka, I. Heinmaa, F. Ritter, W. Assmus, B. Wolf, M. Lang, H. O. Jeschke, R. Valentí, R. Stern, Ch. Rüegg, C. Krellner
     	Stabilization of the tetragonal structure in (Ba1−xSrx)CuSi2O6
     	Phys. Rev. B 93, 174121 (2016) (doi,pdf), (abstract).(hide abstract). We present a structural analysis of the substituted system (Ba1−xSrx)CuSi2O6, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with low-temperature neutron and synchrotron powder diffraction, room-temperature, and cryogenic high-resolution NMR, as well as magnetic- and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x=0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x=0.1 and magnetic measurements prove that the Cu dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einstein-condensation (BEC) of triplons, within the tetragonal crystal structure of I41/acd. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

118. 	H. Lee, H. O. Jeschke, R. Valentí
     	Competition between disorder and Coulomb interaction in a two-dimensional plaquette Hubbard model
     	Phys. Rev. B 93, 224203 (2016) (doi,pdf), (abstract).(hide abstract). We have studied a disordered Nc×Nc plaquette Hubbard model on a two-dimensional square lattice at half-filling using a coherent potential approximation (CPA) in combination with a single-site dynamical mean field theory (DMFT) approach with a paramagnetic bath. Such a model conveniently interpolates between the ionic Hubbard model at Nc=√2 and the Anderson model at Nc=∞ and enables the analysis of the various limiting properties. We confirmed that within the CPA approach a band insulator behavior appears for non-interacting strongly disordered systems with a small plaquette size Nc=4, while the paramagnetic Anderson insulator with nearly gapless density of states is present for large plaquette sizes Nc=48. When the interaction U is turned on in the strongly fluctuating random potential regions, the electrons on the low energy states push each other into high energy states in DMFT in a paramagnetic bath and correlated metallic states with a quasiparticle peak and Hubbard bands emerge, though a larger critical interaction U is needed to obtain this state from the paramagnetic Anderson insulator (Nc=48) than from the band insulator (Nc=4). Finally, we observe a Mott insulator behavior in the strong interaction U regions for both Nc=4 and Nc=48 independent of the disorder strength. We discuss the application of this model to real materials.

117. 	D. Guterding, R. Valentí, H. O. Jeschke
     	Reduction of magnetic interlayer coupling in barlowite through isoelectronic substitution
     	Phys. Rev. B 94, 125136 (2016) (doi,pdf), (abstract).(hide abstract). Materials with a perfect kagome lattice structure of magnetic ions are intensively sought for, because they may exhibit exotic ground states like the a quantum spin liquid phase. Barlowite is a natural mineral that features perfect kagome layers of copper ions. However, in barlowite there are also copper ions between the kagome layers, which mediate strong interkagome couplings and lead to an ordered ground state. Using ab initio density functional theory calculations we investigate whether selective isoelectronic substitution of the interlayer copper ions is feasible. After identifying several promising candidates for substitution we calculate the magnetic exchange couplings based on crystal structures predicted from first-principles calculations. We find that isoelectronic substitution with nonmagnetic ions significantly reduces the interkagome exchange coupling. As a consequence, interlayer-substituted barlowite can be described by a simple two-parameter Heisenberg Hamiltonian, for which a quantum spin liquid ground state has been predicted.

116. 	D. Guterding, M. Altmeyer, H. O. Jeschke, R. Valentí
     	Near-degeneracy of extended s + dx2-y2 and dxy order parameters in quasi-two-dimensional organic superconductors
     	Phys. Rev. B 94, 024515 (2016) (doi,pdf), (abstract).(hide abstract). The symmetry of the superconducting order parameter in quasi-two-dimensional BEDT-TTF organic superconductors is a subject of ongoing debate. We report ab initio density functional theory calculations for a number of organic superconductors containing κ-type layers. Using projective Wannier functions we derive parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random phase approximation spin-fluctuation approach we investigate the evolution of the superconducting pairing symmetry within this model and point out a phase-transition between extended s + dx2-y2 and dxy symmetry. We discuss the origin of the mixed order parameter and the relation between the realistic molecule description and the widely used dimer approximation. Based on our ab initio calculations we position the investigated materials in the obtained molecule model phase diagram and simulate scanning tunneling spectroscopy experiments for selected cases. Our calculations show that many κ-type materials lie close to the phase transition line between the two pairing symmetry types found in our calculation, possibly explaining the multitude of contradictory experiments in this field.

115. 	K. Riedl, D. Guterding, H. O. Jeschke, M. J. P. Gingras, R. Valentí
     	Ab initio determination of spin Hamiltonians with anisotropic exchange interactions: the case of the pyrochlore ferromagnet Lu2V2O7
     	Phys. Rev. B 94, 014410 (2016) (doi,pdf), (abstract).(hide abstract). We present a general framework for deriving effective spin Hamiltonians of correlated magnetic systems based on a combination of relativistic ab initio density functional theory calculations (DFT), exact diagonalization of a generalized Hubbard Hamiltonian on finite clusters and spin projections onto the low-energy subspace. A key motivation is to determine anisotropic bilinear exchange couplings in materials of interest. As an example, we apply this method to the pyrochlore Lu2V2O7 where the vanadium ions form a lattice of corner-sharing spin-1/2 tetrahedra. In this compound, anisotropic Dzyaloshinskii-Moriya interactions (DMI) play an essential role in inducing a magnon Hall effect. We obtain quantitative estimates of the nearest-neighbor Heisenberg exchange, the DMI, and the symmetric part of the anisotropic exchange tensor. Finally, we compare our results with experimental ones on the Lu2V2O7 compound.

114. 	S. M. Winter, Y. Li, H. O. Jeschke, R. Valentí
     	Challenges in Design of Kitaev Materials: Magnetic Interactions from Competing Energy Scales
     	Phys. Rev. B 93, 214431 (2016) (doi,pdf), (abstract).(hide abstract). In this study, we reanalyze the magnetic interactions in the Kitaev spin liquid candidate materials Na2IrO3, α-RuCl3, and α-Li2IrO3 using nonperturbative exact diagonalization methods. These methods are more appropriate given the relatively itinerant nature of the systems suggested in previous works. We treat all interactions up to third neighbours on equal footing. The computed terms reveal significant long range coupling, bond-anisotropy, and/or off-diagonal couplings which we argue naturally explain the observed ordered phases in these systems. Given these observations, the potential for realizing the spin-liquid state in real materials is analyzed, and synthetic challenges are defined and explained.

113. 	M. Altmeyer, H. O. Jeschke, O. Hijano-Cubelos, C. Martins, F. Lechermann, K. Koepernik, A. Santander-Syro, M. J. Rozenberg, R. Valentí, M. Gabay
     	Magnetism, spin texture and in-gap states: Atomic specialization at the surface of oxygen-deficient SrTiO3
     	Phys. Rev. Lett. 116, 157203 (2016) (doi,pdf,supplement), (abstract).(hide abstract). Motivated by recent spin- and angular-resolved photoemission measurements (SARPES) per- formed on the two-dimensional electronic states confined near the (001) surface of SrTiO3 in the presence of oxygen vacancies, we explore their spin structure by means of ab initio density functional theory (DFT) calculations of slabs. Relativistic non-magnetic DFT calculations display Rashba-like spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is in- cluded, bands become spin-split with an energy difference ~100 meV at the Γ point, consistent with SARPES findings. While magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures. Furthermore, we observe an atomic specialization phenomenon, namely two types of electronic contributions; one is from Ti atoms neighboring the oxygen vacancies that acquire rather large magnetic moments and mostly create in-gap states; another comes from the partly polarized t2g itinerant electrons of Ti atoms lying further away from the oxygen vacancy which form the two-dimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.

112. 	D. Guterding, H. O. Jeschke, R. Valentí
     	Prospects for topologically non-trivial electronic and magnetic states in correlated kagome lattice materials
     	Sci. Rep. 6, 25988 (2016) (doi,pdf,supplement), (abstract).(hide abstract). Electronic states with non-trivial topology host a number of novel phenomena with potential for revolutionizing information technology. The quantum anomalous Hall effect provides spin-polarized dissipation-free transport of electrons, while the quantum spin Hall effect in combination with superconductivity has been proposed as the basis for realizing decoherence-free quantum computing. We introduce a new strategy for realizing these effects, namely by hole and electron doping kagome lattice Mott insulators through, for instance, chemical substitution. As an example, we apply this new approach to the natural mineral herbertsmithite. We prove the feasibility of the proposed modifications by performing ab-initio density functional theory calculations and demonstrate the occurrence of the predicted effects using realistic models. Our results herald a new family of quantum anomalous Hall and quantum spin Hall insulators at affordable energy/temperature scales based on kagome lattices of transition metal ions.

111. 	D. Guterding, S. Diehl, M. Altmeyer, T. Methfessel, U. Tutsch, H. Schubert, M. Lang, J. Müller, M. Huth, H. O. Jeschke, R. Valentí, M. Jourdan, H.-J. Elmers
     	Evidence for eight node mixed-symmetry superconductivity in a correlated organic metal
     	Phys. Rev. Lett. 116, 237001 (2016) (doi,pdf,supplement), (abstract).(hide abstract). We report a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. Applying spin-fluctuation theory to a low-energy material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of κ-(BEDT-TTF)2Cu[N(CN)2]Br with the tunneling direction parallel to the layered structure.

110. 	F. Lechermann, H. O. Jeschke, A. Kim, S. Backes, R. Valentí
     	Electron dichotomy on the SrTiO3 defect surface augmented by many-body effects
     	Phys. Rev. B 93, 121103(R) (2016) (doi,pdf), (abstract).(hide abstract). In a common paradigm, the electronic structure of condensed matter is divided into weakly and strongly correlated compounds. While conventional band theory usually works well for the former class, many-body effects are essential for the latter. Materials such as the familiar SrTiO3 (STO) compound that bridge or even abandon this characterization scheme are highly interesting. Here, it is shown, by means of combining density functional theory with dynamical mean-field theory, that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly correlated t2g low-energy quasiparticles and localized “in-gap” states of dominant eg character with a subtle correlation signature. We furthermore touch base with recent experimental work and study the surface instability towards magnetic order.

      2015 

109. 	Y. Iqbal, H. O. Jeschke, J. Reuther, R. Valentí, I. I. Mazin, M. Greiter, R. Thomale
     	Paramagnetism in the Kagome compounds (Zn,Mg,Cd)Cu3(OH)6Cl2
     	Phys. Rev. B 92, 220404(R) (2015) (doi,pdf,supplement), (abstract).(hide abstract). Frustrated magnetism on the kagome lattice has been a fertile ground for rich and fascinating physics, ranging from experimental evidence of a spin liquid to theoretical predictions of exotic superconductivity. Among experimentally realized spin-1/2 kagome magnets, herbertsmithite, kapellasite, and haydeeite [(Zn,Mg)Cu3(OH)6Cl22], are all well described by a three-parameter Heisenberg model, but exhibit distinctly different physics. Matching experimental advances with theoretical concepts is a major challenge; we address it using a novel pseudofermion functional renormalization-group approach, and analyze the low-energy physics in the experimentally accessible parameter range. Our analysis places kapellasite and haydeeite near the boundaries between magnetically ordered and disordered phases, implying that slight modifications could dramatically affect their magnetic properties. Inspired by this, we perform ab initio calculations of (Zn,Mg,Cd)Cu3(OH)6Cl22 at various pressures. Our results suggest that by varying pressure and composition one can traverse a paramagnetic regime between different, magnetically ordered phases.

108. 	S. Backes, H. O. Jeschke, R. Valentí
     	Microscopic nature of correlations in multi-orbital AFe2As2 (A=K, Rb, Cs): Hund′s coupling versus Coulomb repulsion
     	Phys. Rev. B 92, 195128 (2015) (doi,pdf), (abstract).(hide abstract). We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe-pnictides AFe2As2 (A=K, Rb, Cs) as well as the fictitious FrFe2As2 and a-axis stretched CsFe2As2. This choice of systems allows for a systematic analysis of the interplay of Hund′s coupling JH and on-site Coulomb repulsion U in multi-orbital Fe-pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a non-trivial change in the iron 3d hoppings, an increase of orbitally-selective correlations and the presence of incoherent weight at high-binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund′s metal. This is especially prominent in a-stretched CsFe2As2. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on JH and we provide estimates of the coherence scale T∗. We discuss these results in the framework of reported experimental observations.

107. 	R. D. Johnson, S. Williams, A. A. Haghighirad, J. Singleton, V. Zapf, P. Manuel, I. I. Mazin, Y. Li, H. O. Jeschke, R. Valentí, R. Coldea
     	The monoclinic crystal structure of α-RuCl3 and the zigzag antiferromagnetic ground state
     	Phys. Rev. B 92, 235119 (2015) (doi,pdf), (abstract).(hide abstract). The layered honeycomb magnet α-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently-assumed trigonal 3-layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the jeff=1/2 picture once spin orbit coupling and electron correlations are included. We propose that differences in the magnitude of anisotropic exchange along symmetry inequivalent bonds in the monoclinic cell could provide a natural mechanism to explain the spin gap observed in powder inelastic neutron scattering, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as neutron powder diffraction show a single magnetic transition at TN ~ 13K. The analysis of the neutron data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60T show a single transition around 8T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

106. 	E. A. Zvereva, M. I. Stratan, Y. A. Ovchenkov, V. B. Nalbandyan, J.-Y. Lin, E. L. Vavilova, M. F. Iakovleva, M. Abdel-Hafiez, A. V. Silhanek, X.-J. Chen, A. Stroppa, S. Picozzi, H. O. Jeschke, R. Valentí, and A. N. Vasiliev
     	Zigzag antiferromagnetic quantum ground state in monoclinic honeycomb lattice antimonates A3Ni2SbO6 (A=Li, Na)
     	Phys. Rev. B 92, 144401 (2015) (doi,pdf), (abstract).(hide abstract). We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb-lattice monoclinic compounds A3Ni2SbO6 (A=Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic (AFM) long range order with Néel temperatures of ~14 and 16 K for Li3Ni2SbO6 and Na3Ni2SbO6, respectively. The effective magnetic moments of 4.3 μB/f.u. (Li3Ni2SbO6) and 4.4 μB/f.u. (Na3Ni2SbO6) indicate that Ni2+ is in a high-spin configuration (S=1). The temperature dependence of the inverse magnetic susceptibility follows the Curie- Weiss law in the high-temperature region and shows positive values of the Weiss temperature ~8 K (Li3Ni2SbO6) and ~12 K (Na3Ni2SbO6) pointing to the presence of non-negligible ferromagnetic interactions, although the system orders AFM at low temperatures. In addition, the magnetization curves reveal a field-induced (spin-flop type) transition below TN that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni2+ ions are present in the honeycomb planes supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations we propose magnetic phase diagrams for the two compounds.

105. 	D. Guterding, R. Valentí, H. O. Jeschke
     	Influence of molecular conformations on the electronic structure of organic charge transfer salts
     	Phys. Rev. B 92, 081109(R) (2015) (doi,pdf), (abstract).(hide abstract). We report ab-initio calculations for the electronic structure of organic charge transfer salts κ-(ET)2Cu[N(CN)2]Br, κ-(ET)2Cu[N(CN)2]I, κ''-(ET)2Cu[N(CN)2]Cl and κ-(ET)2Cu2(CN)3. These materials show an ordering of the relative orientation of terminal ethylene groups in the BEDTTTF molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene endgroup relative orientations can be used to reversibly tune κ-(ET)2Cu[N(CN)2]Br through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene endgroups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place κ-(ET)2Cu[N(CN)2]Br in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.

104. 	J. K. Glasbrenner, I. I. Mazin, H. O. Jeschke, P. J. Hirschfeld, R. M. Fernandes, R. Valentí
     	Effect of magnetic frustration on nematicity and superconductivity in Fe chalcogenides
     	Nature Physics 11, 953 (2015) (doi,pdf), (abstract).(hide abstract). Over the past few years Fe chalcogenides (FeSe/Te) have advanced to the forefront of Fe-based superconductors (FeBS) research. The most intriguing results thus far are for intercalated and monolayer FeSe, however experimental studies are still inconclusive. Yet, bulk FeSe itself remains an unusual case when compared with pnictogen-based FeBS, and may hold clues to understanding the more exotic FeSederivatives. The FeSe phase diagram is unlike the pnictides: the orthorhombic distortion, which is likely to be of a “spin-nematic” nature in numerous pnictides, is not accompanied by magnetic order in FeSe, and the superconducting transition temperature Tc rises significantly with pressure before decreasing. In this paper we show that the magnetic interactions in chalcogenides, as opposed to pnictides, demonstrate unusual (and unanticipated) frustration, which suppresses magnetic, but not nematic order, favors ferroorbital order in the nematic phase and can naturally explain the nonmonotonic pressure dependence of the superconducting critical temperature Tc(P).

103. 	M. Altmeyer, R. Valentí, H. O. Jeschke
     	Role of layer packing for the electronic properties of the organic superconductor (BEDT-TTF)2Ag(CF3)4(TCE)
     	Phys. Rev. B 91, 245137 (2015) (doi,pdf), (abstract).(hide abstract). The charge transfer compound (BEDT-TTF)2Ag(CF3)4(TCE) crystallizes in three polymorphs with different alternating layers: While a phase with a κ packing motif has a low superconducting transition temperature of Tc=2.6 K, two phases with higher Tc of 9.5 K and 11 K are multi-layered structures consisting of α′ and κ layers. We investigate these three systems within density functional theory and find that the α′ layer shows different degrees of charge order for the two κ-α′ systems and directly influences the electronic behavior of the conducting κ layer. We discuss the origin of the distinct behavior of the three polymorphs and propose a minimal tight binding Hamiltonian for the description of these systems based on projective molecular Wannier functions.

102. 	D. Guterding, S. Backes, H. O. Jeschke, R. Valentí
     	Origin of the superconducting state in the collapsed tetragonal phase of KFe2As2
     	Phys. Rev. B 91, 140503(R) (2015) (doi,pdf,supplement), (abstract).(hide abstract). Recently, KFe2As2 was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under applied pressure of about 20 GPa. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with Tc ~ 12 K, while the tetragonal phase is a Tc ≤ 3.4 K superconductor. We argue that the key difference between the previously known non-superconducting collapsed tetragonal phase in BaFe2As2, CaFe2As2, EuFe2As2, SrFe2As2, and CaFe2P2 and the superconducting collapsed tetragonal phase in KFe2As2 is the qualitatively different electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary (M) and no hole pockets are present in the Brillouin zone center (Γ), the collapsed phase in KFe2As2 has both electron pockets at M and hole pockets at Γ. Our DFT+DMFT calculations show that effects of correlations on the electronic structure are minimal. Within a random phase approximation spin-fluctuation approach we calculate the superconducting pairing symmetry in the collapsed tetragonal phase of KFe2As2 and show that its electronic structure favors an s± symmetry of the superconducting gap function. Finally, we argue that our results are also compatible with a change of sign of the Hall coefficient as observed experimentally.

101. 	H. O. Jeschke, F. Salvat-Pujol, E. Gati, N. H. Hoang, B. Wolf, M. Lang, J. A. Schlueter, R. Valentí
     	Barlowite as a canted antiferromagnet: theory and experiment
     	Phys. Rev. B 92, 094417 (2015) (doi,pdf), (abstract).(hide abstract). We investigate the structural, electronic and magnetic properties of the newly synthesized mineral barlowite Cu4(OH)6FBr which contains Cu2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)6Cl2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F- and Br- compared to Cl-. We perform density functional theory calculations to obtain the Heisenberg Hamiltonian parameters of Cu4(OH)6FBr which has a Cu2+ site coupling the kagome layers. The 3D network of exchange couplings together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound at TN = 15 K as observed by magnetic susceptibility measurements on single crystals.

100. 	D. Guterding, H. O. Jeschke, P. J. Hirschfeld, R. Valentí
     	Unified picture of the doping dependence of superconducting transition temperatures in alkali metal/ammonia intercalated FeSe
     	Phys. Rev. B 91, 041112(R) (2015) (doi,pdf,supplement), (abstract).(hide abstract). In the recently synthesized Lix(NH2)y(NH3)zFe2Se2 family of iron chalcogenides a molecular spacer consisting of lithium ions, lithium amide and ammonia separates layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach Tc ~ 44 K, but the relative importance of the layer separation and effective doping to the Tc enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab-initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of s+--symmetry and explain the experimentally observed limit to Tc in the molecular spacer intercalated FeSe class of materials.

99. 	Y. Li, K. Foyevtsova, H. O. Jeschke, R. Valentí
    	Analysis of the optical conductivity for A2IrO3 (A = Na, Li) from first principles
    	Phys. Rev. B 91, 161101(R) (2015) (doi,pdf), (abstract).(hide abstract). We present results for the optical conductivity of Na2IrO3 within density functional theory by including spin-orbit (SO) and correlation effects (U) as implemented in GGA+SO+U. We identify the various interband transitions and show that the underlying quasi-molecular-orbital nature of the electronic structure in Na2IrO3 translates into distinct features in the optical conductivity. Most importantly, the parity of the quasi-molecular orbitals appears to be the main factor in determining strong and weak optical transitions. We also present optical conductivity calculations for Li2IrO3 and discuss the similarities and differences with Na2IrO3.

98. 	M. Rudloff, K. Ackermann, M. Huth, H. O. Jeschke, M. Tomić, R. Valentí, B. Wolfram, M. Bröring, M. Bolte, D. Chercka, M. Baumgarten, K. Müllen
    	Charge transfer tuning by chemical substitution and uniaxial pressure in the organic complex tetramethoxypyrene–tetracyanoquinodimethane
    	Phys. Chem. Chem. Phys. 17, 4118 (2015) (doi,pdf), (abstract).(hide abstract). In the search for novel organic charge transfer salts with variable charge transfer degree we study the effects of two modiifcations to the recently synthesized donor-acceptor [Tetramethoxypyrene (TMP)]-[Tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by substituting the acceptor TCNQ molecules by F4TCNQ molecules. The second consists in simulating the application of uniaxial pressure along the stacking axis of the system. In order to test the chemical substitution, we have grown single crystals of TMP-F4TCNQ and analyzed its electronic structure via electronic transport measurements, ab initio density functional theory (DFT) calculations and UV/VIS/IR absorption spectroscopy. This system shows an almost ideal geometrical overlap of nearly planar molecules alternately stacked (mixed stack) and this arrangement is echoed by a semiconductor-like transport behavior with an increased conductivity along the stacking direction. This is in contrast to TMP-TCNQ which shows a less pronounced anisotropy and a smaller conductivity response. Our bandstructure calculations confirm the one-dimensional behavior of TMP-F4TCNQ with pronounced dispersion only along the stacking axis. Infrared measurements illustrating the C=N vibration frequency shift in F4TCNQ suggest however no improvement on the degree of charge transfer in TMP-F4TCNQ with respect to TMP-TCNQ. In both complexes about 0.1 is transferred from TMP to the acceptor. Concerning the pressure effect, our DFT calculations on designed TMP-TCNQ and TMP-F4TCNQ structures under different pressure conditions show that application of uniaxial pressure along the stacking axis of TMP-TCNQ may be the route to follow in order to obtain a much more pronounced charge transfer.

97. 	H. O. Jeschke, J. Shen, R. Valentí
    	Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3
    	New J. Phys. 17, 023034 (2015) (doi,pdf), (abstract).(hide abstract). Oxygen vacancies in strontium titanate surfaces (SrTiO3) have been linked to the presence of a twodimensional electron gas with unique behavior. We perform a detailed density functional theory study of the lattice and electronic structure of SrTiO3 slabs with multiple oxygen vacancies, with a main focus on two vacancies near a titanium dioxide terminated SrTiO3 surface. We conclude based on total energies that the two vacancies preferably inhabit the first two layers, i.e. they cluster vertically, while in the direction parallel to the surface, the vacancies show a weak tendency towards equal spacing. Analysis of the nonmagnetic electronic structure indicates that oxygen defects in the surface TiO2 layer lead to population of Ti t2g states and thus itinerancy of the electrons donated by the oxygen vacancy. In contrast, electrons from subsurface oxygen vacancies populate Ti eg states and remain localized on the two Ti ions neighboring the vacancy. We find that both, the formation of a bound oxygen-vacancy state composed of hybridized Ti 3eg and 4p states neighboring the oxygen vacancy as well as the elastic deformation after extracting oxygen contribute to the stabilization of the in-gap state.

96. 	Y. Wang, M. N. Gastiasoro, B. M. Andersen, M. Tomić, H. O. Jeschke, R. Valentí, I. Paul, P. J. Hirschfeld
    	Effects of Lifshitz Transition on Charge Transport in Magnetic Phases of Fe-Based Superconductors
    	Phys. Rev. Lett. 114, 097003 (2015) (doi,pdf), (abstract).(hide abstract). The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe1−xCox)2As2, has been a longstanding puzzle. Here we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Néel temperature TN in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.

     2014 

95. 	U. Tutsch, B. Wolf, S. Wessel, Y. Tsui, H. O. Jeschke, I. Opahle, T. Saha-Dasgupta, R. Valentí, A. Brühl, K. Remović-Langer, T. Kretz, H.-W. Lerner, M. Wagner, M. Lang
    	Evidence of a field-induced Berezinskii–Kosterlitz–Thouless scenario in a two-dimensional spin–dimer system
    	Nature Commun. 5, 5169 (2014) (doi), (abstract).(hide abstract). Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex–antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S=½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

94. 	M. Tomić, H. O. Jeschke, R. Valentí
    	Unfolding of electronic structure through induced representations of space groups: Application to Fe-based superconductors
    	Phys. Rev. B 90, 195121 (2014) (doi,pdf), (abstract).(hide abstract). We revisit the problem that relevant parts of bandstructures for a given cell choice can reflect exact or approximate higher symmetries of subsystems in the cell and can therefore be significantly simplified by an unfolding procedure that recovers the higher symmetry. We show that bandstructure unfolding can be understood as projection onto induced irreducible representations of a group obtained by extending the original group of translations with a number of additional symmetry operations. The resulting framework allows us to define a generalized unfolding procedure which includes the point group operations and can be applied to any quantity in the reciprocal space. The unfolding of the Brillouin zone follows naturally from the properties of the induced irreducible representations. In this context, we also introduce a procedure to derive tight-binding models of reduced dimensionality by making use of point group symmetries. Further, we show that careful consideration of unfolding has important consequences on the interpretation of angle resolved photoemission experiments. Finally, we apply the unfolding procedure to various representative examples of Fe-based superconductor compounds and comment on the interpretation of one-iron versus two-iron Brillouin zone representations.

93. 	J. Diehl, S. Backes, D. Guterding, H. O. Jeschke, R. Valentí
    	Correlation effects in the tetragonal and collapsed tetragonal phase of CaFe2As2
    	Phys. Rev. B 90, 085110 (2014) (doi,pdf), (abstract).(hide abstract). We investigate the role of correlations in the tetragonal and collapsed tetragonal phases of CaFe2As2 by performing charge self-consistent DFT+DMFT (density functional theory combined with dynamical mean-field theory) calculations. While the topology of the Fermi surface is basically unaffected by the inclusion of correlation effects, we find important orbital-dependent mass renormalizations which show good agreement with recent angle-resolved photoemission (ARPES) experiments. Moreover, we observe a markedly different behavior of these quantities between the low-pressure tetragonal and the high-pressure collapsed tetragonal phase. We attribute these effects to the increased hybridization between the iron- and arsenic orbitals as one enters the collapsed tetragonal phase.

92. 	S. Backes, D. Guterding, H. O. Jeschke, R. Valentí
    	Electronic structure and de Haas-van Alphen frequencies in KFe2As2 within LDA+DMFT
    	New J. Phys. 16, 085025 (2014) (doi,pdf), (abstract).(hide abstract). Recent density functional theory (DFT) calculations for KFe2As2 have shown to be insuffcient to satisfactorily describe angle-resolved photoemission (ARPES) measurements as well as observed de Haas van Alphen (dHvA) frequencies. In the present work, we extend DFT calculations based on the full-potential linear augmented plane-wave method by dynamical mean field theory (DFT+DMFT) to include correlation effects beyond the local density approximation. Our results indicate that KFe2As2 is a moderately correlated metal with a mass renormalization factor of the Fe 3d orbitals between 1.6 and 2.7. Also, the obtained shape and size of the Fermi surface are in good agreement with ARPES measurements and we observe some topological changes with respect to DFT calculations like the opening of an inner hole cylinder at the Z point. As a result, our calculated dHvA frequencies differ greatly from existing DFT results and qualitatively agree with experimental data. On this basis, we argue that correlation effects are important to understand the -presently under debate- nature of superconducting state in KFe2As2.

91. 	I. I. Mazin, H. O. Jeschke, F. Lechermann, H. Lee, M. Fink, R. Thomale, R. Valentí
    	Theoretical prediction of a strongly correlated Dirac metal
    	Nature Commun. 5, 4261 (2014) (doi,pdf), (abstract).(hide abstract). Recently, the most intensely studied objects in the electronic theory of solids have been strongly correlated systems and graphene. However, the fact that the Dirac bands in graphene are made up of sp2-electrons, which are subject to neither strong Hubbard repulsion U nor strong Hund’s rule coupling J creates certain limitations in terms of novel, interactioninduced physics that could be derived from Dirac points. Here we propose GaCu3(OH)6Cl2 (Ga-substituted herbertsmithite) as a correlated Dirac-Kagome metal combining Dirac electrons, strong interactions and frustrated magnetism. Using density functional theory (DFT), we calculate its crystallographic and electronic properties, and observe that it has symmetryprotected Dirac points at the Fermi level. Its many-body physics is excitingly rich, with possible charge, magnetic and superconducting instabilities. Through a combination of various many-body methods we study possible symmetry-lowering phase transitions such as Mott- Hubbard, charge or magnetic ordering, and unconventional superconductivity, which in this compound assumes an f-wave symmetry.

90. 	F. Salvat-Pujol, H. O. Jeschke, R. Valentí
    	Determination of magnetic form factors for organic charge transfer salts: a first principles investigation
    	Phys. Rev. B 90, 041101(R) (2014) (doi,pdf), (abstract).(hide abstract). Organic charge transfer salts show a variety of complex phases ranging from antiferromagnetic long-range order, spin liquid, bad metal or even superconductivity. A powerful method to investigate magnetism is spin-polarized inelastic neutron scattering. However, such measurements have often been hindered in the past by the small size of available crystals as well as by the fact that the spin in these materials is distributed over molecular rather than atomic orbitals and good estimates for the magnetic form factors are missing. By considering Wannier functions obtained from density functional theory calculations, we derive magnetic form factors for a number of representative organic molecules. Compared to Cu2+, the form factors |F(q)|2 fall off more rapidly as function of q reflecting the fact that the spin density is very extended in real space. Form factors |F(q)|2 for TMTTF, BEDT-TTF and (BEDT-TTF)2 have anisotropic and nonmonotonic structure.

89. 	S. A. J. Kimber, I. I. Mazin, J. Shen, H. O. Jeschke, S. V. Streltsov, D. N. Argyriou, R. Valentí, D. I. Khomskii
    	Valence bond liquid phase in the honeycomb lattice material Li2RuO3
    	Phys. Rev. B 89, 081408(R) (2014) (doi,pdf,supplement), (abstract).(hide abstract). The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+ cations on cooling below 270°C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a 'valence bond crystal', with a strong bond disproportionation. On warming, x-ray diffraction shows that discrete three-fold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x-rays, show that disordered dimers survive at the nanoscale up to at least 650°C. The high temperature phase of Li2RuO3 is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analogue of the resonating valence bond state often discussed in connection with high Tc cuprates.

88. 	S. Manni, S. Choi, I. I. Mazin, R. Coldea, M. Altmeyer, H. O. Jeschke, R. Valentí, P. Gegenwart
    	Effect of isoelectronic doping on honeycomb lattice iridate A2IrO3
    	Phys. Rev. B 89, 245113 (2014) (doi,pdf), (abstract).(hide abstract). We have investigated experimentally and theoretically the series (Na1−xLix)2IrO3. Contrary to what has been believed so far, only for x≤0.25 the system forms uniform solid solutions. For larger Li content, as evidenced by powder X-ray diffraction, scanning electron microscopy and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na3LiIr2O6 phase with alternating Na3 and LiIr2O6 planes, and a Li-rich phase close to pure Li2IrO3. For x≤0.25 we observe (1) an increase of c/a with Li doping up to x=0.25, despite the fact that c/a in pure Li2IrO3 is smaller than in Na2IrO3, and (2) a gradual reduction of the antiferromagnetic ordering temperature TN and ordered moment. The previously proposed magnetic quantum phase transition at x≈0.7 may occur in a multiphase region and its nature needs to be re-evaluated.

87. 	S. A. J. Kimber, A. Salamat, S. R. Evans, H. O. Jeschke, K. Muthukumar, M. Tomić, F. Salvat-Pujol, R. Valentí, M. V. Kaisheva, I. Zizak, T. Chatterji
    	Giant pressure-induced volume collapse in the pyrite mineral MnS2
    	Proc. Natl. Acad. Sci. 111, 5106 (2014) (doi,pdf,supplement), (abstract).(hide abstract). Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral hauerite (MnS2) undergoes an unprecedented (ΔV∼22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S=1/2 moments are quenched by dimerization. Our results show how the emergence of metal–metal bonding can stabilize giant spin-lattice coupling in Earth’s minerals.

86. 	M. D. Watson, A. McCollam, S.F. Blake, D. Vignolles, L. Drigo, I. I. Mazin, D. Guterding, H. O. Jeschke, R. Valentí, N. Ni, R. Cava, A. I. Coldea
    	Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca10(Pt3As8) ((Fe1-xPtx)2As2)5
    	Phys. Rev. B 89, 205136 (2014) (doi,pdf), (abstract).(hide abstract). We report a high magnetic field study up to 55 T of the nearly optimally doped iron-pnictide superconductor Ca10(Pt3As8) ((Fe1-xPtx)2As2)5 (x=0.078(6)) using magnetic torque, tunnel diode oscillator technique, and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a prominent anomaly in magnetic torque close to 22 T, when the magnetic field is applied perpendicular to the (ab) planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. We suggest that this field-induced transition, observed both in the magnetically ordered parent compound and a nonordered superconducting sample, is a signature of a spin-flop-like transition associated not with long-range order but driven by antiferromagnetic fluctuations of magnetic moments aligned preferentially out of the conducting planes at low temperatures.

85. 	J. Ferber, K. Foyevtsova, H. O. Jeschke, R. Valentí
    	Unveiling the microscopic nature of correlated organic conductors: the case of κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1-x
    	Phys. Rev. B 89, 205106 (2014) (doi,pdf), (abstract).(hide abstract). A few organic conductors show a diversity of exciting properties like Mott insulating behaviour, spin liquid, antiferromagnetism, bad metal or unconventional superconductivity controlled by small changes in temperature, pressure or chemical substitution. While such a behaviour can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic phases in the family of organic systems κ-(BEDT-TTF)2Cu[N(CN)2]BrxCl1-x by a combination of density functional theory calculations and the dynamical mean field theory approach in a new form adapted for organic systems. By computing spectral and optical properties we are able to disentangle the origin of the various optical transitions in these materials and prove that correlations are responsible for relevant features. Remarkably, while some transitions are inherently affected by correlations, others are completely uncorrelated. We discuss the consequences of our findings for the phase diagram in these materials.

84. 	K. Muthukumar, R. Valentí, H. O. Jeschke
    	Dynamics of tungsten hexacarbonyl, dicobalt octacarbonyl and their fragments adsorbed on silica surfaces
    	J. Chem. Phys. 140, 184706 (2014) (doi,pdf), (abstract).(hide abstract). Tungsten and cobalt carbonyls adsorbed on a substrate are typical starting points for the electron beam induced deposition of tungsten or cobalt based metallic nanostructures. We employ first principles molecular dynamics simulations to investigate the dynamics and vibrational spectra of W(CO)6 and W(CO)5 as well as Co2(CO)8 and Co(CO)4 precursor molecules on fully and partially hydroxylated silica SiO2 surfaces. Such surfaces resemble the initial conditions of electron beam induced growth processes. We find that both W(CO)6 and Co2(CO)8 are stable at room temperature and quite mobile on a silica surface saturated with hydroxyl groups (OH), moving up to half an Angstroem per picosecond. In contrast, chemisorbed W(CO)5 or Co(CO)4 ions at room temperature do not change their binding site. These results contribute to gaining fundamental insight into how the molecules behave in the simulated time window of 20 ps and our determined vibrational spectra of all species provide signatures for experimentally distinguishing the form in which precursors cover a substrate.

83. 	H. Lee, Y.-Z. Zhang, H. O. Jeschke, R. Valentí
    	Competition between band and Mott insulators in the bilayer Hubbard model: a dynamical cluster approximation study
    	Phys. Rev. B 89, 035139 (2014) (doi,pdf), (abstract).(hide abstract). We investigate the nature of the insulating phases in a bilayer Hubbard model with intralayer coupling t and interlayer coupling t⊥ at large interaction strength U/t and half-filling. We consider a dynamical cluster approximation with a cluster size of Nc=2×4, where short-range spatial fluctuations as well as on-site dynamical fluctuations are emphasized. By varying the band splitting (t⊥/t), we find that at t⊥/t ≈ 1.5 the Mott behavior is rapidly suppressed in the momentum sectors (π,0) and (0,π). At t⊥/t ≈ 2.5 Mott features dominate in the momentum sectors (π,π) of the bonding band and (0,0) of the anti-bonding band and at t⊥/t ≈ 3.0 a tiny scattering rate is observed in all momentum sectors at the Fermi level, indicating a transition from a Mott to a band insulator. We attribute such a momentum-dependent evolution of the insulating behavior to the competition and cooperation between short-range spatial fluctuations and interlayer coupling t⊥ with the help of the Coulomb interaction U. Finally, we also discuss the possible appearance of non-Fermi liquid behavior away from half-filling.

     2013 

82. 	N. Mevedev, H. O. Jeschke, B. Ziaja
    	Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse
    	Phys. Rev. B 88, 224304 (2013) (doi,pdf), (abstract).(hide abstract). Diamond irradiated with an ultrashort intense laser pulse in the regime of photon energies from soft up to hard x rays can undergo a phase transition to graphite. This transition is induced by an excitation of electrons from the valence band or from atomic deep shells of the material into its conduction band, which is followed by a transient rapid change of the interatomic potential. Such a nonthermal phase transition occurs on a femtosecond time scale, shortly after or even during the laser pulse. In this work we show that the duration of the graphitization depends on the incoming photon energy: the higher the photon energy is, the longer the secondary electron cascading which promotes the electrons into the conduction band will take. The transient kinetics of the electronic and atomic processes during the graphitization is analyzed in detail. The damage threshold fluence is calculated in the broad photon energy range and is found to be always ∼0.7 eV/atom in terms of the average dose absorbed per atom. It is confirmed that the temporal characteristics of a femtosecond laser pulse (at a fixed pulse duration and fluence) do not significantly influence the transient damage kinetics. Finally, the influence of an additional surface layer of high-Z material on the damage within diamond is discussed.

81. 	A.C. Jacko, L. F. Tocchio, H. O. Jeschke, R. Valentí
    	Importance of anisotropy in the spin-liquid candidate Me3EtSb[Pd(dmit)2]2
    	Phys. Rev. B 88, 155139 (2013) (doi,pdf), (abstract).(hide abstract). Organic charge transfer salts based on the molecule Pd(dmit)2 display strong electronic correlations and geometrical frustration, leading to spin liquid, valence bond solid, and superconducting states, amongst other interesting phases. The low energy electronic degrees of freedom of these materials are often described by a single band model; a triangular lattice with a molecular orbital representing a Pd(dmit)2 dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low energy effective model Hamiltonians for a class of Me(4-n)EtnX[Pd(dmit)2]2 (X=As,P,N,Sb) salts and investigate how best to model these systems by using variational Monte Carlo (VMC) simulations. Our findings suggest that the prevailing model of these systems as a t-t′ triangular lattice is incomplete, and that a fully anisotropic triangular lattice (FATL) description produces importantly different results, including a significant lowering of the critical U of the spin-liquid phase.

80. 	J. Gaudin, N. Medvedev, J. Chalupsky, T. Burian, S. Dastjani-Farahani, V. Hajkova, M. Harmand, H. O. Jeschke, L. Juha, M. Jurek, D. Klinger, J. Krzywinski, R. A. Loch, S. Moeller, M. Nagasono, C. Ozkan, K. Saksl, H. Sinn, R. Sobierajski, P. Sovak, S. Toleikis, K. Tiedtke, M. Toufarova, T. Tschentscher, V. Vorlicek, L. Vysin, H. Wabnitz, B. Ziaja
    	Photon energy dependence of graphitization threshold for diamond irradiated with an intense XUV FEL pulse
    	Phys. Rev. B 88, 060101(R) (2013) (doi,pdf), (abstract).(hide abstract). We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultra-violet laser (XUV) pulse of 24 eV to 275 eV photon energy provided by free-electrons lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.

79. 	H. O. Jeschke, F. Salvat-Pujol, R. Valentí
    	First-principles determination of Heisenberg Hamiltonian parameters for the spin-1/2 kagome antiferromagnet ZnCu3(OH)6Cl2
    	Phys. Rev. B 88, 075106 (2013) (doi,pdf), (abstract).(hide abstract). Herbertsmithite (ZnCu3(OH)6Cl2) is often discussed as the best realization of the highly frustrated antiferromagnetic kagome lattice known so far. We employ density functional theory calculations to determine eight exchange coupling constants of the underlying Heisenberg Hamiltonian. We find the nearest neighbour coupling J1 to exceed all other couplings by far. However, next-nearest neighbour kagome layer couplings of 0.019 J1 and interlayer couplings of up to -0.035 J1 slightly modify the perfect antiferromagnetic kagome Hamiltonian. Interestingly, the largest interlayer coupling is ferromagnetic even without Cu impurities in the Zn layer. In addition, we validate our DFT approach by applying it to kapellasite, a polymorph of herbertsmithite which is known experimentally to exhibit competing exchange interactions.

78. 	H. O. Jeschke, I. I. Mazin, R. Valentí
    	Why MgFeGe is not a superconductor
    	Phys. Rev. B 87, 241105(R) (2013) (doi,pdf), (abstract).(hide abstract). The recently synthesized MgFeGe compound is isostructural and isoelectronic with superconducting LiFeAs. Both materials are paramagnetic metals at room temperature. Inspection of their electronic structures without spin polarization reveals hardly any difference between the two. This fact was interpreted as evidence against popular theories relating superconductivity in Fe-based materials with spin fluctuations. We show that in the magnetic domain the two compounds are dramatically different, and the fact that MgFeGe does not superconduct, is, on the contrary, a strong argument in favor of theories based on spin fluctuations.

77. 	A. Ruff, M. Sing, R. Claessen, H. Lee, M. Tomić, H. O. Jeschke, R. Valentí
    	Absence of metallicity in K-doped picene: Importance of electronic correlations
    	Phys. Rev. Lett. 110, 216403 (2013) (doi,pdf,supplement), (abstract).(hide abstract). Potassium-doped picene (Kxpicene) has recently been reported to be a superconductor at x = 3 with critical temperatures up to 18 K. Here we study the electronic structure of K-doped picene films by photoelectron spectroscopy and ab initio density functional theory combined with dynamical mean-field theory (DFT+DMFT). Experimentally we observe that, except for spurious spectral weight due to the lack of a homogeneous chemical potential at low K-concentrations (x ≈ 1), the spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations which provide clear evidence that Kxpicene is a Mott insulator for integer doping concentrations x = 1, 2, and 3. We discuss various scenarios to understand the discrepancies with previous reports of superconductivity and metallic behavior.

76. 	I. I. Mazin, S. Manni, K. Foyevtsova, H. O. Jeschke, P. Gegenwart, R. Valentí
    	Origin of the insulating state in honeycomb iridates and rhodates
    	Phys. Rev. B 88, 035115 (2013) (doi,pdf), (abstract).(hide abstract). A burning question in the emerging field of spin-orbit driven insulating iridates, such as Na2IrO3 and Li2IrO3 is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic jeff = 1/2 band or as a band insulator where the gap is assisted by spin-orbit interaction, or Coulomb correlations, or both. The difference between these two interpretations is that only for the former, strong spin-orbit coupling (λ >~ W; where W is the band width) is essential. We have synthesized the isostructural and isoelectronic Li2RhO3 and report its electrical resistivity and magnetic susceptibility. Remarkably it shows insulating behavior together with fluctuating effective S = 1/2 moments, similar to Na2IrO3 and Li2IrO3, although in Rh4+ (4d5) the spin-orbit coupling is greatly reduced. We show that this behavior has non-relativistic one-electron origin (although Coulomb correlations assist in opening the gap), and can be traced down to formation of quasi-molecular orbitals, similar to those in Na2IrO3.

75. 	K. Foyevtsova, H. O. Jeschke, I. I. Mazin, D. I. Khomskii, R. Valentí
    	Ab initio analysis of the tight-binding parameters and magnetic interactions in Na2IrO3
    	Phys. Rev. B 88, 035107 (2013) (doi,pdf), (abstract).(hide abstract). By means of density functional theory (DFT) calculations [with and without inclusion of spin-orbit (SO) coupling] we present a detailed study of the electronic structure and corresponding microscopic Hamiltonian parameters of Na2IrO3. In particular, we address the following aspects: (i) We investigate the role of the various structural distortions and show that the electronic structure of Na2IrO3 is exceptionally sensitive to structural details. (ii)We discuss both limiting descriptions for Na2IrO3 — quasimolecular orbitals (small SO limit, itinerant) versus relativistic orbitals (large SO limit, localized) — and show that the description of Na2IrO3 lies in an intermediate regime. (iii) We investigate whether the nearest neighbor Kitaev-Heisenberg model is sufficient to describe the electronic structure and magnetism in Na2IrO3. In particular, we verify the recent suggestion of an antiferromagnetic Kitaev interaction and show that it is not consistent with actual or even plausible electronic parameters. Finally, (iv) we discuss correlation effects in Na2IrO3.We conclude that while the Kitaev-Heisenberg Hamiltonian is the most general expression of the quadratic spin-spin interaction in the presence of spin-orbit coupling (neglecting single-site anisotropy), the itinerant character of the electrons in Na2IrO3 makes other terms beyond this model (including, but not limited to, 2nd and 3rd neighbor interactions) essential.

74. 	H. Lee, Y.-Z. Zhang, H. Lee, Y. Kwon, H. O. Jeschke, R. Valentí
    	Semiclassical approximation solved by Monte Carlo integration as an efficient impurity solver for dynamical mean field theory and its cluster extensions
    	Phys. Rev. B 88, 165126 (2013) (doi,pdf), (abstract).(hide abstract). We propose that a combination of the semiclassical approximation with Monte Carlo simulations can be an efficient and reliable impurity solver for dynamical mean field theory equations and their cluster extensions with large cluster sizes. In order to show the reliability of the method, we consider two test cases: (i) the single-band Hubbard model within the dynamical cluster approximation with 4- and 8-site clusters and (ii) the anisotropic two-orbital Hubbard model with orbitals of different band width within the single-site dynamical mean field theory. We compare our results with those obtained from solving the dynamical mean field equations with continuous time and determinant quantum Monte Carlo. In both test cases we observe reasonable values of the metal-insulator critical interaction strength Uc/t and, while some details of the spectral functions cannot be captured by the semiclassical approximation due to the freezing of dynamical fluctuations, the main features are reproduced by the approach.

73. 	A. C. Jacko, H. Feldner, E. Rose, F. Lissner, M. Dressel, R. Valentí, H. O. Jeschke
    	Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions
    	Phys. Rev. B 87, 155139 (2013) (doi,pdf), (abstract).(hide abstract). Using density functional theory, we determine parameters of tight-binding Hamiltonians for a variety of Fabre charge transfer salts, focusing in particular on the effects of temperature and pressure. Besides relying on previously published crystal structures, we experimentally determine two new sets of structures; (TMTTF)2SbF6 at different temperatures and (TMTTF)2PF6 at various pressures. We find that a few trends in the electronic behavior can be connected to the complex phase diagram shown by these materials. Decreasing temperature and increasing pressure cause the systems to become more two-dimensional. We analyze the importance of correlations by considering an extended Hubbard model parameterized using Wannier orbital overlaps and show that while charge order is strongly activated by the inter-site Coulomb interaction, the magnetic order is only weakly enhanced. Both orders are suppressed when the effective pressure is increased.

72. 	N. Medvedev, H. O. Jeschke, B. Ziaja
    	Nonthermal phase transitions in semiconductors induced by a femtosecond extreme ultraviolet laser pulse
    	New J. Phys. 15, 015016 (2013) (doi,pdf), (abstract).(hide abstract). In this paper, we present a novel theoretical approach, which allows the study of nonequilibrium dynamics of both electrons and atoms/ions within free-electron laser excited semiconductors at femtosecond time scales. The approach consists of the Monte-Carlo method treating photoabsorption, highenergy- electron and core-hole kinetics and relaxation processes. Low-energy electrons localized within the valence and conduction bands of the target are treated with a temperature equation, including source terms, defined by the exchange of energy and particles with high-energy electrons and atoms. We follow the atomic motion with the molecular dynamics method on the changing potential energy surface. The changes of the potential energy surface and of the electron band structure are calculated at each time step with the help of the tight-binding method. Such a combination of methods enables investigation of nonequilibrium structural changes within materials under extreme ultraviolet (XUV) femtosecond irradiation. Our analysis performed for diamond irradiated with an XUV femtosecond laser pulse predicts for the first time in this wavelength regime the nonthermal phase transition from diamond to graphite.

71. 	M. Tomić, H. O. Jeschke, R. M. Fernandez, R. Valentí
    	Uniaxial strain effects on the structural and electronic properties of BaFe2As2 and CaFe2As2
    	Phys. Rev. B 87, 174503 (2013) (doi,pdf), (abstract).(hide abstract). Starting from the orthorhombic magnetically ordered phase, we investigate the effects of uniaxial tensile and compressive strains along a, b, and the diagonal a+b directions in BaFe2As2 and CaFe2As2 in the framework of ab initio density functional theory (DFT) and a phenomonological Ginzburg-Landau model. While -contrary to the application of hydrostatic or c-axis uniaxial pressure- both systems remain in the orthorhombic phase with a pressure-dependent nonzero magnetic moment, we observe a sign-changing jump in the orthorhombicity at a critical uniaxial pressure, accompanied by a reversal of the orbital occupancy and a switch between the ferromagnetic and antiferromagnetic directions. Our Ginzburg-Landau analysis reveals that this behavior is a direct consequence of the competition between the intrinsic magneto-elastic coupling of the system and the applied compressive stress, which helps the system to overcome the energy barrier between the two possible magneto-elastic ground states. Our results shed light on the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal and on the changes in the magnetic properties of a system under uniaxial stress.

70. 	F. Salvat-Pujol, H. O. Jeschke, R. Valentí
    	Simulation of electron transport during electron-beam-induced deposition of nanostructures
    	Beilstein J. Nanotechnol. 4, 781 (2013) (doi,pdf), (abstract).(hide abstract). We present a numerical investigation of energy and charge distributions during electron-beam-induced growth of tungsten nanostructures on SiO2 substrates by using a Monte Carlo simulation of the electron transport. This study gives a quantitative insight into the deposition of energy and charge in the substrate and in the already existing metallic nanostructures in the presence of the electron beam. We analyze electron trajectories, inelastic mean free paths, and the distribution of backscattered electrons in different compositions and at different depths of the deposit. We find that, while in the early stages of the nanostructure growth a significant fraction of electron trajectories still interacts with the substrate, when the nanostructure becomes thicker the transport takes place almost exclusively in the nanostructure. In particular, a larger deposit density leads to enhanced electron backscattering. This work shows how mesoscopic radiation-transport techniques can contribute to a model that addresses the multi-scale nature of the electron-beam-induced deposition (EBID) process. Furthermore, similar simulations can help to understand the role that is played by backscattered electrons and emitted secondary electrons in the change of structural properties of nanostructured materials during post-growth electron-beam treatments.

69. 	S. Backes, H. O. Jeschke
    	Finite temperature and pressure molecular dynamics for BaFe2As2
    	Phys. Rev. B 88, 075111 (2013) (doi,pdf), (abstract).(hide abstract). We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe2As2. We use density functional theory-based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T = 5 to 150 K and pressures from P = 0 to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. These transitions are considerably smeared out at finite temperature, with the critical pressure for the first transition increasing with temperature. We also analyze the electronic structure of BaFe2As2 at finite temperature and the effect of the structural oscillations on the band structure and Fermi surface in comparison to known zero-temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe2As2 under high pressure.

68. 	L. F. Tocchio, H. Lee, H. O. Jeschke, R. Valentí, C. Gros
    	Mott correlated states in the underdoped two-dimensional Hubbard model: variational Monte Carlo versus a dynamical cluster approximation
    	Phys. Rev. B 87, 045111 (2013) (doi,pdf), (abstract).(hide abstract). We investigate the properties of the frustrated underdoped Hubbard model on the square lattice using two complementary approaches, i.e., the dynamical cluster extension of the dynamical mean-field theory and the variational Monte Carlo simulations of Gutzwiller-Jastrow wave functions with backflow corrections. We compare and discuss data for the energy and the double occupancies, as obtained from both approaches. At small dopings, we observe a rapid crossover from a weakly correlated metal at low interaction strength U to a non-Fermi-liquid correlated state with strong local spin correlations. Furthermore, we investigate the stability of the correlated state against phase separation. We observe phase separation only for large values of U or very large frustration. No phase separation is present for the parameter range relevant for the cuprates.

     2012 

67. 	I. I. Mazin, H. O. Jeschke, K. Foyevtsova, R. Valentí, D. I. Khomskii
    	Na2IrO3 as a molecular orbital crystal
    	Phys. Rev. Lett. 109, 197201 (2012) (doi,pdf,supplement), (abstract).(hide abstract). Contrary to previous studies that classify Na2IrO3 as a realization of the Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasi-molecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow from the QMO model.

66. 	J. Ferber, H. O. Jeschke, R. Valentí
    	Fermi surface topology of LaFePO and LiFeP
    	Phys. Rev. Lett. 109, 236403 (2012) (doi,pdf,supplement), (abstract).(hide abstract). We perform charge self-consistent LDA+DMFT (density functional theory combined with dynamical mean field theory) calculations to study correlation effects on the Fermi surfaces of the iron pnictide superconductors LaFePO and LiFeP. We find a distinctive change in the topology of the Fermi surface in both compounds where a hole pocket with Fe dz2 orbital character changes its geometry from a closed shape in LDA to an open shape upon inclusion of correlations. The opening of the pocket occurs in the vicinity of the Γ (Z) point in LaFePO (LiFeP). We discuss the relevance of these findings for the low superconducting transition temperature and the nodal gap observed in these materials.

65. 	J. Shen, H. Lee, R. Valentí, H. O. Jeschke
    	Ab initio study of the two-dimensional metallic state at the surface of SrTiO3: importance of oxygen vacancies
    	Phys. Rev. B 86, 195119 (2012) (doi,pdf), (abstract).(hide abstract). Motivated by recent angle-resolved photoemission spectroscopy (ARPES) observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO3 and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state, we analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO3 surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies - independently of the oxygen concentration - redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a 2DEG. We discuss the underlying model emerging from such observations.

64. 	R. Wallauer, S. Voss, L. Foucar, T. Bauer, D. Schneider, J. Titze, B. Ulrich, K. Kreidi, N. Neumann, T. Havermeier, M. Schöffler, T. Jahnke, A. Czasch, L. Schmidt, A. Kanigel, J. C. Campuzano, H. Jeschke, R. Valentí, A. Müller, G. Berner, M. Sing, R. Claessen, H. Schmidt-Böcking, R. Dörner
    	Momentum spectrometer for electron-electron coincidence studies on superconductors
    	Rev. Sci. Instrum. 83, 103905 (2012) (doi,pdf), (abstract).(hide abstract). We present a new experimental setup to study electron-electron coincidences from superconducting surfaces. In our approach, electrons emitted from a surface are projected onto a time- and positionsensitive microchannel plate detector with delayline position readout. Electrons that are emitted within 2 π solid angle with respect to the surface are detected in coincidence. The detector used is a hexagonal delayline detector with enhanced multiple hit capabilities. It is read out with a Flash analog-to-digital converter. The three-dimensional momentum vector is obtained for each electron. The intrinsic dead time of the detector has been greatly reduced by implementing a new algorithm for pulse analysis. The sample holder has been matched to fit the spectrometer while being capable of cooling down the sample to 4.5 K during the measurement and heating it up to 420 K for the cleaning procedure.

63. 	K. Muthukumar, R. Valentí, H. O. Jeschke
    	Simulation of structural and electronic properties of amorphous tungsten oxycarbides
    	New. J. Phys. 14, 113028 (2012) (doi,pdf), (abstract).(hide abstract). Electron beam induced deposition with tungsten hexacarbonyl W(CO)6 as precursors leads to granular deposits with varying compositions of tungsten, carbon and oxygen. Depending on the deposition conditions, the deposits are insulating or metallic. We employ an evolutionary algorithm to predict the crystal structures starting from a series of chemical compositions that were determined experimentally. We show that this method leads to better structures than structural relaxation based on guessed initial structures. We approximate the expected amorphous structures by reasonably large unit cells that can accommodate local structural environments that resemble the true amorphous structure. Our predicted structures show an insulator to metal transition close to the experimental composition at which this transition is actually observed. Our predicted structures also allow comparison to experimental electron diffraction patterns.

62. 	K. Muthukumar, H. O. Jeschke, R. Valentí, E. Begun, J. Schwenk, F. Porrati, and M. Huth
    	Spontaneous Dissociation of Co2(CO)8 and Autocatalytic growth of Co on SiO2 : A Combined Experimental and Theoretical Investigation
    	Beilstein J. Nanotechnol. 3, 546 (2012) (doi,pdf), (abstract).(hide abstract). We present experimental results and theoretical simulations of the adsorption behavior of the metal-organic precursor Co2(CO)8 on SiO2 surfaces after application of two different pre-treatment steps, namely by air plasma cleaning or a focused electron beam pre-irradiation. We observe a spontaneous dissociation of the precursor molecules as well as auto-deposition of cobalt on the pre-treated SiO2 surfaces. We also find that the differences in metal content and relative stability of these deposits depend on the pre-treatment conditions of the substrate. Transport measurements of these deposits are also presented. We are led to assume that the degree of passivation of the SiO2 surface by hydroxyl groups is an important controlling factor in the dissociation process. Our calculations of various slab settings using dispersion corrected density functional theory support this assumption. We observe physisorption of the precursor molecule on a fully hydroxylated SiO2 surface (untreated surface) and chemisorption on a partially hydroxylated SiO2 surface (pre-treated surface) with a spontaneous dissociation of the precursor molecule. In view of these calculations, we discuss the origin of this dissociation and the subsequent autocatalysis.

61. 	K. Medjanik, D. Chercka, P. Nagel, M. Merz, S. Schuppler, M. Baumgarten, K. Müllen, S. A. Nepijko, H. J. Elmers, Gerd Schönhense, H. O. Jeschke, and R. Valentí
    	Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane - A NEXAFS Study
    	J. Am. Chem. Soc. 134, 4694 (2012) (doi), (abstract).(hide abstract). It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a1 of the isolated methoxy group) and acceptor (orbitals b3g, au, b1g, and b2u, all located at the cyano group) with π*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.

60. 	J. Shen, K. Muthukumar, H. O. Jeschke, R. Valentí
    	Physisorption of an organometallic platinum complex on silica. An ab initio study
    	New J. Phys. 14, 073040 (2012) (doi,pdf), (abstract).(hide abstract). The interaction of trimethyl methylcyclopentadienyl platinum (MeCpPtMe3) with a fully hydroxylated SiO2 surface has been explored by means of ab initio calculations. A large slab model cut out from the hydroxylated beta-cristobalite SiO2 (111) surface was chosen to simulate a silica surface. Density functional theory calculations were performed to evaluate the energies of MeCpPtMe3 adsorption to the SiO2 surface. Our results show that the physisorption of the molecule is dependent on both (i) the orientation of the adsorbate and (ii) the adsorption site on the substrate. The most stable configuration was found with the MeCp and Me3 groups of the molecule oriented towards the surface. Finally, we observe that van-der-Waals corrections are crucial for the stabilization of the molecule on the surface. We discuss the relevance of our results for the growth of Pt-based nanostructured materials via deposition processes such as electron beam induced deposition.

59. 	M. Pregelj, H. O. Jeschke, H. Feldner, R. Valentí, A. Honecker, T. Saha-Dasgupta, H. Das, S. Yoshii, T. Morioka, H. Nojiri, H. Berger, A. Zorko, O. Zaharko, D. Arčon
    	Multiferroic FeTe2O5Br: Alternating spin chains with frustrated interchain interactions
    	Phys. Rev. B 86, 054402 (2012) (doi,pdf), (abstract).(hide abstract). A combination of density functional theory calculations, many-body model considerations, and magnetization and electron-spin-resonance measurements shows that the multiferroic FeTe2O5Br should be described as a system of alternating antiferromagnetic S=5/2 chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer model. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization.

58. 	Harald O. Jeschke, Mariano de Souza, R. Valentí, Rudra Sekhar Manna, Michael Lang, and John A. Schlueter
    	Temperature dependence of structural and electronic properties of the spin-liquid candidate κ-(BEDT-TTF)2Cu2(CN)3
    	Phys. Rev. B 85, 035125 (2012) (doi,pdf), (abstract).(hide abstract). We investigate the effect that the temperature dependence of the crystal structure of a two dimensional organic charge-transfer salt has on the low-energy Hamiltonian representation of the electronic structure. For that, we determine the crystal structure of κ-(BEDT-TTF)2Cu2(CN)3 for a series of temperatures between T=5 K and 300 K by single crystal X-ray diffraction and analyze the evolution of the electronic structure with temperature by using density functional theory and tight binding methods. We find a considerable temperature dependence of the corresponding triangular lattice Hubbard Hamiltonian parameters. We conclude that even in the absence of change of symmetry, the temperature dependence of quantities like frustration and interaction strength can be significant and should be taken into account.

57. 	Yu-Zhong Zhang, Hunpyo Lee, Hai-Qing Lin, Chang-Qin Wu, Harald O. Jeschke, and Roser Valentí
    	A General Mechanism for Orbital Selective Phase Transitions
    	Phys. Rev. B 85, 035123 (2012) (doi,pdf), (abstract).(hide abstract). Based on the analysis of a two-orbital Hubbard model within a mean-field approach, we propose a mechanism for an orbital selective phase transition (OSPT) where coexistence of localized and itinerant electrons can be realized. We show that this OSPT exists both at and near half filling even in the absence of crystal field splittings or when bandwidths, orbital degeneracies and magnetic states are equal for both orbitals provided the orbitals have different band dispersions. Such conditions should be generally satisfied in many materials. We find that this OSPT is not sensitive to the strength of Hund′s rule coupling and that heavy doping favors the collinear antiferromagnetic state over the OSPT. We discuss our results in relation to the iron pnictides.

56. 	Johannes Ferber, Kateryna Foyevtsova, Roser Valentí, and Harald O. Jeschke
    	LDA+DMFT study of the effects of correlation in LiFeAs
    	Phys. Rev. B 85, 094505 (2012) (doi,pdf), (abstract).(hide abstract). We discuss the role of electronic correlations in the iron-based superconductor LiFeAs by studying the effects on band structure, mass enhancements, and Fermi surface in the framework of density functional theory combined with dynamical mean field theory calculations. We conclude that LiFeAs shows characteristics of a moderately correlated metal and that the strength of correlations is mainly controlled by the value of the Hund′s rule coupling J. The hole pockets of the Fermi surface show a distinctive change in form and size with implications for the nesting properties. Our calculations are in good agreement with recent angle-resolved photoemission spectroscopy and de Haas-van Alphen experiments.

55. 	Hunpyo Lee, Kateryna Foyevtsova, Johannes Ferber, Markus Aichhorn, Harald O. Jeschke, and Roser Valentí
    	Dynamical cluster approximation within an augmented plane-wave framework: Spectral properties of SrVO3
    	Phys. Rev. B 85, 165103 (2012) (doi,pdf), (abstract).(hide abstract). We present a combination of local-density approximation (LDA) with the dynamical cluster approximation (LDA+DCA) in the framework of the full-potential linear augmented plane wave method, and compare our LDA+DCA results for SrVO3 to LDA with the dynamical mean-field theory (LDA+DMFT) calculations as well as experimental observations on SrVO3. We find a qualitative agreement of the momentum resolved spectral function with angle-resolved photoemission spectra (ARPES) and former LDA+DMFT results. As a correction to LDA+DMFT, we observe more pronounced coherent peaks below the Fermi level, as indicated by ARPES experiments. In addition, we resolve the spectral functions in the K0=(0,0,0) and K1=(π,π,π) sectors of DCA, where band insulating and metallic phases coexist. Our approach can be applied to correlated compounds where not only local quantum fluctuations but also spatial fluctuations are important.

54. 	Milan Tomić, Roser Valentí, and Harald O. Jeschke
    	Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2 and BaFe2As2
    	Phys. Rev. B 85, 094105 (2012) (doi,pdf), (abstract).(hide abstract). We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to superconductivity.

     2011 

53. 	Kaliappan Muthukumar, Ingo Opahle, Juan Shen, Harald O. Jeschke, and Roser Valentí
    	Interaction of W(CO)6 with SiO2 Surfaces -- A Density Functional Study
    	Phys. Rev. B 84, 205442 (2011) (doi,pdf), (abstract).(hide abstract). The interaction of tungsten hexacarbonyl W(CO)6 precursor molecules with SiO2 substrates is investigated by means of density functional theory calculations with and without inclusion of long range van der Waals interactions. We consider two different surface models, a fully hydroxylated and a partially hydroxylated SiO2 surface, corresponding to substrates under different experimental conditions. For the fully hydroxylated surface we observe only a weak interaction between the precursor molecule and the substrate with physisorption of W(CO)6. Inclusion of van der Waals corrections results in a stabilization of the molecules on this surface, but does not lead to significant changes in the chemical bonding. In contrast, we find a spontaneous dissociation of the precursor molecule on the partially hydroxylated SiO2 surface where chemisorption of a W(CO)5 fragment is observed upon removal of one of the CO ligands from the precursor molecule. Irrespective of the hydroxylation, the precursor molecule prefers binding of more than one of its CO ligands. In the light of these results, implications for the initial growth stage of tungsten nano-deposits on SiO2 in an electron beam induced deposition process are discussed.

52. 	H. Lee, Y.-Z. Zhang, H. O. Jeschke, and Roser Valentí
    	Anisotropic two-orbital Hubbard model: Single-site versus cluster Dynamical Mean-Field Theory
    	Ann. Phys. (Berlin) 523, 689 (2011) (doi,pdf), (abstract).(hide abstract). The anisotropic two-orbital Hubbard model with different bandwidths and degrees of frustration in each orbital is investigated in the framework of both single-site dynamical mean-field theory (DMFT) as well as its cluster extension (DCA) for clusters up to four sites combined with a continuous-time quantum Monte Carlo algorithm. This model shows a rich phase diagram which includes the appearance of orbital selective phase transitions, non-Fermi liquid behavior as well as antiferromagnetic metallic states. We discuss the advantages and drawbacks of employing the single-site DMFT with respect to DCA and the consequences for the physical picture obtained out of these calculations. Finally, we argue that such a minimal model may be of relevance to understand the nature of the antiferromagnetic metallic state in the iron-pnictide superconductors as well as the origin of the small staggered magnetization observed in these systems.

51. 	H. O. Jeschke, I. Opahle, H. Kandpal, R. Valentí, H. Das, T. Saha-Dasgupta, O. Janson, H. Rosner, A. Brühl, B. Wolf, M. Lang, J. Richter, S. Hu, X. Wang, R. Peters, T. Pruschke, A. Honecker
    	Multi-step approach to microscopic models for frustrated quantum magnets: The case of the natural mineral azurite
    	Phys. Rev. Lett. 106, 217201 (2011). (doi,pdf,EPAPS), (abstract).(hide abstract). The natural mineral azurite Cu3(CO3)2(OH)2 is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

50. 	P. T. Cong, B. Wolf, M. de Souza, N. Krüger, A. A. Haghighirad, S. Gottlieb-Schönmeyer, F. Ritter, W. Aßmus, I. Ophale, K. Foyevtsova, H. O. Jeschke, R. Valentí, L. Wiehl, M. Lang
    	Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs2CuCl4-xBrx
    	Phys. Rev. B 83, 064425 (2011). (doi,pdf), (abstract).(hide abstract). We report on a systematic study of the magnetic properties on single crystals of the solid solution Cs2CuCl4-xBrx (0 ≤ x ≤ 4), which include the two known end-member compounds Cs2CuCl4 and Cs2CuBr4, classified as quasi-two-dimensional quantum antiferromagnets with different degrees of magnetic frustration. By comparative measurements of the magnetic susceptibility χ(T) on as many as eighteen different Br concentrations, we found that the inplane and out-of-plane magnetic correlations, probed by the position and height of a maximum in the magnetic susceptibility, respectively, do not show a smooth variation with x. Instead three distinct concentration regimes can be identified, which are separated by critical concentrations xc1 = 1 and xc2 = 2. This unusual magnetic behavior can be explained by considering the structural peculiarities of the materials, especially the distorted Cu-halide tetrahedra, which support a site-selective replacement of Cl- by Br- ions. Consequently, the critical concentrations xc1 (xc2) mark particularly interesting systems, where one (two) halidesublattice positions are fully occupied.

49. 	H. Lee, Y. Z. Zhang, H. O. Jeschke, Roser Valentí
    	Orbital selective phase transition induced by different magnetic states: A dynamical cluster approximation study
    	Phys. Rev. B 84, 020401(R) (2011). (doi,pdf), (abstract).(hide abstract). By considering the dynamical cluster approximation combined with the continuous time quantum Monte Carlo algorithm, we analyze the behavior of a degenerate two-orbital anisotropic Hubbard model at half filling where both orbitals have equal bandwidths and one orbital is constrained to be paramagnetic (PM) (PM orbital), while the second one is allowed to have an antiferromagnetic (AF) solution (AF orbital). As the interaction increases, novel orbital selective phase transitions induced by different magnetic states in different orbitals appear regardless of the strength of the Ising Hund′s rule coupling Jz. Moreover, the PM orbital undergoes a transition from a Fermi liquid (FL) to a Mott insulator through an intermediate non-FL phase while the AF orbital shows a transition from a FL to an AF insulator through an intermediate AF metallic phase. Finally, the phase diagram of the model is presented and possible applications of the model to some aspects of the physics of iron pnictides are discussed.

48. 	N.H. Phan, I. Halasz, I. Opahle, E. Alig, L. Fink, J.W. Bats, P.T. Cong, H.-W. Lerner, B. Sarkar, B. Wolf, H.O. Jeschke, M. Lang, Roser Valentí, R. Dinnebier and M. Wagner
    	Thermally induced crystal-to-crystal transformations accompanied by changes in the magnetic properties of a CuII-p-hydroquinonate polymer
    	CrystEngComm 13, 391 (2011). (doi, pdf), (abstract).(hide abstract). In the CuII-p-hydroquinonate coordination polymer 1, the major pathway for antiferromagnetic exchange coupling runs along the hydroquinonate linker. Upon heating, 1 looses its supporting DMF ligands in a two-step sequence; the antiferromagnetic CuII-CuII interaction in the final product 3 is now mediated by two bridging oxygen atoms which results in an increase of the J value by two orders of magnitude.

47. 	K. Foyevtsova, I. Opahle, Y.-Z. Zhang, H. O. Jeschke, Roser Valentí
    	Determination of effective microscopic models for the frustrated antiferromagnets Cs2CuCl4 and Cs2CuBr4 by density functional methods
    	Phys. Rev. B 83, 125126 (2011). (doi, pdf), (abstract).(hide abstract). We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs2CuCl4 and Cs2CuBr4 in the framework of density functional theory. Analysis of the exchange couplings J and J' using the available X-ray structural data corroborates the values obtained from experimental results for Cs2CuBr4 but not for Cs2CuCl4. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs2CuCl4 employing different exchange-correlation functionals. We find that the exchange couplings depend on rather subtle details of the structural optimization and that only when the insulating state (mediated through spin polarization) is present in the structural optimization, we do have good agreement between the calculated and the experimentally determined exchange couplings. Finally, we discuss the effect of interlayer couplings as well as longer-ranged couplings in both systems.

46. 	Y.-Z. Zhang, H. Lee, I. Opahle, H. O. Jeschke, R. Valentí
    	Importance of Fermi Surface Nesting and Quantum Fluctuations for the Magnetism in Iron Pnictides
    	J. Phys. Chem. Solids 72, 324 (2011). (doi, pdf), (abstract).(hide abstract). By applying density functional theory, we find strong evidence for an itinerant nature of magnetism in two families of iron pnictides. Furthermore, by employing dynamical mean field theory with continuous time quantum Monte Carlo as an impurity solver, we observe that the antiferromagnetic metal with small magnetic moment naturally arises out of coupling between unfrustrated and frustrated bands. Our results point to a possible scenario for magnetism in iron pnictides where magnetism originates from a strong instability at the momentum vector (π,π,π) while it is reduced by quantum fluctuations due to the coupling between weakly and strongly frustrated bands.

     2010 

45. 	S. A. J. Kimber, A. H. Hill, Y.-Z. Zhang, H. O. Jeschke, R. Valentí, C. Ritter, I. Schellenberg, W. Hermes, R. Pöttgen, D. N. Argyriou
    	Local moments and symmetry breaking in metallic PrMnSbO
    	Phys. Rev. B 82, 100412(R) (2010). (doi, pdf), (abstract).(hide abstract). We report a combined experimental and theoretical investigation of the layered antimonide which is isostructural to the parent phase of the iron pnictide superconductors. We find linear resistivity near room temperature and Fermi liquid-like T2 behaviour below 150 K. Neutron powder diffraction shows that unfrustrated C-type Mn magnetic order develops below ~ 230 K, followed by a spin-flop coupled to induced Pr order. At T ~ 35 K, we find a tetragonal to orthorhombic (T-O) transition. First principles calculations show that the large magnetic moments observed in this metallic compound are of local origin. Our results are thus inconsistent with either the itinerant or frustrated models proposed for symmetry breaking in the iron pnictides. We show that  is instead a rare example of a metal where structural distortions are driven by f-electron degrees of freedom.

44. 	K. Foyevtsova, H. C. Kandpal, H. O. Jeschke, S. Graser, H.-P. Cheng, Roser Valentí, P. J. Hirschfeld
    	Modulation of pairing interaction in Bi2Sr2CaCu2O8+δ by an O dopant: a density functional theory study
    	Phys. Rev. B 82, 054514 (2010). (doi, pdf), (abstract).(hide abstract). Scanning tunneling spectroscopy measurements on the high temperature superconductor Bi2Sr2CaCu2O8+δ have reported an enhanced spectral gap in the neighborhood of O dopant atoms. We calculate, within density functional theory (DFT), the change in electronic structure due to such a dopant. We then construct and discuss the validity of several tight binding (TB) fits to the DFT bands with and without an O dopant. With the doping-modulated TB parameters, we finally evaluate the spin susceptibility and pairing interaction within spin fluctuation theory. The d-wave pairing eigenvalues are enhanced above the pure system without O dopant, supporting the picture of enhanced local pairing around such a defect.

43. 	J. Ferber, Y.-Z. Zhang, H. O. Jeschke, R. Valentí
    	Analysis of spin density wave conductivity spectra of iron pnictides in the framework of density functional theory
    	Phys. Rev. B 82, 165102 (2010). (doi, pdf), (abstract).(hide abstract). The optical conductivity of LaFeAsO, BaFe2As2, SrFe2As2, and EuFe2As2 in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spinpolarized generalized gradient approximation (GGA) and GGA+U. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, GGA+Ueff with a suitable choice of negative on-site interaction Ueff = U - J was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small U compared to the Hund′s rule coupling J, as well as a strong Holstein-like electron-phonon interaction. Moreover, reminiscent of the fact that GGA+Ueff with a positive Ueff is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative Ueff can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital resolved analysis of the optical conductivity reveals significant contributions from all Fe 3d orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.

42. 	Y.-Z. Zhang, I. Opahle, H. O. Jeschke, R. Valentí
    	Itinerant Nature of Magnetism in Iron Pnictides: A first principles study
    	Phys. Rev. B 81, 094505 (2010). (doi, pdf), (abstract).(hide abstract). Within the framework of ab initio molecular dynamics we investigate the nature of magnetism in various families of Fe-based superconductors. (i) We show that magnetization of stripe-type antiferromagnetic order always becomes stronger when As is substituted by Sb in LaOFeAs, BaFe2As2 and LiFeAs. By calculating Pauli susceptibilities, we attribute the magnetization increase obtained after replacing As by Sb to the enhancement of an instability at (π,π). This points to a strong connection between Fermi surface nesting and magnetism, which supports the theory of the itinerant nature of magnetism in various families of Fe-based superconductors. (ii) We find that within the family LaOFePn (Pn=P, As, Sb, Bi) the absence of an antiferromagnetic phase in LaOFeP and its presence in LaOFeAs can be attributed to the competition of instabilities in the Pauli susceptibility at (π,π) and (0,0), which further strengthens the close relation between Fermi surface nesting and experimentally observed magnetization. (iii) Finally, based on our molecular dynamics and Pauli susceptibility results, we predict that LaOFeSb upon doping or application of pressure should be a candidate for a superconductor with the highest transition temperature among the not yet synthesized compounds LaOFeSb, LaOFeBi, ScOFeP and ScOFeAs while the parent compounds LaOFeSb and LaOFeBi should show at ambient pressure a stripe-type antiferromagnetic metallic state.

41. 	H. Lee, Y.-Z. Zhang, H. O. Jeschke, R. Valentí
    	Possible origin of the reduced ordered moment in iron pnictides: a Dynamical Mean Field Theory study
    	Phys. Rev. B 81, 220506(R) (2010). (doi, pdf), (abstract).(hide abstract). We investigate the phase diagram of a two-band frustrated Hubbard model in the framework of dynamical mean field theory. While a first-order phase transition occurs from a paramagnetic (PM) metal to an antiferromagnetic (AF) insulator when both bands are equally frustrated, an intermediate AF metallic phase appears in each band at different Uc values if only one of the two bands is frustrated, resulting in continuous orbital-selective phase transitions from PM metal to AF metal and AF metal to AF insulator, regardless of the strength of the Ising Hund′s coupling. We argue that our minimal model calculations capture the frustration behavior in the undoped iron-pnictide superconductors as well as local quantum fluctuation effects and that the intermediate phases observed in our results are possibly related to the puzzling AF metallic state with small staggered magnetization observed in these systems as well as to the pseudogap features observed in optical experiments.

40. 	H. Lee, Y.-Z. Zhang, H. O. Jeschke, R. Valentí, H. Monien
    	Dynamical cluster approximation study of the anisotropic two-orbital Hubbard model
    	Phys. Rev. Lett. 104, 026402 (2010). (doi, pdf), (abstract).(hide abstract). We investigate the properties of a two-orbital Hubbard model with unequal bandwidths on the square lattice in the framework of the dynamical cluster approximation (DCA) combined with a continuous-time quantum Monte Carlo (CT QMC) algorithm. We explore the effect of short-range spatial fluctuations on the nature of the metal-insulator transition and the possible occurrence of an orbital-selective Mott transition (OSMT), as a function of cluster size Nc. We observe that for Nc=2 no OSMT is present, instead a band insulator state for both orbitals is stabilized at low temperatures due to the appearance of an artificial local ordered state. For Nc=4 the DCA calculations suggest the presence of five different phases which originate out of the cooperation and competition between spatial fluctuations and orbitals of different bandwidths and a OSMT phase is stabilized. Based on our results, we discuss the nature of the gap opening.

39. 	H. O. Jeschke, H. C. Kandpal, I. Opahle, Y.-Z. Zhang, R. Valentí
    	First principles determination of the model parameters in κ-(ET)2Cu2(CN)3
    	Physica B 405, S224 (2010). (doi, pdf), (abstract).(hide abstract). We present a detailed study of the derivation of the Hubbard model parameters for κ-(ET)2Cu2(CN)3 in the framework of ab initio Density Functional Theory. We show that calculations with different (i) wavefunction basis, (ii) exchange correlation functionals and (iii) tight-binding models provide a reliable benchmark for the parameter values. We compare our results with available extended Hückel molecular orbital calculations and discuss its implications for the description of the properties of κ-(ET)2Cu2(CN)3. The electronic properties of κ-(ET)2Cu(SCN)2 are also briefly discussed.

38. 	S. Thirupathaiah, S. de Jong, R. Ovsyannikov, H. A. Dürr, A. Varykhalov, R. Follath, Y. Huang, R. Huisman, M. S. Golden, Y.-Z. Zhang, H. O. Jeschke, R. Valentí, A. Erb, A. Gloskovskii, J. Fink
    	Orbital character variation of the Fermi surface and doping dependent changes of the dimensionality in BaFe2-xCoxAs2 from angle-resolved photoemission spectroscopy
    	Phys. Rev. B 81, 104512 (2010). (doi, pdf), (abstract).(hide abstract). From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we derive information on the dimensionality and the orbital character of the electronic states of BaFe2As2. Upon increasing Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases. From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Γ-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases.

37. 	Y.-Z. Zhang, I. Opahle, H. O. Jeschke, R. Valentí
    	Pressure-driven phase transitions in TiOCl and the family (Ca, Sr, Ba)Fe2As2
    	J. Phys.: Condens. Matter 22, 164208 (2010). (doi, preprint), (abstract).(hide abstract). Motivated by recent experimental measurements on pressure-driven phase transitions in Mott-insulators as well as the new iron pnictide superconductors, we show that first principles Car-Parrinello molecular dynamics calculations are a powerful method to describe the microscopic origin of such transitions. We present results for (i) the pressure-induced insulator to metal phase transition in the prototypical Mott insulator TiOCl as well as (ii) the pressure-induced structural and magnetic phase transitions in the family of correlated metals AFe2As2 (A=Ca,Sr,Ba). Comparison of our predictions with existing experimental results yields very good agreement.

36. 	Y.-Z. Zhang, K. Foyevtsova, H. O. Jeschke, M. U. Schmidt, R. Valentí
    	Can the Mott Insulator TiOCl be Metallized by Doping? A First-Principles Study
    	Phys. Rev. Lett. 104, 146402 (2010). (doi, pdf), (abstract).(hide abstract). We investigate the effect of Na intercalation in the layered Mott insulator TiOCl within the framework of density functional theory. We show that the system remains always insulating for all studied Na concentrations, and the evolution of the spectral weight upon Na doping is consistent with recent photoemission experiments. We predict the Na-doped superlattice structures, and show that substitutions of O by F, Cl by S, or Ti by V (or Sc), respectively, fail to metallize the system. We propose a description in terms of a multiorbital ionic Hubbard model in a quasi-two-dimensional lattice and discuss the nature of the insulating state under doping. Finally, a likely route for metallizing TiOCl by doping is proposed.

     2009 

35. 	S. A. J. Kimber, A. Kreyssig, Y. Z. Zhang, H. O. Jeschke, R. Valentí, F. Yokaichiya, E. Colombier, J. Yan, T. C. Hansen, T. Chatterji, R. J. McQueeney, P. C. Canfield, A. I. Goldman, D. N. Argyriou
    	Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2
    	Nature Materials 8, 471 (2009). (doi,preprint), (abstract).(hide abstract). The discovery of a new family of high-TC materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

34. 	H.C. Kandpal, I. Opahle, Y.-Z. Zhang, H. O. Jeschke, R. Valentí
    	Revision of model parameters for κ-type charge transfer salts: an ab initio study
    	Phys. Rev. Lett. 103, 067004 (2009). (doi, pdf), (abstract).(hide abstract). Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the κ-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems' description are discussed.

33. 	Q. Feng, Y.-Z. Zhang, H. O. Jeschke
    	Fast impurity solver based on equations of motion and decoupling
    	Phys. Rev. B 79, 235112 (2009). (doi, pdf), (abstract).(hide abstract). In this paper a fast impurity solver is proposed for dynamical mean field theory (DMFT) based on a decoupling of the equations of motion for the impurity Greens function. The resulting integral equations are solved efficiently with a method based on genetic algorithms. The Hubbard and periodic Anderson models are studied with this impurity solver. The method describes the Mott metal insulator transition and works for a large range of parameters at finite temperature on the real frequency axis. This makes it useful for the exploration of real materials in the framework of LDA+DMFT.

32. 	H. O. Jeschke, M. Diakhate, M. E. Garcia
    	Molecular dynamics simulations of laser induced damage of nanostructures and solids
    	App. Phys. A 96, 33 (2009). (doi,pdf), (abstract).(hide abstract). A theoretical approach to treat laser induced femtosecond structural changes in covalently bonded nanostructures and solids is described. Our approach consists in molecular dynamic simulations performed on the basis of time-dependent, many-body potential energy surfaces derived from tight-binding Hamiltonians. The shape and spectral composition of the laser pulse is explicitly taking into account in a non-perturbative way. We show a few examples of the application of this approach to describe the laser damage and healing of defects in carbon nanotubes with different chiralities and the ultrafast nonequilibrium melting of bulk germanium, initiated by the laser induced softening and destabilization of transversal acoustic phonon modes.

31. 	J. Fink, S. Thirupathaiah, R. Ovsyannikov, H.A. Duerr, R. Follath, Y. Huang, S. de Jong, M. S. Golden, Y.-Z. Zhang, H. O. Jeschke, R. Valentí, C. Felser, S. Dastjani Farahani, M. Rotter, D. Johrendt
    	Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy
    	Phys. Rev. B 79, 155118 (2009). (doi, pdf), (abstract).(hide abstract). We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe2As2, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 K and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the Γ-point and an electron pocket at each of the X-points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation of the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

30. 	Y. Z. Zhang, H. C. Kandpal, I. Opahle, H. O. Jeschke, and R. Valentí
    	Microscopic origin of pressure-induced phase transitions in iron-pnictide AFe2As2 superconductors: an ab initio molecular dynamics study
    	Phys. Rev. B 80, 094530 (2009). (doi, pdf), (abstract).(hide abstract). Using ab initio molecular dynamics we investigate the electronic and lattice structure of AFe2As2 (A=Ca, Sr, Ba) under pressure. We find that the structural phase transition (orthorhombic to tetragonal symmetry) is always accompanied by a magnetic phase transition in all the compounds while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe2As2 and its absence in BaFe2As2. We argue that changes of the Fermi surface nesting features dominate the phase transitions under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.

29. 	K. Foyevtsova, Y. Z. Zhang, H. O. Jeschke, and R. Valentí
    	First principles perspective on the microscopic model for Cs2CuCl4 and Cs2CuBr4
    	J. Phys.: Conf. Ser. 145, 012038 (2009). (doi, pdf), (abstract).(hide abstract). We investigate the microscopic model for the frustrated layered antiferromagnets Cs2CuCl4 and Cs2CuBr4 by performing ab initio density functional theory (DFT) calculations and with the help of the tight-binding method. The combination of both methods provide the relevant interaction paths in these materials, and we estimate the corresponding exchange constants. We find for Cs2CuCl4 that the calculated ratio of the strongest in-plane exchange constants J'/J between the spin-1/2 Cu ions agrees well with neutron scattering experiments. The microscopic model based on the derived exchange constants is tested by comparing the magnetic susceptibilities obtained from exact diagonalization with experimental data. The electronic structure differences between Cs2CuCl4 and Cs2CuBr4 are also analyzed.

     2008 

28. 	Y. Z. Zhang, H. O. Jeschke, and R. Valentí
    	Microscopic model for transitions from Mott to spin-Peierls insulator in TiOCl
    	Phys. Rev. B 78, 205104 (2008). (doi, pdf), (abstract).(hide abstract). On the basis of ab initio density-functional-theory calculations, we derive the underlying microscopic model Hamiltonian for TiOCl, a unique system that shows two consecutive phase transitions from a Mott insulator to a spin-Peierls insulator through a structurally incommensurate phase. We show with our model that the presence of magnetic frustration in TiOCl leads to a competition with the spin-Peierls distortion, which results in the unusual incommensurate phase. In addition, our calculations indicate that the spin-Peierls state is triggered by adiabatic phonons, which is essential for understanding the nature of the phase transition.

27. 	Y. Z. Zhang, H. O. Jeschke, and R. Valentí
    	Two pressure-induced transitions in TiOCl: Mott insulator to anisotropic metal
    	Phys. Rev. Lett. 101, 136406 (2008). (doi, pdf), (abstract).(hide abstract). Using Car-Parrinello molecular dynamics we investigate the behavior of the low-dimensional multiorbital Mott insulator TiOCl under pressure. We show that the system undergoes two consecutive phase transitions, first at Pc from a Mott-insulator to a metallic phase in the ab plane with a strong Ti-Ti dimerization along b. At a pressure P'c > Pc the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-tometal transition at Pc is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the t2g bands. Our computed pressure-dependent lattice parameters are consistent with experimental observations and the existing controversy on the change of crystal symmetry at high pressures is discussed.

     2007 

26. 	H. O. Jeschke, L. A. Salguero, B. Rahaman, C. Buchsbaum, V. Pashchenko, M. U. Schmidt, T. Saha-Dasgupta, and R. Valentí
    	Microscopic modeling of a spin crossover transition
    	New J. Phys. 9, 448 (2007). (doi, pdf), (abstract).(hide abstract). In spin crossover materials, an abrupt phase transition between a low-spin state and a high-spin (HS) state can be driven by temperature, pressure or by light irradiation. Of special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S = 0 and 2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of x-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between HS Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

25. 	H. O. Jeschke, A. H. Romero, M. E. Garcia, and A. Rubio
    	Microscopic investigation of laser-induced structural changes in single-wall carbon nanotubes
    	Phys. Rev. B 75, 125412 (2007). (doi, pdf), (abstract).(hide abstract). Extensive excited-state molecular dynamics simulations of femtosecond laser-induced structural transformation in single-walled carbon nanotubes (SWNTs) are presented. We considered in the simulation two limiting cases; one where only a short portion of the tube is irradiated and the other where the whole tube is affected by the laser pulse. We have analyzed the role of chirality (zigzag versus armchair and some chiral tube cases) in the damage threshold as a function of tube diameter. Nontrivial dependence of the damage threshold as a function of diameter has been found. We find that for equal laser parameters, zigzag SWNTs are on average equally stable with respect to laser excitation as armchair SWNTs, but their stabilities show different dependencies on diameter. Due to the higher stiffness of the (n,n) tubes in the direction perpendicular to its axis as compared to the (n,0) tubes, we find the formation of standing waves in the nanotube wall for zigzag and not for armchair tubes. We also studied the role of the laser pulse duration and show that in general longer laser pulses increase the damage threshold. This result is rationalized in terms of electron-ion relaxation times. Implications of laser-induced structural transformations are analyzed.

24. 	L. A. Salguero, H. O. Jeschke, B. Rahaman, T. Saha-Dasgupta, C. Buchsbaum, M. U. Schmidt, and R. Valentí
    	Cu-based metalorganic systems: an ab initio study of the electronic structure
    	New J. Phys. 9, 26 (2007). (doi, pdf), (abstract).(hide abstract). Within a first principles framework, we study the electronic structure of the recently synthesized polymeric coordination compound Cu(II)- 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been suggested to be a good realization of a Heisenberg spin-1/2 chain with antiferromagnetic coupling. By using a combination of classical with ab initio quantum mechanical methods, we design on the computer reliable modified structures of CuCCP aimed at studying effects of Cu-Cu coupling strength variations on this spin-1/2 system. For this purpose, we performed two types of modifications on CuCCP. In one case, we replaced H in the linker by (i) an electron donating group (NH2) and (ii) an electron withdrawing group (CN), while the other modification consisted of adding H2O and NH3 molecules in the structure which change the local coordination of the Cu(II) ions. With the Nth order muffin tin orbital (NMTO) downfolding method, we provide a quantitative analysis of the modified electronic structure and the nature of the Cu-Cu interaction paths in these new structures and discuss its implications for the underlying microscopic model.

23. 	H. O. Jeschke, L. A. Salguero, R. Valentí, C. Buchsbaum, M. U. Schmidt, and M. Wagner
    	Classical and ab initio preparation of reliable structures for polymeric coordination compounds
    	Comptes Rendus Chimie 10, 82 (2007). (doi, pdf), (abstract).(hide abstract). The detailed investigation of electronic and magnetic properties of polymeric coordination materials with accurate ab initio quantum mechanical methods is often computationally extremely demanding because of the large number of atoms in the unit cell. Moreover, usually the available structural data are insufficient or poorly determined, especially when the structure contains hydrogen atoms. In order to be able to perform controlled ab initio calculations on reliable structures, we use a two-step approach to systematically prepare model structures for polymeric coordination compound systems and to relax them to their equilibrium configuration. First, a structure is constructed on the basis of a crystallographic database and optimized by force field methods; in the second step, the structure is relaxed by ab initio quantummechanical molecular dynamics.With this structure,we perform accurate electronic structure calculations. We will apply this procedure to a Fe(II) triazole compound and to a coordination polymer of Cu(II) ions with 2,5-bis (pyrazol-1-yl)-1,4-dihydroxybenzene.

22. 	R. Valentí, T. Saha-Dasgupta, H. O. Jeschke, B. Rahaman, H. Rosner, P. Lemmens, R. Takagi, and M. Johnsson
    	Comparative investigation of the coupled tetrahedra quantum spin systems Cu2Te2O5X2, X=Cl, Br and Cu4Te5O12X4
    	Physica C 460-462, 462 (2007). (doi, pdf), (abstract).(hide abstract). We present a comparative study of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X = Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12X4 (Cu-45124(Cl)) based on ab initio density functional theory calculations. The magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state at a lower critical temperature Tc = 13.6 K than in Cu-2252(Cl) (Tc = 18.2 K) can be well understood in terms of the modified interaction paths. We identify the relevant structural changes between the two systems and discuss the hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio relaxed structure using Car-Parrinello molecular dynamics.

21. 	B. Rahaman, H. O. Jeschke, R. Valentí, and T. Saha-Dasgupta
    	Microscopic model for the frustrated Cu II-spin tetrahedron-based Cu4Te5O12X4 (X=Cl, Br) systems
    	Phys. Rev. B 75, 024404 (2007). (doi, pdf), (abstract).(hide abstract). We present a microscopic study of the electronic and magnetic properties of the recently synthesized spin tetrahedron system Cu4Te5O12Cl4 based on density functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar Cu2Te2O5Cl2. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of Cu4Te5O12Br4 by geometry optimization in an ab initio molecular dynamics calculation and investigate the effect of substituting Cl by Br. The possible magnetic behavior of Cu4Te5O12Br4 in comparison with the recently studied Cu2Te2O5Br2 is also discussed.

     2006 

20. 	T. Dumitrica, M. E. Garcia, H. O. Jeschke, and B. I. Yakobson
    	Breathing coherent phonons and caps fragmentation in carbon nanotubes following ultrafast laser pulses
    	Phys. Rev. B 74, 193406 (2006). (doi, pdf), (abstract).(hide abstract). The response of carbon nanotubes to femtosecond laser pulses is studied with a nonadiabatic simulation technique, which accounts for the evolution of electronic and ionic degrees of freedom, and for the coupling with the external electromagnetic field. As a direct result of electronic excitation, three coherent breathing phonon modes are excited: two radial vibrations localized in the caps and cylindrical body, and one longitudinal vibration coupled to the nanotube length. Under high absorbed energies (but below 2.9 eV/atom, the graphite′s ultrafast fragmentation threshold), the resulting oscillatory motion leads to the opening of nanotube caps. Following the cap photofragmentation the nanotube body remains intact for the rest of the 2 ps simulation time.

19. 	F. Valencia, A. H. Romero, H. O. Jeschke, and M. E. Garcia
    	Large amplitude coherent phonons and inverse Stone-Wales transitions in graphitic systems with defects interacting with ultrashort laser pulses
    	Phys. Rev. B 74, 075409 (2006). (doi, pdf), (abstract).(hide abstract). The mechanical response of a defective graphene layer to an ultrafast laser pulse is investigated through nonadiabatic molecular dynamics simulations. The defects are pentagon-heptagon pairs introduced by a single Stone-Wales transformation in the simulation cell. We found that when the fraction of excited electrons ξ is below 6%, the layer exhibits strong transversal displacements in the neighborhood of the defect. The amplitude of these movements increases with the amount of energy absorbed until the threshold of ξ =6% is reached. Under this condition the layer undergoes a subpicosecond inverse Stone-Wales transition, healing the defect. The absorbed energy per atom required to induce this mechanism is approximately 1.3 eV, a value that is below the laser damage thresholds for the pristine layers. The transition is lead by the electronic entropy and follows a path with strong out-of-plane contributions; it differs from the predicted path for thermally activated transitions, as calculated using standard transition state approaches. The same phenomenon is observed in defective zig-zag and armchair nanotubes. In contrast, for a defective C60 fullerene the mechanism is hindered by the presence of edge-sharing pentagons.

18. 	A. H. Romero, H. O. Jeschke, and M. E. Garcia
    	Laser Manipulation of Nanodiamonds
    	Com. Mat. Sci. 35, 179 (2006). (doi, pdf), (abstract).(hide abstract). We present atomistic simulations of laser induced structural modifications in nanodiamonds. The method used is based on a microscopic approach for the interaction between femtosecond laser pulses and nanostructured materials. It allows us to describe the ultrafast change of bonding type produced in diamond-like nanostructures as a consequence of the laser excitation. In contrast to a thermal processing of nanodiamonds, an ultrashort laser pulse can eliminate all sp3 bonds in the system on a sub-picosecond time scale, opening the possibility for the production of graphitic nanostructures and the creation of nanocontacts between carbon nanoclusters and other nanostructures.

     2005 

17. 	A. H. Romero, M. E. Garcia, F. Valencia, H. Terrones, M.Terrones, and H. O. Jeschke
    	Femtosecond Laser Nanosurgery of Defects in Carbon Nanotubes
    	Nano Lett. 5, 1361 (2005). (doi, pdf), (abstract).(hide abstract). All self-assembled nanostructures, like carbon nanotubes, exhibit structural imperfections that affect their electronic and mechanical properties and constitute a serious problem for the development of novel electronic nanodevices. Very common defects in nanotubes are pentagon-heptagon pairs, in which the replacement of four hexagons by two pentagons and two heptagons disrupts the perfect hexagonal lattice. In this work, we demonstrate that these defects can be eliminated efficiently with the help of femtosecond laser pulses. By performing nonadiabatic molecular dynamics simulations, we show that in the laser-induced electronic nonequilibrium the pentagon-heptagon pair is transformed back into four hexagons without producing any irreversible damage to the rest of the nanotube.

16. 	H. O. Jeschke and G. Kotliar
    	Decoupling method for dynamical mean field theory calculations
    	Phys. Rev. B 71, 085103 (2005). (doi, pdf), (abstract).(hide abstract). In this paper we explore the use of an equation of motion decoupling method as an impurity solver to be used in conjunction with the dynamical mean field self-consistency condition for the solution of lattice models. We benchmark the impurity solver against exact diagonalization, and apply the method to study the infinite U Hubbard model, the periodic Anderson model and the pd model. This simple and numerically efficient approach yields the spectra expected for strongly correlated materials, with a quasiparticle peak and a Hubbard band. It works in a large range of parameters, and therefore can be used for the exploration of real materials using the local density approximation and dynamical mean field theory.

     2004 

15. 	A. H. Romero, H. O. Jeschke, A. Rubio and M. E. Garcia
    	Atomistic simulation of the laser induced damage in single wall carbon nanotubes: Diameter and chirality dependence
    	Appl. Phys. A 79, 899 (2004). (doi, pdf), (abstract).(hide abstract). The effect of high energy laser pulses on single wall carbon nanotubes (SWNT) is studied by a non-equilibrium quantum mechanical model. For the studied laser parameters, we find ablation thresholds that vary between 1.9 eV/atom and 2.3 eV/atom. For zigzag tubes a linear increase of damage thresholds as function of diameter is observed. For armchair tubes, a stability maximum is found at the (10,10) SWNT. We find that below but close to the damage threshold the nanotubes show the presence of standing waves.

14. 	M. E. Garcia, T. Dumitrica and H. O. Jeschke
    	Laser induced coherent phonons in graphite and carbon nanotubes: model and simulations
    	Appl. Phys. A 79, 855 (2004). (doi, pdf), (abstract).(hide abstract). We present a microscopic description of the excitation of coherent phonons as a response to femtosecond laser excitation. Using molecular dynamics simulations based on a tight-binding electronic Hamiltonian we discuss two examples of laser-induced coherent phonons: (1) excitation of the E2g1 phonon mode in graphite under high external pressure and (2) the displacive excitation of two perpendicular phonon modes in carbon nanotubes. We discuss the influence of these coherent phonons on the ablation mechanisms.

13. 	T. Dumitrica, M. E. Garcia, H. O. Jeschke and B. I. Yakobson
    	Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons
    	Phys. Rev. Lett. 92, 117401 (2004). (doi, pdf), (abstract).(hide abstract). We demonstrate the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening.

     2003 

12. 	S. L. Johnson, A. M. Lindenberg, P. A. Heimann, Z. Chang, R. W. Lee, M. E. Garcia, H. O. Jeschke, J. J. Rehr and R. W. Falcone
    	Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy
    	Phys. Rev. Lett. 91, 157403 (2003). (doi, pdf), (abstract).(hide abstract). Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.

11. 	M. E. Garcia and H. O. Jeschke
    	Theoretical approach to the laser induced melting of graphite under different pressure conditions
    	Appl. Surf. Sci. 208-209, 61 (2003). (doi, pdf), (abstract).(hide abstract). We present a theoretical study of the laser-induced femtosecond melting of (1) graphite under high external pressure and (2) ultrathin graphite films under normal conditions. Our approach consists of molecular dynamic simulations performed on the basis of a time-dependent, many-body potential energy surface derived from a tight-binding Hamiltonian. Our results show that the laser-induced melting process occurs in two steps: (i) destruction of the graphite sheets via bond breaking, and (ii) merging of the melted layers. The separation of the two steps is more evident for graphite under pressure (10 GPa), but is also present in graphite films at normal pressure. The melting product is a low-density carbon phase, which remains stable under hig pressure, but is unstable with an ultrashort life-time under normal pressure.

     2002 

10. 	H. O. Jeschke, M. E. Garcia, M. Lenzner, J. Bonse, J. Krüger, and W. Kautek
    	Laser ablation thresholds of Silicon for different pulse durations: theory and experiment
    	Appl. Surf. Sci. 197-198, 839 (2002). (doi, pdf), (abstract).(hide abstract). The ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on molecular dynamics (MD) simulations combined with a microscopic electronic model. We determine the thresholds of melting and ablation for two different pulse durations τ = 20 and 500 fs. Experiments have been performed using 100 Ti:Saphire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25 and 400 fs, respectively. Good agreement is obtained between theory and experiment.

9. 	H. O. Jeschke and M. E. Garcia
   	Theoretical description of the ultrafast ablation of diamond and graphite: dependence of thresholds on pulse duration
   	Appl. Surf. Sci. 197-198, 107 (2002). (doi, pdf), (abstract).(hide abstract). A theoretical description of the ultrafast ablation of diamond and graphite is presented. Laser induced lattice deformations and melting are described with the help of molecular dynamics simulations on time dependent potential energy surfaces derived from a microscopic electronic Hamiltonian. Thermalization effects are explicitly taken into account. We calculate the ablation thresholds as a function of the pulse duration for femtosecond pulses. For both materials we obtain smoothly increasing thresholds for increasing duration. The damage and ablation mechanisms are discussed.

8. 	H. O. Jeschke, M. E. Garcia, and J. A. Alonso
   	Nonthermal fragmentation of C60
   	Chem. Phys. Lett. 352, 154 (2002). (doi, pdf), (abstract).(hide abstract). A theoretical study of the subpicosecond fragmentation of C60 clusters in response to ultrafast laser pulses is presented. We simulate the laser excitation and the consequent nonequilibrium relaxation dynamics of the electronic and nuclear degrees of freedom. The first stages of the nonequilibrium dynamics are dominated by a breathing mode followed by the cold ejection of single C atoms, in contrast to the dimer emission which characterizes the thermal relaxation. We also determine the nonequilibrium damage thresholds as a function of the pulse duration.

7. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Time-dependent energy absorption changes during ultrafast lattice deformation
   	J. Appl. Phys. 91, 18 (2002). (doi, pdf), (abstract).(hide abstract). The ultrafast time dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We use a microscopic theory to describe laser induced structural changes and their influence on the electronic properties. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy.

    2001 

6. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Theory for the ultrafast ablation of graphite films
   	Phys. Rev. Lett. 87, 015003 (2001). (doi, pdf), (abstract).(hide abstract). The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies (.3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold (.2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

    2000 

5. 	M. E. Garcia, H. O. Jeschke, I. Grigorenko, and K. H. Bennemann
   	Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure
   	Appl. Phys. B 71, 361-371 (2000). (doi, pdf), (abstract).(hide abstract). We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses.We analyze the excitation of different types of clusters (Hgn, Agn, Sin, C60 and Xen) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse. For low-intensity pulses (I < 1012 W/cm2) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Sin and C60 clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy. As an example for the case of high intensities (I > 1015 W/cm2), the explosion of clusters under the action of very intense ultrashort laser fields is described.

    1996-1999 

4. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Theory for Laser Induced Ultrafast Phase Transitions in Carbon
   	Appl. Phys. A 69[Suppl.], S49-S53 (1999). (doi, pdf), (abstract).(hide abstract). The response of carbon to femtosecond laser pulses of arbitrary form, different durations, and different intensities is studied theoretically.We perform molecular dynamics simulations based on a microscopic electronic Hamiltonian. We include in our model the theoretical description of the pulse form, the electron thermalization, and diffusion effects explicitly. We apply our method to diamond and C60 crystals. For the diamond case, we show that a femtosecond laser pulse induces a nonequilibrium transition to graphite, which takes place for a wide range of pulse durations and intensities. This ultrafast collective motion of the atoms occurs within a time scale shorter than 100 fs. The laser-induced melting of a C60 crystal under pressure is also analyzed. In this case, an ultrafast melting of the system occurs. We discuss the mechanisms underlying these nonequilibrium phase transitions.

3. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Microscopic analysis of the femtosecond graphitization of diamond
   	Phys. Rev. B 60, R3701-R3704 (1999) (Rapid Communication). (doi, pdf), (abstract).(hide abstract). We present a theoretical study of ultrafast phase transitions induced by femtosecond laser pulses of arbitrary form. Molecular-dynamics simulations on time dependent potential-energy surfaces derived from a microscopic Hamiltonian are performed. Applying this method to diamond, we show that a nonequilibrium transition to graphite takes place for a wide range of laser pulse durations and intensities. This ultrafast transition (~100 fs) is driven by the suppression of the diamond minimum in the potential-energy surface of the laser excited system.

2. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Theory for the Ultrafast Structural Response of Optically Excited Small Clusters: Time Dependence of the Ionization Potential
   	Phys. Rev. A 54, R4601-R4604 (1996) (Rapid Communication). (doi, pdf), (abstract).(hide abstract). Combining an electronic theory with molecular-dynamics simulations we present results for the ultrafast structural changes in small clusters. We determine the time scale for the change from the linear to a triangular structure after the photodetachment process Ag3- → Ag3. We show that the time-dependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion, and that it is sensitive to the initial temperature. We compare with experiment and point out the general significance of our results.

1. 	H. O. Jeschke, M. E. Garcia, and K. H. Bennemann
   	Analysis of the Ultrafast Dynamics of the Silver Trimer upon Photodetachment
   	J. Phys. B 29, L545-L549 (1996) (letter to the editor). (doi, pdf), (abstract).(hide abstract). We study the ultrafast dynamics of Ag3 clusters immediately after the photodetachment of Ag3- by performing molecular dynamics simulations based on a microscopic electronic theory. We determine the time scale for the change from the linear (initial situation) to a triangular structure (potential minimum) of the ground state of Ag3. Our results are in good agreement with experiment. We show how the time dependence of the internal degrees of freedom is reflected in the 'pump and probe' signal. Furthermore, we discuss the application of our results to magnetic systems and chemical reactions.