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Researcher ID: C35072009
Refereed publications
2018
 

 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

 (preprint,pdf),
(abstract).(hide abstract).
We investigate the quantum Heisenberg model on the pyrochlore lattice for a generic spinS in the
presence of nearestneighbor J_{1} and secondnearestneighbor J_{2} 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 J_{2} = 0, which is shown to be robust
against the introduction of breathing anisotropy. The effects of temperature, quantum fluctuations,
breathing anisotropies, and a J_{2} coupling on the nature of the scattering profile, in particular,
the pinch points are studied. For the magnetic phases of the J_{1}J_{2} 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.

 

 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. Valenti, M. Lang 

Evidence for electronicallydriven ferroelectricity in the family of strongly correlated dimerized BEDTTTF molecular conductors

 submitted (preprint,pdf),
(abstract).(hide abstract).
By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ(BEDTTTF)_{2}Hg(SCN)_{2}Cl, we provide evidence for orderdisorder type electronic ferroelectricity which is driven by charge order within the (BEDTTTF)_{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 dimerMott systems at 1/2 filling, and non or weakly dimerized systems at 1/4 filling exhibiting charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated κ(BEDTTTF)_{2}X salts and can create novel states, such as electronicallydriven multiferroicity or chargeorderinduced quasi1D spin liquids.

 

 Y. Iqbal, T. Müller, H. O. Jeschke, R. Thomale, J. Reuther 

Stability of the spiral spin liquid in MnSc_{2}S_{4}

 submitted (preprint,pdf),
(abstract).(hide abstract).
We investigate the stability of the spiral spinliquid phase in MnSc_{2}S_{4} against thermal and quantum fluctuations as well as against perturbing effects of longerrange interactions. Employing ab initio DFT calculations we propose a realistic Hamiltonian for MnSc_{2}S_{4}, featuring second (J_{2}) and third (J_{3}) neighbor Heisenberg interactions on the diamond lattice that are considerably larger than previously assumed. We argue that the combination of strong J_{2} and J_{3} couplings reproduces the correct magnetic Bragg peak position measured experimentally. Calculating the spinstructure factor within the pseudofermion functionalrenormalization group technique we find that close to the magnetic phase transition the sizeable J_{3} 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 spinliquid is realized in MnSc_{2}S_{4}.

 

 S. Chillal, Y. Iqbal, H. O. Jeschke, J. A. RodriguezRivera, R. Bewley, P. Manuel, D. Khalyavin, P. Steffens, R. Thomale, A. T. M. N. Islam, J. Reuther, B. Lake 

A quantum spin liquid based on a new threedimensional lattice

 submitted (preprint,pdf),
(abstract).(hide abstract).
The quantum spin liquid is a highly entangled magnetic state characterized by the absence of longrange magnetic order or any static magnetism in its ground state. Instead the spins are continuously fluctuating in a highly correlated way down to the lowest temperatures. The spin liquid state is very rare and is confined to a few specific cases where the interactions between the magnetic ions cannot be simultaneously satisfied (known as frustration). In particular, lattices with magnetic ions in triangular or tetrahedral arrangements which interact via isotropic antiferromagnetic interactions can generate frustration because it is impossible to connect all the bonds with magnetic moments that are aligned antiparallel. This leads to highly degenerate ground states between which the magnetic moments fluctuate continuously. Threedimensional isotropic spin liquids have mostly been sought in materials where the magnetic ions form pyrochlore or hyperkagome lattices. Here we discover a new type of threedimensional lattice that enables spin liquid behavior called the hyperhyperkagome lattice which manifests in the compound PbCuTe_{2}O_{6}. Using a combination of experiment and theory we show that this system satisfies all the requirements for a quantum spin liquid including the absence of static magnetism, the presence of a degenerate manifold of ground states, and the characteristic continuum of spinon excitations. This result is important because, it points to new ways to engineer spin liquid behavior.

 

 X. Wu, H. O. Jeschke, D. Di Sante, F. O. von Rohr, R. J. Cava, R. Thomale 

Origin of the pressuredependent T_{c} valley in superconducting simple cubic phosphorus

 Phys. Rev. Mater., in press (preprint,pdf),
(abstract).(hide abstract).
Motivated by recent experiments, we investigate the pressuredependent electronic structure and electronphonon (eph) coupling for simple cubic phosphorus by performing firstprinciple calculations within the full potential linearized augmented plane wave method. As a function of increasing pressure, our calculations show a valley feature in T_{c}, followed by an eventual decrease for higher pressures. We demonstrate that this T_{c} valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the bandresolved contributions to the eph coupling. Below the first Lifshitz transition, the phonon hardening and shrinking of the γ Fermi surface with s orbital character results in a decreased T_{c} with increasing pressure. After the second Lifshitz transition, the appearance of δ Fermi surfaces with 3d orbital character generate strong eph interband couplings in αδ and βδ channels, and hence lead to an increase of T_{c}. For higher pressures, the phonon hardening finally dominates, and T_{c} decreases again. Our study reveals that the intriguing T_{c} valley discovered in experiment can be attributed to Lifshitz transitions, while the plateau of T_{c} detected at intermediate pressures appears to be beyond the scope of our analysis. This strongly suggests that besides eph 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 lowenergy orbital weight towards d character, and as such even trigger an enhanced importance of orbitalselective electronic correlations despite an increase of the overall bandwidth.

 

 D. Guterding, H. O. Jeschke 

An efficient GPU algorithm for tetrahedronbased Brillouinzone integration

 submitted (preprint,pdf),
(abstract).(hide abstract).
We report an efficient algorithm for calculating momentumspace 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 orbitalresolved density of states. We benchmark our code on the problem of calculating the orbitalresolved density of states in an ironbased 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).


 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 openshell character in the pressureinduced conductivity in an organic radical DA dyad

 Eur. J. Chem., in press (uncorrected proof)
(abstract).(hide abstract).
Singlecomponent 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 TTFPTM derivative (1) is reported and implications of its crystalline structure on its electrical properties dicussed. On the other hand, the nonradical 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.

 

 M. Kaliappan, H. O. Jeschke, R. Valentí 

Dynamics and fragmentation mechanism of (CH_{3}C_{5}H_{5})Pt(CH_{3})_{3} on SiO_{2} Surfaces

 Beilstein J. Nanotechnol. 9, 711 (2018) (preprint,pdf),
(abstract).(hide abstract).
The interaction of CH_{3}C_{5}H_{5})Pt(CH_{3})_{3} (trimethyl methylcyclopentadienyl platinum) molecules on the fully and partially hydroxylated SiO_{2} 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 CH_{3}C_{5}H_{5})Pt(CH_{3})_{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 SiO_{2} 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 methylcyclopentadienyl 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 CH_{3}C_{5}H_{5})Pt(CH_{3})_{3} precursor on SiO_{2} surfaces, which illustrates that oxidized Pt is formed on the surface, prior to metal deposition.

 
2017

 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 γAl_{2}O_{3}/SrTiO_{3} hosts a twodimensional electron system (2DES) with electron mobilities exceeding those in its allperovskite counterpart LaAlO_{3}/SrTiO_{3} 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 xray 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 SrTiO_{3} layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

 

 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 hightemperature superconductivity in the
thinfilm FeSe/SrTiO_{3} 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 ironselenium distance and, on the other
hand, by increasing the ironselenium distance with unchanged iron
lattice.

 

 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 spin½ pyrochlore molybdate Heisenberg antiferromagnet

 Phys. Rev. Mater. 1, 071201(R) (2017) (doi,pdf,supplement),
(abstract).(hide abstract).
We theoretically investigate the lowtemperature phase of the recently synthesized Lu_{2}Mo_{2}O_{5}N_{2} material, an extraordinarily rare realization of a S=1/2 threedimensional pyrochlore Heisenberg antiferromagnet in which Mo^{5+} are the S=1/2 magnetic species. Despite a CurieWeiss temperature (Θ_{CW}) of −121(1) K, experiments have found no signature of magnetic ordering or spinfreezing 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 nearestneighbors. The analysis of this model via the pseudofermion functional renormalization group method reveals paramagnetic behavior down to a temperature of at least T=Θ_{CW}/100, in agreement with the experimental findings hinting at a possible threedimensional quantum spin liquid. The spin susceptibility profile in reciprocal space shows momentumdependent features forming a "gearwheel" pattern, characterizing what may be viewed as a molten version of a chiral noncoplanar 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 Lu_{2}Mo_{2}O_{5}N_{2}.

 

 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 spinliquid 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 AAg_{2}Cr[VO_{4}]_{2}, with A=Ag, K, or Rb, is layered S = 3/2 triangularlattice (TL) systems in which the magnetic exchange interactions between Cr^{3+}(3d^{3}) ions are mediated by nonmagnetic [VO_{4}]^{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 [CrO_{6}] 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 longrange order (LRO) at T_{N}≈10 K, whereas the highsymmetry 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 spinliquid ground states presumably not related to Ising anisotropy or dimerization.

 

 M. Sing, H. O. Jeschke, F. Lechermann, R. Valentí, R. Claessen 

Influence of oxygen vacancies on twodimensional electron systems at SrTiO_{3}based interfaces and surfaces

 Eur. Phys. J. Special Topics 226, 2457 (2017) (doi),
(abstract).(hide abstract).
The insulator SrTiO_{3} can host highmobility twodimensional electron systems on its surfaces and at interfaces with other oxides. While for the bare surface a twodimensional electron system can only be induced by oxygen vacancies, it is believed that the metallicity of heterostructure interfaces as in LaAlO_{3}/SrTiO_{3} is caused by other mechanisms related to the polar discontinuity at the interface. Based on calculations using density functional and dynamical meanfield 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.


 D. Guterding, H. O. Jeschke, I. I. Mazin, J. K. Glasbrenner, E. Bascones, R. Valentí 

Nontrivial role of interlayer cation states in ironbased 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 lowenergy 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 bondangles 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 ironbased materials may not always be fully understood based on d or dp model Hamiltonians only.

 

 A. J. Kim, H. O. Jeschke, P. Werner, R. Valentí 

Jfreezing and Hund's rules in spinorbitcoupled multiorbital Hubbard models

 Phys. Rev. Lett. 118, 086401 (2017) (doi,pdf,supplement),
(abstract).(hide abstract).
We investigate the phase diagram of the spinorbitcoupled three orbital Hubbard model at arbitrary filling by means of dynamical meanfield theory combined with continuoustime quantum Monte Carlo. We find that the spinfreezing crossover occurring in the metallic phase of the nonrelativistic multiorbital Hubbard model can be generalized to a Jfreezing crossover, with J=L+S, in the spinorbitcoupled case. In the Jfrozen regime the correlated electrons exhibit a nontrivial 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.

 

 Y. Li, S. M. Winter, H. O. Jeschke, R. Valentí 

Electronic excitations in γLi_{2}IrO_{3}

 Phys. Rev. B 95, 045129 (2017) (doi,pdf),
(abstract).(hide abstract).
We investigate the electronic properties of the threedimensional stripyhoneycomb γLi_{2}IrO_{3} 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 twodimensional honeycomb systems αLi_{2}IrO_{3} and Na_{2}IrO_{3} and discuss the results in comparison to γLi_{2}IrO_{3}.


 D. Guterding, S. Backes, M. Tomic, H. O. Jeschke, R. Valentí 

Abinitio perspective on structural and electronic properties of ironbased 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.

 

 M. Knöner, E. Gati, B. Wolf, M. de Souza, A. Haghighirad, M. Kuhnt, W. Assmus, H. O. Jeschke, M. Tomic, R. Valenti, M. Lang 

Combined experimental and theoretical studies of pressure effects in La_{2}Sb

 Phys. Stat. Sol. B 254, 1600168 (2017) (doi),
(abstract).(hide abstract).
The 5 K superconductor La_{2}Sb shares some structural similarities with the ironpnictides and chalgogenides, especially the socalled 122compounds, despite lacking a 3dmetal component. In order to look for similarities and dissimilarities of this layered metal pnictide with the higherT_{c} Febased superconductors, we have performed a combined experimental and theoretical investigation of the effects of hydrostatic pressure. Magnetic measurements reveal a linear increase of T_{c} with pressure at a rate dT_{c}/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 T_{c} is possible within a reasonable range of parameters. Our results indicate that La_{2}Sb, despite its structural similarities to the 122 Febased superconductors, exhibits a BCStype of superconductivity.

 
2016

 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. SaintMartin, A. Fouchet, B. Berini, Y. Dumont, A. J. Kim, F. Lechermann, H. O. Jeschke, M. J. Rozenberg, R. Valentí, A. SantanderSyro 

Hubbard band or oxygen vacancy states in the correlated electron metal SrVO_{3}?

 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 SrVO_{3}. By means of angleresolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dosedependent 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 zerovacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of stateoftheart 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 twodimensional electron gas (2DEG) at the surface of insulating d0 transition metal oxides.

 

 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 DTBDTTCNQ and DTBDTF_{4}TCNQ (where DTBDT is dithieno[2,3d;2′,3′d′] benzo[1,2b;4,5b′]dithiophene) using a combination of nearedge Xray 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 electronhole 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 DTBDTTCNQ and DTBDTF_{4}TCNQ, 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.

 

 M. Souto, H.B. Cui, M. PeñaAlvarez, V. García Baonza, H. O. Jeschke, M. Tomic, R. Valentí, D. Blasi, I. Ratera, C. Rovira, J. Veciana 

PressureInduced Conductivity in a Neutral Nonplanar Spinlocalized Radical

 J. Am. Chem. Soc. 138, 11517 (2016) (doi),
(abstract).(hide abstract).
There is a growing interest in the development of singlecomponent 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 nonplanar and spinlocalized Ccentered 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.

 

 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 twodimensional electron system

 Phys. Rev. B 93, 235117 (2016) (doi,pdf,supplement),
(abstract).(hide abstract).
Twodimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the twodimensional (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 wellcharacterized 2D electron system formed at LaAlO_{3}SrTiO_{3} interfaces and measure a pronounced, unexpected response. Pressure of ∼2 GPa reversibly doubles the 2D carrier density n_{s} at 4 K. Along with the increase of n_{s}, the conductivity and mobility are reduced under pressure. Firstprinciples 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.

 

 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 (Ba_{1−x}Sr_{x})CuSi_{2}O_{6}

 Phys. Rev. B 93, 174121 (2016) (doi,pdf),
(abstract).(hide abstract).
We present a structural analysis of the substituted system (Ba_{1−x}Sr_{x})CuSi_{2}O_{6}, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, roomtemperature, and cryogenic highresolution NMR, as well as magnetic and specificheat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi_{2}O_{6}, is absent for the x=0.1 sample. Furthermore, synchrotron powderdiffraction 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 J_{intra}. Pulsefield magnetization measurements reveal the emergence of a fieldinduced ordered state, tantamount to BoseEinsteincondensation (BEC) of triplons, within the tetragonal crystal structure of I4_{1}/acd. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

 

 H. Lee, H. O. Jeschke, R. Valentí 

Competition between disorder and Coulomb interaction in a twodimensional plaquette Hubbard model

 Phys. Rev. B 93, 224203 (2016) (doi,pdf),
(abstract).(hide abstract).
We have studied a disordered N_{c}×N_{c} plaquette Hubbard model on a twodimensional square lattice at halffilling using a coherent potential approximation (CPA) in combination with a singlesite dynamical mean field theory (DMFT) approach with a paramagnetic bath. Such a model conveniently interpolates between the ionic Hubbard model at N_{c}=√2 and the Anderson model at N_{c}=∞ and enables the analysis of the various limiting properties. We confirmed that within the CPA approach a band insulator behavior appears for noninteracting strongly disordered systems with a small plaquette size N_{c}=4, while the paramagnetic Anderson insulator with nearly gapless density of states is present for large plaquette sizes N_{c}=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 (N_{c}=48) than from the band insulator (N_{c}=4). Finally, we observe a Mott insulator behavior in the strong interaction U regions for both N_{c}=4 and N_{c}=48 independent of the disorder strength. We discuss the application of this model to real materials.

 

 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 firstprinciples calculations. We find that isoelectronic substitution with nonmagnetic ions significantly reduces the interkagome exchange coupling. As a consequence, interlayersubstituted barlowite can be described by a simple twoparameter Heisenberg Hamiltonian, for which a quantum spin liquid ground state has been predicted.

 

 D. Guterding, M. Altmeyer, H. O. Jeschke, R. Valentí 

Neardegeneracy of extended s + d_{x2y2} and d_{xy} order parameters in
quasitwodimensional organic superconductors

 Phys. Rev. B 94, 024515 (2016) (doi,pdf),
(abstract).(hide abstract).
The symmetry of the superconducting order parameter in quasitwodimensional BEDTTTF organic
superconductors is a subject of ongoing debate. We report ab initio density functional theory
calculations for a number of organic superconductors containing κtype layers. Using projective
Wannier functions we derive parameters of a common lowenergy Hamiltonian based on individual
BEDTTTF molecular orbitals. In a random phase approximation spinfluctuation approach we
investigate the evolution of the superconducting pairing symmetry within this model and point out
a phasetransition between extended s + d_{x2y2} and d_{xy} 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 κtype 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.

 

 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 Lu_{2}V_{2}O_{7}

 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 lowenergy 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 Lu_{2}V_{2}O_{7} where the vanadium ions form a lattice of cornersharing spin1/2 tetrahedra. In this compound, anisotropic DzyaloshinskiiMoriya interactions (DMI) play an essential role in inducing a magnon Hall effect. We obtain quantitative estimates of the nearestneighbor Heisenberg exchange, the DMI, and the symmetric part of the anisotropic exchange tensor. Finally, we compare our results with experimental ones on the Lu_{2}V_{2}O_{7} compound.

 

 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 Na_{2}IrO_{3}, αRuCl_{3}, and αLi_{2}IrO_{3} 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, bondanisotropy, and/or offdiagonal couplings which we argue naturally explain the observed ordered phases in these systems. Given these observations, the potential for realizing the spinliquid state in real materials is analyzed, and synthetic challenges are defined and explained.


 M. Altmeyer, H. O. Jeschke, O. HijanoCubelos, C. Martins, F. Lechermann, K. Koepernik, A. SantanderSyro, M. J. Rozenberg, R. Valentí, M. Gabay 

Magnetism, spin texture and ingap states: Atomic specialization at the surface of oxygendeficient SrTiO_{3}

 Phys. Rev. Lett. 116, 157203 (2016) (doi,pdf,supplement),
(abstract).(hide abstract).
Motivated by recent spin and angularresolved photoemission measurements (SARPES) per formed on the twodimensional electronic states confined near the (001) surface of SrTiO_{3} in the presence of oxygen vacancies, we explore their spin structure by means of ab initio density functional theory (DFT) calculations of slabs. Relativistic nonmagnetic DFT calculations display Rashbalike spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is in cluded, bands become spinsplit 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 ingap states; another comes from the partly polarized t_{2g} itinerant electrons of Ti atoms lying further away from the oxygen vacancy which form the twodimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.

 

 D. Guterding, H. O. Jeschke, R. Valentí 

Prospects for topologically nontrivial electronic and magnetic states in correlated kagome lattice materials

 Sci. Rep. 6, 25988 (2016) (doi,pdf,supplement),
(abstract).(hide abstract).
Electronic states with nontrivial topology host a number of novel phenomena with potential for revolutionizing information technology. The quantum anomalous Hall effect provides spinpolarized dissipationfree transport of electrons, while the quantum spin Hall effect in combination with superconductivity has been proposed as the basis for realizing decoherencefree 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 abinitio 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.

 

 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 mixedsymmetry 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 quasitwodimensional organic
superconductor
κ(BEDTTTF)_{2}Cu[N(CN)_{2}]Br. Applying
spinfluctuation theory to a lowenergy materialspecific Hamiltonian
derived from ab initio density functional theory we calculate the
quasiparticle density of states in the superconducting state. We find
a distinct threepeak structure that results from a strongly
anisotropic mixedsymmetry superconducting gap with eight nodes and
twofold rotational symmetry. This theoretical prediction is supported
by lowtemperature scanning tunneling spectroscopy on in situ cleaved
single crystals of
κ(BEDTTTF)_{2}Cu[N(CN)_{2}]Br with the
tunneling direction parallel to the layered structure.

 

 F. Lechermann, H. O. Jeschke, A. Kim, S. Backes, R. Valentí 

Electron dichotomy on the SrTiO_{3} defect surface augmented by manybody 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, manybody effects are essential for the latter. Materials such as the familiar SrTiO_{3} (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 meanfield theory, that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly correlated t_{2g} lowenergy quasiparticles and localized “ingap” states of dominant e_{g} character with a subtle correlation signature. We furthermore touch base with recent experimental work and study the surface instability towards magnetic order.

 
2015

 Y. Iqbal, H. O. Jeschke, J. Reuther, R. Valentí, I. I. Mazin, M. Greiter, R. Thomale 

Paramagnetism in the Kagome compounds (Zn,Mg,Cd)Cu_{3}(OH)_{6}Cl_{2}

 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 spin1/2 kagome magnets, herbertsmithite, kapellasite, and haydeeite [(Zn,Mg)Cu_{3}(OH)_{6}Cl2_{2}], are all well described by a threeparameter 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 renormalizationgroup approach, and analyze the lowenergy 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)Cu_{3}(OH)_{6}Cl2_{2} at various pressures. Our results suggest that by varying pressure and composition one can traverse a paramagnetic regime between different, magnetically ordered phases.

 

 S. Backes, H. O. Jeschke, R. Valentí 

Microscopic nature of correlations in multiorbital 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 holedoped family of Fepnictides AFe_{2}As_{2} (A=K, Rb, Cs) as well as the fictitious FrFe_{2}As_{2} and aaxis stretched CsFe_{2}As_{2}. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling J_{H} and onsite Coulomb repulsion U in multiorbital Fepnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a nontrivial change in the iron 3d hoppings, an increase of orbitallyselective correlations and the presence of incoherent weight at highbinding energies that do not show the typical lower Hubbardband behavior but rather characteristic features of a Hund's metal. This is especially prominent in astretched CsFe_{2}As_{2}. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on J_{H} and we provide estimates of the coherence scale T^{∗}. We discuss these results in the framework of reported experimental observations.

 

 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 αRuCl_{3} and the zigzag antiferromagnetic ground state

 Phys. Rev. B 92, 235119 (2015) (doi,pdf),
(abstract).(hide abstract).
The layered honeycomb magnet αRuCl_{3} has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bonddependent, anisotropic interactions between spinorbit entangled j_{eff}=1/2 Ru^{4+} magnetic moments. Here we report a detailed study of the threedimensional crystal structure using xray 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 inplane offset, with occasional inplane sliding stacking faults, in contrast with the currentlyassumed trigonal 3layer stacking periodicity. We report electronic band structure calculations for the monoclinic structure, which find support for the applicability of the j_{eff}=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 threefold 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 T_{N} ~ 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 inplane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

 

 E. A. Zvereva, M. I. Stratan, Y. A. Ovchenkov, V. B. Nalbandyan, J.Y. Lin, E. L. Vavilova, M. F. Iakovleva, M. AbdelHafiez, 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 A_{3}Ni_{2}SbO_{6} (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 quasitwodimensional (2D) honeycomblattice monoclinic compounds A_{3}Ni_{2}SbO_{6} (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 Li_{3}Ni_{2}SbO_{6} and Na_{3}Ni_{2}SbO_{6}, respectively. The effective magnetic moments of 4.3 μB/f.u. (Li_{3}Ni_{2}SbO_{6}) and 4.4 μB/f.u. (Na_{3}Ni_{2}SbO_{6}) indicate that Ni^{2+} is in a highspin configuration (S=1). The temperature dependence of the inverse magnetic susceptibility follows the Curie Weiss law in the hightemperature region and shows positive values of the Weiss temperature ~8 K (Li_{3}Ni_{2}SbO_{6}) and ~12 K (Na_{3}Ni_{2}SbO_{6}) pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders AFM at low temperatures. In addition, the magnetization curves reveal a fieldinduced (spinflop 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 Ni^{2+} 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.

 

 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 abinitio calculations for the electronic structure of organic charge transfer salts κ(ET)_{2}Cu[N(CN)_{2}]Br, κ(ET)_{2}Cu[N(CN)_{2}]I, κ''(ET)_{2}Cu[N(CN)_{2}]Cl and κ(ET)_{2}Cu_{2}(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)_{2}Cu[N(CN)_{2}]Br through a metalinsulator transition. Using a tightbinding 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)_{2}Cu[N(CN)_{2}]Br in eclipsed and staggered configurations on opposite sides of the metalinsulator
transition.

 

 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 Febased
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 pnictogenbased 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 “spinnematic” nature in numerous pnictides, is not accompanied by magnetic order in FeSe,
and the superconducting transition temperature T_{c} 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 T_{c}(P).

 

 M. Altmeyer, R. Valentí, H. O. Jeschke 

Role of layer packing for the electronic properties of the organic superconductor (BEDTTTF)_{2}Ag(CF_{3})_{4}(TCE)

 Phys. Rev. B 91, 245137 (2015) (doi,pdf),
(abstract).(hide abstract).
The charge transfer compound (BEDTTTF)_{2}Ag(CF_{3})_{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 T_{c}=2.6 K, two phases with higher T_{c} of 9.5 K and 11 K are multilayered 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 κα′ 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.

 

 D. Guterding, S. Backes, H. O. Jeschke, R. Valentí 

Origin of the superconducting state in the collapsed tetragonal phase of KFe_{2}As_{2}

 Phys. Rev. B 91, 140503(R) (2015) (doi,pdf,supplement),
(abstract).(hide abstract).
Recently, KFe_{2}As_{2} 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
T_{c} ~ 12 K, while the tetragonal phase is a T_{c}
≤ 3.4 K superconductor. We argue that the key difference between
the previously known nonsuperconducting collapsed tetragonal phase in
BaFe_{2}As_{2}, CaFe_{2}As_{2},
EuFe_{2}As_{2}, SrFe_{2}As_{2}, and
CaFe_{2}P_{2} and the superconducting collapsed
tetragonal phase in KFe_{2}As_{2} 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 KFe_{2}As_{2}
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
spinfluctuation approach we calculate the superconducting pairing
symmetry in the collapsed tetragonal phase of
KFe_{2}As_{2} 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.

 

 H. O. Jeschke, F. SalvatPujol, 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
Cu_{4}(OH)_{6}FBr which contains Cu^{2+} ions
in a perfect kagome arrangement. In contrast to the spinliquid
candidate herbertsmithite
ZnCu_{3}(OH)_{6}Cl_{2}, 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
Cu_{4}(OH)_{6}FBr which has a Cu^{2+} site
coupling the kagome layers. The 3D network of exchange couplings
together with a substantial DzyaloshinskiiMoriya coupling lead to
canted antiferromagnetic ordering of this compound
at T_{N} = 15 K as observed by magnetic susceptibility
measurements on single crystals.

 

 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
Li_{x}(NH_{2})_{y}(NH_{3})_{z}Fe_{2}Se_{2}
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 T_{c} ~ 44 K, but the relative importance of the
layer separation and effective doping to the T_{c}
enhancement is currently unclear. Using state of the art band
structure unfolding techniques, we construct eightorbital models from
abinitio density functional theory calculations for these
materials. Within an RPA spinfluctuation approach, we show that the
electron doping enhances the superconducting pairing, which is of
s+symmetry and explain the experimentally observed limit
to T_{c} in the molecular spacer intercalated FeSe
class of materials.

 

 Y. Li, K. Foyevtsova, H. O. Jeschke, R. Valentí 

Analysis of the optical conductivity for A_{2}IrO_{3} (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
Na_{2}IrO_{3} within density functional theory by
including spinorbit (SO) and correlation effects (U) as
implemented in GGA+SO+U. We identify the various interband transitions
and show that the underlying quasimolecularorbital nature of the
electronic structure in Na_{2}IrO_{3} translates into
distinct features in the optical conductivity. Most importantly, the
parity of the quasimolecular orbitals appears to be the main factor
in determining strong and weak optical transitions. We also present
optical conductivity calculations for Li_{2}IrO_{3}
and discuss the similarities and differences with
Na_{2}IrO_{3}.


 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 donoracceptor [Tetramethoxypyrene
(TMP)][Tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by
substituting the acceptor TCNQ molecules by F_{4}TCNQ
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
TMPF_{4}TCNQ 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 semiconductorlike transport behavior with an increased
conductivity along the stacking direction. This is in contrast to
TMPTCNQ which shows a less pronounced anisotropy and a smaller
conductivity response. Our bandstructure calculations confirm the
onedimensional behavior of TMPF_{4}TCNQ with pronounced
dispersion only along the stacking axis. Infrared measurements
illustrating the C=N vibration frequency shift in F_{4}TCNQ
suggest however no improvement on the degree of charge transfer in
TMPF_{4}TCNQ with respect to TMPTCNQ. In both complexes
about 0.1 is transferred from TMP to the acceptor. Concerning the
pressure effect, our DFT calculations on designed TMPTCNQ and
TMPF_{4}TCNQ structures under different pressure conditions
show that application of uniaxial pressure along the stacking axis of
TMPTCNQ may be the route to follow in order to obtain a much more
pronounced charge transfer.

 

 H. O. Jeschke, J. Shen, R. Valentí 

Localized versus itinerant states created by multiple oxygen vacancies in SrTiO_{3}

 New J. Phys. 17, 023034 (2015) (doi,pdf),
(abstract).(hide abstract).
Oxygen vacancies in strontium titanate surfaces (SrTiO_{3})
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 SrTiO_{3} slabs
with multiple oxygen vacancies, with a main focus on two vacancies
near a titanium dioxide terminated SrTiO_{3} 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 TiO_{2}
layer lead to population of Ti t_{2g} states and
thus itinerancy of the electrons donated by the oxygen vacancy. In
contrast, electrons from subsurface oxygen vacancies populate
Ti e_{g} states and remain localized on the two
Ti ions neighboring the vacancy. We find that both, the formation of a
bound oxygenvacancy state composed of hybridized Ti
3e_{g} and 4p states neighboring the
oxygen vacancy as well as the elastic deformation after extracting
oxygen contribute to the stabilization of the ingap state.

 

 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 FeBased
Superconductors

 Phys. Rev. Lett. 114, 097003 (2015) (doi,pdf),
(abstract).(hide abstract).
The unusual temperature dependence of the resistivity and its inplane
anisotropy observed in the Febased superconducting materials,
particularly
Ba(Fe_{1−x}Co_{x})_{2}As_{2},
has been a longstanding puzzle. Here we consider the effect of
impurity scattering on the temperature dependence of the average
resistivity within a simple twoband model of a dirty spin density
wave metal. The sharp drop in resistivity below the Néel
temperature T_{N} 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

 U. Tutsch, B. Wolf, S. Wessel, Y. Tsui, H. O. Jeschke, I. Opahle, T. SahaDasgupta, R. Valentí, A. Brühl, K. RemovićLanger, T. Kretz, H.W. Lerner, M. Wagner, M. Lang 

Evidence of a fieldinduced Berezinskii–Kosterlitz–Thouless scenario in a twodimensional spin–dimer
system

 Nature Commun. 5, 5169 (2014) (doi),
(abstract).(hide abstract).
Twodimensional (2D) systems with continuous symmetry lack
conventional longrange order because of thermal
fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and
Thouless (BKT), 2D systems may exhibit socalled 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 molecularbased 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 sodesigned 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.

 

 M. Tomić, H. O. Jeschke, R. Valentí 

Unfolding of electronic structure through induced representations of space groups: Application to Febased 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 tightbinding 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 Febased superconductor
compounds and comment on the interpretation of oneiron versus
twoiron Brillouin zone representations.

 

 J. Diehl, S. Backes, D. Guterding, H. O. Jeschke, R. Valentí 

Correlation effects in the tetragonal and collapsed tetragonal phase of CaFe_{2}As_{2}

 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 CaFe_{2}As_{2} by
performing charge selfconsistent DFT+DMFT (density functional theory
combined with dynamical meanfield theory) calculations. While the
topology of the Fermi surface is basically unaffected by the inclusion
of correlation effects, we find important orbitaldependent mass
renormalizations which show good agreement with recent angleresolved
photoemission (ARPES) experiments. Moreover, we observe a markedly
different behavior of these quantities between the lowpressure
tetragonal and the highpressure collapsed tetragonal phase. We
attribute these effects to the increased hybridization between the
iron and arsenic orbitals as one enters the collapsed tetragonal
phase.

 

 S. Backes, D. Guterding, H. O. Jeschke, R. Valentí 

Electronic structure and de Haasvan Alphen frequencies in KFe_{2}As_{2} within
LDA+DMFT

 New J. Phys. 16, 085025 (2014) (doi,pdf),
(abstract).(hide abstract).
Recent density functional theory (DFT) calculations for
KFe_{2}As_{2} have shown to be insuffcient to
satisfactorily describe angleresolved photoemission (ARPES)
measurements as well as observed de Haas van Alphen (dHvA)
frequencies. In the present work, we extend DFT calculations based on
the fullpotential linear augmented planewave method by dynamical
mean field theory (DFT+DMFT) to include correlation effects beyond the
local density approximation. Our results indicate that
KFe_{2}As_{2} 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.

 

 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 sp^{2}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 GaCu_{3}(OH)_{6}Cl_{2}
(Gasubstituted herbertsmithite) as a correlated DiracKagome 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 manybody
physics is excitingly rich, with possible charge, magnetic and
superconducting instabilities. Through a combination of various
manybody methods we study possible symmetrylowering phase
transitions such as Mott Hubbard, charge or magnetic ordering, and
unconventional superconductivity, which in this compound assumes
an fwave symmetry.

 

 F. SalvatPujol, 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 longrange order, spin liquid, bad metal or
even superconductivity. A powerful method to investigate magnetism is
spinpolarized 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 Cu^{2+}, 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, BEDTTTF and (BEDTTTF)_{2} have anisotropic and
nonmonotonic structure.

 

 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 Li_{2}RuO_{3}

 Phys. Rev. B 89, 081408(R) (2014) (doi,pdf,supplement),
(abstract).(hide abstract).
The honeycomb lattice material Li_{2}RuO_{3} undergoes
a dimerization of Ru^{4+} 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, xray diffraction shows that discrete threefold symmetry is
regained on average, and the dimerization apparently disappears. In
contrast, local structural measurements using highenergy xrays, show
that disordered dimers survive at the nanoscale up to at least
650°C. The high temperature phase of Li_{2}RuO_{3}
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 T_{c} cuprates.

 

 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 A_{2}IrO_{3}

 Phys. Rev. B 89, 245113 (2014) (doi,pdf),
(abstract).(hide abstract).
We have investigated experimentally and theoretically the series
(Na_{1−x}Li_{x})_{2}IrO_{3}. 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 Xray diffraction, scanning electron microscopy and density
functional theory calculations, the system shows a miscibility gap and
a phase separation into an ordered
Na_{3}LiIr_{2}O_{6} phase with alternating Na3
and LiIr_{2}O_{6} planes, and a Lirich phase close to
pure Li_{2}IrO_{3}. 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 Li_{2}IrO_{3}
is smaller than in Na_{2}IrO_{3}, and (2) a gradual
reduction of the antiferromagnetic ordering
temperature T_{N} 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 reevaluated.

 

 S. A. J. Kimber, A. Salamat, S. R. Evans, H. O. Jeschke, K. Muthukumar, M. Tomić, F. SalvatPujol, R. Valentí, M. V. Kaisheva, I. Zizak, T. Chatterji 

Giant pressureinduced volume collapse in the pyrite mineral MnS_{2}

 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 hightemperature superconductivity. In transition metal materials, collapses are usually driven by socalled spinstate transitions, the interplay between the singleion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral hauerite (MnS_{2}) undergoes an unprecedented (ΔV∼22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron Xray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edgesharing 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 lowspin quantum S=1/2 moments are quenched by dimerization. Our results show how the emergence of metal–metal bonding can stabilize giant spinlattice coupling in Earth’s minerals.

 

 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 

Fieldinduced nematiclike magnetic transition in an iron pnictide superconductor, Ca_{10}(Pt_{3}As_{8}) ((Fe_{1x}Pt_{x})_{2}As_{2})_{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 ironpnictide superconductor Ca_{10}(Pt_{3}As_{8}) ((Fe_{1x}Pt_{x})_{2}As_{2})_{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 T_{c} 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 fieldinduced transition, observed both in the magnetically ordered parent compound and a nonordered superconducting sample, is a signature of a spinfloplike transition associated not with longrange order but driven by antiferromagnetic fluctuations of magnetic moments aligned preferentially out of the conducting planes at low temperatures.

 

 J. Ferber, K. Foyevtsova, H. O. Jeschke, R. Valentí 
 Unveiling the microscopic nature of correlated organic conductors: the case of κ(BEDTTTF)_{2}Cu[N(CN)_{2}]Br_{x}Cl_{1x}

 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(BEDTTTF)_{2}Cu[N(CN)_{2}]Br_{x}Cl_{1x} 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.

 

 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 Co_{2}(CO)_{8} and Co(CO)_{4} precursor molecules on fully and partially hydroxylated silica SiO_{2} surfaces. Such surfaces resemble the initial conditions of electron beam induced growth processes. We find that both W(CO)_{6} and Co_{2}(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.

 

 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 halffilling. We consider a dynamical
cluster approximation with a cluster size
of N_{c}=2×4, where shortrange spatial
fluctuations as well as onsite 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 antibonding 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 momentumdependent
evolution of the insulating behavior to the competition and
cooperation between shortrange spatial fluctuations and interlayer
coupling t_{⊥} with the help of the Coulomb interaction
U. Finally, we also discuss the possible appearance of nonFermi
liquid behavior away from halffilling.

 
2013

 N. Mevedev, H. O. Jeschke, B. Ziaja 

Nonthermal graphitization of diamond induced by a femtosecond xray 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 highZ material on the damage within diamond is discussed.

 

 A.C. Jacko, L. F. Tocchio, H. O. Jeschke, R. Valentí 

Importance of anisotropy in the spinliquid candidate
Me_{3}EtSb[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_{(4n)}Et_{n}X[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 tt' 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 spinliquid phase.

 

 J. Gaudin, N. Medvedev, J. Chalupsky, T. Burian, S. DastjaniFarahani, 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 ultraviolet laser (XUV) pulse of 24 eV to 275 eV photon energy provided by freeelectrons lasers. Experimental results obtained show that the irradiated diamond undergoes a solidtosolid 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.

 

 H. O. Jeschke, F. SalvatPujol, R. Valentí 

Firstprinciples determination of Heisenberg Hamiltonian parameters for the spin1/2 kagome antiferromagnet ZnCu_{3}(OH)_{6}Cl_{2}

 Phys. Rev. B 88, 075106 (2013) (doi,pdf),
(abstract).(hide abstract).
Herbertsmithite (ZnCu_{3}(OH)_{6}Cl_{2}) 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 J_{1} to exceed all other couplings by far. However, nextnearest neighbour kagome layer couplings of 0.019 J_{1} and interlayer couplings of up to 0.035 J_{1} 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.

 

 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 Febased 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.


 A. Ruff, M. Sing, R. Claessen, H. Lee, M. Tomić, H. O. Jeschke, R. Valentí 

Absence of metallicity in Kdoped picene: Importance of electronic correlations

 Phys. Rev. Lett. 110, 216403 (2013) (doi,pdf,supplement),
(abstract).(hide abstract).
Potassiumdoped picene (K_{x}picene) 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 Kdoped picene films
by photoelectron spectroscopy and ab initio density functional theory combined with dynamical
meanfield theory (DFT+DMFT). Experimentally we observe that, except for spurious spectral
weight due to the lack of a homogeneous chemical potential at low Kconcentrations (x ≈ 1), the
spectra always display a finite energy gap. This result is supported by our DFT+DMFT calculations
which provide clear evidence that K_{x}picene 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.

 

 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 spinorbit driven
insulating iridates, such as Na_{2}IrO_{3} and
Li_{2}IrO_{3} is whether the observed insulating state
should be classified as a MottHubbard insulator derived from a
halffilled relativistic j_{eff} = 1/2 band or as a
band insulator where the gap is assisted by spinorbit interaction, or
Coulomb correlations, or both. The difference between these two
interpretations is that only for the former, strong spinorbit
coupling (λ >~ W; where W is the band width) is essential. We
have synthesized the isostructural and isoelectronic
Li_{2}RhO_{3} and report its electrical resistivity
and magnetic susceptibility. Remarkably it shows insulating behavior
together with fluctuating effective S = 1/2 moments, similar to
Na_{2}IrO_{3} and Li_{2}IrO_{3},
although in Rh^{4+} (4d^{5}) the spinorbit
coupling is greatly reduced. We show that this behavior has
nonrelativistic oneelectron origin (although Coulomb correlations
assist in opening the gap), and can be traced down to formation of
quasimolecular orbitals, similar to those in
Na_{2}IrO_{3}.

 

 K. Foyevtsova, H. O. Jeschke, I. I. Mazin, D. I. Khomskii, R. Valentí 

Ab initio analysis of the tightbinding parameters and magnetic interactions in Na_{2}IrO_{3}

 Phys. Rev. B 88, 035107 (2013) (doi,pdf),
(abstract).(hide abstract).
By means of density functional theory (DFT) calculations [with and without inclusion of spinorbit (SO)
coupling] we present a detailed study of the electronic structure and corresponding microscopic Hamiltonian
parameters of Na_{2}IrO_{3}. In particular, we address the following aspects: (i) We investigate the role of the various
structural distortions and show that the electronic structure of Na_{2}IrO_{3} is exceptionally sensitive to structural
details. (ii)We discuss both limiting descriptions for Na_{2}IrO_{3} — quasimolecular orbitals (small SO limit, itinerant)
versus relativistic orbitals (large SO limit, localized) — and show that the description of Na_{2}IrO_{3} lies in an
intermediate regime. (iii) We investigate whether the nearest neighbor KitaevHeisenberg model is sufficient to
describe the electronic structure and magnetism in Na_{2}IrO_{3}. 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 Na_{2}IrO_{3}.We conclude that while the KitaevHeisenberg
Hamiltonian is the most general expression of the quadratic spinspin interaction in the presence of spinorbit
coupling (neglecting singlesite anisotropy), the itinerant character of the electrons in Na_{2}IrO_{3} makes other terms
beyond this model (including, but not limited to, 2nd and 3rd neighbor interactions) essential.

 

 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 singleband Hubbard model within the dynamical cluster approximation with 4 and 8site clusters and (ii) the anisotropic twoorbital Hubbard model with orbitals of different band width within the singlesite 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 metalinsulator critical interaction strength U_{c}/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.

 

 A. C. Jacko, H. Feldner, E. Rose, F. Lissner, M. Dressel, R. Valentí, H. O. Jeschke 

Electronic properties of Fabre chargetransfer 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 tightbinding 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)_{2}SbF_{6} at different temperatures and (TMTTF)_{2}PF_{6} 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 twodimensional. 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 intersite Coulomb interaction, the magnetic order is only
weakly enhanced. Both orders are suppressed when the effective pressure is increased.

 

 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
freeelectron laser excited semiconductors at femtosecond time scales. The
approach consists of the MonteCarlo method treating photoabsorption, highenergy
electron and corehole kinetics and relaxation processes. Lowenergy
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 highenergy 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 tightbinding 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.

 

 M. Tomić, H. O. Jeschke, R. M. Fernandez, R. Valentí 

Uniaxial strain effects on the structural and electronic properties of BaFe_{2}As_{2} and
CaFe_{2}As_{2}

 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 BaFe_{2}As_{2} and
CaFe_{2}As_{2} in the framework of ab initio density
functional theory (DFT) and a phenomonological GinzburgLandau
model. While contrary to the application of hydrostatic
or caxis uniaxial pressure both systems remain in the
orthorhombic phase with a pressuredependent nonzero magnetic moment,
we observe a signchanging 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 GinzburgLandau analysis reveals that this behavior is
a direct consequence of the competition between the intrinsic
magnetoelastic coupling of the system and the applied compressive
stress, which helps the system to overcome the energy barrier between
the two possible magnetoelastic ground states. Our results shed light
on the mechanisms involved in the detwinning process of an
orthorhombic ironpnictide crystal and on the changes in the magnetic
properties of a system under uniaxial stress.

 

 F. SalvatPujol, H. O. Jeschke, R. Valentí 

Simulation of electron transport during electronbeaminduced 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 electronbeaminduced growth of tungsten nanostructures
on SiO_{2} 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 radiationtransport techniques can contribute to a model that addresses the multiscale nature of
the electronbeaminduced 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 postgrowth electronbeam treatments.

 

 S. Backes, H. O. Jeschke 

Finite temperature and pressure molecular dynamics for BaFe_{2}As_{2}

 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 BaFe_{2}As_{2}. We use density functional theorybased BornOppenheimer 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 BaFe_{2}As_{2} at finite temperature and the effect of the structural oscillations on the band structure and Fermi surface in comparison to known zerotemperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe_{2}As_{2} under high pressure.

 

 L. F. Tocchio, H. Lee, H. O. Jeschke, R. Valentí, C. Gros 

Mott correlated states in the underdoped twodimensional 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 meanfield theory and the
variational Monte Carlo simulations of GutzwillerJastrow 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 nonFermiliquid
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

 I. I. Mazin, H. O. Jeschke, K. Foyevtsova, R. Valentí, D. I. Khomskii 

Na_{2}IrO_{3} as a molecular orbital crystal

 Phys. Rev. Lett. 109, 197201 (2012) (doi,pdf,supplement),
(abstract).(hide abstract).
Contrary to previous studies that classify Na_{2}IrO_{3} as a realization of
the HeisenbergKitaev model with dominant spinorbit coupling, we show that
this system represents a highly unusual case in which the electronic structure
is dominated by the formation of quasimolecular 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 Na_{2}IrO_{3} naturally follow from the QMO model.


 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
selfconsistent 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 d_{z2} 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.


 J. Shen, H. Lee, R. Valentí, H. O. Jeschke 
 Ab initio study of the twodimensional metallic state at the surface of SrTiO_{3}:
importance of oxygen vacancies

 Phys. Rev. B 86, 195119 (2012) (doi,pdf),
(abstract).(hide abstract).
Motivated by recent angleresolved photoemission spectroscopy (ARPES) observations of a highly
metallic twodimensional electron gas (2DEG) at the (001) vacuumcleaved surface of SrTiO_{3} 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
oxygendeficient SrTiO_{3} surface slabs. We find a significant surface reconstruction after introducing
oxygen vacancies and we show that the charges resulting from surfacelocalized 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.

 

 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. SchmidtBöcking, R. Dörner 

Momentum spectrometer for electronelectron coincidence studies on superconductors

 Rev. Sci. Instrum. 83, 103905 (2012) (doi,pdf),
(abstract).(hide abstract).
We present a new experimental setup to study electronelectron 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
analogtodigital converter. The threedimensional 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.

 

 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.

 

 K. Muthukumar, H. O. Jeschke, R. Valentí, E. Begun, J. Schwenk, F. Porrati, and M. Huth 
 Spontaneous Dissociation of Co_{2}(CO)_{8} and Autocatalytic growth of Co on
SiO_{2} : 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 metalorganic precursor Co_{2}(CO)_{8} on SiO_{2} surfaces after
application of two different pretreatment steps, namely by air plasma cleaning
or a focused electron beam preirradiation. We observe a spontaneous
dissociation of the precursor molecules as well as autodeposition of cobalt on
the pretreated SiO_{2} surfaces. We also find that the differences in metal
content and relative stability of these deposits depend on the pretreatment
conditions of the substrate. Transport measurements of these deposits are also
presented. We are led to assume that the degree of passivation of the SiO_{2}
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 SiO_{2}
surface (untreated surface) and chemisorption on a partially hydroxylated SiO_{2}
surface (pretreated 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.

 

 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. Valenti 
 OrbitalResolved 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 nearedge Xray 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 tetracyanopquinodimethane (TMP/HMPTCNQ)
were grown by vapor diffusion. The oxygen and nitrogen Kedge spectra
are spectroscopic fingerprints of the functional groups in the donor
and acceptor moieties, respectively. The orbital selectivity of the
NEXAFS preedge 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 Kedge
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
6a_{1} of the isolated methoxy group) and acceptor
(orbitals b_{3g}, a_{u}, b_{1g},
and b_{2u}, 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
Kedge (donor) and nitrogen Kedge (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.

 

 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 (MeCpPtMe_{3}) with a fully hydroxylated SiO_{2} surface has been explored by means of ab initio calculations. A large slab model cut out from the hydroxylated betacristobalite SiO_{2} (111) surface was chosen to simulate a silica surface. Density functional theory calculations were performed to evaluate the energies of MeCpPtMe_{3} adsorption to the SiO_{2} 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 Me_{3} groups of the molecule oriented towards the surface. Finally, we observe that vanderWaals corrections are crucial for the stabilization of the molecule on the surface. We discuss the relevance of our results for the growth of Ptbased nanostructured materials via deposition processes such as electron beam induced deposition.

 

 M. Pregelj, H. O. Jeschke, H. Feldner, R. Valentí, A. Honecker, T. SahaDasgupta, H. Das, S. Yoshii, T. Morioka, H. Nojiri, H. Berger, A. Zorko, O. Zaharko, D. Arčon 
 Multiferroic FeTe_{2}O_{5}Br: 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, manybody model considerations, and magnetization and electronspinresonance measurements shows that the multiferroic FeTe_{2}O_{5}Br should be described as a system of alternating antiferromagnetic S=5/2 chains with strong FeOTeOFe bridges weakly coupled by twodimensional 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.


 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 spinliquid candidate κ(BEDTTTF)_{2}Cu_{2}(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 chargetransfer salt has on the lowenergy Hamiltonian representation of the electronic structure. For that, we determine the crystal structure of κ(BEDTTTF)_{2}Cu_{2}(CN)_{3} for a series of temperatures between T=5 K and 300 K by single crystal Xray 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.

 

 YuZhong Zhang, Hunpyo Lee, HaiQing Lin, ChangQin 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 twoorbital Hubbard model within a meanfield 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.


 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 ironbased 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 angleresolved photoemission spectroscopy and de Haasvan Alphen experiments.


 Hunpyo Lee, Kateryna Foyevtsova, Johannes Ferber, Markus Aichhorn, Harald O. Jeschke, and Roser Valentí 
 Dynamical cluster approximation within an augmented planewave framework: Spectral properties of SrVO_{3}

 Phys. Rev. B 85, 165103 (2012) (doi,pdf),
(abstract).(hide abstract).
We present a combination of localdensity approximation (LDA) with the dynamical cluster approximation (LDA+DCA) in the framework of the fullpotential linear augmented plane wave method, and compare our LDA+DCA results for SrVO_{3} to LDA with the dynamical meanfield theory (LDA+DMFT) calculations as well as experimental observations on SrVO_{3}. We find a qualitative agreement of the momentum resolved spectral function with angleresolved 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 K_{0}=(0,0,0) and K_{1}=(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.

 

 Milan Tomić, Roser Valentí, and Harald O. Jeschke 
 Uniaxial versus hydrostatic pressureinduced phase transitions in CaFe_{2}As_{2} and BaFe_{2}As_{2}

 Phys. Rev. B 85, 094105 (2012) (doi,pdf),
(abstract).(hide abstract).
We present uniaxial pressure structural relaxations for CaFe_{2}As_{2} and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions as well as available experimental results. We find that CaFe_{2}As_{2} 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, BaFe_{2}As_{2} 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

 Kaliappan Muthukumar, Ingo Opahle, Juan Shen, Harald O. Jeschke, and Roser Valentí 

Interaction of W(CO)_{6} with SiO_{2} 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 SiO_{2} 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 SiO_{2} 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 SiO_{2} 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 nanodeposits on SiO_{2} in an electron beam induced deposition process are discussed.

 

 H. Lee, Y.Z. Zhang, H. O. Jeschke, and Roser Valentí 
 Anisotropic twoorbital Hubbard model:
Singlesite versus cluster Dynamical MeanField Theory

 Ann. Phys. (Berlin) 523, 689 (2011) (doi,pdf),
(abstract).(hide abstract).
The anisotropic twoorbital Hubbard model with different bandwidths and degrees of frustration in each
orbital is investigated in the framework of both singlesite dynamical meanfield theory (DMFT) as well as
its cluster extension (DCA) for clusters up to four sites combined with a continuoustime quantum Monte
Carlo algorithm. This model shows a rich phase diagram which includes the appearance of orbital selective
phase transitions, nonFermi liquid behavior as well as antiferromagnetic metallic states. We discuss the
advantages and drawbacks of employing the singlesite 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 ironpnictide
superconductors as well as the origin of the small staggered magnetization observed in these systems.

 

 H. O. Jeschke, I. Opahle, H. Kandpal, R. Valentí, H. Das, T. SahaDasgupta, O. Janson, H. Rosner, A. Brühl, B. Wolf, M. Lang, J. Richter, S. Hu, X. Wang, R. Peters, T. Pruschke, A. Honecker 
 Multistep 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 Cu_{3}(CO_{3})_{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 stateoftheart numerical manybody calculations. We propose an effective generalized spin1/2 diamond chain model which provides a consistent description of experiments: lowtemperature 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 manybody methods successfully describes the behavior of this frustrated material.

 

 P. T. Cong, B. Wolf, M. de Souza, N. Krüger, A. A. Haghighirad, S. GottliebSchönmeyer, F. Ritter, W. Aßmus, I. Ophale, K. Foyevtsova, H. O. Jeschke, R. Valentí, L. Wiehl, M. Lang 
 Distinct magnetic regimes through siteselective atom substitution in the frustrated quantum antiferromagnet Cs_{2}CuCl_{4x}Br_{x}

 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 Cs_{2}CuCl_{4x}Br_{x} (0 ≤ x ≤ 4), which include the two known endmember compounds Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4}, classified as quasitwodimensional 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 outofplane 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 x_{c1} = 1 and x_{c2} = 2. This unusual magnetic behavior can be explained by considering the structural peculiarities of the materials, especially the distorted Cuhalide tetrahedra, which support a siteselective replacement of Cl by Br ions. Consequently, the critical concentrations x_{c1} (x_{c2}) mark particularly interesting systems, where one (two) halidesublattice positions are fully occupied.

 

 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 twoorbital 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 J_{z}. Moreover, the PM orbital undergoes a transition from a Fermi liquid (FL) to a Mott insulator through an intermediate nonFL 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.

 

 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 crystaltocrystal transformations accompanied by
changes in the magnetic properties of a Cu^{II}phydroquinonate polymer

 CrystEngComm 13, 391 (2011). (doi, pdf),
(abstract).(hide abstract).
In the Cu^{II}phydroquinonate 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 twostep sequence; the antiferromagnetic Cu^{II}Cu^{II}
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.


 K. Foyevtsova, I. Opahle, Y.Z. Zhang, H. O. Jeschke, Roser Valentí 
 Determination of effective microscopic models for the frustrated antiferromagnets Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4} 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 triangularlattice antiferromagnets Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4} in the framework of density functional theory. Analysis of the exchange couplings J and J' using the available Xray structural data corroborates the values obtained from experimental results for Cs_{2}CuBr_{4} but not for Cs_{2}CuCl_{4}. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs_{2}CuCl_{4} employing different exchangecorrelation 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 longerranged couplings in both systems.

 

 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

 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 liquidlike T^{2} behaviour below 150 K. Neutron powder diffraction shows that unfrustrated Ctype Mn magnetic order develops below ~ 230 K, followed by a spinflop coupled to induced Pr order. At T ~ 35 K, we find a tetragonal to orthorhombic (TO) 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 felectron degrees of freedom.

 

 K. Foyevtsova, H. C. Kandpal, H. O. Jeschke, S. Graser, H.P. Cheng, Roser Valentí, P. J. Hirschfeld 
 Modulation of pairing interaction in Bi_{2}Sr_{2}CaCu_{2}O_{8+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 Bi_{2}Sr_{2}CaCu_{2}O_{8+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 dopingmodulated TB parameters, we finally evaluate the spin susceptibility and pairing interaction within spin fluctuation theory. The dwave pairing eigenvalues are enhanced above the pure system without O dopant, supporting the picture of enhanced local pairing around such a defect.

 

 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, BaFe_{2}As_{2}, SrFe_{2}As_{2}, and EuFe_{2}As_{2} in the spindensity
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+U_{eff} with a suitable choice of negative onsite interaction
U_{eff} = 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
Holsteinlike electronphonon interaction. Moreover, reminiscent of the fact that GGA+U_{eff} with a
positive U_{}eff is a simple approximation for reproducing a gap with correct amplitude in correlated
insulators, a negative U_{eff} 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.

 

 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 Febased superconductors. (i) We show that magnetization of stripetype antiferromagnetic order always becomes stronger when As is substituted by Sb in LaOFeAs, BaFe_{2}As_{2} 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 Febased 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 stripetype antiferromagnetic metallic state.

 

 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 twoband frustrated Hubbard
model in the framework of dynamical mean field theory. While a
firstorder 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 U_{c} values if only one of the two bands is
frustrated, resulting in continuous orbitalselective 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 ironpnictide 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.

 

 H. Lee, Y.Z. Zhang, H. O. Jeschke, R. Valentí, H. Monien 
 Dynamical cluster approximation study of the anisotropic twoorbital Hubbard model

 Phys. Rev. Lett. 104, 026402 (2010). (doi, pdf),
(abstract).(hide abstract).
We investigate the properties of a twoorbital Hubbard model with unequal bandwidths on the square lattice in the framework of the dynamical cluster approximation (DCA) combined with a continuoustime quantum Monte Carlo (CT QMC) algorithm. We explore the effect of shortrange spatial fluctuations on the nature of the metalinsulator transition and the possible occurrence of an orbitalselective Mott transition (OSMT), as a function of cluster size N_{c}. We observe that for N_{c}=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 N_{c}=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.

 

 H. O. Jeschke, H. C. Kandpal, I. Opahle, Y.Z. Zhang, R. Valentí 
 First principles determination of the model parameters in κ(ET)_{2}Cu_{2}(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)_{2}Cu_{2}(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) tightbinding 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)_{2}Cu_{2}(CN)_{3}. The electronic properties of κ(ET)_{2}Cu(SCN)_{2} are also briefly discussed.


 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 BaFe_{2x}Co_{x}As_{2} from angleresolved photoemission spectroscopy

 Phys. Rev. B 81, 104512 (2010). (doi, pdf),
(abstract).(hide abstract).
From a combination of high resolution angleresolved photoemission spectroscopy and density
functional calculations, we derive information on the dimensionality and the orbital character of the
electronic states of BaFe_{2}As_{2}. Upon increasing Co doping, the electronic states in the vicinity of the Fermi level take on increasingly threedimensional character. Both the orbital variation
with k_{z} and the more threedimensional 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 angleresolved photoemission spectroscopy and density functional calculations, we show that BaFe_{2}As_{2} possesses essentially twodimensional electronic states, with a strong change of orbital character of two of the Γcentered Fermi surfaces as a function of k_{z}. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly threedimensional character. Both the orbital variation with k_{z} and the more threedimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases.

 

 Y.Z. Zhang, I. Opahle, H. O. Jeschke, R. Valentí 
 Pressuredriven phase transitions in TiOCl and the family (Ca, Sr, Ba)Fe_{2}As_{2}

 J. Phys.: Condens. Matter 22, 164208 (2010). (doi, preprint),
(abstract).(hide abstract).
Motivated by recent experimental measurements on pressuredriven phase transitions in Mottinsulators as well as the new iron pnictide superconductors, we show that first principles CarParrinello molecular dynamics calculations are a powerful method to describe the microscopic origin of such transitions. We present results for (i) the pressureinduced insulator to metal phase transition in the prototypical Mott insulator TiOCl as well as (ii) the pressureinduced structural and magnetic phase transitions in the family of correlated metals AFe_{2}As_{2} (A=Ca,Sr,Ba). Comparison of our predictions with existing experimental results yields very good agreement.


 Y.Z. Zhang, K. Foyevtsova, H. O. Jeschke, M. U. Schmidt, R. Valentí 
 Can the Mott Insulator TiOCl be Metallized by Doping? A FirstPrinciples 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 Nadoped 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 quasitwodimensional lattice and discuss the nature of the insulating state under doping. Finally, a likely route for metallizing TiOCl by doping is proposed.

2009

 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 BaFe_{2}As_{2}

 Nature Materials 8, 471 (2009). (doi,preprint),
(abstract).(hide abstract).
The discovery of a new family of highTC 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 nonsuperconducting
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 BaFe_{2}As_{2},
namely suppression of the tetragonaltoorthorhombic phase
transition and reduction in the AsFeAs bond angle and FeFe
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 BaFe_{2}As_{2}.

 

 H.C. Kandpal, I. Opahle, Y.Z. Zhang, H. O. Jeschke, R. Valentí 
 Revision of model parameters for κtype 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(BEDTTTF)_{2}X 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.


 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.


 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 timedependent, manybody potential energy surfaces
derived from tightbinding Hamiltonians. The shape and spectral
composition of the laser pulse is explicitly taking into account in a
nonperturbative 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.


 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 BaFe_{2}As_{2} by angleresolved photoemission spectroscopy

 Phys. Rev. B 79, 155118 (2009). (doi, pdf),
(abstract).(hide abstract).
We report high resolution angleresolved photoemission spectroscopy (ARPES) studies of the
electronic structure of BaFe_{2}As_{2}, which is one of the parent compounds of the Fepnictide 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 Γpoint and an
electron pocket at each of the Xpoints. 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.

 

 Y. Z. Zhang, H. C. Kandpal, I. Opahle, H. O. Jeschke, and R. Valentí 
 Microscopic origin of pressureinduced phase transitions in ironpnictide AFe_{2}As_{2} 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 AFe_{2}As_{2}
(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 CaFe_{2}As_{2} and its absence in BaFe_{2}As_{2}.
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.


 K. Foyevtsova, Y. Z. Zhang, H. O. Jeschke, and R. Valentí 
 First principles perspective on the microscopic model for Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4} 
 J. Phys.: Conf. Ser. 145, 012038 (2009). (doi, pdf),
(abstract).(hide abstract).
We investigate the microscopic model for the frustrated layered
antiferromagnets Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4} by performing ab initio density
functional theory (DFT) calculations and with the help of the tightbinding
method. The combination of both methods provide the relevant interaction paths
in these materials, and we estimate the corresponding exchange constants. We
find for Cs_{2}CuCl_{4} that the calculated ratio of the strongest inplane exchange
constants J'/J between the spin1/2 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 Cs_{2}CuCl_{4} and Cs_{2}CuBr_{4} are also analyzed.

 
2008

 Y. Z. Zhang, H. O. Jeschke, and R. Valentí 
 Microscopic model for transitions from Mott to spinPeierls insulator in TiOCl 
 Phys. Rev. B 78, 205104 (2008). (doi, pdf),
(abstract).(hide abstract).
On the basis of ab initio densityfunctionaltheory 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 spinPeierls 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 spinPeierls distortion, which results in
the unusual incommensurate phase. In addition, our calculations indicate that the spinPeierls state is triggered
by adiabatic phonons, which is essential for understanding the nature of the phase transition.


 Y. Z. Zhang, H. O. Jeschke, and R. Valentí 
 Two pressureinduced transitions in TiOCl: Mott insulator to anisotropic metal 
 Phys. Rev. Lett. 101, 136406 (2008). (doi, pdf),
(abstract).(hide abstract).
Using CarParrinello molecular dynamics we investigate the behavior of the lowdimensional multiorbital
Mott insulator TiOCl under pressure. We show that the system undergoes two consecutive phase
transitions, first at P_{c} from a Mottinsulator to a metallic phase in the ab plane with a strong TiTi dimerization
along b. At a pressure P'_{c} > P_{c} the dimerization disappears and the system behaves as a uniform
metal. This second transition has not yet been reported experimentally. We show that the insulatortometal
transition at P_{c} 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 t_{2g}
bands. Our computed pressuredependent lattice parameters are consistent with experimental observations
and the existing controversy on the change of crystal symmetry at high pressures is discussed.

 
2007

 H. O. Jeschke, L. A. Salguero, B. Rahaman, C. Buchsbaum, V. Pashchenko, M. U. Schmidt, T. SahaDasgupta, 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
lowspin state and a highspin (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 xray 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.

 

 H. O. Jeschke, A. H. Romero, M. E. Garcia, and A. Rubio 
 Microscopic investigation of laserinduced structural changes in singlewall carbon nanotubes 
 Phys. Rev. B 75, 125412 (2007). (doi, pdf),
(abstract).(hide abstract).
Extensive excitedstate molecular dynamics simulations of femtosecond laserinduced structural transformation
in singlewalled 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 electronion relaxation times.
Implications of laserinduced structural transformations are analyzed.

 

 L. A. Salguero, H. O. Jeschke, B. Rahaman, T. SahaDasgupta, C. Buchsbaum, M. U. Schmidt, and R. Valentí 
 Cubased 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)
2,5bis(pyrazol1yl)1,4dihydroxybenzene (CuCCP), which has been suggested
to be a good realization of a Heisenberg spin1/2 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 CuCu coupling strength variations on this spin1/2 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 (NH_{2}) and
(ii) an electron withdrawing group (CN), while the other modification consisted
of adding H_{2}O and NH_{3} 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 CuCu interaction paths in these new structures
and discuss its implications for the underlying microscopic model.

 

 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 twostep 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 2,5bis
(pyrazol1yl)1,4dihydroxybenzene.

 

 R. Valentí, T. SahaDasgupta, H. O. Jeschke, B. Rahaman, H. Rosner, P. Lemmens, R. Takagi, and M. Johnsson 
 Comparative investigation of the coupled tetrahedra quantum spin systems Cu_{2}Te_{2}O_{5}X_{2}, X=Cl, Br and Cu_{4}Te_{5}O_{12}X_{4} 
 Physica C 460462, 462 (2007). (doi, pdf),
(abstract).(hide abstract).
We present a comparative study of the coupledtetrahedra quantum spin systems Cu_{2}Te_{2}O_{5}X_{2}, X = Cl, Br (Cu2252(X)) and the
newly synthesized Cu_{4}Te_{5}O_{12}X_{4} (Cu45124(Cl)) based on ab initio density functional theory calculations. The magnetic behavior of
Cu45124(Cl) with a phase transition to an ordered state at a lower critical temperature T_{c} = 13.6 K than in Cu2252(Cl)
(T_{c} = 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 Cu45124(Br) with an ab initio relaxed structure using
CarParrinello molecular dynamics.

 

 B. Rahaman, H. O. Jeschke, R. Valentí, and T. SahaDasgupta 
 Microscopic model for the frustrated Cu IIspin tetrahedronbased Cu_{4}Te_{5}O_{12}X_{4} (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 Cu_{4}Te_{5}O_{12}Cl_{4} based on density functional calculations and on ab initioderived effective
models. In view of these results, we discuss the origin of the observed differences in behavior between this
system and the structurally similar Cu_{2}Te_{2}O_{5}Cl_{2}. Since the Br analog of the title compound has not been
synthesized yet, we derive the crystal structure of Cu_{4}Te_{5}O_{12}Br_{4} by geometry optimization in an ab initio
molecular dynamics calculation and investigate the effect of substituting Cl by Br. The possible magnetic
behavior of Cu_{4}Te_{5}O_{12}Br_{4} in comparison with the recently studied Cu_{2}Te_{2}O_{5}Br_{2} is also discussed.

 
2006

 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.

 

 F. Valencia, A. H. Romero, H. O. Jeschke, and M. E. Garcia 
 Large amplitude coherent phonons and inverse StoneWales 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 pentagonheptagon pairs introduced by a single
StoneWales 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 StoneWales 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 outofplane 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 zigzag and armchair nanotubes. In contrast, for a defective C_{60} fullerene the mechanism is hindered
by the presence of edgesharing pentagons.

 

 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 diamondlike nanostructures as a consequence of the laser excitation. In contrast
to a thermal processing of nanodiamonds, an ultrashort laser pulse can eliminate all sp^{3} bonds in the system on a subpicosecond
time scale, opening the possibility for the production of graphitic nanostructures and the creation of nanocontacts between carbon
nanoclusters and other nanostructures.

2005

 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 selfassembled 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 pentagonheptagon
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 laserinduced electronic nonequilibrium the pentagonheptagon pair is transformed
back into four hexagons without producing any irreversible damage to the rest of the nanotube.


 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 selfconsistency 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

 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 nonequilibrium
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.


 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 tightbinding electronic Hamiltonian we discuss two examples
of laserinduced coherent phonons: (1) excitation of the E_{2g1}
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.


 T. Dumitrica, M. E. Garcia, H. O. Jeschke and B. I. Yakobson 
 Selective cap opening in carbon nanotubes driven by laserinduced 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 laserinduced 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

 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 timeresolved xray absorption spectroscopy 
 Phys. Rev. Lett. 91, 157403 (2003). (doi, pdf),
(abstract).(hide abstract).
Timeresolved xray 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 xray absorption codes.


 M. E. Garcia and H. O. Jeschke 
 Theoretical approach to the laser induced melting of graphite under different pressure conditions 
 Appl. Surf. Sci. 208209, 61 (2003). (doi, pdf),
(abstract).(hide abstract).
We present a theoretical study of the laserinduced 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 timedependent, manybody potential energy surface derived from a tightbinding Hamiltonian. Our results show that the laserinduced 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 lowdensity carbon phase, which remains stable under hig pressure, but is unstable with an ultrashort lifetime under normal pressure.

 
2002

 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. 197198, 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.


 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. 197198, 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.


 H. O. Jeschke, M. E. Garcia, and J. A. Alonso 
 Nonthermal fragmentation of C_{60} 
 Chem. Phys. Lett. 352, 154 (2002). (doi, pdf),
(abstract).(hide abstract).
A theoretical study of the subpicosecond fragmentation of C_{60} 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.


 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann 
 Timedependent 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

 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 timedependent potential energy
surfaces. We show that graphite has the unique property of exhibiting two distinct laserinduced 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

 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, 361371 (2000). (doi, pdf),
(abstract).(hide abstract).
We present a theoretical study of the shorttime relaxation
of clusters in response to ultrafast excitations using
femtosecond laser pulses.We analyze the excitation of different
types of clusters (Hg_{n}, Ag_{n}, Si_{n}, C_{60} and Xe_{n}) and classify
the relaxation dynamics in three different regimes, depending
on the intensity of the exciting laser pulse.
For lowintensity pulses (I < 10^{12} W/cm^{2}) we determine
the timedependent structural changes of clusters upon ultrashort
ionization and photodetachment. We also study the
laserinduced nonequilibrium fragmentation and melting of
Si_{n} and C_{60} 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 >
10^{15} W/cm^{2}), the explosion of clusters under the action of
very intense ultrashort laser fields is described.

 
19961999

 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann 
 Theory
for Laser Induced Ultrafast Phase Transitions in Carbon 
 Appl. Phys. A 69[Suppl.], S49S53 (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 C_{60}
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 laserinduced
melting of a C_{60} 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.

 

 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann 
 Microscopic analysis of the femtosecond graphitization of
diamond 
 Phys. Rev. B 60, R3701R3704 (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. Moleculardynamics simulations on time dependent potentialenergy 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 potentialenergy surface of the laser
excited system.


 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, R4601R4604 (1996) (Rapid
Communication). (doi, pdf),
(abstract).(hide abstract).
Combining an electronic theory with moleculardynamics 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 Ag_{3}^{} → Ag_{3}. We show that the timedependent 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.


 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, L545L549 (1996)
(letter to the editor). (doi, pdf),
(abstract).(hide abstract).
We study the ultrafast dynamics of Ag_{3} clusters immediately after the
photodetachment of Ag_{3}^{} 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 Ag_{3}. 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.

Nonrefereed publications

 H. O. Jeschke and M. E. Garcia 
 Ultrafast structural changes induced by femtosecond laser pulses 
 in Bernhard W. Adams (ed.), Nonlinear Optics, Quantum Optics and Ultrafast Phenomena with Xrays, Kluwer Academic Publishers, Boston/Dordrecht/London, June 2003, pp. 175214. (preprint),
(abstract).(hide abstract).
A theoretical approach to treat laser induced femtosecond structural changes
in covalent solids is described. Our approach consists in molecular dynamic
simulations performed on the basis of a timedependent, manybody potential
energy surface derived from a tightbinding Hamiltonian. The shape and spectral
composition of the laser pulse is explicitly taken into account in a nonperturbative
way. We show a few examples of the application of this approach to describe the
laser excitation of bulk diamond and ultrathin graphite and silicon films.


 K. SokolowskiTinten, S. Kudryashov, V. Temnov, J. Bialkowski, D.
von der Linde, A. Cavalleri, H. O. Jeschke, M. E. Garcia, and K. H. Bennemann 
 Femtosecond laser ablation of graphite, 
 in Ultrafast Phenomena XII, Eds. T.
Elsaesser, S. Mukamel, M. Murnane and N. F. Scherer,
Springer Series in Chemical Physics 66,
276 (Springer, Heidelberg, 2000). (preprint),
(abstract).(hide abstract).
The dynamics of fslaser ablation of graphite has been
investigated experimentally and theoretically. Molecular dynamics
caculations, incorporating changes of the interatomic potentials due to
electronic excitation, support the experimental observation of two
different ablation mechanisms.


 H. O. Jeschke, M. E. Garcia, and K. H. Bennemann 
 Ultrafast Structural Response and Nonlinear Fragmentation
Dynamics of Small Clusters Induced by Optical Excitation 
 in Theory of
Atomic and Molecular Clusters, edited by J. Jellinek, Springer
Series in Cluster Physics, SpringerVerlag, Heidelberg 1999,
pp. 181208,
(abstract).(hide abstract).
The ultrafast relaxation of small clusters immediately after ionization and photodetachment is theoretically studied. Microscopic models are proposed to describe the nonequilibrium dynamics of the clusters. As an example of strong structural response we determine for Ag_{3} the time scale for the change from a linear to triangular structure after the photodetachment process Ag_{3}^{} → Ag_{3}. We show that the timedependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion. We demonstrate that the time scale for bond breaking and formation are temperature dependent. We compare with experiment and point out the general significance of our results. We also study the ultrafast fragmentation dynamics of small Hg_{n} clusters. We determine the fragmentationtime distributions induced by ionization. A dramatic change in the nonequilibrium fragmentation behavior occurs when the temperature before ionization reaches the melting point of the neutral clusters. This new effect could one allow to determine "melting points" of small clusters by pump&probe experiments. The ultrafast dynamics depends nonlinearly on the initial atomic positions and velocities, reflecting the intrinsic chaotic behavior of small clusters. The application of the theoretical models presented here, to the study of other relevant processes like control of chemical reactions, is discussed.

 
Other publications

 Li Jinfa 
 Gefühle 
 ausgewählte Gedichte,aus dem
Chinesischen von Harald Jeschke, Orientierungen 2/1999,
p. 6166. 
