The discovery of the iron oxypnictides [1], a new class
of high Tc superconductors besides the cuprates, has generated
great interest in the condensed matter community. The new class of
materials has similarities to the cuprates like two-dimensionality and
proximity of high temperature superconductivity with magnetic order,
but also differences; the undoped or unpressurized compounds are bad
metals with spin-density wave order and not Mott insulators like the
cuprates [2]. The mechanism of the high temperature
superconductivity in the FeAs materials has not been conclusively
determined; while electron phonon coupling seems to be too small to
account for the high transition temperatures [3], the
spin-lattice coupling seems to be significant [4] so
that lattice degrees of freedom may indirectly play a significant
role. It has been found
experimentally [5,6,7] that the 122
materials CaFe2As2, SrFe2As2 and BaFe2As2 each show
dome shaped superconducting transition temperatures as a function of
pressure, with different pressures marking the onset of
superconductivity in each compound. While a lot of theoretical work
has been done on the electronic properties of the FeAs materials, the
electron-lattice coupling and structural properties under pressure
require careful theoretical investigation.
a) Comparison between structural distortions under pressure and upon doping
An interesting question in the study of iron pnictide superconductors concerns the relative importance of Fermi surface topology and carrier doping for the superconducting transition temperature. As the parent compounds of the iron arsenide superconductors are already metallic, it is not obvious how important the addition of carriers, for example by replacement of Ba2+ by K+ ions in BaFe2As2 is. Careful comparison of the structural properties of BaFe2As2 under pressure and Ba1-xKxFe2As2 as a function of doping x reveals that some structural parameters known to be important for superconductivity evolve in a very similar way. For example, the a lattice parameter that influences the important geometry of the FeAs4 tetrahedra is seen to decrease linearly both under pressure and as function of doping.
This result is published in
Ref. [KKZ+09].
A. I. Goldman, A. Kreyssig, K. Prokes, D. K. Pratt, D. N. Argyriou,
J. W. Lynn, S. Nandi, S. A. J. Kimber, Y. Chen, Y. B. Lee,
G. Samolyuk, J. B. Leao, S. J. Poulton, S. L. Bud'ko, N. Ni,
P. C. Canfield, B. N. Harmon, R. J. McQueeney, Phys. Rev. B 79, 024513 (2009).
