Electronic Structure, Magnetism and Spin-Fluctuations in Fe-As Based Superconductors
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1148-PP02-01
Electronic Structure, Magnetism and Spin-Fluctuations in Fe-As Based Superconductors
David J. Singh1, Mao-Hua Du1, Lijun Zhang1, Alaska Subedi1,2 and Jiming An1,3 1
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6114, USA. 2
Department of Physics, University of Tennessee, Knoxville, TN 37996-1200, USA.
3
Wuhan University of Technology, Wuhan, China
ABSTRACT The physical properties of the layered iron superconductors and related phases are discussed starting from first principles calculations. The electronic structure is described as that of metallic Fe2+ square lattice sheets with substantial direct Fe-Fe hopping and interactions with the neighboring anionic pnictogens or chalcogens. The materials have a semi-metallic band structure, and in particular the Fermi surface consists of small cylindrical electron sections centered at the zone corner, and compensating hole sections at the zone boundary. The density of states N(EF) is high placing the materials near itinerant magnetism in general, and furthermore the small Fermi surface sections are well nested leading to a tendency towards a spin density wave. Comparison of experimental and density functional results imply the presence of exceptionally strong spin fluctuations in these materials. Superconductivity is discussed within this context.
INTRODUCTION The layered iron superconductors, discovered by Kamihara and co-workers [1,2], now comprise a rather large family of materials, with four distinct crystallographic types and remarkable magnetic properties in addition to high temperature superconductivity. Unifying features include (1) the presence of square planar metallic sheets of nominal Fe2+ with approximately tetrahedral coordination by the pnictogens As or P (with lower Tc) or chalcogens [3,4], (2) phase diagrams that show proximity of superconducting and magnetic states, in particular a spin density wave (SDW) state [5,6], with superconductivity generally appearing when the SDW is suppressed by doping or pressure and (3) strong signatures of spin-fluctuations in the normal metal and superconducting states [7-11]. Here, we overview some of the properties of these materials from the point of view of electronic structure. Properties discussed include the band structure and cohesion, Fermi surface, and magnetism. At this time, the mechanism for superconductivity and the symmetry of the superconducting state are yet to be established. However, we discuss the electronic structure and magnetism in relation to superconductivity, supposing a spin-fluctuation mechanism [12-14].
BAND STRUCTURE AND CHEMISTRY As mentioned, superconductivity has now been discovered in four distinct crystallographic types, the oxy-arsenides (prototype LaFeAsO) [1], ThCr2Si2 structure materials (prototype BaFe2As2) [15], LiFeAs [16] and pnictogen-free alpha-PbO structure, FeSe [3,4]. Remarkably, superconductivity can be produced both by doping away from the Fe plane, e.g. using partial substitution of O by F, as in the orig
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