Kourovka-34 Electronic Structure of Iron Based Superconductors: Pnictides vs. Chalcogenides M.V.Sadovskii 1,2 In collaboration with E.Z.Kuchinskii 1 and I.A.Nekrasov 1 1 Institute for Electrophysics, Russian Academy of Sciences, Ekaterinburg, Russia 2 Institute for Metal Physics, Russian Academy of Sciences, Ekaterinburg, Russia
Outline of the talk Electronic structure of Fe-pnictides Fermi surfaces and superconducting gaps Fe-chalcogenides: AFe2Se2 a new class? Fermi surfaces Antiferromagnetism and vacancies DOS and Tc correlation Multiple bands superconductivity Conclusions
Essentially physics of FeAs layers! LiFeAs Li +1 Fe +2 As -3 LaOFeAs BaFe 2 As 2 FeAs tetrahedra form two-dimensional layers surrounded by LaO, Ba or Li. Fe ions inside tetrahedra form a square lattice.
ReOFeAs: phase diagram H. Luetkens et al., arxiv:0806.3533 µsr J. Zhao et al., Nature Materials 7, 953-959 (2008). neutrons neutrons Q. Huang et al., PRB 78, 054529 (2008) LaFeAsO 1-x F x µsr A. J. Drew et al., arxiv:08074876 SmFeAsO 1-x F x
Magnetic properties of 122 Neutron scattering single crystal Ba 1-x K x Fe 2 As 2 Q. Huang et al., arxiv:0806.2776 (2008) H. Chen et al., arxiv:0807.3950 (2008) 142K [220K] 1 T(I4/mmm) O(Fmmm) 142K [220K] 1 AFM order of Fe with 2a 2b 2c cell, stripes along b [a] 1 m Fe =0.87 µ B at 5K for BaFe 2 As 2 m Fe =0.94 µ B at 10K for SrFe 2 As 2 1 for SrFe 2 As 2, J. Zhao et al., PRB 78, 140504 (2008)
LDA band structure of tetragonal LaOFeAs Essentially multiband problem Fe-3d As-4p O-2p I.A. Nekrasov et al., JETP Lett. 87, 560 (2008)
REOFeAs: Rare-Earth Puzzle LiFeAs LaOFeAs BaFe 2 As 2 Pnictogen height? LiFeAs
arxiv:1005.0884
LDA+DMFT: strong or intermediate correlations?
arxiv: 0807.3370 Phonons arxiv: 0807.3172
arxiv: 0806.4806 Three hole cylinders! Band narrowing due to correlations?
arxiv: 0807.0419
Superconducting gap ARPES data Superconducting gap ARPES data arxiv: 0807.0419 Schematic picture of superconducting gaps in Ba 0.6 K 0.4 Fe 2 As 2. Lower picture represents Fermi surfaces (ARPES intensity), upper insert temperature dependence of gaps at different sheets of the Fermi surface.
arxiv: 0809.4455
arxiv:0810.3047 arxiv:0810.3047
arxiv: 0807.2369, 0807.4315, 0807.4775
arxiv: 0809.2058
arxiv:0807.4312
A(A=K,Cs, )Fe2Se2: a New Class? 122 structure ArXiv: 1012.3637 K x +1 Fe +2 2 Se-2 2? Vacancies?
ArXiv: 1012.5552 ArXiv: 1012.5552
ArXiv: 1101.5670
ARPES: Electronic Spectrum and Gaps ArXiv: 1012.5980 No nesting!
ArXiv: 1101.4923 Nothing to nest!
ArXiv: 1101.4556
ArXiv: 1102.1057 No nesting!
A(A=K,Cs, )Fe2Se2: a New AFM Superconductor ArXiv: 1102.0830 K 2 Fe 4 Se 5? K +1 2 Fe+2 4 Se-2 5!
arxiv:1106.0881
Calculated spectrum and FS in the presence of ordered vacancies and AFM
Dependence of bands on anion height As z a approaches 1.37A o Fermi velocity drops due the growth of Fe-As hybridization => Density of states grows.
Tc and Density of States Correlation arxiv:1001.1801 arxiv: 1004.0801
Simple model of multiple band superconductivity V. Barzykin, L.P. Gorkov. Pis'ma ZhETF 88, 142 (2008); arxiv: 0806.1993 i,ν i - a superconducting gap and DOS on the i-th sheet of the Fermi surface! V i,j - intraband and interband pairing coupling constants matrix. λ =V ex,ex = V ey,ey - pairing interactions on the same electronic pockets at point X or Y, µ = V ex,ey - connects electrons of different electronic pockets, u = V h1,h1, u = V h2,h2, w = V h1,h2 - BCS interactions within two hole-like pockets, t = V h,ex = V h,ey - couple electrons at points X and Γ. H.Suhl, B.Matthias, L.Walker Phys.Rev.Lett. 3, 552 (1959) V.Moskalenko FMM 4, 503 (1959) Schematic electronic spectrum and Fermi surfaces of FeAs superconductor in the extended band picture. arxiv: 0901.0164 1/g eff Matrix of dimensionless coupling constants Secular equation, physical solution corresponds to a maximal positive value of g eff, which determines the highest value of T c
Effective coupling from weak to strong? Effective coupling! Effective coupling constant g eff is significantly larger than the pairing constant g on the small hole - like cylinder. It can be said that coupling constants from different cylinders effectively produce additive effect. In fact this can lead to high enough values of T c even for relatively small values of intraband and interband pairing constants. g eff, T c (d x 2 -y 2 pairing) < g eff, T c (s ± pairing) 1. No interband pairing g eff = max(g i ) Value of T c in multiple bands systems is determined by the relations between partial densities of states (and pairing constants) on different sheets of the Fermi surface, not only by the total density of states at the Fermi level. 2. All pairing interactions (both intraband and interband) are just the same - u, and all partial densities of states on all four Fermi surface pockets are also the same - ν 1. Is there a nontrivial optimal band structure (number of bands etc.)?
Gap ratios for for different u /u: T 0 Despite rather large number of free parameters of the model it is not easy to obtain the observable in ARPES experiments values of the ratios 2 / 1 0.5 and 3 / 1 1. In fact it requires small enough attraction (or even repulsion, u > 0) on the large hole - like cylinder.
AFM Superconductors
Ba 123 FeSe Ba +2 (Fe +2 ) 2 (Se -2 ) 3 ArXiv: 1111.7046
Speculations on 1111 FeSe structure? N (P,As,Sb,Bi, ) LaNFeSe La (Nd,Pr,Sm,...) Se (RE) +3 N -3 Fe +2 Se -2
Conclusions Total DOS at the Fermi level directly correlates with T c, but details depend on partial DOS es AFe 2 Se 2 electronic spectrum is significantly different from that of FeAs systems and pure FeSe No Nesting in AFe 2 Se 2! AFe 2 Se 2 metallic despite AFM and vacancy order AFe 2 Se 2 New AFM Superconductor (T N >>T C )