Electronic Structure of Iron Based Superconductors: Pnictides vs. Chalcogenides

Transcription

1 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

2 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

3 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.

4 ReOFeAs: phase diagram H. Luetkens et al., arxiv: µsr J. Zhao et al., Nature Materials 7, (2008). neutrons neutrons Q. Huang et al., PRB 78, (2008) LaFeAsO 1-x F x µsr A. J. Drew et al., arxiv: SmFeAsO 1-x F x

5 Magnetic properties of 122 Neutron scattering single crystal Ba 1-x K x Fe 2 As 2 Q. Huang et al., arxiv: (2008) H. Chen et al., arxiv: (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, (2008)

6 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)

7 REOFeAs: Rare-Earth Puzzle LiFeAs LaOFeAs BaFe 2 As 2 Pnictogen height? LiFeAs

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9 arxiv:

10 LDA+DMFT: strong or intermediate correlations?

11 arxiv: Phonons arxiv:

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13 arxiv: Three hole cylinders! Band narrowing due to correlations?

14 arxiv:

15 Superconducting gap ARPES data Superconducting gap ARPES data arxiv: 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.

16 arxiv:

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18 arxiv: arxiv:

19 arxiv: , ,

20 arxiv:

21 arxiv:

22 A(A=K,Cs, )Fe2Se2: a New Class? 122 structure ArXiv: K x +1 Fe +2 2 Se-2 2? Vacancies?

23 ArXiv: ArXiv:

24 ArXiv:

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26 ARPES: Electronic Spectrum and Gaps ArXiv: No nesting!

27 ArXiv: Nothing to nest!

28 ArXiv:

29 ArXiv: No nesting!

30 A(A=K,Cs, )Fe2Se2: a New AFM Superconductor ArXiv: K 2 Fe 4 Se 5? K +1 2 Fe+2 4 Se-2 5!

31 arxiv:

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34 Calculated spectrum and FS in the presence of ordered vacancies and AFM

35 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.

36 Tc and Density of States Correlation arxiv: arxiv:

37 Simple model of multiple band superconductivity V. Barzykin, L.P. Gorkov. Pis'ma ZhETF 88, 142 (2008); arxiv: 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: /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

38 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.)?

39 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 / and 3 / 1 1. In fact it requires small enough attraction (or even repulsion, u > 0) on the large hole - like cylinder.

40 AFM Superconductors

41 Ba 123 FeSe Ba +2 (Fe +2 ) 2 (Se -2 ) 3 ArXiv:

42 Speculations on 1111 FeSe structure? N (P,As,Sb,Bi, ) LaNFeSe La (Nd,Pr,Sm,...) Se (RE) +3 N -3 Fe +2 Se -2

43 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 )