Electronic Structure of I ron Based Superconductors: Pnictides vs. Chalcogenides

1. “Kourovka-34” Electronic Structure of Iron Based Superconductors: Pnictides vs. Chalcogenides M.V.Sadovskii1,2 In collaboration with E.Z.Kuchinskii1 and I.A.Nekrasov1 1Institute for Electrophysics, Russian Academy of Sciences, Ekaterinburg, Russia 2Institute 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+1Fe+2As-3 LaOFeAs BaFe2As2 FeAs tetrahedra form two-dimensional layers surrounded byLaO, Ba orLi. Fe ions insidetetrahedra form a squarelattice.

4. ReOFeAs: phase diagram neutrons SR H. Luetkens et al., arXiv:0806.3533 J. Zhao et al., Nature Materials 7, 953-959 (2008). neutrons SR A. J. Drew et al., arXiv:08074876 Q. Huang et al., PRB 78, 054529 (2008) LaFeAsO1-xFx SmFeAsO1-xFx

5. Magnetic properties of 122 • 142K [220K]1 – T(I4/mmm)O(Fmmm) • 142K [220K]1 – AFM order of Fewith 2a2b2c cell, stripes along b [a]1 • mFe=0.87 B at 5K for BaFe2As2 • mFe=0.94 Bat 10K for SrFe2As2 Neutron scattering single crystal Ba1-xKxFe2As2 Q. Huang et al., arXiv:0806.2776 (2008) H. Chen et al., arXiv:0807.3950 (2008) 1for SrFe2As2, J. Zhao et al., PRB 78, 140504 (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: LaOFeAs Rare-Earth Puzzle BaFe2As2 LiFeAs LiFeAs Pnictogen height?

9. LDA+DMFT: strong or intermediate correlations?

10. arXiv: 0807.3370 Phonons arXiv: 0807.3172

12. arXiv: 0806.4806 Three hole cylinders! Band narrowing due to correlations?

13. arXiv: 0807.0419

14. Superconducting gap – ARPES data Superconducting gap – ARPES data arXiv: 0807.0419 Schematic picture of superconducting gaps in Ba0.6K0.4Fe2As2. Lower picture represents Fermi surfaces (ARPES intensity), upper insert – temperature dependence of gaps at different sheets of the Fermi surface.

15. arXiv: 0809.4455

16. arXiv: 0807.2369, 0807.4315, 0807.4775

17. arXiv:0807.4312

18. A(A=K,Cs,…)Fe2Se2: a New Class? ArXiv: 1012.3637 122 structure Kx+1Fe+22Se-22? Vacancies?

20. ArXiv: 1101.4923 Nothing to nest!

21. ArXiv: 1102.1057 No nesting!

22. A(A=K,Cs,…)Fe2Se2: a New AFM Superconductor ArXiv: 1102.0830 K2Fe4Se5 ? K+12Fe+24Se-25!

23. arXiv:1106.0881

25. Calculated spectrum and FS in the presence of ordered vacancies and AFM

26. Dependence of bands on anion height AsDzaapproaches1.37AoFermi velocity drops due the growth of Fe-As hybridization => Density of states grows.

27. Tc and Density of States Correlation arXiv:1001.1801 arXiv: 1004.0801

28. Simple model of multiple – band superconductivity V. Barzykin, L.P. Gorkov. Pis'ma ZhETF 88, 142 (2008); arXiv: 0806.1993 i,i- a superconducting gap and DOS on the i-th sheet of the Fermi surface Schematic electronic spectrum and Fermi surfaces of FeAs superconductor in the extended band picture. Vi,j - intraband and interband pairing coupling constants matrix. =VeX,eX= VeY,eY - pairing interactions on the same electronic pockets at point X or Y,  = VeX,eY - connects electrons of different electronic pockets, u = Vh1,h1, u’ = Vh2,h2, w = Vh1,h2 - BCS interactions within two hole-like pockets, t = Vh,eX= Vh,eY - couple electrons at points X and . 1/geff ! H.Suhl, B.Matthias, L.Walker Phys.Rev.Lett. 3, 552 (1959) V.Moskalenko FMM 4, 503 (1959) arXiv: 0901.0164 Secular equation, physical solution corresponds to a maximal positive value of geff, which determines the highest value of Tc Matrix of dimensionless coupling constants

29. Effective coupling – from weak to strong? Effective coupling ! Effective couplingconstant geff is significantly larger than the pairing constant g on the small hole - like cylinder. It can be saidthat coupling constants from different cylinders effectively produce “additive” effect. In fact this can lead tohigh enough values of Tceven for relatively small valuesof intraband and interband pairing constants. • geff , Tc(dx2-y2 pairing)< geff , Tc(spairing) Value of Tcin multiple bands systemsis determined by the relations between partial densitiesof states(and pairing constants) on different sheets of the Fermi surface, not only bythe total density of states at the Fermi level. 1. No interband pairing geff = max(gi) 2. All pairing interactions (both intraband and interband) are just the same - u, andall partial densities of states on all four Fermi surfacepockets are also the same - 1. Is there a nontrivial “optimal” band structure (number of bands etc.)?

30. Ba 123 FeSe Ba+2(Fe+2)2(Se-2)3 ArXiv: 1111.7046

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

32. Conclusions • Total DOS at the Fermi level directly correlates with Tc, but details depend on partial DOS’es • AFe2Se2 electronic spectrum is significantly different from that of FeAs systems and pure FeSe • No Nesting inAFe2Se2 ! • AFe2Se2 – metallic despite AFM and vacancy order • AFe2Se2– New AFM Superconductor (TN>>TC)