High-T c superconductivity in FeSe monolayers: Why T c is so High?

Transcription

1 XV Конференция молодых ученых "Проблемы физики твердого тела и высоких давлений Вишневка, пансионат "Буревестник" сентября 2016 г. High-T c superconductivity in FeSe monolayers: Why T c is so High? M.V. Sadovskii 1,2 1 Institute for Electrophysics, RAS, Ekaterinburg 2 Institute for Metal Physics, RAS, Ekaterinburg

2 Outline of the talk Introduction: Fe pnictides and chalcogenides Experiments on FeSe layers Electronic structure of FeSe layers BCS model multiple Bands What makes Tc so high? Excitonic mechanism Forward scattering Conclusions: Why Tc is so high?

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 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)

5 FeAs systems: LiFeAs LaOFeAs BaFe 2 As 2 Pnictogen height? LiFeAs

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7 LDA+DMFT: strong or intermediate correlations?

8 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.

9 ArXiv: , , ArXiv:

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

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12 ArXiv: No nesting!

13 ArXiv:

14 K x Fe 2-y Se 2 : LDA+DMFT Spectrum I.A.Nekrasov, N.S.Pavlov M.V.Sadovskii, ArXiv: U=3.7 ev J=0.7eV =40 Correlations important! LDA bands narrowing ~5 Strong damping near FS LDA+DMFT LDA+DMFT LDA +DMFT LDA +DMFT ARPES dispersions: Z.X.Shen et al. ArXiv:

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17 ArXiv: ArXiv:

18 ArXiv:

19 I.A. Nekrasov (unpublished) LiOHFeSe FeSe1UC ArXiv:

20 LDA in single layer FeSe I.A. Nekrasov, N.S. Pavlov (unpublished) Wien 2k EF I.A. Nekrasov (unpublished) undoped EF Self doping at the interface? E F shift ~ ev (or 0.2 electron per Fe) e-doped

21 LDA + DMFT in single layer FeSe ARPES: ArXiv: CT-QMC: =40, U=3.5eV, J=0.85eV I.A.Nekrasov, N.S.Pavlov (2016)

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24 ArXiv:

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26 ArXiv:

27 ArXiv:

28 ArXiv:

29 Tc and Density of States Correlation arxiv:

30 BCS multiple band model Multiple bands: 1/g eff Matrix of dimensionless coupling constants V. Barzykin, L.P. Gorkov. Pis'ma ZhETF 88, 142 (2008) Secular equation, physical solution corresponds to a maximal positive value of geff, which determines the highest value of Tc

31 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. E.Z.Kuchinskii, M.V.Sadovskii JETP Lett. 89, 156 (2009) 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 - 1. Is there a nontrivial optimal band structure (number of bands etc.)?

32 Different types of pairing:

33 Gap behavior for different types of pairing:

34 Gap behavior relevant for FeSe on SrTiO3:

35 Gap and Tc evolution during Lifshitz transitions: ArXiv:

36 ArXiv: Magnetic adatoms Non-magnetic adatoms

37 FeSe single layer on SrTiO3 Single Fermi surface? Smaller Tc according to naive BCS analysis?

38 Excitonic Mechanism g eff =VN(0) Increase D and g eff! W.A. Little, V.L. Ginzburg (1964)

39 Excitonic enhancement of Tc? Overestimate!

40 Excitonic enhancement of Tc? g=3.25 SrTiO3 ev in SrTiO ex Negligible effect!

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42 Excitonic enhancement of Tc Optical phonon in SrTiO3? Exciton optical phonon in SrTiO3! g=0.1 ev for opt-ph in SrTiO3 Can we obtain ph ~ for optical phonon in SrTiO3?

43 ArXiv: ArXiv:

44 ArXiv:

45 = = =

46 Antiadiabatic phonon??

47 ArXiv:

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51 Gor kov Analysis ArXiv: ,

52 Gor kov Analysis

53 Gor kov Analysis

54 Gor kov Analysis

55 Gor kov Analysis Conclusions: Why Tc is so high?

56 Gor kov Analysis

57 Gor kov Analysis

58 Conclusions: Why Tc is so high? 1. Role of Doping? 2. Why obvious estimates are wrong? 3. Excitonic enhancement? 4. Antiadiabatic superconductivity? 5. Can we enhance Tc further?