Electronic Structure of FeSe Monolayers: why is so high? M.V. - - PowerPoint PPT Presentation

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Towards Room Temperature Superconductivity: Superhydrides and More Chapman University, Orange, California, USA, May 8-9, 2017 Electronic Structure of FeSe Monolayers: why is so high? M.V. Sadovskii 1,2 1 Institute for Electrophysics, RAS,

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Electronic Structure of FeSe Monolayers: why Тс is so high?

M.V. Sadovskii

1,2 1Institute for Electrophysics, RAS,

Ekaterinburg 2Institute for Metal Physics, RAS, Ekaterinburg

Towards Room Temperature Superconductivity: Superhydrides and More

Chapman University, Orange, California, USA, May 8-9, 2017

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Outline of the talk:

FeSe monolayers - experiment FeSe monolayers – electronic structure

(a) Role of correlations (b) The origin of shallow and “replica” bands

Tc enhancement puzzles:

(a) Doping in multiple band system (b) “Excitonic” mechanism (c) Forward scattering

Higher Tc?

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FeAs tetrahedra form two-dimensional layers surrounded by LaO, Ba or Li. Fe ions inside tetrahedra form a square lattice.

LaOFeAs BaFe2As2 LiFeAs

Essentially physics of FeAs layers!

Li+1Fe+2As-3

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LDA band structure of tetragonal LaOFeAs

Essentially multiband

problem

As- 4p O-2p Fe-3d

I.A. Nekrasov et al., JETP Lett. 87, 560 (2008)

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FeAs systems:

Pnictogen height?

LiFeAs BaFe2As2 LaOFeAs LiFeAs

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LDA+DMFT: strong or intermediate correlations?

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Superconducting gap – ARPES data

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.

arXiv: 0807.0419

Superconducting gap – ARPES data

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ArXiv: 0807.2369, 0807.4315, 0807.4775 ArXiv:0807.4312

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+1Fe+2 2Se-2 2

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ArXiv: 1102.1057

No nesting!

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KxFe2-ySe2: LDA+DMFT Spectrum

I.A.Nekrasov, N.S.Pavlov M.V.Sadovskii, ArXiv:1211.3499

U=3.75 eV J=0.56eV =40

Correlations important! LDA bands narrowing ~5 Strong damping near FS

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LDA+DMFT LDA’+DMFT LDA+DMFT LDA’+DMFT

ARPES dispersions: Z.X.Shen et al. ArXiv:1208.5192

CT-QMC

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Detailed LDA + DMFT in KFeSe system CT-QMC

M. Sunagawa et al.

Journal of the Physical Society of Japan 85, 073704 (2016)

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ArXiv:1201.5694 ArXiv:1402.1357

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ArXiv:1406.3435

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ArXiv:1202.5849

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ArXiv:1505.06361

LiOHFeSe FeSe1UC

I.A. Nekrasov (unpublished)

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ArXiv:1207.6823

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ArXiv:1402.1357

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LDA in single layer FeSe

EF I.A. Nekrasov (unpublished)

undoped e-doped

Self – doping at the interface? EF shift ~ +0.25 eV (or 0.2 electron per Fe)

I.A. Nekrasov, N.S. Pavlov (unpublished)

EF

Wien 2k

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LDA + DMFT in single layer FeSe CT-QMC: =40, U=3.5eV, J=0.85eV

I.A.Nekrasov, N.S.Pavlov (2016)

ARPES: ArXiv:1312.2633

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ArXiv:1606.09358

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LDA + DMFT in single FeSe/STO layer

CT-QMC: =40, U=5eV, J=0.90eV

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LDA + DMFT in single FeSe/STO layer Probably phonons are irrelevant for the formation of the “replica” band?

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LDA + DMFT in single FeSe/STO layer

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Tc and Density of States Correlation

arXiv:1001.1801

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BCS – multiple band model

1/geff

Matrix of dimensionless coupling constants Secular equation, physical solution corresponds to a maximal positive value of geff, which determines the highest value of Tc

Multiple bands:

V. Barzykin, L.P. Gorkov. Pis'ma ZhETF 88, 142 (2008)

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Effective coupling constant geff 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 Tc even for relatively small values of intraband and interband pairing constants.

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Value of Tc 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.

1. No interband pairing

geff = max(gi)

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.

Effective coupling

Is there a nontrivial “optimal” band structure (number of bands etc.)?

Effective coupling – from weak to strong?

E.Z.Kuchinskii, M.V.Sadovskii JETP Lett. 89, 156 (2009)

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Gap and Tc evolution during Lifshitz transitions:

ArXiv:1508.04782

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Gap behavior for different types of pairing:

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Gap behavior relevant for FeSe on SrTiO3:

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FeSe single layer on SrTiO3

Single Fermi surface?

Smaller Tc according to naive BCS analysis?

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ArXiv:1606.01470

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Excitonic Mechanism

Increase D and geff! W.A. Little, V.L. Ginzburg (1964)

geff=VN(0)

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”Excitonic” enhancement of Tc?

Overestimate!

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”Excitonic” enhancement of Tc? 

0.1 0.2 0.3 0.5 ex 0.07 0.13 0.2 0.33

Negligible effect!

SrTiO3

g=3.25 eV in SrTiO3

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Phonons in STO

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”Excitonic” enhancement of Tc – Optical phonon in SrTiO3? Can we obtain ph ~ 0.4-0.5 for optical phonon in SrTiO3? g=0.1 eV for opt-ph in SrTiO3 “Exciton” optical phonon in SrTiO3!

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ArXiv:1508.02461

ArXiv:1312.2633

Electron – phonon interaction at FeSe/STO interface

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1. Forward scattering phonon -> “replica” band:
2. Antiadiabatic phonon!

But can we get (realistically) large enough m?

And probably these phonons are just irrelevant?

3. BCS – BEC crossover regime?

Problems to solve:

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Estimate of Eliashberg coupling constant for FeSe/STO: . .

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Higher Tc ?

1. Role of Doping?
2. Why “obvious” estimates are wrong?
3. “Excitonic” enhancement?
4. Antiadiabatic superconductivity?
5. Can we enhance Tc further?

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