On the Remarkable Superconductivity of FeSe and Its Close Cousins
Abstract
:1 | Introduction | 2 |
2 | Overview | 4 |
2.1 Iron Pnictides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 4 | |
2.2 How FeSe Is Different from Pnictides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 5 | |
2.3 Theoretical Approaches to Pairing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 7 | |
3 | Bulk FeSe | 9 |
3.1 Electronic Structure of FeSe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 9 | |
3.2 Magnetic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 14 | |
3.2.1 Long Range Order. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 14 | |
3.2.2 Spin Fluctuations in Normal State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 15 | |
3.3 Superconducting Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 17 | |
3.3.1 Thermodynamic Probe of Quasiparticle Excitations. . . . . . . . . . . . . . . . . . . . . . . . . . . | 17 | |
3.3.2 STM/ARPES Measurements of Gap Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 19 | |
3.3.3 Orbital Selective Pairing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 20 | |
3.3.4 BCS-BEC Crossover Scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 24 | |
3.3.5 Spin Fluctuations in Superconducting State .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 25 | |
4 | Effects of Physical and Chemical Pressure | 26 |
4.1 FeSe under Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 26 | |
4.2 FeSe under Chemical Pressure: S Substituion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 27 | |
4.3 Diminishing Correlations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 28 | |
4.4 Abrupt Change in Gap Symmetry in Tetragonal Phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 28 | |
4.5 Bogoliubov Fermi Surface Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 29 | |
5 | FeSe/STO Monolayer + Dosing of FeSe Surfaces | 30 |
5.1 Single Layer Films of FeSe on SrTiO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 30 | |
5.1.1 Electronic Structure and Electron Doping. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 30 | |
5.1.2 Structure of the Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 31 | |
5.1.3 Transition Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 31 | |
5.2 Dosing of FeSe Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 32 | |
5.3 Replica Bands and Phonons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 32 | |
5.4 Pairing State in Monolayers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 34 | |
5.4.1 e-Pocket only Pairing: d- and Bonding–Antibonding s-Wave. . . . . . . . . . . . . . . . . . . | 34 | |
5.4.2 SOC Driven Pair States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 34 | |
5.4.3 Incipient Band s± Pairing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 35 | |
5.5 Impurity Experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 36 | |
6 | FeSe Intercalates | 36 |
6.1 Alkali-Intercalated FeSe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 36 | |
6.2 Organic Intercalates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 37 | |
6.3 LiOH Intercalates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 37 | |
7 | Topological Phases of Matter in Iron-Based Superconductors | 39 |
7.1 Basic Properties of FeTeSe .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 39 | |
7.2 Theoretical Proposals for Topological Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 41 | |
7.3 Experimental Evidence for Topological Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 43 | |
7.4 Topological Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 44 | |
7.5 Experimental Evidence for Majorana Zero Modes: Defect States. . . . . . . . . . . . . . . . . . . . . . . | 46 | |
7.6 Experimental Evidence for Majorana Zero Modes: Vortex States. . . . . . . . . . . . . . . . . . . . . . . | 47 | |
7.7 One-Dimensional Dispersive Majorana Modes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 50 | |
7.8 Higher-Order Topological States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 50 | |
8 | Conclusions | 51 |
References | 52 |
1. Introduction
2. Overview
2.1. Iron Pnictides
2.2. How FeSe Is Different from Pnictides
2.3. Theoretical Approaches to Pairing
3. Bulk FeSe
3.1. Electronic Structure of FeSe
3.2. Magnetic Properties
3.2.1. Long Range Order
3.2.2. Spin Fluctuations in Normal State
3.3. Superconducting Gap
3.3.1. Thermodynamic Probe of Quasiparticle Excitations
3.3.2. STM/ARPES Measurements of Gap Structure
3.3.3. Orbital Selective Pairing
3.3.4. BCS-BEC Crossover Scenario
3.3.5. Spin Fluctuations in Superconducting State
4. Effects of Physical and Chemical Pressure
4.1. FeSe under Pressure
4.2. FeSe under Chemical Pressure: S Substituion
4.3. Diminishing Correlations
4.4. Abrupt Change in Gap Symmetry in Tetragonal Phase
4.5. Bogoliubov Fermi Surface Scenario
5. FeSe/STO Monolayer + Dosing of FeSe Surfaces
5.1. Single Layer Films of FeSe on SrTiO
5.1.1. Electronic Structure and Electron Doping
5.1.2. Structure of the Interface
5.1.3. Transition Temperature
5.2. Dosing of FeSe Surface
5.3. Replica Bands and Phonons
5.4. Pairing State in Monolayers
5.4.1. e-Pocket only Pairing: d- and Bonding–Antibonding s-Wave
5.4.2. SOC Driven Pair States
5.4.3. Incipient Band Pairing
5.5. Impurity Experiments
6. FeSe Intercalates
6.1. Alkali-Intercalated FeSe
6.2. Organic Intercalates
6.3. LiOH Intercalates
7. Topological Phases of Matter in Iron-Based Superconductors
7.1. Basic Properties of FeTeSe
7.2. Theoretical Proposals for Topological Bands
7.3. Experimental Evidence for Topological Bands
7.4. Topological Superconductivity
7.5. Experimental Evidence for Majorana Zero Modes: Defect States
7.6. Experimental Evidence for Majorana Zero Modes: Vortex States
7.7. One-Dimensional Dispersive Majorana Modes
7.8. Higher-Order Topological States
8. Conclusions
Funding
Acknowledgments
Conflicts of Interest
References
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