Iron-Based Superconductivity (eBook)

Preface 6
Contents 10
Part I Materials 16
1 Introduction: Discovery and Current Status 17
1.1 A Tale of the Discovery 17
1.1.1 Background Research 17
1.1.2 Electromagnetic Properties of LaTMPnO 19
1.1.3 Emergence of Tc in LaFeAsO 21
1.1.4 What Happens Around 150 K in LaFeAsO? 21
1.2 A Brief History of Fe(Ni)-Based Superconductors at Early Stage 22
1.3 Features of Fe-Based High Tc Superconductors 24
1.4 Recent Progress 26
1.4.1 Discovery of Double Dome Structure in Tc 26
1.4.2 Toward Application 29
1.5 Prospective 30
References 31
2 Synthesis, Structure, and Phase Diagram of Iron-Based Superconductors: Bulk 34
2.1 Crystal Structure 35
2.1.1 FeSe Superconductors 35
2.1.2 Anti-PbFCl-Type Structure 36
2.1.3 ThCr2Si2 Structure 37
2.1.4 ZrCuSiAs-Type Structure 40
2.1.5 Superconductors with Perovskite-Type Blocking Layers 46
2.1.6 Superconductors with Skutterudite Intermediary Layers 48
2.1.7 Relationship Between Structure and Superconductivity 50
2.1.8 Titanium Oxypnictides 51
2.1.9 Composite Superconductor of Iron-Pnictide and Titanium Oxypnictide 53
2.2 Synthesis Method 54
2.2.1 Preparation for Polycrystalline Samples 56
2.2.1.1 Solid-State Method 56
2.2.1.2 High-Pressure Method 58
2.2.1.3 Liquid Ammonia Method 58
2.2.1.4 Hydrothermal Method 59
2.2.2 Growth of Single Crystals 59
2.2.2.1 Bridgman Method 59
2.2.2.2 Flux Method 59
2.3 Phase Diagram 61
2.3.1 Overview 61
2.3.2 “1111” Materials 64
2.3.3 “122” Materials 68
2.3.4 “111” Materials 74
2.3.5 “11” Materials 76
References 78
3 Synthesis, Structure, and Phase Diagram: Film and STM 85
3.1 Introduction 85
3.2 FeSe Thin Films 86
3.2.1 FeSe Films Grown on Graphene 86
3.2.2 Defect Effects on Superconductivity of FeSe Films 90
3.2.2.1 Dumbbell-Like Defects 91
3.2.2.2 Twin Boundary Defects 92
3.2.3 Thickness-Dependent Superconductivity of FeSe Films Grown on Graphene 97
3.2.4 Direct Observation of Nodes and Twofold Symmetry in FeSe Superconductor 98
3.2.5 Interfacial Superconductivity of FeSe Films Grown on STO 104
3.2.5.1 FeSe Films Grown on STO 104
3.2.5.2 Superconductivity of 1-Unit-Cell FeSe Films on STO 104
3.3 KxFe2?ySe2?z Thin Films 108
3.3.1 KxFe2?ySe2 Films on Graphene: Growth, Phase Separation, and Magnetic Order 109
3.3.2 KxFe2?ySe2?z Films on STO: Growth and Phase Diagram 115
3.4 Brief Summary 120
References 120
Part II Characterization 125
4 Electron Spectroscopy: ARPES 126
4.1 Introduction 126
4.1.1 Angle-Resolved Photoemission Spectroscopy 126
4.1.2 kz Measurement in ARPES 127
4.1.3 Polarization Dependence and Orbital-Sensitive Probe 128
4.2 Electronic Structure of Iron-Based Superconductors 129
4.2.1 The Undoped Compounds 129
4.2.2 The Effect of Carrier Doping 131
4.2.3 The Effect of Chemical Pressure 133
4.3 Broken Symmetry Phases 135
4.3.1 Magnetic and Structural Transitions 135
4.3.2 The Coexistence of SDW and Superconductivity 137
4.3.3 Strongly Correlated Electronic Structure in Fe1+yTe 140
4.4 The Superconducting Gap and Pairing Symmetry 141
4.4.1 In-Plane Gap Distributions 142
4.4.2 Gap Distribution Along kz 143
4.4.3 Gap Nodes 143
4.5 Heavily Electron Doped Iron-Chalcogenide 146
4.5.1 Phase Separation in KxFe2-ySe2 146
4.5.2 Superconducting Gap in KxFe2-ySe2 150
4.5.3 Superconductivity in FeSe Thin Film 151
4.6 Summary 155
References 157
5 Magnetic Order and Dynamics: Neutron Scattering 161
5.1 Introduction 161
5.2 Static Antiferromagnetic Order 163
5.3 Spin Waves in Parent Compounds 168
5.4 Spin Excitations in Doped Compounds 175
5.5 Neutron Polarization Analysis of Spin Excitations 183
5.6 Summary 188
References 188
6 Optical and Transport Properties 197
6.1 Introduction 197
6.1.1 Metals 199
6.1.2 Superconductors 201
6.2 Iron-Based Superconductors 202
6.2.1 LaFeAsO1-xFx and Related Materials 203
6.2.2 BaFe2As2 and Related Materials 205
6.2.2.1 (Ba1-xKx)Fe2As2 208
6.2.2.2 Ba(Fe1-xCox)2As2 211
6.2.2.3 BaFe2(As1-xPx)2 215
6.2.3 Fe1+?Te and FeTe1-xSex 217
6.2.4 KxFe2-ySe2 220
6.3 Summary 223
Appendix 224
References 225
Part III Theory 230
7 First-Principles Studies in Fe-Based Superconductors 231
7.1 Introduction 231
7.1.1 Normal State Electronic Structure 232
7.2 Translational Symmetry: One-Fe-Atom Versus Two-Fe-Atom Perspective 235
7.2.1 Change of Representation 235
7.2.2 Important Physical Effects Revealed in One-Fe-Atom Representation 237
7.2.3 Implication to Nodal Structures of Superconductivity Order Parameter 240
7.3 Antiferromagnetic and Ferro-Orbital Correlations 241
7.3.1 Anisotropy and Ferro-Orbital Order 241
7.3.2 Consequence of Ferro-Orbital Order 243
7.4 First Principles Simulations of Disordered Dopants in Fe-Based Superconductors 244
7.4.1 Can Transition Metals Substitutions Dope Carriers in BaFe2As2? 244
7.4.2 Effective Electron Doping by Fe Vacancies in AxFe2-ySe2 248
7.4.3 Can Se Vacancies Electron Dope Monolayer FeSe? 251
7.4.4 Effects of Disordered Ru Substitution in BaFe2As2: Possible Realization of Superdiffusion in Real Materials 256
References 258
8 Itinerant Electron Scenario 262
8.1 Introduction 262
8.2 The Electronic Structure of FeSCs 269
8.3 The Low-Energy Model and the Interplay Between Magnetism and Superconductivity 271
8.3.1 Ladder Approximation 274
8.3.1.1 The SDW Vertex 275
8.3.1.2 The Superconducting Vertex 275
8.3.2 Beyond Ladder Approximation 277
8.3.2.1 How to Get an Attraction in the Pairing Channel? 277
8.4 Interplay Between SDW Magnetism and Superconductivity, Parquet RG Approach 283
8.4.1 Parquet Renormalization Group: The Basics 284
8.4.2 pRG in a 2-Pocket Model 286
8.5 Competition Between Density Wave Orders and Superconductivity 290
8.5.1 Two Pocket Model 291
8.5.1.1 Multi-Pocket Models 296
8.5.2 Summary of the pRG Approach 297
8.6 SDW Magnetism and Nematic Order 297
8.6.1 Selection of SDW Order 298
8.6.1.1 The Action in Terms of X and Y 300
8.6.2 Pre-emptive Spin-Nematic Order 301
8.6.3 Consequences of the Ising-Nematic Order 303
8.7 The Structure of the Superconducting Gap 304
8.7.1 The Structure of s-Wave and d-Wave Gaps in a Multi-Band SC: General Reasoning 304
8.7.1.1 Generic Condition for a Non-zero Tc 308
8.7.2 How to Extract Uij (k,p) from the Orbital Model? 310
8.7.3 Doping Dependence of the Couplings, Examples 312
8.7.3.1 Electron Doping 312
8.7.3.2 Hole Doping 315
8.7.4 LiFeAs 318
8.7.5 Superconductivity Which Breaks Time-Reversal Symmetry 320
8.8 Experimental Situation on Superconductivity 322
8.8.1 Moderate Doping, Gap Symmetry 322
8.8.2 Moderate Doping, s vs s++ 323
8.8.3 Moderate Doping, Nodal vs No-Nodal s Gap 324
8.8.3.1 Hole Doping 324
8.8.3.2 Electron Doping 324
8.8.3.3 Co-existence Region with SDW 325
8.8.3.4 Isovalent Doping 325
8.8.4 Strongly Doped FeSCs 326
8.8.4.1 Electron Doping 326
8.8.4.2 Hole Doping 327
8.8.4.3 FeTe1-xSex 327
8.8.5 Summary 328
References 329
9 Orbital+Spin Multimode Fluctuation Theory in Iron-based Superconductors 337
9.1 Introduction 337
9.2 Orbital Fluctuation Theory 340
9.2.1 Quadrupole Interaction in the RPA 340
9.2.2 Self-consistent VC Method 341
9.2.3 SC-VC? Method 346
9.2.4 Kugel–Khomskii Model 347
9.2.5 Superconductivity in SC-VC? Method 348
9.3 Structural Transition and Softening of C66 350
9.3.1 Two Kinds of Structural Transitions Induced by the AL-VC 350
9.3.2 Softening of C66, Enhancement of Raman Quadrupole Susceptibility ?Raman 351
9.4 Comparison with the 2D RenormalizationGroup Theory 355
9.5 Evidence of S++-Wave State in Iron-Based Superconductors 356
9.5.1 Nonmagnetic Impurity Effect 357
9.5.2 Impurity Induced Nematic State 359
9.5.3 Neutron Scattering Spectrum 362
9.5.4 Gap Functions in BaFe2(As,P)2 366
9.5.4.1 Orbital Independent Gap Function on Hole-Pockets 366
9.5.4.2 Loop-Shape Node on Electron-Pockets Due to Orbital and Spin Fluctuations 368
9.5.5 Superconducting Gap Function in LiFeAs 370
9.6 Summary 376
Appendix 377
Details of the Numerical Calculation of the AL Term 377
Second Order Terms of the VC 378
References 379
10 Coexisting Itinerant and Localized Electrons 383
10.1 Introduction 383
10.1.1 Basic Experimental Evidence 384
10.1.1.1 Itinerant Electrons 385
10.1.1.2 Local Moments 386
10.1.2 Theories for Iron-Based Superconductors 387
10.1.2.1 Itinerant Electron Theory 388
10.1.2.2 Local Moment Theory 389
10.1.2.3 Hybrid Theory 391
10.1.2.4 Orbital Selective Mott Transition 393
10.2 Two-Fluid Description for Iron-BasedSuperconductors 395
10.2.1 Two-Fluid Description Based on the Hybrid Model 395
10.2.2 Low Energy Collective Modes 397
10.2.3 Mean-Field Phase Diagram 398
10.2.4 Spin Dynamics 400
10.2.4.1 NMR Knight Shift 400
10.2.4.2 INS Spectrum 401
10.2.5 Charge Dynamics 403
10.2.5.1 Resistivity 403
10.2.5.2 STM Spectrum 404
10.3 Summary 405
References 407
11 Weak and Strong Correlations in Fe Superconductors 415
11.1 Introduction: Electronic Correlations? 415
11.2 Essentials of the Electronic Structure of Fe-Based Pnictides and Chalcogenides 420
11.3 Overall Correlation Strength: The ``Janus'' Effect of Hund's Coupling 423
11.4 Orbital-Selective Mott Physics: Experimental and Ab Initio Evidences 430
11.5 Orbital Decoupling, the Mechanism of Selective Mottness 433
11.6 Back to Realism: FeSC and Two ``Wrong'' (Yet Instructive) Calculations 437
Appendix: The Slope of the Linear Z?(n?) in the Orbital-Decoupling Regime 442
References 444
Index 448