High-Tc Superconductors Based on FeAs Compounds (eBook)

Izyumov is corresponding member of the Russian Academy of Sciences and head of the Institute for Metal Physics, Ekaterinburg. Kumarev is Scientific Director, Division of Electronic Properties, of the Institute of Metal Physics, Yekaterinburg and holds the State Premium Prize of Russian Federation in Science and Technics.

Preface 6
Contents 8
Acronyms 12
Chapter 1 
13 
Chapter 2 Compounds of the ReOFeAs Type 
16 
2.1 Crystallochemistry and Basic Physical Properties of Doped Compounds 16
2.1.1 Crystal Structure 16
2.1.2 Electron Doping 16
2.1.3 Hole Doping 19
2.1.4 Substitutions on the Fe Sublattice 20
2.1.5 Superconducting Transition Temperature 22
2.1.6 Critical Fields 24
2.1.7 Effect of Pressure on the Tc 25
2.2 Magnetic Properties 29
2.2.1 Magnetic Structure 29
2.2.2 Theoretical Explanation of Long-Range Magnetic Ordering in ReOFeAs 31
2.2.3 Phase Diagrams 35
2.2.4 Magnetic Fluctuations 38
2.3 Electronic Structure 39
2.3.1 Stoichiometric Compounds 39
2.3.2 The Role of Magnetic Ordering and Doping 45
2.3.3 Experimental Studies of the Fermi Surface 49
2.4 Symmetry of the Superconducting Order Parameter 52
2.4.1 Experimental Methods of Determining the Order Parameter 52
2.4.2 Nuclear Magnetic Resonance 53
2.4.3 Point-Contact Andreev Reflection 57
2.4.4 Tunnel and Photoemission Spectroscopies (STS, PES, ARPES) 64
Chapter 3 Compounds of the AFe2As2 (A = Ba,Sr,Ca) Type 
67 
3.1 Crystal and Electronic Structure 67
3.1.1 Crystal Structure 67
3.1.2 LDA Calculations of the Electronic Structure 68
3.1.3 Experimental Studies of the Fermi Surface 73
3.1.4 (Sr3Sc2O5)Fe2As2 and Other Similar Compounds 79
3.2 Superconductivity 82
3.2.1 Doping 82
3.2.2 Coexistence of Superconductivity and Magnetism 87
3.2.3 Effect of Pressure 89
3.2.4 Symmetry of the Superconducting Order Parameter 98
3.2.5 Measurements on the Josephson Contacts 104
3.2.6 Critical Fields 106
3.3 Magnetism 108
3.3.1 Stoichiometric Compounds 108
3.3.2 Doped Compounds 112
3.3.3 Magnetic Excitations 114
Chapter 4 Other FeAs-Based Compounds 
118 
4.1 Compounds of the FeSe, FeTe Type 118
4.1.1 Superconducting Properties 118
4.1.2 Unusual Magnetic Properties 121
4.1.3 Electronic Structure of Stoichiometric Compounds 124
4.1.4 Electronic Structure of Doped Compounds 127
4.1.5 Magnetic Structure of FeTe 129
4.2 Compounds of the LiFeAs Type 131
4.2.1 Superconductivity 131
4.2.2 Electronic Structure 132
4.3 Compounds of the AFFeAs (A=Sr,Ca) Type 135
4.3.1 Primary Experimental Observations 135
4.3.2 Electronic Structure 137
Chapter 5 Theory Models 
139 
5.1 General Properties of Compounds from Different Classes of FeAs-Systems and Corresponding Theory Objectives 139
5.1.1 Crystal and Magnetic Structures 139
5.1.2 Peculiarities of the Electronic Structure 142
5.1.3 Asymmetry of the Electron/Hole Doping 144
5.1.4 Problems of Symmetry of the Superconducting Order Parameter 146
5.1.5 Isotopic Effect 148
5.2 Role of Electron Correlations 149
5.2.1 Dynamical Mean Field Theory (DMFT) 149
5.2.2 LDA+DMFT Calculation for ReOFeAs Compounds 153
5.2.3 LDA+DMFT Calculation on an Extended Basis 156
5.2.4 Comparison with Experiment 160
5.3 A Minimal Two-Orbital Model 166
5.3.1 Formulation of the Model 166
5.3.2 Band Structure of the Spectrum 170
5.3.3 Mean Field Approximation 172
5.3.4 Numerical Calculation for Small Clusters 175
5.4 Multi-Orbital Model 178
5.4.1 Formulation of the Model 178
5.4.2 Equations for a Superconductor in the Fluctuation Exchange (FLEX) Approximation 179
5.4.3 Properties of Superconductors with the s Symmetry of the Order Parameter 182
5.4.4 Three-Orbital Model 186
5.5 Detailed Analysis of the 5-Orbital Model 190
5.5.1 The Hamiltonian of the Model 190
5.5.2 Spin and Charge Susceptibility 192
5.5.3 Pairing of Electrons via Spin Fluctuations 195
5.5.4 Possible Symmetries of the Superconducting Order Parameter 198
5.6 Limit of Weak Coulomb Interaction 202
5.6.1 Renormalization Group Analysis 202
5.6.2 Equations for Superconducting and Magnetic Order Parameters 210
5.6.3 Phase Diagram of the Model 213
5.6.4 Peculiarities of the s-Superconducting State 215
5.7 The Limit of Strong Coulomb Interaction 218
5.7.1 The t-J1-J2-Model 218
5.7.2 Superconductivity with Different Order Parameters 220
5.7.3 Density of States and Differential Tunnel Conductivity 223
5.7.4 The Hubbard Model with the Hund's Exchange 225
5.8 Magnetic Long-Range Order and Its Fluctuations 229
5.8.1 Two Approaches to the Problem 229
5.8.2 The Itinerant Model 232
5.8.3 The Localized Model: Spin Waves 236
5.8.4 The Resonance Mode 241
5.8.5 Unified Models 248
5.8.6 FeAs-Compounds as Systems with Moderate Electron Correlations 253
5.9 Orbital Ordering 255
5.9.1 The Spin-Orbital Model 255
5.9.2 Phase Diagrams with Spin and Orbital Orderings 258
5.9.3 Spectrum of Magnetic Excitations 259
Conclusion 263
References 266
Index 283