Superconductors is neither about basic aspects of superconductivity nor about its applications, but its mainstay is superconducting materials. Unusual and unconventional features of a large variety of novel superconductors are presented and their technological potential as practical superconductors assessed. The book begins with an introduction to basic aspects of superconductivity. The presentation is readily accessible to readers from a diverse range of scientific and technical disciplines, such as metallurgy, materials science, materials engineering, electronic and device engineering, and chemistry. The derivation of mathematical formulas and equations has been kept to a minimum and, wherever necessary, short appendices with essential mathematics have been added at the end of the text. The book is not meant to serve as an encyclopaedia, describing each and every superconductor that exists, but focuses on important milestones in their exciting development.

1. On nes'discovery and one hundred years of superconductors 1

1.1 On nes'discovery 2

1.2 One hundred years of superconductors 3

1.3 Progress with LTS and HTS applications 11

1.4 This book 13

1.5 Summary 13

2. The superconducting state 15

2.1 Electrical conduction in metals and the origin of resistance 15

2.2 Microscopic nature of superconducting state 18

2.3 Summary 27

Appendix 2A: BCS ground state and the energygap 27

3.The superconducting transition and its basic phenomenology 31

3.1 Fundamental characteristics of the superconducting transition 32

3.2 The critical field Hc. 33

3.3 The critical current 33

3.4 Resistive transition 33

3.5 Implications of perfect conductivity 35

3.6 Meissner-Ochs en feld effect 36

3.7 London phenomenology 37

3.8 Penetration depth 38

3.9 De pairing current density 39

3.10 Shortcomings of the London phenomenology 39

3.11 Intermediate state 40

3.12 Filamentary superconductors and Mendelssohn's sponge 40

3.13 Range of coherence and non-local theory 41

3.14 Interface boundary energy 43

3.15 Summary 43

Appendix 3A: Electrodynamics of a perfect conductor and London phenomenology 43

4. Thermodynamics and general properties 47

4.1 Thermodynamic aspects of the transition 47

4.2 Thermal properties 48

4.3 Ultrasonic behaviour 53

4.4 AC and optical properties 54

4.5 Tunnelling in the superconducting state 55

4.6 Summary 60

Appendix 4A 61

      4A.1 Condensation energy 61

      4A.2 Entropy 62

      4A.3 Heat capacity 62

5. Advent of type II superconductors 65

5.1 Ginzburg-Landau phenomenology 65

5.2 Sign of the surface energy and superconductor types 69

5.3 Mixed state and other characteristics 71

5.4 Summary 75

Appendix 5A: Ginzburg-Landauequations 76

6. Critical current and flux pinning 79

6.1 Transport current in the mixed state 79

6.2 Driving force and the critical state 81

6.3 Vortex motion 83

6.4 Stabilisation of superconductors 87

6.5 Pinning centres 89

6.6 Pinning interactions 92

6.7 AC losses 94

6.8 Summary 95

7. Superconductors in abundance 97

7.1 Low-temperature superconductors(LTS) 98

7.2 High-temperature superconductors(HTS)109

7.3 Summary 114

8. Niobium-zirconium and niobium-titanium alloys 115

8.1 The niobium-zirconium system 116

8.2 The niobium-titanium system 119

8.3 Summary 125

9. A-15 superconductors 127

9.1 Crystal structure, stoichiometry, and ordering 128

9.2 Long-range order and Tc 131

9.3 Structural instability at low temperature 132

9.4 Potential binary systems 133

9.5 Pseudo-binaries 134

9.6 A-15 phase formation 135

9.7 Upper critical field and paramagnetic limitation 136

9.8 Critical current density and the nature of pinning centres in A-15s 137

9.9 Strain sensitivity 139

9.10 Summary 140

10. Conductor development of A-15 superconductors 141

10.1 Liquid-solute diffusion 141

10.2 CVD process 142

10.3 The bronze process and formation of A-15 phase by solid state diffusion 143

10.4 Thermodynamics and kinetics of compound-layer formation in the bronze process 147

10.5 Modifications of the bronze process 154

10.6 Fabrication of Nb3Al conductor 157

10.7 Summary 161

11. Chevre l-phase superconductors 163

11.1 Crystal structure and stoichiometry 165

11.2 Occurrence of superconductivity in Chevre l phases 166

11.3 Synthesis of bulk samples 170

11.4 Upper critical field 171

11.5 Critical current density: inherent problems and progress in raising Jc 171

11.6 Conductor development of Chevre l-phase compounds 175

11.7 Nature of superconductivity of Chevre l-phase compounds 176

11.8 Summary 178

12. Rare-earth-based ternary superconductors and quaternary borocarbides 181

12.1 LTS systems with magnetic order 182

12.2 The interplay 183

12.3 Various ternary materials and their interplay behaviour 183

12.4 Quaternary borocarbides 189

12.5 Crystal structure and related aspects 190

12.6 Coexistence and interplay of Tc and Tm 191

12.7 Summary 198

13. Heavy fermion superconductors 201

13.1 Discovery of HF superconductors 201

13.2 Quantum phase transition and quantum critical point 202

13.3 General features of anomalous normal state and unusual superconductivity 204

13.4 Short description of various HF superconductors 207

13.5 Special features of HF superconductors 220

13.6 Summary 226

14. Organic superconductors 227

14.1 Evolution of organic superconducting salts 227

14.2 The(TM)2 family of quasi-one-dimensional superconductors 231

14.3 The(ET)2 family of quasi-two-dimensional superconductors 236

14.4 Superconducting fuller ides 245

14.5 Graphite intercalation compounds(GICs) 251

14.6 Summary 253

15.Superconducting magnesium diboride 255

15.1 Crystal structure and Tc 256

15.2 Conventional superconductivity of MgB2 259

15.3 Band structure and two superconducting gaps 261

15.4 Implications of two gaps 262

15.5 MgB2 for practical applications 263

15.6 Material synthesis 265

15.7 Nanoparticle doping for enhancing Jc 266

15.8 Conductor development: wires and tapes of MgB2 269

15.9 Summary 271

16. High-temperature cuprate superconductors 273

1C2\6.1 Genesis of HTS cup rates 274

16.2 General features of HTS cuprates 277

16.3 Prominent HTS cup rate systems 289

16.4 Substitution studies in HTS 293

16.5 Summary 295

17. Thin-film technology and conductor development of HTS cuprates 297

17.1 Microstructural aspects 297

17.2 Prominent techniques for depositing HTS films 301

17.3 Conductor development 310

17.4 Summary 323

18. Bulk HTS cuprates 325

18.1 General considerations 326

18.2 Melt processing of bulk YBCO samples 326

18.3 Effective pinning centres in bulk HTS 332

18.4 Ternary 123 bulk compounds 334

18.5 Trapped field 335

18.6 Mechanical strengthening 336

18.7 Summary 338

19. Ruthenates and ruthenocuprates 339

19.1 A superconductor in the ruthenate family: Sr2RuO4 339

19.2 Unconventional superconductivity 344

19.3 Summary of the current status of ruthenate superconductors 346

19.4 Superconducting rutheno cuprates 347

19.5 Superconductivity, general features 352

19.6 Magnetic states and coexistence of TM and Tc 358

19.7 Cationic substitutions in Ru-1212 and Ru-1222, effect on Te and TM 363

19.8 Summary 365

20. Iron-based superconductors 367

20.1 Different FBS families, their crystal structures, and their general features 367

20.2 Electronic structure 376

20.3 Phase diagrams 377

20.4 Unconventional superconductivity of FBS 379

20.5 Materials synthesis 383

20.6 Upper critical field, anisotropy,and potential for applications 384

20.7 Summary 389

21. Miscellaneous superconductors 391

21.1 Superconducting bismuthates 391

21.2 Cobaltoxi dehydrate 398

21.3 Intermetallic perovskites free from oxygen: MgCNi3 and related superconducting compounds 406

21.4 Metallonitride halides 410

21.5 Pyrochlore oxides 414

21.6 Layered transition metal chalcogenides 420

21.7 BiS2-based superconductors 426

References 431

Index 471

A. V. Narlikar has been an active researcher in the field of superconductors for over 50 years. He received PhD and ScD degrees in this field from the University of Cambridge, UK. For 30 years Professor Narlikar headed the Superconductivity Division at the National Physical Laboratory, New Delhi. He is presently a Senior Scientist and a Fellow of the Indian National Science Academy, working at UGC-DAE Consortium for Scientific Research in Indore. He is also a Visiting Scientist in the Applied Superconductivity and Cryoscience Group at the Department of Metallurgy and Materials Science, University of Cambridge. He has contributed over 300 papers to international refereed journals and over 50 edited/authored books on superconductors, published by international publishers.

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