This book explores why cobalt oxides have drawn interest as functional materials due to their peculiar physical properties partially originating from a rich variety of the valence and spin state of cobalt ions. The book starts with the basics of condensed matter physics and advances toward the strong electron correlation system stage. It also provides up-to-date information on topics, such as thermoelectric power, superconductivity, solid oxide fuel cells, and nanostructure effect.

Preface vii

Acknowledgments ix

1 Introduction 1

1.1 Prologue 1

1.2 Charge, Spin, and Orbitals 1

1.3 Physical Properties under External Perturbations 12

2 Spin-State Crossover 27

2.1 Prologue 27

2.2 Valence and Spin State of Co Ions 28

2.3 Spin-State Crossover 31

2.4 Metal-Insulator Transition 39

2.5 Thermal Rectifier 40

3 Li Ion Battery 45

3.1 Prologue 45

3.2 History of Batteries 46

3.3 LiCoO2 Batteries 48

4 Huge Thermoelectric Power 57

4.1 Prologue 57

4.2 Thermoelectric Materials 58

4.3 NaxCoO2 61

4.4 Other Co Oxides 63

4.5 Origin of a Huge Thermoelectric Power 64

4.6 Application 71

5 Room-Temperature Ferromagnetism 75

5.1 Prologue 75

5.2 Ferromagnetism in Transition Metal Oxides 78

5.3 Room-Temperature Ferromagnetism of (Sr,Y)CoO3−δ 79

6 Partially Disordered Antiferromagnetic Transition 85

6.1 Prologue 85

6.2 Magnetism of Ca3Co2O6 87

6.3 Magnetism of Quasi-1D An+2Con+1O3n+3 92

7 Superconductivity 109

7.1 Prologue 109

7.2 Bose–Einstein Condensation 110

7.3 High-Tc Cuprate Superconductors 112

7.4 Superconductivity of NaxCoO2·yH2O 115

8 Transport Properties Combined with Charge, Spin, and Orbital: Magnetoresistance and Spin Blockade 121

8.1 Prologue 121

8.2 The Magnetoresistance Effect 122

8.3 Magnetoresistance of RBaCo2O5.5 127

8.4 Extremely Large Magnetoresistance of PdCoO2 130

8.5 Spin Blockade 131

9 Intrinsic Inhomogeneity 137

9.1 Prologue 137

9.2 Spin Cluster 140

9.3 Polaron Cluster 142

9.4 Phase Separation 144

9.4.1 Metallic and Semiconducting Phases 144

9.4.2 Spin-Density Wave and Ferromagnetic Phases 148

10 Move/Diffuse and Charge/Discharge Effect 155

10.1 Prologue 155

10.2 Cathode Material of Solid Oxide Fuel Cell: Sr0.7Y0.3CoO2.63 157

10.3 Oxygen Storage Material: YBaCo4O7+δ 162

Index 165

Tsuyoshi Takami is an assistant professor at Osaka University and editor of the journal Nanothermoelectrics. He previously worked at the University of Texas at Austin and Nagoya University and received the 24th Ando Incentive Prize for the Study of Electronics. He earned his Ph.D. from Nagoya University.

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