Quantum Oscillations and Charge-Neutral Fermions in Topological Kondo Insulator YbB12 (Springer Theses)

Electronic state of every solid is basically classified into two categories according to its electrical responses: insulator or metal. A textbook of modern solid state physics explains that shape of a Fermi surface plays a key role in most physical properties in metals. One of the well-established experimental methods to detect a Fermi surface is measurement of quantum oscillations that is a periodic response of physical quantities with respect to external magnetic fields. As insulators do not host Fermi surface, it is believed that they do not exhibit any quantum oscillations. This book presents a comprehensive review of recent observations of quantum oscillations in the Kondo insulators, SmB6 and YbB12, and discusses how the observations are demonstrated by a newly proposed mechanism where emergent charge-neutral fermions exhibit quantum oscillations instead of bare electrons. It also focuses on topological properties of Kondo insulators, and demonstrates that YbB12 hosts a surface metallic conduction owing to its non-trivial band structure. Further it presents the experiments of specific heat and thermal conductivity in YbB12 down to ultra-low temperature to discuss the possible low-energy excitations from a Fermi surface of neutral fermions. The demonstrated gapless and itinerant fermionic excitations, that is the significant contribution from charge neutral fermions, violates Wiedemann-Franz law. The discoveries point out a highly unconventional phase of quantum state--electrically insulating but thermally metallic--realized in the bulk of topological Kondo insulators Supervisor’s Foreword Publication List Preface Acknowledgements Contents Acronyms 1 Introduction 1.1 Kondo Effect 1.2 Kondo Lattice 1.3 Kondo Insulator 1.4 Electronic Properties of Kondo Lattice Systems 1.4.1 Thermodynamic and Transport Properties 1.4.2 Kadowaki–Woods Ratio 1.4.3 Wilson Ratio 1.4.4 Wiedemann–Franz Law 1.5 Topological Kondo Insulator 1.6 SmB6 1.6.1 Electronic Structure 1.6.2 Topological Surface State 1.7 YbB12 1.7.1 Electronic Structure 1.7.2 Field-Induced Insulator-Metal Transition 1.7.3 Topological Surface State References 2 Quantum Oscillations in SmB6 2.1 Introduction 2.2 Quantum Oscillations 2.3 Results Reported by the Michigan Group 2.4 Results Reported by the Cambridge Group 2.5 Problems Related to the QOs in SmB6 2.6 Theories for the Unconventional Quantum Oscillations 2.6.1 QOs Without a Bulk Fermi Surface 2.6.2 Neutral Quasiparticles as a Source of QOs References 3 Unconventional Quantum Oscillations in YbB12 3.1 Introduction 3.2 Sample Characterization 3.3 QOs in the Insulating Phase of YbB12 3.4 High-Field Exotic Metal of YbB12 3.4.1 SdH Effect in the Kondo Metal State 3.4.2 Fermi Liquid Behavior and Two-Fluid Picture 3.5 Conclusion References 4 Topological Surface Conduction in YbB12 4.1 Introduction 4.2 Experimental 4.3 Metallic Surface Conduction in YbB12 4.4 Magneto Transport of the Microstructure 4.5 Conclusion References 5 Charge-Neutral Fermions in YbB12 5.1 Introduction 5.2 Experimental 5.2.1 Specific Heat 5.2.2 Thermal Transport 5.3 Results and Discussion 5.3.1 Specific Heat 5.3.2 Thermal Conductivity 5.3.3 Mean Free Path of the Neutral Fermions 5.3.4 Thermal Hall Angle 5.4 Conclusion References 6 Conclusion References