Spin Dynamics in Two-Dimensional Quantum Materials: A Theoretical Study (Springer Theses)

✍ Scribed by Marc Vila Tusell

• Publisher: Springer
• Year: 2021
• Tongue: English
• Leaves: 169
• Category: Library

✦ Synopsis

This thesis focuses on the exploration of nontrivial spin dynamics in graphene-based devices and topological materials, using realistic theoretical models and state-of-the-art quantum transport methodologies. The main outcomes of this work are: (i) the analysis of the crossover from diffusive to ballistic spin transport regimes in ultraclean graphene nonlocal devices, and (ii) investigation of spin transport and spin dynamics phenomena (such as the (quantum) spin Hall effect) in novel topological materials, such as monolayer Weyl semimetals WeTe₂ and MoTe₂. Indeed, the ballistic spin transport results are key for further interpretation of ultraclean spintronic devices, and will enable extracting precise values of spin diffusion lengths in diffusive transport and guide experiments in the (quasi)ballistic regime. Furthermore, the thesis provides an in-depth theoretical interpretation of puzzling huge measured efficiencies of the spin Hall effect in MoTe₂, as well as a prediction of a novel canted quantum spin Hall effect in WTe₂ with spins pointing in the yz plane.

✦ Table of Contents

Supervisors’ Foreword
Abstract
Acknowledgements
Contents
Acronyms and Symbols
1 Introduction
1.1 Objectives and Outline
References
2 Fundamentals of Spin Dynamics in Two-Dimensional Materials
2.1 Physics of Spin Dynamics
2.1.1 Spin-Orbit Coupling
2.1.2 Spin Texture of Bloch Bands
2.1.3 Spin Relaxation
2.1.4 Charge-to-Spin Interconversion
2.1.5 Topology in Spintronics
2.2 State-of-the-Art in Two-Dimensional Spintronics
2.2.1 Graphene
2.2.2 Transition Metal Dichalcogenides
References
3 Quantum Transport Methodologies for Spin Transport
3.1 Electronic Structure Modeling
3.1.1 Density Functional Theory
3.1.2 Tight-Binding Models
3.2 Quantum Transport
3.2.1 Landauer-Büttiker Formalism
3.2.2 Kubo-Bastin Formula for the Spin Hall Conductivity
3.2.3 The Kwant Package
3.3 Implementation of Nonlocal Devices in Kwant
3.3.1 Building a Two-Terminal Device
3.3.2 Attachment of the Ferromagnetic Leads
3.3.3 Optimization of Parameters and Geometry
3.3.4 Tuning the Transport Regime
References
4 Nonlocal Spin Dynamics in the Crossover from Diffusive to Ballistic Transport in Graphene
4.1 Diffusive Regime Conditions
4.2 Diffusive Spin Relaxation: New Insights
4.2.1 Effect of the Injector and Detector Polarizations
4.3 Quasiballistic and Ballistic Spin Transport Regimes
4.3.1 Detailed Transition from Diffusive to Quasiballistic Regime
4.4 Conclusions
References
5 Spin Dynamics in Low-Symmetry Transition Metal Dichalcogenides
5.1 Diffusive Regime Conditions
5.2 Spin Relaxation
5.2.1 Origin of the Spin Diffusion Length Anisotropy
5.3 Spin Hall Effect
5.3.1 Origin of the Large Spin Hall Effect
5.3.2 Comparison with Kubo-Bastin Simulations
5.4 Detection of the Oblique Spin Hall Effect
5.5 Comparison Between 1T and 1Td Phases of MoTe2
5.6 Canted Quantum Spin Hall Effect
5.7 Conclusions
References
6 Conclusions and Outlook
References
Appendix A Derivation of the Spin-Diffusion Equation in NSVs Including the Drain and Reference Electrodes
Appendix B Spin Relaxation Induced by Random Magnetic Fluctuations
Appendix C Symmetry-Allowed Spin Hall Conductivity of 1T and 1Td TMDs
Appendix D Derivation of the Spin Hall Angle Formula in Two-Terminal Devices
Appendix E Experimental Setup for Measuring the Oblique Spin Hall Effect
Appendix F Nonlocal Resistance Quantization in the Quantum Spin Hall Effect
Appendix Curriculum Vitae
Appendix Bibliography