Quantum Spin Transport in a Quasi-1D System with Strong Spin-Orbital Interaction

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Quantum Spin Transport in a Quasi-1D System with Strong Spin-Orbital Interaction

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2021-11

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Semiconductor nanowires (NW) with strong spin-orbital coupling, such as InSb NWs and InAs NWs, are highly promising and prevalent material platforms for investigating Majorana zero modes (MZMs) for topological quantum computing devices, spin-orbit qubits, and spin transport in quantum channels because of various properties, including large mobility, strong Rashba spin-orbital interaction, and large g-factors. One of the most attractive topics is the MZM-based quantum computing research. Signatures of MZMs have been observed in such kind of semiconductor NWs with the following elements: 1. proximity-induced superconductivity; 2. spin-orbital interaction; 3. external magnetic field. More specially, to observe MZMs, it is essential to form a spin-helical gap in the semiconductor channel, which can be obtained by finely tuning the latter two elements. However, while the research on MZMs transport is blooming over the past decade, the approach for reliably detecting spin-helical states and exploring their spin texture is currently lacking. In addition to that, the external magnetic field limits the development of Majorana qubit architectures. First, B~ex (external magnetic field) suppresses superconductivity. Second, the requirement of B~ex⊥B~ SO (spin-orbital field) places strict constraints on any future Majorana quantum bit registers since B~ ex is applied globally. These constraints call for the integration of ferromagnetic elements (FMs) with semiconductor NWs, which could replace the global B~ex. These constraints call for the integration of ferromagnetic elements (FMs) with semiconductor NWs, which could replace the global $\Bex$. In this picture, the topological phase could be driven in one of several manners, including local magnetic proximity, non-equilibrium spin injection, or simply local magnetic fields, obtained by integrating a ferromagnetic material with the NWs. Specifically, spins can be injected into NWs electrically through FM contacts to break the spin degeneracy in the NW. Moreover, FMs also offer the possibility to explore the spin texture of the helical states, which cannot be accessed by spin-unpolarized probes, and to utilize such spin-orbital states for future quantum spin-based devices including spin filters and spin modulators, such as the Datta-Das spin transistor. To investigate the spin effects in InSb nanowires towards future topological quantum computing devices, this thesis focuses on the spin transport in InSb NWs through electrical spin-valve contacts, detection of a spin-helical gap, and tight-binding model simulations of the quantum mesoscopic system to illustrate the nature of helical gap and how to realize more efficient experimental detection. Chapter 1 introduces the physics background of this thesis. Topics include, first, nanowires growth and basic quantum transport properties of InSb nanowires; second, recent Majorana zero modes study and the overview of the detection of a spin-helical gap, which is a building block of nanowire-based Majorana physics; third, spin injection and detection in local/ non-local spin-valve geometry. Chapter 2 focuses on the experimental aspects of this thesis. It illustrates the electrical spin injection and spin transport in NWs with both the local spin-valve and the non-local spin-valve geometry. The spin transport picture in a quasi-1D ballistic regime is established by studying the effects of source-drain voltage, global back-gate voltage, electronic interference, and the channel length dependence. Moreover, spin filtering signatures through the NW channel are observed and discussed, which could be evidence of spin transport through the spin-helical gap. Chapter 3 includes the computational part of this thesis. Quantum transport through a quasi-1D structure can be simulated by setting up a tight-binding model using a finite element method. A novel package, KWANT based on Python, could help simulate quantum mesoscopic transport. In this work, by constructing a quasi-1D strip structure, first, a comparison of spin-helical states detection between using normal leads techniques and using spin leads techniques is discussed. Specifically, expected observations in experiments using spin leads are discussed, which could be guidance for future experiments. Also, detection using spin leads is shown to be more robust against interference effects and impurity effects. Chapter 4 is the conclusion part which summarizes the work in the thesis.

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University of Minnesota Ph.D. dissertation. November 2021. Major: Physics. Advisor: Vlad Pribiag. 1 computer file (PDF); xii, 129 pages.

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Yang, Zedong. (2021). Quantum Spin Transport in a Quasi-1D System with Strong Spin-Orbital Interaction. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/260154.

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