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Title: Electron transport in low-dimensional systems with strong spin-orbit interaction
Author: Liu, Wei
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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This thesis experimentally investigates electron low-temperature transport phenomena in one dimensional (1D) quantum wire and two-dimensional (2D) system with strong spin-orbit interaction. In the first experiment, electron transport is studied in a In0.75Ga0.25As/In0.75Al0.25As quasi-1D quantum wire using the top-gated split-gate device at 11 mK. I investigated the hybrid magneto-electric subbands depopulation and the well-defined conductance plateau at partially spin-polarised 0.85(2e 2/h) feature and completely spin-polarised 0.25(2e 2/h) feature. The 0.7 structure is assumed to be spontaneously spin-polarised. The strength of exchange interactions in a 1D quantum wire is also demonstrated by the enhanced effective g-factor which was measured using DC source-drain bias and in-plane parallel magnetic field measurements. In the second experiment, I measured the temperature dependence of the Shubnikov-de Haas (SdH) oscillations in two-dimensional hole gases (2DHG) of a global gated p-type GaAs/AlGaAs heterostructures device at 11 mK. I separately extract the effective masses of the two spin-orbit split subbands from the Inverse Fast Fourier Transform (IFFT). In our system, it can be found that both effective mass for light-heavy-hole (HHl) band and heavy-heavy-hole (HHh) band have a nearly linear dependence on the magnetic field. The top-gate effect on SdH oscillations has been investigated. In the third experiment, I present experimental results of the magnetotransport properties of a two dimensional electron gas formed in an InAs/AlSb quantum well at 1.5 K. I performed the four-terminal resistance measurements to investigate the SdH oscillations and the Quantum Hall Effect (QHE). From the analysis of temperature dependence of SdH oscillations, I estimated the effective mass (0.0239m0), quantum scattering time (0.17 ps), transport time (2.88 ps) and the electron temperature (1.5 K) which are in good agreement with the values expected for InAs. In our InAs/AlSb samples, the transport time was almost 17 times larger than the corresponding quantum scattering time, meaning that long-range and large-angle scattering is dominant. In the last experiment, I studied the characterisation and magnetotransport properties of high-quality and large-scale monolayer MoS2. The structures and morphology of the MoS2 triangular sheet were further characterised by Raman spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images, X-ray photoelectron spectroscopy (XPS). The giant positive magnetoresistance (MR) was observed as high as 48% at 20 mK without any sign of saturation at measured fields up to 8 T. Mott variable range hopping (Mott-VRH) mainly governs the magnetoresistance effect of monolayer MoS2 at temperature range from 20 mK to 2 K. Finally, the plan for the future work is presented, which is mainly on the measurements about the back-gate device and the tilt magnetic field. The corresponding theory simulation by considering the many body effects will be performed to prove the experiment results.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available