Transport properties in two-dimensional GaAs electron systems
Date Issued
2005
Date
2005
Author(s)
Huang, Tsai-Yu
DOI
en-US
Abstract
We have investigated the low-temperature magnetotransport measurements in two-dimensional GaAs electron systems. This dissertation consists of the following four topics.
1. On the low-field insulator-quantum Hall transitions
We studied the insulator-quantum Hall conductor transition which separates the low-field insulator from the quantum Hall state of the filling factor /nu=4 on a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. To enter the /nu=4 quantum Hall state directly from the low-field insulator, the system undergoes a crossover from the low-field localization to Landau quantization. The crossover, in fact, covers a wide range with respect to the magnetic field rather than only a small region near the critical point of the insulator-quantum Hall conductor transition.
2. On the various Hall insulators
We have studied the various Hall insulators (HIs) in a gated two-dimensional GaAs electron system containing self-assembled InAs quantum dots. It is shown that the quantized HI is not necessarily accompanied by the insulator-quantum Hall (I-QH) transition. From our study, the semicircle law can become invalid in the QH liquid so that the quantized Hall plateau is absent at the I-QH transition. The appearance and breakdown of the semicircle law in the insulating phase can be both observed when the QH liquid is destroyed by disorder.
3. Direct measurement of the spin gaps in a gated two-dimensional GaAs electron gas
We have investigated magneto transport in gated GaAs two-dimensional electron gases. From the evolution of spin-split Landau levels (LLs) in the Energy (E)-magnetic field (B) plane, we can perform direct measurements of the spin gap for different LLs. The measured g-factor is greatly enhanced over its bulk value in GaAs (0.44) due to electron-electron (e-e) interactions. As the LL index decreases, the g-factor increases, suggesting that the strength of e-e interactions increases as the number of occupied LL decreases. Moreover, the g-factor determined from the conventional activation energy studies is ~ 2.5 times smaller than that deduced from the direct measurements.
4. “Mobility gap” of a spin-split GaAs two- dimensional electron system
We have performed magnetotransport measurements of the electron g-factor in a two-dimensional GaAs electron gas. In order to obtain the spin gap△S, we measure the spin-split longitudinal resistivity minimum which shows an activated behavior. From the spin gaps at different odd filling factors, we can obtain the effective g-factor which is greatly enhanced over its bare value (0.44) in GaAs. This enhancement is due to many-body electron-electron interactions. Our experimental results provide compelling evidence that conventional activation energy studies yield a “mobility gap” which can be very different from the real spin gap in the energy spectrum.
Subjects
砷化鎵
量子霍爾效應
相變
霍爾絕緣體
電子自旋
GaAs
quantum Hall effect
phase transition
Hall insulator
spin
Type
thesis
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