| Summary: | 博士 === 國立臺灣大學 === 電機工程學研究所 === 93 === In this thesis, I report a systematic studies of the high-frequency magnetotransport properties of low-dimensional electron systems (LDESs) in low-temperature. One of our most important achievement is the development of an ultra-high-sensitivity microwave vector detection system, which delivers an extremely low power level signal to the LDESs under test. The high sensitivity and resolution of our detection system, which have achieved 0.005% and 0.001degree resolutions in amplitude and phase variations at 10 GHz with an average signal power below -100 dBm, together with the coplanar-wave-guide broad band sensor, allow us to probe the very tiny change of the microwave signal due to the presence of the LDES and to explore the underlying new physics. From the measured phase variation, we can distinguish a very tiny change in the induced dipole moment of each quantum wire. Our system outperforms most of the commercial vector network analyzers (VNAs).
Both the real and the imaginary parts of high-frequency conductivity of two-dimensional electron systems in two different heterostructures especially near the center of the integer quantum Hall (IQH) plateau at the frequency range from 100 MHz up to 18 GHz are carefully investigated. Studies of the frequency dependence of real part of the complex conductivity near the center of the IQH plateau shows an
interesting power law dependence between the localization length and the magnetic field difference away from the plateau center with an exponent close to 2.3.
The other important part of this thesis is the high-frequency magnetotransport properties of carriers in a quantum-wire (QW) array at 0.3K. The QW array consists of 7200 QWs fabricated from a 2DES in a GaAs/AlGaAs heterostructure by e-beam lithography and wet etching processes. These QWs are embedded in the gap of a 50 ohm meandering coplanar waveguide (CPW). The microwave spectrum of this QW array is carefully studied. The results exhibit a series of absorption dips, whose frequencies show a bell-shaped magnetic-field dependence corresponding to each IQH plateau. This bell-shaped dependence can be attributed to edge magnetoplasma (EMP) excitations of the QW array, and has been theoretically predicted. Besides that, we also observe a strong absorption extending from the top of each bell-shaped spectrum to the side IQH plateau as the frequency increases, and map out the transition behavior from EMP absorption region to IQHE-typed or Shubnikov-de-Hass-(SdH)-typed adsorption. This observation reveals that the IQH and SdH oscillations are screened at frequencies below the EMP frequencies. At the end, I will present the resonance peaks at fields below that SdH oscillations start. These adsorption peaks may be attributed to the hybrid modes of the cyclotron resonance and the two-dimensional bulk plasma.
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