Electrical Transport in Mesoscopic Quantum Channels and Open Dots

• Main Authors: Liu, Kai-Ming, 劉凱銘
• Other Authors: Hsu, Shih-Ying
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/46118404173592773984

博士 === 國立交通大學 === 電子物理系所 === 99 === In mesoscopic regime where the device dimension is comparable to electron mean free path and coherence length, quantum mechanical features manifest. In this regime, different transport characteristics and physics can be revealed by different device designs and arrangements. This dissertation presents four experiments for assortment of devices that evidence the influence of physical properties of devices on the electrical transport. In the first experiment, the importance of electron-electron interactions on the prominent Zero Bias Anomaly (ZBA) in quasi one dimensional quantum wires (QWs) is verified. Source-drain bias spectroscopy and temperature dependent differential conductance at zero-bias in the range of are demonstrated for a range of wire length and carrier density. The amplitude and width of the ZBA decrease with either decreasing carrier density or increasing channel length, wherein the scattering rate of electrons is expected to increase due to enhanced electron-electron interaction. In addition, the thermal energy scales to the width of ZBA that reveals the essence of electron backscattering. In the second experiment, the influences of impurity on the transport in quasi 1D channel are demonstrated. Lateral shifting technique, imposing an offset between gate voltages applied on a pair of split gates, along with thermal cycling is exploited to vary the impurity arrangement in disordered QWs. Impurity causes distortions and resonances in linear conductance and leads to complicated source-drain bias spectroscopy. The nonlinear conductance presents splitting peaks which resolve back into a single peak with varying gate voltage. This feature may be confusing with the inherent ZBA of QWs, but the result indicates that it is due to resonant states caused by impurities. In the third experiment, dc currents of an open dot generated from two time dependent electric fields in the absence of external bias are studied. Two different electrical setups, charge pump and rectification respectively, were applied to produce the currents. The frequency, environmental coupling, and magnetic field dependences of the dc currents bear little resemblance in response to the different electrical configurations. The two types of currents are indicated to be produced from different mechanisms but not from classical circuitry effects and can be distinguished. In the final experiment, two-terminal conductance and direct transmitting probability across double quantum point contacts (QPCs) are investigated regarding their dependences on carrier density and separation distance. The transport evolves from being ballistic to being classically ohmic as the carrier density is reduced or the separation is increased and the direct transmission probability decreases correspondingly. The results demonstrate that ballistic transport across two quantum systems depends on coherence in between.