介觀量子通道與開放量子點之電性傳輸

Abstract

在介觀系統裡元件的尺寸接近電子的平均自由路徑與同調性長度，因而量子現象得以彰顯。在這個尺度下，不同的物理與電性傳輸能夠經由不同的元件設計來展現與探討。本論文描述了包含多種不同元件的四組電性量測。結果顯現出元件的物理特性，如結構、長度與電子密度，對於電性傳輸的影響。第一組實驗揭示了電子-電子交互作用對於準一維量子線中著名的零偏壓異常的重要性。量測著重在不同通道長度與電子密度的量子線、電導值小於2e2/h的源汲極偏壓能譜圖(source drain bias spectroscopy)與隨溫度變化的線性電導。電子-電子交互作用對傳輸的影響預期因為量子線通道長度變長或密度降低而增大，而實驗結果顯示ZBA的高度與寬度也隨之減小。此外從不同溫度的數據所得到的特徵值與零偏壓異常的寛度呈現出比例關係，這個結果指出電子背向散射是形成零偏壓異常極其重要的原因。在第二組實驗裡，我們探討了在一維通道中雜質對於電子傳輸的影響。側向移動量子通道的技巧與重覆升降溫被運用來改變無序量子通道中雜質的分佈。由於雜質的存在造成量子線的線性電導出現扭曲的階梯與共振振盪，也造成源汲極偏壓能譜圖出現複雜的結構。能譜圖展現出分裂的雙峰結構，但可隨著雜質分佈的改變而逐漸演變單峰結構；這樣的結構很容易與零偏壓異常混淆。但是數據顯示，其能譜圖的變化是來自於雜質所導致的束縛態而非乾淨的準一維通道的固有傳輸特徵。在第三組實驗裡，兩個獨立但同步化的時變電場被用來擾動一開放性量子點。在無外加直流偏壓下，該電場在兩種電路組態-電荷抽運與整流-中產生直流電流。其電流與電場之間的相位差呈現正弦的關係。但不同電路組態下產生的電流對於電場的頻率、環境的耦合與磁場的響應亦有所差異。數據顯示該兩組電流並非由電路效應，如信號耦合或是串擾所造成；而不同的物理機制能由不同的電路組態來分辨。最後一個實驗探討電子連續經過兩個串聯量子接點的傳輸。我們針對兩點電導與直接穿透率(direct transmission)隨電子密度與量子接點間的距離的變化來討論。直接穿透率反應出通過兩量子接點的電流中，電子仍具有同調性的比例。隨著電子密度的降低或是距離的增加，彈道式傳輸逐漸變成歐姆性傳輸，而同時直接穿透率則降至趨近於零。實驗結果印證理論預期—兩個量子系統間的同調性與否決定電子是否能夠以彈道式傳輸通過兩個系統。

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.