量子線中的量子去相干性與自旋極化電流—以並聯耦合雙量子點接觸研究

Quantum decoherence and spin-polarized currents in quantum wires — studied by parallel coupled double quantum point contacts

羅東昇

指導教授 ： 陳正中

國立清華大學/理學院/物理學系/博士(2013年)

摘要

本論文的工作是利用並聯的耦合雙量子點接觸探討量子線中的量子去相干性與自旋極化電流的傳輸行為。我們採用具有二維電子氣的GaAs/Al0.3Ga0.7As半導體異質結構以量子點接觸的方法在二維電子氣層形成量子線的結構，並在極低溫的條件下進行電性傳輸的量測。我們透過觀察量子干涉現象探討量子線系統中的相位同調性，以及利用並聯系統的電導疊加性探討由一對並聯量子線所構成的量子系統中量子線之間的交互作用。首先，我們借由Aharonov-Bohm振盪觀察並聯雙量子線中模態數與量子干涉現象的關係。實驗中我們發現振盪的強度隨系統中模態數減少伴隨著擾動呈現遞減的趨勢。振盪強度的遞減趨勢可視為電子傳送機率減小之結果，而擾動現象推測與相位同調性有關。其次，我們利用Ramsauer-type共振與溫度的關聯探討量子線系統中的量子去相干行為。我們發現Ramsauer-type共振與溫度的關聯可以熱平均效應解釋與分析，而且其隨溫度之變化趨勢與系統的模態數並無關係。最後，我們以並聯系統的電導疊加性作為一個工具，來探討通過並聯耦合雙量子線的自旋極化電流之間的交互作用。在零磁場下電導疊加性是成立的。但當我們加上一平行二維電子氣之強磁場使得所有電子皆自旋極化時，電導疊加性不再成立，並且有新的量子態形成。我們推論電導疊加性的失效以及新量子態之形成為自旋極化電流交互作用之結果。

關鍵字

量子去相干性 ； 自旋極化電流 ； 量子線 ； 並聯耦合雙量子線 ； 量子點接觸 ； 阿哈羅諾夫-玻姆振盪 ； 冉紹爾型共振 ； 電導疊加性

並列摘要

We employ coupled parallel double quantum point contacts to study the quantum decoherence and the transport of spin-polarized currents in quantum wires. The devices are fabricated on the GaAs/Al0.3Ga0.7As heterostructures with two-dimensional electron gas. In the experiment, the structure of quantum wires are formed on the layer of two-dimensional electron gas by means of quantum point contacts, and the electrical transport measurements are done at ultra-low temperatures. We study the phase coherence in a quantum-wire system by quantum interference phenomena, and besides we utilize the conductance additivity in a pair of parallel quantum wires to study the interaction between the quantum wires. Firstly, we use the Aharonove-Bohm oscillation to see how quantum interference depends on the mode number in parallel double quantum wires. We find that the magnitude of the oscillation decreases with the decrease of the mode number, accompanying fluctuation. The decrease of the oscillation magnitude can be understood as the result of the decrease of transmission probability, and the fluctuation possibly relates to the phase coherence. Then, we investigate the temperature dependence of the Ramsauer-type resonance to study the quantum decoherence in a quantum-wire system. We find that the temperature dependence of the Ramsauer-type resonance can be explained and analyzed by thermal averaging effect, and the tendency of the temperature dependence does not vary with the mode number in the system. Finally, we use conductance additivity as a tool to study the interaction between the spin-polarized currents through the coupled parallel double quantum wires. The conductance additivity is valid at zero magnetic field. However, when the electrons are spin-polarized at a high magnetic field parallel to the two-dimensional electron gas, the additivity is failed, and extra quantum states are observed. The breakdown of the additivity and the emergence of the extra states possibly result from the interaction between the spin-polarized currents.

並列關鍵字

quantum decoherence ； spin-polarized current ； quantum wire ； parallel coupled double quantum wires ； quantum point contact ； Aharonov-Bohm oscillation ； Ramsauer-type resonance ； conductance additivity

參考文獻

[53] W. G. van der Wiel, S. De Franceschi, T. Fujisawa, J. M. Elzerman, S. Tarucha, and L. P. Kouwenhoven, Science 289, 2105

[28] Hisashi Aikawa, Kensuke Kobayashi, Akira Sano, Shingo Katsumoto, and Yasuhiro Iye, Phys. Rev. Lett. 92, 176802 (2004)

[19] J. C. Chen, A. M. Chang, and M. R. Melloch, Phys. Rev. Lett. 92, 176801 (2004).

[41] Tung-Sheng Lo, Yiping Lin, and Jeng-Chung Chen, Phys. Rev. B 85, 245416 (2012)

[3] B. J. van Wees, H. van Houten, C. W. J. Beenakker, J. G. Williamson, L. P. Kouwenhoven, D. van der Marel, and C. T. Foxon, Phys. Rev. Lett. 60, 848