參渣二維系統中的自旋與電轉換之研究

Extrinsic Spin-charge Conversion Mechanisms in Disordered Two Dimensional Metals

黃軍笠(Huang, Chunli)

指導教授 ： 米格爾 ； 張奕棟

國立清華大學/理學院/物理學系/博士(2017年)

摘要

資訊通信作為二十一世引領時代進步的科技正面臨著一個非常關鍵的挑戰：降低電子器件中的能源消耗。作為富有潛力的競爭者，石墨烯的自旋傳輸提供一個有效的解決方案，因為它們允許製備低能耗電子器件。在這份論文裡， 我會用不同的理論架構，有系統性的討論自旋在參渣石墨烯的傳輸，並且著重與自旋與電的轉換。首先，我會利用半經典的玻爾茲曼方程去討論參渣石墨烯中的自旋霍爾效應以及共振散射。然後，我會進階的利用量子玻爾茲曼方程去討論自旋在參渣石墨烯中的運動。在這過程中，我們發現了一個新的碰撞機制可以定性的改變自旋與電的轉換。我們把這碰撞機制稱為：各向異性自旋進動 (Anisotropic Spin Precession Scattering)。接著，我們利用非平衡葛林函數也發現了各向異性自旋進動可以在半導體量子井的二維電子氣裡實現。這說明了各向異性自旋進動是一個非常常見的自旋與電轉換機制。最後，我們提出一個非局域電阻實驗去測量各向異性自旋進動。我們發現它會有異常的定量行為。第一，在沒有外加磁場下，非局域電阻可以是負號的。第二，在外加磁場下，非局域電阻會隨著磁場的改變是非對稱的。我們的工作提供了理論基礎去實現自旋傳輸在資訊通信上的應用。

關鍵字

自旋傳輸

並列摘要

Spintronics is one of the most promising applications of the two-dimensional material graphene. Although pristine graphene has negligible spin-orbit coupling (SOC), both theory and experiment suggest that SOC in graphene can be enhanced by extrinsic means, such as functionalization by adatom impurities. Using the semiclassical Boltzmann transport equation, I will first discuss the spin-charge coupled dynamics and the ubiquitous spin Hall effect in adatoms functionalized graphene. Next, I will go beyond the semiclassical theory and discuss a quantum Boltzmann theory that accounts for the spin-coherent dynamics of the carriers. This theory predicts a novel “anisotropic spin precession” (ASP) scattering process in graphene, which contributes to a large current-induced spin polarization and modifies the standard spin Hall effect. More importantly, the ASP scattering couples the electric current directly to the spin density in the spin-continuity equation without any constitutive relationships. Hence, it is a form of direct mangetoelectric coupling.
Next, I will show that ASP scattering can also arise in two-dimensional electron gases lacking inversion symmetry. Therefore, ASP scattering mechanism and the associated direct magnetoelectric coupling is a universal spin transport phenomena that is independent of the microscopic details of the disorder potential. Direct mangetoelectric coupling gives rise to two anomalous features in the nonlocal transport behavior of two-dimensional metallic materials. Firstly, the nonlocal resistance can have negative values and oscillate with distance, even in the absence of a magnetic field. Secondly, the oscillations of the nonlocal resistance under an applied in-plane magnetic field (Hanle effect) can be asymmetric under field reversal. Our study provides theoretical foundations for designing future graphene-based integrated spintronic devices.

並列關鍵字

Spintronics ； Magnetotransport ； Boltzmann-Equation

參考文獻

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