二維拓樸絕緣體的電子自旋操控

Abstract

本論文的研究主題是在二維拓樸絕緣體中操控自旋電子,採用tight binding model配合Non-equilibrium Greens function以及Landauer Buttiker Formalism探討 自旋電子的傳輸行為,並且配合tight binding的能帶計算來研究拓樸絕緣體這一個 有強自旋軌道耦合的材料,其非耗散的表面態受到時間反轉對稱守恆的保護,使 此材料有很大的機會應用在自旋電子元件上面。利用電場來控制電子的傳輸在半 導體工業已經發展得相當成熟。本論文則討論兩種方法用電場來控制在二維拓樸 絕緣體中電子自旋。 第一種方法是利用局部的電場產生量子干涉並且進一步地控制穩固非耗散自旋 流的開關 。我們用電場調控一個放置在工字形二維拓樸絕緣體中間的雜質能量, 電極之間的傳輸係數在不同電極寬度下展現出Fano-like 共振或者Breit-Wigner 共 振,而此兩種共振可以藉由調控電極寬度產生相變。 第二種方法是在二維拓樸絕緣體接薄金屬,二維拓樸絕緣體的表面態會和金屬 的量子井態混成,有兩個特性是值得被討論的,第一個就是此混成造成自旋的分 裂,產生了類似Rashba自旋軌道耦合的效應,第二個則是混成造成能隙的打開, 此混成在正(負)的動量時讓自旋向下(上)電子的能帶產生能隙,因此藉由調 控能量,可以將這個系統變成自旋閥的元件。

This thesis presents a theoretical study of spin manipulation in a two-dimensional topological insulator (2DTI). Non-equilibrium Greens function approach and Landauer Buttiker Formalism cooperates with tight binding band calculation numerically study the electron spin transport properties of two-dimensional electron gas with strong spin-orbit coupling. A topological insulator is a material that a strong intrinsic spin-orbit interaction exists. The non-dispersive edge states make it be a promising material in spintronics application. Adopting non-magnetic field control is predominating in the research field of semiconductor devices, thus, we adopt non-magnetic field to control electron spin in the two-dimensional topological insulator. The thesis provides two ways to manipulate electron spin in a two-dimensional system. First, quantum interference induced by external electric field is adopted. A persistent quantum resonance device is proposed in an H-shaped 2DTI embedded a non-magnetic impurity at the center. Transmissions between each branch of the H-shaped 2DTI shows two kinds of quantum resonance in this device, Breit-Wigner resonance, and Fano-like resonance. These resonances can be realized in the device through modulating the onsite impurity potential. A phase transition between the Fano-like and the Breit-Wigner resonances through modulating the thickness of the 2DTI leads is also presented. Second, we present a band study of a 2DTI-normal metal junction. The helical edge state of 2DTI hybridized with the quantum well state of normal metal is well studied. A systematical study of the band in terms of the coupling strength between 2DTI and normal metal shows there are two interesting phenomena in this junction (i) A helical state induced splitting that similar to Rashba field existed in normal metal. The Rashba-like field generates a spin precession that the precession length can be modulated by the coupling strength. The Rashba-like field can be a promising way to create giant Rashba spin orbital via material manipulation. (ii) The band of spin down opens due to the spin down electron of normal metal penetrated to 2DTI and backward moved restrictedly. Energy band gap opens for one spin channel thus a polarized spin current flows into normal metal in the energy region of the band gap. By modulating the Fermi energy, it is possible to convert the quantum spin hall system into a spin filter.