邊緣態物理、抽運特性與彈道範疇自旋堆積在拓樸絕緣體、Graphene及本質自旋軌道交互作用半導體中的研究

Abstract

國立交通大學電子物理系 計畫名稱: 邊緣態物理、抽運特性與彈道範疇自旋堆積在拓樸絕緣體、graphene、及本質自旋軌道交 互作用半導體中的研究 研究者: 朱仲夏 經費來源: 行政院國家科學委員會我們這次研究一共分成三個主要目標： (一) 探究在新穎的拓樸新材料中的邊緣態物理和Dirac錐相關之物理; (二) 探究在奈米graphene緞帶中之時變的量子傳輸特性; (三) 建立本質的自旋軌道交互作用材料中之量子動能近似模型去計算由於驅動場所產生之空間自旋 堆積。 (一)探究在新穎的拓樸新材料中的邊緣態物理和Dirac錐相關之物理： 我們探究在新穎拓樸絕緣體，例如：HgTe，graphene(二維系統)以及Bi2Se3(三維系統)對外來的微擾，例如：位能障和磁性位障的邊緣態響應。細節分析包括在正常導體和新穎拓墣絕緣體之介面，新穎拓墣絕緣體之PN接面以及各種幾何形狀之樣品的穿透模式。其他物理圖像，例如：鈍化的化學懸鍵，磁性雜質以及由第二近鄰跳躍耦合造成之自旋軌道交互作用將被考慮。主要的焦點是去對我們所設計的新穎材料中之邊緣態物理獲得更深入的見解。 (二)探究在奈米graphene緞帶中之時變的量子傳輸特性： 利用graphene之完美的傳輸性質，去研究由可調交流閘極所產生的時變調制之量子傳輸。我們將考慮高頻和低頻兩種調制位能偏壓加在指狀閘極上的情形。低頻的量子傳輸響應將可以用緩變範疇來處理。高頻指狀閘極偏壓的情形可以用時變模匹配之近似法處理。Graphene奈米緞帶之Dirac錐和量子化特性結合在一起可以預期在傳輸特性上有相當可區別的圖像。我們也將考慮在Graphene奈米緞帶之各種結構之量子幫浦。 (三)建立在本質的自旋軌道交互作用之材料中之量子動能近似模型去計算由所產生之空間自旋堆積： 我們已經從以前的研究中得到許多關於本質自旋軌道交互作用下的自旋堆積Si在擴散範疇中之見解，其中自旋軌道特徵長度lSO遠大於電子平均自由路徑le。我們在這個提案的目標是針對一個不同範疇，即彈道範疇( le > lSO )。在此範疇中，自旋擴散方程並不再適用，因而我們將推導對於Wigner分布函數之量子動能方程式並且建立對於Wigner分布函數的邊界條件。可以預期在此範疇中有較大的自旋密度Si和豐富的Si空間變化情形發生。Wigner分布函數將可以從量子力學在邊界的散射觀念去推導出來。這將會提供一個途徑去了解其他形式的邊界牆之情況

Title of the proposal: A study on the edge states, pumping, and ballistic spin accumulation in topological insulator, graphene, and intrinsic spin-orbit semiconductors There are three major goals in this study: 1. To explore edge-state physics and Dirac cone physics in novel topological material: We are to explore the response of edge states in novel topological insulators (NTI) such as HgTe and graphene (2 dimension), and Bi2Se3 (3 dimension) to external perturbations such as potential barriers and magnetic barriers. Detail analysis also include transmission pattern at the normal conductor – NTI interface, in the NTI PN junction, and in various sample geometries. Other physical features such as passivation of the dangling bonds, magnetic impurities, and SOI due to next nearest neighbor hopping will be considered. The main focus is to obtain deeper insights on the edge-state physics for the designing of new novel materials. 2. To explore time-dependent quantum transport characteristics in graphene nanoribbon (GNR): Making use of the excellent transport properties of graphene, we explore the time-modulated quantum transport due to ac gate-tuning. Both low and high modulation frequencies potential bias applying to the finger-gate will be considered. The low frequency quantum transport response will be treated within the adiabatic regime. The high frequency finger-gate biasing case will be treated within a time-dependent mode-matching approach. The Dirac cone and the quantization in the GNR together are expected to bring forth distinct features in its transport characteristics. Various quantum pumping configurations in GNR will be considered. 3. To establish a quantum kinetic approach for intrinsic spin-orbit interaction (SOI) materials for the calculation of spatial spin accumulation generated by a driving field: Having gained much insight in our previous study on the spatial spin accumulation Si in the diffusive regime of an intrinsic SOI material, where the spin-orbit length lSO >> le , the mean free path, we aim for a different regime in this proposal, namely, the ballistic regime ( le > lSO ). In this regime, spin diffusion equation does not apply, and we will derive a quantum kinetic equation for the Wigner distribution function (WDF) and establish the boundary conditions (BC) for WDF. Both larger Si and richer Si spatial variations are expected in this regime. The WDF BC will be derived from the quantum mechanical scattering at the boundary. This should provide the pathway for other wall profiles.