半導體中場操控之自旋流與自旋堆積

Abstract

本論文旨在探討與了解在半導體中運用數種純電性方式來產生和操控自旋流與自旋堆積之現象。我們研究了在彈道傳輸與擴散範疇中，Rashba和Dresselhaus本質自旋軌道交互作用對電子自旋的影響。在彈道傳輸的Rashba量子窄通道中，我們論證了自旋流可以藉由時變指狀閘極所產生之時變調制的Rashba自旋係數來產生。我們也建構了反彈散射效應對此自旋流堅實性質的影響。在擴散半導體的條狀系統中，我們論證了在自旋霍爾電場中，Dresselhaus自旋軌道交互作用的確會引起條狀系統橫向邊緣的自旋堆積。更進一步，我們探討在外加一個平面弱磁場下，由Dresselhaus自旋軌道交互作用產生之自旋特性。當外加磁場沿著電場方向時，自旋堆積Sz是此磁場的偶函數﹔當外加磁場沿著條狀系統的橫方向時，自旋堆積Sz對磁場的關係變成不對稱。我們發現此一自旋軌道的磁效應可以當作鑑別不同自旋軌道機制的一個工具。類似在一個侷域的散射子附近所形成的Landauer電偶極矩，我們也發現在沒有塊材自旋流存在的Rashba自旋軌道交互作用的系統中，一個正常的散射子附近仍會有自旋偶極的存在，這個結果吻合一般理論結果﹕在擴散範疇中Rashba自旋軌道交互作用的系統中沒有自旋堆積Sz在樣品橫向邊緣。

The thesis of this work is to explore and to understand various ways of generating and manipulating spin flow and spin accumulation in a semiconductor by pure electrical means. Taking the intrinsic spin-orbit interaction (SOI) such as the Dresselhaus SOI and the Rashba SOI as our major wrenches on the electron spin, we focus upon two different regimes: the ballistic and the diffusive regimes. In a ballistic Rashba-type quantum channel, we demonstrate that time-modulation of the Rashba SOI coupling parameter, via an ac-biased finger gate, leads to a dc spin current. That this dc spin current is robust against elastic backscattering is also established. In a diffusive semiconductor stripe, we show that the Dresselhaus SOI does lead to spin accumulation at the lateral edges in a spin-Hall electric field. Further signature of the Dresselhaus SOI is explored by introducing a weak in-plane magnetic field. The spin accumulation Sz is an even function of magnetic field when it is longitudinal, pointing along the driving electric field direction. For a transverse magnetic field, when it is pointing parallel to the width of the stripe, the spin accumulation becomes asymmetric. Our finding thus provide a basis for the establishment of a SOI mechanism diagnostic tool out of the magnetic field effect on the spin accumulation. Furthermore, we find that, in analogy with the Landauer charge dipole around a local scatterer, a spin dipole can exist around a normal scatterer in a Rashba-type 2DEG host where the bulk spin-Hall current is known to be zero. The prediction of these spin dipoles is consistent with another known fact: that no macroscopic spin accumulation occurs at the lateral sample edges in the case of Rashba SOI.