非正規入射型異維度石墨烯接面中的集束性與能谷物理

Abstract

本篇探討在正規與非正規的異維度石墨烯接面中，電子束的傳輸行為。而我們考慮的異維度石墨烯接面(HDGJ)是由扶手型石墨烯奈米帶(AGNR)連接到半無限的大面積石墨烯(semi-infinite graphene)，其集束傳輸以及能谷依賴特性為感興趣的研究主題。針對傳輸波形訂定了三種不同的集束種類，分別是「繞射型」、「能曲引致能谷極化電子束型」，和「類古典集束型」，並且依據入射能量跟奈米帶寬度作出了判定傳輸種類的圖表，當中是透過我們設計的三個定量性指標去評估各類型的比重。 更值得注意的是在非正規接面中，我們發現在入射特定具能隙的次能帶時，存在一段能量範圍使得其中一個能谷電子束的群速方向往回，導致透射波中只含有單一能谷的狀況，將之稱為「能谷阻蔽」(valley-blocking)。

In this work we consider beam injection into a semi-infinite graphene (SIG) sheet. Specifically, we consider both normal and non-normal beam injections in heterodimensional graphene junctions (HDGJ). The HDGJ consists of an armchair graphene nanoribbon (AGNR), which play the role of a beam injection channel, and a semi-infinite graphene sheet. Our major interests are in the collimation characteristics and their valley-dependencies. Three collimation regimes are identified. They are the diffraction regime, the warping-induced valley-polarized beams regime, and classical-like collimation regime. Within a phase space consists of the incident electron energy and the width of the AGNR, we have obtained the phase diagram for the above collimation regimes. This is facilitated by our design of indexes for a quantitative description of the above beam profiles and the identification of the regimes. Most interestingly, we find situations when there is only one valley-polarized beam in the transmission, when the injecting electron is within an energy interval just above a subband band edge in the AGNR. This is due to a valley-blocking mechanism when the group velocity of the blocked beam in the SIG sheet is in the wrong direction.