电子的谷自由度

电子在晶格周期性势场影响下的运动遵循布洛赫定理. 布洛赫电子除了具有电荷和自旋两个内禀自由度外, 还有其他内禀自由度. 能带色散曲线上的某些极值点作为谷自由度, 具有独特的电子结构和运动规律. 本文从布洛赫电子的谷自由度出发, 简单介绍传统半导体的谷电子性质研究现状, 并重点介绍新型二维材料体系, 如石墨烯、硅烯、硫族化合物等材料中谷相关的物理特性. 有效利用谷自由度的新奇输运特性, 将其作为信息的载体可以制作出新颖的纳米光电子器件, 并有望造就下一代纳电子器件的新领域, 即谷电子学(valleytronics).

The valley degree of freedom of an electron

Under the periodic potential of solid, the movement of an electron obeys the Bloch theorem. In addition to the charge and real spin degree of freedom, Bloch electrons in solids are endowed with valley degree of freedom representing the local energy extrema of the Bloch energy bands. Here we will review the intriguing electronic properties of valley degree of freedom of solid materials ranging from conventional bulk semiconductors to two-dimensional atomic crystals such as graphene, silicene, and transition metal dichalcogenides. The attention is paid to how to break the valley degeneracy via different ways including strain, electric field, optic field, etc. Conventional semiconductors usually have multiple valley degeneracy, which have to be lifted by quantum confinement or magnetic field. This can alleviate the valley degeneracy problem, but lead to simultaneously more complex many-body problems due to the remnant valley interaction in the bulk semiconductor. Two-dimensional materials provide a viable way to cope with the valley degeneracy problem. The inequivalent valley points in it are in analogy with real spin as long as the inversion symmetry is broken. In the presence of electric field, the nonvanishing Berry curvature drives the anomalous transverse velocity, leading to valley Hall effect. The valley degree of freedom can be coupled with other degree of freedom, such as real spin, layer, etc, resulting in rich physics uncovered to date. The effective utilization of valley degree of freedom as information carrier can make novel optoelectronic devices, and cultivate next generation electronics–valleytronics.