Npj：轨道耦合—调控谷能带和激子劈裂

由于中心反演对称性破缺和极强的自旋轨道耦合，单层过渡金属硫族化合物MX2(M=Mo,W, X=S, Se,Te)具有两个简并但不等价的能谷，且能谷和自旋耦合。但是要实现其在信息存储领域的应用，就必须打开谷能级简并实现谷极化。一种直接的方法是施加外磁场，但是此方法的器件小型化难以实现，且诱导的谷能级劈裂很小，约为0.2 meV/T。另一种方法是利用磁近邻效应构建MX2/磁性材料异质结构。磁近邻效应不仅是一种有趣的物理现象，而且可以通过克服传统掺杂和功能化的一些限制进而在材料中实现一些定制化物性调控。然而，如何在二维异质结构中实现较大且可调控的谷劈裂仍是亟需解决的问题，需要对磁邻近效应的强度、影响因素及其对激子特性的影响有更深入的认识。

来自清华大学深圳国际研究生院的邹小龙教授研究团队，提出了一种可行的调控方案，通过调控层间轨道杂化来调控能带以及激子的谷劈裂。他们设计构建了由二维室温铁磁VSe2与MSe2（M=Mo,W）组成的范德瓦尔斯异质结，通过第一性原理计算方法证实可以通过改变交换相互作用强度、应变或电场来改变MSe2和 VSe2的能谷之间的能级排列，从而调控轨道间杂化。强的轨道杂化引起单层 MSe2价带中超过30 meV的能谷劈裂和超过 150 meV 的谷激子劈裂。此外，轨道杂化还可以带来谷劈裂符号的改变，为二维过渡金属硫族化合物中谷自由度的调控提供了一种切实有效的方法。

该文近期发表于npjComputational Materials8：239(2022)，英文标题与摘要如下，https://www.nature.com/articles/s41524-022-00932-2可以自由获取论文PDF。

Tunable valley band and exciton splitting by interlayer orbital hybridization

Dan Wang & Xiaolong Zou

Magnetic proximity effect has been demonstrated to be an effective routine to introduce valley splitting in two-dimensional van der Waals heterostructures. However, the control of its strength and the induced valley splitting remains challenging. In this work, taking heterobilayers combining monolayer MSe2 (M = Mo or W) with room-temperature ferromagnetic VSe2 as examples, we demonstrate that the valley splitting for both band edges and excitons can be modulated by the tuning of the interlayer orbital hybridization, achieved by inclusion of different amounts of exact Hartree exchange potential via hybrid functionals. Besides, we show such tuning of orbital hybridization could be experimentally realized by external strain and electric field. The calculations suggest that large valley band splitting about 30 meV and valley exciton splitting over 150 meV can be induced in monolayer MSe2. Our work reveals a way to control proximity effects and provides some guidance for the design of optoelectronic and valleytronic devices.