低维材料电子性质的应变调控：自洽密度泛函紧束缚广义布洛赫方法

For many materials, their electronic properties can be improved through strain Engineering. Often, low dimensional systems such as grapheme are of superior structural flexibility and excellent electromechanical response, i.e., the coupling of electronic properties with structural deformation. This indicates that novel electronic properties can be obtained by strain manipulation on electronic states. The relevant studies had led to an emergent area, i.e., strain engineering. However, theoretical research such as first-principles calculation mainly focuses on homogeneous strain and the exploration on inhomogeneous strain is rare. Largely, this is because that inhomogeneous strain destroys the translational symmetry of crystal which the traditional approaches of electronic energy band calculation rely on. In contrast to homogeneous strain, inhomogeneous strain has distinct impact on electronic properties, e.g., the pseudomagnetic field in grapheme and the effect of strain gradient on the band edge states. Therefore, the development of atomistic-level approach to effectively treat the inhomogeneous strain pattern is of fundamental importance for the study of strain tunable electronic properties of low dimensional materials. In this project, we will develop a new energy band approach named as Generalized Bloch Theorem coupled with self-consistent charge density-functional tight-binding. This new approach can effectively deal with low dimensional materials under torsion or bending deformations. Based on it, we will carry out systematic studies of the strain tunable electronic properties of low dimensional systems including layered transition metal dichalcogenide.

应变调控是改良材料电子性质的一个重要途径。低维材料体系（比如石墨烯）常常具有超强韧性，同时它们的电子性质对结构变形（即应变）的反应敏锐。这意味着可以通过应变调控其电子态，从而获得新颖电子性质。有关的研究已经形成了低维材料研究的一个重要领域，即应变工程。然而，理论研究如第一性原理计算等在目前多关注均匀应变，而对非均匀应变的探讨极少。这主要因为非均匀破坏了主流能带计算方法所依赖的晶体平移对称性。相比于均匀应变，非均匀应变对电子性质的影响具有特殊之处，比如石墨烯等二维体系中的赝磁场效应、氧化锌纳米线中应变梯度对能带边电子的影响等。因此，发展能够有效处理非均匀应变的原子级计算方法，对低维材料物性的应变调控研究具有重要意义。本项目针对低维材料的扭曲和弯曲两种基本非均匀结构变形，结合自洽密度泛函紧束缚，建立广义布洛赫能带计算方法。在此基础上，我们将新方法应用到层状过渡金属硫化物等低维体系。

应变是调控材料电子性质的一个重要手段。在计算模拟方面，理论研究如第一性原理计算等在目前多关注均匀应变，而对非均匀应变的探讨极少。这主要因为非均匀破坏了主流能带计算方法所依赖的晶体平移对称性。然而，理论和实验工作均发现，非均匀应变对低维材料体系（比如石墨烯）的电子性质的影响，迥异于均匀应变，如石墨烯体系中的赝磁场效应。因此，开发新的计算方法，克服现有计算方法在处理非均匀应变时的困难，是一个有意义的题目，具有重要的科学价值。..在本项目中，我们针对低维材料的扭曲和弯曲两种基本非均匀结构变形，结合自洽密度泛函紧束缚，建立广义布洛赫能带计算方法。同时，我们应用该方法研究了（1）弯曲下二维材料条带的自旋电子性质；（2）半导体纳米线（氧化锌，二氧化钛等）在弯曲和扭曲下的能带结构；（3）单晶硅纳米线中扭曲对掺杂元素分布的调控；（4）极性二维材料双层在扭转下平带的形成机制；（5）发展基于广义布洛赫方法的声子计算。..新方法带来新结果。应用该方法，我们在低维材料中获得了（1）应变诱导自旋劈裂，并导致半金属特性；（2）应变诱导单晶材料形成等效第二类能带偏置；（3）应变调控纳米线中的掺杂；（4）在二维材料扭曲双层中获得布洛赫平带；（5）实现了非均因应变下材料的声子计算。这些结果一方面证明了非均匀应变对材料物性的有效调控；另一方面也证明了广义布洛赫方法的有效性。