Cr二维单层薄片中Jahn-Teller效应的第一性原理研究

采用基于密度泛函理论的第一性原理计算，本文对Cr单层薄片的一系列二维结构（长方、正方、六角、斜方和中心长方晶格）进行了结构稳定性和电子性质研究.结果表明，在Cr的二维体系中，对称性较低的斜方晶格和中心长方晶格是稳定的，对称性较高的正方和六角晶格是不稳定的，而长方晶格的形成能很小.Cr二维原子薄片的两种稳定结构都是六角结构畸变的结果，六角晶格的键角减小时会形变成斜方晶格，键角增大时会形变成中心长方晶格，这是由于Jahn-Teller效应使简并能级自发破缺，继而结构产生降低对称性的形变，最终使得体系变得稳定.

First-principles study on Jahn-Teller effect in Cr monolayer film

Computational physics has been used in many scientific research fields, in which first-principles calculation based on density functional theory has made brilliant achievements. Unlike three-dimensional materials, low-dimensional materials present fantastic physical effect, due to the reduction of material dimensions. With the rapid development of two-dimensional materials, people have a more in-depth understanding of them. Requirements for high performance of two-dimensional materials are raised for potential applications, so the exploration of some effects affecting the stability of two-dimensional materials becomes more and more important. Based on the pioneers' work, Jahn-Teller effect is found to have a certain influence on the stabilities of two-dimensional structure of some elements. In the present paper, we explain the stable structure of Cr monolayer film through theoretical calculation, providing a guidance for experimental synthesis. Using first-principles calculation, we study a series of two-dimensional structures (rectangular, square, hexagonal, oblique and centered rectangular) of Cr monolayer film, focusing on the structural stability and electronic properties. Firstly, the equilibrium lattice constant and cohesive energy of each structure are calculated. Then, the bond angle and lattice constant dependence of the total energy are analyzed in detail. Finally, we investigate the energy band structures, total electronic densities of states, charge densities and electron occupation numbers of orbitals. The results show that low-symmetry oblique and centered rectangular lattice are stable in the two-dimensional system of Cr, while high-symmetry square and hexagonal lattices are not stable and the adhesive energy of the rectangular lattices is very small. Two stable structures of Cr monolayer sheet are formed due to hexagonal structure distortion. The hexagonal structure can shape into a centered rectangular structure with the increase of bond angle, while it changes into an oblique structure with the decrease of bond angle. Because of Jahn-Teller effect, the degenerate energy level spontaneously splits. Then the structure deforms into two reduced-symmetry structures, resulting in a stable system. Therefore, we can infer that the Jahn-Teller effect plays a crucial role in the structural stability of monolayer sheet.