| In recent years,material computational simulation has been paid much attention than ever before due to its outstanding role in the development of new materials.The first principles is particularly remarkable in the field of computational physics.In addition,topological insulators have attracted the attention of scientists since they were discovered.From 2D topological insulators to 3D ones,the progress is quicker and quicker,and the topological semi-metal with massless Dirac fermions was further found.While studying the applications of the two kinds of topological states,people are exploring whether the energy gap of a single material can be controlled.Thus,it can transform freely between the two properties and the diverse properties of a single material can be realized.Firstly,we studied the monolayer selenium materials with square structure.The results imply that the monolayer selenium has two structures,the buckled square phase and the buckled rectangular phase.Four anisotropic Dirac cones appear in buckled square structure under spin-orbit coupling.The buckled rectangular structure has a topological phase transition from a 2D topological insulator with two Dirac cones to a trivial insulator,and the phase transition point was a Dirac semi-metal with a single Dirac point.The direct energy gap can also be controlled by strain adjustment to obtain Dirac carriers of various masses.Then,the formation mechanism of Jahn-Teller effect and its influence on the structural properties are illustrated by taking single layer Cr as an example.The band structures of five kinds of structures of Cr has been analyzed,and electronic states and total charge densities are discussed.It can be summarized that Jahn-Teller effect can lead to energy level degeneracy breaking spontaneously,the stable 2D rhombic and 2D rectangular structures are distorted from 2D hexagonal structure.It is proved that Jahn-Teller effect plays a critical role in the structural stability of 2D structures. |
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