The First-principles Study On Topological Materials

In the modern society of economic globalization,great changes have taken place in people's living standard with the improvement of scientific and technological productivity year by year.With the rapid development of science and technology,you can't imagine what people's living standard will be in a few years.Science and technology are the most important factors to promote the development of society.Every time a new technology is introduced,people's lives may undergo a huge change,but it may take decades or even hundreds of years for the corresponding technology to move from the laboratory to people's lives.During this period when new science and technology are gradually approaching to people's life,researchers work hard in obscurity,and even some people will never realize a certain technology in their whole life.However,generation after generation of researchers have no regrets,which is the spirit of scientific research.Two-dimensional materials have been very hot in the past few decades.People have been studying two-dimensional materials for decades,but due to the limitations of the laboratory,no breakthrough has been made.Until 2010,graphene was successfully synthesized,marking the official arrival of two-dimensional materials on the stage.With the continuous research of researchers on two-dimensional materials,good progress has been made in recent years.Two-dimensional materials have gradually entered the fields of metal,semi-metal,topological materials,superconductor and so on.Because of its superior electrochemical properties,two-dimensional materials have been gaining a high position in various fields.In today's society,two-dimensional materials are becoming more and more popular and have become the biggest star materials in the field of scientific research.At the same time,topological materials have attracted the attention of researchers for their unique physical properties in recent decades.As a new quantum state,researchers have enough energy to explore the field of topological materials.With the continuous exploration in the field of topology,many new topological states have appeared,and the topology family is also growing,such as topological insulator,topological Dirac semimetal,topological Weyl semimetal,topological nodal-line semimetal and so on.Bold theoretical predictions continue to be made,and there must be more topological states in the future.Topological materials are favored by scientists because of their superior stability.The topological materials protected by symmetry have strong robustness to perturbations.This excellent stability for the future can enter the field of material parts laid a very solid foundation.At the same time,with the continuous research and bold experiments on topological materials,excellent electrochemical properties of topological materials have also been developed,and they possess excellent physical properties at the high degeneracy point without dissipation near the Fermi level,which will gradually spread to catalysis,electrode materials and other aspects.It is believed that in the future,topological materials and two-dimensional materials can go hand in hand and become the most popular star materials together,contributing to people's lives.The main contents and conclusions of this paper are as follows:In chapter 1,we mainly introduce the quantum Hall effect family and the topological family,and also review their development.In chapter 2,we mainly introduce the basic knowledge of density functional theory and Wannier function theory,as well as some other supporting theories,which provide important theoretical basis for our research.In chapter 3,we study some physical properties of two-dimensional organic magnetic topological insulators.By first principles calculation,considering the spin-orbit coupling effect,we found that two-dimensional organic compounds are magnetic topological insulators.The compound is a Dirac semimetal without considering the spin-orbit coupling effect.After considering the effect of spin-orbit coupling,the Dirac semi-metal opens a band gap near the Fermi level,resulting in the Dirac semi-metal becoming a topological insulator.Because the topological insulator contains magnetic atoms,the symmetry of time inversion in the system will be destroyed,so the quantum anomalous Hall effect will occur.The discovery of this kind of magnetic topological insulators can be applied to the design and synthesis of electronic devices.In chapter 4,topological properties of KAu S and KAu Se materials are studied using first-principles calculations based on density functional theory.The results obtained by first principles calculation are as follows: compound KAu S is a topological nodal line semi-metal without considering spin-orbit coupling;When considering the effect of optional orbital coupling,the band gap of the semi-metal KAu S Fermi level near the topological node line is opened,and KAu S is transformed into topological insulator material.Compound KAu Se is a topological node line semi-metal without considering spin-orbit coupling.The topological node line KAu S is converted into topological Dirac semi-metallic material when the coupling action of optional orbit is considered.