Theoretical Study On Electronic And Topological Properties In Ferromagnets

The combination of magnetism and topological properties can produce a series of novel topological quantum phenomena,which has attracted widespread attention.The introduction of magnetic order in two-dimensional topological insulators can realize the quantum anomalous Hall effect characterized by quantized Hall conductance and chiral edge state under zero magnetic field,which is of great significance for the research of low-energy electronic devices and the development of spintronics.Although the quantum anomalous Hall effect has been experimentally verified,it still faces problems such as the need of extremely low temperature and external magnetic field.In addition,the magnetic Weyl semimetals can achieve a minimum mumber of Weyl points,and the giant anomalous Hall effect presented by them provides a new idea for the design and application of a new generation of Hall devices.Although there have been many explorations of Weyl semimetals,the improvement and development of this field still requires more discovery and research of Weyl semimetal materials with excellent properties.In the paper,based on the first-principles calculation method of density functional theory,we systematically study the switchable quantum anomalous Hall effect in a ferromagnetic topological crystalline insulating NpSb monolayer,as well as the electronic and topological properties of the ferromagnetic Weyl semimetal in BaCrSe2.The paper is divided into five chapters.The first chapter briefly provides an overview of the origin of topological quantum materials and the topological quantum material system.Chapter 2 introduces the theoretical basis,methods,and corresponding software packages of first-principles calculations covered in this paper.Chapter 3 introduces in detail the strain-regulated topological phase transition in NpSb monolayer and the realization of the quantum anomalous Hall effect and explores the possibility of realizing the quantum anomalous Hall effect in NpSb/NaI heterojunction.In Chapter 4,we predict a new class of magnetic Weyl semimetals in BaCrSe2,with detailed descriptions of its Weyl points,Fermi arcs,etc.In Chapter 5,we summarize the main research content and innovation points of the paper and make a prospect on the research field of magnetic topological quantum materials.The main research contents and conclusions of the paper are as follows:(1)NpSb monolayer is a class of intrinsic two-dimensional ferromagnetic topological crystalline insulator.We apply strain to it and under compression or tensile strain within 5%,its magnetic ground state remains unchanged and has high stability.When compressive strain is applied,the topological crystalline insulator phase of the NpSb monolayer remains unchanged,while under tensile stress,the band closes and opens again with the increase of strain,indicating the occurrence of topological phase transition.Taking the 5%tensile strain as an example for calculation and analysis,the band inversion at the high symmetry point,non-zero Chern number and chiral edge state indicate that the system has undergone a topological phase transition from a topological crystalline insulator to a quantum anomalous Hall insulator.We construct a NpSb/NaI heterojunction,rely on the slightly larger lattice constant of NaI to provide certain tensile strain for NpSb monolayer,and realize the quantum anomalous Hall effect,which provides some reference significance for its further experimental implementation.(2)We theoretically predict a class of intrinsic ferromagnetic Weyl semimetal in BaCrSe2,and it has a relatively simple band structure near the Fermi level,and only four pairs of Weyl points in the entire Brillouin zone,which is conducive to the observation of transport phenomena related to the Weyl points.Unlike the previously reported Weyl semimetals,the distance between the Weyl points of BaCrSe2 is very far,about half the length of the inverted lattice vector,so the Weyl points in BaCrSe2 are robust and not easy to be annihilated due to points displacement caused by perturbation.The study of the Fermi arc surface state further proves the highly stable ferromagnetic Weyl semimetal phase of BaCrSe2,which is of great significance for the understanding of basic physical properties of Weyl semimetals and its potential applications.