The Electronic Properties Of Two-Dimensional Kagome Materials

The two-dimensional(2D)materials have become the most promising materials in the fields of technology application and fundamental sciences.The 2D materials have different lattices,such as hexagonal lattice,triangular lattice and Kagome lattice.In recent years,Kagome lattice has attracted much attention.Kagome lattice has an unusual band structure,including two Dirac bands and one flat band,called Kagome band.The former produces massless fermions with very high carrier mobility,while the latter produces heavy fermions.Since the kinetic energy of the electrons in the flat band are suppressed and thus Coulomb interaction becomes more important,which leads to strongly correlated effects,including ferromagnetism,superconductivity,and Wigner lattice.To date,the studies of Kagome band only based on models rather than real materials.In this thesis,we will propose some materials with Kagome lattice and study their physical properties.The thesis divided into five chapters.The first chapter briefly describes the current development of the2D materials.At the same time,the orbital physics and electronic properties,magnetic properties and superconductivity of Kagome crystal materials are introduced.In chapter two,we specifically introduced the first-principle theory and the tight-binding model.In chapter three,we propose a new type of band structure--double Kagome bands,which can realize coexistence of the two kinds of fermions around the fermi level.Moreover,the new band structure is found to exist in a new two-dimensional material,named as phosphorus carbide P2C3.This carbide material shows good stability and unusual electronic properties.Strong magnetism appears in the structure by hole doping of the flat band,which results in spin splitting of the Dirac bands.The edge states induced by Dirac and flat bands coexist around the Fermi level,indicating outstanding transport characteristics.In addition,a possible route to experimentally grow P2C3 on suitable substrates such as the Ag(111)surface is also discussed.In chapter four,we identify that a half-metallic flat band system can be realized when two conditions are satisfied.According to these conditions,we propose two new two-dimensional(2D)covalent organic framework(COF)consisting of five-member rings molecules.Both of the two structures show spin-polarized flat band in case of no electron/hole doping,moreover,both of them are half metals.Due to the limitation of symmetry,the five-member rings molecules in the COFs form a distorted Kagome lattice rather than a perfect lattice.A tight-binding model,describing the distorted Kagome lattice,reveals the origination of the flat band.In chapter five,we summarize the previous studies and we also show the prospect about the studies in the future.