Theoretical Exploration On High-temperature Quantum Anomalous Hall Effect

Topological phases of condensed matter have attracted intensive attention for their unique physical properties such as insulating bulk and gapless surface or edge modes and potential practical applications in dissipationless electronics and spintronics.In two-dimensional systems,e.g.quantum wells,atomic crystal layers of elements from group Ⅲ to Ⅴ,and the transition metal compunds,people have already found many kinds of topologically nontrival phases such as Z2 topological insulators,quantum anoma-lous Hall effect,quantum valley Hall effect and so on.Here we mainly focus on the quantum anomalous Hall effect(QAHE)which is a quantized response of transverse charge current to a longitudinal electric field in the absence of magnetic field.It orig-inates from the joint effect of spin-orbit coupling and intrinsic magnetization.Both graphene and topological insulators are ideal platforms to explore QAHE because of their linear Dirac dispersion.However,currently QAHE can only be experimentally observed at extremely low temperatures based on these two systems,making it unreal-istic for potential device applications.In this dissertation,we propose a promising approach,i.e.compensated n-p codop-ing method,to realize the high-temperature QAHE in both graphene and topological insulators.The dissertation contains five chapters as follows:In chapter 1,we mainly introduce the recent progress in the engineering of QAHE in two-dimensional systems.In chapter 2,we present a new approach to realize the QAHE at high temperatures by n-p codoping in three-dimensional topological insulators.We take vanadium-iodine(V-I)codoped Sb2Te3 as a specific example and numerically demonstrate that the system exhibits a quantized Hall conductance at temperatures of at least～50 K even at low concentrations of～2%Ⅴ and～1%Ⅰ.This proposed approach may shed new light on experimental realization of high-temperature QAHE.In chapter 3,we systematically investigated the adsorption of 3d transition metal atoms on boron-doped graphene by first-principles calculation methods,finding that the 3d transition metal atoms on boron-codoped graphene are much stabler compar-ing with the adsorption on pristine graphene.As a concrete example,the nickel-boron codoped graphene shows a long-range ferromagnetism and can open up a global bulk gap to harbor QAHE.Moreover,for various codoping concentrations,the estimated ferromagnetic Curie transition temperature can reach over 10 K,which indicates the possibility to realize high-temperature QAHE experimentally.In chapter 4,through density functional theory calculations,we systematically study the electronic and especially spintronic properties of 5d transition metal-boroncodoped graphene based on compensated n-p codoping scheme.We find that the elec-trostatic attraction between the n-and p-type dopants effectively enhances the adsorp-tion of the metal adatoms and suppresses their undesirable clutering.Then,the cal-culated Rashba splitting energy in the Re-B and Pt-B codoped graphene systems can reach to 158 and 85 meV respectively,which are several orders of magnitude larger than the reported intrinsic spin-orbit coupling.Moreover,the Hf-B/graphene and Os-B/graphene systems can establish a long-range ferromagnetism and achieve QAHE with the non vanishing Berry curvatures.In chapter 5,we give the summary of this dissertation.