Topological Modulation Of Topological Crystalline Insulator SnTe And Magnetoelectrical Transport Of SnTe And Magnetic Heterostructures

Topological crystalline insulator(TCI)is a new kind of topological materials,whose topological surface states are protected by its own crystal symmetry,which are different from the surface states of topological insulator(TI)protected by time inversion symmetry.Therefore,the surface states of TCI have more advantages than traditional TI in modulating topological surface states by using structural characteristics.SnTe is one of the most common TCIs and it has the potential of adjustable Dirac electron transport characteristic,which is helpful for researchers to observe and study the macroscopic performance of band topology phenomena in electrical transport through experimental methods and it is an ideal material for developing potential quantum electronic devices in the future.An important research subject is to control the Fermi level of SnTe by methods of element doping,in order to study the change of its transport properties.In addition,the interface exchange coupling between TCIs and ferromagnetic insulators can induce magnetic order in TCIs through magnetic proximity effect.This physical phenomenon will help people to understand the magnetic interaction between interfaces,and may lead to new phenomena such as topological magnetoelectrical effect.However,there have been rare reports on the magnetic proximity effect of TCIs and ferromagnetic insulators.Therefore,it is necessary to systematically study the structure and magnetoelectrical transport properties of the heterostructures composed of TCIs and ferromagnetic insulators so as to have a comprehensive understanding on heterostructures consisting of magnetic and topological materials.The aim of this dissertation is to explore the novel magnetoelectrical transport properties of heterostructures composed of TCIs and magnetic single crystal thin films.The magnetic properties of ferromagnetic insulators are used to modulate the structure or magnetoelectrical transport properties of the heterostructures in order to observe strange quantum transport phenomena related to topological surface states.The details are as follows:We have grown single crystalline Pb1-xSnx Te(111)thin films by molecular beam epitaxy(MBE)on BaF2(111)and studied the topological phase transition and different transport characterization through the element doping strategy.The transition from weak antilocalization to weak localization and tunable magnetoelectrical transport properties have been found in high quality Pb1-xSnxTe(111)single crystal films.A weak antilocalization likely related to the topological surface states appears in the transport of Pb1-xSnxTe(x>0.4)thin films,whereas a weak localization is displayed in Pb1-xSnxTe(x<0.4)thin films.This phenomenon is attributed to the open of Dirac gap because of the topological phase transition in TCI Pb1-xSnxTe.By Pb doping,the intrinsic Sn vacancies are greatly decreased and the position of Fermi level of pure TCI SnTe is modulated,which finally lead to the topological phase transition and tunable magnetoelectrical transport properties.We study the transport properties of SnTe thin films grown on ferrimagnetic insulator film Eu3Fe5O12(EuIG)(110)and Y3Fe5O12(YIG)(111)single-crystal substrates by MBE.EuIG(110)and YIG(111)epitaxial thin films were prepared by pulsed laser deposition(PLD)on Gd3Ga5O12(GGG)(110)and GGG(111)substrates,respectively.We observe linear magnetoresistance(at low magnetic field)and non-linear Hall in EuIG/SnTe film which is different with the YIG/SnTe film.Especially,the carrier type changes from hole-dominated to electron-dominated in the non-linear Hall curves of EuIG/SnTe film with increasing the temperature.This is due to the obvious local diffusion of Fe atoms in the EuIG layer to the SnTe layer,which leads to the disorder island growth in the film,and ultimately results in different transport properties of SnTe.Diffuse-free EuIG(110),Tb3Fe5O12(TbIG)(110),EuIG(111)and TbIG(111)epitaxial single crystal films were prepared on GGG(110)and Gd3Sc2Ga3O12(GSGG)(111)substrates by PLD,respectively,and SnTe(100)films were grown on these four films by MBE.The effects of different REIG(RE=Eu,Tb)films on the magnetoelectrical transport properties of TCI SnTe were investigated by comparing the magnetoelectrical transport behaviors of different heterostructure thin film systems.The SnTe grown on the four films presents positive slope linear Hall curve with hole as the main carrier and a metallic transport behavior,and the magnetoresistance curve shows parabolic shape.When SnTe(100)is grown on the diffuse-free EuIG(110)film,no double-carrier transport phenomenon is observed,and no linear magnetoresistance phenomenon is observed at low field either.These phenomena are different from the transport of SnTe(100)films grown on diffusion EuIG(110)films,indicating that the diffusion of Fe atoms in the films has a great influence on the transport properties of SnTe.We study the electrical and magnetoelectrical transport properties of FeTe2(100)epitaxial films grown on MgO(100)substrates by a molecular beam epitaxy system.Through precisely controlling film thickness at 4 nm,8 nm and 12 nm,we find that the epitaxial FeTe2 films exhibit a marcasite phase with orthorhombic structure and present typical semiconductive transport properties with hole as the majority carrier.The magnetoresistance presents the square-law with magnetic field and gradually tends to a linear magnetoresistance when FeTe2 film decreases its thickness to 4 nm at 5 K in low field range,which is mainly ascribed to the gradually disordered domain distribution.The variable range hopping conductive mechanism and the magnetic transition suppression are also revealed and verified by electrical transport measurement and first-principles calculations.This study provides a basis for the development of a new class of magnetic and TCI heterostructures with magnetic thin films as substrates.