Electronic Structures,Magnetism And Topological Properties In Transition-Metal Clusters Adsorbed Topological Insulators

Quantum anomalous Hall effect(QAHE)is a very important research object in the field of condensed matter physics in recent years.Compared to quantum Hall effects,QAHE can be realized in some long-range ferromagnetic and strong spin-orbit coupling materials without external strong magnetic field.For example,it's predicted that ferromagnetic topological insulator films(TIs)can realize QAHE,which is attractive for applications in next-generation low-power consumption electronic devices.With the help of the idea in diluted magnetic semiconductors,a direct way to induce ferromagnetism in TIs is to incorporate magnetic elements into TIs.Thus,Chinese scientists firstly observed the QAHE in Cr doped(Bi,Sb)₂Te₃system in 2013.However,the observed temperature of QAHE in magnetic doped TIs is extreme low(typically about 30 m K).Further studies have found that the inhomogeneous ferromagnetism distribution due to the magnetic doping is a key factor to the low observed temperature of QAHE,which greatly limits the application of QAHE and there is an urgent need to increase it.In addition to the magnetic doping method,QAHE is also predicted by constructing ferromagnetic insulator(FMI)and TI heterostructures.Compared with the magnetic doped method,the heterostructure has a"cleaner"ferromagnetism and a higher Curie temperature.However,the interaction between ferromagnetism and surface states is usually weak,and the observed anomalous Hall resistance cannot be quantized.Therefore,it is difficult to realize high temperature QAHE in such heterostructures.In fact,the third scheme for realizing QAHE exists.The most direct ways of inducing magnetism in TIs is depositing magnetic atoms onto the surfaces of TIs.If the out-of-plane ferromagnetism and strong interaction with the surface state of TIs exist,the QAHE can also be realized.However,a lot of experiments have found that out-of-plane ferromagnetism can't be achieved by directly depositing magnetic transition-metal(TM)atoms onto the surface of TIs.In this thesis,to resolve the experimental problem of TMs onto TIs and realize the high-temperature quantum anomalous Hall effect,electronic structures,magnetic properties and topological properties of some TM clusters adsorbed TIs are investigated by using the first principle method based on density functional theory.Detailed findings are as follows:First,for the problem that a single TM atom is very easily deposited into TIs,and considering that TM clusters with larger size have a dense structure and strong s-d orbital hybridization,the stable adsorption of TM clusters on the surface of the TIs might be possible.However,through the calculations,it is found that the TM clusters cannot be still adsorbed on the surface of TIs due to the layered arrangement of TIs.For example,when V_nclusters are adsorbed on the surface of Bi₂Se₃(001),the metal V atoms and the non-metallic Se atoms of TIs have a strong interaction at the surface of TIs,so the surface structure of TIs is destroyed,where in-plane ferromagnetic structure is formed and the surface band gap can't be opened.Therefore the directly adsorption of TM clusters onto surface of TIs cannot realize the QAHE.Furthermore,it is thought that weakening the interaction between TM atoms and non-metallic atoms of the TIs could solve the problem that TM atoms are easily absorbed into the TIs.Therefore,the TM molecular magnet V_mBz_n(m?2,n?3)is considered The molecular magnet has out-of-plane ferromagnetism and has been successfully prepared by experiments.We constructed a Bi₂Se₃/V_mBz_ncomposite system,and the calculated results found that the system can achieve a stable adsorption structure and solve the problem of TM atoms adsorbing into the TIs.More importantly,the molecular magnet V_mBz_ncan retain the out-of-plane ferromagnetism.Unfortunately,because the weak interaction between TM molecular magnet V_mBz_ninteracts and the surface state of the TI,the system can only open a very small topological non-trivial energy gap.And the similar result is obtained in the Bi₂Se₃/Cr_mBz_nstructure.So high-temperature QAHE can't be realized in the Bi₂Se₃/V_mBz_ncomposite system.Recent research has confirmed that a kind of magnetic TM clusters Ir₂is ferromagnetic after being halogenated.And due to the d-orbital hybridization,the halogenated Ir₂cluster has a larger magnetic anisotropy than the Ir₂cluster,which implicates that a strong interaction between the out-of-plane ferromagnetism and the surface state of the TIs might exist.In addition,the halogenated TM clusters may weaken the interaction with the non-metallic atoms on the surface of the TI,leading to a stable TM cluster structure adsorbed by the TIs.Therefore,we constructed the structure of TIs/Ir₂X_n(X=F/Cl/Br/I)and explored its electronic structure,magnetic and topological properties.From our calculations,it is found that Ir₂X_ncan stably adsorb onto the surface of TIs.Our results also present that the strong magnetic anisotropy of TM clusters after halogenation induces the system to open a larger topological non-trivial energy gap(approximately 15me V).The further analysis shows that the bottom of the conduction band and the top of the valence band near the energy gap are mainly contributed by the Ir-5d orbital from z directions,which means that the out-of-plane ferromagnetic induces system to open the energy gap to realize QAHE.This study proves that the TIs/Ir₂X_n(X=F/Cl/Br/I)system is the good candidates to achieve higher temperature QAHE.