Growth,Electronic Structure And Magnetic Properaties Of Novel 2D Layered Materials

It is found that with the decrease of dimension,the fluctuation effect and correlation effect will be enhanced.Many novel quantum states exist in two-dimensional materials,such as fractional quantum hall effect,charge density wave,high temperature superconductivity,quantum phase transition and quantum critical phenomenon.For the dual purpose of application and fundamental research,in recent years,two-dimensional layered materials have received extensive attention.This paper describes in detail the research work done by the author during the doctoral period in the field of novel two-dimensional layered materials.The work is focused on two physical systems:two-dimensional topological insulators and iron based superconductors.The two-dimensional topological insulator is a new kind of quantum materials discovered in recent years,also known as the quantum spin Hall insulator.Its basic feature is a topologically non-trivial bulk bandgap and helical edge states across the bulk bandgap.Graphene is the first predicted two-dimensional topological insulator.However,due to the small spin-orbit-coupling effect of carbon atoms,the opening gap is too small to be detected experimentally.Then,based on the study of graphene model,theoretical physicists gradually predicted monolayer graphene-like materials(bismuthene,antimonene,germanene,stanene and so on)composed by the elements(Bi,Sb,Ge,Sn and so on)with strong spin-orbit-coupling effect.Their larger topologically non-trivial bulk bandgaps and corresponding conductive edge states have potential application in future.In the first work,we grew the two-dimensional topological insulator,Bi(111)bilayer,on layered insulators BiTeCl.It is found that Bi atoms tend to form islands on the Te terminal surface of BiTeCl,and form 1 bilayer Bi on the Cl terminal surface.The subsequent ARPES measurements,compared to the calculated band structure in the literature,show that a serious charge transfer happens between the Bi film and the surface Cl atoms,which results in the bandgap of Bi film near the Fermi level shifts about 0.9eV to the conductive band of the substrate.At the same time,the valence band top of Bi(111)films strongly hybridizes with the conductive band of the substrate,which causes the Rashba-type spin splitting changing to a special kind of spin splitting with the same spins in the same side.The band structure of Bi(111)film far away from the?point still maintains its own shape.In the second work,we grew predicted large-gap two dimensional topological insulator,stanene,on Bi₂Te₃ substrates,which was thought to has the ability to be chemically modified.In the experiment,we use STM to establish the atomic stacking structure of Sn film grown on the Bi₂Te₃ substrate,which is exactly the same as the structure of desired stanene.Subsequently,the ARPES experiment of variable photon energy and polarization shows that the band structure of stanene is almost the same as the calculated band structure.The only difference is that the gap in the K point at the Brillouin boundary is much larger.The analysis found that this is because the p_z orbits of tin atoms,which constitutes the energy band in the K point,is chemically active and easy to be saturated by absorbates.It confirms the theoretical prediction that chemical groups can be used to saturate the p_z orbits of tin atoms for modifying the band structure of stanene.The iron based superconductor is a new kind of unconventional superconductors,found in the 21st Century.It has a layered structure.The superconductivity happens in the Fe-Pn(Pn=As,Se)layers.FeSe has the simplest crystal structure and is suitable to be grown as thin films by molecular beam epitaxy(MBE).In the third work,we used the spin polarized scanning tunneling microscopy(SPSTM)to explore the magnetic properties of the non-superconducting FeSe multilayer films grown on SrTiO₃(001).Since SPSTM can not directly detect the magnetic Fe atoms below the surface layer formed by Se atoms,we tried to speculate the magnetic properties of the films from the defects.In the experiment,we found that the intrinsic Se vacancy produced by annealing has a paramagnetic magnetization with quasi four-fold symmetry centred in the Se vacancy.Then,the first principle calculation was used to compare the change near the Se vacancy in the theoretically predicted antiferromagnetic ground state and the ground state without any long-term magnetic order.We found that no matter in which antiferromagnetic ground state,due to the pinning effect of the antiferromagnetic background,it is impossible to produce the magnetization distribution with quasi four-fold symmetry which observed in the experiment near the Se vacancy.Only in the ground state without any long-term magnetic order,the oscillating magnetic moments with quasi four-fold symmetry could be produced around the Se vacancy.Therefore,we conclude that,for the multilayer FeSe films grown on SrTiO₃(001),though the superconducting state is suppressed,the FeSe thin films do not form possible completing antiferromagnetic ground state.The content of this thesis is organized as follows:The first chapter will describe the background and significance of the research,and roughly introduce the physical systems of the two materials discussed in main text:topological insulators and iron-based superconductors;the second chapter will introduce in detail the experimental equipment and related experimental methods used in my work;the third,fourth and fifth chapters will respectively describe the above work in detail;The full text will be summarized in the final part.