Theoretical Study On The Topological And Superconducting Properties Of The Low Dimensional Layered Material

Recently,layered materials,as a kind of two-dimensional or quasi-two dimensional low-dimensional materials,show lots of fantastic physical behaviors due to its thickness dependencies,special band structures and easy control of physical properties;therefore,it has become a hot research topic in condensed matter physics.Using first-principles calculations,we have systematically studied the topological and superconducting properties of the low dimensional layered material for the topological electronic?topo-electronic?phase transitions,atomic structure,electronic states and mechanical properties.Topological insulator is a completely new state of quantum matter discovered in recent years,so research in the topological properties of the material receives extensive attention.The interplay between surface effect and quantum confinement effect has a modulation on the topo-electronic properties.Using first-principles calculations,we find that the topo-electronic properties of Sb?111?nanofilms undergo a series of transitions as a function of the reducing film thickness: transforming from a topological semimetal to a topological insulator at 7.8 nm?22 bilayer?,then to a quantum spin hall?QSH?phase at 2.7 nm?8 bilayer?,and finally to a normal?topological trivial?semiconductor at 1.0nm?3 bilayer?.We also find that the film has good mechanical stabilities and weak electron phonon coupling characteristics;moreover,our theoretical findings of the Sb?111?nanofilms provide an ideal testing platform for experimental study of topo-electronic phase transitions and topological superconducting properties.A recent report of Tc around 77 K from scanning tunneling microscopy?STM?measurement on monolayer FeSe grown on the SrTiO₃ is particularly interesting and engenders an in-depth study.STM topography has provided some important information on the atomic and electronic structures of this system.The monolayer FeSe has two main effects: Fe site defects and antiferromagnetic spin arrangement,which dominate at atomic scale.Using the first principles of STM simulation,we demonstrate that due to the joint interaction between these two dominating effects,two types of electronic dimers will present their properties in agreement with STM measurement.Through analyzing the dimer formation,we can study the magnetic ground state of the monolayer FeSe.It gives a new idea of researching the atomic scale phenomenon and the magnetic ground state characteristics of iron-based superconductor films.With the progress of social science and technology,the design and exploitation for special functional materials play a significant role in the application of advanced scientific field.Therefore,using material computational science to search and design a new kind of materials,which enables them to possess good mechanical stabilities,dynamical stabilities and strong electron-phonon coupling characteristics,can provide important reference for the experimental synthesis of new materials.Using first principles calculations,we find a new kind of material,TcB₃.Mechanical property calculations indicate the new structure Tc B3-b is both mechanically and dynamically stable and is superhard with a Vickers hardness of 43.0 GPa;furthermore,our calculations on its electron-phonon coupling predict TcB₃-b is a potential superhard superconductor with a transition temperature about 16?20K.