Theoretical Study Of Superconductivity Of New Two-dimensional Material

Since the discovery of superconductivity in 1911,the exploration of superconductivity mechanism and the search for high-temperature superconductors have become two main directions in the study of superconductivity.With the unremitting efforts of researchers,superconductors of various systems have been widely explored.Next,in 2004,graphene was successfully exfoliated and synthesized experimentally.As a result,a door on the research of two-dimensional（2D）materials has been opened in the field of materials research.Due to their unique physical and chemical properties and easily manipulative structures,2D materials have been argued extensive discussions by researchers.There are many manipulative methods,such as constructing defects,replacing atoms,constructing heterostructures（multilayer structures）,electron\hole doping and strain engineering,which enrich the diversity of their properties and applying.At the same time,in the process,with the rapid development of computer technology,theoretical calculation and simulation methods have been greatly improved and applied.The concept of designing materials by different ways has become more mature from theory to practice.Combining the advantages of a variety of different materials to make up for their shortcomings,and then constructing materials with extremely excellent performance in a specific direction or applying,has always been a very attractive research direction in the field of materials science.To be a conclusion,in the recent years,using theoretical calculation simulation to design and explore novel quantum phenomena in（high-temperature）superconductors of 2D materials has become an important frontier topic in condensed matter physics.At the same time,the continuous in-depth study of quantum materials can not only further deepen the cognition and understanding of the physical world,but also have irreplaceable significance for the development of quantum information technology in the future.Therefore,in the work of this paper,we have made full use of the first principle calculations,density functional theory and other technical and theoretical means to design and explore a variety of different 2D superconducting materials,characterized and studied the novel quantum phenomena in the material system around the above problems,and the obtained major innovative achievements are as follows:（1）Using first-principles calculations,we predict the two-dimensional（2D）monolayer boron-nitrogen compound B₃N,which shows a honeycomb lattice similar to graphene.Its stability is proved by phonon spectra and ab-initio molecular dynamics simulations.Since pristine B₃N is a metal by band structure calculation,we investigate the electron-phonon coupling and possible phonon-mediated superconductivity.Based on the Eliashberg equation,the calculated electron-phonon coupling strength is about0.66,and the superconducting transition temperature T_c is 14.1 K,which is comparable to that of borophene.Furthermore,when 0.04 electron/cell doping and 10%biaxial tensile strain are applied,T_c can be increased to 17.4 K.Thus,the predicted B₃N provides a platform for 2D superconductivity.（2）Theoretically and experimentally,MXenes consisting of Mo and C have aroused much interest due to superconductivity in their films and even monolayer forms.Here,based on first-principles calculations,we systematically calculate the electronic structure,phonon dispersion,and electron–phonon coupling（EPC）of monolayer Mo₂C（both T-and H-phases）,Mo₃C₂,and Mo₃C₃.The results show that H-Mo_xC_y（x=2 or3,y=1–3）always have lower total energies than their corresponding T phase and other configurations.All these two-dimensional（2D）molybdenum carbides are metals and some of them display weak phonon-mediated superconductivity at different superconducting transition temperatures（T_c）.The Mo 4d-orbitals play a critical role in their electronic properties and the Mo atomic vibrations play a dominant role in their low-frequency phonons,EPC,and superconductivity.By comparison,we find that increasing the Mo content can enhance the EPC and T_c.Besides,we further explore the impact of strain engineering on their superconducting related physical quantities.With increasing biaxial stretching,the phonon dispersions are gradually softened to form some soft modes,which can trigger some peaks of a²F（ω）in the low-frequency region and evidently increase the EPCλ.The T_c of H-Mo₂C can be increased up to 11.79 K.Upon further biaxial stretching,charge density waves may appear in T-Mo₂C,H-Mo₃C₂,and H-Mo₃C₃.（3）Since Penta-graphene was given a birth in 2015,two-dimensional（2D）Penta materials have been aroused much interest in a variety of properties.In this work,based on first-principles calculations,we systematically study the electronic structures,phonon dispersions,and electron-phonon coupling（EPC）of the four metallic Penta materials“XY₂”(NC_2,PC₂,BN₂,and BP₂).The results report that two bands crossing the Fermi level dominantly originate from p_z orbitals of“Y”atoms with three coordinations.According to the pure and sample composition near the E_F,using the maximally localized Wannier function method,we also construct the tight-binding model and Hamiltonians to further confirm the two bands with DFT results.In addition,all these four pristine materials display weak phonon-mediated superconductivity with different low superconducting transition temperatures（T_c）,except for the T_c（12 K）of BP₂.Besides,we further study the influence of biaxial tensile strain on their superconducting related physical properties.With increasing of the biaxial tensile strain,the EPC and T_c can be increased effectively.Specifically,theλand T_c of PC₂（BN₂）can be enhanced up to 1.64（2.68）and 49.77（52.29）K,respectively.The results can accord well with those of EPW method.Thus,the paper will provide a platform for searching high-T_c 2D superconductivity.