First-principles Study Of The Structural,Elastic And Electron-phonon Coupling Properties Of The ABC Materials

In recent years,many novel physical properties have been discovered in ABC materials both theoretically and experimentally.For example,the coexistence of surface superconductivity with three-dimensional topological Dirac point,the coexistence of topological semimetallic phase of Dirac point and Weyl point in momentum space.Therefore,ABC materials are expected to become an ideal platform for topological superconductivity research,and how to control the novel physical properties of ABC materials has important research value and significance.In particular,the novel physical properties of the structure are regulated by changing the warping distance of the z-axis of the B-C atomic layer in the186-phase of ABC materials.In this paper,based on first-principles calculations,we systematically studied and discussed the structure,elasticity and electron phonon coupling of CaAgBi and Na Sn Sb 186 phases in the dynamically stable range by means of buckling and pressing.The 186-phase of CaAgBi has been experimentally synthesized,but its elastic properties and the 194-phase structure have not been reported.This paper theoretically proves that the186 phase of CaAgBi is mechanically and dynamically stable under normal pressure,and has metallic and strong covalent properties.By changing the buckling distance of the Ag-Bi atomic layer along the z-axis in the 186-phase structure,the structure can maintain dynamic stability in a small range,but the elastic properties change slightly,and all elastic moduli are rotationally symmetric with respect to the z-axis.However,the 194 phase is dynamically unstable,which explains why this structure with higher symmetry has not been observed experimentally.Under the action of pressure,the buckling distance of the Ag-Bi atomic layer in the CaAgBi 186 phase gradually decreases to zero,and the structure gradually changes from ionic-covalent type to covalent type;the density of states on the Fermi surface gradually increases,and the metallicity also increases;the ability of three atoms to gain or lose electrons gradually decreases,but the elastic modulus and anisotropy gradually increase.With the disappearance of Ag-Bi atomic buckling,the elastic modulus of the structure along the c-axis collapsed,indicating that the Ag-Bi atomic layer is important for the bonding properties of the structure along the z-direction.The structure was transformed into the 194 phase at a pressure of 45 GPa,and the dynamic stability of the structure was destroyed at the same time as the phase transition occurred,but the mechanical stability was not destroyed,which can demonstrate the pressure range and elasticity of the structure under pressure for potential experiments.In addition,the topological material NaSnSb is expected to be a superconductor due to its metallicity.In this paper,it was found that Na Sn Sb is a superconducting material with a superconducting transition temperature of only 56 m K,which provides a theoretical superconductivity basis for Na Sn Sb's topological superconductivity prediction.The superconducting transition temperature of Na Sn Sb can reach 112 m K at a pressure of 1.5 GPa,and the lower acoustic frequency branch contributes the most of its electron phonon coupling coefficient.By buckling its Na-Sb atomic layer,the structure can remain dynamically stable in a small region.Tensiling the Na-Sb atomic layer is beneficial for the log-averaged?_log of the phonon frequency and the Fermi surface density of states N(E_f)in the energy band,but the electron phonon coupling coefficient?of the structure and the superconducting transition temperature Tc are not effective in leaving the ground state.After that,they all decrease rapidly,which indicates that the electron phonon coupling properties are extremely sensitive to the structure.Surprisingly,both compressing the Na-Sb atomic layer and pressure induce the Kohn anomaly of its lower acoustic frequency branch.In this paper,we systematically studied the elastic and electron phonon coupling properties of ABC materials by buckling the B-C atomic layer in the 186-phase of ABC materials and pressurization,which greatly enriches the knowledge of ABC materials,provides a direction for the study of phonon topology,and provides a theoretical basis for potential experiments.