Theoretical Study Of Superconductivity Of Two-dimensional Materials With Light Element

Since the discovery of superconductivity by Onnes in 1911,exploring the superconductivity of materials has become one of the hot research directions in physics.Among them,two-dimensional（2D）superconducting materials have the characteristics of no dissipation,small size and easy regulation,which have important application value in superconducting micro and nano devices.Therefore,the exploration of two-dimensional high-temperature superconductors has become a research hotspot.Based on the Bardeen-Cooper-Schrieffer（BCS）theory of conventional superconductors,superconductors with higher density of states at the Fermi level,stronger electron-phonon coupling（EPC）,and higher vibration frequencies generally have higher superconducting transition temperatures.It is known that the lighter the atomic mass,the higher the phonon frequency.Thus,it is possible to search high-temperature superconductors in systems containing lightweight elements.Based on the above analysis,we studied the superconductivity of the following three graphene-like systems composed of lightweight elements.Ⅰ)The superconductivity of hydrogenated PC₃（HPC₃）was studied based on the first-principles density functional theoretical calculation.The results show that PC₃transforms from semiconductor to metal after hydrogenation,and has strong electron-phonon coupling（EPC）.Based on the EPC mechanism,it is found that the EPC constant is 0.95,which is mainly due to the coupling between the c-πelectron and the in-plane vibration modes of C and H.The superconducting transition temperature T_ccalculated by Mc Millian-Allen-Dynes formula is 31.0 K,which is higher than most two-dimensional superconductors.By applying a further 3%biaxial tensile strain,the T_ccan be raised to 57.3K,exceeding the Mc Millan limit.Ⅱ)The metal-absorpted superconductivity of monolayer hexagonal phosphide（h-BP）compound MBP（M=Li,Na,Ti）were studied.The calculation results show that monolayer h-BP transforms from semiconductor to metal after metal atoms Li,Na and Ti adsorption.Based on the traditional EPC mechanism,it is found that the coupling between electrons and out-of-plane vibration of B and P atoms has a great influence on the EPC intensity.For MBP（M=Li,Na,Ti）,the calculated total EPC constants are 1.76,0.64 and 1.54,respectively,and the superconducting transition temperature T_care 26.38 K,6.05 K and 14.58 K,respectively.Ⅲ)The superconductivity of monolayer hexagonal boron carbon（BC）compound MBC（M=Na,K）adsorbed by metal was studied.The results show that MBC（M=Na,K）is metallic and superconducting.Based on the EPC mechanism,the EPC constants of MBC（M=Na,K）are calculated to be 0.97 and 1.48,respectively.In both Na BC and KBC systems,the EPC results from the coupling between electrons and the in-plane vibrations of C and B.The superconducting transition temperature T_cof MBC（M=Na,K）are 34.1 K and 41.7K,respectively.Under biaxial tensile strain,the T_cof Na BC can reach 45.6 K.Through above three works,it can be seen that the 2D lightweight element system can induce superconductivity through different electronic regulation methods and has a high superconducting transition temperature T_c.These works can provide theoretical basis for the future research on the superconductivity of 2D lightweight element systems.