Study On The Properties Of Electron Phonon Coupling In New Materials

In condensed matter physics,the electrons and phonons are the basic research objects,which can indicate the electronic structure,spectrum of crystal structure and so on.Many material properties are related to the interaction of electrons and phonons.The scatter-ing of electrons by phonons will affect all aspects of the physical properties of materials,such as electronic transport properties and superconductivity.In normal state,the stronger electron-phonon coupling results in the lower phonon-limit carrier mobility and the worse transport properties.However,in the superconducting state,the stronger electron-phonon coupling will lead to the higher superconducting transition temperature.In this thesis,us-ing the first-principles calculations,we study electronic transport properties in the monolayer molybdenum disulfide based on the Boltzmann transport theory,and study superconductivity of monolayer black phosphorus and highly compressed hydrogen sulfide based on the BCS theory.In the thesis,we first introduce the basic knowledge of the electron-phonon coupling and preparations and developments of new materials in recent years,including the two-dimensional materials and the hydrogen-rich materials.Then we briefly introduce the frame-work,development and application of density functional theory,including the basic principle,main calculational process as well as the approximative method.For the density functional perturbation theory,we give the basic principle and calculational process,and describe the calculation of lattice dynamics and electron-phonon coupling matrix element.The main re-search works are summarized as follows.The doping and strain effects on the electronic transport of monolayer MoS₂are sys-tematically investigated using the first-principles calculations within the Boltzmann transport theory.The estimated mobility has a maximum 275 cm²/?V·s?in the low doping level under the strain-free condition.After applying a small strain?³%?,the mobility can improve to a maximum of 1150 cm²/?V·s?and the strain effect is more significant in the high doping level.We demonstrate that the resistance is mainly due to the electron transition between K and Q valleys scattered by the M momentum phonons.However,the strain can effectively suppress this type of electron-phonon coupling by changing the energy difference between the K and Q valleys.This sensitivity of mobility to the external strain may direct the improving electron transport of monolayer MoS₂.The effects of biaxial and uniaxial strains on electron-phonon coupling and supercon-ductivity in monolayer phosphorene are systematically investigated by first-principles calcu-lations.It is found that the electron-phonon coupling primarily comes from the low frequency optical phonon modes around B_3g¹.The biaxial strain gives rise to more obvious increase of density of states around Fermi level and phonon softening in the low frequency regime than the other two types of uniaxial strains do.Therefore,the electron-phonon coupling is more significantly enhanced by the biaxial strain than the uniaxial strains and the superconducting transition temperature T_cincreases sharply from 3 K to 16 K at the typical doping concentra-tion n_2D=3.0×10¹⁴cm^-2when the biaxial strain reaches to 4.0%.We use first-principles calculations to systematically examine the effects of partially sub-stituting the chalcogen atoms on the superconductivity of hydrogen chalcogenides under high pressures.We find detailed trends of how the critical temperature changes with increasing the V-,VI-or VII-substitution rate,which highlight the key roles played by low atomic mass and by strong covalent metallicity.In particular,a possible record high critical temperature of280 K is predicted in a stable H₃S_0.925P_0.075 with the Im?3m structure under 250 GPa.