The Fundamental Properties Of Exciton In Two-dimensional Transition Metal Dichalcogenides Materials

Quantum many-body interaction in semiconductor materials,especially the cou-pling between excitons and optical phonons,relates to the fundamental properties of materials directly and play the dominate role in determining the properties of excitons and its corresponding potential applications in optoelectronic devices.The appearance of two-dimensional semiconductor materials provides a new platform for studying exci-tonic problems.In particular,the unique excitonic effect in monolayer transition metal dichalcogenides(TMDCs),not only enriches the research of excitonic physics,but also further expands the potential applications of excitons.In this paper,we mainly study the influence of exciton coupling with longitudinal optical phonons and surface optical phonons on several fundamental properties of exci-tons in monolayer TMDCs materials by using the variational method and perturbation calculation.We select the structure of Mo Se₂,Mo S₂,WS₂,and WSe₂laying on dif-ferent polar substrates as examples to explore the exciton effect,in which the terms of exciton-optical phonons coupling are simplified by employing Lee-Low-Pines unitary transformation in the effective mass approximation.First,the processes of Lee-Low-Pines unitary transformation,variational operation and perturbation calculation are presented in details based on the total Hamiltonian of exciton-optical phonon coupling,which obtained by constructing a model.Second,we mainly study the effect of exciton-optical phonons coupling on exciton Bohr radius and total mass.the dependences of exciton Bohr radius and the renormal-ization of exciton total mass on Debye cut-off wave vector of optical phonons,polar-ization parameter of materials and internal distance between materials and substrates are given by numerical calculation,respectively.We found that in monolayer TMDCs materials,excitons are prone to coupling with small-wave-vector optical phonons.And both exciton Bohr radius and exciton total mass can be corrected obviously due to the strong coupling between excitons and optical phonons.Meanwhile,the change of total mass influences the critical temperature of excitonic condensed states and the mobility of excitons.Finally,we study the self-energy effect and the correction of Coulomb potential for exciton due to exciton-optical phonon coupling.Moreover,the correction of exciton binding energy is presented.We found that with the exciton-optical phonons coupling increasing,the self-energy of exciton increases gradually and the internal Coulomb po-tential decreases gradually in monolayer TMDCs materials,which finally leads to the decrease of exciton binding energy.These results could shed a light for the deviation between theoretical calculation and experimental measurement for exciton binding en-ergy.