The Fundamental Properties Of Polaron In Two-Dimensional Materials

Carrier movement in ionic crystals or polar semiconductors causes polarization of the surrounding crystal lattice.This polarization field usually can be described by phonons.Carrier and its surrounding phonons form an interacting quasi-particle,which is called a polaron.Polaron effect is very important for studying the basic properties of materials such as optical absorption,electrical conductivity,superconductivity,magnetic resonance thermal effect and spin.Due to quantum confinement effect,the polaron effect becomes more obvious in two-dimensional materials such as graphene and monolayer transition metal dichalcogenides.In this thesis,we focus on the basic properties of polaron states in graphene and monolayer transition metal dichalcogenides on polar substrates,respectively.Based on the Huybrechts model,we focus on the polaron states formed by the coupling between an electron and surface optical phonons induced by polar substrates.In the presence of vertical magnetic field,we propose the special magnetopolaron states(?)(|1>e±|1>ph).namely,the superposition states between one SO phonon and the first landau energy level,whose infrared absorption are studied in detail.We find that the infrared absorption gives rise to two peaks with different intensities originating from the spiling of the resonance state.Moreover,the intensities can be modulated by the magnetic field,the truncated wave-vector of SO phonon,polarity of substrate and internal distance between graphene and substrate.Hence,we provide a vital theoretical approach for the experiment measurement of electron-phonon coupling constant and the validation of polaron state.Using the Devreese-Huybrecht-Lemmens(DHL)model,we study the infrared absorption of the polaron spin state in a monolayer transition metal chalcogenides,in which the polaron spin state is formed by the coupling of the electron with the longitudinal optical(LO)phonon and the induced SO phonon in the presence of the spin-orbit coupling.We find that the magnitudes of optical absorption are different for polaron with spin-up and spin-down states in the same valley.Moreover,the absorption intensities of polaron spin states are adjusted by the strength of Rashba spin-orbit coupling.These results not only provide a theoretical basis for optical resolution and spin state regulation but also a theoretical reference for polaron spin state as a valley information carrier.Based on the linear combination operator method,we study the effective velocities of polaron spin states in monolayer TMDS structure laying on the polar substrate.It is found that the effective velocity of polaron shows the splitting with different magnitudes due to the Rashba spin-orbit coupling.Meanwhile,the effective velocities of polaron spin states appear the reversed distribution in the same valley.Moreover,the reversed points depend on the truncated wave-vector of optical phonon,the polarity of substrate and the internal distance between monolayer TMDS and substrate.The different effective velocities provide a concise theoretical solution to distinguish and modulate the polaron spin states in two-dimensional heterostructures.