Electron-phonon Couplings In The Excited States Of Solids From First Principles

Electron-phonon interactions play a vital role in a variety of physical phenomena including superconductivity,phase transitions,charge density waves,thermal carrier cooling and photon emission.This old tree in condensed matter physics spreads new buds due to the close relationship with ultrafast dynamics and photoinduced phase transition with the development of the experimental facilities.However,the electron-phonon interactions are not directly related to observable quantities such as transient reflectivity,photoconductivity and so on,thus the explicit picture of the nonequilibrium electron-phonon interactions is still unclear.The nonadiabatic molecular dynamics method proved to be a powerful tool to describe photoexcited nonequilibrium kinetic processes.By means of combining the molecular dynamics simulation based on the time-dependent functional theory with frozen phonon approach,in this paper,we investigate the quantitative nonequilibrium electron-phonon coupling under photoexcitation in monolayer Mo S₂,the effects of the photoinduced topological defects in Ti Se₂ and the electron-phonon coupling in self-trapped excitons.We explore the variations of the electron-phonon coupling in non-equilibrium state and its influence on the evolution of electronic state and atomic structure from the perspective of the change of photoexcited state on electron dielectric screening and atomic potential,respectively.The primary contents of this paper are listed as follows:1.First,we take the monolayer Mo S₂ as a prototype to investigate the nonequilibrium electron-phonon coupling under photoexcitation.We quantitatively analyze the variations of the electron-phonon coupling matrix elements of the dominant phonon mode A_1g under photoexcitation.We observe the enhancement of the electron-phonon coupling matrix element due to the suppression of the electronic screening upon phonon perturbation when the electron-hole pairs are introduced during photoexcitation.On the other hand,photoexcitation dramatically increases the phase space of the electron-phonon scattering.These two factors result in growing up of the energy exchange rate from photocarriers to the high energy optical phonon modes.Our work provides a primary perception of the nonequilibrium electron-phonon interactions under photoexcitation and offers a guide to ultrafast quantum control using specific excited coherent phonons.2.The ultrafast phase transition in charge density wave materials induced by amplitude mode has attracted a lot of interests recently,but the impact of the transient topological defects which break the long-range order is still vague.We analyze the photoinduced modulation of the long-range order in Ti Se₂ starting with transient atomic structure and taking the correlation effects into consideration.Using the frozen phonon approach,we show the enhancement of the electron-phonon coupling in the inverted charge density wave structure.By analyzing the orbital composition of the energy band close to Fermi level,we find that this enhancement result from the suppression of long-range Coulomb interactions.We further build a supercell structure to study the variations of the electronic structure in the presence of the topological defects.3.Finally,we study the electron-phonon coupling in Zn O and Cs₂In Ag_0.5Na_0.5Cl₆.We find that the eigenvectors of the character phonon mode such as A₁ mode in Zn O and A_1g mode in Cs₂In Ag_0.5Na_0.5Cl₆ are in line with the distortion of the atomic structure under photoexcitation.This suggests that electron-phonon couplings may directly promote the transition from free excitons to self-trapped excitons.Besides,these high symmetrical phonon modes usually hold strong electron-phonon coupling and will play a dominant role in the atomic vibrations after photoexcitation,which enable the free excitons to cross the barrier.Furthermore,we find the triplet in this material has lower energy than singlet,so the singlet excitons may transfer to a triplet.The Na atom doping also lowers the electronic dimensionality in addition to break the parity forbidden.In summary,electron-phonon couplings essentially boost the transition from free states to trapped states and it is feasible to control the self-trapped state by resonance excitation of the particular phonon mode which is coupling to the lattice distortion.Though the study of the nonequilibrium electron-phonon coupling in these materials,we find photoexcitation can effectively modulate the electron-phonon interactions and can be used for quantum control strategy.Based on the previous knowledge of electron-phonon interactions under thermal equilibrium condition,people can obtain the hidden states under nonthermal transition.These works enlighten the development of new optical storage devices and high efficient light emitting devices.