First-Principle Simulations Of Nonequilibrium Electron-Phonon Dynamics

Under external excitations(such as the photoexcitation),electrons and phonons in condensed-matter systems have nonequilibrium dynamics.Due to the interaction between external excitations and these particles/quasiparticles,the nonequilibrium dynamics can induce properties which are completely different from that in ground state,and even produce new quantum states.With the development of experimental technologies,more and more nonequilibrium phenomena have been revealed.Understanding the dynamic mechanisms behind these novel phenomena not only provides new ideas for condensed-matter physics,but also sheds light on the industrial application of materials.Therefore,it has become a research highlight in recent years.However,limited by Born-Oppenheimer approximation,the traditional density functional theory(DFT)cannot describe the nonequilibrium behaviors of condensed matters sufficiently and accurately.With the help of real-time time-dependent density functional theory(rt-TDDFT),we systematically studied the nonequilibrium dynamics in condensed matters,including the nonadiabatic effect of electron/phonon dynamics,the electron-phonon coupling(EPC)enhancement in photoexcited state,and probing material properties by nonequilibrium dynamics of electrons/phonons.The main contents of this thesis are as follows:1.Taking graphene/graphane as examples,we study the nonadiabatic effect on electron and phonon dynamics in two-dimensional materials.We revealed that the nonadiabatic effect modulates the nonequilibrium electron distributions,renormalizes phonon energy,and corrects the EPC as well as the corresponding superconducting critical temperature.The nonadiabatic corrections originate from the renormalization of EPC matrix and the Fermi surface broadening in nonequilibrium state.A strong isotopic effect is also investigated.Our results show that nonadiabatic effect can significantly affect the electron/phonon nonequilibrium dynamics and properties relevant to EPC(such as superconductivity).These findings will provide a new understanding for the study and application of quantum materials.2.We study the light-induced EPC enhancement in nonequilibrium states of Dirac materials.The analysis on energy exchange between carriers and phonons revealed that the EPC can be significantly enhanced during the ultrafast photocarrier relaxation.This enhancement comes from the nonequilibrium distribution of photocarriers,which provides additional channels for electron-phonon scattering.In addition,by tracking the time-dependent evolution of EPC matrix,we detect the dynamics of nonequilibrium EPC in the excited state.Our research proves that photoexcitation is an effective way to modulate the nonequilibrium characteristics of materials,and provides a strategy for the optical regulation on EPC of materials.3.In addition to the investigations on mechanism of nonequilibrium dynamics,we also apply these nonequilibrium dynamics to probe material properties.Using the nonlinear dynamics of electrons,i.e.,the high harmonic generation(HHG),we realize the reconstruction of bands and EPC in the energy and momentum space,with high resolution and band selectivity.Since the method does not depend on vacuum or electron occupation,which makes up for the shortcomings of angle resolved photoelectron spectroscopy,it is very suitable for the measurement in insulators and materials under extreme conditions(such as high-pressure materials).Moreover,based on the vibration spectrum of nonlinear coherent phonons,we develop the alloptical probing on the interatomic potential,Young’s modulus and thermal expansion coefficient.The technique is not limited by harmonic and adiabatic approximation,which works beyond ground state.These results will help for the all-optical characterization and manipulation on material properties.We explore the mechanism and all-optical probing of nonequilibrium dynamics in condensed matters,from the point view of electrons and phonons.These works provide theoretical guidance for the understanding and further applications on material characterization and device design.