First-principles calculations of the ultrafast dynamics of coupled electrons and phonons

Progress on experiments probing the ultrafast electron dynamics calls for the development of accurate simulations of materials in the time domain. However, widely employed approaches to model the coupled nonequilibrium dynamics of electrons and atomic vibrations (phonons) employ empirical or approximate interactions, or are limited to femtosecond timescales. Here we show a numerical approach to evolve in time the coupled Boltzmann transport equations (BTEs) of electrons and phonons, using ab initio electron-phonon and phonon-phonon interactions together with a parallel algorithm to explicitly time step the BTEs. Our approach can simulate the dynamics up to hundreds of picoseconds (with a femtosecond time resolution) and provide microscopic insight into the scattering mechanisms. The accuracy of the interactions used in the calculations can be validated by computing transport properties. We show example calculations on graphene and semiconductors, for which we compute carrier cooling rates, mode-resolved phonon dynamics, transient absorption, structural snapshots and diffuse X-ray scattering. Possible extensions to include static electric or electromagnetic fields will also be discussed. Our
findings open new avenues for quantitative simulations of ultrafast dynamics in materials.