Ultrafast decay of low-symmetry photo-induced atomic forces.*

Generation and control of atomic forces in optically excited systems is crucial to understanding photocatalysis, renewable energy and laser annealing. E_g-symmetry coherent phonons are excited in group-V semimetals by ultrafast (<100 fs) optical pulses when the radiation is polarised perpendicular to the 3-fold symmetry axis of the crystal. The phonon driving force is consistent with an initially unbalanced occupation of electronic states in symmetry-equivalent regions of the Brillouin zone, which decays to fully-symmetric occupation of the zone on fs timescales. Measured temperature-dependence of the force decay time in Bi and Sb [1] suggests relaxation of the excited electronic distribution by electron-phonon (el-ph) scattering.
We calculate the decay of the E_g-symmetry driving force in Bi, Sb and As within the framework of density functional perturbation theory. We compute the initial excited electronic distribution [2], evolve using el-ph rate equations and calculate the atomic forces at each time-step. We obtain good agreement with experiment, showing that el-ph scattering is a dominant relaxation mechanism for the E_g force.
[1] Li et al PRL 110 047401
[2] Murray, Fahy PRL 114 055502