Ultrafast structural relaxation dynamics of laser-excited graphene: Ab initio molecular dynamics simulations including electron-phonon interactions

The time evolution of the lattice structure in graphene after ultrashort laser excitation results in a complex dynamics of electrons and ions. In particular, a femtosecond laser pulse heats up the electrons, which then couple strongly to particular optical phonon modes, called SCOPs (strongly coupled optical phonons), located around the $\mathrm{\ensuremath{\Gamma}}$ and K (K') points of the Brillouin zone. In this paper, a fully ab initio description of the ultrafast structural response of graphene to femtosecond laser excitation is presented. Our atomistic simulations show that upon intense ultrafast laser excitation, a biexponential decay of the Bragg peaks takes place, in agreement with experiments. The calculated time-dependent phonon energies show that SCOPs equilibrate considerably faster with phonons having frequencies above 10 THz than with lower frequency phonon modes.