Ab initio simulation of coherent phonons in BN-nanotubes

BN nanotubes are isostructural to carbon nanotubes with boron and nitrogen atoms occupying the even and odd sublattices, respectively. An intense ultrashort laser pulse excites the electronic system to very high temperatures, whereas the ions remain close to their initial state. The ensuing laser-induced processes of electronic origin include bond softening, phonon frequency changes (hardening or softening), and the excitation of coherent phonons. We study these processes by means of large-scale molecular dynamics simulations based on density functional theory, including levels of excitation where the nanotube breaks. Our results show a strong radial breathing mode, increasing in amplitude with the laser-induced electronic temperature. We also determine the damage threshold.