Peng Han, Linas Vilciauskas, Gabriel Bester
We perform ab initio Born-Oppenheimer molecular dynamics simulations to study the temperature dependent dynamical processes of a Si10H16 cluster. We calculate the vibrational density of states from the Fourier transform of the velocity auto-correlation function. We benchmark our results against established density functional perturbation theory for the T= 0 K case. While we find the approach to be viable and accurate, the convergence of the vibrational modes, with respect to the number of molecular dynamics simulation time steps, is shown to requires rather long trajectories of around 30 ps. We obtain a blue shift of the Si-Si vibrational modes with transverse acoustic character and a red shift of the other vibrational modes with increasing temperature. These shifts can be linked to the bond length expansion along with the sign of the Grueneisen parameter. All of the vibrational modes (especial the high frequency Si-H modes) are found to be strongly broadened already at moderate temperatures. We link the strong anharmonicity of Si-H modes to the surface effect, which is induced by the low symmetry of the potential of the atoms close to the surface (which represents a large fraction of atoms in our cluster or in small molecules). We extract the time evolution of the energy of selected vibrational modes --related to the phonon (vibron) lifetime-- from the energy auto-correlation functions via first-principles calculation. We find fast vibrational cooling times of around 0.1 ps for high frequency Si-H vibrations, and cooling times of around 1 ps for pure Si-modes, which is close to the values of phonon lifetimes in bulk silicon. We calculate the vibrational cooling time as a function of the temperature and find that low frequency modes (acoustic-like) seem to be more affected by the temperature than high frequency (optic-like) modes.
View original:
http://arxiv.org/abs/1301.6912
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