Anharmonic noninertial lattice dynamics during ultrafast nonthermal melting of InSb

We compute the potential energy surface of femtosecond-laser-excited InSb along the directions in which the crystal becomes soft. Using dynamical simulations the time dependence of the atomic coordinates is obtained. We find that at high excitation densities the anharmonicity of the potential energy surface becomes significant after similar to 100 fs. On the basis of our results we explain recent time-resolved x-ray diffraction experiments. We point out that an alternative model for ultrafast melting [A. M. Lindenberg et al., Science 308, 392 (2005)] is inconsistent with our calculations.