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 $\sim 100\mathrm{fs}$. 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.

Figure 1

Phonon frequencies at the $\Gamma$, $X$, and $L$ points as a function of the electronic temperature. The labels indicate whether the phonons are longitudinal ($L$) or transverse ($T$) and optical ($O$) or acoustic ($A$). Negative frequencies represent repulsive potentials. The lines are a guide to the eye.

Figure 2

Potential energy of laser-excited InSb in the directions of the transverse acoustic phonons at the $X$ point for selected electronic temperatures $T_e$. The horizontal axis shows the displacement of the In atoms. For each temperature, the displacement of the Sb atoms is proportional to that of the In atoms. The proportionality constant varies between 0.81 for the electronic ground state and 0.94 for the highest excited state.

Figure 3

Root-mean-square atomic displacement versus time for two laser-induced electronic temperatures $T_e$. Whereas the top (blue) curves show results within the harmonic approximation, the bottom (red) curves include anharmonic effects. The discrepancies between both approaches demonstrate the importance of anharmonicity for times greater than $\sim 100\mathrm{fs}$. The anharmonicity leads to a deceleration of the atoms.