Density functional perturbation theory calculations have been utilized to characterize the carrier density dependent phonon dispersion of InSb. Similar to prior theoretical studies of Si, these calculations predict that a shear instability develops in the crystal at a carrier density of 3.7% of the valence electron density and the entire transverse acoustic phonon branch becomes unstable over a narrow carrier density range of roughly 1%. Unlike calculations for Si, the shear instability appears first at the $X$ point, rather than the $L$ point. We utilize these calculations to interpret recent ultrafast x-ray diffraction measurements of laser-induced disordering in InSb and find that the time scale and laser fluence dependence of the measured disordering dynamics are consistent with these theoretical predictions. The calculations, however, do not reproduce the experimental anisotropy in the root-mean-square displacement.

• Received 6 February 2008

DOI:https://doi.org/10.1103/PhysRevB.77.195213