Far-infrared phonon absorption in InSb

Far-infrared absorption due to single-phonon and two-phonon processes was investigated in undoped InSb using a high-resolution Fourier-transform spectrometer. Single-phonon absorption by both transverse- and longitudinal-optical modes at the Brillouin-zone center was observed. Two-phonon absorption spectra correlated well with two-phonon density-of-states curves calculated using parameters derived from inelastic-neutron-scattering experiments. Brillouin-zone locations of critical points that gave rise to prominent features on the two-phonon density-of-states curve were identified by investigating phonon frequency contours on symmetry planes. Critical points at (0.6, 0, 0), X, and L for phonon combinations involving transverse modes and on the (111) boundary plane of the Brillouin zone for combinations containing the longitudinal-acoustical phonon gave rise to almost all the prominent features observed on the two-phonon absorption spectra. Derived frequencies for phonon modes at (0.6, 0, 0), X, and L agreed well with values determined directly from inelastic-neutron-scattering experiments. Although strong features, arising from 2TA overtones at X and L, appear in the calculated spectrum, no absorption corresponding to these features was observed.