Imaging of large-k phonons in InSb.

Abstract

Acoustic phonons in InSb with wave vectors up to 30% of the zone boundary are observed to propagate ballistically over a 1 mm distance. An incoherent point source of these large-k phonons gives rise to a distinct pattern of caustics, as detected by phonon imaging. This phonon-focusing pattern is obtained using a frequency-selective phonon detector---a Pb superconducting tunnel junction which is sensitive to frequencies above 650 GHz. The phonon image for these high-frequency phonons is markedly different from that obtained for longer path lengths using an Al bolometer, in an experiment which emphasizes lower frequencies (\ensuremath{\sim}300 GHz). The dispersive phonon image is compared to Monte Carlo simulations employing various lattice-dynamics models as input. Shell, dipole, rigid, and bond-charge (BCM) models with parameters previously optimized for neutron scattering data each predict quite distinct phonon-focusing patterns. Qualitatively, the BCM model agrees best with the phonon-imaging data; however, no effort has been made to adjust the many lattice-dynamics parameters of each model. The dispersive phonon images are in agreement with a Monte Carlo calculation which includes phonon scattering from isotopic defects. The combined effects of isotope scattering in the bulk crystal and frequency selectivity of the Pb tunnel junction result in a quasimonochromatic subset of detected phonons with a frequency distribution that is relatively independent of the heat-source distribution.