Anisotropic phonon-dislocation scattering in deformed LiF

Abstract

By direct imaging of ballistic phonons having equivalent temperatures near 4 K in LiF, we have found that a subset of fast-transverse phonons can propagate without scattering even in heavily deformed samples. The highly anisotropic phonon-scattering cross section is consistent with the concept of fluttering dislocations. The magnitude of the scattering cross section has been obtained for the first time. For small plastic deformation the measured cross section agrees with that calculated theoretically using a measured dislocation density. However, at large deformation the calculated cross section is too small, supporting the speculation that a larger density of dislocation dipoles may provide the dominant scattering. Also consistent with the concept of fluttering dislocations, $\gamma$ irradiation reduces the phonon scattering created by deformation, presumably by pinning of the dislocations. It is demonstrated that the method of phonon imaging used here provides details of phonon scattering processes which are not available from conventional thermal-transport measurements, especially when the scattering is anisotropic.