MODE DEPENDENT SCATTERING OF PHONONS BY DOMAIN-WALLS IN FERROELECTRICKDP

Thermal conductivity and ballistic phonon imaging measurements in KH2P O4 (KDP) at low temperature (T < 3K) indicate that scattering from dom ain walls has a large effect on phonon transport. kDP has a ferroelect ric phase transition from tetragonal to orthorhombic structure at T(c) = 122 K. Below T(c) domains of opposite electric polarization and cry stal orientation form unless the sample is colled in an electric field . Thermal conductivity measured along the [100] (tetragonal) axis drop s 30% when domain walls are present, which is independent of sample si ze and temperature. We attribute this decrease to phonon polarization- dependent scattering at the domain boundaries. This is verified by mea surements of ballistic transport, using phonon imaging techniques, whi ch reveal the phonon polarization and mode dependence of the scatterin g. The scattering is successfully modelled using continuum acoustics w ith simple acoustic mismatch at the domain wall. The interface scatter ing is found to be mode dependent: Caustic structures in the phonon im ages due to slow transverse phonons are most affected by the domain wa ll scattering, which channels these phonons along parallel planes by m ultiple reflections without mode conversion. Mode conversion scatterin g, though possible for a number of phonons, has little effect on the o verall phonon transmission.