Phonon scattering by ferroelectric domain walls in potassium dihydrogen phosphate
Weilert, Mark Anthony
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https://hdl.handle.net/2142/28682
Description
Title
Phonon scattering by ferroelectric domain walls in potassium dihydrogen phosphate
Author(s)
Weilert, Mark Anthony
Issue Date
1993
Doctoral Committee Chair(s)
Anderson, A.C.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
phonon scattering
ferroelectric domain walls
potassium dihydrogen phosphate
Language
en
Abstract
I have studied phonon scattering by ferroelectric domain walls in KH2P04 (KDP) using thermal conductivity and phonon imaging
measurements of samples in both single-domain (poled) and multidomain states. Domains were eliminated by applying a 2- 3kV /em field using evaporated electrodes. Phonon images of a poled (001) sample (at
1.7K) show two-fold symmetric focusing patterns that clearly show the small orthorhombic distortion of the lattice. Poled images are qualitatively reproduced by calculations using continuum acoustics with room temperature tetragonal elastic moduli split to orthorhombic symmetry; a rough set of low temperature orthorhombic moduli is
thereby determined. The presence of domains causes a large change in ST focusing structures but little change in L and FT patterns. There is an apparent decrease in overall intensity for all modes. The images
show strongly mode and polarization dependent phonon scattering. Monte Carlo simulations using acoustic mismatch (phonon reflection) to model phonon/domain wall interactions accurately reproduce the changes seen in the domained images. The low temperature (0.03-3K) thermal conductivity, K, of [100], [110], and [001] (tetragonal basis) oriented samples was measured. K in poled samples shows typical surface-limited behavior which is predicted to 13% by calculations including finite length and anisotropy and using elastic constants from the imaging simulations. Domain walls produce a 40% conductivity decrease in a [110] sample and a 30% decrease in [100) samples which is nearly independent of sample size, suggesting strong scattering of
only a subset of phonons. Domain wall scattering is nearly temperature independent between 0.3K and 1K, but decreases at lower
temperatures, and increases above 1K. The [001] sample couldn't be poled, but the domained K is higher than that expected from sample surface scattering. Domains parallel to the heat flow may be channeling phonons and increasing K. Acoustic mismatch calculations applied to K predict the right order of scattering, but do not reproduce the sample size effects. Orientation effects can be partially explained. The probability of diffuse scattering at the domain boundaries (often seen in Kapitza resistance studies) is at most 1%.
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