Peter Taborek

Phonon focusing catastrophes

We present an elegant application of formal catastrophe theory to phonon focusing phenomena in crystals. The standard analysis of phonon focusing breaks down near singular directions known as caustics. The theory we present, in addition to being numerically accurate, provides a qualitative picture of caustic surfaces which yields important new insights into the results of phonon propagation experiments. Supporting experimental evidence is also presented.

Direct and thermal desorption of ^4He films

We have studied desorption from a ^4He film due to a hot but low-intensity beam of phonons incident from a sapphire substrate. Time of flight spectra of the desorbed atoms revealed two distinct peaks which we attribute to direct desorption by substrate phonons and to thermalization with subsequent desorption.

Phonon reflection at a sapphire-vacuum interface

Theoretical and experimental studies have been conducted of the transmission of phonons in sapphire and their reflection at a crystal-vacuum interface. The effects of crystal anisotropy on the reflection process are discussed in detail. The heat-pulse technique has been used to obtain high-resolution time-of-flight spectra as a function of the angle of incidence in accurately known crystallographic directions. The agreement with predictions from numerical calculations is excellent. The relevance of these observations to studies of the anomalous transmission of energy through a crystal-liquid helium interface is also discussed.

Phonon reflection at noble gas interfaces

High resolution phonon reflection spectra from thin films of noble gases condensed onto a sapphire substrate at 2 K are presented. Phonon reflection signals from films of helium, neon, argon, krypton and xenon are found to be essentially identical, suggesting that quantum effects play no role. We argue, however, that the conventional interpretation of a change in the signal as due to heat escaping from the system cannot be valid for the solidified rare gases.

Control of ultralow pressures: an absolute thermodynamic manometer

A method of accurately manipulating the pressure of helium gas in the range P=10(-1) to 10(-20) Torr, T<5 K is presented. The method requires only a very modest apparatus and makes use of thermodynamic measurements on the 4He-Grafoil system. A calibration curve and a pressure scale for the manometer are developed which will allow convenient application in other laboratories. An experiment which utilizes the technique is briefly disscussed.

Phonon induced desorption from helium films

We have investigated the problem of desorption from thin helium films on sapphire crystals, and find evidence in favor of single phonon processes.

Specular and Diffuse Phonon Reflection Processes at a Crystal Interface

We have performed high resolution phonon reflection experiments in sapphire using the heat pulse technique. Our results1 show as many as eight sharp specular peaks; this structure in the reflection signal cannot be explained using isotropic elastic theory. This is somewhat surprising since the phase velocities of sapphire vary with direction by only 15%.