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https://hdl.handle.net/2142/23740
Description
Title
Thermal vacancies in HCP helium-4
Author(s)
Thorpe, John Clifton
Issue Date
1991
Doctoral Committee Chair(s)
Simmons, R.O.
Department of Study
Physics, Condensed Matter
Discipline
Physics, Condensed Matter
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Activation energies of thermal vacancies have been determined in hcp $\sp4$He as a function of molar volume. A sensitive strain gauge was used to measure the temperature dependence of the pressure in twenty crystals between 18.17 and 20.9 cm$\sp3$/mole. Absolute lattice parameters at low temperatures were measured for many of these crystals. The temperature dependence of the relative lattice parameter was measured in selected crystals. This provided lower bounds on the vacancy activation energy.
The activation energies determined here agree at all reported molar volumes with those from previous measurements of pressure in hcp $\sp4$He. The lower bounds on the activation energies from the relative lattice parameter measurements are consistent with those determined from the pressure. Below 20 cm$\sp3$/mole the activation energies reported here are in disagreement with those from ion mobility, NMR, and plastic deformation experiments. The determination of vacancy activation energies from the measured pressure requires knowledge of the phonon pressure. Difficulties with the present understanding of the phonon pressure are discussed.
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