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https://hdl.handle.net/2142/23861
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
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
thermal vacancies
hcp helium
phonon pressure
Language
en
Abstract
Activation energies of thermal vacancies have been determined in hcp 4He 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 cm3/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 4He. The lower bounds on the
activation energies from the relative lattice parameter measurements are consistent with
those determined from the pressure. Below 20 cm3/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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