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 Title: Thermal vacancies in HCP helium-4 Author(s): Thorpe, John Clifton Doctoral Committee Chair(s): Simmons, R.O. Department / Program: Physics, Condensed Matter Discipline: Physics, Condensed Matter Degree Granting Institution: University of Illinois at Urbana-Champaign Degree: Ph.D. Genre: Dissertation Subject(s): Physics, Condensed Matter 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. Issue Date: 1991 Type: Text Language: English URI: http://hdl.handle.net/2142/23740 Rights Information: Copyright 1991 Thorpe, John Clifton Date Available in IDEALS: 2011-05-07 Identifier in Online Catalog: AAI9124500 OCLC Identifier: (UMI)AAI9124500
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