Vacancies in low-density hcp helium-4 solids

The existence and behavior of ground-state and thermally-activated excitations in helium solids have been extensively examined by a variety of experimental and theoretical studies. Although these excitations are typically identified as vacancies, large discrepancies arise in exactly what their nature and role are in the properties of the solid. At issue here is whether or not the vacancies delocalize and form a band of states at large molar volumes.A new sample chamber has been developed for the simultaneous determination of the vacancy content in helium crystals by two independent techniques; the measurement of the x-ray lattice parameter and the thermodynamic pressure. Reported here are temperature dependent pressure studies on 6 ultra-pure $\sp4$He solid isochores with molar volumes ranging from 20.36 cc/mole to the maximum in allowable molar volume, 20.98 cc/mole. A capacitance strain gauge measures the pressure which is the sum of zero-point, phonon, and thermally-activated defect contributions.The data here was analyzed in terms of a non-localized vacancy bandwidth model in addition to the standard localized vacancy treatment. No significance is attached to this semi-empirical finite bandwidth treatment. The inferred thermal vacancy activation energies determined here through a localized vacancy model are consistent with previous pressure measurements upon higher density hcp $\sp4$He crystals. Comparison of the localized vacancy activation energies determined here with earlier direct measurements of the vacancy content by x-rays in these solids indicate that the vacancies must be delocalized and form a wide band. Recent reanalyses of specific heat data have also come to this conclusion.U of I OnlyETDs are only available to UIUC Users without author permissio