John C. Beamish

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Superfluidity--liquid flow without friction--is familiar in helium. The first evidence for 'supersolidity', its analogue in quantum solids, came from torsional oscillator measurements involving 4He. At temperatures below 200 mK, the torsional oscillator frequencies increased, suggesting that some of the solid decoupled from the oscillator. This behaviour(More)
The recent torsional oscillator results of Kim and Chan show an anomalous mass decoupling, interpreted by the authors as a supersolid phase transition, in solid (4)He. We have used a piezoelectrically driven diaphragm to study the flow of solid helium through an array of capillaries. Our measurements showed no indication of low temperature flow, placing(More)
Torsional oscillator experiments show evidence of mass decoupling in solid 4He. This decoupling is amplitude dependent, suggesting a critical velocity for supersolidity. We observe similar behavior in the elastic shear modulus. By measuring the shear modulus over a wide frequency range, we can distinguish between an amplitude dependence which depends on(More)
Following recent torsional oscillator measurements which appear to show the 'non-classical rotational inertia' which characterizes a supersolid, a number of experiments have searched for evidence of unusual behavior in other properties. We have developed a new technique for measuring the shear modulus of solid helium at low frequencies and small strains. In(More)
Torsional oscillator experiments on solid 4He show frequency changes which suggest mass decoupling, but the onset is broad and is accompanied by a dissipation peak. We have measured the elastic shear modulus over a broad frequency range, from 0.5 Hz to 8 kHz, and observe similar behavior-stiffening and a dissipation peak. These features are associated with(More)
estimates (8, 9) are in favor of there being vacancies at the 10 −4 per site level, even in the pure crystal (SOM text). Repeated efforts by Clark et al. to grow perfect, pure, single crystals have always observed nonclassical rotational inertia (NCRI) on the 10 −4 level relative to the classically expected value (10), and this level or higher has been(More)
We have investigated the adsorption and desorption of helium near its liquid-vapor critical point in silica aerogels with porosities between 95% and 98%. We used a capacitive measurement technique which allowed us to probe the helium density inside the aerogel directly, even though the samples were surrounded by bulk helium. The aerogel's very low thermal(More)
The recent torsional oscillator results of Kim and Chan suggest a supersolid phase transition in solid 4He confined in Vycor. We have used a capacitive technique to directly monitor density changes for helium confined in Vycor at low temperature and have used a piezoelectrically driven diaphragm to study the pressure-induced flow of solid helium into the(More)