Torsional Oscillator Technique Research Articles

Adiabatic fountain resonance

We have detected a new type of resonance in superfluid4He. This involves the adiabatic motion of the superfluid in and out of a superleak sealed at one end. We detect the resonance by measuring the temperature oscillations of the superleak. These measurements yield the superfluid fraction of confined helium. We compare data for a 2113A planar superleak with earlier results obtained using a torsional oscillator technique, and a similar confinement geometry.

Superfluidity in4He adsorbed in porous silica

We have measured the superfluid density of4He adsorbed in silica of 25 A pore diameter for various film coverages using a torsion oscillator technique. The experiment shows that the superfluid density near the full pore coverage is entirely determined by the phonon excitation, with the Kosterlitz-Thouless transition playing no role.

Hydrogen in porous Vycor glass

Hydrogen in porous Vycor glass (pore radiusR p=3.0 nm) has been investigated with a torsional oscillator technique at 7.5 K≤T≤22 K. H2 molecules which are adsorbed in Vycor atT≥T 3 (T 3: triple point of bulk H2) leave the Vycor when decreasing the temperature to below a characteristic valueT c≤T 3;T c depends on the amount of H2 in the Vycor. We conclude that the free energy of solid H2 in the Vycor is larger than that of bulk solid H2. Information on the free energy of H2 confined in the Vycor is important to understand the depression of its freezing temperature in restricted geometries. The mechanism of the mass flow is discussed in the light of the temperature dependence of its speed.

Vortex core size in submonolayer superfluid4He films

The superfluid transition of 4 He films adsorbed on 500 A diameter slip-cast alumina powder is measured with a torsion oscillator technique for films with transition temperatures between 50 and 700 mK. The transitions are found to be controlled by the universal Kosterlitz-Thouless critical line, but a strong increase in the broadening of the transition is observed for the thinnest films. Analysis in terms of vortex pair excitations shows that this broadening results from a vortex core size that increases rapidly as the film is thinned, scaling roughly with the interparticle spacing of the superfluid submonolayer. Third sound modes are identified in the dissipation, in addition to a broad peak from the vortex unbinding. This system provides evidence of a three-dimensional superfluid transition mediated by vortex ezcitations.

Hydrogen in an oscillating porous Vycor glass

We investigate hydrogen in porous Vycor glass with a torsional oscillator technique. Although our primary purpose is searching for a superfluid transition of hydrogen supercooled in Vycor, we find that hydrogen molecules which are adsorbed and liquefied in Vycor at T > T3 (triple point of bulk H2) leave the Vycor when decreasing the temperature to below a characteristic value Tc < T3. We discuss this phenomenon in terms of a free energy balance between solid/liquid hydrogen inside and outside the Vycor.

Vortex Core Size in Submonolayer Superfluid 4He Films.

The superfluid transition of4He films adsorbed on 500 A diameter slip-cast alumina powder is measured with a torsion oscillator technique for films with transition temperatures between 50 and 700 mK. The transitions are found to be controlled by the universal Kosterlitz- Thouless critical line, but a strong increase in the broadening of the transition is observed for the thinnest films. Analysis in terms of vortex pair excitations shows that this broadening results from a vortex core size that increases rapidly as the film is thinned, scaling roughly with the interparticle spacing of the superfluid submonolayer. Third sound modes are identified in the dissipation, in addition to a broad peak from the vortex unbinding. This system provides evidence of a three-dimensional superfluid transition mediated by vortex excitations.

Superfluidity in thin 4He films adsorbed on graphite

We have used the torsional oscillator technique to study supefluidity in 4 He films adsorbed on the basal plane of graphite for coverages between zero and seven atomic layers. The development of superfluidity in this system is significantly different from that observed on disordered substrates such as Mylar and porous glasses. The most unusual phenomenon is a reentrant superfluid phase in the second layer of adsorbed helium. Superfluidity appears at a coverage just above one and one-half layers and then disappears before the completion of the second layer. We observe superfluidity again just above second layer completion, but its evolution with increasing coverage continues to be unusual. Plateaus in the superfluid transition temperature and superfluid signal occur over parts of the third and fourth layers. A periodic structure appears in the measured superfluid mass through the completion of the seventh layer.

Superfluid density of 4He films adsorbed on slip-cast powder

The superfluid density of thin4He films adsorbed in slip-cast samples of 500A˚diameter Al2O3 powder has been measured using a torsion oscillator technique. Calibration of the data shows that the superfluid transition is controlled by the universal Kosterlitz-Thouless line, as found in previous third sound studies. There is no abrupt “jump” in the superfluid density at that point, but a smooth transition that is broadened by the finite grain size of the powder. The slip-cast sample is much more homogenous than previous packed powder samples, but near Tc there is still a rounded tail to the superfluid density of about 10–15 mK.

Onset of superfluidity in4He films adsorbed on a graphite-fiber substrate

A torsional oscillator technique has been employed to investigate the onset of superfluidity in4He adsorbed on graphite fibers. A low temperature capacitance pressure gauge has been used to characterize the layered growth of the film as mass is added to the experimental cell. At T=0.84 K the onset of superfluidity occurs after completion of three layers. A comparison with similar data for a Mylar substrate shows that the dissipation peaks are much sharper for the graphite-fiber substrate.

Kosterlitz-Thouless transition in helium films.

The superfluid response of helium films with transition temperatures ranging from 70 mK to 0.5 K has been studied using a torsional oscillator technique. A detailed analysis of the background indicates that the initial nonsuperfluid layer has an unexpected temperature-dependent period shift and dissipation. The superfluid response obeys a general scaling law which can be derived from the Kosterlitz-Thouless theory. Comparisons with the dynamic theory reveal nonuniversal behavior as a function of coverage.

Superfluidity of 4He Confined within Porous Silica Gel

The torsional oscillator technique is used to study the super fluidity of helium in porous silica gel as a function of temperature and coverage. Although the pore diameter of the silica gel is similar to that of the Vycor glass used in previous studies, remarkable differences in the super fluid behavior are observed near the transition temperature.

Helium-four transition in a restricted geometry below and above the bulk solidification pressure

Abstract Using a torsional oscillator technique, we have investigated the onset of superfluidity of 4He in Vycor at pressures in the liquid region and at higher pressures where bulk 4He is solid. The lower pressure data trace out the onset temperature for superflow in Vycor as a function of pressure, which is roughly parallel to the bulk lambda line but about 0.25K lower. At OK, calculated values of the superfluid fraction are about 0.16 at 1 bar and 0.12 at 25 bar; these numbers contain a geometrical constant. Higher pressure data indicate that superflow can occur in Vycor at pressures as much as 20 bar above bulk solidifcation. An unexplained decrease in period is observed below the bulk solidification temperature.

Viscosity Difference Measurements for Normal and Para Liquid Hydrogen Mixtures

The absence of experimental data in the literature concerning a viscosity difference for normal and equilibrium liquid hydrogen may be attributed to the limited reproducibility of oscillating disk measurements in a liquid-hydrogen environment. Indeed, there is disagreement over the viscosity values for equilibrium liquid hydrogen even without proton spin considerations. Measurements presented here represent the first application of the piezoelectric alpha quartz torsional oscillator technique to liquid-hydrogen viscosity measurements.