One-dimensional Excitations in Superfluid 4^4He and 3^3He-4^4He Mixture Films Adsorbed in Porous Materials

A normal-fluid component varying as T$^{2}$ is observed at very low temperatures in superfluid $^4$He and $^3$He-$^4$He mixture films adsorbed in alumina powder. The normal fluid appears to arise from thermally excited third sound that has one-dimensional propagation characteristics. A Landau model of third sound excitations in an infinite cylindrical pore by Saam and Cole provides good agreement with the experimental measurements over a wide range of $^4$He and $^3$He coverages. However, it is unclear why the powder substrate can be modeled as having cylindrical pores.Comment: 5 pages, 6 figures, revtex4, to appear in PR

Vortex-loop calculation of the specific heat of superfluid ^{4}He under pressure.

Vortex-loop renormalization is used to compute the specific heat of superfluid ^{4}He near the lambda point at various pressures up to 26 bars. The input parameters are the pressure dependence of T_{λ} and the superfluid density, which determine the nonuniversal parameters of the vortex core energy and core size. The results for the specific heat are found to be in good agreement with experimental data, matching the expected universal pressure dependence to within about 5%. The nonuniversal critical amplitude of the specific heat is found to be in reasonable agreement, a factor of four larger than the experiments. We point out problems with recent Gross-Pitaevskii simulations that claimed the vortex-loop percolation temperature did not match the critical temperature of the superfluid phase transition

Rapidly Quenched Kosterlitz-Thouless Superfluid Transitions

Rapidly quenched Kosterlitz-Thouless (KT) superfluid transitions are studied by solving the Fokker-Planck equation for the vortex-pair dynamics in conjunction with the KT recursion relations. Power-law decays of the vortex density at long times are found, and the results are in agreement with a scaling proposal made by Minnhagen and co-workers for the dynamical critical exponent. The superfluid density is strongly depressed after a quench, with the subsequent recovery being logarithmically slow for starting temperatures near T$_{KT}$. No evidence is found of vortices being ''created'' in a rapid quench, there is only decay of the existing thermal vortex pairs.Comment: 4 pages, 5 figures, revtex4, version accepted for PR

Vortex Core Size in 3^3He-4^4He films with Monolayer Superfluid 4^4He

The superfluid transition of $^3$He-$^4$He mixture films adsorbed on alumina powder is studied, with a $^4$He superfluid coverage near one layer. With up to 1.3 layers of $^3$He added, the transition becomes strongly broadened, indicating a linear increase in the vortex core size for $^3$He coverages below one layer. Annealing of the sample mixture at 4.2 K is found to be critically important in ensuring a homogeneous film across the porous substrate.Comment: 2 pages, 2 figures, accepted for LT22 Conference Proceedings, Physica

Vortex Fluctuations in the Critical Casimir Effect of Superfluid and Superconducting Films

Vortex-loop renormalization techniques are used to calculate the magnitude of the critical Casimir forces in superfluid films. The force is found to become appreciable when size of the thermal vortex loops is comparable to the film thickness, and the results for T < Tc are found to match very well with perturbative renormalization theories that have only been carried out for T > Tc. When applied to a high-Tc superconducting film connected to a bulk sample, the Casimir force causes a voltage difference to appear between the film and bulk, and estimates show that this may be readily measurable.Comment: 4 pages, 5 figures, Revtex 4, typo correctio

Non-universal Casimir Effect in Saturated Superfluid 4^4He Films at Tλ_\lambda

Measurements of Casimir effects in $^4$He films in the vicinity of the bulk superfluid transition temperature $T_\lambda$ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. The Kosterlitz-Thouless superfluid onset temperature in the film is found to occur just as the Casimir dip in the film thickness from critical fluctuations becomes evident. Additionally, a new film-thickening effect is observed precisely at $T_\lambda$ when the temperature is swept extremely slowly. We propose that this is a non-universal Casimir effect arising from the viscous suppression of second sound modes in the film.Comment: 5 pages, 6 figures, corrected an equation, small change to fit valu

Is the Soybean Checkoff Program Working?

The primary objective of this study is to answer two key questions regarding the U.S. soybean checkoff program over time: (1) What have been the effects of the soybean checkoff program on U.S. and world soybean and soybean product markets? (2) Has the soybean checkoff program benefited soybean producers? To answer the first key question, the effects of the soybean checkoff program on U.S. and foreign soybean, soybean meal, and soybean oil supplies, demands, prices, and trade over the 1980/81 through 2006/07 time period are measured. Those results are then used to answer the second question in a benefit-cost analysis of the soybean checkoff program to measure the overall return to producers from soybean checkoff and related expenditures over the years. In general, the study concludes that the expenditure of soybean checkoff funds to invest in production research and to promote the demand for soybeans and soybean products at home and abroad has been highly effective in enhancing the profitability, competiveness, and size of the U.S. soybean industry since at least 1980/81.Soybean, Soybean Checkoff, Crop Production/Industries, Demand and Price Analysis, Marketing, Productivity Analysis,

Adsorption properties and third sound propagation in superfluid 4^4He films on carbon nanotubes

We consider the adsorption properties of superfluid $^4$He films on carbon nanotubes. One major factor in the adsorption is the surface tension force arising from the very small diameter of the nanotubes. Calculations show that surface tension keeps the film thickness on the tubes very thin even when the helium vapor is increased to the saturated pressure. The weakened Van der Waals force due to the cylindrical geometry also contributes to this. Both of these effects act to lower the predicted velocity of third sound propagation along the tubes. It does not appear that superfluidity will be possible on single-walled nanotubes of diameter about one nm, since the film thickness is less than 3 atomic layers even at saturation. Superfluidity is possible on larger-diameter nanotube bundles and multi-walled nanotubes, however. We have observed third sound signals on nanotube bundles of average diameter 5 nm which are sprayed onto a Plexiglass surface, forming a network of tubes.Comment: 4 pages, accepted for Journal of Physics: Conference Series (Proceedings of LT25