John D. Reppy

Superfluid helium in porous media

The superfluid properties of 4He absorbed in porous media are discussed. Emphasis is given to the nature of the superfluid transition for helium contained in a variety of different porous structures. Recent heat capacity and superfluid measurements for helium in Vycor glass and other porous media are presented in some detail. The onset of superfluidity as a function of adsorbed helium coverage at zero temperature, or “boson localization,” is touched on briefly. The problem of dissipation of superflow in porous media, especially in the vicinity of the superfluid transition, is discussed.

Disorder and the Supersolid State of Solid He 4

We report torsional oscillator supersolid studies of highly disordered samples of solid

Nonsuperfluid origin of the nonclassical rotational inertia in a bulk sample of solid 4He.

^{4}\mathrm{He}

Probing the upper limit of nonclassical rotational inertia in solid helium 4.

. In an attempt to approach the amorphous or glassy state of the solid, we prepare our samples by rapid freezing from the normal phase of liquid

Density dependence of the transition temperature in a homogeneous bose-einstein condensate

^{4}\mathrm{He}

Viscosity measurements in superfluid 3He-B from 2 to 29 bar

. Less than two minutes is required for the entire process of freezing and the subsequent cooling of the sample to below 1 K. The supersolid signals observed for such samples are remarkably large, exceeding 20% of the entire solid helium moment of inertia. These results, taken with the finding that the magnitude of the small supersolid signals observed in our earlier experiments can be reduced to an unobservable level by annealing, strongly suggest that the supersolid state exists for the disordered or glassy state of helium and is absent in high quality crystals of solid

Onset of superfluidity in {sup 4}He films adsorbed on disordered substrates

^{4}\mathrm{He}

The temperature and pressure dependence of the normal fraction of superfluid3He-B

.

4He as a dilute bose gas

The torsional oscillator experiments described here examine the effect of disorder on the nonclassical rotational inertia (NCRI) of a solid 4He sample. The NCRI increases with increasing disorder, but the period changes responsible for this increase occur primarily at higher temperatures. Contrary to expectations based on a supersolid scenario, the oscillator period remains relatively unaffected at the lowest temperatures. This result points to a nonsuperfluid origin for the NCRI.

The low temperature viscosity of normal liquid3He

We study the effect of confinement on solid 4Hes nonclassical rotational inertia fraction (NCRIF) in a torsional oscillator by constraining it to narrow annular cells of various widths. The NCRIF exhibits an observed maximum value of 20% for annuli of approximately 100 microm width. Samples constrained to porous media or to larger geometries both have smaller NCRIF, mostly below approximately 1%. In addition, we extend the blocked-annulus experiment of Kim and Chan to solid samples with large supersolid fractions. Blocking the annulus suppresses the nonclassical decoupling from 17.1% to below the limit of our detection of 0.8%. This result demonstrates the nonlocal nature of the supersolid phenomena. At 20 mK, NCRIF depends on velocity history showing a closed hysteresis loop in different thin annular cells.