Search for supersolidity in solid 4He using multiple-mode torsional oscillators.

In 2004, Kim and Chan (KC) reported a decrease in the period of torsional oscillators (TO) containing samples of solid (4)He, as the temperature was lowered below 0.2 K [Kim E, Chan MHW (2004) Science 305(5692):1941-1944]. These unexpected results constituted the first experimental evidence that the long-predicted supersolid state of solid (4)He may exist in nature.

Anomalous behavior of solid 4He in porous Vycor glass.

The low temperature properties of solid (4)He contained in porous Vycor glass have been investigated utilizing a two-mode compound torsional oscillator. At low temperatures, we find period-shift signals for the solid similar to those reported by Kim and Chan [Nature (London) 427, 225 (2004)], which were taken at the time as evidence for a supersolid helium phase. The supersolid is expected to have properties analogous to those of a conventional superfluid, where the superfluid behavior is independent of frequency and the ratio of the superfluid signals observed at two different mode periods will depend only on the ratio of the sensitivities of the mode periods to mass loading.

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

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.

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

We study the effect of confinement on solid 4He's 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%.

Observation of classical rotational inertia and nonclassical supersolid signals in solid 4He below 250 mK.

We have confirmed the existence, as first reported by Kim and Chan, of a supersolid state in solid 4He at temperatures below 250 mK. We have employed a torsional oscillator cell with a square cross section to insure a locking of the solid to the oscillating cell. We find that the nonclassical rotational inertia signal is not a universal property of solid 4He but can be eliminated through an annealing of the solid helium sample.