Evidence for Supersolidity in Bulk Solid $^4$He

TL;DR

This study presents low-temperature measurements of bulk solid helium-4 using a specially designed torsional oscillator, providing evidence for a tiny supersolid fraction that is distinct from elastic effects.

Contribution

It demonstrates that a small, frequency-independent period shift in bulk solid helium-4 supports the existence of supersolidity, distinct from elastic effects, at temperatures below 0.25 K.

Findings

Observed period shifts consistent with supersolidity

Signals are two orders smaller than previous reports

Supports existence of a tiny supersolid fraction (~10^-4)

Abstract

We report low temperature measurements of bulk solid $^4$He in a two-frequency compound torsional oscillator with both annular and open cylinder sample geometries. The oscillators were designed to suppress period shifts arising from all known elastic effects of solid $^4$He. At temperatures below 0.25 K, period shift signals similar to those reported by Kim and Chan [Science {\bf 305}, 1941 (2004)] were observed, albeit two orders smaller in magnitude. A sizable fraction of the observed signals are frequency-independent and consistent with the mass-decoupling expected for supersolid $^4$He. This result is in stark contrast with recent works on Vycor-solid-$^4$He system and suggests that a small supersolid fraction on the order of $1 \times 10^{-4}$ may indeed exist in bulk solid $^4$He.