$^4$He Crystal Quality and Rotational Response in a Transparent Torsional Oscillator

TL;DR

This study investigates the rotational response of natural helium-4 crystals in a torsional oscillator, suggesting that observed anomalies are due to changes in stiffness rather than supersolidity, with implications for dislocation behavior.

Contribution

It provides new evidence that helium-4's rotational anomalies are linked to stiffness changes, challenging the supersolidity explanation and highlighting dislocation dynamics.

Findings

Resonant frequency shifts are sample-dependent and reproducible.

Shifts are observed only in single crystals, not polycrystals.

Results suggest stiffness changes, not supersolidity, cause anomalies.

Abstract

We have studied natural purity $^{4}$He single crystals and polycrystals between 10 and 600 mK using a torsional oscillator with a 2 cm$^{3}$ rigid cell made of sapphire with a smooth geometry. As the temperature was lowered, we observed sample dependent but reproducible resonant frequency shifts that could be attributed to a supersolid fraction of order 0.1%. However, these shifts were observed with single crystals, not with polycrystals. Our results indicate that, in our case, the rotational anomaly of solid helium is more likely due to a change in stiffness than to supersolidity. This interpretation would presumably require gliding of dislocations in more directions than previously thought.