Torsional oscillators and the entropy dilemma of putative supersolid He-4

TL;DR

This paper examines the low-temperature specific heat of solid He-4, suggesting that glassy states caused by dislocation defects, rather than supersolidity, explain the observed phenomena and torsional oscillator behavior.

Contribution

It proposes that the linear specific heat and oscillator frequency changes are due to glassy states from dislocation defects, challenging the supersolid interpretation.

Findings

Observed entropy is too small for a supersolid transition.

Linear specific heat term attributed to glassy tunneling states.

Torsional oscillator frequency increase consistent with glass transition.

Abstract

Solid He-4 is viewed as a nearly perfect Debye solid. Yet, recent calorimetry measurements by the PSU group (J. Low Temp. Phys. 138, 853 (2005) and Nature 449, 1025 (2007)) indicate that at low temperatures the specific heat has both cubic and linear contributions. These features appear in the same temperature range where measurements of the torsional oscillator period suggest a supersolid transition. We analyze the specific heat and compare the measured with the estimated entropy for a proposed supersolid transition with 1% superfluid fraction and find that the observed entropy is too small. We suggest that the low-temperature linear term in the specific heat is due to a glassy state that develops at low temperatures and is caused by a distribution of tunneling systems in the crystal. We propose that dislocation related defects produce those tunneling systems. Further, we argue that the reported putative mass decoupling, that means an increase in the oscillator frequency, is consistent with a glass-like transition. The glass scenario offers an alternative interpretation of the torsional oscillator experiments in contrast to the supersolid scenario of nonclassical rotational inertia.