Entropy of solid He4: the possible role of a dislocation glass

TL;DR

This paper investigates the low-temperature specific heat of solid He4, proposing that a dislocation glass state, rather than a supersolid transition, explains observed anomalies and entropy measurements.

Contribution

It introduces the idea that a dislocation glass state causes the linear specific heat term in solid He4, challenging the supersolid transition hypothesis.

Findings

Experimental entropy is less than the predicted for a supersolid.

A glassy state due to dislocation loops explains the linear specific heat.

Mass decoupling aligns with a glass-like transition in the oscillator.

Abstract

Solid He4 is viewed as a nearly perfect Debye solid. Yet, recent calorimetry indicates that its low-temperature specific heat has both cubic and linear contributions. These features appear in the same temperature range ($T \sim 200$ mK) where measurements of the torsional oscillator period suggest a supersolid transition. We analyze the specific heat to compare the measured with the estimated entropy for a proposed supersolid transition with 1% superfluid fraction. We find that the experimental entropy is substantially less than the calculated entropy. 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. It is proposed that small scale dislocation loops produce those tunneling systems. We argue that the reported mass decoupling is consistent with an increase in the oscillator frequency as expected for a glass-like transition.