Intrinsic and dislocation induced elastic behavior of solid helium

TL;DR

This study investigates how dislocations and isotopic impurities influence the elastic properties of solid helium, revealing that dislocation mobility and pinning affect the shear modulus and are related to supersolid phenomena.

Contribution

It provides new insights into the role of dislocations and impurities in the elastic behavior of solid helium, distinguishing intrinsic properties from defect-induced effects.

Findings

High-temperature shear modulus is affected by annealing and stress.

Low-temperature shear modulus remains unchanged, reflecting intrinsic properties.

Dislocations are pinned by 3He impurities at low temperatures, influencing elasticity.

Abstract

Recent experiments showed that the shear modulus of solid 4He stiffens in the same temperature range (below 200 mK) where mass decoupling and supersolidity have been inferred from torsional oscillator measurements. The two phenomena are clearly related and crystal defects, particularly dislocations, appear to be involved in both. We have studied the effects of annealing and the effects of applying large stresses on the elastic properties of solid 4He, using both acoustic resonances and direct low-frequency and low-amplitude measurements of the shear modulus. Both annealing and stressing affect the shear modulus, as expected if dislocations are responsible. However, it is the high temperature modulus which is affected; the low temperature behavior is unchanged and appears to reflect the intrinsic modulus of solid helium. We interpret this behavior in terms of dislocations which are pinned by isotopic 3He impurities at low temperatures and so have no effect on the shear modulus. At higher temperatures they become mobile and weaken the solid. Stressing the crystal at low temperatures appears to introduce new defects or additional pinning sites for the dislocation network but these effects can be reversed by heating the crystal above 500 mK. This is in contrast to dislocations produced during crystal growth, which are only annealed at temperatures close to melting.