Observation of a Dislocation Related Interfacial Friction Mechanism in Mobile Solid $^4$He

TL;DR

This study investigates the temperature and stress dependence of interfacial friction in solid $^4$He, revealing that dislocation climb dominates the friction mechanism and providing insights into the interface dynamics during disordering.

Contribution

It demonstrates that dislocation climb is the primary source of interfacial friction in solid $^4$He and models the interface as a well-oriented slip-stick system based on experimental data.

Findings

Dislocation climb dominates interfacial friction between 0.5K and 1.8K.

The characteristic energy scale of internal friction is between 3K and 6K.

The interface behaves as a well-oriented slip-stick system.

Abstract

We report a study of the temperature and stress dependence of the friction associated with a relative motion of two masses of solid $^4$He in contact. The situation where "two masses" coupled only by friction exists emerges spontaneously during a disordering of a single crystal contained inside a annular sample space of torsional oscillator (TO). Under the torque applied by the oscillating walls of the the TO these "masses" move relative to each other, generating measurable dissipation at their interface. We studied this dissipation between 0.5K and 1.8K in solid samples grown from commercially pure $^4$He and from a 100 ppm $^3$He-$^4$He mixture. The data were analyzed by modelling the TO as a driven harmonic oscillator. In this model, analysis of the resonant frequency and amplitude of the TO yields the temperature dependence of the friction coefficient. By fitting the data to specific forms, we found that over our temperature range, the dominant friction mechanism associated with the interfacial motion results from climb of individual dislocations. The characteristic energy scale associated with this internal friction is between 3K and 6K, depending on the sample. The fact that a single value of this energy accounts for the data of a given sample supports the idea that the interface between the moving and static solid is well oriented. The relative motion of the solid in this case can perhaps be described as the low limit of "slip-stick" motion.