Characterization of the Surface of Moving solid 4He

TL;DR

This study investigates the macroscopic motion of solid helium-4, revealing frictionless flow at low speeds and characterizing its surface orientation using an innovative in-situ vibration detection method.

Contribution

It introduces a novel in-situ flow detection technique to characterize the surface of moving solid helium-4 and demonstrates frictionless flow at low velocities.

Findings

Surface of moving solid is the (0001) basal plane.

Frictionless flow observed below 7 micrometers/sec.

Zero dissipation at 0.5K within measurement accuracy.

Abstract

Crystal grains of solid $^4$He can move in relation to each other even when embedded inside the solid. In this work, we characterize a macroscopic motion of solid hcp $^4$He composed of such grains. Motion is induced by applying an external torque to the solid contained inside an annular channel mounted on a torsional oscillator. In order to characterize the surface of the moving solid, we developed an in-situ flow detection method using a sensitive "microphone" embedded in the wall of the channel. Motion is detected by counting the vibrations induced by rows of He atoms moving past the microphone. Such vibrations were detected only at T=0.5K, our lowest temperature. At this temperature, the measured dissipation associated with the solid He is zero within our accuracy. Our results indicate that the orientation of the surface of the moving solid is the (0001) basal plane of the hcp structure. At T=0.5 K, we found that for speeds < 7 micrometer/sec, the solid flows without detectable friction.