Characterization of the Surface of Moving Solid $$^4$$ 4 He

Abstract

Crystal grains of solid4 He can move in relation to each other even when embedded inside the solid [1,2]. 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 µm/sec, the solid flows without detectable friction.

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

“…It can arise from a superfluid behaviour of the grain boundaries [30],or from dislocation glide. Recent measurements [8] on oriented single crystals show that dislocations glide freely parallel to the basal plane, which is the interface in our experiment [16]. Such motion can induce a slip of the crystal near the interface, which would appear equivalent to a relative motion of the two solid bands.…”

“…This analysis is described in detail in the Appendix of Ref. 16. The result is that various effects related to the shear modulus of solid He affect the resonant period of the TO by 1 to 2 orders of magnitude less than the changes we observe during the decoupling.…”

“…These experiments are described in great detail in previous publications of our group [13,14,15,16]. Here, we highlight the main points.…”

“…Our understanding of the situation in which two bodies of solid He move past each other was developed over several years of systematic experiments done in our lab. These experiments are described in great detail in previous publications of our group [13,14,15,16]. Here, we highlight the main points.…”

Mass superflux in solid helium: The role ofHe3impurities

“…This same reasoning concerning the behavior in response to thermal cycles should also apply to the possibility of liquid channels at grain boundaries unless annealing destroyed them. Recently Livne et al [33] have reported measurements of the macroscopic relative motion of grains of solid 4 He. What relation that evolving work may have to mass flow studies is not yet clear.…”