1. Department of Physics, University of Alberta, Edmonton, Alberta, T6G 2G7 Canada

Correspondence to: John Beamish¹ Correspondence and requests for materials should be addressed to J.B. (Email: beamish@phys.ualberta.ca).

Abstract

Superfluidity—liquid flow without friction—is familiar in helium. The first evidence for 'supersolidity', its analogue in quantum solids, came from torsional oscillator measurements^{1, 2} involving ⁴He. At temperatures below 200 mK, the torsional oscillator frequencies increased, suggesting that some of the solid decoupled from the oscillator. This behaviour has been replicated by several groups^{3, 4, 5, 6, 7}, but solid ⁴He does not respond to pressure differences⁸, and persistent currents and other signatures of superflow have not been seen. Both experiments and theory^{9, 10, 11, 12, 13, 14} indicate that defects are involved; these should also affect the solid's mechanical behaviour. Here we report a measurement of the shear modulus of solid ⁴He at low frequencies and strains. We observe large increases below 200 mK, with the same dependence on measurement amplitude, ³He impurity concentration and annealing as the decoupling seen in the torsional oscillator experiments. We explain this unusual elastic behaviour in terms of a dislocation network that is pinned by ³He at the lowest temperatures but becomes mobile above 100 mK. The frequency changes in the torsional oscillator experiments appear to be related to the motion of these dislocations, perhaps by disrupting a possible supersolid state.

1. Department of Physics, University of Alberta, Edmonton, Alberta, T6G 2G7 Canada

Correspondence to: John Beamish¹ Correspondence and requests for materials should be addressed to J.B. (Email: beamish@phys.ualberta.ca).

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