Low-temperature shear modulus changes in solid He-4 and connection to supersolidity

[摘要] 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 He-4. 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(3-7), but solid He-4 does not respond to pressure differences(8), and persistent currents and other signatures of superflow have not been seen. Both experiments and theory(9-14) 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 He-4 at low frequencies and strains. We observe large increases below 200 mK, with the same dependence on measurement amplitude, He-3 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 He-3 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.

[关键词] HCP HE-4;SOUND-VELOCITY;BCC HE-3;DISLOCATIONS;IMPURITIES;CRYSTALS;HELIUM