Letter | Published:

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

Nature volume 450, pages 853856 (06 December 2007) | Download Citation

Abstract

Superfluidity—liquid flow without friction—is familiar in helium. The first evidence for ‘supersolidity’, its analogue in quantum solids, came from torsional oscillator measurements1,2 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 groups3,4,5,6,7, but solid 4He does not respond to pressure differences8, and persistent currents and other signatures of superflow have not been seen. Both experiments and theory9,10,11,12,13,14 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.

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Acknowledgements

We thank the Natural Sciences and Engineering Research Council of Canada and the University of Alberta for support of this research; we also thank M. H. W. Chan for providing the torsional oscillator data of Figs 1 and 4.

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

    • James Day
    •  & John Beamish

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Correspondence to John Beamish.

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    Supplementary Information

    The file contains Supplementary Figure 1 with Legend, Supplementary Methods, Supplementary Discussion and additional references.  The Supplementary Figure 1 displays a schematic of the experimental apparatus. Also a full description of how measurements were made and the resolution of the experiment are given and the file includes an extended discussion on dislocation motion in helium crystals.

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https://doi.org/10.1038/nature06383

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