Liquid 4He enters the superfluid state and flows without friction below 2.176 K. Thin liquid films adsorbed on solid substrates undergo the same transformation, although at a lower temperature. When the substrate is subjected to oscillatory motion a portion of the film, known as the superfluid fraction, decouples from the oscillation. A similar phenomenon has been observed1,2 in solid 4He, in which a fraction of the solid seems to decouple from the motion of the surrounding lattice. Although this observation has been replicated in various laboratories3,4,5,6, no thermodynamic signature of the possible supersolid transition has been seen. Here we report the finding of a heat capacity peak that coincides with the onset of mass decoupling. This complementary experimental evidence supports the existence of a genuine transition between the normal solid and supersolid phases of 4He.
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We thank J. A. Lipa for the 1 p.p.b. purity helium, and J. Jain, J. S. Kurtz and N. Mulders for their advice. Funding was provided by the National Science Foundation.
Author Contributions X.L., A.C.C. and M.H.W.C. contributed equally to this work.
The authors declare no competing financial interests.
The file contains Supplementary Figure and Legend and Supplementary Discussion. The figure shows the temperature dependence of the specific heat of the four samples with the constant specific heat term of the 10 ppm and 30 ppm samples, deduced from Fig.3, subtracted. We further discuss: 1) Previous heat capacity measurements below 200 mK; 2) Accuracy and uncertainty in the 3He concentration of the samples studied; 3) High temperature deviation from the Debye specific heat; 4) Additional comments on the constant specific heat term found for the 10 ppm and 30 ppm samples; 5) Evidence against phase separation as the origin of the 75 mK peak. (PDF 153 kb)
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Lin, X., Clark, A. & Chan, M. Probable heat capacity signature of the supersolid transition. Nature 449, 1025–1028 (2007). https://doi.org/10.1038/nature06228
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