Probable heat capacity signature of the supersolid transition

Abstract

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 observed 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 laboratories, 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.

A Glassy Contribution to the Heat Capacity of hcp 4He Solids

“…The advantage of an entropy analysis over that of the specific heat lies in the robustness and simplicity of counting states in an equilibrium phase compared to detailed model calculations for the specific heat. We find consistently for specific heat experiments [28,29,30] that the obtained values are 5 to 6 orders of magnitude smaller compared to the theoretical prediction for a supersolid if the entire sample underwent Bose-Einstein condensation (BEC). In the limit of a non-interacting BEC one finds…”

“…We compare our model calculations with experimental data by the Penn State group [30,29] for four different growth processes in ultrapure 4 He with at most 1 ppb of 3 He impurities. We re-analyzed their experiments and obtained quite different assignments for the state of two of their samples: We find two solidliquid coexistence samples with solid ratios 0.34 (SL34) and 0.31 (SL31) [46], while we agree with the assignments for the samples grown with the blocked capillary method (BC).…”

“…The search for such signatures proved to be challenging by the experiments conducted so far, including measurements of the specific heat [26,27,28,29,30,31], pressure dependence of the melting curve [32,33], and pressure-temperature measurements of the solid [34,35]. The main difficulties lie in measuring small signals at low temperatures in the presence of large backgrounds.…”

A Microscopic Study of One-Dimensional Solid4He Using the Static Fluctuation Approximation

“…The specic heat capacity of three-dimensional solid 4 He was measured as a function of T below 200 mK [4,6]. A peak was found.…”

“…A peak in the specic heat has been reported below 200 mK, using a sample cell made of silicon, whose heat capacity is more than ten times lower than that of solid 4 He in the temperature range considered [4,5].…”