Overview

Liquid 4He enters the superfluid state and flows without friction below 2.176 K at zero pressure limit. 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. This phenomenon was first observed in 2004 with torsional oscillator technique that detected the apparent presence of non-classical rotational inertia (NCRI), which can be associated with a new state of matter called a supersolid. There are many theoretical works carried out on this subject, but none of the present models can explain all of the experimental features. More experimental work is needed to understand the microscopic picture of NCRI. If the apparent NCRI stems from a genuine transition between the normal solid and supersolid phases of 4He, it is interesting to consider whether there should be a heat capacity anomaly accompanying this phase change. This question motivated us to carry out high resolution measurements of the specific heat in solid helium. To settle this issue we were required to address the primary problem limiting the sensitivity of previous heat capacity studies, i.e., a large addendum heat capacity of the calorimeter itself. We overcame this obstacle by constructing the calorimeter of silicon. In our study we found a broad peak in specific heat of solid 4He on top of the Debye T3 term. The excess specific heat peaks at a temperature near the onset of NCRI, indicating the likelihood that we have observed the thermodynamic signature related to supersolidity. Solid 3He-4He mixtures were also studied. The specific heat peak was found to be independent of the 3He impurity concentration. With sufficient amounts of 3He, phase separation was observed below 150 mK, together with the specific heat peak, proving that the peak itself is not due to phase separation.

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