Heat capacity of solid He 4 and He 3 - He 4 mixture grown in aerogel

We report heat capacity measurements of solid He4 and He3-He4 solid mixture samples grown in silica aerogel. In addition to the Debye T3 term, the heat capacity of solid He4 samples includes T linear and T2 terms which are attributed to the amorphous and two-dimensional helium layers confined near the silica strands of aerogel. Beside these regular polynomial terms, a broad heat capacity peak is found between 0.1 and 0.35 K. The peak probably has its origin in the vibration of the dislocation lines pinned by the silica strands. Compared to He4 samples, the heat capacity of solid He3-He4 mixture samples shows evidence of phase separation below a characteristic temperature Tc that increases with He3 concentration (X3). Below this temperature the heat capacity shows X31/2 dependence suggesting that the He3 atoms are phase separated from being uniformly distributed in the pore space into two-dimensional patches enfolding the silica strands that are themselves coated with a strongly bound amorphous He4 layer.