Adiabatic Melting of 4He Crystal in Superfluid 3He at Sub-millikelvin Temperatures

Adiabatic melting of 4He crystal to phase separated 3He–4He solution (at T< 2 mK) is probably the most promising method to cool the dilute phase down to temperatures substantially below 0.1 mK. When started well below the superfluid transition temperature Tc of pure 3He, this process allows, in principle, to get the final temperature (Tf) several orders of magnitude less than the initial one (Ti). This work is the first practical implementation of the method below the Tc of 3He. The observed cooling factor was Ti/Tf=1.4 at 0.9 mK, being mainly limited by the bad performance of the superleak filling line, by incomplete solidification of 4He in the cell, and by the improper thermal contact between the cell wall and the liquid.