Thermal conductivity of superfluid ${}^3$He-B in a tubular channel down to 0.1$T_c$ at the ${}^4$He crystallization pressure

Abstract

We studied the thermal conductivity of superfluid ${}^3$He in a 2.5 mm effective diameter and 0.15 m long channel connecting the two volumes of our experimental assembly. The main volume contained pure solid ${}^4$He, pure liquid ${}^3$He and saturated liquid ${}^3$He-${}^4$He mixture at varying proportions, while the separate heat-exchanger volume housed sinter and was filled by liquid ${}^3$He. The system was cooled externally by a copper nuclear demagnetization stage, and, as an option, internally by the adiabatic melting of solid ${}^4$He in the main volume. The counterflow effect of superfluid just below the transition temperature $T_c$ resulted in the highest observed conductivity about five times larger than that of the normal fluid at the $T_c$. Once the hydrodynamic contribution had practically vanished below $0.5T_c$, we first observed almost constant conductivity nearly equal to the normal fluid value at the $T_c$. Finally, below about $0.3T_c$, the conductivity rapidly falls off towards lower temperatures.

11 pages, 4 figures, 1 table