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

## TL;DR

This study measures the thermal conductivity of superfluid $^3$He in a tubular channel down to 0.1$T_c$, revealing how it varies with temperature and the effects of superfluid counterflow and hydrodynamics.

## Contribution

It provides detailed experimental data on superfluid $^3$He thermal conductivity at very low temperatures, including the influence of superfluid counterflow and the transition to normal fluid behavior.

## Key findings

- Maximum conductivity about five times larger than normal fluid at $T_c$
- Conductivity remains nearly constant below 0.5$T_c$
- Rapid falloff of conductivity below 0.3$T_c$

## 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.