Measurements of the Kapitza resistance and Onsager cross-coefficient for the4He crystal-superfluid interface

The effect of a heat current across the atomically rough crystal-superfluid interface in⁴He has been studied below 1 K. The chemical potential difference Δμ and temperature difference ΔT were measured simultaneously, the ΔT by means of a “superfluid melting-curve thermometer.” The results give the mean phonon transmission coefficient across the interface \(\bar \tau \) and a dimensionless quantity β that determines the Onsager coefficient linking Δμ with the heat current. The transmission probability \(\bar \tau \) is proportional toT ² below 0.2 K, in agreement with other experiments and with various theories, and it depends slightly on the crystal orientation, increasing near thec axis. The coefficient β, which is roughly independent of temperature and orientation, agrees with a recent theory of Bowley and Edwards.