We have made determinations of the thermal resistivity $1/\lambda (Q,t)$ ( $t\equiv 1-T/T_{\lambda }$) of a layer of bulk ${}^4\mathrm{He}$ near the superfluid transition temperature $T_{\lambda }$ in heat currents $0.05\lesssim Q\lesssim 300\mu \mathrm{W}/{\mathrm{cm}}^2$. The temperature $T_b$ at the bottom of a layer was measured as a function of the temperature $T_t$ at the top and compared with integrations of models for $\lambda (Q,t)$. For $Q\gtrsim 0.6\mu \mathrm{W}/{\mathrm{cm}}^2$, the data reveal a new dissipative region below $T_{\lambda }\left(Q\right)$ with a small but finite resistivity and a sharp onset which cannot be explained by the usual “mutual friction” due to vortices.

• Received 29 November 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.1300