New dissipative region below the superfluid transition of 4He in a heat current.

Abstract

We have made determinations of the thermal resistivity $1/\ensuremath{\lambda}(Q,t)$ ( $t\ensuremath{\equiv}1\ensuremath{-}T/{T}_{\ensuremath{\lambda}}$) of a layer of bulk ${}^{4}\mathrm{He}$ near the superfluid transition temperature ${T}_{\ensuremath{\lambda}}$ in heat currents $0.05\ensuremath{\lesssim}Q\ensuremath{\lesssim}300\ensuremath{\mu}\mathrm{W}/{\mathrm{cm}}^{2}$. 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 $\ensuremath{\lambda}(Q,t)$. For $Q\ensuremath{\gtrsim}0.6\ensuremath{\mu}\mathrm{W}/{\mathrm{cm}}^{2}$, the data reveal a new dissipative region below ${T}_{\ensuremath{\lambda}}(Q)$ with a small but finite resistivity and a sharp onset which cannot be explained by the usual ``mutual friction'' due to vortices.