NONLINEAR HEAT-TRANSPORT NEAR THE LAMBDA-TRANSITION IN HE-4

The effect of a finite stationary heat current is investigated in the spatially inhomogeneous situation where the heat current induces an interface between normal-liquid and superfluid He-4. The nonlinear temperature profile in the vicinity of the interface and the local thermal conductivity are calculated for T greater-than-or-similar-to T(lambda) within model F up to one-loop order. The field-theoretic renormalization-group approach is employed to describe the critical behavior both in the linear and nonlinear response regime. The finite heat current Q causes a finite temperature gradient at T(lambda) and therefore suppresses the critical divergence of the thermal conductivity. Quantitative predictions are made on the nonlinear Q dependence of the temperature profile and of the thermal conductivity which should be experimentally observable.