THERMAL-CONDUCTIVITY OF HE-4 IN AEROGEL NEAR THE LAMBDA-TRANSITION

We measured the thermal conductivity of He-4 at vapor pressure confine d in base-catalyzed aerogel of density 0.14 g/cm3. As the transition r egion is approached from above, the conductivity increases but remains smaller than that of bulk helium. Below T(lambda) the conductivity is infinite, but for sufficiently large heat current it becomes finite d ue to mutual friction effects. From the power dependence of the conduc tivity below T(lambda) we estimate the effective cross-sectional area of the superfluid path through the sample.