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We report on thermal transport experiments in liquid 4 He near T using heat fluxes 8рQр55 W/cm 2 . We have confirmed the presence of a region near the superfluid transition, reported by Liu and Ahlers ͓Phys. Rev. Lett 76, 1300 ͑1996͔͒, in which thermal dissipation is anomalously small. The temperature transients for reaching a steady state upon entering this region from the superfluid side or cooling back into the superfluid have been studied, and are found to be quite different from each other; one possible explanation for this behavior is proposed which implies that the region of anomalous dissipation has a low thermal diffusivity. We discuss the location of this region in the phase diagram of liquid 4 He.

We report new measurements of the effective thermal comhtetivity K,,t~ and rehtxation t#ne r in dilute mixtures of ~He in superjluid 4He, with mohu" concentrations X <~ 10 s. The temperature range extended fi'om T ~ 1.4 K to T;. Both ~",:t/ and r are found to agree with theoretical predictions, in contrast to previous experhnents where significant diJferences were observed. A new thermal comhtctivi O, cell design was used which ahnost comph, tely eliminates extraneous vohmws and surJ;tces, and the earlier results are e,\plahled h7 relation to these design changes.

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