AC thermal conductivity measurements in dilute mixtures of He-3 in superfluid He-4

In Part III of a three-part study, report measurements of the thermal respo nse, Delta T(omega), of He-3-superfluid-He-4 mixtures to an ac heat flux, Q (t) = Q(0) e(i omega t). These data are for dilute concentrations, X, and t hey show the presence of three separate thermal resistances. One of these i s the bulk-fluid resistance predicted by Khalatnikov and associated,vith th e effective conductivity, kappa(eff) Results for this component of the resi stance are in quantitative agreement with the Khalatnikov predictions. With parallel work by Murphy, and Meyer, these experiments resolve a long-stand ing conflict between theory and experiment. One of the remaining resistance s is the ordinary boundary resistance R-b. The third resistance, R-0, is in dependent of the fluid layer height, d. This resistance is presumably the s ame as that seen in earlier dc measurements. Both the temperature and conce ntration dependences elf this anomalous resistance differ from that of eith er R-fluid or R-b. It has been ascribed recently by Murphy and Meyer to eff ects associated with the narrow gaps usually present in cryogenic thermal c onductivity experiments. We use an ad hoc model as a convenient way to para meterize the extra thermal resistance. The present studies have been carrie d out with an apparatus which permits us to vary d continuously and in situ from zero to 3 mm. This feature and the ac technique ave important for sep arating the various components of the thermal resistance. In two preceeding studies, we considered related aspects of the ac thermal response of liqui d helium. Part I addresses the response of normal liquid helium. Parr II, p rovides the theoretical backdrop for the present experimental study.