Thermal Boundary Resistance between Solids and Helium below 1°K

Results are reported from measurements of the thermal boundary resistance between liquid He³ and copper or epoxy resin, and between solid He³ or liquid He⁴ and copper. Measurements were made in the range 0.05 to 0.9^∘{}K and at pressures up to 395 psi. No quantitative agreement has been found with theory. Above 0.11^∘{}K, the thermal boundary resistance of liquid He³ and liquid He⁴ both have a temperature dependence stronger than T^-3. Below 0.11^∘{}K the thermal boundary resistance of He³ varies as T^-3 over the full pressure range in the liquid. In the case of He⁴ at low pressure, however, the stronger temperature dependence continues to the lowest temperature measured (0.075^∘{}K). It is not clear whether the qualitative difference in the behavior of liquid He³ and liquid He⁴ is associated with zero sound in liquid He³.