a Study of Helium-Three Kapitza Resistance at Millikelvin Temperatures

Experimental measurements of ^3 He Kapitza resistance below 100 mK disagree with each other and with theoretical predictions. An understanding of ^3He Kapitza resistance at low temperatures is important for cryogenic technology and may lead to better theories of the interactions of fermions at surfaces. The Kapitza resistance between liquid ^3He and silver films was measured over two orders of magnitude in temperature using the glass capacitance method developed by Hu, Gramila, and Richardson. The method and an important improvement in the method's thermometry are discussed. The measurements were repeated with a roughly 6 monolayer thick coating of ^4He on the silver. The ^3He Kapitza resistance was also measured at a pressure of 6 bar for temperatures from 45 mK to 230 mK. The experiments confirmed the validity of the glass capacitance method of measuring Kapitza resistance. The measured Kapitza resistance data were in excellent agreement with the modified ^3He acoustic mismatch theory of Bekarevich, Khalatnikov, Fomin, and others for temperatures greater than 50 mK. At lower temperatures the Kapitza resistance was found to be proportional to T^{-1/2}. The 6 bar data also agreed with the theory. The presence of the ^4He layer extended the region where the acoustic theory is valid to lower temperatures. It also increased the Kapitza resistance by as much as an order of magnitude near 3 mK. The increase in Kapitza resistance indicates that the ^4He layer suppressed the dominant channel of heat flow across the interface. Since sound or sound -like channels are not affected by such a thin layer, it is likely that the dominant channel of heat flow at these temperatures is magnetic.