Measuring Kapitza resistance versus interfacial properties at sub-Kelvin temperatures*

For many materials, thermal conductivity at ultra-low temperatures depends almost entirely on transport of phonons. Such phonon transport is generally well-understood in bulk materials but is much less well-explored at material interfaces. Interfaces are commonly the largest limiter of heat transport (especially between dissimilar materials) due to the large amount of scattering that occurs at the interface. At the ultra-low temperatures in a dilution refrigerator, dominant wavelengths can be much longer than interfacial features and it becomes increasingly difficult to understand how finite size effects affect interfacial phonon transport. An experimental apparatus has been constructed to measure heat flow across interfaces at mK temperatures. These experiments will be validated against both atomistic models and macroscopic finite element analysis models and will be used to quantify Kapitza resistance as a function of interfacial properties at sub-Kelvin temperatures. First results using the experimental apparatus will be presented.