He-B and sintered metal*

Understanding the temperature dependence of the thermal boundary resistance, or Kapitza resistance, between liquid helium and sintered metal has posed a problem in low temperature physics for decades. In the ballistic regime of superfluid ³He−B we find the Kapitza resistance can be described via scattering of thermal quasiparticle excitations with a macroscopic geometric area rather than the sintered metal's microscopic area. We estimate that a quasiparticle needs on the order of 1000 collisions to successfully thermalize with the sinter. Finally, we find that the Kapitza resistance is approximately doubled with the addition of two monolayers of solid ⁴He on the sinter surface, which we attribute to an extra magnetic channel of heat transfer closing as the nonmagnetic solid ⁴He replaces the magnetic solid ₃He.