Effect of the boundary condition on the Kapitza resistance between superfluid He3−B and sintered metal

Understanding the temperature dependence of 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 ${}^3\mathrm{He}\text{−}B$, we find the Kapitza resistance can be described via scattering of thermal excitations (quasiparticles) 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 ${}^4\mathrm{He}$ on the sinter surface, which we attribute to an extra magnetic channel of heat transfer being closed as the nonmagnetic solid ${}^4\mathrm{He}$ replaces the magnetic solid ${}^3\mathrm{He}$.