THERMAL-BOUNDARY CONDUCTANCE BETWEEN THE U2D2 SOLID AND B-SUPERFLUID PHASES OF HE-3

We present the first measurement of the thermal boundary conductance, 1/R(B), between the U2D2 spin ordered solid and the B superfluid phases of He-3. We find that 1/R(B) is exceedingly high, roughly seven orders of magnitude greater than that between sintered silver powder and liquid He-3. For T < 0.56 mK, 1/R(B) appeared to be thermally activated, i.e. 1/R(B) approximately exp(-epsilon/k(B)T), with epsilon = 4.5(+/- 0.5) mK, close to the superfluid gap energy. We also present a theory involving magnon-quasiparticle coupling as a model to explain the experimental observations.