HEAT-TRANSPORT IN A NUCLEAR ANTIFERROMAGNET

We report the first measurements of heat transport due to thermal magnons in mono-domain single crystals of U2D2 He-3. Between about 0.95 T(c) and 0.65 T(c) we find clear evidence for umklapp scattering with an activation energy of 5.8 +/- 0.3 mK, with a much weaker temperature dependence at lower T. Our limiting mean free paths varied from 0.3-mu-m to 3-mu-m depending upon crystal orientation. We have also measured the thermal resistance between the U2D2 solid and the bulk B superfluid phase between 0.35 mK and 0.6 mK, where we find an activated conductivity with an energy of 4.5 +/- 0.3 mK. The heat path involves both the solid-liquid boundary resistance and the resistance of superfluid in a 1 mm diameter tube.