THERMAL-CONDUCTIVITY AND GAP STRUCTURE OF THE SUPERCONDUCTING PHASES OF UPT3

We present new measurements of the thermal conductivity (K) of UPt3 do wn to very low temperatures (16 mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt3. The measuremen ts in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don't show the expected lo w field square root dependence, kappa proportional to root B. The disc ontinuity of d kappa/dT at T-c changes drastically when passing from t he high field low temperature C phase to the low field high temperatur e A phase: this is related to the change of the symmetry of the superc onducting order parameter when crossing the A --> C phase transition.