PHASE-DIAGRAM AND ANGULAR-DEPENDENCE OF THE UPPER CRITICAL-FIELD H-C2OF THE HEAVY-FERMION SUPERCONDUCTOR UPT3

The angular dependence of the upper critical field H-c2(Theta,Phi,T) o f UPt3 in magnetic fields up to 3T has been measured. We performed ang ular resolved magnetoresistance measurements to determine the change i n anisotropy of H-c2(Theta) between the hexagonal plane and the c-axis throughout the phase diagram. Furthermore, in single crystals with a rectangular cross-section, a sixfold modulation of the H-c2(Phi) is ob served at the superconducting transition in the basal plane, which can not be related to geometry dependent surface superconductivity effects . This modulation vanishes as the temperature approaches T (the tempe rature of the discontinuity in slope of H-c2(T)) and reappears for T<T but with opposite sign. The results provide new evidence on the unco nventional nature of the superconducting order parameter and its coupl ing with antiferromagnetism.