ANGULAR-DEPENDENCE OF THE UPPER CRITICAL-FIELD OF THE HEAVY-FERMION SUPERCONDUCTOR UPT3

The angular dependence of the upper critical field H-c2 of UPt3 in magnetic fields up to 0.85 T has been measured. In spark cut single crystals a sixfold modulation of the resistivity is observed at the superconducting transition in the basal plane. This modulation vanishes as the temperature approaches T* [temperature of the discontinuity in slope of the H-c2 (T)] and reappears for T < T* but with opposite sign. The results provide important new evidence on the nature of the change in the superconducting order parameter at T* and its possible coupling with the antiferromagnetism.