SUPERCONDUCTING PHASE-DIAGRAM OF UPT3 STUDIED BY THERMAL-EXPANSION AND SPECIFIC-HEAT

The specific heat and thermal expansion of UPt3 were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonal c axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges for H parallel-to c is therefore qualitatively similar to that previously found for H perpendicular-to c, this time with a critical point (or region) around H* = 8 kOe, T* = 0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.