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∥c is therefore qualitatively similar to that previously found for H⊥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.