THERMODYNAMIC STABILITY OF THE SUPERCONDUCTING PHASE-DIAGRAM OF UPT3

The superconducting phase diagram of UPt3, as determined by accurate d ilatometry as function of temperature and magnetic field (B parallel-t o c and B perpendicular-to c), exhibits three superconducting phases w hich meet in a tetracritical point. We show that the measured phase di agrams are thermodynamically stable and that all four phase lines are of second order. Via Ehrenfest relations the pressure dependence of th e various phases has been determined The so-called C phase is the most stable phase under uniaxial pressure along the c-axis (p(c) > 2.5 kba r). The resulting phase diagram in the p(c)-T plane yields stringent c onstraints on the fourth order coupling constants in the Ginzburg-Land au scenario of a vector superconducting order parameter coupled to a s ymmetry breaking field.