Low-temperature phase diagram of the heavy-fermion superconductor UPt3

The transition to the superconducting state of UPt₃ has been studied in specific-heat experiments on polycrystalline and single-crystalline samples in dependence of heat treatment, purity of starting materials and deviations from stoichiometry, as well as on substituted compounds. The concepts behind the splitting of the transition in two anomalies that are 60 mK apart are critically reviewed. The superconducting phase diagram in the B-T plane has been studied with the use of a sensitive dilatometric technique. The three superconducting phases meet at a tetracritical point and all four phase lines are of second order. From the length anomalies at the phase boundaries the pressure derivatives of the phase lines have been determined and the most stable phase under uniaxial pressure along the c-axis has been identified.