The behavior of the heavy-fermion superconductor UPt3 under hydrostatic pressure and in a magnetic field

The (B, T, p) phase diagram for the unconventional superconductor UPt3 is determined from measurements of the specific heat and the magnetocaloric effect. In zero field above the critical pressure p*∼3.5kbar where the two transitions have merged, no re-splitting of the superconducting transition is observed. However, in a magnetic field (B perpendicular to c) there are two superconducting phases: the low-field B phase and the high-field C phase. The results strongly support the concept of a symmetry-breaking field (SBF) due to the weak antiferromagnetism which in zero field and below p* lifts the degeneracy of the two-dimensional order parameter and vanishes at the same critical pressure p* where the two transitions have merged. Above p* the degeneracy of the order parameter is apparently lifted by the magnetic field. ((orig.))