TETRACRITICAL POINTS AND THE SUPERCONDUCTING PHASES OF UPT3 - UNIAXIAL PRESSURE EFFECTS

Using signatures in the longitudinal sound velocity we have mapped for the first time experimentally the pressure dependence of the phase bo undaries in superconducting UPt3 for magnetic field H parallel to the a axis. With increasing pressure P-c parallel to the c axis, the tetra critical point in the H-T plane is observed to shift to lower H and hi gher temperature T. At a critical stress, P-c similar to 2.6 kbar the tetracritical point reaches the H = O, P-c - T plane, and our measurem ents prove coexistence of at least four phases in this plane as well.