We present measurements of longitudinal ultrasonic velocity on single crystals of the heavy-fermion superconductor ${\mathrm{UPt}}_3$. The measurements show clear signatures of second-order phase transitions in the superconducting state, with the velocity anomalies well accounted for by Ginzburg-Landau theory. From these signatures we construct a phase diagram for ${\mathrm{UPt}}_3$ that reveals all the boundary lines that have been identified as possible phase transitions. We are able to track the phase transition lines to a tetracritical point, located on the upper-critical-field curve, to within the width of the normal-superconducting transition.

• Received 27 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2298