Phase diagram of UPt3 from ultrasonic velocity measurements

We present measurements of longitudinal ultrasonic velocity on single crystals of the heavy-fermion superconductor UPt3. 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 UPt3 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.