Acoustic anomalies in UPt₃ at high magnetic fields and low temperatures

Ultrasound velocity and attenuation measurements were performed on single crystals of the heavy fermion compound UPt₃ in magnetic fields up to 33 T and at temperatures ranging from 2.4 K to below 0.1 K. With longitudinal sound propagated in the crystallographic basal plane, parallel to the applied field, the familiar elastic softening is observed at the metamagnetic transition field H∼20.2T. More complicated structure emerges at low temperatures, including quantum acoustic oscillations and a second velocity minimum at ∼21.6 T. A weak frequency dependence (dispersion) is observed in the velocity. The ultrasonic data are analyzed using the Landau-Khalatnikov formalism, from which temperature- and field-dependent relaxation times are deduced.