High-field ultrasound measurements in UPt3 and the single-energy-scale model of metamagnetism

We report longitudinal ultrasound velocity measurements for magnetic fields up to 33 T applied parallel to the a axis of the heavy electron compound UPt3. A characteristic dip in the sound velocity at the metamagnetic critical field H-c = 20 T, reported in earlier work, is reproduced and shown to be independent of temperature at very low temperatures. We show that the single energy scale model [Shivaram et al., Phys. Rev. B 89, 241107(R) (2014)] captures the observed key features of the field dependence in the sound velocity shift delta nu(s). The shift delta nu(s) at H-c is found to be inversely dependent on temperature above similar to 3 K and assumes a fixed value at low T. This "saturation" in delta nu(s) below similar to 3 K is accounted for by level broadening of the uranium spin states.