Ultrasonic attenuation measurements of the flux lattice phase transition in the heavy fermion superconductor UPt3

Attenuation measurements of longitudinal ultrasound in superconducting UPt₃ show a peak, below H_c2 , that depends strongly on the orientation of the field relative to the c-axis. This peak is seen only with longitudinal and not with transverse sound. It is show that, as the field is tipped away from the c axis into the basal plane. H_FL decreases from 1.3 T to 0.58 T (at the lowest temperature). The temperature dependence of H_FL for all orientations of the field is the same-a flat region, followed by a slow decrease in H_FL with increasing temperature