Attenuation measurements of longitudinal ultrasound in superconducting UPt3 show a peak, below Hc2, 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 shown that, as the field is tipped away from the c axis into the basal plane, HFL decreases from 1.3 T to 0.58 T (at the lowest temperature). The temperature dependence of HFL for all orientations of the field is the same--a flat region, followed by a slow decrease in HFL with increasing temperature.
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