Transverse ultrasound revisited: A directional probe of the A phase of UPt3

We have measured the transverse ultrasonic attenuation alpha(q epsilon) in a well-characterized single crystal of UPt3 for propagation direction q in the basal plane and polarization epsilon in and out of the plane. Unlike previous measurements, the sample was of sufficient purity to exhibit three well-defined phases as a function of temperature and magnetic field. We uncover a functional dependence for the attenuation in the (zero-field) high-temperature (A) phase which is distinctly different than that in the low-temperature (B) phase. Our data provide new directional information on the quasiparticle momentum distribution in this largely unstudied phase of UPt3 and direct evidence that the two zero-field phases possess different order parameters.