Anisotropy of the Magnetic-Field-Induced Phase Transition in Superconducting UPt3

Measurements of the attenuation of longitudinal ultrasound in superconducting UPt3, show a peak,below Hc2, which depends strongly on the orientation of the field relative to the c axis. We propose an explanation of this peak in terms of a vortex phase transition in an unconventional superconducting state, which is consistent with recent neutron-scattering data.