We propose an explanation of the large sound-attenuation peak observed at the lower transition, $T_{c2\mathrm{}}$, in superconducting ${\mathrm{U}}_{1-x}{\mathrm{Th}}_x{\mathrm{Be}}_{13}$. It assumes that the transition is between two different anisotropic superconducting phases, and that the low-temperature phase is tetragonally distorted. The domain-wall energy is very small at $T_{c2\mathrm{}}$ and the attenuation due to the motion of walls is very large. This attenuation depends strongly on direction and polarization, and so the model may be easily tested. The implications for the phase diagram of these alloys are discussed.

• Received 29 October 1985

DOI:https://doi.org/10.1103/PhysRevLett.56.1412