λ-Shaped Ultrasound-Attenuation Peak in Superconducting (U,Th)Be13

Abstract

We report measurements of the ultrasound attenuation and the changes in the sound velocity of the heavy-fermion superconductor (U,Th)${\mathrm{Be}}_{13}$. This material has shown evidence of two transitions, a bulk superconducting transition at $T_{c1\mathrm{}}$ and a lower one at $T_{c2\mathrm{}}$. Near $T_{c2\mathrm{}}$ we find a $\lambda$-shaped attenuation peak two orders of magnitude larger than the total contribution from particle-hole scattering, and a dip in the $c_{11}$ elastic constant. These results are consistent with those expected near a magnetic transition. A state of coexisting anisotropic superconductivity and (antiferro-) magnetic order is suggested below $T_{c2\mathrm{}}$.