Ultrasound Attenuation in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$: An Angle-Resolved Study of the Superconducting Gap Function

We present a study of the electronic ultrasound attenuation $\alpha$ in the unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. The power law behavior of $\alpha$ at temperatures down to $T_c/30\mathrm{}$ clearly indicates the presence of nodes in the gap. In the normal state, we find an enormous anisotropy of $\alpha$ in the basal plane of the tetragonal structure. In the superconducting state, the temperature dependence of $\alpha$ also exhibits significant anisotropy. We discuss these results in relation to possible gap functions.