Ultrasound attenuation in Sr(2)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 Sr(2)RuO(4). The power law behavior of alpha at temperatures down to T(c)/30 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.