A ug 2 00 1 Ultrasonic attenuation in magnetic fields for spin - triplet superconducting states with line nodes in Sr 2 RuO

We calculate the ultrasonic attenuation in magnetic fields for spin-triplet superconducting states with line nodes vertical or horizontal relative to the RuO 2 planes. For rotating in-plane field H(θ) the attenuation α(θ) exhibits variations of fourfold symmetry in the rotation angle θ. In the case of vertical nodes, the transverse T100 sound mode yields the weakest (linear) H and T dependence of α , while the longitudinal L100 mode yields a stronger (quadratic) H and T dependence. This is in strong contrast to the case of horizontal line nodes where α is the same for the T100 and L100 modes (apart from a shift of π/4 in field direction) and is roughly a quadratic function of H and T. Thus we conclude that measurements of α in in-plane magnetic fields for different in-plane sound modes may be an important tool for probing the nodal structure of the gap in Sr 2 RuO 4 .