Electromagnetic Response of a $k_x\pm i{k}_y$ Superconductor: Effect of Order Parameter Collective Modes

Effects of order parameter collective modes on electromagnetic response are studied for a clean spin-triplet superconductor with $k_x\pm i{k}_y$ orbital symmetry, which has been proposed as a candidate pairing symmetry for Sr${}_2$RuO${}_4$. It is shown that the $k_x\pm i{k}_y$ superconductor has characteristic massive collective modes analogous to the clapping mode in the A-phase of superfluid ${}^3$He. We discuss the contribution from the collective modes to ultrasound attenuation and electromagnetic absorption. We show that in the electromagnetic absorption spectrum the clapping mode gives rise to a resonance peak well below the pair breaking frequency, while the ultrasound attenuation is hardly influenced by the collective excitations.