Collective Order Parameter Modes and Spin Fluctuations for Spin-Triplet Superconducting State inSr2RuO4

We calculate the collective order parameter modes for possible spin-triplet $p$-wave pairing states in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. The modes are classified in terms of pairing states corresponding to the irreducible representations of $D_{4h}$ for the layered perovskite structure. Besides the phase and amplitude modes, we obtain spin modes which couple to external fields parallel to the basal plane. Observation of the frequencies of the modes can yield information about the nature of the pairing state. We derive also the strong-coupling theory for spin-triplet pairing mediated by exchange of spin fluctuations and make comparison with the corresponding fluctuation exchange approximation for spin-singlet pairing.