Resonance Peak in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$: Signature of Spin Triplet Pairing

We study the dynamical spin susceptibility, $\chi (\mathbf{q},\omega )$, in the normal and superconducting states of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. In the normal state, we find a peak in the vicinity of ${\mathbf{Q}}_i\simeq (0.72\pi ,0.72\mathrm{}\pi )$ in agreement with recent inelastic neutron scattering experiments. We predict that for spin triplet pairing in the superconducting state a resonance peak appears in the out-of-plane component of $\chi$, but is absent in the in-plane component. In contrast, no resonance peak is expected for spin singlet pairing.