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

We study the dynamical spin susceptibility, $\chi (\mathbf{q},\omega )$, in the normal and superconducting state of Sr${}_2$RuO${}_4$. In the normal state, we find a peak in the vicinity of ${\mathbf{Q}}_i\simeq (0.72\pi ,0.72\pi )$ in agreement with recent inelastic neutron scattering (INS) experiments. We predict that for spin triplet pairing in the superconducting state a {\it 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.