Horizontal Line Nodes in Sr2RuO4 Proved by Spin Resonance

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr2RuO4 by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of h omega(res) = 0.56meV centered at a characteristic wavevector Q(res) = o0:3; 0:3; 0:5THORN. The resonance energy corresponds well to the superconducting gap 2 Delta = 0.56meV estimated by the tunneling spectroscopy. The spin resonance shows the L modulation with a maximum at around L = 0.5. The L modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional alpha and beta sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr2RuO4. We also discuss the implications on possible superconducting order parameters.