Spin resonance peak in NaₓCoO₂· yH₂O superconductors: A probe of the pairing symmetry

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Motivated by the recent discovery of superconductivity in layered NaₓCoO₂· yH₂O compound, we investigate theoretically the spin dynamics in the superconducting state with three possible pₓ+ipy-, d+id'-, and f-wave pairing symmetries in two-dimensional triangular lattices. We find that a spin resonance peak, which is elaborated to have a close relevance to the relative phase of the gap function and the geometry of the Fermi surface, appears in both in-plane and out-of-plane components of the spin susceptibility χ for the spin-singlet d+id'-wave pairing, while only in the out-of-plane(in-plane) component of χ for the spin-triplet pₓ+ipy-wave (f-wave) pairing. Because the pairing symmetry in this compound is still hotly debated at present, this distinct feature may be used to probe or determine it unambiguously with future neutron scattering experiments.