Spin-resonance peak in Na xCoO 2·yH 2O superconductors: A probe of the pairing symmetry

Abstract

Spin dynamics in the superconducting state of the newly discovered Na xCoO 2·yH 2O superconductor with three possible pairing symmetries (p x+ip y, d+id′, and f wave) is studied theoretically on a two-dimensional triangular lattice. We find that a spin resonance peak, which is found to have a close relevance to the relative phase of the gap function and the geometry of the Fermi surface, appears in both the in-plane and the out-of-plane components of the spin susceptibility for the spin-singlet (d+id′)-wave pairing, while only in the out-of-plane (in-plane) component for the spin-triplet (p x+ip y)-wave (f-wave) pairing. We also indicate that there is no spin resonance for an s-wave pairing. These distinct features may be used to probe or determine the pairing symmetry in this compound unambiguously.