Spin resonance peak in Na${}_x$CoO${}_2\cdot y$H${}_2$O superconductors: A probe of the pairing symmetry

Abstract

Motivated by the recent discovery of superconductivity in layered Na${}_x$CoO${}_2\cdot y$H${}_2$O compound, we investigate theoretically the spin dynamics in the superconducting state with three possible $p_x+i{p}_y$-, $d+i{d}^{\prime }$-, 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 $\chi$ for the spin-singlet $d+i{d}^{\prime }$-wave pairing, while only in the out-of-plane(in-plane) component of $\chi$ for the spin-triplet $p_x+i{p}_y$-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.

4 pages, 4 figures