Evidence for d-wave pairing in the cobalt oxide superconductor NaxCoO2 1.3H2O
We report the spin susceptibility in the superconducting state of the cobaltoxidesuperconductorNaxCoO2·1.3H2O, via the 59Co Knight shift measurements. Along both a- and c-axis directions, the shift decreases below Tc. At , the shift along the a-axis, Ka, increases with increasing the strength of magnetic field (H), following a variation. Together with the T3 dependence of the spin-lattice relaxation rate, 1/T1, these results indicate unambiguously that the Cooper pairs are in the spin-singlet form and that there exist nodes in the gap function. The data are most consistent with a d-wave symmetry.