Spin singlet pairing in the superconducting state of NaxCoO2•1.3H2O:: Evidence from a 59Co Knight shift in a single crystal

We report a Co-59 Knight-shift measurement in a single crystal of the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O (T-c=4.25 K). We find that the shift due to the spin susceptibility, K-s, is substantially large and anisotropic, with the spin shift along the a-axis K-a(s) being two times that along the c-axis K-c(s). The shift decreases with decreasing temperature (T) down to T approximate to 100 K, then becomes a constant until superconductivity sets in. Both K-a(s) and K-c(s) decrease below T-c. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.