Spin singlet pairing in the superconducting state of Nax Co O2 •1.3 H2 O: Evidence from a Co59 Knight shift in a single crystal

We report a Co59 Knight-shift measurement in a single crystal of the cobalt oxide superconductor Nax Co O2 •1.3 H2 O (Tc =4.25 K). We find that the shift due to the spin susceptibility, Ks, is substantially large and anisotropic, with the spin shift along the a -axis Kas being two times that along the c -axis Kcs. The shift decreases with decreasing temperature (T) down to T≈100 K, then becomes a constant until superconductivity sets in. Both Kas and Kcs decrease below Tc. Our results indicate unambiguously that the electron pairing in the superconducting state is in the spin singlet form.