Superconducting properties under magnetic field in Na0.35CoO2•1.3H2O single crystal

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na(0.35)CoO(2)(.)1.3H(2)O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature T-c of 4.5 K. A sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below T-c indicate the good quality and bulk superconductivity of the single crystal. The upper critical field H-c2 and coherence length xi are obtained from the resistive transitions in the magnetic fields parallel to the c-axis and ab-plane. The anisotropy of xi, xi(ab)/xi(c) = 12 nm / 1.3 nm similar or equal to 9.2, suggests that this material is an anisotropic three-dimensional superconductor. In the field parallel to the ab-plane, H-c2 seems to be suppressed to the Pauli paramagnetic limit. This suggests spin singlet superconductivity in the cobalt oxide.