Superconducting Properties under Magnetic Field in Na₀.₃₅CoO₂·1.3H₂O Single Crystal

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We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na₀.₃₅CoO₂·1.3H₂O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature Tc of 4.5 K. Sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below Tc indicate the good quality and the bulk superconductivity of the single crystal. The upper critical field Hc₂ and the coherence length ξ are obtained from the resistive transitions in magnetic field parallel to the c-axis and the ab-plane. The anisotropy of ξ, ξab / ξc = 12 nm/1.3 nm ≃ 9.2, suggests that this material is considered to be an anisotropic three dimensional superconductor. In the field parallel to the ab-plane, Hc₂ seems to be suppressed to the value of Pauli paramagnetic limit. It may indicate the spin singlet superconductivity in the cobalt oxide.