Superconducting Properties under Magnetic Field in Na_0.35CoO₂·1.3H₂O Single Crystal

Abstract

We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na_0.35CoO₂·1.3H₂O 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 ξ are obtained from the resistive transitions in the magnetic fields parallel to the c-axis and ab-plane. The anisotropy of ξ, ξ_ab⁄ξ_c=12nm⁄1.3nm\\simeq9.2, 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.