Superconducting Properties under Magnetic Field in Na0.35CoO2$\cdot$1.3H2O Single Crystal

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We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na0.35CoO2$\cdot$1.3H2O single crystal. The temperature dependence of the resistivity shows metallic behavior from room temperature to the superconducting transition temperature $T_{\text{c}}$ of 4.5 K. A sharp resistive transition, zero resistivity and almost perfect superconducting volume fraction below $T_{\text{c}}$ indicate the good quality and bulk superconductivity of the single crystal. The upper critical field $H_{\text{c2}}$ 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 $\xi$, $\xi_{ab}/\xi_{\text{c}} = 12\,\text{nm}/1.3\,\text{nm} \simeq 9.2$, suggests that this material is an anisotropic three-dimensional superconductor. In the field parallel to the $ab$-plane, $H_{\text{c2}}$ seems to be suppressed to the Pauli paramagnetic limit. This suggests spin singlet superconductivity in the cobalt oxide.
2004-05-15