Superconducting Properties under Magnetic Field in Na_<0.35>CoO_2・1.3H_2O Single Crystal (Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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We report the in-plane resistivity and magnetic susceptibility of the layered cobalt oxide Na_<0.35>CoO_2・1.3H_2O 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 ≃ 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.
社団法人日本物理学会の論文
2004-05-15