Unconventional Upper- and Lower-Critical Fields and Normal-State Magnetic Susceptibility of the Novel Superconducting Compound Na₀.₃₅ Co O₂ · 1.3 H₂O

Original · EN

Magnetic properties of the novel layered superconductor, Na₀.₃₅CoO₂·1.3H₂O have been investigated. From the temperature dependence and field dependence of the magnetization, the superconducting transition temperature, as well as upper- and lower-critical fields have been estimated to be TC=4.6 K, HC₂(0)=61.0 T and HC₁(0)=28.1 Oe. These values give quite unusual phenomenological parameters, i.e., coherent length, penetration depth and Ginzburg-Landau parameter of ξ=2.32 nm, λ=5.68× 10³ Å and κ≡λ/ξ=244, suggesting an unconventional nature of superconductivity. Normal-state magnetic susceptibility shows an upturn below 130 K, which is confirmed to be inherent by high-field magnetization data. The upturn may have relevance to the mechanism of the superconductivity.