Unconventional upper- and lower-critical fields and normal-state magnetic susceptibility of the superconducting compound Na 0.35 CoO 2 ⋅ 1.3 H 2 O

Magnetic properties of the layered superconductor, Na 0 . 3 5 CoO 2 . 1.3H 2 O have been investigated. From the temperature dependence and field dependence of the magnetization, the superconducting transition temperature, aswell as upper- and lower-critical fields have been estimated to be T C =4.6 K, H C 2 (0)=61.0 T and H C 1 (0)=28.1 Oe, respectively. These values give quite unusual phenomenological parameters, i.e., coherent length, penetration depth and Ginzburg-Landau parameter of Β=2.32 nm, λ=5.68×10 2 nm, and κ≡λ/Β =244, respectively, suggesting an unconventional nature of superconductivity. Additionally, the magnetic susceptibility above T C and the magnetization curve up to 54 T are also reported.