Superconductivity in a layered cobalt oxyhydrate Na0.31CoO2•1.3H2O

We report the electrical, magnetic and thermal measurements on a layered cobalt oxyhydrate Na(0.31)CoO(2)(.)1.3H(2)O. Bulk superconductivity at 4.3 K has been confirmed; however, the measured superconducting fraction is relatively low probably due to the sample's intrinsic two-dimensional characteristic. The compound exhibits weak-coupled and extreme type-II superconductivity with an average energy gap Delta(a)(0) and a Ginzburg-Landau parameter kappa of similar to0.50 meV and 140, respectively. The temperature dependence of the normalized electronic specific heat in the superconducting state gives a clue to the superconducting gap structure.