Phase Diagram of Superconducting Na_xCoO₂·yH₂O

Abstract

We discovered that oxonium (H₃O⁺) ion content is a key parameter to the understanding of unique superconductivity in sodium cobalt oxyhydrate. As Na⁺ ions are substituted by H₃O⁺ ions, keeping the Co valence constant, the superconducting phase is transformed into a magnetically ordered phase, and then into a hitherto hidden superconducting phase. Furthermore, the superconducting phase previously discovered is suppressed by the magnetic phase under application of a magnetic field. These facts indicate that the superconductivity is induced by magnetic interactions which are substantially governed by the oxonium ion content.