Charge ordering, commensurability, and metallicity in the phase diagram of the layered Na_xCoO₂

The phase diagram of the layered Na_xCoO₂ was determined by charge ordering, commensurability and metallicity. The phase diagram revealed a characteristic feature of Na_xCoCO₂ called the strong influence of the Na ions on the electronic properties. The superconducting state produced by intercalation with H₂O evolved from the paramagnetic metal, which was separated from the Curie-Weiss state by the charge-ordered insulator. The large carrier density is ten times larger than in the cuprates doped at 0.3. The larger effective mass may reduce the superfluid density to a value comparable to that observed in the overdoped limit in cuprates.