Extending the Superconducting Phase Diagram of Na_xCoO_2Yh_2O

We have studied the superconducting and structural phase diagram of Na_xCoO_2ot yH_2O as a function of electronic doping x. We find a much wider superconducting region (0.37 < x < 0.28) as a function of x than previously reported [1]. In this region the superconducting transition temperature appears to be weakly coupled to the electronic doping x. Although a band filling behavior analogous to that in the high T_ C cuprates may also play a role in these novel cobalt hydrates, we suggest that it is overshadowed by the strong dependency of T_ C to the inter-planar distance between CoO₂ sheets. [1] Shaak et al. al. Nature 424, 527 (2003).