Revised superconducting phase diagram of hole doped Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O

We have studied the superconducting phase diagram of Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O\space as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state.[1] Our findings are consistent with a recent report that intercalation of H$_{3}$O$^{+}$\space ions into Na$_{x}$CoO$_{2}$, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal T$_{c}$\space is achieved through a Co valence range of 3.24 - 3.35, while T$_{c}$\space decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO$_{2}$ sheets as Co valence is varied into the optimal T$_{c}$\space region is further evidence for this criticality.[1] C.J. Milne $et al.$, $Phys. Rev. Lett.$, in press (2004). Also cond-mat/0401273.