Revised superconducting phase diagram of hole doped   Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O
  We have studied the superconducting phase diagram of \NaH\space as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state. Our findings are consistent with a recent report that intercalation of \oxp\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 \Tc\space is achieved through a Co valence range of 3.24 - 3.35, while \Tc\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 \Tc\space region is further evidence for this criticality. 
C. J. Milne, D. N. Argyriou, A. Chemseddine, N. Aliouane, J. Veira, S.   Landsgesell and D. Alber