Determination Of The Co Valence In Bilayer Hydrated Superconducting Naxcoo2 Center Dot Yh(2)O By Soft X-Ray Absorption Spectroscopy

We addressed the so-far unresolved issue concerning the Co valence in the superconducting bilayer hydrated NaxCoO2 center dot yH(2)O (x similar to 0: 35, y similar to 1: 3) using soft x-ray absorption spectroscopy at the Co-L-2,L-3 and O-K edges. We find that the valence state of the Co lies in a narrow range from +3.3 to +3.4 for all studied NaxCoO2 center dot yH(2)O samples and their deuterated analogue with T-c's ranging from 3.8 to 4.7 K. These valence values are far from the often claimed +3.7, the number based on the Na content only. We propose to modify the phase diagram accordingly, where the basic electronic structure of the superconducting phase is very close to that of the Na0.7CoO2 system, suggesting that the presence of in-plane spin fluctuations could play an important role for the superconductivity.