We addressed the so-far unresolved issue concerning the Co valence in the superconducting bilayer hydrated Na_xCoO₂·yH₂O (x?0.35, y?1.3) using soft x-ray absorption spectroscopy at the Co-L_2,3 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 Na_xCoO₂·yH₂Osamples 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 theNa_0.7CoO₂ system, suggesting that the presence of in-plane spin fluctuations could play an important role for the superconductivity.The research was conducted at the NSRRC beamline 11A1 . 

H. Ohta, K. Yoshimura, Z. Hu, Y. Y. Chin, H. J. Lin, H. H. Hsieh, C. T. Chen, and L. H. Tjeng