Determination of the Co Valence in Bilayer Hydrated Superconducting NaxCoO2·yH2O by Soft X-Ray Absorption Spectroscopy
H. Ohta, K. Yoshimura, Z. Hu, Y. Y. Chin, H. -J. Lin, H. H. Hsieh, C. T. Chen, L. H. Tjeng
DOI 10.1103/PhysRevLett.107.066404 · Physical Review Letters
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Abstract
We addressed the so-far unresolved issue concerning the Co valence in the superconducting bilayer hydrated NaxCoO2·yH2O (x∼0.35, y∼1.3) using soft x-ray absorption spectroscopy at the Co-L2,3 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·yH2O samples and their deuterated analogue with Tc’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.
Source-reported materials — not catalogue approval
| Formula | Reported Tc (K) | Pressure (GPa) | Type |
|---|---|---|---|
| NaxCoO2·yH2O Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 5 | Pressure not reported | unknown |
| NaxCoO2·yH2O Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 4.7 | Pressure not reported | unknown |
| NaxCoO2·yH2O Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | 3.8 | Pressure not reported | unknown |
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