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Superconductivity phase diagram of NaxCoO2·1.3H2O


The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity1 near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO2·1.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO2·1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

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Figure 1: Powder X-ray diffraction patterns (Cu Kα radiation) for NaxCoO2·yH2O samples prepared using different concentrations of the bromine de-intercalant.
Figure 2: Zero field cooled d.c. magnetization for superconducting samples of NaxCoO2·1.3H2O.
Figure 3
Figure 4: The superconducting phase diagram for NaxCoO2·1.3H2O.


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This work was supported by the US National Science Foundation, Division of Materials Research, and the US Department of Energy, Division of Basic Energy Sciences. T.K. thanks the Foundation for Polish Science for support.

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Correspondence to R. J. Cava.

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Schaak, R., Klimczuk, T., Foo, M. et al. Superconductivity phase diagram of NaxCoO2·1.3H2O. Nature 424, 527–529 (2003).

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