2009年03月12日

Patteerning of sodium ions and the control of electrons in sodium bobaltate

Sodium cobaltate (NaxCoO2) has emierged as material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physis of the superconducting copper oxides.(Terasaki,I., Sasago,Y., & Uchinokuma,K. Large thermoelectic power in NaxCoO2 single crystals. Phys. Rev. B56, R12685-12687/1997)(Lee,M.et al. Large enhancement of the thermopower in NaxCoO2 at high Na doing Nture Mater. 5,537-540/2006)(Takada,K. et al. Superconductivity in tow-dimensional NaxCoO2 layers. Nature 422,53-55/2003)

The densty x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers.(Dalmas,C. et al Electrochemical interclation of sodium in NaxCoO2 bronzes. Soild State lonics 3/4,165-169/1981)

Recent electron diffraction measurements revel a kaleidoscope of Na^+ ion patterns as a function of concentration.(Zandbergen,H.W. et al. Sodium ion ordering in NaxCoO2 : Electron diffraction study. Phys. Rev. B70, 024101/2004)

Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-rage three-dimensional superstructures of these ions.

We are goveerned by pure electrostaties, and that the origanizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings.

Our results provide a good startingpoint to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostaric potential from the Na superstructures.(Hubbard,J, Electron correlations in narrow energy bands. Proc. R. Soc Land. A276,238-257/1963)

The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes perferentially occupy the lowest potential regions.

Thus we conclude that the Na^+ ion patterning has a decisive role in the transpot and magnatic properties.


### DataBace ###
nature Vol.445 567-682 Issue no.7128 8 Febuary 2007
Letter p.631 / Patteerning of sodium ions and the control of electrons in sodium bobaltate / M Roger et al. (CEA Saclay)

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