Effect of heavy water intercalation in sodium cobaltate .

Superconductivity in the hydrated compound Na$_{x}$CoO$_{2 }$yH$_{2}$O (x=0.35, y=1.3) is directly related to water intercalation. Nevertheless, so far, no clear explanation of the effect of water on the superconducting mechanism has been given. Here we present our study on deuterated sodium cobaltate Na$_{x}$CoO$_{2 }$yD$_{2}$O (x=0.35, y=1.4) at two different temperatures, T=15K, 100K. Neutron diffraction data were analyzed using the PDF technique. The PDF of Na$_{x}$CoO$_{2 }$yD$_{2}$O was compared with the PDF of heavy water. The diffraction spectra at both temperatures show a weak D-D correlation, and a pronounced D-O correlation. The weakened D-D correlations imply that the dynamics of the hydrogen bond is modified. This increases the dielectric response, and may enhance the electron-phonon coupling. Our results suggest a modification of the geometry of the heavy water molecules inserted in Na $_{x}$CoO$_{2}$, and subsequently a possible active role of water in setting the superconducting state.