The origin of the temperature dependence of the magnetic susceptibility and the large thermoelectric power in metallic layered cobaltites

We perform detailed measurements of the thermoelectric power and the static magnetic susceptibility on metallic Na$_{x}$CoO${_2}$ and Ca$_{3}$Co$_{4}$O$_{9}$, as representatives of layered Co oxides with a triangular Co-lattice. We propose that the observed large thermoelectric power and the Curie-Weiss temperature dependence of the susceptibility have a common origin related to metallic character of these compounds. Thermoelectric power measurements are compared to Boltzmann transport theory calculations. The Curie-Weiss behaviour of the susceptibility is explained within the framework of the self-consistent renormalization theory for spin fluctuations proposed by Moriya for itinerant magnets. Our results clarify the apparent duality in the localised/itinerant behaviour of the electron spin in these systems and provide a unifying view on the physics of metallic layered cobaltites.