Large thermopower in a metallic cobaltite: The layered Tl-Sr-Co-O misfit

The magnetic, transport, and thermoelectric properties of a Tl-based misfit cobaltite have been investigated. Although this material exhibits a metallic behavior, the thermopower (Seebeck) is large, with room temperature values close to +90 μ V/K. Compared to previously studied misfit cobalt oxides $[{\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9,(\mathrm{B}\mathrm{i},\mathrm{}\mathrm{P}\mathrm{b}\left)\mathrm{S}\mathrm{r}\mathrm{C}\mathrm{o}\mathrm{O}\right]$ this Tl-based misfit is the only compound of this family which remains metallic and paramagnetic down to 2 K. On the one hand, this behavior emphasizes the importance of the edge-shared octahedra in the ${\mathrm{CoO}}_2$ layer for the thermoelectric properties of these materials. On the other hand, the absence of a magnetically ordered state at low temperature explains the absence of low temperature negative magnetoresistance. It gives support to a mechanism based on quantum fluctuations stabilizing the paramagnetic state rather than a magnetically ordered state.