Scaling behavior in thermoelectric misfit cobalt oxides

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide [Bi${}_{1.7}$Co${}_{0.3}$Ca${}_2$O${}_4$]${}_{0.6}^{RS}$CoO${}_2$. A large negative magnetothermopower is found to scale with both magnetic field and temperature revealing a significant spin entropy contribution to thermoelectric properties giving rise to a constant S${}_0\approx$ 60 $\mu$V K${}^{-1}$ equal to the high temperature asymptotic value of the spin 1/2 entropy. Low temperature specific heat measurements allow us to determine an enhanced electronic part with $\gamma \approx$ 50 mJ (mol K${}^2$)${}^{-1}$ attesting of strong correlations. Thereby, a critical comparison between [Bi${}_{1.7}$Co${}_{0.3}$Ca${}_2$O${}_4$]${}_{0.6}^{RS}$CoO${}_2$, other cobaltites as well as other materials reveals a universal behavior of the thermopower low temperature slope as a function of $\gamma$ testifying thus a purely electronic origin. This potentially generic scaling behavior suggests here that the high room temperature value of the thermopower in misfit cobalt oxides results from the addition of a spin entropy contribution to an enlarged electronic one.