Scaling Behavior in Thermoelectric Misfit Cobalt Oxides

Abstract

We investigate both thermoelectric and thermodynamic properties of the misfit cobalt oxide $[{\mathrm{Bi}}_{1.7}{\mathrm{Co}}_{0.3}{\mathrm{Ca}}_{2}{\mathrm{O}}_{4}{]}_{0.6}^{\mathrm{RS}}{\mathrm{CoO}}_{2}$. 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 ${S}_{0}\ensuremath{\approx}60\text{ }\text{ }\ensuremath{\mu}\mathrm{V}\text{ }{\mathrm{K}}^{\ensuremath{-}1}$. Specific heat measurements allow us to determine an enhanced electronic part with $\ensuremath{\gamma}\ensuremath{\approx}50\text{ }\text{ }\mathrm{mJ}\text{ }(\mathrm{mol}\text{ }{\mathrm{K}}^{2}{)}^{\ensuremath{-}1}$ attesting to strong correlations. Thereby, the comparison between cobaltites and other materials reveals a universal behavior of the thermopower slope as a function of $\ensuremath{\gamma}$, testifying to 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.