Specific-heat evidence for strong electron correlations in the thermoelectric material (Na,Ca)Co2O4

Abstract

The specific heat of $(\mathrm{N}\mathrm{a},\mathrm{C}\mathrm{a}){\mathrm{Co}}_2{\mathrm{O}}_4$ is measured at low-temperatures to determine the magnitude of the electronic specific-heat coefficient $\gamma ,$ in an attempt to gain an insight into the origin of the unusually large thermoelectric power of this compound. It is found that $\gamma$ is as large as $\sim 48\hspace{0.5em}{\mathrm{m}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{K}}^2,$ which is an order of magnitude larger than $\gamma$ of simple metals. This indicates that $(\mathrm{N}\mathrm{a},\mathrm{C}\mathrm{a}){\mathrm{Co}}_2{\mathrm{O}}_4$ is a strongly-correlated electron system, where the strong correlation probably comes from the low-dimensionality and the frustrated spin structure. We discuss how the large thermopower and its dependence on Ca doping can be understood with the strong electron correlations.