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

The specific heat of (Na,Ca)Co2O4 is measured at low-temperatures to determ ine the magnitude of the electronic specific-heat coefficient gamma, in an attempt to gain an insight into the origin of the unusually large thermoele ctric power of this compound. It is found that gamma is as large as similar to 48 mJ/molK(2), which is an order of magnitude larger than gamma of simp le metals. This indicates that (Na,Ca)Ca2O4 is a strongly-correlated electr on system, where the strong correlation probably comes from the low-dimensi onality and the frustrated spin structure. We discuss how the large thermop ower and its dependence on Ca doping can be understood with the strong elec tron correlations. [S0163-1829(99)01036-X].