Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide ${\mathrm{NaCo}}_{2-x}{\mathrm{Cu}}_x{\mathrm{O}}_4$

Various physical quantities were measured and analyzed for the Cu-substituted thermoelectric oxide ${\mathrm{NaCo}}_{2-x}{\mathrm{Cu}}_x{\mathrm{O}}_4.$ As was previously known, the substituted Cu enhances the thermoelectric power, while it does not increase the resistivity significantly. The susceptibility and the electron specific heat are substantially decreased with increasing x, which implies that the substituted Cu decreases the effective-mass enhancement. Through a quantitative comparison with the heavy-fermion compounds and the valence-fluctuation systems, we have found that the Cu substitution effectively increases the coupling between the conduction-electron and the magnetic fluctuations. The Cu substitution induces a phase transition at 22 K that is very similar to a spin-density-wave transition.