Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo_{2-x}Cu_x$O_4

摘要

Various physical quantities are measured and analysed for the Cu-substitutedthermoelectric oxide NaCo_{2-x}Cu_xO_4. As was previously known, thesubstituted Cu enhances the thermoelectric power, while it does not increasethe resistivity significantly. The susceptibility and the electronspecific-heat are substantially decreased with increasing x, which implies thatthe substituted Cu decreases the effective-mass enhancement. Through aquantitative comparison with the heavy fermion compounds and the valencefluctuation systems, we have found that the Cu substitution effectivelyincreases the coupling between the conduction electron and the magneticfluctuation. The Cu substitution induces a phase transition at 22 K that isvery similar to a spin-density-wave transition.