Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4
Abstract
Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly reviewed. The high thermoelectric properties of this material are attributed to the substantially enhanced effective mass, which comes from the large entropy of Co^{4+} in the low spin state. The large entropy confined in the CoO_2 block causes a spin-density-wave transition at 22 K upon Cu substitution for Co, which can be regarded as ``order from disorder''.