Transport properties and electronic states of the thermoelectric oxide NaCo_2O_4

摘要

Physical properties of the thermoelectric oxide NaCo_2O_4 are brieflyreviewed. The high thermoelectric properties of this material are attributed tothe substantially enhanced effective mass, which comes from the large entropyof Co$^{4+}$ in the low spin state. The large entropy confined in the CoO_2block causes a spin-density-wave transition at 22 K upon Cu substitution forCo, which can be regarded as ``order from disorder''.