  A transition-metal oxide NaCo_2O_4 is a layered oxide in which CoO_2 and Na
alternately stack along the c axis. Recently we have found that this compound
shows unusually large thermopower with low resistivity, which is comparable to
those of Bi_2Te_3. The negative transverse magnetoresistance and the strongly
temperature-dependent Hall coefficient suggest that electron correlation
dominates the conduction mechanism in NaCo_2O_4.
