Nonlinear thermoelectric properties of layered cobalt oxides

We synthesized whisker-crystal and thin-film samples of the layered cobalt oxide (Bi₂Sr₂O₄)_xCoO₂ well known as a candidate of a thermoelectric material, and measured their resistivity, voltage-current characteristics and magnetoresistance with pulsed currents. Resistivity of the whisker sample with a small current density of 2 mA/cm₂ increases with decreasing temperature below 100 K, suggesting development of a pseudogap on the density of states. This increase is suppressed by a large current density of 110 A/cm². The nonlinear voltage-current characteristics are observed both for the whisker and thin-film samples. The negative magnetoresistance of the thin-film sample is suppressed by current density. These suggest that the density of states recovers owing to the suppression of the pseudogap by applying large current density.