We have examined an isovalent Rh substitution effect on the transport properties of the thermoelectric oxide Ca 3 Co 4 O 9 using single-crystalline form. With increasing Rh content x, both the electrical resistivity and the Seebeck coefficient change systematically up to x = 0.6 for Ca 3 Co 4−x Rh x O 9 samples. In the Fermi-liquid regime where the resistivity behaves as ρ = ρ 0 + AT 2 around 120 K, the A value decreases with increasing Rh content, indicating that the correlation effect is weakened by Rh 4d electrons with extended orbitals. We find that, in contrast to such a weak correlation effect observed in the resistivity of Rh-substituted samples, the low-temperature Seebeck coefficient is increased with increasing Rh content, which is explained with a possible enhancement of a pseudogap associated with the short-range order of spin density wave. In high-temperature range above room temperature, we show that the resistivity is largely suppressed by Rh substitution while the Seebeck coefficient becomes almost temperature-independent, leading to a significant improvement of the power factor in Rh-substituted samples. This result is also discussed in terms of the differences in the orbital size and the associated spin state between Co 3d and Rh 4d electrons.

We have carried out the transient nonlinear transport measurements on the layered cobalt oxide Ca 3 Co 4 O 9 , in which a spin density wave (SDW) transition is proposed at T SDW ≃ 30 K. We find that, below T SDW , the electrical conductivity systematically varies with both the applied current and the time, indicating a close relationship between the observed nonlinear conduction and the SDW order in this material. The time dependence of the conductivity is well analyzed by considering the dynamics of SDW which involves a low-field deformation and a sliding motion above a threshold field. We also measure the transport properties of the isovalent Sr-substituted systems to examine an impurity effect on the nonlinear response, and discuss the obtained threshold fields in terms of thermal fluctuations of the SDW order parameter.

We have measured the transport properties of the thermoelectric layered oxide Ca3−xSrxCo4O9 () single crystals to examine the electron correlation effect. Both the electrical resistivity and the Seebeck coefficient systematically vary with increasing Sr content x. In all the present crystals, the resistivity shows a Fermi-liquid behavior of ρ = ρ0 + AT2 at around 120 K, and A increases with increasing Sr content, indicating that the effective mass is enhanced by the substitution. It is also found that the low-temperature Seebeck coefficient increases with increasing Sr content. These results suggest that the present isovalent Sr substitution acts as a negative pressure on this compound and that the two dimensionality is thus enhanced by the substitution, leading to a strong renormalization of the effective mass. We also discuss the Sr-substitution effect on the carrier concentration.