In-plane anisotropy on the transport properties in the modulated Bi₂O₂-based conductors Bi-2212 and Bi-Sr-Co-O

We investigated the in-plane anisotropy on the resistivity and thermopower of the Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) and Bi-Sr-Co-O (BiCo) single crystals. In Bi-2212, the b-axis resistivity is higher than the a-axis resistivity, and is expressed as a sum of the a-axis resistivity and an additional residual resistivity. A downward deviation due to pseudogap is observed below a characteristic temperature T* which is isotropic in the form of conductivity. These results suggest that the modulation structure along the b-axis works as an anisotropic scattering center, but does not affect the pseudogap formation. On the other hand, the anisotropy of the resistivity and the thermopower in Pb-doped BiCo is substantial, probably owing to the misfit structure between the hexagonal CoO₂ layer and the rock-salt Bi₂O₂ layer. However, the anisotropy in the resistivity in Pb-free BiCo is very small, suggesting that the inplane anisotropy is averaged by the modulation structure, whose direction is tilted by 45° from the a- and b-axis.