In-plane anisotropy on the transport properties in the modulated Bi_2O_2-based conductors Bi-2212 and Bi-Sr-Co-O

TL;DR

This study examines the in-plane anisotropy of resistivity and thermopower in Bi-2212 and Bi-Sr-Co-O single crystals, revealing how structural modulations influence anisotropic scattering and pseudogap formation.

Contribution

It provides new insights into how modulation structures affect in-plane anisotropy and pseudogap behavior in Bi-based conductors.

Findings

Resistivity along the b-axis is higher than along the a-axis in Bi-2212.

Pseudogap effects are isotropic despite anisotropic resistivity.

Anisotropy in Pb-doped BiCo is substantial, while it is minimal in Pb-free BiCo.

Abstract

We investigated the in-plane anisotropy on the resistivity and thermopower of the Bi_2Sr_2CaCu_2O_{8+\delta} (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 CoO_2 layer and the rock salt Bi_2O_2 layer. However, the anisotropy in the resistivity in Pb-free BiCo is very small, suggesting that the in-plane anisotropy is averaged by the modulation structure, whose direction is tilted by 45 deg from the a- and b-axes.