In-plane anisotropic transport properties observed in epitaxial Bi ₂(Sr,Ca)₃Cu₂O_x films grown on tilted (001)SrTiO₃ substrate

We report on large in-plane anisotropic transport properties observed in epitaxial Bi₂(Sr,Ca)₃Cu₂O_x films grown on tilted (001)SrTiO₃ substrate. By tilting the surface normal axis about 4°toward [111]SrTiO₃, regular steps and terraces were formed on the substrate surface and perfect alignment of film b axis with incommensurate modulation along [110]SrTiO₃ was realized. The film c axis was perpendicular to the (001)SrTiO₃ terrace, thus the film c axis grew tilted 4°toward [111] from surface normal due to the surface inclination. In those epitaxial configurations, the in-plane resistivity along the step direction(ρ_[110]) involved the contribution from the c-axis(ρ_c) component, and we observed the large resistivity anisotropy between a- and b- direction of the film. The transport along a axis(ρ_a) showed a low resistivity with metallic temperature dependence while the resistivity along the step direction(ρ_[110]) was higher and semiconductive. The ratio of ρ_c/ρ_a ≈ 10⁴ estimated in this experiment agrees well with the anisotropy observed in the bulk single crystal.