Direct measurement of the anisotropy of the resistivity in the a-b plane of twin-free, single-crystal, superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We have measured the resistivity tensor in one nearly twin-free crystal and one twin-free crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. The first crystal was grown with a large twin-free region. We removed the twins in the second crystal by applying a uniaxial stress. Using a modified Montgomery technique, we measured the electrical resistivities in all three crystal directions and found them to be linear in temperature above the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. Our results for ${\mathrm{\rho }}_{\mathit{a}}$ and ${\mathrm{\rho }}_{\mathit{b}}$ at room temperature are among the lowest that have been reported in the literature, indicating that our samples are of high quality. The ratio ${\mathrm{\rho }}_{\mathit{a}}$/${\mathrm{\rho }}_{\mathit{b}}$ is independent of temperature between 150 and 275 K, and its value, 2.2±0.2, indicates that the Cu-O chains contribute 60% of the current when the electric field is parallel to them, with the rest of the current being contributed by the Cu-O planes. This result, at zero frequency, is in good agreement with the recent conductivity at infrared frequencies obtained by Schlesinger et al.