Strongly anisotropic electrical properties of single-crystalYBa2Cu3O7−x

Crystals of orthorhombic, superconducting ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ have been prepared. They are twinned in the nearly tetragonal $\mathrm{ab}$ plane. The resistivity and Hall effect have been measured in the normal state in the $\mathrm{ab}$ plane, the highly conducting plane of the Cu-O layers. We find that the resistivity in the $\mathrm{ab}$ plane, measured with currents exclusively in the plane, is strictly linear in temperature from room temperature down to the fluctuation regime near the superconducting transition. This result confirms previous less direct measurements in which the currents flowed in the much more resistive $c$ direction as well. We find the Hall constant for the magnetic field parallel to $c$ and the currents in the $\mathrm{ab}$ plane, to be $p$ type and inversely proportional to temperature. This latter behavior is extremely unusual and unexplained, but similar to that found in polycrystalline ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ material. It is in contrast to our previous result for the magnetic field in the $\mathrm{ab}$ plane. In that case, the Hall constant is $n$ type and nearly temperature independent. Both the Hall effect and the resistivity demonstrate the extremely anisotropic nature of this system.