Transport studies on single-crystal YBa2Cu3O7-x

This thesis describes anisotropic transport studies on the 90 K superconductor YBa2Cu3O7-x. Resistivity anisotropy, fluctuation conductivity (paraconductivity), and thermal conductivity anisotropy measurements in high-quality single crystals demonstrate highly two-dimensional electronic properties in this material. Parallel to the CuO2 planes the resistivity ρab (300 K) ∼ 150-200 μΩ-cm is clean and metallic, with kFl ∼ 30 and l ∼ 70 angstrom at 100 K. Normal to the planes, however, the resistivity is ∼102 larger, possibly with an anomalous temperature dependence, indicating nonmetallic conduction in this direction. Thermal conductivity measurements also find substantial differences between in-plane and out-of-plane conduction. In-plane paraconductivity measurements show that superconducting fluctuations in the normal state of YBa2Cu3O7-x exhibit two-dimensional Aslamazov-Larkin behavior, with no indication of a crossover to three-dimensional behavior near the superconducting transition. There is also evidence that the critical fluctuation region in YBa2Cu3O7-x is much wider (εc ∼ 10-2) than in conventional superconductors. Two additional experiments are described in the appendices. Thermopower measurements in ceramic YBa2Cu3-xNixO7-x test the effect of Cu substitution on superconductivity and carrier concentration, and resistivity measurements on single-crystals of the 10 K superconductor Bi2Sr2CuO6 find evidence for large anisotropies in this newer family of copper-oxide superconductors