Transport and tunneling studies on single crystals of YBa₂Cu₃O₇

Using a flux technique we have grown crystals of YBa₂Cu₃O₇ which are superconducting above 90 K as-grown. In 10% of these crystals large orthorhombic single domains exceeding 100×100 μm² are observed using polarised light. The conductivity anisotropy (≈100 at 290 K to ≈300 near T_c) indicates a strikingly two-dimensional electronic state. Anisotropy in the thermopower is also reported. Tunneling into these crystals provides direct evidence that the electronic spectrum is different from the BCS spectrum. A (possibly) novel spin-dependent gap which scales linearly with field is described. Finally, paraconductivity studies on crystals with transition widths δT ≈ 0.2 K are summarised.