Orthorhombically mixed s- and dx2-y2-wave superconductivity and Josephson tunneling.

The effect of orthorhombicity on Josephson tunneling in high-${\mathit{T}}_{\mathit{c}}$ superconductors such as ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is studied for both single crystals and highly twinned crystals. It is shown that experiments on highly twinned crystals experimentally determine the symmetry of the superconducting twin boundaries (which can be either even or odd with respect to a reflection in the twinning plane). Conversely, Josephson experiments on highly twinned crystals cannot experimentally determine whether the superconductivity is predominantly s wave or predominantly d wave. The direct experimental determination of the order-parameter symmetry by Josephson tunneling in YBCO thus comes from the relatively few experiments which have been carried out on untwinned single crystals. \textcopyright{} 1996 The American Physical Society.