Josephson tunneling in high-T_c superconductors

This article describes the Josephson tunneling from time-reversal symmetry-breaking states and compares it with that from time-reversal invariant states for both twinned and untwinned crystals and for both $c$-axis and basal-plane currents, in a model for orthorhombic YBCO. A macroscopic invariance group describing the superconducting state of a twinned crystal is introduced and shown to provide a useful framework for the discussion of the results for twinned crystals. In addition, a ring geometry, which allows $s$-wave and $d_{x^2-y^2}$-wave superconductivity in a tetragonal superconductor to be distinguished on the basis of symmetry arguments only, is proposed and analyzed. Finally, an appendix gives details of the experimental Josephson tunneling evidence for a superconducting state of orthorhombic $ux^2+vy^2$ symmetry in YBCO.