Anomalous periodicity of the current-phase relationship of grain-boundary Josephson junctions in high- T c superconductors

The current-phase relation (CPR) for asymmetric 45\ifmmode^\circ\else\textdegree\fi{} Josephson junctions between two d-wave superconductors has been predicted to exhibit an anomalous periodicity. We have used the single-junction interferometer to investigate the CPR for these kinds of junctions in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ thin films. A remarkable amplitude of the $\ensuremath{\pi}$-periodical component of the CPR has been experimentally found, providing an additional source of evidence for the d-wave symmetry of the pairing state of the cuprates.