We have measured the current-phase relationship (CPR) of symmetric 45 degree YBaCuO grain boundary Josephson junctions. Substantial deviations of the CPR from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature T, a local minimum at a temperature T*. (ii) At T approximately equal to T*, the first harmonic of the CPR changes sign. (iii) For T<T*, the second harmonic of the CPR is comparable to the first harmonic, and (iv) the ground state of the junction becomes degenerate. The results are in good agreement with a microscopic model of Josephson junctions between d-wave superconductors.
@article{arxiv.cond-mat/0102404,
title = {Degenerate ground state in a mesoscopic YBaCuO grain boundary Josephson junction},
author = {E. Il'ichev and M. Grajcar and R. Hlubina and R. P. J. IJsselsteijn and H. E. Hoenig and H. -G. Meyer and A. Golubov and M. H. S. Amin and A. M. Zagoskin and A. N. Omelyanchouk and M. Yu. Kupriyanov},
journal= {arXiv preprint arXiv:cond-mat/0102404},
year = {2009}
}