We propose and implement a direct experimental test for subdominant superconducting phases with broken time-reversal symmetry in d-wave superconductors. The critical current of 45-degree asymmetric grain boundary junctions are shown to be extremely sensitive to the predicted onset of a complex order parameter at (110)-surfaces and near magnetic impurities. Measurements in YBCO and Ni-doped YBCO junctions indicate that the symmetry at the surface is consistent with pure d-wave at all temperatures, putting limits on the magnitude and chiral domain structure of any subdominant symmetry component.
@article{arxiv.cond-mat/0105388,
title = {Experimental test for subdominant superconducting phases with complex order parameters in cuprate grain boundary junctions},
author = {W. K. Neils and D. J. Van Harlingen},
journal= {arXiv preprint arXiv:cond-mat/0105388},
year = {2009}
}