Unconventional Superconductor - Junction Experiment Supporting The d Symmetry

Junction Experiment Supporting The d Symmetry

There was a clever experimental design to overcome the muddy situation. An experiment based on flux quantization of a three-grain ring of YBa₂Cu₃O₇ (YBCO) was proposed to test the symmetry of the order parameter in the HTS. The symmetry of the order parameter could best be probed at the junction interface as the Cooper pairs tunnel across a Josephson junction or weak link. It was expected that only for a junction of d symmetry superconductors there could occur a half-integer flux, that is, a spontaneous magnetization. However, even if the junction experiment is the strongest method to determine the symmetry of the HTS order parameter, there have been ambiguous results of the junction experiments. J. R. Kirtley and C. C. Tsuei thought that the ambiguous results came from the defect inside the HTS, so that they designed the experiment where both of clean limit (no defect) and dirty limit (maximum of defects) were simultaneously considered. In the experiment, the spontaneous magnetization was clearly observed in YBCO, which absolutely supported the d symmetry of the order parameter in YBCO. Because YBCO is orthorhombic, it might inherently have an admixture of s symmetry. So, by tuning their technique further, they found that there was an admixture of s symmetry in YBCO within about 3%. Also, they found that there was a pure d_x2-y2 order parameter symmetry in the tetragonal Tl₂Ba₂CuO₆.