RECENT discussions of the mechanism of high-temperature superconductivity have focused on the symmetry of the superconducting order parameter (the wavefunction of the superconducting state) as a means of constraining the nature of the interaction that forms the Cooper pairs1,2. There have been many attempts to measure the symmetry of this parameter in the high-temperature superconductor YBa2Cu3O7 - δ (refs 3–10), but they have not yielded consistent results. Here we show that half-integer flux quantization in superconducting rings with three grain-boundary Josephson junctions, which formed the basis of an earlier, less conclusive symmetry test3, can be used to place firm constraints on the pairing symmetry. For the case of YBa2Cu3O7 - δ, we find that the sign of the order parameter is clearly direction dependent (consistent with a dx2-y2 symmetry) and that the contribution from any out-of-phase isotropic component is less than about 3%.
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Kirtley, J., Tsuei, C., Sun, J. et al. Symmetry of the order parameter in the high-Tc superconductor YBa2Cu3O7- δ. Nature 373, 225–228 (1995). https://doi.org/10.1038/373225a0
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