Nature 373, 225 - 228 (19 January 1995); doi:10.1038/373225a0

Symmetry of the order parameter in the high-T_c superconductor YBa₂Cu₃O_7-

J. R. Kirtley, C. C. Tsuei, J. Z. Sun, C. C. Chi^*, Lock See Yu-Jahnes, A. Gupta, M. Rupp & M. B. Ketchen

IBM Thomas J. Watson Research Center, PO Box 218, Yorktown Heights, New York 10598, USA
^* Present address: Materials Science Center and Department of Physics, National Tsing-Hua University, Taiwan, Republic of China.

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 pairs^1,2. There have been many attempts to measure the symmetry of this parameter in the high-temperature superconductor YBa₂Cu₃O_{7 -} (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 YBa₂Cu₃O_{7 -} , we find that the sign of the order parameter is clearly direction dependent (consistent with a d_x ²-_y ² symmetry) and that the contribution from any out-of-phase isotropic component is less than about 3%.

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