Tunneling Experiments in High‐$T_{\mathrm{c}}$ Superconductors Resolve a Puzzle

A key question regarding high‐temperature superconductors has been the nature of the pairing state of the electrons responsible for the supercurrent: Do the electrons couple in an s‐wave state, as in conventional superconductors, or in a d‐wave state, specifically ${\mathrm{d}}_{x^2{\mathrm{-y}}^2},$ whose wavefunction resembles a four‐leaf clover. The argument was largely settled in favor of d‐wave symmetry when several precise experiments were able to sense the phase of the electron‐pair wavefunction and found that it changed signs, suggestive of the alternating positive and negative lobes of the d‐wave clover leaf. But not all the evidence lined up: One study of the Josephson tunneling by Robert Dynes and his colleagues at the University of California, San Diego, simply wasn’t compatible with a d‐wave interpretation.

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