Theorists trying to explain the mechanism for superconductivity in the copper oxide materials may not agree on much, but they all do believe it involves the coupling of electrons of opposite spin. Paths quickly diverge when it comes to the angular momentum of the pair. Is it a BCS‐like swave (with l = 0)? A d‐wave? Or something else? The answer is linked to the mechanism that couples the electrons. Although early experiments seemed consistent with an swave pairing, recent measurements suggest that the pairing state is at least not isotropic. If the experiments can further delineate the symmetry of the pairing state, they might help narrow the field of contending theories.

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