Since their dramatic debut in 1986, cuprate superconductors have been some of the best-studied materials in existence. Nonetheless, many mysteries about the materials have persisted, including perhaps the key question: What mechanism compels electrons to overcome their repulsion and pair up?
In conventional superconductors, Bardeen-Cooper-Schrieffer (BCS) theory describes how phonon vibrations coax electrons together into Cooper pairs. The material properties of those superconductors often abide by “Matthias’s rules”—no magnetism, no oxides, no insulators. Apart from sulfur hydrides, no BCS superconductor exceeds temperatures of 40 K. None of that has stopped doped copper oxides, whose parent compounds are insulating antiferromagnets, from remaining superconducting at temperatures as high as 135 K. As further evidence against a BCS pairing mechanism, cuprate superconductors are mostly insensitive to changes in phonon frequency.
Cuprate superconductors vary in their chemical formulas, but all contain the same essential building block: planes with one copper atom sandwiched between two oxygen atoms. Hypotheses abound for the mechanism behind cuprates’ superconductivity. Some theorists have suggested spin fluctuations; others believe phonons are the answer. Less than a year after Georg Bednorz and Alex Müller’s discovery of cuprate superconductivity, Philip Anderson proposed that the glue that binds electrons comes from superexchange, in which the spins of copper atoms are coupled, creating a magnetic attraction among their electrons despite the nonmagnetic oxygen atom in between.
Recently, several studies have begun to connect the key factors behind a potential superexchange pairing mechanism. One important factor is the charge-transfer gap (CTG), the energy required (usually a few eV) for an oxygen atom to take an electron from a copper atom. The larger the gap, which exists between the copper d orbital and the oxygen p orbital, the less likely the oxygen is to nab an electron from the copper. Last year, theorists at the University of Sherbrooke in Québec computed the rate at which electron pairing varies with the CTG.
That prediction provided a key target for a team led by J. C. Séamus Davis, who has labs at Oxford University, University College Cork in Ireland, and Cornell University. In a recent study in Proceedings of the National Academy of Sciences (PNAS), Davis and his colleagues report evidence that agrees with the Canadian theorists’ predictions, suggesting the mechanism behind cuprate superconductivity is CTG-mediated superexchange.
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