Since the discovery nearly 25 years ago of high-temperature superconductivity in a family of copper oxides, or cuprates, theorists and experimentalists alike have struggled to understand the mechanism behind the phenomenon. Is it something akin to conventional superconductivity, in which a weak attractive interaction holds the fermionic electrons together in bosonic bound states, which then condense into a superfluid? Or might some fundamentally new theory be required?

In a conventional superconductor, as explained by the Bardeen-Cooper-Schrieffer (BCS) theory, the electrons are held together in pairs by an interaction mediated by lattice phonons. Each electron couples to the spectrum of phonons through its Coulomb attraction to the positively charged nuclei; the result is an attractive interaction between electrons. The attraction is very weak, but as Leon Cooper originally showed, even a weak attraction suffices to create electronic bound states.

The BCS theory is not specific to a phonon-mediated interaction; other excitations...

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