The chemical physics of unconventional superconductivity

Attempts to explain correlated-electron superconductivity (SC) have largely focused on the proximity of the superconducting state to antiferromagnetism. Yet, there exist many correlated-electron systems that exhibit insulator-superconducting transitions where the insulating state exhibits spatial broken symmetry different from antiferromagnetism. Here, we focus on a subset of such compounds which are seemingly very different in which specific chemical stoichiometries play a distinct role, and small deviations from stoichiometry can destroy SC. These superconducting materials share a unique carrier concentration, at which we show there is a stronger than usual tendency to form local spin-singlets. We posit that SC is a consequence of these pseudomolecules becoming mobile as was suggested by Schafroth a few years prior to the advent of the Bardeen-Cooper-Schrieffer (BCS) theory.