Conventional superconductivity, described by BCS theory, arises when electrons form Cooper pairs through an attractive interaction mediated by lattice vibrations (phonons), producing pairs with simple, isotropic (s-wave) symmetry. Unconventional superconductors — including the cuprates, many iron-based superconductors, and several frustrated-magnet materials — show pairing that BCS phonon-mediated attraction does not account for, often with pairing symmetry that changes sign around the Fermi surface (d-wave, or in some proposals the time-reversal-symmetry-breaking d+id combination) and a transition temperature that tracks proximity to a magnetic or otherwise strongly correlated phase rather than the material’s phonon spectrum.
A recurring theoretical proposal is that unconventional superconductivity emerges directly from a nearby strongly correlated parent state — most prominently a quantum spin liquid — rather than from a weakly interacting metal. In the parton language used to describe such spin liquids, superconductivity can appear when the bosonic “chargon” degrees of freedom that carry charge (but were gapped out or otherwise decoupled in the spin-liquid phase) condense, in effect restoring a conventional, non-fractionalized superconducting or charge-ordered state out of the fractionalized parent. Which specific ordered phase — d+id superconductivity, a charge-density wave, or another option — appears is fixed by the symmetry of the parent spin liquid’s own parton construction.
Related papers
- Anderson, “The Resonating Valence Bond State in La2CuO4 and Superconductivity”, Science 235, 1196 (1987) — the original proposal connecting a resonating-valence-bond (spin-liquid-like) state to cuprate superconductivity.