Large Bipolarons and Superconductivity

Abstract

Bipolarons form in materials with exceptionally strong electronlattice interactions. Although small (singlet site) bipolarons readily localize, large (multisite) bipolarons are presumed, in analogy with large polarons, to move itinerantly. Ionic solids with especially displaceable ions are prime candidates for the formation of large bipolarons. In particular, large (mobile) bipolarons can form in materials in which the static dielectric, ε₀, exceeds twice the optical dielectric constant, ε_∞: ε₀ > 2ε_∞. Such unusually large static dielectric constants are found in the insulating parents of oxide superconductors, including those with high T_c’s. With interactions between large bipolarons, their groundstate can become superconducting (having resistanceless flow and a Meissner effect). This superconductivity requires carrier densities great enough to preclude a localized groundstate but small enough that a conventional metal is not formed. The unusual normal-state transport (resistivity, Hall effect and thermopower) resembles that of oxide superconductors. In addition, large bipolarons possess distinctive spectroscopic properties that are in accord with observations in novel superconductors.