Bipolaronic superconductivity

Superconducting properties of narrow-band electrons are examined in the strong-coupling limit. It is shown that bipolarons (localized spatially nonoverlapping Cooper pairs) formed by strong electron-phonon interaction have under certain conditions superconducting properties which are characteristic of superfluid charged Bose systems. They represent an example of the "molecular" superconductivity proposed by Schafroth, Butler, and Blatt [Helv. Phys. Acta 30 93 (1957)]. The Meissner effect and the penetration depth of bipolaronic superconductors are examined. The relationship between Bardeen-Cooper-Schrieffer superconductors and bipolaronic ones is discussed.