Resonating bipolarons
  Electrons coupled to local lattice deformations end up in selftrapped localized molecular states involving their binding into bipolarons when the coupling is stronger than a certain critical value. Below that value they exist as essentially itinerant electrons. We propose that the abrupt crossover between the two regimes can be described by resonant pairing similar to the Feshbach resonance in binary atomic collision processes. Given the intrinsically local nature of the exchange of pairs of itinerant electrons and localized bipolarons, we demonstrate the occurrence of such a resonance on a finite-size cluster made out of metallic atoms surrounding a polaronic ligand center. 
J. Ranninger and A. Romano