Strongly bound yet light bipolarons for double-well electron-phonon   coupling
  We use the Momentum Average approximation (MA) to study the ground-state properties of strongly bound bipolarons in the double-well electron-phonon (el-ph) coupling model, which describes certain intercalated lattices where the linear term in the el-ph coupling vanishes due tosymmetry. We show that this model predicts the existence of strongly bound yet lightweight bipolarons in some regions of the parameter space. This provides a novel mechanism for the appearance of such bipolarons, in addition to long-range el-ph coupling and special lattice geometries. 
Clemens PJ Adolphs, Mona Berciu
cond-mat.str-el