  Light-mass bipolarons in off-diagonally coupled electron-phonon systems
provide a potential route to bipolaronic high-Tc superconductivity. While there
has been numerical progress in the physically relevant limit of slow phonons,
more insights are needed to fully understand to what extent this mechanism
survives at finite densities and in different regimes. We address these
questions using advanced tensor-network methods applied to the
Su-Schrieffer-Heeger model. Studying both the spectral properties of isolated
bipolarons as well as the pairing correlations at small finite densities, we
find evidence that the conventional picture of bipolarons as molecular bound
states which undergo Bose-Einstein condensation may need to be reconsidered.
Instead, our findings suggest correlation-driven formation of a fragmented
condensate with spatially separated polaron pairs, stabilized by strong
repulsive electron-electron interactions at moderate values of the
electron-phonon coupling. These spatially modulated charge clouds exhibit
pairing with a typical length-scale set by the Fermi momentum.
