Spectroscopic signatures of bipolaron dissociation in spectral functions of the Hubbard-Holstein model

We study the spectral functions of the one dimensional Holstein model of electron-phonon coupling at low but finite electronic densities, both for spinless as well as for spinful electrons. In the spinless case, and for small-to-intermediate couplings, we obtain an excellent agreement between variationally exact density matrix renormalization group (DMRG) simulations evaluated directly in frequency-space and a semianalytical generalized Momentum Average variational approximation proposed recently in [M Berciu, J. Phys. Mater. 5 044002 (2022)] which assumed that besides single polarons, the remaining fermions in the system are locked into an inert Fermi sea. In the spinfull case, the DMRG numerics unveils an anomalous dispersive feature at small momenta as a function of Coulomb repulsion strength which we link to the dissociation of the bipolaronic metallic state into a polaronic liquid.