Abstract
We study the two-site Hubbard–Holstein model by using an extended phonon coherentstate. For the nontrivial singlet bipolarons, the double occupancy probability, the fidelityand the entanglement entropy are calculated to characterize the ground-state properties inboth two-site and single-site bipolaron-dominated regimes. We use the localized minimumof the fidelity to define a crossover and plot the bipolaron phase diagram, which separatesthe large and small entanglement region. Furthermore, the relation between the bipolaronentanglement and the correlation functions demonstrates that the large entanglementcorresponds to the large magnitude of lattice deformations induced by electrons.
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