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Formation of strong-coupling (bi)polarons and related in-gap states in lightly-doped cuprate superconductors

The formation of large polarons and bipolarons and related in-gap states in lightly-doped cuprates is studied in the strong electron–phonon coupling regime. The ground-state energies, binding energies and binding radius of strong-coupling large (bi)polarons are calculated using the continuum model and adiabatic approximation taking into account both the short- and long-range electron–phonon interactions. The obtained results show that the binding energies of such large (bi)polarons in the cuprates are progressively increased with decrease in the high frequency dielectric constant [Formula: see text] and ratio [Formula: see text] (where [Formula: see text] is the static dielectric constant). As a result, large (bi)polarons become nearly small (bi)polarons. It is shown that hole carriers strongly interact with acoustic and optical phonons and their self-trapping leads to the formation of (bi)polaronic states in the charge-transfer (CT) gap of the cuprates. As the cuprate system is doped, the CT gap of the parent compound is filled in with low-energy (bi)polaronic states. The calculated energies of such (bi)polaronic states are in good agreement with the experimentally observed energies of new states appearing in the CT gap of the parent cuprates. The calculated values of the binding radius of large polarons are also well consistent with the existing experimental data.