Phonon-mediated attraction between large bipolarons: Condensation to a liquid

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Abstract
The self-trapped carriers of large (multisite) bipolarons redistribute themselves among the sites of their molecular orbitals as their self-trapping potential wells are altered by changing atomic positions. Through this polarizability, large bipolarons reduce the phonon frequencies. The dependence of the lowering of the zero-point vibrational energy on the spatial distribution of large bipolarons produces an intermediate-ranged phonon-mediated attraction between large bipolarons that is comparable to a phonon energy. Coulomb repulsions that oppose the phonon-mediated attractions tend to be suppressed by the large static dielectric constants associated with large-bipolaron formation. However, a strong short-range repulsion between bipolarons, analogous to the hard-core repulsion between helium atoms, persists. With the dominance of the medium-range phonon-mediated attraction and the short-range repulsion, large bipolarons can condense into a liquid. The bipolaronic liquid might be the condensed fluid phase that has been suggested as a precursor to superconductivity in cuprates.