Formation process of a bipolaron and light-absorption bands in halogen-bridged mixed-valence metal complexes

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Abstract
A formation process of a singlet bipolaron from two doped polarons in halogen-bridged mixed-valence metal complexes is studied by using a one-dimensional extended Peierls-Hubbard model with N (≫1) lattice sites and N+2 electrons. The ground and the excited states of this system are calculated within the unrestricted Hartree-Fock theory and by the adiabatic approximation. Two doped polarons are shown to make a singlet bound state as the result of competition between the Coulombic repulsion and the phonon-mediated attraction, and its interpolaron distance becomes a few times the lattice constant. A new light-absorption band due to this bound state is also shown to appear at an energy of about 50% of the intrinsic gap. These theoretical results agree well with recent optical and electron-spin-resonance experiments. A pair of solitons is also studied, and is shown to have no bound state.