Dynamics of bipolarons

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Abstract
Two polarons may interact through the strain field which surrounds each object to form a bound pair. The resulting entity, the bipolaron, may move through the crystal coherently, to form an energy band. It is scattered by phonons, so that at sufficiently high temperatures, it engages in diffusive rather than bandlike motion, in a manner similar to that described by polaron theory. Finally, it may dissociate through multiphonon processes. The question of whether the bipolaron is sufficiently stable to move through the crystal as an entity can be addressed by comparing the dissociation lifetime to the time required to move one lattice constant. We address this issue through the study of the dynamics of bipolaron states in a model where the electron moves in a narrow energy band, coupled to phonons by means of an interaction that is linear in nuclear displacements. Our results may also be applied to the diffusion of light-atom pairs through crystals or on crystal surfaces.