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Two-dimensional Hubbard-Holstein bipolaron

Alexandru Macridin·George. A. Sawatzky·Mark Jarrell
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Abstract

We present a diagrammatic Monte Carlo study of the properties of the Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small Coulomb repulsion, U, and with increasing electron-phonon interaction, and when reaching a value about two times smaller than the one corresponding to the transition of light polaron to heavy polaron, the system suffers a sharp transition from a state formed by two weakly bound light polarons to a heavy, strongly bound on-site bipolaron. Aside from this rather conventional bipolaron a new bipolaron state is found for large U at intermediate and large electron-phonon coupling, corresponding to two polarons bound on nearest-neighbor sites. We discuss both the properties of the different bipolaron states and the transition from one state to another. We present a phase diagram in parameter space defined by the electron-phonon coupling and U. Our numerical method does not use any artificial approximation and can be easily modified to other bipolaron models with longer range electron-phonon and/or electron-electron interaction.

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Alexandru Macridin, George. A. Sawatzky, Mark Jarrell. 2004-01-16. Two-dimensional Hubbard-Holstein bipolaron. https://doi.org/10.1103/physrevb.69.245111

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