First- and second-order phase transitions in the Holstein-Hubbard modelW. Koller1, D. Meyer1, Y. Ono2 and A. C. Hewson1
1 Department of Mathematics, Imperial College - London SW7 2BZ, UK
2 Department of Physics, Niigata University - Ikarashi, Niigata 950-2181, Japan
(Received 16 December 2003; accepted 4 March 2004)
We investigate metal-insulator transitions in the half-filled Holstein-Hubbard model as a function of the on-site electron-electron interaction U and the electron-phonon coupling g. We use several different numerical methods to calculate the phase diagram, the results of which are in excellent agreement. When the electron-electron interaction U is dominant, the transition is to a Mott insulator; when the electron-phonon interaction dominates, the transition is to a localized bipolaronic state. In the former case, the transition is always found to be second order. This is in contrast to the transition to the bipolaronic state, which is clearly first order for larger values of U. We also present results for the quasiparticle weight and the double occupancy as functions of U and g.
71.10.Fd - Lattice fermion models (Hubbard model, etc.).
71.30.+h - Metal-insulator transitions and other electronic transitions.
71.38.-k - Polarons and electron-phonon interactions.
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