Light polarons and bipolarons for a highly inhomogeneous electron-boson couplingM. Berciu and G. A. Sawatzky
Department of Physics and Astronomy, University of British Columbia - Vancouver, BC V6T 1Z1, Canada
received 9 October 2007; accepted in final form 18 January 2008; published March 2008
published online 19 February 2008
We derive the exact polaron and bipolaron Green's functions for a model with a highly inhomogeneous electron-boson coupling , where (1,,1). While the polaron ground-state energy and quasiparticle weight are similar to those of the Holstein polaron with equal effective coupling, the polaron dispersion is very different. Unlike that of a Holstein polaron, which is monotonically increasing with the polaron momentum, the polaron dispersion in this case is folded inside the Brillouin zone. For strong coupling, the polaron and bipolaron effective masses increase linearly with the effective coupling and exhibit no isotope effect, as opposed to an exponential increase and a strong isotope effect, in the Holstein model. As a result, this model exhibits strongly bound yet light bipolarons at strong couplings. Generalizations are discussed.
71.38.-k - Polarons and electron-phonon interactions.
71.38.Mx - Bipolarons.
71.10.Li - Excited states and pairing interactions in model systems.
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