DYNAMICAL MEAN-FIELD THEORY OF THE CDW PHASE

We apply the Dynamical Mean Field Theory to the problem of charge orde ring. In this framework we solve the Holstein model for electrons inte racting with classical phonons in the case of a bipartite lattice. In the normal state as well as in the Charge Density Wave (CDW) state the existence of polarons, i.e. electrons strongly coupled to local latti ce. deformation, is associated to the qualitative properties of the La ttice Polarization Distribution Function (LPDF). Properties of the CDW ordered state are qualitatively different in weak and strong coupling . We investigate the cross-over from weak coupling CDW behavior, chara cterized by a single peak LPDF in each sub-lattice centered on the sub -lattice average polarization, the strong coupling CDW regime which is characterised by a double peak non symmetric LPDF in each sub-lattice . The secondary peak is centered on the average polarization of the ot her sub-lattice indicating a certain amount of defects in the ordered state. This regime can be interpreted as that of a CDW phase resulting from the spatial ordering of preexisting randomly distributed polaron s. The CDW critical temperature exhibits a maximum in the normal-to-po laron crossover region.