Lattice Distortions and Charge Carriers in Cuprates

A phenomenological model with itinerant bands and local states trapped by the lattice on the Cu-sites, is discussed to describe global features of cuprates. Relative energy positions of localized and itinerant states being tuned (thermodynamically or by doping), the system must undergo 1st order Mott metal-insulator transition. Decreasing the local level (from the metallic end of a stoichiometric compound), charge separation instability occurs first before the Mott transition. Crossing and hybridization between local (flat) and itinerant bands introduce a structure in density of states which may account for ``pseudogap'' features in cuprates. Model results in polaronic lattice effects and is rich enough to serve as a phenomenology of cuprates.