A METHOD TO STUDY PHASE-DIAGRAMS FOR HIGH-TEMPERATURE SUPERCONDUCTORSUSING THE EXTENDED HUBBARD-HAMILTONIAN

We derive a method to study the phase diagram for high temperature sup erconductors (HTCS). Our starting point is the Hubbard Hamiltonian wit h a week attractive interaction to obtain the formation of bound pairs . We consider this attractive potential at different positions for dif ferent compounds. We use a BCS type variational method to derive an en ergy gap equation. This simple approach allows us to obtain the critic al temperature as function of the doping concentration which gives ver y good agreement with the experimental phase diagrams of YBaCuO and La (Sr,Ba)CuO compounds.