A MODEL FOR HIGH-TEMPERATURE SUPERCONDUCTORS USING THE EXTENDED HUBBARD HAMILTONIAN

We derive a method to study the phase diagram for high-temperature sup erconductors (HTCS's). Our starting point is the Hubbard hamiltonian w ith a weak attractive interaction to obtain the formation of bound pai rs. We consider this attractive potential at different positions for d ifferent compounds. We then construct a wave function of the BCS type by a variational method using the Fourier transform of this extended H ubbard potential and then derive an energy gap equation which can be a pplied to the s wave and d wave channel. This simple approach allows u s to obtain the critical temperature as a function of the doping conce ntration which gives very good agreement with the experimental phase d iagrams of YBaCuO and La(Sr,Ba)CuO compounds.