Competition between Electron-Electron and Electron-Lattice Interactions —Four-Site-Four-Electron Model—

Abstract

Competition between the electron-electron and the electron-lattice interactions is studied with the 4-site-4-electron model characterized by B (half of the band width), U (on-site Coulomb repulsion) and S (lattice relaxation energy). In the whole range of B, U and S the lowest adiabatic potential is exactly calculated, and all of its minima and their existence conditions are found. The absolute minimum is the antiferromagnetic phase (with antiferromagnetic spin correlation but no lattice distortion) or the charge density wave phase (with alternating lattice distortion). The first-order transition between them occurs along a line close to U=S. Electronic and structural properties of these two phases are studied in detail. System size dependence of the phase diagram is discussed, with the emphasis on the difference between 4n- and (4n+2)-electron systems.