MEAN-FIELD THEORY OF MIXED-VALENCE CONDUCTORS (R1, R2-DCNQI)2 CU

The insulating state of (R1, R2-DCNQI)2Cu salts showing both the mixed valence of Cu+ and Cu++ and the three-fold lattice distortion has been examined by the mean-field theory to the periodic Anderson model where two one-dimensional conduction bands with the electron-phonon interaction are coupled to Cu ions. For the choice of the interaction constant which leads to approximately Cu+4/3 as an average, it is shown that modulations of both charge density and spin density coexist in the ground state and that the phase transition at finite temperature is of the first order.