A mechanism for phase separation in copper oxide superconductors

A two-band Hubbard model is used to describe the band structure and phase separation (PS) in multiband superconductors, especially in cuprates. We predict a large peak in the density of states at the Fermi level in the case of optimum doping, corresponding to the minimum energy difference between the centers of two hole bands. For strong interband hybridization, a metal-insulator transition occurs near this optimum doping level. We suggest a mechanism of PS related to the redistribution of holes between two Hubbard bands rather than to the usual antiferromagnetic correlations. We show that the critical superconducting temperature $T_c$ can be about its maximum value within a wide range of doping levels due to PS.