SUPERCONDUCTING TRANSITION OF 2-DIMENSIONAL 2-BAND SYSTEMS WITH EXCHANGE-LIKE INTERACTION

The exchange-like interaction by which BCS pairs are scattered between two bands is intensified through a process expressed by ladder diagrams. This intensification is reported here to be divergible in the case of two-dimensional two-band systems with electron and hole pockets. In such systems interband polarization function has a peak for wave number equal to the difference of the two Fermi wave numbers. This peak can lead to the divergence of the ladder-diagram process for important wave numbers, leading to s-wave superconductivity. An equation for Tc taking account of this effect is derived and numerically solved. Tc becomes finite and large despite large U when the ladder-diagram process is close to or has the divergence. This mechanism is argued to be substantiated in the high-Tc oxides. Competition with an SDW transition is briefly discussed. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.