Simple expressions for calculation of directions of collinear phase matching in second harmonic generation in biaxial crystals

Abstract

It is shown that an electron-electron attractive interaction, mediated by bosons other than phonons, is impossible at zero energy. This conclusion, which is due IO crystal stability requirements, applies to excitons and also to phonons of the 'metallic excitonium'.High T, superconductivity would be very desirable for its applications, and is physically interesting. The search for it has been proceeding for quite a long time (Little 1964). It has been felt, however, that the known phononic mechanism cannot yield