Charge transfer electron–exciton complexes in single crystals

Abstract

We have considered the excitation spectrum arising from the coherent electron–exciton pairing in crystals at low temperatures. For the pairing processes, three different types of excitons have been considered: Frenkel excitons (tightly bound), Wannier–Mott excitons (loosely bound), and excitons of the intermediate binding. Expressions for the gap functions and transition temperatures at which metal-to-nonmetal phase transitions occur have been derived and discussed for each type of pairing. In the electron pairing with the intermediate exciton, the dispersion relations which determine the exciton and the electron–exciton modes are solved numerically and the derived results are graphically presented. The physical picture for the electron–exciton coupled modes is analogous to that for polaritons and magnon–phonon modes in crystals.