Metal–insulator transitions in systems with electron–phonon and Coulomb interactions

Abstract

We consider a model which includes the electron–phonon coupling to A g or Jahn–Teller H g phonons, the Coulomb interaction U and an exchange integral K. We study the metal–insulator transition for the integer fillings n=3 and 4. We find that the coupling to A g phonons increases the critical value U c where the transition takes place, while the coupling to H g phonons decreases U c. Without electron–phonon coupling the Hund's rule coupling also decreases U c. There is, however, an interesting competition between the Hund's rule coupling and the Jahn–Teller effect. Thus the reduction of U c due to the H g phonons becomes smaller when the Hund's rule coupling is turned on. The implications of this is discussed in the context of A n C 60.