Crossover from largeto small bipolarons

Abstract

A variational procedure is developed to study the properties of thebipolaron in the Fröhlich model including explicitly theelectron band structure and taking into account the long-rangerepulsive electron-electron interaction. Adopting two differentansatze for the longitudinal optical phonon distribution function,the large-bipolaron and small-bipolaron limits are obtained. Theevolution of the bipolaron ground state as a function of theelectron-phonon coupling constant and of the bare-electronic bandwidth is discussed and the bipolaron phase diagram is presented. Aclear crossover from the large-bipolaron to the small-bipolaronregime takes place.