Polaron crystallization and the metal-insulator transition

The Crystallization of polarons at finite density, due to the long-range Coulomb forces - when no bipolarons can be formed - is discussed close to the metal-insulator transition (MIT). As a function of the density, the melting is examined at zero temperature. By calculating the quantum fluctuations of both the electron and the polarization, we show that at strong electron-phonon coupling the dissociation of the polarons at the MIT is favored, rather than the melting towards a polaron liquid. In this regime, we demonstrate that an instability of the transverse vibrational modes of the polaron crystal occurs as the density increases. This provides a new physical mechanism for the MIT in polar materials, for which an experimental signature is predicted.