Polaron crystallization and the metal-insulator transition

The Crystallization of polarons at finite density, due to the long-range Co ulomb forces - when no bipolarons can be formed - is discussed close to the metal-insulator transition (MIT). As a function of the density, the meltin g 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, rat her than the melting towards a polaron liquid. In this regime, we demonstra te that an instability of the transverse vibrational modes of the polaron c rystal occurs as the density increases. This provides a new physical mechan ism for the MIT in polar materials, for which an experimental signature is predicted.