Polaron crystallization and melting: Effects of the long-range Coulomb forces

On examining the stability of a Wigner crystal in an ionic dielectric, two competitive effects due to the polaron formation are found to be important: (i) the screening of the Coulomb force 1/epsilon(s)r, which destabilizes t he crystal, compensated by (ii) the increase of the carrier mass (polaron m ass). The competition between the two effects is carefully studied, and the quantum melting of the polaronic Wigner crystal is examined by varying the density at zero temperature. By calculating the quantum fluctuations of bo th the electron and the polarization, we show that there is a competition b etween the dissociation of the polarons at the insulator-to-metal transitio n (IMT), and a melting towards a polaron liquid. We find that at strong cou pling, a liquid state of dielectric polarons cannot exist, and the IMT is d riven by the polaron dissociation. Next, taking into account the dipolar in teractions between localized carriers, we show that these are responsible f or an instability of the transverse vibrational modes of the polaronic Wign er crystal as the density increases. This provides a new mechanism for the IMT in doped dielectrics, which yields interesting dielectric properties be low and beyond the transition. An optical signature of such a mechanism for the IMT is provided.