Wigner crystallization in a polarizable medium

Abstract

We present a variational study of the two- and three-dimensional Wigner crystal phase of large polarons. The method generalizes that introduced by S. Fratini and P. Quémerais [Mod. Phys. Lett. B 12 1003 (1998)]. We take into account the Wigner crystal normal modes rather than a single mean frequency in the minimization procedure of the variational free energy. We calculate the renormalized modes of the crystal as well as the charge polarization correlation function and polaron radius. The solid phase boundaries are determined via a Lindemann criterion, suitably generalized to take into account the classical-to-quantum crossover. In the weak electron-phonon coupling limit, the Wigner crystal parameters are renormalized by the electron-phonon interaction, leading to a stabilization of the solid phase for low polarizability of the medium. Conversely, at intermediate and strong coupling, the behavior of the system depends strongly on the polarizability of the medium. For weakly polarizable media, a density crossover occurs inside the solid phase when the renormalized plasma frequency approaches the phonon frequency. At low density, we have a renormalized polaron Wigner crystal, whereas at higher densities the electron-phonon interaction is weakened irrespective of the bare electron-phonon coupling. For strongly polarizable media, the system behaves as a Lorentz lattice of dipoles. The abrupt softening of the internal polaronic frequency predicted by Fratini and Quemerais is observed near the actual melting point only at very strong coupling, leading to a possible liquid polaronic phase for a wider range of parameters.