Polaron Crystallization and Melting: Effects of the Long-Range Coulomb Forces

TL;DR

This paper investigates the stability and melting of polaronic Wigner crystals in ionic dielectrics, revealing how Coulomb screening, polaron mass, and dipolar interactions influence the insulator-metal transition and dielectric properties.

Contribution

It introduces a comprehensive analysis of the competing effects of Coulomb screening and polaron mass on crystal stability and identifies a new mechanism for the insulator-metal transition involving dipolar interactions.

Findings

Quantum melting depends on polaron dissociation and density.

Strong coupling prevents the formation of a polaron liquid.

Dipolar interactions cause instability in vibrational modes, influencing the transition.

Abstract

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, which destabilizes the crystal, compensated by (ii) the increase of the carrier mass (polaron mass). 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 both the electron and the polarization, we show that there is a competition between the dissociation of the polarons at the insulator-to-metal transition (IMT), and a melting towards a polaron liquid. We find that at strong coupling, a liquid state of dielectric polarons cannot exist, and the IMT is driven by the polaron dissociation. Next, taking into account the dipolar interactions between localized carriers, we show that these are responsible for an instability of the transverse vibrational modes of the polaronic Wigner crystal as the density increases. This provides a new mechanism for the IMT in doped dielectrics, which yields interesting dielectric properties below and beyond the transition. An optical signature of such a mechanism for the IMT is provided.