Finite-temperature Wigner solid and other phases of ripplonic polarons on a helium film

TL;DR

This paper investigates the phase diagram of electrons on helium, revealing a new ripplopolaron liquid phase and analyzing phase transitions, including reentrant melting, using a finite-temperature variational approach.

Contribution

It introduces a novel ripplopolaron liquid phase and provides detailed phase diagrams for electrons on helium, incorporating finite-temperature effects and varying coupling strengths.

Findings

Identification of a new ripplopolaron liquid phase.

Reentrant melting of the electron solid with temperature.

Agreement of phase regions with recent experiments.

Abstract

Electrons on liquid helium can form different phases depending on density, and temperature. Also the electron-ripplon coupling strength influences the phase diagram, through the formation of so-called "ripplonic polarons", that change how electrons are localized, and that shifts the transition between the Wigner solid and the liquid phase. We use an all-coupling, finite-temperature variational method to study the formation of a ripplopolaron Wigner solid on a liquid helium film for different regimes of the electron-ripplon coupling strength. In addition to the three known phases of the ripplopolaron system (electron Wigner solid, polaron Wigner solid, and electron fluid), we define and identify a fourth distinct phase, the ripplopolaron liquid. We analyse the transitions between these four phases and calculate the corresponding phase diagrams. This reveals a reentrant melting of the electron solid as a function of temperature. The calculated regions of existence of the Wigner solid are in agreement with recent experimental data.