Phase diagram of two-dimensional hard ellipses

TL;DR

This study maps the phase diagram of two-dimensional hard ellipses, revealing various phase transitions and their nature through advanced Monte Carlo simulations, including isotropic, nematic, plastic, and solid phases.

Contribution

It provides the first detailed phase diagram of 2D hard ellipses using replica exchange Monte Carlo, highlighting the nature of phase transitions at different anisotropies.

Findings

Identified four distinct phases: isotropic, nematic, plastic, and solid.

Observed continuous isotropic-nematic transitions.

Supported first order transition scenarios with bimodal probability densities.

Abstract

We report the phase diagram of two-dimensional hard ellipses as obtained from replica exchange Monte Carlo simulations. The replica exchange is implemented by expanding the isobaric ensemble in pressure. The phase diagram shows four regions: isotropic, nematic, plastic, and solid (letting aside the hexatic phase at the isotropic-plastic two-step transition [PRL 107, 155704 (2011)]). At low anisotropies, the isotropic fluid turns into a plastic phase which in turn yields a solid for increasing pressure (area fraction). Intermediate anisotropies lead to a single first order transition (isotropic-solid). Finally, large anisotropies yield an isotropic-nematic transition at low pressures and a high-pressure nematic-solid transition. We obtain continuous isotropic-nematic transitions. For the transitions involving quasi-long-range positional ordering, i. e. isotropic-plastic, isotropic-solid, and nematic-solid, we observe bimodal probability density functions. This supports first order transition scenarios.