Low-temperature crystal structures of the hard core square shoulder model

TL;DR

This study uses simulations and theoretical methods to explore the crystal structures formed by a simple model with two favored particle distances, revealing a variety of stable phases including new lattice types.

Contribution

It introduces a combined Monte Carlo and mean-field approach to map phase diagrams of the square shoulder model, uncovering novel crystal structures.

Findings

Identification of stable crystal phases including BCO lattices

Phase diagrams mapped as functions of density and temperature

Zero-temperature phase diagram for various shoulder widths

Abstract

In many cases, the stability of complex structures in colloidal systems is enhanced by a competition between different length scales. Inspired by recent experiments on nanoparticles coated with polymers, we use Monte Carlo simulations to explore the types of crystal structures that can form in a simple hard-core square shoulder model which explicitly incorporates two favored distances between the particles. To this end, we combine Monte Carlo-based crystal structure finding algorithms with free energies obtained using a mean-field cell theory approach, and draw phase diagrams for two different values of the square shoulder width as a function of the density and temperature. Moreover, we map out the zero-temperature phase diagram for a broad range of shoulder widths. Our results show the stability of a rich variety of crystal phases, such as body-centered orthogonal (BCO) lattices, not previously considered for the square shoulder model.