# Disappearance of the hexatic phase in a binary mixture of hard disks

## TL;DR

This study uses Monte Carlo simulations to investigate how adding small disks to a system of large hard disks affects the melting behavior, revealing the disappearance of the hexatic phase at low concentrations due to fractionation effects.

## Contribution

It demonstrates that the hexatic phase becomes unstable and disappears with minimal addition of small disks, highlighting the fragility of the hexatic phase in binary mixtures.

## Key findings

- Hexatic phase stability shrinks with small disk addition
- Transition changes from continuous to first-order
- Disappearance of hexatic phase at low small disk concentration

## Abstract

Recent studies of melting in hard disks have confirmed the existence of a hexatic phase occurring in a narrow window of density which is separated from the isotropic liquid phase by a first-order transition, and from the solid phase by a continuous transition. However, little is known concerning the melting scenario in mixtures of hard disks. Here we employ specialized Monte Carlo simulations to elucidate the phase behavior of a system of large ($l$) and small ($s$) disks with diameter ratio $\sigma_l/\sigma_s=1.4$. We find that as small disks are added to a system of large ones, the stability window of the hexatic phase shrinks progressively until the line of continuous transitions terminates at an end point beyond which melting becomes a first-order liquid-solid transition. This occurs at surprisingly low concentrations of the small disks, $c\lesssim 1\%$, emphasizing the fragility of the hexatic phase. We speculate that the change to the melting scenario is a consequence of strong fractionation effects, the nature of which we elucidate.