On the metastability of the hexatic phase during the melting of two-dimensional charged particle solids

TL;DR

This study uses molecular dynamics simulations to investigate the stability of the hexatic phase in two-dimensional charged particle systems, concluding that the phase is metastable and dependent on simulation conditions.

Contribution

It demonstrates that the hexatic phase is metastable and highlights the importance of simulation time and size for accurate phase characterization.

Findings

Hexatic phase is metastable and disappears over long simulation times.

Increasing particle number alone does not confirm the existence of the hexatic phase.

Proper thermalization requires increasing both system size and simulation duration.

Abstract

For two-dimensional many-particle systems first-order, second-order, single step continuous, as well as two-step continuous (KTHNY-like) melting transitions have been found in previous studies. Recent computer simulations, using particle numbers in the $\geq 10^5$ range, as well as a few experimental studies, tend to support the two-step scenario, where the solid and liquid phases are separated by a third, so called hexatic phase. We have performed molecular dynamics simulations on Yukawa (Debye-H\"uckel) systems at conditions earlier predicted to belong to the hexatic phase. Our simulation studies on the time needed for the equilibration of the systems conclude that the hexatic phase is metastable and disappears in the limit of long times. We also show that simply increasing the particle number in particle simulations does not necessarily result in more accurate conclusions regarding the existence of the hexatic phase. The increase of the system size has to be accompanied with the increase of the simulation time to ensure properly thermalized conditions.