Phase behavior of a system of particles with core collapse

TL;DR

This study explores the phase behavior of a two-dimensional particle system with a core plus repulsive interaction, revealing diverse ground states and anomalous thermodynamic properties like density maxima and negative slope coexistence lines.

Contribution

It demonstrates that particles with a core and additional repulsion can form various complex ground states, including quasicrystals, and exhibit water-like anomalies in their phase diagram.

Findings

Ground states include squares, chains, hexagons, and quasicrystals.

Presence of a negative slope in the solid-fluid coexistence line.

Fluid phase shows a temperature of maximum density similar to water.

Abstract

The pressure-temperature phase diagram of a one-component system, with particles interacting through a spherically symmetric pair potential in two dimensions is studied. The interaction consists of a hard core plus an additional repulsion at low energies. It is shown that at zero temperature, instead of the expected isostructural transition due to core collapse occurring when increasing pressure, the system passes through a series of ground states that are not triangular lattices. In particular, and depending on parameters, structures with squares, chains, hexagons and even quasicrystalline ground states are found. At finite temperatures the solid-fluid coexistence line presents a zone with negative slope (which implies melting with decreasing in volume) and the fluid phase has a temperature of maximum density, similar to that in water.