Abstract
Active matter has been much studied for its intriguing properties such as collective motion, motility-induced phase separation and giant fluctuations. However, it has remained unclear how the states of active materials connect with the equilibrium phases. For two-dimensional systems, this is also because the understanding of the liquid, hexatic, and solid equilibrium phases and their phase transitions is recent. Here we show that two-dimensional self-propelled point particles with inverse-power-law repulsions moving with a kinetic Monte Carlo algorithm without alignment interactions preserve all equilibrium phases up to very large activities. Furthermore, at high activity within the liquid phase, a critical point opens up a gas–liquid motility-induced phase separation region. In our model, two-step melting and motility-induced phase separation are thus independent phenomena. We discuss the reasons for these findings to be common to a wide class of two-dimensional active systems.
Highlights
Active matter has been much studied for its intriguing properties such as collective motion, motility-induced phase separation and giant fluctuations
Active matter has been intensely studied for its wealth of intriguing properties, such as collective motion[1], motilityinduced phase separation (MIPS)[2], and giant fluctuations away from criticality[3]
We map out the full quantitative phase diagram, and we show that the active system preserves all equilibrium phases
Summary
Active matter has been much studied for its intriguing properties such as collective motion, motility-induced phase separation and giant fluctuations. It has remained unclear how the states of active materials connect with the equilibrium phases. We show that two-dimensional self-propelled point particles with inverse-power-law repulsions moving with a kinetic Monte Carlo algorithm without alignment interactions preserve all equilibrium phases up to very large activities. At high activity within the liquid phase, a critical point opens up a gas–liquid motility-induced phase separation region. At a high enough activity, in the liquid phase, a critical point opens up a gas–liquid MIPS region This demonstrates that the two-step melting and MIPS are independent phenomena. As our model is minimal, we expect this finding to be robust and the independence to be common to a wide class of two-dimensional active systems
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