Phase transitions in hard disk systems with Vicsek type and non-reciprocal interactions

The phase diagram of self-propelled hard disk systems is investigated via extensive event-driven molecular dynamics simulations. These systems are aligned by both the original Vicsek-type interaction between neighbors within a fixed cutoff radius and non-reciprocal interactions. The model incorporates the competition between two intrinsic order parameters: the conventional polar order-disorder transition (representing the collective motion of the velocity field throughout the system) from the original Vicsek model, and the global orientational order in the solid-fluid hard disk phase transition (known as the Alder transition) arising from excluded volume effects of repulsive forces between hard disks at contact. Due to the excluded volume effect of hard disks, phase separation induced by motility is suppressed by the incompressible states at high packing fractions. We observe distinct fluctuations around the phase transition point and investigate anomalous shifts in the transition point by varying noise intensity and packing fraction. Finally, we elucidate the microscopic origin of these anomalous phase transition shifts.