3:30–4:30 pm Maria Geoppert-Meyer
Flocks vs. Droplets: Metastability and Restored Stability in Active Flocks
Sunghan Ro, Harvard University
In equilibrium systems with continuous rotational symmetry, long-range order is not possible in two dimensions due to large fluctuations from Goldstone mode excitations. In contrast, nonequilibrium systems can evade this constraint. In the 90s, Vicsek and colleagues observed flocking with long-range order in self-propelled spins with continuous orientations and polar alignment. Analytical studies have since confirmed that polar order in such models remains stable against large-scale orientation deformations analogous to Goldstone modes.
But how robust are flocking models to abrupt perturbations in alignment? For instance, what happens when a droplet of particles moving against the global order is introduced? In this colloquium, I will present results from various flocking models subjected to such perturbations. The findings indicate that in many cases, flocking is fragile to droplet excitations, rendering its long-range order metastable. However, this instability does not completely destroy long-range order. In continuous models with strong aligning interactions, the interface between the inserted droplet and the ordered phase itself becomes unstable. I will discuss how this 'instability of instability' ultimately leads to the restoration of long-range order.