Anomalous collective diffusion in two-dimensional active particle systems

Anomalous diffusion is an important physical property of active Brownian particles and results mainly from their rich dynamic behavior. We use nonequilibrium molecular dynamics methods to study the mass diffusion of two-dimensional active particles under concentration gradients. It is found that active particles exhibit anomalous collective diffusion behavior that violates Fick's law, whereby their diffusion coefficient diverges as the system size increases. The particles exhibit a flow pattern characterized by large-scale structures, where their movement trajectories and directions are correlated and synchronous. This correlation gives rise to the phenomenon of superdiffusion. This work provides a detailed interpretation of the divergent characteristics and physical mechanisms of active particles based on long-time correlations, velocity spectrum analysis, and local synchronization rates. Our results offer a path for deeply understanding the nonequilibrium dynamic properties of active particles under complex conditions.