Interactions-driven structural morphology and motion in two-dimensional active Brownian systems.

While the structure and dynamics of spherical active Brownian particles are now relatively well understood, the influence of attractive interactions on these particles continues to present intriguing open questions. In this study, we investigate the structural morphology and particle velocity in two-dimensional systems with specific attractive interactions, informed by purely repulsive systems. This exploration is conducted through computer simulations, with variations in the Péclet number and packing fractions. Our systematic exploration reveals a novel non-monotonic evolution of structural and velocity ordering driven by attraction-activity competition, representing a fundamental departure from conventional active matter behavior. In particular, we identify a critical transition from attraction-dominated to activity-dominated mechanisms as activity intensifies, leading to complex structural reorganization and the emergence of distinct dynamical phases. Moreover, our study reveals significant correlations between topological defects, spatial structures, and particle motion, where defects serve as key mediators of the strong interdependence between structural order and velocity alignment. These findings advance fundamental understanding of non-equilibrium physics and provide novel frameworks for controlling collective behavior in active materials, highlighting the substantial impact of attractive forces in active environments beyond traditional motility-induced phase separation.