Research on complex nonequilibrium processes and nonlinear dynamics has seen remarkable growth, revealing rich behavior across classical and quantum systems. Collective effects span wide spatiotemporal scales and arise from intricate interdependencies among many constituents, making their study both challenging and central to modern science. Such phenomena naturally appear in statistical physics, condensed matter, quantum physics, nonlinear optics, chemistry, and materials science, as well as in multidisciplinary areas such as neuroscience, complex networks, climate, and social systems. For example, in the quantum domain, when many degrees of freedom interact through a shared field or mediating medium, their response reorganizes into collective modes absent at the single-particle level. A common thread is the emergence of macroscopic, measurable signatures—scaling of intensities and linewidths, correlation functions, and dynamical response—that reveal how interactions, driving, and dissipation generate quantum collective phenomena.

This Research Topic focuses on the dynamics of nonequilibrium and nonlinear processes across classical and quantum domains. Building on the first edition, Volume II adds a quantum strand on collective effects in large ensembles approaching the thermodynamic limit, where interactions, driving, and dissipation generate emergent behavior and criticality far from equilibrium. With this issue, we aim to gather diverse research that provide fresh insights into collective dynamics in complex systems. The goal is to assemble contributions across disciplines that advance both foundational understanding and applications by (i) clarifying mechanisms of cooperative radiation, ordering, and transport; (ii) developing or benchmarking theoretical and numerical methods; and (iii) establishing clear, testable signatures—scaling laws, correlation statistics, and dynamical response—that connect minimal models to experiments. Work spanning theory, numerical, and experimental approaches is welcome across disciplines. Emphasis is on nonequilibrium pathways to phase transitions and collective critical behavior; finite-size scaling toward the thermodynamic limit; and how structured environments and finite-propagation effects shape memory and effective couplings. By integrating results across subfields, this Special Topic aims to sharpen the conceptual framework for collective dynamics and identify routes toward metrology and device-level implementations.

The topics include, but are not limited to, the following list, grouped into priority and supplementary areas. The priority list of topics outlines the core areas of interest, focusing primarily on fundamental processes in nonequilibrium and nonlinear physics, and the emergence of collective dynamics across a range of physical, chemical, and complex systems in both classical and quantum regimes. The supplementary list of topics reflects broader applications and interdisciplinary connections that enrich and extend these central themes, with particular emphasis on experimental approaches to probing such phenomena.

Priority topics (classical and quantum):
• Nonequilibrium and nonlinear processes in physics and physical chemistry
• Disordered, complex, and chaotic systems
• Collective behavior and self-organization
• Symmetry breaking and pattern formation
• Finite-size scaling toward the thermodynamic limit
• Connecting microscopic models to observables and dynamical response

Supplementary topics:
• Cavity, circuit and waveguide QED
• Magnonic and photonic media, BEC and polariton fluids
• Neuronal avalanche dynamics
• Epidemic spreading and contagion models
• Climatic changes and environmental dynamics
• Development of experimental methods for investigating nonequilibrium and nonlinear behavior

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Nonequilibrium and Nonlinear Processes in Collective Dynamical Phenomena, Volume II