Symmetry-broken states such as magnetic order and superconductivity are the hallmark of complex properties in solids. An intriguing new direction in condensed-matter physics is to manipulate these properties on the fastest possible time scales using ultrashort laser pulses. Theoretically, however, the collective many-particle dynamics that is responsible for the formation and melting of long-range order is associated with many open questions.Here, we combine two state-of-the-art numerical techniques—time-dependent density matrix renormalization group and nonequilibrium dynamical mean-field theory—to create a model system that represents interacting electrons on a bipartite lattice in which electrons can tunnel between sites. We prepare this model such that particles on neighboring sites initially align their magnetic moments in an antiparallel manner (i.e., representing antiferromagnetic order). The particles can then move between lattice sites, which leads to the melting of the magnetic order. We theoretically show that the precise movement mechanism depends strongly on the interaction between the particles: For strong interactions, the system behaves like a collection of localized magnetic moments. For weak interactions, on the other hand, the dynamics reflects the existence of coherent quasiparticles, which are typically restricted to excitations close to the ground state. In our case, these quasiparticles prevail on short times even though the system is strongly excited.Our setup, which is well suited for experiments using cold atoms, has the ability to reveal the crossover between localized and itinerant behavior. In the future, similar studies of systems with several active orbitals may make it possible to better understand how complex solids can relax into entirely new—and possibly thermodynamically hidden—phases
