Driving a many-body system out of equilibrium induces phenomena such as the emergence and decay of transient states, which can manifest itself as pattern and domain formation. The understanding of these phenomena expands the scope of established thermodynamics into the out-of-equilibrium domain. Here, we study the out-of-equilibrium dynamics of a bosonic lattice model subjected to a strong DC field, realized as ultracold atoms in a strongly tilted triangular optical lattice. We observe the emergence of pronounced density wave patterns - which spontaneously break the underlying lattice symmetry - using a novel single-shot imaging technique with single-site resolution in three-dimensional systems, which even resolves the domain structure [1]. We explain the dynamics as arising from resonant pair tunneling processes within an effective description of the tilted Hubbard model [2]. More broadly, we establish the far out-of-equilibrium regime of lattice models subjected to a strong DC field, as an exemplary and paradigmatic scenario for transient pattern formation.
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