Microstructure and superlattices of mesoscopically confined Yukawa solids

We numerically demonstrate the spontaneous formation of superlattices with Moiré patterns in tightly confined Yukawa solids bounded by flat, structure-free walls. Unlike conventional superlattices created by epitaxial growth on patterned substrates or lattice-mismatched interfaces, the ordering reported here arises solely from confinement induced layering and the symmetry breaking of the absence of particles beyond the boundary, especially under long-range interaction. Using molecular dynamics simulations of three-dimensional Yukawa systems under strong confinement, we show that cooling from the liquid state produces particle layering parallel to the boundaries, with the outermost layers exhibiting distinct packing compared to the nearly uniform inner region. The reduced horizontal repulsion at the boundaries leads to a smaller intralayer lattice constant (higher packing density) in the outermost layer, together with a larger interlayer spacing to its adjacent layer. This asymmetry results in nearly equilateral triangular intralayer packing in the first layer and the emergence of superlattices between the first two boundary layers on each side of the system. Systematic analysis across a wide range of screening parameters κ (the ratio of mean Wigner Seitz radius to Debye length) reveals a crossover from large lattice constant and orientation mismatches at small κ, to partial alignment of lattice axes at intermediate κ, and finally to nearly uniform crystalline order at large κ. The transition of inner layers from BCO (body-centered orthogonal) to HCP (hexagonal close packed) and FCC (face-centered cubic) structures is also clarified. These findings uncover a new physical route for superlattice formation in tightly confined long-range particle interacting systems, free of external templating.