Layer-resolved collective dynamics at the free surface of Cu–Ag alloy melts: an intrinsic-sampling molecular dynamics study

The collective dynamics of metallic melts near free surfaces govern interfacial transport, nucleation, and wetting, yet remain far less understood than their bulk counterparts—particularly in chemically complex alloys. Here we combine large-scale molecular dynamics simulations with the intrinsic sampling method (ISM) to resolve the layer-by-layer collective dynamics at the liquid–vapor interface of Cu–Ag alloys. Using the Williams–Mishin–Hamilton embedded-atom potential, we construct slab geometries for pure Cu, pure Ag, and CuxAg1-x alloys (x=0.25,0.5,0.75), identify intrinsic surface layers via intrinsic density profiles, and compute layer-resolved total and partial intermediate scattering functions together with their relaxation time spectra τ(k). For pure Cu and pure Ag, the outermost intrinsic layer exhibits a robust non-monotonic k-dependence: density fluctuations relax more slowly than in the bulk at small k, but are markedly accelerated at intermediate and large k. This reveals a universal coexistence of coordination-reduction-induced slowing-down and short-length-scale speeding-up at metallic free surfaces. In Cu–Ag alloys, strong Ag surface segregation and composition gradients generate a pronounced mismatch of dominant relaxation length scales among layers, manifested as systematic, layer-dependent shifts of the maxima in the total relaxation time τall(k). Partial ISFs further show that majority components preserve the dynamical patterns of the corresponding pure liquids, whereas minority components experience pronounced crowding or dilution, leading to strongly enhanced depth and composition sensitivity and, in some regimes, relaxation behavior opposite to that of the majority species. These results provide, to our knowledge, the first systematic dataset of layer-resolved collective dynamics at a binary metallic free surface, and offer microscopic insight into density-wave relaxation and interfacial transport in multicomponent metallic liquids.