Angle-resolved collective dynamics of the interfacial liquid at an FCC crystal–melt interface

The collective dynamics of liquids near crystal–melt interfaces (CMIs) provide the microscopic input needed to predict anisotropic interface kinetics, yet their dependence on in-plane direction and wavelength remains poorly understood. Here we perform large-scale molecular-dynamics simulations of a Lennard–Jones FCC(100) crystal coexisting with its melt and apply a local formulation of the intermediate scattering function to map the relaxation times τ(q,θ,z) of the interfacial liquid. We focus on two characteristic wave-vector magnitudes corresponding to the principal and next-nearest reciprocal lattice vectors, |K→| and |G→|, and systematically vary the in-plane angle θ and distance z from the interface. For both |K→| and |G→|, we find strong spatial modulation of τ(q,θ,z) within a few atomic layers of the CMI, together with a pronounced in-plane anisotropy that peaks along the diagonal direction θ=π/4. While high-symmetry directions (θ=0 and π/2) exhibit interfacial acceleration of collective relaxation relative to the bulk liquid, the θ=π/4 direction shows a dramatic slowdown, especially for |G→|, where the relaxation time can exceed the bulk value by nearly a factor of two and displays transient solid-like locking. These results reveal a rich and strongly anisotropic dynamical landscape in the interfacial liquid and provide the angle- and wavelength-resolved relaxation times required to incorporate directional collective dynamics into Ginzburg–Landau-type kinetic theories of crystal growth.