Direct evidence of non-acoustic collective modes in dynamics of molten Carbon

Abstract Understanding the structure and dynamics of molten Carbon extends beyond the study of carbon-rich planetary interiors, with perspective applications in nuclear fusion, material science and industrial processes. While the recent X-ray observation of liquid C rekindled the interest in its microscopic structure, it also reminds of the challenges in rationalising the complex dynamics extending beyond the hydrodynamic regime. Primary among them is the theoretical description of non-hydrodynamic processes in one-component liquids. Here, we report collective longitudinal and transverse propagating modes in molten Carbon at T = 5500 K and pressure range 10-40 GPa from ab initio simulations and machine learned molecular dynamics. We observe an unusual two-peak shape of the longitudinal current spectral functions pointing at a branch of non-acoustic propagating modes in the wave number range k > 1 Å −1 . By applying a generalized collective modes framework to recover the time dependence of time correlations in the system, we identify the peak at lower energy as a non-hydrodynamic mode, and ascribe it to out-of-phase motion of particles encapsulated in cages of their nearest neighbors.