Ab initio study of the atomic motion in liquid metal surfaces: comparison with Lennard-Jones systems

Abstract

It is established that liquid metals exhibit surface layering at the liquid-vapor interface, while dielectric simple systems, like those interacting through Lennard-Jones potentials, show a monotonic decay from the liquid density to that of the vapor. First principles molecular dynamics simulations of the liquid-vapor interface of several liquid metals (Li, Na, K, Rb, Cs, Mg, Ba, Al, Tl and Si), and the Na$_3$K$_7$ alloy near their triple points have been performed in order to study the atomic motion at the interface, mainly at the outer layer. Comparison with results of classical molecular dynamics simulations of a Lennard-Jones system shows interesting differences and similarities. The probability distribution function of the time of residence in a layer shows a peak at very short times and a long lasting tail. The mean residence time in a layer increases when approaching the interfacial region, slightly in the Lennard-Jones system, but strongly in the metallic systems. The motion within the layers, parallel to the interface, can be described as diffusion enhanced (strongly, in the case of the outermost layer) with respect to the bulk, for both types of systems, despite its reduced dimensionality in metals.