Diffusion in liquid metals is directed by competing collective modes

TL;DR

This study reveals how competing collective modes influence self-diffusion in liquid metals, showing a crossover in diffusion behavior near 1.4 times the melting temperature due to a shift from cage effects to vortex-like motions.

Contribution

It provides molecular dynamics evidence of a diffusion mechanism transition driven by collective particle modes in liquid metals near melting temperatures.

Findings

Crossover in diffusion coefficient at ~1.4 Tm

Decrease of cage effect with power-law decay in velocity autocorrelation

Transition from dense to fluid-like dynamics around 1.4 Tm

Abstract

The self-diffusion process in a dense liquid is influenced by collective particle movements. Extensive molecular dynamics simulations for liquid aluminium and rubidium evidence a crossover in the diffusion coefficient at about $1.4$ times the melting temperature $T_m$, indicating a profound change in the diffusion mechanism. The corresponding velocity auto-correlation functions demonstrate a decrease of the cage effect with a gradual set-in of a power-law decay, the celebrate {\it long time tail}. This behavior is caused by a competition of density fluctuations near the melting point with vortex-type particle patterns from transverse currents in the hot fluid. The investigation of the velocity autocorrelation function evidences a gradual transition in dynamics with rising temperature. The competition between these two collective particle movements, one hindering and one enhancing the diffusion process, leads to a non-Arrhenius-type behavior of the diffusion coefficient around $1.4~T_m$, which signals the transition from a dense to a fluid-like liquid dynamics in the potential energy landscape picture.