Diffusion in liquid metals is directed by competing collective modes

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>T</a:mi><a:mi>m</a:mi></a:msub></a:math>, indicating a profound change in the diffusion mechanism. The corresponding velocity autocorrelation functions demonstrate a decrease of the cage effect with a gradual set in of a power-law decay, they celebrate 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 <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>1.4</b:mn><b:mspace width="0.16em"/><b:msub><b:mi>T</b:mi><b:mi>m</b:mi></b:msub></b:mrow></b:math>, which signals the transition from a dense to a fluidlike liquid dynamics in the potential energy landscape picture.