Ab initio studies of layering behavior of liquid sodium surfaces and interfaces

摘要

We have studied the liquid surface of sodium with extensive ab initio molecular dynamics simulations based on ensemble density-functional theory. We find clear evidence of layering in the direction perpendicular to the surface that persists to temperatures more than 100 K above the melting point. We also observe clear Friedel oscillations in the electronic density response to the presence of a surface, but their direct effect on atomic layering is ruled out. A careful finite-size effect analysis accompanies our results, showing that liquid slabs 20-25 A thick capture the essential details of the surface structure. We conclude that geometrical confinement is the common cause for layer formation, which is similar to what happens at a liquid-solid interface: at a free liquid surface, the rapid decay of the electronic density from the bulk liquid value to zero in the vapor forms a hard wall against which the atoms pack. Finally, we predict x-ray reflectivities from ab initio molecular dynamics data that include some of the large surface-normal wave vector-transfer regions that, for alkali metals, are not accessible to experiments.