Free surface effects on thermodynamics and glass formation in simple monatomic supercooled liquids

Free surface effects on the thermodynamics and glass formation in simple monatomic supercooled liquids with the Lennard-Jones-Gauss interaction potential were studied by the molecular dynamics simulations. Glass with two free surfaces was obtained by cooling from the melt. We found the following important new results: Free surfaces significantly enhance atomic mobility in the system compared to that of the bulk and induce the formation of so-called layer structure of the interior of both liquid and glassy states. A mobile surface layer in the system exists for a wide temperature range; i.e., the thickness of the mobile surface layer and the discrepancy between atomic mobility in the surface and that in the interior have a tendency to increase with temperature. The atomic mechanism of glass formation in supercooled liquids with free surfaces exhibits heterogeneouslike behavior, unlike the homogeneous behavior observed in the bulk; i.e., the solidlike domain initiates/enhances in the interior and simultaneously grows outward to the surface to form a glassy solid phase. The interior of glass with free surfaces exhibits a stronger local icosahedral order compared to that of the bulk, and it may lead to higher stability of the glassy state compared to that of the latter. In contrast, the surface shell has a more porous structure and contains a large number of undercoordinated sites.