Molecular-dynamics study of surface premelting effects

The thermodynamical and structural behavior of a (110) face of a fcc (12-6) Lennard-Jones solid has been investigated by molecular-dynamics simulation on the solid-gas coexistence line. The temperature dependence of the relevant structural and mass-transport properties shows the following. (a) Despite the high degree of disorder which gradually appears on surface layers when the temperature is increased, the surface retains its solidlike character up to temperatures (T0. 64/k₁) very close to the triple point (Tₓ=0. 68/k₁). This conclusion does not confirm the findings of previous theoretical work predicting the formation of a liquid surface layer well below the bulk melting point. (b) The large concentration of vacancy-adatom pairs, produced at the surface in the high-temperature range, accounts for the high values of the surface diffusivity. (c) The Arrhenius plot of defect concentration indicates a progressive decrease of their formation energy for temperatures ranging from Tₑ=0. 8T₌ to the melting point. Consistently, the order parameter decreases slowly with increasing temperature up to Tₑ but from T=Tₑ to the melting point it decreases much more rapidly than predicted by the extrapolation of the low-temperature data. These results are qualitatively consistent with the onset of a surface-roughening transition, in agreement with recent experimental results obtained from helium scattering on (110) copper surfaces.