LONG-RANGE ORIENTATIONAL CORRELATIONS AT UNDERCOOLED LIQUID-METAL SURFACES

The liquid surface of a reconstructing metal such as Au or Pt is expec ted to exhibit a higher surface lateral density as compared to an equi valent slice of bulk liquid. Using a realistic potential and molecular dynamics, we have analyzed the surface behavior of liquid gold as a f unction of temperature. The density profile along the surface normal d evelops, as a consequence, marked layering oscillations, while surface atoms are forced to a nearly triangular lateral packing. The undercoo led surface, in particular, reveals the onset of very long-range sixfo ld orientational correlation [O-6(r)O-6(0)] similar to exp(-r/xi(6)). At the same time positional correlations do not vary appreciably. The growth of xi(6) upon cooling is dramatic near 1000 K, and the liquid metal surface appears to approach a transition into a hexatic phase at about 940 K. This temperature is, however, inaccessible to simulation , due to surface-initiated recrystallization on the nanosecond time sc ale.