INCOMPLETE MELTING OF PB(001) AND VICINAL SURFACES

Medium energy ion scattering experiments on a cylindrically shaped Pb crystal demonstrate that at temperatures above 500 K, a finite amount of disorder develops at the Pb(001) surface and at vicinal surfaces with 5-degrees and 10-degrees miscut angle. The number of disordered atoms on these surfaces saturates between 1 and 0.05 K below the bulk melting point. The maximum amount of disorder increases with miscut angle from 1.3 ML on Pb(001) to 1.9 and 3.5 ML on the 5-degrees and 10-degrees vicinal surfaces, respectively. The observed scattered ion yields cannot be ascribed to mere anomalously strong surface vibrations. We conclude that these surfaces develop liquid-like disorder which can be interpreted as incomplete melting due to layering effects. The measurements show no sign of a prewetting transition up to (T(m) - 0.05 K). Furthermore, we have no indication of a surface-melting induced faceting transition of the two vicinal surfaces.