Observation of surface-initiated melting

Ion-shadowing and blocking measurements reveal a reversible order--disorder transition at the (110) surface of a Pb crystal well below its melting point T sub m . The transition starts with partial disordering of the surface region at approx 0.75 T sub m . Closer to T sub m a completely disordered film builds up with a thickness that increases rapidly as the temperature approaches T sub m . Electron diffraction patterns show the loss of two-dimensional order of the Pb(110) surface, at a temperature where the surface is nearly completely disordered. A detailed analysis of the surface cleanliness provides evidence against the possible role of surface impurities in the observed effects. 73 ref.--AA