Observation of surface-initiated melting

Ion-shadowing and blocking measurements reveal a reversible order-disorder transition at the (110) surface of a lead crystal well below its melting point $T_m$. The transition starts with partial disordering of the surface region at approximately 0.75$T_m$. Closer to $T_m$ a completely disordered film builds up with a thickness that increases rapidly as the temperature approaches $T_m$. 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.