The temperature dependence of the surface relaxation of Cu(111) and Ag(111) - Evidence for giant anharmonic effects

We have used medium energy ion scattering with channeling and blocking to measure the spacing between the two outermost atomic layers on the (111) faces of Cu and Ag as a function of temperature. The Ag(111) surface undergoes a dramatic transformation from being slightly contracted at room temperature to being strongly expanded (+10% of the bulk spacing) around 900°C. Cu(111) shows similar but smaller effects. The change of the surface lattice constant is paralleled by large enhancements of the surface vibration amplitudes. We therefore conclude that our observations are due to large anharmonic effects at the surface. We further discuss our results in terms of a recent microscopic theoretical analysis.