Explanation of the Force Constant Softening in Noble Metals: the Case of Au(111)

A new mechanism for the well known in-plane force softening on the noble metal surfaces is presented. The mechanism is discussed in terms of a many-body force scheme and is shown to amount to an effective decrease of the lateral atomic ‘size’ of the surface atom. Its strong sensitivity to surface relaxation and reconstruction is emphasized. As a non-trivial illustration we calculate the phonon density of states of Au(111) and show that the large softening leading to the observed anomalous longitudinal resonance is directly related to our proposed mechanism. We also discuss the importance of the surface stress on the statics and dynamics of these surfaces.