The Origin of Force Constant Softening on the Close Packed Noble Metal Surfaces

A qualitatively new mechanism is proposed for the well-known in-plane force softening found on many noble-metal surfaces. The mechanism is discussed in terms of one of the recently developed many-body force schemes and is shown to amount to an effective decrease of the lateral atomic .sizes of the surface atom. Its strong sensitivity to the strength of the manybodyforce terms, as well as to surface relaxation and reconstruction, is emphasized. -As a nontrivial illustration, we calculate the phonons of Au(lll), and show that the exceedingly large softening leading to the recently observed anomalous longitudinal resonance is directly related to our proposed mechanism.