Soft Surface Phonons and Surface Reconstruction

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Abstract
We propose a new mechanism which can lead to reconstruction in the surface region of a large class of materials. The mechanism is based on the fact that the dynamic effective charges present on the atoms near the surface can be different from those on the atoms in the bulk. The long-range interaction of these charges can cause the surface phonon to be lowered in frequency. We present numerical calculations of the normal-mode frequencies of a simple-cubic monatomic slab [(001) faces] of 15 layers when surface-induced dynamic effective charges are present near one of the surfaces. For reasonable values of the short-range atomic force constants, and for values of the dynamic effective charge on the surface layer of the order of 0.5e, the surface is unstable and recontruction occurs. Various patterns of reconstruction can be obtained depending upon the values of the short-range force constants.