Abstract
A microscopic theory of surface elementary excitations is presented, with particular emphasis on semiconductors. Within the framework of this formalism we study the conditions for the appearance of electronic (charge- and spin-density wave) instabilities at surfaces. Quantitative calculations for a Si(111) slab, based on a self-consistent pseudopotential bandstructure, display a pronounced effect of the excitonic (electron-hole) and RPA local-field many-body interactions. Our results show that the ideal paramagnetic surface is unstable with respect to SDW's with wave vectors nearly corresponding to (2×1) and (7×7) superstructures. This is in accordance with recent total energy calculations based on local spin-density pseudopotentials.
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