Renormalized acoustic branch in the vibrational spectrum of theπ-bonded–chain model of Si(111)21

We calculate the low-frequency phonon spectrum of the {π}-bonded--chain model of the Si(111)2×{}1 surface, and find an explanation for the surprising observation, in He-atom scattering experiments, of a low-energy dispersionless surface phonon. This feature results from strong electron polarization effects, which arise from the quasi one-dimensional nature of the {π}-bonded--chain model. The assignment of this mode as an Einstein oscillator is incorrect. Our calculations are based on Chadi's tight-binding theory for structural energies, with the addition of a Hubbard interaction.