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 -bonded--chain model of the Si (111) 21 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 -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.