Dispersion and dipole activity of surface phonons on Si(111) 2×1

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Abstract
Using an empirical tight-binding theory for structural energies we study vibrational excitations on the π-bonded chain model of the Si(111) 2×1 surface. We calculate the phonon spectrum of the surface and find a number of modes that have the form of elementary vibrational excitations of the zigzag chains of surface atoms of the reconstructed surface. In the acoustic part of the spectrum a strong phonon-energy renormalization effect generated by electronic transitions causes an unusual softening of the Rayleigh wave at the zone boundary; this result is consistent with recent He-atom scattering experiments. We also calculate the linearized response of the charge density to lattice displacements, which we use to obtain the phonon-assisted contribution to the surface conductivity. A longitudinal-optical phonon along the surface zigzag chains generates a very large dynamic charge (∼0.75e) and is assigned to the anomalously strong dipole-active surface phonon that has been observed experimentally by high-resolution electron-energy-loss spectroscopy.