Observation of a 10-meV Einstein Oscillator Mode on the Si(111) (2×1) Surface

Surface-phonon dispersion curves have been measured by He-atom inelastic scattering for in situ cleaved Si(111) (2×1). A mode with a flat dispersion curve at 10.5 meV has been observed which can be shown experimentally to be intrinsic to the reconstructed surface. This finding explains the anomaly in the temperature dependence of the optical excitation of electronic surface states but is in contradiction to lattice-dynamical calculations based on the $\pi$-bonded-chain model.