Abstract
The surface phonons along the high symmetry $\overline{\ensuremath{\Gamma}}\overline{J}$ and $\overline{\ensuremath{\Gamma}}{J}^{\ensuremath{'}}$ directions of the $\mathrm{Si}(001)(2\ifmmode\times\else\texttimes\fi{}1)$ surface have been investigated by using high-resolution electron energy loss spectroscopy. Losses are observed at 12, 20, 30, 33, 49, 59, and 64 meV for the $\overline{\ensuremath{\Gamma}}$ point, which are associated with the motions of the surface Si dimers. In particular, the 20- and 64-meV losses are assigned as the dimer-rocking (related with dimer buckling) and dimer-bond stretching modes, respectively. The dimer-rocking mode shows an upward dispersion near the $\overline{\ensuremath{\Gamma}}$ point and characteristic flat dispersion near the ${J}^{\ensuremath{'}}$ point. The dimer-bond stretching mode is dispersionless near the $\overline{\ensuremath{\Gamma}}$ point. The Rayleigh phonons are observed at 8 and 13 meV for the $\overline{J}$ and ${J}^{\ensuremath{'}}$ points, respectively, which are nearly in resonance with the projected density of states of bulk phonons. The present results are in reasonable agreement with the recent lattice dynamical calculations.
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