Surface phonons of the Si(001)(2x1) surface

The surface phonons along the high symmetry <(Gamma)over bar>(J) over bar a nd <(Gamma)over bar>(J) over bar' directions of the Si(001)(2 X 1) surface have been investigated by using high-resolution electron energy loss spectr oscopy. Losses are observed at 12, 20, 30, 33, 49, 59, and 64 meV for the < (Gamma)over bar> point, which are associated with the motions of the surfac e Si dimers. In particular, the 20- and 64-meV losses are assigned as the d imer-rocking (related with dimer buckling) and dimer-bond stretching modes, respectively The dimer-rocking mode shows an upward dispersion near the <( Gamma)over bar> point and characteristic flat dispersion near the (J) over bar' point. The dimer-bond stretching mode is dispersionless near the <(Gam ma)over bar> point. The Rayleigh phonons are observed at 8 and 13 meV for t he (J) over bar and (J) over bar' points, respectively, which are nearly in resonance with the projected density of states of bulk phonons. The presen t results are in reasonable agreement with the recent lattice dynamical cal culations. [S0163-1829(99)00339-2].