New type of surface atomic vibrational modes of the (100) surface of the NaCl-type lattice

On the basis of a lattice-dynamical calculation using the nearest-neighbor force-constant model of a 15-layer slab, we have discussed the optical and acoustical surface phonons of the (100) surface of the NaCl-type lattice. The stiffening of the in-plane force constant of the outermost layer has caused drastic changes of the surface phonon modes associated with the parallel vibrations to the surface. Instead of Lucas modes, the stiffening of 20% results in a new type of surface vibrational modes, which explains qualitatively the recent experimental dispersion curves of the longitudinal surface phonons of the TaC(100) and NbC(100) surfaces.