SHEAR HORIZONTAL VIBRATIONS AT THE (0001) SURFACE OF BERYLLIUM

We report the results of a high-resolution Electron-Energy Loss Spectr oscopy (EELS) investigation of the Be(0001) surface. Two dispersive su rface vibrational modes are observed in the ($) over bar T to ($) over bar M direction: a sagittal-plane mode (the Rayleigh wave) and a shea r horizontal mode. We have compared our experimental results to a 300- layer slab calculation and to a semi-infinite Green's function calcula tion of Sameth and Mele. Both calculations predict a higher energy for the Rayleigh wave and lower energy for the shear horizontal mode than is experimentally measured. These results are consistent with stronge r in-plane bonding and weaker interplanar bonding at the surface, in a ccord with other theoretical predictions.