Shear horizontal vibrations at the (0001) surface of beryllium

The authors report the results of a high-resolution Electron-Energy Loss Spectroscopy (EELS) investigation of Be(0001) surface. Two dispersive surface vibrational modes are observed in the {bar {Gamma}} to {bar M} direction: a sagittal-plane mode (the Rayleigh wave) and a shear horizontal mode. They have compared their experimental results to a 300-layer slab calculation and to a semi-infinite Green`s function calculation of Sameth and Mele. Compared with the experimental results, both calculations predict a higher energy for the Rayleigh wave and lower energy for the shear horizontal mode. These results are consistent with stronger in-plane bonding and weaker interplanar bonding at the surface, in accord with other theoretical predictions.