Cu:Si(111) Incommensurate (5.55 x 5.55) Surface Reconstruction: Helium Beam Measurements of Diffraction and Surface Phonons.

Elastic and inelastic atomic helium scattering has been used to characterize the Cu:Si(111) "quasi-5x5" surface. The surface superlattice is incommensurate with the underlying Si(111) structure, 5.55 times larger, appears to be limited to the outermost layers, yet is not a simple incommensurate overlayer. The helium diffraction indicates a long range coherence of the reconstruction which, while not incompatible with a 2D quasi-crystal, would severely limit any quasi-crystal correlation or tiling rules. The surfaces phonons have intriguing characteristics including a lack of band gaps at fractional order "zone boundaries" and a continuous transition from extended mode to local oscillator mode with increasing wavevector.