Abstract
The high- T c superconductor Bi 2CaSr 2Cu 2O 8 (Bi 2:1:2:2) exhibits a superstructure in the bulk which is manifested at the surface. Helium atom scattering (HAS) from the (001) Bi–O surface has revealed unusual features of the surface phonons, such as the occurrence of apparently rather dispersionless surface modes and an only weak dependence of surface mode dispersion on direction in the surface. We report lattice dynamical calculations of a (001) slab of Bi 2:1:2:2 material, bounded by Bi–O surfaces. The calculations are based on shell model interactions, and take proper account of surface relaxation. We find that the above mentioned features of the surface phonon modes are direct consequences of the anisotropic surface structure.
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