PHONON STRUCTURE OF THIN-FILMS OF THE YBACUO COMPOUND

Lattice dynamics calculations have been performed for thin films of YB a2Cu3O7 using a rigid-ion model. On the basis of parameters coming fro m bulk phonon calculations, the geometry has been optimized by minimiz ing the potential energy for films terminated by either CuO chains or BaO layers. The long-wavelength surface dielectric response of thin fi lms of the YBaCuO compound has been determined for applications to ele ctron-energy-loss spectroscopy and other surface-analysis techniques. For selected values of the film thickness, we have compared the micros copic calculations of the phonon surface response with the predictions of electrostatics which only demands the bulk dielectric function.