Thin epitaxial AgBr(001) films grown on NaCl: Growth, surface structure, and phonon dynamics

Abstract

Thin AgBr͑001͒ films of up to 100 monolayers thickness epitaxially grown on the NaCl͑001͒ single-crystal surface have been investigated with high-resolution elastic and inelastic helium-atom scattering. Diffraction experiments were used to optimize the film growth conditions to assure a microscopically well-ordered AgBr͑001͒ surface under UHV conditions and provided information on the symmetry of the surface unit cell, surface corrugation amplitude, thermal expansion coefficient, and the surface Debye temperature. From timeof-flight spectra the dispersion curves of the surface Rayleigh mode, the longitudinal acoustic, an optical and a crossing mode were measured out to the Brillouin-zone boundary in the two crystallographic directions ͓100͔ and ͓110͔. ͓S0163-1829͑99͒00323-9͔

Anomalous Reconstructions of the (001) Surfaces of AgBr and NaCl

“…From the slope of the nearly perfectly exponential Debye-Waller falloff [20] up to ϳ220 K [ Fig. 2(b)], the surface Debye temperature is calculated to be Q rec D ϳ 100 K, which is very close to the value of ϳ95 K for the unreconstructed surface [19], suggesting that the vibrational properties of the two surfaces do not differ significantly. This is confirmed by measurements of the Rayleigh wave dispersion curves over the entire first Brillouin zone using inelastic HAS timeof-flight spectra.…”

“…This is confirmed by measurements of the Rayleigh wave dispersion curves over the entire first Brillouin zone using inelastic HAS timeof-flight spectra. Except for the expected folding and resulting splittings for the reconstructed surface [19], the dispersion curves are identical out to the first Brillouin zone indicating that even at an atomic level the environments of the surface ions are essentially identical. In Fig.…”

“…20-80 monolayer thick films of silver bromide were epitaxially grown at a substrate temperature T s 400 K as described in Refs. [16,19]. As-prepared AgBr films are terminated with a regular ͑001͒-͑1 3 1͒ surface [19].…”