Phonons in a quasi-two-dimensional solid: Cesium monolayer on Cu(001)

The phonon dispersion curves of a well-ordered Cs monolayer epitaxially grown on Cu(001) were measured over the entire Brillouin zone using high-resolution He atom scattering. In addition to the Rayleigh mode an additional perpendicular resonance near the $\overline{\Gamma }$ point and a longitudinal film mode were observed. The appearance of a distinct longitudinal mode, which was not observed for the corresponding monolayer films of Na or K, is attributed to the small interface corrugation seen by the Cs atoms and the phonon velocity mismatch between the film and substrate. Thus the lateral motion of the film is effectively decoupled from the substrate and reveals a quasi-two-dimensional phonon behavior.