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Zusammenfassung:
The growth of Pb ultrathin films on Cu(111)
has been long studied in connection with electronic quantumsize
effects and for the different temperature-dependent
growth kinetics. At low temperature the formation of a
wetting layer (1 monolayer (ML)), is followed by an instability
of the 2 ML film and a regular layer-by-layer growth is then
only observed for more than two monolayers. The 2 ML film
was, however, shown to be stabilized by alloying Pb with 20%
Tl. This work presents a theoretical study of the dynamics of
the wetting layer as well as for 2 ML Pb0.8Tl0.2, 3 and 4 ML Pb
on Cu(111) in the 4 × 4 commensurate phase, for which
detailed inelastic Helium atom scattering (HAS) spectra have
been measured. The present calculations based on the embedded atom method (EAM) include the dynamics of the substrate.
Besides leading to a detailed interpretation of the HAS experimental data, the present results are compared with a previous
density-functional perturbation theory (DFPT) study for 3 to 7 ML Pb on a rigid substrate. The comparison reveals the role
played by the substrate dynamics at the smallest thicknesses, despite the large mass and stiffness differences between Pb and Cu.
Also the different thermal expansion of the film with respect to the substrate is shown to cause appreciable anomalies in the
temperature and thickness dependence of the phonon dispersion curves.
