Strain and confined resonances in ultrathin alkali-metal films

Abstract

We have investigated the structure and lattice dynamics of ultra-thin alkali-metal films in the systems K/Ni(001) and Cs/Cu(111) using the technique of high-resolution helium atom scattering (HAS). Our results are compared with those from an earlier HAS study of Na/Cu(001). For all of these systems, lattice mismatch at the substrate/film interface introduces strain in the films which is found to be gradually removed during the growth process. Our comparative study shows that relaxation of the alkali-metal films into the bulk alkali metal occurs more rapidly as one proceeds from Na to K and Cs. The presence of strain relief is revealed in our observations of a layer-dependent Debye-Waller effect, the Debye temperature in the film decreasing with increasing film thickness from values close to that of the substrate to the bulk alkali-metal Debye temperature. More detailed information on strain relief in the alkali-metal films is provided by the variations in their vibrational frequencies with increasing film thickness. As for the Na/Cu(001) system, the K and Cs films investigated here exhibit prominent confined vibrational resonances. Our results show that measurements of the vibrations of atoms in the film surface yield information not only on the force constants at the film surface but also on those within the film itself. Comparison of results between the different alkali-metal systems elucidates further the nature of the growth process, the development of strain relief during growth, and the behavior of confined vibrational resonances. © 1996 The American Physical Society.