Dynamic stability of submonolayer structures in the Li/Cu(111) system

The equilibrium crystal structure and vibrational properties of the Li/Cu(111) system have been calculated theoretically using the interatomic potentials obtained within the embedded atom method. The submonolayer $p(3\times3)$, $p(2\times2)$, and $(\surd3\times\surd3)R30^{\circ}$ structures formed at degrees of lithium adsorption from $0.11$ to $0.33$ monolayer and bilayer $(2\times2)$-$3$Li structure have been considered. Surface relaxation data, phonon spectra, and the local distribution density of vibrational states on the adatoms and atoms of the substrate are presented. The calculations have shown that an increase in the degree of submonolayer adsorption of lithium on the Cu(111) surface results in the dependence of the energy of low-frequency longitudinal vibrations of adatoms that is atypical of alkali metals. The energy of such vibrations decreases in the range from $0.11$ to $0.56$ monolayer down to imaginary values in the saturated monolayer coating. A further increase in the degree of adsorption to $0.75$ monolayer leads to the formation of the dynamically stable Cu(111)-$(2\times2)$-$3$Li structure with the strong hybridization of all types of vibrations of Li atoms and the substrate.