Growth of composition-modulated Ag'Co wires on PtÑ997Ö

Sequential deposition of Ag and Co on a Pt(111) stepped surface has been investigated as a means to obtain composition-modulated atomic wires. Thermal energy He atom scattering in grazing incidence conditions allows us to control and characterize the growth of Ag/Co wires on Pt(997) as a function of the substrate coverage and temperature. When Co is deposited first, Ag and Co atoms arrange themselves into regular stripes parallel to the Pt steps. A disordered phase is obtained when Ag is deposited first. We have carried out calculations based on semiempirical potentials to study the equilibrium configuration of a Ag/Co mixture on Pt(997). The experimental observations agree with ground-state calculations of the atomic structure, indicating that the first pure Co row in contact with the Pt step edge is very stable while the second row is occupied by Ag atoms. Free-energy minimization using a mean-field Ising approach at finite temperature of equimolar binary mixtures leads to very consistent results for the Ag/Co system, namely the preferential sequence with nearly pure Co and Ag wires in the first and second rows at the step. Predictive extensions to Co/Cu and Ag/Cu systems are discussed within the same approach. A preferential ordering with Co (Cu) occupying the first row and Cu (Ag) the second row is clearly obtained from the calculations, although the Ag/Cu system can also be frozen in reverse order.