Phase diagrams of simple metals on fcc( 001 ) metal surfaces – an application to Mg on Cu

We study structures of simple metal monolayer adatoms on fcc(0 0 1) metal surface, both on theoretical and experimental sides. (i) Theory: using a simple Hamiltonian, two types of theoretical phase diagram of structures of adatoms are obtained. Then, the following structures are observed in the phase diagrams; (A) c (n 2 × 2 )R45° structures in a coverage range, 5/9 ⩽ σ ⩽2/3; (B) square-like lattice-gas structures at σ =9/13,4/5 and 13/16; (B ′ ) hexagonal arrangement of adatoms at higher coverage or lower amplitude of substrate potential, λ , regions, which is neighboring to the structures (B). (ii) Experiment by low energy electron diffraction (LEED): On Mg on Cu(0 0 1) surface, four-spot patterns and quasi-hexagonal patterns appear with increasing coverage which correspond to structures above (B) and (B ′ ). These are in good agreement with a low λ part of the phase diagrams, while LEED does not exhibit the c(2×2) and (A) patterns. Their origin will be discussed also by using the phase diagrams.