Oxygen chemisorbed on Ni(100): A renormalization-group study of the global phase diagram

Epitaxial adsorption onto a square substrate is studied with a Hamiltonian including nearest-neighbor exclusion, second- and third-neighbor pair and trio interactions. The resulting phase diagrams exhibit $2\times 1$, $2\times 2$, and $\sqrt{2}\times \sqrt{2}$ ordered phases and a disordered phase. Thus all three ordered structures observed with oxygen on nickel (100) are obtained within the same phase diagram in the coverage and temperature variables. Other cross sections of the global phase diagram may be applicable to chemisorbates such as oxygen, sulfur, selenium, or tellurium, on substrates such as nickel, copper, tungsten, or platinum (100). First- and second-order phase boundaries are evaluated, punctuated by tricritical, critical-end, bicritical, and tetracritical points. Reentrant tricriticality is found, yielding closed-loop coexistence regions. The possibility of two distinct 2×2 phases, uniaxial and biaxial, is raised. We use a Migdal-Kadanoff approximation on a cell system, with renormalization-group flows in 16 coupling constants.