Phase Diagram of Selenium Adsorbed on the Ni(100) Surface: A Physical Realization of the Ashkin-Teller Model

We have used electron diffraction to study submonolayers of Se adsorbed on Ni(100); $p(2\mathrm{}\times 2)$, $c(2\mathrm{}\times 2)$, and disordered phases were observed and the boundaries between them located. Symmetry arguments indicate that the phase diagram belongs to the universality class of the Ashkin-Teller model and allow us to predict the critical behavior near the phase boundaries and at the multicritical point where they meet. Our lattice-gas-model calculations support these results. The predicted critical behavior, and thus the universality arguments, should be tested by additional experiments, in particular synchrotron-x-ray diffraction.