Critical behavior of p(2×2) oxygen on Ru(001): An example of four-state Potts critical exponents

Atomic oxygen on the hexagonally close-packed Ru(001) surface forms a p(2×{}2) structure on threefold-coordinated hcp sites. The critical behavior of the structural order-disorder phase transition was studied with low-energy electron diffraction at a relative coverage {Θ}=0.25. Quantitative profile analysis of superstructure beams together with measurements of integrated and peak intensities as a function of temperature in the reduced temperature range 0.004<t<0.1 were used to determine the exponents {α}, {β}, {γ}, and {ν}. They agree within 3% to 10% with the values of the two-dimensional four-state Potts universality class. Conditions and limitations of this determination are discussed.