Oxygen on Ru(001): Critical behavior of ap(2×1) order-disorder transition

The p(2×1) structure of oxygen on the hexagonally close-packed basal plane of Ru at half-monolayer coverage exhibits a reversible order-disorder phase transition with $T_c$ at 555 K. At constant coverage, a quantitative investigation of this phase transition was carried out by analysis of peak and integrated intensities of low-energy-electron-diffraction superstructure spots and of superstructure beam profiles as a function of temperature. Scaling was observed in the range of reduced temperature 0.02<‖t‖<0.2 [t=(T-$T_c$)/$T_c$], whereas finite-size rounding prevents scaling closer to $T_c$. The exponents α=0.30±0.06 and β=0.13±0.02 were determined from integrated and peak intensities as a function of t, respectively. Analyses of amplitudes and half-widths of deconvoluted spot profiles yield the exponents γ=1.35±0.15 and ν=0.74±0.08. They agree with theoretical exponents of the three-state Potts universality class within about 10%. Implications of these results are discussed.