OXYGEN ON RU(001) - CRITICAL-BEHAVIOR OF A P (2X1) 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 Tc 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-Tc)/Tc], whereas finite-size rounding prevents scaling closer to Tc. The exponents =0.30 0.06 and =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 =1.35 0.15 and =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. © 1990 The American Physical Society.