Oxygen on Ru(001): Critical behavior of a<i>p</i>(2×1) order-disorder transition

Abstract

The p(2\ifmmode\times\else\texttimes\fi{}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&lt;\ensuremath{\Vert}t\ensuremath{\Vert}&lt;0.2 [t=(T-${T}_{c}$)/${T}_{c}$], whereas finite-size rounding prevents scaling closer to ${T}_{c}$. The exponents \ensuremath{\alpha}=0.30\ifmmode\pm\else\textpm\fi{}0.06 and \ensuremath{\beta}=0.13\ifmmode\pm\else\textpm\fi{}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 \ensuremath{\gamma}=1.35\ifmmode\pm\else\textpm\fi{}0.15 and \ensuremath{\nu}=0.74\ifmmode\pm\else\textpm\fi{}0.08. They agree with theoretical exponents of the three-state Potts universality class within about 10%. Implications of these results are discussed.