Order-Disorder Critical Behaviour in the System Oxygen on Ru(001)
In a LEED study of the ordering of atomic oxygen on the Ru(001) surface, the phase diagram is constructed and critical exponents are determined. Ordered oxygen phases form (2×2) structures in LEED at coverages between θ = 0.17 and saturation at θ = 0.5. A p(2×2) structure exists up to θ = 0.37. The phase diagram shows two maxima with transition temperatures to disorder of 754 K and 555 K at θ = 0.25 and 0.5, respectively. The critical exponents α,β, γ and ν for θ = 1/4, and α and β for θ = 1/2 are derived from an analysis of LEED superstructure spot intensities versus temperature and from profile analysis. They agree well with theoretically determined exponents of the 4-state Potts universality class at θ=1/4, whereas the situation is less clear at θ=1/2. Finite size effects and specific oxygen overlayer structures are discussed.
KeywordsCritical Exponent Spot Intensity Universality Class Finite Size Effect Diffuse Background
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