Experimental verification of critical exponents in the two-dimensional four-state Potts universality class: Oxygen on Ru(0001)

Order-disorder critical behavior of the adsorption system O/Ru(0001) at (1/4 monolayer, ordered in a p(2×2) structure, is studied by use of LEED. By measuring the profiles—and the peak and integrated intensities—of the (1/2-order oxygen LEED spots as function of temperature for 0.004<‖(T-${\mathrm{T}}_{\mathrm{c}}$)/${\mathrm{T}}_{\mathrm{c}}$‖<0.2, we determine the critical exponents α, β, γ, and ν independently. They agree to within 3% to 10% of the theoretical values for the two-dimensional four-state Potts model universality class.