CRITICAL-BEHAVIOR OF THE P(2 X-2) AND (ROOT-3 X ROOT-3)R30-DEGREES ORDER-DISORDER PHASE-TRANSITIONS OF S RU(001)/

Critical properties of the order-disorder phase transitions of the p(2 x 2) and the (square-root 3 x square-root 3)R30-degrees phases of sul fur on Ru(001) were tested at their optimal coverages analyzing supers tructure spot profiles using high resolution LEED. Both transitions we re found to be continuous, but finite size rounded. Power law behaviou r was observed for 0.01 less-than-or-equal-to Absolute value of t less -than-or-equal-to 0.1 (t = T/T(c) - 1). The exponents nu of the correl ation length fall close to the theoretical values of the 4-state and 3 -state Potts universality classes, respectively. Deviations from the t heoretical values are found for the exponents gamma of the susceptibil ity and beta of the order parameter in both cases.