CRITICAL-BEHAVIOR OF THE P(2X1)-O/W(110) SYSTEM

We have investigated the critical behavior of a two-dimensional phase transition of the p (2 X 1) oxygen overlayer on a W(110) surface, using high-resolution low-energy-electron diffraction (LEED). The temperature dependence of a characteristic (1/2 0) LEED spot of the p (2 X 1 ) structure reveals a power-law divergence for the susceptibility and correlation length of the critical scattering near the transition temperature of 709 K. Two independent exponents estimated from line-shape analyses are beta = 0.19+/-0.05 and gamma- = 1.48+/-0.34 for T < T(c), showing significant deviations from those of the two-dimensional (2D) Ising model. Other exponents estimated by the scaling relations agree qualitatively with experimental values. We further observe that the exponent 77 near T, also shows large deviation from the theoretical value. We find that these nonuniversal values for the exponents are consistent with the 2D XY model with cubic anisotropy.