Critical behavior of thep(2×1)-O/W(110) system

We have investigated the critical behavior of a two-dimensional phase transition of the p(2×1) oxygen overlayer on a W(110) surface, using high-resolution low-energy-electron diffraction (LEED). The temperature dependence of a characteristic (1/20) LEED spot of the p(2×1) 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 β=0.19±0.05 and ${\gamma }^{\mathrm{-}}$=1.48±0.34 for T<${\mathit{T}}_{\mathit{c}}$, 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 η near ${\mathit{T}}_{\mathit{c}}$ 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.