Thermal behavior of c(2X2) layers in the Pb/Cu(110) system and the influence of surface defects on the order-disorder transition

The thermal behavior of a c(2X2) Ph overlayer on Cu(110) surface is studied by thermal energy atom scattering. Diffraction analysis reveals a continuo us reversible order-disorder transition with temperature at 437+/-2.5 K. Tw o of the extracted critical exponents, gamma and nu, are in good agreement with those of the two-dimensional Ising universality class. The third, beta , associated with the disappearance of long-range order in the layer, is, h owever, smaller than the theoretical expectations. The observed transition is discussed in a framework of model, where the deviation of one of the exp onents beta, gamma, and nu from its exact value depends in a different way on the kind of "imperfection" (mean terrace width, domain boundaries, "froz en-in" surface defects, local distortions, and surface alloying) in the rea l physical system. The annihilation of domain walls, created during the sim ultaneous growth of c(2X2) phase on two equivalent sublattices of {110} fcc surface, is found at 310 K. The experimental findings indicate also a noti ceable change in the surface corrugation during the observed order-disorder transition.