Quantitative studies of two-dimensional first- and second-order phase transitions by integrating diffraction methods

Using low-energy electron diffraction (LEED), we show for two classes of sy stems, which are representative for second- and first-order phase transitio ns in adsorbed layers, that quantitative properties of phase transitions ca n be studied also by using integrated diffracted intensities, turning the i nstrument to low resolution in two-dimensional reciprocal space, k(parallel to) For the continuous order-disorder phase transitions of several atomic adsorption systems, critical properties have been studied by determination of the critical exponents alpha (of the specific heat) and eta, the anomalo us critical dimension, in the limit k(parallel to)xi much greater than 1. W e performed systematic tests of the conditions under which these exponents can be determined reliably from the diffracted intensity of superstructure beams. In first-order phase transitions, scaling laws characterize specific mechanisms driving the transitions. As an example of two-dimensional first -order phase transitions, the transitions between a two-dimensional (2D) ga s and the 2D solid of the first monolayer have been studied for the noble g ases Ar, Kr and Xe on a NaCl(100) surface in quasi-equilibrium with the thr ee-dimensional (3D) gas phase. Using linear temperature ramps, we show that the widths of the hysteresis loops of these transitions as a function of t he heating rate, r, scale with a power law proportional to r(x) with x betw een 0.4 and 0.5 depending on the system. The hysteresis loops for different heating rates are similar. The island area of the condensed layer was foun d to grow initially with a time dependence proportional to t(4). These resu lts are in agreement with a model of growth-controlled hysteresis, which pr edicts x = 0.5 and hysteresis loop similarity.