Phase transitions in two-dimensional anisotropic chain systems:: submonolayers of Sr adsorbed on Mo(112)

Ordered phases of Sr on Mo(112) and their phase transitions have been studied up to one physical monolayer as a function of both coverage and temperature using optical LEED. Starting at a coverage of 0.07, islands of a p(8 x 1) structure are formed at 100 K, which coexist with a disordered lattice gas. The formation of incommensurate structures with properties of floating solids starts already at coverages slightly above the completed p(8 x 1) commensurate structure (theta = 0.125). The latter itself behaves like a floating solid and undergoes a depinning transition at T approximate to 125 K, similar to the next commensurate structure, p(5 x 1), which is formed at theta = 0.20. Floating solids are found in the whole coverage range between 0.12 and 0.23. At higher coverage coexistence between p(5 x 1) and c(2 x 2) structures is found, which melt by forming intermediate two-dimensional eutectics, i.e. coexistence regions with their melts, with an eutectic point at theta = 0.37, T-eu = 310 K. Close to a coverage of 0.5 a homogenous phase is formed, which disorders by a continuous phase transition, as explicitly tested by determination of the critical exponents beta of the order parameter and nu of the correlation length. It is shown that the system belongs to the universality class of the Ising model. An incommensurate phase is again formed at higher coverage due to uniaxial compression of the layers. The behaviour at low coverages can be qualitatively understood assuming lateral interactions along the furrows mainly caused by dipole-dipole interactions and electrostatic screening of the adsorbate induced charge redistribution. (C) 2000 Elsevier Science B.V. All rights reserved.