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 studi ed up to one physical monolayer as a function of both coverage and temperat ure 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 g as. The formation of incommensurate structures with properties of floating solids starts already at coverages slightly above the completed p(8 x 1) co mmensurate structure (theta = 0.125). The latter itself behaves like a floa ting 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 th eta = 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 eut ectics, i.e. coexistence regions with their melts, with an eutectic point a t 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 parame ter and nu of the correlation length. It is shown that the system belongs t o the universality class of the Ising model. An incommensurate phase is aga in formed at higher coverage due to uniaxial compression of the layers. The behaviour at low coverages can be qualitatively understood assuming latera l interactions along the furrows mainly caused by dipole-dipole interaction s and electrostatic screening of the adsorbate induced charge redistributio n. (C) 2000 Elsevier Science B.V. All rights reserved.