Adatom pair distribution up to half coverage: O-Pd(100)

• Published in: Surface science Vol. 691; p. 121465
• Main Author: Kappus, Wolfgang
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.01.2020; Elsevier BV
• Subjects: Adatom interactions; Adatoms; Computer simulation; First principles; Integral equations; LEED; Long range order; Mathematical models; Monte Carlo simulation; Nonlinear integral equations; Pair distributions; Power series; Series expansion; Short range order; Short-range adatom order; Substrates; Variation; Short-range adatom order; Adatom interactions; LEED; Nonlinear integral equations; Pair distributions; Monte Carlo simulation
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/j.susc.2019.121465

•Adatom pair distribution calculated from pair interactions.•Monte Carlo simulation for comparison.•Adatoms show local order like glass, no ordered domains. [Display omitted] The superposition approximation and the Born–Green–Yvon (BGY) equation allows calculation of pair distributions in thermal equilibrium from pair potentials. A 2-d variant can be used to derive adatom pair distributions from arbitrary analytical pair potentials. As practical example substrate mediated elastic interactions, fitted previously to first principles (FP) calculations, are used to derive adatom pair distributions of O-Pd(100). The evaluation method utilizes the particle-hole symmetry of the pair interaction lattice gas Hamiltonian. The nonlinear BGY-type integral equation is solved numerically up to half coverage using power series expansion of coverage. The resulting adatom pair- and three-body distributions are used to analyze the short-range order of adatoms. A Monte Carlo (MC) simulation was performed in parallel to compare the current model with established methods. MC derived pair distributions fit well to those found with the analytical model, except in the high coverage region. The Fourier transformed pair correlations are compared with experimental LEED spot data and provide additional insight. Due to significant attraction the model predicts formation of 3rd and 4th nearest neighbors (n.n.) at small 3rd n.n. chains. A long range order of adatoms like a p(2 × 2) lattice is not found with the current model but a glassy structure is proposed as also indicated by the equilibrium Monte Carlo configurations. The glassy structure is assumed to contain fluctuations explaining broad p(2 × 2) LEED spots in the 0.25 monolayer region. The assumptions and limitations of the model towards higher coverages are discussed and open questions are formulated.