Phase diagram of oxygen on Ni(100)

We have studied the coverage-temperature phase diagram of chemisorbed oxygen on Ni(100) with LEED and AES. We find that the oxygen p(2 × 2) structure undergoes a nearly reversible order- disorder phase transition for coverages between 0.15 and 0.30 monolayer (T_c ≈ 560 K for θ = 0.25). For these coverages, it is also found that the chemisorbed oxygen begins dissolving into the bulk nickel as the overlayer disorders. A reversible order-disorder transition is not seen for the oxygen c(2 × 2) structure, but there is evidence that bulk dissolution here also occurs coincident with the overlayer disordering.     

A semiclassical approximation for matrix elements involving non-orthogonal bound states

We analyse the phase diagram of a lattice gas model with both condenseation and order-disorder phase transitions, when the system is confined between two walls. The gas-liquid transition is shifted into the, so called, capillary condensation. The crystallization, both from the gas and from the liquid, is also shifted from the bulk values, but the ordered structure is frustrated or enhanced depending on its commensuration with the walls separation, H. This produces a strong oscillatory dependence of the phase diagram with H.     

Phase diagram of hydrogen adsorbed on Ni(111)

An explanation for the phase diagram of the H/Ni(111) system is proposed, where the existence of the p(2 × 2) phase is assumed in addition to the (2 × 2) phase. Using the position-space renormalization-group theory with prefacing transformation, a lattice gas on a honeycomb lattice, including up to 6th nearest neighbor interactions, is treated. Another explanation previously put forward by Domany et al. is also examined, where the occurrence of the (2 × 2) ordered phase and the gas + p(2 × 2) coexistence phase is assumed. The phase diagrams thus obtained for these two cases are exhibited and discussed.     

On the order-disorder transition of chemisorbed H on Ni (111)

The order-disorder transition of chemisorbed H on Ni (111) has been analyzed by Kittler and Bennemann in terms of the lattice gas model. The main purpose of this paper is to refine the above calculation. The expression with regard to adlayer domains is given for the half-order LEED spot intensity, from which the dependence of the transition temperature and of the LEED spot intensity on H-coverages is calculated. In particular, the H-coverage dependence of the transition-temperature T_c and the LEED intensity I for θ? 0.25 is restudied. We present results obtained by using only pair interactions between the H-atoms and no 3H-interactions. It is shown that the observed asymmetric phase diagram with respect to H-coverages can result from the two kinds of pair interactions between chemisorbed H atoms.     

Surface chemistry of the halogen-metal interface: The interaction of chlorine with ruthenium (100)

Chlorine adsorbs on Ru(100) with high efficiency; UPS, Δφ, TD, AES and LEED data suggest that the low pressure interaction ceases with the formation of a dense chemisorbed layer of atomic Cl. This overlayer desorbs exclusively as Cl(g) with an activation energy of ~ 220 $\text{kJ}\text{mol}$. The failure of the system to grow bulk halide and the absence of molecular chlorine and ruthenium-chlorine species in the desorption products are discussed in thermochemical terms. A series of four ordered surface structures evolves with increasing coverage, and structural models are proposed which are in good quantitative accord with the results from Auger spectroscopy. As coverage deceases, it seems likely that the overlayer relaxes by the growth of ordered vacancies rather than by uniform expansion along a preferred direction.     

The local adsorption site for sulphur on Ni {111} in the low coverage lattice gas

Angle-resolved sulphur L_2.3VV Auger electron spectra have been taken from sulphur adsorbed on Ni {111} at a range of coverages both below and above those corresponding to the ordered (2 × 2) structure. These data indicate that the local adsorption site in the low coverage lattice gas is the same single three-fold hollow site adopted in the ordered overlayer. This contrasts with the low coverage occupation of both three-fold hollow sites for the system I/Ag<{111}.     

Asymmetric phase diagram and coverage dependent effective pair interactions for hydrogen on close-packed metal surfaces

The asymmetry in the phase diagram of the H/Ru(001) system is studied by assuming a lattice gas model for the chemisorbed hydrogen and using the cluster variation method. Ground state analysis of the ordered structures shows that the effective pair interaction for the next-nearest neighbors has to be repulsive. We also found that the order-disorder transition temperatures and hence the phase diagram are very sensitive to v₃, the ratio of the effective next-nearest to nearest neighbor interactions of H adatoms. The asymmetry in the phase diagram, which cannot be accounted for by the adsorbate relaxation model by Persson [Surf. Sci. 258 (1991) 451], is attributed to the coverage dependence of the effective pair interactions. By assuming a simple piecewise linear dependence of v₃ on the chemical potential, we constructed an asymmetric phase diagram which is in excellent agreement with the experimental data. The model studied can be applied to the H/Pd(111) system directly and can be easily generalized for other close-packed metal surfaces.     

Phase diagram and phase transitions of the adsorbate system S/Ru(0001): a Monte Carlo study of a lattice gas model

We investigate the phase diagram and the critical properties of the adsorbate system sulphur/ruthenium(0001) in the coverage region 0 < Θ < 1/3 using Monte Carlo simulations of a lattice gas model on a triangular lattice. From experiments it is known that for low coverages an island phase appears in the phase diagram of this system at low temperatures. To capture this feature we include in our lattice gas model a weak third neighbour attraction in addition to the repulsive first and second neighbour forces. The phase diagram obtained from simulations of this model is in very good agreement with the experimental phase diagram. The critical properties of the lattice gas model are found to be compatible with the results of experiments on the system sulphur/ruthenium(0001). Finer details of the phase diagram, e. g. the location of tricritical points, which may be difficult to assess experimentally will also be discussed.     

The bonding of oxygen to Al(111)

Oxidation of the Al(111) surface is a two-stage process in which the formation of an ordered oxygen overlayer precedes the creation of a bulk-like amorphous oxide. An electronic structure calculation is reported here for the clean and oxygen-covered Al(111) surface and for bulk A1₂O₃. The calculation uses an atomic-orbital basis and the metal surface is modelled by an infinite two-dimensional crystal, containing seven layers of aluminium atoms. Oxygen atoms occupy three-fold sites, with an Al-O separation of 1.9 Å. The oxygen 2p resonance in the (1 × 1) chemisorbed overlayer is about 3 eV wide, compared to 1.9 eV for an equivalent isolated layer of oxygen atoms unhybridized with the metal. The valence band of A1₂O₃ is about 1.5 eV wider than the chemisorbed oxygen resonance, but in both cases most of the states are concentrated in the top 1.5 eV of the band. The results are related to available ultraviolet photoemission spectra, including the recent angular-resolved spectra of Martinson and Flodström.     

CPN−1 coupled to gauge fields: Mean field theory and monte carlo results

We define a two parameter lattice field theory which interpolates between the O (2N) Heisenberg model, pure U(1) gauge theory, and a lattice version of the CP^N?1 model. The phase diagram in space-time dimension d=4 is obtained by Monte Carlo simulation on a 4⁴ lattice, and the nature of the phases is discussed in mean field approximation.     

Site-diluted antiferromagnet in a uniform field

We study a site-diluted Ising antiferromagnet in a square lattice, in an external uniform field (DAFF). We use a real space renormalization group technique to obtain the phase diagram as a function of temperature (T), concentration of magnetic ions (p) and magnetic field (H). At zero temperature the phase diagram, in the H × T plane, has a steplike structure separating the disordered from the ordered antiferromagnetic phase. We also discuss the connection between this problem and that of a ferromagnet in a random field.     

Fluctuation interaction of Ising subsystems

A phase diagram of two Ising subsystems σ and s has been constructed on a Bethe lattice with a coordination number 4 (a simplified analog of a square lattice). In contrast to the known Ashkin-Teller model, the interaction between these two subsystems is of a purely fluctuational nature; i.e., it does not manifest itself in the ground state and nullifies the sums of the products of average spins 〈σ〉〈s〉 (interactions of this type are realized in lattice-type adsorbed systems with dipolelike intermolecular interactions and strong azimuthal angular dependence of the adsorption potential of symmetry C₄). Apart from conventional states, i.e., a high-temperature disordered state (〈σ〉=〈s〉=0) and a low-temperature ordered state (〈σ〉 and 〈s〉 =? 0), this system can also exist in a correlated state (〈σs〉 =? 0 at 〈σ〉=〈s〉=0). In the theory of orientational phase transitions, this state corresponds to a fundamentally different, intermediate (on the temperature axis) phase in which a preferred direction of long molecule axes arises in the absence of spontaneous polarization. The results of Monte Carlo simulation on a square lattice agree with the conclusions obtained on a Bethe lattice. The characteristics of the orientational phase transition in a 2 × 1 monolayer of CO molecules adsorbed on the NaCl(100) surface are discussed.     

Phase structure of U(N→∞) gauge theory on a two-dimensional lattice for a broad class of variant actions

It is pointed out that finding the partition function for U(N) gauge theory on a two-dimensional lattice in the limit N→∞ reduces, for a broad class of single-plaquette actions, to a well-known and solved mathematical problem. The case where in the single plaquette action the matrix U + U⁺ occuring in Wilson's formula is replaced by an arbitrary polynomial in this matrix, is discussed in detail and explicit results for the second-order polynomial are presented. A rich phase structure with second- and third-order phase transitions is found. The results are shown to have at the qualitative level a simple thermodynamical interpretation. They support the view that the phase structure of a lattice gauge theory is an artifact of the lattice action used rather than some reflection of the underlying group structure.     

Progress in lattice gauge theory

Two topics of lattice gauge theory are reviewed. They include string tension and β-function calculations by strong coupling Hamiltonian methods for SU(3) gauge fields in 3 + 1 dimensions, and a 1/N-expansion for discrete gauge and spin systems in all dimensions. The SU(3) calculations give solid evidence for the coexistence of quark confinement and asymptotic freedom in the renormalized continuum limit of the lattice theory. The crossover between weak and strong coupling behavior in the theory is seen to be a weak coupling but non-perturbative effect. Quantitative relationships between perturbative and non-perturbative renormalization schemes are obtained for the O(N) nonlinear sigma models in 1 + 1 dimensions as well as the range theory in 3 + 1 dimensions. Analysis of the strong coupling expansion of the β-function for gauge fields suggests that it has cuts in the complex 1/g²-plane. A toy model of such a cut structure which naturally explains the abruptness of the theory's crossover from weak to strong coupling is presented. The relation of these cuts to other approaches to gauge field dynamics is discussed briefly.The dynamics underlying first order phase transitions in a wide class of lattice gauge theories is exposed by considering a class of models-P(N) gauge theories - which are soluble in the N → ∞ limit and have non-trivial phase diagrams. The first order character of the phase transitions in Potts spin systems for N #62; 4 in 1 + 1 dimensions is explained in simple terms which generalizes to P(N) gauge systems in higher dimensions. The phase diagram of Ising lattice gauge theory coupled to matter fields is obtained in a $\text{1}\text{N}$ expansion. A one-plaquette model (1 time-0 space dimensions) with a first-order phase transitions in the N → ∞ limit is discussed.     

Diffuse LEED intensities of disordered crystal surfaces: II. Multiple scattering on disordered overlayers

The diffraction of low energy electrons from disordered overlayers adsorbed on ordered substrates is treated theoretically by an extension of Beeby's multiple scattering method. A lattice gas model is assumed for the disordered adsorbate layer. Multiple scattering within a certain area around each atom — each atom of the overlayer and within the ordered substrate — is treated self-consistently, the remaining contributions to the total scattering amplitude being averaged. The theory can be used in the limiting cases of random distribution and of long range order within the adsorbate layer.     

Lattice distortion and multiple spin density wave state in CsCl type AuMn

The relation between the tetragonal lattice distortion and the multiple-Q antiferromagnetic ordering and possible phase diagram in the temperature vs. composition plane of CsCl type ordered alloys Au_xMn_1−x (x≈0.5) are discussed on the basis of Landau's phenomenological theory.     

Characteristics electron energy loss studies of molecular halogens adsorbed on Fe(100)

The electron energy loss spectra for several molecular Br₂ monolayers adsorbed on a chemisorbed bromine overlayer on Fe(1 0 0) show a sharp loss at about 3.0 to 3.8 eV. For one or more molecular layers of I₂ adsorbed on a chemisorbed iodine overlayer on Fe (1 0 0), a sharp electron loss feature is observed at 4.4 ± 0.2 eV. It is suggested that the electron energy losses for condensed Br₂ at 3.0 eV and for condensed I₂ at 4.4 eV are a result of a 1π_g to 2σ_u electron excitation.     

High-pressure effect on the crystal and magnetic structure of LuMnO3: Correlation between a distortion of the triangular lattice and the symmetry of the magnetic state in hexagonal frustrated manganites

The effect of a high pressure (up to 6 GPa) on the crystal and magnetic structure of the hexagonal manganite LuMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. It is found that, as the pressure increases, the ordered magnetic moment of Mn ions at T = 10 K decreases noticeably from 2.48 (0 GPa) to 1.98 μ_B (6 GPa). This decrease is due to an enhancement of the geometrical frustration effects on the triangular lattice. At the same time, the symmetry of the triangular antiferromagnetic state (the irreducible representation Γ₂) remains unchanged. A correlation is revealed between the distortion parameter of the triangular lattice formed by Mn ions and the symmetry of the antiferromagnetic state of hexagonal manganites RMnO₃. Based on this correlation, a generalized magnetic phase diagram of these compounds is constructed. The obtained phase diagram provides an explanation for the changes observed in the magnetic state of hexagonal manganites caused by high pressure and chemical substitution.     

States of water molecule adsorbed on Si(111) surface

Calculated electronic energy structure of an overlayer of water molecules chemisorbed on Si(111), with a molecular plane perpendicular to the surface, reveals that 3a₁ molecular state is removed upon adsorption, and the resulting state densities cannot be reconciled with the UPS spectrum. These results, and state densities obtained from different atomic configurations are interpreted to rule out the molecular adsorption on Si(111) surface at room temperature.