Quantized spin waves in antiferromagnetic Heisenberg chains

The quantized stationary spin wave modes in one-dimensional antiferromagnetic spin chains with easy axis on-site anisotropy have been studied by means of Landau-Lifshitz-Gilbert spin dynamics. We demonstrate that the confined antiferromagnetic chains show a unique behavior having no equivalent, neither in ferromagnetism nor in acoustics. The discrete energy dispersion is split into two interpenetrating n and n' levels caused by the existence of two sublattices. The oscillations of individual sublattices as well as the standing wave pattern strongly depend on the boundary conditions. Particularly, acoustical and optical antiferromagnetic spin waves in chains with boundaries fixed (pinned) on different sublattices can be found, while an asymmetry of oscillations appears if the two pinned ends belong to the same sublattice.     

The distribution of the valence electron density in predominantly ionic crystals with different Bravais sublattices

Based on the theory of the local-density functional, self-consistent valence electron densities are calculated for CaO with a rock-salt lattice, CaF₂ with a fluorite lattice, K₂O with an antifluorite lattice, and for their constituent sublattices. It is shown that in the crystals with different Bravais sublattices, the anionic sublattice is a framework with covalent bonds containing a metal sublattice inside of them. The coupling between the sublattices is characterized by the density difference, which is deflned as the difference between the total electron density and the densities of the individual sublattices. The density difference is found to be an order of magnitude smaller than the crystal and sublattice densities, which is evidence of weak hybridization of the sublattices and of the predominately ionic character of the bonding between them.     

Theory of the NO+CO surface-reaction model

We derive a pair approximation (PA) for the NO+CO model with instantaneous reactions. For both the triangular and square lattices, the PA, derived here using a simpler approach, yields a phase diagram with an active state for CO-fractions y in the interval y(1)相似文献   

Atom transport in random two sublattice structures: analogue of the random alloy sum rule

A simple and often used model of atom transport by the vacancy mechanism on two physically distinct interpenetrating sublattices assumes that each atom–vacancy exchange frequency depends only on the species of the atom and the sublattice from which it jumps. In the kinetic theory of this model, the phenomenological coefficients can be expressed as sums of partial correlation functions, each labelled by the sublattices associated with the atoms making the first and last jumps in the sequence of correlated jumps which it represents. A sum rule, a set of exact relations among these partial correlation functions, is derived for the model, assuming arbitrary vacancy content and any number of chemical species. It reduces to a widely used sum rule for the random lattice gas [L.K. Moleko and A.R. Allnatt, Phil. Mag. A 58 677 (1988)] in the limit that atom jump frequencies are made independent of sublattice. For the two sublattice model at very low vacancy contents, a more powerful sum rule is also derived; it is essentially the same as that of Belova and Murch [Defect Diffus. Forum 194/199 547 (2001)]. The efficiencies of the three sum rules are briefly compared. The low vacancy concentration sum rule is illustrated by numerical simulations for a binary two sublattice system.     

Electrochemical promotion of environmentally important catalytic reactions

The performance of conventional heterogeneous metal catalysts may be enhanced by the addition of so-called promoter species that are used to modify the intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical methods provide an alternative, radically different and uniquely efficacious method of catalyst promotion. Substantial and reversible changes in catalyst perfomance can be induced by back-spillover ions pumped from a solid electrolyte to the surface of a catalytically active electrode: one hasin situ control of the working catalyst. Studies of the electrochemical promotion of NO reduction over Pt films supported on β″-alumina (a sodium ion conductor) demonstrate that major enhancements in activity are possible when Na is pumped to the catalyst surface. We have examined the NO+CO reaction and the reaction of NO with propene. Both reactions are relevant to control of automotive and other emissions, and both exhibit strong electrochemical promotion. By simulating lean-burn engine conditions, we have also demonstrated that EP of a Pt catalyst very substantially enhances the ability of NO to oxidise propene in an oxygen-rich atmosphere. Reaction kinetic data obtained as a function of catalyst potential, temperature and gas composition indicate that Na increases the strength of NO chemisorption relative to CO or propene, a process that is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is the critical reaction-initiating step. XP spectroscopy under the appropriate conditions of temperature and catalyst potential confirms that the mode of operation of the elctrochemically promoted Pt film does indeed involve reversible pumping of Na to or from the solid electrolyte. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Monte Carlo simulations of oscillations, chaos and pattern formation in heterogeneous catalytic reactions

Experimental studies employing surface science methods indicate that kinetic oscillations, chaos, and pattern formation in heterogeneous catalytic reactions often result from the interplay of rapid chemical reaction steps and relatively slow complementary processes such as oxide formation or adsorbate-induced surface restructuring. In general, the latter processes should be analysed in terms of theory of phase transitions. Therefore, the conventional mean-field reaction–diffusion equations widely used to describe oscillations in homogeneous reactions are strictly speaking not applicable. Under such circumstances, application of the Monte Carlo method becomes almost inevitable. In this review, we discuss the advantages and limitations of employing this technique and show what can be achieved in this way. Attention is focused on Monte Carlo simulations of CO oxidation on (1 0 0) and (1 1 0) single-crystal Pt and polycrystal Pt, Pd and Ir surfaces and of NO reduction by CO and H₂ on Pt(1 0 0). CO oxidation on supported nanometre-sized catalyst particles and NO reduction on composite catalysts are also discussed. The results show that with current computer facilities the MC technique has become an effective tool for analysing temporal oscillations and pattern formation on the nanometre scale in catalytic reactions occurring on both single crystals and supported particles.     

表面杂质对NO+CO/Pt(100)反应体系振荡动力学的影响

提出了一个格气模型， 探讨催化表面的惰性杂质对NO+CO/Pt(100)反应体系振荡动力学行为的影响．研究发现表面杂质较少时，反应进程中表面重构能够形成相连通的1*1相和持续的振荡行为．当表面杂质不断增加时，反应进程中1*1相只能形成许多孤立的畴，从而在1*1相形成的随机的局域振荡的空间关联被削弱，因而体系全局的持续振荡行为演变为衰减振荡行为．当表面吸附的CO和NO的扩散速率增加时，局域振荡的空间协同又得到加强，体系又呈现出持续振荡行为．     

Surface Restructuring, Kinetic Oscillations, and Chaos in Heterogeneous Catalytic Reactions

Kinetic oscillations in catalytic reactions on single-crystal surfaces often result from the interplay of the purely chemical reaction steps and adsorbate-induced surface restructuring. A classical example is CO oxidation on Pt(100). We survey evolution of the models used to simulate this reaction and show how it can be described self-consistently by employing Monte Carlo simulations combined with the lattice-gas model, taking into account substrate-substrate, substrate-adsorbate and adsorbate-adsorbate lateral interactions. Under the reactive conditions, this approach predicts formation of mesoscopic restructured well ordered islands with atomically sharp boundaries.     

Effects of selective dilution on phase diagram and ground-state magnetizations of an Ising antiferromagnet on triangular and honeycomb lattices

We employ an effective-field theory with correlations in order to study the phase diagram and ground-state magnetizations of a selectively diluted Ising antiferromagnet on triangular and honeycomb lattices. Dilution of different sublattices with generally unequal probabilities results in a rather intricate phase diagram in the sublattice dilution parameters space. In the case of the frustrated triangular lattice antiferromagnet the selective dilution affects the degree of frustration which can lead to some peculiar phenomena, such as reentrant behavior of long-range order or unsaturated sublattice magnetizations at zero temperature. The selectively diluted Ising antiferromagnet on the honeycomb lattice is obtained as a special case when one sublattice of the triangular lattice is completely removed by dilution.     

Standing waves, clustering, and phase waves in 1D simulations of kinetic relaxation oscillations in NO+NH3 on Pt(1 0 0) coupled by diffusion

The Lombardo–Imbihl–Fink (LFI) ODE model of the NO+NH₃ reaction on a Pt(1 0 0) surface shows stable relaxation oscillations with very sharp transitions for temperatures T between 404 and 433 K. Here we study numerically the effect of linear diffusive coupling of these oscillators in one spatial dimension. Depending on the parameters and initial conditions we find a rich variety of spatio-temporal patterns which we group into four main regimes: bulk oscillations (BOs), standing waves (SW), phase clusters (PC), and phase waves (PW). Two key ingredients for SW and PC are identified, namely the relaxation type of the ODE oscillations and a nonlocal (and nonglobal) coupling due to relatively fast diffusion of the kinetically slaved variables NH₃ and H. In particular, the latter replaces the global coupling through the gas phase used to obtain SW and PC in models of related surface reactions. The PW exist only under the assumption of (relatively) slow diffusion of NH₃ and H.     

Surface reconstruction in reactive dynamics: A kinetic Monte Carlo approach

The oscillatory CO oxidation reaction on the restructuring surface of Pt(1 0 0) is studied through a mesoscopic kinetic Monte Carlo (KMC) approach. The present model is an extension of the standard ZGB model with specific attention to the emergence of oscillations in surface reactions. A square and a purely hexagonal lattice are used as substrates on which the CO oxidation reaction steps take place. The dynamics of the reaction on the two substrates exhibit the ZGB kinetic phase transitions, at different kinetic parameter values for each substrate. Surface reconstruction is modelled through switching between the two lattice types. Oscillations are produced in those parametric areas where the steady state concentrations on the two substrates are considerably different. The parametric area where notable oscillations are observed is narrow, but is greatly enhanced when different sticking coefficients of oxygen are taken into account. CO diffusion introduced microscopically to the model on the hexagonal lattice shifts the kinetic transition points and increases considerably the time needed to reach the steady state.     

Surface restructuring, kinetic oscillations, and chaos in heterogeneous catalytic reactions

We present comprehensive Monte Carlo simulations of isothermal kinetic oscillations and chaos in catalytic reactions accompanied by adsorbate-induced surface restructuring. Our analysis is based on the lattice-gas model describing surface restructuring in terms of the statistical theory of first-order phase transitions. As an example, we treat the kinetics of the NO-H2 reaction on the Pt(100) surface. A proposed reduced mechanism of this reaction includes NO adsorption, desorption, and decomposition occurring on the restructured patches of the surface (the decomposition products are rapidly removed from the surface via N2 desorption and H2O formation and desorption). Calculations are performed with a qualitatively realistic ratio between the rates of different elementary steps. In particular, NO diffusion is several orders of magnitude faster compared to the other steps. On the nm scale, the model predicts formation of restructured islands with atomically sharp boundaries. The shape of the islands is found to change dramatically with varying reaction conditions. Despite phase separation on the surface, the transition from almost harmonic oscillations (with relatively small separate islands) to chaos (with merging islands) is demonstrated to occur via the standard Feigenbaum scenario. Near the critical point, the dependence of the amplitude of oscillations on the governing parameter is shown to be close to that predicted for the Hopf supercritical bifurcation.     

Local entropy and structure in a two-dimensional frustrated system

We calculate the local contributions to the Shannon entropy and excess entropy and use these information theoretic measures as quantitative probes of the order arising from quenched disorder in the diluted Ising antiferromagnet on a triangular lattice. When one sublattice is sufficiently diluted, the system undergoes a temperature-driven phase transition, with the other two sublattices developing magnetizations of equal magnitude and opposite sign as the system is cooled.(1) The diluted sublattice has no net magnetization but exhibits spin glass ordering. The distribution of local entropies shows a dramatic broadening at low temperatures; this indicates that the system's total entropy is not shared equally across the lattice. The entropy contributions from some regions exhibit local reentrance, although the entropy of the system decreases monotonically as expected. The average excess entropy shows a sharp peak at the critical temperature, showing that the excess entropy is sensitive to the structural changes that occur as a result of the spin glass ordering.     

Origin of the spatiotemporal oscillations observed during the NO + H2 reaction on a Rh field emitter

The main purpose of the study described in the present paper was to obtain more detailed information on the mechanism of the spatiotemporal oscillations found recently by field emission microscopy for the NO + H₂ reaction on a Rh tip. These oscillations are related to the kinetics of the reaction over a Rh wire. There are two kinetic regions of the steady-state reaction. The oscillations are attributed to the reversible transition between these kinetic regions. The observed sharp moving wave can be ascribed to the hydrogenation of a nitrogen layer, followed by fast combination and/or decomposition of the adsorbed intermediate NH and restoring of its surface coverage by the NO + H₂ interaction. Surface defects and, probably, grain boundaries are responsible for the initiation of the chemical waves in the course of the sustained oscillations.     

Electrochemical promotion of the NO reduction by C2H4 on Pt/YSZ and by CO on Pd/YSZ

The effect of electrochemical promotion was investigated for the catalytic reduction of nitric oxide with ethylene and carbon monoxide on polycrystalline Pt and Pd, respectively, deposited on yttria-stabilized zirconia (YSZ). It was found in both cases that applying negative potentials and thus lowering the catalyst work function results in a pronounced increase in the catalytic rate and in the selectivity to nitrogen. A 7-fold increase was observed for the NO+C₂H₄ reaction on Pt while a 2-fold increase was obtained for the NO+CO reaction on Pd. The induced changes in catalytic rates were found to be 7 to 50 times higher than the rates of ion transfer from the catalyst surface. In both reactions, the observed electrophilic behavior can be attributed to the strengthening of the chemisorptive NO bond and concomitant enhanced dissociation of NO as the catalyst potential and work function is decreased. Forced periodic oscillations of the applied current was investigated and resulted in a enhanced production of CO₂, but an intermediate selectivity towards N₂, as compared to constant current application. The effect of the cycling waveform, frequency and amplitude was studied and provided evidence that the synergy observed during the cycling experiment results from a favorable transient coverage of adsorbed species on the catalyst surface as the catalyst potential oscillates from negative to positive values. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Oscillations, period doublings, and chaos in CO oxidation and catalytic mufflers

Early experimental observations of chaotic behavior arising via the period-doubling route for the CO catalytic oxidation both on Pt(110) and Ptgamma-Al(2)O(3) porous catalyst were reported more than 15 years ago. Recently, a detailed kinetic reaction scheme including over 20 reaction steps was proposed for the catalytic CO oxidation, NO(x) reduction, and hydrocarbon oxidation taking place in a three-way catalyst (TWC) converter, the most common reactor for detoxification of automobile exhaust gases. This reactor is typically operated with periodic variation of inlet oxygen concentration. For an unforced lumped model, we report results of the stoichiometric network analysis of a CO reaction subnetwork determining feedback loops, which cause the oscillations within certain regions of parameters in bifurcation diagrams constructed by numerical continuation techniques. For a forced system, numerical simulations of the CO oxidation reveal the existence of a period-doubling route to chaos. The dependence of the rotation number on the amplitude and period of forcing shows a typical bifurcation structure of Arnold tongues ordered according to Farey sequences, and positive Lyapunov exponents for sufficiently large forcing amplitudes indicate the presence of chaotic dynamics. Multiple periodic and aperiodic time courses of outlet concentrations were also found in simulations using the lumped model with the full TWC kinetics. Numerical solutions of the distributed model in two geometric coordinates with the CO oxidation subnetwork consisting of several tens of nonlinear partial differential equations show oscillations of the outlet reactor concentrations and, in the presence of forcing, multiple periodic and aperiodic oscillations. Spatiotemporal concentration patterns illustrate the complexity of processes within the reactor.     

Topological phases and type-II edge state in two-leg-coupled Su-Schrieffer-Heeger chains

Topological quantum states have attracted great attention both theoretically and experimentally.Here,we show that the momentum-space lattice allows us to couple two Su-Schrieffer-Heeger(SSH)chains with opposite dimerizations and staggered interleg hoppings.The coupled SSH chain is a four-band model which has sublattice symmetry similar to the SSH4.Interestingly,the topological edge states occupy two sublattices at the same time,which can be regarded as a one-dimension analogue of the type-II corner state.The analytical expressions of the edge states are also obtained by solving the eigenequations.Finally,we provide a possible experimental scheme to detect the topological winding number and corresponding edge states.     

Lifted reconstruction as a feedback mechanism in the oscillating CO oxidation on Pt nanofacets: Microscopic evidences

Global and local oscillations in the CO oxidation reaction have been visualized in situ on the apex of a [1 0 0]-oriented Pt field emitter tip used as a well-defined model for catalytically active, nm-sized particles by Field Emission (FEM) and Lithium Field Desorption (Li-FDM) Microscopes. For the first time experimental evidence is provided that the reconstruction feedback mechanism of the self-maintained oscillations for the [1 0 0] and [1 1 0] orientations, which is well established on macroscopic single crystals, is also valid in the heterogeneous, nm-sized system with its different crystallographic orientations which are coupled by surface diffusion.     

Electrochemical promotion in emission control catalysis

Electrochemical promotion for the catalytic reduction of NO by CO and of NO by ethylene over a Pt catalyst are reported for the first time. Both reactions are of importance in the catalytic control of automotive emissions and both exhibit strong rate enhancement when Na is pumped to the Pt catalyst electrode from a β″ solid electrolyte. Complementary data obtained with a Pt(111)/Na model system indicate that electrochemically-pumped Na acts by inducing dissociation of chemisorbed NO, which is the reaction initiating step. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994.     

Iron spin-reorientation transition in NdFeAsO

The low-temperature magnetic structure of NdFeAsO has been revisited using neutron powder diffraction and symmetry analysis using the Sarah representational analysis program. Four magnetic models with one magnetic variable for each of the Nd and Fe sublattices were tested. The best fit was obtained using a model with Fe moments pointing along the c-direction, and Nd moments along the a-direction. This signals a significant interplay between rare-earth and transition metal magnetism, which results in a spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All models that fit the data well, including collinear models with more than one magnetic variable per sublattice, were found to have an Fe moment of 0.5 μ(B) and a Nd moment of 0.9 μ(B), demonstrating that the low-temperature Fe moment is not substantially enhanced compared to the spin-density wave state.