Nearest-neighbor Sites Research Articles

Surface Dynamics of Lead Adsorbates at the Cu(100)–Electrolyte Interface

Direct quantitative studies of the dynamic behavior of individual cationic adsorbates at an electrochemical interface have been performed for Pb on Cu(100) electrode surfaces in Cl-containing electrolyte. Using in situ high-speed scanning tunneling microscopy, the motion of the Pb adsorbates on the c(2 × 2)-Cl covered Cu surface was monitored and analyzed by statistical models. Contrary to Pb on Cu(100) under ultrahigh vacuum conditions, the tracer diffusion of isolated adsorbates can be described by simple activated hopping between neighboring sites within the c(2 × 2) lattice, with an activation energy that increases linearly by 0.5 eV/V with potential. This surprising behavior, which is opposite in sign to that expected for a cation and similar to that for adsorbed sulfide at this interface, suggests a decisive influence of the surrounding Cl coadsorbates on the Pb diffusion. Furthermore, attractive interactions of ∼20 meV were found between Pb adsorbates on nearest and next-nearest neighbor sites of the c(2 × 2) lattice. The effective diffusion barriers are increased in the vicinity of neighboring Pb, resulting in a mutual stabilization of the adsorbates.

Electron Spin Resonance of Chromium–Platinum Pair in Silicon

We have investigated a chromium–platinum pair in silicon by electron spin resonance measurement. A new ESR spectrum originating from a chromium–platinum pair has been detected in both n- and p-type silicons diffused with chromium and platinum. The anisotropic g-tensor obtained by analyzing the angular dependence of the ESR spectrum shows a monoclinic (C1h) symmetry with g-values of g1=4.67, g2=2.99, and g3=1.80. The g1 axis is along the <110 > direction. The g2 and g3 axes are perpendicular to the g1 axis, and the g2 axis is rotated from the <100 > direction to the <111 > direction at an angle of 20°. The anisotropic character of monoclinic (C1h) symmetry results from the nearest-neighbor configuration consisting of a Pt atom at a substitutional site distorted by the displacement of Pt along the <100 > direction and a Cr atom at the nearest-neighbor interstitial site. The ESR measurement under illumination suggests that a chromium–platinum pair forms a donor like electron trap level.

Cr interactions with He and vacancies in dilute Fe-Cr alloys from first principles

We have performed density functional theory calculations to study the effect of Cr in the formation of He-vacancy (He${}_{n}$V${}_{m}$) clusters in a dilute Fe-Cr system. We find a significant repulsive interaction between Cr and He at neighboring tetrahedral sites. The octahedral He close to Cr becomes unstable falling to a nearby tetrahedral site, unlike in pure systems. We note a slight electronic interaction between an interstitial He and the metal atoms, stronger with Cr and responsible of their repulsion. The effect of a Cr atom in the formation of He${}_{n}$V${}_{m}$ clusters is small except for cases with high He-to-vacancy ($n/m$) ratio. In systems with multiple Cr atoms we find that, while interstitial He always repels Cr, a vacancy or a substitutional He may screen the ferromagnetic Cr coupling and promote their clustering around them. Two different behavior regimes may therefore be suggested. When the $n/m$ ratio is close to 1 or lower the Cr atoms seem to be attracted to the He${}_{n}$V${}_{m}$ cluster while when the $n/m$ ratio is greater than 1 the Cr atoms are in general repelled from it. Concerning diffusion, the energy barrier for a tetrahedral He to hop toward a neighboring position of Cr is found to be twice as large as its migration barrier in pure Fe (0.12 vs 0.06 eV). On the other hand, the diffusion of a substitutional He via the dissociative mechanism has a slightly higher dissociation barrier for first nearest neighbor sites of a Cr atom. For the vacancy mechanism we find similar diffusion properties in dilute Fe-Cr systems and in pure Fe except a slightly lower barrier close to Cr that may induce small trapping effects at low temperatures.

Effect of hole–phonon interaction on [formula omitted] superconducting phase in 2D t-J model with phonon mediated interaction

Ground-state properties of the 2D t-J model with phonon mediated interaction are examined by an exact diagonalization method, following the idea that a local lattice distortion occurs in association with hole upon doping. Effect of this phonon mediated interaction on d-wave pairing over the pure t-J model has been shown. Results show an influence of phonon mediated interaction on d-wave pairing state. Formation of a less mobile quasi-particle made of hole–polaron accompanied by a hole at nearest-neighbor site, eventually supports d-wave pairing. It is also shown that increase in coupling strength forms inter-site hole–bipolaron which gradually decrease d-wave pairing strength. Results show that role of phonons in the mechanism of high temperature superconductivity is very much relevant.

Monte Carlo simulations of the dissolution of borosilicate and aluminoborosilicate glasses in dilute aqueous solutions

The aim of this study was to use Monte Carlo simulations to provide atomic-level insights into the dissolution behavior of borosilicate and aluminoborosilicate glasses in dilute aqueous solutions. In the first part of this work, the effects of different structural features, such as the presence of non-bridging oxygens (NBO) or the formation of boroxol rings, on glass dissolution were evaluated separately and led to the following conclusions. (1) The dependence of the dissolution rate on the amount of NBO was found to be linear at all Si/B ratios and the accelerating effect of NBO was shown to increase with increasing Si/B ratio. (2) The formation of boroxol rings and of clusters of boroxol rings resulted in an increase of the dissolution rate at all Si/B ratios and, again, the extent of the rate increase was strongly dependent on the Si/B ratio. (3) For aluminosilicate glasses, the implementation of the aluminum avoidance rule was found to increase the rate of dissolution relative to that obtained for a random distribution. In the second part of this work, the dissolution of the NeB glasses studied by Pierce et al. (2010) was modeled in dilute aqueous solutions. Pierce et al. concluded from their study that either the rupture of the Al–O bonds or that of the Si–O bonds was the rate-limiting step controlling the dissolution of the NeB glasses. The simulations refined this conclusion and showed that, at low B/Al ratios, the rupture of both Al–O–Si and Si–O–Si linkages contributed to the dissolution rate whereas, at high B/Al ratios, the dissolution rate was independent of the rupture of Al–O–Si linkages and was controlled by S 1 sites (silicon sites at the glass–water interface with one connection to nearest-neighbor sites) and dissolution via detachment of clusters.

Lattice gas with nearest- and next-to-nearest-neighbor exclusion

We investigate a hard-square lattice gas on the square lattice by means of transfer-matrix and Monte Carlo methods. The size of the hard squares is equal to two lattice constants, so the simultaneous occupation of nearest-neighbor sites as well as of next-to-nearest-neighbor sites is excluded. Near saturation of the particle density, this system is known to undergo a phase transition to one out of four partially ordered phases. We find that this transition displays strong finite-size corrections to scaling and that the correlation functions deviate from isotropy to rather large distances. In contrast with an earlier study, we find that the critical temperature exponent of the transition is not Ising-like.

Myopic random walkers and exclusion processes: Single and multispecies

A motility mechanism based on a simple exclusion process, where the probability of movement of an agent depends on the number of unoccupied nearest-neighbor sites is considered. Such interacting agents are termed myopic. This problem is an extension of the famous blind or myopic ant in a labyrinth problem. For the interacting agent models considered here, each agent plays the role of an ant in a labyrinth, where the paths of allowed sites though the labyrinth consist of sites not occupied by other agents. We derive a nonlinear diffusion equation for the average occupancy of the discrete agent-based model for myopic agents. In contrast, interacting blind agents have a constant probability of movement to each of their nearest-neighbor sites, giving rise to a linear diffusion equation. Insight into the various terms in the nonlinear diffusion coefficient is obtained from a study of multiple subpopulations of interacting myopic agents, where an advection–diffusion equation for each subpopulation is derived, and from tracking an individual agent within the crowd, where a motility coefficient is extracted. Averaged discrete simulation data compares very well with the solution to the continuum models. We also compare the behavior of myopic and blind agents. The myopic motility mechanism is biologically motivated to emulate information an individual cell gathers from environment cues. The multispecies model developed and investigated here assists with the interpretation of experimental data involving the tracking subpopulations of cells within a total cell population.

First-principles study on surface magnetism in Co-doped (110) SnO 2 thin film

Electronic structure and magnetism of Co-doped (110) SnO 2 surface are investigated using the full-potential linearized augmented plane-wave method. Total energy calculations indicate that Co atoms prefer to surface sites and couple ferromagnetically when they occupy nearest-neighbor sites. Irrespective of the sites that Co dopants occupy, different geometries with ferromagnetic, antiferromagnetic, and nonmagnetic configurations are predicted in Co-doped systems, which provide a key to understand seemingly conflicting experimental results.

Energetic stability of hydrogen-chemisorbed carbon nanotube structures

To optimize the performance of carbon nanotubes in the area of hydrogen storage we explore stable structures that can form upon hydrogenation. Through free energy calculations using first-principles DFT we find that at room temperature the trans-hydride structure, i.e., one in which H-atoms chemisorbed on nearest-neighbor sites alternate between outside and inside of the nanotube is the most stable structure under moderate hydrogen dosage for tubes with diameter >1.2nm. Such structures are energetically stable with respect to recombination into H2 or decomposition into hydrocarbons like benzene or ethylene. We also find that the IR spectrum of the trans-hydrides display striking dependence on nanotube chirality.

Moment ratios and dynamic critical behavior of a reactive system with several absorbing configurations

We determine the critical behavior of a reactive model with many absorbing configurations. Monomers A and B land on the sites of a linear lattice and can react depending on the state of their nearest-neighbor sites. The probability of a reaction depends on temperature of the catalyst as well as on the energy coupling between pairs of nearest-neighbor monomers. We employ Monte Carlo simulations to calculate the moments of the order parameter of the model as a function of temperature. Some ratios between pairs of moments are independent of temperature and are in the same universality class of the contact process. We also find the dynamical critical exponents of the model and we show that they are in the directed percolation universality class whatever the values of temperature.

Effect of atomic structure on migration characteristic and solute segregation of ordered domain interfaces formed in Ni 75Al xV 25- x

Based on the microscopic phase-field model, ordered domain interfaces formed between D0 22 (Ni 3V) phases along [001] direction in Ni 75Al x V 25- x alloys were simulated, and the effects of atomic structure on the migration characteristic and solute segregation of interfaces were studied. It is found that the migration ability is related to the atomic structure of interfaces, and three kinds of interfaces can migrate except the interface (001)//(002) which has the characteristic of L1 2 (Ni 3Al) structure. V atoms jump to the nearest neighbor site and substitute for Ni, and vice versa. Because of the site selectivity behaviors of jumping atoms, the number of jumping atoms during the migration is the least and the jumping distance of atoms is the shortest among all possible modes, and the atomic structures of interfaces are unchanged before and after the migration. The preferences and degree of segregation or depletion of alloy elements are also related to the atomic structure of interface.

How does the resistance threshold in spatially explicit epidemic dynamics depend on the basic reproductive ratio and spatial correlation of crop genotypes?

We examined the fraction of resistant cultivars necessary to prevent a global pathogen outbreak (the resistance threshold) using a spatially explicit epidemiological model (SIR model) in a finite, two-dimensional, lattice-structured host population. Infectious diseases in our model could be transmitted to susceptible nearest-neighbour sites, and the infected site either recovered or died after an exponentially distributed infectious period. Threshold behaviour of this spatially explicit SIR model cannot be reduced to that of bond percolation, as was previously noted in the literature, unless extreme assumptions (synchronized infection events with a fixed lag) are imposed on infection process. The resistance threshold is significantly lower than that of conventional mean-field epidemic models, and is even lower if the spatial configuration of resistant and susceptible crops are negatively correlated. Finite size scaling applied to the resistance threshold for a finite basic reproductive ratio ρ of pathogen reveals that its difference from static percolation threshold (0.41) is inversely proportional to ρ. Our formula for the basic reproductive ratio dependency of the resistance threshold produced an estimate for the critical basic reproductive ratio (4.7) in a universally susceptible population, which is much larger than the corresponding critical value (1) in the mean-field model and nearly three times larger than the critical growth rate of a basic contact process (SIS model). Pair approximation reveals that the resistance threshold for preventing a global epidemic is factor 1/(1− η) greater with spatially correlated planting than with random planting, where η is initial correlation in host genotypes between nearest-neighbour sites. Thus the eradication is harder with a positive spatial correlation ( η>0) in mixed susceptible/resistant plantings, and is easier with a negative correlation ( η<0). The effect of finite field size ( L), which corresponded to the mean distance between sources of infections, is given by the increased resistance threshold (by the amount L −0.75) from its infinite size limit. Implications of these results on effective planting strategies in multi-line control plans are discussed.

Exact two-time correlation and response functions in the one-dimensionalcoagulation–diffusion process by the empty-interval method

The one-dimensional coagulation–diffusion process describes the strongly fluctuatingdynamics of particles, freely hopping between the nearest-neighbour sites of achain, such that one of them disappears with probability one if two particles meet.The exact two-time correlation and response functions in the one-dimensionalcoagulation–diffusion process are derived from a generalization of the empty-intervalmethod. The main quantity is the conditional probability of finding an empty interval ofn consecutive sites,if at distance d a site is occupied by a particle. Closed equations of motion are derived such that theprobabilities needed for the calculation of correlators and responses, respectively, aredistinguished by different initial and boundary conditions. In this way, the dynamicalscaling of these two-time observables is analysed in the long-time ageing regime. Anew generalized fluctuation-dissipation ratio with a universal and finite limit isproposed.

Phenomenological Ginzburg-Landau-like theory for superconductivity in the cuprates

We propose a phenomenological Ginzburg-Landau-like theory of cuprate superconductivity. The free energy is expressed as a functional F of the spin-singlet pair amplitude psi_ij=psi_m=Delta_m exp(i phi_m); i and j are nearest-neighbor sites of the Cu lattice in which the superconductivity is believed to primarily reside and m labels the site at the center of the bond between i and j. The system is modeled as a weakly coupled stack of such planes. We hypothesize a simple form, F[Delta,phi]=sum_m (A Delta_m^2+ B Delta_m^4/2)+C sum_<mn> Delta_m Delta_n cos(phi_m-phi_n), for the functional. The coefficients A, B and C are determined from comparison with experiments. We work out a number of consequences of the proposed functional for specific choices of A, B and C as functions of hole density x and temperature T. There can be a rapid crossover of <Delta_m> from small to large values as A changes sign on lowering T and the crossover temperatures is identified with the observed pseudogap temperature. The superconducting phase-coherence transition occurs at a different temperature T_c, and describes superconductivity with d-wave symmetry for C>0. We calculate T_c(x) which has the observed parabolic shape, being strongly influenced by the coupling between Delta_m and phi_m present in F. The superfluid density, the local gap magnitude, the specific heat (with and without a magnetic field) and vortex properties are obtained using F. We compare our results successfully with experiments. We also obtain the electron spectral density as influenced by the coupling between the electrons and the pair correlation function calculated from F. Features such as temperature dependent Fermi arcs, antinodal pseudogap filling temperature, pseudogapped density of states in different momentum regions of the Fermi surface and `bending' of the energy gap versus momentum on the Fermi surface emerge from the theory.

Berezinskii-Kosterlitz-Thouless-like percolation transitions in the two-dimensionalXYmodel

We study a percolation problem on a substrate formed by two-dimensional XY spin configurations using Monte Carlo methods. For a given spin configuration, we construct percolation clusters by randomly choosing a direction x in the spin vector space, and then placing a percolation bond between nearest-neighbor sites i and j with probability p(ij)=max(0,1-e(-2Ks(i)(x)s(j)(x))), where K>0 governs the percolation process. A line of percolation thresholds K(c)(J) is found in the low-temperature range J≥J(c), where J>0 is the XY coupling strength. Analysis of the correlation function g(p)(r), defined as the probability that two sites separated by a distance r belong to the same percolation cluster, yields algebraic decay for K≥K(c)(J), and the associated critical exponent depends on J and K. Along the threshold line K(c)(J), the scaling dimension for g(p) is, within numerical uncertainties, equal to 1/8. On this basis, we conjecture that the percolation transition along the K(c)(J) line is of the Berezinskii-Kosterlitz-Thouless type.

A-centers and isovalent impurities in germanium: Density functional theory calculations

Abstract In the present study density functional theory calculations have been used to calculate the binding energies of clusters formed between lattice vacancies, oxygen and isovalent atoms in germanium. In particular we concentrated on the prediction of binding energies of A-centers or oxygen interstitials that are at nearest and next nearest neighbor sites to isovalent impurities (carbon, silicon and tin) in germanium. The A-center is an oxygen interstitial atom near a lattice vacancy and is an important impurity-defect pair in germanium. In germanium doped with carbon or silicon, we calculated that most of the binding energy of the cluster formed between A-centers and the carbon or silicon atoms is due to the interaction between the oxygen interstitial atom and the carbon or silicon atoms. For tin-doped germanium most of the binding energy is due to the interaction of the oversized tin atom and the lattice vacancy, which essentially provide space for tin to relax. The nearest neighbor carbon–oxygen interstitial and the silicon–oxygen interstitial pairs are significantly bound, whereas the tin–oxygen interstitial pairs are not. The results are discussed in view of analogous investigations in isovalently doped silicon.

Coherent potential approximation of random nearly isostatic kagome lattice

The kagome lattice has coordination number 4, and it is mechanically isostatic when nearest-neighbor sites are connected by central-force springs. A lattice of N sites has O(√N) zero-frequency floppy modes that convert to finite-frequency anomalous modes when next-nearest-neighbor (NNN) springs are added. We use the coherent potential approximation to study the mode structure and mechanical properties of the kagome lattice in which NNN springs with spring constant κ are added with probability P=Δz/4, where Δz=z-4 and z is the average coordination number. The effective medium static NNN spring constant κ(m) scales as P(2) for P≪κ and as P for P≫κ, yielding a frequency scale ω*~Δz and a length scale l*~(Δz)(-1). To a very good approximation at small nonzero frequency, κ(m)(P,ω)/κ(m)(P,0) is a scaling function of ω/ω*. The Ioffe-Regel limit beyond which plane-wave states become ill-defined is reached at a frequency of order ω*.

Computer simulations of large-scale defect clustering and nanodomain structure in gadolinia-doped ceria

The aggregation of defects in gadolinia-doped ceria (GDC) into clusters and larger domains has been studied by atomistic computer simulation. It is found that sub-nanoscale defect clusters prefer a pyrochlore-type structure in which the dopants and vacancies are at next-nearest-neighbor sites, and have a tendency to grow into larger clusters. It was determined that, as defect clusters grow into nanoscaled domains, the C-type rare earth structure, in which the dopants and vacancies are at nearest-neighbor sites, becomes more stable. Simulations suggest that nanodomains serve as the precursor of phase separation and can be easily formed during synthesis. It is believed that doping concentration limits the size of the nanodomains, and this causes GDC to favor small pyrochlore-type clusters at lower concentrations, but C-type nanodomains at higher concentration. Because of this transition, GDC is expected to show initially an increase in conductivity and then a decrease with increasing doping concentration. The lattice parameter of GDC should also show the same trend and could be used as an indicator of the predominant defect structure. The cation mobility is believed to be another important factor limiting the size of defect clusters, and can be used to control the formation of nanodomains during synthesis and thereby improve the electrolyte performance.

Dimer adsorption on square surfaces with first- and second-neighbor interactions

Dimer adsorption on surfaces simulates the adsorption of particles that bind onto two nearest-neighbor sites. In 1993, we constructed a transfer matrix ( T-matrix) for the study of dimers on stepped surfaces, consisting of M-sites wide square terraces, considering only first-neighbor interaction energies. Here, we consider a more realistic model by including both first- and second-neighbor interaction energies, V and W. The non-trivial construction of the T-matrix to include second-neighbor interactions is used to obtain the low-temperature energy phase diagrams of the dimer system for any M, when first-neighbors are attractive, and for values of M < 7 when first-neighbors are repulsive. New crystallization patterns and phases are observed and extrapolated to infinite M. Monte Carlo simulation techniques confirm our T-matrix results, but the T-matrix method is found to be computationally more efficient and more precise. However, Monte Carlo parallel tempering simulations combined with finite-size scaling, while limited in precision, are more efficient to obtain the critical temperature of the various order–disorder transitions as a function of W / | V | , from the study of the heat capacity and the order parameter as functions of temperature. We also discuss the relevance of these results to experiments.

Modification of Magnetic Stability of Co Single Atoms on Pt(111) by Dimer Formation

Novel magnetic properties of very small nanostructures have attracted broad interests in the fields of nanoscience and nanotechnology. The magnetic stability of such nanostructures relies on the magnetic anisotropy and magnetization dynamics. These properties were investigated for single Co atoms on Pt(111) using low-temperature scanning tunneling microscopy. The atomic manipulation capabilities of scanning tunneling microscopy allow to extend the study to Co dimers. Information on the magnetic anisotropy and lifetime of magnetically excited states of single atoms and dimers was achieved simultaneously with inelastic tunneling spectroscopy. Manufactured dimers consist of adatoms in nearest-neighbor fcc-hcp sites. For adatoms this close, a strong direct exchange interaction of 14 meV/h2 was found, much larger than the weak long-range indirect Ruderman-Kittel-Kasuya-Yoshida interaction. [DOI: 10.1380/ejssnt.2011.237]