Various Numbers Of Atoms Research Articles

Reproducibility of a heterophase structure emergence effect when changing the ignition temperature of SHS in a layered nanosized nonstoichiometric Ti-Al system

The simulation results of SHS process in a package of alternating nanoscale layers of the crystal lattices of Ti and Al atoms for two systems with various numbers of atoms and nonstoichiometric ratios are given. For simulation the LAMMPS package supporting parallel computations and the interatomic interaction potential in the “embedded atom” model (EAM) have been used. For two systems with various numbers of atoms and nonstoichiometric ratios the sets of temperature and density profiles along the layers of the structure at successive instants of time (up to 16 ns) have been obtained. For a system with 453974 atoms and a nonstoichiometric ratio of 1.23 the heterostructure emergence with an alternation of intermetallic phases when changing the SHS “ignition” temperature has been reproduced.

Formation of graphene on BN substrate by vapor deposition method and size effects on its structure

We report MD simulation of the growth of graphene by the vapor deposition on a two-dimensional hBN substrate. The systems (containing carbon vapor and hBN substrate) are relaxed at high temperature (1500 K), and then it is cooled down to room one (300 K). Carbon atoms interact with the substrate via the Lennard-Jones potential while the interaction between carbon atoms is computed via the Tersoff potential. Depending on the size of the model, different crystalline honeycomb structures have been found. Structural properties of the graphene obtained at 300 K are studied by analyzing radial distribution functions (RDFs), coordination numbers, ring statistics, interatomic distances, bond-angle distributions and 2D visualization of atomic configurations. We find that the models containing various numbers of atoms have a honeycomb structure. Besides, differences in structural properties of graphene formed by the vapor deposition on the substrate and free standing one are found. Moreover, the size effect on the structure is significant.

Monte-Carlo tree search for stable structures of planar clusters

Illustrated by the case of the planar clusters, we propose a new method to search the possible stable structures by combining the structural identification and Monte-Carlo tree algorithm. We adopt two kinds of model-potential to describe the interaction between atoms:the pair interaction of Lennard-Jones potential and three-body interaction based on the Lennard-Jones potential. Taking the possible triangular lattice fragment as candidates, we introduce a new nomenclature to distinguish the structures, which can be used for the rapid congruence check. 1) We label the atoms on the triangular lattice according to the distances and the polar angles. where a given triangular structure has a corresponding serial number in the numbered plane. Note that the congruent structures can have a group of possible serial numbers. 2) We consider all the possible symmetrical operations including translation, inversion and rotation, and obtain the smallest one for the unique nomenclature of the structure. In conventional search of magic clusters, the global optimizations are performed for the structures with given number of atoms. Herein, we perform the Monte-Carlo tree search to study the evolution of stable structures with various numbers of atoms. From the structures of given number of atoms, we sample the structures according to their energy with the importance sampling, and then expand the structures to the structures with one more atom, where the congruence check with the nomenclature is adopted to avoid numerous repeated evaluations of candidates. Since the structures various numbers of atoms are correlated with each other, a searching tree will be obtained. In order to prevent the over-expansion of branches, we prove the “tree” according to energy to make the tree asymmetric growth to retain the low energy structure. The width and depth of search is balanced by the control of temperature in the Monte-Carlo tree search. For the candidates with lower energies, we further perform the local optimization to obtain the more stable structures. Our calculations show that the triangular lattice fragments will be more stable under the pair interaction of Lennard-Jones potential, which are in agreement with the previous studies. Under the three body interaction with the specific parameter, the hexagonal lattice fragments will be more stable, which are similar to the configurations of graphene nano-flakes. Combining the congruence check and Monte-Carlo tree search, we provide an effective avenue to screen the possible candidates and obtain the stable structures in a shorter period of time compared with the common global optimizations without the structural identification, which can be extended to search the stable structure for materials by the first-principles calculations.

Cooperative optical response of 2D dense lattices with strongly correlated dipoles.

We study light propagation in dense low-temperature atoms on two-dimensional (2D) square and kagome lattices using a basically exact large-scale numerical computations. In the limit of weak laser intensity, shifts of the resonance line are shown in homogeneously broadened stationary samples with high atom densities, whereas the shifts are not shown in the samples with low densities. We obtain the dependence of shifts on interatomic spacing for square lattices with the various numbers of atoms, and our numerical results are in good agreement with shifts derived using a 2D isotropic infinite lattice model and experimental data for nanometric-thickness atomic ensembles in the literature.

Fluorescent Au nanoclusters: recent progress and sensing applications

Recent progress in nanotechnology has given rise to a new class of fluorescent nanomaterials: fluorescent noble metal nanoclusters. The noble metal nanoclusters include Ag, Au, Cu and Pt and typically consist of a ligand shell and a metal core, which incorporates various numbers of atoms ranging from only a few to several hundred, with diameters <2 nm. Au nanoclusters are the most intensely studied of these materials because of their attractive features, including convenient synthesis, ultra-small size, fluorescent properties, excellent optical/colloidal stability and biocompatibility. This paper summarizes the common synthesis strategies for obtaining water-soluble, highly fluorescent Au nanoclusters using different capping agents, and highlights recent advances in their fluorescent sensing applications with an emphasis on the sensing mechanisms. We also consider the prospects, opportunities and challenges for the synthesis and application of Au nanoclusters.

Validity and size-dependency of Cauchy–Born hypothesis with Tersoff potential in silicon nano-structures

One of the most popular constitutive rules that correlate the continuum and atomic properties in multi-scale models is the Cauchy–Born (CB) hypothesis. Based on this constitutive law of continuum media, it assumes that all atoms follow the deformation subjected to the boundary of crystal. In this paper, the validity and failure of CB hypothesis are investigated for the silicon nano-structure by comparison of the continuum and atomic properties. In the atomistic level, the stresses and position of atoms are calculated using the molecular dynamics (MD) simulation based on the Tersoff inter-atomic potential. The stresses and strains are compared between the atomistic and continuous media to obtain the validity of CB hypothesis. The 3D validity surfaces are derived for various numbers of atoms to display the size-dependency of cubic specimens. It is shown that the CB is valid inside the surface, the deformations are homogeneous, and the material is elastic, while the CB is invalid outside of the surface, the deformations are inhomogeneous, and the material may remain elastic, or plastic.

The electron transfer in the donor-polymer-acceptor molecular complex

The electron transfer in the donor-polymer-acceptor molecular complex was investigated. The carbon chain with various numbers of atoms, which has two possible conformations (homogeneous and dimerizided), was taken as a polymer. A theoretical model that described such systems was proposed, and the probability of detection of the electron at the end of the chain that originally is localized on the first atom was theoretically investigated. Two types of electron transfer were revealed.

Ab initiostudy of thallium nanoclusters onSi(111)−7×7

We have studied the stability of a thallium nanocluster of various numbers of atoms $(N=1,2,\dots{},10)$ on a $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ substrate using density functional theory total energy calculations. We have compared it with clusters of other group III elements (Al, Ga, and In). Thallium is found to be unstable with the triangular cluster, which has been known to be stable for other group III elements. Instead, a slightly different structure, in which Si atop atoms are lower than thallium atoms in height by $2.56\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, was found to be quite stable. Such an abnormal structure originates from the inert pair of $6{s}^{2}$ electrons due to the significant spin-orbit interaction. The initial relaxed $N=6$ Tl cluster continues to grow with increasing $N$ up to $N=9$ in the faulted-half unit cell, which is consistent with experimental observations.

Classical simulations of magnetic structures for chromium clusters: Size effects

Abstract Classical (Heisenberg) simulations show that the total magnetization of the lowest-energy states of clusters made of antiferromagnetically coupled chromium atoms is planar, rather than collinear, depending on the arrangement of the atoms. Although the model Hamiltonian is not restrictive, many cluster configurations of various numbers of atoms do not use all three directions for the spins. This result confirms the conclusion drawn from the local-spin DFT calculation by Kohl and Bertsch that clusters of N≤13 have non-collinear magnetic moments. The present simulations show non-collinear spin ordering also for bigger clusters, designed to be as spherical as possible following the bcc arrangement, when atoms interact both with the nearest and next-nearest neighbours. Depending on the signs of the coupling constants frustration appears. The advantage of the discrete model, despite the simplicity, is that very large clusters and magnetization at finite temperatures can be studied. This model predicts that clusters with specific numbers of atoms interacting only with the nearest neighbours have collinear spins as in the bulk. We also apply the model to simulate the destruction of the anti-ferromagnetic ordering by thermal fluctuations. This model shows no unique magnetization of mixed Fe 0.33 Cr 0.67, which is consistent with experimental observations.

Production of heavy atomic clusters upon interaction of laser radiation with matter

The results of an investigation of the formation of Pb, Th, and U clusters over a broad range of numbers of atoms (from a few atoms to macroparticles) upon interaction of high-power pulsed laser radiation with matter are presented. Clusters of fissionable elements are studied for the first time. A setup for determining the yield of clusters and the number of atoms in them, which is based on the use of several different methods (laser resonance fluorescence, time-of-flight measurements, and counting the number of tracks of fission fragments from the cluster nuclei), is described. The dependence of the yield of clusters with various numbers of atoms on the conditions for their formation is discussed.

The effect of the counter-rotating terms on the Rabi oscillations and squeezing in the Dicke model with cavity losses

The effect of the counter-rotating terms in the Dicke model with cavity losses is examined. Numerical results for the time evolutions of the atomic population inversion, dipole moment, mean-photon number and atomic and field squeezing parameters for an initial coherent field are presented for various numbers of atoms and different cavity dampings. As a consequence of the counter-rotating terms, the appearance of new steady states for atomic population inversion and the mean-photon number is pointed out.

The effect of the counter-rotating terms in the Dicke model

The significance of the counter-rotating terms in the Dicke model (a sample of N identical two-level atoms inside a single-mode cavity) is demonstrated. Numerical results for the time evolution of the atomic population inversion and the mean photon number for various numbers of atoms and different cavity dampings (low- and high-Q cavities) and different photon numbers of the Fock-state field (being present initially in the cavity) are presented. An initial coherent field is also treated. The appearance of new steady states under the influence of the counter-rotating terms is pointed out. Finally, by applying a modified Robertson projection-operator method, developed previously by the author, an analytic solution in the case of strongly damped (low-Q) cavities is derived.

Effect of the counterrotating terms in the many-atom Jaynes–Cummings model with cavity losses

The significance of the counterrotating terms in the many-atom Jaynes–Cummings model with cavity damping is demonstrated. Numerical results for the time evolution of the atomic population inversion for various numbers of atoms and different cavity dampings in the case of spontaneous emission are presented. The appearance of new steady states under the influence of the counterrotating terms is pointed out.

Many-atom Jaynes-Cummings model without rotating-wave approximation

The contribution of the counter-rotating terms in the many-atom Jaynes-Cummings model is examined. Numerical results for the time evolution and time averages of the atomic population inversion for various numbers of atoms and different initial fields (Fock and coherent-state fields) are presented. The results demonstrate the significance of the counter-rotating terms.

Multiple scattering approach to the XANES theory of alkali halide crystals. II. Band structure approach and cluster approximations

AbstractThe conduction band structures of six alkali halide crystals (LiF, NaF, KF, LiCl, NaCl, KCl) are studied by means of the augmented plane wave method (APW). The local partial densities of states (LPDOS) and X‐ray absorption K‐spectra are calculated. The band structure predictions are compared with the results of cluster model calculations obtained within the framework of a Green's function approach employing multiple scattering technique. The effect of cluster size is examined for the clusters containing various numbers of atoms. It is shown that 57‐atom clusters are sufficiently large to represent the band results in XANES region. The hole effect is exhibited as the intensity redistributions and the shifts of the peak positions, although more serious changes in the case of Li K‐spectrum in LiF are found.

Calculation of sputtering of sequentially increasing atom blocks

Abstract Some simplified models are used to computer-calculate the parameters of Cu single crystal sputtering by 1 keV Ar ions. Each of the models is a confined structure of a perfect single crystal with various numbers of atoms (from 2 to 34). The sputtering yields, the energy spectra and spatial distributions of sputtered atoms are calculated at various energies of bombarding ions.

Study of the sputtering process with Rutherford backscattering

The amount of sputtered materials of Au, Al and Si targets bombarded with keV Al, Au and Ge ions has been measured. The backscattering technique allows to determine the various numbers of atoms of the target and of the implanted atoms. Together with the information of the concentration in depth of the different atomic species, measured with the backscattering method or with the secondary ion microscope, the influence of the different ions, the target substrates and the change in composition of the target on the sputtered material is shown and compared with calculations.

Modification of PVC. XXVI. Syntheses and photocrosslinking of polysulfide pendant poly(vinyl chloride)

AbstractPoly(vinyl chloride) pendant with polysulfide (PS–PVC) having various degrees of substitution, various S substituents, and various numbers of atoms in the sulfur chain has been synthesized by the reaction of poly(vinyl chloride) with a thiol, sulfur, and triethylamine in dimethylformamide at 30°C for 0.4–5 hr. The photocrosslinking reaction has been investigated under ultraviolet irradiation at 250–450 mμ. The photocrosslinking reaction of PS–PVC is influenced by the degree of substitution, the nature of the S substituent, and the number of atoms in the sulfur chain. The degree of photocrosslinking r increased in the order, n‐C4H9 &lt; n‐C8H17 &lt; C6H5CH2 &lt; i‐C3H7 &lt; t‐C4H9. On the photocrosslinking of PS–PVC having two different S substituents, r increases in the similar order for aliphatic substituents and in the order NO2C6H4 &lt; ClC6H4 &lt; C6H5CH2 &lt; CH3C6H4 &lt; t‐C4H9C6H4 &lt; C6H5 for the aromatic substituents. Further, r increases markedly with the increase of sulfur chain number for all PS–PVC. The chemical structure of the crosslinks and the crosslinking mechanism are discussed on the basis of the results.