Atomic Density Distribution Research Articles

Phase field crystal simulation of grain boundary annihilation under strain strain at high temperature

Grain boundary (GB) research is always the most fundamental and active study field in interface science. Grain boundary premelting (GBPM) is induced as a consequence of local inner strain around defects in material at high temperature. When GB premelting is under an external stress, it is referred to as stress induced GBPM (SIGBPM). Owing to the fact that the width of a GB usually is a few atoms thick, it is difficult to observe the GBPM directly in experiment, thus the development of computational simulation experiment can make up for the shortcomings in experiment. For this reason, a new method which is named phase field crystal (PFC) model based on density functional theory is proposed. Because the method can be used to simulate the evolution of macroscopic structure of polycrystalline material on a diffusive time and atomic scale, therefore, PFC has a great advantage in simulating the evolution of microstructure. In this paper, PFC method is used to investigate the annihilation process of dislocation pairs of premelted grain boundary under strain at high temperature. Simulated results show that the essence of separation process of sub-GB (SGB) from original GB is that sub-grain structures are generated. The SGB migration is the process of the new grain swallowing up the old one. The annihilation process of GBPM under applied strain at high temperature can be divided into two stage features. The first stage is the stage of system energy increasing, which is corresponding to the process of SGB migration, dislocation gliding; the second stage is the energy decreasing, which corresponds to the interaction of SGBs and annihilation of dislocations, while the speed of annihilation in this process is slow and the peak of energy curve is wide and smooth. According to the changing process of the atomic density distribution projected along the directions of x and y axis with strain increasing, we can reveal that the nature of annihilation of double dislocation pairs at high temperature is the process of two-step annihilations, of which the detailed process is not easy to observe at low temperature due to its fast annihilating speed of dislocation pairs.

Interlayer structure model of tri-hydrated low-charge smectite by X-ray diffraction and Monte Carlo modeling in the Grand Canonical ensemble

The present study aims primarily at refining a structure model for interlayer cations and H2O molecules in tri-hydrated (3W) smectite (d001 = 18-19 Å). The <2 mm fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg2+, Ca2+, Ba2+, or Na+ cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d001 values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H2O content and distribution of interlayer species (cations and H2O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H2O molecules but rather exhibit two sharp planes of H2O molecules wetting the clay surfaces (at ~2.7 Å from the clay layer surface). Additional H2O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H2O molecules with interlayer cation, through their second hydration shell, and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H2O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions.

Increasing the cold atom density in an integrating spherical cavity

A novel method of angled incidence for diffuse laser cooling of Rb-87 is presented to improve the distribution of cold atom density in an integrating sphere. The angled injection scheme could cool more atoms in the middle of the sphere compared to the previous normal injection scheme. The loading time of the cold atoms for the angled injection scheme is twice as that of the normal injection scheme. The SNR and the contrast of the detected signal would be improved in the angled injection scheme. (C) 2014 Elsevier B.V. All rights reserved.

A low-cost dual energy CT system with sparse data

Dual Energy CT (DECT) has recently gained significant research interest owing to its ability to discriminate materials, and hence is widely applied in the field of nuclear safety and security inspection. With the current technological developments, DECT can be typically realized by using two sets of detectors, one for detecting lower energy X-rays and another for detecting higher energy X-rays. This makes the imaging system expensive, limiting its practical implementation. In 2009, our group performed a preliminary study on a new low-cost system design, using only a complete data set for lower energy level and a sparse data set for the higher energy level. This could significantly reduce the cost of the system, as it contained much smaller number of detector elements. Reconstruction method is the key point of this system. In the present study, we further validated this system and proposed a robust method, involving three main steps: (1) estimation of the missing data iteratively with TV constraints; (2) use the reconstruction from the complete lower energy CT data set to form an initial estimation of the projection data for higher energy level; (3) use ordered views to accelerate the computation. Numerical simulations with different number of detector elements have also been examined. The results obtained in this study demonstrate that 1 + 14% CT data is sufficient enough to provide a rather good reconstruction of both the effective atomic number and electron density distributions of the scanned object, instead of 2 sets CT data.

Role of helium in the sliding and mechanical properties of a vanadium grain boundary: A first-principles study

The effects of helium (He) on the sliding and mechanical properties of a vanadium (V)Σ5(310)/[001] grain boundary (GB) have been investigated using a first-principles method. It has been found that He was energetically favorable sitting at the GB region with a segregation energy of −0.27 eV, which was attributed to the special atomic configurations and charge density distributions of the GB. The maximal sliding energy barrier of the He-doped GB was calculated to be 1.73 J/m2, ∼ 35% larger than that of the clean GB. This suggested that the presence of He would hinder the V GB mobility. Based on the thermodynamic criterion, the total energy calculations indicated that the embrittlement of V GB would be enhanced by He segregation.

Ground-State Properties of Superfluid Fermi Gas in Fourier-Synthesized Optical Lattices

By employing the balance condition between the lattice potential and the interatomic interaction, we study the ground state solutions of superfluid Fermi gases in Fourier-synthesized (FS) optical lattices. The average energy of the ground state, the atoms number, and the atom density distribution of the Fermi system are analytically derived along the Bose—Einstein condensation (BEC) side to the Bardeen—Cooper—Schrieffer (BCS) side. We analyze the properties of ground state solutions at both the BEC limit and unitarity in FS optical lattices. It is found that the relative phase α between the two lattice harmonics impacts greatly on the properties of the ground state of the superfluid Fermi gas. Especially in the BCS limit, when α = π/2, the average energy presents an exponential form with the increase of the potential depth of the lattice harmonics υ2. Meanwhile, there exits a minimal value. Moreover, due to the Fermi pressure, the atom density distribution at unitarity is more outstretched than that in the BEC limit. The average energy at unitarity is apparently larger than that in the BEC limit. The properties of the ground state solution exhibit very different behaviors when the system transits from the BEC side to the BCS side.

Development of a high dynamic range spectroscopic system for observation of neutral hydrogen atom density distribution in Large Helical Device core plasma

We report development of a high dynamic range spectroscopic system comprising a spectrometer with 30% throughput and a camera with a low-noise fast-readout complementary metal-oxide semiconductor sensor. The system achieves a 10(6) dynamic range (∼20 bit resolution) and an instrumental function approximated by a Voigt profile with Gauss and Lorentz widths of 31 and 0.31 pm, respectively, for 656 nm light. The application of the system for line profile observations of the Balmer-α emissions from high temperature plasmas generated in the Large Helical Device is also presented. In the observed line profiles, emissions are detected in far wings more than 1.0 nm away from the line center, equivalent to neutral hydrogen atom kinetic energies above 1 keV. We evaluate atom density distributions in the core plasma by analyzing the line profiles.

Numerical analysis of atomic density distribution in arc driven negative ion sources

The purpose of this study is to calculate atomic (H(0)) density distribution in JAEA 10 ampere negative ion source. A collisional radiative model is developed for the calculation of the H(0) density distribution. The non-equilibrium feature of the electron energy distribution function (EEDF), which mainly determines the H(0) production rate, is included by substituting the EEDF calculated from 3D electron transport analysis. In this paper, the H(0) production rate, the ionization rate, and the density distribution in the source chamber are calculated. In the region where high energy electrons exist, the H(0) production and the ionization are enhanced. The calculated H(0) density distribution without the effect of the H(0) transport is relatively small in the upper region. In the next step, the effect should be taken into account to obtain more realistic H(0) distribution.

All-atom molecular dynamics simulation of HPMA polymers

Structural properties such as size, shape and density distribution of a range of N-(2-hydroxypropyl)methacrylamide (HPMA) polymers in various solvent models have been investigated. Common atomistic force fields were compared against rotational barriers and relative conformational energies obtained from ab initio and density functional theory (DFT) data for a monomer and dimer of HPMA. This identified the AMBER99 parameter set as the most appropriate for representing the structures and so AMBER99 was employed for all molecular dynamics simulations. MD trajectories were calculated for a range of polymer sizes from 4 to 265 repeat units (2 to 35 kDa). The time averaged radius of gyration was extracted from trajectories and interpreted in the context of Flory's mean field approach. Comparison with data obtained from small angle neutron scattering (SANS) experiments was then used to differentiate between alternative solvent models. The shape adopted by such polymers was evaluated by fitting structures to ellipsoids, to allow separate analysis of radius and density profile along each axis. The density distribution of atoms was defined using these ellipsoids according to centre of mass or centre of neutron scattering lengths, the latter allowing direct comparison with experimental SANS data. We show that computational simulation of such polymers has practical application in obtaining detailed morphological information of polymer solution structure, and as a benchmark for coarse-grained methods.

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s 3S1, 2s 1S, 2p 3P0 J, and 2p 1P0 created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities n1 in the considered He states along the plasma length L. The field of the theoretical profiles for a series of the n1L parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, n1L corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of n1 could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: n1 av (2s 3S1) = (2.9 ± 1.1) × 1013 cm−3, n1 av (2s 1S) = (1.4 ± 0.5) × 1013 cm−3, n1 av (2p 3P0 J) = (1.1 ± 0.4) × 1013 cm−3, n1 av (2p 1P0) = (4.2 ± 1.6) × 1012 cm−3, and they represent the average populations along the plasma column in the capillary.

Dynamically controlled two-color photonic band gaps via balanced four-wave mixing in one-dimensional cold atomic lattices

We study an ensemble of cold atoms driven to the four-level double-$\ensuremath{\Lambda}$ configuration and trapped in the one-dimensional optical lattices for achieving dynamically controlled photonic band gaps (PBGs) via the balanced four-wave mixing interaction. Numerical results show that a pair of PBGs characterized by high platforms of probe reflectivities and reduced density of photonic states are established, simultaneously, on both probe transitions in one symmetric driving scheme. Such two-color PBGs can be easily manipulated in position and width by modulating, e.g., the balanced Rabi frequencies of two coupling fields and the Gaussian widths of atomic density distributions, to change the equal probe detunings fulfilling both Bragg conditions. Each two-color PBG as predicted in the coherent atomic lattices may be explored to attain robust light entanglement, even for single-probe photons, via an efficient frequency conversion during the nonlinearly correlated reflection.

Structure-induced negatively skewed X-ray diffraction pattern of carbon onions

The idea is discussed that a gradual change of intershell separation in carbon onions may introduce a negative skewness into the most intense X-ray diffraction band. As an example, the experimental X-ray diffraction pattern of carbon onions is analysed using a weighted Lorentzian X-ray profile broadening model for each intershell separation contributing to the profile. The dependence of the mean radius for carbon onion shells on the intershell spacing between the adjacent shells is derived from the model analysis. Comparison with the internal distribution of atomic density in carbon onions measured with high resolution transmission electron microscopy is discussed.

Matter-wave interference versus spontaneous pattern formation in spinor Bose-Einstein condensates

We describe effects of matter-wave interference of spinor states in the ${}^{87}$Rb Bose-Einstein condensate. The components of the $F=2$ manifold are populated by forced Majorana transitions and then fall freely due to gravity in an applied magnetic field. Weak inhomogeneities of the magnetic field, present in the experiment, impose relative velocities onto different ${m}_{F}$ components, which show up as interference patterns upon measurement of atomic density distributions with a Stern-Gerlach imaging method. We show that interference effects may appear in experiments even if gradients of the magnetic field components are eliminated but higher-order inhomogeneity is present and the duration of the interaction is long enough. In particular, we show that the resulting matter-wave interference patterns can mimic spontaneous pattern formation in the quantum gas.

Asymmetric superradiant scattering and abnormal mode amplification induced by atomic density distortion

The superradiant Rayleigh scattering using a pump laser incident along the short axis of a Bose-Einstein condensate with a density distortion is studied, where the distortion is formed by shocking the condensate utilizing the residual magnetic force after the switching-off of the trapping potential. We find that very small variation of the atomic density distribution would induce remarkable asymmetrically populated scattering modes by the matter-wave superradiance with long time pulse. The optical field in the diluter region of the atomic cloud is more greatly amplified, which is not an ordinary mode amplification with the previous cognition. Our numerical simulations with the density envelop distortion are consistent with the experimental results. This supplies a useful method to reflect the geometric symmetries of the atomic density profile by the superradiance scattering.

Kinetic Monte Carlo and density functional study of hydrogen diffusion in magnesium hydride MgH2

In this work, we propose a model to investigate the hydrogen storage ab/desorption kinetic properties for the pure MgH2 using Kinetic Monte Carlo (KMC) simulations. To do such computations, the activation energies for different elementary processes have been estimated. Moreover, the different thermodynamical quantities of interest are determined by performing the ab-initio calculations. Then, we discuss the hydrogen diffusion in magnesium hydride (such as adsorption, diffusion and desorption). More precisely, we study the effect of each elementary mechanism on the diffusion that we characterize through the density distribution of hydrogen atoms including filling ratios, diffusion time, temperature and pressure. Among others, we show that all elementary mechanisms are needed to reproduce a behavior of ab/desorption which correlates well with the experimental results reported in literature. In particular, at high temperature and pressure, the results of simulations indicate that the studied material involves slow kinetics.

Number-resolved imaging of excited-state atoms using a scanning autoionization microscope

We report on a scanning microscopy technique for atom-number-resolved imaging of excited-state atoms. A tightly focused laser beam leads to local autoionization, and the resulting ions are counted electronically. Scanning the beam across the cloud builds up an image of the density distribution of excited atoms, with access to the full counting statistics at each spatial sampling point and an overall detection efficiency of 21 %. We apply this technique to the measurement of a spatially inhomogeneous electric field with a spatial resolution of 50 ?m and a sensitivity to electric field gradients of 0.04 V cm$^{-2}$.

Effect of the solute–solvent interface on small-angle neutron scattering from organic solutions of short alkyl chain molecules as revealed by molecular dynamics simulation

The problem of describing the experimental small-angle neutron scattering (SANS) from diluted solutions of saturated monocarboxylic acids with short chain lengths (myristic and stearic acids) in deuterated decalin is considered. The method of classical molecular dynamics simulation (MDS) is used to obtain the atomic number density distributions, and, as a consequence, the scattering length density (SLD) distribution in the solute–solvent interface area (about 1 nm around the acid molecules), assuming the acid molecules to be rigid and non-associated in the solutions. MDS is performed for solutions in a parallelepiped cell of 5.5 × 5.3 × 5.3 nm (one acid molecule per cell) under normal conditions. The time averaging of the obtained distributions is done over 2 ns (after the system thermalization). It is shown that a specific short-range ordering organization of the solvent molecules in the vicinity of the acid molecules has a significant effect on the scattering, which is mainly determined by a relatively large ratio between the effective size of the solvent molecule and the cross-section diameter of the acid molecule. Various approximations to the simulated SLD distributions, based on the cylinder-type symmetry of the acid molecules, are probed to achieve the best consistency with the experimental SANS curves by varying the residual incoherent background.

A molecular dynamics simulation of boundary lubrication: The effect of n-alkanes chain length and normal load

A molecular dynamics simulation of confined n-alkanes has been conducted to investigate the effects of the chain length of lubricant molecule and normal load on friction and asperity contact. The density distribution and surface coverage of lubricant atoms were calculated. The simulation results indicate that longer chain n-alkanes can provide more atoms for the mono-layer in the asperity contact interface than shorter ones, which, as a result, significantly reduce the friction force when the asperity contact occurs. C64-alkanes molecules can form a mono-layer of lubricant to avoid direct asperity contact at pressures up to 1000MPa.

Probing the interface of doped isotopically mixed helium droplets by the directional anisotropy of interatomic Coulombic decay

Interatomic Coulombic decay (ICD) represents an efficient electronic relaxation mechanism of an ionized or an excited system embedded in an environment. The type of this environment and its size have a great impact on the ICD performance. It is stressed that ICD is sensitive to the arrangement of neighboring atoms when the initially created vacancy has a polarization direction. This is demonstrated in the present paper for the case of a 3p-ionized Ca surrounded by He atoms. Useful explicit expressions are derived for the ICD widths which show that the neighbors located along the polarization direction of the ionized orbital have the largest contribution to the ICD rate. By comparison with ab initio results for small clusters, we also show that in a helium environment, the pairwise approximation represents a reliable approach for computing ICD widths. Using this approximation and the density distribution of the helium atoms obtained within density functional theory, we explore ICD in large isotopically mixed helium droplets doped with Ca. A special emphasis is given to the difference between the ICD widths for the Ca3p orbitals directed perpendicular and parallel to the droplet surface. Depending on the size and isotopic composition of the droplet, Ca resides in the interfacial layer between the (4)He core and the (3)He outer shell. Hence, ICD studies in these droplets may provide valuable information on the properties of this interface.

Mesoscopic properties of interfacial ordering in amorphous germanium on Si(111) determined by quantitative digital image series matching

For the quantitative characterization of atomic ordering in the transition region between crystalline and amorphous materials we have previously described a method based on averaging HREM images along the interface, simulation of averaged images with the use of the averaged projected potential approximation and determination of the atom arrangement by means of an iterative matching procedure for high-resolution focus series. In order to study mesoscopic properties of crystal induced ordering a fully quantitative procedure is developed in this work. For this purpose, the width of the averaging region is defined as a compromise providing necessary accuracy of calculations and desirable locality of characterization of the atom distribution. Fluctuations of the obtained atom distribution on the amorphous side of the interface are estimated by means a of special Monte-Carlo simulation technique. As a result, distribution functions obtained from different regions can be quantitatively compared and statistically significant differences can be identified and related to the atomic structure. The method is applied to investigate the near interfacial atom order at the interface between atomically flat crystalline Si(111) and amorphous Ge. It is shown that significant variations in the atomic density distribution occur on a 5-10nm scale for germanium atoms in the second and third atomic layer lying parallel to the interface.