Atomic Density Distribution Research Articles

Spin-orbit coupling and electronic properties in Pt2MnGa: an ab-initio study

The electronic and magnetic properties of full-Heusler alloy Pt2MnGa with and without the effect of spin–orbit coupling are studied. The calculations have been carried out using ab initio density functional theory. Both the magnetic spin orders of Pt2MnGa, ferromagnetic and antiferromagnetic, are considered. It is found that the ferromagnetic spin arrangement is the most stable spin order at the ground state, regardless of the incorporation of spin–orbit coupling. The density of states and band structure plots are used to validate the obtained ground state structure, which is further validated by the Bader charge analysis and the charge density distribution of the individual atoms. The obtained results of magnetocrystalline anisotropy energy, total magnetic moment, Curie temperature, and the spin-polarization hint at the possible application of this compound in spintronics devices such as bit-patterned media.

Features of Helium-Vacancy Complex Formation at the Zr/Nb Interface.

A first-principles study of the atomic structure and electron density distribution at the Zr/Nb interface under the influence of helium impurities and helium-vacancy complexes was performed using the optimised Vanderbilt pseudopotential method. For the determination of the preferred positions of the helium atom, the vacancy and the helium-vacancy complex at the interface, the formation energy of the Zr-Nb-He system has been calculated. The preferred positions of the helium atoms are in the first two atomic layers of Zr at the interface, where helium-vacancy complexes form. This leads to a noticeable increase in the size of the reduced electron density areas induced by vacancies in the first Zr layers at the interface. The formation of the helium-vacancy complex reduces the size of the reduced electron density areas in the third Zr and Nb layers as well as in the Zr and Nb bulk. Vacancies in the first niobium layer near the interface attract the nearest zirconium atoms and partially replenish the electron density. This may indicate a possible self-healing of this type of defect.

Defect induced nonequilibrium quantum dynamics in an interacting Bose–Hubbard flux ladder

The interacting Bose–Hubbard flux ladder provides an ideal model to probe novel quantum phenomena of many-body systems. Here, we report on the first direct observation of dynamical quantum phase transition (DQPT) in interacting Bose–Hubbard flux ladder induced by defect perturbation, which provides a new scheme for experimental design and manipulation of the DQPT in ultracold atomic system. Under the mean-field approximation, DQPT is identified by resolving the order parameter and the temporal evolution of patterns of atomic density distributions and local current configurations of the system. The threshold for occurrence of DQPT is obtained analytical and the physical mechanism of DQPT is revealed explicitly. Periodic appearance and annihilation of dynamical vortex and the manifestation of symmetry restoration after perturbation from broken-symmetry phase are observed. A thorough connection among the order parameter dynamics, the underlying ground state phase transition and nonequilibrium dynamics is established in real time and real space for the first time. Interestingly, by quenching the defect, the underlying ground state phases are captured, which provides a feasible dynamical measurement scheme for the observation of the underlying ground state phase which is challenging to reach experimentally.

Investigation on spatial distribution of atomic density in a supersonic planar symmetric gas jet by simulations

As a special gas target with large aspect ratio, the planar symmetric gas jet has attracted some interests in High Harmonic Generation (HHG). It can be produced by gas adiabatic expansion through a supersonic slit nozzle into vacuum. In this work, the spatial distributions of atomic density are investigated in detail along the short side, the long side and the gas jet flow of the planar symmetric argon gas jet based on ANSYS Fluent. The results indicate that along the long side, the planar symmetric jet provides the advantages of high atomic density and the large dimension of uniform atomic density distribution. The density distribution profile is similar to the Gaussian one along the short side. The atomic density at the center of the jet outside the nozzle is lower and decreases much faster than that expected by idealized straight streamline model due to the lateral expansion of gas jet at slit nozzle width.

In-situ synthesis of N/S co-doped Cu-based graphene-like nanosheets as high efficiency electrocatalysts for oxygen reduction reaction

Heteroatom-doped transition metal electrocatalysts supported on carbon materials are widely-recognized as the promising catalysts for oxygen reduction reaction (ORR). Here, we describe the one-pot method of preparing the Cu-based PAT1-Cux/KB catalyst with Cu/N/S doped graphene-like nanosheets. For the obtained catalyst, Cu2+ ions catalyzed the polymerized 2-aminothiazole organic monomers into nanosheet and simultaneously in-situ doped Cu, N, S atoms into the carbon matrix. The amount of Cu2+ ions affected the geometry structure, as well as the formation of Cu–N structure and C–S–C bond of PAT1-Cux/KB catalyst. Cu2S crystals with ORR catalytic ability were also generated in the catalysts. The Cu–N structure in the catalyst regulates the d-electron density of inert copper through the synergistic effect of electronic connection between copper and nitrogen atoms. N and S doped in the carbon skeleton changes the spin density and charge distribution of near carbon atoms. Therefore, PAT1-Cu0.33/KB catalyst with graphene-like nanosheets, Cu–N structure, C–S–C bond, and Cu2S crystals exhibits excellent activities with E1/2 = 0.84 V and high limit-current density of 5.25 mA cm−2 for ORR.

Molecular Simulation-Guided Spectroscopy of Imidazolium-Based Ionic Liquids and Effects of Methylation on Ion-Cage and -Pair Dynamics.

Classical molecular dynamics simulations were performed to assess an atomistic interpretation of the ion-probe structural interactions in two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm][NTf2] and 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BDimIm][NTf2] through computational ultrafast spectroscopy. The nitrile stretching vibrations of the thiocyanate anion, [SCN]-, serve as the local mode of the ultrafast system dynamics within the imidazolium-based ionic liquid environment. The wavelet transform of classical trajectories determines the time-varying fluctuating frequencies and the stretch spectral signatures of SCN- in the normalized distribution. However, computational modeling of the two-dimensional (2D) spectra from the wavelet-derived vibrational frequencies yields time evolution of the local molecular structure along with the varied time-dependent dynamics of the spectral diffusion process. We calculated the frequency-frequency correlation functions (FFCFs), time correlations associated with the ion-pair and -cage dynamics, and mean square displacements as a function of time, depicting diffusive dynamics. The calculated results based on the pair correlation functions and the distribution of atomic density suggest that the hydrogen and methylated carbon at the two-position of the imidazolium ring of [BMIm] and [BDimIm] cations, respectively, strongly interact with the probe through the N of the thiocyanate anion rather than the S atom. The center-of-mass center-of-mass (COM-COM) cation-probe radial distribution functions (RDFs) in conjunction with the site-specific structural analysis further reveal well-structured interactions of the thiocyanate ion and [BMIm]+ cation rather than the [BDimIm] cation. In contrast, the anion-probe COM-COM RDFs depict weak interactive associations within the vibrational probe [SCN]- and [NTf2]- ions. Methylation at the two-position of the imidazolium ring predicts slower structural reorganization and breaking and reformation dynamics of the ion pairs and cages within the ionic liquid framework.

Exploring crystal structure by three-dimensional atomic density distribution from molecular dynamics simulations

Predicting crystal structure has been one of the core issues in materials research. X-ray diffraction data can generally be used to fit and refine the positions of atoms within the unit cell. However, for a system with part of its atoms diffusing or hopping below the melting temperature, the atoms inside the crystal begin to vibrate, diffuse, or hop when its temperature gradually rises. Even using the partial-occupancy concept, it becomes difficult or impossible to fit the structure due to the trial-and-error characteristic of refinement. More importantly, the limited partial-occupancy positions are insufficient and inappropriate to describe material systems containing diffusing or hopping atoms. To address this shortcoming, we proposed to perform molecular dynamics simulations and then add up all the positions of diffusing atoms in each time step to form a three-dimensional atomic density distribution into the primitive or conventional unit cell. The maximum atomic density points correspond to the partial-occupancy positions in x-ray diffraction data. The connected atomic density distribution identifies and defines the diffusing paths for mobile atoms within the crystal.

Gas–surface interactions in a large-scale inductively coupled plasma wind tunnel investigated by emission/absorption spectroscopy

Precise prediction of aerothermal loads is significantly limited by the unclear interactions between the thermal protection system surface and the surrounding high-enthalpy gas. To address this, we propose an optical diagnostic method based on optical emission spectroscopy and laser absorption spectroscopy to investigate the gas–surface interactions within the boundary layer. Experiments are conducted in an air plasma flow produced by the 1.2 MW inductively coupled plasma wind tunnel at the China Academy of Aerospace Aerodynamics with an enthalpy of 20 MJ/kg and a heating time of 100 s. The cylindrical samples made of pure silicon carbide are tested, and quartz samples with the same exposed geometry are tested in parallel as a reference material. The optical emission spectroscopy system has four spectrometers to cover the wide wavelength range of 200–1100 nm, providing qualitative, spatially, and spectrally resolved measurements of the multi-species radiative emission adjacent to the sample surface. Laser absorption spectroscopy is deployed at different axial locations to quantify the number density and translational temperature of OI (3s5S) with a 500 Hz scanning rate and 200 kHz acquisition rate. Additionally, the surface temperature of each sample is detected by an infrared pyrometer. Scanning electron microscopy and energy dispersive spectrometry are performed before and after the plasma heating. Our measurement results provide valuable information on surface reaction pathways and catalytic recombination effects on atomic oxygen number density distributions. Finally, these self-consistent results show that the proposed method is reliable to deeply investigate gas–surface interactions within boundary layer in harsh aerothermal environment.

Imaging Moving Atoms by Holographically Reconstructing the Dragged Slow Light

The propagation of light in moving media is dragged by atomic motion. The light-drag effect can be dramatically enhanced by reducing the group velocity with electromagnetically induced transparency (EIT). We develop a systematic procedure to accurately reconstruct the complex wavefront of the slow light with single-shot measurements, enabling precise, photon shot-noise-limited spectroscopic measurements of atomic response across EIT even in the presence of generic atomic number fluctuations. Applying the technique to an expanding cloud of cold atoms, we demonstrate simultaneous inference of the atomic density distribution and the velocity field from the complex imaging data. This inline imaging technique may assist a wide range of cold-atom experiments to access spectroscopic and phase-space information with in situ and minimally destructive measurements.

Measurement of dynamic atomic polarizabilities of Al at 19 wavelengths from 420 nm to 680 nm in electrical exploding wire experiments.

The dynamic polarizabilities of Al atom at 19 wavelengths from 420 nm to 680 nm are measured experimentally for the first time. A 15 µm diameter Al wire is heated to the gas/ microdrop stage using a 3 kA, 25 ns pulsed current, and the energy deposition when the resistive voltage reaches its peak is 4.7 eV·atom-1, which is higher than the atomization enthalpy of Al but much lower than its first ionization potential. Two laser interferometric images of the Al gas are obtained simultaneously using a 532 nm laser and an optical parametric oscillator laser. Then an integrated phase method is used to reconstruct the Al atomic linear density distribution based on the known atomic polarizability at 532 nm, which acts as a bridge between the two interferograms to obtain the relative atomic polarizability value at a specific wavelength with respect to the value at 532 nm. The measured dynamic polarizability of Al atom decreases from 13.5×10-24 cm3 to 9.4×10-24 cm3 as the wavelength increases from 420 nm to 680 nm with a measurement error of approximately ±10%. The experimental result fits well with an uncoupled Hartree-Fock approximation, and the reconstructed static polarizability of 8.13 ± 0.79×10-24 cm3 is well matched with reference measurement results.

On the formulations of the electromagnetic stress–energy tensor

In this work, we review the main existing theories for the electromagnetic stress–energy tensor in dielectric media — a problem directly related to the century-long debate regarding the correct momentum of light in dielectric media, known as Abraham–Minkowski controversy. The theoretical validity of the main existing formulations is analyzed in terms of symmetry and invariance requirements for the tensors. We also present position- and time-dependent numerical simulations of the related electromagnetic forces for a lossless nondispersive medium, highlighting the different expected spatial force distributions. The force densities of different formulations drive forward different momentum and atomic number density distributions of the medium contributing to the total momentum and energy densities associated with the coupled field-medium state of light.

Experimental, DFT study, and in silico molecular docking investigations of dichlorodiphenyltrichloroethane against human estrogen receptor alpha

Advanced computational tools allowed to study a pure commercial sample of dichlorodiphenyltrichloroethane (DDT) prepared in liquid phase in KBr pellets and characterized using FT-IR and GC-MS followed by the application of DDT for molecular docking against human estrogen receptor alpha. The compound was modelled using GaussView software. Using Veda 04 program, the theoretical vibrational energy distributions and experimental vibrational frequencies were compared. Interestingly, C1 and C2�possess the highest atomic charge density distribution (ACDD)�of -0.284e and -0.283e while C21 and C11 have lowest ACDD of -0.064e and -0.063e in a relative manner, since the deactivating power of chlorine atoms decreases charge densities of the bonded carbon. The highest intramolecular interacting perturbation energy is 1121.92 kJ mol�1 occurs between ?*C19�C21 donor orbital and ?*C14�C16 acceptor orbital while the least intramolecular interaction occurs in the lone pair of LPC26 and the�sigma nonbonding (??C1�Cl24) NBO orbitals with E(2) of 32.21�kJ mol�1. Steric interaction was the only interaction found within the complex after the docking.

Experimental, DFT study, and in silico molecular docking investigations of dichlorodiphenyltrichloroethane against human estrogen receptor alpha

Advanced computational tools allowed to study a pure commercial sample of dichlorodiphenyltrichloroethane (DDT) prepared in liquid phase in KBr pellets and characterized using FT-IR and GC-MS followed by the application of DDT for molecular docking against human estrogen receptor alpha. The compound was modelled using GaussView software. Using Veda 04 program, the theoretical vibrational energy distributions and experimental vibrational frequencies were compared. Interestingly, C1 and C2�possess the highest atomic charge density distribution (ACDD)�of -0.284e and -0.283e while C21 and C11 have lowest ACDD of -0.064e and -0.063e in a relative manner, since the deactivating power of chlorine atoms decreases charge densities of the bonded carbon. The highest intramolecular interacting perturbation energy is 1121.92 kJ mol�1 occurs between ?*C19�C21 donor orbital and ?*C14�C16 acceptor orbital while the least intramolecular interaction occurs in the lone pair of LPC26 and the�sigma nonbonding (??C1�Cl24) NBO orbitals with E(2) of 32.21�kJ mol�1. Steric interaction was the only interaction found within the complex after the docking.

Electrically enhanced activity of cationic surfactant for the bubble surface modification of solvent sublation to remove acetaminophen from water

In this article, the molecular dynamics (MD) calculation was carried out to investigate the influence of electric field (EF) on the adsorption of cationic surfactant Dodecyl Trimethyl Ammonium Chloride (DTAC) at the phase interface of bubble and the interaction between acetaminophen (APAP) and DTAC. The main components’ structural parameters, interaction energy, PMF, surface tension of gas–liquid interface and density distribution of marker atoms in DTAC molecules were analyzed. The results show that EF can weaken the restriction of water molecules on DTAC and acetaminophen, increase the van der Waals attraction between DTAC and APAP, reduce the surface tension of phase interface, thereby significantly improving the adsorption of DTAC at the phase interface, and further enhancing the adsorption between APAP and DTAC. In addition, the distribution of DTAC adsorbed on the bubble surface may be controlled through adjusting the direction of EF, which makes it possible to counteract the uneven distribution of adsorbate on the bubble surface caused by liquid velocity and further control the bubble dynamic characteristics in the adsorption process. Our study results are expected to propose a simple and low-cost method for the removal of APAP in water by solvent sublation.

Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics.

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Effects of the Layer Charge Location and Interlayer Cation on Rectorite Swelling Properties: Comparison between Molecular Dynamics Simulations and Experiments

Understanding the swelling properties of clay minerals is of great significance to environmental science and engineering applications, including environmental remediation, oil and gas wellbore collapse, geological disasters, and commercial additives. The vast majority of experimental and molecular simulation studies have mainly focused on pure clays. However, 70% of clays are mixed-layer clays (MLCs), and the most frequent MLCs is illite-montmorillonite (I-MMT). In this research, grand canonical Monte Carlo and molecular dynamics simulations were used to study the effects of the layer charge location (tetrahedral and octahedral substitution), interlayer cation type (Na+, Cs+, and Ca2+), and water contents on the swelling and hydration properties of rectorite (R1 type I-MMT) and compared with the experimentally measured basal spacing by water vapor adsorption and X-ray diffraction. It turned out that the free swelling ratio was positively correlated with water contents. The order of the three cations’ free expansion rate was Na+, Ca2+, and Cs+. The swelling process of rectorite (REC) was revealed from porosity changing with water contents. The surface of tetrahedral substitution (T-substitution) clay was more likely to bind to interlayer cations to form an inner-sphere surface complex (ISSC), the number of fully hydrated cations of the T-substitution clay was less than that of octahedral substitution (O-substitution), and REC was more swelling in O-substitution, which was supported by the basal spacing, atomic density distribution, and radial distribution function. The cation radius, the hydration energy, and the preference of the inner/outer-sphere surface complex (ISSC/OSSC) all affected REC swelling. Because of the presence of asymmetric charges in the REC layer, interlayer cations tended to distribute on the surface of illite and formed an ISSC when they formed 1-, 2-, and 3-layer (1, 2, and 3 W) hydrate states. When the layer charge density of illite and Mmt was constant, only the layer charge position in Mmt was changed, and the REC swelling behavior was transformed into pure Mmt. The interlayer cations and the layer charge location significantly impacted the REC swelling.

Atomistic Insight into the Hydration States of Layered Double Hydroxides.

Effective protective coatings are an essential component of lightweight engineering materials in a large variety of applications as they ensure structural integrity of the base material throughout its whole service life. Layered double hydroxides (LDHs) loaded with corrosion inhibitors depict a promising approach to realize an active corrosion protection for aluminum and magnesium. In this work, we employed a combination of density functional theory and molecular dynamics simulations to gain a deeper understanding of the influence of intercalated water content on the structure, the stability, and the anion-exchange capacity of four different LDH systems containing either nitrate, carbonate, or oxalate as potential corrosion inhibiting agents or chloride as a corrosion initiator. To quantify the structural change, we studied the atom density distribution, radial distribution function, and orientation of the intercalated anions. Additionally, we determined the stability of the LDH systems by calculating their respective hydration energies, hydrogen-bonded network connected to the intercalated water molecules, as well as the self-diffusion coefficients of the intercalated anions to provide an estimate for the probability of their release after intercalation. The obtained computational results suggest that the hydration state of LDHs has a significant effect on their key properties like interlayer spacing and self-diffusion coefficients of the intercalated anions. Furthermore, we conclude from our simulation results that a high self-diffusion coefficient which is linked to the mobility of the intercalated anions is vital for its release via an anion-exchange mechanism and to subsequently mitigate corrosion reactions. Furthermore, the presented theoretical study provides a robust force field for the computer-assisted design of further LDH-based active anticorrosion coatings.

Compact multichannel imaging camera for wide-field imaging of diffused sources

We describe a multichannel camera operating in the visible to near-infrared wavelengths (450 to 900 nm) with an etendue of 0.08 cm2 sr that can be used to image extended sources across multiple optical bands, capable of 3σ detection of 20 Rayleigh signal in 120 s. It has an angular resolution of 0.1 deg and a 35 deg × 25 deg field of view, employing a charge-coupled device detector in a compact 1U CubeSat compatible form factor (10 × 10 × 8 cm3). The design uses commercial off-the-shelf components while offering versatility using a mosaic of bandpass filters to select different spectral channels to address a wide range of remote sensing needs. The specific implementation described here covers the application of the instrument to explore the neutral sodium and potassium atom density distribution in the atmospheres of the Earth or the Moon.

A Highly Efficient Sulfur Host Enabled by Nitrogen/Oxygen Dual‐Doped Honeycomb‐Like Carbon for Advanced Lithium–Sulfur Batteries

High energy density and long cycle life of lithium-sulfur (Li-S) batteries suffer from the shuttle/expansion effect. Sufficient sulfur storage space, local fixation of polysulfides, and outstanding electrical conductivity are crucial for a robust cathode host. Herein, a modified template method is proposed to synthesize a highly regular and uniform nitrogen/oxygen dual-doped honeycomb-like carbon as sulfur host (N/O-HC-S). The unique structure not only offers physical entrapment for polysulfides (LiPSs) but also provides chemical adsorption and catalytic conversion sites of polysulfides. In addition, this structure offers enough space for loading sulfur, and a regular space of nanometer size can effectively prevent sulfur particles from accumulating. As expected, the as-prepared N/O-HC900-S with high areal sulfur loading (7.4mg cm-2 ) shows a high areal specific capacity of 7.35 mAh cm-2 at 0.2 C. Theoretical calculations also reveal that the strong chemical immobilization and catalytic conversion of LiPSs attributed to the spin density and charge distribution of carbon atoms will be influenced by the neighbor nitrogen/oxygen dopants. This structure that provides cooperative chemical adsorption, high lithium ions flux, and catalytic conversion for LiPSs can offer a new strategy for constructing a polysulfide confinement structure to achieve robust Li-S batteries.

Targeted Positioning of Quantum Dots Inside 3D Silicon Photonic Crystals Revealed by Synchrotron X-ray Fluorescence Tomography.

It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize innovative functions. Once the 3D positioning is performed, the challenge arises how to nondestructively verify where the nanoparticles reside in the 3D nanostructure. Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots. The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si–air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, and Pb. The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator. We conclude that X-ray fluorescence tomography is very well suited to nondestructively characterize 3D nanomaterials with photonic and other functionalities.