Internal Atomic Structure Research Articles

Theoretical Study on BiCoO3: Pressure‐Tuned Transformation of Structure, Magnetism, Charge, and Spin State

AbstractThe atomic structures, electronic structures, magnetic properties, and spin states of BiCoO3 multiferroic material under hydrostatic pressure are comprehensively investigated by using first‐principles calculation based on density functional theory. Firstly, we found the pressure‐tuned structural symmetry transition from tetragonal P4 mm (No. 99) to cubic Pmm (No. 221) space group. The structural transformation prompts the Co─O polyhedral coordination transfer from a CoO5 pyramidal to a CoO6 distorted octahedron. Secondly, the pressure‐induced magnetic change from C‐type antiferromagnetic state to ferromagnetic state and then to paramagnetic configuration occurred after continuous compression of the volume. Essentially, the fundamental reason for the transition of the magnetic structure lies in the change of the internal atomic structure, such as the pressure induced abrupt changes of Co─O bond length and <Co─O─Co> bond angle. Meanwhile, an obvious volume collapse was observed when the pressure was applied to 51–56 GPa. Finally, as the pressure increases, the magnetism changes from C‐type antiferromagnetic state to paramagnetic configuration, accompanying the transformation from high spin state with b2g2eg2a1g1b1g1 configuration in pyramid structure to low spin state with t2g6eg0 in octahedral field. Therefore, our results demonstrate that applied hydrostatic pressure in BiCoO3 can trigger the transformation of structure, magnetism, charge, and spin state.

Influence of crystallinity on micro-nano structure response of Zr-based alloys treated by nanosecond laser

Understanding the processes and mechanisms of laser-matter interaction is fundamental to the treatment of materials by using laser-based methods, but it is quite challenging because the response of matter to laser irradiation is influenced by many factors, especially the intrinsic properties of the matter. In this study, the influence of crystallinity on micro-nano structure response of Zr-based alloys (Zr41.2Ti13.8Cu12.5Ni10Be22.5) treated by nanosecond laser is investigated. The microscopic topographies and geometrical dimensions of the laser-induced micro-nano structures formed on the surface of Zr-based alloys with different crystallinity have been explored and compared. The experimental results indicate that the microstructural transformation between micro-concave and micro-convex is observed for both amorphous and crystallized Zr-based alloys, i.e., independent of the arrangement of the internal atomic structure. The geometrical dimensions of the laser-induced micro-nano structures and the critical peak laser power intensity required for the microstructural transformation are different, which can be attributed to the difference in thermal conductivity. Inspired by laser-induced micro-convex, a point-by-point laser patterning technique has been proposed for highly efficient and flexible preparation of micro-nano patterns. Importantly, this patterning technique exhibits ultra-high processing efficiency with a material accumulation rate of more than 1012 μm3/s. This work paves an avenue for understanding the laser-matter interaction and is expected to facilitate the advanced functional applications of Zr-based alloys in many fields.

Analysis method of point defects evolution in fused silica under multi-pulse nanosecond laser irradiation.

Although no optically visible damage is produced in the fused silica under laser irradiation below its laser-induced damage threshold (LIDT), defect proliferation may occur due to the evolution of its internal atomic structure. The escalation in defect content leads to heightened absorption, and resulting in the degradation of the optical performance of the optics. In recent decades, there have been a lot of experimental studies on laser-induced damage and laser conditioning, but there is still a great lack of in-depth understanding and theoretical analysis of the evolution process of point defects in fused silica. In this study, the emphasis is on the evolution of point defects and fatigue damage in fused silica under multi-pulse nanosecond laser irradiation. To address this, a point-defect evolution model is developed, and the coupled evolution law of temperature and defect during laser irradiation is derived by integrating it with a numerical model. The results demonstrate that the model effectively predicts the defect evolution of fused silica under laser irradiation and facilitates the prediction of fatigue damage. It is revealed that the rate of defect evolution in fused silica is more influenced by temperature than stress, and a temperature threshold can be used to judge the condition of damage occurrence. Furthermore, through an analysis of the effect of laser fluence on defect relaxation rate, a defect relaxation method employing variable laser fluence was proposed. This study provides a reliable theoretical analysis method for understanding the fatigue damage induced by multi-pulse laser irradiation in fused silica and offers a new perspective for the annealing treatment of point defects in fused silica.

Molecular dynamics analysis of the effect of temperature on the internal structure transformation of polycrystalline 3 C-SiC nanoindentation

To investigate the structure changes of polycrystalline 3 C-SiC nanoindentation process and the influence of temperature effect on its changes, the nanoindentation experiments of polycrystalline 3 C-SiC are constructed by molecular dynamics method. The initial nanoindentation model of polycrystalline 3 C-SiC and diamond indenter is constructed based on the characteristics of Voronoi method in constructing close-packed spatial lattice. The temperature conditions of 300 K, 500 K, 800 K and 1000 K are selected to analyze the dislocation structure, number and length, internal atomic structure, grain boundary structure and deformation degree of polycrystalline 3 C-SiC during nano-indentation process. The stress and deformation of diamond indenter are analyzed. The results show that the higher the temperature, the higher the degree of dislocation stacking. Temperature has an effect on the internal variation of polycrystalline silicon carbide, particularly on the transverse size of the structure. The central force of the diamond becomes weaker and the force on either side becomes stronger. The temperature effect can change the stacking degree of dislocations, reduce the internal structure transformation rate of polycrystalline 3 C-SiC, and transform fewer discrete atoms and increase the number of cubic lattice atoms.

A numerical code for the analysis of magnetic white-dwarf spectra that includes field effects on the chemical equilibrium

We present a new magnetic-atmosphere model code for obtaining synthetic spectral fluxes of hydrogen-rich magnetic white dwarfs. To date, observed spectra have been analyzed with models that neglect the magnetic field's effects on the atomic populations. In this work, we incorporate state-of-art theory in the evaluation of numerical densities of atoms, free electrons, and ions in local thermodynamical equilibrium under the action of a magnetic field. The energy distribution of atoms is rigorously evaluated for arbitrary field strength. This energy pattern includes going from tightly bound states to metastable or truly bound, highly excited states embedded in the continuum, that is, over the first Landau level. Finite nuclear mass effects and the coupling between the internal atomic structure and the motion of the atom across the magnetic field are also considered. Synthetic fluxes are generated with integrations of numerical solutions of polarized radiative transfer over the visible stellar disk using a spherical $t$-design method. The atmosphere code is tested with observations from the Sloan Digital Sky Survey for a group of known magnetic white dwarfs. Physical stellar parameters are obtained from least-squares fits to the observed energy distribution and compared with results of previous works. We show that the use of zerofield ionization equilibrium in spectral analyses can lead to underestimated effective temperatures for highly magnetic white dwarfs.

Clock transitions versus Bragg diffraction in atom-interferometric dark-matter detection

Atom interferometers with long baselines are envisioned to complement the ongoing search for dark matter. They rely on atomic manipulation based on internal (clock) transitions or state-preserving atomic diffraction. Principally, dark matter can act on the internal as well as the external degrees of freedom to both of which atom interferometers are susceptible. We, therefore, study in this contribution the effects of dark matter on the internal atomic structure and the atom's motion. In particular, we show that the atomic transition frequency depends on the mean coupling and the differential coupling of the involved states to dark matter, scaling with the unperturbed atomic transition frequency and the Compton frequency, respectively. The differential coupling is only of relevance when internal states change, which makes detectors, e.g., based on single-photon transitions sensitive to both coupling parameters. For sensors generated by state-preserving diffraction mechanisms like Bragg diffraction, the mean coupling modifies only the motion of the atom as the dominant contribution. Finally, we compare both effects observed in terrestrial dark-matter detectors.

Effect of surface on internal structure and apparent lattice parameter of nanocrystalline SiC grains modelled by molecular dynamics simulations

SiC nanograins terminated by different types of low index surfaces (100), (110) and (111) with a single or double Si/C atomic planes were simulated by molecular dynamics and their internal atomic structure was examined. Distribution of local strains in the grains volume was traced by following changes of interatomic distances defining nearest-neighbor coordination in SiC lattice. Influence of the grain size, the grain shape and the atomic composition of the grain surface on the apparent lattice parameter and on the internal structure of the grains was discussed.

Effective two-level approximation of a multilevel system driven by coherent and incoherent fields

The numerical simulation of multiple scattering in dense ensembles is the mostly adopted solution to predict their complex optical response. While the scalar- and vectorial-light-mediated interactions are accurately taken into account, the computational complexity still limits current simulations to the low saturation regime and ignores the internal structure of atoms. Here, we propose to go beyond these restrictions, at constant computational cost, by describing a multilevel system by an effective two-level system that best reproduces the coherent and total scattering properties in any saturation regime. The correspondence of our model is evaluated for different experimentally realistic conditions such as the modification of the driving field polarization, the presence of stray magnetic fields, or an incoherent resonant electromagnetic field background. The trust interval of the model is quantified for the ${\mathrm{D}}_{2}$ line of $^{87}\mathrm{Rb}$ atoms, but it could be generalized to any closed transition of a multilevel quantum system.

Preparation of Visible-Light Active Oxygen-Rich TiO2 Coatings Using Low Pressure Cold Spraying

Visible-light active photocatalysts in the form of coatings that can be produced using large-scale methods have attracted considerable attention. Here we show a facile approach to deposit coatings using the low pressure cold spray (LPCS) from oxygen-rich amorphous titanium dioxide, which is a structurally-unconventional feedstock powder for LPCS. We synthesized amorphous TiO2, in which we introduced numerous defects, such as oxide groups (peroxy and superoxy) in volume and hydroxyl groups on the surface. Then we deposited as-prepared powder preserving the presence of active groups, which we demonstrated using Raman spectroscopy. To show the activity of the prepared coatings, we perform methylene blue degradation under visible light. Our research shows that it is worth considering the internal atomic structure and surface chemistry of the powders to be preserved after low pressure cold spraying.

Atomic-Scale Visualization of Polar Domain Boundaries in Ferroelectric In2Se3 at the Monolayer Limit.

Domain boundaries in ferroelectric materials exhibit rich and diverse physical properties distinct from their parent materials and have been proposed for broad applications in nanoelectronics and quantum information technology. Due to their complexity and diversity, the internal atomic and electronic structure of domain boundaries that governs the electronic properties remains far from being elucidated. By using scanning tunneling microscopy and spectroscopy (STM/S) combined with density functional theory (DFT) calculations, we directly visualize the atomic structure of polar domain boundaries in two-dimensional (2D) ferroelectric β'-In2Se3 down to the monolayer limit. We observe a double-barrier energy potential with a width of about 3 nm across the 60° tail-to-tail domain boundaries in monolayer β'-In2Se3. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.

Pulse length effects on autoionizing states under the influence of intense SASE XUV fields

The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

Gold Nanorods: Light Tailoring of Internal Atomic Structure of Gold Nanorods (Small 22/2020)

Gold Nanorods: Light Tailoring of Internal Atomic Structure of Gold Nanorods (Small 22/2020)

Light Tailoring of Internal Atomic Structure of Gold Nanorods.

Laser processing of gold nanorods (Au NRs) relying on light-matter interaction provides great opportunities in various potential applications. Unveiling the light-induced structure change is a crucial goal in order to control the shape and related properties for practical application. However, the internal atomic structure control of metallic NRs has long been a challenge. Here, the concept of internal atomic structure tailored with light is demonstrated and Au NRs with various internal atomic structures including point defects, twin structures, and polycrystalline nanospheres are fabricated. Experimental characterization and theoretical simulation show that light-induced localized energy deposition and dynamic stresses distribution give rise to atomic structure change. Au NRs with internal defects show enhanced potential to improve activity. The concept of light tailoring of internal atomic structure represents a promising strategy for the rational design of metallic NRs to boost wide applications.

Hydrogen ionization equilibrium in magnetic fields

We assess the partition function and ionization degree of magnetized hydrogen atoms at thermodynamic equilibrium for a wide range of field intensities, B ≈ 105–1012 G. Evaluations include fitting formulae for an arbitrary number of binding energies, the coupling between the internal atomic structure and the center-of-mass motion across the magnetic field, and the formation of the so-called decentered states (bound states with the electron shifted from the Coulomb well). Non-ideal gas effects are treated within the occupational probability method. We also present general mathematical expressions for the bound state correspondence between the limits of zero-field and high-field. This let us evaluate the atomic partition function in a continuous way from the Zeeman perturbative regime to very strong fields. Results are shown for conditions found in atmospheres of magnetic white dwarf (MWD) stars, with temperatures T ≈ 5000–80 000 K and densities ρ ≈ 10−12–10−3 g cm3. Our evaluations show a marked reduction of the gas ionization due to the magnetic field in the atmospheres of strong MWDs. We also found that decentered states could be present in the atmospheres of currently known hot MWDs, giving a significant contribution to the partition function in the strongest magnetized atmospheres.

Internal atomic structure of terbium silicide nanowires on Si(001) capped by silicon

In a combined scanning tunneling microscopy and high-resolution transmission electron microscopy study, the internal atomic structure of terbium silicide nanowires on Si(001) capped with silicon is determined. Room temperature capping by amorphous silicon preserves the original nanowire structure, and the nanowires with a usual height of two silicide layers are found to consist of hexagonal TbSi2 with the c-axis in nanowire direction, in contrast to previous assumptions. At larger heights, the nanowires are formed from tetragonal TbSi2. Capping at elevated temperatures results in a shape transition towards higher and more compact nanowires consisting of hexagonal or tetragonal TbSi2 and a crystalline silicon overlayer that shows stacking faults and twin boundaries starting at the nanowires.

Mixture of two unequally charged superfluids in a magnetic field

The artificial magnetic fields engineered for ultracold gases depend on the internal structure of the neutral atoms. Therefore the components of a mixture composed of two atomic gases can exhibit a different response to an artificial magnetic field. Such a mixture can be interpreted as a mixture of two atomic gases, carrying different synthetic charges. We consider such mixtures of two superfluids with unequal synthetic charges in a ring trap subject to a uniform artificial magnetic field. The charge imbalance in such a mixture changes the distribution of excited particles over angular momentum states compared to that of an equally charged mixture. This microscopic difference exhibits macroscopic consequences, such as the occurrence of an angular momentum transfer between two unequally charged components. Due to the interfluid atomic interactions in a ring, the angular momentum transfer can create a counterflowing persistent current in the weakly charged superfluid. Even in the limiting case of a charged and an uncharged superfluid mixture, a persistent current can be induced in the uncharged superfluid, despite the fact that it is not directly coupled to the magnetic field. The stability analysis shows that the induction depends on the interplay between the interfluid interaction and the applied magnetic field. We obtain instability boundaries of the system and construct phase diagrams as a function of the interfluid interaction and the magnetic field. We investigate these properties employing the Bogoliubov approximation.

Теоретическое и эмпирическое обоснование планетарной модели атома водорода

The basis for the hydrogen atom planetary model, which represents the development of the Rutherford-Bohr atom model, is presented. The measurements of the atom polarizability, radius, electric and magnetic moments, as well as the Stark and Zeeman effects are given as an argument. The central paragraph of the Article is a comparison of the binding energy obtained based on wave measurement data (the Rydberg constant) and the binding energy obtained by calculation according to the data of the electron charge and mass fundamental constants and applying the principle of momentum conservation. The substantiation of the atom stability is given proceeding from the atom planetary model. The nature and mechanism of the optical spectrum formation is discussed. The substantiation of the fine structure of the hydrogen atom spectral lines is presented, proceeding from the fundamental association between the electric field strength of the electron charge and the speed of its velocity. The origin of the optical spectrum hyperfine structure is discussed within the planetary model. The difficulty in constructing an acceptable theory of the electronic structure of atoms is primarily due to the complexity of obtaining direct information on the atoms internal structure. The point is that the outermost electron shell of an atom behaves itself like an effective screen for external probing electrical fields. This circumstance sharply limits the possibilities of experimental methods for studying the intra-atomic structure. On the other hand, it is imperative that the research methods being employed be non-destructive ones in order to obtain reliable information concerning the structure of atoms. This means that the action on the atom during measurement process must meet the condition of the electron shell perturbation smallness of the atom or ion being investigated. Concerns associated with the inaccessibility of the electronic structure of atoms served as a pretext for a conclusion of the limited possibilities of the empirical method of knowing the internal structure of atoms at the proper time.

Refined structure model of rare earth silicide nanowires on Si(001)

The internal atomic structure of terbium silicide nanowires on Si(001) is determined by scanning tunneling microscopy and high-resolution transmission electron microscopy. It is found that room temperature capping by amorphous silicon does not affect the original nanowire structure. Atomically resolved transmission electron microscopy data supported by image simulations demonstrate that the nanowires consist of two silicide layers high hexagonal TbSi2 with the c-axis aligned in the nanowire direction, in contrast to previous assumptions. An analogous atomic structure is predicted for related nanowires of other trivalent rare earth elements because of their chemical similarity.

Construction of g-C3N4/ZIF-67 photocatalyst with enhanced photocatalytic CO2 reduction activity

The g-C3N4/ZIF-67 composite has been prepared by encapsulation of ZIF-67 with g-C3N4. The composition of the material and the internal atomic or molecular structure were characterized by XRD and FT-IR. The morphology of the samples was observed by TEM and SEM. In addition, the samples were characterized by UV–vis DRS, PL and N2 adsorption-desorption. Compared with bulk g-C3N4, ethanol production has increased about 3 times. The yield of ethanol decreases by about 6% after 5 cycles, which displayed cycling stability. The enhanced photocatalytic activity towards CO2 reduction was attributed to the synergistic effect of g-C3N4 and ZIF-67, which increases the separation of electrons and holes.

Atomic-Scale Structure and Stress Release Mechanism in Core-Shell Nanoparticles.

Core-shell nanoparticles find applications in catalysts, sensors, and theranostics. The full internal 3D atomic structure, however, cannot be resolved by current imaging and diffraction techniques. We analyzed the atomic positions and stress-release mechanism in a cubic Au-Pd core-shell nanoparticle in approximately 1000 times higher resolution than current experimental techniques using large-scale molecular dynamics simulation to overcome these limitations. The core-shell nanocube of 73 nm size was modeled similarly to solution synthesis by random epitaxial deposition of a 4 nm thick shell of Pd atoms onto a Au core of 65 nm side length using reliable interatomic potentials. The internal structure reveals specific deformations and stress relaxation mechanisms that are caused by the +4.8% lattice mismatch of gold relative to palladium and differential confinement of extended particle facets, edges, and corners by one, two, or three Au-Pd interfaces, respectively. The three-dimensional lattice strain causes long-range, arc-like bending of atomic rows along the faces and edges of the particle, especially near the Au-Pd interface, a bulging deformation of the Pd shell, and stacking faults in the Pd shell at the corners of the particle. The strain pattern and mechanism of stress release were further characterized by profiles of the atomic layer spacing in the principal crystallographic directions. Accordingly, strain in the Pd shell is several times larger in the extended facets than near the edges and corners of the nanoparticle, which likely affects adsorption, optical, and electrochemical properties. The findings are consistent with available experimental data, including 3D reconstructions of the same cubic nanoparticle by coherent diffractive imaging (CDI) and may be verified by more powerful experimental techniques in the future. The stress release mechanisms are representative for cubic core-shell nanoparticles with fcc structure and can be explored for different shapes by the same methods.