Constituent Atoms Research Articles

Influence of the size of iron nanoclusters on their magnetization

The size of iron nanocrystals significantly affects the value of their magnetization. However, an adequate model of the structure of nanocrystalline formations comprising different numbers of iron atoms still does not exist. In this work, spatial models of nanocrystalline iron clusters differing in configuration and the number of their constituent atoms are constructed. Tetrahedrally close-packed cluster assemblies of iron atoms are taken as the basis for the proposed structures of nanocrystals. The spectra of the density of electronic states for the proposed clusters are constructed using the theory of the electron density functional. The calculation was carried out by the method of scattered waves in accordance with the band theory of crystals. The appearance of magnetization in tetrahedral close-packed cluster formations is associated with excited electronic states of atoms located on the surface of the nanocluster. Excited atoms have an increased electron density, that is, electrons are able to transition to states with higher energy, approaching the Fermi energy. In this case, the Stoner criterion necessary for the occurrence of magnetization is fulfilled. The configurations of electrons with spin up and down differ, which is why uncompensated magnetic moments appear. It is shown that the proposed models of iron nanoclusters are in satisfactory agreement with the known experimental data.

Atomic structure and Mott nature of the insulating charge density wave phase of 1T-TaS2

Using x-ray pair distribution function (PDF) analysis and computer modeling, we explore structure models for the complex charge density wave (CDW) phases of layered 1T-TaS2 that both well capture their atomic-level features and are amenable to electronic structure calculations. The models give the most probable position of constituent atoms in terms of 3D repetitive unit cells comprising a minimum number of Ta–S layers. Structure modeling results confirm the emergence of star-of-David (SD) like clusters of Ta atoms in the high-temperature incommensurate (IC) CDW phase and show that, contrary to the suggestions of recent studies, the low-temperature commensurate (C) CDW phase expands upon cooling thus reducing lattice strain. The C-CDW phase is also found to preserve the stacking sequence of Ta–S layers found in the room temperature, nearly commensurate (NC) CDW phase to a large extent. DFT based on the PDF refined model shows that bulk C-CDW 1T-TaS2 also preserves the insulating state of individual layers of SD clusters, favoring the Mott physics description of the metal-to-insulator (NC-CDW to C-CDW) phase transition in 1T-TaS2. Our work highlights the importance of using precise crystal structure models in determining the nature of electronic phases in complex materials.

Electric dipole moments and static dipole polarizabilities of alkali-metal–alkaline-earth-metal molecules: Nonrelativistic versus relativistic coupled-cluster-theory analysis

We analyze the electric dipole moments (PDMs) and static electric dipole polarizabilities of the alkali--alkaline-earth (Alk-AlkE) dimers by employing finite-field coupled-cluster methods, both in the frameworks of non-relativistic and four-component spinfree relativistic theory. In order to carry out comparative analyses rigorously, we consider those Alk-AlkE molecules made out of the lightest to the medium-heavy constituent atoms (Alk: Li to Rb and AlkE: Be through Sr). We present behaviour of electron correlation effects as well as relativistic effects with the size of the molecules. Uncertainties to the above quantities of the investigated Alk-AlkE molecules are inferred by analyzing our results from different form of Hamiltonian, basis set, and perturbative parameter in a few representative molecules. We have also provided empirical relations by connecting average polarizabilities of the Alk-AlkE molecules with their PDMs, and atomic numbers and polarizabilities of the corresponding Alk and AlKE atoms, which can be used to roughly estimate the average polarizabilities of other heavier Alk-AlkE molecules. We finally give our recommended results, and compare them with the literature values.

A pair of carbazate derivatives as novel Schiff base ligands: DFT and POM theory supported spectroscopic and biological evaluation

Schiff bases are mentioned as strongly important molecular scaffolds of industrial and medicinal purposes. Due to wide range applications of carbazate derivatives herein synthesis and characterization of a new Schiff base ligand, (E)-ethyl 2-(4-methoxybenzylidene)hydrazinecarboxylate and 4-(nitrobenzaldehyde)ethylcarbazate are reported. The compound was characterized on the basis of experimental and density functional theory calculations (using the B3LYP and 6-31 G(d,p)formalism combination). Among characterization techniques elemental analysis, FT-IR, UV-Vis and NMR spectroscopic evaluations were mainly employed to carry out the formulation of the compound. In addition to computational validation of characterization other significant molecular parameters were also evaluated including geometry optimization, frontier molecular orbital analysis (FMO) and Columbic interaction of different constituent atoms of the title compound. A good agreement has been found between DFT and experimental outcomes confined to prove the structure of the compound. Moreover, molecular docking and antimicrobial studies have proven the Schiff base as an effective bioactive compound. Communicated by Ramaswamy H. Sarma

Two-dimensional MSi2N4 monolayers and van der Waals heterostructures: Promising spintronic properties and band alignments

A class of septuple-atomic-layer two-dimensional (2D) materials, $\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and $\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, was fabricated recently, and the three constituent atoms may be replaced by other elements in the Periodic Table, thus forming a large family. In the current work, using density functional theory calculations, we systematically investigate 18 $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ (where $M$ denotes groups IV-B, V-B, and VI-B transition metals) monolayers in both H- and T-phases for their stabilities and their electronic, magnetic, and spintronic properties, and we highlight the spintronic properties and their van der Waals heterostructures. The electronic and magnetic properties of these 2D monolayers are correlated with the $d$-levels splitting (from crystal field and exchange field) and electron filling. Spin-dependent band alignment of $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ heterostructures shows that most of them are type II heterostructures for both spin channels. In addition, the valence-band edge of seven $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ monolayers resides at the Brillouin zone center ($\mathrm{\ensuremath{\Gamma}}$-point), which may form momentum-matched heterostructures by stacking with other 2D semiconductors. The ferromagnetic semiconductors (H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, H-$\mathrm{Nb}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$, and H-$\mathrm{Ta}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) and the heterostructures composed of them (e.g., H-$\mathrm{Mo}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ or H-$\mathrm{W}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$/H-$\mathrm{V}{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$) can be a half-semiconductor or half-metal. These results, in addition to the literature reporting high Curie temperatures, indicate that $M{\mathrm{Si}}_{2}{\mathrm{N}}_{4}$ and their heterostructures are promising for room-temperature spintronics and optoelectronics.

Unified theory of atom-centered representations and message-passing machine-learning schemes

Data-driven schemes that associate molecular and crystal structures with their microscopic properties share the need for a concise, effective description of the arrangement of their atomic constituents. Many types of models rely on descriptions of atom-centered environments, which are associated with an atomic property or with an atomic contribution to an extensive macroscopic quantity. Frameworks in this class can be understood in terms of atom-centered density correlations (ACDC), which are used as a basis for a body-ordered, symmetry-adapted expansion of the targets. Several other schemes that gather information on the relationship between neighboring atoms using "message-passing" ideas cannot be directly mapped to correlations centered around a single atom. We generalize the ACDC framework to include multi-centered information, generating representations that provide a complete linear basis to regress symmetric functions of atomic coordinates, and provide a coherent foundation to systematize our understanding of both atom-centered and message-passing and invariant and equivariant machine-learning schemes.

An optical tweezer array of ground-state polar molecules

Fully internal and motional state controlled and individually manipulable polar molecules are desirable for many quantum science applications leveraging the rich state space and intrinsic interactions of molecules. While prior efforts at assembling molecules from their constituent atoms individually trapped in optical tweezers achieved such a goal for exactly one molecule (Zhang J T et al 2020 Phys. Rev. Lett. 124 253401; Cairncross W B et al 2021 Phys. Rev. Lett. 126 123402; He X et al 2020 Science 370 331–5), here we extend the technique to an array of five molecules, unlocking the ability to study molecular interactions. We detail the technical challenges and solutions inherent in scaling this system up. With parallel preparation and control of multiple molecules in hand, this platform now serves as a starting point to harness the vast resources and long-range dipolar interactions of molecules.

Machine learning for accelerated prediction of the Seebeck coefficient at arbitrary carrier concentration

The convenient determination of Seebeck coefficient is a major challenge in the thermoelectric field, either for experimental metrology or theoretical prediction. Taking Heusler compounds as a prototypical class of examples, we propose a neural network model by which the Seebeck coefficient can be quickly evaluated at arbitrary carrier concentration. Using only a few elemental properties of the constituent atoms as input features, the derived model exhibits a high Pearson correlation between the real and predicted Seebeck coefficients for both the n- and p-type systems. Beyond the training data, we have checked the Seebeck coefficients of four randomly selected Heusler compounds and found strong predictive power of our model, which can be in turn utilized to screen a vast number of Heusler compounds for accelerated discovery of new materials with desired Seebeck coefficients. As an addition, we adopt the sure independence screening and sparsifying operator to obtain a three-dimensional descriptor, which is physically interpretable and shows comparable predictive power as our neural network model.

Role of the Rigid Host Structure in Narrow-Band Green Emission of Eu2+ in Rb2Na2(Li3SiO4)4: Insights into Electron-Phonon Coupling.

Eu2+-activated alkali-lithosilicate phosphors exhibit narrow-band emissions that are attractive to high color-rendition and wide color-gamut displays. The microscopic mechanism behind the small emission bandwidth is not presently understood. Here, we report an explicit calculation of the vibronic process occurring in the narrow-band green emission of Rb2Na2[Li3SiO4]4:Eu2+. We show that due to the high rigidity of the host material, the structural strain induced by the localized Eu2+ 4f-5d excitation is distributed among the atoms far beyond the first coordination shell and hence reduces the local structural relaxation around Eu2+. The emission bandshape is thus mainly controlled by the coupling of the electronic transition with the phonon modes associated with motions of host constituent atoms, which was further validated by the good agreement of the calculated bandshape with the experiment. The results provide insights into the generation of narrow-band emission and improve our knowledge on electron-phonon coupling of 4f-5d transitions in phosphors.

Muonic vs electronic dark forces: a complete EFT treatment for atomic spectroscopy

Precision atomic spectroscopy provides a solid model independent bound on the existence of new dark forces among the atomic constituents. We focus on the keV-GeV region investigating the sensitivity to such dark sectors of the recent measurements on muonic atoms at PSI. To this end we develop for the first time, the effective field theory that describes the leading effect of a new (pseudo-)vector or a (pseudo-)scalar particle of any mass at atomic energies. We identify in the Lamb Shift measurement in muonic deuterium (μD) and the 2s Hyperfine Splitting (HFS) in muonic hydrogen (μH) the most promising measurements to probe respectively spin-independent and spin-dependent new forces. Furthermore, we evaluate the expression of the vector force HFS finding that a future measurement of the 2s HFS in regular hydrogen could provide the strongest atomic bound for such a force for masses above 100 MeV.

HyGAL: Characterizing the Galactic Interstellar Medium with Observations of Hydrides and Other Small Molecules. I. Survey Description and a First Look Toward W3(OH), W3 IRS5, and NGC 7538 IRS1

The HyGAL Stratospheric Observatory for Infrared Astronomy legacy program surveys six hydride molecules—ArH+, OH+, H2O+, SH, OH, and CH—and two atomic constituents—C+ and O—within the diffuse interstellar medium (ISM) by means of absorption-line spectroscopy toward 25 bright Galactic background continuum sources. This detailed spectroscopic study is designed to exploit the unique value of specific hydrides as tracers and probes of different phases of the ISM, as demonstrated by recent studies with the Herschel Space Observatory. The observations performed under the HyGAL program will allow us to address several questions related to the life cycle of molecular material in the ISM and the physical processes that impact the phase transition from atomic to molecular gas, such as: (1) What is the distribution function of the H2 fraction in the ISM? (2) How does the ionization rate due to low-energy cosmic rays vary within the Galaxy? (3) What is the nature of interstellar turbulence (e.g., typical shear or shock velocities), and what mechanisms lead to its dissipation? In this overview, we discuss the observing strategy, the synergies with ancillary and archival observations of other small molecules, and the data reduction and analysis schemes we adopted; and we present the first results obtained toward three of the survey targets, W3(OH), W3 IRS5, and NGC 7538 IRS1. Robust measurements of the column densities of these hydrides—obtained through widespread observations of absorption lines—help address the questions raised, and there is a very timely synergy between these observations and the development of theoretical models, particularly pertaining to the formation of H2 within the turbulent ISM. The provision of enhanced HyGAL data products will therefore serve as a legacy for future ISM studies.

Dependency of sliding friction for two-dimensional systems on electronegativity

We study the role of electronegativity in sliding friction for five different two-dimensional (2D) monolayer systems using ab initio calculations within density functional theory with van der Waals corrections. We show that the friction depends strongly on the involved atoms' electronegativity difference. All the studied systems exhibit almost the same magnitude of the friction force when sliding along the nonpolar path, independent of the material and the surface structures. In contrast, for sliding friction along a polar path, the friction force obeys a universal linear scaling law where the friction force is proportional to the electronegativity difference of its constituent atoms. We show that atomic dipoles in the 2D monolayers, induced by the electronegativity difference, enhance the corrugation of the charge distribution and increase the sliding barriers accordingly. Our studies reveal that electronegativity plays an important role in the friction of low-dimensional systems, and provides a strategy for designing nanoscale devices.

Elastic criterion for shear-banding instability in amorphous solids.

In amorphous solids, plastic flow is prone to localization into shear bands via an avalanche of shear-transformation (ST) rearrangements of constituent atoms or particles. However, such banding instability still remains a lack of direct experimental evidence. Using a real 3D colloidal glass under shear as proof of principle, we study STs' avalanches into shear banding that is controlled by strain rates. We demonstrate that, accompanying the emergent shear banding, the elastic response fields of the system, typical of a quadrupole for shear and a centrosymmetry for dilatation, lose the Eshelby-type spatial symmetry; instead, a strong correlation appears preferentially along the banding direction. By quantifying the fields' spatial decay, we identify an elastic criterion for the shear-banding instability, that is, the strongly correlated length of dilatation is smaller than the full length of shear correlation. Specifically, ST-induced free volume has to be confined within the elastic shear domain of ST so that those STs can self-organize to trigger shear banding. This physical picture is directly visualized by tracing the real-space evolution of local dilatation and ST particles. The present work unites the two classical mechanisms: free volume and STs, for the fundamental understanding of shear banding in amorphous solids.

Layered Pt-Co alloys: Bulk, surface and nanoparticle analysis, based on DFT

Pt-based catalysts are commonly used as cathode materials in various types of fuel cells. In this work we apply DFT calculations combined with cluster expansion method to elucidate the most stable structures of Pt-Co bulk alloy across the full range of Pt (or Co) concentrations. Our calculations reveal high stability of alloy configurations with alternating layers in (001) crystallographic planes of face centered cubic (fcc) structure exclusively occupied either by Pt or Co atoms. In particular, we find that Pt1Co1, Pt2Co1 and Pt3Co1 layered structures (periodically repeated 1–3 Pt layers, separated by single Co layers) have exceptionally high stability with Pt2Co1 possessing the lowest energy of formation among all analysed alloy configurations. Employing ab initio thermodynamics approach, we discuss stability of three types of surface terminations (Miller indices (100), (110) and (111)) for these three layered structures and also for highly symmetric fcc Pt3Co1 configuration, previously identified experimentally. Applying the Wulff construction, we obtain the nanoparticle shapes as functions of chemical potentials of constituent atoms (Pt or Co), allowing for design of computational models of Pt-Co nanoparticles and subsequent analysis of their chemical properties.

Quaternary – alloyed capping for strain and band engineering in InAs sub – monolayer quantum dots

In the last few years, a considerable amount of attention has been given to the optimization of several capping layers (CLs) in InAs/GaAs sub – monolayer (SML) quantum dots (QDs) heterostructure for a sustainable room – temperature (RT) operation in 1.3–1.55 μm telecommunication range (O, C – bands). In particular, the quaternary – alloyed capping (Q – CL: Q – Sb: InxGa1−xAsySb1−y, Q – N: InxGa1−xAsyN1−y, and Q – SbN: GaAsxSbyN1−x−y) has been explored in this work to reach the above – stated photoluminescence (PL) emission wavelength and we compared it with the conventional InAs/InxGa1−xAs QDs using Nextnano software. This tool is based on 8 – band k.p model that solves the self – consistent Schrödinger – Poisson equations analytically to obtain the envelope functions of the eigen energy levels (both electrons and holes) and finally the absorption spectra. Although Q – CL help in preserving QD morphology by strain reduction, it also allowed us create deep carrier confinement potential near conduction and valence band offsets i.e. CBO (VBO) where there is a minimum carrier escape probability towards GaAs barriers (more thermal stability) thus fostering a prolonged PL emission wavelength at 300 K. By changing atomic constituent's composition in Q – CLs, the obtained ground – state (GS) band – to – band absorption energy are 1.31 (Q – Sb22%, 1.25 (Q – N1.8%), 1.37 μm (Q – SbN2.2%))). Such deep confinement aid in the dark current reduction and can promote for high temperature operation by improving the PL quantum yield by several orders of magnitude. The inherent material properties of CL composing the surfactant effect of Sb, larger bond strength of tensile – strained dil.N, and phase separation effects of Sb–N alloys are advantageous in this regard to corroborate further PL redshift.

An insight into structural, electronic and optical characteristics of Mo1-xMxO3 (M = Zr, Y, ZrY) for the formation of conducting filaments in optoelectronic memory devices: A first principles study

In the present work, we have theoretically investigated the structural, electronic and optical characteristics of Mo1−xMxO3 (M = Zr, Y, co-doped (ZrY)) for Optoelectronic Resistive Random-Access Memory (ORRAM) devices and associated applications. Density functional theory within Heyd-Scuseria-Ernzerhof (HSE06) functional has been employed to better estimate optoelectronic parameters. The structural outcomes, findings of the energy band structure, spin resolved total density of states (TDOS), three dimensional iso-surface electron charge density, electron localization function and Bader charge analysis unveil thatMo1−x(ZrY)xO3 is comparatively more suitable composite with enhanced charge conductance for ORRAM devices. The partial density of states (PDOS) and Bader charge analysis results reveals that noticeable orbital contribution of the constituent atoms mainly in increasing conductivity through hybridization. Formation energy and phonon calculations confirmed that studied structures are stable. The photo absorption behavior shows that Mo1−x(ZrY)xO3 can absorb a broad electromagnetic radiations wavelength range from ultraviolet (UV) to infrared (IR) coherent with the charge conduction property which has been found a most fitted candidate for ORRAM and associated applications.

Mirror‐Image Magnetic Circularly Polarized Luminescence from Perovskite (M+Pb2+Br3, M+=Cs+ and Amidinium) Quantum Dots

AbstractAmbidextrous magnetic circularly polarized luminescence (MCPL) and redshift in photoluminescence (PL) spectra are observed from two types of achiral soluble diamagnetic perovskite‐Pb‐quantum dots in toluene solution at room temperature, when subjected to external north‐up and south‐up magnetic fields of 1.7 T. The magnitude of the MCPL, gMCPL, is in the order of |gMCPL|=10−3 within a wavelength range of 480–580 nm. The wavelength and the sign of the MCPL spectra can be controlled by the types of perovskite constituent atom and the direction of the magnetic field, respectively.

QED coherence in matter, syntropy and the coherent domains as storing “devices”

Several macroscopic systems, like in particular living organisms, are characterized by stability and high level of order at mesoscopic / macroscopic scale. These peculiar features cannot be explained by considering just short-range interactions among their elementary constituents (atoms / molecules) and the entropic dynamics governing it. The theory of QED coherence predicts that in condensed matter, when suitable boundary conditions about temperature and density are satisfied (as generally happens in living systems), a quantum phase transition spontaneously occurs, driving the system towards a macroscopic coherent quantum state in which phase tuned oscillations between matter and electromagnetic field take place and ensure long-range correlation and order. In 1944, the italian mathematician Luigi Fantappie formulated the “Principi di una teoria unitaria del mondo fisico e biologico”, according to which living systems are governed by “syntropic” spontaneous phenomena, that are dual with respect the “entropic” ones, characterized by a reduction of entropy with time and whose causes are put in the future with respect the occurring events, able to ensure the organization and stability of living systems. In this paper we show how the results of QED coherence in matter are able to reproduce several of the main features of “syntropic” dynamics so giving it a support based on Quantum Field Theory. Finally, the role of water as a gatherer of quantum information, predicted by the QED coherence theory, is discussed as a possible practical realization of syntropic dynamics.

Growth of two-dimensional Janus MoSSe by a single in situ process without initial or follow-up treatments

Two-dimensional (2D) Janus transition metal dichalcogenides (TMDCs) are highly attractive as an emerging class of 2D materials, but only a few methods are available for fabricating them. These methods rely on the initial growth of 2D TMDCs in one process, followed by an additional plasma or high-temperature (T) process. To overcome these drawbacks, we employ the new approach of NaCl-assisted single-process chemical vapor deposition, which consists of three steps that proceed only by altering the temperature in situ. In the first step, MoS2 is deposited onto a SiO2/Si substrate with the Mo and S atoms activated in different temperature zones. In the second step, S vacancies are formed in the upper layer of the grown MoS2 by annealing. In the third step, the vacancies are filled with activated Se atoms. Throughout the steps, NaCl lowers the melting point of the constituent atoms, while the T in each zone is properly controlled. The growth mechanism is clarified by a separate annealing experiment that does not involve a supply of activated atoms. These results highlight a simple and cost-effective approach for growing Janus MoSSe, which is more useful for fundamental studies and device applications.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction.

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔGH* value. Its active site density counts up to 1.46 × 1015 site/cm2, larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H2 spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.