Large Number Of Atoms Research Articles

Large- N limit of Dicke superradiance

We investigate the thermodynamic limit of Dicke superradiance. We find an expression for the system's density matrix that we can prove is exact in the limit of large atom numbers N. This is in contrast to previously known solutions whose accuracy has only been established numerically and that are valid only for a range of times. We also introduce an asymptotically exact solution when the system is subject to additional incoherent decay of excitations as this is a common occurrence in experiment.

Saturation of superradiant light scattering from an atomic grating with a large number of atoms

Superradiance has drawn increasing interest, motivated by the development of quantum technology. In general, a large number of atoms are favored for inducing a strong superradiance. However, our analytical and numerical calculations of light scattering from an atomic grating longitudinally pumped by a laser show that when the atom number is small, superradiant light scattering (SLS) is induced, that is, the reflected light intensity is proportional to the square of the atom number, whereas the SLS is saturated with the increase in the atom number. In our calculation, using the coupled-wave theory, we treat the atomic grating as a dynamical photonic crystal and find rich time-dependent optical properties due to collective atomic recoil motion, which is available within research under current experimental condition. Thus, our paper could also spark an investigation of the optical features of tunable photonic crystals in the time domain.

3D Hierarchical Graphene‐CNT Anode for Sodium‐Ion Batteries: a First‐Principles Assessment

A thermodynamically and kinetically stable 3D hierarchical carbonaceous nanostructure is constructed to examine its potential as an anode for sodium-ion batteries (SIBs). Heptagonal and quadrilateral staggered carbon rings are formed where the graphite layers and the CNT interconnect vertically. Based on first-principles calculations, such a graphite-CNT hybrid with expanded interlayer spacing allows effective binding to Na atoms and contributes a high specific capacity of 324.3 mAh g−1. It maintains a great electronic conductivity for both pristine and Na-adsorbed graphite-CNT along the expanded graphite layers, and constructs a 3D electronic transport network when a large number of Na atoms are adsorbed. This hierarchical nanostructure predicts a fast inner 3D Na-ion transport network, with energy barriers of 0.56 and 0.50 eV along the expanded graphite layer and the CNT channel, respectively. This research manifests the feasibility of obtaining well-performed graphite-based anodes for SIBs through purposeful microcosmic morphology modification.

Interlayer coupling and strain localization in small-twist-angle graphene flakes

Twisted bilayer graphene (TBG) exhibits a wide range of intriguing physical properties, such as superconductivity, ferromagnetism, and superlubricity. Depending on the twist angle, periodic moiré superlattices form in twisted bilayer graphene, with inhomogeneous interlayer coupling and lattice deformation. For a small twist angle (typically < 2°), each moiré supercell contains a large number of atoms (>10,000), making it computationally expensive for first-principles and atomistic modeling. In this work, a finite element method based on a continuum model is used to simulate the inhomogeneous interlayer and intralayer deformations of twisted graphene flakes on a rigid graphene substrate. The van der Waals interactions between the graphene layers are described by a periodic potential energy function, whereas the graphene flake is treated as a continuum membrane with effective elastic properties. Our simulations show that structural relaxation and the induced strain localization are most significant in a relatively large graphene flake at small twist angles, where the strain distribution is highly localized as shear strain solitons along the boundaries between neighboring domains of commensurate AB stacking. Moreover, it is found that there exist many metastable equilibrium configurations at particular twist angles, depending on the flake size. The nonlinear mechanics of twisted bilayer graphene is thus expected to be essential for understanding the strain distributions in the moiré superlattices and the strain effects on other physical properties.

Highly stable cathodes for proton exchange membrane fuel cells: Novel carbon supported Au@PtNiAu concave octahedral core-shell nanocatalyst

Despite the remarkable research efforts, the lack of ideal activity and state-of-the-art electrocatalysts remains a substantial challenge for the global application of fuel cell technology. Herein, is reported the synthesis of Au@PtNiAu concave octahedral core-shell nanocatalysts (Au@PtNiAu-COCS) via solvothermal synthesis modification and optimization approach. The special structure generating a large number of step atoms, enhancing the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activity and stability. The superior ORR mass activity of the Au@PtNiAu-COCS is 11.22 times than the exhibited of Pt/C initially by Pt loading, and 5.11 times by Pt + Au loading. After 30 k cycles the mass activity remains 78.8% (8.83 times the initial Pt/C activity) and the half-wave potential only shifts 12 mV. Au@PtNiAu-COCS has superior half-cell activity and gives ideal membrane electrode assemblies. Furthermore, for MOR the Au@PtNiAu-COCS show enhanced anti-toxic (tolerant) ability in CO. This work provides a new strategy to develop core-shell structure nanomaterials for electrocatalysis.

Bi Layer Properties in the Bi–FeNi GMR-Type Structures Probed by Spectroscopic Ellipsometry

Bismuth (Bi) having a large atomic number is characterized by a strong spin–orbit coupling (SOC) and is a parent compound of many 3D topological insulators (TIs). The ultrathin Bi films are supposed to be 2D TIs possessing a nontrivial topology, which opens the possibility of developing new efficient technologies in the field of spintronics. Here we aimed at studying the dielectric function properties of ultrathin Bi/FeNi periodic structures using spectroscopic ellipsometry. The [Bi(d)–FeNi(1.8 nm)]N GMR-type structures were grown by rf sputtering deposition on Sitall-glass (TiO2) substrates. The ellipsometric angles Ψ(ω) and Δ(ω) were measured for the grown series (d = 0.6, 1.4, 2.0, and 2.5 nm, N = 16) of the multilayered film samples at room temperature for four angles of incidence of 60∘, 65∘, 70∘, and 75∘ in a wide photon energy range of 0.5–6.5 eV. The measured ellipsometric angles, Ψ(ω) and Δ(ω), were simulated in the framework of the corresponding multilayer model. The complex (pseudo)dielectric function spectra of the Bi layer were extracted. The GMR effects relevant for the studied Bi–FeNi MLF systems were estimated from the optical conductivity zero-limit (optical GMR effect). The obtained results demonstrated that the Bi layer possessed the surface metallic conductivity induced by the SOC effects, which was strongly enhanced on vanishing the semimetallic-like phase contribution on decreasing the layer thickness, indicating its nontrivial 2D topology properties.

Investigating Molecular Transformation Processes of Biodiesel Components During Long-Term Storage Via High-Resolution Mass Spectrometry.

Biodiesel is an important building block in renewable energy transformation. The main issue is that during storage biodiesel will undergo transformation processes that can lead to molecular changes, which then can cause applicational problems such as severe motor damage. To prevent this, a detailed understanding of the involved molecules and the emerging aging products is necessary. Biodiesel samples were stored for up to 12 months to monitor molecular changes, and all mixtures were investigated by using ultrahigh‐resolution mass spectrometry (HRMS) with electrospray ionization (ESI). The data revealed that during storage large numbers of oxygen atoms were incorporated into the fatty acid methyl esters (FAMEs). This process was dominant for the first quarter of aging but then became overshadowed by dimerization of oxygenated FAMEs. This means that there are two main pathways for aging in biodiesel: polyoxygenation and oligomerization. These findings greatly pin down the possible causes for sedimentation giving a solid foundation for aging inhibition development.

Will Polycrystalline Platinum Tip Sliding on a Gold(111) Surface Produce Regular Stick–Slip Friction?

Friction measurements by an atomic force microscope (AFM) frequently showed regular stick-slip friction signals with atomic-scale resolutions. Typically, for an AFM metal tip sliding on a metal crystal surface, the microstructure of the tip made from the thermally evaporated metal coating on a silicon cantilever was polycrystalline. Our detailed molecular dynamics(MD) simulations of a polycrystalline Pt tip (R = 10 nm in radius) sliding on an Au(111) surface revealed how the geometry of the polycrystalline tip took effect on the friction behavior at the contact interface. We found that the apex of the Pt tip with multiple grains near the edge of contact could induce severe plastic deformations of the gold substrate, leading to irregular stick-slip frictions upon sliding. Simulation results showed that in order to achieve a clear stick-slip friction signal with single atomic slips, the apex of the Pt tip must adopt a single crystalline protrusion without any neighboring grains involved in the metal contact. We showed that such a single crystalline protrusion, which presumably could be achieved during initial run-in or wear-out of high-energy Pt atoms in the neighboring grains, was passivated by a large number of gold atoms due to metal adhesion in the contact periphery. Using such a crystalline protrusion tip, we demonstrated that the stick-slip friction produced was very "tolerant" to the adhesion of a large number of gold atoms on the tip apex. We further showed that AFM tip mass used in MD simulations also played an important role in determining the transition between friction regimes, which could be well explained by the Prandtl-Tomlinson thermal activation model.

Hydrothermally synthesized lanthanide-incorporated multifunctional zirconia nanoparticles: Potential candidate for multimodal imaging

ObjectivesEnabled zirconium oxide Nanoparticles (NPs) are multifunctional nanoparticles that can be employed for multimodal imaging and can show good biocompatibility. In this work, we have synthesized Dysprosium (Dy) and Holmium (Ho) enabled zirconium oxide nanoparticles (DY/Ho-ZrO2 NPs) and have evaluated their potential as candidates for contrast agents in different imaging modalities such as X-ray computed tomography (CT), Photoluminescence (PL) imaging and Magnetic Resonance Imaging (MRI). Rare earth elements have large atomic numbers, and their incorporation can enhance the X-ray attenuation characteristics of their host along with their optical properties. Rare earth elements exhibit paramagnetic character which can be utilized for MRI contrast enhancement. A combination of these imaging modalities can fulfill the limitations faced while using a single imaging technique. MethodsWe have successfully synthesized Dy and Ho incorporated zirconia nanoparticles (Dy/Ho-ZrO2) by a simple one-step hydrothermal method. ResultsPrepared NPs were characterized for their physical properties. X-ray diffraction (XRD) revealed the crystalline phases present in Dy-ZrO2 and Ho-ZrO2 NPs with a crystallite size of 27 nm and 21.54 nm respectively. Scanning Electron Microscopy (SEM) results revealed the morphology of the nanoparticles, while EDS analysis gave the qualitative as well quantitative nature of synthesized nanocrystals. Photoluminescence (PL) data of Dy/Ho-Zirconia NPs have shown emission peaks near 419 nm when excited at 310 nm. Suspensions of prepared NPs with various concentrations were imaged on a CT machine in the clinical setting for contrast study. High CT contrast was observed for these NPs even at very low concentrations. Dysprosium doped sample was further evaluated for its potential as an MRI contrast agent. Dark cytotoxicity and photo-cytotoxicity results performed using Rhabdomyosarcoma (Rd) cancer cell lines revealed good biocompatibility of prepared NPs. ConclusionsThese results strongly signify the potential of these multifunctional Dy/Ho-Zirconia NPs to act as biocompatible multimodal imaging agents for biomedical applications.

Production of neutron deficient rare earth radionuclides by heavy ion activation

Abstract The attempts to produce neutron deficient radioisotopes of rare Earth elements by heavy ion activation are discussed in this review. The heavy ion induced reaction products have large atomic number difference with that of the target; therefore, radiochemical separation of no-carrier-added radio-lanthanides from the target matrix becomes easier. Heavy ion induced reactions also allow the production of rare Earth radionuclides from non-rare Earth target by tailor-made target-projectile combinations, and in those cases, radiochemical separations become even more easier. In general, the cross sections of heavy ion induced reactions are less than those of light charged particle induced reactions. However, some of the heavy ion induced reactions have comparable cross sections with those of light ion induced reactions. The range of heavy ions is also much smaller in the target matrix than that of lighter charged particles. These points hinder application of heavy ion induced reactions to produce radionuclides for nuclear medicine.

Existence, stability, and dynamics of monopole and Alice ring solutions in antiferromagnetic spinor condensates

In this work we study the existence, stability, and dynamics of select topological point and line defects in anti-ferromagnetic, polar phase, $F=1$ $^{23}$Na spinor condensates. Specifically, we leverage fixed-point and numerical continuation techniques in three spatial dimensions to identify solution families of monopole and Alice rings as the chemical potential (number of atoms) and trapping strengths are varied within intervals of realizable experimental parameters. We are able to follow the monopole from the linear limit of small atom number all the way to the Thomas-Fermi regime of large atom number. Additionally, and importantly, our studies reveal the existence of {\em two} Alice ring solution branches, corresponding to, relatively, smaller and larger ring radii, that bifurcate from each other in a saddle-center bifurcation as the chemical potential is varied. We find that the monopole solution is always dynamically unstable in the regimes considered. In contrast, we find that the larger Alice ring is indeed stable close to the bifurcation point until it destabilizes from an oscillatory instability bubble for a larger value of the chemical potential. We also report on the possibility of dramatically reducing, yet not completely eliminating, the instability rates for the smaller Alice ring by varying the trapping strengths. The dynamical evolution of the different unstable waveforms is also probed via direct numerical simulations.

Finite-volume pionless effective field theory for few-nucleon systems with differentiable programming

Finite-volume pionless effective field theory provides an efficient framework for the extrapolation of nuclear spectra and matrix elements calculated at finite volume in lattice QCD to infinite volume, and to nuclei with larger atomic number. In this work, it is demonstrated how this framework may be implemented via a set of correlated Gaussian wave functions optimized using differentiable programming and via solution of a generalized eigenvalue problem. This approach is shown to be significantly more efficient than a stochastic implementation of the variational method based on the same form of correlated Gaussian wave functions, yielding comparably accurate representations of the ground-state wave functions with an order of magnitude fewer terms. The efficiency of representation allows such calculations to be extended to larger systems than in previous work. The method is demonstrated through calculations of the binding energies of nuclei with atomic number $A\ensuremath{\in}{2,3,4}$ in finite volume, matched to lattice QCD calculations at quark masses corresponding to ${m}_{\ensuremath{\pi}}=806\text{ }\text{ }\mathrm{MeV}$, and infinite-volume effective field theory calculations of $A\ensuremath{\in}{2,3,4,5,6}$ systems based on this matching.

Surface Tension and Kinematic Viscosity of Multicomponent FeCuNbSiB Melt

This work investigated the surface tension and kinematic viscosity of the multicomponent Fe73.5Cu1Nb3Si13.5B9 melt. A relationship was found between surface tension and kinematic viscosity, which manifests itself in a synchronous change in these quantities at temperatures of 1600 and 1780 K. In the temperature range 1600–1780 K, there is a sharp increase in surface tension upon heating and the same decrease upon cooling. The increase in surface tension during heating was explained by the appearance of a large number of free Nb atoms as a result of the dissolution of the residual crystalline phase in the mushy zone, and their diffusion to the melt surface. The drop in the surface tension on cooling below 1780 K is associated with the liquid–liquid structure transition (LLST), which stimulates the outflow of Nb atoms from the surface in order to form new stable clusters. The LLST manifests itself in a change in the activation energy of a viscous flow, which is higher in the high-temperature region and corresponds to the motion of larger clusters with a length scale of about 1 nm.

Evolution of the Periodic Table as We Know it?

The inversion of the Periodic Table to accommodate spatial variation of atomic energy levels relative to the nucleus has subsequently been underwritten by Quantum Field Theory’s U(1) x SU(2) x SU(3) group symmetry and Clifford Algebra resulting in a one-to-one mapping between the Roberts-Janet Nuclear Periodic Table and the Quantum Mechanical Table. A coherent sketch of scientific commentary is attempted in the form of two cycles within the Roberts-Janet Table. Mechanisms of superposition, entanglement, tunnelling and decoherence abound but the pathways of causality remain elusive; whether testable as telescopes and artificial intelligence evolve. Mass number is no longer a fundamental property. It appears unique to each supernova explosion as the ratio of isotopes varies in the nth decimal place as radioactivity proceeds. Goodbye a unique Periodic Table. Hello a large number of Periodic Tables underwritten by the Roberts-Janet mechanism where mass number is merely a by-product. The first cycle reignites radioactivity by mergers between neutron stars and black holes - radioactivity having been extinguished temporarily in the ebb and flow of production of white dwarfs, neutron stars and black holes. Nuclei with large mass numbers and low atomic numbers result; rapidly decaying to nuclei with large atomic numbers. By further decay elements familiar to the Periodic Table appear from higher atomic numbers. Various sizes of black hole production suggest a hierarchy of outcomes which not only produce a reignition of radioactivity but potential creations of other universes from explosions of larger and supermassive black holes as the Planck length is approached and energies increase to a scale giving rise to periods of inflation and condensation that predate the production of quarks. This implies universes can be superimposed on previous universes explaining why some supermassive black holes appear in subsequent universes nearer than current theoretical models. The second cycle indicates a path through abiogenesis, water and trace elements to form life ultimately to be extinguished by the Sun’s expansion thus merging with the first cycle’s production of stars.

Studies on the Crystal Growth and Characterization of Large Size Sr:LCB Single Crystals

Extending the shortest second harmonic generation output wavelength of nonlinear optical crystals into the deep ultraviolet (UV) range is important for their application as frequency conversion devices for an advanced laser. The doping of ions with a large atomic number is believed to be an effective way to realize a shorter SHG output wavelength. In this work, large-sized Sr2+-doped La2CaB10O19 (Sr:LCB) crystals with nominal ratios of 10%, 15% and 30% were grown by the top-seeded solution growth method. The measured lattice parameters of the grown Sr:LCB are nearly the same as that of the LCB crystal, and the rocking curves reveal that the grown Sr:LCB crystals are of high quality. Sr: LCB crystals have a UV cut-off edge of 168 nm. The refractive index of the Sr:LCB crystals was measured, based on which the Sellmeier equations of the Sr:LCB crystals were fitted. The calculated shortest SHG output wavelength for Type I phase matching is 270.5 nm, which is 17.5 nm shorter than that of LCB crystals (288 nm). The characterization results demonstrate that Sr:LCB is a potential nonlinear optical crystal for the deep UV range.

Molecular dynamics simulation of thermal de-icing on a nanochannel with hot fluids

The thermal de-icing is an environmentally-friendly and high-efficiency technique to cope with ice accumulation issues. Herein, the thermal de-icing on a nanochannel with hot fluids is studied using the molecular dynamics simulation. Results show that the ice with a hexagonal network structure is gradually melted to the water with an amorphous structure from the ice mantle to the core. The ice temperature and the ice energy increase while the average number of hydrogen bonds decreases with the change of time. The rapid ice melting benefits from temperature differences between the ice and nanochannel which act as the driving force to promote the heat transfer. A large number of fluid atoms are trapped in low potential energy regions to participate in the interfacial heat transfer, which is conducive to the heat transfer between the ice and hot fluids of the nanochannel. The velocity slip occurs at the wall-fluid interface in the nanochannel. In addition, both the larger hot thermostat temperature and the stronger surface wettability between the ice and nanochannel can shorten the ice melting time, which are beneficial to the thermal de-icing on a nanochannel.

Preparation of conductive Si/C/N/Ni ceramic nanocomposites using phenyl-substituted polysilazane and nickelocene as precursors

In this study, a conductive Si/C/N/Ni nanocomposite was prepared using phenyl-substituted polysilazane and nickelocene (NiCp2) as precursors. Ni-related nanoparticles were uniformly distributed throughout the Si/C/N/Ni nanocomposite without aggregation because of the large number of nitrogen atoms in the silsesquiazane used as a precursor of the matrix. In addition, the nickel silicide crystalline phase and turbostratic carbon phase grew in the matrix of the Si/C/N/Ni nanocomposite at low temperatures because NiCp2 acted as a catalyst. Moreover, NiCp2 contributed to the increase in the turbostratic carbon crystallinity through the dehydrogenation reaction, resulting in an improved electrical conductivity of the Si/C/N/Ni nanocomposite.

Dynamics of large samples of repulsive Fermi gases at nonzero temperatures

We develop a model of a binary fermionic mixture, consisting of a large number of atoms, applicable at nonzero temperatures, in the normal phase. We use this approach to study the dynamics of degenerate Fermi systems under various perturbations. For example, we analyze the spin-dipole oscillations of a two-component fermionic mixture, demonstrating that the ferromagnetic phase shows up at a stronger repulsion between components while the temperature rises. We study as well the radial oscillations of weakly interacting repulsive Fermi gases. We obtain a good agreement with experimental data when available. Otherwise, we compare our results with the outcome of the Hartree-Fock orbital calculations done for a system with a small number of fermions.

A nearsighted force-training approach to systematically generate training data for the machine learning of large atomic structures.

A challenge of atomistic machine-learning (ML) methods is ensuring that the training data are suitable for the system being simulated, which is particularly challenging for systems with large numbers of atoms. Most atomistic ML approaches rely on the nearsightedness principle ("all chemistry is local"), using information about the position of an atom's neighbors to predict a per-atom energy. In this work, we develop a framework that exploits the nearsighted nature of ML models to systematically produce an appropriate training set for large structures. We use a per-atom uncertainty estimate to identify the most uncertain atoms and extract chunks centered around these atoms. It is crucial that these small chunks are both large enough to satisfy the ML's nearsighted principle (that is, filling the cutoff radius) and are large enough to be converged with respect to the electronic structure calculation. We present data indicating when the electronic structure calculations are converged with respect to the structure size, which fundamentally limits the accuracy of any nearsighted ML calculator. These new atomic chunks are calculated in electronic structures, and crucially, only a single force-that of the central atom-is added to the growing training set, preventing the noisy and irrelevant information from the piece's boundary from interfering with ML training. The resulting ML potentials are robust, despite requiring single-point calculations on only small reference structures and never seeing large training structures. We demonstrated our approach via structure optimization of a 260-atom structure and extended the approach to clusters with up to 1415 atoms.

Efficient adsorption of Au(III) from acidic solution by a novel N, S-containing metal–organic framework

Due to the special status of precious metals in the preparation of high-tech materials, the recycling of precious metals has important practical significance. A novel metal–organic framework called as Zn-AMT containing large number of functional atoms (N, S) was prepared as a highly efficient adsorbent to recover Au(III) from acid solution. Compared with the reported adsorbents, Zn-AMT shows remarkable adsorption efficiency towards Au(III) with the maximum adsorption capacity up to 2040 mg·g−1 at 25 °C. Increasing temperature to 42 °C could make the adsorption capacity increased to 2878 mg·g−1. The adsorption ratio of 100% could be achieved for the low Au(III) concentration within 100 mg·L-1. The Zn-AMT shows both excellent selectivity and good reusability. The adsorption kinetics data follow the pseudo-second-order kinetics and the adsorption isotherms data are well fitted by the Langmuir model. The adsorption mechanism was deduced that the Au(III) was firstly adsorbed onto the surface of Zn-AMT through electronic attraction and the coordination interaction of -SH, and then in situ reduced to Au(I). With the prominent adsorption performance, excellent selectivity, and admirable reusability, the Zn-AMT adsorbent could be a promising candidate for gold recovery in practical application.