Atomic Strings Research Articles

Seeding Atomic Silver into Internal Lattice Sites of Transition Metal Oxide for Advanced Electrocatalysis.

Transition metal oxides (TMOs) are widely studied for loading of various catalysts due to their low cost and high structure flexibility. However, the prevailing close-packed nature of most TMOs crystals has restricted the available loading sites to surface only, while their internal bulk lattice remains unactuated due to the inaccessible narrow space that blocks out most key reactants and/or particulate catalysts. Herein, using tunnel-structured MnO2, this study demonstrates how TMO's internal lattice space can be activated as extra loading sites for atomic Ag in addition to the conventional surface-only loading, via which a dual-form Ag catalyst within MnO2 skeleton is established. In this design, not only faceted Ag nanoparticles are confined onto MnO2 surface by coherent lattice-sharing, Ag atomic strings are also seeded deep into the sub-nanoscale MnO2 tunnel lattice, enriching the catalytically active sites. Tested for electrochemical CO2 reduction reaction (eCO2RR), such dual-form catalyst exhibits a high Faradaic efficiency (94%), yield (67.3mol g-1 h-1) and durability (≈48h) for CO production, exceeding commercial Ag nanoparticles and most Ag-based electrocatalysts. Theoretical calculations further reveal the concurrent effect of such dual-form catalyst featuring facet-dependent eCO2RR for Ag nanoparticles and lattice-confined eCO2RR for Ag atomic strings, inspiring the future design of catalyst-substrate configuration.

Diffusion of High-Energy Negatively Charged Particles in the Field Atomic Strings of an Oriented Crystal

The work analyzes the dependence of the diffusion index of high-energy negatively charged particles on the energy of the transverse motion in oriented crystal. The crystal had an axial orientation relative to the direction of particle incidence. The analysis was carried out using the example of π− mesons with a momentum of 100 GeV/c that impinged on a silicon crystal, which corresponds to the conditions achievable on secondary beam of the the CERN SPS accelerator. The analysis showed that the dependence under consideration is not monotonic. It has a minimum in the energy region slightly exceeding the value of the potential energy of particles at the saddle point of the potential of crystal atomic strings. At higher values of the energy of transverse motion of particles E⊥, the diffusion index increases with increasing E⊥, since this increases the average absolute value of the velocity of particle motion in the plane orthogonal to the crystal axis, near which motion takes plase. The increase in the diffusion index at low values of E⊥ is associated with the manifestation of incoherent scattering of particles on thermal vibrations of crystal atoms. The analysis carried out in the work is of interest both for a deeper understanding of the process of high-energy negatively charged particle beams passage through oriented crystals, and for improving methods for charged particle beams steering with a help of straight and bent oriented crystals.

Reassignment of magic numbers for icosahedral Au clusters: 310, 564, 928 and 1426.

Icosahedral Au clusters with three and four shells of atoms are found to deviate significantly from the commonly assumed Mackay structures. By introducing additional atoms in the surface shell and creating a vacancy in the center of the cluster, the calculated energy per atom can be lowered significantly, according to several different descriptions of the interatomic interaction. Analogous icosahedral structures with five and six shells of atoms are generated using the same structural motifs and are similarly found to be more stable than Mackay icosahedra. The lowest energy per atom obtained here is for clusters containing 310, 564, 928 and 1426 atoms, as compared with the commonly assumed magic numbers of 309, 561, 923 and 1415. Some of the vertices in the optimized clusters have a hexagonal ring of atoms, rather than a pentagon, with the vertex atom missing. An inner shell atom in some cases moves outwards by more than an Ångström into the surface shell at such a vertex site. This feature, as well as the wide distribution of nearest-neighbor distances in the surface layer, can strongly influence the properties of icosahedral clusters, for example catalytic activity. The structural optimization is initially carried out using the GOUST method with atomic forces estimated with the EMT empirical potential function, but the atomic coordinates are then refined by minimization using electron density functional theory (DFT) or Gaussian approximation potential (GAP). A single energy barrier is found to separate the Mackay icosahedron from a lower energy structure where a string of atoms moves outwards in a concerted manner from the center so as to create a central vacancy while placing an additional atom in the surface shell.

Incoherent ultrarelativistic channeling particle scattering by electrons

The problem of high-energy charged particle motion in the field of atomic strings and planes of oriented crystals, widely applied to control large accelerator beams and generate intense gamma radiation, is addressed. Following the previously developed theory of channeled particles incoherent scattering by crystal atom nuclei, we consider here the same by crystal atom electrons. The theory developed takes into consideration all the effects of momentum transfer between fast particles and electrons of atoms in a crystal in the range from the nuclear radius up to the many inter-atomic distances. The theory also includes the temperature-dependent Debye – Waller factor, as well as both the atomic form factors and scattering function, evaluated with the detail consideration of atomic structure. All the modifications of electron scattering in crystals are reduced to the value of the effective minimum momentum transfer, which by an order of value exceeds that one, related with the Bethe – Bloch mean atomic energy. Substituting this quantity to the expression for the mean square of the scattering angle of a classically moving particle makes it possible to compare the scattering by electrons and nuclei, while its joint use with the Rutherford cross section allows for the correct simulations of the planar channeling of positively charged particles in the thickest crystals, supposed to be used for the beam extraction from high energy accelerators, measurement of electromagnetic characteristics of short-living particles and development of intense narrow-band X-ray and gamma radiation sources based on crystal undulators.

Quantum coherence for an atom interacting with an electromagnetic field in the background of cosmic string spacetime

We investigate the behaviors of quantum coherence (QC) for an atom coupled to the fluctuating electromagnetic field under the cosmic string spacetime. It is found that vacuum fluctuation, atomic polarization and nontrivial spacetime topology affect the QC behaviors. When deficit angle parameter $$\nu =1$$ and atom is far away from the string, the results restore to that of free Minkowski spacetime. When atom lies in the string and is transversely polarizable, the atom seems like a closed system, and QC completely is not affected by the electromagnetic fluctuation. For nonzero atom–string distance, QC presents oscillatory behaviors as atom–string distance changes, and larger deficit angle parameter causes more prominent oscillation. In a sense, the string can protect the QC to some extent, which is similar to the boundary effect in Minkowski spacetime. Besides, this atom eventually evolves to an incoherent state, which does not appear to be related to the initial state and other various parameters; this means QC cannot keep for long evolution time in the cosmic string spacetime. In principle, atomic polarization, atom–string distance and deficit angle parameter provide us more freedom to steer the QC behaviors, which might be useful to sense various cosmic string spacetime and distinguish the cosmic string spacetime from Minkowski spacetime.

Quantum entanglement in the background of cosmic string spacetime

We examine the entanglement behaviors for two static atoms coupled to a massless scalar field in the cosmic string spacetime. The entanglement measure is calculated for different cases for two atoms initially prepared in the Werner state. It is found that entanglement behaviors depend on vacuum fluctuation, two-atom distance and nontrivial spacetime topology. When deficit angle parameter $$\nu =1$$ , the evolution model reduces to that of Minkowski spacetime. When two-atom distance is sufficiently small, entanglement can be preserved from the impact of vacuum fluctuation to some extent. Large deficit angle parameter can accelerate destruction of entanglement and shorten lifetime of entanglement. Entanglement presents slight oscillating behavior as atom–string distance varies. A large atom–string distance can retard the entanglement decay and extend entanglement lifetime as a whole. Our exploration may be useful to sense the cosmic string spacetime topology structure and property in principle.

On the influence of periodicity in the arrangement of crystalline atomic strings upon the spectral and spectral-angular distribution of high-energy positively charged particle radiation in crystal

Spectral and spectral-angular distribution of the radiated energy in case of positron motion in an oriented crystal and in case of positron motion in the field of randomly located atomic strings (random strings approximation) when the angle between particle momentum and crystal axes is less than the critical angle of axial channeling were calculated. Obtained results showed that the peak in the emission spectrum is related to the quasi-regular motion of the above-barrier positrons in planar channels.

Infinite family of bc8 -like metastable phases in silicon

We show that new silicon crystalline phases, observed in the experiment with the laser-induced microexplosions inside silicon crystals (Rapp et al. // Nat. Commun. 6, 7555 (2015)), are all superstructures of a disordered high-symmetry phase with $Ia\bar{3}d$ cubic space group, as well as known for many years phases $bc8$ (Si-III) and $r8$ (Si-XII). The physics of this phenomenon is rather nontrivial: The $bc8$-like superstructures appear as regularly ordered patterns of switchable atomic strings, preserving everywhere the energetically favorable tetrahedral coordination of silicon atoms. The variety of superstructures arises because each string can be switched between two states independently of the others. An infinite family of different phases can be obtained this way and a number of them are considered here in detail. In addition to the known $bc8$, $bt8$, and $r8$ crystals, 128 tetrahedral structures with 16 (6 phases), 24 (22 phases), and 32 (100 phases) atoms per primitive cell are generated and studied, most of them are new ones. For the coarse-grain description of the structures with two possible states of switchable strings, the black/white (switched/nonswitched) Shubnikov symmetry groups has been used. An ab initio relaxation of the atomic positions and lattice parameters shows that all the considered phases are metastable and have higher density and energy relative to the $bc8$ phase at the ambient pressure. A possible scenario for appearance of those phases from the high-temperature amorphous phase is discussed.

Atomic origin of the morphological evolution of aluminum hydride (AlH3) nanoparticles during oxidation using reactive force field simulations

Metal hydride nanoparticles exhibit different oxidation dynamic characteristics from bulk ones due to size effects as well as the existence of M-H bonds. We used ReaxFF-lg molecular dynamics to elucidate the dehydrogenation and oxidation mechanisms of aluminum hydride nanoparticle (AHNP). Results show that the morphological evolution of AHNP is induced by temperature and structure, and classified into four shapes: sphere, notched sphere, large branch, and small string. The surface dehydrogenation and oxidation are almost co-occurring. The initial dehydrogenation inhibits the rapid oxidation of the surface layer of AHNP. The increase in temperature induces the instability of the oxide shell and provides sufficient kinetic energy for the hydrogen bubbles. As a result, the oxidation shell ruptures, and the internal hydrogen bubbles physically escape from the nanoparticles. Some simultaneous notches appear in the shell, making its appearance to be branchy at 3000 K. AHNP explodes and ejects small strings of atoms at 3500 K. We also investigated the effect of different temperatures and initial oxygen densities on the morphologies of AHNP. Our results emphasize the complicated interplay between the structural evolution of nanoparticles and environmental conditions. It provides insights into the atomic-level dehydrogenation and oxidation mechanism of metal hydride nanoparticles.

Erratum: Quantum features of high energy particle incoherent scattering in crystals [Phys. Rev. Accel. Beams 22 , 054501 (2019)

The coherent process of particle deflection by aligned atomic strings and planes of oriented crystals is accompanied by incoherent scattering by atomic cores. While the coherent particle deflection, described by the axial or planar averaged potential, becomes more and more classical at high energies, the incoherent scattering of relativistic particles remains essentially quantum. Though the latter reminds the scattering by atoms of amorphous medium at high enough momentum transfers, at the smallest ones the incoherent scattering process in crystals experiences some modification by the influence of the inhomogeneity of the atom distribution in the plane, normal to the crystal axis or plane. Considering the axial case as a more general example, we present a consistent theory of high energy particles incoherent scattering in oriented crystals. The latter takes into consideration both the quantum scattering nature and the atom distribution inhomogeneity, revealing the limited applicability of the scattering cross section notion. The way to incorporate the quantum scattering features into the widely used classical trajectory simulations is elaborated using newly introduced mean square scattering angle definition.

Quantum features of high energy particle incoherent scattering in crystals

The coherent process of particle deflection by aligned atomic strings and planes of oriented crystals is accompanied by incoherent scattering by atomic cores. While the coherent particle deflection, described by the axial or planar averaged potential, becomes more and more classical at high energies, the incoherent scattering of relativistic particles remains essentially quantum. Though the incoherent scattering of relativistic particles in crystals reminds the scattering by atoms of an amorphous medium at a large momentum transfer, at small ones the incoherent scattering process in crystals is modified by the influence of the inhomogeneity of the atom distribution in the plane normal to the crystal axis or plane. We present a theory of incoherent scattering of high energy particles in oriented crystals, which takes into consideration both its quantum nature and the impact of the atom distribution inhomogeneity. The axial case is considered as a more general example. The way to incorporate the revealed quantum features into classical trajectory simulations is also outlined.

Destruction of high- n ( n∼300 ) Rydberg atoms in Rydberg-Rydberg collisions

The destruction of high-$n, n\ensuremath{\sim}300$, strontium atoms contained in a hot atomic beam through Rydberg-Rydberg collisions is examined. The Rydberg atoms are initially created, under blockade conditions, in a localized volume and their subsequent motions lead to creation of a string of Rydberg atoms. The Rydberg atoms, however, are formed with a thermal distribution of velocities resulting in Rydberg-Rydberg collisions which lead to their destruction. The experimental data are interpreted using classical trajectory Monte Carlo techniques that simulate the excitation of the Rydberg atoms together with their subsequent motions. Using calculated collisional ionization cross sections and blockade radii, the model yields results in good agreement with experiment. The results highlight the important role collisional destruction can play when attempting to study (long-range) Rydberg-Rydberg interactions in a hot atomic beam.

A study of small impact parameter ion channeling effects in thin crystals

We have recorded channeling patterns produced by 1 to 2 MeV protons aligned with <111> axes in 55 nm thick silicon crystals which exhibit characteristic angular structure for deflection angles up to and beyond the axial critical angle, psi(a). Such large angular deflections are produced by ions incident on atomic strings with small impact parameters, resulting in trajectories which pass through several radial rings of atomic strings before exiting the thin crystal. Each ring may focus, steer or scatter the channeled ions in the transverse direction and the resulting characteristic angular structure beyond 0.6psi(a) at different depths can be related to peaks and troughs in the nuclear encounter probability. Such radial focusing underlies other axial channeling phenomena in thin crystals including planar channeling of small impact parameter trajectories, peaks around the azimuthal distribution at small tilts and large shoulders in the nuclear encounter probability at tilts beyond psi(a).

On the influence of scattering on atomic strings on the stability of planar channeling of high-energy positively charged particles

The problem of the stability of planar channeling of high-energy positively charged particles in a crystal was studied on the basis of analytical calculation and numerical simulation. The evolution of the spectral density of radiation of particles with a decrease of the angle between particle momentum and atomic strings was investigated. The frequency of the peak in radiation, that corresponds to scattering by individual atomic strings in the case of planar channeling, was found. The analytical calculation was carried out in the parabolic planar potential approximation, while in the numerical simulation the Doyle-Turner approximation was used.

Influence of nuclear multiple scattering on axially channeled protons in a bent crystal

The influence of nuclear multiple scattering on axially channeled protons with an energy of 7 TeV through a bent &lt;100&gt; Si crystal, is presented in this paper. The aims of the investigation are the processes correlated to the axial channeling, such as dechanneling, angular distributions and energy loss distribution. The data for these processes are generated via the numerical solution of the proton equations of motion in the transverse plane and the computer simulation method. In the simulations, the crystal thickness is varied from 1 to 5 mm while the bending angle is varied from 0 to 20 mrad. The increasing of the transverse energy of axially channeled protons is due to its multiple scattering by atomic strings and the bending dechanneling mechanism. The analysis of the generated data shows that in the cases we are considering, the dechanneling function, the energy loss spectra, and the angular distributions do not undergo to any significant changes when the effect of nuclear multiple scattering is included in the ion-atom interactions.

Superconducting fluctuations in one-dimensional quasi-periodic “metallic” chains: the Little model of room-temperature superconductivity embodied

It is well known that a purely periodic chain of odd-electron atoms, nominally expected to exhibit metallic behavior, is unstable to charge/spin spatial displacement which lowers its ground-state energy by gapping its highly degenerate Jahn–Teller Fermi surface, in this case consisting of nesting parallel Brillouin zone sheets. It is largely for these reasons that superconductivity has not been observed in highly one-dimensional metals—it is simply energetically more favorable for CDW/SDW gaps to form, via chain dimerization, rather than a BCS state, at the very least one mediated by electron–phonon coupling. In this paper, we explore the hypothetical electronic properties of a nominally “metallic” quasi-periodic chain using both an analytical approach and computationally via density functional theory, searching for configurations which could possibly yield “gap-lets” sufficiently small to permit the formation of BCS pairs as the new energetically favored ground state. The particular embodiment we examine is a proxy structure consisting of a string of odd-electron atoms with interatomic spacing following a Fibonacci sequence, positioned above the surface of an appropriate highly polarizable material substrate. We propose a path to its computational modeling followed by a route to synthesis of such a structure for experimental examination, and thus perhaps leading to an entirely new class of near-room-temperature superconductors.

Scattering of ultrarelativistic electrons in ultrathin crystals

Quantum theory is proposed of high energy electrons scattering in ultrathin crystals. This theory is based upon a special representation of the scattering amplitude in the form of an integral over the surface surrounding the crystal, and on the spectral method of determination of the wave function. The comparison is performed of quantum and classical differential scattering cross-sections in the transitional range of crystal thicknesses, from those at which the channeling phenomenon is not developed up to those at which it is established. It is shown that in this thickness range the quantum scattering cross-section, unlike the classical one, contains sharp peaks corresponding to some specific scattering angles, that is connected with the diffraction of the incident plane wave onto the periodically distributed crystal atomic strings. It is shown that the value of the scattering cross-section in the peaks varies periodically with the change of the target thickness. We note that this must lead to a new interference effect in radiation that is connected with the rearrangement of incident wave packet in transitional area of crystal thicknesses.

On the influence of crystal structure on the electromagnetic shower development in the lead tungstate crystals

The development of high-energy electromagnetic showers in long oriented lead tungstate crystals, accelerated by the effects induced by the strong field of atomic strings, is simulated for the first time. For that the characteristics of pair production and gamma-radiation by electrons or positrons were first simulated by the direct application of Baier-Katkov formulae in a thin PWO crystal to derive the scaling coefficients of the corresponding Bethe-Heitler cross sections to be incorporated into GEANT4 for the simulation of the electromagnetic shower development in a long crystal. Simulation results demonstrate the significant influence of the crystal structure on the e± and gamma-quanta registration processes in the existing homogeneous electromagnetic calorimeters and gamma-telescopes as well as wide possibilities of improving their performance in future developments.

Coherent Backscattering of Light Off One-Dimensional Atomic Strings.

We present the first experimental realization of coherent Bragg scattering off a one-dimensional system-two strings of atoms strongly coupled to a single photonic mode-realized by trapping atoms in the evanescent field of a tapered optical fiber, which also guides the probe light. We report nearly 12% power reflection from strings containing only about 1000 cesium atoms, an enhancement of 2 orders of magnitude compared to reflection from randomly positioned atoms. This result paves the road towards collective strong coupling in 1D atom-photon systems. Our approach also allows for a straightforward fiber connection between several distant 1D atomic crystals.

Wave functions of channeling electrons in regular and chaotic cases

The spectral method applicability to the problem of the charged particles axial channeling is studied. The quantum state characteristics for the electron motion near the atomic string [110] direction of the silicon crystal are obtained using this method of Schrödinger equation numerical solution. The full set of eigenenergies and eigenfunctions for the electron with longitudinal energy of 5 MeV is computed. The results obtained illustrate the difference in the wave function morphology for regular and chaotic cases.