Atoms In Sublattice Research Articles

Unique Domain Structure of Two-Dimensional α-Mo2C Superconducting Crystals.

The properties of two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides are strongly influenced by domain boundaries. Ultrathin transition metal carbides are a class of newly emerging 2D materials that are superconducting and have many potential applications such as in electrochemical energy storage, catalysis, and thermoelectric energy conversion. However, little is known about their domain structure and the influence of domain boundaries on their properties. Here we use atomic-resolution scanning transmission electron microscopy combined with large-scale diffraction-filtered imaging to study the microstructure of chemical vapor deposited high-quality 2D α-Mo2C superconducting crystals of different regular shapes including triangles, rectangles, hexagons, octagons, nonagons, and dodecagons. The Mo atom sublattice in all these crystals has a uniform hexagonal closely packed arrangement without any boundaries. However, except for rectangular and octagonal crystals, the C atom sublattices are composed of three or six domains with rotational-symmetry and well-defined line-shaped domain boundaries because of the presence of three equivalent off-center directions of interstitial carbon atoms in Mo octahedra. We found that there is very small lattice shear strain across the domain boundary. In contrast to the single sharp transition observed in single-domain crystals, transport studies across domain boundaries show a broad resistive superconducting transition with two distinct transition processes due to the formation of localized phase slip events within the boundaries, indicating a significant influence of the boundary on 2D superconductivity. These findings provide new understandings on not only the microstructure of 2D transition metal carbides but also the intrinsic influence of domain boundaries on 2D superconductivity.

Various types of photoactive behavior of the Y3Fe5O12 ferrimagnetic insulator

The photomagnetic behavior of single-crystal yttrium iron garnet Y3Fe5O12 doped with iridium, substituting the cation of iron in the octahedron, is investigated upon illumination at room temperature. It is shown that the photomagnetic properties of Y3Fe4.97Ir0.03O12 samples are to a large degree related to the impurity distortion of the sublattice of iron atoms in octahedral coordination, rather than solely to the possible presence of Fe4+ cations, which are inactive at room temperature and may even be lacking in single crystals doped with iridium. It is concluded that the photoinduced change in the magnetic parameters of this material is determined by the location of impurity cations and increased surface imperfection of the material. The reasons for the different photoactive behavior of this promising material for spintronics, that is, a singlecrystal yttrium iron garnet, are summarized.

Charge and Paramagnetic Spin Susceptibilities of Doped Gapped Graphene-Like Structures

We address spin polarization dependence of dynamical charge and spin susceptibilities of doped gapped graphene-like structures in the context of tight binding model Hamiltonian. The frequency behavior of both longitudinal and transverse spin susceptibilities has been calculated via calculating correlation function of spin density operators. Our results show the increase of electronic concentration corresponding to chemical potential leads to enhance the intensity of charge spectral function. We also show that longitudinal spin susceptibility reduces at fixed frequency with gap parameter associated with on-site energy difference between two types of sublattice atoms. Furthermore, the resonance peak in longitudinal spin susceptibility goes to higher frequencies with gap parameter. The effect of magnetization on the longitudinal spin susceptibility indicates two different behaviors depending on frequency region. Finally, the effects of magnetization and gap parameter on the frequency behavior of transverse spin susceptibility have been studied in details.

Crystal-forming functions of glide planes in the structural organization of a number of (Pb, Bi)-containing sulfides

The reasons for the preference of glide planes in natural crystals are considered by studying the results of an X-ray crystallography analysis of the minerals salzburgite, eldragonite, izoklakeite, and vikingite. The glide planes organize the combination of right- of atomic fragments, which appear during crystallization, and contribute to a uniform distribution of atoms in complex compositions and a more dense and symmetrical packing. The fact that the structures have cation and anion sublattices with similar dimensions and orientations creates a situation similar to that of beats in oscillatory systems and, hence, modulates the intensity of X-ray reflections. The progressively increasing number of structures where individual atomic positions are occupied by two or more types of atoms suggests a stable geometry of the corresponding atomic sublattices with respect to compositional variations. Analysis of these “skeletal” sublattices is a key to understanding the solid-phase transformations, contact interactions, and dynamic processes in mineral associations.

Spin effects in MoS2 and WS2 single layers

magnified imageReplacing the two sublattices of carbon atoms in graphene with transition metal atoms and chalcogenide dimers results in single layers of transition metal dichalcogenides (TMDCs). TMDCs are promising new materials for light and energy harvesting, transistors, sensors and quantum information processing. One way to access the distinctive functionality of these materials is via their optical selection rules. In particular, light with positive or negative helicity is absorbed differently, therefore, understanding the interaction of circularly polarized light with various TMDCs should enable future applications. Using the examples of MoS2 and WS2 we summarize some recent results that illustrate the potential of these materials. First, when optically excited with circularly polarized light, single layers of MoS2 can emit light with an appreciable polarization.Depolarization mechanisms can be subsequently explored by monitoring the polarization of emitted photoluminescence as a function of the excess energy supplied to the system. As the energy of the pumping light increases further from the emission channel, the emission quickly becomes depolarized. The dominant relaxation mechanism is identified as phonon‐assisted intervalley scattering. In single layers of WS2 containing electron gas, the main emission channel is from negatively charged excitons, or trions. In the presence of a two‐dimensional electron gas this trion emission is circularly polarized at zero magnetic field, even when excited with linearly polarized light. This spontaneous circular polarization of the trion has a linear dependence on magnetic field and can be attributed to the existence of a valley polarized state of the two‐dimensional electron gas. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Annealing-induced effects on structural and optical properties of Cd1−xZnxS thin films for optoelectronic applications

AbstractCd1−xZnxS thin films of variable compositions (x = 0.2, 0.4, 0.6, 0.8) were deposited on glass at room temperature by thermal evaporation process. The prepared samples were annealed at two different temperatures (300 °C, 400 °C) for 1 hour in ambient air. The effects of post-annealing on the structural and optical characteristics were investigated using X-ray diffraction (XRD), spectrophotometry, and Raman spectroscopy (RS) methods. XRD studies suggested that the annealed and as-deposited samples belong to wurtzite structure for all Zn concentrations with a preferential orientation along (002) plane. Spectrophotometry analysis of the samples revealed that the energy band gap decreased with annealing temperature. RS investigated different phonon bands and crystalline phases. Two longitudinal optical phonon modes (1-LO, 2-LO) corresponding to monophase hexagonal structure were observed for all Cd1−xZnxS samples. The observed red-shift and anti-symmetrical nature of the 1-LO phonon mode can be associated with annealing, while the overall blue-shift, except for x ⩽ 0.6, was caused by the structural disorders in periodic Cd atomic sub-lattices and broken translational symmetry. The spectroscopic results were strengthened by the XRD studies and their results are consistent.

Role of the tuning parameter at magnetic quantum phase transitions

Heavy-fermion systems, with their competition between Kondo and RKKY interactions, offer a rich variety of materials that may be driven to a magnetic quantum phase transition. Quite often, a quantum critical point can be approached by chemical substitution, notably of isoelectric ligands of Ce, as in CeCu6−x Au x and CePd1−x Ni x Al. While in the former we compare pressure and concentration tuning of the magnetic structure, the latter has the additional feature of geometric frustration due to the distorted kagome sublattice of Ce atoms in the basal plane. We further present the system CeAu2Ge2 where minor structural differences between crystals grown from Sn or Au-Ge flux lead to pronounced differences in the magnetic structure, with several field-induced phases in samples grown from Au-Ge flux. Finally, non-isoelectronic substitution of Ti by V is studied in CeTi1−x V x Ge3 where CeTiGe3 is a ferromagnet, thus allowing the study of ferromagnetic quantum criticality, a rare case for heavy-fermion systems.

Inclusion of the correlation short-range order in Ab initio calculations of the energy of the ground state by example of titanium monoxide TiO1.0

A variant of the distribution of structural vacancies at which the crystal structure of ordered compounds simultaneously has a long-range order and correlation short-range order has been proposed and analyzed. The long-range order determines the fraction of vacancies in the atomic and vacancy sublattices of the superstructure formed because of ordering. The correlation short-range order takes into account energy favorable correlations in the arrangement of vacancies, which exist in disordered phases and are not determined by the long-range order. The possibility of the correlation short-range order has been examined by ab initio methods by example of ordering of vacancies in titanium monoxide TiO1.0 according to the type of monoclinic superstructure M5X5mon. It has been shown that partially ordered modifications with the correlation short-range order are energetically favorable as compared to the modifications without the correlation short-range order.

Lattice vibrations in the Frenkel-Kontorova model. I. Phonon dispersion, number density, and energy

We studied the lattice vibrations of two inter-penetrating atomic sublattices via the Frenkel-Kontorova (FK) model of a linear chain of harmonically interacting atoms subjected to an on-site potential, using the technique of thermodynamic Green's functions based on quantum field-theoretical methods. General expressions were deduced for the phonon frequency-wave-vector dispersion relations, number density, and energy of the FK model system. In addition, as the application of the theory, we investigated in detail cases of linear chains with various periods of the on-site potential of the FK model. Some unusual but interesting features for different amplitudes of the on-site potential of the FK model are discussed. In the commensurate structure, the phonon spectrum always starts at a finite frequency, and the gaps of the spectrum are true ones with a zero density of modes. In the incommensurate structure, the phonon spectrum starts from zero frequency, but at a non-zero wave vector; there are some modes inside these gap regions, but their density is very low. In our approximation, the energy of a higher-order commensurate state of the one-dimensional system at a finite temperature may become indefinitely close to the energy of an incommensurate state. This finding implies that the higher-order incommensurate-commensurate transitionsmore » are continuous ones and that the phase transition may exhibit a “devil's staircase” behavior at a finite temperature.« less

Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study

Because of the advantages of tunability via size, shape, doping, and relatively low level of loss and high extent of spatial confinement, graphene quantum dots (GQDs) are emerging as an effective way to control light by molecular engineering. The collective excitation in GQDs shows high energy plasmon frequency along with frequencies in the terahertz (THz) region, making these systems powerful materials for photonic technologies. Here, we report a systematic study of the linear and nonlinear optical properties of large varieties of GQDs (∼400 systems) in size and topology utilizing the strengths of both semiempirical and first-principles methods. Our detailed study shows how the spectral shift and trends in the optical nonlinearity of GQDs depend on their structure, size, and shape. Among the circular, triangular, stripe, and random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always possess lower HOMO–LUMO gap, broadband absorption, and high nonlinear optical coefficients. Also, we f...

Domains of the phases V8C7 and V3C2 in bulk carbide VC y

The formation of cubic (V8C7) and orthorhombic (V3C2) superstructures at the upper and lower boundaries of the homogeneity interval of nonstoichiometric vanadium carbide VCy has been studied. It has been found that the composition of the ordered phase V8C7 − δ deviates from the composition of the ideal superstructure V8C7 because of the reduced filling factor of 4(a) site of the nonmetal sublattice of carbon atoms. Vanadium atoms forming the octahedral environment □V6 of vacant sites in the phases V8C7 and V3C2 are displaced toward a vacancy. Early stages of the formation of domains of ordered phases have been studied. The effect of the quenching and annealing temperatures on the dimension of domains has been determined. In bulk vanadium carbide, the formation of domains of ordered phases begins at the grain boundaries of disordered carbide VCy and results in the appearance of a domain nanostructure.

Soliton microdynamics of the generation of new-type nonlinear surface vibrations, dissociation, and surfing diffusion in diatomic crystals of the uranium nitride type

The microdynamics of large-amplitude nonlinear vibrations of uranium nitride diatomic lattices has been investigated using the computer simulation and neutron scattering methods at temperatures T = 600–2500°C near the thresholds of the dissociation and destruction of the reactor fuel materials. It has been found using the computer simulation that, in the spectral gap between the frequency bands of acoustic and optical phonons in crystals with an open surface, there are resonances of new-type harmonic surface vibrations and a gap-filling band of their genetic successors, i.e., nonlinear surface vibrations. Experimental measurements of the slow neutron scattering spectra of uranium nitride on the DIN-2PI neutron spectrometer have revealed resonances and bands of these surface vibrations in the spectral gap, as well as higher optical vibration overtones. It has been shown that the solitons and bisolitons initiate the formation and collapse of dynamic pores with the generation of surface vibrations at the boundaries of the cavities, evaporation of atoms and atomic clusters, formation of cracks, and destruction of the material. It has been demonstrated that the mass transfer of nitrogen in cracks and along grain boundaries can occur through the revealed microdynamics mechanism of the surfing diffusion of light nitrogen atoms at large-amplitude soliton waves propagating in the stabilizing sublattice of heavy uranium atoms and in the nitrogen sublattice.

Influence of concentration and position of carboxyl groups on the electronic properties of single-walled carbon nanotubes.

The effects of attaching COOH groups at different sites and in various concentrations on electronic and structural properties of (8,0) single-walled carbon nanotubes (SWNT) were investigated using ab initio calculations. The binding energies and the charge transfers between the COOH functional groups and the tube were calculated for several configurations and a novel feature in the electronic structure of these groups was observed. The electronic character of these systems can be modulated by playing with the concentration and the position of the carboxyl groups bonded on the tube wall. The carboxyl groups bound to different carbon atom sub-lattices are more hybridized than those bound in the same one. These results suggested that SWNT-COOH systems are a playground for engineering electronic properties through a proper chemical functionalization which exploit both the attachment site and concentration of functional groups.

Atomic sublattice decomposition of piezoelectric response in tetragonalPbTiO3,BaTiO3, andKNbO3

The piezoelectric properties of tetragonal perovskites $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$, $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, and $\mathrm{K}\mathrm{Nb}{\mathrm{O}}_{3}$ under uniaxial strain along the [001] direction are calculated from first principles. In order to study the piezoelectric response of individual atomic sublattices, the total polarization $P$ and piezoelectric constant ${d}_{33}$ are decomposed into the contributions of $A$-site and $B$-site atoms. These three materials show different behaviors at the atomic scale. In $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$, the Pb and Ti contribute almost equally to the polarization and ${d}_{33}$. In $\mathrm{K}\mathrm{Nb}{\mathrm{O}}_{3}$, most of the polarization and ${d}_{33}$ are contributed by the ferroelectrically active Nb. But in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$, as the $c$ axis increases, the contribution of Ba to ${d}_{33}$ rises rapidly, reaching 50% of the total ${d}_{33}$ at $c$ = 4.30 \AA{}. This means that the covalent bonding between Ba and O is significant and has an important impact on the piezoelectric response of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$.

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni70Al25Mo5 (composition 1) and Ni75Al20Mo5 (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ∼7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L12) at T ∼ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni3 Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.

Synthesis and crystal structure of silver–gold sulfide AgAuS. Four-fold interpenetrated three-dimensional [(Au,Ag)10S8]-networks

Crystals of the silver(I) thioaurate(I), AgAuS, were obtained by fusing the elements in stoichiometric amounts. The X-ray diffraction analysis showed that the synthesized phase AgAuS represents a new structural type. The unit cell parameters are a = 13.4235(3), c = 9.0873(4) A, V = 1418.07(6) A3 and Z = 24, and the space group is Rm. The sublattice of sulfur atoms has a strongly distorted body-centered cubic packing. All Au atoms are in a typical linear coordination with sulfur ones. Three-quarters of the silver atoms are surrounded by distorted tetrahedra of sulfur atoms, while the rest of the silver atoms are involved in a linear motif typical of Au atoms as well. The resulting motif is composed of four-fold interpenetrating networks of linearly coordinated metals with 2- and 4-connected sulfur atoms: [Au8[2]Ag2[2]S6(2)S2(4)]. The formula of AgAuS can be represented as Ag24[4][Au8[2]Ag2[2]S8]4. The features of the AgAuS structure suggest the possibility of wide variation in the Au : Ag ratio.

Research of Unary Lattice Implication Algebra Equations on Interval Sets

The concepts are re-defined on the interval sets which are filter, prime filter, LI-ideal, dual atom and convex sub-lattice in the lattice implication algebra. Three basic unary lattice implication algebra equations on the interval sets are researched. The necessary and sufficient conditions for existence of solutions for the equations are presented. And some properties of equation sets also are given.

The role of magnetization compensation point for efficient ultrafast control of magnetization in Gd24Fe66.5Co9.5 alloy

The ability of a femtosecond laser pulse to manipulate and reverse the magnetization in a ferrimagnetic Gd24Fe66.5Co9.5 thin film was studied experimentally as a function of temperature. For a fixed energy of the laser pulse, the dynamics of magnetization showed different behavior depending on whether the sample temperature was below or above the magnetization compensation point (TM). The conditions for full ultrafast demagnetization and magnetization reversal were easily achieved below TM, while the same laser excitation caused just 50% demagnetization above TM. This interesting change in magnetization dynamics is qualitatively explained in terms of effective changes in the magnitudes of magnetizations of atomic sublattices.

Nonmonotonic temperature dependence of critical current in superconducting Fe(Se0.3Te0.7)0.82 crystals

A nonmonotonic temperature dependence of the critical current in layered crystals of iron-based superconducting Fe(Se0.3Te0.7)0.82 solid solution has been observed in the direction perpendicular to layers. This behavior is caused by the alternating sign of the superconducting current, which is due to an interaction between the electron spin in Cooper pairs and the magnetic exchange related to the sublattice of interstitial iron atoms. A change in the direction of the applied voltage leads to enhancement of the Josephson current.

Pseudosymmetric Features and Nonlinear Optical Properties of Potassium Titanyl Phosphate Crystals

A number of publications containing structural data, characteristics of nonlinear optical properties of pure and doped crystals of potassium titanyl phosphate (KTP) family have been reviewed to analyze the structural and symmetry conditionality of nonlinear optical properties of these crystals. The pseudosymmetric features of KTP-type crystals with respect to inversion are investigated. Specifically, pseudoinversion distribution maps are calculated; pseudoinversion extrema and coordinates of pseudoinversion centres are found; and the distributions of pure and doped KTP-type structures and their individual atomic sublattices over the degree of pseudoinversion are analyzed. A correlation between the characteristics of nonlinear optical properties of a number of crystals belonging to the KTP family and the degree of pseudoinversion of their atomic structures is demonstrated.