Atoms In Sublattice Research Articles

A simple tight-binding approach to topological superconductivity in monolayer MoS2

Monolayer molybdenum disulfide (MoS2) has a honeycomb crystal structure. Here, with considering the triangular sublattice of molybdenum atoms, a simple tight-binding Hamiltonian is introduced (derived) for studying the phase transition and topological superconductivity in MoS2 under uniaxial strain. It is shown that spin-singlet p + ip wave phase is a topological superconducting phase with nonzero Chern numbers. When the chemical potential is greater (smaller) than the spin–orbit coupling (SOC) strength, the Chern number is equal to four (two) and otherwise it is equal to zero. Also, the results show that, if the superconductivity energy gap is smaller than the SOC strength and the chemical potential is greater than the SOC strength, the zero energy Majorana states exist. Finally, we show that the topological superconducting phase is preserved under uniaxial strain.

Atomic sublattices and basic derivatives in finance

Suppose that E is a vector lattice where the ordering and the lattice operations in E are defined pointwise by a countable family $${\mathcal {F}}=\{f_i|i\in {{\mathbf {N}}}\}$$ of positive linear functional of E and Z is a sublattice of E. Based on algebraic and order properties of E we give necessary and sufficient conditions in order Z to be atomic. Especially we show the existence of a basic sequence $$\{b_n\}$$ of extremal points (atoms) of $$Z_+$$ so that for any $$x\in Z_+$$ a unique sequence $$({\widehat{x}}(n))$$ of real components of x with respect to $$\{b_n\}$$ exists so that $$x=\sup \{{\widehat{x}}(n)b_n\;|\;n\in {{\mathbf {N}}}\}$$ and also $$x=sup_{n}\sum _{i=1}^n{\widehat{x}}(i)b_i$$ . These results give an answer to the problem of the existence of basic derivatives in financial markets.

Spin-glass freezing in single-crystalline Pr2NiSi3

A single crystal of Pr2NiSi3 was examined by means of x-ray diffraction, magnetization, ac susceptibility, heat capacity and electrical resistivity measurements performed in wide ranges of the temperature and external magnetic fields. The compound crystallizes with simple hexagonal AlB2-type unit cell with crystallographic disorder in the non-magnetic atom sublattice. The results indicate formation at low temperatures of a spin-glass state with characteristic freezing temperature Tf=3.2K, which most likely originates from intrinsic crystallographic disorder in the AlB2-type unit cell.

Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

Ferromagnetism in graphene traced to antisymmetric orbital combination of involved electronic states

Based on first principles calculations, we reveal that the origin of ferromagnetism caused by electrons in graphene with vacancies can be traced to electrons partially filling -antibonding and -nonbonding states, which are induced by the vacancies and appear near the Fermi level. Because the spatial wavefunctions of both states are composed of atomic orbitals in an antisymmetric configuration, their spin wavefunctions should be symmetric according to the electron exchange antisymmetric principle, leading to electrons partially filling these states in spin polarization. Since this state originates not from interactions between the atoms but from the unpaired orbitals due to the removal of orbitals on the minority sublattice, the state is constrained, distributed on the atoms of the majority sublattice, and decays gradually from the vacancy as ∼. According to these characteristics, we concluded that the state plays a critical role in magnetic ordering in graphene with vacancies. If the vacancy concentration in graphene is large enough to cause the decay-length regions to overlap, constraining the orbital components as little as possible on the minority sublattice atoms in the overlap regions results in the vacancy-induced states being coherent. The coherent process in the overlap region leads to the wavefunctions in all the involved regions antisymmetrized, consequently causing ferromagnetism according to the electron exchange antisymmetric principle. This unusual mechanism concerned with the origin of -electron magnetism and magnetic ordering has never before been reported and is distinctly different from conventional mechanisms. Consequently, we can explain how such a weak magnetization with such a high critical temperature can be experimentally observed in proton-irradiated graphene.

Developing and programming the algorithm of refinement of the crystal structure of materials with possible isomorphous substitution

In general, the software for analyzing data using the X-ray diffraction method does not include the possibility for using stoichiometric principles between chemical elements and the relations between the occupation of crystallographic positions by atoms. In the article, the algorithm and its program realization for defining the distribution of atoms according to the crystallographic positions using stoichiometric principles in materials with isomorphous substitution are developed. A combination of using the developed algorithm and the FullProf program is proposed for taking into account different conditions that should be satisfied by the distribution of atoms according to the crystallographic positions. It is proposed to estimate the unambiguity of the initially defined distribution of atoms by finding local minima in certain physically substantiated limits of changes in the parameters of the structure. The complex method for minimization of a function of the deviation of the theoretically calculated diffractograms from the experimental ones is given to avoid falling of the objective function to a local minimum. Two ways for minimization of the difference between theoretically calculated and experimental diffractograms are proposed. By the first of them, with the help of the developed algorithm, the occupation of crystallographic positions can be established, and the minimization method built-in in FullProf calculates all other parameters. By the other way, the developed algorithm is used only and the rest of parameters approximately calculated by FullProf before are fixed. The efficiency of the developed algorithm is illustrated by finding the distribution of atoms in sublattices in ferrite-spinels. The developed algorithm can be used for any materials in which isomorphous substitution is possible, such as spinels, garnets, perovskites, and others

Giant magneto-optical Faraday effect of graphene on Co in the soft x-ray range

Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon $1s$ edge of single-layer graphene on Co, reaching Faraday rotation angles of $2.9\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.16em}{0ex}}\mathrm{deg}/\mathrm{mm}$. This value is of the order of those observed at the Co $3p$ and $2p$ edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned \ensuremath{\pi} orbitals of carbon atoms. It is induced by hybridization with the Co $3{d}_{z}2$ orbitals while carbon \ensuremath{\sigma} bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of $0.14\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon ${p}_{\mathrm{z}}$ and metal $3d$ orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co $3{{d}_{z}}^{2}$ orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with $3{d}_{\mathrm{xz}}$ and $3{d}_{\mathrm{yz}}$ orbitals of neighboring Co atoms forming tilted ${p}_{\mathrm{z}}$ bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.

Proton dynamics and the phase diagram of dense water ice.

All the different phases of water ice between 2 GPa and several megabars are based on a single body-centered cubic sub-lattice of oxygen atoms. They differ only by the behavior of the hydrogen atoms. In this study, we investigate the dynamics of the H atoms at high pressures and temperatures in water ice from first-principles molecular dynamics simulations. We provide a detailed analysis of the O-H⋯O bonding dynamics over the entire stability domain of the body-centered cubic (bcc) water ices and compute transport properties and vibrational density-of-states. We report the first ab initio evidence for a plastic phase of water and we propose a coherent phase diagram for bcc water ices compatible with the two groups of melting curves and with the multiple anomalies reported in ice VII around 15 GPa.

Mobility gap and quantum transport in a functionalized graphene bilayer

In a Bernal graphene bilayer, carbon atoms belong to two inequivalent sublattices A and B, with atoms that are coupled to the other layer by bonds belonging to sublattice A and the other atoms belonging to sublattice B. We analyze the density of states and the conductivity of Bernal graphene bilayers when atoms of sublattice A or B only are randomly functionalized. We find that for a selective functionalization on sublattice B only, a mobility gap of the order of 0.5 eV is formed close to the Dirac energy at concentration of adatoms . In addition, at some other energies conductivity presents anomalous behaviors. We show that these properties are related to the bipartite structure of the graphene layer.

Кристаллографический анализ строения LaBiI6·13H2O и NdBiI6·13H2O. Подрешетка атомов иода — общий "скелет" структур

Кристаллографический анализ строения LaBiI6·13H2O и NdBiI6·13H2O. Подрешетка атомов иода — общий "скелет" структур

Synthesis and Stability of Lanthanum Superhydrides.

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. X-ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH10 having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H-H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.

Synthesis and Stability of Lanthanum Superhydrides

AbstractRecent theoretical calculations predict that megabar pressure stabilizes very hydrogen‐rich simple compounds having new clathrate‐like structures and remarkable electronic properties including room‐temperature superconductivity. X‐ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH10 having cages of thirty‐two hydrogen atoms surrounding each La atom. Upon decompression, the fcc‐based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H−H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.

Electron and phonon states localized near the graphene boundary

We perform analytical and numerical analysis of the electronic and phonon spectrum evolution of graphene during formation of a boundary with a “zigzag” chirality. It is determined, that the excited gap wave has a relativistic dispersion near the Fermi level that propagates along the boundary and decays with distance from it. Both properties and formation of the wave is considered. It is shown that the wave propagation occurs only along the atoms of the sub-lattice, which contains atoms with bonds broken during the boundary formation. The gap wave forms narrow resonance peaks in the local density of states of the sublattice atoms. It is shown, that the boundary formation on a graphene layer with this chirality similarly affects the phonon modes polarized normal to the layer, forming narrow maxima with frequencies nearing that of the quasiflexural phonons with the quasiwave vector at the K-point of the first Brillouin zone. This way, the formation of the “zigzag”-boundary increases both the number of charge carriers as well as the number of phonons with a large group velocity that can cause a large contribution to the electron-phonon interactions.

Evaluation of the f-electron rare-earth copper telluride GdCu1+xTe2 as a thermoelectric material

Chalcogenide compounds of composition ABX2 (with A, B, representing metal elements, X as the chalcogenide) have recently been attracting attention as thermoelectric materials. Possible magnetic enhancement of the thermoelectric properties has been proposed for electron-doped chalcopyrite CuFeS2 which exhibits large power factors at room temperature. Promising ZT values close to 1 have been reported for TmAgTe2 or YCuTe2 due to very low thermal conductivities (< 0.5Wm−1K−1 at 800K) resulting from a disordered sublattice of Cu atoms. To further explore the possibilities of the ABX2 compounds, we study GdCu1+xTe2 which is a monoclinic layered compound that crystallizes in the C2/m space group, and has the f-electron Gd as a magnetic element, versus d-electron Fe in CuFeS2. Although the stoichiometric compound could not be stabilized, we report on the properties of this series of compounds sintered by spark plasma sintering (SPS). As for the other materials of ABX2 type, compositions with x > 0.25 show very low thermal conductivities (0.64Wm−1K−1 above 500K for x = 0.25), which, together with a Seebeck coefficient above 0.2mVK−1, allows to reach a moderate ZT value near 0.2 at 540K. We also discuss the dependence with the Cu excess content of the transport properties and the lack of effect of the magnetic moment of Gd on the Seebeck coefficient.

Defect structure and ionic conductivity of as-grown R 1–y Sr y F3–y (R = Ce, Pr, or Nd) crystals with high SrF2 content

The structure of defect nonstoichiometric phases of Ce0.88Sr0.12F2.88, Pr0.85Sr0.15F2.85, and Nd0.85Sr0.15F2.85 single crystals has been investigated by X-ray diffraction at room temperature. Crystals R 1–ySryF3–y (R = Ce, Pr, or Nd) belong to the tysonite structural type (LaF3), which has two forms (α and β). The morphotropic transition from the trigonal β form of the crystals to the hexagonal α form is confirmed, which was previously found for the first time when analyzing the nonstoichiometric phase of La1–ySryF3–y with a SrF2 content above 10 mol % (y ≥ 0.1). Temperature dependences of fluorine-ion conductivity are obtained. It is established that the conductivity of the α form of Ce0.88Sr0.12F2.88 and R0.85Sr0.15F2.85 crystals (R = Pr or Nd) is lower than that for the β form of R0.95Sr0.05F2.95 (R = La–Nd) by 1–2 orders of magnitude. The sublattice of fluorine atoms in the α-form crystals is characterized by an elevated (in comparison with the β form) content of vacancies and more uniform structural and dynamic properties, which leads to a decrease in the mean diffusion mobility of fluorine ions and an increase in the ion-transport activation enthalpy.

Use of Wyckoff position splittings in the supercell model of crystals with point defects

The atomic sublattices occupying different Wyckoff positions in a host crystal are shown to be subdivided in the supercell model owing to the splitting of the occupied Wyckoff positions. The site symmetries of the split Wyckoff positions are in general different, which significantly increases the number of possible occupation schemes for impurities and defects in the supercell model. It is demonstrated that the use of the programs and retrieval tools of the Bilbao Crystallographic Server considerably simplifies the group-theoretical analysis of Wyckoff position splittings in the supercell model of a crystal with a point defect. Ab initio calculations of electronic states of single defects in ZnO (Zn and O neutral vacancies) within the supercell model are performed to demonstrate the influence of Wyckoff position splittings on the results.

A Model Study of Substrate Effect on the Neutron-Scattering Spectra of Ferromagnetic Ordered Graphene

Graphene as a possible new magnetic material has stimulated research interest for application in spintronic devices. Pristine graphene does not show magnetism due to equivalence of the two sub-lattices of carbon atoms. It is predicted that impurity effects, substrate-induced gap, and defects, are expected to produce in-equivalence in two carbon sub-lattices leading to magnetism. The spins on the same sub-lattices are found to exhibit ferromagnetic order and spins of different sub-lattices are found to exhibit anti-ferromagnetic order. We consider here impurity and substrate effects as the cause of generation of magnetism in graphene. We report here a microscopic tight-binding study of frequency-dependent neutron scattering spectra for ferromagnetic ordering in the graphene systems. The tight-binding Hamiltonian consists of electron hoppings upto third nearest neighbors and substrate and impurity effects in the presence of Coulomb interaction of electrons separately at two in-equivalent A and B sub-lattices of graphene. We calculate the two-particle electron Green’s functions by using Zubarev’s Green’s function technique. The frequency-dependent scattering intensity of the system is computed numerically. The spectra displays a sharp peak at the neutron momentum transfer energy at low energies and another higher-energy peak appearing at substrate-induced gap.

Magnetic properties of RCo2 compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo2 ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (ТС), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > ТС. For an ErСо2 sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.

Polarized optical absorption of MnGa2S4 single crystals

Optical absorption of MnGa2S4 single crystals is studied at two light polarizations (E ||C and E⊥ C). The polarization splitting of the absorption edge points to a splitting of the valence band of MnGa2S4. A contribution to the crystal-field splitting is made by two factors, namely, by a difference in the pseudopotential of cationic sublattice atoms and by tetragonal compression of the lattice along the C axis. A scheme of optical transitions in MnGa2S4 in the Brillouin zone center is suggested, according to which the optical transitions Г3 + Г4 → Г1 occur in the polarization E ⊥ C, and the Г2 → Г1 transitions occur in the polarization E || C.

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.