Three-dimensional Character Research Articles

The self-consistent renormalization theory of spin fluctuations for itinerant antiferromagnetism in quasi-one-dimensional metals

We introduce a three-dimensional character to the transfer of one-dimensional conduction electrons within the self-consistent renormalization theory of spin fluctuations for itinerant antiferromagnetism. We study how the three-dimensional character influences the Néel temperature and the temperature dependence of the inverse staggered susceptibility at low temperatures. From these investigations, we find that the Néel temperature and the temperature dependence of the inverse staggered susceptibility are controlled by the three-dimensional character.

Modelling Sieve Tray Hydraulics Using Computational Fluid Dynamics

The ability of computational fluid dynamics (CFD) to model the complex two-phase hydrodynamics of sieve trays is examined. The key to a proper description of the flow is the estimation of the momentum exchange, or drag, coefficient between the gas and liquid phases. In the absence of sound theoretical models, empirical correlations for the average gas fraction on the tray, such as those of Bennet, Agrawal and Cook [AIChEJ, 29 (1983): 434], can be used to estimate the drag coefficient. Transient simulations of sieve trays of 0.3 and 0.9 m in diameter, operating in the bubbly flow regime, reveal the chaotic, three-dimensional character of the flow and the existence of circulation patterns in all three dimensions. The CFD simulations underline the limitations of simpler approaches wherein the flow is assumed to be two-dimensional or where the interaction of the liquid phase with the gas phase is either ignored completely or simplified greatly. The major advantage of the CFD approach is that geometry and scale effects are properly encapsulated and do not require further inputs. It is concluded that CFD can be a powerful investigative and design tool for sieve tray columns.

Dynamics of a matter-wave bright soliton in an expulsive potential

The stability regimes and nonlinear dynamics of bright solitons created in a harmonic potential which is transversely attractive and longitudinally expulsive are presented. This choice of potential is motivated by the recent creation of a matter-wave bright soliton from an attractive Bose-Einstein condensate (L. Khaykovich {\it et al.}, Science 296, 1290 (2002)). The critical branches for collapse due to the three-dimensional character of the gas and explosion caused by the expulsive potential are derived based on variational studies. Particle loss from the soliton due to sudden changes in the trapping potential and scattering length are quantified. It is shown that higher order solitons can also be created in present experiments by an abrupt change of a factor of four in the scattering length. It is demonstrated that quantum evaporation occurs by nonlinear tunneling of particles out of the soliton, leading eventually to its explosion.

A New Conducting Molecular Solid Based on the Magnetic [Ni(dmf) 6] 2+ Cation and on [Ni(dsit) 2] 22− (dsit=1,3-dithiole-2-thione-4,5-diselenolate) Showing an Unprecedented Anion Packing

The synthesis, X-ray structure, magnetic and transport properties of the compound Ni(dmf) 6[Ni(dsit) 2] 2 (dmf=dimethylformamide, dsit=1,3-dithiole-2-thione-4,5-diselenolate) are described. This compound crystallizes in the monoclinic space group P2 1/c, with a=18.709(6), b=22.975(5), c=20.418(5) Å, β=99.31(2)° and Z=6; its structure consists of [Ni(dsit) 2] 2 2− dimers and isolated [Ni(dmf) 6] 2+ cations both centrosymmetric and non-centrosymmetric. The dimers are packed forming chains along the [101] direction with short Se·Se interdimer contacts. Additional interchains S·S contacts render this structure a three-dimensional character, never observed so far in other [Ni(dsit) 2] − salts. This compound exhibits semiconducting behavior with a room temperature conductivity (1 S cm −1) much higher than those reported for other salts of the [Ni(dsit) 2] − anion. Tight-binding band structure calculations were used to analyze the origin of the semiconducting properties of this salt. The magnetic susceptibility shows Curie behavior with C=1.25 emu K mol −1, typical of isolated Ni(II) ions as expected for the octahedrally coordinated [Ni(dmf) 6] 2+ cations.

Study of the quantum-well states in ultra-thin silver films on Si surfaces

The growth morphology and the electronic structures of thin metastable and epitaxial Ag films grown on Si(001)2×1 and Si(111)7×7 surfaces at low temperature were investigated by scanning tunneling microscopy and angle-resolved photoelectron spectroscopy using synchrotron radiation. The morphology of Ag films on Si(001)2×1 exhibits a strong thickness and temperature dependence, indicating an intriguing growth mechanism. The as-deposited film at ∼100 K is composed of nanoclusters with flat tops in a uniform quasi-layer-by-layer film at 2–3 ML and of homogeneous clusters having a more three-dimensional (3D) character above ∼5 ML. By subsequent annealing at 300–450 K, flat epitaxial Ag(111) films are formed at a nominal coverage larger than 5 ML, while a percolating network of 2D islands is formed at a lower coverage. For optimally annealed epitaxial films, discrete Ag 5 s states are observed at binding energies of 0.3–3 eV, together with the Ag(111) surface state. The discrete electronic states are consistently interpreted by a standard description of the quantum-well states (QWSs) based on phase-shift quantization rules. The phase shift, the energy dispersion and the thickness-versus-energy relation of the QWSs of epitaxial Ag(111) films are consistently derived. The in-plane band structures of the QWSs of epitaxial Ag films grown on Si(111)7×7 and Si(001)2×1 surfaces were investigated in detail. In contrast to the expected free-electron-like behavior, the QWSs show intriguing in-plane dispersions, such as (i) a significant enhancement of the in-plane dispersion with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-normal emission. Such unexpected electronic properties of a QWS are obviously related to the substrate band structure. Further, the QWS splitting is explained by the energy-dependent phase shift of the film–substrate interface occurring at the substrate band edge.

Large anisotropy in the optical conductivity ofYNi2B2C

The optical properties of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ are studied by using the first-principles full-potential linearized augmented plane-wave method within the local density approximation. Anisotropic behavior is obtained in the optical conductivity, even though the electronic structure shows three-dimensional character. A large peak in ${\ensuremath{\sigma}}_{z}$ is obtained at 2.4 eV. The anisotropic optical properties are analyzed in terms of interband transitions between energy levels, and it is found that the Ni site plays an important role. The electronic energy loss spectroscopy spectra are also calculated to help elucidate the anisotropic properties in this system.

Charge-density wave and three-dimensional Fermi surface in1T−TaSe2studied by photoemission spectroscopy

We have investigated the electronic structure of the commensurate charge-density-wave (CDW) phase for $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ by Ta $4f$ core-level and the angle-resolved photoemission. From the energy shifts of Ta $4f$ core levels due to a CDW formation, it is found that the CDW amplitude in the $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is smaller than that in $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ reported previously. The band dispersion along the normal to the two-dimensional layers indicates that $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ has a three-dimensional (3D) character in the Fermi surface compared with $1T\ensuremath{-}{\mathrm{TaS}}_{2},$ which has a two-dimensional Fermi surface. The 3D Fermi surface of $1T\ensuremath{-}{\mathrm{TaSe}}_{2}$ is thought to originate from the larger interaction between layers due to a large charge transfer between Ta $5d$ and Se $4p$ orbitals.

Two-domains bulklike Fermi surface of Ag films deposited ontoSi(111)−(7×7)

Thick metallic silver films have been deposited onto $\mathrm{Si}(111)\ensuremath{-}(7\ifmmode\times\else\texttimes\fi{}7)$ substrates at room temperature. Their electronic properties have been studied by using angle-resolved photoelectron spectroscopy (ARPES). In addition to the electronic band dispersion along the high-symmetry directions, the Fermi surface topology of the grown films has been investigated. Using ARPES, the spectral weight distribution at the Fermi level throughout large portions of the reciprocal space has been determined at particular perpendicular electron-momentum values. Systematically, the contours of the Fermi surface of these films reflected a sixfold symmetry instead of the threefold symmetry of the Ag single crystal. This symmetry loss has been attributed to the fact that these films appear to be composed by two sets of domains rotated $60\ifmmode^\circ\else\textdegree\fi{}$ from each other. Extra photoemission features at the Fermi level were also detected, which have been attributed to the presence of surface states and $\mathrm{sp}$-quantum states. The dimensionality of the Fermi surface of these films has been analyzed, studying the dependence of the Fermi surface contours with the incident photon energy. The behavior of these contours measured at particular points along the Ag $\ensuremath{\Gamma}L$ high-symmetry direction puts forward the three-dimensional character of the electronic structure of the investigated films.

Simulating damage growth in a [90/0] s composite laminate using quasi-two-dimensional finite element methods

A computationally efficient numerical method is described for predicting the details of damage growth in cross-plied and angle-plied fiber composite laminates. Separate two-dimensional plane stress meshes are used to represent each layer. Element boundaries are aligned along primary crack paths (parallel to fibers). Element corners in separate layers which share the same planar coordinates are attached to a single through-thickness node. Cracking of a layer parallel to fibers is modeled by combinations of releasing element nodal connections in the layer, creating new nodes, and/or modifying element elastic properties, depending on whether or not the layer is attached to or delaminated from one or both of its neighbors. Delaminations are modeled by releasing nodal connections between layers and creating new nodes. The essential three-dimensional character of the process, which is necessary to incorporate the effects of stacking sequence, is maintained by casting mixed-mode delamination fracture mechanics in terms of interlaminar nodal forces, and including adjacent layer constraint on the fracture mechanics of cracks in layers. Choice of element size is important to the method and has resulted in simplifications to crack propagation and fiber fracture prediction methodologies. Results to-date of predicted static damage growth in notched cross-plied carbon composites are described.

Landau levels in two and three-dimensional electron gases in a wide parabolic quantum well

Shubnikov-de Haas oscillations are measured in a wide parabolic quantum well with 6 subbands in a tilted magnetic fi eld. We find two types of oscillations. The oscillations at low magnetic field are shifted towards higher fields with tilted angles, and can be attributed to the two-dimensional Landau state at the bottom subband. The position of the second type oscillations do not shift with tilted angles indicating a three-dimensional character of the Landau state formed by the highest subbands. The bottom level in the quantum well is not overlapped with the highest subbands due to the enhanced quantum scattering time of the lowest subbands.

Theory of plasmonsin carbon nanotube bundles

We develop a suitable theoretical framework for studyingplasmons in nanotube bundles. In the plane perpendicular to thetubes, the nanotubes form a two-dimensional lattice. We use asimple model of free electron gas confined to arrays ofcylindrical surfaces, however the theoretical framework can alsobe applied to more sophisticated models for carbon nanotubes.Both intrasubband (classical) and intersubband (quantum)plasmons in such nanotube bundle systems are studied. Analytical solutions have been obtained for the case where onlyone subband is occupied, while numerical solutions have beenobtained for the case of many occupied subbands. IntertubeCoulomb coupling is found to play an intricate role, as it canboth harden and soften plasmon modes in the same nanotubebundle. Intertube Coulomb coupling is also responsible forsizable plasmon dispersions in the transverse plane. Allplasmons are found to be undamped by the corresponding singleparticle type electron-hole pair excitations. The classicalplasmon exhibits three-dimensional character in the longwavelength limit, and one-dimensional character in the shortwavelength limit. For quantum plasmons, the plasmon energy canbe significantly larger than the corresponding single particleexcitation energy between subbands. This feature is similar toquantum plasmons in semiconductor quantum wire systems.

Organic superconductor with an incommensurate anion structure: (MDT−TSF)(AuI2)0.44

The degree of charge transfer and the upper critical field of the organic superconductor (MDT-TSF)(AuI 2 ) x have been investigated (MDT-TSF: methylenedithio-tetraselenafulvalene). The x-ray oscillation photograph indicates that the anion lattice is incommensurate with the donor lattice, and the chemical composition is (MDT-TSF)( AuI 2 ) 0 . 4 4 . The charge-transfer degree of this salt is 0.44. The electrical resistivity and the Seebeck coefficient indicate that this salt is a good Fermi liquid above the superconducting transition temperature. The upper critical fields show anisotropic three-dimensional character in spite of the complicated incommensurate structure, and are within the Clogston-Chandrasekhar paramagnetic limit in all directions.

Dynamics of self-interstitial cluster migration in pure α-Fe and Fe-Cu alloys

We report the results of molecular dynamics simulations of self-interstitial cluster diffusion in pure \ensuremath{\alpha}-Fe and in a dilute Fe--1.0 at. % Cu alloy for a number of cluster sizes, $nl~20.$ We find that the effect of this oversized substitutional solute is to enhance the three-dimensional character of small-cluster diffusion and to impact the general cluster diffusion properties. We explain this through a mechanism directly based on the interactions between the atomic displacement field of the Cu atoms and the self-interstitial clusters. Based on these results, we derive simple power laws for the extrapolation of migration energies and diffusion prefactors to larger sizes, required for longer-range microstructural evolution models. These laws represent an improvement over current parametrizations used in previous calculations.

Band structure ofMoS2,MoSe2,andα−MoTe2:Angle-resolved photoelectron spectroscopy andab initiocalculations

In this work the complete valence-band structure of the molybdenum dichalcogenides MoS_2, MoSe_2, and alpha-MoTe_2 is presented and discussed in comparison. The valence bands have been studied using both angle-resolved photoelectron spectroscopy (ARPES) with synchrotron radiation, as well as, ab-initio band-structure calculations. The ARPES measurements have been carried out in the constant-final-state (CFS) mode. The results of the calculations show in general very good agreement with the experimentally determined valence-band structures allowing for a clear identification of the observed features. The dispersion of the valence bands as a function of the perpendicular component k_perp of the wave vector reveals a decreasing three-dimensional character from MoS_2 to alpha-MoTe_2 which is attributed to an increasing interlayer distance in the three compounds. The effect of this k_perp dispersion on the determination of the exact dispersion of the individual states as a function of k_parallel is discussed. By performing ARPES in the CFS mode the k_parallel-component for off-normal emission spectra can be determined. The corresponding k_perp-value is obtained from the symmetry of the spectra along the GammaA, KH, and ML line, respectively.

SIMPLIFIED INTERPRETATION OF AN INTEGRATED PHOTOELASTIC FRINGE PATTERN

It has been shown that with the same load fringe patterns of the Flamant and Boussinesq problems are similar. However, Figs. 2 and 3 show that if the load is concentrated at a point then interpretation of the fringe pattern as if the problem is two-dimensional, leads to erroneous results. Near the upper surface (z < 0.75) simplified interpretation underestimates the value of σ z while at z > 0.75 it is vice versa. The analysis given above may help to understand the meaning of integrated fringe patterns in similar cases and to take into account the three-dimensional character of the stress distribution.

Frequencies and mode shapes for thick truncated hollow cones

The finite element method of analysis is employed to study the vibration of truncated thick hollow cones. The three-dimensional strain–displacement equations are used to formulate the finite element in the conical coordinate system. Axial symmetry is assumed and the formulation is reduced to two dimensions while maintaining the three-dimensional character of the analysis. The accuracy of the analysis is established by comparing results for free–free boundary conditions with existing solutions. The effects of different boundary conditions, including fixed–free and fixed–fixed, are studied to determine their effects on the frequency of vibration. Frequency results are given for an additional boundary condition corresponding to a layer attached to the exterior of a rigid cone. Nondimensional results for an isotropic material with Poisson's ratio of 0.3 are presented in terms of tables, plots of frequency versus cone apex angle and plots of selected mode shapes.

Coexistence of a two- and three-dimensional Landau states in a wide parabolic quantum well

Shubnikov--de Haas oscillations are measured in wide parabolic quantum wells with five to eight subbands in a tilted magnetic field. We find two types of oscillations. The oscillations at low magnetic fields are shifted toward higher field with the tilt angle increasing and can be attributed to two-dimensional Landau states. The position of the oscillations of the second type does not change with increasing the tilt angle which points to a three-dimensional character of these Landau states. We calculate the level broadening due to the elastic scattering rate $\ensuremath{\Gamma}=\ensuremath{\Elzxh}/2\ensuremath{\tau},$ where $\ensuremath{\tau}$ is the quantum time, and the energy separation between two-dimensional subbands, ${\ensuremath{\Delta}}_{\mathrm{ij}}{=E}_{j}\ensuremath{-}{E}_{i},$ in a parabolic well. For all levels we obtain ${\ensuremath{\Gamma}}_{j}\ensuremath{\sim}{\ensuremath{\Delta}}_{\mathrm{ij}},$ which means that the levels overlap, supporting the observation of three-dimensional Landau states. Surprisingly, we find that the lowest subband, which has a smaller energy separation from the higher level, does not overlap with these subbands and forms a two-dimensional state.

Pressure- and photo-induced phase transition in mixed-valence gold complexes

The pressure- and photo-induced phase transition in mixed-valence gold complexes of Cs2Au2X6 (X = Cl, Br, and I) has been investigated by means of the Raman scattering. The Raman-active Au-X stretching modes were deactivated by the pressure, which indicates a pressure-induced phase transition from the mixed-valence (MV) state to the single-valence (SV) state. The electronic phase diagrams of Cs2Au2X6 (X = Cl and Br) as a function of pressure and temperature have been derived. A photoinduced phase transition from the MV state to the SV state has been found for Cs2Au2Br6. The observed time behavior accompanying this phase transition is successfully interpreted by the Avrami model, indicating the three-dimensional character of the MV cluster growth.

Symmetry changes at the Néel point in the high-T c superconductor YbBa 2Cu 3O 7

A Landau-type theory has been applied to the description of the T N=330±20 mK magnetic phase transition in the high temperature superconductor YbBa 2Cu 3O 7. It has been demonstrated that the magnetic order parameter is one-dimensional and that the magnetic ordering has a three-dimensional Ising character. All possible (symmetry allowable) low-temperature spin structures were subsequently determined. They are antiferromagnetic, with the magnetic moments directed along the basis vectors of the Bravais lattice and they exactly fulfil the non-linear differential equations which follow from the variational problem for the thermodynamic potential density. One of the obtained spin structures is identical to that for which B. Roessli et al. obtained the best fit to the experimental diffraction pattern. The domain structure of the superconductor was subsequently determined on the grounds of Landau symmetry considerations.

Three-dimensional structure and decay properties of vortices in shallow fluid layers

Recently, several laboratory experiments on vortex dynamics and quasi-two-dimensional turbulence have been performed in thin (stratified) fluid layers. Commonly, it is tacitly assumed that vertical motions, giving rise to a three-dimensional character of the flow, are inhibited by the limited vertical dimension. However, shallow water flows, which are vertically bounded by a no-slip bottom and a free surface, necessarily possess a three-dimensional structure due to the shear in the vertical direction. This shear may lead to significant secondary circulations. In this paper, the three-dimensional (3D) structure and the decay properties of vortices in shallow layers of fluid, both homogeneous and stratified, have been studied in detail by 3D direct numerical simulations. The quasi-two-dimensionality of these flows is an important issue if one is interested in a comparison of experiments of this type with purely two-dimensional theoretical models. The influence of several flow parameters, like the depth of the fluid and the Reynolds number, has been investigated. In general, it can be concluded that the flow loses its two-dimensional character for larger fluid depth and larger Reynolds number. Furthermore, it is possible to construct a regime diagram that allows the assessment of the parameter regime, where the flow can be considered as quasi-two-dimensional. It is found that the presence of secondary circulations within a planar vortex flow results in a deformation of the radial profile of axial vorticity. In the limiting case of quasi-two-dimensional flow, the vorticity profiles can be scaled according to a simple diffusion model. In a two-layer stratified system, the decay is reduced and three-dimensional motions are significantly inhibited compared to the corresponding flows in a homogeneous layer.