Three-dimensional Character Research Articles

Magnon contribution to specific heat of the magnetic superconductor HoNi 2B 2C

Below 5 K, the quaternary intermetallic borocarbide HoNi 2B 2C shows a coexistence of superconductivity and commensurate antiferromagnetism. Since such a magnetic ground structure consists of ferromagnetic HoC layers that are weakly coupled antiferromagnetically, it is interesting to investigate the character and dimensionality of its low-temperature spin-wave excitations. For that purpose, low-temperature calorimetric studies were carried out on a well-characterized polycrystalline HoNi 2B 2C sample. After a proper subtraction of the nuclear, electronic and lattice contributions, the remaining part of the specific heat below 5 K, which is due solely to the spin-wave excitations, can be expressed as 0.29 T 3exp(−5.3/ T) J/mol K. In agreement with the known magnetic structure, the T 3 factor is a signature of a three-dimensional antiferromagnetic character while the exponential factor emphasizes the presence of an energy gap ( Δ=5.3±0.2 K) in the spin-wave excitation; the magnitude of which equals the strength of the magnetic order ( T N≈5 K).

Modelling of the propagation of passive impurities in Sevastopol bays

We discuss the results of numerical experiments modelling the process of propagation of passive impurities from the Karantinnaya Bay caused by steady wind currents and turbulent diffusion. The currents are computed according to the Felzenbaum model for the cases of northeast and north-west winds [1, 2]. We analyse specific features of the space and time distribution of the impurities caused by the three-dimensional character of the currents.

Growth and electron quantization of metastable silver films on Si(001)

The growth morphology and the electronic structures of thin metastable Ag films grown on the $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface at low temperatures are investigated by scanning tunneling microscopy and angle-resolved photoemission spectroscopy using synchrotron radiation. The morphology of Ag films exhibits a strong thickness and temperature dependence indicating an intriguing growth mechanism. The as-deposited film at \ensuremath{\sim}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 more three-dimensional (3D) character above \ensuremath{\sim}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 the optimally annealed epitaxial films, discrete $\mathrm{Ag}5s$ 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 (QWS's) based on phase-shift quantization. No such well-defined QWS is observed for the films with a coverage less than \ensuremath{\sim}5 ML. The phase shift, the energy dispersion, and the thickness-versus-energy relation of the QWS's of the epitaxial Ag(111) films are consistently derived. The QWS's in photoemission spectra show two distinctive types of the photon-energy dependence in their binding energies; the oscillatory shifts for $h\ensuremath{\nu}=5--15\mathrm{eV}$ and no such shift at $h\ensuremath{\nu}=20--25\mathrm{eV}.$ This can be explained in terms of the different final states in the photoemission process.

Three-Dimensional Aspect of the Surface Tension: An Approach Based on the Total Pressure Tensor

The conditions of mechanical equilibrium were considered, and the generalized notion of the surface tension at an arbitrarily curved surface layer was analyzed on the basis of the total nondiagonal pressure tensor including the external fields and anisotropic even in the bulk phases. It was shown that the transverse surface tension can be eliminated using the selection of a dividing surface; however, in the general case, this surface does not exhibit the properties of the tension surface. On the whole, three-dimensional and nondiagonal character of the tensors of excess surface stresses determined by the integration over the volume and the cross section of the surface layer is retained at any selection of the dividing surface.

Supersonic - subsonic transition in relatively narrow channels

The flow structure in relatively narrow channels has a strong three-dimensional character with complex flow phenomena, including regions of localized separation, various vortical structures, etc., all of which have to be considered when dealing with the problem of supersonic-subsonic transition.

Sol-gel chromium–vanadium mixed oxides as lithium insertion compounds

The sol-gel synthesis of new chromium–vanadium mixed oxides Cr 0.11V 2O 5.16 Cr 0.06VO 2.09 and Cr 0.33V 6O 13.5 is reported. Their electrochemical properties were investigated for Li insertion. In the case of Cr 0.11V 2O 5.16 (V 2O 5 derived structure), the formation of short CrO 6 octahedra chains linking V 2O 5 layers increases its three-dimensional character. The main electrochemical and structural properties are reported, discussed and compared with those for V 2O 5. The attractive behaviour of Cr 0.11V 2O 5.16 both in terms of specific capacity and cycle life (260 A h/kg after 20 cycles at C/10 rate; 3.8–2 V) can be explained by the doping effect played by Cr 3+ in the V 2O 5 structure leading to a lowering of structural changes and a more reversible character of these structural modifications. The Cr 0.06VO 2.09 oxide (VO 2 (B) type) exhibits a maximum reversible capacity of 320 A h/kg in the potential range 4/1.5 V and a good retention on cycling with a specific capacity of 240 A h/kg after 20 cycles at C/10 rate. The Cr 0.33V 6O 13.5 oxide (V 6O 13 type) outperforms stoichiometric and non-stoichiometric V 6O 13.

Hybrid Open Frameworks. Hydrothermal Synthesis, Structure Determinations, and Magnetic Behavior of (VIVO)2(H2O){O3P–(CH2)3–PO3}·2H2O: A New Vanado-alkyldiphosphonate Closely Related to VO(HPO4)·0.5H2O

(VIVO)2(H2O) {O3P–(CH2)3–PO3}·2H2O was hydrothermally synthesized from a mixture of VOSO4·5H2O:H2PO3– (CH2)3–PO3H2:H2O in a molar ratio 1:0.5:250 and heated for 72 hours to 170°C. Its structure was determined from powder X-ray diffraction data. It crystallizes in the orthorhombic space group Immm (No. 71) with a=16.8210(5) Å, b=9.3790(3) Å, c=7.4568(2) Å, V=1176.4(1) Å3, Z=4. The pillared framework is built up from inorganic layers covalently linked via P–C–C–C–P bonds. The inorganic layers are similar to those encountered in a VO(HPO4)·0.5H2O bidimensional compound described previously. The dimers of face-shared VO5(H2O) octahedra are linked by PO3C phosphonate groups instead of PO3(OH) groups in the vanadyl hydrogenphosphate. The terminal hydroxyl groups in the latter are substituted by the carbons of the organic chains which link two layers, giving the three-dimensional character of the structure. The magnetic behaviors of the two compounds are compared.

Estimation of spatially distributed latent heat flux over complex terrain from a Raman lidar

A method is presented in which estimates of evaporation may be made over an area approaching three quarters of a square kilometer, with relatively fine (25 m) spatial resolution, using three-dimensional measurements of water vapor concentration from a scanning Raman lidar. The method is based upon Monin–Obukhov similarity theory applied to spatially and temporally averaged data. Data from the lidar is used to sense the location and orientation of the surface and the location of the water vapor measurements with respect to that surface. Maps of the spatial distribution of evaporation have been produced showing the evaporation rates at regular intervals throughout the day. The method was applied to the SALSA experimental site during the 1997 summer field campaign. The estimates of evaporation rates made during the campaign compare favorably with estimates made using sap flux methods with RMS differences of 18 W/m 2. While the method has certain limitations, the three-dimensional character of the data allows for the detection of anomalous situations so that analysts may alter the analysis technique or reject the estimates from the affected regions. This information can be used in a wide variety of ways to study the spatial variations in evaporation caused by changes in soil type and moisture content, canopy type and topography.

Magneto-optical study of excitonic states inIn0.045Ga0.955As/GaAsmultiple coupled quantum wells

The excitonic states have been investigated in ${\mathrm{In}}_{0.045}{\mathrm{Ga}}_{0.955}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ heterostructures consisting of i quantum wells $(i=1,2,3,4)$ with 7.5 nm well thickness. For a 2.5 nm barrier thickness between the wells, the electronic states are strongly coupled. Because of the coupling, the heavy-hole exciton $\mathrm{ns}\mathrm{hh}$ of each single quantum well is split into ${i}^{2}$ states. The states can be characterized according to their symmetry under a combination of the reflections of the single particles at the quantum-well plane. The energy order of the symmetric and antisymmetric states as a function of quantum-well number is investigated in detail, and compares well to the theoretical calculation. These coupled quantum-well structures exhibit somewhat three-dimensional character based on the study of their exciton binding energies and wave functions. Highly resolved photoluminescence excitation spectra are presented, measured in magnetic fields up to 13 T using circularly polarized light. Strong mixing between light- and heavy-hole excitons causes optical transitions into high-angular-momentum exciton states and strong anticrossing effects. An anticrossing between the $3d{\mathrm{hh}}_{11}$ and ${\mathrm{hh}}_{21}$ exciton is observed. Also, the light-hole exciton is found to possess ${\ensuremath{\Gamma}}_{7g}$ and ${\ensuremath{\Gamma}}_{6g}$ symmetries.

Transition studies on an elliptic cone in Mach 8 flow using Filtered Rayleigh Scattering

Boundary layer transition on a sharp-nosed elliptic cone in a Mach 8 flow was experimentally investigated using CO 2 -enhanced Filtered Rayleigh Scattering. The 4 :1 elliptic cone was tested at Reynolds numbers ranging from 0.44×10 6 to 3.5×10 6 , based on free stream conditions and streamwise distance measured from the cone tip. Boundary layers ranging from fully laminar to late-transitional in character were imaged using single-shot streamwise, spanwise and planform laser orientations. Images taken of the early stages of transition reveal for the first time that transition begins with the emergence of small-scale structures near the centerline axis of the cone (minor axis), rather than in the outboard cross-flow region. The three-dimensional character of these structures is further explored by simultaneously imaging the planform and spanwise planes.

Electronic band structure of CuBa 2Ca 3Cu 4O 10+ x[formula omitted

Electronic band structures of Cu1234O 10 and Cu1234O 11 are calculated by using a full-potential linearized APW method in the local density approximation. Four CuO 2 planes produce four large Fermi surfaces: Two Fermi surfaces are two-dimensional, while the others have a three-dimensional character. The former surfaces are produced by the inner CuO 2 planes of the superconducting layer, and are responsible for the high-temperature superconductivity. The latter surfaces come from the outer CuO 2 planes, and provide the three-dimensional properties in the superconducting state and the normal state. Oxygen doping adds a small amount of holes to only the outer CuO 2 planes, indicating the stable superconducting phase independent of the oxygen content.

Effect of dimensionality on the magnetic properties of Ruddlesden–Popper manganites

We have measured the muon-spin relaxation (μSR) in La 0.6Sr 0.4MnO 3 and in its layered sister system La 1.2Sr 1.8Mn 2O 7 to study the influence of dimensionality on their magnetic properties. Both compounds show ferromagnetic ordering with decreasing temperature. Above their Curie temperatures we followed the spin fluctuations. While the electron spin fluctuations in the three-dimensional La 0.6Sr 0.4MnO 3 slow down critically as the transition temperature is approached they remain finite in the layered material La 1.2Sr 1.8Mn 2O 7. We can fit them to 2D critical behaviour with a lower T C. In addition, we observe a spin precession signal in the ferromagnetically ordered state in La 1.2Sr 1.8Mn 2O 7; the temperature dependence of the muon precession frequency is in agreement with a three-dimensional character of the ordering.

Modelling sieve tray hydraulics using computational fluid dynamics

We develop a computational fluid dynamics (CFD) model for describing the hydrodynamics of sieve trays. The gas and liquid phases are modelled in the Eulerian framework as two interpenetrating phases. The interphase momentum exchange (drag) coefficient is estimated using the Bennett et al. (1983) correlation as basis. Several three-dimensional transient simulations were carried out for a 0.3 m diameter sieve tray with varying superfical gas velocity, weir height and liquid weir loads. The simulations were carried out using a commercial code CFX 4.2 of AEA Technology, Harwell, UK and run on a Silicon Graphics Power Challenge workstation with six R10000 200 MHz processors used in parallel. The CFD simulations reflect chaotic tray hydrodynamics and reveal several liquid circulation patterns, which have true three-dimensional character. The clear liquid height determined from these simulations is in good agreement with the Bennett correlation. It is concluded that CFD can be a powerful tool for modelling and design of sieve trays.

Book review

The strata formed on continental margins record the history of both marine and terrestrial events occurring on Earth. The STRATAFORM Program was created to develop a better understanding of relationships between these events and margin stratigraphy. In order to obtain the maximum information from strata, they are being examined simultaneously on various temporal and spatial scales, and their formation is being studied where events and accumulation are known to be active. The general objectives of STRATAFORM are: to add geological perspective to studies of sedimentary processes, to constrain stratigraphic interpretations better, and to develop numerical models relating processes and stratigraphy. This research is being undertaken on margins off northern California and New Jersey, which provide unique opportunities for investigation of short-term and long-term stratigraphy, respectively. This volume and paper focus upon studies undertaken off northern California, north of Cape Mendocino. This area is tectonically active, with a coastal mountain range, narrow shelf, and input of sediment primarily from the Eel River. Recent estimates indicate that 1 to 3×107 t/yr of mud (silt and clay) are supplied to the continental shelf. Sand accumulates on the inner shelf (<55 m water depth), and the mud rapidly deposits on the middle shelf north of the Eel mouth. An energetic physical regime causes significant seaward transport of muddy sediment, and much (>70%) reaches the continental slope (or deeper). During the previous lowstand of sea level, sediment discharge was ∼25% greater, and probably led to hyperpycnal, debris and other flows transporting sediment down the slope. On the Eel shelf, flood and storm events combine to create normally stratified fine-grained layers with basal sand, and much terrestrial organic debris. These layers are modified through time, but some can be buried and preserved. Subsurface flood layers seem to impact observations of acoustic backscatter on the middle shelf, and subsurface fluids (gas? freshwater?) affect electromagnetic measurements on the inner shelf. Budgets of Eel River mud accumulation for the past century indicate ∼20% trapped on the adjacent shelf and ∼20% on the upper slope (150–600 m). The remaining 60% is dispersed to more distal locations. Over Holocene time scales, Eel River sediment has built a small sandy subaqueous delta, and a subtle bulge of muddy sediment on the middle shelf north of the river. Gullies have been eroded on the open slope during lowstands of sea level, and have accreted upward during highstands. A massive failure has formed the Humboldt slide, due to earthquake activity and unstable substrates (possibly from excess pore pressures and gas). The tectonic setting has impacted margin stratigraphy from small scales (fluid-expulsion pockmarks) to large scales (spatial variation of late Cenozoic sedimentation). The broad dispersal of Eel sediment demonstrates the three-dimensional character of the resulting stratigraphy, including the large-scale clinoform structure developed on the shelf/upper slope. The rollover point (topset/foreset boundary) for this feature is the shelf break (i.e., below sea level), which is important information for stratigraphic interpretations of sea-level change.

Measurement of pressure loss and observation of the flow field in viscoelastic flow through an undulating channel

The viscoelastic flow through undulating channels is experimentally studied by using 0.1 and 1.0 wt % aqueous solutions of polyacrylamide (PAA). In the measurement of the pressure loss, it is confirmed that the excess pressure loss occurs for both concentrations of PAA. However, the magnitude of the excess pressure loss for the 1.0 wt % solution of PAA is much smaller than that for the 0.1 wt % of PAA. The Deborah number at the occurrence of the excess pressure loss is within the range of 0.1–0.4 and it depends on the concentration of PAA and channel geometry. The flow pattern changes to an asymmetric one including a small disturbance when the excess pressure loss occurs. Flow unsteadiness observed in the 0.1 wt % solution of PAA is discussed in connection with the excess pressure loss. In the velocity measurement, an asymmetric character of measured profiles for the velocity provides evidence for its three-dimensional character. The variation of the stretch rate on the center line of an undulating channel indicates a significant phase shift beyond the occurrence of the excess pressure loss.

Ionic conductivities of lithium fluorapatites

The electrical properties of Ca 10− x M x (PO 4) 6− x (SO 4) x F 2 apatite structure where M=Na, Li using the complex impedance method allows us to demonstrate the mixed ionic conduction (anionic and cationic). This effect can be assigned to the three-dimensional character of the diffusion process in these materials. The present study proves several factors are responsible for the diffusion processes particularly the cationic site nature, the size and the polarisability of cations introduced in the system.

Saturated flow in a single fracture: evaluation of the Reynolds Equation in measured aperture fields

Fracture transmissivity and detailed aperture fields are measured in analog fractures specifically designed to evaluate the utility of the Reynolds equation. We employ a light transmission technique with well‐defined accuracy (∼1% error) to measure aperture fields at high spatial resolution (∼0.015 cm). A Hele‐Shaw cell is used to confirm our approach by demonstrating agreement between experimental transmissivity, simulated transmissivity on the measured aperture field, and the parallel plate law. In the two rough‐walled analog fractures considered, the discrepancy between the experimental and numerical estimates of fracture transmissivity was sufficiently large (∼22–47%) to exclude numerical and experimental errors (&lt;2%) as a source. We conclude that the three‐dimensional character of the flow field is important for fully describing fluid flow in the two rough‐walled fractures considered and that the approach of depth averaging inherent in the formulation of the Reynolds equation is inadequate. We also explore the effects of spatial resolution, aperture measurement technique, and alternative definitions for link transmissivities in the finite difference formulation, including some that contain corrections for tortuosity perpendicular to the mean fracture plane and Stokes flow. Various formulations for link transmissivity are shown to converge at high resolution (∼1/5 the spatial correlation length) in our smoothly varying fracture. At coarser resolutions the solution becomes increasingly sensitive to definition of link transmissivity and measurement technique. Aperture measurements that integrate over individual grid blocks were less sensitive to measurement scale and definition of link transmissivity than point sampling techniques.

High-temperature graphitization of the 6H-SiC (0001) face

Silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The gradual graphitization of the 6H-SiC (0001̄) face (carbon termination) has been studied by angle-resolved inverse photoemission spectroscopy (KRIPES). The initial growth of graphite occurs on a 2×2 reconstruction which is best observed by LEED after annealing at 1050°C. Graphite begins to grow at temperatures as low as 1080°C, above which a hexagonal array of tangentially elongated diffraction spots appears superposed with the 2×2 spots of the substrate. The observation by KRIPES of a fingerprint of the three-dimensional character of graphite reveals the growth of several layers as low as 1150°C. Above 1200°C, the elongated spots join to form closed diffraction rings whose brightest parts define a hexagonal lattice corresponding to graphite. The graphite layers grown on the C face are thus essentially azimuthally disordered, although a significant fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 30° with respect to the basis vectors of the SiC lattice. KRIPES reveals that the first graphite layer is strongly bound to the C face since the π∗ states are lacking at low carbon coverage. This strong interaction explains why graphite is not free to orient at the surface, which is best viewed as a mosaic of small crystalline domains with azimuthal disorder, similar to the case of HOPG. A rehybridization of the graphite π∗ states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.

Growth of Hf and HfN on GaN by molecular beam epitaxy

Hf and HfN thin films were grown on n-type GaN(0001̄) by molecular beam epitaxy using a custom built Hf electron beam source and an ammonia leak. The films were characterized by reflection high-energy electron diffraction (RHEED) and atomic force microscopy. It was found that epitaxial growth of Hf is possible even at room temperature. GaN films varying in thickness from 0.6 to 1.8 μm were grown on c-plane sapphire, using ammonia as a precursor, to serve as substrates. Then the films were annealed in ammonia as the temperature was lowered in order to produce a N termination. Hf was then deposited on top of the GaN at temperatures between 20 and 730 °C, both with and without ammonia incident on the sample. Deposition of pure Hf at room temperature revealed an epitaxial two-dimensional (2D) (although with some three-dimensional character) RHEED pattern with sixfold symmetry. The surface is reconstructed with a (2×2) R30° structure. We propose that the pattern is rotated 30° with respect to that of the substrate GaN because of a HfN interlayer between the GaN and Hf layers. When the films were annealed in vacuum up to 730 °C, the 2D pattern became more transmission like. If Hf was deposited at substrate temperatures of 350 °C and higher, a diffraction pattern corresponding to that of a polycrystalline material was observed.

An experimental study of a rib-roughened rotating U-bend flow

Abstract In this paper we report an experimental investigation of turbulent flow through a square-sectioned U-bend of strong curvature, in which the inner and outer walls of the upstream and downstream sections are artificially roughened with square ribs, in a staggered arrangement. The U-bend is either stationary or rotates about an axis parallel to that of curvature, with positive rates, i.e. so that the secondary flows provoked by curvature and rotation are in the same sense. The main objective is to provide CFD validation data for flows which contain most of the flow features encountered in blade-cooling passages, but which are numerically easier to compute, while retaining the modelling challenges provided by a real gas-turbine blade. In earlier investigations we showed that the introduction of ribs in the upstream and downstream sections: (a) raises overall turbulence levels; (b) reduces the size of the separation bubble formed along the inner wall of the U-bend and (c) causes the formation of a large separation bubble along the outer wall, as the flow encounters the first outer-wall rib, after the bend exit. Here we: (a) explore how the location of the first outer-wall rib, after the bend exit, affects the development of the downstream flow and (b) focus on the three-dimensional character of these flows, by providing data along a plane close to the top (flat) wall, in addition to data along the duct symmetry plane. We show that, for both stationary and rotating conditions, as the first outer-wall rib is moved further away from the bend exit, the size of the separation bubble along the outer wall is reduced. The separation bubble along the inner wall, however, increases in size but, overall, turbulence levels are reduced. The flow within and immediately downstream of the U-bend is highly three-dimensional, showing strong variations from the symmetry plane to the top wall. Rotation generates the expected secondary motion in the straight sections that, at the duct centre, convects the faster fluid towards the pressure side, and along the top and bottom (flat) walls, has the opposite effect. Within and downstream of the bend, positive rotation reduces the three-dimensionality of the flow.