Three-dimensional Character Research Articles

An analytical procedure for predicting the response of constant-temperature hot-wire and film anemometers

The resolution of the three-dimensional character of flows using hot-wire/film techniques can be achieved using either single or multiple sensors. Usually, the anemometer response is measured in an unknown flow field, and with predetermined probe calibration data and known directional characteristics, the flow field can be determined. This paper introduces a method capable of predicting the constant-temperature anemometer response of any single wire or film probe in a known flow field. The method is also capable of predicting the probe response at any orientation once the response is known at another orientation. This method will prove particularly useful for checking probe calibration especially if drift is suspected. Voltage drift can be caused by a variety of problems including dirt and bubble formation on the sensor as well as probe aging. Having a mechanism to keep track of the signal will prove extremely useful in identifying external causes of voltage drift. The analysis presented in this paper provides such a mechanism and is shown to be valid for both the linearized and unlinearized signals. The validity of the analysis has been demonstrated by carrying out measurements in a water channel and at the axis of a circular pipe.

STRATAFORM: overview of its design and synthesis of its results

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 3×10 7 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.

Crystallization of a polymer on a surface

We have studied the structure and free energy landscape of a semiflexible lattice polymer in the presence of the surface of a polymer crystal. At low temperatures coexistence of two-dimensional integer-folded crystals is observed. As the temperature is increased there is a transition from these crystalline configurations to a disordered coil adsorbed onto the surface. The polymer then gradually develops a three-dimensional character at higher temperatures. We compute the free energy as a function of increasing crystallinity and compare with the free energy profiles assumed by the Lauritizen–Hoffman surface nucleation theory of polymer crystallization. Our free energy profiles exhibit a “sawtooth” structure associated with the successive formation of chain folds. However, in the early stages of crystallization our profiles significantly deviate from those assumed by surface nucleation theory because the initial nucleus is not a single stem but two incomplete stems connected by a fold. This finding has significant implications for the theoretical description of polymer crystallization.

Magnetooscillations in a wide parabolic well in the presence of an in-plane magnetic field

The magnetoresistance oscillations in a wide parabolic quantum well are investigated at low temperatures in a tilted magnetic field. Magnetooscillations in the presence on an in-plane magnetic field cannot be explained by the diamagnetic Shubnikov–de Haas (Sd H) effect. Three-dimensional character of the electron motion can be responsible for this behaviour.

Effect of twin planes in the magnetization hysteresis loops ofNdBa2Cu3O7single crystals

The angular dependence of the critical currents is investigated in a moderately twinned ${\mathrm{NdBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (NdBCO) single crystal in the temperature range $30&lt;~T&lt;~90\mathrm{K}.$ In intermediate fields around the fishtail peak a reduction of critical current density due to twin structure is observed up to 90 K. This effect implies that vortices retain their three-dimensional character practically up to ${T}_{c}.$ The effect of twin planes disappeared at external fields declined more than $\ensuremath{\theta}\ensuremath{\approx}15\ifmmode^\circ\else\textdegree\fi{}$ from the $c$ axis (twin planes). In this case, the hysteresis loops exhibited a clear fishtail shape. The intermediate peak observed previously in NdBCO single crystal at $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$ is identified with the low-field limit of the twin plane activity.

Pressure dependence of compressibilities in YNi 2B 2C and YPd 2B 2C

The pressure dependence of compressibilities in YNi 2B 2C and YPd 2B 2C is shown together with their equilibrium structural properties, density of states and band structures. This first-principles study was done using the FP-LMTO scheme within the local density approximation to density functional theory and confirms the three-dimensional character of these intermetallic compounds.

Evaluation of bone mineral density using three-dimensional solid state phosphorus-31 NMR projection imaging.

A solid state magnetic resonance imaging technique is used to measure true three-dimensional mineral density of synthetic hydroxyapatite phantoms and specimens of bone ex vivo. The phosphorus-31 free induction decay at 2.0 T magnetic field strength is sampled following application of a short, hard radiofrequency excitation pulse in the presence of a fixed amplitude magnetic field gradient. Multiple gradient directions covering the unit sphere are used in an efficient spherical polar to Cartesian interpolation and Fourier transform projection reconstruction scheme to image the three-dimensional distribution of phosphorus within the specimen. Using 3-6 Gauss/cm magnetic field gradients, a spatial resolution of 0.2 cm over a field of view of 10 cm is achieved in an imaging time of 20-35 minutes. Comparison of solid state magnetic resonance imaging with dual energy X-ray absorptiometry (DXA), gravimetric analysis, and chemical analysis of calcium and phosphorus demonstrates good quantitative accuracy. Direct measurement of bone mineral by solid state magnetic resonance opens up the possibility of imaging variations in mineral composition as well as density. Advantages of the solid state magnetic resonance technique include avoidance of ionizing radiation; direct measurement of a constituent of the mineral without reliance on assumptions about, or models of, tissue composition; the absence of shielding, beam hardening, or multiple scattering artifacts; and its three-dimensional character. Disadvantages include longer measurement times and lower spatial resolution than DXA and computed tomography, and the inability to scan large areas of the body in a single measurement, although spatial resolution is sufficient to resolve cortical from trabecular bone for the purpose of measuring bone mineral density.

Preparation, crystal structures, experimental and theoretical electronic band structures of cobalt tellurides in the composition range

Cobalt tellurides in the composition range crystallize in a -type structure with short intra- and interslab Te-Te contacts indicating a polymeric network with multiple Te-Te bonds explaining the very low values of 1.38 to 1.41 of the hexagonal cells. Single-crystal x-ray investigations performed on confirm the marcasite-type structure in the centrosymmetric space group Pnnm. Experimental valence band spectra (UPS) confirm that the Co tellurides in the composition range (1.3 < x < 2) are metals. The emission at the Fermi level decreases with the Te content and is due to Co 3d and Te 5p states. This assignment is supported by the results of the calculated density of states curve (DOS) which demonstrates that Te p states contribute about 50% in the -type and about 35% in the marcasite-type structure. The Te d states contribute about 15% to the total Te contributions. This behaviour cannot be understood on the basis of a simple tight-binding description, ignoring d-valence states of Te. Core level spectra (XPS) suggest that all samples are best described as intermetallic compounds. Small chemical shifts between the different samples are mainly due to the different Madelung contributions rather than to changes of the electron density located on the Co atoms. An oxidation number for Te <-1 in all samples is deduced, in good agreement with the value of about -1.3 for the Te in that would be deduced from the relation between the Te-Te distances versus oxidation states of the anion in , . The three-dimensional character of the Co tellurides deduced from the crystal structure is further confirmed by the calculated energy dispersion E(k).

A Multiple-Doppler Synthesis and Continuity Adjustment Technique (MUSCAT) to Recover Wind Components from Doppler Radar Measurements

A new approach is presented to account for a simultaneous solution of the three wind components from at least a pair of Doppler radar observations, which could remove potential drawbacks of an iterative (nonsimultaneous) solution of Cartesian dual-Doppler analysis techniques. The multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) is derived from the extended overdetermined dual-Doppler (EODD) variational formalism that contains the basis for a simultaneous (noniterative) solution of a dual- or multiple-equation system and a mass continuity equation. Necessary accommodations are discussed, including the solutions for a plane-to-plane synthesis (as in EODD) instead of a fully three-dimensional and computationally intensive analysis, owing to the three-dimensional character of the continuity equation. The evaluation of MUSCAT is carried out by first considering real data and then performing numerical tests based on simulated radar observations. The comparative study with EODD applications shows that MUSCAT provides a more regular description of the airflow and that EODD may still contain residual errors that make the retrieved wind components inconsistent. Results from the numerical tests definitely reveal the real improvements of MUSCAT in synthesizing Doppler radar data.

Magnetic studies of flux pinning in YBa 2Cu 3O 7− x, YBa 2Cu 4O 8 and Hg-based single crystals

The flux-pinning mechanism in YBa 2Cu 3O 7− x (Y-123), YBa 2Cu 4O 8 (Y-124), HgBa 2CuO 4+ x (Hg-1201) and HgBa 2Ca 2Cu 3O 8+ x (Hg-1223) is compared. Single crystals of Y-123, Y-124 and Hg-1201 show a very pronounced fishtail effect, observed in a wide temperature range. For each compound the irreversibility field and the field at which the peak in the fishtail occurs are well described by a power-law dependence H irr/peak(T) = H irr/peak(0)(1 − T/T c) α with very similar exponents. In Hg-1223 the pinning has a more three-dimensional character at higher temperatures, but becomes rather two dimensional at low temperatures.

Simulation models of salt-marsh morphodynamics: some implications for high-intertidal sediment couplets related to sea-level change

A salt marsh composed of a vegetated platform dissected by creek networks has at any time characters determined by the previous history of the system, because forcing factors constantly change in rate. Results from numerical simulation models of marsh growth are combined with engineering regime theory (channel stability) to form a comprehensive conceptual model which qualitatively describes the three-dimensional character of marshes and the stratigraphic sequences they generate. The morphostratigraphic evolution of marshes is examined for two main patterns of regional relative mean sea-level change. Where the level fluctuates smoothly about an underlying upward trend, there develops a stacked sequence of silt-peat couplets, each of which is symmetrical in facies states but asymmetrical in facies thicknesses. Palaeochannel forms and fills distributed through each couplet show that creek networks expand and densify as sea level rises and the hydraulic duty of the marsh platform increases to its maximum, but shrink and silt-up as sea level temporarily stabilises or falls and the duty declines. Great earthquakes, and also the abandonment of manmade flood defences on aseismic marshes, create catastrophic-episodic increases in relative sea level. Where there is also an underlying upward trend of sea level, stacked silt-peat couplets result that are strongly asymmetrical in both facies states and thickness. The creek networks enlarge rapidly under strong forcing within a short time of the sea-level increase but later, as the hydraulic duty declines, shrink and infill. Under each scenario, the tidal creek networks that functioned during the accumulation of one couplet are unlikely to be inherited by the next, except in the case of channels that also transmit fresh water from the hinterland.

Magnetoelectric effect inR2CuO4(R=Gd, Sm, and Nd)

In strong magnetic fields up to 20 T the magnetoelectric (ME) effect has been measured in the tetragonal antiferromagnets ${\mathrm{R}}_{2}$${\mathrm{CuO}}_{4}$, R = Gd, Sm, and Nd, which are parent compounds for high-${\mathrm{T}}_{\mathrm{c}}$ superconductivity. For the ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ compounds a symmetry analysis yields that the ME effect is associated with the antiferromagnetically ordered rare-earth subsystem. From the magnetic field and temperature dependence of the ME effect in ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ some very detailed conclusions can be drawn about the nature of the magnetic ordering which appear to be in agreement with the magnetic structure revealed in this compound. For ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ in addition to the ME effect a distinct ferroelectric behavior has been observed which is only compatible with a lower crystal symmetry than it has been thought to be. It raises the question of whether superconductivity in Ce-doped ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ coexists with ferroelectricity. The ME-effect data in high magnetic fields show evidence for magnetic phase transitions, associated with a rearrangement of the intrinsic rare-earth magnetic structure. From the obtained (H,T) phase diagrams of ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ and ${\mathrm{Sm}}_{2}$${\mathrm{CuO}}_{4}$ critical exponents [\ensuremath{\mathrm{B}}(Gd)\ensuremath{\approx}0.4 and \ensuremath{\mathrm{B}}(Sm)\ensuremath{\approx}0.5] were found and it can be concluded that the intrinsic ordering of the rare-earth magnetic subsystem is of three-dimensional character. In the case of ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, the adopted crystal and magnetic structure of this compound rules out any kind of ME effect. Nevertheless, we have observed some weak but distinct ME response, which suggests the existence of a spontaneous structural distortion within the ${\mathrm{CuO}}_{2}$ planes.

Meridional profiles of variance-covariance of symmetric dioptric power: classes of variation that are uniform across the meridians of the eye.

Conventional dioptric power, including refractive status and keratometric measurements, can vary in a large variety of ways. The three-dimensional character of this type of power implies that six numbers are required for the complete representation of its variance: three variances and three covariances. Curves (called profiles) that show how these numbers compare in different meridians (or transverse directions) of the eye provide a useful graphical picture of the variation and its nature. This paper explores an important subclass of types of variation in which the variation is the same for all meridians. In this subclass the variation of power is said to be uniform across all meridians of the eye. It turns out that the uniformity may be either complete or partial. In the former case all aspects of the variation are the same. In the latter case certain aspects of the variation are the same across all meridians whereas others are not; in particular the variance of the torsional component of power is the same for every meridian whereas the variance of the curvital component changes from meridian to meridian. There is a range of types of completely uniform variation from spherical variation at one extreme to Jacksonian variation at the other. All types of uniform variation (partial or complete) are characterized by two indices called the jacksonian index and the completeness index. The types can be represented geometrically as points on the triangle of uniformity. Samples of measurements of dioptric power are selected to illustrate various types of uniform variation. Methods are presented for detecting and classifying uniform variation from profiles of variation and also from the variance-covariance matrix. Examples are given of uniform variation of refractive status and of keratometry of an eye. The variation is analyzed and classified. The concepts and methods are proving to be of fundamental importance in the study of the nature and underlying causes of fluctuations of refractive status and keratometric measurements.

Complex three-dimensional magnetic ordering in ultrathin films of fcc Co upon deposition of Cu

We present an ab initio analysis of easy magnetization axis ``switching'' recently discovered in experiments on Co ultrathin films with a deposited submonolayer of Cu. The present large scale calculations show that the nanomagnetic pattern of a stepped Co(001) surface 2--3 monolayers thick with submonolayer quantities of Cu has a complex three-dimensional character. We show that the maximum noncollinearity of spin directions is about 30\ifmmode^\circ\else\textdegree\fi{}, and the ``switching'' has a reentrant character as a function of Co coverage.

Incomplete optical shielding in cold atom traps: three-dimensional Landau-Zener theory

Ultracold atom collisions in the presence of a linearly polarized blueshifted laser field are studied theoretically. An analytical solution is presented within the framework of the Landau-Zener approximation. The effect of incomplete shielding is mostly accounted for by incorporating the three-dimensional character of the collisions, using a partial-wave analysis with space degeneracy. A model of two electronic states, including effects of indistinguishability of the colliding atoms, is used. The model associates the incomplete shielding with a process involving two crossing points. The theory is applied to the case of metastable xenon traps, obtaining a fair agreement with the experimental data. {copyright} {ital 1997} {ital The American Physical Society}

Dynamical behavior in a shallow quantum confinement system

Radiative recombination and relevant dynamic effects are studied in SiGe-based shallow quantum confinement systems where an inherently small barrier height controls the optoelectronic properties. Dynamic localization is demonstrated in a double quantum well carrying a pair of quantum wells with differential potential depths, leading to a remarkable observation of highly efficient “classical” carrier blocking due to an attractive potential as opposed to conventional repulsive potential blocking utilizing the wave character of carriers. The underlying mechanism is controlled by hole re-emission with increasing temperature. In addition, it is shown that shallow-confined electrons are susceptible to electric fields, thereby masking the Stark shifts in type-I quantum wells at low field regime and reducing decay lifetimes due to exciton field ionization. These are shown to arise from extended wavefunction of electrons assuming three-dimensional character. Furthermore, modulation of exciton luminescence due to excess background charges is shown to address the importance of screening in a high pump condition.

An experimental and numerical study of a 90° bifurcation

The flow field of a 90° square duct bifurcation was examined both experimentally and numerically for a Reynolds number of 1200 and equal branch flow rates. Velocity measurements were carried out with a one-component LDV, recording, besides the time mean streamwise velocity component, its time variation due to vortex shedding which took place in the extension of the main duct, as well as secondary flow velocities in the 90° branch. Flow visualization provided additional information about the three-dimensional character of the flow field by recording the paths of neutrally buoyant solid particles. The experimental data were compared with the results of a numerical method, constituting an extension of the well-known SOLA method to a 3-D generalized coordinate system, presented here for the first time. These comparisons were very successful in the vertical branch but less satisfactory in the horizontal branch where unsteadiness was very strong. The characteristic frequency of the velocity fluctuations in the detached shear layer of the horizontal branch was very well predicted but the velocity amplitudes were underestimated. The predicted secondary flow in the vertical branch was compared well with the experimental data presenting maximum values close to the inlet mean velocity.

The trend of magnetic irreversibility for Rb 3C 60 and its implications

The irreversibility magnetic field ( H irr) as a function of the irreversibility temperature ( T irr) for two-dimensional superconductors,3e.g., the ones with a perovskite structure, can be characterized by an exponential law. However, with increased thermal agitation toward T c, the coupling strength between adjacent superconducting (i.e., CuO 2) layers would impart a three-dimensional like character. As a result, the exponential law may be approximated by a simpler power law. The fullerene superconductors are intrinsically isotropic so their H irr trends appear to follow the power law throughout the entire temperature range. For example, our powdered Rb 3C 60 sample showed such a trend with a highly three-dimensional character. Moreover, a well crystallized Rb 3C 60 sample exhibits an ideal isotropic feature as manifested by the linear dependence of H irr on T irr. Furthermore, this linearity may be represented by two superimposed straight lines that intersect the T irr axis at two T 0 values (30.5 K and 60.0 K). For most superconductors, T 0 is indicative of T c. The cause of two T 0 values for our Rb intercalated fullerene crystals is not known. If they represent two T c values of superconducting phases, then, the phase with T c = 30.5 K corresponds to the well-known composition of Rb 3C 60. However, the phase with T c = 60.0 K may possess a different composition designated as Rb x C 60 with x possibly higher than 3.

Revised analysis of the thermal stresses in a shallow spherical shell with a foreign inclusion

We carry out a revised analysis of the concentration of thermal stresses in a shallow spherical shell with a small foreign inclusion. We show that near the interface of the materials the stressed state has a sharply expressed three-dimensional character; thus the stresses differ significantly (both qualitatively and quantitatively) from the shells predicted by the standard theories.

The metal-insulator transition in disordered tungsten bronzes. Results of an Anderson-Mott-Hubbard model

To understand the electronic properties — in particular the metal—insulator transition (MIT) — of cubic tungsten bronzes Na x WO 3 and Na x Ta y W 1 − y O 3, a microscopic model incorporating electron interactions and correlated disorder i presented and treated at the mean-field level of unrestricted Hartree-Fock. The conduction band is found to exhibit a pseudogap at the Fermi level for sufficiently large interaction strengths, in agreement with experiment. The pseudogap has a profound effect on localization properties of Fermi-level states and the position of the MIT. The lower band-edge states are found to be essentially two-dimensional, with a progressive crossover to three-dimensional character with increasing energy. An exception to this behaviour occurs in the pseudogap, where the states are virtually two-dimensional.