Simple Layered Structure Research Articles

Pulsed Laser Deposition Synthesis and Photoemission Study of Superconducting Ba-Cu-O Thin Films

Ba-Cu-O films have been grown by pulsed laser deposition. Investigation of their transport properties and characterization of electronic structure by in-situ photoemission spectroscopy have been carried out. The films grown at 450 ∼ 480 °C from BaCuO2 target had simple infinite layer structure with lattice parameter of a and c about 0.385 and 0.405 nm, respectively. Conductivity of these films were, however, poor. Usage of BaCu0.75Ox target and addition of CO2 into the growth atmosphere resulted in an expansion of c-axis length up to 0.810 ∼ 0.835 nm and a significant rise of conductivity. This structure is represented as ‘‘2c’’. For the 2c phase, conductivity above of 103 S/cm was achieved by adjusting their growth temperature in the range of 500 ∼ 530 °C. Most of the highly conductive 2c phase showed superconductivity. The maximum temperature of onset of the superconducting transition and zero resistance state obtained so far were 60 and 47 K, respectively. Finite spectral weights at Fermi level were successfully observed in in-situ photoemission spectra of the 2c phase. In valence band region, a prominent peak with u 3d character located at a binding energy around 1.6 eV. Though this low binding energy suggested a rather high hole concentration of mobile holes were around middle of 1021 cm−3 range. These results about electronic structure indicate a coexistence of conventional CuO2 planes and distorted ones. The latter structure might play charge-reserving blocks, though some portions of holes should localize in them. Correlations between the properties and electronic structure are to be discussed.

Fabrication of a vertical-type organic transistor with a planar metal base

High-performance vertical-type organic transistors were fabricated using a simple layered structure composed of organic/metal/organic layers. This device could modulate a sheet current between the emitter and collector by a voltage applied to the thin base electrode inserted. When C60 and perylene derivatives were used for the channel layer, the modulated collector current exceeded 300mA∕cm2 by application of only several volts of base voltage. The mechanism for this process is discussed from the viewpoint of metal base transistors.

Waveform tomography of crustal structure in the south San Francisco Bay region

We utilize a scattering‐based seismic tomography technique to constrain crustal structure around the southern San Francisco Bay region (SFBR). This technique is based on coupled traveling wave scattering theory, which has usually been applied to the interpretation of surface waves in large regional‐scale studies. Using fully three‐dimensional kernels, this technique is here applied to observed P, S, and surface waves of intermediate period (3–4 s dominant period) observed following eight selected regional events. We use a total of 73 seismograms recorded by a U.S. Geological Survey short‐period seismic array in the western Santa Clara Valley, the Berkeley Digital Seismic Network, and the Northern California Seismic Network. Modifications of observed waveforms due to scattering from crustal structure include (positive or negative) amplification, delay, and generation of coda waves. The derived crustal structure explains many of the observed signals which cannot be explained with a simple layered structure. There is sufficient sensitivity to both deep and shallow crustal structure that even with the few sources employed in the present study, we obtain shallow velocity structure which is reasonably consistent with previous P wave tomography results. We find a depth‐dependent lateral velocity contrast across the San Andreas fault (SAF), with higher velocities southwest of the SAF in the shallow crust and higher velocities northeast of the SAF in the midcrust. The method does not have the resolution to identify very slow sediment velocities in the upper approximately 3 km since the tomographic models are smooth at a vertical scale of about 5 km.

Interface formation in K doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes

Manufacturing of Al/K/OC1C10 poly(p-phenylene vinylene)/indium–tin–oxide light emitting diode structures by physical vapor deposition of K onto the emissive polymer layer has been characterized by electroluminescence and ion spectroscopy. Varying the deposited K areal density from 3.9×1012 to 1.2×1014 atoms cm−2 the external efficiency rises from 0.01 to 1.2 Cd A−1. Spectra obtained by ion scattering analysis demonstrate the overall absence of K at the polymer outermost surface layer, and diffusion up to a depth of 200 Å. Depth profiles have been derived, and were modeled using an irreversible first order “trapping” reaction. Trapping may stem from confinement of the electron at a conjugated segment, that was donated through charge transfer typical for alkali/π-conjugated systems. This study demonstrates that evaporation of low work function metals onto organic systems should not be depicted as simple layered stacking structures. The enhanced electroluminescence with submonolayer K deposition is attributed to the shift of the recombination zone away from the Al cathode, which is demonstrated to prevail over the known exciton quenching mechanism due to the formation of gap states.

Reconstruction and magnetic structure of ultrathinγ-Fe films on Cu(111)

We report ab initio investigations of the magnetic structure and reconstruction of ultrathin films of $\ensuremath{\gamma}$-Fe on Cu(111) substrates. For this system striking differences in the magnetic properties of films produced by thermal deposition and pulse-laser deposition have been reported [J. Shen et al. J. Phys.: Condens. Matter $15,$ R1 (2003)]. We find that unlike for $\ensuremath{\gamma}$-Fe films on Cu(001), the geometrical and magnetic structures of Fe/Cu(111) films cannot be explained in terms of simple ferromagnetic or layered antiferromagnetic structures stacked along the surface normal. Instead, we find that the (bi)layer antiferromagnetic structures oriented along [001] representing the magnetic ground state of Fe/Cu(001) films with four or more monolayers also determine the magnetic structure of Fe/Cu(111), even in the monolayer limit. In both cases, the stability of the bilayer antiferromagnetic structure is related to the stability of this type of magnetic ordering for nearly cubic or tetragonally distorted bulk $\ensuremath{\gamma}$-Fe. However, a ferromagnetic state of films with 2--4 ML (monolayers) is only about 6 meV/Fe atom higher in energy. Ferromagnetic ordering is coupled to strong monoclinic shear distortions in the bulk as well as in thin Fe/Cu(001) films. In Fe/Cu(111), however, the close packing in the layer leads to a quite large elastic energy for the large lateral displacements coupled to the ferromagnetic ordering so that the antiferromagnetic structure is marginally more stable from 1 to 4 ML. The coupling between magnetic structure and surface reconstruction could open a way for an experimental verification of our predictions.

Development of Organic Recording Media for Blue High Numerical Aperture Optical Disc System

Organic write once media is developed for the Blu-ray disc format, namely, a system comprising a blue wavelength laser diode of 405 nm and a high numerical aperture (NA) lens of 0.85. The first generation Blu-ray disc format employs phase change media. Thus, the organic write once media was designed with a reflectivity modulation scheme in order to retain compatibility. A very simple layer structure is proposed, SiO2/Dye/SiO2 without any metal reflective layer, and the media showed sufficient modulation amplitude and signal quality. The optical constants before and after the recording were measured to clarify the recording mechanism, and the result matched well with the readout waveform in the optical disc system. The measured jitter level after random pattern recording at a density of 23.3 GB per disc was 6.7% using a limit equalizer, which satisfies the Blu-ray disc format.

Critical steps in the early evolution of the isocortex: insights from developmental biology.

This article proposes a comprehensive view of the origin of the mammalian brain. We discuss i) from which region in the brain of a reptilian-like ancestor did the isocortex originate, and ii) the origin of the multilayered structure of the isocortex from a simple-layered structure like that observed in the cortex of present-day reptiles. Regarding question i there have been two alternative hypotheses, one suggesting that most or all the isocortex originated from the dorsal pallium, and the other suggesting that part of the isocortex originated from a ventral pallial component. The latter implies that a massive tangential migration of cells from the ventral pallium to the dorsal pallium takes place in isocortical development, something that has not been shown. Question ii refers to the origin of the six-layered isocortex from a primitive three-layered cortex. It is argued that the superficial isocortical layers can be considered to be an evolutionary acquisition of the mammalian brain, since no equivalent structures can be found in the reptilian brain. Furthermore, a characteristic of the isocortex is that it develops according to an inside-out neurogenetic gradient, in which late-produced cells migrate past layers of early-produced cells. It is proposed that the inside-out neurogenetic gradient was partly achieved by the activation of a signaling pathway associated with the Cdk5 kinase and its activator p35, while an extracellular protein called reelin (secreted in the marginal zone during development) may have prevented migrating cells from penetrating into the developing marginal zone (future layer I).

Capacitance–voltage study of single-crystalline Si dots on ultrathin buried SiO2 formed by nanometer-scale local oxidation

By applying nanometer-scale local oxidation process to a silicon-on-insulator (SOI) wafer with an ultrathin (2 nm) buried oxide, single-crystalline Si dots were fabricated on the tunnel SiO2, and their electronic properties were studied by capacitance–voltage (C–V) measurements. The C–V curves were primarily interpreted by electron tunneling between the dots and the base substrate. More importantly, the gap states at the dot/SiO2 interface were always observed as a shoulder in the C–V curve. Since such a shoulder was not observed for a simple layered SOI structure, it is strongly suggested that the dot structure inevitably accompanies a large number of the interface gap states, probably due to the undulation of the interface.

Mechanism of the gasochromic coloration of porous WO 3 films

Films of WO 3 were coated with platinum and coloured by exposure to diluted hydrogen gas. Bleaching was achieved with diluted oxygen. Because of its simple layer structure, this so-called gasochromic device is suitable for large-area window applications. By variation of several parameters, such as film thickness and gas concentration, and by considering the bleaching in vacuum and argon, we found a model for the mechanism of the reaction. It is strikingly different to the ideas of the double injection model, and there are indications that the gasochromic and electrochromic reactions are similar for porous films. According to our model, H 2 is dissociated on the platinum, transferred into a pore or grain boundary of the WO 3 and creates water and an oxygen vacancy. The O vacancy diffuses into the interior of the grain and the water slowly leaves the film. The bleaching in argon or vacuum is assumed to be the reverse reaction. The bleaching by oxygen is suggested to occur by by the dissociation of the O 2 on the platinum, transfer into a pore and recombination with the oxygen vacancy.

Scalar mixing and dissipation rate in large-eddy simulations of non-premixed turbulent combustion

Predictions of scalar mixing and the scalar dissipation rate from large-eddy simulations of a piloted nonpremixed methane/air diffusion flame (Sandia flame D) using the Lagrangian-type flamelet model are presented. The results obtained for the unconditionally filtered scalar dissipation rate are qualitatively compared with general observations of scalar mixing from experiments in non-reactive and reactive jets. In agreement with experimental data, provided the reaction zone has an inward direction, regions of high scalar dissipation rate are organized in layerlike structures, inwardly inclined to the mean flow and aligned with the instantaneous reaction zone. The analysis of single-point time records of the mixture fraction reveals ramplike structures, which have also been observed experimentally and are believed to indicate large-scale turbulent structures. The probability density function (pdf) of the instantaneous resolved scalar dissipation rate at stoichiometric mixture evaluated at cross sections normal to the the nozzle axis is shown to be described accurately by a lognormal pdf with σ=1. A new model for the conditionally averaged scalar dissipation rate has been proposed and is shown to account for local deviations from the simple mixing layer structure. The stabilizing effect of the pilot flame in the present configuration is also discussed. Finally, the influence of the resolved fluctuations of the scalar dissipation rate on the flame structure is investigated, revealing only a weak influence on temperature and nitric oxide predictions. However, the model requires further refinement for situations in which local extinction events become important.

Synthesis and Cation Distribution of the Spinel Cobaltites CuxMyCo3-(x+y)O4 (M = Ni, Zn) Obtained by Pyrolysis of Layered Hydroxide Nitrate Solid Solutions

A series of hydroxide nitrate solid solutions with a simple layered structure and general formula CumMnCo2-(m+n)(OH)3NO3, (M = Ni, Zn) has been synthesized by double jet precipitation technique, from the corresponding aqueous metal nitrate solutions and NaOH. X-ray diffraction, infrared spectroscopy, and thermal analysis studies have shown that they are single phases which decompose thermally at about 200 °C, yielding the corresponding single-phase spinel-type oxides CuxMyCo3-(x+y)O4 (x = 3m/2, y = 3n/2). The cation distribution in these complex spinels has been calculated on the basis of the tetrahedral and octahedral M−O distances determined by X-ray Rietveld refinement of their structures.

AlGaInAs/InP strained-layer quantum well lasers at 1.3 µm grown by solid source molecular beam epitaxy

AlxGayIn1−x−yAs/InP strained-layer multiple-quantum-well lasers emitting at 1.3 µm have been grown by solid source molecular beam epitaxy, and the performance characteristics have been studied. The lasers contain 4, 5, or 6 compressively strained quantum wells in the active region. They exhibit low transparency current densities, high gain coefficients, and high characteristic temperatures compared to conventional GaInAsP/InP quantum well lasers. The results show that desired lasing features can be achieved with relatively simple layer structures if the doping profiles and waveguide structures are properly designed and the material is grown to high structural perfection.

Novel piezoelectric structures for sensor and actuator applications

The piezoelectric behaviour of ferroelectric ceramics is commonly utilised for sensors and actuators. Simple shapes (bimorph beams, moonies and rainbows), thin films, composite and layered structures have been used thus far. Processing technologies developed at the University of Birmingham allow the production of novel complex shapes which, as well as improving the mechanical strength of the ceramic, may well allow the application of piezoelectric elements into areas previously unexplored. This paper concentrates on circular cross-sectional shapes in the form of helical springs, and describes the preliminary investigation carried out to characterise the response of springs to both dynamic and static impulses. The application of such geometries to sensor and actuator devices is discussed and possible areas of further work to realise the potential of the processing technology are also outlined.

Translaminar Reinforced Composites: A Review

A trans-laminar-reinforced (TLR) composite may be defined as a composite laminate with up to 5% volume of fibrous reinforcement oriented in a translaminar or through-thickness direction. The TLR can be continuous threads as in stitched laminates, or it can be discontinuous rods or pins as in Z-fiber materials. These materials may be considered a subset of 3D composites, with the distinction that the material structure is a simple layered structure with only a few percent volume of reinforcement through-the-thickness. It has been repeatedly documented in the literature that adding either type of TLR to an otherwise two-dimensional laminate results in the following advantages: substantially improved compression-after-impact response, considerably increased fracture toughness in Mode I (double cantilever beam) and Mode II (end notch flexure), and severely restricted size and growth of impact damage and edge delamination. TLR has also been used to eliminate catastrophic stiffener disbonding in stiffened structures, and in cocured structures it may be used as a substitute for mechanical fasteners. TLR directly protects the Achilles’ heel of laminated composites, that is, delamination. As little as 1% volume of TLR significantly alters the mechanical response of laminates. While there is a significant volume of literature, which is reviewed in this paper, there is still a very incomplete understanding of the mechanisms and parameters affecting those mechanisms responsible for the significant changes in the laminate response.

Layered Structure Analysis of Multilayers by Use of X-Ray Reflectometry and X-Ray Fluorescence Analysis

A technique of layered structure analysis using X-ray reflectometry and X-ray fluorescence analysis was investigated for multi-layered thin films. Standard samples consisting of a simple layered structure Si/Ta (50)/X (x)/Ta (100A) (X = Cu, NiFe, CoFe, PdPtMn, x=10-300A) were fabricated to calibrate the elemental sensitivity coefficients of X-ray fluorescence measurement. The stuctural parameters of the standard samples, such as the thicknesses, densities, and interface widths of each laser, were determined by X-ray reflectometry. Using the evaluated values of thicknesses and densities for the standard samples, elemental sensitivity coefficients whose accuracy was within 5% were calibrated at a suitable level for thin films. We found that the thickness of each layer of a spin-valve film can be evaluated within an accuracy of 5% by X-ray fluorescence measurement, and the thicknesses obtained by X-ray fluorescence measurement agreed well with those given by cross-sectional TEM and X-ray reflectometry.

Pulsed second-harmonic generation in nonlinear, one-dimensional, periodic structures

We present a numerical study of second-harmonic (SH) generation in a one-dimensional, generic, photonic band-gap material that is doped with a nonlinear ${\ensuremath{\chi}}^{(2)}$ medium. We show that a 20-period, 12-\ensuremath{\mu}m structure can generate short SH pulses (similar in duration to pump pulses) whose energy and power levels may be 2--3 orders of magnitude larger than the energy and power levels produced by an equivalent length of a phase-matched, bulk medium. This phenomenon comes about as a result of the combination of high electromagnetic mode density of states, low group velocity, and spatial phase locking of the fields near the photonic band edge. The structure is designed so that the pump pulse is tuned near the first-order photonic band edge, and the SH signal is generated near the band edge of the second-order gap. This maximizes the density of available field modes for both the pump and SH field. Our results show that the ${\ensuremath{\chi}}^{(2)}$ response is effectively enhanced by several orders of magnitude. Therefore, mm- or cm-long, quasi-phase-matched devices could be replaced by these simple layered structures of only a few micrometers in length. This has important applications to high-energy lasers, Raman-type sources, and frequency up- and down-conversion schemes.

Cortical development: Layers of complexity

Studies of spontaneous mutant mice with neurological phenotypes, particularly the cloning and analysis of the genes responsible, are shedding light on the complex processes that lead to formation of the deceptively simple layered structure of the cerebral cortex.

Characterizing biological tissue using scanning laser acoustic microscopy

The scanning laser acoustic microscopy (SLAM) technique offers high resolution for determining the properties of individual layers. To introduce the SLAM technique as a method for measuring the speed of sound in tubular layered organs, and possibly distinguish between the layers by this measure, the authors chose the guinea-pig urethra, due to its relatively simple layered structure. To obtain more data of the acoustic properties of tissue, the attenuation coefficient was determined for the wall in toto.

Variations of multi-dimensional supramolecular structures built of the two-dimensional [Cd(py)2{Ag(CN)2}2] n network: Three-dimensional textile structures of catena-poly[trans-bis(pyridine)cadmium(II)-di-?-{dicyanoargentato(I)-N,N?}�G (G = benzene or pyrrole) and two-dimensional layer structure of catena-poly[trans-bis(pyridine)cadmium(II)-di-?-{dicyanoargentato(I)-N,N?}

The three title supramolecular structures of [Cd(py)2{Ag(CN)2}2]·C6H61a, [Cd(py)2·{Ag(CN)2}2]·C4H5N1b and [Cd(py)2{Ag(CN)2}2]1c (py = pyridine) have been determined by single crystal X-ray diffraction.1a: tetragonalI4122,a = 12.7816(8),c = 13.062(1) A,U = 2133.9(3) A,Z = 4,R = 0.030 for 958 reflections;1b: orthorhombic,I212121,a = 12.861(2),b = 12.438(3),c = 13.105(2) A,U = 2096.3(5) A3,Z = 4,R = 0.045 for 1272 reflections;1c:C2/ma = 8.665(2),b = 14.139(1),c = 7.549(1) A, β = 93.15(2)°,U = 923.3(3) A3,Z = 2,R = 0.043 for 977 reflections. All the structures involve a two-dimensional (2D) network of [Cd(-NCAgCN-Cd1/4)4]n with a rhombus mesh from which network a pair of unidentate py ligands protrude at the trans positions of Cd. In clathrates1a and1b the 3D textile hosts interwoven by the layers of the 2D networks accommodate a benzene and pyrrole molecule at every centre of the mesh, respectively.1c has a simple layer structure stacked by the 2D networks the mesh of which is penetrated by the py ligands from the upper and lower layers.

Neural networks for statistical recognition of continuous speech

In recent years there has been a significant body of work, both theoretical and experimental, that has established the viability of artificial neural networks (ANN's) as a useful technology for speech recognition. It has been shown that neural networks can be used to augment speech recognizers whose underlying structure is essentially that of hidden Markov models (HMM's). In particular, we have demonstrated that fairly simple layered structures, which we lately have termed big dumb neural networks (BDNN's), can be discriminatively trained to estimate emission probabilities for an HMM. Recently simple speech recognition systems (using context-independent phone models) based on this approach have been proved on controlled tests, to be both effective in terms of accuracy (i.e., comparable or better than equivalent state-of-the-art systems) and efficient in terms of CPU and memory run-time requirements. Research is continuing on extending these results to somewhat more complex systems. In this paper, we first give a brief overview of automatic speech recognition (ASR) and statistical pattern recognition in general. We also include a very brief review of HMM's, and then describe the use of ANN's as statistical estimators. We then review the basic principles of our hybrid HMM/ANN approach and describe some experiments. We discuss some current research topics, including new theoretical developments in training ANN's to maximize the posterior probabilities of the correct models for speech utterances. We also discuss some issues of system resources required for training and recognition. Finally, we conclude with some perspectives about fundamental limitations in the current technology and some speculations about where we can go from here.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>