Well-defined single-walled carbon nanotube fibers as quantum wires: Ballistic conduction over micrometer-length scales

We report quasi-ballistic conduction in single-walled carbon nanotube (SWNT) fibers at room temperature, with a measured mean free path of 1.0–2.6 μm. The dynamic submersion of vertical SWNT fibers into liquid mercury (Hg) electrode shows plateaus and steps indicative of quasi-ballistic electron transport. This response is described with a newly developed network model that uses surface impurities to simplify the parallel conducting channels. The quasi-ballistic SWNT fibers exhibit a resistance per unit length of 2.5–6.5 kΩ/μm and a mean free path exceeding 1 μm, a length longer than typical via dimensions in existing Si-chip technologies. These results highlight that SWNT fiber conductivity can be enhanced by increasing the metallic to semiconducting SWNT ratio and reducing the surface impurities.     

Single-element ultrasonic transducer modeling using a hybrid FD–PSTD method

In a recent publication [E. Filoux, S. Callé, D. Certon, M. Lethiecq, F. Levassort, Modeling of piezoelectric transducers with combined pseudospectral and finite-difference methods, J. Acoust. Soc. Am. 123 (6) (2008) 4165–4173], a new finite-difference/pseudospectral time-domain (FD–PSTD) algorithm was presented and used to model the generation of acoustic waves by a piezoelectric resonator and their propagation in the structure and the surrounding water. In this paper, the model has been extended to simulate the two-dimensional behaviour of a complete single-element transducer, composed of the resonator, a backing and a front matching layer. This further version of the model takes into account the mechanical loss in materials, and enables the calculation of electrical impedance, which is a characteristic of high interest to optimize the performance of ultrasonic transducers. The impedance curves of a PZT [URL: http://www.ferroperm-piezo.com (last viewed 04/2008); B. Jaffe, R.S. Roth, S. Marzullo, Piezoelectric properties of lead zirconate-lead titanate solid-solution ceramics, J. Appl. Phys. 25 (1954) 809–810] plate-based high-frequency transducer, with a 50 MHz thickness resonant frequency, were compared to those of a KLM model [R. Krimholtz, D.A. Leedom, G.L. Matthei, New equivalent circuit for elementary piezoelectric transducers, Electron. Lett. 6 (1970) 398–399] in the one-dimensional case. The acoustical properties were also found to be in good agreement with those obtained using the finite element (FE) method of ATILA^® software in two-dimensional configuration.     

The impact of GARCH on asymmetric unit root tests

Using Monte Carlo simulation, threshold autoregressive (TAR) and momentum-threshold autoregressive (MTAR) asymmetric unit root tests are examined in the presence of generalised autoregressive conditional heteroskedasticity (GARCH). It is shown that TAR and MTAR unit root tests exhibit greater size distortion than the original (implicitly symmetric) Dickey–Fuller unit root test when applied to series exhibiting GARCH. Importantly, it is found that the use of consistent-threshold estimation increases the oversizing of the resulting asymmetric unit root test whether based upon the TAR or the MTAR model. The extent of oversizing of all tests considered is shown to be positively dependent upon the size of the volatility parameter of the GARCH model. The relevance of the simulation analysis conducted is supported by GARCH modelling of the term structure of US interest rates. The results of the current analysis indicate that if GARCH behaviour is suspected in economic or financial data, practitioners should interpret the results of asymmetric unit root tests with care to avoid drawing a spurious inference of stationarity. The paper concludes by suggesting future areas of research prompted by the present findings.     

Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine

Four conformers of tryptamine have been detected in a supersonic expansion and characterized by laser ablation molecular beam Fourier transform microwave spectroscopy LA-MB-FTMW in the 5–10 GHz frequency range. The quadrupole hyperfine structure originated by two ¹⁴N nuclei has been completely resolved for all conformers and used for their unambiguous identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group involved in N–H?π interactions: to the π electronic system of the pyrrole unit in the Gauche-Pyrrole conformers (GPy) or to the phenyl unit in the Gauche-Phenyl ones.     

Modelling of nonlinear crack–wave interactions for damage detection based on ultrasound—A review

The past decades have been marked by a significant increase in research interest in nonlinearities in micro-cracked and cracked solids. As a result, a number of different nonlinear acoustic methods have been developed for damage detection. A general consensus is that – under favourable conditions – nonlinear effects exhibited by cracks are stronger than crack-induced linear phenomena. However, there is still limited understanding of physical mechanisms related to various nonlinearities. This problem remains essential for implementation of nonlinear acoustics for damage-detection applications. This paper reviews modelling approaches used for nonlinear crack–wave interactions. Various models of classical and nonclassical crack-induced elastic, thermo-elastic and dissipative nonlinearities have been discussed.     

In vivo comparison of three ultrasound vector velocity techniques to MR phase contrast angiography

The objective of this paper is to validate angle independent vector velocity methods for blood velocity estimation. Conventional Doppler ultrasound (US) only estimates the blood velocity along the US beam direction where the estimate is angle corrected assuming laminar flow parallel to vessel boundaries. This results in incorrect blood velocity estimates, when angle of insonation approaches 90° or when blood flow is non-laminar. Three angle independent vector velocity methods are evaluated in this paper: directional beamforming (DB), synthetic aperture flow imaging (STA) and transverse oscillation (TO). The performances of the three methods were investigated by measuring the stroke volume in the right common carotid artery of 11 healthy volunteers with magnetic resonance phase contrast angiography (MRA) as reference. The correlation with confidence intervals (CI) between the three vector velocity methods and MRA were: DB vs. MRA: R = 0.84 (p < 0.01, 95% CI: 0.49–0.96); STA vs. MRA: R = 0.71 (p < 0.05, 95% CI: 0.19–0.92) and TO vs. MRA: R = 0.91 (p < 0.01, 95% CI: 0.69–0.98). No significant differences were observed for any of the three comparisons (DB vs. MRA: p = 0.65; STA vs. MRA: p = 0.24; TO vs. MRA: p = 0.36). Bland–Altman plots were additionally constructed, and mean differences with limits of agreements (LoA) for the three comparisons were: DB vs. MRA = 0.17 ml (95% CI: −0.61–0.95) with LoA = −2.11–2.44 ml; STA vs. MRA = −0.55 ml (95% CI: −1.54–0.43) with LoA = −3.42–2.32 ml; TO vs. MRA = 0.24 ml (95% CI: −0.32–0.81) with LoA = −1.41–1.90 ml. According to the results, reliable volume flow estimates can be obtained with all three methods. The three US vector velocity techniques can yield quantitative insight into flow dynamics and visualize complex flow patterns, which potentially can give the clinician a novel tool for cardiovascular disease assessment.     

Low-loss optical magnetic metamaterials on Ag–Au bimetallic fishnets

We have designed a low-loss magnetic metamaterial with double-fishnet structures of silver and gold operating at near-infrared wavelength. A Particle swarm optimization algorithm was employed to optimize the geometry dimensions of Ag–dielectric–Au unit cell. The Ag–Au bimetallic fishnet produces a maximum figure of merit value of 21, transmission as high as 62%, moderate negative permeability Re(μ) of −0.5, and Re(n) of −1 at 1463 nm. The negative permeability Re(μ) reaches minimum value of −5 at 1573 nm, where the antisymmetric charge distribution in fishnet metamaterials was observed. The antisymmetric plasmonic resonance at 1573 nm is a mixed mode of asymmetric magnetic dipole and symmetric electrical quadrupole in Ag–MgF₂–Au fishnet structures.     

Reduced basis method for multi-parameter-dependent steady Navier–Stokes equations: Applications to natural convection in a cavity

This work focuses on the approximation of parametric steady Navier–Stokes equations by the reduced basis method. For a particular instance of the parameters under consideration, we are able to solve the underlying partial differential equations, compute an output, and give sharp error bounds. The computations are split into an offline part, where the values of the parameters are not yet identified, but only given within a range of interest, and an online part, where the problem is solved for an instance of the parameters. The offline part is expensive and is used to build a reduced basis and prepare all the ingredients – mainly matrix–vector and scalar products, but also eigenvalue computations – necessary for the online part, which is fast.     

Studies on the properties of sputter-deposited Al-doped ZnO films

ZnO is a well known material; however, the research interest in this material is still high enough because ZnO is one of the materials with the most potential for optoelectronics due to its promising properties of high conductivity as well as good transparency. In this work, aluminum doped zinc oxide films (ZnO:Al) were deposited by RF magnetron sputtering on glass and silicon substrates with different deposition times of 2, 3 and 4 h. The aim of this work is the study of the deposition time effect on the properties of ZnO:Al films. It is shown that films grow with the hexagonal c$c$-axis perpendicular to the substrate surface. The morphological characteristics show a granular and homogenous surface and the cristallinity of the films is enhanced with increased deposition time. The deposited films show good optical transmittance (80%–90%) in the visible and near infrared spectrum. The calculated band gap is about 3.3 eV. The electrical ZnO:Al/Si(p) junction properties were investigated using the Capacitance–Voltage (C–V$C\text{–}V$) dependence. Calculations of the built-in potential from classical 1/C²–V$1/C^2\text{–}V$ characterization give values between 0.54 and 0.71 V.     

Ultrasound-assisted extraction of resveratrol from functional foods: Cookies and jams

Rapid extraction techniques with ultrasonic systems have been established to extract resveratrol from two functional foods, namely cookies and jams. The ultrasound-assisted extraction (UAE) was optimized using a full factorial design with three variables: solvent composition (10–70% and 30–90% methanol in water for cookies and jams, respectively), solvent-to-solid ratio (10:1–40:1), and ultrasonic probe diameter (2 and 7 mm). The extraction kinetics (5–30 min) were studied to confirm the full recovery of resveratrol from the functional foods. The resveratrol was quantified by Ultrahigh Performance Liquid Chromatography with a Fluorescense Detector (UPLC-FD).     

How random is a random vector?

Over 80 years ago Samuel Wilks proposed that the “generalized variance” of a random vector is the determinant of its covariance matrix. To date, the notion and use of the generalized variance is confined only to very specific niches in statistics. In this paper we establish that the “Wilks standard deviation” –the square root of the generalized variance–is indeed the standard deviation of a random vector. We further establish that the “uncorrelation index” –a derivative of the Wilks standard deviation–is a measure of the overall correlation between the components of a random vector. Both the Wilks standard deviation and the uncorrelation index are, respectively, special cases of two general notions that we introduce: “randomness measures” and “independence indices” of random vectors. In turn, these general notions give rise to “randomness diagrams”—tangible planar visualizations that answer the question: How random is a random vector? The notion of “independence indices” yields a novel measure of correlation for Lévy laws. In general, the concepts and results presented in this paper are applicable to any field of science and engineering with random-vectors empirical data.     

A Langevin approach to the Log–Gauss–Pareto composite statistical structure

The distribution of wealth in human populations displays a Log–Gauss–Pareto composite statistical structure: its density is Log–Gauss in its central body, and follows power-law decay in its tails. This composite statistical structure is further observed in other complex systems, and on a logarithmic scale it displays a Gauss-Exponential structure: its density is Gauss in its central body, and follows exponential decay in its tails. In this paper we establish an equilibrium Langevin explanation for this statistical phenomenon, and show that: (i) the stationary distributions of Langevin dynamics with sigmoidal force functions display a Gauss-Exponential composite statistical structure; (ii) the stationary distributions of geometric Langevin dynamics with sigmoidal force functions display a Log–Gauss–Pareto composite statistical structure. This equilibrium Langevin explanation is universal — as it is invariant with respect to the specific details of the sigmoidal force functions applied, and as it is invariant with respect to the specific statistics of the underlying noise.     

Short- and midterm repeatability of magnetic resonance elastography in healthy volunteers at 3.0 T

The purpose of this study was to evaluate the short- and midterm repeatability of liver stiffness measurements with magnetic resonance elastography (MRE) in healthy subjects at 3.0 T. Twenty-two healthy volunteers were enrolled in this prospective study. The stiffness measurements were obtained from three slices with three repeated acquisitions for each slice (session 1) by two independent raters. After a mean period of 7 ± 2 days (session 2) and 195 ± 15 days (session 3), each subject was scanned again using the same protocol and MR system. The liver stiffness differences were calculated between sessions or raters. The intraclass correlation coefficient (ICC) was calculated to assess interrater agreement and intersession agreement. The stiffness differences over the short- and midterm intervals was (− 0.004 ± 0.086) kPa for sessions 1–2, lower than (− 0.055 ± 0.150) kPa for sessions 1–3 and (− 0.051 ± 0.173) kPa for sessions 2–3. The liver stiffness was more repeatable for the short-term interval with the mean overall ICC of 0.96 (sessions 1–2) (95% confidence interval [CI]: 0.90–0.98) compared with 0.91 (sessions 1–3) (95% CI: 0.78–0.96) and 0.87 (sessions 2–3) (95% CI: 0.69–0.95) for the midterm intervals. The overall ICC of interrater agreement was excellent at 0.987 (95% CI: 0.983 to 0.990). These results confirm that MRE is a reproducible technique for liver stiffness quantification over short- and midterm intervals up to 6 months in a healthy population at 3.0 T.     

Green photoluminescence from highly oriented ZnO thin film for photovoltaic application

We report here the fabrication of ZnO nanoparticles embedded on glass substrate by sol–gel and spin coating technique. Transmission electron microscope images revealed that the thin film is composed of ZnO nanoparticles. X-ray diffraction data confirms that the fabricated ZnO nanoparticles have hexagonal unit cell structure. The ZnO nanocrystals of the thin film are oriented along the c-axis of the hexagonal unit cell. UV–vis absorption spectroscopy shows that the absorption occurring at 373 nm in the ZnO thin film. The band gap was calculated from the absorption data and found to be 3.76 eV. This band gap enhancement occurs due to size effect in the nanoscale regime. Room temperature photoluminescence spectrum shows strong green emission at 530 nm owing to the singly ionized oxygen vacancy. This green emission was further investigated by annealing the thin film at different temperature. This singular green emission will be very useful in optoelectronic and nanophotonic devices.     

Controlled fabrication of nano-scale double barrier magnetic tunnel junctions using focused ion beam milling method

The controlled fabrication method for nano-scale double barrier magnetic tunnel junctions (DBMTJs) with the layer structure of Ta(5)/Cu(10)/Ni₇₉Fe₂₁(5)/Ir₂₂Mn₇₈(12)/Co₆₀Fe₂₀B₂₀(4)/Al(1)–oxide/Co₆₀Fe₂₀B₂₀(6)/Al(1)–oxide/Co₆₀Fe₂₀B₂₀(4)/Ir₂₂Mn₇₈(12)/Ni₇₉Fe₂₁(5)/Ta(5) (thickness unit: nm) was used. This method involved depositing thin multi-layer stacks by sputtering system, and depositing a Pt nano-pillar using a focused ion beam which acted both as a top contact and as an etching mask. The advantages of this process over the traditional process using e-beam and optical lithography in that it involve only few processing steps, e.g. it does not involve any lift-off steps. In order to evaluate the nanofabrication techniques, the DBMTJs with the dimensions of 200 nm×400 nm, 200 nm×200 nm nano-scale were prepared and their R–H, I–V characteristics were measured.     

HCC-to-liver contrast on arterial-dominant phase images of EOB-enhanced MRI: comparison with dynamic CT

The purpose of this study was to assess the efficacy of arterial-dominant phase images of gadolinium–ethoxybenzyl–diethylenetriamine pentaacetic acid (EOB)-enhanced magnetic resonance imaging (MRI) for evaluation of arterial blood supply in hepatocellular carcinoma (HCC) in comparison with that of multiphasic dynamic computed tomography (CT). This study comprised 30 patients (22 men and 8 women, mean age: 68.0 years) with 40 pathologically proven HCCs (well differentiated: 3, moderately differentiated: 30, poorly differentiated: 7, mean diameter: 45.1 mm), all of whom underwent EOB-enhanced MRI and dynamic CT preoperative assessment. Regions of interest were placed over HCCs and the adjacent normal liver, and signal intensities or CT values were measured by two experienced abdominal radiologists on the arterial-dominant phase images of EOB-enhanced MRI and dynamic CT images. HCC-to-liver contrasts [Michelson's contrast: C_M=(S_HCC− S_Liver)/(S_HCC+ S_Liver)] were calculated and compared among the modalities. HCC-to-liver contrasts were also visually scored on a 5-point scale and compared. The mean C_M and visual score for dynamic CT were significantly higher than those for EOB-enhanced MRI. Good agreements were obtained among the two observers. Dynamic CT is a more suitable modality than EOB-enhanced MRI for evaluation of arterial blood supply in HCC. This should be taken into account for diagnosis and management of HCC.     

Giant magnetostriction in rapidly quenched Fe–Ga?

Substituting Fe by nonmagnetic Ga causes a dramatic increase of the magnetostriction. The reason for this effect is related to structure and also due to softening of the elastic properties. Of special interest is that in literature “giant” magnetostriction values (up to 2100 ppm) for rapidly quenched Fe–Ga (15–20% Ga) ribbons were reported. In this work, careful investigations using a strain gauge method as well as a capacitance cell were performed. Especially for the case applying an external field perpendicular to the ribbon plane, it is demonstrated that bending effects can occur and they are difficult to avoid without introducing any stress into the sample. This effect leads to large signals in the strain gauge of more than ±3000 ppm, which sign depends on the occurrence of strain or stress. Experiments on a 25-μm-thin Fe foil leads to similar results. Avoiding bending by gluing ribbons or thin foils or splat-cooled thin pure Ni on a thin plastic plate, gave magnetostriction values close to those of polycrystalline bulk materials.     

Structural and DC conduction studies on vacuum evaporated BaTiO3 thin films prepared from BaTiO3 nano particles synthesised by wet chemical method

BaTiO₃ nanoparticles prepared by wet chemical method were thermally grown onto well cleaned glass substrates under the vacuum of 2 × 10⁻⁵ Torr, using 12A4 Hind Hivac coating unit. An Al–BaTiO₃–Al sandwich structure has been used for electrical conduction properties in the temperature range 303–423 K. The composition of nanoparticles and thin films were identified by EDS spectrum. The structural studies have been performed by the X-ray diffraction (XRD) technique. The X-ray analysis showed that the nano particle has a tetragonal structure and deposited films at a lower thickness amorphous in nature, whereas the crystallinity increases with increase of thickness. In the DC conduction studies, the current–voltage characteristics of the films showed ohmic conduction in the low voltage region. In the higher voltage region, a space charge limited conduction (SCLC) takes place due to the presence of the trapping level. The activation energy was estimated and the values found to decrease with increasing applied voltage. The zero field value of the activation energy is found to be 0.31 eV. The free carrier mobility, carrier density and trap density values were calculated and reported in this paper.     

Anomalous is ubiquitous

Brownian motion is widely considered the quintessential model of diffusion processes—the most elemental random transport processes in Science and Engineering. Yet so, examples of diffusion processes displaying highly non-Brownian statistics–commonly termed “Anomalous Diffusion” processes–are omnipresent both in the natural sciences and in engineered systems. The scientific interest in Anomalous Diffusion and its applications is growing exponentially in the recent years. In this Paper we review the key statistics of Anomalous Diffusion processes: sub-diffusion and super-diffusion, long-range dependence and the Joseph effect, Lévy statistics and the Noah effect, and 1/f noise. We further present a theoretical model–generalizing the Einstein–Smoluchowski diffusion model–which provides a unified explanation for the prevalence of Anomalous Diffusion statistics. Our model shows that what is commonly perceived as “anomalous” is in effect ubiquitous.     

Effects of composition of catalysts on the preparation of single-walled carbon nanotubes synthesized over W–Co–MgO catalysts

A series of W–Co–MgO catalysts were prepared for the first time by decomposing a mixture of magnesium nitrate, ammonium paratungstate, citric acid, and cobalt nitrate. Single-walled carbon nanotubes (SWCNTs) were synthesized over these W–Co–MgO catalysts and the effects of the quantity of metal in the catalysts on the synthesis of SWCNTs were investigated by Raman spectroscopy and transmission electron microscopy (TEM). The results show that, among W–Co–MgO catalysts, the W1–Co5 catalyst was found to be most effective for synthesizing SWCNTs. The diameter distribution of as-grown SWCNTs prepared over the W1–Co5 catalyst was estimated to range from 0.72–1.64 nm. When the molar ratios of W:MgO and Co:MgO in the catalysts are more than 2:100 and 5:100, respectively, the amorphous carbon content or defect concentration of SWCNTs may be increased with the increase of the quantity of metal in the catalysts. The dependence of the diameter distribution of SWCNTs on the quantity of W in the catalysts is small. However, the proportion of SWCNTs with larger diameter is increased as the quantity of Co in the catalysts is increased owing to the increase in the number of larger active sites.