bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of the ACE Inhibitor Quinapril on Echocardiographic Variables in Horses with Mitral Valve Insufficiency
Authors: Gehlen H. , Vieht J. C. , Stadler P.
Published: January 2004

Twenty horses with mitral valve insufficiency, but without signs of congestive heart failure, and five horses without signs of heart disease were examined before and after medication with an angiotensin‐converting enzyme (ACE) inhibitor. The examination included echocardiography assessment as well as heart catheterization. The echocardiographic examination included B‐mode, M‐mode, conventional and colour Doppler techniques. For 8 weeks, all horses were treated with Accupro 20®(active substance: Quinapril) at an oral dose rate of 120 mg/horse/day. A follow‐up of the horses with mitral valve insufficiency after 8 weeks revealed a statistically significant increase in the stroke volume and the cardiac output as well as a decrease in regurgitation velocity time integral (VTI). The regurgitation blood velocity remained the same. The severity of mitral valve insufficiencies revealed a moderate improvement in five horses, from moderate to mild, after therapy. Significant changes of cardiac dimension (B‐mode) and shortening fraction (M‐mode) before and after treatment could not be observed. The owners’ judgement of the horses’ performance was that of a minor improvement. In the horses without clinical findings the results of examination before and after treatment remained the same.
The future of the cuffed endotracheal tube
Authors: Fine Gavin F. , Borland Lawrence M.
Published: January 2004

It has been traditionally taught that only uncuffed endotracheal tubes (ETTs) should be used for intubation in children younger than 8, or even 10, years old. However, recent literature suggests that the advantages of using uncuffed ETTs in children may be just another myth of paediatric anaesthesia. Using an uncuffed ETT does allow a tube of larger internal diameter to be used, minimizing resistance to airflow and the work of breathing in the patient who is breathing spontaneously. However, this advantage does not hold for ventilated patients, for whom ventilator settings can be adjusted to provide optimal airflow. Longer duration of intubation and a poorly fitted ETT are risk factors for mucosal damage, whether the ETT is cuffed or uncuffed. Furthermore, a properly sized, positioned, and inflated modern (low‐pressure, high‐volume) cuffed ETT can offer many advantages over an uncuffed ETT, including greater ease of intubation, better control of air leakage, lower rate and better control of flow of anaesthetic gases, and decreased risk of aspiration and infection.
Meta‐Analysis of Linkage Studies for Complex Diseases: An Overview of Methods and a Simulation Study
Authors: Dempfle A. , Loesgen S.
Published: January 2004

Linkage genome scans for complex diseases have low power with the usual sample sizes, and hence meta‐analysis of several scans for the same disease might be a promising approach. Appropriate data are now becoming accessible. Here we give an overview of the available statistical methods and current applications. In a simulation study, we compare the power of different methods to combine multipoint linkage scores, namely Fisher's p‐value combination, the truncated product method, the Genome Search Meta‐Analysis (GSMA) method and our weighting methods. In particular, we investigate the effects of heterogeneity introduced by different genetic marker sets and sample sizes between genome scans. The weighting methods explicitly take those differences into account and have more power in the simulated scenarios than the other methods.
On the Power of Affected Relative Pair Designs for Linkage Studies
Authors: LI Z. , GASTWIRTH J. L.
Published: January 2004

Using the recurrence risk ratio (λ), Risch (1990b) indicated that affected pairs of distant relatives are preferable to affected sib‐pairs for linkage analysis when λ is large and the mode of inheritance is additive. By using the optimum test for affected sib‐pairs instead of the test used by Risch (1990b), the range of values of λ for which the affected sib‐pair design has larger or smaller power than other pairs is clarified. Risch's conclusion remains true when λO > 2.5, however, sib‐pairs have larger power for lower values. As affected sib‐pairs occur smore frequently than other relative pairs, when ascertainment costs are non‐negligible, they may be the most cost‐effective relative pairs to use.
Statistics of Poincaré recurrences for maps with integrable and ergodic components
Authors: Hu H. , Rampioni A. , Rossi L. , Turchetti G. , Vaienti S.
Published: January 2004

Recurrence gives powerful tools to investigate the statistical properties of dynamical systems. We present in this paper some applications of the statistics of first return times to characterize the mixed behavior of dynamical systems in which chaotic and regular motion coexist. Our analysis is local: we take a neighborhood 𝒜 of a point x and consider the conditional distribution of the points leaving 𝒜 and for which the first return to 𝒜, suitably normalized, is bigger than t. When the measure of 𝒜 shrinks to zero the distribution converges to the exponential e−t for almost any point x, if the system is mixing and the set 𝒜 is a ball or a cylinder. We consider instead a system, a skew integrable map of the cylinder, which is not ergodic and has zero entropy. This map describes a shear flow and has a local mixing property. We rigorously prove that the statistics of first return is of polynomial type around the fixed points and we generalize around other points with numerical computations. The result could be extended to quasi-integrable area preserving maps such as the standard map for small coupling. We then analyze the distribution of return times in a region which is composed by two invariants subdomains: one with a mixing dynamics and the other with an integrable dynamics given by our shear flow. We show that the statistics of first return in this mixed region is asymptotically given by the exponential law, but this limit is attained by an intermediate regime where exponential and polynomial laws are linearly superposed and weighted by some factors which are proportional to the relative sizes of the chaotic and regular regions. The result on the statistics of first return times for mixed regions in the phase space can provide a basis to analyze such a property for area preserving maps in mixed regions even when a rigorous result is not available. To this end we present numerical investigations on the standard map which confirm the results of the model.
The rheology of a dilute suspension of Brownian dipolar spheroids in a simple shear flow under the action of an external force
Authors: Asokan K. , Ramamohan T. R.
Published: January 2004

The effect of rotational Brownian motion on the rheology of a dilute suspension of dipolar spheroids in a simple shear flow under the action of an external force field, is investigated through a generalized Langevin equation approach. The force field is assumed to be either constant or periodic. In the case of constant external fields earlier results in the literature are reproduced, while for the case of periodic forcing certain parametric regimes corresponding to weak Brownian diffusion are identified where the rheological parameters evolve chaotically and settle onto a low dimensional attractor. The response of the system to variations in the strengths of the force field and diffusion is also analyzed through numerical experiments. These results correspond to the region of weak Brownian motion where usual methods render the problem intractable.
Impaction of a droplet on an orifice plate
Authors: Hsu C. F. , Ashgriz N.
Published: January 2004

A novel method of generating secondary droplets using the impaction of a liquid droplet on an orifice plate is proposed. Here, a numerical simulation of this problem is provided using a one fluid volume-of-fluid-based method without considering air in the environment. The impacting droplet may partly spread on the surface of the plate, and partly penetrate through the orifice. The penetrated part may breakup forming a daughter droplet, as well as smaller satellite droplets. The sizes of the generated droplets are related to the volume of the liquid immediately on top of the orifice upon impact. The outcome of the impaction is discussed based on the sizes of the daughter and satellite droplet formed, their velocities, the breakup time, and the breakup length of the penetrated ligament. The influence of the following parameters on the outcome of the impaction is considered: liquid viscosity, liquid surface tension, initial droplet size and velocity, orifice diameter, and the liquid contact angle. A critical Rec number exists for any given We number, under which droplet generation is impossible. Generally, the size of the generated droplet is weakly influenced by the liquid viscosity, and strongly influenced by the liquid surface tension. The size of the daughter droplet increases with the orifice size and reduces with the liquid viscosity, whereas the size of the satellite droplet increases with the viscosity.
Ferroelectricity in BaTiO3 nanoscopic structures
Authors: Stachiotti M. G.
Published: January 2004

The ground-state polarization of BaTiO3 nanosized films and cells is studied using an atomic-level simulation approach based on a shell model with parameters obtained from first-principles calculations. We demonstrate that the critical thickness for ferroelectricity in a free-standing BaTiO3 stress-free film is 3.6 nm, and a decrease in lateral size to nanometric dimensions does not spoil the ferroelectric properties of the film. Nanocells with different lateral faces, BaO or TiO2 planes, present a different domain structure and polarization due to a strong surface effect.
Velocity oscillations in turbulent Rayleigh–Bénard convection
Authors: Qiu X.-L. , Shang X.-D. , Tong P. , Xia K.-Q.
Published: January 2004

A systematic study of velocity oscillations in turbulent thermal convection is carried out in small aspect-ratio cells filled with water. Local velocity fluctuations and temperature-velocity cross-correlation functions are measured over varying Rayleigh numbers and spatial positions across the entire convection cell. These structural measurements reveal how the thermal plumes interact with the bulk fluid in a closed cell and provide an interesting physical picture for the dynamics of the temperature and velocity oscillations in turbulent convection.
Slow flow through a brush
Authors: Tachie M. F. , James D. F. , Currie I. G.
Published: January 2004

This paper reports velocity measurements of slow flow through a model brush. The flow field was created in the annulus between two concentric cylinders, which was filled with a viscous oil. The inner cylinder was stationary and the outer one was installed on a turntable and rotated at a constant speed. A brush was modeled by an array of uniformly spaced rods mounted horizontally onto the inner cylinder. With a generous gap between the rod ends and the outer cylinder, the flow outside the array was circular Couette flow. Three brushes were made and the external shear flow penetrated these because the solid volume fractions were small, namely, 0.025, 0.05, and 0.10. Particle image velocimetry was used to study the velocity field in the penetration region, and from these measurements the velocity at the interface, “the slip velocity,” was determined. The slip velocity was found to be close to the value predicted by Brinkman’s equation, and to be higher than velocities found previously for rod arrays in other configurations.
Drop fall-off from the vibrating ceiling
Authors: Kim Ho-Young
Published: January 2004

This work investigates a scheme to rapidly remove liquid drops from a ceiling by vibrating the solid surface. We show that there exist resonant frequencies at which the minimum vibration amplitudes inducing a fall-off of the pendant drops are remarkably less than the detaching amplitudes at neighboring frequencies. These frequencies are shown to correspond to the resonance frequencies of different drop oscillation modes.
Aspects of wake vortex control through base blowing/suction
Authors: Arcas D. R. , Redekopp L. G.
Published: January 2004

Numerical simulations are used to explore the effect of blowing/suction on the characteristics of vortex shedding from a plane forebody with a rectangular base. The distribution of blowing/suction is varied to assess its role in either enhancing or suppressing the strength of shed vortices in the wake. It is shown that the amplitude of vortices in the near wake, and the required fluxes to suppress vortex shedding, scale differently depending on whether the control is exercised with blowing or with suction. These differences are rationalized in terms of local instability properties of mean velocity profiles in the near wake.
Highly efficient red phosphorescent light-emitting diodes based on ruthenium(II)-complex-doped semiconductive polymers
Authors: Xia Hong , Zhang Chengbo , Qiu Song , Lu Ping , Zhang Jingying , Ma Yuguang
Published: January 2004

Red electrophosphorescence from light-emitting devices based on ruthenium(II)-complex [Ru(4,7-Ph2-phen)3]2+-doped wide-band-gap semiconductive polymers, i.e., poly(vinylcarbazole) (PVK), polydihexylfluorene (PDHF), and ladderlike polyphenylene (LPPP), as the emitting layers are reported. However, only highly efficient energy transfer was investigated in a PVK system, not only because of the relatively longer lifetime of its excited state compared with PDHF and LPPP, but also because of the good chemical compatibility of [Ru(4,7-Ph2-phen)3]2+ with PVK. The EL spectra show the characteristic spectrum of [Ru(4,7-Ph2-phen)3]2+, at a peak of 612 nm and Commission Internationale del’Eclairage of (0.62, 0.37). The optimized device indium tin oxide/PVK: 5 wt % [Ru(4,7-Ph2-phen)3]2+/PBD/Alq3/LiF/Al shows the maximum luminance efficiency and power efficiency as 8.6 cd/A and 2.1 lm/W, respectively.
Revisit of viscous sublayer scaling law
Authors: Park Ju Yeop , Chung Myung Kyoon
Published: January 2004

Recent study has shown that the valid region of linear velocity profile in the viscous sublayer of turbulent channel flow is only the asymptotic limit y+→0 even at a very high Reynolds number. In the view of similarity, this result implies that the conventional complete similarity assumption, which is the basis of linear velocity profile, is not valid. In terms of extended similarity argument of G. I. Barenblatt [J. Eng. Math. 36, 361 (1999)], it is shown that the plausible similarity assumption for the viscous sublayer is the incomplete similarity in y+ and the complete similarity in h+(=huτ/ν), where h is channel half width. The resulting velocity profile based on these assumptions has a quartic term of y+ from Reynolds stress in addition to a linear term of y+ from viscous stress, which greatly enlarges the extent of valid region of similarity.
Stability of symmetric and asymmetric vortex pairs over slender conical wings and bodies
Authors: Cai Jinsheng , Luo Shijun , Liu Feng
Published: January 2004

Theoretical analyses are presented for the stability of symmetric and asymmetric vortex pairs over slender conical wings and bodies under small perturbations in an inviscid incompressible flow at high angles of attack and sideslip. The three-dimensional problem of a pair of vortices over slender conical wings and bodies is reduced to a problem in two dimensions by using the conical flow assumption and classical slender-body theory. The stability of symmetric and asymmetric vortex pairs over flat-plate delta wings, slender circular cones, and elliptic cones of various thickness ratios are examined. Results are compared with available experimental data.
Experimental measurement of the scale-by-scale momentum transport budget in a turbulent shear flow
Authors: Marié L. , Daviaud F.
Published: January 2004

We report measurements linking velocity fluctuations with the turbulent drag in a turbulent closed flow, namely the von Kármán flow. Making use of the angular momentum balance equation in integral form, we obtain a simple expression for the torque applied by the forcing mechanism, which we check against quantitative laser Doppler velocimetry measurements. We then decompose the angular momentum flux into contributions coming from the different spectral components of the flow. We provide evidence of the fact that the turbulent drag is dominantly generated by coherent structures at the largest scales of the flow.
The motion of a vortex near a gap in a wall
Authors: Johnson E. R. , McDonald N. Robb
Published: January 2004

Two models are presented for the motion of a vortex near a gap in an infinitely long straight barrier: a problem of geophysical significance. The first model considers a line vortex for which the trajectories are obtained analytically and are generalized to include simple ambient flows. The criterion determining whether in the absence of background flow a vortex originating far from the gap passes through or leaps across the gap is derived. The second model considers a finite area patch of constant vorticity and is based on conformal mapping and the numerical method of contour surgery. The two models enable a comparison of the trajectories of line vortices and vortex patches. In most examples the centroid of the patch is constrained to follow closely the trajectory of a line vortex of the same circulation. An exception occurs when flow through the gap forces the vortex patch close to one of the edges of the barrier where it splits into two with only one part of the vortex passing through the gap.
Direct numerical simulation and analysis of a spatially evolving supersonic turbulent boundary layer at M=2.25
Authors: Pirozzoli S. , Grasso F. , Gatski T. B.
Published: January 2004

A spatially developing supersonic adiabatic flat plate boundary layer flow (at M∞=2.25 and Reθ≈4000) is analyzed by means of direct numerical simulation. The numerical algorithm is based on a mixed weighted essentially nonoscillatory compact-difference method for the three-dimensional Navier–Stokes equations. The main objectives are to assess the validity of Morkovin’s hypothesis and Reynolds analogies, and to analyze the controlling mechanisms for turbulence production, dissipation, and transport. The results show that the essential dynamics of the investigated turbulent supersonic boundary layer flow closely resembles the incompressible pattern. The Van Driest transformed mean velocity obeys the incompressible law-of-the-wall, and the mean static temperature field exhibits a quadratic dependency upon the mean velocity, as predicted by the Crocco–Busemann relation. The total temperature has been found not to be precisely uniform, and total temperature fluctuations are found to be non-negligible. Consistently, the turbulent Prandtl number is not unity, and it varies between 0.7 and 0.8 in the outer part of the boundary layer. Nonetheless, a modified strong Reynolds analogy is still verified. In agreement with the low Mach number results, the streamwise velocity component and the temperature are only weakly anti-correlated. The turbulent kinetic energy budget also shows similarities with the incompressible case provided all terms of the equation are properly scaled; indeed, the leading compressibility contributions are negligible throughout the boundary layer.
Local spectrum of commutation error in large eddy simulations
Authors: Vasilyev Oleg V. , Goldstein Daniel E.
Published: January 2004

In this Brief Communication we present a new mathematical tool, which we call local spectrum analysis, that can be used to obtain information about local spectral content of the commutation error in large eddy simulations and its dependence on the filter shape and the non-uniformity of the filter width. To illustrate these theoretical findings, the local commutation spectrum analysis is applied to the results of 2563 direct numerical simulation of forced homogeneous turbulence at Reλ=168. The results confirm strong dependence of the spectral content of the commutation error on the filter shape: the spectrum is wide for smooth filters like Gaussian, while for filters, that are close to the sharp cut-off, the spectral content is localized. It is also demonstrated that the amplitude of the commutation error is linearly proportional to the filter width stretching factor.
Polarization reversal anti-parallel to the applied electric field observed using a scanning nonlinear dielectric microscopy
Authors: Morita Takeshi , Cho Yasuo
Published: January 2004

Ultrahigh-density storage devices consisting of poling reversed nanodots are examined using a scanning nonlinear dielectric microscope (SNDM). By using the SNDM, real-time observation of the poling direction was attempted and an unexpected phenomenon was discovered. For lithium tantalite films thicker than 350 nm, poling directions were aligned anti-parallel to the poling electric field. The critical thickness is thought to be dependent on the material properties, the probe radius, the applied voltage and the pulse duration. This anti-parallel poling phenomenon disagrees with previous poling reversal mechanisms from the conventional plate capacitor model. At present, the reason for and details of anti-parallel poling reversal are unclear, but may be related to the concentrated electric field near the cantilever.
High-efficiency tandem organic light-emitting diodes
Authors: Liao L. S. , Klubek K. P. , Tang C. W.
Published: January 2004

Tandem organic light-emitting diodes (OLEDs), with multiple electroluminescent (EL) units connected electrically in series, have been fabricated. Using an optically transparent doped organic “p-n” junction as the connecting unit between adjacent EL units, excellent light out-coupling and carrier-injection properties have been realized. The luminous efficiency is found to scale almost linearly with the number of EL units in the stack, giving values as high as 32 or 136 cd/A for a three-unit tandem OLED using a fluorescent or a phosphorescent emitter, respectively.
Diffusion phenomena in a Pt/IrO2/Ir/TiN/W multilayer structure during annealing in oxygen
Authors: Alberti A. , Borzı̀ A. M. , Ravesi S.
Published: January 2004

The process generating hillock structures at the surface of a Pt/IrO2/Ir/TiN/W multilayer stack has been studied at a temperature of 700 °C. This phenomenon has been related to a structural modification of the barrier that involves iridium and platinum layers, and consists of platinum atoms diffusing downwards while iridium atoms move upwards. Once in the surface proximity, iridium has been oxidized forming large grains, the hillocks, that have grown and protruded up to the sample surface. Nevertheless, oxygen was not able to deeply penetrate the barrier stack, and therefore, the inner TiN and W layers have been preserved from oxidation.
Measurement of ion flow in a negative ion source using a Mach probe
Authors: Tanga A. , Bandyopadhyay M. , McNeely P.
Published: January 2004

Measurements of ion flow in an ion source made for negative ion extraction are reported in this letter. The ion flow has been measured using the Mach probe as two single probes compared with the results as a double probe. The measured values of the Mach number lie between 0.2 and 0.4. The maximum value of the Mach number is observed near the radio-frequency excitation coil. The flow shows a dominant direction toward the extraction grid; however, the flow pattern, away from the central axis of the source, shows a direction reversal and, therefore, convection. The presence of an ion flow has a strong influence on negative ion motion toward the extraction grid and on the sheath potential. The ion flow will play a dominant role in the ion dynamics and negative ion beam overall efficiency.
Reynolds dependence of third-order velocity structure functions
Authors: Gagne Yves , Castaing Bernard , Baudet Christophe , Malécot Yann
Published: January 2004

We study the experimental dependence of the third-order velocity structure function on the Taylor based Reynolds number, obtained in different flow types over the range 72⩽Rλ⩽2260. As expected, when the Reynolds number is increasing, the third-order velocity structure functions (plotted in a compensated way) converge very slowly to a possible −4/5 plateau value according to the Kolmogorov 41 theory. Actually, each of these normalized third-order functions exhibits a maximum, at a scale close to the Taylor microscale λ. In this Brief Communication, we show that experimental data are in good agreement with the recent predictions of Qian and Lundgren. We also suggest that, from an experimental point of view, a log-similar plot suits very well to study carefully the behavior of the third-order velocity structure functions with the flow Reynolds number.
Approximate solution for the natural convection flow around an array of obstacles on a vertical wall
Authors: Higuera F. J.
Published: January 2004

A combination of an asymptotic multiscale technique and numerical computation is used to describe the natural convection flow of a high Prandtl number fluid around an array of identical obstacles on a vertical wall. The method relies on the existence of a spatially periodic limiting solution for infinitely high Rayleigh number and reduces the problem to the computation of the flow around a single obstacle. It is applied to the cooling of a fuel with a temperature-dependent viscosity in contact with a cold wall fitted with shelf-like obstacles.
Decay of spin-polarized hot carrier current in a quasi-one-dimensional spin-valve structure
Authors: Pramanik S. , Bandyopadhyay S. , Cahay M.
Published: January 2004

We study the spatial decay of spin-polarized hot carrier current in a spin-valve structure consisting of a semiconductor quantum wire flanked by half-metallic ferromagnetic contacts. The current decays because of D’yakonov-Perel’ spin relaxation in the semiconductor caused by Rashba and Dresselhaus spin–orbit interactions in multi-channeled transport. The associated relaxation length is found to decrease with increasing lattice temperature (in the range from 30 to 77 K) and exhibit a nonmonotonic dependence on the electric field driving the current. The relaxation lengths are several tens of microns which are at least an order of magnitude larger than what has been theoretically calculated for two-dimensional structures at comparable temperatures, spin-orbit interaction strengths, and electric fields. This improvement is a consequence of one-dimensional carrier confinement that does not necessarily suppress carrier scattering, but nevertheless suppresses D’yakonov-Perel’ spin relaxation.
Infrared dielectric function and vibrational modes of pentacene thin films
Authors: Schubert M. , Bundesmann C. , Jacopic G. , Maresch H. , Arwin H.
Published: January 2004

Generalized infrared spectroscopic ellipsometry over the wave-number range from 300 to 2000 cm−1 is used for precise determination of the dielectric function, frequency, amplitude, and broadening parameters of 27 infrared active modes for polarization parallel to the growth surface of pentacene thin films obtained by molecular-beam deposition on glass. No in-plane anisotropy was detected, which is indicative for a random orientation of crystallites around the growth direction supporting previous x-ray diffraction results.
Velocity overshoot in ultrathin double-gate silicon-on-insulator transistors
Authors: Gámiz F.
Published: January 2004

Concerning electron velocity overshoot in ultrathin double-gate silicon-on-insulator transistors, as a function of the silicon thickness and the electron sheet density, it is proved that for very small silicon thicknesses (smaller than 5 nm), velocity overshoot behavior is dominated by the average conduction effective mass; that is, the lower the average conduction effective mass, the higher the velocity overshoot peak. Thus, the velocity overshoot peak increases as the silicon thickness decreases, unlike low-field electron mobility, which diminishes abruptly at silicon thicknesses below 5 nm. This fact enables further reduction in the device channel length, in contrast to what might be supposed from the low-field mobility behavior.
Microcavity-coupled optical Stark effect: Application to ultrafast all-optical modulation
Authors: Sanchez S. , De Matos C. , Pugnet M.
Published: January 2004

We propose a modeling of the microcavity-coupled optical Stark effect that we compare to degenerate pump-probe experimental results obtained at room temperature on a bulk GaAs microcavity. The cavity energy mode is adjusted 15 meV below the edge of the GaAs bandgap. With a pump intensity as low as 0.4 MW/cm2, we could demonstrate an instantaneous reflectivity modulation with a 5:1 contrast ratio. The good agreement between experiment and modeling is promising to design structures well suited to all-optical modulation with low switching intensities.
Structural perturbations within Ge nanocrystals in silica
Authors: Cheung A. , de M. Azevedo G. , Glover C. J. , Llewellyn D. J. , Elliman R. G. , Foran G. J. , Ridgway M. C.
Published: January 2004

Extended x-ray absorption fine structure (EXAFS) spectroscopy was used to identify structural perturbations in Ge nanocrystals produced in silica by ion implantation and annealing. Although the nanocrystals retained tetrahedral coordination, both the short- and medium-range orders were perturbed relative to bulk crystalline material. Equivalently, the nanocrystal interatomic distance distribution deviated from that of bulk crystalline Ge, exhibiting enhanced structural disorder of both Gaussian and non-Gaussian forms in the first, second, and third nearest-neighbor shells. The relative influences of nanocrystal size, bonding distortions, multiple phases, and a matrix-induced compression were considered.
Resistance changes similar to ballistic magnetoresistance in electrodeposited nanocontacts
Authors: Svedberg Erik B. , Mallett Jonathan J. , Ettedgui Hanania , Gan Li , Chen P. J. , Shapiro Alexander J. , Moffat Thomas P. , Egelhoff William F.
Published: January 2004

We have studied the behavior of electrodeposited Ni and Fe nanocontacts in magnetic fields and the changes in resistivity (ΔR) that occur. Metallic particles suspended in plating solution, created and collected from the electroplating bath of a nanocontact that later exhibited high values of ΔR/R, have been transferred to a second set of electrodes, in which similar high values of ΔR/R were measured without any plating process being performed. We attribute this effect to a mechanical reorientation of magnetic nanoparticles at the junction between the electrodes as the field is close to zero, and relate this work to present work with ballistic magnetoresistance in nanocontacts. We also show that Fe whiskers brought in close contact can produce this effect as well.
Diode laser coupled to an atomic line by incoherent optical negative feedback
Authors: da Rocha A. F. A. , Segundo P. C. S. , Chevrollier M. , Oriá M.
Published: January 2004

We report on experiments using optical negative feedback with orthogonal polarization to control the emission frequency of single-mode AsAlGa diode lasers. We obtain linewidth reduction and absolute frequency stabilization by injecting into the semiconductor laser a beam whose intensity is controlled by a Cs–D2 absorption line shape.
Improving the ratio of the grain-boundary and bulk critical currents in YBa2Cu3O7−δ film
Authors: Vlasko-Vlasov V. K. , Claus H. , Welp U. , Gray K. E. , Ma B. , Balachandran U.
Published: January 2004

Magneto-optical images of flux patterns in a ring of epitaxial YBa2Cu3O7−δ film with an artificial 10° grain boundary are used to determine the ratio of the grain-boundary critical current to the bulk critical current. This ratio which first increases slowly with temperature is found to make a sharp upturn above 77 K showing that the grain-boundary critical current quickly converges to the bulk value at higher temperatures. From the measured ratio and the grain-boundary critical current obtained from magnetization, the temperature dependence of the bulk critical current is recovered.
Observation of the apparent metal–insulator transition of high-mobility two-dimensional electron system in a Si/Si1−xGex heterostructure
Authors: Lai K. , Pan W. , Tsui D. C. , Xie Ya-Hong
Published: January 2004

Field-effect transistors are fabricated from the Si/Si1−xGex heterostructures. The density of the two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.13×1011 to 4.24×1011 cm−2. The temperature dependence of the resistivity of the 2DES was measured and the apparent metal–insulator transition was observed. Its main features are similar to those reported in other semiconductor-based two-dimensional systems.
Growth of high-quality single-wall carbon nanotubes without amorphous carbon formation
Authors: Lacerda R. G. , Teh A. S. , Yang M. H. , Teo K. B. K. , Rupesinghe N. L. , Dalal S. H. , Koziol K. K. K. , Roy D. , Amaratunga G. A. J. , Milne W. I. , Chhowalla M. , Hasko D. G. , Wyczisk F. , Legagneux P.
Published: January 2004

We report an alternative way of preparing high-quality single-wall carbon nanotubes (SWCNTs). Using a triple-layer thin film of Al/Fe/Mo (with Fe as a catalyst) on an oxidized Si substrate, the sample is exposed to a single short burst (5 s) of acetylene at 1000 °C. This produced a high yield of very well graphitized SWCNTs, as confirmed by transmission electron microscopy and Raman spectroscopy. We believe that the high temperature is responsible for the high crystallinity/straightness of the nanotubes, and the rapid growth process allows us to achieve a clean amorphous carbon (a-C) free deposition which is important for SWCNT device fabrication. The absence of a-C is confirmed by Auger electron spectroscopy, Raman spectroscopy, and electrical measurements.
Morphology and composition of highly strained InGaAs and InGaAsN layers grown on GaAs substrate
Authors: Patriarche G. , Largeau L. , Harmand J.-C. , Gollub D.
Published: January 2004

We have studied the morphology and the composition of highly strained InGaAs and InGaAsN quantum wells (QWs) by transmission electron microscopy. 002 dark-field images show that two symmetrical interfacial layers of about 1.5 nm border the QWs. The selected-area electron diffraction technique gives further evidence of these layers since two extra spots are always observed near the high-index spots on the diffraction patterns. From the position of these extra spots, we determine the strain of the interfacial layers as well as the strain of the middle part of the wells. The comparison of InGaAs and InGaAsN reveals that the morphology of the quaternary alloy QWs is deteriorated and its lateral fluctuation of composition is increased.
Spectroscopic observation of oxidation process in InN
Authors: Kurimoto E. , Hangyo M. , Harima H. , Yoshimoto M. , Yamaguchi T. , Araki T. , Nanishi Y. , Kisoda K.
Published: January 2004

Spectroscopic observations of high-quality wurtzite InN have shown that oxygen is easily incorporated in the crystal by thermal treatments in the air. Incorporation of oxygen may play a key role in determining the apparent properties of InN including the bandgap and the lattice constant. It is shown that Raman scattering is a sensitive tool to probe the oxygen incorporation process and associated deterioration in crystallinity.
Transparent conducting Sb-doped SnO2 thin films grown by pulsed-laser deposition
Authors: Kim H. , Piqué A.
Published: January 2004

Antimony-doped tin oxide (SnO2:Sb) thin films (100–480 nm thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical, and optical properties of these films have been investigated as a function of doping amount, substrate temperature, and oxygen partial pressure during deposition. Films were deposited at temperatures ranging from 25 to 600 °C in O2 partial pressures ranging from 10 to 100 mTorr. The films (300 nm thick) deposited at 300 °C in 45 mTorr of oxygen show electrical resistivities as low as 9.8×10−4 Ω cm, an average visible transmittance of 90%, a refractive index of 1.98 (at 550 nm), and an optical band gap of 4.21 eV.
Metal/semiconductor superlattices containing semimetallic ErSb nanoparticles in GaSb
Authors: Hanson M. P. , Driscoll D. C. , Kadow C. , Gossard A. C.
Published: January 2004

We demonstrate the growth by molecular beam epitaxy of a metal/semiconductor composite consisting of epitaxial semimetallic ErSb particles in a GaSb matrix. The ErSb nucleates in an island growth mode leading to the spontaneous formation of nanometer-sized particles. These particles are found to preferentially grow along a [011] direction on a (100) GaSb surface. The particles can be overgrown with GaSb to form an epitaxial superlattice consisting of ErSb particles between GaSb spacer layers. The size of the ErSb particles increases monotonically with the deposition. The carrier concentrations in the superlattices are found to be dependent on both the size and density of the ErSb particles. Smaller particles and closer layer spacings reduce the hole concentration in the film.
Carrier-induced refractive index in quantum dot structures due to transitions from discrete quantum dot levels to continuum states
Authors: Uskov A. V. , O’Reilly E. P. , McPeake D. , Ledentsov N. N. , Bimberg D. , Huyet G.
Published: January 2004

The carrier-induced refractive index in quantum dot (QD) structures due to optical transitions from QD levels to continuum states is considered. It is shown that, for large photon energies, the refractive index change is given asymptotically by the Drude formula. Calculations of the linewidth enhancement factor, α, show that α∼1 due to this contribution to the total refractive index. Furthermore, for highly localized QD states, the absorption coefficient at the photon energies ∼0.8–1.0 eV due to these transitions can be on the order of 103 m−1.
Electronic and magnetic properties of zinc blende half-metal superlattices
Authors: Fong C. Y. , Qian M. C. , Pask J. E. , Yang L. H. , Dag S.
Published: January 2004

Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0μB and 14.0μB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.
Multicolor electroluminescent devices using doped ZnS nanocrystals
Authors: Manzoor K. , Vadera S. R. , Kumar N. , Kutty T. R. N.
Published: January 2004

Alternate-current electroluminescent (ac EL) devices based on doped ZnS nanocrystals emitting blue, green, and orange-red colors are reported. ZnS nanocrystals doped with Cu+–Al3+ and Cu+–Al3+–Mn2+ combinations were synthesized by wet chemical method at room temperature. The ZnS:Cu+, Al3+ nanocrystals show blue (462 nm) and green (530 nm) EL emissions depending upon the presence and absence of sulphur vacancies, respectively. The orange EL emission (590 nm) is realized from ZnS:Cu+, Al3+, Mn2+ nanoparticles by way of nonradiative energy transfer from AlZn–CuZn pairs to MnZn. The EL devices show low turn-on voltage of ∼10 V ac @100 Hz. The mechanism of ac EL in ZnS nanocrystals has been explained wherein the excitation is attributed to the electric-field-assisted injection of electron-hole pairs from the surface regions into the interiors and their subsequent recombination therein causes emission.
Contact resistance in organic thin film transistors
Authors: Blanchet Graciela B. , Fincher C. R. , Lefenfeld Michael , Rogers J. A.
Published: January 2004

This letter reports on the unexpected dependence of contact resistance on the dielectric layer for pentacene thin film transistors with printed organic conducting electrodes. While the intrinsic mobility is weakly reliant on the dielectric, the contact resistance does vary considerably with dielectric layer. We show that while morphological changes are not apparent, contact resistances vary by an order of magnitude. This result suggests that the barrier to charge injection may depend not only on interactions at the complex triple interface but also on the details of the electronic structure at the semiconductor/dielectric interface.
Second-harmonic efficiency of ZnO nanolayers
Authors: Neumann Uwe , Grunwald Ruediger , Griebner Uwe , Steinmeyer Günter , Seeber Wolfgang
Published: January 2004

Pure and doped nanocrystalline ZnO films of 0.1–1.0 μm thickness were prepared by spray pyrolysis techniques. Structural properties of these thin films were investigated by electron microscopy, force microscopy, x-ray diffractometry, ellipsometry, and optical spectroscopy. Using Ti:sapphire laser pulses, the second-harmonic efficiency of the films was measured. A correlation between measured efficiency and the grain shape in the polycrystalline layers is reported. Moreover, measurements of the angular dependence of the second-harmonic intensity are indicative of the dominant role of bulk effects over surface effects for the second-harmonic generation in such films.
Simultaneous inductive determination of grain and intergrain critical current densities of YBa2Cu3O7−x coated conductors
Authors: Palau A. , Puig T. , Obradors X. , Pardo E. , Navau C. , Sanchez A. , Usoskin A. , Freyhardt H. C. , Fernández L. , Holzapfel B. , Feenstra R.
Published: January 2004

An inductive methodology simultaneously enabling the determination of grain- and intergrain critical current densities of YBa2Cu3O7−x coated conductors is developed. This noninvasive method is based on the identification of a clear peak in the reverse branch of the magnetization loop at a positive magnetic field, as a signature of the electromagnetic granularity inherent to these materials. A quantitative evaluation of the return magnetic field at the grain boundaries allows us to understand the existence of this magnetization peak and quantify the grain critical current density. This methodology is envisaged to sort out granularity effects from vortex pinning effects on coated conductors.
Complex admittance analysis for La2Hf2O7/SiO2 high-κ dielectric stacks
Authors: Apostolopoulos G. , Vellianitis G. , Dimoulas A. , Hooker J. C. , Conard T.
Published: January 2004

A method of analyzing the complex admittance of metal-insulator-semiconductor (MIS) structures has been developed with the aim to extract the density and capture cross section of interface traps from combined ac capacitance–voltage and conductance–voltage measurements at different frequencies. The procedure is applied to study dielectric stacks based on La2Hf2O7 high-κ dielectric, which could be considered as a SiO2 replacement for future transistors.
Tuning quantum-cascade lasers by postgrowth rapid thermal processing
Authors: Anders S. , Schrenk W. , Roch T. , Pflügl C. , Strasser G.
Published: January 2004

We report on postgrowth heat treatment of quantum-cascade lasers. Intermixing of the atoms at the barrier–well interfaces in the temperature range between 850 and 875 °C shifts the energy levels and thus the gain of the structure. The achieved emission wavelength shift is from 10.3 to 11.9 μm. X-ray rocking curves confirm Al–Ga interdiffusion at the interfaces. A fit to the data yields interface widths of 1–2 nm.
Room-temperature continuous-wave operation of InAsSb quantum-dot lasers near 2 μm based on (001) InP substrate
Authors: Qiu Yueming , Uhl David , Keo Sam
Published: January 2004

Single-stack InAsSb self-assembled quantum-dot lasers based on (001) InP substrate have been grown by metalorganic vapor-phase epitaxy. The narrow ridge waveguide lasers lased at wavelengths near 2 μm up to 25 °C in continuous-wave operation. At room temperature, a differential quantum efficiency of 13% is obtained and the maximum output optical power reaches 3 mW per facet with a threshold current density of 730 A/cm2. With increasing temperature the emission wavelength is extremely temperature stable, and a very low wavelength temperature sensitivity of 0.05 nm/°C is measured, which is even lower than that caused by the refractive index change.
Ultraviolet detection with ultrathin ZnO epitaxial films treated with oxygen plasma
Authors: Liu Mingjiao , Kim Hong Koo
Published: January 2004

We have investigated the effects of oxygen plasma treatment on the UV detection properties of ultrathin (∼20-nm-thick) ZnO epitaxial films. Highly epitaxial ZnO films grown on sapphire were exposed to oxygen-radical-rich, inductively coupled plasma, and then their UV detection properties were characterized at 325 nm wavelength using a photoconductor structure. The oxygen plasma treatment is found to dramatically enhance the UV detection properties of ZnO, reducing the decay time constant (to below 50 μs) and increasing the on/off ratio of photocurrent (to over 1000) with high UV responsivity (1–10 A/W). This result, in conjunction with the microstructural and electrical characterization results, indicates that the plasma treatment efficiently suppresses the chemisorption sites (primarily the oxygen deficiency sites) on surface and also the oxygen vacancies in ZnO, therefore results in major reduction of the chemisorption effects and the dark current, respectively.
Er
3+
–
Yb
3+
codoped polymeric optical waveguide amplifiers
Authors: Wong W. H. , Pun E. Y. B. , Chan K. S.
Published: January 2004

Codoping of erbium (Er3+) and ytterbium (Yb3+) rare-earth ions in polymer was studied for optical amplifier applications. The absorption spectrum confirms that the presence of Yb3+ ions enhances the absorption efficiency of Er3+ ions. Typical Er3+ luminescence at ∼1540 nm wavelength was observed, and the full width at half maximum bandwidth is ∼47 nm wide. Multimode Er3+–Yb3+ codoped polymeric channel waveguides were fabricated using electron beam direct writing. With an input signal power of <−18 dB m, an optical gain of 13 dB at a wavelength of 1533 nm was measured in an 18 mm long multimode channel waveguide.
Narrow, deep level cathodoluminescence emission from semi-insulating GaAs
Authors: Radhakrishnan J. K. , Salviati G.
Published: January 2004

Cathodoluminescence investigations on bulk undoped semi-insulating GaAs samples taken from different sources reveal the presence of a deep level emission at 0.9 eV at 77 K, with a narrow full width at half maximum of 8 meV. The temperature-dependent and beam-parameter-dependent studies indicate that, the probable origin for this emission may be some nanocluster/quantum-dot like structures with a band gap of 0.9 eV, present inside the semi-insulating GaAs crystal lattice. One possibility for such structures in semi-insulating GaAs are clusters/nanoprecipitates of arsenic.
Propagation effects in apertureless near-field optical antennas
Authors: Wang Kanglin , Barkan Adrian , Mittleman Daniel M.
Published: January 2004

We report a direct observation of the electromagnetic wave propagation on optical antennas. A free-space broadband terahertz pulse is coupled into a propagating mode along the shaft of an optical antenna acting as an apertureless near-field probe. We determine the spatial extent of this guided mode and its velocity. In addition, we consider the possibility of multiply reflected modes propagating along the near-field probe, an effect which will be significant in near-field spectroscopic measurements.