bluuebunny/crossref_metadata_2025 huggingface dataset:
A forward modeling approach to paleoclimatic interpretation of tree‐ring data
Authors: Evans M. N. , Reichert B. K. , Kaplan A. , Anchukaitis K. J. , Vaganov E. A. , Hughes M. K. , Cane M. A.
Published: August 2006

We investigate the interpretation of tree‐ring data using the Vaganov‐Shashkin forward model of tree‐ring formation. This model is derived from principles of conifer wood growth, and explicitly incorporates a nonlinear daily timescale model of the multivariate environmental controls on tree‐ring growth. The model results are shown to be robust with respect to primary moisture and temperature parameter choices. When applied to the simulation of tree‐ring widths from North America and Russia from the Mann et al. (1998) and Vaganov et al. (2006) data sets, the forward model produces skill on annual and decadal timescales which is about the same as that achieved using classical dendrochronological statistical modeling techniques. The forward model achieves this without site‐by‐site tuning as is performed in statistical modeling. The results support the interpretation of this broad‐scale network of tree‐ring width chronologies primarily as climate proxies for use in statistical paleoclimatic field reconstructions, and point to further applications in climate science.
A multiscale finite element linearization scheme for the unsaturated flow problems in heterogeneous porous media
Authors: He Xinguang , Ren Li
Published: August 2006

Numerical solution of unsaturated water flow in heterogeneous porous media is difficult not only because of the spatial variability in the parameters used to characterize the relevant physical properties of the natural porous media but also because of the mathematical problems that arise in dealing with the nonlinearities. In this study a multiscale finite element linearization scheme is presented for effectively simulating unsaturated flow in heterogeneous porous media spanning over many scales. The central goal of this method is to obtain the large‐scale solution of Richards' equation with heterogeneous coefficients accurately and efficiently on a coarse grid without resolving all the small‐scale details. The underlying idea is to employ the Slodicka linear relaxation approximation scheme to address the nonlinear characteristics of the equation and to use the multiscale finite element base functions to account for the spatial variability in the equation coefficients. We describe the principle for constructing such a method and give an algorithm for implementing it. Numerical experiments are carried out for the unsaturated flow equation with both periodic and randomly generated lognormal hydraulic conductivity to demonstrate the efficiency and accuracy of the proposed method.
Functional Uncoupling of Hemodynamic from Neuronal Response by Inhibition of Neuronal Nitric Oxide Synthase
Authors: Stefanovic Bojana , Schwindt Wolfram , Hoehn Mathias , Silva Afonso C
Published: August 2006

The cerebrovascular coupling under neuronal nitric oxide synthase (nNOS) inhibition was investigated in α-chloralose anesthetized rats. Cerebral blood flow (CBF), cerebral blood volume (CBV), and blood oxygenation level dependent (BOLD) responses to electrical stimulation of the forepaw were measured before and after an intraperitoneal bolus of 7-nitroindazole (7-NI), an in vivo inhibitor of the neuronal isoform of nitric oxide synthase. Neuronal activity was measured by recording somatosensory-evoked potentials (SEPs) via intracranial electrodes. 7-Nitroindazole produced a significant attenuation of the activation-elicited CBF ( P < 10−6), CBV ( P < 10−6), and BOLD responses ( P < 10−6), without affecting the baseline perfusion level. The average Δ CBF was nulled, while Δ BOLD and Δ CBV decreased to ~30% of their respective amplitudes before 7-NI administration. The average SEP amplitude decreased ( P < 10−5) to ~60% of its pretreatment value. These data describe a pharmacologically induced uncoupling between neuronal and hemodynamic responses to functional activation, and provide further support for the critical role of neuronally produced NO in the cerebrovascular coupling.
5-Hydroxy-l-[β-11C]tryptophan versus α-[11C]Methyl-l-tryptophan for Positron Emission Tomography Imaging of Serotonin Synthesis Capacity in the Rhesus Monkey Brain
Authors: Lundquist Pinelopi , Hartvig Per , Blomquist Gunnar , Hammarlund-Udenaes Margareta , Långström Bengt
Published: August 2006

The purpose of this study was to compare two positron emission tomography (PET) tracers that were developed to follow serotonin (5HT) synthesis by performing sequential PET scanning of the same rhesus monkey ( n = 4) on the same day. α-[11C]Methyl-l-tryptophan ([11C]AMT) and 5-Hydroxy-l-[β-11C]tryptophan ([11C]HTP) are substrates in the first and second enzymatic steps, respectively, in the biosynthesis of 5HT. Regional net accumulation rate constants were derived from kinetic (two-tissue compartment model with irreversible tracer trapping) and graphic (Patlak) analyses, using the arterial plasma concentrations as input. The kinetic data analysis showed that the rate constant for the transfer of [11C]HTP into the brain ( K1) was higher than that for [11C]AMT in the striatum and thalamus but was similar in other brain regions. The rate constant for tracer trapping ( k3) was also higher for [11C]HTP than for [11C]AMT in the striatum (0.046 ± 0.024 versus 0.019 ± 0.006 min−1) and thalamus (0.039 ± 0.013 versus 0.016 ± 0.007 min−1). In agreement with previously reported regional HTP accumulation rates, the net accumulation rate constant ( Kacc) for [11C]HTP was also higher in these regions than in other brain regions; this is in contrast to the uniform distribution of [11C]AMT Kacc values. This suggests that the regional net accumulation rates obtained with these two PET tracers will be of different magnitude, which might be related to the activity of each targeted enzyme.
Brain Changes to Hypocapnia Using Rapidly Interleaved Phosphorus-Proton Magnetic Resonance Spectroscopy at 4 T
Authors: Friedman Seth D , Jensen J Eric , Frederick Blaise B , Artru Alan A , Renshaw Perry F , Dager Stephen R
Published: August 2006

Substantial controversy persists in the literature concerning the physiologic consequences hypocapnia, or low partial pressure of carbon dioxide (PaCO2). Invasive animal studies have demonstrated large pH increases (>0.25 U), phosphocreatine (PCr) decreases (>30%), and adenosine triphosphate (ATP) decreases (>10%) after hyperventilation (HV) (20 mm Hg PaCO2). However, using magnetic resonance spectroscopy, HV studies in awake humans have demonstrated only small pH changes (∼0.05 U) and no changes in PCr or ATP. It remains important to ascertain whether this failure to detect PCr changes in human studies reflects a true absence of changes, or a limitation in data fidelity. The present study used a rapidly interleaved phosphorus-proton spectroscopy acquisition from large samples at high magnetic field (4 T), to measure pH, PCr, inorganic phosphate, β-ATP, and lactate changes with high temporal and signal sensitivity. Five of six subjects had usable data. During 20 mins HV, PaCO2 reached a minimum at 16 mins (17 mm Hg); however, the maximum pH change (+0.047) peaked earlier (14 mins). Maximal lactate increases were measured at 15 mins. By 10 mins, maximum changes were observed for PCr (−3.4%) and inorganic phosphate (+6.4%). No changes in β-ATP were observed. The peak in pH, despite continued decreases in PaCO2, suggests active buffering during HV. These data, and the small magnitude of early PCr and inorganic phosphate changes, do not support substantial energy compromise during HV. Other mitigating factors, such as anesthesia-induced deregulation of the cerebrovasculature, might have contributed to the exaggerated metabolic changes observed in previous animal investigations.
Interpretation of polarization pinning due to scattering loss differentiation in asymmetric vertical-cavity surface-emitting laser cavities
Authors: Nhan Elbert , Riyopoulos Spilios
Published: August 2006

The asymmetric aperture geometry in vertical-cavity surface-emitting laser cavities has long been known through experiments to effectively pin the polarization state of the output laser emission. In this article, we present a theoretical explanation for the observed effects due to noncircular aperture shape. The crux of our theory is that the unequal scattering losses of the laser beam between orthogonal polarizations of the same cavity eigenmodes lead to polarization pinning. Polarization-dependent calculations for elliptic apertures show that the scattering losses are lower when the polarization mode is aligned with the longest aperture axis, in agreement with experimental observations. This imbalance in scattering losses is higher than any intrinsic gain-related polarization dependence, and we suggest that it is the determining factor for polarization selection above a few-percent asymmetry in cavity geometry.
Effect of the shape anisotropy on the magnetic configuration of (Ga,Mn)As and its evolution with temperature
Authors: Hamaya K. , Taniyama T. , Koike T. , Yamazaki Y.
Published: August 2006

We study the effect of the shape anisotropy on the magnetic domain configurations of a ferromagnetic semiconductor (Ga,Mn)As∕GaAs(001) epitaxial wire as a function of temperature. Using magnetoresistance measurements, we deduce the magnetic configurations and estimate the relative strength of the shape anisotropy compared with the intrinsic anisotropies. Since the intrinsic anisotropy is found to show a stronger temperature dependence than the shape anisotropy, the effect of the shape anisotropy on the magnetic domain configuration is relatively enhanced with increasing temperature. This information about the shape anisotropy provides a practical means of designing nanostructured spin electronic devices using (Ga,Mn)As.
Hydrogen-implant-induced polarization loss and recovery in IrO2∕Pb(Zr,Ti)O3∕Pt capacitors
Authors: Cross J. S. , Kurihara K. , Sakaguchi I. , Haneda H.
Published: August 2006

Hydrogen was implanted into IrO2 (200nm)∕Pb(Zr,Ti)O3∕Pt thin film capacitors at 26keV with a flux of 2×1015H+ions∕cm2 and also implanted at 20keV with a flux of 1×1015H+ions∕cm2 into IrO2 (50nm)∕Pb(Zr,Ti)O3∕Pt. The implanted samples showed a net loss in polarization of 20μC∕cm2 as a result of the hydrogen implant and the hysteresis loop was severely distorted. Transmission electron microscopy cross-sectional images and microdiffraction patterns showed no physical damage in the Pb(Zr,Ti)O3 after implanting. Secondary ion mass spectroscopy (SIMS) analysis revealed that a maximum concentration of H of 4×1020atom∕cm3 after implantation. Permittivity measurements also showed a decrease after implantation of 20% or more. After annealing the implanted samples in air at 350°C for 5min the remanent polarization loop and dielectric properties were restored. SRIM-2003 software simulation of the implanted hydrogen revealed that 1% oxygen vacancies are created by implanting using these implant conditions. In addition, hydrogen implanted in the Pb(Zr,Ti)O3 lattice can inhibit the switching of Ti∕Zr by binding with oxygen. The recovery at 350°C indicates that the polarization decrease created by hydrogen implantation is relatively minor and suggests that the degradation mechanism is due to implanted hydrogen in the Pb(Zr,Ti)O3 films.
Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. III. Dielectric relaxation near the transition temperature
Authors: Raymond O. , Font R. , Portelles J. , Suárez-Almodovar N. , Siqueiros J. M.
Published: August 2006

Dielectric relaxation processes occurring near the ferroelectric-paraelectric phase transition of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors are discussed using microstructural and equivalent circuit modeling and the impedance spectroscopy technique. The frequency-temperature response was obtained from room temperature to 300°C and from 20Hzto1Mz. In correspondence with a previous structural, morphological, and temperature response study, appropriate microstructural, and equivalent circuit models were established. The frequency response study was carried out by means of the simultaneous analysis of the complex dielectric constant ε̃ and admittance Ỹ functions and the dielectric loss, tanδ. A strong absorption near the transition temperature region at a frequency around 1MHz is discussed and is attributed with relaxation processes associated with domain reorientation, domain wall (DW) motion, and the dipolar behavior of ferroelectric materials. Such processes were found to take place inside the grain, and their low characteristic frequencies are explained by clamping effects of the DW due to the thermally activated diffusion of oxygen vacancies. At frequencies before relaxation, the high polarization values are due to small polaron mechanisms associated with the presence of Fe2+. The relaxation processes are very much conditioned by the grain and domain sizes, the degree of deformation of the lattice and the crystallites, as well as the potential barriers in the grain boundaries. Values of the activation energy corresponding to the different relaxation processes were determined from fitting of experimental data, identifying thus the involved mechanisms, and an excellent agreement with those obtained from the temperature response [Raymond et al., J. Appl. Phys. 97, 084107 (2005)] was found. The relaxation processes studied here are an evidence of domain structure.
Detecting clandestine material with nuclear resonance fluorescence
Authors: Pruet J. , McNabb D. P. , Hagmann C. A. , Hartemann F. V. , Barty C. P. J.
Published: August 2006

We study the performance of a class of interrogation systems that exploit nuclear resonance fluorescence (NRF) to detect specific isotopes. In these systems the presence of a particular nuclide is inferred by observing the preferential attenuation of photons that strongly excite an electromagnetic transition in that nuclide. Estimates for the false positive/negative error rates, radiological dose, and detection sensitivity associated with discovering clandestine material embedded in cargo are presented. The relation between performance of the detection system and properties of the beam of interrogating photons is also considered. Bright gamma-ray sources with fine energy and angular resolution, such as those based on Thomson upscattering of laser light, are found to be associated with uniquely low radiological dose, scan times, and error rates. For this reason a consideration of NRF-based interrogation systems may provide impetus for efforts in light source development for applications related to national security and industry.
Highly aligned coiled nitrogen-doped carbon nanotubes synthesized by injection-assisted chemical vapor deposition
Authors: Liu J. , Webster S. , Carroll D. L.
Published: August 2006

Large quantities of coiled nitrogen-doped carbon nanotubes with high purity and uniform diameter of approximately 25nm have been synthesized by pyrolysis of pyridine with the assistance of sublimated melamine using injection-assisted chemical vapor deposition. The coiled tubes were vertically aligned with high regularity over large areas. The pitch and coil diameter varied between (1.1–1.3μm)/revolution and 0.4–0.5μm, respectively. The overall nitrogen concentration of the coiled carbon nanotubes has been measured to be approximately 3.5%. We suggest that the growth mechanism of these coiled nitrogen-doped carbon nanotubes are related to asymmetric growth rates resulting from sublimated melamine.
Small mode-size waveguides in quantum-dot-doped glasses by Ag-film ion exchange
Authors: Auxier Jason M. , Honkanen Seppo , Morrell Michael M. , Leigh Matthew A. , Sen Sabyasachi , Borrelli Nicholas F. , Schülzgen Axel
Published: August 2006

We present a silver-sodium ion-exchange process for fabricating small mode-size waveguides in glass doped with PbS semiconductor quantum dots. We show that the process does not alter the optical properties of the quantum dots by comparing the waveguide and bulk luminescence spectra. We also show that the optical mode is highly confined. This field confinement produces high optical intensities, which are essential for nonlinear optical applications.
Energetics and dynamics of Cu(001)-c(2×2)Cl steps
Authors: van Dijk Frank R. , Zandvliet Harold J. W. , Poelsema Bene
Published: August 2006

The energetics of the step faceting transition of Cu(001) [copper (001) surface] upon Cl (chloride) adsorption in contact with HCl (hydrogen chloride) solution is modeled in terms of a solid-on-solid model that incorporates both nearest-neighbor and next-nearest-neighbor interactions. It is shown that faceting of the closed packed [110] Cu(001) steps into step segments oriented along [010] directions of the Cu(001)-c(2×2)Cl can be explained by a strongly repulsive interaction between next-nearest-neighbor Cl atoms in the c(2×2) adlayer. The dynamics of kink motion along the [010]-oriented steps of the Cl-covered Cu(001) surface via attachment/detachment is studied with scanning tunneling microscopy. At some kink sites local dissolution takes place, whereas at other kink sites growth occurs. After correcting for the net local dissolution (or growth) process the kinks perform a one-dimensional random walk along the [010] oriented step segments.
Influence of hydrogenation on surface morphologies, transport, and optical properties of InN epifilms
Authors: Fu S. P. , Lin T. J. , Su W. S. , Shieh C. Y. , Chen Y. F. , Chang C. A. , Chen N. C. , Chang P. H.
Published: August 2006

We report the investigation of surface morphologies and transport and optical properties of hydrogenated InN epifilms. The average rms surface roughness decreases from 24nm on the as-grown sample to 13.2nm after hydrogenation. The free electron concentration can be increased or decreased depending on the duration of hydrogenation. The linewidth of the photoluminescence spectra can be reduced, and the peak intensity can be enhanced by about three times. All our results indicate that the physical properties of InN films can be improved by hydrogenation. Possible origins of the underlying mechanism have been proposed to explain the improvement.
Enhancement of the maximum energy product of α-Fe∕Nd2Fe14B nanocomposite magnets by interfacial modification
Authors: Li Wei , Li Xiaohong , Li Lanlan , Zhang Jingwu , Zhang Xiangyi
Published: August 2006

Due to the existence of vacancy-sized free volumes in the interfacial structure of α-Fe∕Nd2Fe14B nanocomposite magnets, their energy product is thought to be enhanced by interfacial modification. To improve the interfacial structure, in the present study, we produce an interfacial amorphous phase with a volume fraction of 14%–16% in α-Fe∕Nd2Fe14B nanocomposites by a proper addition of Nb in the melt-spun NdPrFeCoB. The nanocomposites show a higher maximum energy product (BH)max=23.2MGOe and a higher remanence Mr=0.78Ms compared with the one without interfacial modification, (BH)max=16.1MGOe and Mr=0.75Ms.
Effect of substrate growth temperatures on H diffusion in hydrogenated Si∕Si homoepitaxial structures grown by molecular beam epitaxy
Authors: Shao Lin , Lee J. K. , Wang Y. Q. , Nastasi M. , Thompson Phillip E. , David Theodore N. , Alford T. L. , Mayer J. W. , Chen Peng , Lau S. S.
Published: August 2006

We have investigated hydrogen diffusion in hydrogenated ⟨100⟩ Si∕Si homoepitaxial structures, which were grown by molecular beam epitaxy at various temperatures. The substrate growth temperature can significantly affect the H diffusion behavior, with higher growth temperatures resulting in deeper H diffusion. For the Si∕Si structure grown at the highest temperature of 800°C, H trapping occurs at the epitaxial Si∕Si substrate interface, which results in the formation of (100) oriented microcracks at the interface. The mechanism of H trapping and the potential application of these findings for the development of a method of transferring ultrathin Si layers are discussed.
Synthesis and characterization of ZnO thin film grown by electron beam evaporation
Authors: Agarwal D. C. , Chauhan R. S. , Kumar Amit , Kabiraj D. , Singh F. , Khan S. A. , Avasthi D. K. , Pivin J. C. , Kumar M. , Ghatak J. , Satyam P. V.
Published: August 2006

Highly transparent, conducting, highly oriented, and almost single phase ZnO films have been deposited by simple e-beam evaporation method, and the deposition parameters were optimized. The films were prepared by (a) evaporation of ZnO at different substrate temperatures and (b) evaporation of ZnO at room temperature and subsequent annealing of the films in oxygen ambient at different temperatures. The characterizations of the film were performed by optical absorption spectroscopy (UV-visible), Fourier transform infrared spectroscopy, resistivity measurement, transmission electron microscopy (TEM), photoluminescence, and x-ray diffraction measurement. Absorption spectra revealed that the films were highly transparent and the band gap of the pre- and postannealed films was in good agreement with the reported values. The band gap of the films increases on increasing the substrate temperature as well as annealing temperature, whereas the resistivity of the film decreases with substrate temperature and increases with annealing temperature. Fourier transform infrared spectroscopy of ZnO films confirms the presence of Zn–O bonding. X-ray diffraction, electron diffraction, and TEM images with high resolution and Raman spectra of the films showed the formation of crystalline ZnO having wurtzite structure.
Electrical conduction behavior of La and Mn substituted strontium titanate
Authors: Singh Prakash , Parkash Om , Kumar Devendra
Published: August 2006

Attempts were made to synthesize various compositions in the system Sr1−xLaxTi1−xMnxO3 (x⩽0.50) by solid state ceramic route. A few compositions were also synthesized by citrate nitrate route. Solid solution forms in all the compositions investigated. Crystal structure remains cubic up to x=0.10 similar to SrTiO3, while for compositions with x⩾0.20, it is orthorhombic similar to LaMnO3. Average grain size is small in all the compositions. Average grain size decreases with increasing x. Seebeck coefficient (α) and dc resistivity were measured as a function of temperature. For compositions with x⩽0.30, Seebeck coefficient is initially positive. It becomes negative at higher temperature. For x=0.50, it is negative over the entire temperature range. Based on Seebeck coefficient and activation energy of conduction in different temperature ranges, it is proposed that for x=0.01, at higher temperatures conduction occurs due to migration of doubly ionized oxygen vacancies. In all the compositions conduction occurs either by hopping of holes among Mn4+ and Mn3+ or electrons among Mn2+ and Mn3+ sites. For compositions with x⩽0.30, at low temperature, conduction occurs due to hopping of holes, it changes to conduction due to hopping of electrons at higher temperature. For composition with x=0.50 conduction occurs due to hopping of electrons.
Singlet excited state absorption of porphyrin molecules for pico- and femtosecond optical limiting application
Authors: Barbosa Neto N. M. , Oliveira S. L. , Misoguti L. , Mendonça C. R. , Gonçalves P. J. , Borissevitch I. E. , Dinelli L. R. , Romualdo L. L. , Batista A. A. , Zilio S. C.
Published: August 2006

This work employs the Z-scan technique with 120fs pulses to investigate the singlet excited state absorption spectra of tetrapyridyl and tetrasulfonatophenyl porphyrins. We have used a three-energy-level model to adjust Z-scan curves in order to obtain the singlet excited absorption cross section from 460to800nm. Starting from these values, we determine the spectra of the ratio between excited and ground singlet state absorption cross sections, whose values are as good as the best found in the literature for reverse saturable absorbers. The results obtained point these porphyrins as good candidates for applications in optical limiting of ultrashort pulses.
Toluene oxidation in a plasma-catalytic system
Authors: Magureanu M. , Mandache N. B. , Gaigneaux E. , Paun C. , Parvulescu V. I.
Published: August 2006

Oxidative removal of toluene in a dielectric barrier discharge reactor combined with manganese catalysts downstream was investigated. Toluene input concentration was varied in the range of 415–2227ppm. The discharge was operated in pulsed mode, with short pulses of 23–35kV peak voltage. At 7W average power, toluene conversion was 60%–70%, independent on the toluene input concentration and on the total gas flow rate in the range of 110–330SCCM (SCCM denotes cubic centimeter per minute at STP). Toluene total oxidation was favored at high residence time of the gas in the discharge zone and low toluene concentration, when the main reaction product was CO2 with selectivities of 80%–85%. The addition of the catalysts led to a 15%–20% increase in toluene conversion with respect to the values obtained in the plasma, due to oxidation with ozone on the catalyst surface.
Ion-beam induced effects at 15K in α-Al2O3 of different orientations
Authors: Schnohr C. S. , Wendler E. , Gärtner K. , Wesch W. , Ellmer K.
Published: August 2006

Ion-beam induced effects in α-Al2O3 of c, a, and r orientations were studied by Rutherford backscattering spectrometry (RBS) in channeling configuration using 1.4MeV He ions. 150keV Ar, 150keV K, or 80keV Na ions were step by step implanted at 15K followed immediately by the RBS analysis without changing the sample environment. Defect annealing was observed during the RBS measurement, which is attributed to the electronic energy loss of the He ions. A similar effect occurs due to the electronic energy loss of the implanted ions, resulting in a reduced defect concentration between surface and profile maximum. The electronic energy loss of ions may change the charge state of defects, thus enhancing their mobility and causing defect annealing. The results suggest that within the collision cascade of individual ions in perfect sapphire only point defects are produced, the concentration of which is well reproduced by SRIM calculations taking into account suggested values of the displacement energies of EdAl=20eV and EdO=50eV for aluminum and oxygen, respectively. The lower efficiency for point defect production measured in c-oriented material can be explained by the heavily reduced visibility of Al atoms sitting on vacant octahedral sites, which are hidden in this direction. Point defect recombination is observed when the collision cascades start to overlap. Above a critical concentration point defects are altered into clusters which rapidly grow during further irradiation until a saturation is reached.
Microstructure, ferromagnetism, and magnetic transport of Ti1−xCoxO2 amorphous magnetic semiconductor
Authors: Song Hong-Qiang , Mei Liang-Mo , Yan Shi-Shen , Ma Xiu-Liang , Liu Jia-Ping , Wang Yong , Zhang Ze
Published: August 2006

Ti O 2 -based magnetic semiconductors with high Co doping concentrations (Ti1−xCoxO2) were synthesized under thermal nonequilibrium condition by sputtering machine. Microstructure and composition analysis by transmission electron microscopy, x-ray photoelectron spectroscopy, and electron energy-loss spectroscopy indicated that Co element was incorporated into TiO2 to form Ti1−xCoxO2 compound. The direct evidence for the compositional inhomogeneity of the Ti1−xCoxO2 compound was given. Room temperature ferromagnetism with high magnetization was obtained, which could be attributed to the intrinsic properties of the amorphous magnetic semiconductor. The electrical transport in a low temperature range was explained by spin-dependent Efros’s variable range hopping, and correspondingly an exponential function of the magnetoresistance versus T−1∕2 was found.
Effect of various annealing environments on electrical and optical properties of indium tin oxide on polyethylene napthalate
Authors: Han H. , Mayer J. W. , Alford T. L.
Published: August 2006

The effect of different annealing ambients on the electrical and optical properties of indium tin oxide (ITO) on polyethylene napthalate (PEN) has been investigated. ITO layers of different carrier concentrations have been prepared by radio frequency sputtering followed by annealing in either 5% H2∕Ar, oxygen, or air at 150°C for 2h. The carrier concentration of the ITO films increased with heat treatment in the 5% H2∕Ar ambient. However, the carrier concentration of the ITO films decreased with heat treatment in oxygen and air atmospheres, respectively. Different annealing ambients affect the electrical properties of ITO/PEN samples by changing the oxygen vacancy in the ITO layer. Electrical mobilities were found to be affected by the carrier concentration. These experimental mobility values were in good agreement with those obtained from a theoretical ionized impurity scattering mechanism model. Optical transmittances of ∼86% were obtained for the ITO thin films and the optical band gap is about 3.15eV for all ITO films on PEN substrates.
Optical and electrical transport properties of facing-target sputtered Al doped ZnO transparent film
Authors: Li Z. Q. , Zhang D. X. , Lin J. J.
Published: August 2006

Al doped zinc oxide thin film was prepared by dc facing-target sputtering method and its structural, optical, and electrical transport properties have been investigated. The average transmittance of the films is greater than 90% in the wavelength region of 450–700nm while the resistivity is as high as 3×10−3Ωcm. The band gap energy derived from the transmission data is 3.76eV, which is higher than that of pure ZnO thin film. This band gap growth phenomenon cannot be explained in terms of the Burstein-Moss effect. The resistivity and Hall effect measurements suggest that the interaction between the charge carriers and phonons plays a key role in the electrical transport properties of the film between 60 and 300K. The film exhibits negative magnetoresistance at low temperatures, which can be well described by a semiempirical expression that takes into account the third order s-d exchange Hamiltonians describing a negative part and a two-band model for positive contribution.
Assessment of microscopic lattice structures in dilute (AlGaIn)NAs laser materials by local mode spectroscopy and numerical simulations
Authors: Talwar D. N.
Published: August 2006

In the framework of a rigid-ion model we report the results of a comprehensive Green’s function analyses of the Fourier transform infrared (FTIR) absorption and Raman scattering data on localized vibrational modes (LVMs) to investigate the microscopic lattice structures related with the nitrogen incorporation in (AlGaIn)NAs alloys. Contrary to the outcome of a recent FTIR report of LVMs on GaInNAs multiple-quantum well structures, our group-theoretical analyses of impurity vibrations support the earlier IR and Raman scattering experiments providing corroboration to the fact that upon annealing and/or by increasing In(Al) composition in GaIn(Al)NAs there occur structural changes causing N environment to transform from NAsGa4 to NAsIn(Al)Ga3 and/or NAsIn2(Al2)Ga2 microstructures. From the force variation correlation with bond covalency for the closest mass acceptor CAs(a−) and isoelectronic NAs(i) defects in GaAs we have obtained the corrected value of u for Al–N bond predicting NAs local mode in AlAs (∼510cm−1) at a higher frequency than that of GaAs:N (471cm−1). Theoretical results of impurity modes for Al–N complexes in GaAlNAs are compared and discussed with the existing Raman data.
Distortion of a received radar pulse due to high target velocity
Authors: Boehm James A. , Debroux Patrick S.
Published: August 2006

This paper presents a method to determine the signal received for the general radar problem, i.e., velocity, translation/rotation, displacement, and modifications caused by the target. In addition to the velocity and spatial/rotation, displacement effects of the radar receiver are included. The narrow-band restriction of the signal does not apply. Once the received signal is determined, it can be compared to the original transmitted signal and the changes in the radar parameters can be determined. These changes are incorporated in the matched filter processing to produce the same output wave forms as if the transmitted wave form was not modified by the motion of the target. The method uses the Poincare transform and a mathematical representation of the reflection and modifications created by the moving target. The solution for the monostatic radar case is presented in detail. Three examples are given to demonstrate the use of the method in the monostatic radar case.
Structure-film thickness relationship study of sputtered NiO∕Ni bilayers using depth profiling and atomic force microscopy techniques
Authors: Abbey Brian , Lipp John D. , Barber Zoe H. , Rayment Trevor
Published: August 2006

Ni O ∕ Ni thin film bilayers have been grown on Si (100) substrates using low temperature dc reactive magnetron sputtering. The samples were grown under identical process conditions but with different amounts of NiO deposited in order to determine film quality as a function of thickness. In order to investigate the structural properties of the NiO overlayers a synchrotron detector has been developed to make energy-resolved electron yield x-ray absorption spectroscopy measurements at ambient pressure. From these studies we have been able to construct a complete depth profile of the NiO∕Ni bilayers and, by modeling of the electron multiplication/propagation processes within the films, extract quantitative information about them. In combination with atomic force microscopy measurements we have determined that there exists a clear variation in the structural and morphological properties of the NiO thin films as a function of thickness. The densest overlayers with the most conformal surface are observed for film thicknesses <20nm and >100nm. We rationalize these results in terms of the underlying morphology of the Ni film and the effects of misfit strain between the layers.
Nonequilibrium phonon generation in coupled Wannier-Stark ladders from a semiconductor superlattice in a three-terminal device
Authors: Rizzi Francesco , Manenti Maurizio , Di Carlo Aldo , Lugli Paolo
Published: August 2006

We present an experimental and a theoretical study on the midinfrared electroluminescence associated with transitions between electric-field-induced conduction states, forming Wannier-Stark (WS) ladders, in a strongly coupled GaAs∕AlAs superlattice inserted in a heterostructure bipolar transistorlike device. Electroluminescence shows wide tunability (100–210meV) of the diagonal intra-WS radiative transition, up to the merging with the vertical inter-WS emission, at constant injected emitter current. Intra-WS linewidth analysis suggests electric field homogeneity on the whole superlattice for F⩾160kV∕cm, until intervalley electronic scattering limits the radiative efficiency above 230kV∕cm. In this range, the integral optical intensity is constant, as expected by a transistorlike optoelectronic device. Monte Carlo simulations give a good agreement between experimental and theoretical electroluminescence spectra. Theoretical investigation indicates that intersubband scattering via LO phonon is responsible for the population of the states of the excited ladder. It shows that transitions between WS ladder states are responsible for hot phonon generation at fixed momenta by inter- and intrasubband nonradiative transitions.
Phenomenological modeling of diffusion profiles: Sn in GaAs
Authors: Gonzalez Debs M. , Kuech T. F.
Published: August 2006

A phenomenological methodology has been used to characterize the intrinsic diffusion of tin in GaAs based on the coupled motion of substitutional and interstitial Sn atoms. Both the rapid diffusivity of interstitial Sn atoms and their transformation to substitutionals by occupying gallium vacancies are processes that lead to an anomalous “double-profile” diffusion curve observed in highly doped samples. The model was developed for samples annealed over a range of 973–1123K. The model developed while developed and perhaps only applicable over this temperature range may be extendable to a larger temperature range provided that the underlying mechanism is unchanged. The methodology consists of using key aspects of the diffusion profiles along with the mass conservation criterion to predetermine bounds which prescribe the values of the parameters used to model the experimental data. This fitting procedure allowed for a more rapid assessment of the model parameters by providing physical bounds on their values as derived from the experimental profiles. The proposed diffusion mechanism combines the simultaneous diffusion of both interstitial and substitutional Sn atoms as well as key defect reactions characterizing their mutual equilibrium with the background vacancy concentration.
Temperature dependence of nonlinear susceptibilities near ferroelectric phase transition of a lead germanate single crystal
Authors: Miga S. , Dec J. , Molak A. , Koralewski M.
Published: August 2006

The results of measurements of the linear and nonlinear dielectric susceptibilities of a lead germanate, Pb5Ge3O11, single crystal are presented. The change in the sign of the third-order dielectric susceptibility χ3 as predicted by the thermodynamic theory of the continuous phase transitions is confirmed experimentally. Both the change in the sign of χ3 and the temperature independence, within the temperature range of T⩽TC+8K above the phase transition temperature point, of the nonlinearity coefficient B confirm that the lead germanate belongs to the Landau universality class.
Perovskite (Na0.5K0.5)1−x(LiSb)xNb1−xO3 lead-free piezoceramics
Authors: Zang Guo-Zhong , Wang Jin-Feng , Chen Hong-Cun , Su Wen-Bin , Wang Chun-Ming , Qi Peng , Ming Bao-Quan , Du Juan , Zheng Li-Mei , Zhang Shujun , Shrout Thomas R.
Published: August 2006

Lead-free potassium sodium niobate piezoelectric ceramics substituted with lithium and antimony (Na0.5K0.5)1−x(LiSb)xNb1−xO3 have been synthesized by conventional solid state sintering method. Compositionally engineered around the orthorhombic-tetragonal polymorphic phase transition, the dielectric and piezoelectric properties were further enhanced with the addition of lithium and antimony substituted into the perovskite structure. The combined effects of lithium and antimony additions resulted in a downward shift in the orthorhombic-tetragonal (TO-T) without significantly reducing TC. The dielectric, piezoelectric, and electromechanical properties were found to be ε∕ε0>1300, d33>260pC∕N, and kp>50%, while maintaining low dielectric loss. The enhanced polarizability associated with the polymorphic TO-T transition and high TC transition (∼390°C) should provide a wide range of temperature operation.
Al composition dependent properties of quaternary AlInGaN Schottky diodes
Authors: Liu Y. , Jiang H. , Egawa T. , Zhang B. , Ishikawa H.
Published: August 2006

Pd Schottky barrier diodes were fabricated on undoped AlxIn0.02Ga0.98−xN∕GaN with x less than 20%. The material properties, which were characterized by photoluminescence, x-ray diffraction, and atomic force microscopy, indicated that the quaternary samples were coherently grown on GaN template with high crystalline quality. The flatband barrier height obtained by capacitance-voltage (C-V) measurement increased with increasing Al mole fraction (increasing the band gap of the quaternary) up to 2.06eV, in agreement with the predictions of the Schottky-Mott theory. However, current-voltage (I-V) measurements revealed that the barrier height decreased from 1.32to1.12eV, which was accompanied by an increase in ideality factor from 1.04 to 1.73. The large difference of barrier height between I-V and C-V measurements could not be quantitatively explained by the traditional electron transport mechanisms of Schottky diode, such as tunneling effect, image force effect, and barrier inhomogeneity theory. Strong polarization effect in strained AlxIn0.02Ga0.98−xN∕GaN heterostructure was proposed to account for the experimental results, since similar phenomena had been observed extensively in strained AlyGa1−yN∕GaN heterojunction structures.
Temperature dependence of the resistance of magnetic tunnel junctions with MgO barrier
Authors: Kou X. , Schmalhorst J. , Thomas A. , Reiss G.
Published: August 2006

The temperature dependent tunneling resistance of magnetic tunnel junctions with MgO barriers was characterized. In the junctions prepared by magnetron sputtering, the tunnel magnetoresistance decreases with increasing temperature. Various contributions to the tunnel conductance are discussed using different models. Not only the direct elastic tunneling contributes to the temperature dependence of tunnel magnetoresistance, but also the assisted, spin-independent tunneling plays an important role in determining the temperature dependent behavior in our magnetic tunneling junctions. The process is further investigated assuming magnon and phonon assisted tunneling and compared to junctions with alumina tunnel barrier.
Analysis of limitations to wallplug efficiency and output power for quantum cascade lasers
Authors: Vurgaftman I. , Meyer J. R.
Published: August 2006

We have carried out detailed optical and thermal simulations of quantum cascade laser (QCL) operation as a function of material and device parameters. The main objective of the study is to evaluate the potential for improvements in the cw wallplug efficiencies and output powers beyond the current state of the art, when such parameters as the internal loss, series resistance, doping level in the active region, number of QCL stages, ridge width, cavity length, current density, and operating temperature are varied within reasonable ranges. For the test case of a narrow-ridge λ=4.8μm QCL mounted epitaxial side down, we project that a maximum wallplug efficiency of >10% and cw output power of >1W may be feasible at room temperature, if the net internal loss can be reduced by 30% from its best current value.
Reverse-bias emission sheds light on the failure mechanism of degraded vertical-cavity surface-emitting lasers
Authors: McElfresh D. K. , Lopez L. D. , Vacar D.
Published: August 2006

The phenomenon of reverse-bias electroluminescence (EL) has been exploited extensively to elucidate the physics of failure of Si-based semiconductor devices. In this article, we study the reverse-bias emission of vertical-cavity surface-emitting lasers (VCSELs) that have been stressed under accelerated temperature, humidity, and bias. Using quantitative emission microscopy in conjunction with other electrical and optical measurements, we show that the dark line defect dislocation network responsible for VCSEL degradation can be imaged directly as reverse-bias EL and that the spectra of the reverse-bias EL provide further insight into the failure mechanism.
Terahertz plasma wave generation in ultrashort-channel field effect transistors: The essential role of carrier drift velocity saturation
Authors: Cheremisin M. V. , Samsonidze G. G.
Published: August 2006

Taking into account both the carrier scattering and the velocity saturation, we examine the plasma wave instability of the high-density two-dimensional electron gas in a field effect transistor. It is shown that both the carrier scattering and the velocity saturation lead to a strong damping of the plasma wave instability. A threshold diagram of the instability is calculated. The device parameters required for a plasma wave generation are shown to be more strict than those reported in up-to-date subterahertz emission experiments.
Superparamagnetic behavior in ultrathin CoNi layers of electrodeposited CoNi∕Cu multilayer nanowires
Authors: Tang X.-T. , Wang G.-C. , Shima M.
Published: August 2006

We present evidence that in a very thin regime the magnetic layers become discrete islands and superparamagnetic in multilayered CoNi(1–17nm)∕Cu(4.2nm) nanowires grown by pulsed electrodeposition using a hole pattern of anodized alumina templates. Magnetic hysteresis loops measured at room temperature using a vibrating sample magnetometer show that superparamagnetism appears at t(CoNi)<1.7nm, due to a volumetric reduction of the CoNi layers that may result in discontinuity of the layer or formation of islands. The magnetic hysteresis loops for the superparamagnetic nanowires can be represented by the Langevin function. The temperature dependence of coercivity data obtained for the superparamagnetic nanowires using a superconducting quantum interference device indicates that the magnetization reversal can be consistently explained by the Stoner-Walfarth model for coherent rotation. The volumetric reduction accounted for the observed superparamagnetism is probably due to an electrochemical exchange reaction between CoNi and Cu species at the interface during each Cu deposition cycle. The exchange reaction may cause partial dissolution of the CoNi layers at the interface which is eventually stabilized by cementation with Cu. The effects of the nucleation and growth process on the formation of superparamagnetic islands are also discussed.
Nonphenomenological damping constant due to eddy current losses in uniformly magnetized samples
Authors: Martinez E. , Lopez-Diaz L. , Torres L.
Published: August 2006

By coupling Maxwell’s equations with Landau-Lifshitz’s gyromagnetic precession in a uniformly magnetized sample with finite conductivity we show that, within plausible approximations, the effect of eddy currents is equivalent to a linear dissipation term. From this term, a simple expression for the damping constant αeddy in terms of the material parameters and the particle’s volume and shape can be derived. As a test for the accuracy of this expression, magnetization reversal of a nanocube is computed assuming that eddy currents are the only dissipation mechanism in the system and the results are compared with a full micromagnetic simulation in which eddy currents are calculated explicitly. An excellent agreement between both approaches is found.
Low temperature epitaxial growth of In0.25Ga0.75N on lattice-matched ZnO by pulsed laser deposition
Authors: Kobayashi Atsushi , Ohta Jitsuo , Fujioka Hiroshi
Published: August 2006

We have grown In0.25Ga0.75N films on nearly lattice-matched ZnO (0001¯) substrates with atomically flat surfaces at low substrate temperatures using pulsed laser deposition. We have found that InGaN having a stepped and terraced structure on its surface grows at low substrate temperatures. We have also found that InGaN grows in the layer-by-layer mode, even at room temperature (RT), while maintaining straight steps and atomically flat terraces from the first monolayer. This phenomenon exhibits a singular contrast to the case of RT epitaxy of GaN on ZnO, where a considerable number of defects are observed on the stepped structure of the first several layers. This difference can be attributed to a smaller lattice mismatch between InGaN and ZnO.
Structural and magnetic properties of Co∕Pt(111) with Ni buffer layers
Authors: Ho H. Y. , Chen Y. J. , Chen R. H. , Shern C. S.
Published: August 2006

Magneto-optical Kerr effect, low-energy electron diffraction, and Auger electron spectroscopy were used to investigate the magnetic properties and the growth mode of ultrathin Co films deposited on 1 ML (monolayer) Ni∕Pt(111) at room temperature. No Kerr signal was observed when the thickness of Co film was below 3 ML, and the easy axis of the magnetization was in plane when the thickness of Co was greater than 3 ML at room temperature. The polar Kerr signal appeared after the sample was annealed at 450K for 1 ML of Co. Further studies in the magnetic properties and surface composition of 3 ML Co/1 ML Ni∕Pt(111) during an annealing process showed that the out-of-plane magnetization enhances significantly when Co and Ni atoms diffuse into the Pt substrate. The coercivity of the system can be adjusted by changing the annealing temperature. Measuring at room temperature, the coercivity increases when the annealing temperature rises. The possible mechanism of the evolution of magnetic properties with the structure of the interface is discussed.