bluuebunny/crossref_metadata_2025 huggingface dataset:
NONPARAMETRIC ESTIMATION OF REGRESSION FUNCTIONS WITH DISCRETE REGRESSORS
Authors: Ouyang Desheng , Li Qi , Racine Jeffrey S.
Published: January 2009

We consider the problem of estimating a nonparametric regression model containing categorical regressors only. We investigate the theoretical properties of least squares cross-validated smoothing parameter selection, establish the rate of convergence (to zero) of the smoothing parameters for relevant regressors, and show that there is a high probability that the smoothing parameters for irrelevant regressors converge to their upper bound values, thereby automatically smoothing out the irrelevant regressors. A small-scale simulation study shows that the proposed cross-validation-based estimator performs well in finite-sample settings.
COVARIANCE-BASED ORTHOGONALITY TESTS FOR REGRESSORS WITH UNKNOWN PERSISTENCE
Authors: Maynard Alex , Shimotsu Katsumi
Published: January 2009

This paper develops a new test of orthogonality based on a zero restriction on the covariance between the dependent variable and the predictor. The test provides a useful alternative to regression-based tests when conditioning variables have roots close or equal to unity. In this case standard predictive regression tests can suffer from well-documented size distortion. Moreover, under the alternative hypothesis, they force the dependent variable to share the same order of integration as the predictor, whereas in practice the dependent variable often appears stationary and the predictor may be near-nonstationary. By contrast, the new test does not enforce the same orders of integration and is therefore capable of detecting a rich set of alternatives to orthogonality that are excluded by the standard predictive regression model. Moreover, the test statistic has a standard normal limit distribution for both unit root and local-to-unity conditioning variables, without prior knowledge of the local-to-unity parameter. If the conditioning variable is stationary, the test remains conservative and consistent. Simulations suggest good small-sample performance. As an empirical application, we test for the predictability of stock returns using two persistent predictors, the dividend-price ratio and short-term interest rate.
A GENERALIZED PORTMANTEAU TEST FOR INDEPENDENCE BETWEEN TWO STATIONARY TIME SERIES
Authors: Shao Xiaofeng
Published: January 2009

We propose generalized portmanteau-type test statistics in the frequency domain to test independence between two stationary time series. The test statistics are formed analogous to the one in the paper by Chen and Deo (2004,Econometric Theory20, 382–416), who extended the applicability of the portmanteau goodness-of-fit test to the long memory case. Under the null hypothesis of independence, the asymptotic standard normal distributions of the proposed statistics are derived under fairly mild conditions. In particular, each time series is allowed to possess short memory, long memory, or antipersistence. A simulation study shows that the tests have reasonable size and power properties.
LASSO-TYPE GMM ESTIMATOR
Authors: Caner Mehmet
Published: January 2009

This paper proposes the least absolute shrinkage and selection operator–type (Lasso-type) generalized method of moments (GMM) estimator. This Lasso-type estimator is formed by the GMM objective function with the addition of a penalty term. The exponent of the penalty term in the regular Lasso estimator is equal to one. However, the exponent of the penalty term in the Lasso-type estimator is less than one in the analysis here. The magnitude of the exponent is reduced to avoid the asymptotic bias. This estimator selects the correct model and estimates it simultaneously. In other words, this method estimates the redundant parameters as zero in the large samples and provides the standard GMM limit distribution for the estimates of the nonzero parameters in the model. The asymptotic theory for our estimator is nonstandard. We conduct a simulation study that shows that the Lasso-type GMM correctly selects the true model much more often than the Bayesian information Criterion (BIC) and another model selection procedure based on the GMM objective function.
Reported foodborne outbreaks due to noroviruses in Belgium: the link between food and patient investigations in an international context
Authors: BAERT L. , UYTTENDAELE M. , STALS A. , VAN COILLIE E. , DIERICK K. , DEBEVERE J. , BOTTELDOORN N.
Published: January 2009

The Belgian data for foodborne norovirus (NoV) outbreaks became available for the first time with the introduction of an extraction and detection protocol for NoV in the National Reference Laboratory for foodborne outbreaks in September 2006. In 2007, 10 NoV foodborne outbreaks were reported affecting 392 persons in Belgium. NoV became the most detected agent in foodborne outbreaks followed bySalmonella(eight foodborne outbreaks). The major implicated foods were sandwiches (4/10), where food handlers reported a history of gastroenteritis in two outbreaks. A food handler was implicated in the limited number of Belgian NoV outbreaks which is in accord with internationally recorded data. Forty foodborne and waterborne outbreak events due to NoV, epidemiological and/or laboratory confirmed, from 2000 to 2007 revealed that in 42·5% of the cases the food handler was responsible for the outbreak, followed by water (27·5%), bivalve shellfish (17·5%) and raspberries (10·0%).
Bend Sinister: How the Constitution Saved the Republic
and Lost Itself
Authors: Lowi Theodore J.
Published: January 2009

This is a history of illegitimacy in America. And here is my text,
drawn from three Russians, who understand illegitimacy better than
anyone else.
Emotion perception in obsessive–compulsive disorder
Authors: BOZIKAS VASILIS P. , KOSMIDIS MARY H. , GIANNAKOU MARIA , SAITIS MIHALIS , FOKAS KOSTAS , GARYFALLOS GEORGE
Published: January 2009

The purpose of the present study was to investigate the ability to perceive facial and vocal affect in a group of patients with obsessive–compulsive disorder (OCD) and to explore the specific emotions that might be troublesome for them. Participants were 25 patients with OCD and 25 healthy controls, matched for age, education, and gender. They were assessed with computerized tests of affect perception using visual faces [Kinney’s Affect Matching Test (KAMT)], visual everyday scenarios [Fantie’s Cartoon Test (FCT)], and prosody [Affective Prosody Test (APT)], as well as a facial recognition test [Kinney’s Identity Matching Test (KIMT)]. Severity of OCD symptoms in the patient group was measured with the Yale–Brown Obsessive Compulsive Scale. Patients with OCD were not impaired in the perception of emotion, in either the visual [still photographs (KAMT) or sketches of everyday scenarios (FCT)] or the vocal (APT) modality, as compared with age-, sex-, and education-matched healthy individuals. Moreover, patients with OCD did not differ from healthy individuals in discriminating facial identity (KIMT). With regard to each emotion type separately, patients performed equally well as healthy individuals in all the emotions examined. Emotion processing of both facial expressions and prosody does not appear to be deficient in patients with OCD (JINS, 2009, 15, 148–153).
Increasing winter baseflow and mean annual streamflow from possible permafrost thawing in the Northwest Territories, Canada
Authors: St. Jacques Jeannine‐Marie , Sauchyn David J.
Published: January 2009

Increasing surface air temperatures from anthropogenic forcing are melting permafrost at high latitudes and intensifying the hydrological cycle. Long‐term streamflow records (≥30 yrs) from 23 stream gauges in the Canadian Northwest Territories (NWT) indicate a general significant upward trend in winter baseflow of 0.5–271.6 %/yr and the beginning of significant increasing mean annual flow (seen at 39% of studied gauge records), as assessed by the Kendall‐τ test. The NWT exports an average discharge of ≥308.6 km3/yr to the Beaufort Sea, of which ≥120.9 km3/yr is baseflow. We propose that the increases in winter baseflow and mean annual streamflow in the NWT were caused predominately by climate warming via permafrost thawing that enhances infiltration and deeper flowpaths and hydrological cycle intensification. To provide hydroclimatic context, we present evidence of a statistically significant positive link between the Northern annular mode and annual NWT streamflow at the interannual‐to‐decadal timescales.
Polarimetric remote sensing of aerosols over land
Authors: Waquet F. , Cairns B. , Knobelspiesse K. , Chowdhary J. , Travis L. D. , Schmid B. , Mishchenko M. I.
Published: January 2009

We present a new approach to retrieve the aerosol properties over land that uses accurate polarization measurements over a broad spectral (410–2250 nm) and angular (±60° from nadir) ranges. The approach uses longer wavelength observations to accurately estimate the surface effects, and it is incorporated into an optimal estimation framework for retrieving the particle number density and a detailed aerosol microphysical model: effective radius, variance, and complex refractive index. A sensitivity analysis shows that the uncertainties in aerosol optical thickness (AOT) increase with AOT while the uncertainties in the microphysical model decrease. The uncertainty in the single scattering albedo (SSA) is notably less than 0.05 by the time the AOT is greater than 0.2. We find that calibration is the major source of uncertainty and that perfect angular and spectral correlation of calibration errors reduces the uncertainties in retrieved quantities. Finally, we observe that shorter wavelength (<500 nm) observations are crucial for determining the aerosols vertical extent and imaginary refractive index from polarization measurements. The retrieval approach is tested under pristine and polluted conditions using observations made by the Research Scanning Polarimeter during the Aerosol Lidar Validation experiment and over California Southern wild fires. In both cases we find that the retrievals are within the combined uncertainties of the retrieval and the Aerosol Robotic Network Cimel products and Total Ozone Mapping Spectrometer Aerosol Index. This demonstrates the unique capability of polarization measurements to accurately retrieve AOTs under pristine conditions and provide estimation of the SSA at higher AOTs.
Functional analysis reveals effects of tobacco alternative oxidase gene (NtAOX1a) on regulation of defence responses against abiotic and biotic stresses
Authors: Zhang Yi , Xi Dongmei , Wang Jian , Zhu Dongfang , Guo Xingqi
Published: January 2009

Mitochondrial AOX (alternative oxidase) is the terminal oxidase of the CN (cyanide)-resistant alternative respiratory pathway in plants. To investigate the role of the tobacco AOX gene (NtAOX1a) (where Nt is Nicotiana tabacum) under deleterious conditions which could induce ROS (reactive oxygen species) accumulation, we generated and characterized a number of independent transgenic tobacco (N. tabacum) lines with altered NtAOX1a gene expression and AP (alternative pathway) capacity. AOX efficiently inhibited the production of low-temperature-induced H2O2 and might be a major enzyme for scavenging H2O2 at low temperature. Furthermore, NtAOX1a may act as a regulator of KCN-induced resistance to TMV (tobacco mosaic virus) through the regulation of H2O2. Notably, a moderate accumulation of H2O2 under the control of NtAOX1a was crucial in viral resistance. Analysis of seed germination indicated an important role for NtAOX1a in germination under H2O2-induced oxidative stress when the CP (cytochrome pathway) was inhibited. These results demonstrate that NtAOX1a is necessary for plants to survive low temperature, pathogen attack and oxidative stress by scavenging ROS under these adverse conditions when the CP is restricted.
Neutralization efficiency estimation in a neutral beam source based on inductively coupled plasma
Authors: Vozniy O. V. , Yeom G. Y.
Published: January 2009

This study examined the optimal conditions of neutral beam generation to maintain a high degree of neutralization and focusing during beam energy variation for a neutral beam source based on inductively coupled plasma with a three-grid ion beam acceleration system. The neutral beam energy distribution was estimated by measuring the energy profiles of ions that “survived” the neutralization after reflection. The energy measurements of the primary and reflected ions showed narrow distribution functions, each with only one peak. At higher beam energies, both the ratio of the ion energy loss to the primary energy and the degree of energy divergence decreased, confirming the precise alignment of the neutral beam. The neutralization efficiency of the neutral beam source with a three-grid acceleration system was found to be affected mainly by the beam angle divergence rather than by the particle translation energy.
Structural changes during the natural aging process of InN quantum dots
Authors: González D. , Lozano J. G. , Herrera M. , Browning N. D. , Ruffenach S. , Briot O. , García R.
Published: January 2009

The natural aging process of InN nanostructures by the formation of indium oxides is examined by transmission electron microscopy related techniques. Uncapped and GaN-capped InN quantum dots (QDs) on GaN/sapphire substrates were grown under the same conditions and kept at room temperature/pressure conditions. The GaN capping layer is found to preserve the InN QDs in the wurtzite phase, avoiding the formation of group-III oxides, while in the uncapped sample, a thin layer of cubic phases are formed that envelops the nucleus of wurtzite InN. These cubic phases are shown to be mainly bcc-In2O3 for long aged samples where the nitrogen atoms in the InN surface layers have been substituted by atmospheric oxygen. This process implies the gradual transformation of the In sublattice from hcp to a quasi-fcc structure. Metastable zinc-blende InN phases rich in oxygen atoms are proposed to act as intermediate phases and they are evinced in samples less aged. The large concurrence of interplanar spaces, the twin formation, and the existence of a free surface that facilitates the transformation support this mechanism and would explain the high instability of the InN nanostructures at ambient conditions.
Birational mappings and matrix subalgebra from the chiral Potts model
Authors: Preissmann E. , Anglès d’Auriac J.-Ch. , Maillard J.-M.
Published: January 2009

We study birational transformations of the projective space originating from lattice statistical mechanics, specifically from various chiral Potts models. Associating these models with stable patterns and signed patterns, we give general results which allow us to find all chiral q-state spin-edge Potts models when the number of states q is a prime or the square of a prime, as well as several q-dependent family of models. We also prove the absence of monocolor stable signed pattern with more than four states. This demonstrates a conjecture about cyclic Hadamard matrices in a particular case. The birational transformations associated with these lattice spin-edge models show complexity reduction. In particular, we recover a one-parameter family of integrable transformations, for which we give a matrix representation when the parameter has a suitable value.
Microstructure and luminescent properties of novel InGaP alloys on relaxed GaAsP substrates
Authors: Mori M. J. , Fitzgerald E. A.
Published: January 2009

We present a metal organic chemical vapor deposition (MOCVD) growth study of unconventional alloys of InGaP (with In fraction of 0.2–0.4) grown on fully relaxed GaAsP virtual substrates for the application of high performance visible light emitting diodes (LEDs) and lasers in the green to red range of the visible spectrum. Several defects which are harmful to optical performance were identified, characterized, and removed. These include CuPt–B order (which lowers band gap), phase separation or short range order (which contributes to leakage currents and reduces luminescence) and undulation of the virtual substrate surface (which prevents high quality epitaxial growth). Each of these defects is understood through a two-step growth model which describes the formation and subsequent randomization of defects during growth. Through control of MOCVD parameters including growth temperature, V/III ratio, growth rate, and surfactant we demonstrate growth of extremely high quality InGaP heterostructures which hold promise for fabrication of light emitting devices.
Fabrication and optical properties of C/β-SiC/Si hybrid rolled-up microtubes
Authors: Huang G. S. , Mei Y. F. , Cavallo F. , Baunack S. , Coric E. , Gemming T. , Bertram F. , Christen J. , Fu R. K. Y. , Chu Paul K. , Schmidt O. G.
Published: January 2009

C / β -SiC / Si hybrid microtubes have been fabricated by releasing prestressed C/Si bilayer structures and treating with a postannealing process. Detailed characterization reveals the synthesis of β-SiC via a solid phase reaction at the C/Si interface. Remarkably, the production of β-SiC is promoted in the tube wall by rolled-up bonding of adjacent windings, which increases the area of the C/Si interface by a factor of 2. The Raman spectra acquired from the hybrid microtubes disclose peaks pertaining to the optical phonon modes of β-SiC that exhibit obvious downshifts due to surface effects on the SiC nanoparticles. Moreover, two light emission bands are detected from a hybrid microtube and their origin is discussed based on spectral analyses.
Optical index profile of nonuniform depth-distributed silicon nanocrystals within SiO2
Authors: Barba D. , Dahmoune C. , Martin F. , Ross G. G.
Published: January 2009

Optical properties of silicon nanocrystals (Si-ncs) prepared by silicon implantation into silicon oxide have been investigated by photoluminescence measurements and spectroscopic ellipsometry. The dielectric function associated with Si-nc uniformly and nonuniformly depth distributed has been determined by means of the Tauc–Lorentz (TL) model, using the Bruggemann effective medium approximation. The evolution of the Si-nc sublayer dielectric response as a function of the ion fluence has been established for volume fractions of Si excess varying between 9.1% and 50.4%. Comparison between the depth profile of optical indices determined by ellipsometry and TRIM calculations shows that for implanted Si volume fraction lower than 30%, the center and the width of the optical index profile agree with the spatial distribution of the implanted Si when both the swelling and the ion sputtering effects are taken into account. This is also valid in systems having two separate Si-nc sublayers, where the geometric characterization of the optical index variations has been computed from a data extrapolation. For a volume fraction of 50.4%, where the ion implantation performed at high fluence can activate the oxygen depletion from the material surface, the spatial distribution of the optical refractive index is deeper and narrower than the Si excess profile.
Atomic force microscope force spectroscopy study of the electric double layer at a liquid crystal interface
Authors: Škarabot M. , Muševič I.
Published: January 2009

Electric force spectroscopy on an atomic force microscope has been used to determine the electric field distribution in the electric double layer at a liquid-crystal–glass interface. The separation-dependence of the electric force has been studied inside the liquid crystal interface, and screening of the surface electric field was observed. The results were compared with a simple theoretical analysis and a relatively good quantitative agreement was found. The method provides simple, accurate, and straightforward measurement of the Debye screening length, while the determination of the surface electric potential is less accurate. The observed Debye screening lengths are of the order of 50nm and change when the interface is illuminated with UV light.
Control of cavity cross section in microplasma devices: Luminance and temporal response of 200×100 and 320×160 arrays with parabolic Al2O3 microcavities
Authors: Kim K. S. , Kim T. L. , Yoon J. K. , Park S.-J. , Eden J. G.
Published: January 2009

Intense green luminance and luminous efficacies approaching 4 lm/W have been observed for large (50×50 to 320×160) arrays of microplasma devices with parabolic cross-sectional Al2O3 microcavities and conformal aluminum electrodes, operating in Ne/Xe gas mixtures. Precise control of the cross-sectional geometry and surface morphology of the cavities within Al/Al2O3 microplasma devices having a dielectric barrier structure has been achieved with a sequence of wet electrochemical processes. Continuous variation of the cavity cross section between a linear taper and parabolic geometry can be specified and all dimensions controlled to within ±2%. Aluminum electrodes encompassing each cavity are azimuthally symmetric and the inner face of each electrode is conformal to the Al2O3 microcavity wall. Arrays comprising 20 000 devices (in a 200×100 configuration) with a parabolic microcavity wall profile and an emitting aperture 160±2 μm in diameter produce a green luminance >1800 cd/m2 and a peak luminous efficacy of 3.9 lm/W in Ne/30% Xe gas mixtures at a total pressure of 500 Torr. Temporal response measurements show the visible emission rise time of 200–250 ns to be limited only by the rise time of the voltage waveform itself.
Fedosov quantization of Lagrange–Finsler and Hamilton–Cartan spaces and Einstein gravity lifts on (co) tangent bundles
Authors: Anastasiei Mihai , Vacaru Sergiu I.
Published: January 2009

We provide a method of converting Lagrange and Finsler spaces and their Legendre transforms to Hamilton and Cartan spaces into almost Kähler structures on tangent and cotangent bundles. In particular cases, the Hamilton spaces contain nonholonomic lifts of (pseudo) Riemannian∕Einstein metrics on effective phase spaces. This allows us to define the corresponding Fedosov operators and develop deformation quantization schemes for nonlinear mechanical and gravity models on Lagrange–and Hamilton–Fedosov manifolds.
On exchangeable continuous variable systems
Authors: König Robert , Wolf Michael M.
Published: January 2009

We investigate permutation-invariant continuous variable quantum states and their covariance matrices. We provide a complete characterization of the latter with respect to permutation invariance and exchangeability and representing convex combinations of tensor power states. On the level of the respective density operators this leads to necessary criteria for all these properties which become necessary and sufficient for Gaussian states. For these we use the derived results to provide de Finetti-type theorems for various distance measures.
Solving non-Born–Oppenheimer Schrödinger equation for hydrogen molecular ion and its isotopomers using the free complement method
Authors: Hijikata Yuh , Nakashima Hiroyuki , Nakatsuji Hiroshi
Published: January 2009

The Schrödinger equations for the hydrogen molecular ion (H2+) and its isotopomers (D2+, T2+, HD+, HT+, and DT+) were solved very accurately using the free iterative complement interaction method, which is referred to in short as the free complement (FC) method, in the non-Born–Oppenheimer (non-BO) level, i.e., in the nonrelativistic limit. Appropriate complement functions for both electron and nuclei were generated automatically by the FC procedure with the use of the non-BO Hamiltonian, which contains both electron and nuclear operators on an equal footing. Quite accurate results were obtained not only for the ground state but also for the vibronic excited states. For example, we obtained the ground-state energy of H2+ as −0.597 139 063 123 405 074 834 134 096 025 974 142 a.u., which is variationally the best in literature. The difference in the nuclear spin states of S1 (para) and P3 (ortho) of H2+ and some physical expectation values for several of the isotopomers shown above were also examined. The present study is the first application of the FC method to molecular systems with the non-BO Hamiltonian.
Organic light-emitting diodes with carbon nanotube cathode-organic interface layer
Authors: Liu Deang , Fina Michael , Guo Jinghua , Chen Xiaobo , Liu Gao , Johnson Stephen G. , Mao Samuel S.
Published: January 2009

Improved performance of organic light-emitting diodes (OLEDs) was achieved by implementing a carbon nanotube (CNT) layer at the cathode-organic interface, spin coated between the organic layer and the cathode. The small geometry of CNTs could enable the enhancement of the electric field around them, thus increasing electron injection efficiency from the cathode to the organic layer. In addition, as measured from the x-ray absorption and emission spectroscopy, incorporation of CNT could reduce the lowest unoccupied molecular orbital of the organic material at the cathode-organic interface, thus effectively decreasing the barrier for electron injection. Increased electron injection and luminance characteristics were demonstrated for both polymer and small molecule based OLED devices.
Absence of amorphous phase in high power femtosecond laser-ablated silicon
Authors: Rogers Matthew S. , Grigoropoulos Costas P. , Minor Andrew M. , Mao Samuel S.
Published: January 2009

As femtosecond lasers emerge as viable tools for advanced microscale materials processing, it becomes increasingly important to understand the characteristics of materials resulting from femtosecond laser microablation or micromachining. We conducted transmission electron microscopy experiments to investigate crater structures in silicon produced by repetitive high power femtosecond laser ablation. Comparable experiments of nanosecond laser ablation of silicon were also performed. We found that an amorphous silicon layer that is typically produced in nanosecond laser ablation is absent when the material is irradiated by high power femtosecond laser pulses. Instead, only a defective single crystalline layer was observed in the high power femtosecond laser-ablated silicon crater. Possible mechanisms underlying the formation of the defective single crystalline phase are discussed.
An impact rotary motor based on a fiber torsional piezoelectric actuator
Authors: Han W. X. , Zhang Q. , Ma Y. T. , Pan C. L. , Feng Z. H.
Published: January 2009

A prototype small impact rotary motor has been fabricated based on a newly developed torsional actuator which is 15.0 mm long and 1.0 mm in diameter. The motor can rotate when it is powered with a saw-shaped voltage. The experimental results show that its angular speed is proportional to both the driving voltage’s amplitude and the frequency under 1 kHz. The large nonlinearity occurs at higher driving frequency due to the resonance of the partial mechanical structure of the motor. The motor can rotate at a speed of 90 rpm with a saw-shaped driving voltage of 600Vp.-p. at 8 kHz, and produce a stall torque of 80 μN m with 1000Vp.-p. at 3 kHz.
How to track protists in three dimensions
Authors: Drescher Knut , Leptos Kyriacos C. , Goldstein Raymond E.
Published: January 2009

We present an apparatus optimized for tracking swimming micro-organisms in the size range of 10–1000 μm, in three dimensions (3Ds), far from surfaces, and with negligible background convective fluid motion. Charge coupled device cameras attached to two long working distance microscopes synchronously image the sample from two perpendicular directions, with narrow band dark-field or bright-field illumination chosen to avoid triggering a phototactic response. The images from the two cameras can be combined to yield 3D tracks of the organism. Using additional, highly directional broad-spectrum illumination with millisecond timing control the phototactic trajectories in 3D of organisms ranging from Chlamydomonas to Volvox can be studied in detail. Surface-mediated hydrodynamic interactions can also be investigated without convective interference. Minimal modifications to the apparatus allow for studies of chemotaxis and other taxes.
X-ray reflectivity and total reflection x-ray fluorescence study of surface oxide evolution in a GaAs/AlAs multilayer system
Authors: Colombi Paolo , Bontempi Elza , Depero Laura E. , Azuma Yasushi , Fujimoto Toshiyuki
Published: January 2009

Modifications of GaAs/air interface under x-ray exposure have been investigated by means of x-ray reflectometry (XRR). For this purpose a GaAs/AlAs multilayer system was continuously monitored for 90 h while acquiring XRR profiles. Three different starting models for the oxide/contaminants layer were tested for the analysis of XRR data and discussed. The main effect of x-ray exposure, revealed by the three different approaches, is an increase in the thickness of the gallium and arsenic oxides until a saturation value (about 3.0 nm) is reached after about 50 h. Total reflection x-ray fluorescence analysis performed on a couple of twin samples, either irradiated and nonirradiated, confirms that the oxidation process is promoted by x-ray exposure and indicates the presence of a richer As oxide phase at the surface of the x-ray irradiated samples.
Deep-level transient spectroscopy of low-energy ion-irradiated silicon
Authors: Kolkovsky Vl. , Privitera V , Larsen A. Nylandsted
Published: January 2009

During electron-gun deposition of metal layers on semiconductors, the semiconductor is bombarded with low-energy metal ions creating defects in the outermost surface layer. For many years, it has been a puzzle why deep-level transient spectroscopy spectra of the as-deposited, electron-gun evaporated, n-type Schottky diodes are so simple displaying only one peak consisting of the merged E center and single-acceptor divacancy peaks, and no A center and double-acceptor divacancy peaks. With reference to a recent publication, we demonstrate that this is not due to a reduced production of divacancies and A centers in this situation but to the localization of these defects in highly defected regions.
Experimental study of femtosecond laser-stimulated electrical discharges in small gaps and surface modifications
Authors: Chen J. , Farson D. F. , Rokhlin S. I.
Published: January 2009

Femtosecond laser-stimulated discharges in nanoscale and microscale gaps between etched nanoprobe tip cathodes and gold film anodes with applied dc potential were experimentally studied to define parameter ranges for their controlled formation and resulting surface modifications. For appropriate values of gap length, applied potential, and laser irradiance, breakdown discharges could be reliably stimulated by femtosecond laser pulses and the mean breakdown field was approximately an order of magnitude smaller than for breakdown without laser stimulation. For 500 nm gaps, discharges were observed for applied potentials as small as 20 V and controllable gold film surface melting was detected for applied potential of 27.5 V. Minor cathode tip ablation could be observed for femtosecond laser pulses that reliably stimulated discharges, suggesting that cathode material played an important role in stimulation of breakdown discharges in nanoscale gaps. Surface melting produced features as small as 60 nm on gold film when discharge current was limited by 1 MΩ series resistor.
Effects of the shape of elongated magnetic particles on the coercive field
Authors: Ott F. , Maurer T. , Chaboussant G. , Soumare Y. , Piquemal J.-Y. , Viau G.
Published: January 2009

Magnetic nanowires could be the building bricks in the fabrication of composite magnetic materials because of their large intrinsic shape anisotropy. We investigate the relation between the detailed shape of a magnetic nanowire and its magnetic coercivity. We have performed three-dimensional micromagnetic simulations on various types of nanowires synthesized during the past few years such as cylinders, dumbbells, or diabolos. The calculations, performed on individual model objects, show that the wire tip plays a key role in the reversal mechanism and on the magnitude of the coercive field and that the aspect ratio plays a much lesser role. Ellipsoidal or cylindrical shapes favor a coherent rotation of the magnetization and thus large coercive fields. Complex tip shapes act as nucleation points and significantly reduce the coercive field. Thus, in order to optimize the shape of magnetic nanowires for permanent magnet applications, the focus should be put on the detailed shape of the wire tips and thus on the growth mechanism rather than on the aspect ratio. The numerical modeling results on individual wires are compared with the experimental data obtained on various types of wires synthesized by soft chemistry methods.
Enhanced amplified spontaneous emission using layer-by-layer assembled cowpea mosaic virus
Authors: Li Na , Deng Zhaoqi , Lin Yuan , Zhang Xiaojie , Geng Yanhou , Ma Dongge , Su Zhaohui
Published: January 2009

Layer-by-layer assembly technique was used to construct ultrathin film of cowpea mosaic virus (CPMV) by electrostatic interactions, and the film was employed as a precursor on which an OF8T2 film was deposited by spin coating. Amplified spontaneous emission (ASE) was observed and improved for the OF8T2 film. Compared with OF8T2 film on quartz, the introduction of CPMV nanoparticles reduced the threshold and loss, and remarkably increased the net gain. The threshold, loss, and gain reached 0.05 mJ/pulse, 6.9 cm−1, and 82 cm−1, respectively. CPMV nanoparticles may enormously scatter light, resulting in a positive feedback, thus the ASE is easily obtained and improved.
Interfaces in poly(vinylidene fluoride) terpolymer/ZrO2 nanocomposites and their effect on dielectric properties
Authors: Chu Baojin , Lin Minren , Neese Bret , Zhang Qiming
Published: January 2009

Enhancement of weak and high field dielectric responses was observed in poly(vinylidene fluoride) terpolymer/ZrO2 nanocomposites. A frequency-dependent interfacial polarization mechanism (related to the interfacial area between polymer and nanoparticles) is responsible for the enhancement of the dielectric constant at the low field. Due to the improvement of chain mobility and stabilization of the polar phase in the polymer/nanoparticle interface regions, the high field dielectric response of nanocomposites was also enhanced in the certain particle loading. Based on the high-field polarization saturation phenomenon, the dimension of interface regions in the nanocomposites was estimated as 20–40 nm.
The effect of demagnetization on the magnetocaloric properties of gadolinium
Authors: Bahl C. R. H. , Nielsen K. K.
Published: January 2009

Gadolinium displays a strong magnetocaloric effect at temperatures close to room temperature making it useful in the field of room temperature magnetic refrigeration. We discuss the importance of including the effects of the demagnetization field when considering the magnetocaloric properties of gadolinium. The adiabatic temperature change ΔTad of gadolinium sheets upon application of a magnetic field has been measured at a range of applied magnetic fields and sample orientations. A significant dependence of ΔTad on the sample orientation is observed. This can be accounted for by the demagnetization factor. Also, the temperature dependence of ΔTad has been measured experimentally and modeled by mean field theory. Corrections to mean field theory modeling due to the demagnetization field are proposed and discussed.
Properties of Na2SiF6:Mn4+ and Na2GeF6:Mn4+ red phosphors synthesized by wet chemical etching
Authors: Xu Yan Kai , Adachi Sadao
Published: January 2009

We report a simple method of synthesizing red phosphors, Na2SiF6:Mn4+ and Na2GeF6:Mn4+, by wet chemical etching of Si wafers and Ge shots in aqueous HF solution with the addition of an oxidizing agent NaMnO4. The x-ray diffraction analysis suggests that the red phosphors have the trigonal structure (space group=D32−P321). The optical properties of these phosphors are investigated using photoluminescence, photoluminescence excitation, optical absorption, and diffuse reflectance spectroscopy. The spectral features show no clear difference between Na2SiF6:Mn4+ and Na2GeF6:Mn4+ phosphors. They are well explained by the Tanabe–Sugano energy-level diagram with the crystal-field and Racah parameters of Dq∼1970 cm−1, B∼775 cm−1, and C∼3490 cm−1 with C/B∼4.5.
Density-dependent energy relaxation of hot electrons in InN epilayers
Authors: Yang M. D. , Liu Y. W. , Shen J. L. , Chen C. W. , Chi G. C. , Lin T. Y. , Chou W. C. , Lo M. H. , Kuo H. C. , Lu T. C.
Published: January 2009

This work investigates the dependence of the hot-electron energy relaxation in InN epilayers on electron density. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons was determined. Acoustic phonons have an important role in the energy relaxation of the hot electrons. The density-dependent electron energy loss rate in InN can be explained by a combination of longitudinal optical and acoustic phonon emissions. A slowing of energy loss rate at high electron densities was observed and attributed to piezoelectric coupling to acoustic phonons.
Thermoelectric performance of half-Heusler compounds TiNiSn and TiCoSb
Authors: Wang L. L. , Miao L. , Wang Z. Y. , Wei W. , Xiong R. , Liu H. J. , Shi J. , Tang X. F.
Published: January 2009

The electronic structures of half-Heusler compounds TiNiSn and TiCoSb are investigated by using the full-potential linearized augmented plane-wave method. When the spin-orbital coupling is included in the calculations, there is only a slight change in the energy band structures. The transport coefficients (Seebeck coefficient, electrical conductivity, and power factor) are then calculated within the Boltzmann theory, and further evaluated as a function of chemical potential assuming a rigid band picture. Our calculations offer a valuable insight on how to improve the thermoelectric performance of these two compounds.
Transient photorefraction in a-Se films
Authors: Reznik A. , Klebanov M. , Lyubin V.
Published: January 2009

The kinetics of photoinduced transparency changes has been studied experimentally for amorphous selenium (a-Se) films at different intensities of irradiating 632.8 nm light. The effect of transient reversible photorefraction with comparatively short relaxation time compared to other photoinduced effects is shown to exist in these films in addition to the transient photodarkening phenomenon studied previously.
Silver-induced layer exchange for the low-temperature preparation of intrinsic polycrystalline silicon films
Authors: Scholz M. , Gjukic M. , Stutzmann M.
Published: January 2009

The preparation of large grained continuous polycrystalline silicon layers by metal-induced crystallization is reported. The macroscopic layer exchange of an amorphous silicon precursor layer in contact with a silver layer was observed for temperatures below the softening point of glass. This process is quite similar to the well-known aluminum-induced layer exchange. However, due to the use of silver as a catalyst, the recrystallized layers are electrically intrinsic rather than highly doped with Al acceptors. The resulting polycrystalline silicon layers show a good crystalline quality as deduced from Raman scattering, x-ray diffraction, and UV-reflectance measurements.
Fourier-transform resonance shear measurement for studying confined liquids
Authors: Sakuma Hiroshi , Kurihara Kazue
Published: January 2009

The resonance shear measurement we recently developed is an efficient tool for investigating the rheological and tribological properties of liquid nanofilms confined between solid surfaces with varying film thicknesses. However, the previously employed resonance shear measurement measured shear responses at various oscillation frequencies for one film thickness (the frequency scanning method), so it required at least several minutes to obtain a reliable resonance curve. The fast Fourier-transform resonance shear method was developed to rapidly acquire the resonance curves. The obtained curves were very similar to those obtained by the frequency scanning method. The time necessary for the FFT method was 2–10 s, much shorter than the time required for the scanning method. This technique is easy to use and extends the applicability of the method to volatile liquids and unstable samples.
Controlling high-mobility conduction in SrTiO3 by oxide thin film deposition
Authors: Herranz G. , Basletić M. , Copie O. , Bibes M. , Khodan A. N. , Carrétéro C. , Tafra E. , Jacquet E. , Bouzehouane K. , Hamzić A. , Barthélémy A.
Published: January 2009

SrTiO 3 becomes a high-mobility metallic conductor when doped with oxygen vacancies at low concentrations (≥1016 cm−3). We show that the vacancy concentration in the SrTiO3 single crystal substrates could be controllably tuned by changing the thickness of oxide films (deposited by pulsed laser deposition at high temperature and low oxygen pressure). The obtained variation in the carrier density strongly influences the transport properties. The quantitative analysis of the experimental results leads toward new and accurate strategies for the design of multifunctional oxide heterostructures for electronics and spintronics.
Organic thin film transistors using 6,13-bis(tri-isopropylsilylethynyl)pentacene embedded into polymer binders
Authors: Kwon Jae-Hong , Shin Sang-Il , Kim Kyung-Hwan , Cho Min Ju , Kim Kyu Nam , Choi Dong Hoon , Ju Byeong-Kwon
Published: January 2009

The active channel material of an organic thin film transistor (OTFT), 6,13-bis(tri-isopropylsilylethynyl)pentacene (TIPS pentacene), is a functionalized pentacene designed to enhance both the solubility and solid-state packing of the pentacene. In this work, in order to improve device performance, three types of polymer binders—poly(α-methylstyrene) (PAMS), poly(4-vinylbiphenyl) (PVBP), and poly(triarylamine) (PTAA)—were employed to fabricate OTFT devices with organic soluble TIPS pentacene. These binders improved film formation in a large area uniformly and helped the TIPS pentacene to form a stronger binding between source/drain electrodes onto dielectric layer. Thus, device performance was highly improved due to improvement of interfacial contact and an increase in the charge transfer in the active channel. OTFTs using TIPS pentacene with PAMS, PVBP, and PTAA for field effect mobilities in the saturation regime have 5×10−3, 8×10−3, and 2.7×10−2 cm2/V s, respectively.
A first-principles study of enhanced dielectric responses in Ti and Ce doped HfO2
Authors: Dutta Gargi
Published: January 2009

In this letter, the dielectric response in Ti and Ce doped hafnia (HfO2) has been investigated by density functional theory calculations based on pseudopotentials, plane wave basis, and local density approximation. It is found that Ti doped samples give greater dielectric response than Ce doped HfO2, with Hf0.5Ti0.5O2 having the highest response. The Born effective charges on atoms and infrared-active phonon modes have been studied; soft phonon modes cause enhancement in the responses. The materials remain insulating upon doping of HfO2 and are of potential importance for applications as gate oxide.
Field and polarity dependence of time-to-resistance increase in Fe–O films studied by constant voltage stress method
Authors: Eriguchi Koji , Wei Zhiqiang , Takagi Takeshi , Ohta Hiroaki , Ono Kouichi
Published: January 2009

Constant voltage stress (CVS) was applied to Fe–O films prepared by a sputtering process to investigate a stress-induced resistance increase leading to a fundamental mechanism for switching behaviors. Under the CVS, an abrupt resistance increase was found for both stress polarities. A conduction mechanism after the resistance increase exhibited non-Ohmic transport. The time-to-resistance increase (tr) under the CVS was revealed to strongly depend on stress voltage as well as the polarity. From a polarity-dependent resistance increase determined by a time-zero measurement, the voltage and polarity-dependent tr were discussed on the basis of field- and structure-enhanced thermochemical reaction mechanisms.
Ellipsometric study of vertically aligned nematic liquid crystals
Authors: Marino A. , Santamato E. , Bennis N. , Quintana X. , Otón J. M. , Tkachenko V. , Abbate G.
Published: January 2009

The director tilt angle distribution in vertically aligned nematic liquid crystal displays has been investigated by means of variable angle spectroscopic ellipsometry. Liquid crystal vertical alignment has been realized by thermal evaporation of SiOx. By changing the deposition angle, it is possible to control the pretilt angle. The director profile inside the sample was inferred by reflection and transmission ellipsometric measurements. The tilt angle distribution inside the cell versus the applied voltage is reported and eventually, comparing it with the simulations from the elastic theory, the anchoring energy has been obtained.
Multiple scattering effects in strain and composition analysis of nanoislands by grazing incidence x rays
Authors: Richard M.-I. , Favre-Nicolin V. , Renaud G. , Schülli T. U. , Priester C. , Zhong Z. , Metzger T.-H.
Published: January 2009

Experiments and numerical simulations based on finite element modeling show that the x-ray intensity scattered by comparatively large nanostructures on a substrate is not simply related to their strain in experiments using either grazing incidence or exit because of multiple scattering effects. However, whatever the nanostructure size, the composition profiles are correctly extracted from grazing incidence multiwavelength anomalous scattering. These effects are illustrated for the structural analysis of Ge dome-shaped islands grown on Si(001).
Combining magnetic resonance imaging and ultrawideband radar: A new concept for multimodal biomedical imaging
Authors: Thiel F. , Hein M. , Schwarz U. , Sachs J. , Seifert F.
Published: January 2009

Due to the recent advances in ultrawideband (UWB) radar technologies, there has been widespread interest in the medical applications of this technology. We propose the multimodal combination of magnetic resonance (MR) and UWB radar for improved functional diagnosis and imaging. A demonstrator was established to prove the feasibility of the simultaneous acquisition of physiological events by magnetic resonance imaging and UWB radar. Furthermore, first in vivo experiments have been carried out, utilizing this new approach. Correlating the reconstructed UWB signals with physiological signatures acquired by simultaneous MR measurements, representing respiratory and myocardial displacements, gave encouraging results which can be improved by optimization of the MR data acquisition technique or the use of UWB antenna arrays to localize the motion in a focused area.
Blech effect in Pb-free flip chip solder joint
Authors: Lu Minhua , Shih Da-Yuan , Lauro Paul , Goldsmith Charles
Published: January 2009

The Blech effect in electromigration is studied in flip-chip-like Pb-free solder joint structures. The results from two different studies indicate that the Blech limit (J×L)c (current density times solder length) is close to 30 A/cm in Sn1.8 Ag solders, where the dominating degradation mechanism is Sn self-diffusion. For Sn0.7 Cu solders, where the failure is driven by interstitial diffusion, the Blech effect is not observed. When Blech product is approaching the Blech limit, a steady increase in resistance is replaced by a near-zero resistance change. This saturation in resistance shift significantly extends the electromigration lifetime in SnAg solders.
Photoluminescence intensity modulation by charge carrier injection in silicon nanocrystals at room temperature
Authors: Lacombe A. , Beaudoin F. , Koshel D. , Barba D. , Martin F. , Ross G. G.
Published: January 2009

Hysteretic intensity modulation of the photoluminescence (PL) of silicon nanocrystals (Si-NCs) embedded in silicon oxide (SiO2) is observed in metal-oxide-semiconductor (MOS) structures at room temperature during gate voltage sweeps of ±3 V. This PL intensity modulation is correlated with simultaneous current density measurements in the different operating regimes of these MOS devices. In particular, PL intensity enhancement is shown to result from electron injection into the oxide. The different mobilities of the charge carriers in SiO2 and the competing effects of charge trapping in the Si-NC and the surrounding oxide defects are used to explain the observed PL modulation.