bluuebunny/crossref_metadata_2025 huggingface dataset:
Huge nasopharyngeal angiofibroma with intracranial extension: change in the dura mater and choice of surgical management
Authors: Yi ZX , Li ZC , Cheng JM , Zhang R , Lin Chang , Zhou AD , Fan ZM
Published: February 2007

We aimed to review (1) the imaging changes in the dura mater in cases of huge, lobulated juvenile nasopharyngeal angiofibroma, and (2) the choice of surgical management.

Imaging from four cases of juvenile nasopharyngeal angiofibroma showed extrapharyngeal extension of the tumour. The sphenoid sinus, sella turcica and clivus were extensively eroded, and the tumour had spread deep into the cranial fossa. In three cases, intracranial exploration was performed to treat the intracranial tumour lobule. Subsequently, the tumours were removed using extracranial approaches. No perforation of the dura mater was found in these three cases, although the dura mater in the superior orbital fissure was congested, haemorrhagic and solid. Pre-operative imaging for two cases (i.e. the first operation for one and the second operation for the other) revealed no dura mater perforation. A transantral approach via a midfacial degloving incision was used to remove these tumours completely.

We conclude that change in the dura mater is a crucial indication for the choice of management. If the dura mater is intact, a transantral approach via a midfacial degloving incision may remove the tumour successfully.
Fluctuating hearing loss with otorrhoea in Hughes syndrome
Authors: Jovanovic-Bateman L , Warrington G
Published: February 2007

We present an unusual case of bilateral aural symptoms in a 34-weeks pregnant woman with Hughes syndrome. This report underscores the unpredictable nature of this syndrome and its ENT complications. No previous case of fluctuating hearing loss and otorrhoea in Hughes syndrome has been described.
Grisel syndrome: a delayed presentation in an asymptomatic patient
Authors: Doshi J , Anari S , Zammit-Maempel I , Paleri V
Published: February 2007

Grisel syndrome is a rare condition characterised by atlanto-axial subluxation following an inflammatory process in the head and neck region. It occurs more commonly in children and usually presents with cervical pain and torticollis, in addition to symptoms of the primary infection. We present the case of an asymptomatic 78-year-old man who was incidentally found to have atlanto-axial subluxation on a routine follow-up computed tomography scan, three months following successful treatment of a skull base infection. This case emphasises the importance of appropriate follow-up imaging for patients with skull base infections, even if they respond clinically to medical treatment.
Isolated hypoglossal nerve palsy: a harbinger of malignancy
Authors: Khoo S G , Ullah I , Wallis F , Fenton J E
Published: February 2007

The hypoglossal nerve is an underrated nerve usually consigned to a few words in anatomical text books, under the last four cranial nerves. However, paralysis of this nerve may be the first indication of a serious underlying disorder. Excluding previous surgery, radiotherapy and trauma, 50 per cent of cases of isolated hypoglossal nerve palsy are idiopathic. A further 20 per cent are malignant, 20 per cent are vascular and 10 per cent are due to miscellaneous causes. Presentation of an isolated hypoglossal nerve palsy is therefore an ominous sign. There is confusion over both cause and investigation, and management protocols for isolated hypoglossal nerve palsy are ill-defined. We present a case of isolated hypoglossal palsy which was due to a metastatic skull base deposit. This case illustrates the fact that magnetic resonance imaging is the investigation of choice in assessing the entire course of the hypoglossal nerve.
Small-amplitude electrostatic solitary waves in a dusty plasma with variable charge resonant trapped dust particles
Authors: GOUGAM LEILA AIT , TRIBECHE MOULOUD , MEKIDECHE FAWZIA
Published: February 2007

Small-amplitude electrostatic solitary waves are investigated in unmagnetized dusty plasmas with variable charge resonant trapped dust particles. It is found that under certain conditions spatially localized structures, the height and nature of which depend sensitively on the plasma parameters, can exist. The effects of dust grain temperature, equilibrium dust charge, trapping parameter, and dust size on the properties of these solitary waves are briefly discussed. A neural network with a given architecture and learning process, and which may be useful to interpret experimental data, is outlined. Our investigation may be taken as a prerequisite for the understanding of the solitary dust waves that may occur in space as well as in laboratory plasmas.
Variation as accessing ‘non-optimal’ candidates
Authors: Coetzee Andries W.
Published: February 2007

This paper argues that rather than just select the best candidate, EVAL imposes a harmonic rank-ordering on the full candidate set. Language users have access to this enriched information, and it shapes their performance. This paper applies this idea to variation. The claim is that language users can access the full candidate set via the rank-ordering imposed by EVAL. In variation, more than one candidate is well-formed enough to count as grammatical. Consequently, language users will access more than just the best candidate from the rank-ordering. However, the accessibility of a candidate depends on its position on the rank-ordering. The higher the position a candidate occupies, the more likely it is to be selected. In a variable process, variants that appear higher on the rank-ordering (i.e. are more well-formed) will therefore also be the more frequent variants. This model is applied to variation in the phonology of Faialense Portuguese and Ilokano.
A variational approach for an inverse dynamical problem for composite beams
Authors: MORASSI ANTONIO , NAKAMURA GEN , SHIROTA KENJI , SINI MOURAD
Published: February 2007

This paper deals with a problem of nondestructive testing for a composite system formed by the connection of a steel beam and a reinforced concrete beam. The small vibrations of the composite beam are described by a differential system where a coupling takes place between longitudinal and bending motions. The motion is governed in space by two second order and two fourth order differential operators, which are coupled in the lower order terms by the shearing,k, and axial, μ, stiffness coefficients of the connection. The coefficientskand μ define the mechanical model of the connection between the steel beam and the concrete beam and contain direct information on the integrity of the system. In this paper we study the inverse problem of determiningkand μ by mixed data. The inverse problem is transformed to a variational problem for a cost function which includes boundary measurements of Neumann data and also some interior measurements. By computing the Gateaux derivatives of the functional, an algorithm based on the projected gradient method is proposed for identifying the unknown coefficients. The results of some numerical simulations on real steel-concrete beams are presented and discussed.
Entwicklung eines Hochschul-Rankingverfahrens mittels Mixed-Rasch-Modell und Mehrebenenanalyse am Beispiel der Psychologie
Authors: Mutz Rüdiger , Daniel Hans-Dieter
Published: February 2007

Zusammenfassung. In diesem Beitrag soll für den Bereich von Lehre und Studium ein auf studentischen Befragungen basiertes Verfahren zur Beurteilung der Studienbedingungen im Fach Psychologie entwickelt und empirisch erprobt werden mit dem Ziel, eine Rangordnung der Hochschulen (Hochschul-Ranking) zu erstellen. Datengrundlage bildet eine deutschlandweite Befragung im Jahr 2000 an N = 1 490 Studierenden, die im Wintersemester 1999/2000 ihr Studium begonnen haben. In einem ersten Schritt wird mittels Mixed-Rasch-Modellen eine eindimensionale Skala zu den Studienbedingungen erstellt, die innerhalb von 2 Klassen unterschiedlichen Antwortverhaltens (Tendenz zur Mitte, Tendenz zu Extremwerten) raschkonform ist. In einem zweiten Schritt kann mittels Mehrebenenanalyse ein signifikanter Einfluss von Bias-Variablen (z.B. Lebensalter, Einwohnerzahl der Hochschulstadt) auf die Skala und das Ranking der Hochschulen nachgewiesen werden. Für einen fairen Hochschulvergleich sollten daher adjustierte Rankings auf der Basis empirischer Bayes-Schätzungen verwendet werden, die grafisch mit Goldstein-adjustierten Konfidenzintervallen dargestellt werden sollten, und in denen die Hochschulen zusätzlich zu Ranggruppen zusammengefasst sind.
FTC Review of Generic Drug Mergers — Four Recent Consents Provide Significant Guidance on Mode of Analysis and Divestiture Requirements
Authors: Silber Seth C.
Published: February 2007

This paper details the Federal Trade Commission's (FTC) analysis in four recent generic drug merger consents: Watson/Andrx, Barr/Pliva, Teva/Ivax, and Novartis/Eon. It discusses the FTC's basis for requiring divestitures in each of these mergers. The factual analysis is followed by some general guidance on the mode of analysis for FTC review of generic drug mergers. Based on this guidance, companies entering into such transactions can better anticipate under what circumstances the Commission will require divestitures.
Floating probe for electron temperature and ion density measurement applicable to processing plasmas
Authors: Lee Min-Hyong , Jang Sung-Ho , Chung Chin-Wook
Published: February 2007

A floating-type probe and its driving circuit using the nonlinear characteristics of the probe sheath was developed and the electron temperature and the plasma density which is found from the ion part of the probe characteristic (ion density) were measured in inductively coupled plasmas. The floating-type probe was compared with a single Langmuir probe and it turned out that the floating-type probe agrees closely with the single probe at various rf powers and pressures. The ion density and electron temperature by the floating-type probe were measured with a film on the probe tip coated in CF4 plasma. It is found that the ion density and electron temperature by the floating-type probe were almost the same regardless of the coating on the probe tip while a single Langmuir probe does not work. Because the floating-type probe is hardly affected by the deposition on the probe tip, it is expected to be applied to plasma diagnostics for plasma processing such as deposition or etching.
Dynamics of chaotic driving: Rotation in the restricted three-body problem
Authors: Vanyó József , Tél Tamás
Published: February 2007

We investigate the rotation of a small nonspherical body in the planar restricted three-body problem along periodic, quasi-periodic, and chaotic orbits of the small body’s center of mass. The rotation dynamics is chaotic in all three cases, but a systematic overview of it via stroboscopic mappings is possible only in the periodic case. We propose to explore the structured phase space patterns by following an ensemble of trajectories, a droplet, in the phase space. The temporal evolution of the pattern can be characterized by a time-dependent fractal dimension. It is shown to converge exponentially to a time-independent value for long times. In the presence of dissipation, the droplet typically converges to a so-called snapshot chaotic attractor whose shape might change chaotically in time, but whose asymptotic fractal dimension is constant.
On the classification of buoyancy-driven chemo-hydrodynamic instabilities of chemical fronts
Authors: D’Hernoncourt J. , Zebib A. , De Wit A.
Published: February 2007

Exothermic autocatalytic fronts traveling in the gravity field can be deformed by buoyancy-driven convection due to solutal and thermal contributions to changes in the density of the product versus the reactant solutions. We classify the possible instability mechanisms, such as Rayleigh-Bénard, Rayleigh-Taylor, and double-diffusive mechanisms known to operate in such conditions in a parameter space spanned by the corresponding solutal and thermal Rayleigh numbers. We also discuss a counterintuitive instability leading to buoyancy-driven deformation of statically stable fronts across which a solute-light and hot solution lies on top of a solute-heavy and colder one. The mechanism of this chemically driven instability lies in the coupling of a localized reaction zone and of differential diffusion of heat and mass. Dispersion curves of the various cases are analyzed. A discussion of the possible candidates of autocatalytic reactions and experimental conditions necessary to observe the various instability scenarios is presented.
Molecular dynamics for surfactant-assisted protein refolding
Authors: Lu Diannan , Liu Zheng , Wu Jianzhong
Published: February 2007

Surfactants are widely used to refold recombinant proteins that are produced as inclusion bodies in E. Coli. However, the microscopic details of the surfactant-assisted protein refolding processes are yet to be uncovered. In the present work, the authors aim to provide insights into the effect of hydrophobic interactions of a denatured protein with surfactant molecules on the refolding kinetics and equilibrium by using the Langevin dynamics for coarse-grained models. The authors have investigated the folding behavior of a β-barrel protein in the presence of surfactants of different hydrophobicities and concentrations. It is shown that the protein folding process follows a “collapse-rearrangement” mechanism, i.e., the denatured protein first falls into a collapsed state before acquiring the native conformation. In comparison with the protein folding without surfactants, the protein-surfactant hydrophobic interactions promote the collapse of a denatured protein and, consequently, the formation of a hydrophobic core. However, the surfactants must be released from the hydrophobic core during the rearrangement step, in which the native conformation is formed. The simulation results can be qualitatively reproduced by experiments.
Modeling photoreflectance of quantum well heterostructures: A comprehensive approach
Authors: Mayhew Laurel M. , Trivedi Deborah Lehr , Anderson Neal G.
Published: February 2007

We present a comprehensive approach to modeling the photoreflectance (PR) spectra of semiconductor quantum wells embedded in layered heterostructures. Near-gap PR spectra are obtained directly from the calculated variation of surface reflectance spectra induced by modulation of an internal electric field. The field-dependent reflectance spectra are themselves obtained from a transfer matrix model of a quantum well heterostructure (QWH) in which the quantum well layer is treated in detail using electric-field-dependent optical absorption calculations and all surrounding higher-gap layers are treated as lossless dielectric slabs. The model is described in detail and is applied to unstrained GaAs/AlGaAs and compressively strained InGaAs/GaAs single-well QWHs for which both experimental data and other calculations are available for comparison. This model can serve as a tool for interpretation of experimental PR spectra, and should be particularly useful for analysis of dense spectra with overlapping features that would be difficult to analyze using empirical fitting schemes. The approach can be used to model electroreflectance without modification.
A unified approach to attractor reconstruction
Authors: Pecora Louis M. , Moniz Linda , Nichols Jonathan , Carroll Thomas L.
Published: February 2007

In the analysis of complex, nonlinear time series, scientists in a variety of disciplines have relied on a time delayed embedding of their data, i.e., attractor reconstruction. The process has focused primarily on intuitive, heuristic, and empirical arguments for selection of the key embedding parameters, delay and embedding dimension. This approach has left several longstanding, but common problems unresolved in which the standard approaches produce inferior results or give no guidance at all. We view the current reconstruction process as unnecessarily broken into separate problems. We propose an alternative approach that views the problem of choosing all embedding parameters as being one and the same problem addressable using a single statistical test formulated directly from the reconstruction theorems. This allows for varying time delays appropriate to the data and simultaneously helps decide on embedding dimension. A second new statistic, undersampling, acts as a check against overly long time delays and overly large embedding dimension. Our approach is more flexible than those currently used, but is more directly connected with the mathematical requirements of embedding. In addition, the statistics developed guide the user by allowing optimization and warning when embedding parameters are chosen beyond what the data can support. We demonstrate our approach on uni- and multivariate data, data possessing multiple time scales, and chaotic data. This unified approach resolves all the main issues in attractor reconstruction.
Wetting-layer formation mechanisms of surface-directed phase separation under different quench depths with off-critical compositions in polymer binary mixture
Authors: Yan Li-Tang , Xie Xu-Ming
Published: February 2007

Focusing on the off-critical condition, the quench depth dependence of surface-directed phase separation in the polymer binary mixture is numerically investigated by combination of the Cahn-Hilliard-Cook theory and the Flory–Huggins–de Gennes theory. Two distinct situations, i.e., for the wetting, the minority component is preferred by the surface and the majority component is preferred by the surface, are discussed in detail. The simulated results show that the formation mechanism of the wetting layer is affected by both the quench depth and the off-critical extent. Moreover, a diagram, illustrating the formation mechanisms of the wetting layer with various quench depths and compositions, is obtained on the basis of the simulated results. It is found that, when the minority component is preferred by the surface, the growth of the wetting layer can exhibit pure diffusion limited growth law, logarithmic growth law, and Lifshitz-Slyozov growth law. However, when the majority component is preferred by the surface, the wetting layer always grows logarithmically, regardless of the quench depth and the off-critical extent. It is interesting that the surface-induced nucleation can be observed in this case. The simulated results demonstrate that the surface-induced nucleation only occurs below a certain value of the quench depth, and a detailed range about it is calculated and indicated. Furthermore, the formation mechanisms of the wetting layer are theoretically analyzed in depth by the chemical potential gradient.
A density functional model for the binary crystal of hard spheres with vacancies
Authors: Singh Sunil P. , Das Shankar P.
Published: February 2007

We study the stability of a binary mixture of hard spheres in the crystalline state in which a small fraction of lattice sites in the solid structure are vacant. The optimum vacancy concentration is obtained by minimizing the free energy of the inhomogeneous solid state. We use the modified weighted density functional approximation to compute the free energy. The necessary input for the theory is the thermodynamic properties of the homogeneous state of the mixture and is obtained from the solutions of the corresponding Percus–Yevick integral equations for the binary system. We compute the free energy for the crystal having two kinds of ordered structures in which (i) both the species lie in a disordered manner on a single face-centered-cubic lattice and (ii) each of the two species lie on a separate cubic lattice. Our theoretical model obtains equilibrium vacancy fraction of O(10−5) near the freezing point in both cases. The vacancy concentration decreases with the increase of density in both cases.
Phase separations of single-crystal nanowires grown by self-catalytic chemical vapor deposition method
Authors: He Maoqi , Motayed Abhishek , Noor Mohammad S.
Published: February 2007

The fundamentals of phase separations of single-crystal III-V nitride nanowires grown by self-catalytic chemical vapor deposition method have been studied. Experimental tools, such as high resolution transmission electron microscopy and scanning electron microscopy, have been used to characterize the nanowires. The study indicates that nanowires with diameters exceeding about 100nm undergo phase transitions and/or crystal structure deterioration. The study highlights a relationship between the crystal structure and the kinetics of growth of nanowires.
Characterizing the intrinsic stability of gas-phase clusters of transition metal complex dianions with alkali metal counterions: Counterion perturbation of multiply charged anions
Authors: Burke Ruth M. , Boxford William E. , Dessent Caroline E. H.
Published: February 2007

The authors report the gas-phase generation and characterization of a series of cation-dianion clusters, e.g., M+∙PtCl62−, M+∙PtCl42−, M+∙Pt(CN)62−, and M+∙Pd(CN)42−, where M+=Na+,K+,Rb+, as model systems for investigating gas-phase contact ionpairs. Low-energy collisional excitation of these systems isolated within a quadrupole ion trap reveals that the fragmentation products are determined by the dianion and are independent of the counterion. This indicates that cation-dianion clusters represent gaseous ion-pair complexes, in line with recent findings for K+∙Pt(CN)n2−, n=4,6 [Burke et al., J. Chem. Phys. 125, 021105 (2006)]. The relative fragmentation energies of several cation-dianion systems are obtained as a function of the counterion to explore the nature of ion-pair binding. For most of the systems studied, e.g., M+∙PtCl62−, the fragmentation energy increases as the cation size decreases, in line with a simple electrostatic description of the cation-dianion binding. However, the M+∙Pt(CN)42− clusters displayed the reverse trend with the fragmentation energy increasing as the cation size increases. Density functional theory calculations of the cation-dianion fragmentation potential energy surfaces reveal the existence of a novel double-minima surface, separated by a repulsive Coulomb barrierlike feature at short range. The experimentally observed trends in the fragmentation energies can be fully understood with reference to the computed surfaces, hence providing strong support for the existence of the double-minima surface.
Competition between tensile and compressive stress creation during constrained thin film island coalescence
Authors: Bhandari Abhinav , Sheldon Brian W. , Hearne Sean J.
Published: February 2007

Various analytical models have been proposed to predict the tensile stress created when discrete islands contact during a Volmer-Weber thin film growth. Past efforts to experimentally validate these models have been hindered by the stochastic nucleation of islands, which results in coalescence over a large distribution of times and length scales. To avoid this we systematically varied island geometries using electrodeposition of Ni islands on lithographically patterned conductive substrates (Au film on Si), which allowed for independent control of island size and growth rate. Using this technique, we previously demonstrated that most of the coalescence stress occurred after the initial contact of the neighboring islands, reaching a steady state when the film surface became nearly planar. In this work, we expand on these initial results to examine the kinetics of the coalescence process and to systematically evaluate the stress transition from discrete islands to a planar film. The steady state stress in planar films increased with growth rate, but asymptotically approached a limiting value for higher growth rates that depended on the island size. We attribute this to the competition between the kinetically limited compressive stress generation and tensile coalescence stress processes. The interaction of these mechanisms is consistent with both the observed transient stress evolution during the initial stages of island coalescence and the steady state stress evolution later in the process. The instantaneous stress at both the initial contact and at longer times decreased with increasing island size, as predicted in the literature. However, the existing models predict significantly larger grain size effects than those observed in these experiments.
Dispersion of confined optical phonons in semiconductor nanowires in the framework of a continuum approach
Authors: Comas F. , Camps I. , Marques G. E. , Studart Nelson
Published: February 2007

Confined optical phonons are discussed for a semiconductor nanowire of the Ge (Si) prototype on the basis of a theory developed some years ago. In the present work this theory is adapted to a nonpolar material and generalized to the case when the phonon dispersion law involves both linear and quadratic terms in the wave vector. The treatment is considered along the lines of a continuous medium model and leads to a system of coupled differential equations describing oscillations of mixed nature. The nanowire is modeled in the form of an infinite circular cylinder and the solutions of the fundamental equations are found. We are thus led to a description of long-wavelength optical phonons, which should show a closer agreement with experimental data and with calculations along atomistic models. The presented theory is applied to the calculation of optical phonons in a Ge nanowire. We have found the dispersion curves for various optical phonon modes. We also normalize the modes and discuss the electron-phonon interaction within the deformation potential approximation.
Broadband SiGe∕Si quantum dot infrared photodetectors
Authors: Lin C.-H. , Yu C.-Y. , Peng C.-Y. , Ho W. S. , Liu C. W.
Published: February 2007

The broadband absorption of metal-oxide-semiconductor SiGe∕Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption.
Effect of channel positioning on the 1∕f noise in silicon-on-insulator metal-oxide-semiconductor field-effect transistors
Authors: von Haartman Martin , Östling Mikael
Published: February 2007

p -channel metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on silicon-on-insulator (SOI) substrates with an ultrathin (∼20nm) lightly p-doped Si body were found to show about an order of magnitude lower 1∕f noise than that in conventional bulk Si pMOSFETs when biased in strong inversion. In order to investigate the origin of the 1∕f noise and find an explanation for the 1∕f noise reduction, the 1∕f noise in the SOI devices was studied as a function of the back gate voltage. The 1∕f noise was found to increase with increasing back gate voltage, which acts to push the carriers closer towards the front gate oxide interface. The average distance of the inversion carriers from the gate oxide interface was obtained from simulations and used to interpret the 1∕f noise behavior. The Hooge parameter, extracted for several different 1∕f noise experiments where one or two terminal voltages were varied, exhibited a general behavior similar for both the SOI and bulk Si pMOSFETs. The Hooge parameter was shown to increase markedly when the average carrier-oxide separation is around 2nm. Possible explanations of the results were discussed in terms of the mobility fluctuation noise model.
Crossover from diffusive to quasi-ballistic transport
Authors: Csontos Dan , Ulloa Sergio E.
Published: February 2007

We present a detailed study of quasi-ballistic transport in submicron semiconductor channels. The electron distribution in such channels differs significantly from a near-equilibrium, shifted Maxwell-Boltzmann distribution function, and displays a large broadening, as well as pronounced features, peaks and shoulders, in the high-velocity tail of the distribution associated with the nonequilibrium and quasi-ballistic nature of the electron transport. For submicron channel lengths, analyses of the spatial dependence of the velocity distribution function show that scattering in the entire channel is important and that the scattering efficiency has a strong spatial dependence. In this article, we particularly study the crossover between the diffusive and quasi-ballistic regimes of transport and find characteristic signatures in (i) the electrostatics, which manifests as a redistribution of the voltage drop from the channel to the contact regions; (ii) the electron density, where exponential and linear spatial dependences of the source-injected and channel- and drain-backscattered electron densities are signatures of diffusive and quasi-ballistic transport, respectively, and (iii) the electron distribution function, where the source-injected ballistic peak observed in the quasi-ballistic regime disappears at the onset of diffusive transport.
A chromophoric switch based on pseudorotaxanes
Authors: Liu Yu , Li Chun-Ju , Zhang Heng-Yi , Wang Li-Hua , Luo Qian , Wang Ge
Published: February 2007

In a three-component system containing dibenzo-24-crown-8 (1), diamino-dibenzo-24-crown-8 (2), and 1,2-bis(4,4′-dipyridyl)ethane (32+), axle 32+ can shuttle between wheels 1 and 2 by acid-base control, accompanying color changes from yellow to red. This system could not only be taken as a chromophoric supramolecular switch, but also exhibit a function as INHIBIT logic gate.
Molecular vibrations of [n]oligoacenes (n=2−5 and 10) and phonon dispersion relations of polyacene
Authors: Yamakita Yoshihiro , Kimura Jin , Ohno Koichi
Published: February 2007

As model compounds for nanosize carbon clusters, the phonon dispersion curves of polyacene are constructed based on density functional theory calculations for [n]oligoacenes (n=2−5, 10, and 15). Complete vibrational assignments are given for the observed Fourier-transform infrared and Raman spectra of [n]oligoacenes (n=2−5). Raman intensity distributions by the 1064-nm excitation are well reproduced by the polarizability-approximation calculations for naphthalene and anthracene, whereas several bands of naphthacene and pentacene at 1700−1100 cm−1 are calculated to be enhanced by the resonance Raman effect. It is found from vibronic calculations that the coupled ag modes between the Kekulé deformation and joint CC stretching give rise to the Raman enhancements of the Franck–Condon type, and that the b3g mode corresponding to the graphite G mode is enhanced by vibronic coupling between the L1a(B11u) and B1b(B12u) states. The phonon dispersion curves of polyacene provide a uniform foundation for understanding molecular vibrations of the oligoacenes in terms of the phase difference. The mode correlated with the defect-sensitive D mode of the bulk carbon networks is also found for the present one-dimensional system.
Analysis of semiconductor laser dynamics under gigabit rate modulation
Authors: Ahmed Moustafa , Yamada Minoru , Mahmoud Safwat W. Z.
Published: February 2007

A theoretical study of the dynamics of semiconductor lasers subjected to pseudorandom digital modulation at gigabit rates is presented. The eye diagram, turn-on jitter (TOJ), and power fluctuations in the modulated laser wave form are analyzed. The study is based on numerical large-signal analysis of the laser rate equations. Influences of the biasing and modulation currents on the eye diagram and TOJ are examined. The degree of eye opening is measured in terms of a Q factor of the laser signal analogous to the Q factor determining the bit-error rate in transmission systems. Influence of optimizing both the sampling and decision times on the signal Q factor is modeled. We show that the most eye opening corresponds to shortening the sampling time associated with lengthening the decision time. We also assess the relative contributions of the laser intrinsic noise and pseudorandom bit pattern to the TOJ. The results show that the bit pattern is the major contributor to the TOJ when the setting time of the relaxation oscillation is longer than the bit slot.
Tip artifact in atomic force microscopy observations of InAs quantum dots grown in Stranski–Krastanow mode
Authors: Shiramine Ken-ichi , Muto Shunichi , Shibayama Tamaki , Sakaguchi Norihito , Ichinose Hideki , Kozaki Tamotsu , Sato Seichi , Nakata Yoshiaki , Yokoyama Naoki , Taniwaki Masafumi
Published: February 2007

The tip artifact in atomic force microscopy (AFM) observations of InAs islands was evaluated quantitatively. The islands were grown in the Stranski–Krastanow mode of molecular beam epitaxy. The width and height of the islands were determined using transmission electron microscopy (TEM) and AFM. The average [1¯10] in-plane width and height determined using TEM excluding native oxide were 22 and 7nm, respectively; those determined using AFM including the oxide were 35 and 8nm, respectively. The difference in width was due to the oxide and the tip artifact. The sizes including the oxide were deduced from TEM observations to be a width of 27nm and a height of 6nm with correction for the thickness of the oxide. The residual difference of 8nm between the width determined using AFM and that determined using TEM including the oxide was ascribed to the tip artifact. The results enable us to determine the actual size of the islands from their AFM images.
Field emission from nanoforest carbon nanotubes grown on cobalt-containing amorphous carbon composite films
Authors: Zhang Y. B. , Lau S. P. , Li H. F.
Published: February 2007

Nanoforest carbon nanotubes (CNTs) were synthesized by biased thermal chemical vapor deposition on cobalt-containing amorphous carbon composite films. The composite films were deposited on silicon by filtered cathodic vacuum arc using a Co-containing graphite target and then CNTs were grown at 580°C using the composite film as a catalyst layer. The nanoforest multiwalled CNTs have thin diameters between 10 and 20nm. Their tips point upwards while the rest is densely tangled. They exhibit good field emission properties with a low threshold field of 1.6V∕μm and a high current density of 2.1mA∕cm2 at 3V∕μm.
Multilayer route to iron nanoparticle formation in an insulating matrix
Authors: Wang Feng , Malac Marek , Egerton Ray F. , Meldrum Alkiviathes , Zhu Xiaobin , Liu Zhigang , Macdonald Nicole , Li Peng , Freeman Mark R.
Published: February 2007

Well-protected, isolated bcc-iron nanoparticles embedded in silicon dioxide were prepared by e-beam evaporation and postannealing of multilayers in an ultrahigh vacuum system. The spherical shape and isolation of the particles were confirmed by plan-view and cross-sectional transmission electron microscopy. Oxidation was evaluated from the electron energy-loss near edge structure. In this technique, a postedge peak of 40eV above the iron L3 threshold, originating from backscattering of oxygen atoms, provides a clear indication of iron oxide. The white-line ratio (WLR), measuring the 3d-orbital occupancy, is used to estimate the oxidation-layer thickness. In the samples of large iron nanoparticles (with average diameter larger than 10nm), a very thin surface layer appears to be the oxide maghemite, approximately one atomic layer according to the WLR evaluations. The evolution of the coercivity with particle size, as measured by the magneto-optical Kerr effect, shows that the reversal process is dominated by the surface anisotropy and is also affected by the dipole interaction, particularly in samples with large volume-filling factor.
Real time cantilever signal frequency determination using digital signal processing
Authors: Obukhov Yu. , Fong K. C. , Daughton D. , Hammel P. C.
Published: February 2007

We describe a digital signal processing method for high precision frequency evaluation of approximately sinusoidal signals based on a computationally efficient method. We demonstrate frequency measurement enabling sensitive measurement of the oscillatory force exerted on a micromechanical cantilever. We apply this technique to detection of the force signal arising in a micromechanically detected magnetic resonance force microscopy electron spin resonance signal. Our frequency detection measurements agree well with the theoretical noise analysis presented here, and we find that due to the excellent sensitivity of optical displacement detection, our sensitivity is limited only by the thermal displacement noise of the cantilever.
Strong perpendicular magnetic anisotropy of Fe–Pd nanocrystalline particles enhanced by Co addition
Authors: Kovács András , Sato Kazuhisa , Hirotsu Yoshihiko
Published: February 2007

L 1 0 - Pd Co Fe nanoparticles were prepared by vapor deposition of the components and characterized by transmission electron microscopy and by superconducting quantum interference device magnetometry. Co addition to the Fe–Pd nanocrystals resulted in a strong perpendicular magnetic anisotropy and a reduction in the formation temperature of the ordered L10 phase. Formation of the ordered phase was studied by in situ annealing of specimens inside the transmission electron microscope. L10 phase formation started at 698K, as evidenced by the appearance of 110 superlattice reflections in electron diffraction patterns. Co additions enhanced the magnetic easy axis alignment perpendicular to the film plane. The perpendicular maximum coercivity at room temperature was 1.75kOe for L10-PdCoFe nanoparticles with an average size of 8nm, and the magnetic coercivity decreased by increasing the Co concentration in the L10 nanoparticles.
Inhibition of nonlinear acoustic cavitation dynamics in liquid CO2
Authors: van Iersel Maikel M. , Cornel Jeroen , Benes Nieck E. , Keurentjes Jos T. F.
Published: February 2007

The authors present a model to study ultrasound-induced cavitation dynamics in liquid carbon dioxide (CO2), which includes descriptions for momentum, mass, and energy transport. To assist in the interpretation of these results, numerical simulations are presented for an argon cavity in water. For aqueous systems, inertia effects and force accumulation lead to a nonlinear radial motion, resulting in an almost adiabatic compression of the cavity interior. The simulations for liquid CO2 suggest that transport limitations impede nonlinear cavitation dynamics and the corresponding temperature rise. Consequently, in liquid CO2 the ultrasound-induced formation of radicals appears improbable.
Free energy of liquid water from a computer simulation via cell theory
Authors: Henchman Richard H.
Published: February 2007

A method to calculate the free energy of water from computer simulation is presented. Based on cell theory, it approximates the potential energy surface sampled in the simulation by an anisotropic six-dimensional harmonic potential to model the three hindered translations and three hindered rotations of a single rigid water molecule. The potential is parametrized from the magnitude of the forces and torques measured in the simulation. The entropy of these six harmonic oscillators is calculated and summed with a conformational term to give the total entropy. Combining this with the simulation enthalpy yields the free energy. The six water models examined are TIP3P, SPC, TIP4P, SPC/E, TIP5P, and TIP4P-Ew. The results reproduce experiment well: free energies for all models are within 1.6kJmol−1 and entropies are within 3.6JK−1mol−1. Approximately two-thirds of the entropy comes from translation, a third from rotation, and 5% from conformation. Vibrational frequencies match those in the experimental infrared spectrum and assist in their assignment. Intermolecular quantum effects are found to be small, with free energies for the classical oscillator lying 0.5–0.7kJmol−1 higher than in the quantum case. Molecular displacements and vibrational and zero point energies are also calculated. Altogether, these results validate the harmonic oscillator as a quantitative model for the liquid state.
Errors in the application of the JWKB method to calculating the survival factor in dissociative electron attachment using the local complex potential
Authors: Gallup G. A.
Published: February 2007

The accuracy of the JWKB method for determining the survival factor defined for dissociative electron attachment (DEA) processes is examined for a range of electronic resonance lifetimes within the local complex potential approximation. The author concludes that the accuracy is inadequate for molecules with properties commonly found for shape resonance induced DEA. More accurate methods using the uniform Airy function approximation give much better results, but the direct numerical integration of Schrödinger’s equation appears simpler still.
Second harmonic ion cyclotron heating of sloshing ions in a straight field line mirror
Authors: Moiseenko V. E. , Ågren O.
Published: February 2007

A qualitative analysis of second harmonic heating is carried out, in which a fast magnetosonic wave is launched from a location near the magnetic mirror (where the magnetic field is stronger than the second harmonic resonance field) and directed to the midplane of the open trap. The analysis shows that there is no “magnetic beach” heating in contrast to the case with minority heating on the fundamental harmonic. Conversion to the ion Bernstein wave would distort the heating pattern, and the condition for this conversion is estimated. The scenario of second harmonic heavy ion heating is examined numerically. In the scenario chosen, the regime of global resonance overlapping is achieved that provides good heating performance. The computations show that the power deposition is core, the amount of deposited power does not depend sensitively on the parameters of the discharge, and the range of plasma beta at which the heating is efficient is not narrow. The estimated antenna Q is noticeably low and, therefore, the antenna performance is high.
Simulation of inversion motion and N–H stretching overtone spectra of aniline
Authors: Hänninen Vesa , Halonen Lauri
Published: February 2007

A curvilinear internal coordinate Hamiltonian is used to simulate the N–H stretching overtone spectra and the associated inversion splittings in aniline. A simple local mode type model is applied to the N–H stretching and H–N–H bending modes. Geometric algebra is employed to derive the kinetic energy operator for the large amplitude inversion motion. Electronic structure calculations at the Møller-Plesset second order perturbation theory and correlation consistent aug-cc-pVTZ basis set level are used to obtain model parameters, some of which have been optimized with the least-squares method using experimental vibrational term values as data. The observed N–H stretching overtone vibrational levels and the inversional tunneling splittings are well reproduced with our approach.
Evolution of the electronic properties of Snn− clusters (n=4–45) and the semiconductor-to-metal transition
Authors: Cui Li-Feng , Wang Lei-Ming , Wang Lai-Sheng
Published: February 2007

The electronic structure of Snn− clusters (n=4–45) was examined using photoelectron spectroscopy at photon energies of 6.424eV (193nm) and 4.661eV (266nm) to probe the semiconductor-to-metal transition. Well resolved photoelectron spectra were obtained for small Snn− clusters (n⩽25), whereas more congested spectra were observed with increasing cluster size. A distinct energy gap was observed in the photoelectron spectra of Snn− clusters with n⩽41, suggesting the semiconductor nature of small neutral tin clusters. For Snn− clusters with n⩾42, the photoelectron spectra became continuous and no well-defined energy gap was observed, indicating the onset of metallic behavior for the large Snn clusters. The photoelectron spectra thus revealed a distinct semiconductor-to-metal transition for Snn clusters at n=42. The spectra of small Snn− clusters (n⩽13) were also compared with those of the corresponding Sin− and Gen− clusters, and similarities were found between the spectra of Snn− and those of Gen− in this size range, except for Sn12−, which led to the discovery of stannaspherene (the icosahedral Sn122−) previously [L. F. Cui et al., J. Am. Chem. Soc. 128, 8391 (2006)].
Photodissociation dynamics of nitrobenzene and o-nitrotoluene
Authors: Lin Ming-Fu , Lee Yuan T. , Ni Chi-Kung , Xu Shucheng , Lin M. C.
Published: February 2007

Photodissociation of nitrobenzene at 193, 248, and 266nm and o-nitrotoluene at 193 and 248nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO2 elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO2 elimination channels were determined to be NO∕NO2=0.32±0.12(193nm), 0.26±0.12(248nm), and 0.4±0.12(266nm) for nitrobenzene and 0.42±0.12(193nm) and 0.3±0.12(248nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.
Probing amyloid fibril formation of the NFGAIL peptide by computer simulations
Authors: Melquiond Adrien , Gelly Jean-Christophe , Mousseau Normand , Derreumaux Philippe
Published: February 2007

Amyloid fibril formation, as observed in Alzheimer’s disease and type II diabetes, is currently described by a nucleation-condensation mechanism, but the details of the process preceding the formation of the nucleus are still lacking. In this study, using an activation-relaxation technique coupled to a generic energy model, we explore the aggregation pathways of 12 chains of the hexapeptide NFGAIL. The simulations show, starting from a preformed parallel dimer and ten disordered chains, that the peptides form essentially amorphous oligomers or more rarely ordered β-sheet structures where the peptides adopt a parallel orientation within the sheets. Comparison between the simulations indicates that a dimer is not a sufficient seed for avoiding amorphous aggregates and that there is a critical threshold in the number of connections between the chains above which exploration of amorphous aggregates is preferred.
Improvement of GaInNAs p-i-n photodetector responsivity by antimony incorporation
Authors: Loke W. K. , Yoon S. F. , Tan K. H. , Wicaksono S. , Fan W. J.
Published: February 2007

Deep-level transient spectra (DLTS) and photoresponsivity were measured for Ga0.90In0.10N0.033As0.967∕GaAs and Ga0.96In0.04N0.028As0.967Sb0.005∕GaAs p-i-n photodetector structures. The GaInNAs and GaInNAsSb layers were grown closely lattice matched to GaAs substrate at 460°C using molecular beam epitaxy. Two hole-trap levels were observed in the DLTS spectra of the GaInNAs sample with activation energies of 0.152 and 0.400eV (labeled as H-1 and H-2 peak, respectively). The lower activation energy is believed to be associated with nitrogen-related defects and the higher activation energy is associated with arsenic antisite defects (AsGa). Following the incorporation of Sb into GaInNAs, the H-1 peak vanished from the DLTS spectra of the GaInNAsSb sample, and the AsGa defect-related DLTS signal was significantly reduced. Analysis of the DLTS data also showed that the trap concentration related to AsGa was reduced from 2.15×1015to2.58×1014cm−3. The DLTS results are in good agreement with the photoresponsivity results, in which the GaInNAsSb sample showed 10× higher photoresponse compared to the GaInNAs sample. This indicates the incorporation of Sb into GaInNAs has effectively improved the p-i-n photodetector device performance.