bluuebunny/crossref_metadata_2025 huggingface dataset:
Subband gap impact ionization and excitation in carbon nanotube transistors
Authors: Guo Jing , Alam Muhammad A. , Ouyang Yijian
Published: March 2007

Impact excitation (IE) and impact ionization (II) play important roles in carbon nanotube (CNT) optoelectronics and device reliability. The Boltzmann transport equation (BTE) in both the real and k spaces is solved to study subband gap II and IE in a CNT metal-oxide-semiconductor field-effect transistor (MOSFET). We show that even when the band bending is smaller than the CNT band gap, considerable II or IE can occur. The subband gap II rate varies exponentially with the applied drain voltage, but the current varies linearly due to a small amount of excess carriers. In contrast, solving the BTE by assuming a constant electric field indicates that both the II rate and current varies exponentially. Subband gap II or IE explains why considerable light emission was observed even when the potential drop is smaller in a recent experiment on bright CNT light emitters. The observed exponential variation of light intensity versus the linear variation of current alone, however, is not sufficient to distinguish between subband gap II and subband gap IE. The results also indicate that the bias condition for the maximum hot carrier degradation in CNT MOSFETs is the same as in Si MOSFETs.
Particle manipulation with nonadiabatic ponderomotive forces
Authors: Dodin I. Y. , Fisch N. J.
Published: March 2007

Average, or ponderomotive potentials effectively seen by particles in oscillating fields allow advanced techniques of particle manipulation inaccessible with static potentials. In strongly inhomogeneous fields the ponderomotive force is phase dependent, and the particle dynamics resembles that of a quantum object in a conservative barrier. Probabilistic transmission through a ponderomotive potential is then possible and can be used for particle beam slicing. Resonant fields can also cool and trap particles exhibiting natural oscillations (e.g., Larmor rotation), as well as transmit them asymmetrically; hence, acting as one-way walls. An approximate integral of particle motion is found for this case and a new ponderomotive potential is introduced accordingly.
Coupling of global toroidal Alfvén eigenmodes and reversed shear Alfvén eigenmodes in DIII-D
Authors: Van Zeeland M. A. , Austin M. E. , Gorelenkov N. N. , Heidbrink W. W. , Kramer G. J. , Makowski M. A. , McKee G. R. , Nazikian R. , Ruskov E. , Turnbull A. D.
Published: March 2007

Reversed shear Alfvén eigenmodes (RSAEs) are typically thought of as being localized near the minima in the magnetic safety factor profile, however, their spatial coupling to global toroidal Alfvén eigenmodes (TAEs) has been observed in DIII-D discharges. For a decreasing minimum magnetic safety factor, the RSAE frequency chirps up through that of stable and unstable TAEs. Coupling creates a small gap at the frequency degeneracy point forming two distinct global modes. The core-localized RSAE mode structure changes and becomes temporarily global. Similarly, near the mode frequency crossing point, the global TAE extends deeper into the plasma core. The frequency splitting and spatial structure of the two modes throughout the various coupling stages, as measured by an array of internal fluctuation diagnostics, are in close agreement with linear ideal MHD calculations using the NOVA code. The implications of this coupling for eigenmode stability is also investigated and marked changes are noted throughout the coupling process.
Quantitative experiments with electrons in a positively charged beam
Authors: Molvik A. W. , Kireeff Covo M. , Cohen R. , Friedman A. , Lund S. M. , Sharp W. , Vay J-L. , Baca D. , Bieniosek F. , Leister C. , Seidl P.
Published: March 2007

Intense ion beams are difficult to maintain as non-neutral plasmas. Experiments and simulations are used to study the complex interactions between beam ions and (unwanted) electrons. Such “electron clouds” limit the performance of many accelerators. To characterize electron clouds, a number of parameters are measured, including total and local electron production and loss for each of three major sources, beam potential versus time, electron line-charge density, and gas pressure within the beam. Electron control methods include surface treatments to reduce electron and gas emission, and techniques to remove electrons from the beam or block their capture by the beam. Detailed self-consistent simulations include beam-transport fields and electron and gas generation and transport; these compute unexpectedly rich behavior, much of which is confirmed experimentally. For example, in a quadrupole magnetic field, ion and dense electron plasmas interact to produce multi-kV oscillations in the electron plasma and distortions of the beam velocity space distribution, without the system becoming homogeneous or locally neutral.
Basic studies of the generation and collective motion of pair-ion plasmas
Authors: Oohara W. , Hatakeyama R.
Published: March 2007

A fullerene pair-ion plasma without electrons is generated and electrostatic modes propagating along magnetic-field lines are externally excited in the range of low frequencies. It is found that four kinds of wave modes, including theoretically unexpected ones, exist in the plasma, and the phase lag between the density fluctuations of positive and negative ions strongly depends on the frequency. In order to illuminate further collective motion of pair-ion plasmas in the range of high frequencies, a concept of a hydrogen pair-ion plasma consisting of only H+ and H− is proposed and an experimental configuration is presented. On the basis of the production of a hydrogen plasma by Penning ionization gauge discharge, the principles of ion cyclotron resonance and E×B drift motion are shown to be effective for ion-species analysis/selection and separated electron detection from negative ions in the generation of pure hydrogen pair-ion plasmas.
Dynamics and control of the expansion of finite-size plasmas produced in ultraintense laser-matter interactions
Authors: Peano F. , Martins J. L. , Fonseca R. A. , Silva L. O. , Coppa G. , Peinetti F. , Mulas R.
Published: March 2007

The strong influence of the electron dynamics provides the possibility of controlling the expansion of laser-produced plasmas by appropriately shaping the laser pulse. A simple irradiation scheme is proposed to tailor the explosion of large deuterium clusters, inducing the formation of shock structures, capable of driving nuclear fusion reactions. Such a scenario has been thoroughly investigated, resorting to two- and three-dimensional particle-in-cell simulations. Furthermore, the intricate dynamics of ions and electrons during the collisionless expansion of spherical nanoplasmas has been analyzed in detail using a self-consistent ergodic-kinetic model. This study clarifies the transition from hydrodynamic-like to Coulomb-explosion regimes.
BARCOS, an automation and remote control system for atmospheric observations with a Bruker interferometer
Authors: Neefs E. , De Mazière M. , Scolas F. , Hermans C. , Hawat T.
Published: March 2007

In order to make long-term monitoring of the atmospheric composition using commercial Bruker Fourier transform spectrometers more cost effective, a system called BARCOS has been developed. The system enables one to perform the operation of the spectrometric atmospheric observations in a remotely controlled or autonomous way, without human presence at the measuring site. Several observation geometries are foreseen, including solar and lunar absorption spectrometry. BARCOS is built using existing commercial hardware and software components, including the Bruker software for the operation of the spectrometer (OPUS) and runs in a personal computer (Microsoft) environment. It includes a small meteorological station. It is a flexible system, allowing manual interventions at any time. To run BARCOS effectively, the only prerequisite is that internet access is available at the site of operation. This article describes the BARCOS system hardware and software configurations.
Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields
Authors: Owocki Stan , Townsend Rich , ud-Doula Asif
Published: March 2007

Hot, luminous stars (spectral types O and B) lack the hydrogen recombination convection zones that drive magnetic dynamo generation in the sun and other cool stars. Nonetheless, observed rotational modulation of spectral lines formed in the strong, radiatively driven winds of hot stars suggests magnetic perturbations analogous to those that induce “co-rotating interaction regions” in the solar wind. Indeed, recent advances in spectropolarimetric techniques have now led to direct detection of moderate to strong (100–10 000G), tilted dipole magnetic fields in several hot stars. Using a combination of analytic and numerical magnetohydrodynamic models, this paper focuses on the role of such magnetic fields in channeling, and sometimes confining, the radiatively driven mass outflows from such stars. The results show how “magnetically confined wind shocks” can explain the moderately hard x-ray emission seen from the O7V star Theta-1 Ori C, and how the trapping of material in a “rigidly rotating magnetosphere” can explain the periodically modulated Balmer line emission seen from the magnetic B2pV star Sigma Ori E. In addition, magnetic reconnection heating from episodic centrifugal breakout events might explain the occasional very hard x-ray flares seen from the latter star. The paper concludes with a brief discussion on the generation of hot-star fields and the broader relationship to other types of magnetospheres.
Novel design methods for magnetic flux loops in the National Compact Stellarator Experiment
Authors: Pomphrey N. , Lazarus E. , Zarnstorff M. , Boozer A. , Brooks A.
Published: March 2007

Magnetic pickup loops on the vacuum vessel (VV) can provide an abundance of equilibrium information for stellarators. A substantial effort has gone into designing flux loops for the National Compact Stellarator Experiment (NCSX) [Zarnstorff et al., Plasma Phys. Controlled Fusion 43, A237 (2001)], a three-field period quasi-axisymmetric stellarator under construction at the Princeton Plasma Physics Laboratory. The design philosophy, to measure all of the magnetic field distributions normal to the VV that can be measured, has necessitated the development of singular value decomposition algorithms for identifying efficient loop locations. Fields are expected to be predominantly stellarator symmetric (SS)—the symmetry of the machine design—with toroidal mode numbers per torus (n) equal to a multiple of 3 and possessing reflection symmetry in a period. However, plasma instabilities and coil imperfections will generate non-SS fields that must also be diagnosed. The measured symmetric fields will yield important information on the plasma current and pressure profile as well as on the plasma shape. All fields that obey the design symmetries could be measured by placing flux loops in a single half-period of the VV, but accurate resolution of nonsymmetric modes, quantified by the condition number of a matrix, requires repositioning loops to equivalent locations on the full torus. A subarray of loops located along the inside wall of the vertically elongated cross section was designed to detect n=3, m=5 or 6 resonant field perturbations that can cause important islands. Additional subarrays included are continuous in the toroidal and poloidal directions. Loops are also placed at symmetry points of the VV to obtain maximal sensitivity to asymmetric perturbations. Combining results from various calculations which have made extensive use of a database of 2500 free-boundary VMEC equilibria, has led to the choice of 225 flux loops for NCSX, of which 151 have distinct shapes.
Strong correlations and Fickian water diffusion in narrow carbon nanotubes
Authors: Mukherjee Biswaroop , Maiti Prabal K. , Dasgupta Chandan , Sood A. K.
Published: March 2007

The authors have used atomistic molecular dynamics (MD) simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solidlike ordering at room temperature, which they quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not subdiffusive. They also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. They find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal mode diffusion inside the nanotube. They attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length.
Car-Parrinello treatment for an approximate density-functional theory method
Authors: Rapacioli Mathias , Barthel Robert , Heine Thomas , Seifert Gotthard
Published: March 2007

The authors formulate a Car-Parrinello treatment for the density-functional-based tight-binding method with and without self-consistent charge corrections. This method avoids the numerical solution of the secular equations, the principal drawback for large systems if the linear combination of atomic orbital ansatz is used. The formalism is applicable to finite systems and for supercells using periodic boundary conditions within the Γ-point approximation. They show that the methodology allows the application of modern computational techniques such as sparse matrix storage and massive parallelization in a straightforward way. All present bottlenecks concerning computer time and consumption of memory and memory bandwidth can be removed. They illustrate the performance of the method by direct comparison with Born-Oppenheimer molecular dynamics calculations. Water molecules, benzene, the C60 fullerene, and liquid water have been selected as benchmark systems.
“Metric” complexity for weakly chaotic systems
Authors: Galatolo Stefano
Published: March 2007

We consider the number of Bowen sets necessary to cover a large measure subset of the phase space. This introduces some complexity indicator characterizing different kinds of (weakly) chaotic dynamics. Since in many systems its value is given by a sort of local entropy, this indicator is quite simple to calculate. We give some examples of calculations in nontrivial systems (e.g., interval exchanges and piecewise isometries) and a formula similar to that of Ruelle-Pesin, relating the complexity indicator to some initial condition sensitivity indicators playing the role of positive Lyapunov exponents.
Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation
Authors: Scheerbaum Nils , Hinz Dietrich , Gutfleisch Oliver , Skrotzki Werner , Schultz Ludwig
Published: March 2007

Composites consisting of magnetic shape memory (MSM) particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni50.9Mn27.1Ga22.0 (at. %) fibers. The crystal structure of the martensite is tetragonal (5M) with c<a=b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression (height reduction), the MSM particles have a (004)-fiber texture in the direction of compression. This is unambiguous evidence for stress induced twin boundary motion within the MSM particles.
Simulation and design study of cryogenic cone shell target for Fast Ignition Realization Experiment project
Authors: Nagatomo Hideo , Johzaki Tomoyuki , Nakamura Tatsufumi , Sakagami Hitoshi , Sunahara Atsushi , Mima Kunioki
Published: March 2007

In the fast ignition (FI) scheme, at first, high-density fuel core plasma is assembled by implosion laser, and is then heated by petawatt laser to achieve a fusion burning condition. The formation of high-density fuel core plasma is one of the key issues for FI. A typical target for FI is a shell fitted with a reentrant gold cone to make a pass for heating laser. The ablated plasma of gold cone interferes with the implosion dynamics, which is quite different from that of the conventional central-hot-spot approach. Therefore, the dynamics of a nonspherical implosion must be controlled to assemble high density and high areal density. Numerical simulations are performed to study radiation hydrodynamics of cone-guided implosions. In the results, the effect of the cone on implosion dynamics is clarified. The cone surface is irradiated by the radiation and ablated plasma affects the imploding shell. Coating on the cone, which tamps the gold plasma, is effective to improve the implosion performance, although the result does not satisfy the condition of core plasma for the first stage of the Fast Ignition Realization Experiment [K. Mima et al., Proceedings of the IAEA Fusion Energy Conference, Lyon, 2002 (IAEA, Vienna, 2002), Paper No. IAEA-CN-94/IF/03].
Dependence of Langmuir probe data on distance from the axis of a collisionless plasma
Authors: Knappmiller Scott , Robertson Scott
Published: March 2007

A cylindrical Langmuir probe in a low-density, collisionless plasma (density ~108 cm−3, electron temperature 0.2 eV) has been scanned radially through the presheath region to determine the effect of distance from the axis on the current-voltage characteristic. In the ion part of the probe characteristic, the collected ion current decreases with distance from the axis as a consequence of ion acceleration by the presheath. The part of the ion current from charge-exchange collisions remains relatively constant. In the electron part of the probe characteristic, the collected current decreases with distance from the axis, consistent with the existence of a small potential barrier from the presheath between the axis and the probe. The electron temperature from the slope of the probe characteristic is nearly constant across the presheath region. The plasma potential from the Langmuir probe characteristic is also nearly constant, indicating that the probe analysis finds the plasma potential on the axis, even when the probe is not on the axis. The plasma potential from an emissive probe shows an approximately parabolic profile. The plasma potential from the emissive probe and the Boltzmann relation give nearly the same density profile in the presheath that is obtained from the Langmuir probe data.
Stress-induced deceleration of electromigration-driven void motion in metallic thin films
Authors: Gungor M. Rauf , Maroudas Dimitrios
Published: March 2007

We report effects of biaxially applied stress on the electromigration-driven motion of morphologically stable voids in elastically deforming metallic thin films based on self-consistent numerical simulations of void evolution. We find that under certain electromechanical conditions, the applied stress can cause substantial deceleration of the electromigration-driven void motion, as measured by the migration speed of morphologically stable voids translating along the metallic thin film. This effect reveals a new aspect of the complex evolution of voids in metallic thin films under the action of external fields: deceleration of electromigration-driven void motion and even complete inhibition of such motion may be possible through application of mechanical stress.
Linear-scaling symmetric square-root decomposition of the overlap matrix
Authors: Jansík Branislav , Høst Stinne , Jørgensen Poul , Olsen Jeppe , Helgaker Trygve
Published: March 2007

We present a robust linear-scaling algorithm to compute the symmetric square-root or Löwdin decomposition of the atomic-orbital overlap matrix. The method is based on Newton-Schulz iterations with a new approach to starting matrices. Calculations on 12 chemically and structurally diverse molecules demonstrate the efficiency and reliability of the method. Furthermore, the calculations show that linear scaling is achieved.
Gyration-radius dynamics in structural transitions of atomic clusters
Authors: Yanao Tomohiro , Koon Wang S. , Marsden Jerrold E. , Kevrekidis Ioannis G.
Published: March 2007

This paper is concerned with the structural transition dynamics of the six-atom Morse cluster with zero total angular momentum, which serves as an illustrative example of the general reaction dynamics of isolated polyatomic molecules. It develops a methodology that highlights the interplay between the effects of the potential energy topography and those of the intrinsic geometry of the molecular internal space. The method focuses on the dynamics of three coarse variables, the molecular gyration radii. By using the framework of geometric mechanics and hyperspherical coordinates, the internal motions of a molecule are described in terms of these three gyration radii and hyperangular modes. The gyration radii serve as slow collective variables, while the remaining hyperangular modes serve as rapidly oscillating “bath” modes. Internal equations of motion reveal that the gyration radii are subject to two different kinds of forces: One is the ordinary force that originates from the potential energy function of the system, while the other is an internal centrifugal force. The latter originates from the dynamical coupling of the gyration radii with the hyperangular modes. The effects of these two forces often counteract each other: The potential force generally works to keep the internal mass distribution of the system compact and symmetric, while the internal centrifugal force works to inflate and elongate it. Averaged fields of these two forces are calculated numerically along a reaction path for the structural transition of the molecule in the three-dimensional space of gyration radii. By integrating the sum of these two force fields along the reaction path, an effective energy curve is deduced, which quantifies the gross work necessary for the system to change its mass distribution along the reaction path. This effective energy curve elucidates the energy-dependent switching of the structural preference between symmetric and asymmetric conformations. The present methodology should be of wide use for the systematic reduction of dimensionality as well as for the identification of kinematic barriers associated with the rearrangement of mass distribution in a variety of molecular reaction dynamics in vacuum.
Prediction of B–Sii–F complex formation and its role in B transient enhanced diffusion suppression and deactivation
Authors: Harrison Scott A. , Edgar Thomas F. , Hwang Gyeong S.
Published: March 2007

Gradient corrected density functional calculations are used to examine the interaction of boron and fluorine in crystalline silicon. We have determined the formation of a stable boron-silicon-fluorine (Bs–Sii–Fi) complex in which the B and F atoms are indirectly connected through a Si interstitial, while the direct B–F bonding interaction is likely to be insignificant. Depending on dissociation reactions, the binding energy of the Bs–Sii–Fi complex is predicted to be 1.82–1.91eV relative to the corresponding products in the neutral state. We also show the atomic structure and bonding mechanism of Bs–Sii–Fi and discuss the potential role of Bs–Sii–Fi formation in B transient enhanced diffusion suppression and deactivation.
Plasma acceleration processes in an ablative pulsed plasma thruster
Authors: Koizumi Hiroyuki , Noji Ryosuke , Komurasaki Kimiya , Arakawa Yoshihiro
Published: March 2007

Plasma acceleration processes in an ablative pulsed plasma thruster (APPT) were investigated. APPTs are space propulsion options suitable for microspacecraft, and have recently attracted much attention because of their low electric power requirements and simple, compact propellant system. The plasma acceleration mechanism, however, has not been well understood. In the present work, emission spectroscopy, high speed photography, and magnetic field measurements are conducted inside the electrode channel of an APPT with rectangular geometry. The successive images of neutral particles and ions give us a comprehensive understanding of their behavior under electromagnetic acceleration. The magnetic field profile clarifies the location where the electromagnetic force takes effect. As a result, it is shown that high density, ablated neutral gas stays near the propellant surface, and only a fraction of the neutrals is converted into plasma and electromagnetically accelerated, leaving the residual neutrals behind.
Self-organization of gold nanoclusters on hexagonal SiC and SiO2 surfaces
Authors: Ruffino F. , Canino A. , Grimaldi M. G. , Giannazzo F. , Bongiorno C. , Roccaforte F. , Raineri V.
Published: March 2007

Very thin Au layers were deposited on SiC hexagonal and SiO2 substrates by sputtering. The Au surface diffusion, clustering, and self-organization of Au nanoclusters on these substrates, induced by thermal processes, were investigated by Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. On both types of substrates, clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory allowed us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process. The system Au nanoclusters/SiC and Au nanoclusters/SiO2 are proposed as nanostructured materials for nanoelectronic and nanotechnology applications.
Phase transition of ultrathin AlN interlayer at AlGaN∕GaN interface
Authors: Cai Duanjun , Kang Junyong
Published: March 2007

The authors report the wurtizite to zinc-blende phase transition of ultrathin AlN interlayer at the interface of AlGaN∕GaN. The shape transformation of the pinhole from hexagon to dodecagon exhibits the structural transition of the interfacial layer, where exists an AlN segregation interlayer. By transmission electron diffraction, the structure of AlN interlayer is determined to be zinc blende. Ab initio simulations demonstrate that the epitaxial strain favors the formation of the high Al segregation and the phase transition.
Intermittent character of interplanetary magnetic field fluctuations
Authors: Bruno Roberto , Carbone Vincenzo , Chapman Sandra , Hnat Bogdan , Noullez Alain , Sorriso-Valvo Luca
Published: March 2007

Interplanetary magnetic field magnitude fluctuations are notoriously more intermittent than velocity fluctuations in both fast and slow wind. This behavior has been interpreted in terms of the anomalous scaling observed in passive scalars in fully developed hydrodynamic turbulence. In this paper, the strong intermittent nature of the interplanetary magnetic field is briefly discussed comparing results performed during different phases of the solar cycle. The scaling properties of the interplanetary magnetic field magnitude show solar cycle variation that can be distinguished in the scaling exponents revealed by structure functions. The scaling exponents observed around the solar maximum coincide, within the errors, to those measured for passive scalars in hydrodynamic turbulence. However, it is also found that the values are not universal in the sense that the solar cycle variation may be reflected in dependence on the structure of the velocity field.
Use of the point defect model to interpret the iron oxidation kinetics under proton irradiation
Authors: Lapuerta S. , Moncoffre N. , Jaffrézic H. , Millard-Pinard N. , Bérerd N. , Esnouf C. , Crusset D.
Published: March 2007

This article concerns the study of iron corrosion in wet air under mega-electron-volt proton irradiation for different fluxes at room temperature and with a relative humidity fixed to 45%. Oxidized iron sample surfaces are characterized by ion beam analysis (Rutherford backscattering spectrometry and elastic recoil detection analysis), for the elemental analysis. The structural and physicochemical characterization is performed using the x-ray photoelectron spectroscopy and transmission electron microscopy techniques. We have also measured the iron oxidation kinetics. Radiation enhanced diffusion and transport processes have been evidenced. The modeling of the experimental data shows that the apparent oxygen diffusion coefficient increases whereas the oxygen transport velocity decreases as function of flux. Finally, the point defect model has been used to determine the electric field value in the samples. Results have shown that the transport process can be attributed to the presence of an electrical potential gradient.
Characterization of metamorphic InxAl1−xAs∕GaAs buffer layers using reciprocal space mapping
Authors: Lee D. , Park M. S. , Tang Z. , Luo H. , Beresford R. , Wie C. R.
Published: March 2007

The depth profiles of metamorphic InxAl1−xAs (0.05<x<1) buffer layers grown on GaAs substrates were characterized using the x-ray reciprocal space mapping. Three types of metamorphic samples were investigated and compared: step grade, single-slope linear grade, and dual-slope linear grade. The lattice mismatch, residual strain, crystallographic tilt, tilt azimuth, and the full width at half maximum were obtained from the reciprocal space maps. The tilt angle of linearly graded buffer layers stayed low at low In compositions until In≈60%, at which composition the tilt angle increased abruptly. All linear-grade samples had an untilted relaxed structure in the low In region (below 60% In) and a tilted structure in the upper, high In region (above 60% In). The average lattice mismatch between the untilted relaxed structure and the tilted structure determines the tilt angle. The tilt angle of the step-graded layers increased at a near-linear rate as the In composition was increased. The tilt azimuth was intermediate between the ⟨100⟩ and ⟨110⟩ in-plane directions. The x-ray full width at half maximum generally increased with the In composition, but tended lower toward surface. We suggest a possible design strategy for the linear-grade metamorphic buffer layer based on our result.
The effect of nitrogen incorporation on the bonding structure of hydrogenated carbon nitride films
Authors: Camero M. , Buijnsters J. G. , Gómez-Aleixandre C. , Gago R. , Caretti I. , Jiménez I.
Published: March 2007

This work describes the composition and bonding structure of hydrogenated carbon nitride (a-CNx:H) films synthesized by electron cyclotron resonance chemical vapor deposition using as precursor gases argon, methane, and nitrogen. The composition of the films was derived from Rutherford backscattering and elastic recoil detection analysis and the bonding structure was examined by infrared (IR) spectroscopy and x-ray absorption near edge spectroscopy (XANES). By varying the nitrogen to methane ratio in the applied gas mixture, polymeric a-CNx:H films with N/C contents varying from 0.06 to 0.49 were obtained. Remarkably, the H content of the films (∼40 at. %) was rather unaffected by the nitrogenation process. The different bonding states as detected in the measured XANES C(1s) and N(1s) spectra have been correlated with those of a large number of reference samples. The XANES and IR spectroscopy results indicate that N atoms are efficiently incorporated into the amorphous carbon network and can be found in different bonding environments, such as pyridinelike, graphitelike, nitrilelike, and amino groups. The nitrogenation of the films results in the formation of N-H bonding environments at the cost of C-H structures. Also, the insertion of N induces a higher fraction of double bonds in the structure at the expense of the linear polymerlike chains, hence resulting in a more cross-linked solid. The formation of double bonds takes place through complex C=N structures and not by formation of graphitic aromatic rings. Also, the mechanical and tribological properties (hardness, friction, and wear) of the films have been studied as a function of the nitrogen content. Despite the major modifications in the bonding structure with nitrogen uptake, no significant changes in these properties are observed.
The effect of deposition rate on the intrinsic stress in copper and silver thin films
Authors: Del Vecchio A. L. , Spaepen F.
Published: March 2007

The effect of changing the deposition rate on the development of stress in evaporated copper and silver thin films deposited on oxidized silicon was examined. In situ stress measurements were made during deposition in ultrahigh vacuum using a scanning laser curvature system. In some experiments, the deposition rate was alternated without interruption of deposition. For copper thin films, a change in deposition rate has no effect on the development of the tensile stress, while the magnitude of the postcoalescence compressive stress decreases with increasing deposition rate. In silver films, the film thickness at the tensile maximum increases slightly with increasing deposition rate, while the magnitude of the postcoalescence compressive stress again decreases with increasing deposition rate. Analysis of the heat flow during deposition shows that the radiative heating and condensation contribute roughly equally to the temperature rise of the sample.
A comparative investigation of thickness measurements of ultra-thin water films by scanning probe techniques
Authors: Opitz A. , Scherge M. , Ahmed S. I.-U. , Schaefer J. A.
Published: March 2007

The reliable operation of micro- and nanomechanical devices necessitates a precise knowledge of the water film thickness present on the surfaces of these devices with accuracy in the nanometer range. In this work, the thickness of an ultra-thin water film was measured by distance tunneling spectroscopy and distance dynamic force spectroscopy during desorption in an ultra-high vacuum system, from about 2.5 nm up to complete desorption at 10−8 mbar. The tunneling current and the amplitude of vibration and the normal force were detected as a function of the probe-sample distance. In these experiments, a direct comparison of both methods was possible. It was determined that dynamic force spectroscopy provides the most accurate values. The previously reported tunneling spectroscopy, which requires the application of significantly high voltages generally leads to values that are 25 times higher than values determined by dynamic force spectroscopy.
Studies on structural, electrical, and optical properties of Cu doped As–Se–Te chalcogenide glasses
Authors: Hu Juejun , Sun Xiaochen , Agarwal Anuradha M. , Viens Jean-Francois , Kimerling Lionel C. , Petit Laeticia , Carlie Nathan , Richardson Kathleen C. , Anderson Troy , Choi Jiyeon , Richardson Martin
Published: March 2007

Cu doped chalcogenide (ChG) glassy films in the As–Se–Te glass system have been prepared using thermal evaporation techniques. Single-source evaporation from bulk (1−x) As0.40Se0.35Te0.25+x Cu glasses with x=0.05, 0.075, 0.10, 0.125, and 0.15, as well as dual-source coevaporation from As-chalcogenide and Cu-chalcogenide binary glasses as source materials, has been explored. We have shown that it is not possible to deposit high concentration Cu doped ChG glassy films, from the Cu doped bulk samples using single-source evaporation. However, using the dual-source coevaporation technique, we have demonstrated that the films can be doped with high concentrations of Cu. Micro-Raman spectroscopy has been utilized to verify that Cu is introduced into the glass network without disrupting the basic As-chalcogen units. Optical measurements have shown that introduction of Cu decreases the band gap of As–Se–Te glasses. The electrical properties of the investigated films have been measured at different temperatures and it has been shown that Cu incorporation in the As–Se–Te glass system vastly improves electrical conductivity. Moreover, we have shown that the temperature dependence of electrical conductivity can be fitted assuming variable range hopping between states near the Fermi level.
X-ray reciprocal space maps and x-ray scattering topographic observation of GaN layer on GaAs (001) in plasma-assisted molecular beam epitaxy
Authors: Suzuki Yoshifumi , Shinbara Masakazu , Kii Hideki , Chikaura Yoshinori
Published: March 2007

We have characterized plasma-assisted N+ molecular beam epitaxy-grown polymorphous GaN epitaxial layer on GaAs by x-ray reciprocal mapping using four-circle x-ray diffractometer and a personal computer controlled x-ray scattering topography system by ourselves. x-ray reciprocal mapping indicates that GaN wurtzite epitaxial film was grown along only [1¯1¯1] direction. While GaN wurtzite and zinc-blende crystals were contracted along the surface normal, those of lattice constants were expanded along lateral direction. The values of expansion were larger than our instrumental resolution. The lateral expansion rate of lattice constants in GaN wurtzite was larger than that in GaN zinc blende. It was found that zinc-blende phase was unevenly distributed, but wurtzite one was uniformly distributed by growth condition.
Regrowth of diluted magnetic semiconductor GaMnAs on InGaP (001) surfaces to realize freestanding micromechanical structures
Authors: Choi Hyung Kook , Lee Joon Sue , Cho Sung Woon , Lee Won Oh , Shim Seung Bo , Park Yun Daniel
Published: March 2007

Low temperature molecular beam epitaxy regrowths of Ga1−xMnxAs (x≈0.04) diluted magnetic semiconductors on GaAs∕In1−yGayP∕GaAs(001) and In1−yGayP∕GaAs(001) (y≈0.51) heterostructures prepared by metal-organic chemical vapor deposition are described. The resulting Ga1−xMnxAs properties are comparable to epitaxial films grown directly on GaAs (001) substrates from in situ reflection high-energy electron diffraction, x-ray diffraction, magnetometry, and transport measurements with magnetic ordering temperature of as-grown films to range between ∼50 and ∼60K. Postgrowth low temperature annealing enhances both magnetic and transport properties. Perfect etch selectivity between Ga1−xMnxAs∕GaAs and In1−yGayP is utilized to realize suspended Ga1−xMnxAs∕GaAs doubly clamped beam micromechanical freestanding structures.
Streaked optical pyrometer system for laser-driven shock-wave experiments on OMEGA
Authors: Miller J. E. , Boehly T. R. , Melchior A. , Meyerhofer D. D. , Celliers P. M. , Eggert J. H. , Hicks D. G. , Sorce C. M. , Oertel J. A. , Emmel P. M.
Published: March 2007

The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of approximately megabar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented.
Improved parallel scan method for nanofriction force measurement with atomic force microscopy
Authors: Wang Yu-Liang , Zhao Xue-Zeng , Zhou Fa-Quan
Published: March 2007

Based on Ruan and Bhushan’s study [J. Ruan and B. Bhushan, J. Tribol. 116, 378 (1994)], an improved method for quantitative nano/microfriction force measurements with the atomic force microscope (AFM) is presented. The related theoretical derivation is given in detail. The coefficient of friction can be calculated by scanning in the direction parallel to the long axis of the AFM cantilever. Then conversion factor, which can convert the lateral deflection response of the photodetector into corresponding friction force, is identified with the Meyer and Amer method [G. Meyer and N. M. Ame, Appl. Phys. Lett. 57, 2089 (1990)]. Like Ruan and Bhushan method, the advantage of this approach is that the coefficient of friction can be obtained with the plan-view geometry of AFM cantilevers and some common uncertainties, such as thickness, coating, and material properties, are not necessary. The result of the experiments performed utilizing rectangular cantilevers of different lengths shows that this improved method produces an accurate agreement for cantilevers of different lengths, thus the method can be used to measure nano/microfriction force.
Measurement of the Dα spectrum produced by fast ions in DIII-D
Authors: Luo Y. , Heidbrink W. W. , Burrell K. H. , Kaplan D. H. , Gohil P.
Published: March 2007

Fast ions are produced by neutral beam injection and ion cyclotron heating in toroidal magnetic fusion devices. As deuterium fast ions orbit around the device and pass through a neutral beam, some deuterons neutralize and emit Dα light. For a favorable viewing geometry, the emission is Doppler shifted away from other bright interfering signals. In the 2005 campaign, we built a two channel charge-coupled device based diagnostic to measure the fast-ion velocity distribution and spatial profile under a wide variety of operating conditions. Fast-ion data are acquired with a time resolution of ∼1ms, spatial resolution of ∼5cm, and energy resolution of ∼10keV. Background subtraction and fitting techniques eliminate various contaminants in the spectrum. Neutral particle and neutron diagnostics corroborate the Dα measurement. Examples of fast-ion slowing down and pitch angle scattering in quiescent plasma and fast-ion acceleration by high harmonic ion cyclotron heating are presented.
Noise-rejection techniques for impedance and dielectric spectrometers using ubiquitous test and measurement equipment
Authors: Lukacs Stephen J.
Published: March 2007

This work encompassed the development for a frequency-domain impedance and dielectric spectrometer using ubiquitous test and measurement equipment, i.e., signal generators and digital oscilloscopes. Various methods of amplification, noise rejection, and computations were employed to achieve the desired goals. The frequency range of 100mHz–1MHz was tested using air capacitors of 3.7 and 14.5pF and an applied voltage range of 10–300mV. The multichannel instrument produced a stable and reproducible dual-phase (real and imaginary or magnitude and phase) current sensitivity of 250fA with an average phase stability of less than 0.5° (tanδ<10−2) and a single-phase (magnitude only) current sensitivity of 60fA.
Liquid-crystal-based linear polarization rotator
Authors: Ren Hongwen , Wu Shin-Tson
Published: March 2007

A liquid-crystal (LC)-based polarization rotator which can rotate the polarization axis of an incident linearly polarized light from 0° to 90° is demonstrated. In the LC cell, the top substrate has a uniform rubbing but the bottom substrate has two orthogonal rubbings which are separated by a nonrubbing zone. Between these two rubbed strips, the LC directors twist continuously from 0° to 90°. As a result, the optic axis of the incident linearly polarized light can be rotated continuously depending on the beam position.
Near-infrared thermography using a charge-coupled device camera: Application to microsystems
Authors: Teyssieux D. , Thiery L. , Cretin B.
Published: March 2007

Using near-infrared thermography microscopy and a low-cost charge-coupled device (CCD) camera, we have designed a system which is able to deliver quantitative submicronic thermal images. Using a theoretical model based on Planck’s law and CCD sensor properties allowed us to determine a minimal theoretical detection temperature and an optimal temperature sensitivity of our system. In order to validate this method, we show a good relationship between a theoretical study and a thermal measurement of a microsample.
Reflection properties of hybrid quarter-wavelength silicon microstructures
Authors: Barillaro Giuseppe , Annovazzi-Lodi Valerio , Benedetti Mauro , Merlo Sabina
Published: March 2007

The authors present experimental and numerical results relative to the polarization-resolved spectral reflectivity of one-dimensional periodic microstructures, evaluated in the near-infrared region at non-normal incidence. The tested hybrid quarter-wavelength microstructures, fabricated by electrochemical deep etching of silicon, consist of arrays of silicon walls and air gaps, with 3 and 4μm periods and aspect ratio of up to 100. A theoretical Monte Carlo analysis taking into account the presence of a Gaussian statistical distribution for the structure porosity has been carried out and the calculated wavelength dependence of the reflectivity at non-normal incidence has been confirmed by experimental data.
Effective attraction interactions between like-charge macroions bound to binary fluid lipid membranes
Authors: Shi Xia-qing , Ma Yu-qiang
Published: March 2007

Using integral equation theory of liquids to a binary mixed fluid lipid membrane, the authors study the membrane-mediated interactions between binding macroions and the redistribution of neutral and charged lipids due to the macroions. The authors find that when the concentration of binding macroions is infinitely dilute, the main contribution to the attractive potential between macroions is the line tension between neutral and charged lipids of the membrane. As the relative concentration of charged lipids is increased, the authors observe a repulsive-attractive-repulsive potential transition due to the competition between the line tension of mixed lipids and screened electrostatic macroion-macroion interactions. For the finite concentration of macroions, the main feature of the attraction is similar to the infinite-diluted case. However, the corresponding line tension of binary lipids under single macroion is lowered with the formation of multicomplexes by the charged lipids and the macroions, and the maximum of attractive potential will shift toward the higher values of charged lipid concentration.
Dynamic behaviors on zadaxin getting into carbon nanotubes
Authors: Liu Ying-Chun , Wang Qi
Published: March 2007

The dynamic behaviors of drug zadaxin getting into carbon nanotubes (CNTs) in different water surroundings were investigated by molecular dynamics simulation. It was found that the diameter (1.9nm) of (14, 14) CNT is the critical size for inserting zadaxin into CNT at the present conditions. In addition, the length of CNTs is another factor for inserting. A certain length is needed. It implies that interactions of zadaxin with both the CNT and the water molecules are competitive in the insertion process. The CNT-zadaxin attractive interaction is found to be the main driving force with the lower density of water molecules in the surroundings, while the zadaxin-water interaction becomes dominant with the higher density. The study of the authors suggests that biomolecules-CNT systems can be further exploited for the potential applications to drugs, vaccines, and gene delivery.
Dynamics of mode competition in a gigawatt-class magnetically insulated line oscillator
Authors: Kim D. H. , Jung H. C. , Min S. H. , Shin S. H. , Park G. S.
Published: March 2007

An axial mode competition is observed in a 1GHz magnetically insulated line oscillator operating at gigawatt power level with a pulse duration of 130ns. A fast-growing axial mode adjacent to desired π-mode starts up first and hops to the slow-growing and stable π mode. The dynamics of the mode competition is found to be strongly dependent on the time-varying axial velocity of the magnetically insulated electron beam. The experimental observation is verified by the particle-in-cell simulation using a time-frequency analysis.