bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of flow shear on temperature gradient driven short wavelength modes
Authors: Gao Zhe , Dong J. Q. , Sanuki H.
Published: May 2004

The effects of flow shear on the temperature gradient driven short wavelength ion (SWITG) modes and electron temperature gradient (ETG) modes are investigated in a sheared slab. The SWITG mode can be stabilized at arbitrary β when the E×B velocity shear, VE′, reaches above a critical value. Since the SWITG mode has a lower frequency, a lower VE′ is needed to stabilize the SWITG mode than to stabilize the conventional ITG mode. However, the critical values of VE′ for stabilization of both SWITG and conventional ITG modes are much less than vti/Ln, where vti and Ln are ion thermal speed and the scale length of density gradient. Contrastively, the ETG mode cannot be stabilized until the VE′ is larger than vti/Ln. Similarly, a parallel velocity shear with order vti/Ln has significant effects on the SWITG mode but is too small to influence the ETG mode. The different behaviors of flow shear effects on the SWITG and ETG modes may indicate that the ETG mode is more reasonable than the SWITG as the candidate responsible for anomalous electron thermal transport.
Single particle motion near an X point and separatrix
Authors: Weitzner Harold , Chang Choong-Seock
Published: May 2004

The motion of a single particle near an X point or separatrix in given static axisymmetric electric and magnetic fields is studied. It is shown that large classes of particles are lost on a time scale given by the distance scale on which the fields vary divided by the thermal speed. In a low beta version of the problem the results are more specific and the classes of particles are described which are either lost or confined. A preliminary, simpler problem is also treated with model fields in rectangular geometry. Although the losses are similar, the orbits near the X point exhibit a more complicated structure in phase space.
The appearance of the resolved singular hypersurface in the classical phase space of the Lie group SU(n)
Authors: Paul Samir K. , Sen Siddhartha
Published: May 2004

A classical phase space with a suitable symplectic structure is constructed together with functions which have Poisson brackets algebraically identical to the Lie algebra structure of the Lie group SU(n). In this phase space we show that the orbit of the generators corresponding to the simple roots of the Lie algebra give rise to fibers that are complex lines containing spheres. There are n−1 spheres on a fiber and they intersect in exactly the same way as the Cartan matrix of the Lie algebra. This classical phase space bundle, being compact, has a description as a variety. Our construction shows that the variety containing the intersecting spheres is exactly the one obtained by resolving the singularities of the variety x0x1−x2n=0 in C3. A direct connection between this singular variety and the classical phase space corresponding to the Lie group SU(n) is thus established.
Histories electromagnetism
Authors: Burch Aidan
Published: May 2004

Working within the HPO (History Projection Operator) Consistent Histories formalism, we follow the work of Savvidou on (scalar) field theory [J. Math. Phys. 43, 3053 (2002)] and that of Savvidou and Anastopoulos on (first-class) constrained systems [Class. Quantum Gravt. 17, 2463 (2000)] to write a histories theory (both classical and quantum) of Electromagnetism. We focus particularly on the foliation-dependence of the histories phase space/Hilbert space and the action thereon of the two Poincaré groups that arise in histories field theory. We quantize in the spirit of the Dirac scheme for constrained systems.
Losses of neutral beam injected fast ions due to adiabaticity breaking processes in a field-reversed configuration
Authors: Takahashi Toshiki , Inoue Koji , Iwasawa Naotaka , Ishizuka Takashi , Kondoh Yoshiomi
Published: May 2004

Losses of neutral beam (NB) injected fast ions from the confinement region of a field-reversed configuration (FRC) with a strong magnetic mirror are numerically analyzed for parameters relevant to NB injection experiments on the FIX (FRC injection experiment) device [T. Asai et al., Phys. Plasmas 7, 2294 (2000)]. Ionization processes of beam particles are calculated by the Monte Carlo method. The confinement of beam ions is discussed with the concept of accessible regions that restrict the ion excursion and are determined from two constants of motion, the kinetic energy and canonical angular momentum, in the case of an axisymmetric and a steady state FRC without an electrostatic field. From the calculation of the accessible regions, it is found that all the fast ions suffer from the orbit loss on the wall surface and/or the end loss. Single particle orbits are also calculated to find a difference of confinement properties from the results by employing the accessible regions. The magnetic moment is observed to show nonadiabatic motions of the beam ions, which cause a gradual orbit loss on the wall even in a case that a strong magnetic mirror is applied. The results show that the correlation of the magnetic moment disappears as the fast ions experience the density gradient around the separatrix surface and the field-null points.
On dust dynamics in tokamak edge plasmas
Authors: Krasheninnikov S. I. , Tomita Y. , Smirnov R. D. , Janev R. K.
Published: May 2004

It is shown that in a tokamak edge plasma dust particles can move with high speed and traverse distances comparable to tokamak radii. As a result, the dust deposition areas on the wall structures can be far away from the origin of the dust. Presented estimates indicate that it is very likely that transport of dust particles can be an important mechanism of core plasma contamination by impurities.
A molecular-dynamics study of structural and physical properties of nitromethane nanoparticles
Authors: Alavi Saman , Thompson Donald L.
Published: May 2004

The structural and physical properties of nanoparticles of nitromethane are studied by using molecular dynamics methods with a previously developed force field. [Agrawal et al., J. Chem. Phys. 119, 9617 (2003).] This force field accurately predicts solid- and liquid-state properties as well as melting of bulk nitromethane. Molecular dynamics simulations of nanoparticles with 480, 240, 144, 96, 48, and 32 nitromethane molecules have been carried out at various temperatures. The carbon–carbon radial distribution function, dipole–dipole correlation function, core density, internal enthalpy, and atomic diffusion coefficients of the nanoparticles were calculated at each temperature. These properties were used to characterize the physical phases and thus determine the melting transitions of the nanoparticles. The melting temperatures predicted by the various properties are consistent with one another and show that the melting temperature increases with particle size, approaching the bulk limit for the largest particle. A size dependence of melting points has been observed in experimental and theoretical studies of atomic nanoparticles, and this is a further demonstration of the effect for large nanoparticles of complex molecular materials.
Regularities in molecular properties of ground state stable diatomics
Authors: Zavitsas Andreas A.
Published: May 2004

A simple relationship is reported between vibrational frequencies, bond lengths, and reduced masses for many families of stable, ground state diatomics: the frequency is proportional to the reciprocal of the product of the bond length and the square root of the reduced mass. This is demonstrated with each of the following related families: the alkali metal diatomics, the group 15 diatomics, the group 16 diatomics, the halogen diatomics, the alkali metal hydrides, the alkaline earth oxides, the group 14 oxides and their sulfides, the diatomics of carbon, of silicon and of germanium with group 16 elements, the hydrogen halides, the halides of lithium, of sodium, of potassium, of rubidium and of cesium, the chlorides of the alkali metals and of silver, and the polyatomic hydrides of groups 14 and 15. Although correlation coefficients of 0.99 or greater in each of the 21 families examined demonstrate the validity of the correlation, the deviations found are significantly larger than can be attributed to experimental uncertainties.
Study of electromagnetic microinstabilities in helical systems with the stellarator expansion method
Authors: Sugama H. , Watanabe T.-H.
Published: May 2004

Electromagnetic microinstabilities in helical systems are studied by numerically solving integral eigenmode equations, which are derived from the ion gyrokinetic equation, the quasineutrality equation, the Ampère’s law, and the massless electron approximation. The stellarator expansion technique is used to evaluate finite-beta effects on the guiding-center drift in the helical configuration, where the toroidal plasma shift and the magnetic shear strongly influence the magnetic curvature and accordingly the stability of both magnetohydrodynamics (MHD) and kinetic modes. The kinetic integral equations are shown to reduce to the ideal MHD ballooning mode equation in the fluid limit, from which the Mercier criterion is obtained. For helical geometry like the Large Helical Device (LHD) [Motojima, et al., Nucl. Fusion 43, 1674 (2003)], it is confirmed that, when increasing the beta value, the ion temperature gradient mode is stabilized while the kinetic ballooning mode (KBM) is destabilized due to the unfavorable geodesic curvature resulting from the negative magnetic shear combined with the toroidal plasma shift. Also, dependencies of these kinetic-mode properties on the poloidal wave number and the magnetic shear are investigated. It is found that the KBM-unstable parameter region is narrower than the Mercier-unstable region in the LHD-like configuration.
Large amplitude solitary electromagnetic waves in electron-positron plasmas
Authors: Verheest Frank , Cattaert Tom
Published: May 2004

Waves in electron-positron plasmas have fundamentally different dispersion characteristics due to the equal charge-to-mass ratios between negative and positive charges, which mix different timescales, and are of interest in understanding aspects of pulsars and active galactic nuclei, where astrophysical electron-positron plasmas occur. Earlier systematic nonlinear treatments of parallel propagating electromagnetic waves via a reductive perturbation analysis had indicated unusual results, namely a vector equivalent of the modified Korteweg–de Vries equation. The latter is nonintegrable except in the case of linear polarization when it becomes equivalent to the scalar (integrable) modified Korteweg–de Vries equation. Here large amplitude purely stationary nonlinear solitary waves are studied in their own reference frame via the McKenzie approach. The behavior of the wave magnetic field can be expressed through an energy integral that involves the Mach number of the structure. Possible solitons are super-Alfvénic and occur symmetrically for positive or negative fields, owing to the obvious symmetry between positive and negative charges with the same mass. The limits on the allowable Mach numbers and soliton amplitudes have also been computed.
Laser light and hot electron micro focusing using a conical target
Authors: Sentoku Y. , Mima K. , Ruhl H. , Toyama Y. , Kodama R. , Cowan T. E.
Published: May 2004

The laser light propagation inside the conical target had been studied by three-dimensional particle-in-cell simulations. It is found that the laser light is optically guided inside the conical target and focused at the tip of the cone. The intensity increases up to several tens of times in a several micron focal spot. It is the convergence of hot electrons to the head of the cone that is observed as a consequence of the surface electron flow guided by self-generated quasistatic magnetic fields and electrostatic sheath fields. As a result, the hot electron density at the tip is locally ten times greater than the case of using a normal flat foil.
Investigation of electrical breakdown characteristics in the electrodes of cylindrical geometry
Authors: Li Shou-Zhe , Uhm Han S.
Published: May 2004

Analytical solutions of Laplace’s equation in the space between electrodes of cylindrical geometry are derived by conformal mapping. Electrical breakdown properties of the two cylindrical electrodes are investigated by making use of the Townsend criterion and the electric potential obtained analytically. The characteristics of electrical breakdown are investigated in terms of the geometrical configurations, the cylinder radii, and the distance between the two axes of cylinders. The present theoretical expression of the breakdown analysis is applied to the special cases to discuss the well-known electrode configurations such as the two parallel plane electrodes with infinite curvature radii, the wire-plane electrodes, and the coaxial cylindrical electrodes. It is shown that the electrical field distribution determined by electrodes arrangement plays a pivotal role in the electrical breakdown phenomena.
Symmetric informationally complete quantum measurements
Authors: Renes Joseph M. , Blume-Kohout Robin , Scott A. J. , Caves Carlton M.
Published: May 2004

We consider the existence in arbitrary finite dimensions d of a positive operator valued measure (POVM) comprised of d2 rank-one operators all of whose operator inner products are equal. Such a set is called a “symmetric, informationally complete” POVM (SIC–POVM) and is equivalent to a set of d2 equiangular lines in Cd. SIC–POVMs are relevant for quantum state tomography, quantum cryptography, and foundational issues in quantum mechanics. We construct SIC–POVMs in dimensions two, three, and four. We further conjecture that a particular kind of group-covariant SIC–POVM exists in arbitrary dimensions, providing numerical results up to dimension 45 to bolster this claim.
Fine gradings of o(4,C)
Authors: Patera Jiřı́ , Pelantová Edita , Svobodová Milena
Published: May 2004

A grading of a Lie algebra is called fine if it cannot be further refined. Fine gradings provide basic information about the structure of the algebra. There are six fine gradings of the semisimple Lie algebra of type A1×A1 over the complex number field. An explicit description of all the fine gradings of A1×A1 is given in terms of the four-dimensional representation o(4,C) of the algebra.
Relativistic quantum field theory with a fundamental length
Authors: Brüning E. , Nagamachi S.
Published: May 2004

Since there are indications (from string theory and concrete models) that one must consider relativistic quantum field theories with a fundamental length the question of a suitable framework for such theories arises. It is immediately evident that quantum field theory in terms of tempered distributions and even in terms of Fourier hyperfunctions cannot meet the (physical) requirements. We argue that quantum field theory in terms of ultra-hyperfunctions is a suitable framework. For this we propose a set of axioms for the fields and for the sequence of vacuum expectation values of the fields, prove their equivalence, and we give a class of models (analytic, but not entire functions of free fields).
Tilt angle dependent three-dimensional-position detection of a trapped cylindrical particle in a focused laser beam
Authors: Kress Holger , Stelzer Ernst H. K. , Rohrbach Alexander
Published: May 2004

We investigated theoretically the applicability of an optically trapped cylindrical particle as a local probe in photonic force microscopy. To do this we calculated the far-field scattering from a subwavelength-sized dielectric cylinder in a highly focused laser field. From this we obtained interferometric three-dimensional-position detection signals and compared these to signals calculated for a spherical particle. We have calculated the accuracy to which the position of an optically trapped cylinder can be determined, as a function of the cylinder’s orientational fluctuations. The position accuracy is better than a few nanometers for tilt angle fluctuations up to several degrees. Our study is relevant for trapping experiments, where the influence of angle fluctuations needs to be estimated.
Pseudoelasticity of shape-memory titanium–nickel films subjected to dynamic nanoindentation
Authors: Ma X.-G. , Komvopoulos K.
Published: May 2004

Surface force microscopy was used to examine the dynamic indentation response of austenitic titanium–nickel (TiNi) films sputtered on oxidized silicon substrates. Results demonstrate the significance of indentation cycles, time at maximum contact load, and loading/unloading rate on the nanoscale pseudoelastic behavior of the TiNi films. The dynamic behavior of the films is interpreted in terms of the mechanisms responsible for the energy dissipated in the absence of irreversible deformation. The significant amount of mechanical work dissipated without permanent deformation illustrates the potential of TiNi films as structural materials for dynamic microdevice applications.
Magneto-optical Kerr effect in Fe21Ni79 films on Si(100): Quantum behavior for film thicknesses below ∼6 nm
Authors: Talmadge J. M. , Gao J. , Riley M. P. , Roth R. J. , Kim S.-O. , Eden J. G. , Pudonin F. A. , Mel’nikov I. V.
Published: May 2004

The magneto-optical Kerr effect (MOKE) has been observed and characterized in 1–80 nm thick Fe21Ni79 films deposited onto Si(100), for an external magnetic field (variable in strength up to 400 G) oriented parallel or orthogonal to the magnetization axis of the film. A measurable response is observed for film thicknesses (d) as small as 2 nm and, if the external magnetic field lies in the plane of the film, two-dimensional quantum behavior is evident for d≲6 nm. A precipitous decline in the magnitude of the MOKE response is accompanied by an increase in the coercivity and, when the external field is perpendicular to the film magnetization axis, a rapid rise in the saturation field. Experiments also confirm the existence of a component of the film magnetization oriented out of the plane of the film, a result consistent with the prediction of computational studies [T. Trunk et al., J. Appl. Phys. 89, 7606 (2001)] that the transition between Bloch and Néel wall domain structure occurs in FeNi films for film thicknesses of ≈30 nm.
Dip-pen nanopatterning of photosensitive conducting polymer using a monomer ink
Authors: Su Ming , Aslam Mohammed , Fu Lei , Wu Nianqiang , Dravid Vinayak P.
Published: May 2004

Controlled patterning of conducting polymers at a micro- or nanoscale is the first step towards the fabrication of miniaturized functional devices. Here, we introduce an approach for the nanopatterning of conducting polymers using an improved monomer “ink” in dip-pen nanolithography (DPN). The nominal monomer “ink” is converted, in situ, to its conducting solid-state polymeric form after patterned. Proof-of-concept experiments have been performed with acid-promoted polymerization of pyrrole in a less reactive environment (tetrahydrofuran). The ratios of reactants are optimized to give an appropriate rate to match the operation of DPN. A similar synthesis process for the same polymer in its bulk form shows a high conductance and crystalline structure. The miniaturized conducting polymer sensors with light detection ability are fabricated by DPN using the improved ink formula, and exhibit excellent response, recovery, and sensitivity parameters.
Symmetry classification of KdV-type nonlinear evolution equations
Authors: Güngör F. , Lahno V. I. , Zhdanov R. Z.
Published: May 2004

Group classification of a class of third-order nonlinear evolution equations generalizing KdV and mKdV equations is performed. It is shown that there are two equations admitting simple Lie algebras of dimension three. Next, we prove that there exist only four equations invariant with respect to Lie algebras having nontrivial Levi factors of dimension four and six. Our analysis shows that there are no equations invariant under algebras which are semi-direct sums of Levi factor and radical. Making use of these results we prove that there are three, nine, thirty-eight, fifty-two inequivalent KdV-type nonlinear evolution equations admitting one-, two-, three-, and four-dimensional solvable Lie algebras, respectively. Finally, we perform a complete group classification of the most general linear third-order evolution equation.
Phase-space Green’s functions for modeling time-harmonic scattering from smooth inhomogeneous objects
Authors: Melamed T.
Published: May 2004

The paper deals with inhomogeneous medium Green’s functions in the phase-space domain by which the phase-space (local) spectral distributions of the field, scattered by a high contrast object due a genetic time-harmonic incidence, are evaluated. Two forms of phase-space Green’s functions are considered: one that links induced sources in the configuration-space to phase-space distributions of the scattered field, while the other one directly links the phase-space distribution of the incident field to phase-space distributions of the scattered field. The scattering mechanism is described in terms of local samplings of the object function which are localized in the object domain according to the scattered- and incidence-processing parameters. Applications in the field of inverse scattering may be expected to yield fast and efficient algorithms, due to the capability of analytically evaluating (forward) scattering Green’s functions.
Traveling wave solutions of a generalized modified Kadomtsev–Petviashvili equation
Authors: Schürmann H. W. , Serov V. S.
Published: May 2004

A method to generate traveling wave solutions of the generalized modified Kadomtsev–Petviashvili equation is reported and several physical solutions, including conditions of their existence, are presented.
Quantizing the line element field
Authors: Roberts Mark D.
Published: May 2004

A metric with signature (−+++) can be constructed from a metric with signature (++++) and a double-sided vector field called the line element field. Some of the classical and quantum properties of this vector field are studied.
Method to sense single-particle motion using a tapered-gap microcapacitor
Authors: Kapon Ruti , Sagiv Ilan , Shappir Joseph , Mazorski Noa , Ziv Guy , Shahar Dan , Reich Ziv
Published: May 2004

We present an approach to study the real-time dynamics of single molecules using capacitance measurements. The method is based on a nonparallel-plate microcapacitor, which has a tapered-gap geometry. A particle moving within such a capacitor induces capacitance changes that depend on its position. Monitoring these changes allows motion to be traced at a resolution which is higher than the smallest fabricated feature of the device. The detection scheme also enables the distinction between particles of different dielectric constants and the exertion of dielectrophoretic forces on the particles. This approach provides a means for studying various aspects of single-particle dynamics at high resolution, in real time, and under conditions compatible with biological systems.
The relativistic Dirac–Morse Green’s function
Authors: Alhaidari A. D.
Published: May 2004

Using a recently developed approach for solving the three-dimensional Dirac equation with spherical symmetry, we obtain the two-point Green’s function of the relativistic Dirac–Morse problem. This is accomplished by setting up the relativistic problem in such a way that makes comparison with the nonrelativistic problem highly transparent and results in a mapping of the latter into the former. The relativistic bound states energy spectrum is easily obtained by locating the energy poles of the Green’s function components in a simple and straightforward manner.
A technique to identify solvable dynamical systems, and a solvable generalization of the goldfish many-body problem
Authors: Calogero Francesco
Published: May 2004

A simple approach is discussed which associates to (solvable) matrix equations (solvable) dynamical systems, generally interpretable as (interesting) many-body problems, possibly involving auxiliary dependent variables in addition to those identifying the positions of the moving particles. We then focus on cases in which the auxiliary variables can be altogether eliminated, reobtaining thereby (via this unified approach) well-known solvable many-body problems, and moreover a (solvable) extension of the “goldfish” model.
Scanning tunneling spectroscopy of suspended single-wall carbon nanotubes
Authors: LeRoy B. J. , Lemay S. G. , Kong J. , Dekker C.
Published: May 2004

We have performed low-temperature scanning tunneling microscopy measurements on single-wall carbon nanotubes that are freely suspended over a trench. The nanotubes were grown by chemical vapor deposition on a Pt substrate with predefined trenches etched into it. Atomic resolution was obtained on the freestanding portions of the nanotubes. Spatially resolved spectroscopy on the suspended portion of both metallic and semiconducting nanotubes was also achieved, showing a Coulomb-staircase behavior superimposed on the local density of states. The spacing of the Coulomb blockade peaks changed with tip position reflecting a changing tip-tube capacitance.
Vacuum response and gas leak detection in piezoelectrically driven sound-resonance cavity
Authors: Dong Shuxiang , Bai Feiming , Yan Li , Cao Hu , Li Jie-Fang , Viehland D.
Published: May 2004

It has been found that a piezoelectrically driven sound-resonance cavity (PSRC) is quite sensitive to low-pressure changes: a change from ambient to vacuum (67 Pa) conditions results in a 89° phase shift and a −81% voltage decrease. Under vacuum conditions, the PSRC is highly sensitive (∼7 ppm) to leakage of H2 gas.
Nonlinear microwave properties of ferroelectric thin films
Authors: Ott R. , Lahl P. , Wördenweber R.
Published: May 2004

The nonlinear microwave properties of ferroelectric SrTiO3 thin films are characterized via complex analysis of the intermodulation distortion (IMD) signals up to high microwave power. The measurements reveal an unusual dependence of the IMD signals on the input power, which indicates the presence of two different nonlinear properties being responsible for the generation of IMD, i.e., the nonlinear permittivity and conductivity at low and high rf power levels, respectively. The IMD signal strength cannot be explained in terms of the classical description based on a Taylor expansion of the nonlinear parameter. In contrast, simulations of the frequency spectra using more appropriate descriptions of two nonlinear parameters yield an excellent agreement between theory and experiment, and, thus, demonstrate that the IMD experiments together with the simulation might provide further insight into the mechanism of nonlinear behavior of these materials.
Boron diffusion in amorphous silicon and the role of fluorine
Authors: Duffy R. , Venezia V. C. , Heringa A. , Pawlak B. J. , Hopstaken M. J. P. , Maas G. C. J. , Tamminga Y. , Dao T. , Roozeboom F. , Pelaz L.
Published: May 2004

We demonstrate that boron diffuses at high concentrations during low-temperature thermal annealing in amorphous silicon pre-amorphized by germanium ion implantation. For a typical boron ultrashallow junction doping profile, concentrations as high as 2×1020 cm−3 appear to be highly mobile at 500 and 600 °C in the amorphous silicon region before recrystallization. In crystalline silicon at the same temperatures the mobile boron concentration is at least two orders of magnitude lower. We also show that boron diffusivity in the amorphous region is similar with and without fluorine. The role of fluorine is not to enhance boron diffusivity, but to dramatically slow down the recrystallization rate, allowing the boron profile to be mobile up to the concentration of 2×1020 cm−3 for a longer time.
Effects of finite deformed length in carbon nanotubes
Authors: Lu Jun-Qiang , Wu Jian , Duan Wenhui , Gu Bing-Lin
Published: May 2004

The effect of finite deformed length is demonstrated by squashing an armchair (10,10) single-walled carbon nanotube with two finite tips. Only when the deformed length is long enough, an effectual metal–semiconductor–metal heterojunction can be formed in the metallic tube. The effect of finite deformed length is explained by the quantum tunneling effect. Furthermore, some conceptual designs of nanoscale devices are proposed from the metal–semiconductor–metal heterojunction.
Investigation of forward and reverse current conduction in GaN films by conductive atomic force microscopy
Authors: Spradlin J. , Doǧan S. , Xie J. , Molnar R. , Baski A. A. , Morkoç H.
Published: May 2004

We have used conductive atomic force microscopy (C–AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C–AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current–voltage spectra taken by C–AFM on and off such hillocks indicate Frenkel–Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxial GaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, that films with more rectifying nominal Schottky behavior (less reverse leakage current) produce forward and reverse bias C–AFM images with strong asymmetry.
Visible cathodoluminescence from Eu-implanted single- and polycrystal c-BN annealed under high-temperature, high-pressure conditions
Authors: Vetter Ulrich , Hofsäss Hans , Taniguchi Takashi
Published: May 2004

Red and red/blue cathodoluminescence in the temperature range 12 to 300 K was obtained from single- and polycrystal cubic boron nitride bulk samples implanted with europium and annealed under high-temperature, high-pressure conditions. All observed radiative intra-4f electron transitions of Eu3+ can be assigned to transitions starting from the 5D0 level of Eu3+. Additionally, radiative 4fN−15d→4fN electron transitions related to Eu2+ were detected in the polycrystal c-BN specimens. The higher-resolution Stark level as well as the time-resolved cathodoluminescence spectroscopy suggest that the Eu ions occupy at least two different sites in the c-BN host.
Characterization and reduction of unexplained noise in superconducting transition-edge sensors
Authors: Ullom J. N. , Doriese W. B. , Hilton G. C. , Beall J. A. , Deiker S. , Duncan W. D. , Ferreira L. , Irwin K. D. , Reintsema C. D. , Vale L. R.
Published: May 2004

The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance.
An imaging fiber-based optical tweezer array for microparticle array assembly
Authors: Tam Jenny M. , Biran Israel , Walt David R.
Published: May 2004

In this letter, we present a method to generate and regenerate arrays of microspheres by optically trapping through optical imaging fiber bundles. In this method, a laser beam is coupled into the proximal end of an optical fiber bundle, and the light energy is distributed across the face of the fiber. Each illuminated individual fiber in the array propagates light to the distal face of the bundle, where light focusing elements at the end of each fiber focus the laser light and form optical traps. These optical traps are capable of capturing and arraying microspheres in parallel. The number of optical traps is determined by the number of fibers in the optical fiber bundle and is capable of creating a dense array (∼5×104 traps/mm2) of optical tweezers.
Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets
Authors: Wood Tiffany A. , Gleeson Helen F. , Dickinson Mark R. , Wright Amanda J.
Published: May 2004

A detailed study is presented that evaluates the relative importance of wave plate behavior, scattering processes and absorption phenomena in transferring optical torque from circularly polarized light to optically trapped nematic droplets. A wide range of parameters is considered: droplet diameters between 1 and 15 μm, birefringence values from 0.15 to 0.26 and trapping beam powers from 50 mW to 400 mW. Wave plate behavior is verified through the dependence of torque on droplet diameter and material birefringence. The dependence of the magnitude of the torque on material birefringence confirms the additional importance of the scattering mechanism. Absorption processes are found to be negligible.
Conformal sensor skin approach to the safety-monitoring of H2 fuel tanks
Authors: Dong Shuxiang , Cao Hu , Bai Feiming , Yan Li , Li J. F. , Viehland D. , Gao Yongkang
Published: May 2004

A conformal sensor skin approach has been developed for safety monitoring of H2 fuel tanks. Small piezoelectrically driven sound resonance cavities were embedded in a porous polymer. When placed on a structural composite plate, it was found feasible to detect the leakage of small concentrations of H2 in real time.
Electrically injected InGaAs/GaAs quantum-dot microcavity light-emitting diode operating at 1.3 μm and grown by metalorganic chemical vapor deposition
Authors: Tasco V. , Todaro M. T. , De Vittorio M. , De Giorgi M. , Cingolani R. , Passaseo A. , Ratajczak J. , Katcki J. W.
Published: May 2004

We present a quantum-dot microcavity light-emitting diode emitting at 1.3 μm at room temperature. The long wavelength emission is achieved by using InGaAs quantum dots directly grown on GaAs, by metalorganic chemical vapor deposition. The device exhibits electroluminescence bright emission, peaked at 1298 nm and with a full width at half maximum of 6.5 meV.
Doping effects on the valence band of Tl2Mn2O7 pyrochlores: Relation to magnetoresistance
Authors: Sánchez-Benı́tez J. , de Andrés A. , Prieto C. , Ávila J. , Martı́n-Carrón L. , Martı́nez J. L. , Alonso J. A. , Martı́nez-Lope M. J. , Casais M. T.
Published: May 2004

Photoemission and x-ray absorption near edge spectroscopy measurements have been performed in Tl2Mn2O7 oxides doped with 10% different ions, Bi, Cd, or Sb, which produce colossal changes in the magnetoresistance values. The contributions to the valence band related to Mn, O, and the doping ions have been obtained. We found that the paramagnetic phase of doped Tl pyrochlores is charge-transfer insulator type with oxygen character of the upper edge of the valence band. Bi 6s and Cd 4d orbitals lie also at the upper edge of the valence band. Mn valence is identical for all samples while oxygen content varies to compensate for the charge introduced by doping. The density of carriers, which is correlated to the magnetoresistance values, is determined by the density of states near the Fermi level provided by Tl and O content.
Efficient excitation transfer from polymer to nanocrystals
Authors: Chang Tung-Wah Frederick , Musikhin Sergei , Bakueva Ludmila , Levina Larissa , Hines Margaret A. , Cyr Paul W. , Sargent Edward H.
Published: May 2004

We quantify experimentally the efficiency of excitation transfer from a semiconducting polymer matrix to quantum dot nanocrystals. We study 5±0.5 nm PbS nanocrystals embedded in MEH-PPV (poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)]) polymer. We determine the excitation transfer efficiency from normalized photoluminescence excitation measurements. When the composites are made using as-synthesized PbS nanocrystals capped by oleate ligands, the excitation transfer efficiency is about 20%. Replacing these ligands with shorter chains results in a factor-of-3 enhancement in the excitation transfer efficiency. Our findings provide guidance to the realization of efficient electroluminescent devices.
Temperature stability of the refractive index and the direct bandedge in TlInGaAs quaternary alloys
Authors: Imada A. , Lee H.-J. , Fujiwara A. , Mukai T. , Hasegawa S. , Asahi H.
Published: May 2004

TlInGaAs quaternary alloy layers were grown on InP substrates by gas-source molecular-beam epitaxy. Refractive index dispersions were determined at the temperature range of 300–340 K in the photon-energy region below and a little above the direct bandedge E0 by the optical reflectance measurements. The temperature dependence of the refractive index was analyzed with the first-order Sellmeier equation. The temperature dependence of the E0 edge was also determined by the absorption measurements. It was found that the temperature coefficients of both refractive index and E0 edge of TlInGaAs are much smaller than those for InGaAs. These results facilitate the fabrication of the temperature-stable-wavelength optoelectronic devices using this alloy system.
Unpinning of Fermi level in nanocrystalline semiconductors
Authors: Malagù Cesare , Guidi Vincenzo , Carotta Maria Cristina , Martinelli Giuliano
Published: May 2004

A theoretical model has been developed to interpret the size dependent behavior of nanostructured metal-oxide semiconductors. It is based on the determination of the surface-state density, which pins the Fermi level of the semiconductor, thus removing the linear relationship between the work function and the Schottky barrier. To provide the model with numerical input, measurements of the Schottky barrier height were performed at different temperatures on nanocrystalline SnO2 and TiO2 films. The obtained solution predicts the unpinning of the Fermi level when the material can be considered as nanostructured, and the evidence confirms it.
Reduced dielectric dispersion in ferroelectric (Pb,La)TiO3/(Pb,Ca)TiO3 thin-film multilayer heterostructures due to a mechanical stress relaxation mechanism
Authors: Poyato Rosalı́a , Calzada M. Lourdes , Pardo Lorena
Published: May 2004

The dielectric dispersion of (Pb,Ca)TiO3/(Pb,La)TiO3/(Pb,Ca)TiO3 and (Pb,La)TiO3/(Pb,Ca)TiO3/(Pb,La)TiO3 ferroelectric thin-film multilayer heterostructures onto Si-based substrates has been studied and compared with that of identically prepared (Pb,La)TiO3 and (Pb,Ca)TiO3 films. Grazing incidence x-ray diffraction analysis reveals that the tetragonal distortion of the heterostructures is higher than the ones for the single-component films, which evidences a mechanical stress relaxation mechanism in the heterostructures. A lower dielectric dispersion has been obtained in the heterostructures with respect to the single-component films. This is also a consequence of the stress relaxation by plastic deformation through vacancy diffusion present in the former. Vacancy diffusion decreases the number of VPb–VO defect-dipoles, which could contribute to the dielectric permittivity at low frequencies in the heterostructures.
Lasing mode pattern of a quantum cascade photonic crystal surface-emitting microcavity laser
Authors: Srinivasan Kartik , Painter Oskar , Colombelli Raffaele , Gmachl Claire , Tennant Donald M. , Sergent A. Michael , Sivco Deborah L. , Cho Alfred Y. , Troccoli Mariano , Capasso Federico
Published: May 2004

The identification of the lasing mode within a quantum cascade photonic crystal microcavity laser emitting at λ∼8 μm is presented. The symmetry of the lasing mode is determined by the position of nodal lines within micro-bolometer camera measurements of its polarized spatial distribution. Full three-dimensional finite-difference time-domain simulations are also performed, and the resulting vertically emitted radiation field pattern is seen to follow the experimental results closely.
Control of surface roughness in amorphous thin-film growth
Authors: Elsholz F. , Schöll E. , Rosenfeld A.
Published: May 2004

The growth of optical layers of SiO2 and Nb2O5 on amorphous substrates is investigated. We develop a kinetic Monte Carlo model which mimics the amorphous structure by randomly fluctuating binding energies. The resulting surface profiles are characterized by their root-mean-square roughness, height–height correlation functions, and growth exponents. For strong random fluctuations, the growth exponents exceed the value of 0.5, in good agreement with experiment.
Spatial distribution of the red luminescence in pristine, γ rays and ultraviolet-irradiated multimode optical fibers
Authors: Girard S. , Meunier J.-P. , Ouerdane Y. , Boukenter A. , Vincent B. , Boudrioua A.
Published: May 2004

In this letter, a confocal microscopy setup was used to evaluate, with a resolution of 2 μm, the nonuniform spatial distributions of the red photoluminescence (640–750 nm) in pristine, γ rays (∼1 MeV, 1.2 kGy, and 0.33 Gy/s) and ultraviolet (244 nm and 127 J/cm2) irradiated multimode optical fibers. In pristine samples, the Raman scattering is predominant and the emitting centers are only present at low concentration in the fiber cladding. However, these centers are generated by both irradiations in the whole fiber cross sections, in particular near the core–cladding interface. The spectral deconvolution of the luminescence spectra showed that the Non-Bridging Oxygen Hole Centers are mainly responsible for this red luminescence. For both irradiation types, these centers seem to be predominantly created from precursor sites: strained Si–O–Si bonds which could be induced during the fiber drawing process.
Organic small molecule solar cells with a homogeneously mixed copper phthalocyanine: C60 active layer
Authors: Uchida Soichi , Xue Jiangeng , Rand Barry P. , Forrest Stephen R.
Published: May 2004

An efficient organic solar cell with a vacuum codeposited donor–acceptor copper phthalocyanine (CuPc):C60 mixed layer is described. A device with a structure of indium tin oxide/330 Å CuPc:C60(1:1)/100 Å C60/75 Å 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/Ag has a series resistance of only RS=0.25 Ω cm2, resulting in a current density of ∼1 A/cm2 at a forward bias of +1 V, and a rectification ratio of 106 at ±1 V. Under simulated solar illumination, the short circuit current density increases linearly with light intensity up to 2.4 suns. The maximum power conversion efficiency is ηP=(3.6±0.2)% at 0.3 suns (AM1.5G simulated solar spectrum) and ηP=(3.5±0.2)% at 1 sun. Although the fill factor decreases with increasing intensity, a power efficiency as high as ηP=(3.3±0.2)% is observed at 2.4 suns intensity.
First-principles study of magnetism in (112̄0) Zn1−xMnxO thin film
Authors: Wang Q. , Jena P.
Published: May 2004

First-principles calculations of total energies and magnetism of Zn1−xMnxO thin film are performed by simulating it with a slab consisting of seven layers along (112̄0). It is shown that a single Mn atom shows very little preference for the site it occupies. This is consistent with the experimental finding that Mn atoms are homogeneously distributed in ZnO films. As the concentration of Mn atoms increases, antiferromagnetic coupling between Mn atoms becomes more favorable, and there is a tendency for Mn atoms to form clusters around oxygen, in agreement with recent experiments.
1.5-μ
m
-wavelength light guiding in waveguides in square-lattice-of-rod photonic crystal slab
Authors: Tokushima Masatoshi , Yamada Hirohito , Arakawa Yasuhiko
Published: May 2004

We have experimentally demonstrated the guiding of light at a 1.5 μm wavelength in straight and 90°-bent line-defect waveguides in two-dimensional square-lattice-of-rods photonic crystal slabs. The light was guided by being confined in a row of the rods that were thinner than the surrounding ones. A new structural design to greatly facilitate their fabrication process without degrading the guiding property was used. The propagation loss measured for a 1.8-mm-straight waveguide was 4.8 dB/mm, which is small enough to allow us to proceed to integrated optical circuit application.
All-optical polymeric interferometeric wavelength converter comprising an excited state intramolecular proton transfer dye
Authors: Kang Jae-Wook , Kim Sehoon , Park Soo Young , Kim Jang-Joo
Published: May 2004

We designed and demonstrated an all-optical wavelength converter using a polymeric Mach–Zehnder interferometer (MZI) comprised of an excited state intramolecular proton transfer (ESIPT) dye, 2,2′-{oxybis[4-(4-methoxyphenyl)quinoline-6,2-diyl]}bis(5-methoxyphenol) (MQ). This MZI wavelength converter is composed of the MQ dye-doped polymeric waveguide and a thick light blocking metal film. A feature of this device is that one arm of the MZI can be irradiated by 355 nm pulses (signal beam), while the other arm was not, thus allowing a differential phase shift in the submicrosecond time scale. Because of the refractive index change of the ESIPT dye in one arm of interferometer upon irradiation with the signal beam, phase modulation of the continuous-wave probe light propagating in the irradiated arm of the MZI takes place, leading to the intensity modulation at the output defined by the signal beam, resulting in an all-optical wavelength converter, that is, the conversion of the signal modulation to output signal modulation of the probe light of the MZI. The characteristics of the wavelength converter are well described by a simple kinetic model.
Effects of stacking on the structural and optical properties of self-organized GaN/AlN quantum dots
Authors: Gogneau N. , Fossard F. , Monroy E. , Monnoye S. , Mank H. , Daudin B.
Published: May 2004

We report on the effect of vertical correlation on GaN/AlN quantum dots grown by plasma-assisted molecular-beam epitaxy using the modified Stranski–Krastanow growth mode. When increasing the number of GaN periods, we observe a homogenization of the island distribution and a redshift of the luminescence line. This redshift is attributed to an increase of the quantum Stark effect due to the increase of the piezoelectric contribution to the internal electric field.
Role of intericosahedral chains on the hardness of sputtered boron carbide films
Authors: Jacobsohn L. G. , Averitt R. D. , Wetteland C. J. , Schulze R. K. , Nastasi M. , Daemen L. L. , Jenei Z. , Asoka-Kumar P.
Published: May 2004

The relationship between the structure and mechanical properties of sputter-deposited boron carbide films was investigated. Changes in the structure induced by annealing were characterized in terms of chemical composition, chemical bonding, and concentrations of defects and trapped impurities. The creation of intericosahedral chains for higher annealing temperatures was revealed by infrared and Raman measurements, and the intensity of the infrared band at 1500 cm−1 was found to be related to the hardness. The presence of residual trapped Ar atoms and of open-volume defects is insensitive to relatively high annealing temperatures and does not influence the recovery of the hardness. Our results suggest postdeposition annealing as a pathway to enhance the mechanical properties of boron carbide films.
Anomalous magnetic properties and Hall effect in ferromagnetic Co2MnAl epilayers
Authors: Chen Y. J. , Basiaga D. , O’Brien J. R. , Heiman D.
Published: May 2004

Epitaxial films of the full Heusler alloy Co2MnAl were grown on GaAs substrates by molecular-beam epitaxy. The close lattice matching and similar structure to GaAs make this material a good candidate for a spin injector. Magnetization measurements reveal complex ferromagnetic transitions near the Curie point at TC≃800 K. At room temperature, magnetotransport studies show metallic behavior and a giant Hall effect driven by the extraordinary Hall effect.
Relationship between the Li ionic conduction and the local structures in LiyLa(1−y)/3NbO3
Authors: Nakayama Masanobu , Ikuta Hiromasa , Uchimoto Yoshiharu , Wakihara Masataka
Published: May 2004

In order to demonstrate the importance of local structural study for the lithium ionic conductor, we investigate the local structure of the Li+ conductor with perovskite structure, LiyLa(1−y)/3NbO3 (0⩽y⩽0.25) using the extended x-ray absorption fine structure (EXAFS) technique. The results of EXAFS measurement imply that changes of the local structure are more crucial for the property on ionic conductivity than that of the lattice parameter obtained by x-ray diffraction technique.