bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics simulations of aqueous NaCl and KCl solutions: Effects of ion concentration on the single-particle, pair, and collective dynamical properties of ions and water molecules
Authors: Chowdhuri Snehasis , Chandra Amalendu
Published: July 2002

We have performed a series of molecular dynamics simulations of aqueous NaCl and KCl solutions at different concentrations, ranging from 0 M to 4.5 M, to investigate the effects of ion concentration on the single-particle, pair, and collective dynamical properties of aqueous electrolyte solutions. The SPC/E model is used for water and the ions are modeled as charged Lennard-Jones particles. The single-particle dynamics is investigated by calculating the self-diffusion coefficients of ions and water molecules and also the orientational relaxation times. The pair dynamics is studied by evaluating the ion–water residence and water–water hydrogen bond time correlation functions. The relaxation of relative velocity autocorrelation function and the cross velocity correlation function of two hydrogen bonded water molecules are also investigated at varying ion concentration. Finally, we explore the collective dynamical properties by calculating the frequency dependent dielectric function and conductivity. It is found that the self and relative diffusion coeffcients decrease and the orientational relaxation times increase with ion concentration. The residence times of water molecules near ions and also the structural relaxation time of water–water hydrogen bonds show an increasing trend as the ion concentration is increased. The dielectric relaxation time is found to decrease with ion concentration for the solutions investigated here. The static conductivity of concentrated solutions shows significant departure from the Nernst–Einstein behavior due to formation of ion pairs. With an increase of frequency, the conductivity first increases substantially and then decreases at very high frequency. The initial increase of conductivity is attributed to the disruption of ion pairs on application of high frequency electric fields.
Plasma potential in the presence of an external electric field
Authors: Kurosawa Hitoshi , Hasegawa Shuichi , Suzuki Atsuyuki
Published: July 2002

We studied the mechanism and behavior of a plasma in the presence of an external electric field with a one-dimensional fluid model. We used a two-fluid model, treating both electron and ion motions, rather than a commonly used one-fluid model assuming the electron thermal equilibrium. In particular, we investigated the following issues: (1) the motion of plasma electrons caused by an external static electric field and the formation of a plasma potential; (2) influence of parameters on the plasma potential, which are plasma electron temperature, plasma density, and applied voltage. The plasma potential rises and reaches a value nearly equal to the anode potential in a few nanoseconds. It is higher for a higher anode potential, a higher electron temperature, and a lower plasma density.
Spectroscopic properties of CaF2:U4+ as a saturable absorber
Authors: Gruber John B. , Sardar Dhiraj K. , Merkle Larry D. , Zandi Bahram , Jarman Richard , Hutchinson J. Andrew
Published: July 2002

The spectroscopic properties of crystals of CaF2:U4+ have been examined in light of recent interest expressed for this material as a saturable absorber in the passive Q switching of near infrared solid state lasers. The detailed crystal-field splitting of the energy levels of U4+(5f2 ) is analyzed for the ion in different charge-compensated sites. Identification of the site is based on laser excited site-selective spectroscopic studies reported earlier. Experimental Stark levels are compared with calculated levels based on lattice-sum models that include charge compensation in the lattice structure. The final set of atomic parameters for U4+ is in good agreement with values reported for the ion in UF4. The predicted non-Kramers’ doublet as the ground-state Stark level for U4+ in C3V sites is in agreement with the spectroscopic analysis and with results reported earlier from electron spin-resonance studies of CaF2:U4+. Using pump–probe methods, we have investigated excited state absorption (ESA) at 1.54 μm. Consistent with the interpretation of the absorption spectra, we find that the measured ESA is quite small in these crystals. In turn, we use this conclusion to model CaF2:U4+ as a passive Q switch, with good results.
Numerical simulation of particle-laden turbulent channel flow
Authors: Li Yiming , McLaughlin J. B. , Kontomaris K. , Portela L.
Published: July 2002

This paper presents results for the behavior of particle-laden gases in a small Reynolds number vertical channel down flow. Results will be presented for the effects of particle feedback on the gas-phase turbulence and for the concentration profile of the particles. The effects of density ratio, mass loading, and particle inertia will be discussed. The results were obtained from a numerical simulation that included the effects of particle feedback on the gas phase and particle–particle collisions. The resolution of the simulation was comparable to the smallest scales in the particle-free flow, but the grid spacings were larger than the particle size. Particle mass loadings up to 2 and both elastic and inelastic collisions were considered. Particle feedback causes the turbulent intensities to become more anisotropic as the particle loading is increased. For small mass loadings, the particles cause an increase in the gas flow rate. It will be shown that the particles tend to increase the characteristic length scales of the fluctuations in the streamwise component of velocity and that this reduces the transfer of turbulent energy between the streamwise component of velocity and the components transverse to the flow. Particle–particle collisions greatly reduce the tendency of particles to accumulate at the wall for the range of mass loadings considered. This was true even when the collisions were inelastic.
Energy amplification in channel flows with stochastic excitation
Authors: Bamieh Bassam , Dahleh Mohammed
Published: July 2002

We investigate energy amplification in parallel channel flows, where background noise is modeled as stochastic excitation of the linearized Navier–Stokes equations. We show analytically that the energy of three-dimensional streamwise-constant disturbances achieves O(R3) amplification. Our basic technical tools are explicit analytical calculations of the traces of solutions of operator Lyapunov equations, which yield the covariance operators of the forced random velocity fields. The dependence of these quantities on both the Reynolds number and the spanwise wave number are explicitly computed. We show how the amplification mechanism is due to a coupling between wall-normal velocity and vorticity disturbances, which in turn is due to nonzero mean shear and disturbance spanwise variation. This mechanism is viewed as a consequence of the non-normality of the dynamical operator, and not necessarily due to the existence of near resonances or modes with algebraic growth.
Thermal evolution of the magnetization in nanocrystalline Fe particles investigated by electron holography
Authors: Bonetti E. , Del Bianco L. , Pasquini L. , Matteucci G. , Beeli C. , Signoretti S.
Published: July 2002

Micrometric, irregularly shaped Fe particles with a nanocrystalline structure have been prepared by mechanical attrition through ball-milling. Electron holography has been employed to visualize the stray field emerging from isolated Fe particles, both at 300 K and at selected temperatures T⩽1200 K, from which indirect information on the magnetic domain configuration has been inferred. By complementary x-ray diffraction and transmission electron microscopy investigations a relationship has been established between the changes of the leakage field and of the microstructure upon annealing: it indicates that the structural evolution is accompanied by strong modifications in the interior magnetization pattern. This relationship finds explanation in the framework of the random anisotropy model, including temperature-induced reversible variations in the exchange correlation length and saturation magnetization. Moreover, the role played by the overall geometrical features of the particles in the determination of the actual domain configuration has been investigated.
Factors responsible for the stability and the existence of a clean energy gap of a silicon nanocluster
Authors: Liu Lei , Jayanthi C. S. , Wu Shi-Yu
Published: July 2002

We present a critical theoretical study of electronic properties of silicon nanoclusters, in particular the roles played by symmetry, relaxation, and hydrogen passivation on the stability, the gap states and the energy gap of the system using the order N [O(N)] nonorthogonal tight-binding molecular dynamics and the local analysis of electronic structure. We find that for an unrelaxed cluster with its atoms occupying the regular tetrahedral network, the presence of undistorted local bonding configuration is sufficient for the appearance of a small clean energy gap. However, the energy gap of the unrelaxed cluster does not start at the highest occupied molecular orbital (HOMO). In fact, between the HOMO and the lower edge of the energy gap, localized dangling bond states are found. With hydrogen passivation, the localized dangling bond states are eliminated, resulting in a wider and clean energy gap. Relaxation of these hydrogen passivated clusters does not alter either the structure or the energy gap appreciably. However, if the silicon clusters are allowed to relax first, the majority of the dangling bonds are eliminated but additional defect states due to bond distortion appear, making the energy gap dirty. Hydrogen passivation of these relaxed clusters will further eliminate most of the remnant dangling bonds but no appreciable effect on the defect states associated with bond distortions will take place, thus still resulting in a dirty gap. For the hydrogen-passivated SiN nanoclusters with no bond distortion and no overall symmetry, we have studied the variation of the energy gap as a function of size of the cluster for N in the range of 80<N<6000. The dependence of the energy gap on the size shows similar behavior to that for silicon nanoclusters with no bond distortion but possessing overall symmetry.
Electrodeposition of compound semiconductors in polymer channels of 100 nm diameter
Authors: Engelhardt R. , Könenkamp R.
Published: July 2002

We report the growth of a transparent, p-type semiconductor, CuSCN, in nanometer sized channels of a thin polyester foil. High energy heavy ion irradiation and subsequent etching is used to prepare vertical cylindrical channels in polymers with diameters in the range 30–3000 nm. Electrodeposition is then used to fill these channels with polycrystalline CuSCN. The embedding is robust against mechanical stress on the foil. Since the bulk modulus of the inorganic CuSCN is larger than that of the polymeric matrix, only small forces act on the embedded semiconductor, resulting in low electrical changes upon bending or flexing of the foil. Dissolving the polyester matrix results in free-standing semiconductor columns with height-to-diameter ratios in excess of 100.
Initial formation and growth of an amorphous phase in Al–Pt thin films and multilayers: Role of diffusion
Authors: Gas P. , Labar J. , Clugnet G. , Kovacs A. , Bergman C. , Barna P.
Published: July 2002

Despite their technological importance for nanoscale technologies, the initial stages of reaction at interfaces (and their dependence on experimental conditions) are still poorly understood. In this article we analyze the initial stages of solid state reaction (SSR) in Al/Pt multilayers (period: 54 nm, overall composition: Al4Pt) and compare the results to those recently obtained by high temperature sequential deposition (HTSD). These two methods differ in several aspects, the most important being the state of reacting Pt (solid in SSR, vapor in HTSD) and its flux (limited in HTSD by the deposition rate). The chemical driving force for the Al/Pt reaction are thus different. We have shown that: (i) The first reaction product observed during SSR or HTSD is the same: namely, a metastable amorphous Al2Pt phase (a-Al2Pt): (ii) This amorphous phase grows in a layer by layer mode. Its growth is diffusion controlled during SSR and stops when all the available Pt is consumed. In the case of HTSD the layer growth is limited by the Pt evaporation rate and diffusion only controls the “critical” thickness that the amorphous layer can reach. (iii) The Al diffusion coefficients in a-Al2Pt deduced from the a-Al2Pt growth rate (SSR) or “critical” thickness (HTSD), using the Nernst–Einstein equation in order to take into account the difference in the chemical driving forces, are similar.
Effect of membrane viscosity on the dynamic response of an axisymmetric capsule
Authors: Diaz A. , Barthès-Biesel D. , Pelekasis N.
Published: July 2002

The role of membrane surface viscosity is investigated in the transient response of an axisymmetric capsule following a sudden start of external flow. Results obtained in a recent study for capsules with a hyperelastic membrane [Phys. Fluids 12, 948 (2000)] are extended to account for membrane viscosity. The common feature between hyperelastic and viscoelastic membranes is the time evolution of capsule deformation which can be fitted to an exponential function and thus characterized with only two parameters, the deformation at steady state D∞ and the response time τs. Membrane and bulk viscosity have quasilinear and independent effects on the transient response.
N-type negative differential conductance in quasiballistic single-barrier heterostructures
Authors: Aleshkin V. Ya. , Reggiani L. , Reklaitis A.
Published: July 2002

We propose a mechanism for negative differential conductance in nanometric single barrier heterostructures when transport is controlled by tunneling processes and space charge effects. Theoretical expectations are confirmed by Monte Carlo simulations of a model GaAs–GaAlAs–GaAs semiconductor heterostructure.
Chemical order influence on the phase transition in the relaxor Pb(Sc1/2Nb1/2)O3
Authors: Bidault O. , Perrin C. , Caranoni C. , Menguy N.
Published: July 2002

The transitions from a relaxor state to a rhombohedral ferroelectric phase are studied in different Pb(Sc1/2Nb1/2)O3 ceramics characterized by varied chemical ordering on the B site of the perovskite structure. The transition may arise at different temperatures, eventually in two stages in the same sample. The dielectric response reflects the density of polar domains and changes from relaxorlike (numerous polar nanodomains in the nearly fully ordered sample) to the relaxor-normal ferroelectric transition (in the disordered sample). The chemical order degree, as well as the spatial inhomogeneity of this order have to be taken into account to understand this behavior.
Conductivity and photoconductivity in boron doped diamond films: Microwave measurements
Authors: Gevrey Frédéric , Gire Alain , Gaudiot David , Théobald Jean-Gérard , Gheeraert Etienne , Bernard Matthieu , Torrealba-Anzola Freddy
Published: July 2002

We used the microwave technique to measure electrical properties of heavily boron doped diamond films. This technique overcomes problems such as the presence of parasitic resistance due to the electrical contacts. The conductivity of these films is rather high (about 104 S m−1). The measured microwave conductivity has the same values as the dc conductivity. The conductivity varies with the doping. The highest value of conductivity does not appear for the highest value of doping, because of the presence of a parasitic phase. This will be proved by electron spin resonance and Raman spectra. Theoretical calculation rules out the skin effect in our measurements. It will be also possible to perform microwave photoconductivity measurements. The photoconductivity behavior varies in the opposite way to the conductivity. One explanation, based on the recombination time which decreases with the conductivity, will be presented.
Existence and atomic arrangement of the CuPt-type ordered structure near the ZnTe/GaAs heterointerface due to residual impurities
Authors: Lee H. S. , Lee J. Y. , Kim T. W. , Lee D. U. , Choo D. C. , Park H. L.
Published: July 2002

Transmission electron microscopy (TEM) and selected area electron diffraction pattern (SADP) measurements were carried out to investigate the ordered structures near ZnTe/GaAs heterointerfaces, and Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) measurements were performed to determine the compositions of the ZnTe/GaAs interfacial layer. The SADP showed two sets of {12 12 12} extra spots with symmetrical intensity, and the corresponding high-resolution TEM image showed doublet periodicity in contrast of the {111} lattice planes. The results of the SADP and the high-resolution TEM measurements showed that a CuPt-type ordered (Cd, Zn)Te structure was observed near the ZnTe/GaAs heterointerface, and the AES and SIMS results showed that the ordered structure was formed due to the diffusion of Cd atoms into the ZnTe layer. Two variants, one for each direction of the doublet periodicity on the {111} lattice, were observed in the ordering, and each variant had its own domain structure with a similar probability. The formation of the CuPt-type ordered structure near the ZnTe/GaAs heterointerface originated from both the existence of the Cd residual impurities during the initial growth stage of the ZnTe epilayer and the strain relaxation of the ZnTe epilayer. These results can help to improve the understanding of the microstructural properties of the ZnTe/GaAs heterointerface.
Strong localization of Bloch photons in dual-periodic dielectric multilayer structures
Authors: Shimada Ryoko , Koda Takao , Ueta Tsuyoshi , Ohtaka Kazuo
Published: July 2002

We predict that singular photonic Bloch modes with extremely narrow bandwidths occur in dual-periodic dielectric multilayer structures. The electromagnetic field of these modes is shown to be strongly localized around the troughs of undulated periodic dielectric structures, which leads to an extraordinary high-Q value effect in the dual-periodic one-dimensional photonic crystals. We also propose several real systems suitable for experimental confirmation of the features predicted.
Switchable yttrium–hydride mirrors grown on CaF2(111): A x-ray photoelectron spectroscopy and diffraction study
Authors: Hayoz J. , Schoenes J. , Schlapbach L. , Aebi P.
Published: July 2002

The epitaxial growth of Y (hydride) films on CaF2(111) has been investigated using x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, and low energy electron diffraction (LEED). For Y deposition at 700 °C the formation of high-quality epitaxial hcp(0001) oriented Y films is observed. Whenever the Y films showed good surface quality, the surface is rich in F. Only when the deposition temperature was chosen so low that the LEED reflexes were very broad was no fluorine detected. This is a strong indication that F acts as a surfactant. For Y deposition at room temperature under a H2 partial pressure of 5×10−6 mbar we observe the formation of a F-free, transparent YH2.3 film of a red/yellow color and poor crystallinity. Hydrogen unloading is accomplished by annealing to 600 °C. The film loses its transparency, the poorly ordered fcc(111) lattice converts to a well-ordered hcp(0001) lattice, and F contamination is restored.
Another regime of operation for a 18.2 nm recombination laser using a capillary-discharged carbon plasma
Authors: Lee K. , Kim D.
Published: July 2002

Another regime of operation for the significant amplification of C VI Hα radiation in a capillary-discharged carbon plasma is proposed. This suggested regime of operation features the existence of gain at the axis of a capillary, the large gain region (several hundred micrometers in diameter), and the hollow electron density profile for the better guiding of the radiation being amplified. All these features help to overcome the previous problems and favor the high amplification of the radiation.
Scattering of surface waves by a semi-infinite floating elastic plate
Authors: Sahoo T. , Yip T. L. , Chwang Allen T.
Published: July 2002

A new inner product is developed based on the Fourier analysis to study the scattering of surface waves by a floating semi-infinite elastic plate in a two-dimensional water domain of finite depth. The eigenfunctions for the plate-covered region are orthogonal with respect to this new inner product. The problem is studied for various wave and geometrical conditions. Especially, the influence of different edge conditions on the hydrodynamic behavior is investigated and compared. The edge conditions considered in the present study involve (i) a free edge, (ii) a simply supported edge, and (iii) a built-in edge. The hydrodynamic performance of an elastic plate is characterized for various conditions in terms of wave reflection and transmission, plate deflection, and surface strain. It is observed that the hydrodynamic behavior depends on the wave conditions, the geometrical settings, and the edge conditions. The built-in edge condition induces the maximum wave reflection and the minimum wave transmission. The free edge condition leads to the maximum plate deflection.
Characteristics of electron-beam-gun-evaporated Er2O3 thin films as gate dielectrics for silicon
Authors: Mikhelashvili V. , Eisenstein G. , Edelmann F.
Published: July 2002

Structural properties of an ultrathin, 4.5 nm, erbium-oxide film and electrical properties of metal–oxide–semiconductor structure based on it are described. The evolution of the dielectric constant, total charge density, breakdown electric field, and leakage current density with annealing temperature in an oxygen environment are reported. The dielectric constant in the as-deposited state is relatively low, ∼7, possibly because the initial deposition forms ErO (with low polarizibility) rather than Er2O3. Annealing causes a transformation of ErO to Er2O3 but at the same time it initiates the growth of an interfacial SiO2 layer so that the effective dielectric constant is reduced to 5.5. Using the 4.5 nm film following annealing at up to 750 °C, we demonstrate an effective oxide thickness in the range 2.4–3.2 nm, with a leakage current density as low as 1–2×10−8 A/cm2 at an electric field of 106 V/cm and a breakdown electric field of 0.8–1.7×107 V/cm. A shift of the flat band voltage to the positive side and lowering of the total positive charge density down to 1012 cm−2 with annealing temperature are observed and can be explained by a charge compensation mechanism between the charges accumulated at the SiO2/Er2O3 and Si/SiO2 interface.
Experimental studies of helium droplets
Authors: Northby J. A.
Published: July 2002

In this report I will review experimental studies of free helium droplets, with the exception of spectroscopic studies of helium droplets that contain impurities. This particular topic, as well as theoretical studies of helium droplets, will be reviewed separately elsewhere in this issue.
Interactions of excited activators in rare earth and transition metal doped phosphors and their role in low energy cathodoluminescence
Authors: Seager C. H. , Tallant D. R.
Published: July 2002

We have measured the time decay of spectrally resolved, pulsed cathodoluminescence (CL) and photoluminescence (PL) in several phosphors activated by rare earth and transition metal impurities, including Y2O3:Eu, Y2SiO5:Tb, and Zn2SiO4:Mn. Activator concentrations ranged from ∼0.003% to 10%. The CL decay curves are almost always nonlinear on a log (CL)-linear(time) plot—i.e., they deviate from first-order decay kinetics. These deviations are always more pronounced at short times and larger activator concentrations and are largest at low beam energies where the decay rates are noticeably faster. PL decay is always slower than that seen for CL, but these differences disappear after most of the excited species have decayed. PL decays display higher order decay kinetics and resemble CL decay profiles when the laser excitation intensity is increased. We have also measured the dependence of steady state CL efficiency on beam energy and find that larger activator concentrations accelerate the drop in CL efficiency seen at low beam energies. These effects are largest for the activators which interact more strongly with the host lattice. While activator–activator interactions are known to limit PL and CL efficiency in most phosphors, the present data suggest that a more insidious version of this mechanism is partly responsible for poor CL efficiency at low beam energies. This “enhanced” concentration quenching is due to the interaction of nearby excited activators. These interactions can lead to nonradiative activator decay, hence lower steady state CL efficiency. We suggest that this excited state “clustering” arises from the large energy loss rate of low energy primary electrons.
Imaging three-dimensional polarization in epitaxial polydomain ferroelectric thin films
Authors: Ganpule C. S. , Nagarajan V. , Hill B. K. , Roytburd A. L. , Williams E. D. , Ramesh R. , Alpay S. P. , Roelofs A. , Waser R. , Eng L. M.
Published: July 2002

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90° domain wall is observed. Nucleation of new reversed 180° domains at the 90° domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time.
Border traps in 6H-SiC metal–oxide–semiconductor capacitors investigated by the thermally-stimulated current technique
Authors: Ólafsson H. Ö. , Sveinbjörnsson E. Ö. , Rudenko T. E. , Tyagulski I. P. , Osiyuk I. N. , Lysenko V. S.
Published: July 2002

We demonstrate the usefulness of the thermally-stimulated current (TSC) technique for investigating shallow interface state defects in silicon carbide metal–oxide–semiconductor (MOS) structures. For dry oxides, low-temperature TSC measurements reveal a high density of near-interfacial oxide traps (border traps) close to the band edges of 6H–SiC. Furthermore we find that annealing the SiC/SiO2 interface in pyrogenic steam at 950 °C (reoxidation) essentially reduces the density of deep interface states, while it increases the density of shallow states. Our results agree with observations of the appearance of a negative oxide charge in reoxidized MOS capacitors and the corresponding increase of the threshold voltage in n channel metal–oxide–semiconductor field-effect transistors.
Fast organic electronic circuits based on ambipolar pentacene field-effect transistors
Authors: Schön Jan Hendrik , Kloc Christian
Published: July 2002

Inverter circuits and ring oscillators have been prepared based on the organic semiconductor pentacene. High-purity pentacene single crystals and thin films exhibit ambipolar transport properties, i.e., n- and p-channel transistor activity. This feature is very attractive for complementary logic electronic circuits, since only one organic material has to be used. Switching frequencies up to 11 MHz have been achieved for single crystal circuits. Thin film devices prepared on flexible polyimide substrates revealed delays of 0.1 μs per stage resulting in oscillation frequencies up to 900 kHz.

At the request of the authors and editor, this article is being retracted effective 24 February 2003.
Optically pumped polarized H− ion source for RHIC spin physics
Authors: Zelenski A. , Alessi J. , Briscoe B. , Dutto G. , Huang H. , Kponou A. , Kokhanovski S. , Klenov V. , Lehrach A. , Levy P. , LoDestro V. , Mori Y. , Okamura M. , Raparia D. , Ritter J. , Takeuchi T. , Wight G. , Zoubets V.
Published: July 2002

A new optically pumped polarized H− ion source (OPPIS) was developed for the RHIC polarization program and successfully used for the first polarized beam commissioning at RHIC. The OPPIS produces in excess of 1.0 mA H− ion current at about 80% polarization. An ECR primary proton source development and a new 29 GHz microwave power supply are described. A new type sodium-jet ionizer cell is biased to −32 kV to produce a 35 keV polarized beam ready for injection to the RFQ. Higher current and higher polarization were also obtained with the biased jet-cell in comparison with an old oven-type ionizer cell.
Bouncing motion of spherical particles in fluids
Authors: Gondret P. , Lance M. , Petit L.
Published: July 2002

We investigate experimentally the bouncing motion of solid spheres onto a solid plate in an ambient fluid which is either a gas or a liquid. In particular, we measure the coefficient of restitution e as a function of the Stokes number, St, ratio of the particle inertia to the viscous forces. The coefficient e is zero at small St, increases monotonically with St above the critical value Stc and reaches an asymptotic value at high St corresponding to the classical “dry” value emax measured in air or vacuum. This behavior is observed for a large range of materials and a master curve e/emax=f(St) is obtained. If gravity is sufficient to describe the rebound trajectory (after the collision) in a gas, this is not the case in a liquid where drag and added-mass effect are important but not sufficient: History forces are shown to be non-negligible even at large Reynolds number.
Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates
Authors: Sheraw C. D. , Zhou L. , Huang J. R. , Gundlach D. J. , Jackson T. N. , Kane M. G. , Hill I. G. , Hammond M. S. , Campi J. , Greening B. K. , Francl J. , West J.
Published: July 2002

We have fabricated organic thin-film transistor (OTFT)-driven active matrix liquid crystal displays on flexible polymeric substrates. These small displays have 16×16 pixel polymer-dispersed liquid crystal arrays addressed by pentacene active layer OTFTs. The displays were fabricated using a low-temperature process (<110 °C) on flexible polyethylene naphthalate film and are operated as reflective active matrix displays.
Structural characterization of cup-stacked-type nanofibers with an entirely hollow core
Authors: Endo M. , Kim Y. A. , Hayashi T. , Fukai Y. , Oshida K. , Terrones M. , Yanagisawa T. , Higaki S. , Dresselhaus M. S.
Published: July 2002

Straight long carbon nanofibers with a large hollow core obtained by a floating reactant method show a stacking morphology of truncated conical graphene layers, which in turn exhibit a large portion of open edges on the outer surface and also in the inner channels. Through a judicious choice of oxidation conditions, nanofibers with increased active edge sites are obtained without disrupting the fiber’s morphology. A graphitization process induces a morphological change from a tubular type to a reversing saw-toothed type and the formation of loops along the inner channel of the nanofibers, accompanied by a decrease in interlayer spacing.
Empirical mapping of Ni–Mn–Ga properties with composition and valence electron concentration
Authors: Jin X. , Marioni M. , Bono D. , Allen S. M. , O’Handley R. C. , Hsu T. Y.
Published: July 2002

A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K <Tmart<TC, has been explored only at the margins.
Stress level in Finemet materials studied by impedanciometry
Authors: Carara Marcos , Baibich M. N. , Sommer R. L.
Published: July 2002

In this work, a study of the stress relief in Finemet ribbons, Fe73.5Cu1Nb3Si16.5B6, as a function of the annealing temperature is presented. The as melt-spun samples are amorphous and become partially crystallized after annealing at appropriate temperatures. For temperatures TA⩾480 °C the samples are nanocrystalline, with a microstructure composed by α-Fe1−xSix (x∼0.2) crystallites (10 nm average diameter) embedded in an amorphous matrix. Nanocrystallization, associated with stress relief effects, improves the soft magnetic properties of this kind of material. The stress level was quantified using magnetorestriction (measured by SAMR), magnetoelastic anisotropy, and domain wall energy data obtained from impedance spectra measurements. A reduction of the internal stress from 15 to 0.2 MPa was verified when comparing the as-cast to the sample annealed at 580 °C. Improvement of the magnetic softness of the samples was also followed by the increase of the domain wall and magnetization rotation contributions to the overall effective permeability.
The pure rotational spectrum of NaC in its X 4Σ− state: Observation and interpretation
Authors: Sheridan P. M. , Xin J. , Ziurys L. M. , Beaton Sara A. , Kermode Stephen M. , Brown John M.
Published: July 2002

The rotational spectrum of the NaC radical in the v=0 level of its X 4Σ− state has been recorded using millimeter/submillimeter-wave absorption spectroscopy. This work is the first spectroscopic observation of this species. The molecule was generated under dc discharge conditions by the reaction of sodium vapor with methane; the vapor was generated in a Broida oven. Thirteen rotational transitions were measured in the frequency range from 231 (N=9←8) to 539 GHz (N=21←20). The quartet fine structure was partially resolved for each transition. The rotational transition frequencies were modeled with an effective Hamiltonian. An ambiguity in the fitting procedure produced two alternative values of the spin–spin coupling constant λ, one large and the other small in magnitude. A simple theoretical estimate of this parameter, based on atomic orbitals on the C− atom, suggests that the latter fit is the correct solution to the problem. The same ambiguity occurs in the fit of the rotational spectrum of KC in its X 4Σ− state published earlier [Xin and Ziurys, J. Chem. Phys. 110, 4797 (1999)]. The revised, preferred fit of the data is given in this paper.
Study of terahertz radiation from InAs and InSb
Authors: Gu Ping , Tani Masahiko , Kono Shunsuke , Sakai Kiyomi , Zhang X.-C.
Published: July 2002

Terahertz radiation from InSb and InAS, which are typical narrow band-gap semiconductors, was investigated using time-resolved THz emission measurements. When we compared between the polarity of the THz waveforms of these narrow band-gap semiconductors with that of InP, which is a wide bandgap semiconductor, we concluded that the ultrafast buildup of the photo-Dember field is the main mechanism for the emission of THz radiation in both InAs and InSb. The emission efficiency of InSb is approximately one-hundredth of that of InAs, although the electron mobility in InSb is higher than in InAs. Wavelength-dependent measurements implied that the anomalously low THz emission efficiency of InSb might be due to a reduction in transient mobility resulting from the scattering of electrons into the low-mobility L valley.
Transient photovoltage and dielectric relaxation time in porous silicon
Authors: Duzhko V. , Koch F. , Dittrich Th.
Published: July 2002

Transient photovoltage (PV) measurements have been carried out on free-standing porous silicon of different porosity. For porous silicon of low porosity, fast charge-carrier separation takes place in the surface space-charge region. A retarded component arises in the PV transients due to limitation of the separation of charge carriers by interparticle transport. For nanoporous silicon, the fast component is absent and the PV transients are retarded strongly in time. The maximum of the retarded PV transients is correlated with the dielectric relaxation time which was obtained by using dc conductivity measurements. This opens opportunities to a contactless electrical characterization of porous semiconductors and its application.
Quantum control of molecular handedness in a randomly oriented racemic mixture using three polarization components of electric fields
Authors: Hoki Kunihito , González Leticia , Fujimura Yuichi
Published: July 2002

A new laser control scenario is presented for obtaining substantial amounts of enantiomeric enrichment from a randomly oriented racemic mixture. This is carried out by using three polarization components of electric fields; one is used for orientation, the other two for controlling the chirality. The effectiveness is demonstrated by numerical simulations on the enantiomeric enrichment of the axial chiral H2POSH molecule.
Effects of rapid thermal annealing on the ferromagnetic properties of sputtered Zn1−x(Co0.5Fe0.5)xO thin films
Authors: Cho Young Mok , Choo Woong Kil , Kim Hyojin , Kim Dojin , Ihm YoungEon
Published: July 2002

We have investigated the effects of rapid thermal annealing under vacuum on the CoFe-doped ZnO [Zn1−x(Co0.5Fe0.5)xO] films grown by reactive magnetron co-sputtering. At least up to x=0.15, the films have the single phase of the same wurtzite structure as pure ZnO. Ferromagnetism was observed for the CoFe-doped ZnO films. We found that rapid thermal annealing leads to a remarkable increase in the spontaneous magnetization of the CoFe-doped ZnO as well as the electron concentration. The annealing also leads to a significant increase in the Curie temperature (TC), resulting in room temperature ferromagnetism with TC>300 K for the CoFe-doped ZnO films.
Reduction of reverse-bias leakage current in Schottky diodes on GaN grown by molecular-beam epitaxy using surface modification with an atomic force microscope
Authors: Miller E. J. , Schaadt D. M. , Yu E. T. , Poblenz C. , Elsass C. , Speck J. S.
Published: July 2002

The characteristics of dislocation-related leakage current paths in an AlGaN/GaN heterostructure grown by molecular-beam epitaxy and their mitigation by local surface modification have been investigated using conductive atomic force microscopy. When a voltage is applied between the tip in an atomic force microscope (AFM) and the sample, a thin insulating layer is formed in the vicinity of the leakage paths where current is observed. As the insulating layer reaches a thickness of 2–3 nm, the leakage current is blocked and subsequent growth of the layer is prevented. Although conductive screw or mixed dislocations are observed, dislocations with a screw component that do not conduct current are also apparent. The reverse-bias leakage current is reduced by a factor of two in a large-area diode fabricated on an area modified in this manner with an AFM compared to typical diodes fabricated on unmodified areas with comparable series resistances, confirming that dislocation-related leakage current paths are a major component of the reverse-bias leakage current in Schottky diodes fabricated on nitride material.
Molecular organic light-emitting diodes using highly conducting polymers as anodes
Authors: Kim W. H. , Mäkinen A. J. , Nikolov N. , Shashidhar R. , Kim H. , Kafafi Z. H.
Published: July 2002

Films fabricated from commercially available poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) aqueous dispersions have been widely used in many electronic and optoelectronic applications. Previous attempts to utilize them as anodes in organic light-emitting diodes (OLEDs) were not satisfactory due to their low conductivity. In this letter we report on the fabrication and characterization of an OLED device made using a highly conductive form of PEDOT:PSS as anode and demonstrate its superior performance relative to that of a similar device using the commercial conducting polymer as an anode. An external electroluminescence quantum efficiency of ∼0.73% was measured at 100 A/m2.
Luminescence enhancement of core-shell ZnS:Mn/ZnS nanoparticles
Authors: Cao Lixin , Zhang Jiahua , Ren Shanling , Huang Shihua
Published: July 2002

Mn 2+ -doped ZnS nanoparticles and Mn2+-doped ZnS nanoparticles coated with a shell of ZnS were prepared in reverse micelles. Mn2+ emission at 580 nm in the coated nanoparticles was seven times stronger than that in the uncoated ones. Ultraviolet light treatment enhanced the luminescence at 580 nm in the uncoated nanoparticles, but led to no luminescence enhancement in the coated ones. Some calculations indicated that Mn2+ ions were not randomly distributed in the particle, but preferred to occupy the sites close to the surface. The experiment results indicated that ZnS shell and UV treatment could block the nonradiative transition paths and led to the enhancement of luminescence.
Piezoresponse force microscopy for polarity imaging of GaN
Authors: Rodriguez B. J. , Gruverman A. , Kingon A. I. , Nemanich R. J. , Ambacher O.
Published: July 2002

The polarity distribution of GaN based lateral polarity heterostructures is investigated by piezoresponse force microscopy (PFM). Simultaneous imaging of surface morphology, as well as the phase and magnitude of the piezoelectric response, is performed by PFM on a GaN film with patterned polarities on a c-Al2O3 substrate. We demonstrate that the polarity distribution of GaN based lateral polarity heterostructures can be deduced from the phase image of the piezoresponse with nanometer scale spatial resolution.
Detection of a single magnetic microbead using a miniaturized silicon Hall sensor
Authors: Besse Pierre-A. , Boero Giovanni , Demierre Michel , Pott Vincent , Popovic Radivoje
Published: July 2002

Using a highly sensitive silicon Hall sensor fabricated in a standard complementary metal–oxide–semiconductor (CMOS) technology, we detect a single magnetic microbead of 2.8 μm in diameter. The miniaturized sensor has an active area of 2.4×2.4 μm2, a sensitivity of 175 V/AT and a resistance of 8.5 kΩ. Two detection methods, both exploiting the superparamagnetic behavior of the bead, are experimentally tested and their performances are compared. This work opens the way to the fabrication of low cost microsystems for biochemical applications based on the use of dense arrays of silicon Hall sensors and CMOS electronics.
Fractional kinetic equations: solutions and applications
Authors: Saichev Alexander I. , Zaslavsky George M.
Published: July 2002

Fractional generalization of the diffusion equation includes fractional derivatives with respect to time and coordinate. It had been introduced to describe anomalous kinetics of simple dynamical systems with chaotic motion. We consider a symmetrized fractional diffusion equation with a source and find different asymptotic solutions applying a method which is similar to the method of separation of variables. The method has a clear physical interpretation presenting the solution in a form of decomposition of the process of fractal Brownian motion and Lévy-type process. Fractional generalization of the Kolmogorov–Feller equation is introduced and its solutions are analyzed.
Directions in ferrohydrodynamics (invited)
Authors: Rosensweig R. E.
Published: July 2002

Ferrohydrodynamics treats the flow and thermodynamics of magnetically polarizable fluid in response to applied magnetic field. Following its introduction twenty years ago, ferro-hydrodynamics has developed as a branch of mechanics similar to magnetohydrodynamics and electrohydrodynamics. Unique phenomena of ferrofluids and related media are manifested in hydrostatics, inviscid and viscous flows, magnetically induced flow instability, magnetic stabilization of flow, generation of antisymmetric stress, and magnetic multiphase flow. Certain of the phenomena serve as bases of developed technology in seals, dampers, accelerometers, and cooling techniques with new applications under active development in tribology, acoustics, printing, instrumentation, and other areas. Ferrohydrodynamic principles underlie also the development of magnetic multiphase systems yielding turbulence prevention of moving-bed operations in the process industry. Major concepts and advances of the field are presented in the context of the continuum mechanical framework.
Domain-wall surface energy derived from the complex impedance of Metglas ribbons traversed by ac currents
Authors: Agarwala A. K. , Berger L.
Published: July 2002

An electric current I crossing plane domain walls causes a distortion which increases wall area, hence total wall energy. This stored energy ∝ I2 manifests itself as a contribution to the imaginary part of the complex impedance Z of a ribbon. We can obtain the wall surface energy σ per unit area from measurements of Z. We have measured Z in a 21 μm×361 μm×6.6 mm piece of nonmagnetostrictive Metglas 2705X between 200 Hz and 50 kHz. The sample has been mechanically polished and annealed in a magnetic field transverse to the length.
Electroluminescence of III–V single-crystal semiconducting electrodes
Authors: Decker Franco , Prince Francisco , Motisuke Paulo
Published: July 2002

The luminescence generated by hole injection from an aqueous electrolyte into GaAs and InP has been investigated. Solid-state properties are responsible for the variations encountered in different samples and for most differences observed between photoluminescence and electroluminescence.
Effect of dislocations on the efficiency of thin-film GaAs solar cells on Si substrates
Authors: Yamaguchi Masafumi , Yamamoto Akio , Itoh Yoshio
Published: July 2002

Recombination loss at dislocations is the predominant loss mechanism in thin-film GaAs solar cells on Si substrates. Cell parameters are calculated based on a simple model in which dislocations act as recombination centers. Excellent agreement is observed between theory and experiment. It is indicated that one could fabricate thin-film GaAs solar cells with an efficiency of 17–18% on Si substrates if the dislocation density is less than 5×105 cm−2.
High-pressure and high-temperature crystal chemistry of beryllium oxide
Authors: Hazen R. M. , Finger L. W.
Published: July 2002

The crystal structure of synthetic BeO, bromellite, has been determined at several pressures to 5.0 GPa and several temperatures to 1183 K. The single variable atomic-positional parameter, z of oxygen, does not vary significantly with pressure, but does undergo a small increase with temperature. The hexagonal axial ratio c/a is constant with pressure, but decreases slightly from 1.624±0.001 to 1.623±0.001 between 300 and 1183 K. The observed crystal bulk modulus is 212±3 GPa, if K′ is assumed to be 4. The bulk modulus of the beryllium tetrahedron in BeO is 210 GPa, identical to that of the crystal, and close to the value observed for beryllium tetrahedra in other beryllium minerals. The average volume thermal expansivity of the BeO beryllium tetrahedron between 298 and 1183 K is 2.5±0.2×10−5 K−1, compared to 2.66±0.10×10−5 K−1 for the crystal.