bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of temperature on luminescence of terbium ions in LiNbO3
Authors: Ryba-Romanowski W. , Gołab S. , Dominiak-Dzik G. , Palatnikov M. N. , Sidorov N. V.
Published: July 2002

Single crystals of LiNbO3 doped with terbium were grown by the Czochralski method and their optical properties were examined. It has been found that, in contrast to isostructural LiTaO3:Tb, the terbium ions in LiNbO3 exhibit intense luminescence at low temperatures only, up to about 150 K. At this temperature, a luminescence quenching mechanism with activation energy of 0.22 eV is switched on. As a consequence, the luminescence of LiNbO3:Tb is reduced to a negligible level at room temperature.
Theoretical study of the protolytic dissociation of HCl in water clusters
Authors: Milet Anne , Struniewicz Cezary , Moszynski Robert , Wormer Paul E. S.
Published: July 2002

Reaction mechanisms for the acidic dissociation of HCl in water clusters are considered. Intermediates in the reaction are obtained from stationary points on the potential energy surface of the systems HCl–(H2O)n with n=4 and 5. These points have been determined by the B3LYP density functional method in an aug-cc-pVDZ atomic orbital (AO) basis. The total energies of the stationary points are checked by the coupled cluster single-double-triple [CCSD(T)] method in the same AO basis. For the case of n=4 a multibody analysis of the interaction energies is performed by the CCSD(T) method as well as by symmetry adapted perturbation theory. The clusters have a completely dissociated form as their energetically lowest minimum.
Josephson junctions and superconducting quantum interference devices made by local oxidation of niobium ultrathin films
Authors: Bouchiat V. , Faucher M. , Thirion C. , Wernsdorfer W. , Fournier T. , Pannetier B.
Published: July 2002

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3–6.5 nm thick under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated: lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4 K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length ξ which is of the order of 10 nm.
Multifold enhancement of the thermoelectric figure of merit in p-type BaBiTe3 by pressure tuning
Authors: Meng J. F. , Shekar N. V. Chandra , Badding J. V. , Chung D-Y. , Kanatzidis M. G.
Published: July 2002

As in the combinatorial approach to materials discovery, pressure tuning allows the phase space of interaction parameters that determine materials’ properties to be explored more rapidly than by traditional means. Both the thermoelectric power and electrical conductivity of BaBiTe3 are found to increase upon compression. As a result there is a dramatic increase in the dimensionless thermoelectric figure of merit ZT from ∼0.1 to ∼0.8. We suggest a mechanism for the enhancement involving an electronic topological transition. Such transitions can be induced both by means of pressure tuning and alloying, offering a potential route to reproducing the pressure induced improved behavior reported here by chemical means.
Rapid calculation of the Coulomb component of the stress tensor for three-dimensional systems with two-dimensional periodicity
Authors: Kawata Masaaki , Mikami Masuhiro , Nagashima Umpei
Published: July 2002

Computationally efficient formulas were developed for calculating the Coulomb component of the stress tensor in Ewald summations for three-dimensional systems with two-dimensional periodicity. The computational efficiency and accuracy of the formulas were compared with existing computational methods for water systems in the shapes of cubes, quadratic prisms, and slabs, all with two-dimensional periodicity. For these three systems, the formulas in our method significantly reduced the computational time required, and yielded sufficiently accurate evaluations of the Coulomb component of the stress tensor.
Electron temperature analysis of two-gas-species inductively coupled plasma
Authors: Bai K. H. , Chang H. Y. , Uhm H. S.
Published: July 2002

The electron energy distribution functions and electron temperatures are measured in Ar/He and Ar/Xe inductively coupled plasma with various mixing ratios. The electron temperature does not change linearly with the mixing ratios; instead it increases abruptly near PHe/PAr+He=1 and decreases rapidly near PXe/PAr+Xe=0. A simple model using a two-ion-species fluid model is suggested to explain the electron temperature variations, and it agrees well with the experimental results.
Gold nanoparticle single-electron transistor with carbon nanotube leads
Authors: Thelander Claes , Magnusson Martin H. , Deppert Knut , Samuelson Lars , Poulsen Per Rugaard , Nygård Jesper , Borggreen Jørn
Published: July 2002

We have used carbon nanotubes to electrically contact a 7 nm gold particle by scanning-probe manipulation. The result was a single-electron transistor showing a periodic modulation of the current as a function of gate voltage for temperatures up to ∼200 K, with the particle responsible for the main features of the electron transport. This interpretation could be verified when the particle was removed and the two nanotubes were moved into electrical contact.
Quantum-mechanical reaction rate constants from centroid molecular dynamics simulations
Authors: Geva Eitan , Shi Qiang , Voth Gregory A.
Published: July 2002

It has been shown recently that in order for real-time correlation functions obtained from centroid molecular dynamics (CMD) simulations to be directly related, without further approximations, to the corresponding quantum correlation functions, one of the operators should be linear in the position and/or momentum [Jang and Voth, J. Chem. Phys. 111, 2357 (1999)]. Standard reaction rate theory relates the rate constant to the flux–Heaviside or the flux–flux correlation functions, which involve two nonlinear operators and therefore cannot be calculated via CMD without further approximations. We present an alternative, and completely equivalent, reaction rate theory which is based on the position–flux correlation function. The new formalism opens the door to more rigorously using CMD for the calculation of quantum reaction rate constants in general many-body systems. The new method is tested on a system consisting of a double-well potential bilinearly coupled to a harmonic bath. The results obtained via CMD are found to be in good agreement with the numerically exact results for a wide range of frictions and temperatures.
High-power and long-lifetime InAs/GaAs quantum-dot laser at 1080 nm
Authors: Liu Hui-Yun , Xu Bo , Wei Yong-Qiang , Ding Ding , Qian Jia-Jun , Han Qin , Liang Ji-Ben , Wang Zhan-Guo
Published: July 2002

High power and long lifetime have been demonstrated for a semiconductor quantum-dot (QD) laser with five-stacked InAs/GaAs QDs separated by an InGaAs strain-reducing layer (SRL) and a GaAs spacer layer as an active medium. The QD lasers exhibit a peak power of 3.6 W at 1080 nm, a quantum slope efficiency of 84.6%, and an output-power degradation rate of 5.6%/1000 h with continuous-wave constant-current operation at room temperature. A comparative reliability investigation indicates that the lifetime of the InAs/GaAs QD laser with the InGaAs SRL is much longer than that of a QD laser without the InGaAs SRL. This improved lifetime of the QD laser could be explained by the reduction of strain in and around InAs QDs induced by the InGaAs SRL.
Anisotropic diffusion of tagged spheres near the isotropic-nematic phase transition
Authors: Vasanthi R. , Ravichandran S. , Bagchi Biman
Published: July 2002

Molecular dynamics simulations of diffusion of isolated tagged spheres in liquid crystalline molecules have been carried out. While the diffusion is isotropic at density slightly below the isotropic-nematic phase transition, it is found to become strongly anisotropic very close to the I-N transition where the diffusion perpendicular to the direction of the order parameter g(D⊥) decouples from the parallel (D∥) component. The diffusion coefficient of the sphere also shows a marked slowing down as the I-N transition approaches, in accordance with the mode coupling theory analysis. The variation of the orientational order parameter with the density shows a sharp transition at a reduced density, ρIN* where plot of the ratio, D∥/D⊥ against the reduced density, ρ* clearly shows the emergence of anisotropy. D⊥ could be fitted to a form (ρc*−ρ*)p with a ρc* which is slightly larger than ρIN*. The diffusion shows a strong non-Stokesian dependence on the radius of the spheres, in agreement with recent experiments.
Sound–ultrasound interaction in bubbly fluids: Theory and possible applications
Authors: Khismatullin D. B. , Akhatov I. Sh.
Published: July 2002

The interaction between sound and ultrasound waves in a weakly compressible viscous liquid with gas bubbles is considered. Using the method of multiple scales one- and two-dimensional nonlinear interaction equations are derived. The degeneracy of the interaction is found in bubbly fluids. This phenomenon lies in the fact that the interaction coefficients vanish at a certain frequency of ultrasound. We demonstrate that the integrable Davey–Stewartson I (DSI) system of equation can describe the two-dimensional sound-ultrasound evolution. The DSI equations are remarkable by their solutions referred to as dromions. In bubbly fluids the dromion represents the localized focused ultrasound wave which can alter the direction of its motion under changes in the boundary conditions for the sound wave. The condition of singular focusing of ultrasound in bubbly fluids is obtained. By numerical analysis of the interaction models, we reveal such processes as intensification of ultrasound by sound, nonlinear instability of a sound profile, and prove the validity of the singular focusing condition. Finally, possible applications of the results are outlined.
Formation of ordered Ge quantum dots on the Si(111)–(7×7) surface
Authors: Zhang Y. P. , Yan L. , Xie S. S. , Pang S. J. , Gao H.-J.
Published: July 2002

We present a pathway for the formation of ordered Ge quantum dots on Si(111)–(7×7) substrate. Self-assembled growth of Ge quantum dots on the Si(111)–(7×7) surface has been investigated using scanning tunneling microscopy. The Ge is grown on the substrate by solid phase epitaxy at room temperature. It has been found that the deposited submonolayer Ge can aggregate and form ordered Ge quantum dots on the surface through controlling the annealing temperature. The formation of ordered Ge quantum dots is due to the preferential adsorption sites of Ge on Si(111)–(7×7). The formed Ge quantum dots may have a great potential in the application of nanodevices.
Design of a miniature picosecond low-energy electron gun for time-resolved scattering experiments
Authors: Karrer R. , Neff H. J. , Hengsberger M. , Greber T. , Osterwalder J.
Published: July 2002

We present the design and performance tests of a miniaturized pulsed low-energy electron gun. Electrons photoemitted from a gold cathode are accelerated over a distance of 75 μm and then collimated by a microchannel plate. According to calculations, this novel concept will allow the time spread of the electron pulses to be kept below 5 ps for kinetic energies as low as 100 eV. The achievement of a minimum angular beam divergence (≈1°) along with an energy resolution of 1.1 eV has to be paid for by low signal intensities. We demonstrate the performance of the gun and the high electron-beam coherence by presenting low-energy-electron diffraction images taken from a submonolayer of lead adsorbed on the germanium (111) surface. We anticipate that this electron gun will open up new possibilities for following structural changes on solid surfaces in real time.
Growth of strongly biaxially aligned MgB2 thin films on sapphire by postannealing of amorphous precursors
Authors: Berenov A. , Lockman Z. , Qi X. , MacManus-Driscoll J. L. , Bugoslavsky Y. , Cohen L. F. , Jo M.-H. , Stelmashenko N. A. , Tsaneva V. N. , Kambara M. , Babu N. Hari , Cardwell D. A. , Blamire M. G.
Published: July 2002

MgB 2 thin films were cold grown on sapphire substrates by pulsed laser deposition followed by postannealing in mixed, reducing gas, Mg rich, Zr gettered, environments (pO2∼10−24 atm) at 750 and 950 °C. The films had Tc in the range 29–34 K, Jc (20 K, H=0) in the range 3×104–3×105 A cm−2, and irreversibility fields H* at 20 K of 4–6.2 T. An inverse correlation was found between Tc and H*. The films had grain sizes of ∼0.1–1 μm and a strong biaxial alignment was observed in the 950 °C annealed film. (111) oriented MgO was also observed. Mg coating of films during crystallization appeared to improve film Tc.
All optical integrated upconversion fluorescence-based point temperature sensing system using Er3+-doped silica-on-silicon waveguides
Authors: Messias D. N. , Vermelho M. V. D. , Gouveia-Neto A. S. , Aitchison J. S.
Published: July 2002

In this work an optically integrated and compact point temperature sensor based upon upconversion fluorescence emission in an Er3+-doped silica-on-silicon waveguide is proposed. The sensing system relies upon the temperature information residing in the intensity ratio of two thermally coupled upconversion excited fluorescence emission signals around 530 and 550 nm due to the Er3+ transitions H11/22→4I15/2 and S3/24→4I15/2, respectively. The system was tested in the range of temperature from 20 °C to 140 °C. Under this circumstance the system have shown sensitivity close to 9.0×10−3/°C with a coupled pump power around 800 nm as low as 7 mW and a maximum sensitivity of 0.010/°C is expected at 250 °C.
Global minimizer for the Ginzburg-Landau functional of an inhomogeneous superconductor
Authors: Kim Kwang Ik , Liu Zuhan
Published: July 2002

In this paper, we prove that the global minimizer of the Ginzburg-Landau functional of an inhomogeneous superconductor in an external magnetic field below the first critical field Hc1 is the vortexless solution. Here, Hc1 is defined as the value of hex, the applied field, for which the minimal energy among vortexless configurations is equal to the minimal energy among single-vortex configurations.
Boron carbide nanolumps on carbon nanotubes
Authors: Lao J. Y. , Li W. Z. , Wen J. G. , Ren Z. F.
Published: July 2002

Boron carbide nanolumps are formed on the surface of multiwall carbon nanotubes by a solid-state reaction between boron and carbon nanotubes. The reaction is localized so that the integrity of the structure of carbon nanotubes is maintained. Inner layers of multiwall carbon nanotubes are also bonded to boron carbide nanolumps. These multiwall carbon nanotubes with boron carbide nanolumps are expected to be the ideal reinforcing fillers for high-performance composites because of the favorable morphology.
Observation of a magic discrete family of ultrabright Si nanoparticles
Authors: Belomoin G. , Therrien J. , Smith A. , Rao S. , Twesten R. , Chaieb S. , Nayfeh M. H. , Wagner L. , Mitas L.
Published: July 2002

We demonstrate that electrochemically etched, hydrogen capped SinHx clusters with n larger than 20 are obtained within a family of discrete sizes. These sizes are 1.0 (Si29), 1.67 (Si123), 2.15, 2.9, and 3.7 nm in diameter. We characterize the particles via direct electron imaging, excitation and emission optical spectroscopy, and colloidal crystallization. The band gaps and emission bands are measured. The smallest four are ultrabright blue, green, yellow and red luminescent particles. The availability of discrete sizes and distinct emission in the red, green and blue (RGB) range is useful for biomedical tagging, RGB displays, and flash memories.
Enhancement of exciton binding energies in ZnO/ZnMgO multiquantum wells
Authors: Sun H. D. , Makino T. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

The effect of confinement on the exciton binding energies has been systematically investigated for two series of ZnO/ZnMgO multiquantum wells with various well widths and barrier heights. The exciton binding energies were extracted from the energy difference between the stimulated emission band induced by inelastic exciton–exciton scattering and the free exciton absorption band. The binding energies of excitons are found to be sensitively dependent on the well widths. The experimental results of the well width dependence of binding energies are in good agreement with Coli and Bajaj’s theoretical calculations for these structures [G. Coli and K. K. Bajaj, Appl. Phys. Lett. 78, 2861 (2001)]. The remarkable reduction in coupling strength between excitons and longitudinal optical phonons is closely correlated with the enhancement of the exciton binding energy, indicating that the stability of excitons is greatly increased by the enhancement of exciton binding energy in quantum wells.
Hysteresis anomalies and exchange bias in 6.6 nm CuO nanoparticles
Authors: Punnoose A. , Seehra M. S.
Published: July 2002

In the 6.6 nm particles of CuO with Néel temperature TN≃40 K, hysteresis loops are observed up to 330 K. With decrease in temperature, the coercivity Hc increases slowly up to TN below which rapid increases in Hc and exchange bias He are observed. At 5 K, the ratio He/Hc≃2, decreasing to zero as T→TN. At 10 K, the variation of He, Hc, and the ratio He/Hc with the cooling field show behavior similar to that of the weak ferromagnetic moment resulting from the uncompensated surface spins.
Effect of a titanium cap in reducing interfacial oxides in the formation of nickel silicide
Authors: Tan W. L. , Pey K. L. , Chooi Simon Y. M. , Ye J. H. , Osipowicz T.
Published: July 2002

Interfacial silicon oxide present at the Ni–Si interface hampers the silicidation between Ni and Si. In this work we present findings of the interaction of a Ti cap layer on top of Ni to remove the interfacial native oxide and chemically grown silicon oxide at several annealing temperatures. It was found that at 500 °C, Ti diffuses through the Ni layer and segregates at the Ni/Si interface, which subsequently reduces the interfacial silicon oxide and enables nickel monosilicide (NiSi) formation at 600 °C. The thickness of the Ti cap layer was found to strongly influence the temperature of the onset of nickel silicidation. A thin Ti cap layer resulted in the onset temperature of nickel silicidation being the same as that without a Ti cap layer, whereas a thick Ti cap layer lowered the onset temperature of the nickel silicidation.
High-silicon steel produced by hot dipping and diffusion annealing
Authors: Ros-Yañez T. , Houbaert Y. , Gómez Rodrı́guez V.
Published: July 2002

Steels with high Si content (up to 6.5% Si) are excellent soft magnetic materials, however, as the Si content is increased, the material becomes extremely brittle and it is very difficult to produce thin sheet by conventional rolling. An alternative production route has been developed through hot dipping in a high Si-bath followed by diffusion annealing. Experiments were carried out in a hot dip simulator using as substrate a steel with 0.35 mm thickness and 3.2% Si. Samples were dipped in an Al–Si hypereutectic bath at 800 °C for different times. After dipping, the coating consists of Fe–Al–Si phases and primary silicon crystals within a matrix of eutectic Al–Si. During a first annealing inside the hot dip simulator, diffusion of Si and Al into the substrate and phase transformation occurs in the layer producing a series of ternary intermetallics of the Fe–Si–Al system. A second annealing was performed outside the hot dip simulator under vacuum at 1250 °C with different holding times. This diffusion annealing allows the homogenisation of the composition obtaining a homogeneous concentration of 6%–7% Si and/or 2%–5% Al over the thickness in function of the previous hot-dip parameters. The material was characterized by SEM. Magnetic properties were measured and it was shown that the magnetic losses after hot dipping and diffusion annealing were reduced at 50 Hz from 3.7 W/kg (original material) to 0.64 W/kg (1 T), and at 400 Hz from 47 W/kg (original material) to 10 W/kg (1 T).
Solitary waves on inclined films: Flow structure and binary interactions
Authors: Malamataris N. A. , Vlachogiannis M. , Bontozoglou V.
Published: July 2002

The downstream evolution of disturbances, introduced at the inlet of a liquid film flowing along an inclined plane wall, is studied numerically by solving the full, time-dependent Navier–Stokes equation. Computational results are validated against the predictions of spatial linear stability analysis and against detailed data of the entire evolution process. The structure of the flow field below the waves is analyzed, and the results are used to test assumptions frequently invoked in the theoretical study of film flow by long-wave equations. An interesting prediction is that solitary waves exhibit strongly nonparabolic velocity profiles in front of the main hump, including a slim region of backflow. The computational scheme is subsequently used to study solitary wave interactions. It is predicted that coalescence (the inelastic collision of two humps) is not inevitable but occurs only when the waves differ appreciably in height. Waves of similar size repel monotonically, whereas for intermediate differences in height a strong oscillatory interaction between the two humps is predicted. Encouraging qualitative agreement with the limited experimental information available is noted.
Magnetic properties of iron-rich mixed rare-earth Sm2−xTbxFe17−ySiy compounds
Authors: Kamaraju K. , Yang J. B. , Yelon W. B. , Pringle O. A. , James W. J. , I’Heritier Ph.
Published: July 2002

A series of Sm2−xTbxFe17−ySiy solid solutions with x=0, 1, and 1.5 and y=1, 2, and 3 were prepared by induction melting stoichiometric amounts of high purity elements. The x-ray diffraction data confirm that the postannealed samples are 2:17 intermetallics of the R3̄m space group. The lattice parameters and the unit cell volumes were calculated using a modified Rietveld program. The fitted intensities showed behavior related to a disordered rhombohedral structure as inferred from neutron data. It was observed that for a particular y, the unit cell volume decreased almost linearly with increasing Tb content (x=0, 1, and 1.5). The unit cell volume decreases with increasing Si content (y) for a particular x as is also observed for the single rare-earth 2:17 compounds. The Curie temperature measurements show that with increasing Tb content x a decrease in the Curie temperatures of about 3% for y=1, 10% for y=2, and 15% for y=3 was observed. For a particular x, a nearly linear type increase in the Curie temperature resulted with increasing silicon content (y⩽3). It was observed that with increasing x there was a decrease of about 40% in the magnetization for y=1 and 2, whereas there was a decrease of about 35% for y=3. For a particular x, the magnetization decreased almost linearly with increasing Si content (y=1–3).
Domain wall permeability limit for the giant magnetoimpedance effect
Authors: Barandiarán J. M. , Garcı́a-Arribas A. , Muñoz J. L. , Kurlyandskaya G. V. , Valenzuela R.
Published: July 2002

The magnetoimpedance (MI) effect is based on the change of inductance and resistance under the effect of an external magnetic field. In bulk homogeneous wires or ribbons these two components of the impedance are related to the penetration depth of the electromagnetic field in the material. From simple considerations it is shown that the maximum MI ratio is then proportional to the square root of the relative permeability of the material. That limit is reached for frequencies at which the penetration depth is of the order of the transverse dimensions of the sample. At low operating frequencies, typical of thick geometries, the permeability is limited by the microeddy currents associated with domain wall displacements. The permeability relaxation equivalent to these local eddy currents can be calculated and used in the classical expressions for MI. The real part of the permeability is highly reduced at the relaxation frequency and gives rise to a decrease of the inductance, while the imaginary part contributes to the resistance, with a maximum at the same frequency.
Semiconducting and ferromagnetic behavior of sputtered Co-doped TiO2 thin films above room temperature
Authors: Park Wan Kyu , Ortega-Hertogs Ricardo J. , Moodera Jagadeesh S. , Punnoose Alex , Seehra M. S.
Published: July 2002

We have investigated Co-doped TiO2 thin films grown by reactive co-sputtering. X-ray diffraction showed a single phase polycrystalline rutile structure, without any segregation of Co into particulates within the instrumental resolution limit. The atomic content of Co ranged from 1% to 12%. The temperature dependence of resistivity showed an extrinsic semiconducting behavior. From optical absorption measurements, the band gap Eg≈3.25±0.05 eV was found, independent of the Co concentration, and in agreement with a literature value. Room temperature M-H loops showed a ferromagnetic behavior for Co content higher than 3%. The magnetic moment per Co atom was estimated to be about 0.94 μB, suggesting a low spin configuration of Co ions. The temperature dependence of remanent magnetization revealed a Curie temperature higher than 400 K for Co content of 12%.
Magnetoelastic coupling in epitaxial Cu/Ni90Fe10/Cu/Si(001) thin films
Authors: Ciria M. , Ha K. , Bono D. , O’Handley R. C.
Published: July 2002

Magnetic anisotropy energy (MAE) and magnetoelastic (ME) stress in epitaxial Cu(4 nm)/Ni90Fe10/Cu(160 nm)/Si(001) films have been studied at room temperature as a function of the permalloy film thickness (2 nm⩽tPm⩽50 nm). Magnetostatic energy keeps the magnetization within the film plane, although surface and magnetoelastic anisotropy energy favor magnetization normal to the film plane. The direct measurement of the magnetoelastic stress shows the ME coefficients to depend linearly on the strain, ε, for the ME coefficient. As a result, a second-order magnetoelastic contribution, proportional to ε2, has to be included in the MAE. Using both sets of measurement we determine two second-order ME coefficients, M1γ,2=−0.3×107 J/m3 and M2γ,2=8.3×107 J/m3, and the surface magnetic anisotropy constant, Ksur=0.4 mJ/m2.
Electroluminescence mechanism in SiOx layers containing radiative centers
Authors: Bae H. S. , Kim T. G. , Whang C. N. , Im S. , Yun J. S. , Song J. H.
Published: July 2002

Luminescent silicon oxides containing radiative centers were obtained by using two different techniques. Silicon rich silicon oxides (SRSOs) were fabricated by rf magnetron sputter deposition and Ge-implanted SiO2 films were fabricated by ion implantation following the thermal oxidation of Si. Blue and violet photoluminescence were observed from the SRSO and the Ge-implanted SiO2, respectively. However, the electroluminescence (EL) spectra from both oxides exhibited red and near-infrared luminescence bands. Strong EL was observed only under reverse bias conditions on metal-luminescent oxide–semiconductor structures. The EL intensity and peak position were varied with applied voltages. According to the EL and current–voltage measurements, it is concluded that the possible EL mechanism is the impact ionization of ground state electrons in the radiative centers.
Effect of oxygen vacancies on the magnetic structure of the La0.6Sr0.4FeO3−δ perovskite: A neutron diffraction study
Authors: Chu Z. , Yelon W. B. , Yang J. B. , James W. J. , Anderson H. A. , Xie Y. , Malik S. K.
Published: July 2002

Magnetic interactions in perovskite compounds of the type La1−xSrxMO3−δ (M=3d transition such as Mn and Fe) are presumed to arise through a super exchange between 3d electrons of the magnetic ions via oxygen orbitals. The magnetic structure of La0.6Sr0.4FeO3−δ has been studied with neutron diffraction. Oxygen vacancies were created by annealing samples under various gases including N2, air and mixtures of CO/CO2. All La0.6Sr0.4FeO3−δ compounds maintain the rhombohedral structure (space group R3̄c). The air- or oxygen-annealed samples have almost no oxygen vacancies while those made in the reducing atmosphere show 7%–11% oxygen vacancies. The rhombohedral distortion decreases in the reduced samples. All the samples exhibit antiferromagnetic ordering at room temperature, although a small ferromagnetic moment may also be present. The samples with little or no oxygen vacancies show a room temperature magnetic moment of ∼1.4 μB at the Fe site while those having >7% oxygen vacancies show a moment of ∼4.0 μB. Magnetization measurements reveal a much higher magnetic ordering temperature in samples with oxygen vacancies
Metabolic network dynamics in open chaotic flow
Authors: Károlyi György , Scheuring István , Czárán Tamás
Published: July 2002

We have analyzed the dynamics of metabolically coupled replicators in open chaotic flows. Replicators contribute to a common metabolism producing energy-rich monomers necessary for replication. The flow and the biological processes take place on a rectangular grid. There can be at most one molecule on each grid cell, and replication can occur only at localities where all the necessary replicators (metabolic enzymes) are present within a certain neighborhood distance. Due to this finite metabolic neighborhood size and imperfect mixing along the fractal filaments produced by the flow, replicators can coexist in this fluid system, even though coexistence is impossible in the mean-field approximation of the model. We have shown numerically that coexistence mainly depends on the metabolic neighborhood size, the kinetic parameters, and the number of replicators coupled through metabolism. Selfish parasite replicators cannot destroy the system of coexisting metabolic replicators, but they frequently remain persistent in the system.
980 nm excited upconversion in an Er-doped ZnO–TeO2 glass
Authors: Vetrone Fiorenzo , Boyer John-Christopher , Capobianco John A. , Speghini Adolfo , Bettinelli Marco
Published: July 2002

In this letter, we investigate the upconversion properties of 19ZnO–80TeO2–1Er2O3 glass after excitation into the I11/24 level using 980 nm radiation. At an excitation power density of 880 W/cm2, green emission [(2H11/2, 4S3/2)→4I15/2] dominates the upconversion spectrum with an efficiency of 0.16%. Temporal studies reveal that the I11/24 level is the intermediate state by which the two-step upconversion process occurs. Excited-state absorption and phonon-assisted energy transfer are discussed as possible mechanisms for the upconversion.
Single-wall carbon nanotube atomic force microscope probes
Authors: Snow E. S. , Campbell P. M. , Novak J. P.
Published: July 2002

We examine the factors that govern the stability of imaging using single-wall C nanotubes as probes for atomic force microscopy. Nonvertical alignment of the nanotubes causes such probes to bend in response to the surface–nanotube interaction forces during imaging. For long nanotubes, this elastic response causes the nanotube tip to jump into contact with the surface and renders it unsuitable for imaging. For short nanotubes, stable noncontact-mode imaging can be achieved using a small cantilever vibration amplitude. However, the bending response is enhanced on highly textured surfaces, which limits the ability to image nonplanar features.
Gate leakage effects and breakdown voltage in metalorganic vapor phase epitaxy AlGaN/GaN heterostructure field-effect transistors
Authors: Tan W. S. , Houston P. A. , Parbrook P. J. , Wood D. A. , Hill G. , Whitehouse C. R.
Published: July 2002

The gate leakage behavior in AlGaN/GaN heterostructure field-effect transistors was studied as a function of applied bias, temperature, and surface periphery. A surface hopping conduction mechanism with an activation energy of 0.21 eV is proposed for the gate–drain leakage for voltages that exceed pinchoff. The reverse breakdown voltage of the device exhibited a negative temperature coefficient of −0.11 V K−1, suggesting that a breakdown mechanism other than impact ionization, such as thermal runaway, may be responsible.
Thermophysical properties and glass forming ability in the alloy series (Ti100−xZrx)45(Ni14.5Cu85.5)55
Authors: Hays C. C. , Glade S. C.
Published: July 2002

Measurements of the structural and thermophysical properties of bulk metallic glass forming alloys in the series (Ti100−xZrx)45(Ni14.5Cu85.5)55 (at. %) are presented for 25⩽x⩽75. The bulk glass forming ability was found to terminate abruptly for x>50 due to the nucleation of NiTiZr and Zr2Cu symmetry phases with increasing x. Calorimetric measurements reveal that crystallization is facilitated by a decomposition process in the under-cooled liquid.
Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applications
Authors: Chen J. X. , Markus A. , Fiore A. , Oesterle U. , Stanley R. P. , Carlin J. F. , Houdré R. , Ilegems M. , Lazzarini L. , Nasi L. , Todaro M. T. , Piscopiello E. , Cingolani R. , Catalano M. , Katcki J. , Ratajczak J.
Published: July 2002

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot (QD) grown under Stranski–Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9×1010 to 0.6×1010 cm−2 as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 μm with the excited state transition wavelength of around 1.3 μm has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 μm but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%.
Nickel clusters: The influence of adsorbates on magnetic moments
Authors: Knickelbein Mark B.
Published: July 2002

Magnetic moments have been measured for bare, isolated nickel clusters Nin and their association complexes with carbon monoxide, oxygen, and hydrogen using a molecular beam deflection method. The moments measured for bare Nin are in general agreement with those previously reported by Apsel et al. [Phys. Rev. Lett. 76, 1441 (1996)], lying slightly lower overall. It is found that adsorbed carbon monoxide and hydrogen decrease magnetic moments of Nin. The reduction in moments can be substantial for smaller clusters: the moment of Ni8 is decreased by approximately 6 bohr magnetons by a single absorbed CO molecule. In general, the adsorbate-induced changes in magnetic moments diminish with increasing cluster size. The present results are in qualitative accord with previous measurements of the magnetization quenching effects of CO and H on nickel nanoparticles and thin nickel films. Atomic oxygen decreases cluster moments of some nickel clusters and increases those of others, an effect attributed to adsorbate-induced reconstruction. The experimental results are compared to detailed electronic structure calculations of nickel cluster–adsorbate complexes and to the predictions of the semiempirical bond order-rigid band model of Fourier and Salahub [Surf. Sci. 238, 330 (1990)].
Photoluminescence properties of Eu3+-doped ZnS nanocrystals prepared in a water/methanol solution
Authors: Qu S. C. , Zhou W. H. , Liu F. Q. , Chen N. F. , Wang Z. G. , Pan H. Y. , Yu D. P.
Published: July 2002

Monodispersed ZnS and Eu3+-doped ZnS nanocrystals have been prepared through the co-precipitation reaction of inorganic precursors ZnCl2, EuCl3, and Na2S in a water/methanol binary solution. The mean particle sizes are about 3–5 nm. The structures of the as-prepared ZnS nanoparticles are cubic (zinc blende) as demonstrated by an x-ray powder diffraction. Photoluminescence studies showed a stable room temperature emission in the visible spectrum region for all the samples, with a broadening in the emission band and, in particular, a partially overlapped twin peak in the Eu3+-doped ZnS nanocrystals. The experimental results also indicated that Eu3+-doped ZnS nanocrystals, prepared by controlling synthetic conditions, were stable.
Drift diffusion and Einstein relation for electrons in silicon subjected to a high electric field
Authors: Arora Vijay K.
Published: July 2002

We present a theory that makes an explicit connection between scattering-limited Ohmic mobility and quantum-emission-limited saturation velocity. The theory is applied to electrons in bulk silicon by taking a quantum equal to the energy of an optical phonon. A modification in the mean-free path because of this quantum emission is indicated, an effect that appears only in the high-field regime. This modification is shown to lead to electric-field-induced degradation of the diffusion coefficient. The theory presented agrees well with the drift-diffusion experimental data and empirical relations used in modeling devices. The theory makes connections with an alternate description in terms of electron temperature under ac and dc conditions. Because drift-diffusion processes are central in performance evaluation of submicron-scale devices in which high fields are necessarily present, these results contribute significantly to reshaping thinking processes in the high-field regime.
Comprehensive modeling of the electro-optical-thermal behavior of (AlGaIn)(AsSb)-based 2.0 μm diode lasers
Authors: Rattunde M. , Mermelstein C. , Schmitz J. , Kiefer R. , Pletschen W. , Walther M. , Wagner J.
Published: July 2002

Strained triple-quantum-well, large-optical-cavity GaInSb/AlGaAsSb/GaSb diode lasers emitting at 1.98 μm at 300 K are investigated with regard to their high-power capability. As the heating of the active region is a limiting factor for these devices, a quantitative model is derived to simulate the performance of these lasers including thermal effects. The standard laser parameters, deduced from measurements on ridge waveguide lasers, and the measured thermal resistance of the mounted devices were then taken as input parameters. The output power and power efficiency of the lasers calculated using the presented model. Good agreement was found between calculated data and the measurements for different heatsink temperatures as well as for different laser geometries and mounting techniques. The maximum output power achieved for p-side down mounted 1000×150 μm2 broad-area laser was 1.7 W at 300 K in cw operation.
Extraordinary magnetoresistance effect in a microstructured metal–semiconductor hybrid structure
Authors: Möller C. H. , Kronenwerth O. , Grundler D. , Hansen W. , Heyn Ch. , Heitmann D.
Published: July 2002

We have fabricated hybrid structures consisting of a metallic thin film and of a microstructured two-dimensional electron system in an InAs heterostructure. The devices are found to exhibit a huge magnetoresistance (MR) effect in magnetic fields ⩽1 T. At low temperature, a value of ΔR/R=[R(B=1 T)−R(B=0)]/R(B=0) as high as 115 000% is measured. The value of ΔR/R has been studied as a function of the electron mobility, the electron density and the lateral width of the semiconductor. We find that the MR effect can be tailored by these different parameters and technological relevant devices can be realized.
Tunable microfluidic optical fiber
Authors: Mach P. , Dolinski M. , Baldwin K. W. , Rogers J. A. , Kerbage C. , Windeler R. S. , Eggleton B. J.
Published: July 2002

We describe a class of active, tunable optical fiber that incorporates multiple microfluidic plugs into interior fiber microchannels. The propagation characteristics of certain optical modes of these fiber waveguides can be usefully manipulated by controlling the positions and optical properties of these plugs, using actuators and pumps formed on the fiber surface. These hybrid microfluidic/silica structures offer versatile tuning capabilities in a format that preserves the advantages of conventional, passive optical fiber. As an example, we report an all-fiber, narrowband filter, whose transmission wavelength and attenuation are independently adjustable via microfluidic tuning.
Investigation of determinism in heart rate variability
Authors: Gomes M. E. D. , Souza A. V. P. , Guimarães H. N. , Aguirre L. A.
Published: July 2002

The article searches for the possible presence of determinism in heart rate variability (HRV) signals by using a new approach based on NARMA (nonlinear autoregressive moving average) modeling and free-run prediction. Thirty-three 256-point HRV time series obtained from Wistar rats submitted to different autonomic blockade protocols are considered, and a collection of surrogate data sets are generated from each one of them. These surrogate sequences are assumed to be nondeterministic and therefore they may not be predictable. The original HRV time series and related surrogates are submitted to NARMA modeling and prediction. Special attention has been paid to the problem of stationarity. The results consistently show that the surrogate data sets cannot be predicted better than the trivial predictor—the mean—while most of the HRV control sequences are predictable to a certain degree. This suggests that the normal HRV signals have a deterministic signature. The HRV time series derived from the autonomic blockade segments of the experimental protocols do not show the same predictability performance, albeit the physiological interpretation is not obvious. These results have important implications to the methodology of HRV analysis, indicating that techniques from nonlinear dynamics and deterministic chaos may be applied to elicit more information about the autonomic modulation of the cardiovascular activity.
Multiple precision pseudospectral computations of the radiation coefficient for weakly nonlocal solitary waves: Fifth-order Korteweg–DeVries equation
Authors: Boyd John P.
Published: July 2002

By performing multiple precision pseudospectral calculations using two different basis sets, we compute the radiation coefficient α(ε) for very small ε to resolve discrepancies between earlier numerical work of the author’s and a small-ε perturbation theory of Grimshaw and Joshi. Multiple precision is needed because α is asymptotically proportional to exp(−π/2ε), and is therefore below the single precision roundoff threshold when ε<1/25. Because α decreases exponentially with 1/ε, we use a numerical method whose error decreases exponentially fast with the number of grid points N: a pseudospectral method or finite differences of as high as twenty-fourth order. Richardson iteration, preconditioning by high order finite differences, parity symmetry, Aitken and Richardson extrapolation, and the fast Fourier transform are all crucial in reducing our longest runs to about 24 h on a Unix workstation. Although preconditioning by second order differences is the norm, we find that high order preconditioning—as large as 14th order—is more efficient for our one-dimensional problem. We find that the discrepancies are mostly due to differences in convention for (i) definition of the parameter ε and (ii) choice of the far field phase. When these are accounted for, we obtain very good agreement with Grimshaw and Joshi’s first order term. However, their second order term, which is predicted to be π2/2≊4.94, is about O(0.0 to −0.2) in our computations. The reason for this difference is still a mystery. © 1995 American Institute of Physics.
Simulation and study of Fresnel diffraction for arbitrary two-dimensional apertures
Authors: Dauger Dean E.
Published: July 2002

A stable, efficient algorithm to calculate numerically a Fresnel diffraction image, given any two-dimensional aperture or obstacle, is introduced. The algorithm predicts both the intensity and relative phase of the image. An alternate faster algorithm is presented for a limited class of apertures. Also examined are images formed from a variety of aperture shapes. Using this computational technique, plots on the complex plane are shown to give insight into Fresnel diffraction. With additional modifications to accept parameters from an experimental apparatus, the algorithms model Fresnel diffraction for laboratory situations. In addition, the algorithms are suitable for numerical implementation on personal computers. © 1996 American Institute of Physics.
Relation between barrier height and work function in contacts to selenium
Authors: Champness C. H. , Chan A.
Published: July 2002

In Se–Tl, Se–Cd, Se–Au, and Se–Te contacts prepared by evaporation on a crystallized selenium layer, Schottky barrier heights were measured from junction capacitance. These were found to decrease systematically with the work function of the contacting metal. From this variation, the density of interface states was estimated to be about 1014 cm−2 eV−1, assuming an interfacial layer thickness of 10 Å. The neutral level of the states was estimated to lie close to the valence band edge.
Optical properties of amorphous silicon and silicon dioxide
Authors: Ravindra N. M. , Narayan J.
Published: July 2002

A detailed study of the optical properties of amorphous silicon prepared by various methods is presented here. Comparisons of the various major optical parameters like the refractive index and the optical gap and related properties are then made for amorphous silicon films prepared by glow-discharge, chemical vapor deposition, and sputtered films. Experimental observations of a linear variation of the static refractive index with the Urbach tail parameter for boron-doped chemically vapor deposited films have been analyzed. Wherever relevant, spectroscopic and band-structural models like those of Penn and Wemple and DiDomenico are employed to evaluate the optical parameters of the materials. We also examine briefly the optical properties of amorphous silicon dioxide. Extrapolation of conventional models are then made to derive the relevant optical parameters of SiO2.
Iron nitride thin films prepared by facing targets sputtering
Authors: Morisako A. , Takahashi K. , Matsumoto M. , Naoe M.
Published: July 2002

The iron nitride thin films have been deposited using a facing targets sputtering (FTS) system and a conventional dc diode magnetron sputtering system, and their crystallographic characteristics and magnetic properties have been investigated. The total gas pressure (PN2+PAr) was set at 2 mTorr. The films deposited by the FTS system without heating the substrate at PN2 below 0.3 mTorr possessed α-Fe-like structure with an increased lattice constant. At PN2 of around 0.5 mTorr, a two-phase mixture of α-Fe and Fe2–3N was formed in the films prepared in the FTS system, while ζ-Fe2N phase was formed in the films prepared by dc magnetron sputtering. The films prepared by FTS at PN2 of 0.1 mTorr possessed α-Fe and γ′-Fe4N structures when a substrate temperature (Ts) was in the range from 80 to 300 °C. The films deposited at Ts of about 150 °C possessed a saturation magnetization (Ms) of about 1700 emu/cm3, which is larger than that of pure iron films and coercivity (Hc) of about 1.5 Oe.