bluuebunny/crossref_metadata_2025 huggingface dataset:
Light-induced negative differential resistance in planar InP/InGaAs/InP double-heterojunction p-i-n photodiode
Authors: Ho Chong-Long , Wu Meng-Chyi , Ho Wen-Jeng , Liaw Jy-Wang
Published: July 2002

Negative differential resistance (NDR) was observed in the dc photocurrent-reverse bias characteristics at 1.55 μm wavelength of a planar InP/InGaAs/InP double-heterojunction p-i-n photodiode, which is designed for high-speed operation. In the bias range of measurements, two NDR regions can be observed, which are suggested to be mainly caused by the drop of average electron velocities in both the InGaAs absorption and InP buffer layers, i.e., the transferred electron effect. For providing evidence, power dependence of these NDR regions was investigated. Besides, another high-responsivity p-i-n photodiode was fabricated and characterized for comparison.
Depth profiling of GaN by cathodoluminescence microanalysis
Authors: Fleischer K. , Toth M. , Phillips M. R. , Zou J. , Li G. , Chua S. J.
Published: July 2002

We present the results of a depth-resolved cathodoluminescence (CL) and transmission electron microscopy study of autodoped GaN grown on sapphire. Depth-resolved CL analysis can be used for depth profiling of the yellow luminescence (YL) center concentration which was found to increase with depth. The results are consistent with the (ON–VGa)2− complex model of YL centers [J. Neugebauer and C. G. Van de Walle, Appl. Phys. Lett. 69, 503 (1996) and T. Mattila and R. M. Nieminen, Phys. Rev. B 55, 9571 (1996)]. Depth profiling of the near-edge emission in GaN layers thicker than ∼0.5 μm is not possible due to strong self-absorption.
Evidence of strong flux pinning in melt-processed ternary (Nd–Eu–Gd)Ba2Cu3Oy superconductors
Authors: Pradhan A. K. , Muralidhar M. , Koblischka M. R. , Murakami M. , Nakao K. , Koshizuka N.
Published: July 2002

The flux pinning behavior of ternary melt-processed (Nd–Eu–Gd)Ba2Cu3Oy superconductors is studied with varying defect concentrations using magnetotransport and magnetization measurements. A huge field-induced bump feature in the resistivity with increasing defect density, field-induced magnetization kink and linear voltage-current (V-I) characteristics over the whole transition temperature regime favor the phenomenon of vortex entanglement in the liquid phase. The Nd/Ba substitution sites along with the fine second-phase particles are one of the possible reasons for the vortex entanglement and these defects give rise to high critical current density and reduced dissipation at low temperature with significant enhancement in pinning.
Oxygen diffusion into SiO2-capped GaN during annealing
Authors: Pearton S. J. , Cho H. , LaRoche J. R. , Ren F. , Wilson R. G. , Lee J. W.
Published: July 2002

SiO 2 layers were deposited on p-GaN (hole concentration 9×1017 cm−3) by inductively coupled plasma chemical vapor deposition using an O17-enriched O2 precursor. The samples were then annealed at 500–900 °C and the SiO2 was removed. Secondary ion mass spectrometry profiling showed significant indiffusion of O17 into the GaN under these conditions, with an incorporation depth of ∼0.18 μm after the 900 °C anneal. The O17 diffusion profiles indicate that the high dislocation density in the GaN strongly affects the effective penetration depth. The GaN remained p type upon incorporation of the oxygen.
Two-dimensional growth of InSb thin films on GaAs(111)A substrates
Authors: Kanisawa K. , Yamaguchi H. , Hirayama Y.
Published: July 2002

Heteroepitaxy of high-quality InSb films was performed directly on GaAs surfaces by using molecular beam epitaxy. Despite the 14.6% lattice mismatch, two-dimensionally grown InSb on GaAs(111)A substrates were obtained from the initial stage, but not on (001) substrates. A conductive layer was formed from the early stage of the growth on the (111)A surface, and the mobilities and carrier concentrations of InSb on (111)A substrates suggested a low defect density due to confinement of the dislocations to the interface.
Polymeric electro-optic modulator based on 1×2 Y-fed directional coupler
Authors: An Dechang , Shi Zan , Sun Lin , Taboada John M. , Zhou Qingjun , Lu Xuejun , Chen Ray T. , Tang Suning , Zhang Hua , Steier William H. , Ren Albert , Dalton Larry R.
Published: July 2002

We have demonstrated a polymeric electro-optic modulator based on a 1×2 Y-fed directional waveguide coupler. The symmetric geometry of the 1×2 Y-fed directional coupler provided the modulator unique characteristics of intrinsic 3 dB operating point and two complementary output ends. A low switching voltage of 3.6 V and a high extinction ratio of 26 dB were obtained with the modulator operating at a wavelength of 1.34 μm. The modulator was fabricated with a novel electro-optic polymer that was synthesized from polyurethane cross-linking with a chromophore.
Highly efficient pure blue electroluminescence from polyfluorene: Influence of the molecular weight distribution on the aggregation tendency
Authors: Weinfurtner Karl-Heinz , Fujikawa Hisayoshi , Tokito Shizuo , Taga Yasunori
Published: July 2002

A dependency between the molecular weight distribution of polyfluorene (PFO) and its aggregation tendency on operation in organic light-emitting devices (OLED) is described. As a result of these findings, low molecular parts of the polymer could be made responsible for the aggregation in liquid crystalline PFO. Consequently, a procedure was developed which leads to PFO-based OLEDs which do not show aggregation on operation anymore but exhibit highly efficient and stable blue electroluminescence.
Characterization of threading dislocations in GaN epitaxial layers
Authors: Hino T. , Tomiya S. , Miyajima T. , Yanashima K. , Hashimoto S. , Ikeda M.
Published: July 2002

We investigated Si-doped GaN epitaxial layers on a (0001)-sapphire substrate using a HCl vapor-phase etching technique, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. Three kinds of distinctive etch pits correspond to three different types of threading dislocations, edge, mixed, and screw types. Photoluminescence intensity increases with the decrease in the number of etch pits corresponding to mixed and screw dislocations. The number of etch pits corresponding to edge dislocations, however, did not change. We concluded, therefore, that threading dislocations having a screw-component burgers vector act as strong nonradiative centers in GaN epitaxial layers, whereas edge dislocations, which are the majority, do not act as nonradiative centers.
High-temperature photoluminescence in sol-gel silica containing SiC/C nanostructures
Authors: Li Guangming , Burggraf Larry W. , Shoemaker James R. , Eastwood DeLyle , Stiegman Albert E.
Published: July 2002

Silicon carbide and carbon nanostructures were produced by pyrolysis of organosilane or aromatic compounds in nanoporous sol-gel silica glasses. Intense photoluminescence was observed in the visible and the near infrared regions, depending on material processing. Emission bands at 2.97, 2.67, 2.53, 2.41, 2.24, 2.09, 1.93, 1.13, 1.00, and 0.85 eV were observed in samples prepared at temperatures between 870 and 1220 K. Phosphorescence emission showed two lifetime components at 300 K: a 0.03 s component and a very long component of 0.5–4 s that depends on the precursors and sample processing. These lifetimes approximately doubled at 77 K. The visible emission increased significantly as the temperature was elevated from 77 to 950 K, suggesting thermally assisted light emission from sites in the silica glasses containing SiC/C nanostructures. Surface SiC vacancy defects modeled using integrated ab initio quantum mechanics/molecular mechanics calculations suggest phosphorescence may originate from C vacancy (Si–Si dimers) in the visible and Si vacancy in the near infrared.
Billiard representation for multidimensional cosmology with intersecting p-branes near the singularity
Authors: Ivashchuk V. D. , Melnikov V. N.
Published: July 2002

Multidimensional model describing the cosmological evolution of n Einstein spaces in the theory with l scalar fields and forms is considered. When electromagnetic composite p-brane ansatz is adopted, and certain restrictions on the parameters of the model are imposed, the dynamics of the model near the singularity is reduced to a billiard on the (N−1)-dimensional Lobachevsky space HN−1, N=n+l. The geometrical criterion for the finiteness of the billiard volume and its compactness is used. This criterion reduces the problem to the problem of illumination of (N−2)-dimensional sphere SN−2 by pointlike sources. Some examples with billiards of finite volume and hence oscillating behavior near the singularity are considered. Among them examples with square and triangle two-dimensional billiards (e.g., that of the Bianchi-IX model) and a four-dimensional billiard in “truncated” D=11 supergravity model (without the Chern–Simons term) are considered. It is shown that the inclusion of the Chern–Simons term destroys the confining of a billiard.
Blastfurnace slag cements: A construction material with very unusual nuclear spin relaxation behavior during hardening
Authors: Nestle Nikolaus , Dakkouri Marwan , Geier Oliver , Freude Dieter , Kärger Jörg
Published: July 2002

In this contribution, nuclear magnetic resonance (NMR) relaxometry results on hardening hydraulically activated blastfurnace slag pastes are presented. The results are found to be very different from the behavior of other hydraulic building materials such as cement and gypsum. A mechanism based on water diffusion in inner field gradients is suggested in order to explain the observed relaxation behavior and corroborated by additional experimental evidence from magnetic susceptibility measurements and NMR relaxation measurements on slag pastes prepared from nonwatery fluids.
High-pressure, high-temperature synthesis of SiC–diamond nanocrystalline ceramics
Authors: Ekimov E. A. , Gavriliuk A. G. , Palosz B. , Gierlotka S. , Dluzewski P. , Tatianin E. , Kluev Yu. , Naletov A. M. , Presz A.
Published: July 2002

Dense and entirely nanocrystalline diamond–SiC ceramics were synthesized by the infiltration of liquid Si into the nanodiamond body under high-pressure (77 kbar) and high-temperature (1400–2000 °C) conditions. Based on x-ray diffraction and transmission electron microscopy observations, a model of as-synthesized material is proposed, where individual polycrystalline diamond particles are bonded via SiC–diamond nanolayers. The nanolayers provide a very high hardness of the entire specimen up to 51 GPa. The phenomenon of self-stop Si infiltration was detected in this system. This phenomenon can be explained by the closure of pores near the boundary between molten Si and diamond nanopowder due to the formation of SiC inside the pores.
Separation of thermal wave induced displacements and thermoreflectance variations using a rotated beamsplitter cube interferometer
Authors: Fiedler C. J. , Wagner J. W. , Henseler K. E.
Published: July 2002

A rotated beamsplitter cube interferometer has been developed which can measure both the change in reflectivity and the displacement of nontransparent thin films caused by thermal waves. Thermal waves are used to characterize material properties, especially those of thin films, by monitoring the heat propagation in a specimen. Thermal waves cause both a change in the reflectivity of the specimen and a displacement of the specimen surface. By varying the phase difference between the reference and signal arms of an interferometer, a method for detecting and separating the contributions of each of these effects has been developed. This enables the determination of the thermal expansion coefficient of the thin film independent of substrate properties. Data demonstrating the application of this technique on Cu and TiN thin films are presented.
Helium doped hydrogen or deuterium beam as cost effective and simple tool for plasma spectroscopy
Authors: Falter H. D. , Proschek M. , Menhart S. , Aumayr F. , Winter HP. , Ciric D. , Cox S. , Dines A. , Godden D. , Hawkes N. , Jones T. T. C.
Published: July 2002

Energetic neutral particles from neutral beam heating systems are widely used for active spectroscopic measurements of key plasma parameters in fusion experiments. Both the plasma discharges and the neutral beam systems are normally operated with hydrogen or deuterium. Helium beams are used in dedicated diagnostic beam lines as they offer deeper penetration and are subject to less background radiation and enable resonant double charge exchange with alpha particles. Neutral beam systems using pure helium either require specialized helium gas pumping with a pumping speed in excess of 1000 m3/h or are restricted to short pulses (normally less than 1 s). A doped hydrogen/helium beam combines the requirements for plasma heating and diagnostics without the need for sophisticated helium pumping. A small flow of helium gas is injected into the plasma source for the time helium particles are required. The helium current is typically 10% of the total extracted current. The reduction in heating power of the doped beam can be kept below 5%. The small amount of helium gas does not cause an excessive pressure rise along the beam line and does not reduce the reliability of the beam heating system. Doped deuterium/helium beams have been successfully tested and routinely used at JET. The Hel beam emission spectra obtained with a doped deuterium/helium beam produce sufficiently strong visible lines for spectroscopic applications. Furthermore, the simultaneous availability of helium and hydrogenic particles in the beam allows us to extend spectroscopic measurements to another atomic system and hence cross-check results from helium beams with those from hydrogenic beams. The only investment required is an additional helium gas inlet system into the ion source.
Radio frequency ion source for plasma diagnostics in magnetic fusion experiments
Authors: Ivanov A. A. , Davydenko V. I. , Deichuli P. P. , Kreter A. , Mishagin V. V. , Podminogin A. A. , Shikhovtsev I. V. , Schweer B. , Uhlemann R.
Published: July 2002

Low-divergent quasistationary neutral beams are often applied in modern magnetic fusion devices as a diagnostic tool providing unique information about plasma parameters. The most important requirements of these beams are sufficiently large current and energy of the particles, so that the beam can penetrate to the plasma core. Also the duration of the beams must be long enough, i.e., close to that of a plasma discharge, amounting to at least a few seconds for large fusion devices. We developed a neutral beam injector for plasma diagnostics in the tokamak TEXTOR-94 which is capable of meeting these requirements. The maximum beam energy is 50 keV and the source operated in hydrogen delivers an ion current of up to 2 A with a pulse duration of up to 4 s. The low divergent beam (∼0.5°– 0.6°) is geometrically focused 4 m downstream from the source having a 1/e width of ∼ 70 mm at the focal point. The beam can be modulated with a frequency variable up to 500 Hz. The ion source plasma is produced by a radio frequency discharge in hydrogen or helium. The ion beam is extracted by a four-grid system with 163 single holes. The measured beam parameters were compared with those predicted by simulations.
Automatic and autonomous infrared system for remote sensing of forest fires
Authors: Ugarte M. F. , de Castro A. J. , Briz S. , Aranda J. M. , López F.
Published: July 2002

The proposed sensor is a new application of the well-known ratiometric techniques to photoconductor array. In this case, the sensor is a photoconductor multiplexer array of 64×4 pixels that covers a sector area 5 km range by 20° azimuth. The proposed system is an automatic and autonomous staring system with only one fixed lens and no mobile parts in order to reduce costs and maintenance. This article presents the fundamental contributions of the new configuration system, especially the adaptive background capability. This configuration can compensate large temperature variations, nonuniformity, and ambient interference without complex algorithms or complex mechanical system.
Improved stop cylinder loading scheme for a centrifuge pellet accelerator
Authors: Lang P. T. , Cierpka P. , Horton L. , Lorenz A. , Watson M. J. , Twynam P. , Walden A. , Wijetunge S. , Willis B. , Hosogane N. , Hiratsuka H. , Kizu K.
Published: July 2002

In order to perform plasma refueling experiments with cryogenic hydrogen pellets at the midsize tokamak ASDEX Upgrade, a centrifuge injector was developed based on the novel stop cylinder principle. Following the demonstration of excellent performance with this instrument, similar injectors were constructed for two of the world’s largest tokamaks, JET and JT-60U. The extended fuel consumption in these tokamaks enforced the employment of continuous extruders instead of a storage cryostat, requiring an increase in pellet temperature from 8 to 14 K. Degradation of the injector performance was found at these higher pellet temperatures due to increased gas evaporation rates but could be eliminated by a revised acceleration scheme. Applying the adapted setup, reliable and stable operation has been realized with JET and JT-60U injectors.
Approximating the basis set dependence of coupled cluster calculations: Evaluation of perturbation theory approximations for stable molecules
Authors: Dunning Thom H. , Peterson Kirk A.
Published: July 2002

The coupled cluster CCSD(T) method provides a theoretically sound, accurate description of the electronic structure of a wide range of molecules. To obtain accurate results, however, very large basis sets must be used. Since the computational cost of CCSD(T) calculations formally increases with the seventh power of the number of basis functions (N7), the CCSD(T) method can only be applied to a restricted range of molecules. In this work we show that the basis set dependence of the CCSD(T) method is well described by perturbation theory. Starting with CCSD(T)/aug-cc-pVTZ calculations, use of the MP3 method to simulate the effect of increasing the basis set to aug-cc-pV5Z leads to average absolute errors, relative to the full CCSD(T)/aug-cc-pV5Z calculations, of less than ±0.4 kcal/mol (De), ±0.0002 Å (re), ±2 cm−1 (ωe), 0.1 kcal/mol (IPe), and 0.2 kcal/mol (EAe) for the test set of diatomic molecules considered here. Although the corresponding MP2 approximation does not provide this high level of accuracy, it also should be useful for many molecular studies. When properly implemented, the savings in computer time should be significant since the MP3 method formally scales as N6, while the MP2 method scales as only N5.
Continuum percolation in macromolecular fluids
Authors: Chatterjee Avik P.
Published: July 2002

A heuristic treatment of the connectedness Ornstein–Zernike equation is developed for macromolecular fluids within the framework of the polymer reference interaction site model (PRISM). Results are presented for the critical volume fraction at the percolation threshold and for the mean number of interchain contacts per molecule for athermal rodlike and Gaussian coil-like particles. The results for rodlike particles are in qualitative agreement with prior investigations based on fully numerical solutions of the PRISM equations, and with computer simulations of ellipsoids. The method proposed here leads to the physically reasonable result that the percolation threshold for direct connectivity on the length scale of the molecular dimensions is closely related to the semidilute crossover concentration as usually defined, and can be generalized directly to multicomponent systems.
Thermal stress and glass transition of ultrathin polystyrene films
Authors: Zhao Jie-Hua , Kiene Michael , Hu Chuan , Ho Paul S.
Published: July 2002

The thermal stress of thin and ultrathin polystyrene (PS) films on Si substrate has been studied and the glass transition temperature (Tg) is determined from the thermal stress data. Tg of PS turned out to be thickness independent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains (<100 nm). The thermal stress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates that the product of the biaxial modulus E/(1−ν) and the coefficient of thermal expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease significantly with respect to the bulk value.
The infrared spectroscopy of hydrogen-bonded bridges: 2-pyridone-(water)n and 2-hydroxypyridine-(water)n clusters, n=1,2
Authors: Florio Gina M. , Gruenloh Christopher J. , Quimpo Robert C. , Zwier Timothy S.
Published: July 2002

The water-containing clusters of the two tautomers 2-hydroxypyridine (2HP) and 2-pyridone (2PYR) are studied in the hydride stretch region of the infrared using the techniques of resonant ion-dip infrared spectroscopy (RIDIRS) and fluorescence-dip infrared spectroscopy (FDIRS). The results on 2PYR-(water)n build on previous high-resolution ultraviolet spectroscopy [Held and Pratt, J. Am. Chem. Soc. 115, 9708 (1993)] on the n=1,2 clusters and the infrared depletion spectra of Matsuda et al. [J. Chem. Phys. 110, 8397 (1999)] on the n=1 cluster. The 2PYR-W2 FDIR spectrum reflects the consequences of extending and strengthening the H-bonded bridge between N–H and C=O sites in 2PYR. The spectrum shows evidence of strong coupling along the bridge, both in the form of the hydride stretch normal modes and in the breadth of the observed infrared transitions. RIDIR spectra of the 2HP-Wn clusters are compared with those of 2PYR-Wn in order to assess the spectroscopic consequences of forming the analogous water bridges in the lactim tautomer. Density functional theory calculations are compared with the RIDIR spectra to deduce that the 2HP-Wn clusters are indeed water-containing bridge structures closely analogous to their 2PYR counterparts. The IR spectra of the 2HP-Wn clusters bear a striking resemblance to those of 2PYR-Wn. Potential reasons for the unusual breadth of the bridge XH stretches are discussed.
Side-by-side silicon carbide–silica biaxial nanowires: Synthesis, structure, and mechanical properties
Authors: Wang Z. L. , Dai Z. R. , Gao R. P. , Bai Z. G. , Gole J. L.
Published: July 2002

Elevated-temperature synthesis has been used to generate side-by-side biaxially structured silicon carbide–silica nanowires. The axial growth direction approaches [311] for nanowires with a high density of microtwins and is [211] for defect-free nanowires. The structure of these nanowires, their cross-sectional shape, and their structural transformation between a biaxial and coaxial configuration have been studied by transmission electron microscopy. The Young’s modulus of the biaxially structured nanowires was measured to be 50–70 GPa depending on the size of the nanowire.
Capacity of quantum channels using product measurements
Authors: King Christopher , Ruskai Mary Beth
Published: July 2002

The capacity of a quantum channel for transmission of classical information depends in principle on whether product states or entangled states are used at the input, and whether product or entangled measurements are used at the output. We show that when product measurements are used, the capacity of the channel is achieved with product input states, so that entangled inputs do not increase capacity. We show that this result continues to hold if sequential measurements are allowed, whereby the choice of successive measurements may depend on the results of previous measurements. We also present a new simplified expression which gives an upper bound for the Shannon capacity of a channel, and which bears a striking resemblance to the well-known Holevo bound.
A single Lanczos propagation method for calculating transition amplitudes. II. Modified QL and symmetry adaptation
Authors: Chen Rongqing , Guo Hua
Published: July 2002

The recently proposed single Lanczos propagation method [J. Chem. Phys. 111, 9944 (1999)] for calculating multiple transition amplitudes is made more efficient in several aspects. It is shown that the amplitudes can be calculated without the explicit calculation and storage of the Lanczos eigenvectors, thus significantly reducing the computational costs, particularly for long propagation. It is also shown that symmetry adaptation can be implemented in the Lanczos propagation in a straightforward manner. In particular, eigenspectra in multiple irreducible representations can be determined from a single recursion by projecting the symmetrized Lanczos states at each step. The accuracy and efficiency of the improved methods are ascertained by numerical tests of realistic triatomic and tetratomic systems.
Pseudodonor nature of the DI defect in 4H-SiC
Authors: Storasta L. , Carlsson F. H. C. , Sridhara S. G. , Bergman J. P. , Henry A. , Egilsson T. , Hallén A. , Janzén E.
Published: July 2002

We use the recent findings about the pseudodonor character of the DI defect to establish an energy-level scheme in the band gap for the defect, predicting the existence of a hole trap at about 0.35 eV above the valence band. Using minority carrier transient spectroscopy, we prove that the DI defect indeed is correlated to such a hole trap. In addition, we show that the DI defect is not correlated to the Z1/2 electron trap, in contrast to what was previously reported.
Phase equilibrium in a quadrupolar hard sphere interaction site model of benzene
Authors: Schroer J. W. , Monson P. A.
Published: July 2002

We present a study of the phase diagram of a quadrupolar hard-sphere interaction site model of benzene using Monte Carlo simulation. Extensive calculations were made for the fluid phase and three solid phases for several values of the quadrupole moment. For an appropriate value of the quadrupole moment the model yields a volume change on freezing in good agreement with the experimental value for benzene. The results indicate that the relative stability of the high- and low-pressure solid phases of benzene can be understood qualitatively in terms of a competition between packing effects associated with short-range repulsions and electrostatic (quadrupole–quadrupole) interactions. A theoretical approach in which thermodynamic perturbation theory is used for the fluid phase and the cell theory for the solid phases gives a quite accurate description of the phase behavior in the model system.
Large eddy simulation of a turbulent reacting jet with conditional source-term estimation
Authors: Steiner H. , Bushe W. K.
Published: July 2002

The Conditional Source-term Estimation (CSE) method was recently proposed to close the chemical source terms occurring in the spatially filtered transport equations of species and enthalpy for Large Eddy Simulation (LES) of nonpremixed reacting flows  [W. K. Bushe and H. Steiner, Phys. Fluids 11, 1896 (1999)]. The model is based on the Conditional Moment Closure hypothesis, which provides fairly accurate predictions for the conditional averages of the chemical reaction rates as functions of the conditionally averaged composition vector and temperature with the mixture fraction being an appropriate conditioning variable. In CSE the conditionally averaged composition vector and temperature are obtained by mapping the corresponding spatially filtered scalar fields resolved by the LES into the conditioning (i.e., mixture fraction) space. After the conditional averages of the chemical reaction rates are approximated in mixture fraction space, these are mapped into the physical space to close the source terms in the LES transport equations for the reactive scalars. The present simulation of a turbulent reacting jet is the first test of this new closure in a self-sustained predictive LES. A two-step reduced chemical kinetic mechanism for methane–air flames was used. The results of the simulation, which are in reasonable agreement with available experimental data, prove the model’s predictive capabilities as well as its robustness and feasibility for LES.
Particle Monte Carlo simulation of quantum phenomena in semiconductor nanostructures
Authors: Tsuchiya Hideaki , Ravaioli Umberto
Published: July 2002

Quantum effects in semiconductor devices are usually described in terms of wave functions obtained from the solution of the Schrödinger equation. However, it is difficult to simulate practical devices where clear semiclassical and quantum features coexist, as is the case for nanoscale devices at normal temperatures. We present here a particle description of quantum phenomena derived starting from Wigner’s transport formalism, where the dynamics of particles are treated semiclassically, but with an effective force added to account for quantum effects. The resulting model is solved by using a particle Monte Carlo approach, which we apply to describe transport across a single tunneling barrier. Results of the numerical calculations indicate that size quantization and tunneling effects can be well resolved by the combined Monte Carlo/quantum force approach, yielding quantitative agreement with Schrödinger equation results.
Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion
Authors: Palamaru M. , Lalanne Ph.
Published: July 2002

An accurate model for the out-of-plane radiation losses occurring when a guided wave propagating in a conventional waveguide impinges on a photonic crystal waveguide is presented. The model makes clear that the losses originate from insertion losses resulting from a mode mismatch. A generic taper structure realizing an adiabatic modal conversion is proposed and validated through numerical computations for cavities with large Q’s and large peak transmission.
Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure
Authors: Yanagi Hiroshi , Hase Tomomi , Ibuki Shuntaro , Ueda Kazushige , Hosono Hideo
Published: July 2002

A transparent oxide semiconductor with delafossite structure, CuInO2, was found to exhibit both p-type and n-type conduction by doping of an appropriate impurity and tuning of proper film-deposition conditions. Thin films of Ca-doped or Sn-doped CuInO2 (optical band gap=∼3.9 eV) were prepared on α-Al2O3(001) single crystal substrates by pulsed laser deposition method. The films were deposited at 723 K in O2 atmosphere of 1.0 Pa for the Ca-doped films or 1.5 Pa for the Sn-doped films. The positive sign of the Seebeck coefficient demonstrated p-type conduction in the Ca-doped films, while the Seebeck coefficient of the Sn-doped films was negative indicating n-type conductivity. The electrical conductivities of Ca-doped and Sn-doped CuInO2 thin films were 2.8×10−3 S cm−1 and 3.8×10−3 S cm−1, respectively, at 300 K.
Conductivity of deionized two-component colloidal suspensions
Authors: Wette Patrick , Schöpe Hans-Joachim , Biehl Ralf , Palberg Thomas
Published: July 2002

The low frequency ac-conductivity of deionized aqueous suspensions comprising of charged latex spheres is investigated. For the one-component cases σ increases linearly with particle number density n, irrespective of the suspension structure. Two-component mixtures are found to form substitutional crystals and no phase separation is observed for small size differences. Then σ is proportional to the sum of the individual conductivity contributions. Further at fixed composition the linear increase with n is retained. The effects can be well described with an extension of Hessinger’s conductivity model to two-component systems.
Effect of dipolar interactions on the ferromagnetic resonance properties in arrays of magnetic nanowires
Authors: Encinas-Oropesa A. , Demand M. , Piraux L. , Ebels U. , Huynen I.
Published: July 2002

The microwave absorption properties of arrays of magnetic nanowires electrodeposited in nanoporous membranes have been studied using a strip transmission line configuration. From the measurements, the ferromagnetic resonance properties of the magnetic wires have been extracted at frequencies ranging from 100 MHz to 40 GHz for various angles between the wire axis and an external static magnetic field up to 8 kOe. We report on extensive data obtained on the ferromagnetic resonance properties in arrays of nanowires of Ni, NiFe, and NiFeCo alloys with various diameters and packing densities. In such arrays, apart from the shape anisotropy, a strong dipolar coupling between wires is present which can be modulated by the wire density. Both ferromagnetic resonance spectra and hysteresis loops show that this coupling acts as an additional uniaxial anisotropy favoring an easy axis perpendicular to the wires. In particular, it is shown that the easy axis of magnetization can be tuned parallel or perpendicular to the wire axis by changing the wire packing density. Furthermore, ferromagnetic resonance occurs even at zero field and the resonance frequency is fixed by the saturation magnetization of the material independent of the packing density.
Scanning superconducting quantum interference device susceptometry
Authors: Gardner Brian W. , Wynn Janice C. , Björnsson Per G. , Straver Eric W. J. , Moler Kathryn A. , Kirtley John R. , Ketchen Mark B.
Published: July 2002

We report a scanning superconducting quantum interference device (SQUID) microsusceptometer with a spatial resolution of 8 μm, tested by measuring the susceptibility of individual 3 μm diam tin disks. Images of the disks agree well with numerical modeling based on the known geometry of the SQUID microsusceptometers. The low-field spin sensitivity between 1.5 and 6 K is 1×105 μB/Hz while scanning.
A numerical study of the evolution of a solitary wave over a shelf
Authors: Liu Philip L.-F. , Cheng Yinguo
Published: July 2002

Using Reynolds averaged Navier–Stokes (RANS) equations, we have conducted a series of numerical experiments to investigate the evolution of a solitary wave propagating over a step. Both nonbreaking and breaking solitary waves are studied. To study breaking waves, the RANS equations are coupled with k–ε turbulence equations. For the nonbreaking wave case the numerical solutions are compared with available experimental data. The numerical experiments demonstrate that both nonbreaking and breaking solitary waves disintegrate into several solitons over the step. However, the fission processes for generating the second and third soliton are quite different for nonbreaking and breaking solitary waves.
Millimeter-wave intracavity-jet OROTRON-spectrometer for investigation of van der Waals complexes
Authors: Surin L. A. , Dumesh B. S. , Lewen F. , Roth D. A. , Kostromin V. P. , Rusin F. S. , Winnewisser G. , Pak I.
Published: July 2002

A highly sensitive intracavity millimeter-wave spectrometer was developed for the investigation of the absorption spectra of van der Waals complexes in a supersonic jet. The key element of the spectrometer is a tunable oscillator, called OROTRON, which generates the millimeter-wave radiation through the interaction of an electron beam with the electromagnetic field of a high quality (Q≈104) Fabry–Perot resonant cavity. This cavity consists of a movable spherical mirror and a fixed planar mirror with the periodic structure imprinted on its surface. The electron beam moves along the periodic structure of the plane mirror. This part separated from the rest of the resonator by a mica foil is kept under ultrahigh vacuum conditions. The molecular jet is injected by a pulsed valve into the other part of the resonator. The absorption in the jet is sensitively detected by measuring the electric current in a special collector circuit of the OROTRON. The spectral purity of the OROTRON radiation is 10–15 kHz providing the capability of sub-Doppler spectral resolution without phase locking. An increase in sensitivity of a factor of about 100 in comparison with the usual single pass arrangement was evaluated from the measurements of the absorption lines of the CO rare isotopomers, the Ar–CO and Ne–CO van der Waals complexes. The high sensitivity, wide spectral range, and simple tunability of the spectrometer make it a very efficient tool for the searching of weakly absorbing species in a jet.
Time-resolved probing of the Purcell effect for InAs quantum boxes in GaAs microdisks
Authors: Gayral B. , Gérard J.-M. , Sermage B. , Lemaı̂tre A. , Dupuis C.
Published: July 2002

We studied, by time-resolved microphotoluminescence (PL), the spontaneous emission dynamics of InAs/GaAs quantum boxes coupled to high Purcell factor (up to 155) whispering gallery modes of microdisks under nonresonant excitation. A global shortening of the PL decay by a factor of up to 12 was observed for the collection of quantum boxes on resonance with the modes. This behavior is well understood when the random distribution of the quantum boxes and the finite relaxation rate of the carriers are taken into account.
Temperature dependence of the radiative recombination zone in InGaN/GaN multiple quantum well light-emitting diodes
Authors: Lee Chia-Ming , Chuo Chang-Cheng , Dai Jing-Fu , Zheng Xian-Fa , Chyi Jen-Inn
Published: July 2002

The temperature dependence of the radiative recombination zone in InGaN/GaN multiple quantum well light-emitting diodes is investigated. From the electroluminescence spectra measured at various temperatures, it is found that there are two peaks at about 400 and 460 nm, which can be assigned as Mg-related and quantum well transitions, respectively. The behavior of these two peaks with temperature is modeled by the two rate equation. Based on this model, we deduce the activation energy of Mg in GaN films to be about 126 meV, which is consistent with reported results obtained by other techniques.
Guided ion beam study of collision-induced dissociation dynamics: integral and differential cross sections
Authors: Muntean Felician , Armentrout P. B.
Published: July 2002

The low energy collision-induced dissociation (CID) of Cr(CO)6+ with Xe is investigated using a recently modified guided ion beam tandem mass spectrometer, in the energy range from 0 to 5 eV in the center-of-mass (CM) frame. The additions to the instrument, updated with a double octopole system, and the new experimental methods available are described in detail. Integral cross sections for product formation are presented and analyzed using our standard modeling procedure. A slightly revised value for the bond dissociation energy of (CO)5Cr+–CO of 1.43±0.09 eV is obtained, in very good agreement with literature values. Axial and radial velocity distributions for primary and product ions are measured at 1.3, 2.0, and 2.7 eV, in the threshold region for product formation. The resulting velocity scattering maps are presented and discussed. Evidence of efficient energy transfer is observed from angular scattering of CID products. Experimental distributions of residual kinetic energies are derived and extend to zero, the point of 100% energy deposition. This indicates that energy transfer is nonimpulsive and probably associated with transient complex formation. For the first time, the experimental residual kinetic energy distributions are compared with the predictions of the empirical model used in integral cross section analyses. Good agreement is observed within experimental uncertainties. A model for the distribution of deposited energy during collisional activation is derived on the basis of these experimental observations.
Fiber optic sensor for dual measurement of temperature and strain using a combined fluorescence lifetime decay and fiber Bragg grating technique
Authors: Wade S. A. , Forsyth D. I. , Grattan K. T. V. , Guofu Q.
Published: July 2002

A fiber optic sensor device has been developed incorporating a short length of erbium doped fiber fused in close proximity to a single-fiber Bragg grating, to measure both the fluorescent lifetime decay and the wavelength shift in these respective elements, for temperature and strain determination. Calibration results obtained from this simple, low cost, intrinsic sensor scheme show standard deviation errors of 20.4 με and 1.2 °C over strain and temperature ranges of 22–1860 με and 25–120 °C, respectively.
Magnetic-field-tunable photoluminescence transitions in antiferromagnetic EuTe epilayers layers with an effective g factor of 1140
Authors: Heiss W. , Prechtl G. , Springholz G.
Published: July 2002

Narrow photoluminescence transitions of excitons in antiferromagnetic layers of EuTe grown by molecular beam epitaxy are reported. At low temperatures, two excitonic peaks are observed at around 1.9 eV with an additional broad emission band at 1.5 eV that is attributed to defects. With applied magnetic field, the excitonic transitions shift linearly by −34 meV/T to smaller energies with a total shift of more than 240 meV at 7.2 T. At T=2 K, the observed magnetic field dependence corresponds to an effective g factor of 1140, which is independent of applied field. The observed magnetic field tunability of the excitonic transitions is explained by the formation of magnetic polarons.
Multiwavelength thermal lens determination of fluorescence quantum efficiency of solids: Application to Nd3+-doped fluoride glass
Authors: Lima S. M. , Andrade A. A. , Lebullenger R. , Hernandes A. C. , Catunda T. , Baesso M. L.
Published: July 2002

A multiwavelength thermal lens (TL) method was developed to determine the fluorescence quantum efficiency of solids. The experiments have been performed in Nd3+-doped fluoride glasses, using five different excitation wavelengths: 514, 576, 741, 796, and 867 nm. In this work, the absolute value of fluorescence quantum efficiency can be obtained by the linear dependence of the TL signal with the excitation wavelength. Our results suggest that the method can be applied to study a wide range of fluorescent materials, with a special ability to be used for measurements of highly transparent materials with very low TL effect.
Conditional mode-elimination and the subgrid-modeling problem for isotropic turbulence
Authors: McComb W. D. , Hunter A. , Johnston C.
Published: July 2002

It is shown analytically that different subgrid eddy viscosities are required to characterize the evolution of the velocity field, the energy spectrum and the dissipation rate, respectively. Also, a subgrid viscosity calculated recursively from the conditionally averaged “subgrid–subgrid” stress can only be used to renormalize the dissipation relation. It is also shown, by direct numerical simulation, that the spectral correlations of subgrid and resolved stresses are dominated by “resolved–subgrid” terms, in agreement with existing conclusions from energy transfer or other more global methods. We also note the low level of correlation between the exact subgrid stress and the eddy–viscosity model for all explicit–scales wave numbers. Lastly, we report results from a large-eddy simulation (LES) based on a recursively calculated effective viscosity and note that the results are about as good as for other subgrid models but underline the need to take account of phase effects as well as the dissipation rate.
Role of laser pulse duration and gas pressure in deposition of AlN thin films
Authors: Gyorgy Eniko , Ristoscu Carmen , Mihailescu I. N. , Klini Argyro , Vainos N. , Fotakis C. , Ghica C. , Schmerber G. , Faerber J.
Published: July 2002

We investigated the relative merits and limits of pulsed laser deposition from AlN targets in vacuum and low-pressure nitrogen in obtaining stoichiometric and crystalline aluminum nitride thin films. We used two UV excimer laser sources (λ=248 nm): a nanosecond system (τFWHM=30 ns) and, a subpicosecond (τFWHM=450 fs) system. The obtained structures were characterized by x-ray diffraction, electron microscopy in cross section, selected area electron diffraction, and profilometry. We demonstrated that the best results are obtained with the sub-ps laser source in vacuum and in low pressure nitrogen when the AlN thin films are very pure, crystalline, clearly exhibiting a tendency to epitaxy. Metallic Al is present in the films deposited with the ns laser source. We believe this is an effect of the gradual decomposition of AlN inside the crater on the target surface under multipulse laser irradiation.
Low-noise 1 THz superconductor–insulator–superconductor mixer incorporating a NbTiN/SiO2/Al tuning circuit
Authors: Jackson B. D. , Baryshev A. M. , de Lange G. , Gao J.-R. , Shitov S. V. , Iosad N. N. , Klapwijk T. M.
Published: July 2002

Low-noise heterodyne mixing at 1 THz is demonstrated in a quasioptical mixer incorporating Nb superconductor–insulator–superconductor tunnel junctions and a NbTiN/SiO2/Al tuning circuit. Receiver noise temperatures as low as 250 K at 850 GHz, 315 K at 980 GHz, and 405 K at 1015 GHz are measured—a factor of 2 improvement in sensitivity versus state-of-the-art 1 THz receivers, which incorporate normal metal tuning circuits. An analysis of the receiver sensitivity at 980 GHz demonstrates that NbTiN is low loss up to ∼1 THz.
Structure dependence of ferroelectric properties of bismuth layer-structured ferroelectric single crystals
Authors: Irie Hiroshi , Miyayama Masaru , Kudo Tetsuichi
Published: July 2002

Ferroelectric properties along the a(b) axis and c axis in single crystals of various bismuth layer-structured ferroelectrics (BLSFs) were investigated. By measuring saturated P-E hysteresis curves of BLSFs, values of the saturated remanent polarization and the coercive electric field were found to be related to the Curie temperature and the number of BO6 octahedra (m) between bismuth layers, respectively, along the a(b) axis. The saturated remanent polarization was larger in the BLSF with a higher Curie temperature. This is attributed to a large atomic displacement accompanied by a high Curie temperature. In contrast, the saturated coercive electric field was smaller in the BLSF with a larger number of m. This phenomenon is assumed to be caused by the decrease in the strain energy of the octahedra from the bismuth layer, which leads to an easy movement of the octahedral cations in the direction of an applied external electric field. Along the c axis, the BLSFs with an odd number of m had the same relationship, that is, the saturated remanent polarization was larger with the high Curie temperature, and the saturated coercive field was smaller with the large m number. However, no remanent polarization was confirmed in the BLSFs with an even number of m.
Direct observation of structural changes in organic light emitting devices during degradation
Authors: Kolosov Dmitry , English Douglas S. , Bulovic Vladimir , Barbara Paul F. , Forrest Stephen R. , Thompson Mark E.
Published: July 2002

A method for studying the degradation of organic light emitting devices (OLEDs) in real time is described. Transparent OLEDs allow for the spatial correlation of cathode topographic images with optical images (transmission, photoluminescence, and electroluminescence) of the devices throughout the degradation process. In this study we focused on the evolution of nonemissive, “dark” spots during device operation. We conclude that the electroluminescent dark spots originate as nonconductive regions at the cathode/organic interface and expand or grow as a result of exposure to atmosphere. We propose a mechanism of dark spot growth involving aerobic oxidation of the cathode/organic interfacial region, leading to a highly resistive, carrier blocking interface at the dark spot locations. No initial defects on the cathode surface, which might be responsible for the formation of dark spots, were detected by atomic force microscopy. Structural changes, such as degradation of organic materials and the cathode surface, occur well after the formation and growth of the dark spots.