bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-consistent analysis of three-dimensional uniformly charged ellipsoid with zero emittance
Authors: Batygin Yuri K.
Published: July 2002

A self-consistent treatment of a three-dimensional ellipsoid with negligible emittance in the time-dependent external field is performed. Envelope equations describing the evolution of an ellipsoid boundary are discussed. For a complete model, it is required that the initial particle momenta be linear functions of the coordinates. Numerical examples and verification of the problem by three-dimensional particle-in-cell simulations are given.
Novel continuously tunable high spectral resolution optical filter for two-dimensional imaging
Authors: Mukherjee Pritish , Chen Shudong , Witanachchi Sarath
Published: July 2002

Despite the existence of a variety of optical filters for the separation of spectral components in a multicolor image, a filter that is both continuously wavelength tunable and capable of high spectral resolution while preserving the spatial integrity of a two-dimensional image is not currently available. We present, in this article, the introduction of a novel optical filtering concept that permits the development of such a system. Both the concept and its implementation in an optical-fiber-based prototype that converts two-dimensional images to a one-dimensional array followed by interconversion for image reconstruction are presented. The performance of the prototype is analyzed using both a xenon arc lamp as a standard broadband illumination source as well as He–Ne and Ar lasers as sources of coherent radiation. An unoptimized throughput efficiency of approximately 30% and a bandwidth of 6 Å without spectral leakage or spatial crosstalk is obtained over the entire investigated tuning range from 430 to 807 nm. Potential applications of such an optical filtration system, with wavelength tunability on the angstrom scale and potential spatial resolutions in the micrometer range, using suitable optical imaging are discussed.
A novel geometric embedding algorithm for efficiently generating dense polymer structures
Authors: Müller M. , Nievergelt J. , Santos S. , Suter U. W.
Published: July 2002

A new algorithm for generating starting polymer structures for molecular simulations (e.g., MD) in dense phase is presented. The algorithm yields structures that fulfill to a large extent rotational isomeric state (RIS) probabilities and avoid atomic overlap. The heuristic search bases on the new parallel-rotation (ParRot) technique. We tested the performance of the algorithm on two polymeric systems: Atomistic polyethylene and polystyrene. The algorithm permits to tackle the problem of packing chains into large boxes of size up to 50 Å in a couple of hours on common workstations. Moreover, our packing algorithm is applicable for general polymer systems. The algorithm requires CPU effort scaling with a power 2.8 in the chain length, and with a power 1.5 in the number of chains.
Diffraction based optical particle sizer for on-line monitoring in hostile environments of low concentration particle laden flows
Authors: Golinelli E. , Martinelli P. , Musazzi S. , Perini U. , Trespidi F. , Paganini E.
Published: July 2002

We present an optical particle sizer conceived for the on-line analysis of low concentration particle-laden flows. It belongs to the wide class of instruments based on the detection of the light scattered at small angle in the forward direction. Innovative solutions have been adopted both in the optical configuration and in the detection scheme which enable the instrument to operate at very low concentration regimes (i.e., at extinction values as small as 10−5). Particle classification is made over 31 size classes in the range of diameters 0.9–90 μm. The whole system has been designed for applications in hostile environments. Preliminary measurements have been carried out in a coal fired power plant on a flue duct downstream the electrostatic precipitators (average temperature 150 °C, particle concentration smaller than 50 mg/Nm3).
Lightweight fiber optic microphones and accelerometers
Authors: Bucaro J. A. , Lagakos N.
Published: July 2002

We have designed, fabricated, and tested two lightweight fiber optic sensors for the dynamic measurement of acoustic pressure and acceleration. These sensors, one a microphone and the other an accelerometer, are required for active blanket sound control technology under development in our laboratory. The sensors were designed to perform to certain specifications dictated by our active sound control application and to do so without exhibiting sensitivity to the high electrical voltages expected to be present. Furthermore, the devices had to be small (volumes less than 1.5 cm3) and light (less than 2 g). To achieve these design criteria, we modified and extended fiber optic reflection microphone and fiber microbend displacement device designs reported in the literature. After fabrication, the performances of each sensor type were determined from measurements made in a dynamic pressure calibrator and on a shaker table. The fiber optic microbend accelerometer, which weighs less than 1.8 g, was found to meet all performance goals including 1% linearity, 90 dB dynamic range, and a minimum detectable acceleration of 0.2 mg/Hz. The fiber optic microphone, which weighs less than 1.3 g, also met all goals including 1% linearity, 85 dB dynamic range, and a minimum detectable acoustic pressure level of 0.016 Pa/Hz. In addition to our specific use in active sound control, these sensors appear to have application in a variety of other areas.
Landau damping effects on classical electron–ion Coulomb bremsstrahlung in dense plasmas
Authors: Jung Young-Dae , Kim Chang-Geun
Published: July 2002

Landau damping effects on the classical electron–ion Coulomb bremsstrahlung process in dense Maxwellian plasmas are investigated. The electron–ion dynamic interaction potential is obtained by introduction of the longitudinal plasma dielectric function. The classical trajectory method is applied to represent the differential bremsstrahlung radiation cross section as a function of the impact parameter, photon energy, projectile velocity, electron thermal velocity, and Debye length. The Landau damping effect in the soft photon case is found to be more important than that in the hard photon case. It is also found that the Landau damping effect is important when the projectile velocity is equal to the electron thermal velocity (vT/ve=1), for example, the Landau damping effect is about 3.4% for Ep/Ee=0.1.
A frequency-modulated injection-locked diode laser for two-frequency generation
Authors: Kowalski R. , Root S. , Gensemer S. D. , Gould P. L.
Published: July 2002

We demonstrate that two-frequency laser light can be generated by rapid current modulation of a diode laser which is injection locked to a separate frequency-stabilized diode laser. First-order sidebands containing at least 2% of the power in the carrier are produced for modulation frequencies>7 GHz. This configuration is a convenient and inexpensive alternative to using two separate frequency-stabilized diode lasers or an electro-optic modulator.
On the relation between Long’s vortex and general self-similar vortex cores
Authors: Stein Carl Fredrik
Published: July 2002

We have found that the existence/nonexistence behavior of self-similar axisymmetric vortices studied by Fernandez-Feria, Fernandez de la Mora, and Barrero [J. Fluid Mech. 305, 77 (1995)], is not particularly different from that for Long’s vortex. In fact, in both cases the existence of a solution appears to depend exclusively on the value of the flow force in a disk of radius r at the inflow divided by the square of the circulation of the inflow at the same radius. However, for Long’s vortex the value of this quotient does not affect the ratio of the azimuthal and axial velocities at the edge of the layer, which it does for all other self-similar axisymmetric vortices. This result provides a simple explanation of the existence properties for the general self-similar axisymmetric vortices, quite consistent with the corresponding properties for Long’s vortex.
The combined multiconfigurational self-consistent-field/molecular mechanics wave function approach
Authors: Poulsen Tina D. , Kongsted Jacob , Osted Anders , Ogilby Peter R. , Mikkelsen Kurt V.
Published: July 2002

We present theory and implementation for a new approach for studying solvent effects: the multiconfigurational self-consistent-field/molecular mechanics method. With this method it is possible to describe ground, excited, and ionized states of molecules in solution. The approach is tested by investigating the effect of solvent on H2O in aqueous solution. For the calculated energies we find that polarization effects are significant.
Stress-induced leakage current in ultrathin SiO2 layers and the hydrogen dispersive transport model
Authors: Houssa M. , Stesmans A. , Carter R. J. , Heyns M. M.
Published: July 2002

The time dependence of the current density variation ΔJ(t) observed during constant gate voltage stress of metal–oxide–semiconductor capacitors with ultrathin gate oxide and oxynitride layers is investigated. The generation of bulk neutral defects in the SiO2 layer is calculated within a dispersive transport model, assuming that these defects are induced by the random hopping of H+ ions in the gate oxide layer. It is shown that the stress–voltage and gate-oxide-thickness dependence of ΔJ(t) can be quite well explained by this dispersive hydrogen transport model.
Scalar vortex dynamics of incompressible fluids and plasmas in Lagrangian space
Authors: Schamel H. , Das Nilakshi
Published: July 2002

The vortex dynamics of two-dimensional, incompressible, inviscid flows is shown to possess a self-consistent, canonical Hamiltonian structure in Lagrangian space. The general set of evolution equations is derived for a Cartesian and a cylindrical coordinate system.
Reversible electron transfer in photochemistry and electrochemistry
Authors: Burshtein A. I. , Neufeld A. A. , Ivanov K. L.
Published: July 2002

A complete set of integral equations is used to describe the kinetics of reversible photoionization after instantaneous excitation, including geminate and bimolecular charge recombination, to either the ground or excited states of neutral products. The normalized distribution of ionization products, calculated taking into account reverse electron transfer, differs from that for irreversible photoionization. At low reorganization energy and slow diffusion, the reversibility of the quasiresonant ionization reduces its quantum yield, but does not affect the charge separation quantum yield. The excitations restored by bimolecular recombination of ions produce the delayed fluorescence which goes to zero as a second power of time. The quantum yield of the electroluminescence detected after injection of ions into solution has a sharp free energy dependence, like that observed experimentally.
The Szilard engine revisited: Entropy, macroscopic randomness, and symmetry breaking phase transitions
Authors: Parrondo Juan M. R.
Published: July 2002

The role of symmetry breaking phase transitions in the Szilard engine is analyzed. It is shown that symmetry breaking is the only necessary ingredient for the engine to work. To support this idea, we show that the Ising model behaves exactly as the Szilard engine. We design a purely macroscopic Maxwell demon from an Ising model, demonstrating that a demon can operate with information about the macrostate of the system. We finally discuss some aspects of the definition of entropy and how thermodynamics should be modified to account for the variations of entropy in second-order phase transitions.
Enhanced thermal stability of silica-encapsulated metal nanoshells
Authors: Radloff C. , Halas N. J.
Published: July 2002

A silica-encapsulating layer of a 60–70 nm thickness is fabricated around composite nanoparticles consisting of a silica core and a gold shell (metal nanoshells). The outer silica layer provides greatly enhanced thermal stability to the nanoparticle, effectively raising its melting temperature by 300° relative to uncoated nanoshells. Both spectroscopic and microscopic structural evaluations are used to assess changes in the nanostructure when subjected to potentially destructive heating cycles. This encapsulation method may prove valuable in enhancing the thermal stability of other types of assembled nanostructures and nanoscale photonic materials.
Long-wavelength HgCdTe negative luminescent devices
Authors: Ashley T. , Gordon N. T. , Nash G. R. , Jones C. L. , Maxey C. D. , Catchpole R. A.
Published: July 2002

We have investigated the negative luminescent properties of a HgCdTe device, fabricated from a 1 mm diameter array of photodiodes having peak emission at a wavelength of 8.5 μm. This long-wavelength luminescence is of sufficient efficiency and area to be useful in device applications.
Effects of boron-interstitial silicon clusters on interstitial supersaturation during postimplantation annealing
Authors: Solmi S. , Mancini L. , Milita S. , Servidori M. , Mannino G. , Bersani M.
Published: July 2002

Boron marker-layer structures have been used to investigate the effects of B doping on the evolution of the implantation damage and of the associated transient enhanced diffusion. The samples were damaged by Si implants at different doses in the range 2×1013–1×1014 cm−2 and annealed at 740 °C for times between 2 s and 4 h. The values of interstitial supersaturation, from the beginning of the annealing up to the complete damage recovery, have been determined for the different Si doses for a given B doping level. Damage removal has been followed by double crystal x-ray diffraction. Our results confirm that the formation of boron-interstitial silicon clusters traps a relevant fraction of the interstitials produced by the implantation. This trapping action gives rise to a strong reduction of the interstitial supersaturation, prevents the interstitial clusters from being transformed in {113} defects and modifies the time evolution of the transient enhanced diffusion. X-ray analyses indicate also that the size of the boron-interstitial silicon clusters remains below 2 nm.
Influence of dose rate and temperature on ion-beam-induced defect evolution in Si investigated by channeling implantation at different doses
Authors: Posselt M. , Bischoff L. , Teichert J.
Published: July 2002

A focused ion beam system is applied to study the dose dependence of the shape of Ge channeling implantation profiles at two very different dose rates (1018 and 1011 cm−2 s−1), and for implantation temperatures of 250 °C and room temperature (RT). A simple model for the buildup of radiation damage is developed to describe the dechanneling effect of defects formed during ion bombardment. The use of this model in atomistic computer simulations yields Ge depth profiles which agree well with measured data. The lifetime of ion-beam-induced defects at 250 °C was estimated to be in the order of 100 s. At RT, some defect relaxation was found between 10 μs and 100 s after ion impact.
Excited-state orientation-dependent irreversible interconversion and fluorescence depolarization in organized molecular media. I. Theory
Authors: Fisz J. J. , Buczkowski M.
Published: July 2002

Fluorescence depolarization in organized molecular media probed by the fluorophores undergoing an excited-state orientation-dependent irreversible inter conversion is discussed. The effect of the angular dependence of the excited-state interconversion on the order parameters and correlation functions is examined in a systematic way by considering different angular dependence for the interconversion kinetic rate. The expected manifestation of this effect in the time-resolved fluorescence depolarization experiments is discussed on the basis of the synthetic decays for the experiments on membrane vesicles suspension and for variable-angle experiments on macroscopically ordered samples. The main properties of the model and its experimental applications are discussed.
Concentration dependence of the density of states in the Pauli paramagnets YNi5−xCux
Authors: Gurevich A. M. , Eropkin V. N. , Prentslau N. N. , Terekhov A. V. , Dmitriev V. M. , Ermolenko A. S. , Kuchin A. G.
Published: July 2002

The temperature dependence and concentration dependence of the heat capacity C and surface resistance Rs are measured in the compounds YNi5−xCux (x=0, 0.1, 0.2, 0.25, 0.6, and 1.25). The measured values of C(x) and Rs(x) are used to determine the concentration dependence of the density of states N(E,x). A maximum is observed on N(E,x) at x=0.2. The results constitute direct experimental confirmation of the nonmonotonic dependence of N(E,x) that has previously been predicted theoretically. It is found that at certain values of x the low-temperature heat capacity of the compounds YNi5−xCux increases with decreasing temperature, and the temperature dependence of the electrical resistance has a minimum at T=10 K. Below 10 K the temperature dependence of the resistance obeys a logarithmic law.
Particle image velocimetry measurements of flow over a wavy wall
Authors: Nakagawa Shinji , Hanratty Thomas J.
Published: July 2002

The velocity field in a plane that is oriented in the flow direction and perpendicular to the wall was measured for flow over a sinusoidal wavy boundary. The conditions were such that the surface may be considered as fully rough. Visual observations, as well as two-point correlations of the fluctuating velocity field, reveal that the turbulent structure is similar to what is found for flow over a smooth surface even though the mechanisms by which the wall maintains the turbulence are quite different. This provides support for the notion that, far enough away from the wall, the turbulence is universal.
Chaotic oscillation of a bubble in a weakly viscous dielectric fluid under electric fields
Authors: Oh J. M. , Kim P. J. , Kang I. S.
Published: July 2002

The dynamics of a bubble in a weakly viscous dielectric fluid under electric fields is studied. The dynamical equations for the volume and shape mode oscillations are derived using the domain perturbation method with first-order accuracy in deformation. For the volume mode oscillation, we obtain the modified Rayleigh–Plesset equation which includes a forcing term due to the effect of electric field. For the shape mode oscillations, the Prosperetti–Seminara equation [Phys. Fluids 21, 1465 (1978)] is also extended. The dynamical equations are analyzed with two types of electric fields: the uniform field and the axisymmetric straining field. Equilibrium analysis is performed to find the equilibrium points in the phase planes and their stabilities in static electric fields. Then, the effects of time-periodic electric fields on the bubble dynamics are considered at two levels of viscosity effect [the inviscid limit and the case of Re=O(10)]. The nonlinear dynamics theory is used for analysis of the complicated volume and shape mode oscillations.
Stabilization of predissociating nitric oxide Rydberg molecules using microwave and radio-frequency fields
Authors: Murgu Elena , Martin J. D. D. , Gallagher T. F.
Published: July 2002

We present three techniques for suppressing predissociation of the nitric oxide Rydberg states normally excited in pulsed-field ionization zero-kinetic-energy photoelectron spectroscopy. By applying a combination of appropriate dc and microwave fields it is possible to inhibit predissociation by resonantly mixing Stark states of adjacent principal quantum number n, with similar parabolic quantum number k. Lifetime enhancement is also obtained by using an appropriate radio-frequency field to resonantly mix Stark states of the same n. Finally, in the absence of dc fields, microwaves are used to stabilize optically excited nf Rydberg states, by inducing transitions to higher angular momentum states with longer lifetimes, specifically to the n±1, l⩾4 states.
Space-dependent self-diffusion processes in molten copper halides: A molecular dynamics study
Authors: Alcaraz Olga , Trullàs Joaquim
Published: July 2002

This work is concerned with single ion dynamics in molten copper halides (CuI and CuCl) which exhibit fast ionic conduction before melting. The self-dynamic structure factor of the two ionic species in each melt have been calculated by molecular dynamics simulations and the corresponding effective wavelength-dependent self-diffusion coefficients have been studied. The results have been compared with those obtained for molten alkali halides (KCl and RbCl).
Ground- and excited-state cusp conditions for the electron density
Authors: Nagy Á. , Sen K. D.
Published: July 2002

Higher-order cusp relations are derived for the wave function and the electron density of the ground and the excited states of atoms, ions or molecules. The total energy is expressed in terms of the electronic potential and density terms defined at the nucleus. It is proved that the linear term of the spherical part in the expansion of the Kohn–Sham potential, the classical Coulomb and the exchange correlation potentials around a nucleus are all equal to zero. A relationship involving the values of the density and its second and third derivatives at the nucleus is derived.
Thermal redistribution of localized excitons and its effect on the luminescence band in InGaN ternary alloys
Authors: Li Q. , Xu S. J. , Cheng W. C. , Xie M. H. , Tong S. Y. , Che C. M. , Yang H.
Published: July 2002

Temperature-dependent photoluminescence measurements have been carried out in zinc-blende InGaN epilayers grown on GaAs substrates by metalorganic vapor-phase epitaxy. An anomalous temperature dependence of the peak position of the luminescence band was observed. Considering thermal activation and the transfer of excitons localized at different potential minima, we employed a model to explain the observed behavior. A good agreement between the theory and the experiment is achieved. At high temperatures, the model can be approximated to the band-tail-state emission model proposed by Eliseev et al. [Appl. Phys. Lett. 71, 569 (1997)].
Vibrational spectroscopy of matrix-isolated, mass-selected cyanoacetylene cations
Authors: Smith-Gicklhorn A. M. , Lorenz M. , Kołos R. , Bondybey V. E.
Published: July 2002

Recent infrared spectroscopic results from a new setup for the deposition of mass-selected ions in neon matrices are presented. The strong infrared signal observed for the asymmetric stretch of CNC+ and the appearance of the corresponding band for CNC+13 in its natural isotopic abundance, demonstrates the sensitivity and usefulness of the apparatus. Evidence of a combination band of CNC+ involving the low-frequency bend is also obtained. Using this setup, the infrared spectra of the cyanoacetylenic cations, H(D)C3N+, and C4N2+ are observed for the first time. The assignment of the stretching vibrations of these linear cations are well-supported by density-functional calculations.
Molecular-beam epitaxial growth of one-dimensional rows of InAs quantum dots on nanoscale-patterned GaAs
Authors: Lee S. C. , Dawson L. R. , Malloy K. J. , Brueck S. R. J.
Published: July 2002

Single-dot-wide rows of InAs quantum dots (QDs) aligned along a [011̄] direction on a 180-nm-period nanoscale-patterned (nanopatterned) GaAs(100) substrate are reported. The nanopatterned substrate is realized by interferometric lithography along with the selective growth mode of GaAs. Orientation-dependent migration and incorporation of In atoms from (111)A to (100) facets on the nanopatterned substrate localizes QD formation exclusively along a 30–40-nm-wide (100) facet defined by neighboring (111)A-type facets within each period. These aligned QDs form face-to-face multi-QDs analogous to multi-quantum-well structures, in a one-dimensional configuration. Spatially controlled formation of QDs with an improved size uniformity on the nanopatterned substrate is presented.
Effects of vibrational excitation of target N2 molecule in charge–transfer reaction of He+ with N2 at thermal energy
Authors: Tsuji Masaharu , Tsuji Takeshi , Hamagami Taro , Nakano Kousuke
Published: July 2002

The effects of vibrational excitation of target N2 molecule in the charge-transfer (CT) reaction He++N2(X:v″)→N2+(C)+He has been studied at thermal energy using a flowing–afterglow method. The vibrational distribution of N2(X:v″⩾0), produced from a microwave discharge of Ar/N2 or He/N2 mixtures, was determined using N2+(B–X) emission resulting from the He(2 3S)/N2(X:v″) Penning ionization. Although the initial vibrational distribution of N2+(C) produced from the He+/N2(X:Nv″=0=100) reaction was Nv′=3:Nv′=4=100±5:28±2, it was Nv′=3:Nv′=4:Nv′=5:Nv′=6=100±5:60±3:41±2:49±2 in the He+/N2(X:Nv″=0:Nv″=1:Nv″=2:Nv″=3=91±5:100±5:100±5:18±1) reaction. The N2+(C:v′=3) level, which is the most favorable level in the thermal He+/N2(X:v″=0) reaction, is still favored in the He+/N2(X:v″⩾0) reaction. This indicates that the deviation from the energy-resonance rule becomes large by the vibrational excitation of N2(X:v″). The rotational distributions of v′=3 and 4 were similar between the He+/N2(X:v″=0) and He+/N2(X:v″⩾0) reactions, demonstrating that V→R transfer is insignificant for N2(X)→N2+(C) ionization in the He+/N2(X:v″⩾0) reaction.
Melt viscosities of lattice polymers using a Kramers potential treatment
Authors: Dürr O. , Frisch H. L. , Dieterich W.
Published: July 2002

Kramers relaxation times τK and relaxation times τR and τG for the end-to-end distances and for center-of-mass diffusion are calculated for dense systems of athermal lattice chains. τK is defined from the response of the radius of gyration to a Kramers potential which approximately describes the effect of a stationary shear flow. It is shown that within an intermediate range of chain lengths N the relaxation times τR and τK exhibit the same scaling with N, suggesting that N-dependent melt-viscosities for nonentangled chains can be obtained from the Kramers equilibrium concept.
Ultrafast electron trapping times in low-temperature-grown gallium arsenide: The effect of the arsenic precipitate spacing and size
Authors: Loukakos P. A. , Kalpouzos C. , Perakis I. E. , Hatzopoulos Z. , Logaki M. , Fotakis C.
Published: July 2002

The role and interplay of basic structure parameters of arsenic precipitates on the ultrafast trapping of conduction band electrons has been studied in a series of low-temperature-grown GaAs epilayers grown at various temperatures ranging from 170 up to 325 °C and annealed at 600 °C. Cross sectional electron-transmission characterization was used to determine the density and size of the precipitated arsenic clusters with growth temperature. The dependence of the electron trapping times (τ) on the spacing (R) and radius (α) of the arsenic precipitates has been systematically studied by time-resolved pump-probe transient transmission spectroscopy. The present work demonstrates that the electron trapping time follows a τ∝R3/α law.
Prediction of transition state barriers and enthalpies of reaction by a new hybrid density-functional approximation
Authors: Kang Jeung Ku , Musgrave Charles B.
Published: July 2002

We present a new hybrid density-functional method which predicts transition state barriers with the same accuracy as CBS-APNO, and transition state barriers and enthalpies of reaction with smaller errors than B3LYP, BHandHLYP, and G2. The accuracy of the new method is demonstrated on 132 energies, including 74 transition state barriers and 58 enthalpies of reaction. For 40 reactions with reliable experimental barriers, the absolute mean deviations of the transition state barriers are 0.9, 1.0, 3.1, 3.5, and 3.6 kcal/mol for the new method and the CBS-APNO, G2, B3LYP, and BHandHLYP methods, respectively. The absolute mean deviations of the enthalpies of reaction for 38 reactions with reliable experimental enthalpies are 1.2, 1.4, 3.0, and 5.9 kcal/mol for the new method and the G2, B3LYP, and BHandHLYP methods, respectively. For the new method the maximum absolute deviations for the barriers and enthalpies of reaction are 2.6 and 5.6 kcal/mol, respectively. In addition, we present a simple scheme for a high-level correction that allows accurate determination of atomization energies. The accuracy of this scheme is demonstrated on the 55 atomization energies of the G2 test set [J. Chem. Phys. 94, 7221 (1992)].
Geometry and strain effects on single-electron charging in silicon nano-crystals
Authors: Thean A. , Leburton J. P.
Published: July 2002

We investigate by numerical simulation the effects of geometry and strain on single-electron charging in 100-Å-diameter silicon (Si) nano-crystals of various shapes, embedded in silicon dioxide (SiO2). Our approach accounts for the Si band structure within the effective mass approximation and a continuum strain model based on the deformation potential theory. Electron–electron interactions in nano-crystals are treated within the density-functional theory. We show that the interplay between the Si nano-crystal geometry and the particular spatial symmetry generated by the strain potential enhances confinement in the quantum-dot and dramatically influences the electronic structure and single-electron charging behavior. In particular, for a Si(001)-oriented truncated-sphere and hemispherical nano-crystals, the 001-oriented orbitals (YY′ states) are relatively insensitive to nano-crystal strain compared to orbitals originating from conduction band valleys oriented in the other directions.
Understanding third-order dipolar effects in solution nuclear magnetic resonance: Hahn echo decays and intermolecular triple-quantum coherences
Authors: Warren W. S. , Huang S. Y. , Ahn S. , Lin Y. Y.
Published: July 2002

Dipolar effects in solution nuclear magnetic resonance lead to additional peaks in two-dimensional experiments. These peaks, which have the experimental properties of intermolecular multiple-quantum coherences, have been used in a variety of applications. Most efforts have focused on intermolecular zero-quantum or double-quantum coherences, which originate in two-spin terms from the equilibrium density matrix. In this paper, we characterize the “third-order experiments” (Hahn echo decay and triple-quantum CRAZED, which both originate in the three-spin terms in the equilibrium density matrix) both theoretically and experimentally. For example, in the coupled-spin picture, Hahn echo decays in concentrated solutions arise initially from intermolecular, 3-spin, −1-quantum coherences, which are partially converted to 3-spin, +1-quantum coherences by the second pulse, and hence survive the 1:1 coherence transfer echo. Such terms require two dipolar couplings to become observable. We discuss the general properties of both of these sequences, and show that they only give information that is already present in the “second-order” double-quantum and zero-quantum experiments. Finally, we also show that relaxation and diffusion can be introduced into the coupled-spin picture in a straightforward manner.
Band-gap energies of sol-gel-derived SrTiO3 thin films
Authors: Bao Dinghua , Yao Xi , Wakiya Naoki , Shinozaki Kazuo , Mizutani Nobuyasu
Published: July 2002

Band-gap energies of sol-gel-derived SrTiO3 thin films were studied in terms of annealing temperature and film thickness. The band-gap energies of highly crystallized films were comparable to those of single crystals reported, whereas for poor-crystallized films, their band-gap energy values were much larger than those of single crystals. The larger band-gap energy shift was believed to be mainly due to both quantum size effect and existence of amorphous phase in thin films. The band-gap energies also showed a strong dependence on film thickness. There was a critical film thickness (∼200 nm), above which the films had band-gap energies close to those of crystals or bulks, but below that, the values shifted largely, which can be attributed to the influence of crystallinity of thin films. Such a thickness effect of band-gap energy should be of high interest in optical device applications.
Self-assembled GaIn(N)As quantum dots: Enhanced luminescence at 1.3 μm
Authors: Hakkarainen T. , Toivonen J. , Sopanen M. , Lipsanen H.
Published: July 2002

Self-assembled GaIn(N)As quantum dots are fabricated on GaAs by atmospheric pressure metalorganic vapor-phase epitaxy using dimethylhydrazine (DMHy) precursor as a nitrogen source. The incorporation of nitrogen into the islands is observed to be negligible. However, the areal density of the islands is increased by up to one order of magnitude compared to that of the respective GaInAs islands. The GaIn(N)As island size can also be controlled by varying the DMHy flow. An enhancement of the room-temperature luminescence at 1.3 μm is observed in the GaIn(N)As samples grown with DMHy.
Blue up-conversion luminescence and energy transfer process in Nd3+-Yb3+-Tm3+ co-doped ZrF4-based glasses
Authors: Qiu Jianbei , Kawamoto Yoji
Published: July 2002

Up-conversion luminescence properties and energy transfer processes in Nd3+, Yb3+, and Tm3+ co-doped ZrF4-based fluoride glasses have been studied under 800 nm light excitation. Blue up-converted emission around 478 nm which can be assigned to the Tm3+: 1G4→3H6 transition, was strongly observed. Up-conversion luminescence intensity exhibited an YbF3-concentration dependence. Among Nd3+, Yb3+, and Tm3+, both Nd3+ and Tm3+ have ground state absorption bands due to the (2H9/2, F5/24) ←I9/24 and F43←3H6 transitions, respectively, which can be directly pumped by 800 nm light. However, no emissions were observed in Tm3+ singly doped and Tm3+-Yb3+ doubly doped glasses under 800 nm excitation. Therefore, a possible up-conversion mechanism may be proposed as follows: Energy transfer firstly occurs from Nd3+ to Yb3+ when Nd3+ is excited by 800 nm light, then the energy is transferred from Yb3+ to Tm3+ which is on the excited state and, finally, blue up-conversion emission of Tm3+ is observed through the Tm3+: 1G4→3H6 transition. It was also demonstrated that the energy on the Tm3+: 1G4 level was back-transferred to Nd3+ in the Nd3+ high-concentration region, quenching the up-conversion luminescence.
Study of the conversion of the VO to the VO2 defect in silicon heat-treated under uniform stress conditions
Authors: Londos C. A. , Antonova I. V. , Potsidou M. , Misiuk A. , Bak-Misiuk J. , Gutacovskii A. K.
Published: July 2002

The VO defect is one of the major defects produced by irradiation in Cz-grown Si. Its presence in the infrared spectra is manifested by a localized vibration mode (LVM) band at 829 cm−1. Upon annealing, the decay of this band is accompanied by the emergence in the spectra of another LVM band at 890 cm−1 generally attributed to the VO2 defect. The annealing of the VO center is discussed in the literature by considering mainly two reaction processes in neutron irradiated material, that is, VO+SiI→Oi and VO+Oi→VO2, which could occur in parallel. There are some points, however, which cannot be explained within the above reaction scheme. In this article we report infrared, x-ray, transmission electron microscopy and selective etching investigations on the annealing behavior of the VO defect, in neutron-irradiated Cz-grown Si samples, subjected to various high temperature–high pressure (HTHP) treatments prior to the irradiation. The contribution of each of the above two reactions to the whole annealing process of the VO defect and its conversion to the VO2 defect is studied. The results are discussed by taking into account that the state of the self-interstitials, adjacent to the oxygen precipitates and the structural defects formed due to the HTHP treatment, is different for each sample because of the different treatments.
Photoluminescence and photoelectron spectroscopic analysis of InGaAsN grown by metalorganic chemical vapor deposition
Authors: Chang W. , Lin J. , Zhou W. , Chua S. J. , Feng Z. C.
Published: July 2002

InGaAsN films and InGaAsN/GaAs quantum wells were grown by metalorganic chemical vapor deposition using 1,1-dimethylhydrazine and tetrirybutylarsice as N and As sources, respectively. A photoluminescence peak at 1.22 μm wavelength at low temperature was observed for the In0.26Ga0.74As1−yNy/GaAs quantum wells. X-ray photoelectron spectroscopy investigation on the InGaAsN films demonstrated the success of nitrogen incorporation and provided evidence of the existence of two principle N configurations, indicating the formation of N–In and N–O,H bonds. The addition of N atoms increases the In concentration in InGaAsN wafers, whereas postgrowth annealing results in In diffusion in surface region.
In situ growth monitoring of distributed GaN–AlGaN Bragg reflectors by metalorganic vapor phase epitaxy
Authors: Schenk H. P. D. , de Mierry P. , Vennéguès P. , Tottereau O. , Laügt M. , Vaille M. , Feltin E. , Beaumont B. , Gibart P. , Fernández S. , Calle F.
Published: July 2002

A series of distributed GaN-AlGaN Bragg reflectors (DBR) has been grown on Al2O3(0001) substrates by metalorganic vapor phase epitaxy. The growth of the GaN template as well as of the GaN–AlxGa1−xN quarter-wave stack has been monitored by laser reflectometry. The evolution of the in situ reflectivity as well as DBR reflection spectra are discussed as function of the AlxGa1−xN composition x.
Accurate dissociation energies of O–H⋯O hydrogen-bonded 1-naphthol⋅solvent complexes
Authors: Wickleder Claudia , Henseler Debora , Leutwyler Samuel
Published: July 2002

Accurate O–H⋯O hydrogen-bond dissociation energies were measured for the supersonic-jet-cooled complexes 1-naphthol⋅S with S=D2O, ethanol, oxirane, and oxetane. A mass-selective pump–dump–probe method was used, combining stimulated emission pumping with resonant two-photon ionization and ion-dip techniques. The ground-state dissociation energies D0(S0) are 5.83±0.13 kcal/mol for d1-1-naphthol⋅D2O, 7.94±0.02 kcal/mol for 1-naphthol⋅ethanol, 7.71±0.14 kcal/mol for 1-naphthol⋅oxirane and >8.17 kcal/mol for 1-naphthol⋅oxetane. The D0’s increase by 5%–7% upon excitation of 1-naphthol to the S1 state. These dissociation energies are compared to those of the analogous complexes with S=H2O, methanol, NH3, and ND3 [Chem. Phys. Lett. 246, 291 (1996)]. The trends in D0 are compared to the electric dipole moments μ, molecular polarizabilities ᾱ, and gas-phase proton affinities of the H bond acceptor molecules. For the O-containing acceptors, the D0’s correlate well with ᾱ, but the only good overall correlation for both O- and N-containing acceptors was found between the dissociation energies and proton affinities.
Crystalline Si/SiO2 quantum wells
Authors: Lu Z. H. , Grozea D.
Published: July 2002

Based on epitaxial layer transfer silicon on insulator wafers, crystalline Si/SiO2 quantum wells have been made. The chemical composition of the quantum wells was studied by Auger electron spectroscopy depth profile. The crystalline structure of the well was confirmed by x-ray photoelectron diffraction. Band-edge shifts in the conduction-band minimum and in the valence-band maximum were measured by using synchrotron x-ray absorption near-edge spectroscopy and by using laboratory x-ray photoelectron spectroscopy, respectively. It is found that the energy gaps of the quantum wells increase as the Si layer is reduced to below 3 nm, as expected for well-defined quantum wells. The experimental data provide direct evidence of a quantum-confined band-gap increase, which has been theoretically predicted for the crystalline Si/SiO2 quantum-well system.
Energy landscape diversity and supercooled liquid properties
Authors: Stillinger Frank H. , Debenedetti Pablo G.
Published: July 2002

Families of model “rugged landscape” potential energy functions have been constructed and examined, in order to clarify the molecular-level basis for the relationship between thermodynamic and kinetic behaviors of glassforming substances. The general approach starts by forming elementary basin units, each of which contains a single local minimum (inherent structure). These units are then spliced together to create a continuous composite potential with the requisite number of basins, upper and lower limits, and boundary conditions. We demonstrate by example that this approach creates wide topographic diversity. Specifically, many pairs of model potential functions exist that share identical thermodynamic properties (depth distribution of minima), but drastically different kinetics (overall topography). Thus, within the confines of this purely mathematical exercise, the “strong” versus “fragile” classifications of thermodynamics and of kinetics are logically disconnected. We conclude that the empirically-observed correlation between thermodynamic and kinetic behaviors embodied, for example, in the Adam–Gibbs equation, must rest upon an additional physical principle involving details of interparticle interactions, transcending the purely mathematical aspects of potential energy landscape topography.
A systematic Monte Carlo study of self-assembling amphiphiles in solution
Authors: Kenward M. , Whitmore M. D.
Published: July 2002

In this paper, we present a systematic Monte Carlo study of the self-assembly of nonionic, amphiphilic, chainlike molecules in dilute solution. The focus is on the regime in which the molecules form relatively weakly segregated micelles, which are in equilibrium with small submicellar aggregates. We study the size and shape distributions of the aggregates, and the structure of the aggregates’ cores and surfaces. In some cases, spherical micelles, relatively large nonspherical micelles, and submicellar aggregates, all coexist. The size distributions of the spherical micelles are approximately Gaussian, while the nonspherical micelles contribute non-Gaussian tails at relatively large aggregation numbers. The simulation results are interpreted in terms of a simple theory of spherical micelles, and the size distributions are compared with its predictions. For the cases where the agreement is good, we combine the simulations and the theory to calculate the critical micelle concentration as functions of the chain lengths and solvent quality. In cases where there are nonspherical aggregates, the asphericity is quantified using the principal radii of gyration of the micelles, and the size distributions are compared with mean field predictions that account for both spherical and nonspherical aggregates.
Electron binding energies of dipole-bound anions at the coupled cluster level with single, double, and triple excitations: HCN− and HNC−
Authors: Peterson Kirk A. , Gutowski Maciej
Published: July 2002

The electron binding energies for the weak dipole-bound anions HCN− and HNC− were found to be 13.2 and 35.7 cm−1, respectively, at the coupled cluster level of theory with single, double, and triple excitations [CCSDT]. A more approximate approach, in which the triples contribution is treated perturbatively [CCSD(T)], provides an electron binding energy which is underestimated for HCN− by 25% and overestimated for HNC− by 19%. The new results provide benchmarks for model potentials aiming to reproduce dynamical correlation effects in electron–molecule interactions.
Note on the role of the dimensionality in the structural glass transition
Authors: Thalmann Fabrice
Published: July 2002

Mézard and Parisi recently proposed a method for investigating the thermodynamics of a supercooled liquid around and below its glass transition. This approach assumes the existence of a true phase transition at a (finite) Kauzmann temperature TK, which is determined, and gives a quantitative estimate of the jump of specific heat ΔCv at TK. Using the simplest possible scheme, we apply this formalism to a system of pure “soft-spheres” fluid, when the dimension d of the embedding space is lower than 3 (and possibly noninteger). Looking for a lower critical dimension (which we do not find), we assess the mean-field character of the approximation used in the above mentioned approach.
Enhancement of exchange field and reduction of GMR in PtMn-based spin valves by ion irradiation
Authors: Lai Chih-Huang , Yang C. H. , Wang Y. J. , Niu H. , Hou Chunhong , Mao S.
Published: July 2002

C-ion irradiation was performed on PtMn-based spin valves and the effects of ion irradiation on magnetoresistance (MR) and on the exchange field were investigated. When ion irradiation was performed prior to post-annealing, the exchange field was substantially enhanced after annealing, especially for an annealing temperature below 240 °C. The enhancement could be related to the formation of PtMn(C) phase, which might accelerate the formation of ordered PtMn phase and might increase the anisotropy of PtMn. Although the majority of C ions located in capping layers Ta and PtMn, small amounts of C ions may go into the interface of CoFe/Cu/CoFe, leading to a small MR.
Evaluation of modulating field of photoreflectance of surface-intrinsic-n+ type doped GaAs by using photoinduced voltage
Authors: Lee W. Y. , Chien J. Y. , Wang D. P. , Huang K. F. , Huang T. C.
Published: July 2002

Photoreflectance (PR) of surface-intrinsic-n+ type doped GaAs has been measured for various power densities of pump laser. The spectra exhibited many Franz–Keldysh oscillations, whereby the strength of electric field F in the undoped layer can be determined. The thus obtained Fs are subject to photovoltaic effect and are less than built-in field Fbi. In the previous work we have obtained the relation F≈Fbi−δF/2 when δF≪Fbi by using electroreflectance to simulate PR, where δF is the modulating field of the pump beam. In this work a method was devised to evaluate δF by using photoinduced voltages Vs and, hence, the relation can be verified by PR itself. The δFs obtained by Vs are also consistent with those of using imaginary part of fast Fourier transform of PR spectra.
A quantum mechanics-molecular mechanics study of dissociative electron transfer: The methylchloride radical anion in aqueous solution
Authors: Soriano Alejandro , Silla Estanislao , Tuñón Iñaki
Published: July 2002

The dissociative electron transfer reaction CH3Cl+e−→CH3•+Cl− in aqueous solution is studied by using a QM/MM method. In this work the quantum subsystem (a methylchloride molecule plus an electron) is described using density functional theory while the solvent (300 water molecules) is described using the TIP3P classical potential. By means of molecular dynamics simulations and the thermodynamic integration technique we obtained the potential of mean force (PMF) for the carbon–chlorine bond dissociation of the neutral and radical anion species. Combining these two free energy curves we found a quadratic dependence of the activation free energy on the reaction free energy in agreement with Marcus’ relationship, originally developed for electron transfer processes not involving bond breaking. We also investigated dynamical aspects by means of 60 dissociative trajectories started with the addition of an extra electron to different configurations of a methylchloride molecule in solution. The PMF shows the existence of a very flat region, in which the system is trapped during some finite time if the quantum subsystem quickly losses its excess kinetic energy transferring it to the solvent molecules. One of the most important factors determining the effectiveness of this energy transfer seems to be the existence of close contacts (hydrogen bonds) between the solute and the solvent.