bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrafast differential transmission spectroscopy of excitonic transitions in InGaN/GaN multiple quantum wells
Authors: Chen Fei , Cheung M. C. , Sweeney Paul M. , Kirkey W. D. , Furis M. , Cartwright A. N.
Published: April 2003

Room-temperature carrier dynamics in InGaN/GaN multiple quantum wells are studied by employing ultrafast pump–probe spectroscopy. Specifically, the observed differential spectral signatures are characteristic of changes in the absorption coefficient through both a reduction of the quantum-confined Stark shift due to the photoinduced in-well field screening (low carrier densities) and excitonic absorption quenching (high carrier densities). The comparison of the differential absorption spectra at different injected carrier densities allows us to separate field screening from excitonic bleaching. The estimated in-well field at the transition point between field screening and excitonic bleaching is consistent with the theoretical value of the piezoelectric field in the strained InGaN well.
Understanding and predicting the temporal response of laser-induced incandescence from carbonaceous particles
Authors: Michelsen H. A.
Published: April 2003

This paper describes a model for analyzing and predicting the temporal behavior of laser-induced incandescence (LII) from combustion-generated soot, carbon black, and other carbonaceous particles on a nanosecond time scale. The model accounts for particle heating by absorption of light from a pulsed laser and cooling by sublimation, conduction, and radiation. The model also includes mechanisms for oxidation, melting, and annealing of the particles and nonthermal photodesorption of carbon clusters from the particle surface. At fluences above 0.1 J/cm2, particle temperatures during the laser pulse are determined by the balance between absorption and sublimation, whereas at lower fluences particle temperatures do not reach the sublimation temperature, and temperatures are predominantly controlled by absorption and conduction. After the laser pulse, temperatures are predominantly controlled by conductive cooling rates. Oxidative heating may compete with conductive cooling on these time scales. Annealing of the particles to a more ordered phase of carbon is predicted to occur at fluences as low as 0.02 J/cm2. Annealing may strongly influence sublimation rates, and changes in emissivity during annealing are predicted to increase signal decay rates. Supersonic expansion of the carbon clusters sublimed from the surface is calculated to occur at fluences above 0.12 J/cm2. When compared with LII measurements recorded in a flame at atmospheric pressure, the model reproduces the shapes and relative magnitudes of LII temporal profiles over a wide range of laser fluences. Comparisons between model predictions and experimental observations suggest that the particles do not melt at laser fluences that lead to melting of bulk graphite. These comparisons also indicate that the energy released during particle annealing is much smaller than that released during annealing of neutron- or electron-irradiated graphite. Despite good agreement between model and experimental results, large uncertainties exist for input parameters used to calculate annealing rates and rates of oxidation, conduction, absorption, emission, and photolytic desorption of carbon clusters for both the initial and annealed particles.
Closed loop learning control to suppress the effects of quantum decoherence
Authors: Zhu Wusheng , Rabitz Herschel
Published: April 2003

This paper explores the use of laboratory closed loop learning control to suppress the effects of decoherence in quantum dynamics. Simulations of the process are performed in multilevel quantum systems strongly interacting with the environment. A genetic algorithm is used to find an optimal control field which seeks out transition pathways to achieve a minimum influence of decoherence upon the system at a target time. The simulations suggest that decoherence may be optimally managed in the laboratory through closed loop operations with a suitable cost that is sensitive to the coherence of the dynamics. The case studies of dimension N=4 and N=10 with strong system–environment coupling indicate that the additional complexity with increasing system dimension can make it more difficult to manage decoherence.
Effective Hamiltonians for the nonorthogonal basis set
Authors: de Andrade Paulo C. P. , Freire José A.
Published: April 2003

We consider the problem of building an effective Hamiltonian in a subset 𝒫 of the full Hilbert space in the case where there is an overlap between the states in 𝒫 and the states in its complement 𝒬. In this case the projectors onto these subspaces are non-Hermitian and one has various possible effective Hamiltonians. We show how these can be constructed directly from the Schrödinger equation and relate them to projections of the Green function operator. In the context of a simple electron-transfer model we discuss the dependence of the matrix elements of the effective Hamiltonians on the distance between orbitals and on the choice of the tunneling energy parameter. We also investigate with what accuracy the effective Hamiltonians estimate the exact eigenenergies of the problem.
Photoinduced magnetic uniaxial anisotropy and molecular orientation of polymer containing azobenzene side groups with free radical end
Authors: Xu Z. D. , Chen X. F. , Fan X. H. , Wan X. H. , Zhou Q. F.
Published: April 2003

A polymer containing azobenzene side groups each with an organic free radical end was synthesized. Before irradiation by an Ar+ laser beam, the polymer was magnetically isotropic. However, with the irradiation of a polarized Ar+ laser beam, the photoisomerization and reorientation of the azobenzenes took place bringing about a magnetic anisotropy in the radical containing polymer film. The magnetic anisotropy was studied by using a superconducting quantum interference device. It was found that when the applied magnetic field H≈1 Oe, the magnetization Mz(H) in the direction perpendicular to the polymer film was 3.63×10−4 emu/g, while those parallel to the polymer film were Mx(H)=My(H)=−1.1×10−5 emu/g.
Hysteretic properties of array of soft cylindrical particles
Authors: Grebenshchikov Yu. B. , Usov N. A.
Published: April 2003

Magnetization curves and hysteresis loops for a dense array of soft cylindrical particles are studied by means of numerical simulation. A stray magnetic field of a neighboring particle is approximated by means of a magnetic field of a certain macrodipole located at the neighboring particle. This approach is justified comparing the results of exact numerical simulation for a cluster of two interacting particles with the corresponding data obtained by means of the macrodipole approximation. The well-known Ewald’s method [P. Ewald, Ann. Phys. 64, 250 (1921)] is used to take into account slowly decreasing contributions of distant particles into a total demagnetizing field of an array. It is found that various types of stable nonuniform micromagnetic states can participate in the magnetization reversal process in soft magnetic particles, namely, one-vortex state, multivortex states, and bending type states. The bending state originates due to instability of quasiuniform magnetization when external magnetic field decreases after array saturation. It is found that for a dense array a characteristic field for array saturation changes considerably as a function of the lattice period, and the shape of the hysteresis loop depends significantly on the direction of the in-plane external magnetic field with respect to the array axes.
Optoelectronic properties of thick SiGe layers grown as small mesas by low pressure chemical vapor deposition
Authors: Stoica T. , Vescan L.
Published: April 2003

Arrays of Si0.80Ge0.20/Si(001) square mesas were epitaxially grown by low pressure chemical vapor deposition to optimize the light emission in the near infrared range. To study the influence of mesa size on light emission the current–voltage characteristics, the spectral photocurrent, and the electroluminescence of p-i-n structures were measured. While the plastic relaxation has a strong influence on the electroluminescence spectra, the current–voltage characteristics are only slightly changed. At low temperatures, a tunneling current was observed and its possible location is discussed. Due to the high SiGe thickness, both the contributions of the no-phonon and transversal optical phonon-assisted transitions to the photocurrent spectra could be observed. Direct evidence of the higher band gap of relaxed SiGe was obtained from electroluminescence studies.
Structural and magnetic effects of Ni addition in FeN films
Authors: Zhuge L. J. , Yao W. G. , Wu X. M.
Published: April 2003

Fe–Ni–N films were prepared on glass substrates by dual ion beam sputtering. The effect of nickel (Ni) addition on the structure, magnetic properties, and thermal stability of Fe–N films was investigated. In the γ′-(Fe,Ni)4N phase, the addition of Ni influences the growth, nitrogen content, and the preferred orientation of the film. The (200) plane of the γ′-phase in the Fe–Ni–N films grows dominantly parallel to the substrate plane with increasing Ni content, indicating the possibility of controlling the crystal orientation. A single γ′-(Fe,Ni)4N phase film with all grains oriented in the (200) direction was actually grown for a Ni content of 21.2 at. %. The coercivity HC of Ni-containing films was lower than that of the Ni-free films. The film with Ni content of 6.7 at. % showed good soft magnetic properties, with HC=0.68 kA/m and MS=2.04 T. The thermal stability, however, was weakened due to the existence of Ni in the Fe–N films.
Electronic structure and x-ray magnetic circular dichroism in Gd5(Si2Ge2)
Authors: Harmon B. N. , Antonov V. N.
Published: April 2003

The electronic structure and x-ray magnetic circular dichroism in Gd5(Ge2Si2) are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA) as well as the LSDA+U approximation. The x-ray absorption spectra as well as the x-ray circular magnetic dichroism at the K edges for Ge and Si sites and L2,3, M2,3, M4,5, N2,3, N4,5, and O2,3 edges for Gd sites are calculated.
A comparative study of the hydration of Na+ and K+ with refined polarizable model potentials
Authors: Carrillo-Tripp Mauricio , Saint-Martin Humberto , Ortega-Blake Iván
Published: April 2003

The hydrations of Na+ and K+ were investigated by means of Monte Carlo simulations with refined ab initio based potentials. These interaction potentials include intramolecular relaxation, polarizability and many-body nonadditive effects. Care was taken to ensure proper convergence of the MC runs and that the statistical samples were large enough. As a result, agreement was attained with all experimental data available for the hydration of the ions. The water molecules in the first hydration shell were found to have the same intramolecular geometries and dipole moments as those of the bulk. Furthermore, their dipoles were not aligned to the electric field produced by the ion, but quite tilted. The hydration number for the sodium was found to be 5 or 6 water molecules, whereas the potassium’s hydration number had a probability distribution ranging from 5 to 10. From an analysis of the energetic contributions of each hydration shell to the total enthalpy of hydration we propose that the hydrated ions have a distinct behavior. Sodium has a stronger interaction with its first hydration shell than potassium, giving the latter a more flexible structure.
Quasi-classical trajectory study of H2 elimination in the photodissociation of difluoroethylenes at 193 nm
Authors: Vázquez S. A. , Aoiz F. J. , Bañares L. , Santamarı́a J. , Martı́nez-Núñez E. , Fernández-Ramos A.
Published: April 2003

Direct quasi-classical trajectories were carried out to calculate translational energy distributions, P(ET), for the elimination of molecular hydrogen from 1,1- and 1,2-difluoroethylene (DFE) at 193 nm. A reasonably good agreement is found between the calculated and the experimental distributions. The calculations corroborate that the elimination from 1,1-DFE takes place through the expected (direct) three-center mechanism (leading to :C=CF2 and H2), which essentially has no exit barrier. For the photodissociation of 1,2-DFE, the results strongly support that the most important pathway for H2 elimination involves a previous isomerization to 1,1-DFE, which then dissociates via the three-center mechanism. A minor contribution from a pathway having a large exit channel barrier is responsible for the long tail decay of the P(ET) in comparison to that of 1,1-DFE.
General formulation of locally designed coherent control theory for quantum system
Authors: Sugawara M.
Published: April 2003

A general local control theory for manipulating quantum system dynamics is developed. Basic concept of the present theory is lying in the realization of monotonous increasing condition of the performance index, which is locally (in time domain) defined to major how the present quantum state satisfies the current objective. The local control field is designed to satisfy the above condition taking into account the equation of motion of the system. It is found, through the formulation, that the monotonous increasing condition can be achieved as long as the performance index is given as a function of expectation values of time-dependent observable operators, whose equation of motion is governed by the field-free system Hamiltonian or Liouvillian. It is also shown that the present theory is a generalization of the local optimization approach which has been successfully applied to many of molecular dynamics control problems. As for the special cases, performance indices for “transition path control,” “population distribution control,” and “wave packet shaping” are proposed. The theory is applied to vibrational control problems of the one-dimensional model system of hydrogen fluoride. The results show that the present method works effectively for the population dynamics control as well as the wave packet shaping.
Amide I modes in the N-methylacetamide dimer and glycine dipeptide analog: Diagonal force constants
Authors: Ham Sihyun , Cho Minhaeng
Published: April 2003

The local amide I mode frequency of a peptide has been found to be strongly affected by the interpeptide interaction, because the electronic and molecular structures of the peptide bond change due to the electrostatic interaction with surrounding peptides. Ab initio vibrational analyses of three different series of N-methylacetamide dimers and glycine dipeptide analog in α-helical and β-sheet conformations have been performed. It is found that the diagonal force constant shift originates from the electronic structure change of a given peptide in combination with the cubic anharmonicity of the local amide I mode.
Theory of sequence-dependent DNA elasticity
Authors: Coleman Bernard D. , Olson Wilma K. , Swigon David
Published: April 2003

The elastic properties of a molecule of duplex DNA are strongly dependent on nucleotide sequence. In the theory developed here the contribution ψn of the nth base-pair step to the elastic energy is assumed to be given by a function ψ̃n of six kinematical variables, called tilt, roll, twist, shift, slide, and rise, that describe the relative orientation and displacement of the nth and (n+1)th base pairs. The sequence dependence of elastic properties is determined when one specifies the way ψ̃n depends on the nucleotides of the two base pairs of the nth step. Among the items discussed are the symmetry relations imposed on ψ̃n by the complementarity of bases, i.e., of A to T and C to G, the antiparallel nature of the DNA sugar–phosphate chains, and the requirement that ψ̃n be independent of the choice of the direction of increasing n. Variational equations of mechanical equilibrium are here derived without special assumptions about the form of the functions ψ̃n, and numerical solutions of those equations are shown for illustrative cases in which ψ̃n is, for each n, a quadratic form and the DNA forms a closed, 150 base-pair, minicircle that can be called a DNA o-ring because it has a nearly circular stress-free configuration. Examples are given of noncircular equilibrium configurations of naked DNA o-rings and of cases in which the interaction with ligands induces changes in configuration that are markedly different from those undergone by a minicircle of intrinsically straight DNA. When a minicircle of intrinsically straight DNA interacts with an intercalating agent that upon binding to DNA causes a local reduction of intrinsic twist, the configuration that minimizes elastic energy depends on the number of intercalated molecules, but is independent of the spatial distribution of those molecules along the minicircle. In contrast, it is shown here that the configuration and elastic energy of a DNA o-ring can depend strongly on the spatial distribution of the intercalated molecules. As others have observed in calculations for Kirchhoff rods with intrinsic curvature, an o-ring that has its intrinsic twist reduced at a single base-pair step can undergo large deformations with localized untwisting and bending at remote steps, even when the amount α of twist reduction is less than the amount required to induce supercoiling in rings of intrinsically straight DNA. We here find that the presence in the functions ψ̃n of cross-terms coupling twist to roll can amplify the configurational changes induced by local untwisting to the point where there can be a value of α at which a first-order transition occurs between two distinct stable noncircular configurations with equal elastic energy.
Density functional theory based effective fragment potential method
Authors: Adamovic Ivana , Freitag Mark A. , Gordon Mark S.
Published: April 2003

The effective fragment potential (EFP) method, is a discrete method for the treatment of solvent effects, originally formulated using Hartree–Fock (HF) theory. Here, a density functional theory (DFT) based implementation of the EFP method is presented for water as a solvent. In developing the DFT based EFP method for water, all molecular properties (multipole moments, polarizability tensors, screening parameters, and fitting parameters for the exchange repulsion potential) are recalculated and optimized, using the B3LYP functional. Initial tests for water dimer, small water clusters, and the glycine–water system show good agreement with ab initio and DFT calculations. Several computed properties exhibit marked improvement relative to the Hartree–Fock based method, presumably because the DFT based method includes some dynamic electron correlation through the corresponding functional.
Local energy equation for two-electron atoms and relation between kinetic energy and electron densities
Authors: March N. H.
Published: April 2003

In early work, Dawson and March [J. Chem. Phys. 81, 5850 (1984)] proposed a local energy method for treating both Hartree–Fock and correlated electron theory. Here, an exactly solvable model two-electron atom with pure harmonic interactions is treated in its ground state in the above context. A functional relation between the kinetic energy density t(r) at the origin r=0 and the electron density ρ(r) at the same point then emerges. The same approach is applied to the Hookean atom, in which the two electrons repel with Coulombic energy e2/r12, with r12 the interelectronic separation, but are still harmonically confined. Again the kinetic energy density t(r) is the focal point, but now generalization away from r=0 is also effected. Finally, brief comments are added about He-like atomic ions in the limit of large atomic number.
Excited states of beryllium isoelectronic series from explicitly correlated wave functions
Authors: Gálvez F. J. , Buendı́a E. , Sarsa A.
Published: April 2003

A study of the first excited states of some members of the beryllium isoelectronic series is performed. An explicitly correlated wave function is used to describe the different bound states. It consists of a correlation Jastrow factor and a configuration interaction model wave function calculated from a generalization of the optimized effective potential method to deal with multiconfiguration wave functions. Accurate values for the binding energy of the different states are obtained. Several other properties are also calculated and discussed. A systematic analysis of the Hund’s rules through the isoelectronic series in terms of the one- and two-body radial densities and some of their radial moments is carried out. The optimization of the wave function and the calculation of the different quantities is carried out by means of the variational Monte Carlo method.
Structures and properties the lead-doped carbon clusters PbCn/PbCn+/PbCn− (n=1–10)
Authors: Li Guoliang , Xing Xiaopeng , Tang Zichao
Published: April 2003

A systemic density functional theory study of the lead-doped carbon clusters PbCn/PbCn+/PbCn− (n=1–10) has been carried out using B3LYP method with both CEP-31G and TZP+ basis sets. For each species, the electronic states, relative energies and geometries of various isomers are reported. According to these calculations, the Pb-terminated linear or quasilinear isomer is the most stable structure for PbCn/PbCn+/PbCn− clusters except for PbC2/PbC2+ and PbC10/PbC10+. Both PbC2 and PbC2+ have bent ground state structure. For neutral PbC10, the global minimum possesses a Pb-containing 11-membered ring structure, while for cationic PbC10+, the Pb-side-on C10 monocyclic configuration has lowest energy. Except for the smallest PbC, PbC+, and PbC−, the electronic ground state is alternate between Σ3 (for n-odd member) and Σ1 (for the n-even member) for linear PbCn and invariably Π2 for linear PbCn+ and PbCn−. The incremental binding energy diagrams show that strong even–odd alternations in the cluster stability exist for both neutral PbCn and anionic PbCn−, with their n-even members being much more stable than the corresponding odd n−1 and n+1 ones, while for cationic PbCn+, the alternation effect is less pronounced. These parity effects also reflect in the ionization potential and electron affinity curves. The even–odd alternation predicted by theoretical studies for anionic PbCn− is in good agreement with the even–odd alternation mass distribution observed in the time-of-flight mass spectra. By comparing with the fragmentation energies accompanying various channels, the most favorable dissociation channel for each kind of the PbCn/PbCn+/PbCn− clusters are given.
A theoretical study of the NiO2 species
Authors: Deng Ke , Yang Jinlong , Zhu Qingshi
Published: April 2003

The neutral, cationic and anionic NiO2 species are studied using the density-functional theory with the local spin density approximation and generalized gradient approximation methods. The most stable geometry of the NiO2 species is found to be a linear ONiO (D∞h) for the neutral and anion and to be a cyclic Ni(O2) (C2v) for the cation. The electronic structure analyses show the Ni–O bondings in the NiO2 species have both covalent and ionic character. The low-lying excited states for the ONiO and Ni(O2) isomers are calculated and used to assign the features in the photoelectronic spectrum. Our results compare well with the available experimental results.
Sound wave propagation in multiwall carbon nanotubes
Authors: Yoon J. , Ru C. Q. , Mioduchowski A.
Published: April 2003

This article studies transverse sound wave propagation in individual multiwall carbon nanotubes. The present model predicts that there exist (N-1) critical frequencies (within terahertz range) for an N-wall carbon nanotube. When the frequency is below all critical frequencies, vibrational mode is almost coaxial and the associated sound speed can be predicted satisfactorily by the existing single-elastic beam model. However, when the frequency is higher than at least one of the critical frequencies, non-coaxial vibrational modes emerge which propagate at various speeds significantly higher or lower than the speed predicted by the single-elastic beam model. Hence, terahertz sound waves in multiwall carbon nanotubes exhibit complex phenomena and are essentially noncoaxial. In particular, terahertz sound waves in multiwall carbon nanotubes propagate at various speeds, depending not only on the frequency but also on the noncoaxial vibrational modes.
Dopant density determination in disordered organic field-effect transistors
Authors: Meijer E. J. , Detcheverry C. , Baesjou P. J. , van Veenendaal E. , de Leeuw D. M. , Klapwijk T. M.
Published: April 2003

We demonstrate that, by using a concentric device geometry, the dopant density and the bulk charge-carrier mobility can simultaneously be estimated from the transfer characteristics of a single disordered organic transistor. The technique has been applied to determine the relation between the mobility and the charge density in solution-processed poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transistors. The observation that doping due to air exposure takes place already in the dark, demonstrates that photoinduced oxygen doping is not the complete picture.
In situ observations of weld pool solidification using transparent metal-analog systems
Authors: Trivedi R. , David S. A. , Eshelman M. A. , Vitek J. M. , Babu S. S. , Hong T. , DebRoy T.
Published: April 2003

The dynamics of weld solidification were observed in situ using a laser welding process on transparent organic materials systems. Succinonitrile was used to simulate a pure metal system and succinonitrile with 1.2 wt. % acetone was used to simulate an alloy system. Observed weld pool shapes in succinonitrile were in good agreement with theoretical heat transfer calculations. The dynamics of weld pool shape in the succinonitrile-acetone system were related to complex interactions between grain orientation, grain selection, and dendrite orientations, which depend strongly on welding speed. An increase in welding speed leads to a transition from a steady-state to a nonsteady-state weld pool shape. Several other phenomena, including epitaxial growth, grain selection process, grain boundary melting, and porosity formation, were also observed.
Modifying the growth of organic thin films by a self-assembled monolayer
Authors: Liu Xiangdong , Mohamed S. H. , Ngaruiya J. M. , Wuttig Matthias , Michely Thomas
Published: April 2003

A systematic study comparing the growth of perylene films on an Au substrate and on an Au substrate coated additionally with a self-assembled monolayer (SAM) of 1-octadecanethiol is presented. The films have been characterized by atomic force microscopy and x-ray diffraction. Compared to the Au substrate, on the SAM substrate, smaller island sizes and a better c-axis texture result. Possible mechanisms for these changes in the perylene film growth are discussed.
Modeling gas flow through microchannels and nanopores
Authors: Roy Subrata , Raju Reni , Chuang Helen F. , Cruden Brett A. , Meyyappan M.
Published: April 2003

Microchannel based systems have emerged as a critical design trend in development of precise control and maneuvering of small devices. In microelectronics, space propulsion and biomedical areas, these systems are especially useful. Nanoscale pores are recently becoming of great interest due to their beneficial drag and heat transfer properties. However it is difficult to predict the flow performance of these microsystems and nanosystems numerically since the standard assumptions of using Navier–Stokes equations break down at micrometer scales, while the computational times of applicable molecular-dynamics codes become exorbitant. A two-dimensional finite-element based microscale flow model is developed to efficiently predict the overall flow characteristics up to the transition regime for reasonably high Knudsen number flow inside microchannels and nanopores. Presented two-dimensional numerical results for Poiseuille flow of a simple fluid through the microchannel are comparable to the numerical and experimental data published in the literature. The nanopore solutions are also validated with presented experimental data.
Optical wire-grid polarizers at oblique angles of incidence
Authors: Yu X. J. , Kwok H. S.
Published: April 2003

Nanotechnology enables the fabrication of wire-grid polarizers (WGP) in the visible optical region. At oblique angles of incidence, WGP can be used as polarizing beam splitters (PBS). As such, they have the advantages of large numerical aperture and high-extinction ratios in both transmission and reflection. Because of these properties, WGP is being explored as PBS replacement in projectors. In this article, we present a complete theoretical investigation of the WGP. Rigorous diffraction theory, exact lowest-order eigenmode effective-media theory, and form birefringence theory are discussed. These theories are compared with experimental measurement of T(θ) and R(θ) as a function of the polarization state of the input light and as a function of the incident angle θ. It is shown that only the rigorous diffraction theory can fit the data for all incident angles. Using diffraction theory we provide a calculation relating the optical properties of the WGP to the physical dimensions of the wire grids. Thus, a framework for optimizing the optical properties of the WGP for various applications and requirements is provided.
Theoretical analysis of nonlinear pulse propagation in ferrite-dielectric-metal structures based on the nonlinear Schrödinger equation with higher order terms
Authors: Kindyak Alexander S. , Scott Mark M. , Patton Carl E.
Published: April 2003

An analysis of nonlinear magnetostatic surface wave pulse propagation in planar ferrite-dielectric-metal (FDM) structures has been performed. The analysis was based on numerical solutions to the higher-order nonlinear Schrödinger (HONLS) equation, with third-order (D3) and nonlinear (Q) dispersion terms taken into account. The analysis focuses on (1) the crossover dispersion region for FDM structures and the point in wave-number k where the second-order dispersion parameter D2 is a positive maximum, the Lighthill criterion for envelope soliton propagation is satisfied, and D3 is close to zero, and (2) the end points of this crossover region where D2 is zero. All operational HONLS equation parameters were evaluated from analytical dispersion expressions for the FDM structure and for magnetic field and structure parameters which match experiments. For (1), the pulse results indicate nondispersive propagation consistent with envelope solitons. The only effect of the Q term is to decrease slightly the propagation speed. For (2), both end points give nondispersive propagation as well. At the low-k crossover point, this result is consistent with condition D3Q<0 and the existence of an analytic soliton solution. For the high-k crossover point, the nondispersive propagation is likely due simply to the relatively small value of the third-order dispersion D3 parameter. In both cases, one finds a rapidly oscillating wave-packet structure which propagates ahead of the main pulse. For the low-k crossover point, this structure travels at approximately twice the main pulse velocity. For the high-k crossover point, the structure travels with the main pulse. The Q term has a small effect on the pulse amplitude for the low-k crossover point, and serves to reduce the pulse amplitude by 20%–30% at the high-k crossover point.
Possible origin of ferromagnetism in (Ga,Mn)N
Authors: Zaja̧c M. , Gosk J. , Grzanka E. , Kamińska M. , Twardowski A. , Strojek B. , Szyszko T. , Podsiadło S.
Published: April 2003

Ferromagnetic behavior of GaN doped with Mn (Ga1−zMnzN) grown by the ammonothermal and chemical transport methods is discussed in terms of a second phase (ferromagnetic one) produced during the growth process. The reference manganese nitride samples grown by the same method as (Ga,Mn)N reveal room-temperature ferromagnetic behavior, depending on the growth details. Different MnxNy phases are suggested to be responsible for ferromagnetic behavior of (Ga,Mn)N.
Antireflection coatings from analogy between electron scattering and spin precession
Authors: Sprung D. W. L. , Morozov Gregory V. , Martorell J.
Published: April 2003

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band.
Experimental study of the oxidation of silicon germanium alloys
Authors: Kilpatrick S. J. , Jaccodine R. J. , Thompson P. E.
Published: April 2003

We report on the in situ oxidation of a series of Si–Ge alloy layers grown epitaxially on a (100) Si substrate. The alloy compositions of the layers varied in Ge composition from 1.8% to a high of 16.8%. Oxidations were carried out in situ in a Scientia ESCA-300 system between 400 and 800 °C. In order to explore the transition from pure SiO2 to mixed SiO2−GeO2 films, dry oxidations were carried out under partial pressures which were varied by almost nine orders of magnitude. State of the art oxidations were also carried out in a double walled oxidation furnace at one atmosphere to compare with some of the published results. In situ oxidations and analyses were done at each stage of the process from initial cleaning to monolayer absorption and oxide formation. Films grown in the thin regime up to 10 nm were analyzed by angle resolved x-ray spectroscopy. These studies were carried out to explore the influence of alloy composition on the distribution of subbonded states of Si and their influence on interfacial properties. Comparisons were made between films that were grown as pure SiO2 and those that resulted in mixed SiO2−GeO2 with regard to the areal density of these states. In spite of the strong pileup of Ge at the interface during oxide growth, there was less than expected influence of alloy composition on the interface and some evidence that interfacial annealing was taking place. It was also confirmed that mixed oxide interfaces were poorer and more highly defective than those of pure SiO2 on the same alloy.
Organic surface emitting laser based on a deep-ultraviolet photopolymer containing thiocyanate groups
Authors: Kranzelbinder Gerald , Toussaere Eric , Zyss Joseph , Kavc Thomas , Langer Gregor , Kern Wolfgang
Published: April 2003

Gratings for laser applications are being optically recorded in poly(styrene-co-4-vinylbenzyl thiocyanate) (PST-co-VBT) by deep-ultraviolet lithography (λ<300 nm). The gratings writing configuration is based on interference patterning of this holographic material. Grating structures can be further stabilized and modified by selective amine reaction from the gas phase. Gratings based on refractive index contrast (Δn∼+0.02) and on thickness modulation (Δd up to 70 nm) are obtainable from this photoreactive polymer. Holographically recorded structures in films of PST-co-VBT doped with a laser dye are designed for second order distributed feedback operation resulting in low threshold organic laser devices (wL,th=0.25 μJ cm−2) with laser emission output (TE-mode: λL=632 nm) directed perpendicular to the film plane.
Analysis of the multireference state-universal coupled-cluster Ansatz
Authors: Paldus Josef , Li Xiangzhu
Published: April 2003

An algorithm for the multireference (MR) cluster analysis of configuration interaction (CI) wave functions, based on the state-universal (SU) coupled-cluster (CC) Ansatz of Jeziorski and Monkhorst, is formulated. It is illustrated on two often-studied model systems that consist of two (the so-called H4 model) and four (the H8 model), slightly stretched, hydrogen molecules and enable one to vary the degree of quasidegeneracy via a single geometric parameter. The role played by higher-than-two-body clusters is examined and their importance in the MR effective Hamiltonian formalism, particularly in the presence of intruder states, is explored in detail. It is pointed out that in contrast with the single-reference CC methods, the MR SU CC approach with singles and doubles (CCSD) cannot yield exact energies even when exact one- and two-body clusters are available. It is shown that this limitation, which is particularly crucial in the presence of intruder states, can be remedied by employing a small subset of higher-than-pair-cluster amplitudes, extracted via cluster analysis from the MR CISD wave functions that are based on the same model space as the SU CCSD method, not unlike in the reduced MR CCSD method.
Molecular orbital study of mechanisms of the reactions of alkyl bromides with O(3P) atoms
Authors: Stevens Jonathan E. , Kaufman Marina S. , Morokuma Keiji
Published: April 2003

Ab initio electronic structure computations have been performed to study the singlet and triplet potential energy surfaces involved in the gas-phase reactions of O(3P) with the alkyl bromides C2H5Br and 1,2-C2H4Br2 to produce spin-allowed OBr and spin-forbidden HOBr products. The computations explore the effects of multiple alkane bromination on the reaction path. Our calculations indicate that high barriers to OBr formation exist on the triplet potential energy surface of alkyl-bromides reacting with O atoms. We compute spin–orbit couplings between the singlet and triplet surfaces at the lowest points where the surfaces intersect. We conclude that OBr products may be observed at sufficiently high collision energies via the triplet path while there is low probability of production of OBr or HOBr from a singlet state.
Combining density-functional calculations with kinetic models: NO/Rh(111)
Authors: Hermse C. G. M. , Frechard F. , van Bavel A. P. , Lukkien J. J. , Niemantsverdriet J. W. , van Santen R. A. , Jansen A. P. J.
Published: April 2003

We present a dynamic Monte-Carlo model involving lateral interactions and different adsorption sites (top, fcc and hcp). Using this model in combination with kinetic parameters from UHV experiments and lateral interactions derived from DFT calculations we have reproduced the ordering behavior of NO on Rh(111) during adsorption and the temperature programmed desorption (TPD) of NO from Rh(111) under UHV conditions. The formation of c(4×2)-2NO domains at 0.50 ML coverage is shown to depend strongly on the next-next-nearest-neighbor repulsion between the NO adsorbates in our model. The formation of the (2×2)-3NO structure at higher coverage follows from the avoidance of the strong next-nearest-neighbor repulsion in favor of the occupation of the top sites. A single-site model was able to reproduce the experimental TPD, but the lateral interactions were at odds with the values of the DFT calculations. A three-site model resolved this problem. It was found that all NO dissociates during TPD for initial coverages of NO below 0.20 ML. The nitrogen atoms recombine at higher temperatures. For NO coverages larger than 0.20 ML, 0.20 ML NO dissociates while the rest desorbs. This is due to a lack of accessible sites on the surface, i.e., sites where a molecule can bind without experiencing large repulsions with neighboring adsorbates. For NO coverages above 0.20 ML, the dissociation of NO causes a segregation into separate NO and N+O islands. The dissociation causes the surface to be filled with adsorbates, and the adsorbates are therefore pushed closer together. NO on one hand can easily be compressed into islands of 0.50 ML coverage, because there is no large next-next-nearest-neighbor repulsion. N+O on the other hand form islands with a lower coverage (0.30–0.35 ML) due to the considerable next-next-nearest-neighbor repulsion. Top bound NO (above 0.50 ML initial coverage) does not dissociate during TPD. It desorbs in a separate peak at 380 K.
Photonic band-gap effects on photoluminescence of silicon nanocrystals embedded in artificial opals
Authors: Valenta J. , Linnros J. , Juhasz R. , Rehspringer J.-L. , Huber F. , Hirlimann C. , Cheylan S. , Elliman R. G.
Published: April 2003

Si nanocrystals were formed in synthetic opals by Si-ion implantation and their optical properties studied using microphotoluminescence and reflection techniques. The properties of areas with high crystalline quality are compared with those of disordered regions of samples. The photoluminescence spectrum from Si nanocrystals embedded in silica spheres is narrowed by the inhibition of emission at wavelengths corresponding to the opal photonic pseudoband gap (∼690 nm). Measurements of photoluminescence spectra from individual implanted silica spheres is also demonstrated and the number of emitting Si nanocrystals in single brightly emitting spheres is estimated to be of the order of one thousand.
Characterization of the electro-optical behavior of Zn2Si0.5Ge0.5O4:Mn thin-film electroluminescent devices
Authors: Stuyven Gert , De Visschere Patrick , Neyts Kristiaan , Kitai Adrian
Published: April 2003

Zn 2 Si 0.5 Ge 0.5 O 4 : Mn was investigated as a green-emitting oxide phosphor for thin-film electroluminescent (EL) display applications. Detailed electro-optical characteristics of thin-film EL devices with SrTiO3 dielectrics on a glass substrate (TFEL) and EL devices with a single ceramic insulator (CIEL) have been analyzed. The steeper brightness–voltage dependency of the TFEL device is ascribed to the capability to provide high current densities and efficient light generation for both voltage polarities. The CIEL device on the other hand suffered from asymmetrical behavior, related with the lack of a top dielectric. It was found that the use of high-permittivity dielectrics in the TFEL device results in a small voltage drop over the insulating layers, enhancing the amount of charge transferred through the phosphor. Transient charge–voltage (αQp–V) measurements on the TFEL device evidence that a high positive space charge density of ∼0.68 μC/cm2 is present in the oxide phosphor during steady-state operation. In addition, the transient behavior after ultraviolet illumination indicates the presence of shallow electron traps in the phosphor, giving rise to a conductive current flow at electric fields below the threshold.
Efficient method for the calculation of ballistic quantum transport
Authors: Mamaluy D. , Sabathil M. , Vogl P.
Published: April 2003

We present an efficient method to calculate the ballistic transmission function and current of an arbitrarily shaped, multiterminal two- or three-dimensional open device. It is shown that the calculation of the energy dependent transmission function can be reduced to a single calculation of some stationary states of the isolated device and the inversion of a small matrix that is energy dependent. The size of this matrix is shown to be governed by the size of the boundary region between the leads and the device. The method that we term contact block reduction method is illustrated by a numerical example.
High-field quantum transport in the inversion layer of a metal–oxide–semiconductor field effect transistor
Authors: Abu-Safe Husam H.
Published: April 2003

The presence of high electric fields, both in the gate as well as in the channel in a metal–oxide–semiconductor field effect transistor (MOSFET), degrades the carrier mobility, resulting in the breakdown of Ohm’s law behavior and ultimately saturating the carrier’s velocity. Using an asymmetrical distribution wave function, we show that the velocity saturation arises due to the realignment of carriers by the longitudinal electric field along the channel of the MOSFET. Velocity is, then, limited by the thermal velocity for the nondegenerate carriers, and becomes independent from the scattering-limited mobility. This ultimate velocity is further reduced by carrier quantum emission. It is also shown, that the gate electric field degradation of the mobility arises from a quantum-confinement effect, rather than the usually-assumed mechanism of gate heating. The impact of these processes on the performance of a MOSFET is presented.
Preparation and optical characterization of Au/SiO2 composite films with multilayer structure
Authors: Liao H. B. , Wen Weijia , Wong G. K. L.
Published: April 2003

An investigation of Au/SiO2 composite multilayer films with a large third-order optical nonlinearity is reported in this article. Multilayer films containing Au nanoparticles with narrow size and shape distributions can be obtained by controlling the thickness of the Au layer, as well as the annealing temperature and time duration. It is found that the peak of surface plasmon resonance (SPR) of films with mean nanoparticle size of 30 nm is located at 2.35 eV with a width of 350 meV. The SPR peak of films containing larger or smaller particles red-shifts and broadens due to the particle size and shape effects. The third-order optical nonlinear susceptibility χ(3) of these films has a maximum value of about 5.1×10−6 esu at 532 nm when the mean nanoparticle size is around 30 nm. We attribute the observed size dependence of χ(3) to the competition between the enhancement of the local field factor and the skin-depth effect.
Gas temperature measurement in CH4/CO2 dielectric-barrier discharges by optical emission spectroscopy
Authors: Luque Jorge , Kraus Martin , Wokaun Alexander , Haffner Ken , Kogelschatz Ulrich , Eliasson Baldur
Published: April 2003

The gas temperatures were determined by optical emission in a dielectric-barrier discharge at atmospheric pressure. The feed gases were either pure CH4 to yield higher hydrocarbons or CH4/CO2 mixtures to yield synthesis gas (H2/CO). The monitored emission was from the CH radical A 2Δ–X 2Π electronic system and the gas temperature range characterized was from 300 to 600 K. The technique described in this article enables the measurement of the neutral gas temperature in the discharge that is not accessible via conventional methodology using thermocouples. A bimodal rotational population distribution in the CH A 2Δ v′=0 state was determined in the investigated gas mixtures of CO2/CH4 and in pure methane. Most of the rotational population was at temperatures from 300 to 600 K depending on experimental conditions, which are only slightly higher than the set temperature of the reactor. A small fraction of the emitting species was found to have a much higher rotational temperature of ∼4000 K for the pure methane gas and the mixture of CO2 and CH4. The low temperature rotational distribution correlated with changes in the ambient conditions and is used as a thermometer, while the high rotational temperature component and the vibrational temperature reflect the excess of energy during the CH radical formation by electron impact dissociative excitation of methane, and the extent of collisional relaxation before emission takes place.