bluuebunny/crossref_metadata_2025 huggingface dataset:
Separation of a benzene and nitric oxide mixture by a molecule prism
Authors: Zhao Bum Suk , Lee Sung Hyup , Chung Hoi Sung , Hwang Sungu , Kang Wee Kyung , Friedrich Bretislav , Chung Doo Soo
Published: October 2003

In molecule optics, a matter wave of molecules is manipulated by a molecule-optical component made out of external, typically radiative, fields. The molecule-optical index of refraction, n, for a nonresonant IR laser pulse focused onto a molecular beam can be obtained from the energy conservation and wave properties of molecules. Experimentally measured values of n for benzene and nitric oxide agreed well with the calculated values. Since n depends on the properties of molecules as well as those of the laser field, a molecule prism composed of the focused nonresonant laser field can separate a multi-component molecular beam into several components according to their molecule-optical refractive indices n. We obtained a chromatographic resolution of 0.62 for the spatial separation of a mixture beam of benzene and nitric oxide using a focused Nd:YAG laser pulse as a molecule prism.
Translational–rotational interaction in the dynamics and thermodynamics of a 2D atomic crystal with a molecular impurity
Authors: Antsygina T. N. , Poltavskaya M. I. , Chishko K. A.
Published: October 2003

The interaction between the rotational degrees of freedom of a diatomic molecular impurity and the phonon excitations of a two-dimensional atomic matrix commensurate with a substrate is investigated theoretically. It is shown that the translational–rotational interaction changes the form of the rotational kinetic energy operator as compared to the corresponding expression for a free rotator, and also renormalizes the parameters of the crystal field without altering in its initial form. The contribution of the impurity rotational degrees of freedom to the low-temperature heat capacity for a dilute solution of diatomic molecules in an atomic two-dimensional matrix is calculated. The possibility of experimental observation of the predicted effects is discussed.
Influence of rotational motion of molecules on the thermal conductivity of solid SF6, CHCl3, C6H6, and CCl4
Authors: Purskii O. I. , Zholonko N. N. , Konstantinov V. A.
Published: October 2003

The thermal conductivity of solid SF6, CHCl3, C6H6, and CCl4 is investigated by the linear-flow method under saturated vapor pressures in the temperature range from 80 K to the respective melting temperatures, and the values obtained are then adjusted to a constant density of the samples. The contributions of the phonon–phonon and phonon–rotation interactions to the total thermal resistance are separated using the modified method of reduced coordinates. It is shown that the phonon–rotation contribution to the thermal resistance of the crystals decreases as the rotational motion of the molecules attains more freedom.
Molecular dynamics simulation of the phase behavior of lamellar amphiphilic model systems
Authors: Guo Hongxia , Kremer Kurt
Published: October 2003

Using efficient simplified continuum amphiphile models-AB dimers, A2B2 tetramers, and A4B4 octamers-as models for diblock copolymers, surfactants and liquid crystals, the phase behavior of the corresponding lamellar systems is investigated by large-scale parallelized dissipative particle dynamics type of molecular dynamics simulation. We not only observe the first order nature of the order–disorder transition, but also find that this first order phase transition becomes weaker as the block length of the models increases. The theoretically predicted “chain stretching” effect is reproduced as the like-monomer attractive potential well depth is increased. At the order–disorder transition point, a singular discontinuous jump in the stretching amplitude is accompanied with a distinctive orientation-induced stretching effect in the layer normal direction and an orientation induced-compressing effect in the layer plane directions. The configuration of the lamellar phase is very sensitive to the commensurability of its equilibrium layer spacing with the cell size. Too large layer spacings lead to the formation of a slightly tilted lamellar structure in the “quasistatic-cooling” process and to an undulation instability in the “quasistatic-heating” process. The relation between equilibrium layer spacing and like-monomer attraction is examined.
Grand canonical Monte Carlo simulation study of water structure on hydrophilic mesoporous and plane silica substrates
Authors: Puibasset Joël , Pellenq Roland J.-M.
Published: October 2003

The radial distribution functions obtained by grand canonical Monte Carlo simulations of water adsorbed on disordered mesoporous and ordered plane silica surfaces at 300 K are investigated. The plane surfaces are crystallographic faces of cristobalite. The disordered mesoporous Vycor-like glass was previously obtained by off-lattice reconstruction known to reproduce in a realistic way the geometrical complexity of the real Vycor of high specific surface, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra. The SPC model is used for water, and water–substrate interactions are described by the PN-TrAZ model without any adjustment of parameter. It is shown that the water confined in Vycor is slightly more structured than bulk water, mainly due to the interaction of the first layer with the substrate. The vapor pressure is reduced so as to obtain a mono-layer coverage for each surface. On ordered crystalline faces, the water mono-layer has long range correlations, that are absent in the case of a disordered silica (Vycor-like sample): The main change being the displacement of the second peak towards larger distances (4.5 Å in bulk, 5 Å in mono-layer), characteristic of bi-dimensional structure of water.
Resonant nonlinear polarizabilities in the time-dependent density functional theory
Authors: Tretiak S. , Chernyak V.
Published: October 2003

The response of the density matrix to an external field is calculated in the adiabatic time-dependent density functional (TDDFT) theory by mapping the equation of motion for the driven single-electron density matrix into the dynamics of coupled harmonic oscillators. The resulting nonlinear response functions and the closed expressions for nonlinear frequency-dependent polarizabilities are derived. These expressions include transition densities and frequencies calculated in the linear response TDDFT, and higher order functional derivatives of the exchange-correlation functional. Limitations of the applicability of the traditional sum over states approach for computing the nonlinear response to the TDDFT are discussed.
Distinct interfaces of poly (9,9-dioctylfluorene-co-benzothiadiazole) with cesium and calcium as observed by photoemission spectroscopy
Authors: Fung M. K. , Lai S. L. , Tong S. W. , Bao S. N. , Lee C. S. , Wu W. W. , Inbasekaran M. , O’Brien J. J. , Lee S. T.
Published: October 2003

We report that the green-emitting polymer—poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), a copolymer of poly(9,9-dioctylfluorene) (F8), formed two distinct interfaces with calcium and cesium, respectively, as observed in photoemission measurements. Ultraviolet photoemission spectroscopy showed that Ca formed a stable interface with F8BT without significantly changing the electronic structure of F8BT, which is in contrast to the Cs/F8BT interface where bipolaron states occurred in the forbidden gap as a result of charge transfer processes. X-ray photoemission spectroscopy revealed that Ca covalently bonded with the sulfur atoms whereas Cs preferably interacted with the nitrogen in the F8BT. The results are useful to account for the undesirable device performance using CsF/Al as a cathode in the F8BT-based polymer light-emitting device. The exposure of Cs/F8BT interface to residual gases at a pressure of 2.0×10−9 mbar for 2 and 12 h, respectively, slightly and largely eliminated the gap states. Deposition of a thin layer of Ag onto the Cs/F8BT interface protected the films from the influence of residual gases and hence retained the affected valence-band structures.
Angular anisotropy of electron energy distributions in inductively coupled plasmas
Authors: Vasenkov Alex V. , Kushner Mark J.
Published: October 2003

The noncollisional electron transport that is typical of low-pressure (<10 mTorr) and low-frequency (<10 MHz) inductively coupled plasmas (ICPs) has the potential to produce highly anisotropic angle-dependent electron energy distributions (AEEDs). The properties of AEEDs in axially symmetric ICPs were investigated using a Monte Carlo simulation (MCS) embedded in a two-dimensional plasma equipment model. A method was developed to directly compute the coefficients for a Legendre polynomial expansion of the angular dependence of the distributions during advancement of the trajectories of pseudoelectrons in the MCS. We found significant anisotropy in the AEEDs for transport in the azimuthal–radial plane for a wide range of pressures and frequencies, and attributed this behavior to the superposition of both linear and nonlinear forces. The angular anisotropy of AEEDs in the radial–axial plane in the bulk plasma was found to be significant only when the skin layer was anomalous and nonlinear Lorentz forces are large.
Gas-phase hydrogen atom abstraction reactions of S− with H2, CH4, and C2H6
Authors: Angel Laurence A. , Dogbevia Moses K. , Rempala Katarzyna M. , Ervin Kent M.
Published: October 2003

Reaction cross sections, product axial velocity distributions, and potential energy surfaces are presented for the hydrogen atom abstraction reactions S−+RH→R+HS− (R=H, CH3, C2H5) as a function of collision energy. The observed threshold energy, E0, for S−+H2→H+HS− agrees with the reaction endothermicity, ΔrH0. At low collision energies, the H+HS− products exhibit symmetric, low-recoil-velocity scattering, consistent with statistical reaction behavior. The S−+CH4→CH3+HS− and S−+C2H6→C2H5+HS reactions, in contrast, show large excess threshold energies when compared to ΔrH0. The excess energies are partly explained by a potential energy barrier separating products from reactants. However, additional dynamical constraints must account for more than half of the excess threshold energy. The observed behavior seems to be general for collisional activation of anion-molecule reactions that proceed through a tight, late transition state. For RH=CH4 and C2H6, the HS− velocity distributions show anisotropic backward scattering at low collision energies indicating small impact parameters and a direct rebound reaction mechanism. At higher collision energies, there is a transition to HS− forward scattering and high velocities consistent with grazing collisions and a stripping mechanism.
Interface formation in K doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes
Authors: Gommans H. H. P. , Denier van der Gon A. W. , Andersson G. G. , van IJzendoorn L. J. , Pijper R. M. T. , Brongersma H. H.
Published: October 2003

Manufacturing of Al/K/OC1C10 poly(p-phenylene vinylene)/indium–tin–oxide light emitting diode structures by physical vapor deposition of K onto the emissive polymer layer has been characterized by electroluminescence and ion spectroscopy. Varying the deposited K areal density from 3.9×1012 to 1.2×1014 atoms cm−2 the external efficiency rises from 0.01 to 1.2 Cd A−1. Spectra obtained by ion scattering analysis demonstrate the overall absence of K at the polymer outermost surface layer, and diffusion up to a depth of 200 Å. Depth profiles have been derived, and were modeled using an irreversible first order “trapping” reaction. Trapping may stem from confinement of the electron at a conjugated segment, that was donated through charge transfer typical for alkali/π-conjugated systems. This study demonstrates that evaporation of low work function metals onto organic systems should not be depicted as simple layered stacking structures. The enhanced electroluminescence with submonolayer K deposition is attributed to the shift of the recombination zone away from the Al cathode, which is demonstrated to prevail over the known exciton quenching mechanism due to the formation of gap states.
Confinement on energy transfer between luminescent centers in nanocrystals
Authors: Chen X. Y. , Zhuang H. Z. , Liu G. K. , Li S. , Niedbala R. S.
Published: October 2003

The luminescence dynamics of optical centers in nanocrystals depends critically on the phonon density of states (PDOS), which is quite distinct from that of bulk materials. It is shown that energy transfer (ET) in nanocrystals is confined by discrete PDOS as well as direct size restriction. Temperature-, concentration-, and size-dependence of the fluorescence decay from the S3/24 state of Er3+ in Y2O2S nanocrystals have been investigated using laser spectroscopic experiments and computational simulations. A set of microscopic rate equations that govern the evolution of the excitation probability Pi(t) are solved iteratively using a Monte Carlo method. The simulations of ET based on a theoretical model with five parameters are in good agreement with the experimental results. It is shown that phonon-assisted ET processes in Er3+:Y2O2S nanocrystals contribute partly to the fluorescence decay at 295 K, and is negligible at 5 K. For applications, the nanoconfinement effects on ET may significantly reduce the efficiency of sensitized or upconversion luminescence due to the lack of low-frequency phonon modes and restricted excitation migration in nanophosphors.
Surface optical phonons in spherically capped quantum-dot/quantum-well heterostructures
Authors: Comas F. , Trallero-Giner C.
Published: October 2003

Surface optical (SO) phonons are studied in the case of a spherical quantum-dot/quantum-well (QD/QW) heterostructure in the framework of a dielectric continuum approach. The prototypical case is a QD/QW of CdS/HgS in the form of a spherical shell of HgS sandwiched between a spherical core of CdS and a cap layer of CdS. Structures of this type appear to be of present interest to the scientific community and have been intensively investigated in the latter years. It is assumed that the QD/QW heterostructure is surrounded by a host material considered as an infinite dielectric medium which does not participate of the polar optical vibrations. We study the SO phonons, their frequency dependence on the geometrical and material parameters, as well as the electron–phonon interaction Hamiltonian. A detailed discussion of the SO phonons physical properties and of the electron–phonon interaction strength is also made.
Solute mobility and packing fraction: A new look at the Doolittle equation for the polymer glass transition
Authors: Budzien Joanne , McCoy John D. , Adolf Douglas B.
Published: October 2003

Molecular dynamics simulations of the diffusion of penetrants in bead-spring polymers were performed along both constant volume and constant pressure paths at several temperatures. By using an effective hard sphere diameter, the packing fractions of these systems were calculated. It was found that all the data for a given polymer type collapsed to a single curve that can be well described using a modification of the Doolittle equation. Data on polymer simulations from the literature was analyzed in this manner and showed similar results. In this work we demonstrate that the packing fraction analysis that is used to describe hard sphere and colloidal systems can also be extended to polymeric systems.
Theoretical studies of the mechanical unfolding of the muscle protein titin: Bridging the time-scale gap between simulation and experiment
Authors: Li Pai-Chi , Makarov Dmitrii E.
Published: October 2003

Brute-force, fully atomistic simulations of single molecule mechanical unfolding experiments are not feasible because current simulation time scales are about six orders of magnitude shorter than the time scales explored by experiments. To circumvent this difficulty, we have constructed a model, in which the unfolding dynamics of the I27 domain of the muscle protein titin is described as diffusive motion along a single unfolding coordinate R (equal to the domain extension) in the presence of an external driving potential and the potential of mean force G(R). The effect of the remaining degrees of freedom is described in terms of a viscous force with a friction coefficient η. The potential of mean force G(R) is computed from a series of equilibrium molecular dynamics trajectories performed with constrained values of R and η is extracted from a series of steered molecular dynamics simulations, in which R is increased at a constant rate and the mechanical response of the molecule is monitored as a function of time. The estimated G(R) allows us to calculate the force-dependent unfolding rate via transition-state theory and—by performing kinetic Monte Carlo simulations—to predict unfolding force distributions in experimentally relevant regimes. We compare the computed unfolding free energy profile with that deduced from atomic force microscopy studies of titin and find that, while the unfolding free energy barrier at zero force is nearly identical to the experimental value, the force dependence of the barrier is nonlinear, in contrast to most phenomenological models of titin unfolding. Because of this, the value ku(0) of the unfolding rate extrapolated to zero unfolding force, as well as the location of the unfolding transition state, differ from those previously estimated from experimental data. In particular, our estimate of ku(0) is several orders of magnitude lower than the unfolding rate measured in chemical denaturation experiments, suggesting that the two experimental techniques may probe different unfolding pathways. At the same time, the distribution of the unfolding force as well as its dependence on the pulling rate predicted by our simulations are found to be in agreement with atomic force microscopy experiments.
Dispersion of refractive properties of solvents: Chloroform, toluene, benzene, and carbon disulfide in ultraviolet, visible, and near-infrared
Authors: Samoc Anna
Published: October 2003

Refractive index dispersion formulas have been derived for chloroform, toluene, benzene, and carbon disulfide from a broad range of the experimental refractive index data at 20 °C. The data were examined with the linear least-squares method. The dispersion equations allow one to calculate the values of refractive index required for analysis of nonlinear optical measurements in the UV, visible, and near-IR wavelength range (0.3–2.5 μm) in these liquids and solutions. The indices were compared to those estimated from quantum chemical calculations. A survey of the experimental data revealed that the measurement results published recently [Opt. Mater. 20, 81 (2002); Rev. Sci. Instrum. 65, 2056 (1994); 66, 38 (1995); 69, 1243 (1998)] were significantly different from other data reported in the literature.
Water formation reaction on Pt(111): Near edge x-ray absorption fine structure experiments and kinetic Monte Carlo simulations
Authors: Nagasaka Masanari , Kondoh Hiroshi , Amemiya Kenta , Nambu Akira , Nakai Ikuyo , Shimada Toru , Ohta Toshiaki
Published: October 2003

The catalytic water formation reaction was investigated by the energy dispersive near-edge x-ray absorption fine structure (dispersive NEXAFS) spectroscopy. An oxygen covered Pt(111) surface with the (2×2) structure was exposed to gaseous hydrogen (5.0×10−9 Torr) at constant surface temperatures (120–140 K). O K-edge NEXAFS spectra were measured during the reaction with a time interval of 35 s. Quantitative analyses of the spectra provided the coverage changes of the adsorbed species (O, OH, and H2O). The reaction is composed of three steps, which are characterized by an induction period (I), fast increase in coverage of OH and H2O with consuming O (II), and slow conversion of OH to H2O after the complete consumption of O (III). It was also found that the maximum OH coverage becomes smaller at a higher temperature. The kinetic Monte Carlo simulation has reproduced the three characteristic reaction steps; in the first step OH domains are created through two-dimensional aggregation of H2O (I), after the nucleation process the second step sets in where the OH domains propagate by the autocatalytic cycle until they contact with each other (II), and finally the merged OH domains convert to H2O (III). The reaction diffusion method was also applied to this system. It explained the reaction behavior in a wide surface area.
Structure and photoluminescence properties of ZnO microrods
Authors: Zhao Dongxu , Liu Yichun , Shen Dezhen , Lu Youming , Zhang Ligong , Fan Xiwu
Published: October 2003

ZnO microrods (whiskers) were fabricated by a simple thermal oxidation method. The morphologies of the samples were detected by the scanning electron microscope, which showed the ZnO microrods were about 30 μm long with a diameter of 1–2 μm. In the photoluminescence (PL) spectra, only the exciton related emission bands in the ultraviolet region could be obtained at room temperature. And, a new emission band due to the exciton–exciton collision process was observed at the low energy side of the free exciton emission under the excitation intensity of 2.1 kW/cm2. This emission band increased nonlinearly. When the excitation intensity was increased above 16 kW/cm2 some fine structures could also be seen clearly in the PL spectra. These fine structures originated from the cavity modes of the Fabry–Pérot étalon.
Effect of germanium concentration and tunnel oxide thickness on nanocrystal formation and charge storage/retention characteristics of a trilayer memory structure
Authors: Ho V. , Teo L. W. , Choi W. K. , Chim W. K. , Tay M. S. , Antoniadis D. A. , Fitzgerald E. A. , Du A. Y. , Tung C. H. , Liu R. , Wee A. T. S.
Published: October 2003

The effect of germanium concentration and the rapid thermal oxide (RTO) layer thickness on the nanocrystal formation and charge storage/retention capability of a trilayer metal–insulator–semiconductor device was studied. We found that the RTO and the capping oxide layers were not totally effective in confining the Ge nanocrystals in the middle layer when a pure Ge middle layer was used for the formation of nanocrystals. From the transmission electron microscopy and secondary ion mass spectroscopy results, a significant diffusion of Ge atoms through the RTO and into the silicon substrate was observed when the RTO layer thickness was reduced to 2.5 nm. This resulted in no (or very few) nanocrystals formed in the system. For devices with a Ge+SiO2 cosputtered middle layer (i.e., lower Ge concentration), even though a higher charge storage capability was obtained from devices with a thinner RTO layer, the charge retention capability was poorer as compared to devices with a thicker RTO layer.
Polymer thin films containing Eu(III) complex as lanthanide lasing medium
Authors: Hasegawa Yasuchika , Wada Yuji , Yanagida Shozo , Kawai Hideki , Yasuda Naoki , Nagamura Toshihiko
Published: October 2003

Direct evidence of lanthanide(III) lasing using Eu(III) complex in polymer thin films (threshold level <0.05 mJ) is reported. The thin film consists of polystyrene containing Eu(III) complexes based on two criteria: (1) Higher emission quantum yield of Eu(III) complexes, which increases the rs (energy density), and (2) faster radiation rate at large B (Einstein coefficient). The microcavity was constructed by coating a glass substrate with a film having a high refractive index. The film thickness was found to be 1.71 mm. The threshold level for laser transmission was found to be <0.05 mJ.
Theory of laser-induced adiabat shaping in inertial fusion implosions: The decaying shock
Authors: Anderson K. , Betti R.
Published: October 2003

The evolution of a decaying shock driven by a laser prepulse in inertial confinement fusion (ICF) capsules is investigated including the effects of mass ablation and residual ablation pressure. The adiabat (entropy) profile left behind by a decaying shock is calculated and compared with the results of one-dimensional simulations. It is found that the adiabat shape follows a simple power law of the areal density α=αinn(mshell/m)δa, where αinn is the shell inner surface adiabat, mshell is the initial total shell areal density, and m is the local shell areal density. The power law index δa varies from 1.12 for prepulses in the 100 ps range to 1.20 for prepulses in the 300 ps range. This adiabat profile accurately reproduces the simulation results to within a few percent. A simple procedure aimed at designing a laser pulse inducing the maximum adiabat ratio between the outer- and inner-shell surface is also developed. ICF capsules with a shaped adiabat are expected to benefit from improved hydrodynamic stability while maintaining the same one-dimensional performance as flat adiabat shells.
Optimal control of tokamak resistive wall modes in the presence of noise
Authors: Sen A. K. , Nagashima M. , Longman R. W.
Published: October 2003

A scheme of optimal control based on “state” feedback for the resistive wall modes in tokamaks is described. The important effects of ever present magnetohydrodynamic noises, which have been neglected in the past, are included in the stochastic formulation of the basic equations. The optimality of the system is obtained both in terms of minimization of the stabilized fluctuation levels of the modes and the control energy. It is found that the stabilized fluctuation level and control signal level are of the order of the noise level of the system in the steady state.
Apparent trap-controlled mobility evaluation in insulating polymers through depolarization characteristics derived by space charge measurements
Authors: Mazzanti G. , Montanari G. C. , Palmieri F. , Alison J.
Published: October 2003

This article illustrates a method for the estimation of apparent trap-controlled mobility of charges (i.e., the mobility of charges trapped at different states due to chemical-physical features) in polymeric insulation, based on the results of space charge measurements. The information provided by the so-called depolarization characteristic, obtained measuring space charge with short-circuited electrodes after a given poling time, is used for this purpose. Two approximate expressions for apparent-trap controlled mobility estimation are compared, the first based on rough assumptions (essentially unipolar injection and uniform charge density within insulation thickness), the second obtained following an approach based on space charge profile conservation under depolarization. These expressions are applied to the results of space charge measurements performed on unaged and aged insulating polymers. The two approaches provide apparent mobility estimates versus depolarization time in substantial agreement, and of the same order of magnitude of apparent mobility data reported in the literature for similar materials. This seems to support the validity of the key idea of the work, i.e., deriving apparent trap-controlled mobility estimates from space charge observations, particularly when such estimates are used to compare different materials or to infer the degradation level of aged insulation for diagnostic purposes.
Optimization on the performance of a harmonic quantum Brayton heat engine
Authors: Lin Bihong , Chen Jincan
Published: October 2003

The cycle model of an irreversible quantum heat engine working with many noninteracting harmonic oscillators is established. The engine cycle consists of two adiabatic and two constant-frequency processes and is referred to as the harmonic quantum Brayton cycle. The general performance characteristics of the cycle are investigated, based on the quantum master equation and semigroup approach. Expressions for several important performance parameters, such as the efficiency, power output, and rate of the entropy production, are derived. By using numerical solutions, the power output of the heat engine subject to finite cycle duration is optimized. The maximum power output and the corresponding parameters are calculated numerically. The optimal regions of the efficiency and the optimal ranges of temperatures of the working substance and times spent on the two constant-frequency processes are determined. Moreover, the optimal performance of the cycle in the high-temperature limit is compared with that of a classical Brayton heat engine working with an ideal gas. The results obtained here show that in the high-temperature limit, a harmonic quantum Brayton cycle may be equivalent to a classical Brayton cycle.
Hydrogen release from magnesium-doped GaN with clean ordered surfaces
Authors: Wampler W. R. , Myers S. M.
Published: October 2003

We examine physical processes controlling thermal release of hydrogen (H) from magnesium-doped gallium nitride, GaN(Mg). H release from GaN(Mg) was measured under ultra-high-vacuum conditions with surface cleaning and characterization to give clean well-ordered (0001) hexagonal (1×1) surfaces. Second-order release kinetics were observed, indicating recombinative desorption of H2 is a rate determining process. Observed H release is compared with diffusion–reaction theory, which incorporates current information about H in GaN from theory and previous experiments. Based on previous studies of thermal desorption of H from GaN surfaces, the release model includes recombinative desorption from neutral H states at the surface. We conclude that the formation energy (at T=0) of this surface H state is close to that of a H2 molecule in vacuum, and that the formation energy of MgH is about 1.1 eV/atom below that of H in the surface state. This implies the surface H coverage must be very low during thermal release of H from GaN(Mg). This, together with a high activation barrier for desorption from the surface, accounts for the high temperatures required to remove H from GaN(Mg) and activate p-type conductivity.
Edge localized magnetosonic eigenmodes in the ion cyclotron frequency range
Authors: Hellsten T. , Laxåback M.
Published: October 2003

Edge localized magnetosonic modes are analyzed in toroidal plasmas. For low and medium high toroidal mode numbers they are found to propagate poloidally around the plasma. A model of these modes is developed and benchmarked against a global wave code. The eigenvalues are found to be almost independent of the aspect ratio for constant minor radius, but sensitive to the ellipticity. Magnetosonic modes localized to an annular region in the poloidal plane have been proposed as an explanation for ion cyclotron emission from the edge and strongly localized mode conversion. The appearance of such modes will also give rise to parasitic absorption at the edge during ion cyclotron resonance heating.
Thermal annealing effects on low-frequency noise and transfer behavior in magnetic tunnel junction sensors
Authors: Liu Xiaoyong , Xiao Gang
Published: October 2003

We have investigated the low-frequency noise and transfer curve behavior in NiFe/AlOx/NiFe magnetic tunnel junction sensors as a function of the annealing temperature. The magnetoresistance reaches a maximum of 35% with a maximal field sensitivity of 5%/Oe at an optimal annealing temperature of 170 °C. The origin of 1/f noise is explored by measuring the magnetic field dependence of noise at low frequencies. Our data indicate that samples annealed near this temperature exhibit 1/f noise that originates from magnetization fluctuations, whereas electronic noise due to charge trapping is dominant for other temperatures.
Photoluminescence of polycrystalline ZnO under different annealing conditions
Authors: Hur Tae-Bong , Jeen Gwang Soo , Hwang Yoon-Hwae , Kim Hyung-Kook
Published: October 2003

We investigated polycrystalline zinc oxide (ZnO) with different annealing conditions in air by x-ray photoelectron spectroscopy and photoluminescence. We found that the concentration of antisite oxide (OZn) increases when ZnO ceramics were in an O-rich condition. As the concentration of antisite oxide (OZn) increased, the photoluminescence intensity of the green band emission increased. The crossover temperature of the free and bound excitons was roughly estimated as 100 K.
Stress measurements of germanium nanocrystals embedded in silicon oxide
Authors: Wellner A. , Paillard V. , Bonafos C. , Coffin H. , Claverie A. , Schmidt B. , Heinig K. H.
Published: October 2003

Ge nanocrystals embedded in thermal SiO2 on top of a Si substrate are investigated using Raman spectroscopy and transmission electron microscopy. We observe that the Raman peak frequency of the Ge nanocrystals is strongly affected by compressive stress. In the case of large particles for which the phonon confinement-induced Raman shift can be neglected, the stress is measured taking into account isotopic composition effects induced by the ion implantation process used to produce the nanocrystals. The stress is proposed to originate from a liquid–solid phase transition in Ge.
Multiplicative noise: A mechanism leading to nonextensive statistical mechanics
Authors: Anteneodo Celia , Tsallis Constantino
Published: October 2003

A large variety of microscopic or mesoscopic models lead to generic results that accommodate naturally within Boltzmann–Gibbs statistical mechanics [based on S1≡−k∫du p(u)ln p(u)]. Similarly, other classes of models point toward nonextensive statistical mechanics [based on Sq≡k[1−∫du[p(u)]q]/[q−1], where the value of the entropic index q∈R depends on the specific model]. We show here a family of models, with multiplicative noise, which belongs to the nonextensive class. More specifically, we consider Langevin equations of the type u̇=f(u)+g(u)ξ(t)+η(t), where ξ(t) and η(t) are independent zero-mean Gaussian white noises with respective amplitudes M and A. This leads to the Fokker–Planck equation ∂tP(u,t)=−∂u[f(u)P(u,t)]+M∂u{g(u)∂u[g(u)P(u,t)]}+A∂uuP(u,t). Whenever the deterministic drift is proportional to the noise induced one, i.e., f(u)=−τg(u)g′(u), the stationary solution is shown to be P(u,∞)∝{1−(1−q)β[g(u)]2}1/(1−q) [with q≡(τ+3M)/(τ+M) and β=(τ+M/2A)]. This distribution is precisely the one optimizing Sq with the constraint 〈[g(u)]2〉q≡{∫du [g(u)]2[P(u)]q}/{∫du [P(u)]q}=const. We also introduce and discuss various characterizations of the width of the distributions.
Nanocavities in He implanted InP
Authors: Chicoine M. , Roorda S. , Masut R. A. , Desjardins P.
Published: October 2003

The formation of nanocavities in InP(001) by room-temperature He implantation and subsequent thermal annealing was studied using a combination of high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) analyses. The nanocavities size and depth distributions were measured as a function of He ion dose φHe (1×1016 to 9×1016 cm−2) and ion energy E (25 to 70 keV), as well as annealing temperature Ta (600 to 750 °C) and time ta (5 to 25 min). HRXRD scans from annealed samples indicate an expansion of the InP lattice, contrary to what is usually observed following heavy-ion implantation. The critical φHe and Ta values for the formation of nanocavities were found by XTEM analysis to be between 1 and 2×1016 cm−2 and between 600 and 620 °C, respectively. Cavities of diameter 4–50 nm with {110}, {101}, and {001} facets were obtained. Increasing Ta and ta resulted in larger cavities and increasing φHe produced a larger number of cavities. Furthermore we find that nanocavities are metastable as their size first increases with annealing temperature and time but then decreases until they disappear for ta>25 min at Ta=640 °C or ta>10 min at Ta=750 °C. Results are compared with similar work carried out on He-implanted silicon and differences between the two materials are explained in terms of defect diffusivity and surface energy, higher diffusivity enhancing cavity collapse and lower surface energy enhancing cavity growth.
On anode effects in explosive emission diodes
Authors: Saveliev Yuri M. , Sibbett Wilson , Parkes David M.
Published: October 2003

Planar explosive emission diodes operating at moderate pulse energy densities of a few joules per square centimeter conform normally with the Child–Langmuir law in its basic unipolar form. Several recent publications [D. A. Shiffler et al., IEEE Trans. Plasma Sci., 30, 1232, (2002)] suggest, however, that the bipolar flow develops in the diode at comparatively low electron current densities of ∼10 A/cm2 and pulse durations of ∼50 ns. Understanding the nature of a charged particle flow (unipolar or bipolar) in explosive emission diodes is of paramount importance for interpretation of the experimental results. We have therefore investigated the operation of a diode having velvet cathodes at voltages in the 40–170 kV range, electron current densities of 4–16 A/cm2, beam power densities of 0.3–2.2 MW/cm2, and pulse energy densities of 0.5–3.3 J/cm2. Within these latitudes of operating parameters, unipolar flow was always observed and bipolar flow was achieved only when specific measures for generation of anode plasma were introduced. Prolonged operation of the CsI coated cathodes may lead to a deposition of thin CsI films on the anode and, potentially, other elements of the high voltage structure. In interpreting the plasma images, care must therefore be taken to distinguish between the light coming from the anode plasma and the luminescence from CsI films.
Instability and dynamics of thin slipping films
Authors: Sharma Ashutosh , Kargupta Kajari
Published: October 2003

The linear stability analysis of the full Navier–Stokes equations shows that the surface instability and dynamics of thin liquid films are profoundly altered by the presence of slippage on the substrate. For example, the exponents for the length scale (λm∝h0n; h0 is film thickness) and time scale of instability (tr∝h0m) change nonmonotonically with slippage [for van der Waals force induced instability, n∈(1.25,2), m∈(3,6)]. Slippage always encourages faster rupture and can greatly reduce the number density of holes for moderate to strong slip. Thus, any interpretation of thin film experiments, including determination of intermolecular forces from the length and time scales, needs to account for the possibility of slippage.
Photon–photon scattering in a plasma channel
Authors: Shen Baifei , Yu M. Y. , Wang Xiaofang
Published: October 2003

The Heisenberg–Euler correction due to photon–photon scattering, a still unverified quantum electrodynamics effect, on electromagnetic wave interaction inside a plasma channel is investigated theoretically. From a signal laser beam in the relativistic overdense plasma channel, photon–photon scattering can produce a detectable output beam of different frequency and polarization.
Field-induced charge transport at the surface of pentacene single crystals: A method to study charge dynamics of two-dimensional electron systems in organic crystals
Authors: Takeya J. , Goldmann C. , Haas S. , Pernstich K. P. , Ketterer B. , Batlogg B.
Published: October 2003

A method has been developed to inject mobile charges at the surface of organic molecular crystals, and the dc transport of field-induced holes has been measured at the surface of pentacene single crystals. To minimize damage to the soft and fragile surface, the crystals are attached to a prefabricated substrate which incorporates a gate dielectric (SiO2) and four probe pads. The surface mobility of the pentacene crystals ranges from 0.1 to 0.5 cm2/V s and is nearly temperature independent above ∼150 K, while it becomes thermally activated at lower temperatures when the induced charges become localized. Ruling out the influence of electric contacts and crystal grain boundaries, the results contribute to the microscopic understanding of trapping and detrapping mechanisms in organic molecular crystals.
Scale calculus and the Schrödinger equation
Authors: Cresson Jacky
Published: October 2003

This paper is twofold. In a first part, we extend the classical differential calculus to continuous nondifferentiable functions by developing the notion of scale calculus. The scale calculus is based on a new approach of continuous nondifferentiable functions by constructing a one parameter family of differentiable functions f(t,ε) such that f(t,ε)→f(t) when ε goes to zero. This led to several new notions as representations: fractal functions and ε-differentiability. The basic objects of the scale calculus are left and right quantum operators and the scale operator which generalizes the classical derivative. We then discuss some algebraic properties of these operators. We define a natural bialgebra, called quantum bialgebra, associated with them. Finally, we discuss a convenient geometric object associated with our study. In a second part, we define a first quantization procedure of classical mechanics following the scale relativity theory developed by Nottale. We obtain a nonlinear Schrödinger equation via the classical Newton’s equation of dynamics using the scale operator. Under special assumptions we recover the classical Schrödinger equation and we discuss the relevance of these assumptions.
Gate leakage suppression and contact engineering in nitride heterostructures
Authors: Wu Yuh-Renn , Singh Madhusudan , Singh Jasprit
Published: October 2003

We present a self-consistent approach to examine current flow in a general metal–polar heterostructure junction. The approach is applied to examine properties of three classes of junctions that are important in devices: (i) GaN/AlGaN structures that are used in nitride heterojunction field effect transistors; (ii) GaN/AlGaN/high-κ insulator structures for potential application in very small gate devices to suppress gate tunneling current; and (iii) GaN/AlGaN/polar insulator junctions with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if a dielectric thicknesses of ∼20 Å can be achieved.
High-frequency magnetic properties of W-type barium–ferrite BaZn2−xCoxFe16O27 composites
Authors: Li Z. W. , Chen Linfeng , Ong C. K.
Published: October 2003

The static and high frequency magnetic properties have been studied for BaW ferrites BaZn2−xCoxFe16O27 with x=0, 0.5, 0.7, 1.0, 1.5, and 2.0. The results showed that Co ions are able to modify the anisotropy from c axis to c plane at x=0.6–0.7. For the BaW composites with c-plane anisotropy, as the Co concentration x increases, the resonance frequencies fR are shifted to higher frequency, from 2.5 GHz at x=0.7 to 12.0 GHz at x=1.5. The permeabilities μ0′ and μmax″ are 2.4–2.2 and ∼0.8, respectively, for composites with 35% (by volume) BaW ferrite powders at x=0.7 and 1.0. The parameters indicate that BaZn2−xCoxFe16O27 composites have suitable high frequency properties for use as electromagnetic materials with low reflectivity at microwave frequencies. In addition, for BaW composites with c-axis anisotropy, there exist both the domain wall and natural resonances. However, only natural resonance is observed for composites with c-plane anisotropy.
Resonant Raman scattering of double wall carbon nanotubes prepared by chemical vapor deposition method
Authors: Ci Lijie , Zhou Zhenping , Yan Xiaoqin , Liu Dongfang , Yuan Huajun , Song Li , Gao Yan , Wang Jianxiong , Liu Lifeng , Zhou Weiya , Wang Gang , Xie Sishen , Tan Pingheng
Published: October 2003

Resonant Raman spectra of double wall carbon nanotubes (DWCNTs), with diameters from 0.4 to 3.0 nm, were investigated with several laser excitations. The peak position and line shape of Raman bands were shown to be strongly dependent on the laser energies. With different excitations, the diameter and chirality of the DWCNTs can be discussed in detail. We show that tubes (the inner or outer layers of DWCNTs) with all kinds of chiralities could be synthesized, and a DWCNT can have any combination of chiralities of the inner and outer tubes.
Purely electronic zero-phonon line as the foundation stone for high-resolution matrix spectroscopy, single-impurity-molecule spectroscopy, and persistent spectral hole burning. Recent developments
Authors: Rebane K. K.
Published: October 2003

A few examples of recent progress in the study and applications of purely electronic zero-phonon line (ZPL) and its offshoots are briefly considered: new experimental values of the narrowest ZPL; time-and-space-domain holography in the femtosecond domain, and the realization of a femtosecond Taffoli gate by it; single-impurity-molecule spectroscopy, its relation to single-photon interference and to the realization of quantum computing; the promises of quantum computing compared to what has already been done in holography.
Luminescence and formation of alkali-halide ionic excimers in solid Ne and Ar
Authors: Śliwiński G. , Frankowski M. , Schwentner N.
Published: October 2003

Transitions from ionic states A2+X− of the alkali halides CsF, CsCl, and RbF isolated in solid Ne and Ar films recorded under pulsed e-beam excitation are studied. The B(2Σ1/2)–X(2Σ1/2) and C(2Π3/2)–A(2Π3/2) luminescence bands of Cs2+F− (196.5 nm, 227 nm), Cs2+Cl− (220.1 nm, 249.2 nm) and Rb2+F− (136 nm) in Ne, and a weaker B–X emission of Cs2+F− (211.2 nm) in Ar are identified. For CsF the depopulation of the A2+X− state is dominated by the radiative decay. The ratio of the recorded exciplex emission intensities, I(CsF)/I(CsCl)/I(RbF)=20/5/1, reflects the luminescence efficiency, and for RbF and CsCl a competitive emission channel due to predissociation in the A2+X−(B2Σ1/2) state is observed. For those molecules the efficient formation of the X2* state is confirmed through recording of the the molecular D′(3Π2g)–A′(3Π2u) transition. A strong dependence of the luminescence intensities on the alkali-halide content reveals quenching at concentrations higher than 0.7%.
Exciton self-trapping into diatomic and triatomic molecular complexes in xenon cryocrystals
Authors: Ogurtsov A. N. , Savchenko E. V. , Sombrowski E. , Vielhauer S. , Zimmerer G.
Published: October 2003

Our recent study of molecular trapped centers in Xe cryocrystals is extended to triatomic self-trapped excitons. Time- and spectrally-resolved molecular luminescence is measured in the temperature range 5–60 K. The processes of intrinsic exciton self-trapping into diatomic and triatomic molecular complexes and extrinsic exciton trapping at lattice imperfections are separated by selective photoexcitation of Xe cryocrystals with synchrotron radiation. The temperature dependences of the triplet lifetimes of molecular exciton subbands are measured for the first time.
Circular dichroism in the photoelectron angular distribution from randomly oriented enantiomers of camphor
Authors: Garcia Gustavo A. , Nahon Laurent , Lebech Mogens , Houver Jean-Christophe , Dowek Danielle , Powis Ivan
Published: October 2003

Circular dichroism in the angular distribution of valence photoelectrons emitted from randomly oriented chiral molecules has been observed in experiments that use circularly polarized VUV synchrotron radiation. Photoionization of the outermost carbonyl oxygen lone pair electrons from pure enantiomers of the prototype chiral molecule camphor is shown to have an asymmetry in the forward–backward scattering of photoelectrons (relative to the propagation direction of the light beam) of magnitude approaching 3% at 9.2 eV photon energy. The asymmetry reverses on exchange of either the helicity of the radiation or of the molecular enantiomer, confirming theoretical predictions of an effect that arises in the pure electric-dipole approximation.
Linear and nonlinear dust-hydromagnetic waves
Authors: Mamun A. A. , Shukla P. K.
Published: October 2003

A self-consistent and complete theory for linear, weakly nonlinear, and highly nonlinear dust-hydromagnetic waves in a magnetized dust-ion plasma has been presented. Oblique propagation as well as two typical special cases, propagation along and across the ambient magnetic field direction, are considered. The linear dispersion properties are analyzed by using the normal mode analysis. The weakly nonlinear properties are studied by employing the reductive perturbation method, which is valid for small but finite amplitude waves. The highly nonlinear properties are investigated by means of a pseudopotential approach, which is valid for arbitrary amplitude waves. The implications of our results to some space and astrophysical dusty plasmas are briefly mentioned.