bluuebunny/crossref_metadata_2025 huggingface dataset:
Bonding modifications in carbon nitride films induced by thermal annealing: An x-ray absorption near edge study
Authors: Jiménez I. , Tong W. M. , Shuh D. K. , Holloway B. C. , Kelly M. A. , Pianetta P. , Terminello L. J. , Himpsel F. J.
Published: July 2002

The thermal stability of nonstoichiometric carbon nitride films has been studied by x-ray absorption near edge spectroscopy. Amorphous carbon nitride thin films were annealed in vacuum up to 1150 °C revealing the presence of nitrogen in different bonding configurations. Annealing to 450 °C results in the loss of ∼50% of the nitrogen. The remaining nitrogen is bonded to carbon within a graphitic framework and it evolves into a more stable configuration with increasing temperature without significant N loss up to 820 °C. Beyond this temperature, nitrogen loss occurs without important structural changes.
Strain dependent gap nature of epitaxial β-FeSi2 in silicon by first principles calculations
Authors: Miglio Leo , Meregalli V. , Jepsen O.
Published: July 2002

In this letter, we show that the gap nature in β-FeSi2 is turned from indirect to direct when a suitable strain field is induced in the structure. Such a lattice deformation corresponds to a full lattice matching for the epitaxial relationship β-FeSi2(110)//Si(111), which is one of the most common orientations occurring to β-FeSi2 precipitates in silicon.
Photothermal fixation of laser-trapped polymer microparticles on polymer substrates
Authors: Won Jaihyung , Inaba Takanori , Masuhara Hiroshi , Fujiwara Hideki , Sasaki Keiji , Miyawaki Shigeru , Sato Setsuya
Published: July 2002

Photothermal fixation of polymer microparticles onto a polymer substrate is demonstrated by using a laser manipulation technique. Individual microparticles were sequentially trapped by a 1064 nm Nd3+:yttrium–aluminum–garnet (YAG) laser light and pressed on a polymer substrate. Dye molecules that are doped into a substrate or dissolved into a solvent absorb the second harmonic pulse of a Nd3+:YAG laser and convert the absorbed light to thermal energy. It is considered that the local temperature elevation led to the local melting of the microparticle and substrate, resulting in mutual adhesion. The processes and microfixation conditions are presented, and the mechanism is discussed.
Dynamic dc voltage band observed within each current branch in the transition from static to dynamic electric-field domain formation in a doped GaAs/AlAs superlattice
Authors: Wang Jiannong , Sun Baoquan , Wang Xiangrong , Wang Yuqi , Ge Wekun , Wang Hailong
Published: July 2002

A dynamic dc voltage band was found emerging from each sawtooth-like branch of the current–voltage characteristics of a doped GaAs/AlAs superlattice in the transition process from static to dynamic electric-field domain formation caused by increasing the sample temperature. As the temperature increases, these dynamic dc voltage bands expand within each sawtooth-like branch, squeeze out the static regions, and join up together to turn the whole plateau into dynamic electric-field domain formation. These results are well explained by a general analysis of stability of the sequential tunneling current in superlattices.
Scanning apertureless microscopy below the diffraction limit: Comparisons between theory and experiment
Authors: Hill C. J. , Bridger P. M. , Picus G. S. , McGill T. C.
Published: July 2002

The exact nature of the signal in scanning apertureless microscopy techniques is the subject of much debate. We have sought to resolve this controversy by carrying out simulations and experiments on the same structures. Simulations of a model of tip–sample coupling are shown to exhibit features that are in agreement with experimental observations at dimensions below the diffraction limit. The simulation of the optical imaging process is carried out using atomic force microscope data as a topographical template and a tip–sample dipole coupling model as the source of optical signal. The simulations show a number of key fingerprints including a dependence on the polarization of the external laser source, the size of the tip, and index of refraction of the sample being imaged. The experimental results are found to be in agreement with many of the features of the simulations. We conclude that the results of the dipole coupling theory agree qualitatively with experimental data and that apertureless microscopy measures optical properties, not just topography.
Near-field optical spectroscopy using an incoherent light source
Authors: Aigouy L. , Andréani F. X. , Boccara A. C. , Rivoal J. C. , Porto J. A. , Carminati R. , Greffet J.-J. , Mégy R.
Published: July 2002

We present a method to perform near-field optical spectroscopy. It consists of an apertureless near-field optical microscope combined with an incoherent light source and a monochromator. We show that the optical response of the tip depends both on the wavelength and the tip apex shape. The experimental optical response of the tips is in good agreement with the predicted theoretical one. The near-field optical spectrum obtained on a gold island in the visible spectrum range (λ=400–600 nm) is presented and shows that the method is reliable to perform spectroscopic measurements on submicrometer-sized objects.
Magnetic nanoscale dots on colloid crystal surfaces
Authors: Li S. P. , Lew W. S. , Xu Y. B. , Hirohata A. , Samad A. , Baker F. , Bland J. A. C.
Published: July 2002

We demonstrate that uniform, ordered, single-domain magnetic nanoscale dots can be fabricated on concentrated colloid surfaces. The substrate consists of compact silica nanosphere arrays grown on a glass wafer. Through the subsequent deposition and oxidation treatment of a Co film, monodisperse magnetic Co nanoscale dot arrays with controlled magnetic properties and size were obtained. We suggest that magnetic dots deposited on colloidal surfaces might open a way of developing artificially nanostructured materials for fundamental studies in nanomagnetism and for applications such as patterned magnetic recording media.
Photoluminescence spectra from δ-doped barrier layers in modulation-doped InAlAs/InGaAs field-effect transistor structures
Authors: Watanabe Kazuo , Yokoyama Haruki
Published: July 2002

The photoluminescence (PL) spectra from the Si δ-doped InAlAs barrier layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with contact and recess-etch-stopper layers are investigated. A PL peak associated with the δ-doped layer (δ peak) is found. The δ-peak position shifts to shorter wavelength with increasing δ-doping concentration. The intensity reduction of the δ peak due to raising the temperature from 6 K is considerably slower than that of the exciton peak from the InAlAs buffer layer at about 8100 Å, to which confined electrons and holes probably contribute. As a result, the δ-peak overwhelms the exciton peak and becomes the main peak of the InAlAs PL at temperatures higher than 40 K. The δ peak is not detected from HFET structures without the contact and recess-etch-stopper layers. The δ peak is attributed to the recombination of electrons from the δ-doped layer and photogenerated holes confined in the upper side of the barrier layer.
Kinetics of YBa2Cu3O7 film growth by postdeposition processing
Authors: Solovyov Vyacheslav F. , Wiesmann Harold J. , Wu Li-jun , Zhu Yimei , Suenaga Masaki
Published: July 2002

The ex situ growth of thick textured YBa2Cu3O7 films on SrTiO3 substrates was investigated in order to find the factors, which influence growth kinetics. It was found that the growth rate of YBa2Cu3O7 films remains constant during heat treatment and is proportional to the square root of the H2O partial pressure in the processing atmosphere. Using transmission electron microscopy, we observed that the growth of the YBa2Cu3O7 films proceeds from a thin, ∼7-nm-thick, layer of liquid located at the growth front. Chemical analysis of this layer and the adjacent material suggests that decomposition of an oxifluoride compound in the precursor film occurs at the liquid layer-precursor film interface. The rate of this decomposition reaction defines the growth rate of YBa2Cu3O7. Using a simple model we show that rate of this reaction is limited by the out-diffusion of the decomposition product HF.
Ab initio study of phonons in wurtzite AlxGa1−xN alloys
Authors: Bungaro Claudia , de Gironcoli Stefano
Published: July 2002

We present a theoretical study of the zone-center optical phonons in wurtzite AlxGa1−xN alloys over the whole compositional range from pure GaN to pure AlN. The phonon modes are broadened upon alloying and their frequencies display a blue shift with increasing Al concentration. The E2 and E1(TO) modes display a two-mode like behavior and do not preserve a well-defined symmetry in the alloy but rather are mixed and have a large broadening. The LO modes, instead, display a one-mode behavior and have a well-defined symmetry, small broadening, and a pronounced dependence of the frequency upon alloy composition. Therefore, we propose them as the best candidates for the compositional characterization of these materials.
Improvement of wet-oxidized AlxGa1−xAs (x∼1) through the use of AlAs/GaAs digital alloys
Authors: Pickrell G. W. , Epple J. H. , Chang K. L. , Hsieh K. C. , Cheng K. Y.
Published: July 2002

A comparison of the water vapor oxidation characteristics of AlAs, Al0.98Ga0.02As, and an AlxGa1−xAs digital alloy was performed. The AlxGa1−xAs digital alloy consists of periods of 49 monolayers of AlAs and 1 monolayer of GaAs and has an equivalent composition of x=0.98. Oxidation rates and the structural integrity of the three layers were compared. When oxidized in water vapor, the AlxGa1−xAs digital alloy and the AlAs have similar oxidation rates, both of which are twice as fast as the Al0.98Ga0.02As layer. Post-oxidation annealing of these samples at 450 °C showed severe delamination at the oxide/GaAs interface in the AlAs sample while the AlxGa1−xAs digital alloy sample was not damaged.
Directed assembly of carbon nanotube electronic circuits
Authors: Wei Y. Y. , Eres Gyula
Published: July 2002

We report on a method for directed assembly of integrated carbon nanotube circuits using selective area chemical vapor deposition on prepatterned catalyst electrodes. The circuits consist of a multiwall carbon nanotube bridging a pair of electrodes, forming a metal/carbon nanotube/metal structure. Electron-beam lithography was used to define electrode sets separated by a desired distance on a SiO2/Si substrate. Following metal evaporation and lift-off, molecular jet chemical vapor deposition was used for selective growth of carbon nanotubes on the catalyst electrodes. The carbon nanotubes eventually form a bridge between nearby electrodes consisting of one, or in some cases more than one multiwall nanotubes. The room temperature resistance of the carbon nanotube circuits measured at the electrode leads is typically less than 100 kΩ. The carbon nanotube circuits were characterized by electronic transport measurements in the temperature range from room temperature to 2 K.
Pump-probe measurement of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells
Authors: Asano T. , Tamura M. , Yoshizawa S. , Noda S.
Published: July 2002

Modulation of interband-resonant light (∼800 nm) by intersubband-resonant light (5–7 μm) was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation with a recovery time of ∼1 ps is observed in a plainer-type modulation device at room temperature. The modulation of interband absorption coefficient is ∼1000 cm−1 when the energy density of the intersubband light pulse is ∼4 fJ/μm2. The modulation efficiency indicates that 99% modulation can be achieved with a control pulse energy of ∼1 pJ when a conventional waveguide-type device structure is utilized. The mechanism which determines the modulation speed is discussed in terms of carrier relaxation process. It is shown that the modulation speed is mainly determined by the inter- and intrasubband relaxation times, where the latter is influenced by hot phonon effects.
Dynamically stable growth of strained-layer superlattices
Authors: Shilkrot L. E. , Srolovitz D. J. , Tersoff J.
Published: July 2002

In heteroepitaxy, misfit stress leads to a well-known instability of planar films against “roughening.” In contrast, we find that growth of a strained-layer superlattice is dynamically stable under a range of growth conditions. Outside the stable range, the modulations of successive layers may be in phase, out of phase, or more complex, as summarized in a dynamical phase diagram. This remarkable behavior results from the collective influence of the buried interfaces, via their strain fields, on the evolution of the surface morphology.
Effect of three-body forces on the phase behavior of charged colloids
Authors: Wu J. Z. , Bratko D. , Blanch H. W. , Prausnitz J. M.
Published: July 2002

Statistical-thermodynamic theory for predicting the phase behavior of a colloidal solution requires the pair interaction potential between colloidal particles in solution. In practice, it is necessary to assume pairwise additivity for the potential of mean force between colloidal particles, but little is known concerning the validity of this assumption. This paper concerns interaction between small charged colloids, such as surfactant micelles or globular proteins, in electrolyte solutions and the multibody effect on phase behavior. Monte Carlo simulations for isolated colloidal triplets in equilateral configurations show that, while the three-body force is repulsive when the three particles are near contact, it becomes short-ranged attractive at further separations, contrary to a previous study where the triplet force is attractive at all separations. The three-body force arises mainly from hard-sphere collisions between colloids and small ions; it is most significant in solutions of monovalent salt at low concentration where charged colloids experience strong electrostatic interactions. To illustrate the effect of three-body forces on the phase behavior of charged colloids, we calculated the densities of coexisting phases using van der Waals-type theories for colloidal solutions and for crystals. For the conditions investigated in this work, even though the magnitude of the three-body force may be as large as 10% of the total force at small separations, three-body forces do not have a major effect on the densities of binary coexisting phases. However, coexisting densities calculated using Derjaguin–Landau–Verwey–Overbeek theory are much different from those calculated using our simulated potential of mean force.
Toluene: Structure, dynamics, and barrier to methyl group rotation in its electronically excited state. A route to IVR
Authors: Borst David R. , Pratt David W.
Published: July 2002

Rotationally resolved fluorescence excitation spectra of several torsionally active bands in the S1–S0 electronic transitions of toluene and toluene-d3 have been recorded in the collision-free environment of a molecular beam. Analyses of these data provide accurate values of the internal rotor constants F; the barrier heights V6; the frame rotational constants AF; the overall rotational constants B and C; and the torsion-rotation coupling constants AF′; in the m=0 and m=±1 levels of the S0 state and the m=0, ±1, and 3+ levels of the S1 state. Comparison of the AF, B, and C values in the m=0 levels of the two states shows that S1 toluene is quinoidal in form, with shorter ring “parallel” C–C bonds than “perpendicular” ones, unlike the S0 state. The preferred conformation of the methyl group is staggered in both states, but the V6 values are significantly different; V6(S0)=−4.874 and V6(S1)=−26.376 cm−1. Comparison of the F, AF, and AF′ values in the different torsional levels of the S1 state shows that, below the barrier, the methyl group tilts and the ring bond lengths change with increasing displacements along the torsional coordinate. Above the barrier, the precessional motion of the CH3 is quenched but larger ring distortions are observed. Thus, the data are consistent with an enhanced hyperconjugative interaction between the benzene ring and the methyl group in the S1 state. This interaction is substantially modulated by the relative motion of the two attached groups, providing a facile route to IVR.
Spectral decomposition and resolvent kernel for a magnetic Laplacian in Cn
Authors: Askour N. , Mouayn Z.
Published: July 2002

Spectral decomposition and resolvent kernel for a magnetic Laplacian in Cn are given. As an application we obtain the corresponding Schrödinger propagator and wave kernel.
Temperature dependence of the dielectric function of C6H6(l) and C6H5CH3(l) measured with THz spectroscopy
Authors: Ro/nne Cecilie , Jensby Kasper , Loughnane Brian J. , Fourkas John , Nielsen O. Faurskov , Keiding So/ren R.
Published: July 2002

We report on an experimental investigation of the temperature dependence of the intermolecular dynamics in liquid benzene and toluene. With the use of THz time domain spectroscopy we measured the complex dielectric function (0.2–3.3 THz) of the liquids, at temperatures between −6 °C and 75 °C. By analyzing the dielectric loss (as opposed to the absorption coefficient) we found three contributions to the dielectric function for toluene and two for benzene. In the dipolar liquid toluene we observed a contribution from rotational diffusion at lower frequencies in addition to the two high-frequency librations also observed in benzene. The temperature and density dependence were different for the two librational bands, probably due to the different effect of three-body interactions for the two processes. Furthermore, we present measurements of the low-frequency depolarized Raman spectra as a function of temperature for benzene and toluene. These have been compared with the dielectric loss at similar temperatures. Moreover, for benzene a similar comparison is made between the dielectric loss and the reduced Raman susceptibility obtained by optical Kerr effect spectroscopy. Libration of the symmetric top axis is found in all spectra.
Calibration of piezoelectric bimorphs for experiments in a surface forces apparatus
Authors: Bae Sung Chul , Granick Steve
Published: July 2002

A simple interferometric method is described to measure the time-dependent deformation of piezoelectric bimorphs driven by ac voltage. Applying this method to a bimorph assembly used to measure viscoelastic forces in a surface forces apparatus, we find that a sinusoidal-shaped harmonic drive voltage generates sinusoidal deformation but that a triangular-shaped harmonic drive voltage fails to produce triangular-shaped deformation if the drive frequency exceeds 1/100 of the resonance frequency. Sinusoidal displacement amplitudes as small as 0.1 nm were resolved.
Adaptive feedback control of ultrafast semiconductor nonlinearities
Authors: Kunde J. , Baumann B. , Arlt S. , Morier-Genoud F. , Siegner U. , Keller U.
Published: July 2002

We experimentally demonstrate that adaptive feedback optical pulse shaping can be used to control ultrafast semiconductor nonlinearities. The control scheme is based on an evolutionary algorithm, which directs the modulation of the spectral phase of 20 fs laser pulses. The algorithm has optimized the broadband semiconductor continuum nonlinearity measured in differential transmission experiments. Our results show that insight into light–semiconductor interaction is obtained from the optimum laser pulse shape even if the interaction is too complex to predict this shape a priori. Moreover, we demonstrate that adaptive feedback control can substantially enhance ultrafast semiconductor nonlinearities by almost a factor 4.
Time-dependence of luminescence of nanoparticles of Eu2O3 and Tb2O3 deposited on and doped in alumina
Authors: Gedanken A. , Reisfeld R. , Sominski L. , Zhong Z. , Koltypin Yu. , Panczer G. , Gaft M. , Minti H.
Published: July 2002

Nanoparticles of Eu2O3 and Tb2O3 have been prepared using ultrasound radiation and were deposited sonochemically on microspherical alumina particles. In a different sonochemical reaction Eu2O3 and Tb2O3 were doped into nanophased alumina particles. For both systems the decay times of the fluorescence was measured. The luminescence of the alumina substrate was found to be much shorter than that of the rare-earth oxides. Differences between the decay times of the deposited and doped materials are accounted for by the stronger guest-host interaction and absence of concentration quenching in the doped material.
Charge trapping in very thin high-permittivity gate dielectric layers
Authors: Houssa M. , Stesmans A. , Naili M. , Heyns M. M.
Published: July 2002

The trapping of charge carriers in very thin SiOx/ZrO2 and SiOx/TiO2 gate dielectric stacks during constant gate voltage stress of metal–oxide–semiconductor capacitors has been investigated. The increase of the gate current density observed during the gate voltage stress has been analyzed, taking into account both the buildup of charges in the layer as well as the stress-induced leakage current contribution. From data analysis, the cross section of traps generated during the electrical stress is estimated. It is suggested that these traps are probably ZrOH and TiOH neutral centers that are related to the breaking of bridging O bonds by mobile H+ protons followed by the trapping of these protons at ZrO or TiO sites.
Room- and low-temperature voltage tunable electroluminescence from a single layer of silicon quantum dots in between two thin SiO2 layers
Authors: Photopoulos P. , Nassiopoulou A. G.
Published: July 2002

Room- and low-temperature electroluminescence (EL) in the visible range was observed from a single layer of silicon nanocrystals in between two thin SiO2 layers. The EL peak wavelength exhibited tunability from the red (∼800 nm) to the yellow (∼600 nm) depending on the excitation voltage. By decreasing the temperature while keeping the excitation voltage constant, an increase in EL intensity was observed together with a blueshift in EL peak position. This blueshift was much larger than that observed under optical excitation. Nonradiative Auger recombination, Coulomb charging effects, and/or the quantum-confined Stark effect are considered accountable for this behavior.
Anomalous magnetotransport in Pr0.5Ca0.5Mn0.99Cr0.01O3
Authors: Mahendiran R. , Maignan A. , Hervieu M. , Martin C. , Raveau B.
Published: July 2002

Pr 0.5 Ca 0.5 Mn 0.99 Cr 0.01 O 3 shows several anomalous properties: Field-cooled (FC) resistivity at 10 K is several orders of magnitude lower than the zero-field cooled (ZFC) value, magnetoresistance isotherm at 125 K is asymmetric about the origin (H=0 T) and resistivity and magnetization are hysteretic between field cooling and field warming. No clear difference between ZFC and FC magnetization up to 5 T was found. We suggest that nanometric-size ferromagnetic clusters coexist with the charge-ordered antiferromagnetic matrix in H=0 T and the matrix breaks into domains upon field cooling which leads to low resistivity.
Direct iterative solution of the generalized Bloch equation. IV. Application to H2, LiH, BeH, and CH2
Authors: Meißner Holger , Paldus Josef
Published: July 2002

The state selective, one- and two-determinantal versions of the recently proposed direct iterative approach to the solution of the generalized Bloch equation [H. Meißner and J. Paldus, J. Chem. Phys. 113, 2612 (2000); Part II] are applied to the ground and excited states of several test molecular ab initio models for which the exact full configuration interaction results are available. Both closed- and open-shell states of different spin multiplicity are considered, as well as the excited states belonging to the same symmetry species as does the ground state. The versatility and flexibility of this approach stems from the possibility of introducing the cluster expansion based approximations of highly excited clusters at various levels of the iterative scheme, leading to a sequence of approximations including both the configuration interaction and coupled cluster methods with singles and doubles as a special case. Both the reliability and the efficiency of these various approximations is examined, and potentially promising approximation schemes are identified.
Characterization of a subpicosecond x-ray streak camera for ultrashort laser-produced plasmas experiments
Authors: Gallant P. , Forget P. , Dorchies F. , Jiang Z. , Kieffer J. C. , Jaanimagi P. A. , Rebuffie J. C. , Goulmy C. , Pelletier J. F. , Sutton M.
Published: July 2002

We present results of the characterization of an ultrafast x-ray streak camera, based on Photonis (formerly Philips Photonics) P860 tube, developed for use in ultrashort laser-produced plasma research. The streak camera presented here (called PX1) has been extensively characterized with continuous and pulsed x-ray sources. Time resolution of 350 fs in the keV x-ray range has been achieved, while maintaining a high spatial resolution of 40 μm along a direction perpendicular to the time dispersion axis. It is shown that the streak camera response is lower when the photocathode is illuminated by a pulsed source than when used with a continuous one. This effect seems to be related to a change in the phosphor response. The camera has been used to achieve high-resolution subpicosecond time-resolved spectroscopy of ultrashort laser plasmas allowing the measurements of K-shell line emission durations of 700 fs.
Homogeneous nucleation rates of n-pentanol in nitrogen measured in a piston-expansion tube
Authors: Graßmann A. , Peters F.
Published: July 2002

Homogeneous nucleation rates of n-pentanol in nitrogen are presented. They are obtained from a piston-expansion tube (pex-tube) involving the nucleation pulse method which generates a limited number of nuclei that grow into droplets. The detection of the droplets is achieved by a new counting method developed on the basis of a CCD camera in combination with a laser light sheet. Nucleation rates between 104 and 109 cm−3 s−1 are covered for three nucleation temperatures 250, 260, and 270 K. The rates are plotted as isotherms vs supersaturation. Influence of the initial expansion temperature and the nucleation pressure on the nucleation rate is identified. Comparison with results available in literature and classical nucleation theory (CNT) is provided.
Photoelectron spectroscopy of ammonia: Mode-dependent vibrational autoionization
Authors: Bacon J. A. , Pratt S. T.
Published: July 2002

Photoelectron spectroscopy was used to study the mode dependence of vibrational autoionization in high-Rydberg states of NH3. Two-color, two-photon resonant, three-photon excitation via the C̃′ 1A1′(1300) intermediate state was used to populate selected autoionizing Rydberg states between the (1200) and (1300) ionization thresholds of the NH3+ X̃ 2A2″ ground electronic state, and the ionic vibrational distributions were determined from the photoelectron spectra. Excitation of Rydberg states in which two different vibrational modes are excited allowed the direct comparison of the autoionization efficiencies for the two modes. Autoionization via the loss of one quantum of vibrational energy from the nontotally symmetric “umbrella” mode, ν2, was found to be the dominant autoionization process. Vibrational branching fractions obtained from the ionic vibrational distributions indicate that, for the Rydberg states accessed via the C̃′ 1A1′(1300) intermediate state, the ν2 mode is approximately 25 times more efficient at promoting autoionization than the totally symmetric stretching mode, ν1.
Magnetic field of a relativistic nonlinear plasma wave
Authors: Khachatryan Arsen G.
Published: July 2002

Longitudinal and transverse behavior of the magnetic field of a relativistic nonlinear three-dimensional plasma wave is investigated. It is shown that the magnetic field of the wave is different from zero and performs higher frequency oscillations compared to the plasma electron frequency. An increase in the nonlinearity leads to strengthening of the magnetic field. The oscillations of the magnetic field in the transverse direction arise, which are caused by the phase front curving of a nonlinear plasma wave. The numerical results conform well to predictions of the analytical consideration of a weakly-nonlinear case.
Evidence for nonideality in the fundamental liquid mixture (36Ar+40Ar)
Authors: Rebelo L. P. N. , Dias F. A. , Lopes J. N. C. , Calado J. C. G. , da Ponte M. Nunes , Jancsó G.
Published: July 2002

Positive excess molar Gibbs energies were detected in liquid mixtures of (36Ar+40Ar) using a high-accuracy double differential manometric technique. The vapor pressure differences between a mixture of (36Ar+40Ar) and Ar40 and between Ar36 and Ar40 were measured simultaneously with the vapor pressure of Ar40. Four different mixtures were selected and 80 temperatures were chosen in the 85–96 K temperature range. The results show that the vapor pressure of an equimolar liquid mixture is greater than what would be expected if the solution were strictly ideal, by approximately 2.5 Pa in the 85–96 K temperature range. The calculated excess Gibbs energy for the equimolar mixture at 85 K is G1/2E=(0.021±0.004) J mol−1. For the excess molar enthalpy a value of H1/2E=(0.12±0.04) J mol−1 was found. The corresponding deviations from Raoult’s law are therefore extremely small (3 to 4 orders of magnitude smaller than those typically found in binary mixtures of nonisotopic constituents). The evidence of nonideal behavior in such a fundamental and simple system is relevant for the interpretation of the properties of liquid mixtures of more general importance. The pitfalls of the determination of activity coefficients through the comparison between vapor pressure isotope effect data and liquid–vapor isotope fractionation factors are discussed in some detail. The results were used to test three theoretical models, namely, the vdW-1 fluid theory, the 1cLJ perturbation theory, and the statistical theory of isotope effects in mixtures. While the first two proved to be inadequate, the isotope effect theory agrees reasonably well with experiment. The excess Gibbs energy of monatomic isotopic mixtures are related with the molar volume isotope effect and with the mean Laplacian of the potential energy in the liquid phase.
Electromagnetic vortices in streaming pair plasmas
Authors: Vranješ J. , Kono M. , Lazzaro E. , Lontano M.
Published: July 2002

Two coupled nonlinear equations for a perturbed electromagnetic field in an electron–positron streaming plasma which is placed in a nonuniform magnetic field are derived and solved analytically, yielding stationary solutions in the form of vortices consisting of monopolar and quadrupolar parts. It is shown that vortices are created in and carried by a specific given linear shear flow profile and a given nonuniformity of the magnetic shear.
Role of collective effects in dominance of scattering off thermal ions over Langmuir wave decay: Analysis, simulations, and space applications
Authors: Cairns Iver H.
Published: July 2002

Langmuir waves driven to high levels by beam instabilities are subject to nonlinear processes, including the closely related processes of scattering off thermal ions (STI) and a decay process in which the ion response is organized into a product ion acoustic wave. Calculations of the nonlinear growth rates predict that the decay process should always dominate STI, creating two paradoxes. The first is that three independent computer simulation studies show STI proceeding, with no evidence for the decay at all. The second is that observations in space of type III solar radio bursts and Earth’s foreshock, which the simulations were intended to model, show evidence for the decay proceeding but no evidence for STI. Resolutions to these paradoxes follow from the realization that a nonlinear process cannot proceed when its growth rate exceeds the minimum frequency of the participating waves, since the required collective response cannot be maintained and the waves cannot respond appropriately, and that a significant number of e-foldings and wave periods must be contained in the time available. It is shown that application of these “collective” and “time scale” constraints to the simulations explains why the decay does not proceed in them, as well as why STI proceeds in specific simulations. This appears to be the first demonstration that collective constraints are important in understanding nonlinear phenomena. Furthermore, applying these constraints to space observations, it is predicted that the decay should proceed (and dominate STI) in type III sources and the high beam speed regions of Earth’s foreshock for a specific range of wave levels, with a possible role for STI alone at slightly higher wave levels. Deeper in the foreshock, for slower beams and weaker wave levels, the decay and STI are predicted to become ineffective. Suggestions are given for future testing of the collective constraint and an explanation for why waves in space are usually much weaker than in the simulations.
Laser optogalvanic and jet spectroscopy of germylene (GeH2): New spectroscopic data for an important semiconductor growth intermediate
Authors: Smith Tony C. , Clouthier Dennis J. , Sha Wei , Adam Allan G.
Published: July 2002

The Ã 1B1–X̃ 1A1 electronic transition of germylene has been reinvestigated. A room temperature absorption spectrum of the central portion of the 000 band of GeH2 has been obtained using the technique of laser optogalvanic spectroscopy. A rotationally resolved spectrum of the 000 band of jet-cooled GeD2 has been recorded with a pulsed discharge source. Analysis of these spectra has yielded ground and excited state rotational constants for the GeH274, GeH272, GeH270, GeD276, GeD274, GeD272, and GeD270 isotopomers and approximate equilibrium structures of: r″(Ge–H)=1.5883(9) Å, θ″(H–Ge–H)=91.22(4)°, r′(Ge–H)=1.5471(6) Å, and θ′(H–Ge–H)=123.44(2)°. The ground state ν1 and ν2 vibrational frequencies have been determined from wavelength-resolved fluorescence spectra of jet-cooled GeH2 and GeD2. There is good evidence that GeH2 rotational levels with Ka′>1 are so strongly predissociated that lifetime broadening makes them diffuse, severely restricting the information that can be obtained from absorption and laser-induced fluorescence experiments.
Breakdown of the k-conservation rule in Si1−xGex alloy nanocrystals: Resonant photoluminescence study
Authors: Fujii Minoru , Kovalev Dmitri , Diener Joachim , Koch Frederick , Takeoka Shinji , Hayashi Shinji
Published: July 2002

Resonant photoluminescence from Si1−xGex alloy nanocrystals as small as 4 nm in diameter embedded in SiO2 thin film matrices was studied. In pure Si nanocrystals, phonon structures were clearly observed, indicating that optical transitions are assisted by momentum-conserving phonons. These structures are quenched by adding a small amount of Ge atoms in Si nanocrystals, i.e., due to the formation of Si1−xGex alloy. Furthermore, the lifetime of the radiative recombination became much faster in doped nanocrystals. These results suggest that Si1−xGex alloy formation leads to a disorder in the translation invariance of the crystalline lattice, thus resulting in the breakdown of the k-conservation rule.
Nonlinear optical response in acceptor-substituted carotenoids: A theoretical study
Authors: Balakina M. Yu. , Li J. , Geskin V. M. , Marder S. R. , Brédas J. L.
Published: July 2002

Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 107–108 V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and α, β, and γ. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molecular polarizabilities is assessed.
Theory of nonoverlapping low frequency modes in axisymmetric toroidal plasmas
Authors: Rogister André L.
Published: July 2002

The theory of low frequency instabilities in axisymmetric toroidal plasmas is presented from the point of view of the two-fluids equations, assuming the standard drift wave ordering. Attention is focused on the limit in which neighboring rational surfaces (corresponding to a fixed toroidal mode number) are sufficiently far apart that mode overlapping can be neglected. Owing to field line bending, poloidal side bands m±1,… coexist with the primary mode m, enhancing noticeably the role of the parallel ion dynamics. The electron and ion branches are investigated systematically under those conditions. It is found that the radial widths of the eigenmodes increase with respect to the slab values; the shear damping rate of the electron branch, respectively, the growth rate of the ion branch increase correspondingly. Other interesting new results are obtained concerning, in particular, the frequency, the growth rate and the poloidal asymmetry of the ion mode fluctuations. It is mentioned that those appear to be directly relevant to various experiments and, in particular, to internal transport barriers. Shortcomings of the standard ballooning formalism are pointed out.
Waves and coherent structures in the turbulent plasma of a simple magnetized torus
Authors: Riccardi Claudia , Fredriksen Åshild
Published: July 2002

A systematic analysis of electrostatic fluctuations by means of Beall’s technique has been performed in a helium plasma of the simple magnetized torus Blaamann [Rypdal et al., Plasma Phys. Controlled Fusion 35, 1099 (1994)]. A comparison between the wave-like description furnished by Beall’s technique and the method based on conditional sampling, has been carried out. In agreement with previous results in Blaamann using the conditional sampling technique, coherent structures of the vortex type have been identified and analyzed measuring the bicoherence and the phase between the density and potential fluctuations. Moreover, with Beall’s technique it was also possible to analyze the role of coherent structures in particle transport and identify another instability driving collisional drift waves. Fluctuations can produce particle transport in the edge of the coherent structures but not in the core of the vortical structure. Also, drift modes were detected at high magnetic fields, for which the density gradient was more pronounced. In addition, it was observed that increased neutral pressure produced a decrease of the spatial and cross coherence leading to dissipation of coherent structures and to an increase of the particle transport.
Nonexponential decay of velocity correlations in surface diffusion: The role of interactions and ordering
Authors: Vattulainen I. , Hjelt T. , Ala-Nissila T. , Ying S. C.
Published: July 2002

We study the diffusive dynamics of adparticles in two model systems with strong interactions by considering the decay of the single-particle velocity correlation function φ(t). In accordance with previous studies, we find φ(t) to decay nonexponentially and follow a power-law φ(t)∼t−x at intermediate times t, while at long times there is a crossover to an exponential decay. We characterize the behavior of the decay exponent x in detail in various ordered phases and in the vicinity of phase boundaries. We find that within the disordered phase, the behavior of x can be rationalized in terms of interaction effects. Namely, x is typically larger than two in cases where repulsive adparticle–adparticle interactions dominate, while attractive interactions lead to x<2. In ordered phases, our results suggest that the behavior of x is mainly governed by ordering effects that determine the local structure in which adatoms diffuse. Then the decay is characterized by 1<x<2 under conditions where diffusion is truly two-dimensional, while in phases where adatoms diffuse in a one-dimensional fashion along ideal rows of vacancies, we find a regime characterized by x<1. Also, changes in the qualitative behavior of x are closely related to phase boundaries and local ordering effects. Our studies suggest that φ(t) can be used to obtain information about the ordering of the system and about the nature of predominant interactions between adparticles. Our predictions can be tested experimentally by techniques such as scanning tunneling microscopy, in which φ(t) can be measured in terms of discrete adparticle displacements as shown in this work. Finally, our studies suggest that the decay of velocity correlations in collective diffusion follows, qualitatively, the same behavior as the decay of single-particle velocity correlations in tracer diffusion.
Systematic study of Ga1−xInxAs self-assembled quantum wires with varying interfacial strain relaxation
Authors: Li Liang-Xin , Sun Sophia , Chang Yia-Chung
Published: July 2002

A systematic theoretical study of the electronic and optical properties of Ga1−xInxAs self-assembled quantum wires (QWRs) made of short-period superlattices with strain-induced lateral ordering is presented. The theory is based on the effective bond-orbital model combined with a valence-force-field (VFF) model. Valence-band anisotropy, band mixing, and effects due to local strain distribution at the atomistic level are all taken into account. Several structure models with varying degrees of alloy mixing for lateral modulation are considered. A VFF model is used to find the equilibrium atomic positions in the QWR structure by minimizing the lattice energy. The strain tensor at each atomic (In or Ga) site is then obtained and included in the calculation of electronic states and optical properties. It is found that different local arrangement of atoms leads to very different strain distribution, which in turn alters the optical properties. In particular, we find that in model structures with thick capping layer the electrons and holes are confined in the Ga-rich region and the optical anisotropy can be reversed due to the variation of lateral alloy mixing, while for model structures with thin capping layer the electrons and holes are confined in the In-rich region, and the optical anisotropy is much less sensitive to the lateral alloy mixing.
Beam scattering and low-frequency fluctuations in a beam-driven strongly turbulent plasma
Authors: McFarland M. D. , Wong A. Y.
Published: July 2002

The scattering of an electron beam in angle and energy in an unmagnetized, statistically stable, strongly turbulent plasma is investigated experimentally and the results are compared to a theoretical model derived using the Born approximation and the two-component model of strong Langmuir turbulence. It is found that for a typical wave intensity level 〈W〉∼0.04 the scattering in angle is θ∼3°, and in energy is ΔU∼25 eV for a 400 eV beam. The scattering of the beam in both angle and energy is found to agree well with the theory. The low-frequency (ω∼ωpi) fluctuations are also measured in this investigation and their magnitude Δn/n is compared to the two-component model of strong Langmuir turbulence. It is found that 〈Δn2〉/n2∝〈W〉β where β∼1.3 for the experiment and is predicted to be β∼1.98 by the model. The results indicate that the two-component model, and the Zakharov equations to which the theory was originally verified, give an accurate description of strong Langmuir turbulence. In addition, the validity of transit time scattering theory, based on the Born approximation, is confirmed.
Three-dimensional conformal irradiation with a multilayer energy filter for proton therapy
Authors: Sakae Takeji , Nohtomi Akihiro , Maruhashi Akira , Sato Masaru , Hosono Katsuhisa , Terunuma Toshiyuki , Kohno Ryosuke , Akine Yasuyuki , Hayakawa Yoshinori
Published: July 2002

The first experimental evidence of three-dimensional conformal irradiation is performed by using a new type of filter developed for charged particle radiotherapy. The new filter can yield a static irradiation field where the width of the spread-out Bragg peak is adjusted to the target as a two-dimensional continuous function in the transverse plane. The filter is made of many layers produced by using stereolithography. In the filter, a structure with two regions with different shaped miniaturized ridges is adopted to get a flat dose profile on the spread-out peak. A conformal field around a spherical target is realized by a filter that has a shape corresponding to the outward form of the target.
Nonadiabatic transition-state theory: A Monte Carlo study of competing bond fission processes in bromoacetyl chloride
Authors: Marks Alison J.
Published: July 2002

Nonadiabatic Monte Carlo transition-state theory is used to explore competing C–Cl and C–Br bond fission processes in a simple model of [n,π1*(CO)] photoexcited bromoacetyl chloride. Morse potentials are used to represent bond stretching coordinates, and the positions and magnitudes of nonadiabatic coupling between excited state potentials are modeled using ab initio data. The main effect of nonadiabaticity is to favor C–Cl fission over C–Br, despite a larger barrier to C–Cl dissociation. The absolute values of the rate constants are smaller than observed experimentally, but the calculated branching ratios are close to the experimental value. For C–Cl fission, it is shown that the minimum energy crossing point is not sufficient to describe the rate constant, suggesting that care must be taken when using alternative models which make this assumption.
Formation and sustainment of electrostatically driven spheromaks in the resistive magnetohydrodynamic model
Authors: Sovinec C. R. , Finn J. M. , del-Castillo-Negrete D.
Published: July 2002

The nonlinear time-dependent equations of resistive magnetohydrodynamics are solved in simply connected domains to investigate spheromak formation and sustainment with electrostatic current drive. Spheromak magnetic fields are generated in three-dimensional computations as the nonlinear state resulting from an unstable pinch. Perturbations convert continuously supplied toroidal magnetic flux into poloidal magnetic flux, leading to “flux amplification” of field embedded in the electrodes. Relaxation of the axisymmetric component of the parallel current profile can be substantial, and the final nonlinear state is steady over a wide range of parameters. However, for sufficiently large values of Lundquist number or sufficiently large applied potential, nonsteady final states are observed with periodic relaxation events in some cases. Under most conditions, the saturated configuration exhibits chaotic scattering of the magnetic field lines. Conditions just above the marginal point of pinch instability sustain large closed flux surfaces in steady state; a weakly kinked pinch current threads the toroidal region of closed flux surfaces and imposes stellarator-like helical transform. Closed flux surfaces also form during decay, due to reduced fluctuation levels and average toroidal current driven directly by inductive electric field.