bluuebunny/crossref_metadata_2025 huggingface dataset:
A two-chain path integral model of positronium
Authors: Larrimore L. , McFarland R. N. , Sterne P. A. , Bug Amy L. R.
Published: July 2002

We have used a path integral Monte Carlo technique to simulate positronium (Ps) in a cavity. The primitive propagator is used, with a pair of interacting chains representing the positron and electron. We calculate the energy and radial distribution function for Ps enclosed in a hard, spherical cavity, and the polarizability of the model Ps in the presence of an electrostatic field. We find that the positron distribution near the hard wall differs significantly from that for a single particle in a hard cavity. This leads to systematic deviations from predictions of free-volume models which treat Ps as an effective, single particle. A virial-type estimator is used to calculate the kinetic energy of the particle in the presence of hard walls. This estimator is found to be superior to a kinetic-type estimator given the interaction potentials, cavity sizes, and chain lengths considered in the current study.
Optical properties of a Fabry–Pérot microcavity with Er-doped hydrogenated amorphous silicon active layer
Authors: Dukin A. A. , Feoktistov N. A. , Golubev V. G. , Medvedev A. V. , Pevtsov A. B. , Sel’kin A. V.
Published: July 2002

Fabry–Pérot hydrogenated amorphous silicon (a-Si:H)/amorphous-SiOx:H microcavities with an erbium-doped a-Si:H active region are fabricated by a plasma-enhanced chemical-vapor deposition technique in a single technological cycle without exposure to air between the intermediate operations. A metalorganic compound is used to incorporate erbium in the active a-Si:H layer. Transmission, reflection, and photoluminescence spectra of the microcavities are measured. The experimental data are compared to theoretical calculations performed in terms of field amplitudes generated by stochastic excitation sources.
Construction of a near-field spectrum analysis system using bent tapered fiber probes
Authors: Chen Sy-Hann , Chen Yung-Fu
Published: July 2002

We take advantage of a combination of laser heating and pulling and electric arc bending to fabricate bent tapered fiber probes. The bent angles can be varied from 30° to 70° and tip diameters fall within a few tens of nanometers. These bent fiber probes can easily be adapted into any dynamic mode atomic force microscope. By proper manipulation of the bent angles, a spatial resolution of up to 60 nm is achievable. After coating the bent fiber probes with a thin layer of Pt/Pd film by ion sputtering, the transmission efficiency is measured to be around 10−5, which is applicable for near-field spectrum analysis experiment.
Mixing quantum and classical dynamics using Bohmian trajectories
Authors: Gindensperger E. , Meier C. , Beswick J. A.
Published: July 2002

A novel time-dependent hybrid quantum/classical propagation scheme based on Bohmian quantum trajectories is presented. The quantum subsystem is described by a wave packet depending on the quantum variables x and, via the total potential energy of the system, parametrically on the classical trajectories X(t). The wave packet is used to calculate de Broglie–Bohm quantum trajectories x(t) which are used to calculate the force acting on the classical variables. Quantum corrections of the classical equation of motion are also included. The method is applied to a simple case of two coupled oscillators. Comparison between exact quantum and approximate results demonstrates that these MQCB (Mixed Quantum/Classical Bohmian) trajectories provide a good description of the energy exchange between the two oscillators.
Electrical spin injection across air-exposed epitaxially regrown semiconductor interfaces
Authors: Park Y. D. , Jonker B. T. , Bennett B. R. , Itskos G. , Furis M. , Kioseoglou G. , Petrou A.
Published: July 2002

We have fabricated spin-polarized light-emitting diode structures via epitaxial regrowth of Zn1−xMnxSe on air-exposed surfaces of AlyGa1−yAs/GaAs quantum wells. No passivation procedures were used to protect or prepare the III–V surface. The electroluminescence is strongly circularly polarized due to the electrical injection of spin-polarized electrons from the ZnMnSe contact into the GaAs quantum well. An analysis of the optical polarization yields a lower bound of 65% for the spin injection efficiency. These results demonstrate the robustness of the spin injection process in the diffusive transport regime, and attest to the practicality of manufacturing semiconductor-based spin injection devices.
Low temperature diamond growth using CO2/CH4 plasmas: Molecular beam mass spectrometry and computer simulation investigations
Authors: Petherbridge James R. , May Paul W. , Pearce Sean R. J. , Rosser Keith N. , Ashfold Michael N. R.
Published: July 2002

Microwave plasma enhanced chemical vapor deposition has been used to grow diamond films at substrate temperatures down to 435 °C using CO2/CH4 gas mixtures. An Arrhenius plot of growth rate as a function of substrate temperature yields a value for the activation energy for the growth step of 28 kJ mol−1. This is lower than that measured previously for CH4/H2 systems and hints at a different gas-surface chemistry when using CH4/CO2 plasmas. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of plasma gas mixtures (0%–80% CH4, balance CO2). The CHEMKIN computer package has also been used to simulate the experimental results in order to gain insight into the major reactions occurring within the microwave plasma. The calculated trends for all species agree well with the experimental observations. Using these data, the model for the gas phase chemistry can be reduced to only four overall reactions. Our findings suggest that CH3 radicals are likely to be the key growth species when using CO2/CH4 plasmas and provide a qualitative explanation for the narrow concentration window for diamond growth.
A complete active space self-consistent field multiconfiguration reference configuration interaction study of the potential energy curves of the ground and excited states of CCl
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The potential energy curves have been examined for the ground (X 2Π) and excited states (1 2Δ, 1 2Σ+, 2 2Σ+, and 1 Σ−) of CCl using complete active space SCF (CASSCF) and multiconfiguration reference internally contracted configuration interaction (MRCI) with the aug-cc-pVTZ basis set. The active spaces are comprised of all valence molecular orbitals and include some nonvalence virtual molecular orbitals, which are used to further confirm the nature of the excited states. The anharmonic vibrational energy levels are also obtained for the ground and excited states based on the potential energy curves calculated with different active spaces. A comparison with experimental results is made and discussed.
New diamond anvil cell for single-crystal analysis
Authors: Yamanaka Takamitsu , Fukuda Tomoo , Hattori Takanori , Sumiya Hitoshi
Published: July 2002

A new diamond anvil cell was devised for the single-crystal structure analyses under high pressure. The cell is characterized by large single-crystal diamond plates, which are used for the windows and fixed on the table planes of diamond anvils. The diamond plate windows have great advantages for single-crystal diffractometry, such as low x-ray absorption, much higher pressures up to 50 GPa, no powder rings from the window, and wide 2θ-angle (2θ<70°). Great progress of the structure analyses could be made by the combination with synchrotron radiation (SR). The short wavelength of λ=0.407 72 Å (E=30.388 keV) of SR source with 8 GeV 100 mA at SPring-8 was used for the diffraction intensity measurement of SiO2 stishovite (rutile type). These advantages could measure the intensities of much higher order reflections and the precise electron density distribution as a function of pressure was discussed.
Texture transformations in reactive metal films deposited upon amorphous substrates
Authors: Dunn D. N. , Hull R. , Ross F. M. , Tromp R. M.
Published: July 2002

We present results that demonstrate how interfacial reactions between a metal film and substrate during deposition affect microstructural evolution. In particular, we investigate Ti films deposited on amorphous SiO2 using ultrahigh vacuum transmission electron microscopy. Ti films were deposited in situ at room temperature and were examined using Auger electron spectroscopy and transmission electron microscopy. An initial [hk0] preferred orientation developed in films up to 2.5 nm in thickness. Films between 2.5 and 5.0 nm developed a [001] preferred orientation that persisted in films up to 20.0 nm thick. These data, in conjunction with Auger electron spectra and dark-field microscopy, suggest that growth of Ti films on SiO2 is directly affected by reactions at the Ti/SiO2 interface and that this reaction is responsible for the observed change in preferred orientation.
Quantum point contact transistor with high gain and charge sensitivity
Authors: Kurdak Ç. , Farina L. , Lewis K. M.
Published: July 2002

We analyze the potential performance of quantum point contact (QPC) devices in charge detection applications. For the standard QPC structure we show that the charge sensitivity is strongly dependent on gate geometry and can be close to the quantum limit, and that the gain parameter is less than one under bias conditions where the charge sensitivity is optimized. We propose a novel QPC device consisting of two split gates for defining the QPC and a third gate which can be used to filter out hot electrons that are emitted from the QPC. We show that this proposed device can have a high gain and a charge sensitivity close to that of single electron transistors. The device can be realized using high quality GaAs/AlGaAs with a two-dimensional electron gas and standard nanofabrication techniques. Unlike single electron transistors, the gain of the proposed device does not depend on the charge configuration near the active region of the device. Therefore the device can be used as an electrometer without a feedback charged locked loop and multiple devices can easily be integrated.
Preparation of electrically conductive diamond-like carbon films using i-C4H10/N2 supermagnetron plasma
Authors: Kinoshita Haruhisa , Hando Takuya , Yoshida Masahiro
Published: July 2002

Electrically conductive diamond-like carbon (DLC) films were deposited by supermagnetron plasma chemical vapor deposition. The deposition was made on Si and glass wafers using mixed isobutane (i-C4H10) and N2 gases. The physical properties of deposited film were measured and analyzed. Fourier transform infrared spectroscopy measurements revealed that the absorption due to N–H, C–N, and C≡N bonds increased with increases in N2 gas concentration. The increase in electrical conductivity could be attributed to C–N and C≡N bond creation in the DLC films. The lowest resistivity, 0.17 Ω cm, was achieved at an N2 concentration of 70%, gas pressure of 50 mTorr, lower electrode temperature of 160 °C, and rf powers of 1 kW/1 kW. The lowest resistivity film was 1750 kg/mm2 hard, harder than glass (1340 kg/mm2). Raman spectroscopy measurements revealed two peak D and G bands, and the D band was more intense than the G band. The optical band gap decreased with increases in the N2 concentration. Hall measurements showed that the carrier was n type and both carrier density and Hall mobility increased with rf powers.
Controlled photoluminescence in amorphous-silicon-nitride microcavities
Authors: Serpengüzel Ali , Tanriseven Selim
Published: July 2002

Narrow-band and enhanced photoluminescence have been observed in hydrogenated amorphous-silicon-nitride microcavities. The distributed Bragg reflectors were fabricated using alternating layers of hydrogenated amorphous-silicon nitride and hydrogenated amorphous-silicon oxide. The microcavity resonance wavelength was designed to be at the maximum of the bulk hydrogenated amorphous-silicon-nitride luminescence spectrum. At the microcavity resonance, the photoluminescence amplitude is enhanced, while the photoluminescence linewidth is reduced with respect to the bulk hydrogenated amorphous-silicon nitride.
Magnification relations in gravitational lensing via multidimensional residue integrals
Authors: Dalal Neal , Rabin Jeffrey M.
Published: July 2002

We investigate the so-called magnification relations of gravitational lensing models. We show that multidimensional residue integrals provide a simple explanation for the existence of these relations, and an effective method of computation. We illustrate the method with several examples, thereby deriving new magnification relations for galaxy lens models and microlensing (point mass lensing).
Self-diffusion near the liquid–vapor critical point
Authors: Drozdov Alexander N. , Tucker Susan C.
Published: July 2002

While detailed descriptions of critical anomalies are well known for thermodynamic variables, the effect of critical fluctuations on microscopic properties is much less well understood. Herein we use the results of molecular dynamics simulation, for the distribution of local densities around a tagged particle in a neat Lennard-Jones fluid, to evaluate the behavior of the self-diffusion coefficient in the critical region (Tr≈1.05; 0.4<ρr<1.6). We find a weakly anomalous behavior at near critical densities which we attribute to both the broadening of this local-density distribution and the enhancement of mean local density.
Temperature dependence of electric-field induced photoluminescence from an InGaN-based light-emitting diode
Authors: Kudo Hiromitsu , Tanabe Tomoyuki , Ishibashi Hiroki , Zheng Ruisheng , Yamada Yoichi , Taguchi Tsunemasa
Published: July 2002

Temperature dependence of reverse-biased photoluminescence has been investigated for understanding the radiative recombination mechanism in an InGaN single-quantum-well light-emitting diode. It is found that the applied-voltage dependence of luminescence intensities is strongly affected by temperature from 17 to 100 K, and a dramatic decrease in the luminescence intensity is observed over 100 K. The model of a field ionization of excitons cannot explain this dramatic decrease in the luminescence intensity. It is therefore suggested that the free-carrier recombination process becomes dominant over 100 K. Two emission components are found on the condition of reverse bias. The lower-energy component becomes strongly dependent on reverse-bias voltage with increasing temperature, and fully disappears under the applied voltage of only −2 V at 100 K.
Computing the inertia operator of a rigid body
Authors: Lawton Wayne , Noakes Lyle
Published: July 2002

We prove that the inertia operator A of a rigid body is generically determined, up to a scalar multiple, by the curve Ω in R3 that describes its angular velocity in the body. The precise condition is that Ω not be contained in a two-dimensional subspace of R3. We derive two indirect methods to compute A from the values of Ω over an arbitrary interval, and a direct method to compute A from the second- and fourth-order moments of Ω. The direct method utilizes moment identities derived from symmetries in Euler’s equation.
The hydrophobic hydration of methane as a function of temperature from histogram reweighting Monte Carlo simulations
Authors: Hernández-Cobos J. , Mackie A. D. , Vega L. F.
Published: July 2002

A microscopic explanation for the experimental observations of the solvation of methane in water is proposed by way of free energy and structural information estimated using computer simulations. In particular, the temperature dependence of both the free energy of solvation as well as structural information is calculated. The free energy is estimated by reweighting histograms from canonical and grand canonical Monte Carlo simulations along a thermodynamic path. The results for the free energy and structural changes are found to be in qualitative agreement with experimental data. We propose that the decrease in entropy found in the solvation process is brought about by excluded volume effects on mixing the two components rather than any local “freezing” of the water around the methane. At the same time the enthalpy of solvation is determined by the solute-solvent interaction. A weak dependence of the hydration free energy with temperature is found. A possible explanation lies with the relatively low temperature sensitivity of the methane–water radial distribution functions.
Dynamics of a condensing liquid film under conjoining/disjoining pressures
Authors: Oron Alexander , Bankoff S. George
Published: July 2002

The dynamics of a condensing apolar ultrathin liquid film is studied in the framework of long-wave theory in the cases of both horizontal and slightly tilted solid coated surfaces. When condensation is slow, the film on a horizontal substrate passes through the stages of hole opening driven by the “reverse reservoir effect,” hole closing, eventual thickness equilibration and further spatially uniform growth of the condensate. When condensation is faster and the resistance to phase change is lower, secondary droplet(s) may emerge within the hole. During the film evolution the thickness of the microlayer covering the hole remains practically constant due to the “reverse reservoir effect.” The total heat flux across the condensate film is found to decrease with the absolute value of the condensation constant. When the solid substrate is tilted, the film dynamics exhibits the formation of multidrop structures and their coarsening along with the stages typical for the horizontal case. The increase of the tilt angle leads to faster transition from dropwise to filmwise condensation and to the increase of the total heat flux through the condensate.
Density profile evolution and nonequilibrium effects in partial and full spreading measurements of surface diffusion
Authors: Nikunen P. , Vattulainen I. , Ala-Nissila T.
Published: July 2002

We study the nature of nonequilibrium effects in the collective diffusion coefficient DC(θ) vs the coverage θ as extracted from Boltzmann–Matano analysis of spreading coverage profiles. We focus on the temporal behavior of the profiles and study how the corresponding nonequilibrium effects in DC(θ) depend on the initial density gradient and the initial state from which the spreading starts. To this end, we carry out extensive Monte Carlo simulations for a lattice-gas model of the O/W(110) system. Studies of submonolayer spreading from an initially ordered p(2×1) phase at θ=12 reveal that the spreading and diffusion rates in directions parallel and perpendicular to rows of oxygen atoms are significantly different within the ordered phase. Aside from this effect, we find that the degree of ordering in the initial phase has a relatively small impact on the overall behavior of DC(θ). Also, although we find that nonequilibrium effects are clearly present in submonolayer spreading profiles, DC(θ) determined from such data approaches its asymptotic equilibrium behavior much more rapidly than in the case of full spreading. Nevertheless, in both cases there are noticeable deviations from equilibrium results that persist even at very long times and are strongest in ordered phases and in the vicinity of phase boundaries. These conclusions are confirmed by complementary studies of the temporal behavior of the order parameter φ(θ). Finally, we use DC(θ) and φ(θ) to determine the locations of phase boundaries and find such data to be clearly time dependent during full spreading. We conclude that nonequilibrium effects seem to be an inherent feature in profile evolution studies of surface diffusion in all cases where ordering plays a prominent role. This warrants particular care to be taken with profile spreading experiments.
Investigation of dead-layer thickness in SrRuO3/Ba0.5Sr0.5TiO3/Au thin-film capacitors
Authors: Sinnamon L. J. , Bowman R. M. , Gregg J. M.
Published: July 2002

Thin-film capacitors, with barium strontium titanate (BST) dielectric layers between 7.5 and 950 nm in thickness, were fabricated by pulsed-laser deposition. Both crystallography and cation chemistry were consistent with successful growth of the BST perovskite. At room temperature, all capacitors displayed frequency dispersion such that ε100 kHz/ε100 Hz was greater than 0.75. The dielectric constant as a function of thickness was fitted, using the series capacitor model, for BST thicknesses greater than 70 nm. This yielded a large interfacial di/εi ratio of 0.40±0.05 nm, implying a highly visible parasitic dead layer within the capacitor structure. Modeled consideration of the dielectric behavior for BST films, whose total thickness was below that of the dead layer, predicted anomalies in the plots of d/ε against d at the dead-layer thickness. In the capacitors studied here, no anomaly was observed. Hence, either (i) 7.5 nm is an upper limit for the total dead-layer thickness in the SRO/BST/Au system, or (ii) dielectric collapse is not associated with a distinct interfacial dead layer, and is instead due to a through-film effect.
Tritium depth dependence of tritium imaging
Authors: Youle I. , Haasz A. A.
Published: July 2002

A Monte Carlo simulation of tritium β particle motion in a matrix material indicates that the probability of escape through the material surface is greater than 50% for near-surface tritium, but drops quite rapidly with increasing depth. Beyond an areal density depth of 2×10−5 g/cm2, the decline is close to exponential, with an e-folding length of approximately 3×10−5 g/cm2, the exact value varying slightly with the material. The sensitivity of the tritium imaging technique will decrease at a similar rate with increasing depth of tritium. Experimentally, the image intensity of a tritium-implanted graphite specimen was observed to decrease exponentially with coating thickness as an aluminum layer was evaporated onto its surface, with an exponent that was within 30% of the predicted value. The Monte Carlo simulation also indicated that the limit of lateral resolution of the tritium imaging technique will be slightly less than the depth of the tritium, subject always to Recknagel’s limit of resolution due to “chromatic aberration” of the electron optics, which is of the order of 200 nm. If tritium is uniformly distributed through the material, surface tritium so dominates image formation that Recknagel’s limit inevitably applies.
Helical polyacetylene heavily doped with iodine: Magnetotransport
Authors: Suh D.-S. , Kim T. J. , Aleshin A. N. , Park Y. W. , Piao G. , Akagi K. , Shirakawa H. , Qualls J. S. , Han S. Y. , Brooks J. S.
Published: July 2002

The magnetoconductivity of two kinds of iodine doped helical polyacetylene, R- and S-polyacetylene, were investigated to understand the effect of the intrafibril and the interfibrillar interaction in the polyacetylene system. The zero-field resistivity ratio, ρr=ρ(1.2 K)/ρ(300 K), is comparable to that of stretch-oriented high-density polyacetylene film, which indicates the partial alignment of chains inside a polymer fiber. At low magnetic fields, the small negative component of magnetoconductivity (positive magnetoresistance) was observed and its magnitude increases as the ρr value increases. In the high field region, the magnetoconductivity is positive and it clearly shows the linear dependence on the magnetic field up to H=30 T. The linear field dependence of magnetoconductivity is different from what is expected in the three-dimensional localization-interaction picture. For the same ρr value samples, the magnitude of negative magnetoconductivity of S-polyacetylene is much bigger than that of R-polyacetylene, which could be attributed to the difference in the degree of helicity determining the strength of interfibrillar interaction.
Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. II. Numerical calculations
Authors: Ohta Kaoru , Larsen Delmar S. , Yang Mino , Fleming Graham R.
Published: July 2002

We model recent experimental wavelength dependent Three Pulse Photon Echo Peak Shift (WD-3PEPS) and Transient Grating (WD-TG) signals considering both solvation dynamics and vibrational contributions. We present numerical simulations of WD-3PEPS and WD-TG signals of two probe molecules: Nile Blue and N,N-bisdimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide to investigate the influence of intramolecular vibrations in the signals. By varying the excitation wavelength, we show that the different initial conditions for the vibrational wave packets significantly affect the signals, especially through the contributions associated with high frequency modes, often neglected in experimental analyses. We show that the temporal properties of both WD-TG and WD-3PEPS signals display sensitivities to both the excitation wavelength and the vibronic structure of the specific probe molecule used. Several mechanisms for generating vibronic modulations in the signals are discussed and their effects on the signals are described. Quantitative agreement between experiment and simulated signals requires accurate characterization of the laser pulses, specifically the magnitude and sign of chirp has a significant effect on the initial temporal properties of the signals. We provide a description of the experimental considerations required for accurate determination of molecular dynamics from 3PEPS and TG experiments and conclude with a brief discussion of the implications of our results for previous analyses of such experiments.
The role of electron density in magnetic turbulence
Authors: Terry P. W. , McKay Cavendish , Fernandez Eduardo
Published: July 2002

Inertial range energy transfer, decorrelation, and energy spectra are studied analytically and numerically for strongly anisotropic magnetohydrodynamic (MHD) turbulence augmented by electron density evolution. The model is relevant to interstellar turbulence and magnetic turbulence in fusion devices. For long wavelengths (compared to the ion gyroradius), magnetic and kinetic energies are equipartitioned through interactions that decorrelate on the Alfvénic time scale. Internal energy transfer is governed by advection and decorrelates on the eddy turnover time scale. For short wavelengths, the roles of internal and kinetic energy reverse. Magnetic and internal energies are equipartitioned by the kinetic Alfvén interaction, while kinetic energy evolves under a decoupled fluid straining interaction. The spectral indices for magnetic, kinetic, and internal energies are −3/2, −3/2, and −7/4 for long wavelengths, and −2, −5/3, and −2 for short wavelengths.
Three-dimensional secondary instability in plane current-vortex sheets
Authors: Dahlburg R. B. , Einaudi G.
Published: July 2002

It is shown that three-dimensional ideal instabilities can occur for the plane current-vortex sheet in the magnetically dominated regime. For the parameters under consideration, increasing the mean flow strength leads to decreased secondary instability growth rates. A calculation is presented which tracks the evolution of the secondary instability into the fully nonlinear regime, in which it is found that spanwise modes are dominant.
Fermi level position at metal Si1−x−yGexCy interfaces
Authors: Aubry-Fortuna V. , Barthula M. , Tremblay G. , Meyer F. , Warren P. , Lyutovitch K.
Published: July 2002

In this work, we have investigated the Schottky barrier heights on n- and p-type Si1−x−yGexCy alloys with Zr, Ti, W, Ni and Pt as metals (ΦBn and ΦBp, respectively). Contacts on Si1−xGex alloys showed various behaviors depending on the metal work function Φm. For low-Φm metals (Zr, Ti), ΦBn increases with x, while ΦBp(x) decreases. For higher Φm metals (Pt), ΦBn strongly decreases with x. In the particular case of W (intermediate Φm value), ΦBp follows exactly the decrease of the SiGe band gap with x, while ΦBn remains constant. Nevertheless, whatever the metal, the reduction of the sum ΦBn+ΦBp gives the band-gap variation as a function of x, and the Fermi level is located at the same position for both n and p-type layers. A weaker effect of Φm on the Schottky barrier heights is observed compared to pure Si: the position of the Fermi level tends to remain in the range 0.60–0.65 eV below the conduction band, as soon as Ge is adding in Si. W contacts on Si1−x−yGexCy alloys evidenced the strong effect of C on ΦBn and ΦBp. The variations of ΦBn(y) or ΦBp(y) cannot be correlated to the band gap. In addition, the position of the Fermi level at the interface depends on the type of the alloy. Nevertheless, as in the case of the binary alloy SiGe, a weaker dependence on Φm compared to that observed for pure Si is shown. High values of the ideality factor with increasing the C content may evidence the presence of interfacial inhomogeneities, which could be correlated to C short range order. The present results have been compared to existing published results.
Asymmetric nanoscale switching in ferroelectric thin films by scanning force microscopy
Authors: Gruverman A. , Kholkin A. , Kingon A. , Tokumoto H.
Published: July 2002

Scanning force microscopy (SFM) has been used to perform nanoscale studies of the switching behavior of Pb(Zr, Ti)O3 thin films via the direct observation of their domain structures. The study revealed a significant asymmetry of a switching pattern which is a function of the voltage polarity and original domain structure of individual grains. The phenomenon of asymmetric switching is attributed (1) to the presence of an internal built-in electric field at the bottom interface and (2) to the mechanical stress exerted by the SFM tip. The former effect results in incomplete 180° switching, while the latter effect leads to a 90° rotation of the polarization vector. The resulting shear stress deformation of the grain underneath the tip combined with the applied field effect propels polarization reversal in the adjacent grains.
On velocity structure functions and the spherical vortex model for isotropic turbulence
Authors: Aivazis Keri A. , Pullin D. I.
Published: July 2002

We investigate a stochastic model for homogeneous, isotropic turbulence based on Hill’s spherical vortex. This is an extension of the method of Synge and Lin [Trans. R. Soc. Can. 37, 45 (1943)], to the calculation of higher even-order velocity structure functions. Isotropic turbulence is represented by a homogeneous distribution of eddies, each modeled by a spherical vortex. The cascade process of eddy breakdown is incorporated into the statistical model through an average over an assumed log-normal distribution of vortex radii. We calculate the statistical properties of the model, in particular order-n velocity structure functions defined by rank-n tensors for the ensemble average of a set of incremental differences in velocity components. We define Di⋯s=〈(ui(x+ξ)−ui(x))⋯(us(x+ξ)−us(x))〉, where 〈⋯〉 denotes the ensemble average. Specifically Dij, Dijkl, and the longitudinal component of Dijklmn are calculated directly from the spherical vortex ensemble. Matching the longitudinal components of Dij and Dijkl with experimental results fixes two independent model parameters. The lateral and mixed components of Dijkl and the longitudinal component of Dijklmn are then model predictions.
Compressed-shell integrity measurements in spherical implosion experiments
Authors: Smalyuk V. A. , Yaakobi B. , Delettrez J. A. , Marshall F. J. , Meyerhofer D. D.
Published: July 2002

The shell integrity near peak compression of spherical implosions using the 60-beam, 30-kJ UV OMEGA laser system [Opt. Commun. 133, 495 (1997)] has been measured. Hot core emission backlights a shell with a thin titanium-doped layer that is imaged at x-ray photon energies above and below the titanium K edge. The x-ray intensity ratio between the two images is related to perturbations in the cold, or absorbing, part of the shell. The measured cold-shell areal-density modulations, integrated over the time of peak compression, are of the order of 25% to 50% with nonuniformity spectra peaked at spatial wavelengths of 30 to 50 μm and with the smallest detectable nonuniformity features extending down to spatial wavelengths of 12 to 15 μm. Hot-shell areal-density modulations of the emitting part of the shell (inner edge) are of the order of 13% to 20%. The measured shell modulations are in agreement with the results of two-dimensional simulations that include initial shell perturbations, imprinted shell modulations due to nonuniformities in a single laser beam, and a beam-to-beam energy imbalance in the laser drive.
“Voids” and phase separation in complex (dusty) plasmas
Authors: Avinash K.
Published: July 2002

By comparing the Helmhotz free energy for the homogeneous and the mixed (inhomogeneous) states a phase diagram for dusty plasma showing gas, fluid and mixed phases of dust grains is obtained. The thermophoretic force provides an additional attractive force between the grains. States with “voids” appear as phase separated states near the fluid boundary. A number of observations related to voids are explained. Further pressure–density isotherms for dust grains are plotted to show the existence of critical point and first order phase transitions.
Charge transfer of carriers by interband photoexcitation in asymmetric GaAs/AlGaAs coupled quantum wells
Authors: Levy M. , Khait Yu. L. , Beserman R. , Sa’ar A. , Thierry-Mieg V. , Planel R.
Published: July 2002

The influence of interband photoexcitation intensity on the photoluminescence line shape was investigated in GaAs/AlGaAs modulation-doped asymmetric structure composed of a wide quantum well weakly coupled to a narrow quantum well. The emission spectra show a broadening and a narrowing of the linewidths from the wide quantum well and from the narrow quantum well, respectively, with increasing laser power. In addition, we observe a fast increase of the emission intensity from the narrow quantum well with respect to that from the wide quantum well. These processes are shown to be the result of carrier transfer into the narrow quantum well. We propose a charge transfer model to explain semiquantitatively our results.
Low temperature tool for photoluminescence mapping with submicron resolution
Authors: De Vittorio M. , Melcarne A. , Rinaldi R. , Cingolani R.
Published: July 2002

We have developed a local probe technique in order to realize photoluminescence maps with submicron resolution at temperatures as low as 25 K. To this end a closed cycle He cryostat has been modified in order to damp mechanical vibrations to avoid spatial resolution losses. Both the optical laser pump and the collected signal are fiber-optic coupled. Photoluminescence maps are provided by a motorized X-Y translation stage that scans the microscope objective over the sample surface. The overall resolution of the microphotoluminescence (μ-PL) system is ∼500 nm, by considering the contributions of the laser focused spot size (λ=325 nm), the cryostat vibrations, and the motorized stage resolution. The system is described and two low temperature μ-PL experiments on quantum wires and quantum dot nanostructures are presented and discussed.
Preparation and evaluation of tungsten tips relative to diamond for nanoindentation of soft materials
Authors: Grunlan Jaime C. , Xia Xinyun , Rowenhorst David , Gerberich William W.
Published: July 2002

A simplified electrochemical etching technique has been developed to produce conical tungsten tips with radii of curvature ranging between 0.05 and 3 μm. These tips were characterized using scanning electron microscopy. Following characterization, the tungsten tips were mounted into tip holders and used in nanoindentation experiments on bulk aluminum, polystyrene, and polyethylene samples. Calculated modulus values for the polymers, based upon indentation data and known tip radii, agreed very well with modulus obtained using more macroscopic techniques (e.g., stress-strain apparatus). In the case of aluminum, calculated modulus was far lower than expected due to tip deformation during indentation. Calculated hardness values for the polymers tended to rise with indentation depth due to pressure effects that lead to densification and/or phase transformation. These results were also compared to modulus values, on the same three samples, obtained using a diamond tip with a comparable radius of curvature. For all of the samples the diamond tip data yielded larger modulus and hardness values than did the tungsten data. The diamond-based modulus and hardness values for the polymers were larger than expected due to greater adhesion between tip and sample relative to tungsten, which resulted in a larger than expected contact area. Using a new analytical method that accounts for differences in adhesion between the tip and sample, the modulus calculated using diamond and tungsten indenters was found to agree.
A bright metastable atom source at 80 K
Authors: Lu W. , Hoogerland M. D. , Milic D. , Baldwin K. G. H. , Buckman S. J.
Published: July 2002

We describe a high flux source of cold (80 K) metastable helium atoms. The source employs a direct current nozzle discharge which produces in excess of 1015 atoms/steradian/s. Liquid nitrogen cooling of the discharge source yields atomic velocities below 900 ms−1. Such a source has practical applications for experiments concerned with laser cooling and trapping of metastable helium atoms.
On the importance of parallel heat conduction and magnetic geometry for multifaceted radiation from the edge (MARFE)
Authors: Tokar M. Z.
Published: July 2002

An importance of parallel heat conduction in plasma for formation of multifaceted radiation from the edge (MARFE) is assessed analytically by taking into account the magnetic geometry of the tokamak. It is demonstrated for circular nonconcentric due to Shafranov shift magnetic surfaces that toroidally symmetric perturbations of a MARFE-like structure with a maximum at the high-field side have the largest growth rate. Both the threshold and characteristic poloidal width of these perturbations depend essentially on plasma parallel heat conduction and geometric characteristics.
The spatial correlations in the velocities arising from a random distribution of point vortices
Authors: Chavanis Pierre-Henri , Sire Clément
Published: July 2002

This paper is devoted to a statistical analysis of the velocity fluctuations arising from a random distribution of point vortices in two-dimensional turbulence. Exact results are derived for the correlations in the velocities occurring at two points separated by an arbitrary distance. We find that the spatial correlation function decays extremely slowly with the distance. We discuss formal analogies with the statistics of the gravitational field in stellar systems.
A mode-coupling approach to the attractive interaction effect on the solute diffusion in liquids
Authors: Yamaguchi T. , Matubayasi N. , Nakahara M.
Published: July 2002

The effect of the solute–solvent and the solvent–solvent attractive interactions on the diffusion coefficient of the solute is studied by the mode-coupling theory. The mode-coupling part of the friction coefficient in our calculation is compared with that of molecular dynamics simulation. The theory qualitatively reproduces the increase in the friction coefficient with the solute–solvent attractive interaction, although the absolute value is larger than the simulation. A theoretical scheme is further provided to divide the memory function into attractive and repulsive components, and a mode-coupling approximation is introduced to both components separately. It is found that the solute–solvent interaction affects the attractive friction mainly, in agreement with the simulation. Further analyses reveal the presence of the large cross term of the attractive and repulsive random forces. When the solvent–solvent attractive interaction is weakened while keeping the solute–solvent interaction fixed, the mode-coupling friction is increased, which is opposite to the simulation results.
Measurement of the Auger lifetime in GaInAsSb/GaSb heterostructures using the photoacoustic technique
Authors: Riech I. , Gomez-Herrera M. L. , Dı́az P. , Mendoza-Alvarez J. G. , Herrera-Pérez J. L. , Marı́n E.
Published: July 2002

We have studied GaxIn1−xAsySb1−y/GaSb heterostructures for x=0.84 and y=0.14 using the photoacoustic technique with the heat transmission configuration. A theoretical model, which includes all the possible nonradiative recombination mechanisms that contribute to heat generation, was developed to calculate the photoacoustic signal for this type of heterostructure. The Auger recombination lifetime τAuger was determined by fitting our experimental results to the calculated frequency dependence of the theoretical photoacoustic signal. The obtained value for τAuger is compatible with those reported in the literature for semiconductors with band-gap energies below and above 0.5 eV, the energy region where there is a lack of experimental τAuger values.
Er
3+
photoluminescence properties of erbium-doped Si/SiO2 superlattices with subnanometer thin Si layers
Authors: Ha Yong Ho , Kim Sehun , Moon Dae Won , Jhe Ji-Hong , Shin Jung H.
Published: July 2002

The effect of the Si layer thickness on the Er3+ photoluminescence properties of the Er-doped Si/SiO2 superlattice is investigated. We find that the Er3+ luminescence intensity increases by over an order of magnitude as the Si layer thickness is reduced from 3.6 nm down to a monolayer of Si. Temperature dependence of the Er3+ luminescence intensity and time-resolved measurement of Er3+ luminescence intensity identify the increase in the excitation rate as the likely cause for such an increase, and underscore the importance of the Si/SiO2 interface in determining the Er3+ luminescence properties.
Capillary-gravity wave drag
Authors: Sun Shu-Ming , Keller Joseph B.
Published: July 2002

Drag due to the production of capillary-gravity waves is calculated for an object moving along the surface of a liquid. Both two and three dimensional objects, moving at large Froude and Weber numbers, are treated.
Single-electron charging effect in individual Si nanocrystals
Authors: Baron T. , Gentile P. , Magnea N. , Mur P.
Published: July 2002

We present a detailed study of the electronic properties of individual silicon nanocrystals (nc-Si) elaborated by low-pressure chemical vapor deposition on 1.2 nm thick SiO2 grown on Si (100). The combination of ultrathin oxide layers and highly doped substrates allows the imaging of the hemispherical dots by scanning tunneling microscopy. Spectroscopic studies of single dots are made by recording the I(V) curves on the Si nanocrystal accurately selected by a metallic tip. These I(V) curves exhibit Coulomb blockade and resonant tunneling effects. Coulomb pseudogaps between 0.15 and 0.2 V are measured for different dots. Capacitances between 0.2 and 1 aF and tunnel resistances around 5×109 Ω are deduced from the width and height of the staircases. The charging and confinement energies deduced from the I(V) curves are in good agreement with a modified orthodox model which includes the quantification of electronic levels.
Femtosecond laser energy deposition in strongly absorbing cluster gases diagnosed by blast wave trajectory analysis
Authors: Zweiback J. , Ditmire T.
Published: July 2002

An intense ultrafast laser pulse can be very strongly absorbed in a moderate density gas composed of van der Waals bonded clusters. In this paper, the deposition of the energy of intense 30 fs light pulses in a gas of deuterium clusters has been diagnosed using a technique based on analysis of the trajectories of the resulting cylindrically symmetric blast waves. Using the well-known relation between blast wave velocity and energy deposition in gas, the laser energy deposited per unit length as a function of distance in gas jet plume was measured. These measurements were conducted in jets containing either deuterium clusters or simple deuterium molecules.
Asymptotic behavior of three-dimensional bubbles in the Richtmyer–Meshkov instability
Authors: Abarzhi S. I.
Published: July 2002

We report an analysis to the problem of nonlinear motion of bubbles and spikes generated by the Richtmyer–Meshkov instability. The flow is three-dimensional (3D), periodic and anisotropic in the plane normal to the direction of shock. We show that in the traditional Layzer-type approach, regular asymptotic solutions to the problem are absent in the general case. We propose yet another approach and find a family of regular asymptotic solutions parameterized by the principal curvatures at the bubble top. In the expanded functional space the interplay of harmonics is well captured. For solutions of this family, a bubble with a flattened surface is faster than a bubble with finite curvatures in both 3D and two-dimensional (2D) cases, while highly symmetric 3D bubbles are faster than anisotropic 3D and 2D bubbles. For nearly symmetric 3D flows, the Layzer-type solution is the point of bifurcation.
Electromagnetic micromotor for microfluidics applications
Authors: Barbic Mladen , Mock Jack J. , Gray Andrew P. , Schultz S.
Published: July 2002

An electromagnetic micromotor has been developed that combines microcoils and magnetic microtips as stator elements and individual permanent magnet single domain particles as rotors. The three-phase stator microcoils (with poles separated by 100 μm) are positioned outside the fluid, and the rotor (a 40 μm long, 1 μm diam permanent magnet) spins in aqueous solution 50 μm from the stator. The small size and essentially disposable rotors make the magnetic micromotor attractive for use in microfluidics systems for various physical, chemical, and biological applications.
ZnO–CoO solid solution thin films
Authors: Yoo Y.-Z. , Fukumura T. , Jin Zhengwu , Hasegawa K. , Kawasaki M. , Ahmet P. , Chikyow T. , Koinuma H.
Published: July 2002

Co-doped ZnO(Zn1−xCoxO) and Co, Al codoped ZnO(Zn1−xCoxO:Al) films were grown on c-plane sapphire (0001) substrates by pulsed laser deposition using a KrF excimer laser. In order to enhance homogeneous substitution of Co2+ for Zn2+ in the ZnO film, an alternating deposition (AD) method was employed for doping. ZnO films doped with the same elements were grown without employing the AD method under the same fabrication conditions to compare with the corresponding AD films. Despite Co incorporation, the Zn1−xCoxO film by AD methods showed better crystallinity than pure ZnO film. Also, the crystallinity of Zn1−xCoxO:Al films by AD methods was better than those of Zn1−xCoxO and ZnO:Al films by conventional doping methods. Root mean square roughnesses of the films by AD methods were less than 1.6 nm. Absorption peaks caused by d–d transitions were observed at 1.88, 2.01, and 2.19 eV in Zn1−xCoxO films. The electric conductivity of Zn1−xCoxO:Al film by AD methods was comparable to that of ZnO:Al film by conventional methods in spite of Co doping and lower Al concentration.
Degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes
Authors: Chang Shun-Chi , He Gufeng , Chen Fang-Chung , Guo Tzung-Fang , Yang Yang
Published: July 2002

The degradation mechanism of phosphorescent-dye-doped polymer light-emitting diodes (PLEDs) is investigated. The active medium of our PLED is a polymer blend comprising poly(vinylcarbazole) (PVK), [2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole] (t-PBD), and platinum(II)-2,8,12,17-tetraethyl-3,7,13,18-tetramethylporphyrin (PtOX). The cyclic voltammetry result shows that the reductive reversibility of PtOX is poor. This result suggests that PLED doped with PtOX is not stable if PtOXs trap electrons and turn into anionic PtOX species. This was indeed verified by fabricating single-layer PLEDs with various amounts of electron-transporting material, t-PBD. A slower degradation rate was observed from the devices with higher concentration of t-PBD, because of the reduction of the electron accumulation at the PtOX sites. The half decay lifetime of our phosphorescent polymer LED has been improved by a factor of ∼40, from 1.2 to 45 h.