bluuebunny/crossref_metadata_2025 huggingface dataset:
Transitions in ZnS and CdSe quantum dots and wave-function symmetry
Authors: Zorman B. , Friesner R. A.
Published: March 2003

Excitation energies for wurtzite spherical ZnS and CdSe quantum dots in the range of 40–4000 atoms were calculated using empirical pseudopotentials and a real-space basis. The energies are compared to experiments and other pseudopotential models. For ZnS quantum dots, squared transition dipole sums were computed efficiently, without the need for full wave functions of the excited states; and some transition dipole calculations include the effects of an approximate electron-hole Coulomb potential. Squared transition dipole sums from the highest energy linear dipole like valence states to the lowest excited state were computed as a function of dot size. The model predicts that the per atom dipole transition sum decreases with quantum dot size for those transitions. The mixing of even and odd angular components and charge asymmetry of the wave functions affect the dipole transition strengths. The total oscillator strength for the lowest energy transition region increases with size at small radii, resembling the pattern recently observed experimentally for CdSe quantum dots. We examined the role of wave-function angular momentum for transitions to conduction band surface states.
Theoretical investigation of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation
Authors: Woywod Clemens , Scharfe Sandra , Krawczyk Robert , Domcke Wolfgang , Köppel Horst
Published: March 2003

The spectroscopic and dynamic aspects of Jahn–Teller and pseudo-Jahn–Teller interactions in the ammonia cation are investigated within an ab initio based vibronic-coupling model approach. Multireference second-order perturbation theory (CASPT2) has been employed to obtain the potential energies of the ground state and the first excited state of NH3+ as a function of symmetry-coordinate displacements. Vibronic-coupling parameters determining the Franck–Condon, Jahn–Teller, and pseudo-Jahn–Teller activity of the normal modes have been obtained from the ab initio data. The vibronic structures of the X̃ 2A1 and Ã 2E photoelectron bands of ammonia have been calculated by numerical diagonalization of the vibronic Hamiltonian matrix. All six vibrational degrees of freedom are taken into account. The effects of Jahn–Teller and pseudo-Jahn–Teller interactions on the band shape of the Ã 2E photoelectron band are analyzed. The calculation of the time-dependent population probability of the Ã 2E state reveals a radiationless decay process on a time scale of 30 fs caused by a conical intersection of the X̃ and Ã potential-energy surfaces, which arises from the combined effect of the Jahn–Teller splitting of the Ã 2E state and the X̃–Ã pseudo-Jahn–Teller interaction. In the X̃ 2A1 band, the X̃–Ã pseudo-Jahn–Teller coupling results in the weak excitation of a single quantum of the degenerate bending mode. This theoretical result corroborates the earlier assignment of the vibronic structure of the X̃ 2A1 photoelectron band of NH3 by Edvardsson et al. [J. Phys. B 32, 2583 (1999)].
A spin–orbit coupling study on the spin inversion processes in the direct methane-to-methanol conversion by FeO+
Authors: Shiota Yoshihito , Yoshizawa Kazunari
Published: March 2003

Possible spin inversion processes in the direct conversion of methane to methanol by the bare FeO+ complex are discussed by means of spin–orbit coupling (SOC) calculations. This reaction proceeds via two transition states (TSs) in the following way; FeO++CH4→FeO+(CH4)→[TS1]→HO–Fe+−CH3→[TS2]→Fe+(CH3OH)→Fe++CH3OH. B3LYP density functional theory calculations show that the potential energies in the quartet and sextet states lie close and involve three crossing seams that can provide a chance of spin-forbidden transition. The spin-forbidden transition leads to a significant decrease in the barrier heights of TS1 and TS2 that correspond to the hydrogen atom abstraction and the methyl shift, respectively. To evaluate the spin-forbidden transition in the reaction pathway, the SOC matrix elements are calculated along the intrinsic reaction coordinate of the reaction. The SOC analysis along the IRC is useful to look at how the FeO+/CH4 reacting system changes its spin multiplicity between the sextet and quartet surfaces. The strength of the SOC between the low-lying quartet state and the sextet state is 133.6 cm−1 in the reactant complex FeO+(CH4), 21.4 cm−1 in the hydroxo intermediate HO–Fe+–CH3, and 0.3 cm−1 in the product complex Fe+(CH3OH). Since the SOC value decreases along the oxidation process, the ease of spin inversion probability is the first crossing seam, the second crossing seam, and the third crossing seam, in this order.
Continuum quantum systems as limits of discrete quantum systems. IV. Affine canonical transforms
Authors: Barker Laurence
Published: March 2003

Affine canonical transforms, complex-order Fourier transforms, and their associated coherent states appear in two scenarios: finite-discrete and continuum. We examine the relationship between the two scenarios, making systematic use of inductive limits, which were developed in the preceding articles in this series.
Rectification of thermodynamic inequalities
Authors: Ben-Amotz Dor , Honig J. M.
Published: March 2003

The Clausius inequality is converted to an equality in which a positive quantity with units of energy, termed the deficit function, is used to track entropy production and other effects of irreversible processes. This inequality rectification procedure is shown to yield particularly transparent new derivations of several important results based on the Second Law, with some consequences that have apparently not been previously recognized. For example, the Helmholtz free energy of any system is shown to decrease towards a minimum as the result of any spontaneous processes which begin and end at the same temperature and involve no work exchange, without the need to invoke the stricter requirements of fixed system volume and/or chemical composition.
Generating generalized distributions from dynamical simulation
Authors: Barth Eric J. , Laird Brian B. , Leimkuhler Benedict J.
Published: March 2003

We present a general molecular-dynamics simulation scheme, based on the Nosé thermostat, for sampling from arbitrary phase space distributions. We formulate numerical methods based on both Nosé–Hoover and Nosé–Poincaré thermostats for two specific classes of distributions; namely, those that are functions of the system Hamiltonian and those for which position and momentum are statistically independent. As an example, we propose a generalized variable temperature distribution that is designed to accelerate sampling in molecular systems.
Cavity formation and preferential interactions in urea solutions: Dependence on urea aggregation
Authors: Weerasinghe Samantha , Smith Paul E.
Published: March 2003

A series of models for 8 M urea solutions was investigated using molecular dynamics simulations. The models differed only in their charge distributions and displayed various degrees of urea aggregation. The relationship between urea aggregation and the thermodynamics of the solution was established using Kirkwood–Buff theory. It was observed that high urea aggregation resulted in lower predicted values for the solution activity, and that Kirkwood–Buff theory provided a sensitive test for the properties of a particular force field. The free energy for formation of repulsive cavities in the different solutions was also investigated. The free energy was more unfavorable than in pure water, but independent of the extent of urea aggregation. However, the preferential exclusion of urea from the cavities was very sensitive to the degree of urea aggregation and varied by more than an order of magnitude in response to changes in the activity derivatives. A simple explanation for these observations is presented.
Apparent exponents for the chain length dependence of the volume fraction in critical polymer solutions
Authors: Yelash Leonid V. , Kraska Thomas , Imre Attila R. , Rzoska Sylwester J.
Published: March 2003

The dependence of the critical volume fraction at constant pressure as a function of the chain length of a polymer/solvent system can be described by a power law. The exponent of this power law is investigated based on an equation of state model and experimental data for various chain-molecule solutions here. The results are compared to recent molecular simulation data taken from the literature and analytical models. The theoretical models, simulation, and experimental data show that the exponent depends on the chain length of the dissolved chain molecules. The power law with a constant exponent is therefore not a universal relationship for this dependence. Based on the investigation of the chain length dependence a correlation for the critical volume fraction is proposed here. This function generalizes the Flory and a renormalization group model and is applied to the correlation of the experimental data. This more general relationship includes the power law with the exponent obtained from the Flory theory as limiting behavior. Some additional experimental data for oligomer solutions which are necessary for an investigation of the short chain length limit have been measured.
On the role of connectivity in the relative stability of crystal types for model polymeric solids
Authors: Sushko Nazar , van der Schoot Paul , Michels M. A. J.
Published: March 2003

We study the relative stability of two different lattice types for model polymeric solids, using a recently developed density functional theory for freely-hinged, polymeric chains of hard globular segments. The most stable crystal-lattice type appears to be determined by the mean length and the stretching stiffness of the bonds as well as by the size of the segments. We discuss the possible implications of this for the crystallization of chemically realistic polymers.
Modified Zakharov equations in dusty plasmas with variable charges on dust particles
Authors: Tribeche Mouloud , Zerguini Taha Houssine
Published: March 2003

The nonlinear coupling between large amplitude Langmuir waves and dust ion-acoustic waves in a charge-varying dusty plasma is considered. By employing a two-time-scale three fluid approach, modified Zakharov equations are derived. A normal mode analysis of these nonlinearly coupled equations is performed to obtain a general dispersion relation that is useful for studying parametric instabilities. The results show that the impurities can significantly alter the characteristics and damping of these instabilities by reducing their phase velocity and causing charging related damping.
Transport properties of two-dimensional electron gas in different subbands in triangular quantum wells at AlxGa1−xN/GaN heterointerfaces
Authors: Zheng Z. W. , Shen B. , Gui Y. S. , Jiang C. P. , Tang N. , Zhang R. , Shi Y. , Zheng Y. D. , Guo S. L. , Zheng G. Z. , Chu J. H. , Someya T. , Arakawa Y.
Published: March 2003

Magnetotransport properties of modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated by means of magnetoresistance measurements at low temperatures and high magnetic fields. Strong Shubnikov–de Haas oscillations with the double periodicity are observed. The mobility spectrum is obtained, which demonstrates that the mobilities of the two-dimensional electron gas (2DEG) in the two subbands in the triangular quantum well at heterointerface. It is found that the mobility of the 2DEG in the second subband is much higher than that in the first one. This is explained that interface roughness scattering and alloy disorder scattering have much stronger influence on transport properties of the 2DEG in the first subband than that in the second subband in AlxGa1−xN/GaN heterostructures.
Nonlinear electromagnetic fields in 0.5 MHz inductively coupled plasmas
Authors: Ostrikov K. , Tsakadze E. , Xu S. , Vladimirov S. V. , Storer R.
Published: March 2003

Radial profiles of magnetic fields in the electrostatic (E) and electromagnetic (H) modes of low-frequency (∼500 kHz) inductively coupled plasmas have been measured using miniature magnetic probes. In the low-power (∼170 W) E-mode, the magnetic field pattern is purely linear, with the fundamental frequency harmonics only. After transition to higher-power (∼1130 W) H-mode, the second-harmonic nonlinear azimuthal magnetic field Bφ2ω that is in 4–6 times larger than the fundamental frequency component Bφω, has been observed. A simplified plasma fluid model explaining the generation of the second harmonics of the azimuthal magnetic field in the plasma source is proposed. The nonlinear second harmonic poloidal (r−z) rf current generating the azimuthal magnetic field Bφ2ω is attributed to nonlinear interactions between the fundamental frequency radial and axial magnetic fields with the azimuthal rf current.
Ideal stability of an elliptical plasma column in the presence of external feedback currents
Authors: Coelho Rui , Porcelli Franco
Published: March 2003

The magnetohydrodynamic stability of a plasma column with an elliptical cross section to axisymmetric, rigid plasma displacements is analyzed. The considered plasma equilibrium, characterized by the presence of a double-null magnetic separatrix, is found to be unstable. An application of the well-known Energy Principle [I. B. Bernstein, E. A. Frieman, M. D. Kruskal, and R. M. Kulsrud, Proc. R. Soc. London, Ser. A 244, 17 (1958)] is presented, taking into account the effect of external localized electric currents on the mode stability. It is shown that such currents can stabilize the axisymmetric displacements if their values exceed a critical threshold.
Dust acoustic solitary waves and double layers in a dusty plasma with an arbitrary streaming ion beam
Authors: El-Labany S. K. , El-Taibany W. F.
Published: March 2003

The effects of variable dust charge, dust temperature and an arbitrary streaming ion beam on small amplitude dust acoustic waves are investigated. It is found that both compressive and rarefactive solitons as well as double layers exist. There exists a critical ion beam velocity below which the ion beam is unable to generate solitons. Korteweg–de Vries (KdV) equations with third and fourth-order nonlinearity at the critical values are derived and the properties of dust acoustic solitary waves are discussed. In the vicinity of the critical values, KdV-type with mixed nonlinearity is obtained. The quite dense positive ion, the dust temperature, the ion beam density, mass ratio and temperature govern the existence of dust acoustic waves. The findings of this investigation may be useful in understanding laboratory plasma phenomena and astrophysical situations.
Parallel and poloidal fluxes in turbulent non-ohmic plasmas: An ion-cyclotron resonance heating case
Authors: Pometescu N. , Weyssow B.
Published: March 2003

The parallel component of the vectorial fluxes, in particular the ion parallel particle flux h∥i(1) and the ion parallel heat flux h∥i(3), in an electrostatic turbulent magnetically confined plasma submitted to an external rf heating are obtained analytically in terms of the thermodynamic forces. Their expressions are derived using the methodology of neoclassical transport theory, the standard model of magnetic field and assuming a non-ohmic heating in the ion cyclotron resonance heating range for which a decoupling of the electron to the ion dynamics may be admitted as a first approximation. It is shown that all but one of the ion parallel turbulent transport coefficients related to a spectrum of electrostatic fluctuations are drastically reduced when the rf power density measured in terms of the Stix parameter is increased. The remaining coefficient, absent for an ohmic plasma increases with the Stix parameter.
Absolute chlorine and hydrogen atom quantum yield measurements in the 193.3 nm photodissociation of CH3CFCl2 (HCFC-141b)
Authors: Läuter Almuth , Suresh Dhanya , Volpp Hans-Robert
Published: March 2003

The dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH3CFCl2 (HCFC-141b), were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence (LIF) “pump-and-probe” technique. Nascent ground-state Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) as well as H(2S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl(2P3/2), Cl*(2P1/2), and H atom concentrations, values for the chlorine-atom spin–orbit branching ratio [Cl*]/[Cl]=0.36±0.08, the total chlorine atom quantum yield (ΦCl+Cl*=1.01±0.14), and the H atom quantum yield (ΦH=0.04±0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be ET(Cl)=157±12 kJ/mol and ET(Cl*)=165±12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH3CFCl+Cl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH3CFCl2→CH3CFCl+Cl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C–Cl bond cleavage is the most important primary fragmentation mechanism for CH3CFCl2 after photoexcitation in the first absorption band.
Lifshitz tails for random acoustic operators
Authors: Najar Hatem
Published: March 2003

This paper is devoted to the study of Lifshitz tails for random acoustic operators of the form Aω=−∇(1/ϱω)∇. We prove that the integrated density of states of Aω has a Lifshitz behavior at the edges of internal spectral gaps if and only if the integrated density of states of a well-chosen periodic operator is nondegenerate at the same edges.
Persistent photoconductivity in InxAlyGa1−x−yN quaternary alloys
Authors: Chen C. H. , Hang D. R. , Chen W. H. , Chen Y. F. , Jiang H. X. , Lin J. Y.
Published: March 2003

The optical properties of InxAlyGa1−x−yN quaternary alloys were investigated by photoconductivity (PC), persistent photoconductivity (PPC), photoluminescence (PL), and photoluminescence excitation (PLE) measurements. Quite interestingly, persistent photoconductivity was observed. Through the combination of our optical studies, we show that the PPC effect arises from composition fluctuations in InxAlyGa1−x−yN quaternary alloys. From the analysis of the decay kinetics, the localization depth caused by composition fluctuations was determined. A comparison between the PL, PLE, and PC measurements gives a direct access to the Stokes’ shift. The Stokes’ shift can be explained in terms of localization due to the existence of nanoscale clusters, and it is consistent with the PPC result. The results shown here provide concrete evidence to support our previously proposed model that the existence of InGaN-like clusters is responsible for the strong luminescence in InxAlyGa1−x−yN quaternary alloys.
The envelope equation of oblique Alfvén waves
Authors: Lee Nam C. , Parks George K.
Published: March 2003

An equation is derived to describe the evolution of envelope of electromagnetic waves having arbitrary propagation directions and modulation in cold plasma. The ambient magnetic field is assumed constant and uniform. The derived equation is applied to Alfvén waves propagating obliquely with respect to the ambient magnetic field and it is shown that it can have both a kink-type solitary wave as well as a bump-type solitary wave solution.
Dynamics of a plasma channel created by the wakefield of a short laser pulse
Authors: Gorbunov L. M. , Mora P. , Solodov A. A.
Published: March 2003

A new physical effect of a plasma channel formation by the ponderomotive force of a wakefield excited by a short laser pulse with duration of the order of electron plasma oscillation period ωp0−1 (ωp0 is the plasma frequency) is discussed. The hydrodynamic and particle numerical codes, including plasma ion response, are used to simulate the long-term wakefield behavior. It is found that the wakefield creates a channel with a radial profile depending on the laser pulse width. Particularly, for a narrow pulse, wherein the width is less than c/ωp0 (c is the speed of light), the channel has an annular form with on-axis density maximum. The depth of the channel increases with the distance from the pulse until fine-scale mixing arises and the wake starts to break. Particle simulations show that wave breaking results in emergence of fast electrons taking an essential part of the wake energy during a few plasma periods. Quasilinear fluid equations describing self-consistently, the laser wakefield generation, and plasma channel formation are derived. The wave-breaking conditions are obtained in the geometrical optics approximation. The results of numerical simulations for high-intensity laser pulses are in good agreement with theoretical predictions. The scaling laws for wave breaking are discussed.
The Balian–Low theorem for the symplectic form on R2d
Authors: Benedetto John J. , Czaja Wojciech , Maltsev Andrei Ya.
Published: March 2003

In this paper we extend the Balian–Low theorem, which is a version of the uncertainty principle for Gabor (Weyl–Heisenberg) systems, to functions of several variables. In particular, we first prove the Balian–Low theorem for arbitrary quadratic forms. Then we generalize further and prove the Balian–Low theorem for differential operators associated with a symplectic basis for the symplectic form on R2d.
Attractive wake field formation due to an array of dipolar projectiles in a multi-component dusty plasma
Authors: Ali S. , Nasim M. H. , Murtaza G.
Published: March 2003

The dielectric response function for modified dust acoustic waves is studied, incorporating the dipole moment and moment of inertia of finite sized elongated dust grains. Using this dielectric constant, generalized expressions for the Debye potential and for the wake potential are obtained due to a cluster of N2 dust grain projectiles moving with a constant velocity along the z-axis through a multi-component dusty plasma. For illustration, numerical results due to single, three, and six projectiles are presented with and without a dipole effect. A negative wake potential is observed behind each projectile in different geometeries.
On the poloidal beta evolution of fixed-q heating in tokamaks
Authors: Kissick M. W. , Leboeuf J.-N. , Kruger S. E.
Published: March 2003

For typical tokamak safety factor, q, profiles and aspect ratio, A, fixed-q heating leads to a limit for equilibrium code convergence near a poloidal beta value of 1+A. Also presented is a simple tanh functional fit that saturates just above the 1+A limit. This poloidal beta limit does not depend on the q-profile, but the total beta achieved at this critical poloidal beta does depend strongly on q. In this paper we confirm very early computations of the flux-conserving tokamak that routinely achieved a poloidal beta >A.
Local particle flux reversal under strongly sheared flow
Authors: Terry P. W. , Newman D. E. , Ware A. S.
Published: March 2003

The advection of electron density by turbulent E×B flow with linearly varying mean yields a particle flux that can reverse sign at certain locations along the direction of magnetic shear. The effect, calculated for strong flow shear, resides in the density-potential cross phase. It is produced by the interplay between the inhomogeneities of magnetic shear and flow shear, but subject to a variety of conditions and constraints. The regions of reversed flux tend to wash out if the turbulence consists of closely spaced modes of different helicities, but survive if modes of a single helicity are relatively isolated. The reversed flux becomes negligible if the electron density response is governed by electron scales while the eigenmode is governed by ion scales. The relationship of these results to experimentally observe flux reversals is discussed.
Response to “Comment on ‘Parallel electric fields in the upward current region of the aurora: Numerical solutions’ ” [Phys. Plasmas 10, 1175 (2003)]
Authors: Ergun R. E. , Andersson L. , Main D. , Su Y.-J. , Newman D. L. , Goldman M. V. , Carlson C. W. , McFadden J. P. , Mozer F. S.
Published: March 2003

The Comment on our article contains a general statement and three more specific points to which we reply. We hold that the general statement of the Comment, “a fully dynamic, multidimensional analysis is needed for even the lowest order solution (of an oblique double layer)” is not true in the example that we presented. The general statement is based on an argument that the first adiabatic moment of H+ is violated as the ion traverses the double layer. We demonstrate here that this argument is erroneous and that, in fact, the first adiabatic moment of H+ is largely conserved. Our article identifies several areas where a dynamic simulation is needed to fully understand the observations of the auroral double layer and the ion (mainly O+) dynamics.
Wigner functions for curved spaces. II. On spheres
Authors: Alonso Miguel Angel , Pogosyan George S. , Wolf Kurt Bernardo
Published: March 2003

The form of the Wigner distribution function for Hamiltonian systems in spaces of constant negative curvature (i.e., hyperboloids) proposed in M. A. Alonso, G. S. Pogosyan, and K. B. Wolf, “Wigner functions for curved spaces. I. On hyperboloids” [J. Math. Phys. 43, 5857 (2002)], is extended here to spaces whose curvature is constant and positive, i.e., spheres. An essential part of this construction is the use of the functions of Sherman and Volobuyev, which are an overcomplete set of plane-wave-like solutions of the Laplace–Beltrami equation for this space. Rotations that displace the poles transform these functions with a multiplier factor, and their momentum direction becomes formally complex; the covariance properties of the proposed Wigner function are understood in these terms. As an example for the one-dimensional case, we consider the energy eigenstates of the oscillator on the circle in a Pöschl–Teller potential. The standard theory of quantum oscillators is regained in the contraction limit to the space of zero curvature.
Space-charge-controlled conductivity in low-density polyethylene
Authors: Neagu Eugen R. , Marat-Mendes Jose N.
Published: March 2003

We studied the dc conductivity of low-density polyethylene in the temperature range from 20 to 90 °C for electric fields from 4 kV m−1 to 20 MV m−1. The isochronal data measured after 1 h are in agreement with the literature. For a long time (6 days), the conduction mechanism is dominated by the space charge trapped in the material. The current does not attain a steady-state value after 29 days at 50 °C and 8 MV m−1. It oscillates continuously, the variations are less regular and the conductivity decreases significantly as the sample thickness increases. The activation energy decreases from 0.8 to 0.58 eV when determined from long time measurements. An explanation is proposed considering the constraints imposed by the trapped space charge on charge injection and transport. We propose to call the observed mechanism space-charge-controlled conductivity.
Analysis of breakdown probabilities in avalanche photodiodes using a history-dependent analytical model
Authors: Wang Shuling , Ma Feng , Li Xiaowei , Karve Gauri , Zheng Xiaoguang , Campbell Joe C.
Published: March 2003

The breakdown probabilities of avalanche photodiodes (APDs) working in the Geiger mode are analyzed using a history-dependent analytical impact-ionization model [R. J. McIntyre, IEEE Trans. Electron Devices 46, 1623 (1999)]. The breakdown sharpness in devices with thin and thick multiplication regions is found to follow the same trend in GaAs, InAlAs, and InP material systems. Breakdown characteristics of InP and InAlAs are compared for their applications in photon counting at telecommunication wavelengths.
Polariton emission from polysilane-based organic microcavities
Authors: Takada Noriyuki , Kamata Toshihide , Bradley Donal D. C.
Published: March 2003

We report the observation of strong coupling between exciton and photon modes in a conjugated polymer-based semiconductor microcavity. Thin films of the σ-conjugated poly[bis(p-butylphenyl)silane] (PBPS) were inserted between metal and dielectric mirrors to form the microcavity structures. Variation of the PBPS film thickness between 80 and 140 nm allowed the cavity photon resonance to be tuned in the vicinity of the free exciton energy. The expected anticrossing behavior, with intensity and linewidth averaging, was observed at room temperature in the cavity reflection spectra and the vacuum Rabi splitting was found to be ⩽430 meV. This large value is consistent with the expectations of transfer matrix reflectivity calculations performed with optical constants data derived from a Kramers–Kronig analysis of the PBPS absorption spectrum. Angle-dependent photoluminescence measurements were performed for the microcavity with a 120 nm thickness PBPS layer. Unlike the emission from a standard, weakly coupled, cavity, the polariton emission shows almost no blueshift with angle, a desirable feature for potential display applications.
Electromagnetic modes in dusty plasmas with charge and mass distributions
Authors: Verheest Frank , Cattaert Tom
Published: March 2003

Electromagnetic modes in dusty plasmas are studied for polydisperse dust grains with a distribution in charge and mass. Owing to the different charge and mass weightings of the velocities, there occurs an infinite chain of equations of motion, coupled through the magnetic part of the Lorentz force. Depending on the frequency and associated convergence regimes, one has to close the chain in ascending or in descending order. Both series together lead to the polydisperse dispersion law that is a generalization of monodisperse or size distributed dust. Power-law distributions typical for heliospheric plasmas are discussed as an application.
Stability formalism of a flowing two-fluid plasma
Authors: Yamada Hideaki , Katano Takayuki , Ishida Akio , Steinhauer Loren C.
Published: March 2003

An improved formalism for a stability analysis of flowing two-fluid equilibria with constant density is developed. The two-fluid formalism, in which the generalized vorticity of each species is introduced as characteristic quantity, extends the usual single-fluid formalism. A new relation between the perturbed generalized vorticity and the displacement is found for each species. The spectral formalism is developed for stability of axisymmetric equilibrium. The missing elements in the single-fluid analysis of Frieman and Rotenberg [Rev. Mod. Phys. 32, 898 (1960)] are identified.
Spectroscopy of laser-plasma accelerated electrons: A novel concept based on Thomson scattering
Authors: Tomassini P. , Galimberti M. , Giulietti A. , Giulietti D. , Gizzi L. A. , Labate L.
Published: March 2003

The spectrum of relativistic electron bunches with large energy dispersion, like the ones usually generated with laser-plasma acceleration processes, is difficult to obtain with conventional methods. A novel spectroscopic concept, based on the analysis of the photons generated by Thomson scattering of a probe laser pulse by the electron bunch, is presented. The feasibility of a single-pulse spectrometer, using an energy-calibrated charge coupled device as detector, is investigated. Numerical simulations performed in conditions typical of a real experiment show the effectiveness and accuracy of the new method.
Transport barriers and edge localized modes-like bursts in a plasma model with turbulent equipartition profiles
Authors: Naulin V. , Rasmussen J. Juul , Nycander J.
Published: March 2003

Self-consistent development of transport barriers is investigated analytically and numerically in flux driven interchange turbulence with highly intermittent turbulent fluxes. Numerical simulations on a bounded domain show turbulence leading to a homogenization of Lagrangian invariants by mixing, resulting in quasisteady pressure profiles predicted by turbulent equipartition. Below a critical aspect ratio α=Ly/Lx—for our parameters ≈3.8—which is related to the rotational transform, large scale poloidal flows develop. They reduce the energy in the turbulence, prevent mixing, and constitute transport barriers for the turbulent fluxes, but are intermittently disrupted by strong bursts in the transport, which may be related to the strong edge localized modes observed in toroidal devices.
Periodic long-range transport in a large volume dc glow discharge dusty plasma
Authors: Thomas Edward , Amatucci William E. , Compton Christopher , Christy Brian , Jackson Jon David
Published: March 2003

In an earlier paper, the authors reported on observations of a variety of particle transport phenomena observed in DUPLEX—the DUsty PLasma EXperiment at the Naval Research Laboratory [E. Thomas, Jr., W. E. Amatucci, C. Compton, and B. Christy, Phys. Plasmas 9, 3154 (2002)]. DUPLEX is a large, transparent polycarbonate cylinder that is 40 cm in radius and 80 cm in height. dc glow discharge argon plasmas are generated in DUPLEX. In this paper, the authors expand upon one particular feature of particle transport in DUPLEX, the long-range (i.e., greater than 15 cm), periodic (T∼2.5 min) transport of suspended alumina particles through the plasma. A detailed description of this particle motion through the plasma is presented. Finally, a qualitative model describing the phenomena that lead to this transport is also given.
Cyclic identities for Jacobi elliptic and related functions
Authors: Khare Avinash , Lakshminarayan Arul , Sukhatme Uday
Published: March 2003

Identities involving cyclic sums of terms composed from Jacobi elliptic functions evaluated at p equally shifted points on the real axis were recently found. These identities played a crucial role in discovering linear superposition solutions of a large number of important nonlinear equations. We derive four master identities, from which the identities discussed earlier are derivable as special cases. Master identities are also obtained which lead to cyclic identities with alternating signs. We discuss an extension of our results to pure imaginary and complex shifts as well as to the ratio of Jacobi theta functions.
Improvement in electrical characteristics of graded manganese doped barium strontium titanate thin films
Authors: Jain M. , Majumder S. B. , Katiyar R. S. , Miranda F. A. , Van Keuls F. W.
Published: March 2003

Highly (100) textured graded manganese (Mn) doped Ba0.5Sr0.5TiO3 [BST (50/50)] thin films were deposited on lanthanum aluminate substrates using sol-gel technique. We have demonstrated that the graded acceptor doping is a promising technique to reduce the temperature coefficient of capacitance (TCC), loss tangent, and leakage current of BST thin films. In the temperature range between 175 and 260 K the reported TCC of Mn graded BST (50/50) films is less than 5.55×10−4/K, which is comparable to the best capacitors known so far. The lower temperature coefficient of the capacitance of the Mn graded films has been argued to be due to the induced compositional heterogeneity resulting into a distribution of the Curie temperature.
Superconducting properties of single-crystal whiskers of (Y0.86Ca0.14)Ba2Cu3Ox grown from precursors containing calcium and tellurium
Authors: Nagao Masanori , Sato Mitsunori , Maeda Hiroshi , Yun Kyung Sung , Takano Yoshihiko , Hatano Takeshi , Kim SangJae
Published: March 2003

Single-crystal whiskers of YBa2Cu3Ox (Y-123) phase with a length of about 2.5 mm were grown from sintered precursor powders with a nominal composition of Y1Ba2Cu3Te0.5Ca1.0Ox. Electron probe microanalysis analyses showed that the whiskers contain Ca but no Te with a composition of (Y0.86Ca0.14)Ba2Cu3Ox in which 14%Y is substituted by Ca. The precursor powder is composed of Ca-doped Y-123 and (BaCa)3TeO6 phases. The presence of the (BaCa)3TeO6 plays an important role in enhancing the growth of the whiskers. The whiskers show a critical temperature Tc of about 83 K and a critical current density, Jc of 4.4×104 A/cm2 at 82 K and self-field along the c axis. The whiskers exhibit interesting intrinsic Josephson effects along the c axis.
p-type doping of II–VI heterostructures from surface states: Application to ferromagnetic Cd1−xMnxTe quantum wells
Authors: Maślana W. , Kossacki P. , Bertolini M. , Boukari H. , Ferrand D. , Tatarenko S. , Cibert J. , Gaj J. A.
Published: March 2003

We present a study of p-type doping of CdTe and Cd1−xMnxTe quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding 2×1011 cm−2. Surface doping was applied to obtain samples with Cd1−xMnxTe quantum well with up to x=9.3% containing hole gas. We could also increase the growth temperature up to 280 °C, which results in sharper photoluminescence lines, when compared to the similar nitrogen doped samples. Carrier-induced ferromagnetism was observed in surface doped samples.
Writing process on a permeable recording layer: A fundamental issue in perpendicular recording
Authors: Li Shaoping , Wang Lei
Published: March 2003

Normally, the head field is calculated without considering the influence of the recording layer, causing some inaccuracies on certain occasions. In this work, we describe the writing process on a permeable, perpendicular recording medium from both a simple reluctance analysis and complete micromagnetic simulations. It is found that: (i) the initial writing field is always reduced in perpendicular recording scheme, and the head-field rise-time could be somewhat prolonged in the presence of the permeable recording media, (ii) the writing field at saturation could be enhanced up to 10%–20%.
Photovoltaic efficiency enhancement through thermal up-conversion
Authors: Ekins-Daukes N. J. , Ballard I. , Calder C. D. J. , Barnham K. W. J. , Hill G. , Roberts J. S.
Published: March 2003

Anti-Stokes emission is observed experimentally from a series of quantum well p–i–n structures and is attributed to a quasithermal equilibrium being established between the quantum well and barrier material. A device is described whereby the anti-Stokes emission from a quantum well p–i–n structure can be used to increase the short-circuit current of a second conventional solar cell, essentially coupling sub-band-gap sunlight to the ambient thermal reservoir. For a GaAs p/n cell at 300 K, this effect is calculated to raise the one-sun power conversion efficiency from 30.0% to 31.3%. Greater efficiency increases are possible if a thermal gradient is established across the structure.
Growing evanescent waves in negative-refractive-index transmission-line media
Authors: Grbic Anthony , Eleftheriades George V.
Published: March 2003

We show the enhancement of evanescent waves by a realizable negative-refractive-index (NRI) medium consisting of a periodic 2-D L,C loaded transmission-line (TL) network. This network is referred to as a dual TL structure. Growing evanescent waves within the dual TL structure are predicted analytically and demonstrated through simulation. These findings confirm that the dual TL structure is not simply a phase compensator that corrects the phase of propagating waves, but is in fact a NRI medium, since it also enhances the amplitudes of evanescent waves. This structure is a likely candidate for microwave subwavelength focusing and imaging applications.
Local impedance imaging and spectroscopy of polycrystalline ZnO using contact atomic force microscopy
Authors: Shao Rui , Kalinin Sergei V. , Bonnell Dawn A.
Published: March 2003

A current detection scanning probe technique is developed that quantifies frequency-dependent local transport properties. The approach, referred to as nanoimpedance microscopy/spectroscopy (NIM), is based on impedance spectroscopy with a conductive atomic force microscopy (AFM) tip. NIM is applied to study the quality of a tip/surface contact and transport behavior of individual grains and grain boundaries in polycrystalline ZnO. Impedance spectra were measured in the frequency range 40 Hz to 110 MHz, and the grain boundary properties are studied by nonlinear fitting of experimental data to an equivalent circuit. Two-terminal measurements are performed in the vicinity of a single ZnO grain boundary and the Cole–Cole plot indicates two major relaxation processes attributed to grain boundary relaxation and tip/surface contact.
Hamiltonian mapping of magnetic reconnection during the crash stage of the sawtooth instability
Authors: Pavlenko I. , Rapoport B. , Weyssow B. , Carati D.
Published: March 2003

Since the determination of the magnetic field topology and of the safety factor profile during the crash stage of the sawtooth instability is a difficult problem both theoretically and experimentally, a complementary approach based on a mapping technique usually referred to as the Tokamap [Balescu et al., Phys. Rev. E 58, 951 (1998)] is proposed to reconstruct the stochastic magnetic field evolution. It is shown that this method, when combined with the constraints on the magnetic fluxes provided by the theories of the sawtooth instability, is able to generate poloidal cross sections of the magnetic field topology during the crash phase of the instability that have behavior similar to the experimental ones. The method is applied to both the complete and the incomplete reconnection of the magnetic field.
Nanoscale investigation of longitudinal surface acoustic waves
Authors: Yang Jianshu , Voigt P. U. , Koch R.
Published: March 2003

Scanning tunneling microscopy of longitudinally polarized surface acoustic waves (SAWs) yields amplitude and phase images with nanometer resolution. The eccentricity of the surface oscillation calculated from the experimental data by the model of Chilla et al. is 88°±5° in good agreement with the macroscopic value for high velocity pseudo SAWs. Our study reveals that reliable amplitude and phase information can be deduced even from atomic scale features; however, the local surface geometry and tip shape need to be treated in more detail.
Synthesis and electronic transport studies of CdO nanoneedles
Authors: Liu Xiaolei , Li Chao , Han Song , Han Jie , Zhou Chongwu
Published: March 2003

Single-crystalline, needle-shaped CdO nanostructures were synthesized using a chemical vapor deposition method and characterized using a variety of techniques. Devices consisting of individual CdO nanoneedles were fabricated and high conductance as well as high carrier concentrations was observed. The temperature dependence of the conductance revealed thermal excitation as the dominating transport mechanism. Our devices exhibited good sensitivity to both infrared light and diluted NO2 gas, indicating potential applications as infrared photodetectors and toxic gas sensors.
Fluorocarbon plasma etching and profile evolution of porous low-dielectric-constant silica
Authors: Sankaran Arvind , Kushner Mark J.
Published: March 2003

To achieve shorter RC-delay times in integrated circuits low-dielectric-constant (low-k) materials are being investigated for interconnect wiring. Porous silicon dioxide (PS) is one such material. To address scaling issues during fluorocarbon plasma etching of PS, a feature profile model has been integrated with a plasma equipment model. To focus on issues related to the morphology of porous materials, the PS was treated as stoichiometric SiO2. The model was validated by comparison to experiments for PS etching in CHF3 plasmas sustained in an inductively coupled reactor. We found that etch rates (ER) for PS are generally higher than for SiO2 due to the inherent smaller mass density, although ER do not necessarily scale linearly with pore size or porosity. Mass-corrected ER can be either larger or smaller than that of solid SiO2. For example, in polymerizing environments, at high porosities and large pore radii, there is a reduction in ER due to pore filling with polymer. Profile scaling parameters, such as for tapering, observed for solid SiO2, are generally applicable to PS.
Epitaxial growth of CoSi2 on hydrogen-terminated Si(001)
Authors: Ishida K. , Miura Y. , Hirose K. , Harada S. , Narusawa T.
Published: March 2003

We demonstrate that CoSi2 grows epitaxially on H-terminated Si(001) and present the growth mechanism. It was found that direct reaction of Co with Si is suppressed on H-terminated Si below 400 °C. Thus, the hydrogen at the Co/Si interface hinders the formation of Co2Si and CoSi. Upon thermal desorption of hydrogen at around 400–550 °C, CoSi2, which is closely lattice-matched to Si(001), grows on Si(001) and thus, thin epitaxial CoSi2 films are formed on Si(001). The {111}-faceting was completely suppressed in the epitaxial CoSi2/Si(001), leading to the atomically flat interface.
Electrical and thermal properties of single-crystalline (Ca2CoO3)0.7CoO2 with a Ca3Co4O9 structure
Authors: Shikano Masahiro , Funahashi Ryoji
Published: March 2003

Single crystals of (Ca2CoO3)0.7CoO2 were grown using a modified strontium chloride flux technique, and their electrical and thermal properties were determined. Growth conditions were established for obtaining plate-like single crystals of a relatively large size (5×10×0.05 mm3). At 973 K, thermoelectric power S and electrical resistivity ρ of the specimens are ≈240 μV K−1 and ≈2.3×10−5 Ω m, respectively, and their thermal conductivity κ is nearly 3 W m1 K−1, as determined by mathematical extrapolation. The figure of merit ZT=S2T/ρκ derived therefrom is ≈0.87 at 973 K. The relatively low κ is likely the result of a misfit structure between the CoO2 layer and the Ca2CoO3 slab.
Silver thick-film contacts on highly doped n-type silicon emitters: Structural and electronic properties of the interface
Authors: Ballif C. , Huljić D. M. , Willeke G. , Hessler-Wyser A.
Published: March 2003

The properties of Ag thick-film contacts screen-printed on P-diffused [100] Si wafers have been investigated. In cross-sectional transmission electron microscopy, the interface is found to be composed of 200–500-nm-diameter Ag crystallites penetrating the silicon on average by up to 130 nm. The smaller crystallites are in epitaxial relation with the Si substrate, indicating their growth from the glass frit melt. A quasicontinuous glassy layer is present between the interface Ag crystallites and the larger Ag grains, which form the bulk of the contact. Conductive atomic force microscopy of cross sections shows that the interface crystallites form a low contact resistivity with the Si emitter in the range of 10−7 Ω cm2. We discuss the mechanisms of contact formation and propose a model in which the current flow from the emitter into the contact is not uniform, but occurs via a few isolated Ag crystallites that are directly connected or in close distance to the Ag grains forming the contact bulk.
Cu-filled carbon nanotubes by simultaneous plasma-assisted copper incorporation
Authors: Zhang G. Y. , Wang E. G.
Published: March 2003

Carbon nanotubes are filled with copper by means of chemical vapor deposition. Control of the density of copper in the plasma is crucial and is accomplished by adjusting the length of the copper electrodes that are exposed to the tungsten heating filament. Both simple and branched Cu-filled nanotubes were grown. Most of the branched nanotubes branch only once to form a regular trilateral starburst structure; others branch repeatedly in simple dendritic patterns. A model to account for the filling and growth characteristics is proposed based on observations made with a high-resolution transmission electron microscope and selected area electron diffraction.