bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetics of ultrathin FePt nanoparticle films
Authors: Held G. A. , Zeng Hao , Sun Shouheng
Published: January 2004

We present magnetization data for polymer-mediated, self-assembled films of 4 nm FePt nanoparticles with thicknesses between one and four nanoparticle layers. As synthesized and deposited, the assemblies contain weakly magnetic material whose volume is equivalent to an outer 0.5 nm shell of the nanoparticles. During a 580 °C anneal, this material is incorporated into the magnetic domains of the nanoparticles. The fraction of nanoparticles transformed by annealing into ferromagnetic L10 FePt, the structure essential for magnetic storage applications, is found to vary with sample thickness; samples thinner than four nanoparticle layers show no significant fraction of ferromagnetic (at 300 K) L10 structured nanoparticles under these annealing conditions, while in films comprised of four nanoparticle layers, less than half of the particles are ferromagnetic at 300 K. Possible causes of these observed results are discussed.
Size extensive modification of local multireference configuration interaction
Authors: Venkatnathan Arun , Szilva Andrew B. , Walter Derek , Gdanitz Robert J. , Carter Emily A.
Published: January 2004

We recently developed a reduced scaling multireference configuration interaction (MRCI) method based on local correlation in the internal (occupied) and external (virtual) orbital spaces. This technique can be used, e.g., to predict bond dissociation energies in large molecules with reasonable accuracy. However, the inherent lack of size extensivity of truncated CI is a disadvantage that in principle worsens as the system size grows. Here we implement an a priori size-extensive modification of local MRCI known as the averaged coupled pair functional (ACPF) method. We demonstrate that local MR-ACPF recovers more correlation energy than local MRCI, in keeping with trends observed previously for nonlocal ACPF. We test the size extensivity of local ACPF on noninteracting He atoms and a series of hydrocarbons. Basis set and core correlation effects are explored, as well as bond breaking in a variety of organic molecules. The local MR-ACPF method proves to be a useful tool for investigating large molecules and represents a further improvement in predictive accuracy over local MRCI.
On symmetry breaking in BNB: Real or artifactual?
Authors: Kalemos Apostolos , Dunning Thom H. , Mavridis Aristides
Published: January 2004

The ground state of the linear BNB molecule has been examined with multireference-based ab initio methods coupled with quantitative basis sets. Previous computational studies on BNB, even those using highly correlated single reference-based methods, e.g., the CCSD(T) and BDT methods, suggested that the two BN bond lengths were unequal. In this paper, the BN(X 3Π)+B(2Pu) potential energy curve is constructed using a state-averaged multireference-based correlated method (SA-CASSCF+PT2). The four lowest states of BN were included in the state averaging procedure. These calculations reveal no symmetry breaking along the antisymmetric stretching mode of the molecule.
A global investigation of excited state surfaces within time-dependent density-functional response theory
Authors: Wanko M. , Garavelli M. , Bernardi F. , Niehaus T. A. , Frauenheim T. , Elstner M.
Published: January 2004

This work investigates the capability of time-dependent density functional response theory to describe excited state potential energy surfaces of conjugated organic molecules. Applications to linear polyenes, aromatic systems, and the protonated Schiff base of retinal demonstrate the scope of currently used exchange-correlation functionals as local, adiabatic approximations to time-dependent Kohn–Sham theory. The results are compared to experimental and ab initio data of various kinds to attain a critical analysis of common problems concerning charge transfer and long range (nondynamic) correlation effects. This analysis goes beyond a local investigation of electronic properties and incorporates a global view of the excited state potential energy surfaces.
Sigma–pi separation of the electron localization function and aromaticity
Authors: Santos J. C. , Tiznado W. , Contreras R. , Fuentealba P.
Published: January 2004

The electron localization function (ELF) has been separated in its sigma and pi components. The topological analysis of the new ELFσ and ELFπ functions has been used to quantify the concept of resonance. The highest bifurcation values of these functions describe in a correct way the aromaticity of classical ring molecules and some new aromatic compounds as B6CO6, Al42−, and N5−. In the case of Al42−, an important σ delocalization contribution has been found, which is in agreement with previous interpretation.
The mutual diffusion coefficient for (meth)acrylate monomers as determined with a nuclear microprobe
Authors: Leewis Christian M. , Mutsaers Peter H. A. , de Jong Arthur M. , van IJzendoorn Leo J. , de Voigt Martien J. A. , Ren Min Q. , Watt Frank , Broer Dirk J.
Published: January 2004

The value of the mutual diffusion coefficient DV of two acrylic monomers is determined with nuclear microprobe measurements on a set of polymer films. These films have been prepared by allowing the monomers to diffuse into each other for a certain time and subsequently applying fast ultraviolet photo-polymerization, which freezes the concentration profile. The monomer diffusion profiles are studied with a scanning 2.1 MeV proton microprobe. Each monomer contains a marker element, e.g., Cl and Si, which are easily detected with proton induced x-ray emission. From the diffusion profiles, it is possible to determine the mutual diffusion coefficient. The mutual diffusion coefficient is dependent of concentration, which is concluded from the asymmetry in the Cl- and Si-profiles. A linear dependence of the mutual diffusion coefficient on the composition is used as a first order approximation. The best fits are obtained for a value of b=(0.38±0.15), which is the ratio of the diffusion coefficient of 1,3-bis(3-methacryloxypropyl)-1, 1,3,3-tetramethyldisiloxane in pure 2-chloroethyl acrylate and the diffusion coefficient of 2-chloroethyl acrylate in pure 1,3-bis(3-methacryloxypropyl)-1,1,3,3-tetramethyldisiloxane. Under the assumption of a linear dependence of the mutual diffusion coefficient DV on monomer composition, it follows that DV=(2.9±0.6)⋅10−10 m2/s at a 1:1 monomer ratio. With Flory–Huggins expressions for the monomer chemical potentials, one can derive approximate values for the individual monomer diffusion coefficients.
Monte Carlo simulation methodology of the ghost interface theory for the planar surface tension
Authors: Moody Michael P. , Attard Phil
Published: January 2004

A novel “ghost interface” expression for the surface tension of a planar liquid–vapor interface is derived in detail from consideration of the free energy of the system, and a methodology for utilization of this new technique is given. An augmented Monte Carlo computer simulation procedure is developed specifically for the ghost interface, including derivation of long-range corrections resulting from potential truncation and a modified Gibbs ensemble technique for the simulation of adjacent coexisting phases. Results generated from the ghost interface theory for the surface tension are presented and found to be in good quantitative agreement with those resulting from the Kirkwood–Buff equation. Applications of this new approach to curved and to supersaturated systems are also discussed.
Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle
Authors: Pal Subrata , Balasubramanian Sundaram , Bagchi Biman
Published: January 2004

Dielectric relaxation of aqueous solutions of micelles, proteins, and many complex systems shows an anomalous dispersion at frequencies intermediate between those corresponding to the rotational motion of bulk water and that of the organized assembly or macromolecule. The precise origin of this anomalous dispersion is not well-understood. In this work we employ large scale atomistic molecular dynamics simulations to investigate the dielectric relaxation (DR) of water molecules in an aqueous micellar solution of cesium pentadecafluorooctanoate. The simulations clearly show the presence of a slow component in the moment–moment time correlation function [ΦMW(t)] of water molecules, with a time constant of about 40 ps, in contrast to only 9 ps for bulk water. Interestingly, the orientational time correlation function [Cμ(t)] of individual water molecules at the surface exhibits a component with a time constant of about 19 ps. We show that these two time constants can be related by the well-known micro-macrorelations of statistical mechanics. In addition, the reorientation of surface water molecules exhibits a very slow component that decays with a time constant of about 500 ps. An analysis of hydrogen bond lifetime and of the rotational relaxation in the coordinate frame fixed on the micellar body seems to suggest that the 500 ps component owes its origin to the existence of an extended hydrogen bond network of water molecules at the surface. However, this ultraslow component is not found in the total moment–moment time correlation function of water molecules in the solution. The slow DR of hydration water is found to be well correlated with the slow solvation dynamics of cesium ions at the water-micelle interface.
Molecular dynamics simulation of the coalescence of nanometer-sized water droplets in n-heptane
Authors: Zhao Liyan , Choi Phillip
Published: January 2004

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 Å. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized.
Experimental and simulation study of neon collision dynamics with a 1-decanethiol monolayer
Authors: Isa N. , Gibson K. D. , Yan T. , Hase W. , Sibener S. J.
Published: January 2004

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV.
Resonant Raman spectroscopy of PAH–Os self-assembled multilayers
Authors: Tognalli N. , Fainstein A. , Bonazzola C. , Calvo E.
Published: January 2004

We present a resonant Raman scattering study of (PAH–Os/PVS)n and (PAH–Os/GOx)m self-assembled multilayers (n=1–11 and m=1–3). These Os polymer multilayers can be used in electrodes as efficient molecular wires for biomolecular recognition. The Raman intensity dependence on the number of self-assembly cycles provides information on the deposition process. The spectra are identical to that observed for PAH–Os in aqueous solution, indicating that the PAH–Os metal complex structure is conserved in the multilayers. We observe at ∼500 nm incoming and outgoing Raman resonances of osmium and bipyridine vibrational modes. These resonances are associated to the metal-to-ligand charge transfer (MLCT) transition. We study the evolution of these Raman modes as a function of the Os oxidation state during in situ electrochemistry. During the oxidation process, Os(II)→Os(III), the Raman resonance related to the MLCT disappears and the bipyridine related modes harden by ∼10 cm−1. These results are correlated with optical transmission measurements which show the disappearance of the visible region absorption when the Os complex is oxidized. We also find partial quenching of the Raman mode intensity after in situ voltamperometric cycles which demonstrates the existence of photo-electro-chemical processes.
Solvation force for long-ranged wall–fluid potentials
Authors: Maciołek A. , Drzewiński A. , Bryk P.
Published: January 2004

The solvation force of a simple fluid confined between identical planar walls is studied in two model systems with short ranged fluid–fluid interactions and long-ranged wall–fluid potentials decaying as −Az−p,z→∞, for various values of p. Results for the Ising spins system are obtained in two dimensions at vanishing bulk magnetic field h=0 by means of the density-matrix renormalization-group method; results for the truncated Lennard-Jones (LJ) fluid are obtained within the nonlocal density functional theory. At low temperatures the solvation force fsolv for the Ising film is repulsive and decays for large wall separations L in the same fashion as the boundary field fsolv∼L−p, whereas for temperatures larger than the bulk critical temperature fsolv is attractive and the asymptotic decay is fsolv∼L−(p+1). For the LJ fluid system fsolv is always repulsive away from the critical region and decays for large L with the the same power law as the wall–fluid potential. We discuss the influence of the critical Casimir effect and of capillary condensation on the behavior of the solvation force.
Optical control over photoconductivity in polyferrocenylsilane films
Authors: Tzolov M. , Cyr P. W. , Sargent E. H. , Manners I.
Published: January 2004

We report the study and elucidate the origin of the photoconductivity of polyferrocenylsilanes achieved through photooxidation performed by ultraviolet irradiation in the presence of chloroform. The persistence over months of the changes in the optoelectronic properties allowed more detailed studies of the charge photogeneration process. The photocurrent spectrum mimics that of the absorption indicating that the photooxidized material is not a mechanical mixture of oxidized and unoxidized polymer units. Photomodulation spectroscopy revealed the existence of long-lived photoexcited states with a lifetime in the millisecond range. They have been interpreted as trapped excitons at the oxidized sites where the polymer is deformed due to the presence of the chloroform derived counter ions. Because of the relatively long lifetime of the trapped excitons they can dissociate and the formed charge carriers can be separated in an externally applied electric field. The effect of the polymer chain deformation around the oxidized unit extends over the neighboring polymer units. In light of the potential applications of this class of polymers in various electronic and photonic devices, the clarification of such a basic process as the photocurrent generation will be a key factor for further technological development.
Structures of BaF2–CaF2 heterolayers and their influences on ionic conductivity
Authors: Jin-Phillipp N. Y. , Sata N. , Maier J. , Scheu C. , Hahn K. , Kelsch M. , Rühle M.
Published: January 2004

Recently, artificial ion conductors have been prepared by growing epitaxial heterolayers consisting of BaF2–CaF2 using molecular beam epitaxy. The ionic conductivity of these heterolayers shows a strong dependence on the layer thickness [N. Sata, S. Eberman, K. Eberl, and J. Maier, Nature 408, 996 (2000)]. In this paper three such heterolayers with different spacings (sample A: 80 nm, sample B: 10 nm, sample C: 1 nm) are investigated by conventional transmission electron microscopy and high-resolution transmission electron microscopy. The spacings are chosen such that they fall into the three conductivity regimes observed in N. Sata et al. (l>50 nm; 8<l<50 nm; l<8 nm). In accordance with conductivity studies, the samples with spacings of 10 nm or greater (A,B) are epitaxial and continuous, whereas in the case of extremely small spacing (C) the continuity of the layers is destroyed by formation of a column-like structure. Analytical electron microscopy reveals that, instead of forming multilayers, Ca and Ba separate in different columns in sample C. The structure properties of sample A (large l) are quite ideal: Planar interfaces with regular arrays of misfit dislocations with their Burgers vectors on the interface are observed. In the case of sample B (medium l) the lattice misfit is accommodated, in addition, by wavy interfaces associated with dislocations characterized by a Burgers vector that makes a large angle to the interfaces. The (111) lattice spacing very close to the interfaces is markedly changed due to this novel relaxation mechanism in the multilayer. The influences of the crystallographic defects on the ionic conductivity are also discussed.
Interfacial tension and wetting in colloid–polymer mixtures
Authors: Aarts D. G. A. L. , Dullens R. P. A. , Lekkerkerker H. N. W. , Bonn D. , van Roij R.
Published: January 2004

We calculate the interfacial tension and the wetting behavior in phase separated colloid–polymer mixtures both for ideal and excluded volume interacting polymers. Within the recently developed extension of the free volume theory to include polymer interactions [Aarts, Tuinier, and Lekkerkerker, J. Phys.: Condens. Matter 14, 7551 (2002)] the interfacial tension of the free interface is calculated by adding a van der Waals squared gradient term. The wetting behavior at a hard wall is calculated following a Cahn–Fisher–Nakanishi approach taking the one- and two-body colloid-wall interactions into account. Comparing results for interacting polymers with those for ideal polymers we find that for interacting polymers the interfacial tension does not increase as steeply as a function of the gas–liquid colloid density difference. Furthermore, the wetting transition shifts to higher polymer concentrations, even to above the triple line. The predictions for both the interfacial tension and the wetting are compared to recent experiments.
Modeling liquid crystal bilayer structures with minimal surfaces
Authors: Enlow J. D. , Enlow R. L. , McGrath K. M. , Tate M. W.
Published: January 2004

This paper describes a new convenient and accurate method of calculating x-ray diffraction integrated intensities from detailed cubic bilayer structures. The method is employed to investigate the structure of a particular surfactant system (didodecyldimethylammonium bromide in a solution of oil and heavy water), for which single-crystal experimental data have recently been collected. The diffracted peak intensities correlate well with theoretical structures based on mathematical minimal surfaces. Optimized electron density profiles of the bilayer are presented, providing new insight into key features of the bilayer structure.
Squeezing wetting and nonwetting liquids
Authors: Samoilov V. N. , Persson B. N. J.
Published: January 2004

We present molecular-dynamics results for the squeezing of octane (C8H18) between two approaching solid elastic walls with different wetting properties. The interaction energy between the octane bead units and the solid walls is varied from a very small value (1 meV), corresponding to a nonwetting surface with a very large contact angle (nearly 180 degrees), to a high value (18.6 meV) corresponding to complete wetting. When at least one of the solid walls is wetted by octane we observe well defined molecular layers develop in the lubricant film when the thickness of the film is of the order of a few atomic diameters. An external squeezing-pressure induces discontinuous, thermally activated changes in the number n of lubricant layers (n→n−1 layering transitions). With increasing interaction energy between the octane bead units and the solid walls, the transitions from n to n−1 layers occur at higher average pressure. This results from the increasing activation barrier to nucleate the squeeze-out with increasing lubricant-wall binding energy (per unit surface area) in the contact zone. Thus, strongly wetting lubricant fluids are better boundary lubricants than the less wetting ones, and this should result in less wear. We analyze in detail the effect of capillary bridge formation (in the wetting case) and droplets formation (in the nonwetting case) on the forces exerted by the lubricant on the walls. For the latter case small liquid droplets may be trapped at the interface, resulting in a repulsive force between the walls during squeezing, until the solid walls come into direct contact, where the wall–wall interaction may be initially attractive. This effect is made use of in some practical applications, and we give one illustration involving conditioners for hair care application.
Ionization from a double bond: Rovibronic photoionization dynamics of ethylene, large amplitude torsional motion and vibronic coupling in the ground state of C2H4+
Authors: Willitsch S. , Hollenstein U. , Merkt F.
Published: January 2004

Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X̃→X̃+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm−1. The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm−1 in C2H4+ and of 37.1(5) cm−1 in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the X̃+ and Ã+ states mediated by the torsional mode ν4 and a weaker mixing between the X̃+ and B̃+ states mediated by the symmetric CH2 out-of-plane bending mode ν7. The vibrational intensities could be accounted for quantitatively using a Herzberg–Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm−1 and 84 913.3(14) cm−1, respectively.
Ab initio rate constants from hyperspherical quantum scattering: Application to H+CH4→H2+CH3
Authors: Kerkeni Boutheı̈na , Clary David C.
Published: January 2004

A general and practical procedure is described for calculating rate constants for chemical reactions using a minimal number of ab initio calculations and quantum-dynamical computations. The method exploits a smooth interpolating functional developed in the hyperspherical representation. This functional is built from two Morse functions and depends on a relatively small number of parameters with respect to conventional functionals developed to date. Thus only a small number of ab initio points needs to be computed. The method is applied to the H+CH4→H2+CH3 reaction. The quantum scattering calculations are performed treating explicitly the bonds being broken and formed. All the degrees of freedom except the breaking and forming bonds are optimized ab initio and harmonic vibrational frequencies and zero-point energies for them are calculated at the MP2(full) level with a cc-pVTZ basis set. Single point energies are calculated at a higher level of theory with the same basis set, namely CCSD(T, full). We report state-to-state cross sections and thermal rate constants for the title reaction and make comparisons with previous results. The calculated rate constants are in good agreement with experiments.
Probing lattice distortions in mixed CH3I1−cBrc by methyl rotational tunneling
Authors: Prager M.
Published: January 2004

Methyl iodide alloyed with methyl bromide is studied for low methyl bromide concentrations c⩽0.3 by rotational tunneling spectroscopy with neutrons. The appearance of three tunneling bands, their shift with respect to the pure materials and their broadening is explained semiquantitatively on the basis of the crystal structure and global and local changes of interatomic distances based on the r-dependence of intermolecular interactions. Besides the overall reduction of the lattice parameter local free volume around guest molecules is important. A local relaxation of the atomic position by 1.3% towards guest molecules is found.
Electron-spin polarization of photoions produced through photoionization from the laser-excited triplet state of Sr
Authors: Yonekura Nobuaki , Nakajima Takashi , Matsuo Yukari , Kobayashi Tohru , Fukuyama Yoshimitsu
Published: January 2004

We report the detailed experimental study on the production of electron-spin-polarized Sr+ ions through one-photon resonant two-photon ionization via laser-excited 5s5p 3P1 (MJ=+1) of Sr atoms produced by laser-ablation. We have experimentally confirmed that the use of laser-ablation for the production of Sr atoms prior to photoionization does not affect the electron-spin polarization. We have found that the degree of electron-spin polarization is 64±9%, which is in good agreement with our recent theoretical prediction. As we discuss in detail, we infer, from a simple analysis, that photoelectrons, being the counterpart of electron-spin-polarized Sr+ ions, have approximately the same degree of electron-spin polarization. Our experimental results demonstrate that the combined use of laser-ablation technique and pulsed lasers for photoionization would be a compact and effective way to realize a pulsed source for spin-polarized ions and electrons for the studies of various spin-dependent dynamics in chemical physics.
Near neighbor approximation in nuclear magnetic resonance
Authors: Zhang Shanmin
Published: January 2004

A new way to deal with the excitation by multiple effective RF fields with interference is presented using the coherent averaging theory. It significantly simplifies the calculation of the effect of RF interference that occurs in the excitations by periodic pulses and phase-incremented pulses (PIPs). This approach shows that each neighboring RF field contributes to an excitation profile an offset shift, which is termed the Bloch–Siegert offset shift (BSOS). The BSOS depends not only on the strengths of both RF fields that interfere with each other but also on their relative phase between the two RF fields. Consequently, it can be positive, negative, and zero. In addition, the BSOS is also inversely proportional to the frequency separation of the two RF fields. Therefore, only a few near neighbors need to be taken into account in most cases, resulting in a near neighbor approximation (NNA). The BSOS for two multiband excitation profiles, one by a periodic pulse and the other by a PIP, are calculated using the NNA. The results are in good agreement with the computer simulated ones.
Simulated experiment for elimination of chemical and biological warfare agents by making use of microwave plasma torch
Authors: Hong Yong C. , Kim Jeong H. , Uhm Han S.
Published: January 2004

The threat of chemical and biological warfare agents in a domestic terrorist attack and in military conflict is increasing worldwide. Elimination and decontamination of chemical and biological warfare (CBW) agents are immediately required after such an attack. Simulated experiment for elimination of CBW agents by making use of atmospheric-pressure microwave plasma torches is carried out. Elimination of biological warfare agents indicated by the vitrification or burnout of sewage sludge powders and decomposition of toluene gas as a chemical agent stimulant are presented. A detailed characterization for the elimination of the simulant chemicals using Fourier transform infrared and gas chromatography is also presented.
Observation of collisionless inward propagation of electrons into helical vacuum magnetic surfaces via stochastic magnetic fields
Authors: Himura H. , Wakabayashi H. , Fukao M. , Yoshida Z. , Isobe M. , Okamura S. , Suzuki C. , Nishimura S. , Matsuoka K. , Toi K. , Yamada H.
Published: January 2004

Electrons are injected into a stochastic magnetic region (SMR) of a stellarator vacuum configuration. Remarkably, when the SMR is present, some field-following electrons in the SMR move inwardly across the last closed flux surface. This inward propagation occurs in a collisionless process, but it is never observed for cases where the SMR is lost, nor is the electron density small in the SMR. These suggest the existence of cross-field transport that is associated with free-streaming of electrons along the stochastically wandering field lines in the SMR.
Generalized electron Bernstein modes in a plasma with a kappa velocity distribution
Authors: Mace R. L.
Published: January 2004

Perpendicularly propagating electron Bernstein modes in a uniformly magnetized plasma having an isotropic kappa velocity distribution are investigated within the framework of a fully electromagnetic plasma model but one which ignores particle relativistic effects. The dispersion relations for the Bernstein mode waves are found to be significantly dependent on the spectral index, κ, of the electron kappa distribution. In particular, waves with frequencies exceeding the upper hybrid frequency are seen to occupy a diminishing range of frequencies above the nearest cyclotron harmonic as κ is reduced. The Bernstein mode wave whose frequency lies closest to the upper hybrid frequency is found to couple to the cold plasma, electromagnetic Z mode, as it does in a Maxwellian plasma. For waves whose frequencies lie below the upper hybrid frequency, diminishing κ gives rise to an increasingly weak dependence of frequency on wave number and a slower frequency fall off with this parameter, but the frequency occupies the entire intraharmonic band here. All Bernstein modes are observed to become significantly electromagnetic at very long wavelengths, or small wave numbers, having in general elliptical polarization whose elliptical eccentricity depends on κ and wave number. At smaller wavelengths the modes are found to be electrostatic to a very good approximation, irrespective of κ value. The significance of the results to the interpretation of banded emissions in planetary magnetospheres is briefly discussed.
Effects of niobium doping on the piezoelectric properties of sol–gel-derived lead–zirconate–titanate films
Authors: Kwok K. W. , Tsang R. C. W. , Chan H. L. W. , Choy C. L.
Published: January 2004

Sol–gel-derived lead–zirconate–titanate (PZT) films doped with various amounts of niobium (Nb) have been spin coated on silicone substrates, and their remanent polarization Pr, and effective longitudinal and transverse piezoelectric coefficients (d33,c and e31,c) as well as the pyroelectric coefficient p, have been measured. The Zr/Ti ratio of the films is 53/47. Our results reveal that the Nb-dopant effects on the PZT films are very similar to the experimentally known effects on the corresponding bulk ceramics, i.e., enhancing both the longitudinal and transverse piezoelectric properties. However, because of the substrate clamping effect, the exact enhancement in the longitudinal piezoelectric properties cannot be evaluated by the d33,c measurement on the film samples. Accordingly, the observed d33,c value of the Nb-doped PZT films remains almost unchanged while the observed −e31,c (as well as Pr and p) increases with increasing Nb concentration, showing an optimum Nb concentration of 2 mol %. For the PZT film doped with 2% Nb, the observed values of Pr, d33,c, −e31,c, and p are about 30 μC/cm2, 95 pm/V, 18 C/m2, and 350 μC/m2 K, respectively.
Extended Cauchy equations for the refractive indices of liquid crystals
Authors: Li Jun , Wu Shin-Tson
Published: January 2004

The extended Cauchy equations are derived based on the Vuks equation for describing the wavelength- and temperature-dependent refractive indices of liquid crystal compounds and mixtures. This model fits experimental data well in the off-resonance spectral region. Correlations between the Cauchy coefficients for the nematic and isotropic phases are derived and validated by experiments.
Noise performance of the radio-frequency single-electron transistor
Authors: Roschier Leif , Hakonen P. , Bladh K. , Delsing P. , Lehnert K. W. , Spietz Lafe , Schoelkopf R. J.
Published: January 2004

We have analyzed a radio-frequency single-electron-transistor (RF-SET) circuit that includes a high-electron-mobility-transistor (HEMT) amplifier, coupled to the single-electron-transistor (SET) via an impedance transformer. We consider how power is transferred between different components of the circuit, model noise components, and analyze the operating conditions of practical importance. The results are compared with experimental data on SETs. Good agreement is obtained between our noise model and the experimental results. Our analysis shows, also, that the biggest improvement to the present RF-SETs will be achieved by increasing the charging energy and by lowering the HEMT amplifier noise contribution.
Spark-eroded particles: Influence of processing parameters
Authors: Carrey J. , Radousky H. B. , Berkowitz A. E.
Published: January 2004

Ni particles were prepared by spark erosion in a fixed-gap apparatus, and in the usual “shaker-pot” assembly, in an investigation of the influence of various processing parameters on the particles’ properties. The sizes of the particles were studied as functions of spark energies ranging from 10 μJ to 1 J, and a scaling relation derived from a simple model was verified. Several different static and rotating electrode configurations were compared with respect to their suitability for producing significant yields of small particles. The advantages of stirring the dielectric with the fixed-gap apparatus and of rotating the electrodes were demonstrated. Water, kerosene, and liquid argon and nitrogen were used as dielectric liquids. When compounds were formed, the reaction with the dielectric proceeded inversely with particle size. Spark erosion in kerosene at low spark energies, followed by annealing, proved to be an effective method to produce fine nickel particles.
Optical absorption in terahertz-driven quantum wells
Authors: Mi X. W. , Cao J. C. , Zhang C.
Published: January 2004

The optical absorption spectra in a quantum well driven both by an intense terahertz (THz) and by an optical pulse are theoretically investigated within the theory of density matrix. We found that the optical absorption spectra and the splitting of the excitonic peaks splitting can be controlled by changing the THz field intensity and/or frequency. The Autler–Towns splitting is a result of the THz nonlinear dynamics of confined excitons, which is in agreement with the experiments. In addition, the dependence of the optical absorption on the quantum well width and the carrier density is also discussed.
Phosphorescence from iridium complexes doped into polymer blends
Authors: Gong Xiong , Lim Sang-Hyun , Ostrowski Jacek C. , Moses Daniel , Bardeen Christopher J. , Bazan Guillermo C.
Published: January 2004

Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, tris[9,9-dihexyl-2-(phenyl-4′-(-pyridin-2″-yl))fluorene] iridium (III) [Ir(DPPF)3], is investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy. A redshifted PL and slow fluorescence decay are due to the formation of an exciplex in PVK-PBD blends. A decrease in intensity in polymer blends observed at 425 nm with increasing concentrations of Ir(DPPF)3 and an evident rising feature observed in films with 1 wt % Ir(DPPF)3 in the range of 578 to 615 nm within a 200 ns timescale indicate that efficient Förster energy transfer from exciplex to Ir(DPPF)3 occurs. The electrophosphorescent light-emitting diodes fabricated with PVK-PBD doped with Ir(DPPF)3 have external quantum efficiency of 8% ph/el, luminous efficiency of 29 cd/A and brightness greater than 3500 cd/m2 at 1 wt % Ir(DPPF)3. The devices exhibited no electroluminescence (EL) emission from PVK or PBD even at a low concentration of Ir(DPPF)3 (0.1 wt %), which indicates that the dominant mechanism in EL is charge trapping rather than energy transfer.
Optical quenching of photoconductivity in undoped n-GaN
Authors: Cai S. , Parish G. , Umana-Membreno G. A. , Dell J. M. , Nener B. D.
Published: January 2004

The optical quenching of photoconductivity in undoped n-type GaN has been investigated. It was observed that for increasing 360 nm background intensity, at constant 632 nm quenching illumination intensity, the quenching increased for intensities up to 1012 photons cm−2 s−1 and decreased thereafter. At low background illumination intensities, the transient in photoconductivity induced by the quenching illumination exhibits a sharp rise followed by a slow decay; removal of the quenching illumination was also noted to induce a sharp photocurrent drop followed by a slow recovery to the photocurrent level induced by the background illumination alone. The quenching effect at constant background and quenching illumination wavelength and intensity is noted to keep nearly constant at low temperature and decrease rapidly when temperature is higher than 270 K. Furthermore, measurement of the spectral distribution of the quenching effect indicates the existence of a broadband hole trap centered around 1.5 eV above the valence band. To explain the observed experimental behavior, the conventional hole trap and recombination center model was extended to include an electron trap. This extended model was found to accurately predict the experimental results.
Gated nonlinear transport in organic polymer field effect transistors
Authors: Hamadani B. H. , Natelson D.
Published: January 2004

We measure hole transport in poly(3-hexylthiophene) field effect transistors with channel lengths from 3 μm down to 200 nm, from room temperature down to 10 K. Near room temperature effective mobilities inferred from linear regime transconductance are strongly dependent on temperature, gate voltage, and source–drain voltage. As T is reduced below 200 K and at high source–drain bias, we find transport becomes highly nonlinear and is very strongly modulated by the gate. We consider whether this nonlinear transport is contact limited or a bulk process by examining the length dependence of linear conduction to extract contact and channel contributions to the source–drain resistance. The results indicate that these devices are bulk limited at room temperature, and remain so as the temperature is lowered. The nonlinear conduction is consistent with a model of Poole–Frenkel-like hopping mechanism in the space-charge limited current regime. Further analysis within this model reveals consistency with a strongly energy dependent density of (localized) valence band states, and a crossover from thermally activated to nonthermal hopping below 30 K.
Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure
Authors: Ma B. S. , Wang X. D. , Su F. H. , Fang Z. L. , Ding K. , Niu Z. C. , Li G. H.
Published: January 2004

The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states.
Morphology and transport properties of nanostructural gold on silicon
Authors: Pal S. , Sanyal M. K. , Hazra S. , Kundu S. , Schreiber F. , Pflaum J. , Barrena E. , Dosch H.
Published: January 2004

Nanometer sized Au clusters deposited on a silicon substrate forming Au–SiO2–Si structure are important for the development of contacts in nanotechnology. Systematic x-ray reflectivity, scanning probe microscopy, and scanning tunneling spectroscopy measurements were done to understand the relationship between morphology and electrical transport properties of this nanostructural metal–insulator–semiconductor system. The presence of an interfacial layer at the metal–insulator interface dictates the tunneling current through this structure and exhibits a gap leading to a suppression of current. Local density of states and electron density/thickness of the interfacial layer have been extracted from the measurements to understand the evolution of metallicity of this Au–SiO2–Si structure.
Laser–plasma interactions in fused silica cavities
Authors: Zeng Xianzhong , Mao Xianglei , Mao Samuel S. , Yoo Jong H. , Greif Ralph , Russo Richard E.
Published: January 2004

The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a cavity with aspect ratio of 4 increased faster with irradiance after the laser irradiance reached a threshold of 5 GW/cm2. The threshold irradiance of particulate ejection was lower for laser ablation in a cavity compared with on a flat surface; the greater the cavity aspect ratio, the lower the threshold irradiance. The ionization of silicon becomes saturated and the crater depths were increased approximately by 1 order of magnitude after the irradiance reached the threshold. Phase explosion was discussed to explain the large change of both plasma characteristics and mass removal when irradiance increased beyond a threshold value. Self-focusing of the laser beam was discussed as being responsible for the decrease of the threshold in cavities.
Anisotropic optical properties and molecular orientation in vacuum-deposited ter(9,9-diarylfluorene)s thin films using spectroscopic ellipsometry
Authors: Lin Hao-Wu , Lin Chun-Liang , Chang Hsin-Hua , Lin Yu-Ting , Wu Chung-Chih , Chen You-Ming , Chen Ruei-Tang , Chien Yuh-Yih , Wong Ken-Tsung
Published: January 2004

This article reports on the investigation of anisotropic optical properties of vacuum-deposited thin films of high-efficiency blue-emitting ter(9,9-diarylfluorene)s using variable-angle spectroscopic ellipsometry. Under deposition conditions typical for thin-film organic devices, both real and imaginary parts of refractive indices of vacuum-deposited ter(9,9-diarylfluorene) films exhibit rather significant uniaxial anisotropy with the optical axis along the surface normal. In particular, for the absorption associated with the π–π* transition of the terfluorene backbone, they show substantially larger in-plane extinction coefficients than the out-of-plane extinction coefficients. It is thus inferred that the vacuum-deposited ter(9,9-diarylfluorene) molecules tend to align their molecular axes and π–π* transition dipole moments along the substrate surface as observed previously in spin-coated films of alkyl-substituted polyfluorenes or oligofluorenes, even though the present ter(9,9-diarylfluorene)s have rigid and bulky aryl substituents on C9, relatively shorter oligomer chains, and very different deposition conditions.
Electronic and thermoelectric transport in semiconductor and metallic superlattices
Authors: Vashaee Daryoosh , Shakouri Ali
Published: January 2004

A detailed theory of nonisothermal electron transport perpendicular to multilayer superlattice structures is presented. The current–voltage (I–V) characteristics and the cooling power density are calculated using Fermi–Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in a thermionic emission will increase. This has a significant effect on the I–V measurements as well as the cooling characteristics. Theoretical calculations are compared with the experimental dark current characteristics of quantum well infrared photodetectors and good agreement over a wide temperature range for a variety of superlattice structures is obtained. In contrast with earlier studies, it is shown that lateral momentum is conserved for the case of electron transport in planar semiconductor barriers.
Nonlinear refraction and absorption of Mg doped stoichiometric and congruent LiNbO3
Authors: Pálfalvi L. , Hebling J. , Almási G. , Péter Á. , Polgár K. , Lengyel K. , Szipöcs R.
Published: January 2004

The light induced change of refraction is studied in pure and Mg doped LiNbO3 with congruent and stoichiometric compositions by the Z-scan method using all-lines visible argonion laser, up to MW/cm2 intensity level. In Mg-doped congruent and stoichiometric crystals with Mg concentrations above threshold a positive change in the refractive index was found, in contrast to all other cases where beam fanning and negative change of the refractive index were observed. The beam distortion in the samples doped above threshold was related to thermal lensing, while below it to the photorefractive effects. It was also shown that for thermal lensing nonlinear absorption plays a dominant role. The Z-scan method was found to be an alternative technique to decide whether the Mg dopant level is above or below the photorefractive threshold. The damage resistance of the Mg doped samples above threshold was higher for the stoichiometric crystal than for the congruent one and increased with the amount of the built-in Mg concentration.
Annealing of phosphorus-doped Ge islands on Si(001)
Authors: Kamins T. I. , Medeiros-Ribeiro G. , Ohlberg D. A. A. , Williams R. Stanley
Published: January 2004

To study the effect of phosphorus on the evolution of Ge islands on Si(001), phosphorus-doped islands were annealed in a H2 ambient and undoped islands were annealed in a PH3/H2 ambient. In both cases phosphorus stabilizes the island structure and reduces coarsening during annealing. The changes of island shape during annealing are consistent with phosphorus influencing the thermodynamic properties of the islands, while the slower decrease in the density of islands when phosphorus is present is consistent with phosphorus kinetically retarding surface diffusion of Ge atoms.