bluuebunny/crossref_metadata_2025 huggingface dataset:
A statistical mechanical model for the calculation of the permittivity of water in hydrated polymer electrolyte membrane pores
Authors: Paul Reginald , Paddison Stephen J.
Published: July 2002

An equilibrium statistical mechanical model is derived to compute the spatial variation in the permittivity of water within the hydrated pores of ion-containing polymeric membranes. The fixed anionic groups within the pore are modeled as periodic arrays of point charges. The Helmholtz free energy is calculated from a total Hamiltonian of the pore that includes energy from (1) interactions between the fields generated by the fixed charge groups and the dipoles of the water molecules, (2) “hard core” interactions between the water molecules, and (3) dipole–dipole interactions between the water molecules. The free energy is divided into two parts: (a) a reference free energy associated with five water molecules in a cluster interacting with each other through the hard core potentials and with the fixed charge groups and (b) an excess free energy due to the dipolar interactions between the water molecules in two cluster units. In the present work we calculate the polarization and corresponding permittivity from this reference free energy. We first show that our calculations, even at this level of sophistication, go beyond all the traditional approaches. Furthermore, with our model we compute radial profiles of the permittivity in the pores of the sulfonic acid–based Nafion® and 65% sulfonated poly ether ether ketone ketone polymer electrolyte membranes at several different hydration levels. These numerical results and predictions are in agreement with known experimental measurements.
Nanopolar reorientation in ferroelectric thin films
Authors: Hubert C. , Levy J. , Rivkin T. V. , Carlson C. , Parilla P. A. , Perkins J. D. , Ginley D. S.
Published: July 2002

The influence of varying oxygen pressure P(O2) during the growth of Ba0.4Sr0.6TiO3 thin films is investigated using dielectric and local optical probes. A transition from in-plane to out-of-plane ferroelectricity is observed with increasing P(O2). Signatures of in-plane and out-of-plane ferroelectricity are identified using dielectric response and time-resolved confocal scanning optical microscopy (TRCSOM). At the crossover pressure between in-plane and out-of-plane polarization (Pc=85 mTorr), TRCSOM measurements reveal a soft, highly dispersive out-of-plane polarization that reorients in plane under modest applied electric fields. At higher deposition pressures, the out-of-plane polarization is hardened and is less dispersive at microwave frequencies, and the dielectric tuning is suppressed. Nanopolar reorientation is believed to be responsible for the marked increase in dielectric tuning at P(O2)=Pc.
Piezoelectric photothermal study of AlxGa1−xAs epitaxial layer (x=0.22, 0.28, and 0.5) grown on semi-insulating GaAs substrate
Authors: Fukuyama Atsuhiko , Fukuhara Hironori , Tanaka Shin-ichi , Memon Aftab A. , Sakai Kentaro , Akashi Yoshito , Ikari Tetsuo
Published: July 2002

Piezoelectric photothermal measurements of an AlxGa1−xAs (x=0.22, 0.28, and 0.5) epitaxial layer grown on a GaAs substrate were carried out in the temperature range of 297 to 80 K. In addition to the band gap signal of the GaAs substrate, the direct transition gaps of AlGaAs were clearly observed in the higher photon energy region. It was experimentally confirmed that the temperature coefficient of the direct transition gap of AlxGa1−xAs alloy decreases with increasing Al mole fraction. By conducting the quenching light illumination measurements at 80 K we concluded that the photoexcited electrons in the AlGaAs epitaxial layer drifted under the influence of an electric field present at the AlGaAs/GaAs interface. The drifted electrons eventually recombined with the ionized EL2 centers in the SI GaAs substrate.
Transport of carriers in metal/porous silicon/c-Si device structures based on oxidized porous silicon
Authors: Balagurov L. A. , Bayliss S. C. , Kasatochkin V. S. , Petrova E. A. , Unal B. , Yarkin D. G.
Published: July 2002

Current–voltage (I–V) and impedance measurements of oxidized metal/porous silicon (PS)/c-Si sandwich structures were performed at various temperatures. The I–V dependence at relatively small bias was found to be determined by the resistance of the PS layer. When the reverse bias is increased the injection of carriers commences. A power-law space charge limited current in the I–V dependence was observed at high forward bias. An exponential energy distribution of localized states with a characteristic energy of 60 meV was calculated from these. The existence of an inversion (n-type) layer with high conductivity was established in the p-type c-Si substrate adjacent to the PS layer. The presence of this inversion layer leads to an increase of the active device area, capacitance, and reverse current.
Ab initio CASPT2//CASSCF study of the O(1D)+H2O(X 1A1) reaction
Authors: Sayós R. , Oliva Carolina , González Miguel
Published: July 2002

The ground potential energy surface (PES) of the O(1D)+H2O system was studied with the CASPT2//CASSCF ab initio method. We analyzed the degree of validity of an earlier ab initio study by us that used the Møller–Plesset (MP) method. Both the present CASPT2//CASSCF calculations and the highest level MP calculations (PUMP4//UMP2) showed that the main reaction channel (OH+OH) has no energy barrier along the minimum energy path. This result is consistent with the absence of experimental activation energy. The CASPT2//CASSCF and PUMP4//UMP2 results, however, show important differences, mainly concerning the energy, due to the dominant open-shell singlet character of the ground PES. To make an accurate general description of this system, ab initio calculations using multireference methods like the one discussed here are required. Nevertheless, the earlier PUMP4//UMP2 calculations can be taken as a reasonable starting point for characterizing the ground PES of this system. Moreover, the pseudotriatomic (O(1D)+H–(OH)) analytical potential energy surface derived in the previous work to interpret the experimental results is a reasonable model for describing the O(1D)+H2O→2 OH reaction.
Transient nucleation distributions and fluxes at intermediate times and sizes
Authors: Shneidman Vitaly A.
Published: July 2002

General interpolating expressions, valid for near- and arbitrary overcritical sizes of clusters, are proposed for the nucleation fluxes and distributions. Results are expressed in terms of the deterministic growth rates, and are characterized by a non-Gaussian dependence on the size of nuclei. In a sense, the proposed approximations combine the positive aspects of the parabolic model by Trinkaus and Yoo [Philos. Mag. A 55, 269 (1987)] and of the boundary layer (“matched asymptotic”) solution earlier described by the author [Sov. Phys. Tech. Phys. 32, 76 (1987); 33, 1338 (1988)]. Specifications of the general results are made for several mainstream nucleation models via selection of appropriate growth rates. Examples include surface- and diffusion-limited nucleation in the continuous (Zeldovich–Frenkel) and discrete (Becker–Döring, Turnbull–Fisher) versions of the nucleation equation.
Nanospectrofluorometry inside single living cell by scanning near-field optical microscopy
Authors: Lei F. H. , Shang G. Y. , Troyon M. , Spajer M. , Morjani H. , Angiboust J. F. , Manfait M.
Published: July 2002

Near-field fluorescence spectra with subdiffraction limit spatial resolution have been taken in the proximity of mitochondrial membrane inside breast adenocarcinoma cells (MCF7) treated with the fluorescent dye (JC-1) by using a scanning near-field optical microscope coupled with a confocal laser microspectrofluorometer. The probe–sample distance control is based on a piezoelectric bimorph shear force sensor having a static spring constant k=5 μN/nm and a quality factor Q=40 in a physiological medium of viscosity η=1.0 cp. The sensitivity of the force sensor has been tested by imaging a MCF7 cell surface.
InGaN composition and growth rate during the early stages of metalorganic chemical vapor deposition
Authors: Gerthsen D. , Neubauer B. , Rosenauer A. , Stephan T. , Kalt H. , Schön O. , Heuken M.
Published: July 2002

Transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were applied to study the metalorganic chemical vapor deposition of InGaN and the correlation between the structural properties and luminescence of GaN/InxGa1−xN-quantum well structures. A series of samples was grown varying only the growth duration for the InGaN under otherwise unaltered growth conditions. Composition analyses were carried out by measuring local lattice parameters from TEM images, which are directly related to the local In concentration. A rising average In concentration from 6.5% to 15.4% and a decreasing growth rate are observed with increasing growth duration. All samples show an inhomogeneous In distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of some 10 nm. The redshift of the PL peak energy with increasing quantum well thickness indicates that the luminescence is predominantly determined by the piezoelectric field.
Kinetic study on replacement of hydrogen by deuterium at (100)Si/SiO2 interfaces
Authors: Cheng Kangguo , Hess Karl , Lyding Joseph W.
Published: July 2002

The kinetics for the replacement process of hydrogen by deuterium at the (100)Si/SiO2 interface is studied. Metal–oxide–semiconductor devices were initially annealed in hydrogen and then reannealed in deuterium at various temperatures for various durations. The deuterium concentration CD, defined as the percentage of interface defects passivated by deuterium, was then determined by a technique that is solely based on electrical measurements. From fundamental kinetics, activation energy E for replacing hydrogen by deuterium at the (100)Si/SiO2 interface is determined to be 1.84 eV. This result suggests that the replacement of hydrogen by deuterium involves a process of the dissociation of molecular deuterium at the interface.
Millisecond dynamics of thermal expansion of mechanically controllable break junction electrodes studied in the tunneling regime
Authors: Kolesnychenko O. Yu. , Toonen A. J. , Shklyarevskii O. I. , van Kempen H.
Published: July 2002

The thermal expansion dynamics of W, Pt–Ir, and Au mechanically controllable break junction electrodes was studied in the millisecond range. By measuring a transient tunnel current as a function of time, we found that, at low temperatures, the electrode elongation Δs∼t1/2 due to the large values of thermal diffusivity of metals. The magnitude of Δs varies in direct proportion to the power P dissipated in the electrodes.
Self-assembled epitaxial and polycrystalline magnetic nickel nanocrystallites
Authors: Kumar D. , Zhou H. , Nath T. K. , Kvit Alex V. , Narayan J.
Published: July 2002

Nanocrystalline nickel particles were embedded in amorphous alumina and crystalline TiN matrices using a pulsed-laser deposition process to investigate the effect of texturing on magnetic properties of nickel nanocrystallites. The crystalline quality of both the matrix and magnetic particles were investigated by cross-sectional high-resolution transmission electron microscopy. The embedded Ni nanocrystals were found to be epitaxial in case of TiN matrix and polycrystalline in Al2O3 amorphous matrix. The Ni nanocrystals on TiN/Si grow epitaxially because the TiN acting as a template grows epitaxially on Si substrate via domain epitaxy. On the other hand, Ni nanocrystals in Al2O3 matrix are polycrystalline because of the amorphous nature of the alumina matrix. Magnetization versus temperature measurements have shown that the blocking temperature, above which the samples lose magnetization-field (M–H) hysteretic behavior, of Ni–TiN sample (∼190 K) is significantly higher than that of Ni–Al2O3 sample (∼30 K) with a similar size distribution of embedded magnetic particles. A comparison of the values of coercivity (Hc) of the two samples, measured from M–H data, indicates that epitaxial Ni nanocrystals also exhibit significantly higher coercivity than polycrystalline Ni particles. The high values of TB and Hc of Ni–TiN samples with respect to TB of Ni–Al2O3 samples are believed to be associated with preferred alignment of nanocrystallites.
Microstructure and magnetic properties of sputtered spin valve systems
Authors: Langer J. , Mattheis R. , Ocker B. , Maaß W. , Senz S. , Hesse D. , Kräußlich J.
Published: July 2002

Different sputtering methods, viz. rf-diode, rf-magnetron, and dc-magnetron sputtering, have been used to increase the sensitivity of Co/Cu/Co/FeMn top spin valve systems by more than a factor of 2. This improvement is due to an enhanced magnetoresistive effect ΔR/R and a reduced anisotropy field Hk for the best sample. In the present paper the transport and magnetic properties are discussed in the context of microstructure and interfacial quality of the multilayers evaluated by comprehensive transmission electron microscopy and x-ray reflectometry investigations. The sample with the highest sensitivity (dc-magnetron sputtered) shows less structural defects and locally smoother interfaces than the other samples. Additionally it is shown that the ferromagnetic interlayer exchange field is not affected by the applied sputtering methods and can be well understood in terms of orange peel coupling.
A cluster model study of the electron–phonon interaction in magnesium diborate
Authors: Yoshizawa Kazunari , Kondo Masakazu
Published: July 2002

Orbital–vibronic coupling constants were calculated and analyzed using cluster models with D6h symmetry at the B3LYP level of density functional theory to aid the understanding of the electron–phonon interaction mechanism in magnesium diborate, a new binary superconductor with Tc of 39 K. How the frontier orbitals couple with the E2g modes of molecular vibration in the cluster models is discussed with an emphasis on the symmetry of the relevant molecular orbitals and vibrations.
Growth and magneto-optical properties of sub 10 nm (Cd, Mn)Se quantum dots
Authors: Kratzert P. R. , Puls J. , Rabe M. , Henneberger F.
Published: July 2002

We report on the formation of self-assembled semimagnetic (Cd, Mn)Se quantum dots via thermally activated reorganization of an initially two-dimensional film. Incorporation of Mn causes a marked decrease of the dot density, while the size is only slightly affected. Magneto-optical data are consistent with a homogeneous distribution of the magnetic ions. We observe g factors as large as 200, enabling control of individual carrier spins on a sub 10 nm length scale.
Comparison of inherent, instantaneous, and saddle configurations of the bulk Lennard-Jones system
Authors: Shah Pooja , Chakravarty Charusita
Published: July 2002

The configurational energies, order parameters and normal mode spectra associated with inherent structure, inherent saddle, and instantaneous configurations of the bulk Lennard-Jones system are compared. Instantaneous structures are generated by sampling configurations from an isothermalisobaric ensemble Monte Carlo simulation. Local minimization of the potential, starting from a given instantaneous configuration is used to determine the corresponding inherent structure. The inherent saddles are obtained by local minimization on a pseudo-potential surface defined in terms of the square magnitude of the potential gradient. In the solid phase, no stationary points of order greater than zero are sampled and minimizations of both the potential, as well as of the pseudo-potential, always lead to the same global minimum energy crystalline configuration. The energies of instantaneous configurations of the solid show a clear negative correlation with the second-order bond orientational parameters. The instantaneous normal mode spectrum of the solid close to melting has a fairly prominent imaginary branch and is sufficiently smoothed out by local disorder that it qualitatively resembles the liquid phase INM spectrum. In the liquid phase, the inherent, saddle, and instantaneous structures form distinct sets of configurations. The thermal averages of the saddle energies and force constants lie between that of the instantaneous and inherent structures. The temperature dependence of the mean saddle energy and force constant is essentially parallel to that of the corresponding instantaneous quantities. The fraction of imaginary modes for the saddle configurations is approximately half that of the instantaneous configurations. The most striking similarity between the instantaneous and saddle configurations is the linear relationship between the index density and the configurational energy. The most notable difference between the two sets of configurations is the reduction to zero of the fraction of imaginary modes of the saddle configurations on freezing, making the saddle normal mode spectra qualitatively different in the liquid and solid phases.
Investigation of electronegativity in a radio-frequency Xe/SF6 inductively coupled plasma using a Langmuir probe
Authors: Kimura T. , Ohe K.
Published: July 2002

The ratio of negative ion to electron densities (electronegativity) has been investigated using a Langmuir probe technique in a radio-frequency Xe/SF6 inductively coupled plasma, where the electron density is in the order of 1016 m−3 and the electron and negative ion temperatures are about 3.5–5 and 0.4 eV, respectively, which are weakly dependent on SF6 content and power injected into plasma. The electronegativity, which is between 5 and 10, does not strongly depend on SF6 content, while it decreases gradually with the increase of electron density.
Linear and nonlinear waves on the charged surface of liquid hydrogen
Authors: Brazhnikov M. Yu. , Kolmakov G. V. , Levchenko A. A. , Mezhov-Deglin L. P.
Published: July 2002

The results of research on the properties of linear and nonlinear waves on the charged surface of liquid hydrogen in a cylindrical cell are reported. It is found that the spectrum of oscillations of linear waves softens with increasing applied electric field. Weak turbulence in a system of capillary waves on the charged surface of liquid hydrogen is investigated. The formation of a Kolmogorov cascade is observed in the inertial interval from 100 Hz to 10 kHz. It is found that the correlation function of the deviation of the surface from its flat equilibrium state can be described by a power-law function of the frequency, with an exponent m=−3.7±0.3 when the surface is excited at a single resonance frequency, and m=−3.0±0.3 in the case of two-frequency excitation. The results of these studies are in qualitative agreement with the theoretical predictions.
Magnetic behavior of Fe:Al2O3 nanocomposite films produced by pulsed laser deposition
Authors: Dempsey N. M. , Ranno L. , Givord D. , Gonzalo J. , Serna R. , Fei G. T. , Petford-Long A. K. , Doole R. C. , Hole D. E.
Published: July 2002

Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al2O3) have been grown by sequential pulsed laser deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5×1015 atoms/cm2. Increasing the Fe content leads first to the formation of elongated nanocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this case, a reduced moment per Fe atom (1.4 μB/atom) with respect to the value for α-Fe (2.2 μB/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4×106 J/m3) and the low temperature coercivity in these samples are attributed to a strong surface contribution to anisotropy, whereas the temperature dependence of coercivity is attributed to thermal activation.
Synchrotron infrared photoacoustic spectroscopy
Authors: Michaelian Kirk H. , Jackson Richard S. , Homes Christopher C.
Published: July 2002

The use of synchrotron radiation (SR) as a far- and mid-infrared source in the measurement of photoacoustic Fourier transform infrared spectra of solids is demonstrated for the first time in this work. Initial experiments were performed at beamline U10A at the National Synchrotron Light Source, Brookhaven National Laboratory. For synchrotron photoacoustic spectroscopy to be feasible, it must yield results superior to those obtained with a conventional thermal (Globar®) source; accordingly, SR and Globar® photoacoustic spectra recorded under similar conditions were compared in detail. The intensities of SR far-infrared photoacoustic spectra were found to be consistently greater than the corresponding Globar® spectra. At shorter wavelengths, SR always underfills the effective aperture (or, alternately, sample size); SR is a superior source in a spectral region that is a function of this aperture. The high wave number limit of this region exhibits a power-law dependence on aperture size. This investigation also showed that the entire mid-infrared photoacoustic spectrum is more intense using SR and apertures smaller than approximately 0.5 mm.
An ultrahigh vacuum dual-tip scanning tunneling microscope operating at 4.2 K
Authors: Okamoto Hiroshi , Chen Dongmin
Published: July 2002

The design and performance of an ultrahigh vacuum compatible cryogenic dual-tip scanning tunneling microscope is described. The microscope is attached at the bottom of a low-loss liquid helium Dewar and can be operated down to 4.2 K. The coarse positioning system consists of five linear steppers driven by piezo-tubes. The displacement of each stepper can be monitored by its own embedded capacitive position sensor with a submicron resolution, thus allowing accurate control of the tip navigation process. An alignment procedure, using a specimen made of three mutually nonparallel planes, is introduced to bring the two tips into overlapped scan ranges without the help of an additional guiding device such as an electron microscope. The overall system exhibits good mechanical rigidity and atomic resolution has been achieved with either tip. This instrument is well suited for investigating low temperature quantum properties of atomically clean nanostructures in a three-terminal configuration.
Linear oscillations and stability of a liquid bridge in an axial electric field
Authors: Pelekasis N. A. , Economou K. , Tsamopoulos J. A.
Published: July 2002

Small amplitude oscillations of viscous, capillary bridges are studied in the presence of an electric dc field. The electric field is proposed as a means to maintain bridges longer than their perimeter and of uniform cylindrical shape. This is desired in the fabrication of semiconductor crystals. The material of the bridge and the surrounding medium is modeled either as a perfect or as a leaky dielectric. The frequency and the damping rate of the oscillations are calculated numerically by solving a generalized eigenvalue problem. It is shown that they depend on the ratios of the dielectric constants, ε=εin/εout, and conductivities, S=σin/σout, of the two materials, the aspect ratio of the bridge, Λ=πR̃/L̃, the ratio of viscous to the capillary force, Oh=Re−1, which can also be viewed as the inverse Reynolds number of the flow, and, finally, the electrical Bond number, Cel, which is the ratio of the electric stresses to the capillary force. The stability limit of an initially cylindrical bridge is determined with respect to varicose disturbances. In agreement with previous studies it is shown that, if both materials are perfect dielectrics, application of an electric field has a stabilizing effect on the bridge, in the sense that the minimum value, Λmin, of the aspect ratio for the bridge to remain stable drops below 0.5, irrespective of the specific value of the ratio ε. If both materials are leaky dielectrics, bridge stability is determined by the sign of (S−ε) and (S−1)(ε−1), with the positive sign indicating bridge stabilization. The factor (S−ε) arises due to the appearance of a tangential electric stress in the perturbed state for leaky dielectrics. For both cases of leaky and perfect dielectrics, the most unstable mode is the one leading to amphora shaped bridges. It was also found that, when application of an electric field stabilizes the bridge, leaky dielectrics require a lower field than perfect dielectrics and that a large enough field tends to stabilize the bridge for almost the entire range of values of the aspect ratio Λ. These findings concur with earlier analytical results for the stability of jets in longitudinal electric fields and, in conjunction with certain experimental observations, point to the usefulness of the leaky dielectric model pertaining to the stability of bridges.
Structural and optical studies of vertically aligned InAs/GaAs self-assembled quantum dots
Authors: Migliorato M. A. , Wilson L. R. , Mowbray D. J. , Skolnick M. S. , Al-Khafaji M. , Cullis A. G. , Hopkinson M.
Published: July 2002

We report a structural and optical spectroscopic investigation of multiple layer InAs/GaAs self-assembled quantum dots, studied as a function of the GaAs thickness between the quantum dot layers. With decreasing GaAs thickness the positions of dots in different layers exhibit a transition from no correlation to full correlation. Optically the dots in uncorrelated and fully correlated structures are found to exhibit very distinct and different properties. With increasing laser power the photoluminescence of the correlated structure exhibits a high energy, asymmetrical broadening, an effect absent in the uncorrelated structure. In photoluminescence excitation multiple-LO-phonon carrier relaxation features are observed in the spectra of the uncorrelated structure but not in the spectra of the correlated structure. These differences are explained in terms of nonresonant carrier tunneling between the dots in the correlated dot structures.
Carrier doping into MgIn2O4 epitaxial thin films by proton implantation
Authors: Miyakawa M. , Noshiro R. , Ogawa T. , Ueda K. , Kawazoe H. , Ohta H. , Orita M. , Hirano M. , Hosono H.
Published: July 2002

Epitaxial thin films of MgIn2O4 were grown on MgO(100) single-crystal substrates through the pulsed laser deposition technique. X-ray diffraction measurements revealed film orientations MgIn2O4(h00)//MgO(h00) and MgIn2O4(0k0)//MgO(0k0), respectively. Proton implantation was applied to generate carrier electrons in the films. The electrical conductivity of the as-deposited films is below ∼10−7 S cm−1 at room temperature. The maximum conductivity of ∼70 S cm−1 was obtained by the implantation. Hall voltage measurements revealed that H+ implantation causes carrier generation in proportion to H+ fluence without reduction of electron mobility. Following the post-annealing process resulted in further enhancement of the conductivity in each H+-implanted film, as conductivity and generation efficiency were found to increase up to ∼2×102 S cm−1 and ∼95% at the maximum, respectively. This differs from the behavior of polycrystalline films in which conductivity decreased by post-annealing due to a decrease in the Hall mobility of electrons. Thus it is concluded that crystal quality is crucial for heavy carrier doping by ion implantation, especially when utilizing post-annealing treatments to enhance the carrier generation efficiency without reduction of the Hall mobility.
Magnetic nanoelements for magnetoelectronics made by focused-ion-beam milling
Authors: Xiong Gang , Allwood D. A. , Cooke M. D. , Cowburn R. P.
Published: July 2002

Focused-ion-beam (FIB) milling has been used to structure magnetic nanoelements from 5 nm thick films of permalloy. We have used focused 30-keV Ga+ ions to define small arrays (6 μm×6 μm) of wires, circles, and elongated hexagons in the size range 100–500 nm. High-sensitivity magneto-optical measurements combined with atomic force microscopy show that very high quality magnetic nanostructures can be fabricated by FIB milling even in thin films of soft magnetic materials. This finding could be significant for the future commercialization of certain aspects of magnetic nanotechnology and magnetoelectronics.
Photodissociation dynamics of CF3Br at 234 nm: An implication of symmetry reduction during photodissociation
Authors: Kim Tae Kyu , Park Moon Soo , Lee Keon Woo , Jung Kyung-Hoon
Published: July 2002

The photodissociation dynamics of CF3Br in the A-band has been investigated utilizing a two-dimensional photofragment ion-imaging technique coupled with a state-selective resonance-enhanced multiphoton ionization scheme. The total translational energy distributions for the Br(2P3/2) and Br*(2P1/2) channels are well characterized by Gaussian functions with average translational energies of 183 and 151 kJ/mol, respectively. The recoil anisotropies were measured to be β=0.66 for Br and 1.83 for Br*. It was found that Br* production is preferred, with a relative quantum yield of 0.80. The reduction in the recoil anisotropy for Br results from nonadiabatic coupling between the Q11 and Q03 states. The fraction of molecules that dissociate via a distorted pathway induced by symmetry reduction from C3v to Cs is estimated to be 0.11.
Over 30-fold enhancement of light extraction from free-standing photonic crystal slabs with InGaAs quantum dots at low temperature
Authors: Ryu H. Y. , Lee Y. H. , Sellin R. L. , Bimberg D.
Published: July 2002

Highly efficient extraction of photoluminescence is observed from two-dimensional photonic crystal slabs employing InGaAs quantum dots as active material. The introduction of quantum dots reduces diffusion of carriers and thereby suppresses the surface recombination at the air-hole sidewalls of the photonic crystal. Around the normalized frequency of 0.7, over thirty-fold enhancement of the photoluminescence extraction is achieved at 78 K, indicating strong coupling to leaky modes of the free-standing photonic crystal slab. In addition, when the photoluminescence spectra overlaps with a photonic band gap, enhanced light extraction originating from the photonic band gap is observed experimentally.
Quantum-well profile optimization for maximal Stark effect and application to tunable infrared photodetectors
Authors: Radovanović J. , Milanović V. , Ikonić Z. , Indjin D.
Published: July 2002

Two procedures are described for quantum-well shape tailoring, such that both the Stark effect and the intraband absorption on the 1→2 transition in a prescribed range of bias fields are maximized. One of them relies on the isospectral transform of the Hamiltonian, and delivers quantum wells with continuously graded composition. The other uses the simulated annealing algorithm and delivers globally optimal stepwise-graded wells with a preset number of layers. Numerical calculation is performed for wells based on the AlxGa1−xAs alloy.
Temperature dependent switching properties of patterned 200 nm Ni81Fe19 elements
Authors: Li Jian , Shi Jing , Tehrani Saied
Published: July 2002

Quasistatic magnetization switching of patterned 200 nm wide Ni81Fe19 elements has been systematically studied over a wide range of temperatures. In large aspect-ratio elements, the switching field between two bistable magnetization states decreases linearly as the temperature increases, in good agreement with thermally activated nucleation theory. In small aspect-ratio elements, however, switching occurs through a third stable state: the magnetization vortex state. Although the switching mechanism is drastically different in these small aspect-ratio structures, the switching fields also are linearly dependent on temperature.
Nonlinear optical dynamics and Eu3+ spectral holeburning in strontium barium niobate thin film grown by pulsed laser deposition
Authors: Liu H. , Liu G. K. , Li S. T. , Beitz J. V. , Fernandez F. E.
Published: July 2002

Optical quality SrxBa1−xNb2O6 (SBN) thin films, both undoped and Eu3+-doped, of thickness less than 0.5 μm have been successfully grown on fused quartz substrates using a pulsed laser deposition technique. Optical properties of these films were characterized in high-resolution spectroscopic experiments in time and frequency domains. For undoped SBN thin films, broadband emission in the UV region extending to the visible was observed following excitation at 355 nm. This emission is attributed to exciton luminescence of the SBN film. Nonlinear optical response in the picosecond regime and the third-order nonlinear susceptibility, χ(3), were studied using degenerate four-wave-mixing methods. In transverse alignment, χ(3) is enhanced by two orders of magnitude in comparison with its bulk counterpart. A thermal annealing process, monitored via changes in spectral properties of Eu3+, was employed to convert the as-grown amorphous film into a polycrystalline film. High-resolution spectroscopic measurements in the frequency domain were conducted on a 200-nm-thick film of Eu3+-doped SBN. Our spectroscopic results suggest that Eu3+ ions may substitute for Nb, thereby occupying a normally six-fold coordinated lattice site. At liquid helium temperature, spectral holes in the F07–D05 optical transition were burned in the thermally annealed films. Typical observed hole widths were 70–100 MHz and hole depths were as large as 30% of the peak fluorescence intensity.
Perturbative study of spectral line shapes involving line-mixing and collision-duration asymmetry
Authors: Wang W. F. , Sirota J. Marcos
Published: July 2002

A perturbative theory has been developed to model the spectral line shapes affected by line-mixing and finite duration-of-collision. The line profile was given in an explicit form tractable for dealing with the line interference and asymmetry problems. The coupling of line-mixing and collision-duration was studied. The simplified model was conditionally applied to the CO2 ν3 high-frequency wing beyond the bandhead, and was found to reproduce the experimental absorption coefficients well. The effective collision-duration parameters were obtained from the fit of the ν3 band edge and from the typical CO2–CO2 collision time of 1 ps. These parameters were then used to calculate the Q-branch core region of the CO2 ν2 band near 667 cm−1. The coupling effect from line-mixing and finite-duration was theoretically shown to be discernible at the pressure of 1 amagat. This effect grows with increased pressures and, expectedly, becomes significant for higher pressures.
Infrared multiphoton absorption and alignment of diatomic molecules in a continuous wave field
Authors: Van Leuven P. , Malvaldi M. , Persico M.
Published: July 2002

We compute multiphoton absorption spectra of HF and LiH in a continuous wave field, as generic examples of diatomic vib–rotors. Absorbed energy and other observables are determined by numerical integration of the time-dependent Schrödinger equation, taking into account the coupling of both permanent dipole and electronic polarizability with radiation. Three peculiar effects of strong laser fields are investigated: first, the frequency shift of the absorption peaks and its dependence on laser intensity, vibrational and rotational quantum numbers of the final state, and molecular parameters; second, the existence of subharmonic resonances, strongly influenced by the electronic polarizability; third, the considerable degree of alignment (with negligible orientation) along the laser polarization axis.
Inter- and intragrain transport measurements in YBa2Cu3O7−x deformation textured coated conductors
Authors: Feldmann D. M. , Larbalestier D. C. , Verebelyi D. T. , Zhang W. , Li Q. , Riley G. N. , Feenstra R. , Goyal A. , Lee D. F. , Paranthaman M. , Kroeger D. M. , Christen D. K.
Published: July 2002

Using photolithography, links for transport measurement have been placed across individual Ni grain boundaries and within individual Ni grains on several coated conductor samples. The typical Ni grain size is ∼50 μm, while the YBa2Cu3O7−x grains are submicron in size. It is found that the intragrain Jc(0 T,77 K) can exceed 5 MA/cm2, thus showing that present coated conductor Jc values are not significantly limited by the intragrain Jc. Inter- and intragrain Jc values ranged from one-half to more than four times full-width measured values, demonstrating that current percolates through the conductor. The misorientation angle dependence of Jc fits well with previous studies of [001] tilt SrTiO3 bicrystals.
Thermal stability of exchange coupling in permalloy/FeMn bilayers and its dependence on the antiferromagnetic layer thickness
Authors: Li H. Y. , Chen L. Y. , Zhou S. M.
Published: July 2002

For permalloy/FeMn bilayers, the exchange field and coercivity exponentially decay in time with the application of an applied field antiparallel to the unidirectional anisotropy direction. The relative change of the exchange field, 1−HE(t=∞)/HE(t=0), also decays in an exponential manner with the FeMn layer thickness. This dependence can be explained by using a thermal fluctuation theory. Moreover, calculations show that the thermal stability of the exchange coupling is independent of the permalloy layer thickness. These findings may be helpful to the design of the spin-valve giant magnetoresistance devices.
Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy
Authors: Kirilyuk V. , Hageman P. R. , Christianen P. C. M. , Larsen P. K. , Zielinski M.
Published: July 2002

The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition.
Direct observation of the coexistence of coherent and incoherent InAs self-assembled dots by x-ray scattering
Authors: Malachias A. , Magalhães-Paniago R. , Neves B. R. A. , Rodrigues W. N. , Moreira M. V. B. , Pfannes H.-D. , de Oliveira A. G. , Kycia S. , Metzger T. H.
Published: July 2002

In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth.
Feedback studies on resistive wall modes in the reversed field pinch
Authors: Paccagnella R. , Schnack D. D. , Chu M. S.
Published: July 2002

A single fluid nonlinear magnetohydrodynamic cylindrical model is considered in this paper in order to study the problem of feedback stabilization of the nonresonant m=1 ideal kinks in a reversed field pinch. The ideal growth rates are “passively” reduced by the presence of a resistive wall within the radius for perfectly conducting wall stabilization of these modes. In this work we consider cases with up to two resistive walls. Moreover the feedback system is assumed to be able to react to any given Fourier harmonic, with an “ideal response,” in the sense that no spurious harmonics are generated. Successful feedback schemes are shown to be possible. However, a careful choice of the gains, along with the simultaneous feedback on at least 4 or 5 modes, is found to be necessary.
Excitation mechanisms in dye-doped organic light-emitting devices
Authors: Pschenitzka F. , Sturm J. C.
Published: July 2002

In an organic light-emitting device with a polymeric matrix concurrently doped with two different dyes, the photoluminescence (PL) and electroluminescence (EL) spectra are observed to be very different, with both dyes emitting in PL and only one in EL at room temperature. A simple model based on charge trapping and thermal excitation is introduced to explain this observation. The EL spectral change of a device at 77 K is consistent with this model. In addition, using the same model, the strong dependence of the EL efficiency (but not PL) on the concentration of a single dye in an organic film can be understood. The materials used in this experiment are the polymer poly(9-vinylcarbazole) combined with electron transport molecules, and the dyes coumarin 47, coumarin 6, and Nile red.
Monte Carlo simulations of charged platelet systems
Authors: Léger Daniel , Levesque Dominique
Published: July 2002

Systems of thin platelets are studied by Monte Carlo simulations. The platelets interact by a hard core pair potential and Coulomb interactions associated with a distribution of electrical charges. According to the relative values of the charges localized in the central part and on the periphery of the platelets, it is shown that, at constant density, the system can evolve from an homogeneous fluid phase to a phase where the platelets are associated in clusters which percolate through the simulation cell. We discuss the possibility that this simple model can be a first approximation able to describe qualitatively the behavior of kaolin suspensions when the pH value of these solutions decreases from basic to acid.
Emission zone in organic light-emitting devices having a single layer of polyphenylenevinylene derivatives
Authors: Matsumura Michio , Manabe Kaoru
Published: July 2002

Light-emitting properties of devices fabricated from a copolymer of p-phenylenevinylene derivatives, 98% 3-[4′-(3,7-dimethyloctyloxy)phenylene]-p-phenylenevinylene and 2% 2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene (MEH–PV), were investigated. The emission efficiency of the devices showed strong oscillatory dependence on the film thickness, and the frequency agreed with that expected from a localized emission near the indium–tin–oxide (ITO) electrode. Although holes are injected from the ITO electrode into the copolymer layer, they are probably trapped by the MEH–PV unit of the copolymer. On the other hand, electrons are easily transported through the copolymer layer. These are probably the reasons for the confinement of the light-emitting zone near the ITO/copolymer interface. The confined light-emitting zone leads to high emission. For comparison, the properties of devices based on a homopolymer were examined. In this case, the emission zone was distributed over a wide region, and the light-emission efficiency was low.
Investigation of the thermal degradation mechanism for Cu/Au Schottky contacts to the InGaP layer
Authors: Liu Day-Shan , Lee Ching-Ting
Published: July 2002

The thermal degradation mechanism of Cu/Au contacts to the InGaP layer has been investigated using current–voltage measurement, x-ray diffractometry (XRD) measurement and cross-sectional transmission electron microscopy with energy dispersive x-ray spectroscopy (EDS). The diffraction peaks of Cu, Au, and InGaP could be observed from the XRD measurement and an evident interface of Cu/InGaP also could be found from the cross-sectional microstructure. The apparent interface of Cu/InGaP was responsible for the high performance of Cu/Au Schottky contacts to the InGaP layer. The thermal stability endurance for the high performance of the Schottky contact can be maintained up to 450 °C. However, the Schottky contact performance became inferior at temperatures higher than 450 °C. At a temperature of 500 °C, the peaks of Cu and Au disappeared, the XRD measurement revealing the diffraction peaks of CuP2 and Cu3Au2 alloys. According to the cross-sectional microstructure and the EDS measurements at 500 °C, the Cu layer had been released, penetrated into the InGaP layer, and outdiffused to the Au layer to form an intermetallic layer. As a result, the thermal degradation mechanism is attributed to the release of the Cu layer and the indiffusion of Cu element to the InGaP layer.
Qualitative and quantitative descriptions on the localized electronic structure in single-walled carbon nanotubes
Authors: Zhou Gang , Duan Wenhui , Gu Binglin , Kawazoe Yoshiyuki
Published: July 2002

The localized electronic structure of single-walled carbon nanotubes (SWNTs) is qualitatively and quantitatively described by use of the first-principles calculations, and the localized molecular orbitals are distilled and the corresponding work functions are calculated within the framework of linear combination of atomic orbitals for molecular orbital. We find that the localized molecular orbital and localized work function are sensitive to the morphology of carbon nanotubes as like local density of states. With the localized electronic structures (i.e., localized work function, localized molecular orbital, and local density of states) of carbon nanotubes with various morphologies, we cannot only easily clarify the field emission characteristics of SWNTs, but also successfully interpret why open-ended SWNTs have higher chemical reactivity than capped ones.
Probe diameter and probe–specimen distance dependence in the lateral resolution of a scanning Kelvin probe
Authors: McMurray H. N. , Williams G.
Published: July 2002

The ability of a scanning Kelvin probe (SKP) to resolve lateral differences in specimen surface potential is investigated by two routes. First, electrostatic calculations are used to obtain analytical expressions for the maximum lateral resolution attainable by a “point probe” of negligible physical dimensions, as a function of probe–specimen distance. Second, lateral resolution is measured experimentally by scanning plane-ended cylindrical probes of varying diameters at varying heights across a linear edge, delimiting two electrically continuous coplanar areas of dissimilar metal. Finally the two approaches are combined to obtain a semiempirical relationship between probe diameter, probe–specimen distance, and lateral resolution for plane-ended cylindrical probes. An expression is also developed for the minimum error to be expected in a surface potential measurement when this is associated with a specimen feature exhibiting a diameter comparable with the probe–specimen distance.
Optical amplification in nematics doped with carbon nanotubes
Authors: Lee Wei , Yeh Sheng-Long
Published: July 2002

We present results on the orientational photorefractive effect in a mesogenic nematic doped with multiwalled carbon nanotubes. Efficient coherent-beam amplification of a signal beam by energy exchange with a pump beam was observed in homogeneous cells of the doped liquid crystal E7 at electric fields of ∼0.5 V/μm. Photorefractive gains as high as 103 cm−1 are achieved in the Raman–Nath regime. It is found that the gain coefficient depends strongly on the externally applied dc voltage, the pump/probe intensity ratio and the total input intensity.
Exchange coupling mechanism of polycrystalline Co/CrMnPt films
Authors: Soeya Susumu
Published: July 2002

This article discusses the mechanism of enhancing unidirectional anisotropy Ke of exchange coupled Co/CrMnPt films by thermal annealing, the exchange coupling mechanism, and the physical origin of CrMnPt lattice distortion. The results obtained were: (i) positive evidence that Ke enhancement by thermal annealing is due to CrMnPt antiferromagnetic anisotropy KAF enhancement associated with CrMnPt lattice distortion, (ii) Ke appeared to be proportional to KAF, which seems to be consistent with the law that has been predicted based on the planar domain wall model’s mode coupling, and (iii) assuming that the planar domain wall model is suitable, the physical origin of CrMnPt lattice distortion could be attributed to exchange striction.