bluuebunny/crossref_metadata_2025 huggingface dataset:
Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v⩽5) detected with time-resolved Fourier transform spectroscopy
Authors: Lin Shiaw-Ruey , Lin Shih-Che , Lee Yu-Chang , Chou Yung-Ching , Chen I-Chia , Lee Yuan-Pern
Published: July 2002

Following photodissociation of vinyl fluoride (CH2CHF) and vinyl bromide (CH2CHBr) at 193 nm, fully resolved vibration–rotational emission spectra of HF and HBr in spectral regions 3050–4900 and 2000–2900 cm−1, respectively, are temporally resolved with a step–scan Fourier transform spectrometer. With a data acquisition window 0–5 μs suitable for spectra with satisfactory ratio of signal-to-noise, emission from HX (with X = F or Br) up to v=6 is observed. All vibrational levels show bimodal rotational distributions. For CH2CHF, these two components of HF have average rotational energies ∼2 and 23 kJ mol−1 and vibrational energies ∼83 and 78 kJ mol−1, respectively; the values are corrected for small quenching effects. For CH2CHBr, these two components of HBr correspond to average rotational energies ∼4 and 40 kJ mol−1, respectively, and similar vibrational energies ∼68 kJ mol−1. The separate statistical ensemble (SSE) model is suitable for three-center (α, α) elimination of HX because of the loose transition state and a small exit barrier for this channel; predicted vibrational energy distributions of HX are consistent with those observed for the high-J component. An impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts substantial rotational excitation for three-center elimination of HX but little rotational excitation for four-center (α, β) elimination; observed rotational energies of low-J and high-J components are consistent with those predicted for four-center and three-center elimination channels, respectively. The model also explains why observed rotational energy of HF produced via three-center elimination of CH2CHF is smaller than that of HCl from CH2CHCl. Ratios of rate coefficients (0.66:0.34 and 0.88:0.12) predicted for three-center or four-center elimination channels based on Rice–Ramsberger–Kassel–Marcus theory are consistent with estimated branching ratios ∼0.75:∼0.25 and ∼0.81:0.19 determined based on counting vibrational distribution of HF and HBr, respectively, to v⩽5 for high-J and low-J components and considering possible quenching effects within 5 μs. Hence we conclude that, similar to photolysis of CH2CHCl, observed high-J and low-J components correspond to HX (v,J) produced from three-center and four-center elimination channels, respectively. The results are compared with those from photolysis of vinyl chloride at 193 nm.
Electronic structure of pristine and sodium doped poly(p-pyridine)
Authors: Greczynski G. , Johansson N. , Lögdlund M. , Pettersson L. A. A. , Salaneck W. R. , Horsburgh L. E. , Monkman A. P. , dos Santos D. A. , Brédas J. L.
Published: July 2002

The electronic structure of pristine and sodium-doped poly(p-pyridine) has been studied using both ultraviolet and x-ray photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum-chemical calculations. Electronic band-structure calculations are performed for isolated chains with different connectivity patterns (head-to-tail and head-to-head), using the valence effective Hamiltonian (VEH) method, with geometries derived from optimizations using the Austin Model 1 Hamiltonian. The density-of-valence-states are derived directly from the VEH band structure. Excellent agreement is obtained between the theoretical simulations and the experimental data, which allows for a detailed assignment of the different peaks in the spectra. The C(1s) and N(1s) shake-up spectra of poly(p-pyridine) are analyzed on the basis of corresponding data for pyridine in the gas phase. Upon sodium doping of poly(p-pyridine), new states are observed within the otherwise forbidden energy gap. These new states can be assigned to the formation of bipolarons.
Effects of microwave power on the structural and emission properties of hydrogenated amorphous silicon carbide deposited by electron cyclotron resonance chemical vapor deposition
Authors: Cui J. , Rusli  , Yoon S. F. , Yu M. B. , Chew K. , Ahn J. , Zhang Q. , Teo E. J. , Osipowicz T. , Watt F.
Published: July 2002

Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films have been deposited using an electron cyclotron resonance chemical vapor deposition system. The effects of varying the microwave power from 100 to 1000 W on the deposition rate, optical band gap, film composition, and disorder were studied using various techniques such as Rutherford backscattering spectrometry, spectrophotometry, Fourier-transform infrared absorption, and Raman scattering. Samples deposited at 100 W are found to have a carbon fraction (x) of 0.49 which is close to that of stoichiometric SiC, whereas samples deposited at higher microwave powers are carbon rich with x which are nearly independent of the microwave power. The optical gaps of the films deposited at higher microwave powers were noted to be related to the strength of the C–Hn bond in the films. The photoluminescence (PL) peak emission energy and bandwidth of these films were investigated at different excitation energies (Eex) and correlated to their optical band gaps and Urbach tail widths. Using an Eex of 3.41 eV, the PL peak energy was found to range from 2.44 to 2.79 eV, with the lowest value corresponded to an intermediate microwave power of 600 W. At increasing optical gap, the PL peak energy was found to be blueshifted, accompanied by a narrowing of the bandwidth. Similar blueshift was also observed at increasing Eex, but in this case accompanied by a broadening of the bandwidth. These results can be explained using a PL model for amorphous semiconductors based on tail-to-tail states radiative recombination. A linear relation between the full width at half maximum of the PL spectra and the Urbach energy was also observed, suggesting the broadening of the band tail states as the main factor that contributes to the shape of the PL spectra observed.
The calculation of thermal rate constants for gas phase reactions: A semiclassical flux–flux autocorrelation function (SCFFAF) approach
Authors: Runge Keith , Cory Marshall G. , Bartlett Rodney J.
Published: July 2002

A semiclassical approach to the calculation of thermal rate constants, based on the flux–flux autocorrelation function method, is presented with its applications. The autocorrelation function is generated along classical trajectories using a classical interpretation of the Boltzmannized flux operator. The activation energies for considered reactions are calculated using the G2/MP2 procedure. The forces are generated using a new parametrization of the PM3 NDDO Hamiltonian optimized for accurate gradients. Thermal rate constants for hydrogen abstraction from ethane and haloethanes by hydroxyl radical serve as a first test of this approach. Calculated results are in good agreement with cumulative rate constants for all systems considered over a range of temperature including room temperature. The approach is able to distinguish between α and β abstraction with a result for fluoroethane at room temperature that is consistent with the available experiment and trends that are in line with those expected.
The isotope effect and orientational potentials of methane molecules in gas hydrates
Authors: Gutt Christian , Press Werner , Hüller Alfred , Tse John S. , Casalta Helene
Published: July 2002

We report an inelastic neutron scattering experiment at T=2 K of a fully deuterated methane hydrate CD4⋅5.75 D2O. In the experiment the J=0→J=1 rotational transition of an almost free quantum CD4 rotor at an energy transfer of 0.4 meV has been found. No line splitting due to the two different kinds of cages, which are occupied by the methane molecules in the host lattice, has been found. Using a simple electrostatic model of the orientational interaction between the methane molecules and the water molecules in the host lattice we are able to explain the measured spectra of both the deuterated and protonated samples. The intrinsic linewidth found for the rotational transition in deuterated and protonated samples can be attributed to the frozen in disorder of the water dipole moments in the cages. The contribution of the van der Waals interaction has also been calculated on the basis of empirical atom–atom interaction parameters and has been found to be one order of magnitude smaller than the electrostatic interaction. In addition the calculated orientational scattering length density of the CD4 molecules is found to be in good agreement with experimental values. It is shown, that previous diffraction experiments lead to only limited information about the orientational distribution function due to the proton disorder.
Molecular dynamics simulation of the liquid–vapor interface of dipolar fluids under different electrostatic boundary conditions
Authors: Mecke Matthias , Fischer Johann , Winkelmann Jochen
Published: July 2002

Molecular dynamics simulations are performed for a liquid film of strongly and moderate dipolar fluids in equilibrium with its vapor in both, three- and two-dimensional (2-D) periodic systems. In the three-dimensional periodic system, the long-range dipolar interactions are treated by the conventional Ewald summation, where the shape of the basic simulation cell is modified in order to study the influence of periodicity on the interfacial properties. In the two-dimensional periodic system, an alternative method is used that is based on dividing the 2-D-lattice sum into a lattice sum of the position vectors of the molecules and additional terms describing the dipole orientations independently of the positions. The results from both techniques agree very well in case that in the 3-D treatment the distance between the liquid films is sufficiently large. The phase equilibrium data obtained from the simulations are verified by a comparison with a recent equation of state. The surface tension results are compared to experimental values using the corresponding state principle. Moreover, the orientation of strongly dipolar elongated molecules and Stockmayer molecules at the interface is found to be parallel to the interface on the liquid as well as on the vapor side.
Deformations of the metaplectic representations
Authors: Lesimple Marc , Pinczon Georges
Published: July 2002

We describe all possible “natural” deformations of the metaplectic representation of the Lie superalgebras osp(1,2n) and of its natural extension and show in particular that in most cases for the metaplectic representation itself, it is rigid.
A harmonic adiabatic approximation to calculate highly excited vibrational levels of “floppy molecules”
Authors: Lauvergnat David , Nauts André , Justum Yves , Chapuisat Xavier
Published: July 2002

The harmonic adiabatic approximation (HADA), an efficient and accurate quantum method to calculate highly excited vibrational levels of molecular systems, is presented. It is well-suited to applications to “floppy molecules” with a rather large number of atoms (N>3). A clever choice of internal coordinates naturally suggests their separation into active, slow, or large amplitude coordinates q′, and inactive, fast, or small amplitude coordinates q″, which leads to an adiabatic (or Born–Oppenheimer-type) approximation (ADA), i.e., the total wave function is expressed as a product of active and inactive total wave functions. However, within the framework of the ADA, potential energy data concerning the inactive coordinates q″ are required. To reduce this need, a minimum energy domain (MED) is defined by minimizing the potential energy surface (PES) for each value of the active variables q′, and a quadratic or harmonic expansion of the PES, based on the MED, is used (MED harmonic potential). In other words, the overall picture is that of a harmonic valley about the MED. In the case of only one active variable, we have a minimum energy path (MEP) and a MEP harmonic potential. The combination of the MED harmonic potential and the adiabatic approximation (harmonic adiabatic approximation: HADA) greatly reduces the size of the numerical computations, so that rather large molecules can be studied. In the present article however, the HADA is applied to our benchmark molecule HCN/CNH, to test the validity of the method. Thus, the HADA vibrational energy levels are compared and are in excellent agreement with the ADA calculations (adiabatic approximation with the full PES) of Light and Bačić [J. Chem. Phys. 87, 4008 (1987)]. Furthermore, the exact harmonic results (exact calculations without the adiabatic approximation but with the MEP harmonic potential) are compared to the exact calculations (without any sort of approximation). In addition, we compare the densities of the bending motion during the HCN/CNH isomerization, computed with the HADA and the exact wave function.
Liquid-crystal behavior of hard ellipsoid dimers
Authors: Wu Guang-Wen , Sadus Richard J.
Published: July 2002

Liquid crystals exhibit orientation-dependent phases ranging from a disordered (isotropic) phase to a highly ordered crystalline phase. In between these extremes, increasing order can result in nematic and smectic phases. Typically, molecular simulation studies of liquid-crystal behavior use a nonspherical hard-body monomer. In this work, molecular simulation is used to study dimers of hard prolate ellipsoids. The results indicate that dimers of hard prolate ellipsoids exhibit a rich diversity of liquid-crystal behavior including smectic phases. In some cases, the dimer model may be a more realistic alternative to the conventional monomer model for liquid-crystal behavior.
Nonintegrability of Bianchi VIII Hamiltonian system
Authors: Maciejewski Andrzej J. , Strelcyn Jean-Marie , Szydłowski Marek
Published: July 2002

In this paper we study the Bianchi VIII cosmological dynamical system. Our aim is to show that this system is nonintegrable. To show this we use an extension of Ziglin theory made by Morales-Ruiz and Ramis.
Low-energy electron scattering by C2HF5
Authors: Bettega M. H. F. , Winstead C. , McKoy V.
Published: July 2002

We report elastic and electronically inelastic cross sections for low-energy electron scattering by pentafluoroethane, C2HF5. Our calculations were performed using the Schwinger multichannel method. For elastic scattering, we calculated integral, differential, and momentum transfer cross sections for energies from 5 to 50 eV. In the inelastic case, we obtained integral and differential cross sections for electron-impact excitation of the 1 1,3A′ and 2 1,3A′ excited states at the three-channel level of approximation. At higher energies, the elastic differential cross sections are quite similar to existing theoretical results for C2F6. Limited electronic-structure calculations were carried out to explore the dissociation behavior of the excited states.
⊥-order automorphisms of Hilbert space effect algebras: The two-dimensional case
Authors: Molnár Lajos , Páles Zsolt
Published: July 2002

It is well known that the ⊥-order automorphisms of the effect algebra of a Hilbert space of dimension not less than three are implemented by unitary or antiunitary operators. The aim of this paper is to show that the same assertion also holds true in the two-dimensional case.
A comparison of glass fluorescers used to measure a pulsed ultraviolet image at F/2
Authors: Deniz A. V. , Stamper J. A.
Published: July 2002

We built and characterized an instrument to measure the output profile of a pulsed KrF laser. The laser beam is focused onto a ∼1-mm-thick fluorescer, and the resulting fluorescence is imaged by a charge coupled device (CCD) camera. We tested 21 materials for efficiency, linearity, and fidelity. The best material is Schott BG-26 color filter glass. It has acceptable linearity for a fluence of 50 mJ/cm2, and a spatial resolution of ∼50 μm with a laser convergence angle of 30° (F/2), when the fluorescence is imaged with F/3 optics. We present a list of the materials tested and comparisons of profiles measured by using the fluorescer and by direct recording with a CCD camera.
Ab initio study of the vibronic spectrum in the X 2Π electronic state of HCCS
Authors: Perić M. , Marian C. M. , Peyerimhoff S. D.
Published: July 2002

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the X 2Π state are calculated by means of an extensive ab initio approach. Particular attention is paid to calculating accurate three-dimensional potential surfaces involving variations of two bending and torsional coordinates, which play the central role in vibronic interactions (Renner–Teller effect), determining the structure of spectra of this radical. In the second part of this paper we use these potential surfaces and the ab initio computed spin–orbit coupling constant to calculate vibronic spectra of HCCS and DCCS in the framework of a theoretical model developed in our laboratory. The results of the present study are in excellent agreement with those derived by Tang and Saito [J. Chem. Phys. 105, 8020 (1996)] and thus strongly support the interpretation of their experimental findings.
Acoustic resonance in tetramethoxysilane matrices: A new tool to characterize the gel formation
Authors: Senouci B. , Serfaty S. , Griesmar P. , Gindre M.
Published: July 2002

Several techniques have been used to approach the sol–gel transition. This article develops a new technique based on the evolution of a transmitted acoustic wave through the tetramethoxysilane (TMOS) bulk. The applied frequencies lie within the lowest part of the audible range. The sol–gel transition is assessed by the evolution of resonance peaks. The temporal evolution of the associated resonance frequencies is depicted through a number of curves, each one representing a harmonic mode. Moreover, the shape of these curves is very similar to that obtained in alcogels for the evolution of the complex shear modulus. They converge towards a unique intersection point with the time axis, which corresponds to the gelation time. An appropriate numerical treatment shows that the building process of the network in gel phase is not influenced by the silicon initial concentration and hydrolysis molar ratio range. However, above a certain silicon initial concentration, departures from the unique curve take place. These departures are assumed to be due to aggregations between precursor molecules.
Triple drift ion beam bunching systems for enhanced bunching efficiency
Authors: Sarma P. R. , Pandit V. S. , Bhandari R. K.
Published: July 2002

To obtain high efficiency in bunching ions one generally uses a multiharmonic buncher or a double drift buncher. Here we have explored the possiblity of using a triple drift bunching system to obtain still higher bunching efficiency. It consists of three bunchers separated in space. The first buncher is excited with the fundamental rf wave whereas the other two bunchers are driven by the second harmonic. We have optimized the parameters of such a system and have shown that by using only two rf frequencies one can obtain bunching efficiency higher than what can be obtained with a multiharmonic buncher consisting of nine harmonics.
An improved decoupled model for spin relaxation in 4-n-alkyloxy-4′-cyanobiphenyls
Authors: Dong Ronald Y.
Published: July 2002

An improved decoupled model has recently been proposed by us to account for the correlated internal rotations in flexible alkyl(oxy) chains of liquid crystals. In this model, the three-bond motions in the chain are disallowed, while both gauche migration and gauche pair production can now occur in the chain. Preliminary results from the analysis of deuteron spin relaxation rates in the liquid crystal 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) were encouraging. We apply this motional model to interpret the existing relaxation data of 4-n-octyloxy-4′-cyanobiphenyl and 6OCB. Transition rates for the chain dynamics are obtained for both liquid crystals in conjunction with their overall rotational diffusion constants. The derived rotational diffusion constants are compared with those obtained from dielectric relaxation studies. A consistent picture emerges in the present study for the two members of a homologous series when the order director fluctuations are included in the nematic phases of these liquid crystals.
Magneto-optical imaging of transient vortex states in superconductors
Authors: Giller D. , Kalisky B. , Shaulov A. , Tamegai T. , Yeshurun Y.
Published: July 2002

A high temporal resolution magneto-optical system is employed to follow the crystallization process of the quasiordered vortex state in Bi2Sr2CaCu2O8+δ crystals following a sudden change in the applied magnetic field. Two types of experiments are performed. In the first one the sample is suddenly exposed to a steady magnetic field smaller than the vortex order–disorder transition field, Bod. In the second type of experiment the sample is initially exposed to an external field larger than Bod, and then the field is suddenly reduced. The two types of experiments reveal growth of the quasiordered state proceeding in opposite directions: from the sample center toward its edge in the first experiment, and from the sample edge toward the center in the second experiment. This motion enables tracing of the time evolution of the thermodynamic quasiordered vortex phase in the early stages of its formation.
Thermal annealing study of spin valves
Authors: Wu Yihong , Han Guchang , Chong Towchong
Published: July 2002

The thermal stability of spin valves was studied through measuring the magnetoresistance response curves at different temperatures with the applied field set at different angles from the pinning field direction. According to a simple coherent rotation model, the magnetoresistance versus applied field curves should be symmetrical with respect to the zero field line when the applied field is perpendicular to the pinning field. The deviation from symmetry should be attributed to the canting of the pinning field direction. The deviation from symmetry should be attributed to the canting of the pinning field direction. Experiments have been carried out on spin valves with a structure of Si/SiO2/Ta(2 nm)/Ni80Fe20(2.5 nm)/CoFe(1 nm)/Cu(2 nm)/CoFe(2 nm)/IrMn(5 nm)/Ta(2 nm). The canting of the pinning field was seen clearly from the change of the magnetoresistance curves while the temperature was varied from 50 to 100 °C. A quantity (the relative change in the asymmetry of the response curve) was newly defined to estimate the temperature window for the onset of the canting process.
Muon spin relaxation study of spin dynamics in a polysaccharide iron complex
Authors: van Lierop J. , Ryan D. H. , Pumarol M. E. , Roseman M.
Published: July 2002

Polysaccharide iron complex, a ferritin analog, has been examined with zero-field muon spin-relaxation at temperatures from 2 to 40 K. Spectra exhibit a clear separation of static moments and collective excitations at low temperatures. At intermediate temperatures, superparamagnetic relaxation is observed and a blocking temperature of TB=10±2.5 K is measured, in agreement with transmission Mössbauer spectra and frequency dependent χac data. Superparamagnetic relaxation rates of 20–150 MHz are in agreement with those extrapolated from Mössbauer spectra using a multilevel magnetic relaxation model.
Application of artificial neural networks and genetic algorithms to modeling molecular electronic spectra in solution
Authors: Lilichenko Mark , Kelley Anne Myers
Published: July 2002

A novel approach is presented for finding the vibrational frequencies, Franck–Condon factors, and vibronic linewidths that best reproduce typical, poorly resolved electronic absorption (or fluorescence) spectra of molecules in condensed phases. While calculation of the theoretical spectrum from the molecular parameters is straightforward within the harmonic oscillator approximation for the vibrations, “inversion” of an experimental spectrum to deduce these parameters is not. Standard nonlinear least-squares fitting methods such as Levenberg–Marquardt are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here we employ a genetic algorithm to force a broad search through parameter space and couple it with the Levenberg–Marquardt method to speed convergence to each local minimum. In addition, a neural network trained on a large set of synthetic spectra is used to provide an initial guess for the fitting parameters and to narrow the range searched by the genetic algorithm. The combined algorithm provides excellent fits to a variety of single-mode absorption spectra with experimentally negligible errors in the parameters. It converges more rapidly than the genetic algorithm alone and more reliably than the Levenberg–Marquardt method alone, and is robust in the presence of spectral noise. Extensions to multimode systems, and/or to include other spectroscopic data such as resonance Raman intensities, are straightforward.
A scanning superconducting quantum interference device microscope with high spatial resolution for room temperature samples
Authors: Gruhl Friederike , Mück Michael , von Kreutzbruck Marc , Dechert Jörg
Published: July 2002

We have developed a scanning superconducting quantum interference device (SQUID) microscope based on niobium tunnel junction direct current (dc) SQUIDs. It employs either a SQUID magnetometer or a planar first order gradiometer with an effective area of 40×40 μm2 and a magnetic field resolution on the order of 1 pT/√Hz at frequencies above a few hertz. The gradiometer has a base line of 1 mm. The SQUIDs are mounted inside the insulation vacuum of a fiberglass helium Dewar, and are thermally coupled to the helium bath via a brass block. A sapphire window with a thickness of about 50 μm separates the SQUIDs from the room temperature sample. We have also investigated different window materials, such as tungsten or iridium, and found them equally useful. The stand-off distance between SQUID and room temperature sample can be as low as 75 μm, and the spatial resolution of the microscope is about 50–100 μm. The SQUID sensor is read out using conventional dc SQUID electronics with a bandwidth of 1 kHz and a slew rate of 200 mT/s. A computer controlled x–y stage scans the sample below the microscope. With this microscope, we have investigated magnetic signatures of magnetic storage media, ferromagnetic and paramagnetic inclusions in geological and biological samples as well as platinum coins, and have also performed nondestructive testing of stainless steel plates and aircraft parts with high spatial resolution.
Theory and simulation of electronic relativistic parametric instabilities for ultraintense laser pulses propagating in hot plasmas
Authors: Héron A. , Adam J. C. , Laval G. , Mora P.
Published: July 2002

The dispersion relation for electronic parametric instabilities of a circularly polarized laser wave is solved in the case where the distribution function is supposed to be cold in the transverse direction and to be a linear combination of a cold distribution function and of a Maxwellian in momentum in the longitudinal direction. Only densities below the critical density are considered. It is shown that the longitudinal temperature as expected reduces the growth rate, but that the existence of a hot tail is not sufficient to significantly reduce the instability. It is the bulk of the distribution function that must be heated to efficiently stabilize the system. Another important effect of the heating is to reduce the backscattered component of the instability. An example of a one-dimensional particle simulation performed in the exact conditions of validity of the theory is discussed.
Magnetic properties and interactions of single-domain nanomagnets in a periodic array
Authors: Abraham M. C. , Schmidt H. , Savas T. A. , Smith Henry I. , Ross C. A. , Ram R. J.
Published: July 2002

Macroscopic and microscopic switching characteristics are obtained for 100 nm period arrays of Ni nanomagnets with a mean switching field (coercive field) of 710 Oe. Magnetic force microscopy in combination with micromagnetic theory shows that inhomogeneities in the particle shapes result in an intrinsic standard deviation in switching fields of 105 Oe, while the interactions between neighboring nanomagnets broaden the distribution to 276 Oe, equivalent to a squareness of 0.8 in the bulk hysteresis loop. The switching field distribution is consistent with curling as the switching mechanism.
Three-dimensional simulation of nanocrystal Flash memories
Authors: Iannaccone G. , Coli P.
Published: July 2002

We have developed a code for the detailed simulation of nanocrystal Flash memories, which consist of metal–oxide–semiconductor field-effect transistors (MOSFETs) with an array of semiconductor nanocrystals embedded in the gate dielectric. Information is encoded in the MOSFET threshold voltage, which depends on the amount of charge stored in the nanocrystal layer. Nanocrystals are charged through direct tunneling of electrons from the channel. Such memories are promising in terms of shorter write–erase times, larger cyclability, and lower power consumption with respect to conventional nonvolatile memories. We show results obtained from the self-consistent solution of the Poisson–Schrödinger equation on a three-dimensional grid, focusing on the charging process and on the effect of charge stored in the nanocrystals on the threshold voltage.
An effective Hamiltonian for correlations in interfaces
Authors: Stecki J.
Published: July 2002

The effective Hamiltonian Hcap for a liquid–vapor interface is constructed by using the “experimental” direct correlation function obtained from molecular-dynamics simulation. The density fluctuation is parametrized according to: (a) The standard shift of the density profile, parallel and small; (b) the parallel shift; and (c) the shift with rotation as recently introduced, used in a density functional theory and compared with experiment. We find a universal shape of Hcap, discussed in some detail. The shape strongly suggests the existence of two regions of low wave-vector q and of high q corresponding to microscopic distances. Although bending and nonzero curvature of the fluctuating interface are present, the apparent bending (rigidity) coefficient is always negative, thus supporting the contention that the membrane Hamiltonians are not applicable to normal liquid–vapor interfaces. Several definitions of the momentum-dependent surface tension are also discussed.
Fabrication of wirelike InAs quantum dots on 2°-off GaAs (100) substrates by changing the thickness of the InAs layer
Authors: Kim Hyo Jin , Park Young Ju , Park Young Min , Kim Eun Kyu , Kim Tae Whan
Published: July 2002

Wirelike InAs quatum dots (QDs) grown on 2°-off (100) GaAs substrates by changing the thickness of the InAs layer were successfully fabricated. The sizes of the InAs QDs along the step lines increased with increasing the thickness of the InAs layer, and their increases were attributed to transform of the InAs QDs into the wirelike InAs QDs. The optimal thicknesses of the InAs layers for the wirelike QDs and the interval of the wirelike QDs were significantly affected by the terrace width resulting from the bunching effect due to the thickness variations of the GaAs buffer layers grown on 2°-off (100) GaAs substrates. These results indicate that these wirelike InAs QDs are useful for applications in nanoelectronic devices, such as wrap gate single electron transistors.
Effects of external electric field upon the photonic band structure in synthetic opal infiltrated with liquid crystal
Authors: Meng Q.-B. , Fu C.-H. , Hayami S. , Gu Z.-Z. , Sato O. , Fujishima A.
Published: July 2002

Based on the electrically controlled birefringence effects of a nematic liquid crystal, a tunable photonic band gap crystal has been fabricated by infiltrating nematic liquid crystal into the voids of synthetic opal composed of silica spheres. A reversible shift of the photonic band gap position and a change of transmittance through the composite opal can be induced by applying an external electric field.
Structural studies of Cu–III–VI2 chalcopyrite semiconductor heteroepitaxial films grown by low-pressure metalorganic vapor phase epitaxy
Authors: Harada Yoshiyuki , Nakanishi Hisayuki , Chichibu Shigefusa F.
Published: July 2002

Growth orientation, domain structures, and residual strain of Cu(Al,Ga)(S,Se)2 epitaxial films grown by low-pressure metalorganic vapor phase epitaxy on various substrates were systematically investigated. All epilayers grew in such a manner that the lattice mismatch between the epilayer and the substrate became minimum. Domain structure of the epilayer changed depending on the type of substrate. Most residual strain in the epilayer was assigned as being due to the pseudomorphic stress for the epilayers having small lattice mismatch (<1%) against the substrate and to the thermal stress for large lattice mismatch (>1%) ones.
Anomalous low-voltage tunneling current characteristics of ultrathin gate oxide (∼2 nm) after high-field stress
Authors: Huang Chia-Hong , Hwu Jenn-Gwo
Published: July 2002

The effect of oxide barrier shape change caused by stress-induced interface trap charges on the low-voltage tunneling current (LVTC) characteristics of ultrathin gate oxide (∼2 nm) is studied in this work. It was found that for an ultrathin gate oxide working in the direct tunneling regime, the LVTC behavior is strongly dependent on the barrier shape of the oxide. After high-field stress, anomalous LVTC phenomenon is observed. There is an invariant point existing in current–voltage curves. For a bias smaller than the value of the invariant point, the gate current decreases with stress time. However, for a bias larger than that, the gate current increases with stress time and then saturates. This phenomenon cannot be explained by conventional trap-assisted tunneling conduction, but by the change of tunneling probability due to barrier shape variation. An interface trap charge model is proposed to explain the observed invariant point mentioned above. From this, one can find the voltage corresponding to the midgap bias and, therefore, the initial effective oxide charge number density.
Infrared-induced nonlinear optical effects in chalcogenide glasses
Authors: Kityk I. V. , Sahraoui B.
Published: July 2002

The second-order nonlinear optical effects such as second harmonic generation (SHG) and linear electrooptics (LEO) in the middle infrared spectral range (5–15 μm) are experimentally observed for the first time. We have found that the novel As2Te3–BaBr2–BiCl2 chalcogenide glasses, possessing transmission windows within the 3–45 μm spectral range, have both photoinduced SHG equal to 0.0012 pm/V and comparable values of photoinduced electrooptics effect at wavelength of excitation (λ=10.6 μm). The photoinduced anharmonic electron–phonon interaction play the major role in the observed phenomena. A good correlation between the IR-induced nonlinear optical susceptibilities and the photoinduced anharmonic electron-quasi-phonon interactions is demonstrated. The pump–probe experiment shows that the intensity-dependent maximum of the LEO is shifted towards the higher delaying times comparing with the SHG ones. Comparison of theoretical simulations with experimental measurements indicate dominant role of the electron–phonon anharmonic subsystem in the observed effects.
Thermal activation of magnetization reversal in spin-valve systems
Authors: Hughes T. , Laidler H. , O’Grady K.
Published: July 2002

Magnetization reversal in the pinned layer of exchange biased spin valves is a complex process due to the exchange interaction between the ferromagnetic layer and the antiferromagnetic layer. This interaction results in progressive reversal of the antiferromagnetic layer as the magnetization of the ferromagnetic layer changes direction. This reversal of the antiferromagnet will effect the subsequent reversal of the ferromagnet. It is known that this process is thermally activated but time dependence measurements are difficult to interpret, as the exchange field is nonconstant at many positions along the hysteresis curve. Measurements have been made of the time dependence of the reversal of the antiferromagnetic layer by measuring the recoil loops, following different times spent with the ferromagnetic layer saturated in the negative direction. In this manner, the exchange field can be assumed to be constant during the reversal of the antiferromagnet. These measurements show a shift of the loop of the pinned layer towards positive fields. This shift in the loop is interpreted as being the result of reordering of the antiferromagnet. Increasing the temperature during the time spent at saturation shows that the process is driven by thermal activation. Close examination of the degree of loop shift with time spent at saturation shows behavior consistent with thermal activation governed by a distribution of activation energies. At longer times and elevated temperatures, the behavior of the antiferromagnet reversal suggests that this distribution is complex and may be multimodal. The reversal process is, however, reversible even at high temperatures indicating that the elevated temperatures do not significantly change the structure of the ferromagnetic–antiferromagnetic layers or the interface between them. Finally, measurements at 77 K show that the active portion of the energy barrier distribution will change significantly at low temperatures.
Low current electroplastic effect in ferromagnetic materials
Authors: Parkansky N. , Molotskii M. , Alterkop B. , Goldsmith S. , Boxman R. L. , Rosenberg Y. , Barkay Z.
Published: July 2002

The effect of an electric current on the indentation of nickel and copper samples was studied. Rectangular bars of nickel (99.7% pure) or copper (99.9% pure) with dimensions of 3×5×70 mm were polished and mounted on a sample holder which provided electrical contacts at the sample ends. A dc current I in the range of 0–15 A was injected parallel to the bar axis during Vickers indentation tests with loads of 0.25, 0.5 and 1 N. The tested surfaces were examined by scanning electron microscopy, and by x-ray diffraction. The nickel microhardness increased as a function of the current up to 10% for 1⩽3–6 A, depending on the indenter loading, and then decreased monotonically by as much as 20–25%. The microhardness of the copper samples varied by less than 5% with I, except at a load of 0.25 N, where it increased up to 17% for I⩽3 A, then decreased for 3<I⩽9 A, and then was approximately constant with further increasing of I. A theoretical model is proposed in which the decrease of the Ni hardness is explained by the interaction between the current-induced magnetic field and magnetic domain walls.
Rotational analysis of the origin and the inversion bands of the S1←S spectrum of acetaldehyde
Authors: Jalviste Erko , Berden Giel , Drabbels Marcel , Wodtke Alec M.
Published: July 2002

Fully rotationally resolved spectra of the two lowest-frequency bands (the origin (000) and the inversion (1401) band) of the S1(nπ*)←S0 transition of jet-cooled (Trot≈6 K) acetaldehyde, CH3CHO, have been recorded with a resolution of ≈0.01 cm−1 using a pulsed dye amplified continuous wave (cw) laser. In modeling the spectra a nonperturbative solution of the rotational–torsional coupling problem was used, but the torsion–inversion and the rotation–inversion couplings were neglected. All the lines of the origin band were reproduced with a model using the same rotational–torsional Hamiltonian for the ground- and excited electronic state. The inversion band could not be described with this model, since the ordering of the torsional levels in the excited state is reversed. The measured spectrum was reproduced by using a rigid asymmetric rotor Hamiltonian for the two torsional levels in the excited state. Some rotational levels of 00 and 141 states were found to be shifted from their predicted energy values. These shifts were explained by an accidental resonance between the excited singlet level and some higher rovibronic triplet level. The relative intensities of the ab-type and c-type torsional subbands and the Herzberg–Teller–induced transition dipole moment direction, characterizing the ab-type subband, were determined by an intensity fit.
Intrinsic exchange biasing in MnAs epilayers grown on (001) GaAs
Authors: Chun S. H. , Potashnik S. J. , Ku K. C. , Berry J. J. , Schiffer P. , Samarth N.
Published: July 2002

We have studied the magnetic properties of MnAs epilayers grown in two preferred orientations on (001) GaAs by molecular-beam epitaxy. Samples with the (1̄100) MnAs parallel to (001) GaAs (“type A”) show asymmetric hysteresis loops and an anomalous temperature dependence of the coercive field. We attribute these anomalies to intrinsic exchange biasing effects arising from the presence of a strain-induced antiferromagnetic β-MnAs phase that coexists with the ferromagnetic α-MnAs phase. These unusual effects vanish with decreasing sample thickness, suggesting a strategy for suppressing the antiferromagnetic β-MnAs phase.
Molecular lens applied to benzene and carbon disulfide molecular beams
Authors: Chung Hoi Sung , Zhao Bum Suk , Lee Sung Hyup , Hwang Sungu , Cho Keunchang , Shim Sang-Hee , Lim Soon-Mi , Kang Wee Kyung , Chung Doo Soo
Published: July 2002

A molecular lens of the nonresonant dipole force formed by focusing a nanosecond IR laser pulse has been applied to benzene and CS2 molecular beams. Using the velocity map imaging technique for molecular ray tracing, characteristic molecular lens parameters including the focal length (f ), minimum beam width (W), and distance to the minimum beam width position (D) were determined. The laser intensity dependence of the observed lens parameters was in good agreement with theoretical predictions. W was independent of the laser peak intensity (I0), whereas f and D varied linearly with 1/I0. The differences in lens parameters between the molecular species were well correlated with the polarizability per mass values of the molecules. A high chromatographic resolution of Rs=0.84 was achieved between the images of benzene molecular beams undeflected and deflected by the lens. The possibilities for a new type of chromatography are discussed.
Dynamic characteristics of dislocations in highly boron-doped silicon
Authors: Yonenaga I. , Taishi T. , Huang X. , Hoshikawa K.
Published: July 2002

The dynamic behavior of dislocations in highly boron (B)-doped Si crystals with concentration up to 2.5×1020 cm−3 is investigated using the etch pit technique. Suppression of the generation of dislocations from a surface scratch is found for B-doped Si and the critical stress for dislocation generation increases with B concentration, which is interpreted in terms of dislocation locking due to impurity segregation. The velocity of dislocations in B-doped crystals is revealed to increase by increasing the B concentration.
Conservative antiphase boundary in SrTiO3 films on LaAlO3 substrates with SrRuO3 buffer layers
Authors: Wu J. S. , Jia C. L. , Urban K. , Hao J. H. , Xi X. X.
Published: July 2002

We have studied the microstructure of SrTiO3 films on LaAlO3 substrates with the SrRuO3 buffer layer using high-resolution transmission electron microscopy. While high density of defects due to lattice mismatch were found at the SrRuO3/LaAlO3 interface, no misfit dislocation was observed at the SrTiO3/SrRuO3 interface. The {111} stacking fault in the SrRuO3 buffer layer propagates into the SrTiO3 film, giving rise to a type of antiphase boundary on the {110} plane with a crystallographic shear vector of a/2〈001〉. The boundary is a conservative one which does not lead to any charge defects. A model based on dislocation interactions is proposed to explain the generation mechanism of the antiphase boundary.
Density functional theory of the kinetics of crystallization of a hard sphere suspension: Coupling structure to density
Authors: Wild Robert , Harrowell Peter
Published: July 2002

A theoretical study is presented on the kinetics of crystallization of a colloidal suspension in a fixed volume based upon the use of time dependent density functional theory incorporating conserved particle and nonconserved structure dynamics. This is a continuation of previous work done with conserved particle dynamics alone. The constraints of fixed number and volume lead to nonuniform solutions to the time independent equations of motion. One of the nonuniform solutions is found to have the minimum free energy and is identified as the stable equilibrium coexistence of crystalline and disordered suspension. Numerical integration is used to follow the time dependent motion of a range of initial crystallites. A broadband of stationary states, additional to those identified analytically, are located by the numerical integration. We show that these solutions arise from pinning induced by the discretization of space. The normal and tangential osmotic pressure fields are given and the growing crystallite is shown to be isolated from the higher pressure of the surrounding disordered suspension by the nonequilibrium depletion zone that surrounds it. These results are compared with recent light scattering studies.
Dynamic coercivity measurements of antiferromagnetically coupled magnetic media layers
Authors: Lohau J. , Moser A. , Margulies D. T. , Fullerton E. E. , Schabes M. E.
Published: July 2002

We have performed dynamic coercivity measurements on a series of antiferromagnetically coupled (AFC) magnetic recording media utilizing a static write/read tester. The samples consist of two magnetic layers, which are antiferromagnetically coupled by a nonmagnetic layer. The investigated samples have a fixed top layer thickness and variable bottom layer thickness, such that the composite remanent magnetization thickness product (MRtAFC) varies over a large range, 0.17–0.30 memu/cm2. We find that the ratio between anisotropy energy and thermal energy (≡ stability ratio, C−1), and the intrinsic switching field H0 are, within the experimental error, constant for the series. This suggests that the top magnetic layer to first order determines the stability ratio of the AFC media and that MRtAFC can be varied over a large range without decreasing the stability or increasing the write field requirements.
Nonlinear direct and indirect third harmonic generation in piezoelectric resonators by the intermodulation method
Authors: Albareda Alfons , Kayombo Jean Hubert , Gorri José A.
Published: July 2002

Nonlinear characterization in power piezoelectric devices requires a knowledge of what mechanisms are involved in the nonlinearity. For resonant devices, it is considered that the origin of these nonlinearities is mechanical, and their effects are: the increase of losses, the shift of the resonant frequency, and the frequency hysteresis. These effects, which can be understood by the analysis of the impedance increase near the resonance, can arise from third harmonic generation (second overtone) due to the nonlinearities. This third harmonic can be generated by the cubic terms of the nonlinear constitutive equations (direct contribution), or by the quadratic terms when they perform twice consecutively (indirect contribution). The present work presents a quantitative evaluation of these two effects, using an intermodulation method that allows us to know every harmonic generated by the nonlinearity, whether quadratic or cubic. The conclusion of this work is that for the studied piezoelectric ceramics, with low and high losses, the indirect effect is negligible compared with the direct contribution, being about 1000 times lower.