bluuebunny/crossref_metadata_2025 huggingface dataset:
Near-field photoluminescence study of GaNAs alloy epilayer at room and cryogenic temperature
Authors: Matsuda K. , Saiki T. , Takahashi M. , Moto A. , Takagishi S.
Published: July 2002

We have measured the spatial distribution of the optical properties of a GaNAs (N∼0.8%) epilayer to investigate the carrier recombination mechanism at both room temperature and cryogenic temperature using a near-field scanning optical microscope. A difference between the macro and near-field photoluminescence (PL) spectra at room temperature was not observed. At low temperature, we found spatial inhomogeneity of the optical properties and sharp features in the near-field PL spectrum. These findings indicate that the dominant emission mechanism changes from recombination of delocalized carriers at room temperature to recombination of localized carriers (excitons) trapped in the local potential minimum due to compositional fluctuation at low temperature.
Collisionless magnetic reconnection in a toroidal cusp
Authors: Egedal J. , Fasoli A. , Tarkowski D. , Scarabosio A.
Published: July 2002

Fast collisionless magnetic reconnection is driven and observed in a toroidal magnetic cusp. For low values of the toroidal (guide) magnetic field, reconnection occurs without the formation of a macroscopic current channel. In the absence of a current channel, the measured plasma potential and poloidal flows are consistent with predictions based upon single particle theory. Only for large values of the toroidal magnetic field compared to cusp poloidal field is a current channel present. Also in this case single particle orbit theory provides a good description of the anomalous resistivity observed experimentally.
Micromagnetic simulation of tunneling magnetoresistance junctions with parallel hard bias
Authors: Gibbons M. R. , Sin K. , Funada S. , Mao M. , Rao D. , Chien C. , Tong H. C.
Published: July 2002

Tunneling magnetoresistance (TMR) films and devices were simulated to understand the response of the free layer with a parallel hard bias. In order to determine the effect of the granular hard bias material micromagnetic simulation was used to model both the hard bias and TMR material. Minimizing hysteresis and Barkhausen jumps in the response of the device involves an optimization of the spacing between the free layer and the hard bias coupled with the shape of the device edges.
Oxygen recoil implant from SiO2 layers into single-crystalline silicon
Authors: Wang G. , Chen Y. , Li D. , Oak S. , Srivastav G. , Banerjee S. , Tasch A. , Merrill P. , Bleiler R.
Published: July 2002

It is important to understand the distribution of recoil-implanted atoms and the impact on device performance when ion implantation is performed at a high dose through surface materials into single crystalline silicon. For example, in ultralarge scale integration impurity ions are often implanted through a thin layer of screen oxide and some of the oxygen atoms are inevitably recoil implanted into single-crystalline silicon. Theoretical and experimental studies have been performed to investigate this phenomenon. We have modified the Monte Carlo ion implant simulator, UT-Marlowe (B. Obradovic, G. Wang, Y. Chen, D. Li, C. Snell, and A. F. Tasch, UT-MARLOWE Manual, 1999), which is based on the binary collision approximation, to follow the full cascade and to dynamically modify the stoichiometry of the Si layer as oxygen atoms are knocked into it. CPU reduction techniques are used to relieve the demand on computational power when such a full cascade simulation is involved. Secondary ion mass spectrometry (SIMS) profiles of oxygen have been carefully obtained for high dose As and BF2 implants at different energies through oxide layers of various thicknesses, and the simulated oxygen profiles are found to agree very well with the SIMS data.
Giant magnetoresistance enhancement in spin valves with nano-oxide layers
Authors: Lai Chih-Huang , Chen C. J. , Chin T. S.
Published: July 2002

The magnetoresistance (MR) ratio is enhanced by 35% by inserting the nano-oxide layer (NOL) at the Ta/Co interface in the FeMn-based top spin valves (Ta/NOL/Co/Cu/Co/FeMn/Ta). The enhancement is attributed to specular reflection, resulting in a large resistance change and small sheet resistance. However, the formation of NOL at the interface of Ta/Co suppresses the (111) texture, resulting in small exchange fields. Top spin valves with NOLs show good thermal stability up to 200 °C annealing. The MR ratio is further increased after annealing at temperatures below 200 °C. Enhancement of the MR ratio by 61% can be achieved by annealing at 150 °C. For bottom spin valves (Ta/NiFe/FeMn/Co/Cu/NiFe/Ta), NOLs formed at FeMn/Co and NiFe/Ta interfaces increase MR ratios, but NOLs at Co/Cu or Cu/NiFe deteriorate the differential spin scattering and significantly reduce MR ratios.
Temperature dependent specular scattering enhancement in Cu-wedge spin valves
Authors: Uhlig W. C. , Mao M. , Yiu V. , Li J. , Shi Jing
Published: July 2002

Magneto-transport properties have been systematically studied in Cu-wedge regular and specular spin valves prepared by magnetron sputtering. In the specular spin valve, a thin oxide layer is inserted between the Ta underlayer and the free layer to provide a specular interface. Up to 50% enhancement in giant magnetoresistance (GMR) is realized by the specular interface at room temperature. The observed Cu layer thickness dependence of both the resistivity and GMR in both types of the spin valves has been satisfactorily modeled using a semiclassical Boltzmann approach. Temperature dependence of the GMR has been measured from 325 to 15 K for several Cu-layer thicknesses. The specular scattering enhancement initially increases as the temperature is lowered from 325 K, but rapidly decreases below 100 K. This decrease is correlated with the reduced degree of the full antiparallel alignment between the magnetic moments of the pinned and free layers in the specular spin valve, which is caused by the antiferromagnetic ordering of the oxide layer.
Exchange bias phenomena in ferrimagnetic based bilayers
Authors: Canet F. , Mangin S. , Bellouard C. , Piécuch M. , Schuhl A.
Published: July 2002

We present positive and negative exchange bias (EB) phenomena observed in two ferrimagnetic-based amorphous bilayers systems: FeSn/FeGd and FeGd/TbFe. In both cases, magnetization measurements exhibit a shift of the loop of one of the ferrimagnetic layer towards a positive or negative field. In FeSn/FeGd, the coupling at the interface between the net magnetization of the two layers is antiferromagnetic, which leads to a positive shift. However in FeGd/TbFe bilayers, the coupling is ferromagnetic and the shift is negative. The EB phenomenon is attributed to the occurrence of a magnetic domain wall present at the interface, which can be “compressed” or “decompressed” by an external applied field. A simple model based on the competition between the Zeeman energy and the interface exchange interaction energy permits a good quantitative evaluation of the observed EB fields.
Magnetic and magneto-optical properties of Pd/Cr/Co multilayers
Authors: Tang Y. J. , Wong C. Y. , Zhou X.
Published: July 2002

Recent studies found that the (Pt/Co/Pt) trilayers can be used as a unit in combination with nonmagnetic or magnetic layer, X (X=Pd, Ag, Cu, and Ni), to enhance the perpendicular magnetic anisotropy of the films, reduce the Curie temperature, and alter the magneto-optical properties. The effects of intercalating Cr into Pd/Co multilayers on the magnetic and magneto-optical properties are studied in this article. The perpendicular magnetic anisotropy Ku and the coercivity Hc of the system decrease rapidly with increasing the Cr thickness (XCr) up to 0.4 nm, and change slightly when Cr thickness further increases. The dependence of the coercivity Hc on the Cr thickness, which obeys the law: Hc (XCr)=XCr−2.66, indicates that the magnetization reversal is controlled by domain wall moving, mainly due to the interface roughness. Large decrease of the Kerr rotation θk of the Pd/Cr/Co multilayers compared with pure Pd/Co multilayers is also found in the wavelength ranging from 200 to 800 nm. As it is well known, the large anisotropy and Kerr rotation in Pd/Co system are mainly caused by the polarization of Pd atoms due to nearby Co atoms. As the intercalating of Cr layer between Pd and Co layer, the average polarization of Pd atoms will be reduced largely. As a matter of fact, the Cr atoms can also be polarized by nearby Co atoms, which, however, seems to take a minor effect on the anisotropy and Kerr rotation of the system.
Charge correlations in the magnetoresistive oxide La0.7Ca0.3MnO3 (invited)
Authors: Lynn J. W. , Adams C. P. , Mukovskii Y. M. , Arsenov A. A. , Shulyatev D. A.
Published: July 2002

Neutron scattering has been used to study the nature of the spin dynamics and charge correlations in a single crystal of the colossal magnetoresistive perovskite La0.7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Magnetic fields are found to suppress this polaron formation. The temperature dependence of the polaron correlations follows the same behavior as both the resistivity and the anomalous quasielastic component in the magnetic fluctuation spectrum, indicating that they have a common origin.
Observation of reversed hysteresis loops and negative coercivity in granular GaAs–Fe hybrid structures
Authors: Fumagalli P. , Sommer G. , Lippitz H. , Haneda S. , Munekata H.
Published: July 2002

We have studied GaAs–Fe granular magnetic-semiconductor hybrid structures by magneto-optic spectroscopy in a photon-energy range from 0.7 to 5 eV at temperatures from 7 to 300 K in magnetic fields up to 1.8 T. The structures have been grown by alternative molecular-beam deposition of GaAs and Fe. At the chosen substrate temperature of Ts=580 °C the Fe precipitates into clusters of nanometer to submicron size which are in part ferromagnetically ordered at 300 K. The polar Kerr spectra at 300 and 7 K show for the hybrid structures with high Fe content a broad negative peak with a Kerr rotation of −0.2° at a photon energy of 0.9 eV. The spectral dependence is very similar to pure Fe. Looking at the polar Kerr hysteresis loops at 300 K, a striking behavior is found. The hysteresis loops are reversed, i.e., the magneto-optic hysteresis curves decrease faster when decreasing the fields than the situation is when closing the hysteresis loop on the way back. This peculiar behavior is discussed in the context of local exchange-bias like coupling between ferromagnetic Fe clusters and an antiferromagnetic FeAs surrounding. An alternative model of two superimposed Kerr hysteresis loops is proposed as well.
Micromagnetic simulation in two antiferromagnetically coupled ferromagnetic layers separated by a spacer
Authors: Wang Y. J. , Wang J. P. , Hee C. H. , Chong T. C. , Ng Vivian
Published: July 2002

On the basis of a micromagnetic model, magnetization reversal in two antiferromagnetically coupled ferromagnetic layers (10 nm t1/0.9 nmM/t2 nm films with t2=2, 3, 4, 5 nm, here t1 and t2 are the top and bottom layers, respectively, and M the spacer) with the same random anisotropy arrangement can be represented by a computer simulation. The calculation indicates that the appearance of a full antiferromagnetic coupling at remanence requires the antiferromagnetic coupling constant j=−3.5 erg/cm2 for the 10 nm t1/0.9 nmM/3 nm t2, films and it needs a large j value if t2 increases for the case of Ku2=Ku1=1×106 erg/cm3 and Ms1=Ms2=400 emu/cm3. Hc follows the equation Hc=−0.755j/Mst1 (j<0). Why the calculated Hc values deviate from Hcmax=−j/Mst1 is discussed.
Control of tearing modes in toroidal fusion experiments using “designer” error fields
Authors: Fitzpatrick Richard , Rossi Enrico
Published: July 2002

It is demonstrated that a magnetic island chain formed by a saturated tearing instability in a toroidal magnetic fusion device can lock to a special class of externally generated magnetic perturbation in a stabilizing phase. The theoretical apparatus needed to design such perturbations is outlined. These special perturbations—which are termed “designer” error fields—could be used to control the amplitudes of tearing modes in toroidal magnetic fusion experiments without the requirement of fast phase modulation. This type of control would be far more feasible in a reactor environment than conventional active feedback control via external magnetic perturbations.
Evidence for the onset of three-body decay in photodissociation of vibrationally excited CHFCl2
Authors: Chen Xiangling , Marom Ran , Rosenwaks Salman , Bar Ilana , Einfeld Tina , Maul Christof , Gericke Karl-Heinz
Published: July 2002

Excitation of C–H stretch overtones of CHFCl2 followed by ∼235 nm photodissociation was applied to investigate the effect of internal parent excitation on the dynamics of two- and three-body photofragmentation. The ∼235 nm photons also tagged ground Cl 2P3/2 [Cl] and spin–orbit excited Cl 2P1/2 [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer, and monitored their time-of-arrival profiles. These profiles revealed the product velocities and angular distributions of Cl35 and Cl*35 and suggest the contribution of three-body decay in photodissociation of CHFCl2 pre-excited with five quanta of C–H stretch. This is the first evidence for three-body decay in photodissociation of vibrationally excited molecules.
Turbulence and beam size effects on reflectometry measurements
Authors: Estrada T. , Sánchez J. , Zhuravlev V. , de la Luna E. , Brañas B.
Published: July 2002

A two-dimensional code based on the WKB (Wenzel–Kramers–Brillouin) approximation is used to simulate reflectometry measurements in plasmas with turbulence. In this work we aim to understand, first the role of turbulence in the determination of density profiles with reflectometry and, second, the ability of reflectometry techniques to give reliable information on the characteristics of the turbulence itself. The effects on the profile determination of a rotating turbulence structure with nonperpendicular reflection are analyzed. The influence of the turbulence level, fluctuation wavelengths and antenna beam size on the density profile determination has been studied in static plasma with perpendicular launching. The code has been used to simulate correlation measurements. The results show the correlation of the reflectometry signals for different turbulence parameters. Errors in the correlation length increase when two-dimensional effects become important, though the homodyne signal works better than the phase. The correlation simulations also show the way for new methods to determine the group delay and therefore the density profile.
Green, red and infrared Er-related emission in implanted GaN:Er and GaN:Er,O samples
Authors: Monteiro T. , Soares J. , Correia M. R. , Alves E.
Published: July 2002

Er-related luminescence near 1.54 μm (∼805 meV) is observed under below band gap excitation at 4.2 K in GaN:Er and GaN:Er,O implanted samples. The spectrum of the recovered damage samples is a multiline structure. So far, these lines are the sharpest ones reported for GaN. Well-resolved green and red luminescences are observed in implanted samples. The dependence of luminescence on the excitation energy as well as the influence of different nominal fluence and annealing conditions is discussed. Combining the results obtained from photoluminescence and Rutherford backscattering spectrometry, different lattice sites for the optical active Er-related centers are identified.
IR excitation of ethylene molecules and clusters embedded in He4 droplets
Authors: Kunze Martina , Reuss Jörg , Oomens Jos , Parker David H.
Published: July 2002

Ethylene and ethylene clusters embedded in superfluid He4 droplets have been spectroscopically investigated with continuous wave line tunable CO2 and N2O lasers, utilizing the strong absorption of the ν7 mode of C2H4. The monomer and the different clusters of ethylene absorb in the same spectral region and show significant overlap of their spectra. By measuring the dependence of the signal on the pick-up pressure and fitting the resulting curves with a Poisson distribution and by further studying the spectral dependence on the droplet size, it was possible to assign two peaks. The origin of the 10 cm−1 broad gas phase dimer spectrum has been the subject of much discussion; we show that the broad peak is not due to homogeneous broadening, because the low temperature of the He droplet leads to a dimer full width at half maximum width of about 1 cm−1.
Band strengths for C–H stretching polyads of CHBr3 calculated by use of a two-dimensional electric dipole moment surface from density functional theory
Authors: Lin Hai , Yuan Lan-Feng , He Sheng-Gui , Wang Xiao-Gang
Published: July 2002

Band strengths of the Fermi resonance polyads involving C–H stretching and bending vibrations for the CHBr3 molecule are calculated with a one-dimensional dipole function and a two-dimensional dipole surface. These are obtained by the ab initio density functional method. Both treatments reproduce the interpolyad band strengths quite well, while the two-dimensional dipole moment surface is superior to the one-dimensional one in modeling the intrapolyad band strength pattern, indicating the importance of the bending vibrations in dipole moment surface and intensities calculations. The different behavior of interpolyad band strengths of CHBr3 compared to CHCl3, with an intensity anomaly occurring in the latter case, is illustrated by “dipole–weighted” overlap integrals.
Molecular dynamics simulation of supersaturated vapor nucleation in slit pore. II. Thermostatted atomic-wall model
Authors: Kholmurodov K. T. , Yasuoka K. , Zeng X. C.
Published: July 2002

Molecular dynamics simulations of nucleation of a supersaturated Lennard-Jones vapor in slit nanopores are carried out. In this study we extend a previous work [K. Yasuoka, G. T. Gao, and X. C. Zeng, J. Chem. Phys. 112, 4279 (2000)] in that the walls of the slit are treated as actual atomic walls serving as both the confining solid surfaces and a thermostat. The walls are fixed in place in a fcc lattice structure and wall atoms are subjected to a stiff biharmonic potential thereby bounded to lattice sites. The two walls of the slit have an identical surface [fcc (100)], but different strength of attractive interaction with the vapor particles—one is strongly adsorbing and another is weakly adsorbing. Heterogeneous nucleation of the supersaturated vapor in the slit is investigated and events of nucleus formation are monitored in real time. A comparison with the previous simulation (using rigid structureless walls) leads to useful insight into the influence of the wall model to the nucleus formation. In particular, it is found that although the adsorbed particles on the structureless wall diffuse faster than those on the atomic wall, the rate of nucleus formation on the structureless wall is actually about one order of magnitude lower. A detailed analysis of particle and cluster-formation flux indicates that the rate of nucleus formation on the wall is more sensitive to the kinetics of adsorption of gas particles onto the wall than the diffusion rate of adsorbed particles. The higher flux of cluster formation on the atomic wall is apparently due to the higher rate of deposition of monomers onto the wall.
Two-dimensional modeling of a microcell plasma in a mixture of Ne/Xe driven by a capacitively coupled high-frequency source
Authors: Kurihara Masaru , Makabe Toshiaki
Published: July 2002

The basic characteristics of a micro-cell plasma in a gas mixture sustained by a high-frequency voltage source with a ring-shaped electrode are described in this article. The key to maintaining a microcell plasma is to reduce wall loss and increase plasma production. The advantage of a gas mixture of Ne/Xe is the increase of the plasma production rate in a low electric field compared to that in pure gas. As a result, a microcell plasma can be sustained by a lower applied voltage by using a gas mixture of Ne/Xe under the same power condition as compared with that in pure Xe.
Comparison of dielectric dispersion of Al2O3 and Se thin films
Authors: Deǧer D. , Ulutaş K.
Published: July 2002

We investigate the frequency and temperature dependence of the dielectric conductivity and dielectric constant of Al2O3 thin films having ionic structure and compare the results with those obtained for Se thin films having homopolar structure in the 0.1–100 kHz frequency range and in the 100–400 K temperature range. Our samples have a metal–insulator–metal configuration. Oxide-layer thickness ranges between 50 and 1550 Å for Al2O3 films and 150 and 8500 Å for Se films. The existence of a minimum in the imaginary part of dielectric constant versus frequency curves for each thickness of Al2O3 films leads us to conclude that two types of polarization mechanisms are dominant in different frequency regions, depending on the structure of Al2O3 samples, but that only one mechanism dominates in Se samples, which do not have such minima in the imaginary part of their dielectric constant versus frequency curves.
Ab initio potential-energy surfaces for the reactions OH+H2↔H2O+H
Authors: Yang Minghui , Zhang Dong H. , Collins Michael A. , Lee Soo-Y.
Published: July 2002

Accurate global ab initio surfaces are presented for the title reactions and competing exchange processes. The ab initio calculations were based on the quadratic configuration interaction treatment with all single and double excitations and perturbative account for triple excitations together with a 6-311++G(3df,2pd) basis, the multi-reference configuration interaction method with the Davidson correction procedure together with an aug-cc-pVTZ basis, and the unrestricted coupled-cluster treatment with all single and double excitations and perturbative account for triple excitations together with an aug-cc-pVQZ basis. The surfaces are interpolations of ab initio data for energies, energy gradients, and second derivatives at configurations scattered throughout the relevant configuration space.
Electronic structure of polycrystalline polyamine copper dinitrate complexes investigated by photoacoustic and electron paramagnetic resonance spectroscopy
Authors: Guskos N. , Papadopoulos G. J. , Likodimos V. , Majszczyk J. , Typek J. , Wabia M. , Grech E. , Dziembowska T. , Perkowska A. , Aidinis K.
Published: July 2002

Photoacoustic and electron paramagnetic resonance (EPR) spectroscopies have been applied to resolve the electronic structure in powder polycrystalline samples of three biogenic polyamine copper complexes, spermine copper dinitrate, aqua norspermine copper dinitrate, and homospermine copper dinitrate. The fine structure of the intense absorption band in the photoacoustic spectra is assigned to the d–d transitions between the crystal field split levels of copper ions, that cannot be discriminated in the UV/vis solution absorption spectra. Combination with the EPR results allows one to probe the variation of the electronic properties and bonding interaction at the copper site, consistent with the structural data for the crystalline complexes and further supports the reliability of the photoacoustic method to resolve the d–d transition band. A dominant contribution of the in-plane ligand field due to equatorial nitrogen atoms is deduced for the complexes of polyamines with copper salts.
A radiation transport coupled particle-in-cell simulation. II. Simulation results in a one-dimensional planar model
Authors: Lee Hae June , Verboncoeur J. P.
Published: July 2002

The radiation-transport coupled particle-in-cell model is applied to one-dimensional planar model to investigate the radiation trapping phenomena in relatively high- and low-pressure Ar glow discharges. The radiation intensity spectra and the total radiation flux are calculated from the radiative excited-state profile. The simulation results for Doppler and resonance collision broadenings are compared with the eigenmode description of the Holstein equation, with good agreement. Observed are the effects of radiation trapping of photons, diffusion, collisional quenching, and step ionization from excited states on the discharge properties.
Relative stabilities of the two isomers of the methanol-water dimer: The effects of the internal rotations of the hydroxyl and methyl groups of methanol
Authors: Moskowitz Jules W. , Bačić Zlatko , Sarsa Antonio , Schmidt Kevin E.
Published: July 2002

The ground-state energies of the two isomers of the methanol-water dimer, with water-donor (WM) and methanol-donor (MW) structures, have been calculated using the diffusion Monte Carlo (DMC) method with constraint dynamics. Unlike the rigid-body DMC, this method permits the internal rotations of the hydroxyl and methyl groups of methanol about the C–O bond. The DMC calculations were performed for the isotopomers CH3OH⋯H2O, CH3OD⋯H2O, and CD3OH⋯H2O. The calculations with the internal rotation of the methyl and hydroxyl groups of methanol included resulted in a much larger ground-state energy gap between the WM and MW isomers than those in which these internal rotations were frozen. This result demonstrated the critical importance of including the internal hydroxyl and methyl rotations in the DMC calculations aimed at predicting accurately the relative stabilities of the two isomers of the methanol-water dimer.
Br
(
2
P
j
)
and Cl(2Pj) atom formation dynamics of allyl bromide and chloride at 234 nm
Authors: Park Moon Soo , Lee Keon Woo , Jung Kyung-Hoon
Published: July 2002

The photodissociation dynamics of allyl bromide and chloride have been investigated at 234 nm using a two-dimensional photofragment ion imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme. After absorbing a photon, allyl bromide dissociates into C3H5+Br(2Pj;j=1/2,3/2) exclusively via the repulsive surfaces. The enhanced contribution of the singlet state to the initial transition is attributed to intensity borrowing from the nearby (π,π*)1 state. Trimodal translational energy distributions of Cl(2Pj;j=1/2,3/2) have been observed after the photolysis of allyl chloride. Low-velocity components with Boltzmann shapes are produced via internal conversion between the initially pumped (π,π*)1 state and the vibrationally excited ground state. Middle-velocity components with Gaussian shapes originated from curve crossing between the bound (π,π*)1 state and the dissociative (π,σ*)1 state. High-velocity components are produced via curve crossing from the (π,π*)1 state to the (π,σ*)1 state. The enhanced curve crossings in exit channels are attributed to the non-planar geometry of the parent molecule and torsional torque induced by the initial (π,π*)1 transition. The location of the curve crossing between the (π,π*)1 state and the (π,σ*)1 state has been estimated to be ≈40 000 cm−1 based on the localized available energy.
Saturated defect densities of hydrogenated amorphous silicon grown by hot-wire chemical vapor deposition at rates up to 150 Å/s
Authors: Mahan A. H. , Xu Y. , Nelson B. P. , Crandall R. S. , Cohen J. D. , Palinginis K. C. , Gallagher A. C.
Published: July 2002

Hydrogenated amorphous-silicon (a-Si:H) is grown by hot-wire chemical vapor deposition (HWCVD) at deposition rates (Rd) exceeding 140 Å/s (∼0.8 μm/min). These high rates are achieved by using multiple filaments and deposition conditions different than those used to produce our standard 20 Å/s material. With proper deposition parameter optimization, an AM1.5 photo-to-dark-conductivity ratio of 105 is maintained at an Rd up to 130 Å/s, beyond which it decreases. In addition, the first saturated defect densities of high Rd a-Si:H films are presented. These saturated defected densities are similar to those of the best HWCVD films deposited at 5–8 Å/s, and are invariant with Rd up to 130 Å/s.
The Si–H stretching–bending overtone polyads of SiHF3: Assignments, band intensities, internal coordinate force field, and ab initio dipole moment surfaces
Authors: Lin Hai , Bürger Hans , MKadmi El Bachir , He Sheng-Gui , Yuan Lan-Feng , Breidung Jürgen , Thiel Walter , Huet Thérèse R. , Demaison Jean
Published: July 2002

Fourier transform overtone spectra of SiHF3 were recorded in the region of 2500–9000 cm−1 and vibrationally assigned. Experimental intensities were estimated. The 3ν1 overtone band at 6753 cm−1 was observed to be more than 10 times weaker than the 4ν1 band. A reduced three-dimensional Hamiltonian model in terms of internal coordinates was employed to study the Si–H stretching and bending vibrations including 5ν1 and 6ν1 which were recently recorded using optoacoustic spectroscopy. Potential energy parameters were optimized by fitting to experimental band centers. The Fermi resonance between the Si–H stretching and bending motions was found to be insignificant. Band intensities were computed using ab initio one- and three-dimensional dipole moment surfaces (DMS) expanded to polynomials in terms of symmetrized internal coordinates. The intensity anomaly of 3ν1 is understood as resulting from cancellation of contributions by the linear and quadratic terms in the DMS expansion. The behavior of X–H stretching overtone intensities as excitation increases was also studied in the low and medium energy regions. Whether a rapid or a slow decrease of intensity occurs with increasing excitation depends strongly on the nonlinearity of the DMS. For some molecules, there is an almost complete cancellation of contributions from the lower order terms in the DMS so that the accuracy of the computed overtone intensities is mainly limited by the uncertainty of the higher order expansion coefficients in the DMS.
Nuclear magnetic resonance of J-coupled quadrupolar nuclei: Use of the tensor operator product basis
Authors: Kemp-Harper R. , Philp D. J. , Kuchel P. W.
Published: July 2002

In nuclear magnetic resonance (NMR) of I=1/2 nuclei that are scalar coupled to quadrupolar spins, a tensor operator product (TOP) basis set provides a convenient description of the time evolution of the density operator. Expressions for the evolution of equivalent I=1/2 spins, coupled to an arbitrary spin S>1/2, were obtained by explicit algebraic density operator calculations in Mathematica, and specific examples are given for S=1 and S=3/2. Tensor operators are described by the convenient quantum numbers rank and order and this imparts to the TOP basis features that enable an intuitive understanding of NMR behavior of these spin systems. It is shown that evolution as a result of J coupling alone changes the rank of tensors for the coupling partner, generating higher-rank tensors, which allow efficient excitation of S-spin multiple-quantum coherences. Theoretical predictions obtained using the TOP formalism were confirmed using multiple-quantum filtered heteronuclear spin-echo experiments and were further employed to demonstrate polarization transfer directly to multiple-quantum transitions using the insensitive nucleus enhancement by polarization transfer pulse sequence. This latter experiment is the basis of two-dimensional heteronuclear correlation experiments and direct generation of multiple-quantum S-spin coherences can therefore be exploited to yield greater spectral resolution in such experiments. Simulated spectra and experimental results are presented.
Near-infrared absorption spectrum of the Ar–HD complex: A theoretical study of predissociation effects
Authors: Mrugała Felicja
Published: July 2002

Results of close-coupling calculations on the near-infrared absorption by the Ar–HD(v=0,j=1) complex, exploiting the best available empirical and theoretical models of intermolecular potential and of induced-dipole moment for Ar–H2, are reported. The first quantitative description of the S1(1) band of the spectrum is given, including transition energies, linewidths, and intensities, and the description of the Q1(1) band, included in the recent study on Ar–HD [J. Chem. Phys. 109, 10823 (1998)], is completed with calculation of the spectrum shape. A pattern of pairs of lines of distinct widths and heights is found in the T- and N-transition regions of the S1(1) band. The narrow lines in the pairs (widths ∼0.1–0.2 cm−1) result from transitions between bound and predissociating states. The wider lines (about 6–10 times) appear due to transitions between predissociating states. The relations between heights of lines in the pairs differ qualitatively from the relative intensities of the corresponding lines in the S1(1) band of the absorption spectrum of Ar–H2. In the Q1(1) band, transitions between states predissociating rotationally are found to be the only ones which have discernible impact on the line contour. Results of this work and the recent results on the Ar–HD(v=0,j=0) complex are summarized in a discussion of effects of the asymmetric isotopic substitution on the absorption spectrum shape.
Electrical properties of LaAlO3/Si and Sr0.8Bi2.2Ta2O9/LaAlO3/Si structures
Authors: Park Byung-Eun , Ishiwara Hiroshi
Published: July 2002

Lanthanum aluminate (LaAlO3) films were deposited on Si(100) substrates by evaporating single-crystal pellets in vacuum using an electron-beam gun. Then, they were annealed in N2 ambience at 700 °C for 10 min using an electric furnace. X-ray diffraction analysis showed that the LaAlO3 films were amorphous even after the annealing process. No hysteretic characteristics were observed in the capacitance–voltage (C–V) measurement and the dielectric constant of the LaAlO3 films was estimated to be 21–25. It was also found that the leakage current density decreased by about three orders of magnitude after the annealing process. On these films, Sr0.8Bi2.2Ta2O9 films with 210 nm thickness were deposited by a sol–gel method. All samples annealed in O2 atmosphere at temperatures ranging from 650 to 750 °C showed hysteretic C–V characteristics, and the memory window width in the sample annealed at 750 °C for 30 min was about 3.0 V for a voltage sweep of ±10 V. It was also found that the capacitance values biased in the hysteresis loop were unchanged over 12 h.
Development of an infrared absorption measurement method using the photothermal deflection effect of thallium bromide iodide (KRS-5): Measurement with a stepped-scan Fourier transform infrared spectrometer
Authors: Minato Hideyuki , Ishido Yoshinari
Published: July 2002

An optical system was produced experimentally for infrared absorption measurements based on the photothermal deflection effect of solid material (KRS-5), which does not require a cell for the medium as does absorption measurement using the photothermal deflection effect of liquids. In this study, it was shown that the photoelastic effect of photothermal beam deflection exists in KRS-5 crystals, and the necessity of regulating the polarization direction of the laser beam is described. Infrared absorption spectral measurements were done on several samples using a stepped-scan Fourier transform infrared spectrometer for the evaluation of this technique. The results agreed well with the results of transmittance measurements of the same samples. In conclusion, this new technique seems to be useful for the measurement of infrared absorption.
Green’s function calculation of electron spin polarization. V. Quasi-equilibrium and fast mixing approximations for micellar radical pairs
Authors: Neufeld A. A. , Doktorov A. B. , Pedersen J. Boiden
Published: July 2002

Simple analytical formulas are derived for the electron spin polarization (CIDEP) generated in neutral radicals pairs in micellar solutions at high magnetic fields. The formulas are applicable for a locally weak exchange interactions, which could be integrally strong, and are based on the general analytic expressions obtained in the preceeding article of this series. We introduce two approximation schemes, for slow and for fast singlet–triplet mixing. For practical purposes these two cover the complete range of mixing rates. The approximation for slow mixing is similar to the quasi-equilibrium approximation, but we have derived more accurate expressions and explicitly evaluated all quantities. For fast singlet–triplet mixing we suggest a new and highly accurate approximation, which replaces the micellar RP with an effective freely diffusing RP with a redefined value of the exchange interaction. The accuracy of the approximations is documented by a comparison with numerically exact results. A specific feature for micelles is that the polarization as a function af the singlet–triplet mixing rate (q) has two maxima, where the low field maximum can be as large as 1/2 of the maximun possible value even for very small values of the exchange interaction.
Digital nuclear magnetic resonance spectrometer
Authors: Gengying Li , Yu Jiang , Xiaolong Yan , Yun Jiang
Published: July 2002

A digital nuclear magnetic resonance (NMR) spectrometer based on a PC is described. The instrument works in a digital manner not only in rf frequency synthesis but also in receiver detection and filtering. A control program and a data processing procedure are developed using the Delphi 5 for Windows ’95/’98 operating system. Compared with a traditional spectrometer, the present design is rather simple in its hardware parts. As a consequence, building a NMR spectrometer using the proposed design is simple and low cost.
Wave packet dynamics of the N(4S)+O2(X 3Σg−)→NO(X 2Π)+O(3P) reaction on the X 2A′ potential energy surface
Authors: Defazio Paolo , Petrongolo Carlo , Gray Stephen K. , Oliva Carolina
Published: July 2002

We report three-dimensional quantum calculations of total angular momentum J=0 reaction probabilities, J-shifting cross sections, and rate constants of the title reaction. Employing the real wave packet approach, we propagate wave packets corresponding to several O2(v,j) initial levels on the X 2A′ potential surface of Sayós et al. As collision energy increases, the average probabilities first increase monotonically and then become nearly constant, while the cross sections rise in the overall energy range. Numerous probability resonances point out the formation of NOO collision complexes and NO final states. Rotational excitation in O2 decreases the collisional energy thresholds and enhances the state-resolved rate constants, mainly at low temperature. O2 vibrational excitation inhibits the reactivity, although the energy thresholds are still reduced. With respect to previous quasiclassical and mixed quantum-classical studies, we obtain lower thresholds and cross sections but similar rate constants, which are however lower than experimental rates. By inspection of the average properties of the wave packets, we suggest a qualitative reaction mechanism, we propose a modified J-shifting approximation, and we find a possible explanation of the low quantum reactivity on the present potential.
Molecular beam and infrared spectroscopic studies of the thermodynamics of CO on clean and vinylidene-covered Pd(111)
Authors: Stacchiola D. , Wu G. , Kaltchev M. , Tysoe W. T.
Published: July 2002

A differentially pumped, capillary array molecular beam source is used to study the reversible adsorption of CO on CO/Pd(111) [Θ(CO)=0.55] and vinylidene/Pd(111) [Θ(vinylidene)=1.0] at 300 K. Differentially pumping allows the beam to equilibrate rapidly (in ∼2 s) while maintaining good beam uniformity. The isosteric heat of adsorption of reversibly held CO on a surface precovered with 0.55 monolayers of chemisorbed CO is 5.6±0.2 kcal/mol at low excess coverages but decreases linearly with coverage so that an additional 2% of a monolayer of CO reduces the isosteric heat of adsorption to 3.0±0.2 kcal/mol. CO adsorbs reversibly on vinylidene-saturated Pd(111) with an isosteric heat of adsorption of 1.0±0.5 kcal/mol for coverages up to ∼1% of a monolayer of CO. Infrared spectra of CO on vinylidene-covered Pd(111) at higher pressures (several Torrs) reveal that CO adsorbs on the metal surface. Assuming that the heat of adsorption of CO on vinylidene-covered Pd(111) decreases with CO coverage at higher coverages shows that the low-coverage data measured using molecular beam methods are consistent with the isotherm measured using infrared spectroscopy at higher pressures.
Multiple fragmentation dynamics after laser excitation: A simple theoretical approach
Authors: Rubner O. , Engel V.
Published: July 2002

A model to describe molecular fragmentation into several fragments is presented. The case where dissociation is induced by the absorption of a single or several photons from ultrashort pulses is considered. Time-dependent perturbation theory is employed to describe the excitation process and the subsequent fragmentation dynamics is treated using various theories treating direct and indirect break-up processes. The multiple fragmentation of the Fe(CO)5 molecule serves as a numerical example. Different experiments are simulated and, regarding the simplicity of the model, an overall excellent agreement between theory and experiment is found, thus providing an insight into the excitation and fragmentation dynamics under various conditions.
Vibrational modes of graphitic fragments and the nucleation of carbon nanotubes
Authors: Volpe Manuela , Cleri Fabrizio
Published: July 2002

We study the nucleation of carbon nanotubes based on the hypothesis that a cylindrical seed can be formed by the spontaneous rolling of a small graphite fragment under thermal fluctuations. We calculate the vibrational spectra of fragments of different sizes by means of a tight-binding model, and compare the spectra to that of an infinite graphite sheet, as a function of temperature. The spectra of finite-size fragments contain an increasing density of low-frequency, large amplitude modes with increasing temperature; it is shown that such modes tend to fold over the fragment into a cylindrical shape. Finally, direct tight-binding molecular-dynamics simulations of the high-temperature fluctuation of parallel graphite fragments demonstrate that nanotube seeds closed at one end can spontaneously form.
X-ray diffraction line broadening under elastic deformation of a polycrystalline sample: An elastic-anisotropy effect
Authors: Singh Anil K. , Balasingh Chellayyan
Published: July 2002

A homogeneous stress field imposed on a polycrystalline sample containing elastically anisotropic crystallites produces an inhomogeneous strain field. The average strain causes a shift of the diffraction-line position, and the variance of the strain is a measure of line broadening. Though the shift of the line is commonly observed, earlier attempts to measure the broadening caused by elastic anisotropy were not conclusive. In the article, expressions have been derived for the average strain and variance of strain for a polycrystalline sample (cubic system) subjected to uniaxial elastic stress. The (310) and (222) lines of beta brass under an uniaxial load have been recorded using Co Kα and Cu Kα radiations, respectively. The average strain and variance of strain derived from the measured diffraction line profiles are in good agreement with those predicted by the theory. The present measurements provide conclusive evidence for the diffraction line broadening caused purely by elastic anisotropy.
Electric-field-effect thermoelectrics
Authors: Sandomirsky V. , Butenko A. V. , Levin R. , Schlesinger Y.
Published: July 2002

A significantly large thermoelectric “figure of merit” in a bipolar semiconductor is achieved by converting it, by doping, into an essentially monopolar semiconductor. We show here, that for a bipolar semiconductor film, having a thickness smaller than the screening length, there is an alternative to doping to obtain a practically monopolar semiconductor. The electric-field effect (EFE) or the ferroelectric-field effect can be used to quench the concentration of one type of charge carriers. We show that this method is particularly suited for narrow-gap semiconductors and for semimetals, having sufficiently high dielectric permittivity. We also show that this “EFE doping” is free from the drawbacks of conventional doping. Our analysis demonstrates that increased values of the figure of merit are feasible. We present here the theory and the figure-of-merit calculation for typical cases of the bipolar semiconductor thermoelectric film under EFE doping. Numerical results for a film of the PbTe type are presented.
Refractive correction measurement in an ophthalmic lens
Authors: Trujillo-Schiaffino Gerardo , Malacara-Hernández Daniel
Published: July 2002

An ophthalmic lens is designed so that its refractive power is constant when the observer looks through the center of the lens, as well as when the observation is made through the edge of the lens. This state of correction of an ophthalmic lens can be confirmed by direct measurements of the dioptric refractive power by means of a typical lens meter, if some special precautions are taken. Here we describe a method to perform these measurements.
Controlling probe-sample separation with aerodynamic floating
Authors: Popovic Zoran , Junjinger Johann , Waldron Philip , Graham John
Published: July 2002

We have recently developed a noncontact technique capable of detecting microscopic variations in the surface potential of charged dielectric films. The technique is based on measuring the charge induced on a small capacitive probe held at a constant distance from a charged sample surface. Distance control is achieved by aerodynamic floating, which is an inexpensive and simple passive feedback system capable of maintaining a constant probe-sample separation despite minor variations in sample morphology. We have used the technique to detect the presence of microscopic electrostatic defects in organic photoreceptors, such as charge deficient spots, which are responsible for image degradation in xerographic copiers and printers. However, aerodynamic floating could be implemented in other applications requiring accurate control over probe-sample separation.
Using six moments of Boltzmann’s transport equation for device simulation
Authors: Grasser Tibor , Kosina Hans , Gritsch Markus , Selberherr Siegfried
Published: July 2002

As has been frequently pointed out the distribution function of hot carriers in state-of-the-art devices is insufficiently described using just the average carrier energy. In this work the distribution function is characterized by six moments to obtain a more accurate description of hot carrier phenomena. A transport model based on six moments is derived and compared to a previously published model. A detailed comparison of results obtained from the model with Monte-Carlo data shows excellent agreement provided proper models for the relaxation times are used.
Growth and field emission of carbon nanotubes on electroplated Ni catalyst coated on glass substrates
Authors: Kim Jaemyung , No Kwangsoo , Lee Cheol Jin
Published: July 2002

Carbon nanotubes are grown on Ni catalyst coated on soda-lime glass substrates using chemical vapor deposition of C2H2 gas at 550 °C. Ni film is coated on the surface of Ag film using the electroplating method. Ni was etched by ammonia (NH3) gas in order to form nanometer sized catalytic particles before carbon nanotube growth. Pd film is applied as a gas activator to decrease the growth temperature of carbon nanotubes. The carbon nanotubes grown on Ni catalyst particles showed a multiwalled structure with defective graphite sheets at the wall. The turn-on voltage was about 2.8 V/μm with an emission current density of 10 μA/cm2, and the threshold voltage was about 4.0 V/μm with an emission current density of 10 mA/cm2. The Fowler-Nordheim plot showed a good linear fit, indicating that the emission current of carbon nanotubes follows Fowler-Nordheim behavior. The calculated field enhancement factor was 2850.
Deep-level transient spectroscopy of dislocation-related defects in epitaxial multilayer structures
Authors: Thor E. , Mühlberger M. , Palmetshofer L. , Schäffler F.
Published: July 2002

Dislocated Si, SiGe, SiC, and SiGeC n-type heterostructures, grown by molecular beam epitaxy, were characterized by capacitance–voltage profiling and deep-level transient spectroscopy. Exclusively dislocation-related defects were found in the different layers, which correspond to the well-known defects found in plastically deformed silicon.