bluuebunny/crossref_metadata_2025 huggingface dataset:
Absolute calibration of laser-induced fluorescence experiments by optical depth correction
Authors: Franke St. , Dinklage A. , Wilke C.
Published: July 2002

Laser-induced fluorescence measurements are absolutely calibrated by correction of relative spatial fluorescence profiles for optical depth. The method is applicable if optical depths of fluorescence transitions are sufficiently high (mean optical depth τ̄≳0.3). For calibration two relative spatial density profiles are required. Measurements of absolute particle densities in a dc glow discharge are performed and compared to results of laser absorption spectroscopy. Absolute errors are found to be less than 20%. The calibration method is expected to be useful in experimental environments that are not accessible for other calibration methods, e.g., due to high optical depth.
Quantitative measurement system of molecular orientation by coaxial optical Kerr effect spectroscopy
Authors: Sakai Keiji , Ikeda Yasuhiro , Takagi Kenshiro
Published: July 2002

We have developed a new system for quantitative measurement of laser induced molecular orientation spectroscopy with continuous wave lasers operating in a coaxial configuration. An exciting Ar+ laser and a probing He–Ne laser are superimposed coaxially and incident to the liquid sample of anisotropic molecules, in which the former drives the molecules into the direction of light polarization, while the latter picks up the induced molecular orientation as the Kerr effect. The new system realizes long interaction lengths of the exciting and probing beams, which enables us to measure the molecular orientation with high sensitivity even for materials with small Kerr constant. Quantitative estimation of the Kerr constant also becomes possible by measuring accurate profiles of the laser beams. The strength of the Kerr effect was theoretically calculated on the basis of the optics of the Gaussian beam propagation and the relation between the expected signal intensity and the optical configuration was derived. The experimental result obtained for liquid carbon disulfide shows good agreement with theory.
Equation of motion of the magnetization vector in materials with fluctuating valence
Authors: Suhl Harry
Published: July 2002

In micromagnetics, it is common practice to describe the motion of the magnetization vector in terms of the Landau–Liftshitz–Gilbert equations. In these, the strength of the dissipative torque is measured by a phenomenological parameter, and the form of the torque is chosen so as to lead to a reversion of the magnetic vector to a stable or metastable position. The microscopic origin of the parameter, as well as the correct form of the torque is not generally discussed. Retaining the mathematical description of all the degrees of freedom to which the magnetization vector is coupled, would give correct results. However, it is obviously desirable to formally eliminate these degrees of freedom, resulting in equations for the magnetization vector alone. In this article, the necessary procedure is carried out for the particular case in which the loss mechanism arises from valence fluctuations changing electron occupation numbers, and thereby the magnetic moments, of the various sites. Since the fluctuations are not instantaneous, the total magnetization vector develops time lags that manifest themselves as a loss mechanism. Apart from a numerical measure of this damping, we find that the structure of the torque term does not, in general, have one of the familiar forms. Furthermore, this results in certain changes in the switching characteristics of small magnetic particles. This particular origin of dissipation was discussed in the early 1950’s by Clogston, Gate, and others in the context of domain wall motion and small-signal ferromagnetic resonance, but no attempt was made at that time to re-express the entire process in terms of an equation of motion of the magnetization vector alone.
Elimination of amplitude-phase crosstalk in frequency domain near-infrared spectroscopy
Authors: Morgan S. P. , Yong K. Y.
Published: July 2002

Changes in phase that occur with changes in amplitude impose severe limitations on the accuracy of frequency domain near-infrared spectrometers. Phase is related to the photon pathlength in tissue and phase errors introduced by the instrument can be interpreted as changes in tissue oxygenation. The instrument described in this article employs a reference radio frequency modulated laser diode to eliminate the effects of amplitude-phase crosstalk and requires no feedback. Light from the reference laser diode does not pass through the medium under investigation but passes directly onto the detector. The reference and medium signals follow a common path through the detector and so the same phase error is imposed on both. Summing the reference and medium phase eliminates the crosstalk and enables the resultant to be attributed only to the photon pathlength within the medium. It is also demonstrated that elimination of amplitude-phase crosstalk is a natural consequence of a phased array configuration.
Production of HCO from propenal photolyzed at 193 nm: Relaxation of excited states and distribution of internal states of fragment HCO
Authors: Kao Ya-Ting , Chen Wei-Chen , Yu Chin-Hui , Chen I-Chia
Published: July 2002

The dynamics of photodissociation of propenal at 193 nm are studied by detecting laser-induced fluorescence of nascent fragment HCO in its transition B̃ 2A′–X̃ 2A′. Rotational states up to N=30 and K=3 of HCO X̃ 2A′ are populated and vibrational states (000), (010), and (001) are detected. The Ka=1 doublet states and the two spin states for all vibrational levels detected are nearly equally populated. Much less rotational excitation is observed than the distributions calculated on a statistical model—phase space theory. This implies that dissociation occurs from the triplet channel with a small exit barrier. Small rotational excitation arises from the repulsive part of the exit barrier and the geometry of the transition state on the triplet surface. Experimental data yield an energy partitioning with translation, rotation, and vibration of HCO at 3.0, 1.3, and 1.5 kcal/mol, respectively, in total accounting for 11.5% of available energy. These results indicate that the other fragment C2H3 has 3.2 kcal/mol of translation and 42.5 kcal/mol of internal energy; hence, most C2H3 is expected to undergo secondary dissociation to C2H2 and H. Because the appearance of HCO is faster than that calculated based on the Rice–Ramsperger–Kassel–Marcus theory, other decay pathways dominate the pathway of the radical channel from the triplet surface.
Infrared radiometry-based background-compensated thermometric instrument for noncontact temperature and friction measurements
Authors: Li L. , Mandelis A. , Garcia J. , Eccles C.
Published: July 2002

The design and performance of a novel thermometric instrument featuring thermalemission-intensity harmonic modulation, noncontact infrared radiometric detection, and stray background suppression is described. The instrumental principle depends on thermal (blackbody) emission of Planck radiation from a heated surface. It was developed to measure small temperature rises caused by frictional heating. A low-power He–Ne heating laser was used to investigate the sensitivity and estimate a figure-of-merit (FOM) for the instrument. Background compensation leading to signal baseline suppression was partly achieved with a differential mechanical chopper blade, designed to induce destructive interference of infrared radiation superposition from heated and reference spots on a ceramic sample coated with a metallic thin film. Additional background suppression was achieved by lock-in amplifier signal amplitude and phase compensation through an externally superposed wave at the same chopping frequency. The FOM of the noncontact thermometric instrument was 159.9±8.5. The system sensitivity (minimum temperature rise) for the particular thin-film/ceramic material was estimated to be 0.18–0.23 °C.
Dynamics of the O induced reconstruction of the Rh(110) surface: A scanning tunnelling microscopy study
Authors: Africh C. , Esch F. , Comelli G. , Rosei R.
Published: July 2002

The oxygen adsorption on the unreconstructed Rh(110) surface has been studied by scanning tunnelling microscopy (STM), by imaging the evolution of the surface reconstruction in situ during oxygen uptake at 390 K. The dynamics terminate with the formation of a strained (10×2)–O structure with high local oxygen coverage. Initially, kink atoms are detached from the steps and diffuse over the surface, most probably as Rh–O units, as inferred by comparison to x-ray photoelectron spectroscopy (XPS) data, until they condense in zigzag structures characteristic for the (2×2)p2mg–O structure (0.5 ML). At oxygen doses above 0.12 L, missing rows begin to be dug along the [11̄0] direction into the steps, detaching further Rh atoms. The reconstruction proceeds as a missing/added row reconstruction. With ongoing surface reconstruction, a second reconstruction mechanism of local nature sets in: Rh atoms start to be ejected out of the terraces and condense at the ends of the so formed troughs. Dosing further oxygen on the (2×2)p2mg–O structure leads to the formation of the strained (10×2)–O structure. The ejection of two of every 10 Rh atoms proceeds only if the Rh rows are not neighbored by troughs.
Isothermal binodal curves near a critical endpoint
Authors: Kim Young C. , Fisher Michael E. , Barbosa Marcia C.
Published: July 2002

Thermodynamics in the vicinity of a critical endpoint with nonclassical exponents α, β, γ, δ, … , is analyzed in terms of density variables (mole fractions, magnetizations, etc.). The shapes of the isothermal binodals or two-phase coexistence curves are found at and near the endpoint for symmetric and nonsymmetric situations. The spectator- (or noncritical-) phase binodal at T=Te is characterized by an exponent (δ+1)/δ (≃1.21) with leading corrections of relative order 1/δ (≃0.21), θ4/βδ (≃0.34) and 1−(βδ)−1 (≃0.36); in contrast to classical (van der Waals, mean field, etc.) theory, the critical endpoint binodal is singular with a leading exponent (1−α)/β (≃2.73) and corrections which are elucidated; the remaining, λ-line binodals also display the “renormalized exponent,” (1−α)/β but with more singular corrections. [The numerical values quoted here pertain to (d=3)-dimensional-fluid or Ising-type systems.]
Global linear gyrokinetic simulations in quasi-symmetric configurations
Authors: Jost G. , Tran T. M. , Cooper W. A. , Villard L. , Appert K.
Published: July 2002

First global linear study of electrostatic drift waves in two realistic quasisymmetric configurations, namely the Quasi-Axially symmetric Stellarator with three fields periods (QAS3) [P. Garabian and L. P. Ku, Phys. Plasma 6, 645 (1999)] and the Helically Symmetric eXperiment (HSX) [F. S. B. Anderson et al., Trans. Fusion Technol. 27, 273 (1995)], are presented. Effects of the shape of the plasma on the growth rate and frequency of the ion temperature gradient (ITG) driven mode are investigated by varying the quasi-symmetric configurations to an equivalent symmetric system. The calculations have been performed using a three-dimensional (3D) global gyrokinetic code in the magnetic configurations provided by the magnetohydrodynamic (MHD) equilibrium code VMEC [S. P. Hirshman and D. K. Lee, Comput. Phys. Commun. 39, 161 (1986)]. The plasma is modeled by gyrokinetic ions and adiabatic electrons. In QAS3, results are very close to those obtained for a tokamak. The drift waves are only slightly affected by the shape of the plasma or the local magnetic shear. On the other hand, results for the HSX configuration show a clear 3D effect, namely a strong toroidal variation of the drift wave mode structure. This variation is a clear structure of the 3D plasma shape. However, first results show that the growth rate of the ITG driven mode is largely unaffected by this effect.
Configuration interaction study of singlet excited state of thiophene and its cyano derivative oligomers
Authors: Chakraborty D. , Lagowski J. B.
Published: July 2002

Using the singles configuration interaction (CIS) theoretical approach, we investigated the nature of the geometric conformations and electronic transitions in π-conjugated oligo(thiophene)s, and their cyano derivatives. Geometry optimizations were carried at the restricted Hartree–Fock (RHF/3-21G*) level for the ground-state configurations, which were followed by calculations of first (lowest) singlet excitation energies using the CIS (CIS/3-21G*) method. In agreement with several previous theoretical/experimental studies, this investigation indicates that the cyano substituted thiophene polymer has a considerably smaller intrinsic band gap than its parent polymer. CIS calculations produce singlet excitation energies in excellent agreement with the experimental values (∼0.1–0.5 eV). They also show that delocalized first singlet excited states (CIS) are accompanied by a strong geometry relaxation in comparison to their ground-state (HF) geometries. The shape and the location of the relaxation depends on the oligomer length.
Interaction of the small carboxylic acids CH3COOH and C2H5COOH with Pt(111) studied with helium atom scattering
Authors: Graham Andrew P.
Published: July 2002

High resolution helium atom scattering (HAS) has been used to measure the adsorption, diffraction and low frequency vibrations of acetic and propionic acids adsorbed on the Pt(111) surface at 170 K. Acetic and propionic acid adsorb at 170 K as isolated molecules, initially, with evidence for the dissociation of a fraction of the molecules to produce CO. At the completion of the monolayer helium diffraction shows that the molecules are locally ordered with characteristic length scales of 3.6 Å and 4.5 Å. By comparing the HAS time-of-flight spectra for the hydrogenated and deuterated adsorbed molecules the parallel wagging mode frequencies for the acetate and propionate bidentate components could be determined to be 12.6 meV and 8.9 meV, respectively.
3ω
method for specific heat and thermal conductivity measurements
Authors: Lu L. , Yi W. , Zhang D. L.
Published: July 2002

We present a 3ω method for simultaneously measuring the specific heat and thermal conductivity of a rod- or filament-like specimen using a way similar to a four-probe resistance measurement. The specimen in this method needs to be electrically conductive and with a temperature-dependent resistance, for acting both as a heater to create a temperature fluctuation and as a sensor to measure its thermal response. With this method, we have successfully measured the specific heat and thermal conductivity of platinum wire specimens at cryogenic temperatures, and measured those thermal quantities of tiny carbon nanotube bundles some of which are only ∼10−9 g in mass.
Electronic transport by small polarons in La0.5Sr0.5MnO3
Authors: Quenneville E. , Meunier M. , Yelon A. , Morin F.
Published: July 2002

Electrical conductivity measurements on La0.5Sr0.5MnO3 (LSM5) thin films as a function of temperature are presented. These are used to demonstrate that the electronic transport in LSM5 is well described by the Emin–Holstein adiabatic small polaron model. Measurements have also been performed on bulk samples. Even if the conductivity behaves somewhat differently in the latter case, the same polaronic model still applies. The polaron densities extracted from conductivity measurements are very similar, ranging from 7 to 8×1021 cm−3 for thin films and are 9.1×1021 cm−3 for bulk samples. These results agree quite well with the nominal polaron density for LSM5, 8.4×1021 cm−3. We have also derived important quantities for transport in LSM5. For thin films, we obtained hopping energies, WH, ranging between 73 and 99 meV and almost constant, at 34 meV, for bulk samples. By fitting conductivity measurements from 50 to 1123 K, we also find the zero point interaction constant, γ0, to be 0.35 in thin films. These conductivity results are compared with the literature.
Quantum-mechanical effects on electron–electron scattering in dense high-temperature plasmas
Authors: Jung Young-Dae
Published: July 2002

Quantum-mechanical effects on electron–electron scattering cross sections are investigated in dense high-temperature plasmas. An effective pseudopotential model taking into account both quantum-mechanical effects and plasma screening effects is applied to describe electron–electron interactions in dense high-temperature plasmas. The Born approximation and the total spin states are considered to obtain high-energy electron–electron scattering cross sections. The results show that the differential scattering cross sections are significantly reduced with increasing the thermal de Broglie wavelength. It is also found that the quantum-mechanical effects are especially important at the scattering angle θlab=π/4.
On Taylor-series expansions of residual stress
Authors: Pruett C. David , Sochacki James S. , Adams Nikolaus A.
Published: July 2002

Although subgrid-scale models of similarity type are insufficiently dissipative for practical applications to large-eddy simulation, in recently published a priori analyses, they perform remarkably well in the sense of correlating highly against exact residual stresses. Here, Taylor-series expansions of residual stress are exploited to explain the observed behavior and “success” of similarity models. Specifically, the first few terms of the exact residual stress τkl are obtained in (general) terms of the Taylor coefficients of the grid filter. Also, by expansion of the test filter, a similar expression results for the resolved turbulent stress tensor Lkl in terms of the Taylor coefficients of both the grid and test filters. Comparison of the expansions for τkl and Lkl yields the grid- and test-filter dependent value of the constant cL in the scale-similarity model of Liu et al. [J. Fluid Mech. 275, 83 (1994)]. Until recently, little attention has been given to issues related to the convergence of such expansions. To this end, we re-express the convergence criterion of Vasilyev et al. [J. Comput. Phys. 146, 82 (1998)] in terms of the transfer function and the cutoff wave number of the filter. As a rule of thumb, the less dissipative the filter (e.g., the higher the cutoff), the faster the rate of convergence.
Small time controllability of systems on compact Lie groups and spin angular momentum
Authors: D’Alessandro Domenico
Published: July 2002

In this article, we develop some general results on the properties of the reachable sets for right invariant bilinear systems with state varying on compact Lie groups. The main results consist of a characterization of the set of states reachable in arbitrary time from the identity of the group. This, under suitable assumptions, is proved to be a Lie subgroup of the underlying Lie group. We apply these results to the analysis of the controllability of particles with spin. The results are motivated by and generalize the results in another work [D. D’Alessandro, Sys. Control Lett. 41, 213–221 (2000)], where the specific model of a spin 12 particle system in an electro-magnetic field was considered.
Magnetic field effects on cross section of inelastic electron–ion collisions
Authors: Tavassoli H. , Sohbatzadeh F. , Latifi H. , Shokri B.
Published: July 2002

The magnetic field effects on the cross section of inelastic electron–ion collisions on 1s→2p dipole transition of hydrogenic ions are calculated. An atmospheric pressure plasma in thermodynamic equilibrium imbedded in an external, constant, and homogeneous magnetic field is considered. The magnetic field and plasma screening effects on the bound states of hydrogenic ion are included. The longitudinal dielectric function for magnetoactive plasmas is used to describe the interaction potential. The semiclassical straight-line trajectory approximation is used to the motion of the projectile electron. The partial cross section of electron-impact excitation of hydrogenic ions is derived as a function of the impact parameter, projectile energy, Larmor frequency, and plasma parameters. The scaled partial cross section of the electron-impact excitation including the magnetic field effect is always smaller than one without the magnetic field. When the Larmor frequency increases, the magnetic field effect decreases. For larger projectile velocity the differences between the partial cross section of electron-impact excitation for different magnitude of magnetic field becomes more significant.
Thermohydrodynamics for a van der Waals fluid
Authors: Español Pep
Published: July 2002

Following a cell method of van Kampen for the calculation of a coarse-grained free-energy functional for the van der Waals gas, we compute a corresponding entropy functional from microscopic principles. This entropy functional is one of the building blocks of the recently developed GENERIC framework [H. C. Öttinger and M. Grmela, Phys. Rev. E 56, 6633 (1997)]. This framework allows us to obtain, in a thermodynamically consistent way, the continuum hydrodynamic equations for a fluid able to display liquid–vapor coexistence. Surface tension appears naturally and the resulting model describes interfaces as diffuse regions, much in the same spirit as the gradient theory for equilibrium situations. We suggest that using interfacial forces in the integral form obtained in the microscopic derivation instead of third order derivatives of the density field might represent an advantage from a computational point of view.
Dynamics of inhomogeneous cross-linked polymers consisting of domains of different sizes
Authors: Gurtovenko A. A. , Gotlib Yu. Ya.
Published: July 2002

The theoretical approach is developed to describe the dynamics of inhomogeneous cross-linked polymers consisting of cross-link agglomerations. An inhomogeneous polymer is treated as an ensemble of noninteracting cross-linked regions (domains) of different sizes. We model an internal architecture of the domains in a rather regular way and assume a power law decay of the relaxation modulus inside the domains, a decay usual for a broad class of cross-linked materials on microscopic scales. Assuming a broad size distribution of the domains in cross-linked polymers due to a random character of cross linking, we demonstrate a stretched exponential time behavior of the relaxation modulus on scales larger than the average size of inhomogeneities in the polymer. We apply this general approach to some special cases of cross-linked polymers, namely to polydisperse polymer networks, to inhomogeneous meshlike networks, and to inhomogeneously cross-linked polymeric gels.
Kaluza’s theory in generalized coordinates
Authors: Garcı́a-Perciante Ana Laura , Sandoval-Villalbazo Alfredo , Garcı́a-Colı́n L. S.
Published: July 2002

Maxwell’s equations can be obtained in generalized coordinates by considering the electromagnetic field as an external agent. The work presented here shows how to obtain the electrodynamics for a charged particle in generalized coordinates eliminating the concept of external force. Based on Kaluza’s formalism, the one presented here extends the 5×5 metric into a 6×6 space–time giving enough room to include magnetic monopoles in a very natural way.
Filtered random variables, bialgebras, and convolutions
Authors: Lenczewski Romuald
Published: July 2002

We introduce the filtered *-bialgebra which is a multivariate generalization of the unital *-bialgebra C〈X,X′,P〉 of polynomials in noncommuting variables X=X*, X′*=X′ and a projection P=P*=P2, endowed with the coproduct Δ(X)=X⊗1+1⊗X, Δ(X′)=X′⊗P+P⊗X′, with P being group-like. We study the associated convolutions, random walks and filtered random variables. The GNS representations of the limit states lead to filtered fundamental operators which are the CCR fundamental operators on the multiple symmetric Fock space Γ(ℋ) over H=L2(R+,G), where 𝒢 is a separable Hilbert space, multiplied by appropriate projections. The importance of filtered random variables and fundamental operators stems from the fact that by addition and strong limits one obtains from them the main types of noncommutative random variables and fundamental operators, respectively, regardless of the type of noncommutative independence.
Dynamical systems embedded into Lie algebras
Authors: Campoamor-Stursberg O. R. , Gascon F. G. , Peralta-Salas D.
Published: July 2002

Analytical and geometrical information on certain dynamical systems X is obtained under the assumption that X is embedded into a certain real Lie algebra.
Ionization potentials, electron affinities, and vibrational frequencies of Gen (n=5–10) neutrals and charged ions from density functional theory
Authors: Li Si-Dian , Zhao Zhi-Gang , Wu Hai-Shun , Jin Zhi-Hao
Published: July 2002

Geometrical and electronic properties of Gen (n=5–10) neutrals, cations, and anions have been investigated using the density functional method of Becke’s three-parameter hybrid functional with the Perdew/Wang 91 expression. Berny structural optimization and frequency analyses are performed with the basis of 6-311G(d) for both the neutrals and charged ions. Cohesive energies, ionization potentials, and electron affinities calculated at the optimized ground-state structures agree satisfactorily with recent experimental values. Frequency analyses indicate that the bicapped antitetragonal prism, which was previously proposed as the ground-state structure of Ge10−, is in fact a first-order stationary point with an imaginary frequency at 95i (b2). The optimized ground-state structure of Ge10− obtained in this work is a distorted, bicapped antitetragonal prism with the symmetry of C1. It is a typical Jahn–Teller distortion. Prominent charge-induced structural changes are also determined for Ge5+, Ge6−, Ge7+, Ge8+, Ge8−, and Ge9−.
Combinatorial identities for binary necklaces from exact ray-splitting trace formulas
Authors: Blümel R. , Dabaghian Yu.
Published: July 2002

Based on an exact trace formula for a one-dimensional ray-splitting system, we derive novel combinatorial identities for cyclic binary sequences (Pólya necklaces).
An expansion of the hypergeometric function in Bessel functions
Authors: Nagel Bengt
Published: July 2002

An expansion of the hypergeometric function F12(a,b,c+1;−z2/4ab) in Bessel functions of argument z is derived. This expansion can be used to obtain an asymptotic expansion of the hypergeometric function for large absolute values of a and b.
Dissociation dynamics from a de Broglie–Bohm perspective
Authors: Wang Z. S. , Darling G. R. , Holloway S.
Published: July 2002

Within the framework of Bohm’s reformulation of quantum physics we revisit the activated dissociation of hydrogen molecules at metal surfaces. The quantum-mechanical force, which accounts for most of quantum effects in the method, and is caused by nonlocal, topographical properties of the wave function, is computed using time-dependent wave packets obtained using conventional, spectral methods. Driven by a combination of the classical force together with the quantum force, trajectories carrying probability density either succeed in overcoming the barrier for dissociation or are scattered back into the gas phase. The Bohmian picture for the dissociation process has enabled us to develop a novel mechanism to account for vibrationally enhanced molecular dissociation. This is relevant to the recently observed promotion of dissociation of very highly vibrationally excited NO molecules at Cu surfaces.
Some exact results for mid-band and zero band-gap states of associated Lamé potentials
Authors: Khare Avinash , Sukhatme Uday
Published: July 2002

Applying certain known theorems about one-dimensional periodic potentials, we show that the energy spectrum of the associated Lamé potentials, a(a+1)msn2(x,m)+b(b+1)m cn2(x,m)/dn2(x,m), consists of a finite number of bound bands followed by a continuum band when both a and b take integer values. Further, if a and b are unequal integers, we show that there must exist some zero band-gap states, i.e., doubly degenerate states with the same number of nodes. More generally, in case a and b are not integers, but either a+b or a−b is an integer (a≠b), we again show that several of the band-gaps vanish due to degeneracy of states with the same number of nodes. Finally, when either a or b is an integer and the other takes a half-integral value, we obtain exact analytic solutions for several mid-band states.
Backscattering of ultrashort high intensity laser pulses from solid targets at oblique incidence
Authors: Andreev A. A. , Platonov K. Yu. , Salomaa R. R. E.
Published: July 2002

Backreflection of short, intense laser pulses at oblique incidence on solid targets is explained with a model where a periodic electron density modulation acts as a diffraction grating. The pump and reflected electromagnetic waves drive through the ponderomotive force the grating, and the coupled system becomes parametrically unstable. The basic equations governing this situation are given. A linearized stability analysis is used to obtain the instability growth rate in a homogeneous plasma and the convective gain coefficients for the inhomogeneous case. The results support the feasibility of the suggested mechanism. An absolute instability is predicted to set on at a threshold intensity of about 1016 W/cm2 for a typical laser pulse with a length of 100 fs and a spot size of 30 μm. The instability is shown to saturate at a level of a few percent, because the higher harmonics in the electron density modulation make the diffraction more diffuse, thus reducing both the ponderomotive force and the backreflection coefficient.
Si δ-doped GaAs structure with different dopant distribution models
Authors: Ozturk E. , Ergun Y. , Sari H. , Sokmen I.
Published: July 2002

We have theoretically investigated the diffusion of donor impurities of single Si δ-doped GaAs inserted into a quantum well at T=0 K. The spread of the impurities is taken into account in three different models: (i) a uniform distribution, (ii) a triangular distribution and (iii) a nonuniform distribution. In this article, the nonuniform distribution is different from the Gaussian distribution used by other authors. The electronic structures were calculated by solving the Schrödinger and Poisson equations self-consistently. We also found the confining potential, the charge density, the subband energies, the subband occupations and the Fermi energy. In this study, we have seen that the electronic structures are quite sensitive to the type of donor distribution when the thickness of the donor distribution (Δz) is large.
Structural and magnetic properties of R(Fe1−yCoy)12−xNbx compounds
Authors: Wang J. L. , Tang N. , Shen Y. P. , Yang D. , Fuquan B. , Wu G. H. , Yang F. M. , de Boer F. R. , Brück E. , Buschow K. H. J.
Published: July 2002

The magnetic properties of RFe12−xNbx and R(Fe1−yCoy)11.3Nb0.7 compounds with R=Y, Tb, and Dy have been investigated in the concentration region defined by 0.6<x<0.8 and y⩽0.3, where the compounds are single phase with the ThMn12-type of structure. The Curie temperature TC of the RFe12−xNbx compounds is almost independent of the Nb content whereas the saturation magnetization Ms decreases with increasing Nb content. The spin-reorientation temperature Tsr of the TbFe12−xNbx and DyFe12−xNbx compounds decreases monotonically with increasing x. Substitution of Co for Fe in RFe11.3Nb0.7 leads to a remarkable increase of TC and the appearance of a maximum in the Co-concentration dependence of Ms. In contrast, Tsr decreases monotonically with increasing Co content for both R=Tb and Dy. The modification of T- and R-sublattice anisotropy originating from a change of the Nb content and from substitution of Co for Fe was analyzed by combining crystalline electric field theory and the individual-site model.
Comment on “Diffusion measurements with the pulsed gradient nonlinear spin echo method” [J. Chem. Phys. 112, 5275 (2000)]
Authors: Jeener J.
Published: July 2002

An undocumented approximation has been found in the discussion of the effects of molecular diffusion in the CRAZED experiment, as presented in the reference quoted in the title. A detailed discussion shows the subtle origin of the problem. Reference is given to discussions which avoid this approximation by using the extended Bloch equation.
Cross-correlation image tracking for drift correction and adsorbate analysis
Authors: Mantooth B. A. , Donhauser Z. J. , Kelly K. F. , Weiss P. S.
Published: July 2002

A digital image tracking algorithm based on Fourier-transform cross-correlation has been developed to correct for instrumental drift in scanning tunneling microscope images. A technique was developed to eliminate cumulative tracking errors associated with fractional pixel drift. This tracking algorithm was used to monitor conductance changes associated with different conformations in conjugated molecular switch molecules and to trace the diffusion of individual benzene molecules on Ag{110}. Molecular motions have been tracked for up to 25 h (400 images) of acquisition time.
Low voltage stress induced leakage currents and surface states in ultrathin (1.2–2.5 nm) oxides
Authors: Meinertzhagen A. , Petit C. , Zander D. , Simonetti O. , Maurel T. , Jourdain M.
Published: July 2002

It has been shown recently that the low voltage gate current in ultrathin oxide metal–oxide–semiconductor devices is very sensitive to electrical stresses. Therefore it can be used as a reliability monitor when the oxide thickness becomes too small for traditional electrical measurements to be used. This paper presents a thorough study of the low voltage gate current variation for different uniformed or localized electrical stress conditions at or above room temperature, and for various oxide thicknesses ranging from 1.2 to 2.5 nm. As it has been proposed recently that this current could be due to electron tunneling through Si/SiO2 interface states, the results obtained in the thicker oxides for the gate current have been compared with the corresponding surface state density variations measured by charge pumping. It is shown that there is no clear relation between low voltage gate current increase after stress and that of surface state density, and that soft or hard oxide breakdown happens when the low voltage current reaches a critical value independently of the stress created interface state density.
Early-stage modification of a silicon oxide surface in fluorocarbon plasma for selective etching over silicon
Authors: Ishikawa Kenji , Sekine Makoto
Published: July 2002

During the etching of the film of silicon dioxide in an Ar-diluted octafluoro cyclobutane (c-C4F8) plasma, infrared spectra were obtained every half a second by using in situ time-resolved attenuated total reflection infrared (IR-ATR) spectroscopy. With IR-ATR, it was possible to separately observe the bands of absorption by carbon fluorine and silicon oxygen. These time-resolved spectroscopic observations enabled us to characterize the formation of the film of amorphous fluorinated carbon (a-C:F) during the etching process. Experimental results for the etching of a-C:F on the silicon oxide indicated that the thickness of the film of a-C:F on silicon-oxide surface reached steady-state value that was one fifth of the steady-state value on a silicon surface. The modification of the surface in the early stages was characterized in terms of the balance between the rates of deposition and sputtering of the film of a-C:F. Infrared analysis is shown to be a powerful tool for characterizing the film of a-C:F film that is formed during oxide etching.
Excitation spectra of nitrogen bound excitons in 4H- and 6H-SiC
Authors: Egilsson T. , Ivanov I. G. , Henry A. , Janzén E.
Published: July 2002

We report photoluminescence excitation spectra of the nitrogen (N) donor bound excitons (BE) in 4H- and 6H-SiC. The spectra reveal several excited states of the N-BEs. An attempt is made in the article to classify the N-BE states according to a simple shell model.
Noise characteristics and modeling of silicon-on-insulator insulated-gate pn-junction devices
Authors: Wakita Shigeyuki , Omura Yasuhisa
Published: July 2002

This article describes the noise characteristics of various silicon-on-insulator insulated-gate pn-junction devices with anode-offset regions. The static characteristics of these devices are modeled for the noise analysis; the model is composed of a series of a metal-oxide-semiconductor field-effect transistors and the pn junction. It is shown experimentally that the noise power of the devices is proportional to In (n>0), where I is the anode current. Since the noise characteristics are not explained by the conventional theory, a phenomenological model is proposed. It is shown that the proposed basic model, which is compatible with the conventional Hooge model, can explain the experimental results. The influence of anode-offset length is also discussed and modeled.
Using computer modeling analysis in single junction a-SiGe:H p–i–n solar cells
Authors: Rubinelli F. A. , Jiménez R. , Rath J. K. , Schropp R. E. I.
Published: July 2002

In this article we discuss basic aspects of single junction a-SiGe:H p–i–n solar cells by coupling computer simulations with experimental characteristics. We are able to fit the dark illuminated current–voltage characteristics and the spectral response curves of a-SiGe:H p–i–n structures in the initial state, modeling the density of dangling bonds in each device layer by using either uniform density profiles or the defect pool model. Although we can fit these experimental curves with any of these two electrical models, band gap profiling in the a-SiGe:H intrinsic layer leads to improvement of the solar cell performance only when the defect pool model is implemented in our simulations. A U-shaped band gap profile is tailored in our samples by a staircase band gap profile composed of (i) several front band gap graded steps, (ii) one lowest band gap region, and (iii) several back band gap graded steps. Only by using the defect pool model are we able to predict an optimum thickness for the front band gap graded steps and for a buffer layer located at the p/i interface. Furthermore, using the defect pool model, the simulation predicts that higher efficiencies in single junction a-SiGe:H p–i–n solar cells can be achieved by depositing nonuniform graded steps, i.e., thicker band gap graded layers besides the lowest band gap a-SiGe:H layer and thinner band gap graded layers besides the p/i and i/n interfaces.
Evidence for the physical basis and universality of the elimination of particulates using dual-laser ablation. II. Dynamic time-resolved target reflectivity of metals and film growth of Zn
Authors: Mukherjee Pritish , Chen Shudong , Cuff John B. , Witanachchi Sarath
Published: July 2002

We have presented, in Part I of this series of two articles, the applicability of dynamic melt studies of the surface of a target under pulsed CO2 laser radiation to determine the onset of melt in these targets. Determination of this time and the accurate synchronization of an excimer (KrF) laser to coincide with the onset of melt on the target surface was shown to lead to particulate-free film deposition for Y2O3 and ZnO films. A key feature of the pump–probe reflectivity studies was the dynamic enhancement of the reflected probe signal, indicating the onset of melt. Some metallic targets, depending on the diameter of the precursor powder globules, such as the zinc target used in this study, do not yield such observable enhancements. In this article (Part II) we present the determination of the time for ablation of a variety of metallic targets under pulsed CO2 laser radiation, by monitoring the dynamic target reflectivity. The melt time is either directly determined or, in the absence of observable melt, estimated based on a simple thermal model for the absorption of the laser radiation by the target. Correlation of the calculated melt times with the morphological quality of particulate-free film growth of Zn is demonstrated. The effect of variable CO2 laser fluence on the deposited films is also demonstrated. The universality of applicability of the target reflectivity studies to a wide range of dissimilar materials, as well as the physical basis for the removal of particulates in dual-laser ablation is established.
Synergistic effects of the safety factor and shear flows on development of internal transport barriers in reversed shear plasmas
Authors: Wang A. K. , Dong J. Q. , Qu W. X. , Qiu X. M.
Published: July 2002

A new suppression mechanism of turbulent transport, characteristic of the synergism between safety factor and shear flows, is proposed to explain the internal transport barriers (ITBs) observed in neutral-beam-heated tokamak discharges with reversed magnetic shear. It is shown that the evolution of turbulent transport with the strength of the suppression mechanism reproduces the basic features of the formation and development of ITBs observed in experiments. In addition, the present analyses predict the possibility of global ion and electron heat transport barriers.
Accurate control of the misorientation angles in direct wafer bonding
Authors: Fournel Frank , Moriceau Hubert , Aspar Bernard , Rousseau Karine , Eymery Joël , Rouvière Jean-Luc , Magnea Noël
Published: July 2002

A direct wafer bonding process has been developed to accurately control both the bonding interface twist and tilt angles between a monocrystalline layer and a bare monocrystalline wafer. This process is based on the bonding of twin surfaces produced by splitting a single wafer, using for instance the Smart Cut® process. A targeted control of ±0.005° is obtained for the twist angle without any crystallographic measurement. Moreover, pure twist-bonded interfaces have been artificially made between two (001) bonded silicon surfaces.
Singular flow dynamics in three space dimensions driven by advection
Authors: Karimov A. R. , Schamel H.
Published: July 2002

The initial value problem of an ideal, compressible fluid is investigated in three space dimensions (3D). Starting from a situation where the inertia terms dominate over the force terms in Euler’s equation we explore by means of the Lagrangian flow description the basic flow properties. Special attention is drawn to the appearance of singularities in the flow pattern at finite time. Classes of initial velocity profiles giving rise to collapses of density and vorticity are found. This paper, hence, furnishes evidence of focused singularities for coherent structures obeying the 3D Euler equation and applies to potential as well as vortex flows.
Terahertz electroluminescence from superlattice quantum cascade structures
Authors: Colombelli Raffaele , Straub Axel , Capasso Federico , Gmachl Claire , Blakey Myrtle I. , Sergent A. Michael , Chu S. N. George , West Ken W. , Pfeiffer Loren N.
Published: July 2002

Intersubband electroluminescence is reported in a quantum-cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range (λ≈80 μm). Comparison with a structure based on a “vertical transition” in a single quantum well shows an increased full width at half maximum (2.8 vs 0.9 meV) of the emission line. In both cases the dependence of the optical power on the injected current is linear or sublinear and remains in the pW range.
Modeling magnetostriction in RCu2 compounds using McPhase
Authors: Rotter M. , Doerr M. , Loewenhaupt M. , Svoboda P.
Published: July 2002

In RCu2 compounds (R=rare earth) magnetostriction results from two different sources, the single ion crystal field contribution and the two ion exchange interaction. The crystal field contribution can also be observed above the magnetic ordering temperature and leads to giant magnetostriction in high magnetic fields at some members of the series. The exchange contribution has to be considered additionally when describing the magnetostriction in the magnetically ordered state. A new feature of the mean field–Monte Carlo simulation program McPhase (http://www.mcphase.de) allows to calculate the magnetic phase diagram and model the exchange contribution. We present new data of thermal expansion and the longitudinal and transversal magnetostriction of NdCu2 with magnetic field applied along the a, b, and c direction of the orthorhombic single crystal. These data are compared to the results of a McPhase simulation, which is based on exchange parameters derived from measurements of the magnetic excitations. The magnetoelastic interaction is compared to the case of GdCu2. The analysis can be extended to other RCu2 compounds.
Interparticle interactions in perpendicularly and longitudinally oriented Ba-ferrite particulate media
Authors: Tateno Yasuo
Published: July 2002

Interparticle interactions in perpendicularly and longitudinally oriented Ba-ferrite particulate media are studied experimentally. In order to make clear the effect of the orientation on the interparticle interactions in the media, the angular dependence of the interaction fields is measured by compensating for the demagnetizing fields at each measurement direction. It is found that perpendicularly and longitudinally oriented Ba-ferrite particulate media exhibit quite different angular dependence of the interaction fields, even though both of the media were prepared using the same dispersion. The study of the magnetic anisotropy derived from the interparticle interactions, which is significantly sensitive to the arrangement of the particles in the media, is considered to be useful for estimating the morphology of the magnetic layer of the media.
Numerical and experimental approbation of extended application of light figures
Authors: Shim Yonggu , Mamedov Nazim , Yamamoto Nobuyuki
Published: July 2002

The light figure based method proposed recently for determination of the relative dispersion of the optical constants has been applied to the uniaxial material, CaCO3, with accurately known refraction indices in a wide spectral range. The dispersion term determined directly from the spectral data for the ordinary refraction index and the term calculated from the approximation given by the light figure based approach coincide with high accuracy. Distribution of light intensity in light figures of 1.225×103 μm thick CaCO3 has also been measured and analyzed to ascertain the obtained value of the dispersion term. Agreement is very good. The use of the plate with known optical constants is shown to enable calibration of the deployed optical system.
Kinetic electron closures for electromagnetic simulation of drift and shear-Alfvén waves. II
Authors: Cohen Bruce I. , Dimits Andris M. , Nevins William M. , Chen Yang , Parker Scott
Published: July 2002

An electromagnetic hybrid scheme (fluid electrons and gyrokinetic ions) is elaborated in example calculations and extended to toroidal geometry. The scheme includes a kinetic electron closure valid for βe>me/mi (βe is the ratio of the plasma electron pressure to the magnetic field energy density). The new scheme incorporates partially linearized (δf ) drift-kinetic electrons whose pressure and number density moments are used to close the fluid momentum equation for the electron fluid (Ohm’s law). The test cases used are small-amplitude kinetic shear-Alfvén waves with electron Landau damping, the ion-temperature-gradient instability, and the collisionless drift instability (universal mode) in an unsheared slab as a function of the plasma βe. Attention is given to resolution and convergence issues in simulations of turbulent steady states.
Mechanical detection of ferromagnetic resonance spectrum in a normally magnetized yttrium–iron–garnet disk
Authors: Charbois V. , Naletov V. V. , Ben Youssef J. , Klein O.
Published: July 2002

The ferromagnetic resonance spectrum of a normally magnetized yttrium–iron–garnet disk, with thickness of 4.75 μm and radius of 80 μm, is measured at room temperature both by magnetic resonance force microscopy and by standard detection of the microwave susceptibility. The comparison indicates that magnetic resonance force microscopy represents one of the most potent means of obtaining the complete ferromagnetic resonance spectra of micron-size samples. In the weak coupling regime, the measured data can be quantitatively understood within the framework of the Damon and Eshbach model.