bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties and origins of different stacking faults that cause degradation in SiC PiN diodes
Authors: Jacobson H. , Bergman J. P. , Hallin C. , Janzén E. , Tuomi T. , Lendenmann H.
Published: January 2004

The electrical degradation of 4H–SiC PiN diodes has recently attracted much interest and is a critical material problem for high power applications. The degradation is caused by stacking faults observed as an increased forward voltage drop after forward injection operation. In this article we have combined electrical, optical, and structural techniques to study the formation and growth of the stacking faults causing degradation. We will show three different sources causing two different types of stacking fault properties.
Coupled core–shell model of magnetoimpedance in wires
Authors: Melo L. G. C. , Ménard D. , Ciureanu P. , Yelon A. , Cochrane R. W.
Published: January 2004

Magnetoimpedance (MI) has been studied extensively in soft magnetic wires and plates. Although a general theoretical basis has evolved, several details remain poorly understood. In particular, the amplitude of the effect in the low field region has proven impossible to fit within current models which assume a uniform static magnetization within the material. In this article, we present magnetization and MI data on CoFeSiBNb melt-extracted wires and conclude that the behavior of these materials can be analyzed on the basis of a core–shell magnetic structure. This approach introduces a nonuniform magnetization into the MI theory in such wires. We calculate the static magnetic configuration in the presence of an exchange coupling between the two regions and use it to solve for the dynamical magnetization of the outer shell using the Landau–Lifshitz and Maxwell equations to obtain the impedance as a function of the applied field and frequency. The agreement for the MI between theory and experiment is greatly improved from that of previous models.
Mapping the interaction energy surfaces of cyclic alkanes: Evaluating the transferability of an ab initio based potential model
Authors: Jalkanen Jukka-Pekka , Pakkanen Tapani A. , Rowley Richard L.
Published: January 2004

Detailed interaction energy maps are computed for symmetric cyclopropane and tetrahedrane dimer systems using ab initio methods. Interaction energies of cubane and cyclohexane dimers are also reported. The global minimum energy structures of cyclopropane and tetrahedrane systems are both D3d structures with energies of −1.850 and −2.171 kcal mol−1. The ability of NIPE potential model, based on ab initio nonbonding data of neopentane (N), isobutane (I), propane (P), ethane (E) and all their combinations [J.-P. Jalkanen, T. A. Pakkanen, Y. Yang, and R. L. Rowley, J. Chem. Phys. 118, 5474 (2003)] to predict the pair interaction energies of these strained cyclic hydrocarbons is also investigated. The difference between the energies predicted by NIPE and those obtained from the ab initio calculations increases with ring strain In general, NIPE values are in close agreement with the ab initio results for alkane ring structures having low ring strain.
Continuum description of ionic and dielectric shielding for molecular-dynamics simulations of proteins in solution
Authors: Egwolf Bernhard , Tavan Paul
Published: January 2004

We extend our continuum description of solvent dielectrics in molecular-dynamics (MD) simulations [B. Egwolf and P. Tavan, J. Chem. Phys. 118, 2039 (2003)], which has provided an efficient and accurate solution of the Poisson equation, to ionic solvents as described by the linearized Poisson–Boltzmann (LPB) equation. We start with the formulation of a general theory for the electrostatics of an arbitrarily shaped molecular system, which consists of partially charged atoms and is embedded in a LPB continuum. This theory represents the reaction field induced by the continuum in terms of charge and dipole densities localized within the molecular system. Because these densities cannot be calculated analytically for systems of arbitrary shape, we introduce an atom-based discretization and a set of carefully designed approximations. This allows us to represent the densities by charges and dipoles located at the atoms. Coupled systems of linear equations determine these multipoles and can be rapidly solved by iteration during a MD simulation. The multipoles yield the reaction field forces and energies. Finally, we scrutinize the quality of our approach by comparisons with an analytical solution restricted to perfectly spherical systems and with results of a finite difference method.
Box length search algorithm for molecular simulation of systems containing periodic structures
Authors: Schultz A. J. , Hall C. K. , Genzer J.
Published: January 2004

We have developed a box length search algorithm to efficiently find the appropriate box dimensions for constant-volume molecular simulation of periodic structures. The algorithm works by finding the box lengths that equalize the pressure in each direction while maintaining constant total volume. Maintaining the volume at a fixed value ensures that quantitative comparisons can be made between simulation and experimental, theoretical or other simulation results for systems that are incompressible or nearly incompressible. We test the algorithm on a system of phase-separated block copolymers that has a preferred box length in one dimension. We also describe and test a Monte Carlo algorithm that allows the box lengths to change while maintaining constant volume. We find that the box length search algorithm converges at least two orders of magnitude more quickly than the variable box length Monte Carlo method. Although the box length search algorithm is not ergodic, it successfully finds the box length that minimizes the free energy of the system. We verify this by examining the free energy as determined by the Monte Carlo simulation.
Modeling of microdischarge devices: Pyramidal structures
Authors: Kushner Mark J.
Published: January 2004

Microdischarge (MD) devices are plasma sources typically operating at 100s Torr to atmospheric pressure with dimensions of 10s–100s μm. Their design in based on pd (pressure×characteristic dimension) scaling; smaller dimensions are enabled by higher operating pressures with typical devices operating with pd=1–10 Torr cm. MD devices have exhibited behavior that resemble both Townsend and hollow-cathode discharges, with bulk and beam electrons providing the dominant excitation, respectively. In this article, results from a two-dimensional computational study of MD devices operating in neon using a pyramidal cathode structure are discussed. Pressures of 400–1000 Torr and device dimensions of 15–40 μm are investigated. The onset of behavior resembling negative glow discharges with decreasing pressure correlates with an extension of cathode fall accelerated beam electrons into the bulk plasma. For constant applied voltage, peak electron densities increase with increasing pressure as the beam electrons are slowed in more confined regions. The MD devices typically require higher applied voltages to operate at lower pressures, and so resemble discharges obeying Paschen’s curve for breakdown. MD devices having similar magnitudes and spatial distributions of plasma and excited state densities can be obtained to dimensions of <15 μm by keeping pd and current density constant, and having a cathode fall thickness small compared to the characteristic dimension.
Observation of transition metals at shunt locations in multicrystalline silicon solar cells
Authors: Buonassisi T. , Vyvenko O. F. , Istratov A. A. , Weber E. R. , Hahn G. , Sontag D. , Rakotoniaina J. P. , Breitenstein O. , Isenberg J. , Schindler R.
Published: January 2004

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescence microscopy, transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cell materials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.
Light diffraction from a bilayer lattice of microspheres enhanced by specular resonance
Authors: Miyazaki Hideki T. , Miyazaki Hiroshi , Jimba Yoji , Kurokawa Yoichi , Shinya Norio , Miyano Kenjiro
Published: January 2004

Anomalously strong diffraction from a bilayer lattice of dielectric microspheres, previously reported by the authors, has been attributed to the enhancement of diffraction by specular resonance in constituent bispheres. On the basis of rigorous calculations and experiments, specular resonance from bispheres is found to be dominant even in the scattering from a cluster of several microspheres. As a consequence, a diffraction model, in which a bilayer lattice of microspheres is viewed as a two-dimensional array of bispheres, is constructed. By incorporating the specular resonance into a diffraction theory of two-dimensional lattices as a structure factor, observed diffraction behavior is explained. In the diffraction from a bilayer lattice of microspheres, the specular resonance functions as a blazing mechanism. Possible configurations and suitable parameters for efficient gratings are discussed, and a self-assembled bilayer lattice of dielectric microspheres with a diameter 1.6–3.2 times as large as the wavelength is found to be promising as a low-cost and highly efficient transmission grating. The spheres can be replaced with cylinders or lenses. Tunable diffraction devices with efficiency exceeding that of conventional gratings would be realized by microelectromechanical systems comprised of two optimally designed microlens arrays.
Two breakdown mechanisms in ultrathin alumina barrier magnetic tunnel junctions
Authors: Oliver Bryan , Tuttle Gary , He Qing , Tang Xuefei , Nowak Janusz
Published: January 2004

Two breakdown mechanisms are observed in magnetic tunnel junctions having an ultrathin alumina barrier. The two breakdown mechanisms manifest themselves differently when considering large ensembles of nominally identical devices under different stress conditions. The results suggest that one type of breakdown occurs because of the intrinsic breakdown of a well-formed oxide barrier that can be described by the E model of dielectric breakdown. The other is an extrinsic breakdown related to defects in the barrier rather than the failure of the oxide integrity. The characteristic of extrinsic breakdown suggests that a pre-existing pinhole in the barriers grows in area by means of dissipative (Joule) heating and/or an electric field across the pinhole circumference.
Separation of copper ion-induced and intrinsic polymer instabilities in polyarylether using triangular voltage sweep
Authors: Mallikarjunan A. , Murarka S. P. , Lu T.-M.
Published: January 2004

A low dielectric constant polyarylether (PAE) polymer was investigated for resistance to copper drift. Positive bias-temperature stressing (BTS) of metal/PAE/SiO2/Si capacitors led to large initial flat-band voltage shifts toward the left-hand side in the capacitance–voltage curve for all gate metals investigated (Pt, Al, and Cu). Upon triangular voltage sweep (TVS) testing, Pt/PAE/SiO2/Si capacitors exhibited no peak, whereas a single peak (indicative of mobile ions) was seen for Cu/PAE/SiO2/Si capacitors. The results indicate that an intrinsic polymer instability source, such as polarization (under bias), was the major cause for the BTS instability of the polymer. From TVS studies, Cu+ ion drift could be characterized in the temperature range of 150–250 °C, and mobility of Cu+ in PAE was extracted from Hillen’s analysis. A combination of BTS and TVS techniques is thus more useful for characterization of copper drift, and helps to resolve the intrinsic polymer effect from that of metal drift-related instability.
Photoluminescence properties of single CdS nanorods
Authors: Kulik D. , Htoon H. , Shih C. K. , Li Yadong
Published: January 2004

We present low-temperature studies of the optical properties of single CdS nanorods in relation to their morphology. Keeping track of the same nanorods in the photoluminescence (PL) study and during scanning electron microscope imaging, we were able to distinguish intrinsic spectral features of the nanorods from those of nanocluster aggregates. We observed strong PL polarization in the direction perpendicular to the nanorod axis. The sharp peaks in the “blue” region of the PL spectra are redshifted relative to those of bulk CdS and of CdS microparticles in a glass matrix. Finally, we observe a very interesting “spectral diffusion” of the main emission peaks with the following characteristics: (a) Energy–energy correlation of the two main peaks, and (b) correlation and anticorrelation between energy and intensity for the two peaks. Possible mechanisms behind this phenomenon are discussed.
First-principles study on field evaporation for silicon atom on Si(001) surface
Authors: Ono Tomoya , Hirose Kikuji
Published: January 2004

The simulations of field-evaporation processes for silicon atoms on various Si(001) surfaces are implemented using the first-principles calculations based on the real-space finite-difference method. We find that the atoms which locate on atomically flat Si(001) surfaces and at step edges are easily removed by applying an external electric field, and the threshold value of the external electric field for evaporation of atoms on atomically flat Si(001) surfaces, which is predicted between 3.0 and 3.5 V/Å, is in agreement with the experimental data of 3.8 V/Å. In this situation, the local field around an evaporating atom does not play a crucial role. This result is instead interpreted in terms of the bond strength between an evaporating atom and surface.
In situ transmission electron microscopy study of the crystallization of Ge2Sb2Te5
Authors: Kooi B. J. , Groot W. M. G. , De Hosson J. Th. M.
Published: January 2004

Crystallization of amorphous Ge2Sb2Te5 films (10, 40, and 70 nm thick) was studied by in situ heating in a transmission electron microscope (TEM). Electron irradiation-induced crystallization is possible at room temperature using a 400 kV electron beam where the reciprocal of the incubation time for crystallization scales linearly with the current density during electron irradiation. Without electron-beam exposure, crystallization starts at 130 °C. Using a 200 kV beam, crystallization also occurred in the temperature interval between 70 and 130 °C. In principle, electron irradiation always affects the crystallization kinetics, strongly promoting nucleation and probably not hampering growth. At 130 °C without electron-beam exposure, 400 nm diameter colonies of 10–20 nm grains develop in the 40 and 70 nm thick films showing clear symmetric bending contour contrast. These spherulites prefer to have in their center the 〈111〉 zone axis of the Fm3̄m structure perpendicular to the surface of the film and show a typical tilt variation of ±10°. At 340 °C, the transition from the metastable to the stable trigonal (P3̄m1) crystal structure takes place. Fast and excessive grain growth occurs with the [0001] axis perpendicular to the film surface of the film. Also shown is that oxidation of the Ge2Sb2Te5 film strongly influences its crystallization; its critical temperature decreases from 130 to 35 °C.
Effective dielectric response of nonlinear composites with external ac and dc electric field
Authors: Wei En-Bo , Song Jin-Bao , Gu Guo-Qing
Published: January 2004

The perturbation method is developed to investigate the effective nonlinear dielectric response of Kerr composites when the external ac and dc electric field is applied. Under the external ac and dc electric field Eapp=Ea(1+sin ωt), the effective coupling nonlinear response can be induced by the cubic nonlinearity of Kerr nonlinear materials at the zero frequency, the finite basic frequency ω, the second and the third harmonics, 2ω and 3ω, and so on. As an example, we have investigated the cylindrical inclusions randomly embedded in a host and derived the formulas of the effective nonlinear dielectric response at harmonics in dilute limit. For a higher concentration of inclusions, we have proposed a nonlinear effective-medium approximation by introducing the general effective nonlinear response. With the relationships between the effective nonlinear response at harmonics and the general effective nonlinear response, we have derived a set of formulas of the effective nonlinear dielectric responses at harmonics for a larger volume fraction.
Fishtail effect studied by ac susceptibility in ErBa2Cu3O7−δ single crystal
Authors: Cardoso C. A. , de Lima O. F.
Published: January 2004

The second magnetization peak (SMP), also known as the fishtail effect, is studied by a scaling procedure for ac susceptibility measurements. From this scaling law the frequency dependence of the critical current J(ν) and the flux creep exponent n can be determined. A striking correlation between n and the SMP is observed, which indicates a clear change in the rate of flux creep around the peak position. This result points to the relevance of the dynamical contribution for the peak formation. However, we could not observe a transition between two different regimes of vortex motion at the SMP. The dependence of the SMP with sample dimensions is also discussed.
Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system
Authors: Cótica L. F. , Zanatta S. C. , Rocha M. A. , Santos I. A. , Paesano A. , da Cunha J. B. M. , Hallouche B.
Published: January 2004

The system (α-Fe2O3)x(α-Al2O3)1−x was subjected to 24 h of high-energy ball-milling varying its nominal concentration, x. The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. Mössbauer studies were also performed in the temperature range 250–6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low α-Fe2O3 initial concentration (i.e., x⩽0.12), the paramagnetic solid solution α-(FeΔYAl1−ΔY)2O3, the α-Fe and the FeAl2O4 phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For x>0.12, the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, α-(Fe1−ΔWAlΔW)2O3, could be seen to increase with increasing x. This solid solution was shown to be transitional, at room temperature, between two ordering states, weak ferromagnetic and superparamagnetic, the latter resulting from the milling induced particle size reduction. Low temperature Mössbauer spectra revealed the magnetic ordering of both solid solutions, α-(FeΔYAl1−ΔY)2O3 and α-(Fe1−ΔWAlΔW)2O3, and indicated the suppression of the Morin transition for the iron-rich solid solution. The magnetization versus magnetic field curves obtained for samples with x⩾0.12 revealed, besides a general superparamagnetic character, some hysteretic behavior due to the magnetic phases eventually existing.
Microstructure and dielectric properties of Mg-doped barium strontium titanate ceramics
Authors: Su B. , Button T. W.
Published: January 2004

The dielectric properties of Ba0.8Sr0.2TiO3 ceramics doped with Mg from 0.5 to 10 mol % have been investigated systematically. Two effects of Mg doping on the dielectric properties of the Ba0.8Sr0.2TiO3 ceramics have been observed. At low Mg doping concentrations (<2 mol %), Mg mainly acts as an acceptor dopant to replace Ti in the B site of ABO3 perovskite structure, leading to a shift of Curie point (TC) to lower temperatures and a diffused phase transition. The permittivity and dielectric loss are suppressed gradually as the Mg content increases and the grain size decreases drastically. At higher Mg doping concentrations (⩾2 mol %), further suppression of permittivity and losses without further TC shift are observed, which indicates a “composite” mixing effect. The grain size remains almost constant. The solubility limit of Mg in the Ba0.8Sr0.2TiO3 appears to be ∼2 mol %.
Amorphization/recrystallization of buried amorphous silicon layer induced by oxygen ion implantation
Authors: de Souza J. P. , Cima C. A. , Fichtner P. F. P. , Boudinov H.
Published: January 2004

In this paper we discuss the structural modifications observed in a buried amorphous Si (a-Si) layer containing high oxygen concentration level (up to ∼3 at. %) after being implanted at elevated temperature with O+16 ions. For implants conducted at temperatures lower than 150 °C, the a-Si layer expands via layer by layer amorphization at the front and back amorphous–crystalline (a–c) interfaces. When performed at temperatures above 150 °C, the implants lead to the narrowing of the buried a-Si layer through ion beam-induced epitaxial crystallization at both a–c interfaces. Cross section transmission electron microscopy analysis of samples implanted at 400 °C revealed an array of microtwins and a dislocation network band in the recrystallized material. In samples implanted at 550 °C, only a buried dislocation network band is observed.
Molecular dynamics study of atomic transport properties in rapidly cooling liquid copper
Authors: Chen F. F. , Zhang H. F. , Qin F. X. , Hu Z. Q.
Published: January 2004

Based on Mei’s embedded atom model [Mei et al., Phys. Rev. B 43, 4653 (1991)] molecular dynamics simulations have been performed to investigate the rapidly cooling processes of Cu. The atomic transport property, namely the self-diffusion coefficient, is computed in the liquid state, and the results near the melting point of Cu are in good agreement with experimental data and other computational values. The atom diffusion movements during the long period of relaxation have been also studied around the solidification temperature Tc. To describe the complex microstructural evolutions during the rapidly cooling processes and the long relaxation processes, the pair correlation function and the pair analysis technique are used. It is demonstrated that the crystallization of amorphous Cu is caused by the atomic diffusion.
Molecular dynamic simulation of the hydration and diffusion of chloride ions from bulk water to polypyrrole matrix
Authors: López Cascales J. J. , Otero T. F.
Published: January 2004

A molecular dynamic simulation of wet polypyrrole film was carried out, in both oxidized and reduced state. The system was modeled by two layers of polypyrrole, water and chloride ions (as counterions required for charge balance in the oxidized state) in atomic detail to provide an insight into some dynamic and steady properties of the system. Our simulations pointed to a swelling of the polymer matrix after oxidation due to electrostatic repulsions between charged sites of the oxidized polypyrrole, followed by penetration of the polypyrrole by counterions to maintain the electroneutrality of the system. Associated with this penetration of counterions toward the core of the oxidized polypyrrole, dehydration of the counterions was observed. This dehydration was compensated (in part) by a strong coordination with the charged sites of the polymer. The remaining hydrophobicity inside the polymer also contributed to the dehydration of these counterions. The translational diffusion coefficient of chloride ions was also calculated at different positions of the polypyrrole/water interface, from bulk water to the inner polymer matrix. A value of 4.1×10−5 cm2 s−1 was measured in the bulk water compared to 5×10−7 cm2 s−1 inside the polymer, representing a diminution of two orders of magnitude for the translational diffusion coefficient from bulk water to the core of a oxidized polypyrrole matrix. These results were in good agreement with experimental data.
Polymers confined between two parallel plane walls
Authors: Hsu Hsiao-Ping , Grassberger Peter
Published: January 2004

Single three-dimensional polymers confined to a slab, i.e., to the region between two parallel plane walls, are studied by Monte Carlo simulations. They are described by N-step walks on a simple cubic lattice confined to the region 1⩽z⩽D. The simulations cover both regions D≪RF and D≫RF (where RF∼Nν is the Flory radius, with ν≈0.587), as well as the cross-over region in between. Chain lengths are up to N=80 000, slab widths up to D=120. In order to test the analysis program and to check for finite size corrections, we actually studied three different models: (a) ordinary random walks (mimicking Θ polymers); (b) self-avoiding walks; and (c) Domb–Joyce walks with the self-repulsion tuned to the point where finite size corrections for free (unrestricted) chains are minimal. For the simulations we employ the pruned-enriched-Rosenbluth method with Markovian anticipation. In addition to the partition sum (which gives us a direct estimate of the forces exerted onto the walls), we measure the density profiles of monomers and of end points transverse to the slab, and the radial extent of the chain parallel to the walls. All scaling laws and some of the universal amplitude ratios are compared to theoretical predictions.
A doubly nudged elastic band method for finding transition states
Authors: Trygubenko Semen A. , Wales David J.
Published: January 2004

A modification of the nudged elastic band (NEB) method is presented that enables stable optimizations to be run using both the limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) quasi-Newton and slow-response quenched velocity Verlet minimizers. The performance of this new “doubly nudged” DNEB method is analyzed in conjunction with both minimizers and compared with previous NEB formulations. We find that the fastest DNEB approach (DNEB/L-BFGS) can be quicker by up to 2 orders of magnitude. Applications to permutational rearrangements of the seven-atom Lennard-Jones cluster (LJ7) and highly cooperative rearrangements of LJ38 and LJ75 are presented. We also outline an updated algorithm for constructing complicated multi-step pathways using successive DNEB runs.
Full-dimensional quantum calculations of vibrational spectra of six-atom molecules. I. Theory and numerical results
Authors: Yu Hua-Gen
Published: January 2004

Two quantum mechanical Hamiltonians have been derived in orthogonal polyspherical coordinates, which can be formed by Jacobi and/or Radau vectors etc., for the study of the vibrational spectra of six-atom molecules. The Hamiltonians are expressed in an explicit Hermitian form in the spatial representation. Their matrix representations are described in both full discrete variable representation (DVR) and mixed DVR/nondirect product finite basis representation (FBR) bases. The two-layer Lanczos iteration algorithm [H.-G. Yu, J. Chem. Phys. 117, 8190 (2002)] is employed to solve the eigenvalue problem of the system. A strategy regarding how to carry out the Hamiltonian-vector products for a high-dimensional problem is discussed. By exploiting the inversion symmetry of molecules, a unitary sequential 1D matrix-vector multiplication algorithm is proposed to perform the action of the Hamiltonian on the wavefunction in a symmetrically adapted DVR or FBR basis in the azimuthal angular variables. An application to the vibrational energy levels of the molecular hydrogen trimer (H2)3 in full dimension (12D) is presented. Results show that the rigid-H2 approximation can underestimate the binding energy of the trimer by 27%. Finally, it is demonstrated that the two-layer Lanczos algorithm is also capable of computing the eigenvectors of the system with minor effort.
Isotopic effect on the cage-induced quenching of OH(A)/OD(A) inside small argon clusters
Authors: Kanaev A. , Museur L. , Edery F. , Laarmann T. , Möller T.
Published: January 2004

In this paper we report on the isotopic effect on the cage-induced excited-state quenching inside small Arm clusters (m<102) solvated in large NeN clusters (N≈7.5×103). Excited OH(A)/OD(A) fragments are produced by photodissociation of H2O and D2O molecules and the quenching agents are correspondingly H or D atoms. The decrease of the fluorescence yield with the size of the cluster m>m0 is observed in both cases and it is attributed to the formation of the cage of argon atoms around the doped molecule. Interestingly, more atoms are needed to induce the fluorescence quenching of OD*(A) fragments, m0=21±3, compared to the electronically excited state quenching of OH*(A) molecules, 11±2. A diffusion model containing two free parameters, the quenching cross section σq and the number of argon atoms forming the cage m0, explains the effect in terms of the residence time of the hydrogen atom inside the cage. We suggest that the melting of the doped rare gas clusters is responsible for the different predissociation dynamics. The quenching cross section obtained from the experimental data is in good agreement with former experiments.
A combined crossed beam and theoretical investigation of O(3P)+C3H3→C3H2+OH
Authors: Lee Hohjai , Joo Sun-Kyu , Kwon Lee-Kyoung , Choi Jong-Ho
Published: January 2004

The radical–radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated in a crossed beam configuration. The radical reactants O(3P) and C3H3 were produced by the photodissociation of NO2 and the supersonic flash pyrolysis of precursor propargyl bromide, respectively. A new exothermic channel of O(3P)+C3H3→C3H2+OH was identified and the nascent distributions of the product OH in the ground vibrational state (X 2Π:ν″=0) showed bimodal rotational excitations composed of the low- and high-N″ components without spin–orbit propensities. The averaged ratios of Π(A′)/Π(A″) were determined to be 0.60±0.28. With the aid of ab initio theory it is predicted that on the lowest doublet potential energy surface, the reaction proceeds via the addition complexes formed through the barrierless addition of O(3P) to C3H3. The common direct abstraction pathway through a collinear geometry does not occur due to the high entrance barrier in our low collision energy regime. In addition, the major reaction channel is calculated to be the formation of propynal (CHCCHO)+H, and the counterpart C3H2 of the probed OH product in the title reaction is cyclopropenylidene (1c-C3H2) after considering the factors of barrier height, reaction enthalpy and structural features of the intermediates formed along the reaction coordinate. On the basis of the statistical prior and rotational surprisal analyses, the ratio of population partitioning for the low- and high-N″ is found to be about 1:2, and the reaction is described in terms of two competing addition-complex mechanisms: a major short-lived dynamic complex and a minor long-lived statistical complex. The observed unusual reaction mechanism stands in sharp contrast with the reaction of O(3P) with allyl radical (C3H5), a second significant conjugated hydrocarbon radical, which shows totally dynamic processes [J. Chem. Phys. 117, 2017 (2002)], and should be understood based upon the characteristic electronic structures and reactivity of the intermediates on the potential energy surface.
Excitonic coupling in polythiophenes: Comparison of different calculation methods
Authors: Beenken Wichard J. D. , Pullerits Tõnu
Published: January 2004

In conjugated polymers the optical excitation energy transfer is usually described as Förster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers.
Thermal instability of ionized dusty plasmas
Authors: Bora Madhurjya P.
Published: January 2004

We investigate the role of thermal instability, arising from radiative cooling of an optically thin, ionized, dusty plasma, by linear stability analysis. The corresponding isobaric stability condition for condensation mode is found to be modified significantly by the concentration of finite sized, relatively heavy, and negatively charged dust particles. It has been shown that the radiation condensation mode is severely affected by the presence of dust particles. A distinct departure from the classical behavior (Field 1965) is that the existence of unstable acoustic modes depends on the dust-charge fluctuation parameter and is not affected by the cooling through radiation.
Low-frequency instabilities in a laboratory magnetized plasma column
Authors: Gravier E. , Brochard F. , Bonhomme G. , Pierre T. , Briançon J. L.
Published: January 2004

A detailed analysis of the phase velocity of unstable low-frequency waves is performed in a laboratory magnetized plasma column. The measurement of the radial profiles of the density, electron temperature and plasma potential allows to determine the radial profile of the electric drift velocity and electron diamagnetic drift for increasing values of the magnetic field. In the case of a large diameter plasma column, only diamagnetic drift waves without E×B Doppler shift occur. On the other hand, in the case of a restricted diameter plasma column, the radial electric field induces a rotation of the plasma column. At low magnetic field the recorded unstable waves are in that case flute modes propagating azimuthally at the E×B drift velocity.
Postdeposition reactivity of sputter-deposited high-dielectric-constant films with ambient H2O and carbon-containing species
Authors: Gougousi Theodosia , Parsons Gregory N.
Published: January 2004

The room temperature reactivity of group III and IV based high-dielectric-constant films during long-term (>1 year) exposure to ambient atmosphere (H2O- and C-containing species such as CO2) has been studied in order to assess the stability of the materials. We prepare the films by physical vapor deposition of metal on Si (100) surfaces which is followed by oxidation in N2O (1 atm), and monitor the formation of carbonate/alkoxylate and hydroxide species using Fourier transform infrared spectroscopy. For La-based films we find that the oxidation temperature has a strong effect on the reactivity with the ambient. For Hf- and Zr-based films, formation of various carbonate or alkoxylate species is confirmed within minutes of exposure, while substantial reaction with H2O is not detected. Hf-based films, in general exhibit superior stability upon long-term exposure.
Structure, magnetic, and transport properties of the perovskites Bi0.5Ca0.5Mn1−xCrxO3
Authors: Xiong C. M. , Sun J. R. , Li R. W. , Zhang S. Y. , Zhao T. Y. , Shen B. G.
Published: January 2004

Structure, magnetic, and transport properties of the compounds Bi0.5Ca0.5Mn1−xCrxO3 (BCMCO) with 0⩽x⩽0.15 have been experimentally investigated. The substitution of Cr3+ ion for Mn3+ ion suppresses the charge-ordering (CO) state obviously, leading to a gradual reduction of the CO transition temperature (TCO). In contrast, the antiferromagnetic (AF) ordering temperature is essentially unaffected until x=0.024. A spin-glasslike behavior appears above x=0.024 due to the competition between the antiferromagnetic and ferromagnetic correlations. The latter emerges and develops with the incorporation of Cr. The AF phase disappears completely for x⩾0.1, whereas the CO phase is still perceptible for x=0.15. A further analysis indicates that Cr doping produces a similar effect on TCO for the Bi-based and lanthanide-based manganites: TCO decreases at the same rate of ∼8 K for 1% Cr, despite the different initial values of TCO. A striking observation of the present work is that for the manganites, there exists a characteristic temperature 180 K, and the CO state will be unstable when TCO<180 K. As a result, a heavier Cr doping is required to completely destroy the CO phase with a higher TCO. For BCMCO, the critical Cr content is estimated to be about 0.2. Electrical study shows that the conduction of BCMCO proceeds via variable-range hopping of charge carriers, and a huge magnetoresistance can be produced by Cr doping.
Nanoscale structural change in a sputter-deposited SiO2/a-Si/SiO2 sandwich
Authors: Li B. Q. , Xu W. T. , Fujimoto T. , Kojima I.
Published: January 2004

We report direct evidence that annealing a sputter-deposited SiO2/a-Si/SiO2 sandwiched structure on Si(100) results in the Si layer to expand. X-ray reflectivity (XRR) revealed the thickness of the Si layer to be temperature-dependent. Auger depth profiling (ADP) detected marked difference in the annealing behavior between the SiO2/a-Si and a-Si/SiO2 interfaces and oxygen diffusion into the Si layer at the a-Si/SiO2 interface. High-resolution electron microscopy observation further identified the formation of Si nanocrystals, the increase in thickness of the Si layer, and the presence of oxygen-rich nanocavities at the Si/SiO2 interface, which confirms the XRR and ADP measurements.
All-organic thin-film transistors made of poly(3-butylthiophene) semiconducting and various polymeric insulating layers
Authors: Parashkov R. , Becker E. , Ginev G. , Riedl T. , Johannes H.-H. , Kowalsky W.
Published: January 2004

We have fabricated fully patterned all-organic thin-film transistors with a variety of organic polymer insulators. Poly(3-butylthiophene) deposited by spin coating was used as the active organic layer. We have built top-gate structures with gates printed on top of the gate dielectric layer. The field enhanced current is weak with poly(4-vinyl phenol), but much stronger with polyvinyl alcohol and cyanoethylpullulan. Carrier mobilities as large as 0.04 cm2/V s were measured in the case of cyanoethylpullulan. A strong correlation is found between the solvents used for the dielectrics, dielectric constant of the insulator, and the field-effect mobility.
Shape stability of TiSi2 islands on Si (111)
Authors: Yang W.-C. , Ade H. , Nemanich R. J.
Published: January 2004

The evolution of the shape and size of TiSi2 islands on Si (111) surfaces is explored with real time ultraviolet photoelectron emission microscopy. During continuous deposition of Ti at elevated temperatures, individual islands in a dilute surface distribution grow larger without island–island interactions. As they increase in size, symmetric islands transform into elongated shaped islands with high length-to-width aspect ratios. An extremely elongated island shows a ratio of 85:1 and is ∼17 μm long and ∼0.2 μm wide. The individual elongated islands have different widths regardless of their length. The width of the growing islands is determined at the initial transition stage and remains essentially constant with increasing length. We propose that the various widths of the elongated islands are determined by the degree of strain relaxation, possibly through the nucleation of dislocations at the island interface. In addition, it is found that the elongated islands display a prism-like shape or a truncated prism-like shape. We propose that the shape evolution of the elongated islands is related to both strain relaxation and growth kinetics.
Functional separation in two frequency operation of an inductively coupled plasma
Authors: Denda T. , Miyoshi Y. , Komukai Y. , Goto T. , Petrović Z. Lj. , Makabe T.
Published: January 2004

Measurements of densities of excited atoms and metastables were performed in pure Ar and in mixtures of Ar and CF4 in inductively coupled plasma sustained by a high frequency (13.56 MHz) source and biased by a low frequency (500 kHz) voltage applied to the wafer supporting electrode. The measurements are made in front of the biased electrode with a goal to understand the effects of different parameters on the plasma profile and to test whether functional separation between plasma sustaining and biasing voltage is achieved. We find a very efficient separation with small or no observable effects of biasing voltage both in pure argon and in mixtures. These results have been achieved at all pressures (5–50 mTorr) and were confirmed by additional microwave measurements of electron density. The effect of flow rate, pressure, power, and distance from the biased electrode was studied from the spatial profiles of short lives excited states and metastable states of argon. We have also compared the profiles close to the biasing electrode, close to the coil and in extended processing chamber, and found a slight increase of metastable density close to the biasing electrode due to reduced electron quenching far from plasma source.
Stretched exponential relaxation in perovskite ferroelectrics after cyclic loading
Authors: Lupascu Doru C. , Fedosov Sergei , Verdier Cyril , Rödel Jürgen , von Seggern Heinz
Published: January 2004

The switching kinetics in fatigued polycrystalline bulk ferroelectrics do not exhibit relaxation with a singular time constant but rather stretched exponential behavior. Several microscopic mechanisms are discussed to explain this effect for a polycrystalline material. Some of the mechanisms may apply equally well to thin films, others are characteristic of a macroscopic sample or a thick film.
Temperature dependence of the piezoelectric response in lead zirconate titanate films
Authors: Wolf R. A. , Trolier-McKinstry S.
Published: January 2004

The temperature dependence of the effective transverse piezoelectric coefficient (e31,f) in lead zirconate titanate (PZT) thin films was measured between −55 and 85 °C. e31,f was calculated by simultaneously monitoring the piezoelectric charge output and strain in the film during wafer flexure. This method was used to characterize the temperature dependence of e31,f in PZT films with 2, 4, and 6 μm thickness and 40/60, 52/48, and 60/40 Zr/Ti ratios. |e31,f| was found to increase with temperature and average increases were 46%, 32%, and 12% for films with PZT 60/40, 52/48, and 40/60 compositions, respectively. Measurement uncertainty ranged from ±3%–12%. The measured temperature dependences of e31,f were consistent with the rapid rise in intrinsic d31 as Tc is approached, suggesting that they were controlled by intrinsic contributions. Additional contributors to the measured variation in the PZT film piezoelectric response over the measured temperature range were identified. Changes in film elastic properties could have decreased measured e31,f values by to 1%–4% in heating from −55 to 85 °C. Changes in the biaxial film stress with temperature were expected to account for 1%–3% of the measured increase in e31,f from −55 to 85 °C.
Photothermal investigation of the thermal shock behavior of alumina ceramics for engine components
Authors: Li Bincheng , Mandelis Andreas , Kish Zoltan Z.
Published: January 2004

The photothermal radiometric technique is used to measure the thermal properties of alumina ceramic materials (96% Al2O3/3% SiO2/0.9% MgO) treated by applying high temperature and high pressure, a process known as ASPRO conversion technology. Alumina ceramics subjected to ASPRO treatment have shown much higher thermal shock resistance than corresponding untreated ceramics. A theoretical model for thermal conduction in a three-layered sample, in which the thermal resistance at grain boundaries is taken into account, is developed to interpret the experimental data. The experimental results with both untreated and ASPRO treated ceramic samples show that the improvement in thermal shock behavior is the result of the reduction of thermal resistance between ceramic grain boundaries. The good agreement of the experimental results to the three-layered theoretical model indicates that the thermal-wave behavior of these samples is consistent with the presence of an inter-grain thermal boundary impedance which controls the thermal shock behavior of the alumina ceramics.
Quantum-mechanical theory of atom-molecule and molecular collisions in a magnetic field: Spin depolarization
Authors: Krems R. V. , Dalgarno A.
Published: January 2004

A theory for quantum-mechanical calculations of cross sections for atom-molecule and molecular collisions in a magnetic field is presented. The formalism is based on the representation of the wave function as an expansion in a fully uncoupled space-fixed basis. The systems considered include S1-atom-Σ2-molecule, S1-atom-Σ3-molecule, Σ2-molecule–Σ2-molecule, and Σ3-molecule–Σ3-molecule. The theory is used to elucidate the mechanisms for collisionally induced spin depolarization.
Kinetics of C2(a3Πu) radical reactions with alkanes by LIF
Authors: Huang Cunshun , Zhu Zhiqiang , Xin Yao , Pei Linsen , Chen Congxiang , Chen Yang
Published: January 2004

The reactions of C2(a3Πu) radicals with a series of alkanes have been studied at room temperature and 6.5 torr total pressure using the pulsed laser photolysis/laser-induced fluorescence technique. C2(a3Πu) radicals were generated by photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd: YAG laser at 266 nm. The relative concentration of C2(a3Πu) radicals was monitored on the (0,0) band of the C2(d3Πg←a3Πu) transition at 516.5 nm by laser-induced fluorescence. From the analysis of the relative concentration-time behavior of C2(a3Πu) under pseudofirst-order conditions, the rate constants for the reactions of C2(a3Πu) with alkanes (C1–C8) were determined. The rate constant increases linearly with the increasing of the number of CH2 groups in the alkanes. The experimental results indicate that the reaction of C2(a3Πu) with small alkanes (C1–C8) follows the typical hydrogen abstraction process. Based on the correlation of the experimental results with the bond dissociation energy of the alkanes, the reactions of C2(a3Πu) with small alkanes likely proceed via the mechanism of hydrogen abstraction.
Stereographic projections path integral for inertia ellipsoids: Applications to Arn–HF clusters
Authors: Russo M. F. , Curotto E.
Published: January 2004

The DeWitt formula for inertia ellipsoids mapped by stereographic projection coordinates is developed. We discover that by remapping the quaternion parameter space with stereographic projections, considerable simplification of the differential geometry for the inertia ellipsoid with spherical symmetry takes place. The metric tensor is diagonal and contains only one independent element in that case. We find no difficulties testing and implementing the DeWitt formula for the inertia ellipsoids of asymmetric tops mapped by stereographic projections. The path integral algorithm for the treatment of Rm⊗S2 manifolds based on a mixture of Cartesian and stereographic projection coordinates is tested for small Arn–HF clusters in the n=2 to n=5 range. In particular, we determine the quantum effects of the red shift and the isomerization patterns at finite temperatures. Our findings are consistent with previously reported computations and experimental data for small Arn–HF clusters.
Effect of pressure on dynamic heterogeneity in dendrimeric alkyd resin
Authors: Paluch M. , Sekula M. , Maślanka S. , Mańczyk K. , Sułkowski W. W. , Rzoska S. J. , Ziolo J.
Published: January 2004

Broadband dielectric spectroscopy is employed to investigate the non-Debye relaxation behavior in a dendrimeric alkyd resin. From temperature-dependent measurements at ambient pressure, we found a very broad distribution of relaxation times. This is attributed to the complex geometrical topology of the molecule. However, compression significantly reduces the non-Debye character of the dielectric response; thus, pressure induces dynamic homogeneity in the dendrimeric alkyd resin.