bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterogeneous structure of poly(vinyl chloride) as the origin of anomalous dynamical behavior
Authors: Arbe A. , Moral A. , Alegrı́a A. , Colmenero J. , Pyckhout-Hintzen W. , Richter D. , Farago B. , Frick B.
Published: July 2002

We have investigated the thermal evolution of the structure and the dynamics of poly(vinyl chloride) (PVC) in a wide temperature range. Corroborating earlier findings, small angle neutron scattering revealed the presence of structural heterogeneities. On the other hand, the single chain form factor corresponds to that of Gaussian chains. Gradually with increasing temperature the system becomes homogeneous. A simple description of the heterogeneities in terms of microcrystallites is forwarded. The dynamical behavior of PVC has been investigated combining broadband dielectric spectroscopy (DS) with coherent and incoherent neutron scattering. In a wide temperature range broadband DS facilitated a precise determination of the dynamic response related to the segmental relaxation. Close to the glass transition temperature the line shape strongly deviates from the usual Kohlrausch–Williams–Watts functional form of common glassforming systems. Moreover, the characteristic relaxation time observed by incoherent scattering displays an anomalous dependence on momentum transfer indicating the possible existence of heterogeneities in the sample. Based on the structural and dynamical results, a model is proposed, that considers the coexistence of regions with different dynamical properties leading to a distribution of characteristic relaxation times. The model accounts for the experimental observations, assuming for all regions the same functional form for the α-relaxation. It may be univocally determined from the coherent scattering data at the first static structure peak. The distribution of relaxation times found is compatible with the distribution of only one variable, the glass transition temperature.
Assessment of the Handy–Cohen optimized exchange density functional for organic reactions
Authors: Baker Jon , Pulay Peter
Published: July 2002

We have investigated the performance of the new optimized exchange functional (OPTX) developed by Handy and Cohen [Mol. Phys. 99, 403 (2001)] for predicting geometries, heats of reaction, and barrier heights for twelve organic reactions (six closed-shell and six radical). OPTX has been used in conjunction with, among others, the well-known Lee–Yang–Parr (LYP) correlational functional to form two new functionals, OLYP and O3LYP. These are similar to the well-established BLYP and B3LYP functionals, respectively, with OPTX replacing the standard Becke exchange functional, B88. Our results strongly support claims made by their developers that OLYP is superior to BLYP, and essentially renders it obsolete. The computed OLYP heats of reaction, barrier heights, and even molecular geometries (with larger basis sets), are comparable with, if not better than, the corresponding B3LYP values. The O3LYP functional is overall better than B3LYP, albeit not by much. Both OLYP and O3LYP are among the best functionals currently available; the performance of OLYP in particular is noteworthy given that this functional includes no exact exchange.
Dynamics of proton-acetylene collisions at 30 eV
Authors: Malinovskaya S. A. , Cabrera-Trujillo R. , Sabin John. R. , Deumens E. , Öhrn Y.
Published: July 2002

Collisions of protons with ground state acetylene molecules at 30 eV are studied using the electron nuclear dynamics (END) theory. This time-dependent methodology for the study of molecular processes is a nonadiabatic approach to direct dynamics, which has been successfully applied to ion–atom and ion–molecule reactive collisions. Using the minimal END theory, we calculate the direct and charge-transfer differential cross sections. Different initial conditions lead to diverse product channels, such as charge transfer, proton exchange, and collision induced dissociation. Projectile energy loss is analyzed in terms of transfer into target electronic, translational, and rovibrational excitations. The comparison of the computed results with time-of-flight measurements is discussed.
Reconstruction of interatomic vectors by principle component analysis of nuclear magnetic resonance data in multiple alignments
Authors: Hus Jean-Christophe , Brüschweiler Rafael
Published: July 2002

A general method is presented for the reconstruction of interatomic vector orientations from nuclear magnetic resonance (NMR) spectroscopic data of tensor interactions of rank 2, such as dipolar coupling and chemical shielding anisotropy interactions, in solids and partially aligned liquid-state systems. The method, called PRIMA, is based on a principal component analysis of the covariance matrix of the NMR parameters collected for multiple alignments. The five nonzero eigenvalues and their eigenvectors efficiently allow the approximate reconstruction of the vector orientations of the underlying interactions. The method is demonstrated for an isotropic distribution of sample orientations as well as for finite sets of orientations and internuclear vectors encountered in protein systems.
A microbeam collimator for high resolution x-ray diffraction investigations with conventional diffractometers
Authors: Papaioannou Dimitrios , Spino José
Published: July 2002

A collimating system has been developed for condensing hard x rays, providing very thin, intense, and low divergent beams. The primary radiation is compressed down to the micrometer scale by multiple total reflections in the channel between two oblong and flexible metallic mirrors with exit aperture 4 mm×15 μm. The flexible mirrors permit variation of the channel profile and opening for the incoming radiation, adjusted for maximum transmitted x-ray intensities. The condenser, due to the high brilliance gain of the obtained beam compared to the uncompressed radiation going through a slit of the same size, can be operated even with conventional x-ray tubes, e.g., common x-ray diffractometers without the need for expensive high intensity synchrotron radiation sources, as demanded usually by the glass monocapillaries. A prototype, being mounted on a commercial theta–theta diffractometer, has been thoroughly tested for intensity gain, divergence, and spatial resolution and utilized for accurate structure determinations on very thin regions (only some tens of microns) of solid materials. This article gives an extensive description of the apparatus and presents the obtained results of some representative investigations.
A direct interrogation of superfluidity on molecular scales
Authors: Benderskii A. V. , Eloranta J. , Zadoyan R. , Apkarian V. A.
Published: July 2002

Time-resolved, pump–probe measurements are used to directly interrogate dissipative fluid dynamics in bulk He-II, on molecular scales, as a function of temperature and pressure. The Rydberg transitions of the triplet He2* excimers, which solvate in bubble states in liquid helium, are used as nanoscale transducers to initiate and to directly monitor the motion of the fluid in the form of damped oscillations of a 13 Å spherical bubble. The oscillations are damped out after one period, with a temperature-dependent period that directly tracks the normal fraction. As such, the bubble oscillator acts as a nanoviscosimeter. Through simulations of the observed signals, it is established that the coherent response of the bath obeys hydrodynamic equations of motion of a continuum subject to two-fluid flow. Dissipation occurs through two distinct channels: (a) Radiation of sound in the farfield, driven by the acceleration of volume in the compressible fluid; (b) temperature-dependent drag in the near-field. The drag can be considered to be strictly viscous in origin, or due to ballistic scattering of rotons from the bubble edge. The experiments do not distinguish between these two microscopic models. With this caveat in mind, it can be concluded that for these breathing modes of bubble states, the macroscopic concepts of superfluidity scale down to molecular dimensions. The simulations also yield effective potentials that describe the coupling between the compressible Rydberg electron and the compressible fluid.
Effects of air exposure on amorphous carbon nitride surfaces
Authors: Ripalda J. M. , Dı́az N. , Montero I. , Rueda F. , Galán L.
Published: July 2002

The possible application of amorphous carbon nitride thin films to prevent the multipactor effect is discussed. The main difficulty lies in the increase of the secondary electron emission coefficient after air exposure. Adsorption processes on amorphous carbon nitride surfaces due to air exposure and subsequent changes due to thermal annealing have been studied by means of x-ray photoemission spectroscopy with the purpose of understanding the changes of secondary electron emission characteristics.
Detection and electrical characterization of hidden layers using time-domain analysis of terahertz reflections
Authors: Geltner I. , Hashimshony D. , Zigler A.
Published: July 2002

We use a time-domain analysis method to characterize the outer layer of a multilayer structure regardless of the inner ones, thus simplifying the characterization of all the layers. We combine this method with THz reflection spectroscopy to detect nondestructively a hidden aluminum oxide layer under opaque paint and to measure its conductivity and high-frequency dielectric constant in the THz range.
Optimization of Al interlayer thickness for the multilayer diffusion barrier scheme in Cu metallization
Authors: Datta Arindom , Nam Ki Tae , Kim Soo-Hyun , Kim Ki-Bum
Published: July 2002

The TiN(5 nm)/Al/TiN (5 nm) structure is fabricated by sequential sputtering for the diffusion barrier application against Cu diffusion. Al is used as an interlayer with an expectation that it will favor stuffing of TiN grain boundaries by reacting with O2 in the structure. In one phase of experiments, the upper TiN layer is exposed to air and preannealed in N2/O2 ambient for better stuffing. In another phase, the preannealing is not performed for the sake of reducing process steps. The Al layer thickness is varied to find out the optimized value. The diffusion barrier performance is investigated by x-ray diffraction, sheet resistance measurement, secco etching and cross sectional transmission electron microscopy after stressing the structure in temperature range of 350–700 °C at vacuum for 30 min. It is found that the barrier property is improved remarkably with the introduction of an Al layer between two TiN layers as compared to that of a single TiN layer with the same thickness. The results indicate that the barrier performance is enhanced proportionally with increasing Al thickness (from 0.5 to 2 nm) in the case that upper TiN is preannealed, showing the best result for 2 nm thick Al interlayer. On the other hand, in not preannealed upper TiN, barrier property is at its best when Al thickness is 1 nm but above this value the barrier performance deteriorates drastically.
Optical properties of PbTiO3, PbZrxTi1−xO3, and PbZrO3 films deposited by metalorganic chemical vapor on SrTiO3
Authors: Moret M. P. , Devillers M. A. C. , Wörhoff K. , Larsen P. K.
Published: July 2002

Epitaxial PbZrxTi1−xO3 (PZT) films have been prepared by metalorganic chemical vapor deposition on SrTiO3 substrates. Two sets of films of thicknesses 50–100 and 700–1400 nm, containing 0%, 40%, 60%, and 100% Zr, were prepared and investigated. The refractive index n was determined by ellipsometry for the thin films and by reflectivity for the thicker films. Results were obtained over the energy range from 1.55 to 3.72 eV, and with a Cauchy-fit extrapolation down to 0.62 eV. The refractive-index curves show a systematic variation with composition. For all compositions, n is close to 3.2 at 3.72 eV (333 nm), while at 1.55 eV (800 nm) n is 2.35 for PZ (x=1) and 2.61 for PT (x=0). In agreement with previous results we find that the optical band gap is essentially independent of composition for PZT. We obtained 3.6±0.1 eV. The n(E) results were analyzed by a Wemple–DiDomenico dispersion analysis, yielding results for the dispersion region in the ultraviolet. Unlike the band gap, which is insensitive to composition in PZT, the dispersion energy Ed decreases from PT to PZ in the same fashion as the refractive index in the transparent region.
Observations of modulated shock waves in solid targets driven by spatially modulated laser beams
Authors: Boehly T. R. , Collins T. J. B. , Gotchev O. , Kessler T. J. , Knauer J. P. , Sangster T. C. , Meyerhofer D. D.
Published: July 2002

The growth of surface perturbations due to nonuniformities in the drive laser is an important subject in laser–matter interactions. We present results of experiments using drive lasers with known, single-mode modulations to produce nonuniform shocks that propagate into planar plastic (CH) targets. An optical probe beam is used to measure the arrival of these modulated shocks at various surfaces in the target. Experiments at moderate laser intensities (≲1013 W/cm2) exhibit behavior predicted by hydrocodes and simple scaling laws. This technique will be used to observe various dynamic effects in laser-produced plasmas and shock-wave propagation.
Amorphous silicon waveguides for microphotonics
Authors: de Dood M. J. A. , Polman A. , Zijlstra T. , van der Drift E. W. J. M.
Published: July 2002

Amorphous silicon a-Si was made by ion irradiation of crystalline silicon with 1×1015 Xe ions cm−2 at 77 K in the 1–4 MeV energy range. Thermal relaxation of the amorphous network at 500 °C for 1 h leads to an amorphous layer with a refractive index of n=3.73, significantly higher than that of crystalline silicon (n=3.45 at λ=1.55 μm). a-Si can thus serve as a waveguide core in Si based optical waveguides. Channel waveguides were made by anisotropic etching of a 1.5 μm silicon-on-insulator structure that was partly amorphized. Transmission measurements of these waveguides as function of the amorphous silicon length show that the a-Si part of the waveguides exhibit a modal propagation loss of 70 cm−1 (0.03 dB μm−1) and a bulk propagation loss of 115 cm−1 (0.05 dB μm−1). Losses due to sidewall roughness are estimated, and are negligible compared to the modal loss.
Singlet–triplet conversion induced by external magnetic field in gaseous oxalylfluoride excited to different single rotational levels of the Ã 1Au state. I. Excitation to the SRLs of the 81 vibronic level
Authors: Makarov Vladimir I.
Published: July 2002

Fluorescence decay of the rotationally cooled oxalylfluoride excited to the different single rotational levels of the Ã 1Au(81) state was measured as a function of an external magnetic field. On excitation to this level, the dynamics in both zero and nonzero fields may be described using the intermediate-molecule limit, with the fluorescence exhibiting biexponential decay. The fast component decay rate constant is the field-independent parameter, while that of the slow component depends on the magnetic field strength. The results obtained were explained by the indirect (electronic and nuclear-spin-decoupling) mechanism, proposed earlier.
A helium atom scattering study of the H/NiAl(110) adsorption system
Authors: Farı́as D. , Patting M. , Rieder K. H.
Published: July 2002

Hydrogen adsorption on NiAl(110) has been investigated by means of helium atom scattering. In agreement with previous studies, hydrogen adsorption was found to be activated on this surface. Two ordered adsorbate structures could be identified after exposure of atomic hydrogen at 90 K: a c(2×2) and a (1×1)H saturation phase. The structures of the clean and hydrogen covered surfaces were determined from a quantitative analysis of the corresponding He-diffraction spectra. For the clean surface, a very shallow corrugation in which only the topmost Al atoms appear is obtained from He-diffraction data, whereas both Ni and Al atoms are visible in the corrugation determined with Ne diffraction. Surprisingly, the total corrugation amplitude determined for the c(2×2) structure was as shallow as the one of the clean surface. This very unusual fact may be taken as an evidence for the existence of delocalized hydrogen on this phase. The hydrogen adsorption site in the (1×1)H phase was determined to be the Ni–Ni bridge site, with a H–Ni bond length of 1.75 Å, in agreement with previous ab initio calculations.
Analytical solutions for hole-like equilibria for arbitrary aspect ratio high β tokamaks
Authors: Warrier M. , Srinivasan R. , Kaw P. K.
Published: July 2002

Analytical solutions for ultrahigh beta (εβp≫1), high safety factor q [q>O(1)], hole-like equilibria with shaped boundary have been derived for arbitrary aspect ratio tokamaks. These solutions are similar to those derived by Cowley et al. [Phys. Fluids B 3, 2066 (1991)] in the large aspect ratio limit. A high beta, high qscaling (similar to that used by Hsu et al. [Phys. Plasmas 3, 266 (1996)]) is used to obtain the various features of finite aspect ratio tokamak equilibrium with a core region occupying most of the volume where ψ≡ψ(R) and a boundary layer region of thickness 1/q. It is seen from the analytical solution that there exists a critical polodial beta value (βc) beyond which one can have hole-like equilibria. The dependency of βc on the inverse aspect ratio (ε) is also studied.
First-principles investigations of the orbital magnetic moments in CrO2
Authors: Jeng Horng-Tay , Guo G. Y.
Published: July 2002

The electronic and magnetic properties of the ferromagnetic half-metal CrO2 have been studied by using the full-potential linear muffin-tin orbital method within the local-spin-density approximation (LSDA) and the LSDA+U approach. The orbital magnetic moments are investigated by including the spin-orbit coupling in both schemes. Compared with the orbital contributions to magnetization in other transition metal oxides, the orbital magnetic moment of the Cr atom is quenched, while the O atoms exhibit a relatively significant orbital moment in CrO2. The LSDA gives the orbital moments of −0.037μB/Cr and −0.0011μB/O. They are too small as compared with the moments of −0.05μB/Cr and −0.003μB/O obtained from x-ray magnetic circular dichroism measurements. By taking the on-site Coulomb energy U into account, both the spin and orbital magnetic moments of Cr and O are dramatically enhanced. For a Hubbard U of 3 eV, the LSDA+U gives orbital moments of −0.051μB/Cr and −0.0025μB/O, in good agreement with the experimental orbital moments. Both the spin and orbital moments are found to increase more or less linearly with increasing U. Similar trends have also been found in CoO and NiO.
Phase space structure and short distance behavior of local quantum field theories
Authors: Mohrdieck Stephan
Published: July 2002

In this article a general relation between the short distance structure of quantum field theories and their phase space properties is exemplified by the simple class of generalized free fields. As is known, theories with decent phase space properties, resulting from a finite or moderately increasing infinite particle spectrum, always have non-trivial scaling (short distance) limits [cf. D. Buchholz and R. Verch, Rev. Math. Phys. 10, 775 (1998)]. But, whereas in the finite particle case the phase space properties of the limit theories comply with strong nuclearity conditions, they violate in the infinite particle case even rather mild compactness assumptions. These results provide further evidence to the effect that relevant information on the short distance structure of a theory can be obtained by phase space analysis.
The permanent electric dipole moments of iron monocarbide, FeC
Authors: Steimle Timothy C. , Virgo Wilton L. , Hostutler David A.
Published: July 2002

Numerous branch features in the (0,0) [12.0] Ω=2←X 3Δ3 and (0,0) [13.1] Φ43←X 3Δ3 band systems of the iron monocarbide, FeC, have been studied using optical Stark spectroscopy. The electric field induced splittings in the high resolution laser induced fluorescence spectra were analyzed to produce values for the permanent electric dipole moments, μ, of 4.02(6) D, 4.44(6) D, and 2.36(3) D for the [12.0] Ω=2, [13.1] Φ43, and X 3Δ3 states, respectively. A comparison with other iron containing molecules and theoretical predictions is made.
Piezoelectric and strain properties of Ba(Ti1−xZrx)O3 ceramics
Authors: Yu Zhi , Ang Chen , Guo Ruyan , Bhalla A. S.
Published: July 2002

The polarization and strain behavior of Ba(Ti1−xZrx)O3 (x=0–0.3) ceramics are studied. The unipolar strain levels of ∼0.18% at 40 kV/cm and of ∼0.25% at ∼120 kV/cm with small hysteresis were obtained for the ceramics with 0.03⩽x⩽0.08. The remnant polarization is ∼13–18 μC/cm2 for 0.03⩽x⩽0.08 and below 2 μC/cm2 for 0.15⩽x⩽0.3 at room temperature. The electromechanical coupling coefficient K33=56.5% and the piezoelectric stain coefficient d33=236 pC/N were obtained for the ceramics with x=0.05. These results indicated that the Ba(Ti1−xZrx)O3 system is a promising lead-free high strain material for applications.
Reduction of HKT-structures
Authors: Grantcharov Gueo , Papadopoulos George , Poon Yat Sun
Published: July 2002

KT-geometry is the geometry of a Hermitian connection whose torsion is a 3-form. HKT-geometry is the geometry of a hyper-Hermitian connection whose torsion is a 3-form. We identify nontrivial conditions for a reduction theory for these types of geometry.
A quantum dynamics study of H2+OH→H2O+H employing the Wu–Schatz–Lendvay–Fang–Harding potential function and a four-atom implementation of the real wave packet method
Authors: Goldfield Evelyn M. , Gray Stephen K.
Published: July 2002

We carry out numerous six-dimensional wave packet propagations for H2+OH→H2O+H on the ab initio based, Wu–Schatz–Lendvay–Fang–Harding potential energy function. For comparison, some calculations are also carried out on the older but more widely studied potential function of Walch, Dunning, Schatz, and Elgersma. The energy dependence of the total angular momentum J=0 cumulative reaction probability is obtained and J-shifting is used to estimate the bimolecular rate constant as a function of temperature. Some J>0 calculations are also carried out. A novel J-shifting procedure, designed to more accurately describe the effects of angular momentum, is introduced. We compare our results with transition state theory calculations and experiment. An important feature of our work is the development of an efficient, four-atom, parallel implementation of the real wave packet method, augmented with a recently developed finite difference method.
Decay processes of the C2 2Π1/2u states of Ar2+, Kr2+, and Xe2+
Authors: Yoshii H. , Hayaishi T. , Onuma T. , Aoto T. , Morioka Y. , Ito K.
Published: July 2002

The threshold photoelectron photoion coincidence spectra of the rare gas dimers Ar2, Kr2, and Xe2 have been measured in the photon energy region between the first and second dissociation limits of the dimer ions. It has been found that the C2 2Π1/2u states of Kr2+ and Xe2+ completely dissociate via an optical transition to the repulsive B2 2Π1/2g state, while that of Ar2+ does not dissociate.
Determination of mesoscale crystallization by collection-mode polarization modulated near-field optical microscopy
Authors: Wei Pei-Kuen , Chiu Sheng-Yu , Chang Wei-Lun
Published: July 2002

We present a new setup of polarization modulated near-field optical microscopy for actually measuring the mesoscale crystallization of thin films. In comparison to previous transmission mode polarization near-field optical microscopy, our new collection mode microscopy provides no axial polarization, free of fiber birefringence and flexibility for tuning wavelength. The Jones matrix calculation verifies that the crystallization of the sample can be obtained by simply subtracting the polarization vector from anisotropy of the tip. The crystallization domains for thermal annealed naphthaphenylene benzidine thin films are observed by the system, stripe domains with averaged ∼15% dichroic ratios are first measured.
X-ray scattering microscope with a Wolter mirror
Authors: Takano Hidekazu , Aoki Sadao , Kumegawa Masaru , Watanabe Norio , Ohhigashi Takuji , Aota Tatsuya , Yamamoto Kimitake , Yokosuka Hiroki , Tanoue Ryuichi , Tsujita Yuji , Ando Masami
Published: July 2002

Full-field x-ray scattering microscopic images were obtained with a Wolter mirror (×10 magnification). A synchrotron radiation white beam (4–20 keV) from a bending magnet beamline at the Photon Factory was used to obtain x-ray scattering images. The system was available for multi-kilo-electron-volt x-ray range (4–12 keV) with the Wolter mirror. The image was formed only with scattered x rays from the object, which is kind of a dark-field image. Very low absorptive materials could be imaged with this microscope. Sensitivity of the system was evaluated and also the detection limit was estimated.
Optimal three-layer cylindrical magnetic shield sets for scientific applications
Authors: Burt E. A. , Ekstrom C. R.
Published: July 2002

We present results from modeling axial shielding effectiveness for various configurations of three-layer cylindrical magnetic shield sets. In particular, we examine the effect on shielding of spacing between shield layers, end cap geometry, and end cap holes as well as gaps between mating surfaces. We find that shield spacing is critical and that increased radial spacing continues to improve overall shielding at least up to the point where the ratio of the outer shield diameter to its length approaches 1. The analysis suggests that further increases in shield diameter would improve the shielding, but since this quickly becomes uneconomical, we present an approach that optimizes the cost-benefit ratio of a shield set. Finally, we describe a three-layer shield set design based on this analysis and present results demonstrating an axial shielding effectiveness of 3.5(4)×104.
Ultracompact explosive-driven high-current source of primary power based on shock wave demagnetization of Nd2Fe14B hard ferromagnetics
Authors: Shkuratov S. I. , Talantsev E. F. , Dickens J. C. , Kristiansen M.
Published: July 2002

A new type of explosive driven high-current pulsed source utilizing a shock wave demagnetization of a Nd2Fe14B hard ferromagnetic energy carrier was developed. The design of the ferromagnetic energy carrier, which was made a hollow cylinder, has made it possible to reduce dramatically to 1 g the amount of the explosive providing a complete demagnetization of Nd2Fe14B energy carrier of weight 64 g. The developed generator is capable of producing high-current [up to 1.9 kA, 100 μs full width at half maximum (FWHM)] and high-power pulses (up to 42 kW, 2.8 μs FWHM).
Long-term drift laser frequency stabilization using purely optical reference
Authors: Rossi A. , Biancalana V. , Mai B. , Tomassetti L.
Published: July 2002

We describe an apparatus for the stabilization of laser frequencies that prevents long-term frequency drifts. A Fabry–Perot interferometer is thermostated by referencing it to a stabilized He–Ne laser (master), and its length is scanned over more than one free spectral range allowing the analysis of one or more lines generated by other (slave) lasers. A digital acquisition system makes the detection of the position of all the laser peaks possible, thus producing both feedback for the thermostat and the error signal used for stabilizing the slave lasers. This technique also allows for easy, referenced scanning of the slave laser frequencies over range of several hundred MHz, with a precision of the order of a few MHz. This kind of stabilization system is particularly useful when no atomic or molecular reference lines are available, as in the case of rare or short lived radioactive species.
A plasma focus driven by a capacitor bank of tens of joules
Authors: Silva Patricio , Soto Leopoldo , Moreno José , Sylvester Gustavo , Zambra Marcelo , Altamirano Luis , Bruzzone Horacio , Clausse Alejandro , Moreno César
Published: July 2002

As a first step in the design of a repetitive pulsed neutron generator, a very small plasma-focus device has been designed and constructed. The system operates at low energy (160 nF capacitor bank, 65 nH, 20–40 kV, and ∼32–128 J). The design of the electrode was assisted by a computer model of Mather plasma focus. A single-frame image converter camera (5 ns exposure) was used to obtain plasma images in the visible range. The umbrellalike current sheath running over the end of the coaxial electrodes and the pinch after the radial collapse can be clearly observed in the photographs. The observations are similar to the results obtained with devices operating at energies several orders of magnitude higher. The calculations indicate that yields of 104–105 neutrons per shot are expected with discharges in deuterium.
Detection efficiency of large-active-area NbN single-photon superconducting detectors in the ultraviolet to near-infrared range
Authors: Verevkin A. , Zhang J. , Sobolewski Roman , Lipatov A. , Okunev O. , Chulkova G. , Korneev A. , Smirnov K. , Gol’tsman G. N. , Semenov A.
Published: July 2002

We report our studies on spectral sensitivity of meander-type, superconducting NbN thin-film single-photon detectors (SPDs), characterized by GHz counting rates of visible and near-infrared photons and negligible dark counts. Our SPDs exhibit experimentally determined quantum efficiencies ranging from ∼0.2% at the 1.55 μm wavelength to ∼70% at 0.4 μm. Spectral dependences of the detection efficiency (DE) at the 0.4 to 3.0-μm-wavelength range are presented. The exponential character of the DE dependence on wavelength, as well as its dependence versus bias current, is qualitatively explained in terms of superconducting fluctuations in our ultrathin, submicron-width superconducting stripes. The DE values of large-active-area NbN SPDs in the visible range are high enough for modern quantum communications.
Fast driving technique for integrated thermal bimorph actuator toward high-throughput atomic-force microscopy
Authors: Akiyama T. , Staufer U. , de Rooij N. F.
Published: July 2002

A driving scheme for improving the effective response of a microfabricated cantilever with an integrated thermal bimorph actuator is proposed for applications in high-throughput atomic-force microscopy. The essential part of the proposed scheme is a tuned boost filter (BSF), which is inserted in the setup for constant force imaging and which boosts the servo signal according to its frequency. Using this setup, an imaging bandwidth of 5 kHz was obtained within an actuation range of 1.7 μm. Constant force imaging with a tip velocity of 0.62 mm/s is demonstrated and the effectiveness of the BSF is verified. The improved bandwidth is gained at the expense of the achievable deflection range. An optimized system, therefore, requires not only a BSF but also a thermal bimorph with a large power to deflection efficiency.
CoSi
2
formation in the presence of carbon
Authors: Detavernier C. , Van Meirhaeghe R. L. , Bender H. , Richard O. , Brijs B. , Maex K.
Published: July 2002

Silicide formation was studied for Co/C/Si and C/Co/Si bilayers. Phase formation and preferential orientation of the CoSi2 phase is strongly dependent on the C layer thickness. For thin interlayers and capping layers, CoSi2 nucleation is delayed to higher temperature and epitaxially aligned nuclei are formed preferentially. To explain these findings, we propose a model based on heterogeneous nucleation theory: C is present as a contaminant in the CoSi and is able to influence CoSi2 nucleation by enhancing the CoSi grain-boundary cohesion and reducing grain-boundary diffusion. For thicker interlayers (>1 nm), the carbon acts as a diffusion mediating interlayer, and epitaxial CoSi2 is formed by slow outdiffusion of Co from an amorphous Co–C alloy.
Electrical resistivity and interdiffusion in Gd1−xCox/Co multilayers
Authors: González J. A. , Andrés J. P. , Arranz M. A. , López de la Torre M. A. , Riveiro J. M.
Published: July 2002

The interdiffusion process between Gd and Co has been investigated in several series of multilayered films consisting of Co and different GdCo alloys (Gd1−xCox)/Co (x=0, 0.37, and 0.60). Grazing incidence x-ray profiles provided a preliminary insight into the structure of the samples. Electrical resistivity measurements were performed in a temperature range between 8 and 300 K. For the three series, the room temperature electrical resistivity shows a clear fall once a critical Co thickness is surpassed, which we interpret as due to the growth of pure polycrystalline Co between the alloy layers. A simple model allows us to quantify the amount of Co diffused into the alloy layer. The results will be discussed considering the possible decrease of interdiffusion effects when Co is deposited onto the more stable GdCo alloys. This leads to a well defined multilayered structure, something that is essential for practical applications and for the theoretical study of its underlying physics.
Prevention of the cathode induced electrochemical degradation of [Ru(bpy)3](PF6)2 light emitting devices
Authors: Rudmann Hartmut , Shimada Satoru , Rubner Michael F. , Oblas Daniel W. , Whitten James E.
Published: July 2002

Light emitting electrochemical cells based on the tris(2,2′ bipyridyl) ruthenium(II) complex show improved performance if electrochemically stable materials such as Ag are used as the cathode material. In contrast, if Al is used as the cathode such devices undergo degradation when stored in the off-state in inert atmosphere. In this work, the mechanism of the aluminum-induced degradation is investigated. X-ray photoelectron spectroscopy shows that some of the Ru(II) complexes are reduced in the presence of the Al cathode to Ru(I). In addition, secondary ion mass spectrometry depth profiles indicate degradation of the indium tin oxide in devices with Al cathodes. Because of the mixed-valent Ru(II)/(I) states, devices with Al cathodes exhibit unipolar charge injection at voltages below the turn-on voltages. The unipolar charge injection can be described by a theory of charge hopping in mixed-valent redox systems. In addition, impedance analysis data at 0 V bias of devices with Al or Ag cathodes can be fit using simple equivalent electric circuits. In the case of Al devices, the equivalent electric circuit was modified to account for the redox conduction at 0 V bias and an oxide layer at the cathode interface.
Raman scattering and photoluminescence of nitrogenated amorphous carbon films
Authors: Papadimitriou D. , Roupakas G. , Dimitriadis C. A. , Logothetidis S.
Published: July 2002

The structural and optical properties of nitrogenated amorphous carbon films, grown by rf-magnetron sputtering on silicon substrates, were studied by Raman and photoluminescence (PL) spectroscopy as a function of the nitrogen concentration and the substrate bias voltage Vb. For films deposited with Vb=10 V, the photoluminescence emission was most intense at nitrogen concentrations in the carrier gas of 25% (75% Ar), while the intensity ratio I(D)/I(G) of the Raman bands of disordered graphite (D band) and graphite (G band) partially substituted by nitrogen exhibited a minimum simultaneously observed with a minimum of G-band frequency and a maximum of G-band width. Changes in spectral characteristics of Raman scattering at a concentration of 25% (≅30 at %) are indicative of an increase of sp3-bonded fraction and disorder. PL-enhancement coincides, in this case, with structural changes and is probably correlated to the substitution of nitrogen in the tetrahedraly bonded amorphous matrix. In the case of films deposited in a pure nitrogen atmosphere, N2=100%, no significant PL-intensity changes appeared to exist between films deposited at low positive (10 V) and highly negative (−200 V) substrate bias. After several months of sample storage in air, samples grown at negative Vb were found to preserve their structural and optical properties, while films grown at positive bias (Vb=10) and nitrogen concentrations in the carrier gas above 70% (≅40 at %) delaminated.
New insight into the orientational order of water molecules at the water/1,2-dichloroethane interface: A Monte Carlo simulation study
Authors: Jedlovszky Pál , Vincze Árpád , Horvai George
Published: July 2002

The preferential orientation of the water molecules near the water/1,2-dichloroethane interface is analyzed in detail at different distances from the interface on the basis of a grand canonical ensemble Monte Carlo simulation. The orientation of the individual water molecules is described by the angular polar coordinates of the interface normal vector in a local coordinate frame fixed to the particular water molecule, and the bivariate joint distribution of the two polar angles is calculated. It is found that water molecules have two distinct orientational preferences, and these two preferences exist simultaneously among the water molecules penetrating farthest into the organic phase. In the first preferred orientation the plane of the molecule is parallel to the interface, whereas in the second the molecular plane is aligned perpendicularly to the interface and the molecular dipole vector declines from the plane parallel to the interface by about 30° pointing toward the organic phase. The first of the two preferred orientations is found to be present in the entire interfacial region and also, to a smaller extent, in the subsurface water layer adjacent to the interface. The second orientational preference is only present among the water molecules penetrating farthest into the organic phase. The two orientations correspond to the alignment of a hydrogen bonded pair of water molecules, in which the molecule located toward the aqueous phase has the first, whereas the one on the organic side the second of the two preferred orientations. The obtained picture is in a clear contrast with the findings of previous studies, in which the orientation of the water molecules was described by monovariate distributions of the alignment of one or more selected molecule-fixed vectors. In order to understand the origin of the difference between the present results and earlier findings we also calculate the monovariate distributions of the direction of three of such molecular vectors, i.e., the dipole vector of the water molecule, the vector joining the two H atoms, and the vector perpendicular to the molecular plane. The comparison of the obtained monovariate distributions with the bivariate joint distribution of the two polar angles reveals that the averaging of the bivariate distribution over any of its two angles completely obscures the dual orientational preference. The present study clearly points out the importance of choosing appropriate statistical distributions in the analysis of simulation results and demonstrates the pitfalls of averaging over too many variables.
Virial pressure of periodic systems with long range forces
Authors: Winkler Roland G.
Published: July 2002

The virial expression of the pressure of systems with periodic boundary conditions is discussed for long range Coulomb interactions. It is shown, that the mechanical definition of the pressure leading to the virial pressure agrees with the thermodynamical definition for such systems.
Dynamics and rheology of a supercooled polymer melt in shear flow
Authors: Yamamoto Ryoichi , Onuki Akira
Published: July 2002

Using molecular dynamics simulations, we study dynamics of a model polymer melt composed of short chains with bead number N=10 in supercooled states. In quiescent conditions, the stress relaxation function G(t) is calculated, which exhibits a stretched exponential relaxation on the time scale of the α relaxation time τα and ultimately follows the Rouse dynamics characterized by the time τR∼N2τα. After application of shear γ̇, transient stress growth σxy(t)/γ̇ first obeys the linear growth ∫0tdt′G(t′) for strain less than 0.1 but saturates into a non-Newtonian viscosity for larger strain. In steady states, shear thinning and elongation of chains into ellipsoidal shapes take place for shear γ̇ larger than τR−1. In such strong shear, we find that the chains undergo random tumbling motion taking stretched and compact shapes alternatively. We examine the validity of the stress–optical relation between the anisotropic parts of the stress tensor and the dielectric tensor, which are violated in transient states due to the presence of a large glassy component of the stress. We furthermore introduce time-correlation functions in shear to calculate the shear-dependent relaxation times, τα(T,γ̇) and τR(T,γ̇), which decrease nonlinearly as functions of γ̇ in the shear-thinning regime.
Raman spectroscopic study of glassy water in dilute lithium chloride aqueous solution vitrified under pressure
Authors: Suzuki Yoshiharu , Mishima Osamu
Published: July 2002

We vitrify dilute lithium chloride (LiCl) aqueous solutions (2.0–10.0 mol % solute) at about 0.5 GPa using a needle-type pressure device developed for cooling a liquid material rapidly, and measure Raman spectra of the vitrified solution at 1 atm and 25 K. The OH stretching vibrational modes of the vitrified solution seem to consist of a OH stretching vibrational mode of high density amorphous ice (HDA) of pure water and that of glass of highly concentrated LiCl aqueous solution. The Raman profile resembles more and more that of HDA as the solute concentration decreases. When the temperature of the vitrified solution increases at 1 atm, the Raman profile suddenly changes to that of low density amorphous ice around 135 K and the transformed profile goes to that of Ic around 160 K. This transformation behavior seems to correspond to a transformation behavior of HDA. These results suggest that the portion of solvent water in the dilute LiCl aqueous solution vitrified under pressure may form HDA-type molecular arrangements.
Suppression of interfacial atomic diffusion in InGaNAs/GaAs heterostructures grown by molecular-beam epitaxy
Authors: Peng C. S. , Pavelescu E.-M. , Jouhti T. , Konttinen J. , Fodchuk I. M. , Kyslovsky Y. , Pessa M.
Published: July 2002

We have studied the effects of annealing of InGaNAs/GaAs heterostructures on diffusion at the interfaces and the resultant changes in optical and structural properties. Interdiffusion between In and Ga was found to be very significant. Inserting a thin compressively strained layer of InxdGa1−xdNydAs1−yd on either side of an InxqGa1−xqNyqAs1−yq quantum well (QW) suppressed this interdiffusion significantly. As a consequence, a blue shift of the photoluminescence signal after annealing remained small and the optical activity was largely improved. It was also found that a small amount of N incorporated in InGaAs QWs embedded in GaAs increased the In/Ga interdiffusion and that increased mechanical stresses enhanced the interdiffusion.
Monte Carlo calculation of electronic noise under high-order harmonic generation
Authors: Shiktorov P. , Starikov E. , Gružinskis V. , Reggiani L. , Varani L. , Vaissière J. C.
Published: July 2002

The time and frequency behavior of hot-carrier velocity fluctuations in bulk semiconductors subjected to strong periodic electric fields is analyzed by using two complementary approaches based on the correlation function and the finite Fourier transform. Monte Carlo calculations performed for GaAs, InP, and InN show that semiconductor materials with a high value of the threshold field for the Gunn effect are characterized by a high value of the signal-to-noise ratio under high-order harmonics generation and, hence, they are promising materials for microwave generation in the terahertz frequency range by high-order harmonic extraction.
Far-infrared stimulated emission from optically excited bismuth donors in silicon
Authors: Pavlov S. G. , Hübers H.-W. , Rümmeli M. H. , Zhukavin R. Kh. , Orlova E. E. , Shastin V. N. , Riemann H.
Published: July 2002

Far-infrared stimulated emission from optically pumped neutral Bi donors in silicon has been obtained. Lasing with wavelengths of 52.2 and 48.6 μm from the intra-center 2p±→1s(E:Γ8),1s(T2:Γ8) transitions has been realized under CO2 laser pumping. The population inversion mechanism is based on fast optical-phonon-assisted relaxation from the 2p0 and 2s excited states directly to the ground 1s(A) state leading to relatively small population in the intermediate 1s(E), 1s(T2) excited states.
Thermoelectric properties of (Zn1−yMgy)1−xAlxO ceramics prepared by the polymerized complex method
Authors: Katsuyama S. , Takagi Y. , Ito M. , Majima K. , Nagai H. , Sakai H. , Yoshimura K. , Kosuge K.
Published: July 2002

( Zn 1−y Mg y ) 1−x Al x O powders were synthesized by the polymerized complex method and then consolidated by spark plasma sintering apparatus. The microscopic structure and thermoelectric properties were examined comparing with the experimental results of the samples prepared by the conventional solid-state reaction method. A small amount of ZnAl2O4 spinel phase as the second phase was observed in the sintered samples with x⩾0.02 by x-ray diffraction and a scanning electron microscope. The grain size of the samples prepared by the polymerized complex method is much smaller than that of the samples prepared by the conventional solid-state reaction method. The absolute values of the Seebeck coefficient and electrical resistivity decrease with increasing x up to about x=0.01, but above x=0.01 they are almost independent of x. This result indicates that the solubility limit of Al in Zn1−xAlxO is about x=0.01, which is also confirmed by Al27 nuclear magnetic resonance spectroscopy. At a fixed composition of x, the absolute values of the Seebeck coefficient and electrical resistivity for the samples prepared by the polymerized complex method are smaller than those for the samples prepared by the solid-state reaction method, which indicates that the doping of the carrier into the material can be more easily realized in the samples prepared by the polymerized complex method. The thermal conductivity decreases with increasing x, but the further suppression of the thermal conductivity was attained by the additional substitution on the Zn site by Mg. The Seebeck coefficient of (Zn1−yMgy)1−xAlxO is almost independent of Mg content y, but the electrical resistivity increases with increasing y. As a result, (Zn0.90Mg0.10)0.9975Al0.0025O shows a maximum dimensionless figure of merit of 0.10 at 1073 K.
High-resolution magnetic Co supertips grown by a focused electron beam
Authors: Utke I. , Hoffmann P. , Berger R. , Scandella L.
Published: July 2002

We present a technique for local growth of high-resolution, high-aspect-ratio magnetic tips and thin adherent magnetic cap coatings on top of batch fabricated scanning force microscopy silicon tips. A focused electron beam of a scanning electron microscope is used for decomposition of a directed cobalt carbonyl vapor flux. Exposure parameters determine the tip geometry and tip length. Deposits consist of cubic Co clusters of 2–5 nm in size dispersed in a stabilizing carbonaceous matrix. Magnetic force microscope sensors having magnetic tip apex diameters between 50 and 240 nm were produced. Tracks of magnetic transitions written in recording media of hard disks were used to characterize tip performance.
A method for measuring barrier heights, metal work functions and fixed charge densities in metal/SiO2/Si capacitors
Authors: Zafar Sufi , Cabral Cyril , Amos R. , Callegari A.
Published: July 2002

A method for measuring metal barrier heights, work function and fixed charge densities in metal/SiO2/Si capacitors is developed and verified. This technique is based on theoretical studies of tunneling phenomenon through a potential barrier and requires measurement of current versus voltage sweeps at two different temperatures. Unlike the commonly used capacitance method, this method does not require a set of capacitors with different gate oxide thickness for determining work functions and fixed charge densities in metal/SiO2/Si capacitors. Hence, this method provides a fast means for investigating metal work function and fixed charge densities in metal-gated SiO2 capacitors.