bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry
Authors: Eiceman G. A. , Nazarov E. G. , Rodriguez J. E. , Stone J. A.
Published: July 2002

Mobility spectra for positive ions, created from a Ni63 foil in purified air at ambient pressure (660 Torr) with 0.15 ppm moisture, were obtained with a drift tube with a discrete drift ring design at 250 °C as electric fields for components were individually and independently varied. Peak area, peak width, baseline intensity, drift times, and reduced mobilities (Ko) were used to measure the function and performance of each component and findings were interpreted using a model for the transport of thermalized ions in weak electric fields at ambient pressure. Transit times and intensities for ions in drift tubes at ambient pressure can be understood through a detailed knowledge of the fields local to a component and derivations from theory of ion transport. Prolonged ion residence in the drift region resulted in ion transformations even for highly purified gases of low moisture at high temperature. These findings suggest that mobility spectra may be obtained with uniformly high quality and reproducibility only under conditions when ion residence time is the primary point of reference in obtaining spectra. Other regions of the drift tube were optimized and newly observed chemistry occurred in the aperture to detector region. The sampling of ions by such an ion shutter was found to inherently bias the ion distributions and alter actual lengths of drift regions. Consequently, drift lengths measured from physical configurations of drift tubes will be inadequate for precise measurements of drift times. These studies establish baseline measurements for evaluating drift tubes that should be generally applicable for optimizing performance in other drift tubes with discrete drift ring designs. Also, these results demonstrate that precise measurements in ion mobility spectrometry (IMS) will require attention to detail not heretofore carefully regarded in modern analytical IMS.
High resolution infrared spectroscopy andab initiocalculations of HCN–H2/D2 binary complexes
Authors: Moore D. T. , Ishiguro M. , Oudejans L. , Miller R. E.
Published: July 2002

High-resolution infrared laser spectroscopy has been used to study HCN–H2 and HCN–D2 complexes in the gas phase. The experimental results are compared with ab initio calculations that are also reported here. The latter calculations reveal two prominent minima on the potential surface, one corresponding to a “T-shaped” complex with the H2 at the hydrogen end of the HCN and the other a “linear” complex with the H2 H-bonded to the nitrogen. The latter minimum is the global minimum on the surface, in agreement with the fact that this structure is observed experimentally for both o-H2 and p-D2.
Hardness and elasticity in cubic ruthenium dioxide
Authors: Léger J. M. , Djemia P. , Ganot F. , Haines J. , Pereira A. S. , da Jornada J. A. H.
Published: July 2002

The Knoop hardness of the highly incompressible cubic phase of ruthenium dioxide was found to be 19–20 GPa from indentation tests. This value scales well with the shear modulus approximated by the elastic constant C44 of 144 GPa obtained from Brillouin scattering measurements. This work provides evidence that the shear modulus is a better indicator of hardness than the bulk modulus for ionic and covalent materials.
Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating
Authors: Petridis C. , Lindsay I. D. , Stothard D. J. M. , Ebrahimzadeh M.
Published: July 2002

We report 80 GHz of mode-hop-free tuning from an extended cavity diode laser (ECDL) without the need for antireflection coating on the diode facet. A large continuous tuning range was achieved by using a simple electronic circuit which simultaneously varies the diode laser current and external cavity length in an appropriate ratio. The ECDL employs a commercial, single mode AlGaAs diode laser, operating at 814 nm, in a Littrow external cavity configuration. The applicability of this technique to the most commonly available single-mode diode lasers without the requirement of antireflection coating, makes it an attractive approach for the attainment of mode-hop-free tuning over extended ranges.
Heat Capacity of Liquids: Critical Review and Recommended Values. Supplement I
Authors: Zábranský Milan , Růžička Vlastimil , Domalski Eugene S.
Published: July 2002

A study was carried out in which new experimental data on heat capacities of pure liquid organic and some inorganic compounds were compiled, critically evaluated, and recommended values provided. Compounds included in the compilation have a melting point below 573 K. The bulk of the compiled data covers data published in the primary literature between 1993 and 1999 and some data of 2000. However, some data from older sources were also included. The data were taken from almost 1030 literature references. Parameters of correlating equations for temperature dependence of heat capacities of liquids were developed. This paper is an update of a two volume monograph entitled Heat Capacity of Liquids: Critical Review and Recommended Values (96ZAB/RUZ) that was published in 1996 in the Journal of Physical and Chemical Reference Data as Monograph No. 6 and was the product of the IUPAC Project No. 121/11/87.
Dynamics of Parkinsonian tremor during deep brain stimulation
Authors: Titcombe Michèle S. , Glass Leon , Guehl Dominique , Beuter Anne
Published: July 2002

The mechanism by which chronic, high frequency, electrical deep brain stimulation (HF-DBS) suppresses tremor in Parkinson’s disease is unknown. Rest tremor in subjects with Parkinson’s disease receiving HF-DBS was recorded continuously throughout switching the deep brain stimulator on (at an effective frequency) and off. These data suggest that the stimulation induces a qualitative change in the dynamics, called a Hopf bifurcation, so that the stable oscillations are destabilized. We hypothesize that the periodic stimulation modifies a parameter affecting the oscillation in a time dependent way and thereby induces a Hopf bifurcation. We explore this hypothesis using a schematic network model of an oscillator interacting with periodic stimulation. The mechanism of time-dependent change of a control parameter in the model captures two aspects of the dynamics observed in the data: (1) a gradual increase in tremor amplitude when the stimulation is switched off and a gradual decrease in tremor amplitude when the stimulation is switched on and (2) a time delay in the onset and offset of the oscillations. This mechanism is consistent with these rest tremor transition data and with the idea that HF–DBS acts via the gradual change of a network property.
On the relationship between two-body and three-body interactions from nonequilibrium molecular dynamics simulation
Authors: Marcelli Gianluca , Todd B. D. , Sadus Richard J.
Published: July 2002

Nonequilibrium molecular dynamics (NEMD) simulations are performed for argon at different strain rates using accurate two-body and three-body intermolecular potentials. The contributions of two- and three-body interactions to the configurational energy of argon at different strain rates are reported. The NEMD data indicate that there is the same simple relationship between two- and three-body interactions as reported previously [Marcelli and Sadus, J. Chem. Phys. 112, 6382 (2000)] from equilibrium Monte Carlo simulations. The relationship is largely independent of strain rate. NEMD calculations using this relationship for shear viscosity at different strain rates indicate good agreement with full two-body+three-body calculations. This means that the effect of three-body interactions on transport properties might be achieved in a conventional two-body NEMD simulation without incurring the computational penalty of three-body calculations.
Elimination of nonuniformities in thick GaN films using metalorganic chemical vapor deposited GaN templates
Authors: Valcheva E. , Paskova T. , Abrashev M. V. , Paskov P. P. , Persson P. O. Å. , Goldys E. M. , Beccard R. , Heuken M. , Monemar B.
Published: July 2002

Thick hydride vapor phase epitaxial GaN layers are grown on metalorganic chemical vapor deposited GaN template layers as well as directly on sapphire, with the aim of investigating the effect of the template on the strain relaxation and spatial distribution of free carriers in the overgrown GaN films. Spatially resolved cross-sectional micro-Raman spectroscopy, cathodoluminescence, and transmission electron microscopy show improved crystalline quality for growth on metalorganic chemical vapor deposited GaN templates. The highly doped and highly defective near-substrate layer composed of columnar domains, typically present in hydride vapor phase epitaxial GaN films grown directly on sapphire, is absent in the layers grown on templates. Consequently, this results in elimination of the nonuniformities of free electron distribution, a lower residual free carrier concentration (<1017 cm−3), and improved strain relaxation.
On the theory of the strange and unconventional isotopic effects in ozone formation
Authors: Gao Yi Qin , Marcus R. A.
Published: July 2002

The strange mass-independent isotope effect for the enrichment of ozone and the contrastingly unconventional strong mass-dependent effect of individual reaction rate constants are studied using statistical (RRKM)-based theory with a hindered-rotor transition state. Individual rate constant ratios of recombination reactions and enrichments are calculated. The theory assumes (1) an “η-effect,” which can be interpreted as a small deviation from the statistical density of states for symmetric isotopomers, compared with the asymmetric isotopomers, and (2) weak collisions for deactivation of the vibrationally excited ozone molecules. A partitioning effect controls the recombination rate constant ratios. It arises from small differences in zero-point energies of the two exit channels of dissociation of an asymmetric ozone isotopomer, which are magnified into large differences in numbers of states in the two competing exit channel transition states. In enrichment experiments, in contrast, this partitioning factor disappears exactly [Hathorn and Marcus, J. Chem. Phys. 112, 9497 (2000)], and what remains is the η-effect. Both aspects can be regarded as “symmetry driven” isotopic effects. The two experiments, enrichments and rate constant ratios, thus reveal markedly different theoretical aspects of the phenomena. The calculated low-pressure ozone enrichments, the low-pressure recombination rate constant ratios, the effects of pressure on the enrichment, on the individual recombination rate constant ratios, and on the recombination rate constant are consistent with the experimental data. The temperature dependence of the enrichment and of the recombination rate constant ratios is discussed and a variety of experimental tests are proposed. The negative temperature dependence of the isotopic exchange rate constant for the reaction O+16O18O→18O16O+18O18 at 130 K and 300 K is used for testing or providing information on the nature of a variationally determined hindered-rotor transition state. The theory is not limited to ozone formation but is intended to apply to other reactions where a symmetrical stable or unstable gas phase molecule may be formed.
Finite element analysis of domain structures in epitaxial PbTiO3 thin films
Authors: Lee Kilho , Lee Kyeong Seok , Baik Sunggi
Published: July 2002

Equilibrium domain structures commonly observed in epitaxial Pb-based ferroelectric thin films are analyzed by the finite element method (FEM) using a commercial package, ABAQUS (Hibbit, Karlsson & Sorensen, Inc., 1080 Main Street, Pawtucket, RI 02860-4847). Structures of periodic 90° domains in epitaxial PbTiO3 thin films on cubic single crystalline substrates are analyzed as a function of decreasing temperature in order to simulate cooling process after the film deposition at elevated temperature (TG). The degree of c-axis orientation (α) is determined as a function of temperature below the Curie temperature and compared to the experimental results. It is then possible to calculate the magnitude of misfit strain during film growth and its relaxation due to dislocation generation. The effect of PZT composition on c-domain abundance is also simulated. FEM simulation is performed with the assumption that the major driving force for such domain formation is thermoelastic strains arising from the film–substrate interaction and the cubic-tetragonal phase transformation. The FEM analysis also suggests that initial misfit stress at TG is not fully relaxed and the unrelaxed misfit stress is inversely related to final c-domain abundance.
A novel approach to computing the orientation moments of spheroids in simple shear flow at arbitrary Péclet number
Authors: Asokan K. , Ramamohan T. R. , Kumaran V.
Published: July 2002

A Langevin approach to computing the orientation moments of a dilute suspension of spheroids in a simple shear flow at arbitrary Péclet number is presented. In this method we obtain the equations governing the time evolution of the orientation averages using a generalized Langevin equation approach and develop a computational technique for computing the evolution of the moments from these equations. These results are compared with those available in the literature obtained from other methods and show good agreement. The approach presented here can be easily generalized to a number of similar systems such as forced suspensions of dipolar particles in shear flows and can be applied to other flow problems governed by appropriate Fokker–Planck equations.
A Mössbauer spectral study of the magnetic properties of Ho2Fe17 and Ho2Fe17D3.8
Authors: Long Gary J. , Isnard Olivier , Grandjean Fernande
Published: July 2002

The Mössbauer spectra of Ho2Fe17 and Ho2Fe17D3.8 have been measured between 4.2 and 295 K and analyzed with a model which takes into account both the disordered nonstoichiometric hexagonal Th2Ni17-like structure and the basal orientation of the iron magnetic moments. The isomer shifts of the five crystallographically inequivalent iron sites in both Ho2Fe17 and Ho2Fe17D3.8 follow the sequence of Wigner–Seitz cell volumes and their temperature dependence follows the typical second-order Doppler shift. An increase in the weighted average isomer shift upon deuterium insertion results from the lattice expansion. The sequence of the site-averaged hyperfine fields is in agreement with the increasing number of iron near neighbors. The five quadrupole interactions agree with those observed in the paramagnetic spectra of other R2Fe17 compounds. For the dumbbell 4e and 4f sites, the asymmetry parameter and the θ angle are zero and 90° in agreement with the site point symmetry and the basal orientation of the iron magnetic moment. For the 6g site, the asymmetry parameter is zero in agreement with the site point symmetry. In both compounds the 12j site asymmetry parameter is 0.9 and for the 12k site this parameter increases from 0.0 in Ho2Fe17 to 0.8 in Ho2Fe17D3.8 as a result of the insertion of an additional deuterium near neighbor. The θ angles for the magnetically inequivalent 12j and 12k sites differ by 60° as expected in the hexagonal symmetry. The variations with rare-earth of the weighted average isomer shifts and hyperfine fields in R2Fe17, R2Fe17Hx, and R2Fe17N3, are discussed in terms of the lanthanide contraction and lattice expansion upon insertion of hydrogen, or deuterium, or nitrogen and the axial and basal magnetic anisotropy.
Hydrogen-induced transport properties of holes in diamond surface layers
Authors: Nebel C. E. , Sauerer C. , Ertl F. , Stutzmann M. , Graeff C. F. O. , Bergonzo P. , Williams O. A. , Jackman R.
Published: July 2002

Three hydrogen-terminated diamonds with different surface roughness and morphologies have been investigated by conductivity and Hall experiments in the temperature regime 0.34–350 K. The sheet hole densities are weakly temperature dependent above a critical temperature Tc (20 K⩽Tc⩽70 K), below Tc carriers freeze out. The mobilities of holes show a minimum at Tc increasing towards higher and even stronger towards lower temperatures significantly up to 400 cm2/V s. A transport model is introduced where holes propagate in the valence band where a disorder-induced tail of localized states is present.
Charge transformer to enhance noise performance of single-electron transistor amplifiers in high-capacitance applications
Authors: Lewis K. M. , Kurdak Ç. , Krishna S. , Bhattacharya P.
Published: July 2002

A device, called a charge transformer, is proposed for noise matching single-electron transistor amplifiers to high-capacitance devices. The operation principle is demonstrated on a prototype charge transformer with four capacitors made using a GaAs/AlGaAs heterostructure with a two-dimensional electron gas.
Orientation-controlled epitaxy and anisotropic properties of La2−xSrxNiO4 with 0.5⩽x⩽1.5 covering the insulator–metal transition
Authors: Shinomori S. , Kawasaki M. , Tokura Y.
Published: July 2002

The a- and c-axis-oriented epitaxial thin films of layered nickelate La2−xSrxNiO4 (0.5⩽x⩽1.5), of which bulk single crystals are not available, were grown on LaSrAlO4 (100) and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (001) substrates, respectively. Due to the in-plane anisotropy of the lattice mismatch, the lattice parameters of the a-axis-oriented films vary with x in a different manner from those of the ceramics. In the a-axis-oriented films, resistivity, and optical spectra show large anisotropy between the b and c axes, indicating the quasi-two-dimensional nature of the electronic structure in La2−xSrxNiO4.
Refractive optics using lithium metal
Authors: Arms D. A. , Dufresne E. M. , Clarke R. , Dierker S. B. , Pereira N. R. , Foster D.
Published: July 2002

Thanks to its low x-ray absorption, lithium should be the material of choice for x-ray refractive lenses. This article discusses some of the measurements done to verify lithium’s relevant properties. Both x-ray transmission and refraction are consistent with expectations. The lens gain suffers from broadening that is related to small-angle scattering.
Measurements of the turbulent energy dissipation rate
Authors: Pearson B. R. , Krogstad P.-Å. , van de Water W.
Published: July 2002

The one-dimensional surrogate for the dimensionless energy dissipation rate Cε is measured in shear flows over a range of the Taylor microscale Reynolds number Rλ, 70≲Rλ≲1217. We recommend that Cε should be defined with respect to an energy length scale derived from the turbulent energy spectrum. For Rλ≳300, a value of Cε≈0.5 appears to be a good universal approximation for flow regions free of strong mean shear. The present results for Cε support a key assumption of turbulence—the mean turbulent energy dissipation rate is finite in the limit of zero viscosity.
Evidence for bandedge lasing in a two-dimensional photonic bandgap polymer laser
Authors: Moll N. , Mahrt R. F. , Bauer C. , Giessen H. , Schnabel B. , Kley E. B. , Scherf U.
Published: July 2002

Optically pumped organic semiconductor lasers are fabricated by spincoating a thin film of a methyl-substituted ladder-type poly-(para-phenylene) onto a nanopatterned circular grating distributed Bragg reflector. Varying the period of the grating allows for tunability of the laser wavelength. Comparing the emission spectra below and above the threshold shows that the lasing is spectrally located at the bandedge of the Bragg dip. This is in excellent agreement with the calculated dispersion relation of the laser system.
Evidence of polymorphous amorphous-to-quasicrystalline phase transformation in Zr66.7Pd33.3 metallic glass
Authors: Jiang J. Z. , Saksl K. , Saida J. , Inoue A. , Franz H. , Messel K. , Lathe C.
Published: July 2002

The amorphous-to-quasicrystalline phase transformation and the pressure effect on the transformation in a Zr66.7Pd33.3 metallic glass have been investigated by in situ x-ray diffraction measurements using synchrotron radiation. It is found that the transformation is a polymorphous reaction and external pressure enhances the onset temperature for the formation of quasicrystals with a rate of 22 K/GPa while the temperature interval for the stability of quasicrystals remains almost unchanged in the pressure range of 0–4 GPa. The enhancement of the onset temperature for the formation of quasicrystals has been further discussed with the nucleation theory.
Effect of magnetic field on the electrode potential of metals
Authors: Perov N. S. , Sheverdyaeva P. M. , Inoue M.
Published: July 2002

The effect of a magnetic field on the electrode potential of Fe, Co, Ni, and Cu in different electrolytes was investigated. The electrolytes used were iron(III) chloride (0.02–0.6 M/l), iron (III) nitrate (0.06 M/l), caustic soda (8 M/l), hydrochloric acid (0.1 M), and sulphuric acid (0.1 M). All measurements were made at room temperature in magnetic fields of up to 4 kOe. Results from experiments with nickel and cobalt ferromagnetic electrodes and different electrolytes showed no effect of magnetic field. On the contrary, magnetic field caused a proportional change in the potential of the ferromagnetic iron electrode in solutions of iron(III) chloride and nitrate, and caustic soda. A significant long-time drift of the potential was found in all electrolytes. The electrode magnetization change gives the change of the potential, which is not proportional to magnetization.
Structure and defects of vapor-phase-grown diamond nanocrystals
Authors: Jiang X. , Jia C. L.
Published: July 2002

Diamond nanocrystalline films are prepared by the ion bombardment-assisted gas vapor synthesis technique. The phase quality, morphology and microstructure of the films are investigated by means of Raman spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. The grain size in the film ranges from several nanometers to several tens of nanometers. There is a high density of lattice defects, mainly stacking faults and twin boundaries in the nanocrystals. Lattice distortions, vacancies, and dislocations are observed. The growth of nanocrystals and the mechanism of defect formation are discussed in light of the ion impact effect.
The electronic influence of a third body on resonance energy transfer
Authors: Daniels Gareth J. , Andrews David L.
Published: July 2002

This investigation concerns an analysis of the effect, on the rate of resonance energy transfer between a pair of chromophores (the donor and acceptor), of a third chromophore species in their vicinity. Particularly when it is polar, the third body can exert an electrostatic influence on the energy-exchanging species that may result in a significant increase or decrease of their transfer rate. Furthermore it is shown how the presence of the third species can effectively “switch on” the energy transfer process, under circumstances where conventional energy transfer is forbidden due to orientational effects or on symmetry grounds. For example it is indicated that it is possible to instigate energy transfer when the salient acceptor or donor transitions are one-photon forbidden. In addition to results for structurally rigid systems, a rotationally averaged result applicable to fluids or disordered systems is presented.
Stereodynamics and rovibrational effect for H+CH4(v,j,K,n)→H2+CH3 reaction
Authors: Wang Mingliang , Zhang J. Z. H.
Published: July 2002

In this work, we employ the semirigid vibrating rotor target (SVRT) model to study the influence of rotational and vibrational excitation of the reagent on reactivity for the benchmark reaction H+CH4(v,j,K,n). The excitation of the pseudo H–CH3 stretching vibration of the SVRT model gives significant enhancement of reaction probability, consistent with the later position of the reaction barrier on the potential energy surface. The vibrationally thermal-averaged rate constant is much larger than the rate constant of the ground vibrational state. Detailed study of the influence of initial rotational states on reaction probability shows strong steric effect. The reaction probability is directly correlated with the angular distribution of the initial wave function determined by different angular momentum relationships among three vectors j, R, and r. The steric effect of polyatomic reactions, treated by the SVRT model, is more complex and richer than theoretical calculations involving linear molecular models.
Structure, bonding, and anharmonic librational motion of CO on Ir{100}
Authors: Titmuss S. , Johnson K. , Ge Q. , King D. A.
Published: July 2002

A combined low energy electron diffraction (LEED)—density functional theory (DFT) study of the structure of the Ir{100}-c(2×2)-CO phase provides a comparison of the two techniques for a simple molecular adsorbate. Both studies clearly identify atop adsorption and agree on the key structural parameters: a strong buckling of the first Ir layer, a short Ir–C bond length, and a slight lengthening of the CO bond. The molecule is found to be adsorbed in an upright configuration, although an incomplete treatment of the correlated vibrational motion of the CO molecule across the surface in the LEED analysis results in an apparent tilt of 8° from the surface normal. The DFT study determines a high adsorption energy of 2.65 eV for the c(2×2) phase which can be associated with the relief of the high tensile stress of the metastable Ir{100}-(1×1) phase and can be correlated with the short Ir–C bond. The 0.25 ML p(2×2)-CO phase displays an almost identical local bonding geometry but has a slightly lower adsorption energy of 2.61 eV, indicative of an attractive nearest neighbor interaction in the c(2×2) phase. The potential-energy surface for displacement of the CO molecule away from the atop position is found to display quartic anharmonicity. The resulting vibrational amplitude of 0.19 Å can be associated with a harmonic frequency of 8 meV, in good agreement with previous EELS measurement. The level of agreement between the LEED and DFT determined structures is sufficiently good to demonstrate that the two techniques are capable of converging on very similar structures. Furthermore, this study clearly demonstrates the future role for low-temperature LEED measurements and DFT studies in achieving an understanding of the structure, bonding, and energetics of molecules adsorbed at surfaces.
Coulomb motor by rotation of spherical conductors via the electrostatic force
Authors: Wistrom Anders O. , Khachatourian Armik V. M.
Published: July 2002

Three spherical conductors fixed in space and held at constant potential produces a rotational force that causes the conductors to rotate about their axis. The motor is described by an expression for the moment of force given by Coulomb’s law complemented by Gauss’ law of the electric potential. The observed rotation is likely to be general and apply to machines of all size scales where the electrostatic force is the dominant operative force. This would include systems ranging in size from molecular to macroscopic and be useful for devices that require rotational motion.
Symmetry breaking to a rotating wave in a lid-driven cylinder with a free surface: Experimental observation
Authors: Hirsa A. H. , Lopez J. M. , Miraghaie R.
Published: July 2002

A systematic experimental investigation of the flow in an open cylinder, driven by the constant rotation of the bottom endwall, shows that axisymmetry is spontaneously broken via a supercritical Hopf bifurcation to a rotating wave with azimuthal wave number 4. The physical mechanism responsible for the symmetry breaking is shown to be due to the instability of the shear layer that is produced by the boundary layer on the bottom rotating endwall being turned into the interior by the stationary sidewall. Comparison with other experiments and numerical studies (restricted to axisymmetric subspaces) sheds new light on disparate observations in the literature and helps distinguish between spontaneous and forced (via imperfections) symmetry breaking.
Multiscaling in a long semiflexible polymer chain in two dimensions
Authors: Yoshinaga Natsuhiko , Yoshikawa Kenichi , Kidoaki Satoru
Published: July 2002

Using atomic force microscopy, full visualization of a single giant T4 DNA molecules (166 kilo base pairs), the contour length of which is sufficient to examine the scaling property, was achieved. Fluorescence microscopic measurement was performed on exactly the same T4 DNA molecules. The results showed that there are three distinguishable regions in the scaling property R∼Lν, where R, L, and ν are the end-to-end distance, contour length and scaling exponent, respectively: (i) ν≃1 when L<0.10 μm, (ii) ν≃0.5 when 0.10 μm<L<4 μm, and (iii) ν≃0.75 when L>4 μm. This conformational behavior is discussed in relation to self-avoiding walk in 2D.
Four-component relativistic configuration-interaction calculation using the reduced frozen-core approximation
Authors: Watanabe Yoshihiro , Matsuoka Osamu
Published: July 2002

The reduced frozen-core approximation (RFCA) is a kind of frozen-core approximation (FCA). In our previous study [J. Chem. Phys. 109, 8182 (1998)], we found that Dirac–Fock–Roothaan calculations using RFCA had some desirable features compared to the all-electron and usual FCA calculations. In this study, in addition to these features, we found that they also describe well unoccupied as well as occupied orbitals. We developed a fully relativistic multireference configuration-interaction (CI) program that incorporates these features of RFCA, and report CI calculations for ThO and AuH. The calculated spectroscopic constants agree well with experimental values.
CuIn
1−x
Al
x
Se
2
thin films and solar cells
Authors: Paulson P. D. , Haimbodi M. W. , Marsillac S. , Birkmire R. W. , Shafarman W. N.
Published: July 2002

CuIn 1−x Al x Se 2 thin films are investigated for their application as the absorber layer material for high efficiency solar cells. Single-phase CuIn1−xAlxSe2 films were deposited by four source elemental evaporation with a composition range of 0⩽x⩽0.6. All these films demonstrate a normalized subband gap transmission >85% with 2 μm film thickness. Band gaps obtained from spectroscopic ellipsometry show an increase with the Al content in the CuIn1−xAlxSe2 film with a bowing parameter of 0.62. The structural properties investigated using x-ray diffraction measurements show a decrease in lattice spacing as the Al content increases. Devices with efficiencies greater than 10% are fabricated on CuIn1−xAlxSe2 material over a wide range of Al composition. The best device demonstrated 11% efficiency, and the open circuit voltage increases to 0.73 V.
Residual dipolar coupling for the assessment of cross-link density changes in γ-irradiated silica-PDMS composite materials
Authors: Maxwell Robert S. , Balazs Bryan
Published: July 2002

We have measured changes in transverse relaxation times (T2e), residual dipolar couplings (〈Ωd2〉), and the mean-squared fluctuations in the residual dipolar couplings (〈δΩd2〉), associated with cross-link density changes in a complex silica-filled polydiphenylpolysiloxane/polydimethylsiloxane (PDPS/PDMS) block copolymer composite material. The crosslinks were induced by both chemical modification of the base polymer and by radiolytic aging. We have detected H1 NMR responses from polymer chains directly interacting with the silica filler (〈δΩd2〉≫2×106 rad2 sec−2), network polymer chains not directly interacting with the silica filler (〈Ωd2〉∼2×106 rad2 sec−2 and 〈δΩd2〉∼2×106 rad2 sec−2) and non-network, low molecular weight chains and chain ends (〈δΩd2〉∼1×105 rad2 sec−2). The network domain and the non-network domain were observed to exchange spin magnetization with a correlation time of 1 sec. No evidence of spin-exchange effects on the stimulated echo were observed between the PDMS and PDPS blocks, although the blocks were observed to be in spatial proximity by double quantum NMR methods. The residual dipolar couplings change in a straightforward manner with radiation and chemically induced cross-linking of the polymer network. The strength of the filler-polymer interaction was seen to affect only the residual dipolar couplings and the transverse relaxation times and not directly the mean-squared fluctuations of the residual dipolar couplings. Dipolar correlation effect NMR shows direct evidence for surface adsorbed species, however, and has measured changes in the amount of surface adsorption due to irradiation. The results suggest that siloxane polymer cross-linking was preceded by an initial disruption of the hydrogen bond interaction between the polymer backbone and the silica silanol groups at the polymer/silica interface and that noticeable radiation induced cross-linking then occurs at dosages above 100 kGray. The work reported here shows that detailed characterization of the relaxation processes of the various nuclei in the siloxane polymers under static conditions has the potential to provide detailed insight into changes in the mechanisms and energetics of motional processes brought about by polymer aging processes.
Multiprobe magnetoresistance measurements on isolated magnetic nanowires
Authors: Vila L. , Piraux L. , George J. M. , Faini G.
Published: July 2002

We present magnetotransport measurements on single cobalt nanowires fabricated by electrodeposition into nanoporous membranes and contacted by an electron beam lithography technique. Isolated nanowires (up to 20 μm long) are extracted from the nanoporous template and spread over a silicon substrate. Electron-beam lithography allows us to make nanosized multiprobe connections on isolated wires and perform measurements on parts of the nanowire as small as 500 nm. Measurements of different segments of a single 60 nm Co nanowire show the inhomogeneity of magnetization reversal processes along the same wire.
External forcing of spiral waves
Authors: Zykov Vladimir S. , Steinbock Oliver , Müller Stefan C.
Published: July 2002

The effect of an external rhythm on rotating spiral waves in excitable media is investigated. Parameters of the unperturbed medium were chosen, such that the organizing spiral tip describes meandering (hypocyclic) trajectories, which are the most general shape for the experimentally observed systems. Periodical modulation of excitability in a model of the Belousov–Zhabotinsky (BZ) reaction forces meandering spiral tips to describe trajectories that are not found at corresponding stationary conditions. For different modulation periods, two types of resonance drift, phase-locked tip motion, a spectrum of hypocyclic trajectories, and complex multifrequency patterns were computed. The computational results are complemented by experimental data obtained for periodically changing illumination of the photosensitive BZ reaction. The observed drastic deformation of the tip trajectory is considered as an efficient means to study and to control wave processes in excitable media.
Multirhythmic bursting
Authors: Butera Robert J.
Published: July 2002

A complex modeled bursting neuron [C. C. Canavier, J. W. Clark, and J. H. Byrne, J. Neurophysiol. 66, 2107–2124 (1991)] has been shown to possess seven coexisting limit cycle solutions at a given parameter set [Canavier et al., J. Neurophysiol 69, 2252–2259 (1993); 72, 872–882 (1994)]. These solutions are unique in that the limit cycles are concentric in the space of the slow variables. We examine the origin of these solutions using a minimal 4-variable bursting cell model. Poincaré maps are constructed using a saddle-node bifurcation of a fast subsystem such as our Poincaré section. This bifurcation defines a threshold between the active and silent phases of the burst cycle in the space of the slow variables. The maps identify parameter spaces with single limit cycles, multiple limit cycles, and two types of chaotic bursting. To investigate the dynamical features which underlie the unique shape of the maps, the maps are further decomposed into two submaps which describe the solution trajectories during the active and silent phases of a single burst. From these findings we postulate several necessary criteria for a bursting model to possess multiple stable concentric limit cycles. These criteria are demonstrated in a generalized 3-variable model. Finally, using a less direct numerical procedure, similar return maps are calculated for the original complex model [C. C. Canavier, J. W. Clark, and J. H. Byrne, J. Neurophysiol. 66, 2107–2124 (1991)], with the resulting mappings appearing qualitatively similar to those of our 4-variable model. These multistable concentric bursting solutions cannot occur in a bursting model with one slow variable. This type of multistability arises when a bursting system has two or more slow variables and is viewed as an essentially second-order system which receives discrete perturbations in a state-dependent manner.
Role of hydroxide impurities in the thermoluminescent behavior of lithium fluoride
Authors: Stoebe T. G. , DeWerd L. A.
Published: July 2002

The influence of OH ion impurities on thermoluminescent sensitivity and supralinearity in LiF:Mg, Ti is analyzed. Available evidence is shown to be consistant with the presence of Ti-OH and Mg-OH complexes. The track interaction model is used to explain the data, with competing centers decreasing sensitivity and supralinearity at high concentrations.
On the theory of electron-beam-induced current contrast from pointlike defects in semiconductors
Authors: Jakubowicz A.
Published: July 2002

An approximate solution of the problem of the electron-beam-induced current contrast from point-like defects in a semi-infinite semiconductor is given, which differs from that of Donolato [Optik (Stuttgart) 52, 19 (1978/79)]. The ‘‘strength of the defect’’ is characterized by an effective radius. The contrast is shown to be a nonlinear function of the strength of the defect. The results are in qualitative agreement with new experimental data of Ourmazd, Wilshaw, and Booker [in Proceedings of the 41st Annual Meeting of the Electron Microscopy Society of America, edited by G. W. Bailey (San Francisco Press, San Fransisco, 1983), p. 142].
Ferromagnetic resonance in a Ni-Mo superlattice
Authors: Pechan M. J. , Salamon M. B. , Schuller Ivan K.
Published: July 2002

Ferromagnetic resonance (FMR) measurements, at room temperature and at 4.2 K, have been made on a layered Ni (249 Å)-Mo(83 Å) superlattice. We have examined the resonance position as a function of the angle between the film normal and the applied field. The measured g value agrees with that of bulk Ni, but the magnetization is lower than that obtained for bulk Ni and also for this sample using both light scattering and direct measurement techniques. This low magnetization contrasts with FMR measurements on compositionally modulated Ni-Cu samples, where the magnetization was reported to be greater than that of bulk Ni. We show that a reduced value of the magnetization is consistent with perpendicular uniaxial anisotropy. When the applied field is less than 20° from the surface normal, additional lines appear that move to higher fields than the main resonance. These lines are consistent with the existence of nonuniform regions of distinct magnetization. An observed resonance, which is suggestive of a spin-wave mode, is discussed.
A model for time dependence in shock-induced thermal radiation of light
Authors: Boslough Mark B.
Published: July 2002

High-speed optical pyrometry has seen increasing application in the measurement of shock temperatures in initially transparent solids and liquids; however, the information contained in the time-dependent intensity of the emitted light has frequently been overlooked. A model has been developed for this time dependence in the observed intensity of light emitted from materials undergoing high-pressure shock loading. Most experimental observations of this time dependence can be explained on the basis of geometric effects only, without having to invoke intrinsic time dependences of the source intensity (due to changes in temperature, emissivity or shock-wave structure). By taking advantage of this fact, observed time dependences can be used to determine the absorption coefficient of shocked materials and their effective emissivities, thereby providing more precise temperature measurements. The model is invoked under various limiting conditions to explain time dependences previously observed in NaCl, CaO, Mg2SiO4 (forsterite), SiO2 (quartz), MgO, and CaAl2Si2O8 (anorthite) glass. As an example, the linear absorption coefficient at 650 nm of NaCl shocked to 75 GPa is found to be 13 cm−1, close to previously published values based on a similar but less general model.
Resonant tunneling transistor with quantum well base and high-energy injection: A new negative differential resistance device
Authors: Capasso Federico , Kiehl Richard A.
Published: July 2002

We propose a new negative conductance device consisting of a heterojunction bipolar transistor with a quantum well and a symmetric double barrier or a superlattice in the base region. The key difference compared to previously studied structures is that resonant tunneling is achieved by high-energy minority carrier injection into the quantum state rather than by application of an electric field. Thus this novel geometry maintains the crucial, structural symmetry of the double barrier, allowing unity transmission at all resonance peaks and higher peak-to-valley ratios and currents compared to conventional resonant tunneling structures. Both tunneling and ballistic injection in the base are considered. These new functional devices have significant potential for a variety of signal processing and multiple-valued logic applications and for the study of the physics of transport in superlattices.
Electrical properties of planar rf discharges for dry etching
Authors: van Roosmalen A. J. , van den Hoek W. G. M. , Kalter H.
Published: July 2002

rf impedance, dc self-bias, and ion sheath (dark space) thicknesses are measured in an O2 discharge for 7–53 Pa pressure and 50–800 W rf power (13.56 MHz). Special attention is paid to corrections for reactor stray impedances. It is concluded that the discharge can be described as a capacitance (the ion sheath) with both a parallel and a series resistance, the series element being the more important one. Good agreement is found between optical and electrical measurements of the ion sheath thickness. Evidence is presented that the dc potential difference between plasma and ground and rf electrode can be estimated with reasonable accuracy from the dc self-bias and the optical dark space thicknesses. Positive ion acceleration in the ion sheath and electron-neutral collisions in the bulk of the plasma glow account for only part of the total rf power transfer. It is suggested that significant dissipation takes place near the glow-sheath boundary, although a quantitative description cannot be given yet.
Photoemission from transition metals and their compounds
Authors: Davis L. C.
Published: July 2002

Photoemission experiments on 3d transition metals are reviewed. The emphasis is on understanding the results of experiments, not on experimental details and methods. Extensive use is made of simple models. Much of the review pertains to resonances associated with the autoionization 3p53dn+1 →3p63dn−1 +e and their implications for electronic structure. Nonresonant ultraviolet and x-ray photoemission spectroscopies are also discussed. Photoemission and photoabsorption of transition-metal atoms are discussed first. Results for Mn are described at length to establish the validity of the autoionization mechanism. The results from atomic spectroscopy are used to interpret experiments on solids. The role of atomiclike excitations in solids is examined. Compounds of transition metals are analyzed in terms of ligand-field theory, which is shown to be inadequate. Newer theories involving configuration interaction are shown to agree better with experiment. Various mechanisms for the excitation of photoemission satellites are presented. In the metallic state, effects similar to those observed for the compounds occur. The existence of two-bound-hole final states is demonstrated. Their importance in Auger spectroscopy, valence- and core-emission satellites, and resonant photoemission is discussed. The effects of closely related electron correlations on the band structure are described.
A study of thermal depolarization of polyvinylidene fluoride using x-ray pole-figure observations
Authors: Bur A. J. , Barnes J. D. , Wahlstrand K. J.
Published: July 2002

Measurements of piezoelectric and pyroelectric activity, density, and x-ray pole figures were used to study the effect of thermal aging on the state of polarization in polyvinylidene fluoride. A rolled and poled  β-phase specimen of polyvinylidene fluoride was subjected to thermal aging which consisted of temperature cycling between room temperature and successively higher maximum temperatures, TMAX, where TMAX ranged from room temperature to 164 °C. We found that the room temperature piezo and pyroelectric activity decreased linearly as a function of TMAX from 75 °C to 164 °C at which temperature the specimen had 30% of its original activity; a linear extrapolation of these data to zero activity yielded a temperature Tc=207 °C. From density measurements at room temperature, crystallinity was calculated and found to remain constant during thermal cycling. X-ray pole-figure observations of the (200) (110) composite diffraction of the  β-phase crystal showed single-crystal texture of the rolled specimen and confirmed the six-site model of dipole orientation. Changes in x-ray intensity at the six sites on the pole figure, as a function of thermal aging, were associated with the depolarization process which occurs via a 60° rotation of dipoles away from the direction of primary polarization. Based on these data, we propose a model which describes the state of polarization in polyvinylidene fluoride and from which we calculate the fraction of dipoles in the crystalline state contributing to the polarization.
High-temperature thermodynamic properties of alpha and gamma lanthanum sesquisulfides and related compounds
Authors: Amano T. , Beaudry B. J. , Gschneidner K. A.
Published: July 2002

The high-temperature heat contents of La2S3(α), La2S3(γ), La3S4, and La2.65Eu0.35S4 were measured from 400 to 1800 K by using a copper block drop calorimeter. The heat and entropy of transition of the α-to-γ transformation of La2S3 was determined. Equations for the thermodynamic functions H0T−H0298.15, Cp, S0T−S0298.15 and −(F0T−H0298.15)/T were derived in terms of the least-squares fit parameters of the heat content data.
Computer modeling of carrier transport in (Hg,Cd)Te photodiodes
Authors: Summers C. J. , Darling B. , Martin B. G.
Published: July 2002

A numerical technique has been used to solve the carrier transport equations for several (Hg,Cd)Te photodiode configurations, namely n+pp+, n+np, and pin. Of particular interest are the fundamental recombination mechanisms of radiative and Auger. Results clearly demonstrate the importance of Auger type 1 and 7 mechanisms on the n and p sides of the junction, respectively, in limiting carrier lifetimes. For example, it was found that for defect-free Hg1−xCdxTe alloys with x=0.2 and hole concentration less than 4×1014 cm−3 adjacent to the depletion region, the Auger recombination rate can be reduced below the radiative rate. An analysis of the spatial dependence of the electron mobility shows that the presence of high carrier concentrations and electric field strengths can reduce the mobility and consequently have an effect on the sensitivity and temporal response of the photodiode.
Magnetic permeability of a simple cubic lattice of conducting magnetic spheres
Authors: Lam John
Published: July 2002

The effective magnetic permeability of a simple cubic lattice of conducting magnetic spheres is calculated in the quasistatic limit including the eddy current effect. The problem is formulated explicitly in terms of an infinite matrix equation which is solved by iteration. A general analytical formula for the effective magnetic permeability is derived as a series expansion in powers of the volume fraction of the spheres. The formula contains that for the effective dielectric constant as a special case. The results of sample numerical calculations are presented.