bluuebunny/crossref_metadata_2025 huggingface dataset:
Heterodyned fifth-order two-dimensional IR spectroscopy: Third-quantum states and polarization selectivity
Authors: Ding Feng , Fulmer Eric C. , Zanni Martin T.
Published: September 2005

A heterodyned fifth-order two-dimensional (2D) IR spectrum of a model coupled oscillator system, Ir(CO)2(C5H7O2), is reported. The spectrum is generated by a pulse sequence that probes the eigenstate energies up to the second overtone and combination bands, providing a more rigorous potential-energy surface of the coupled carbonyl local modes than can be obtained with third-order spectroscopy. Furthermore, the pulse sequence is designed to generate and then rephase a two-quantum coherence so that the spectrum is line narrowed and the resolution improved for inhomogeneously broadened systems. Features arising from coherence transfer processes are identified, which are more pronounced than in third-order 2D IR spectroscopy because the transition dipoles of the second overtone and combination states are not rigorously orthogonal, relaxing the polarization constraints on the signal intensity for these features. The spectrum provides a stringent test of cascading signals caused by third-order emitted fields and no cascading is observed. In the Appendix, formulas for calculating the signal intensities for resonant fifth-order spectroscopies with arbitrarily polarized pulses and transition dipoles are reported. These relationships are useful for interpreting and designing polarization conditions to enhance specific spectral features.
Synthesis of TiO2 nanoparticles on the Au(111) surface
Authors: Biener Jürgen , Farfan-Arribas Enrique , Biener Monika , Friend Cynthia M. , Madix Robert J.
Published: September 2005

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O2 at 300K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600to900K. At the same time, the Au(111) surface assumes a serrated ⟨110⟩-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900K results in the formation of stoichiometric TiO2 nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO2 on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.
Time autocorrelation function analysis of master equation and its application to atomic clusters
Authors: Zhang Chi , Berry R. Stephen
Published: September 2005

We derive the energy fluctuation Δ2E, and the time autocorrelation κ(τ) and its Fourier transformation—the fluctuation spectra S(ω)—of the master-equation transition matrix. The contribution from each eigenmode of the transition matrix to these fluctuation quantities reveals the relevant importance of the individual mode in the relaxation processes. The time scales associated with these relaxation processes are determined by the corresponding eigenvalues. Unlike traditional time evolution analysis, the autocorrelation function and fluctuation spectra analysis does not involve an arbitrary initial population. It is also more suitable for analyzing the underlying dynamic, kinetic behavior near the equilibrium and the behavior of the long-time-scale rare events. We utilize our technique to analyze the solid-liquid phase coexistence of the 13-atom Morse cluster and the fcc-to-icosahedral structure transition of the 38-atom Lennard-Jones cluster. For the processes studied, the fluctuation spectra from the master equation simplify the analysis of the transition matrix, and the important relaxation modes are easily extracted.
Collision-induced desorption in 193-nm photoinduced reactions in (O2+CO) adlayers on Pt(112)
Authors: Han Song , Ma Yunsheng , Matsushima Tatsuo
Published: September 2005

The spatial distribution of desorbing O2 and CO2 was examined in 193-nm photoinduced reactions in O2+CO adlayers on stepped Pt(112)=[(s)3(111)×(001)]. The O2 desorption collimated in inclined ways in the plane along the surface trough, confirming the hot-atom collision mechanism. In the presence of CO(a), the product CO2 desorption also collimated in an inclined way, whereas the inclined O2 desorption was suppressed. The inclined O2 and CO2 desorption is explained by a common collision-induced desorption model. At high O2 coverage, the CO2 desorption collimated closely along the (111) terrace normal.
Electric-field generated by the combustion of titanium in nitrogen
Authors: Martirosyan K. S. , Setoodeh Mona , Luss Dan
Published: September 2005

A short temporal electrical impulse (duration of 30–150ms) was generated during the nitridation of mixtures of titanium and titanium nitride by a high temperature moving reaction front. The maximum voltage and current were generated in the combustion front region, in which the conversion of Ti to TiN was incomplete. The electric field (voltage up to 2V and current up to 60mA) decayed and vanished before the maximum combustion temperature was attained. The generation of an electric field during a rapid high-temperature nitridation is most probably due to the different diffusion velocities of charge carriers through the growing titanium nitride shell during the initial stage of the reaction. When the reactant mixture contained a high percentage of pure titanium (larger than 60wt%), partial melting led to irreproducibility in the amplitude and duration of the electrical signal.
Frequency-dependent hyperpolarizabilities of the Ne, Ar, and Kr atoms using the approximate coupled cluster triples model CC3
Authors: Høst Stinne , Jørgensen Poul , Köhn Andreas , Pawłowski Filip , Klopper Wim , Hättig Christof
Published: September 2005

The frequency-dependent electric field-induced second harmonic generation (ESHG) second hyperpolarizabilities γ of neon, argon, and krypton are calculated using the approximate coupled cluster triples model CC3. Systematic basis set investigations are carried out to establish basis set limits, and scalar relativistic effects are accounted for by direct perturbation theory. To estimate higher-order correlation effects, full configuration-interaction results are used to benchmark the accuracy of CC3. The best theoretical estimates obtained thereby for the static second hyperpolarizabilities γ0 are 107.4, 1159, and 2589 a.u. for neon, argon, and krypton, respectively. These values as well as the results for the dispersion curve of the parallel component γ‖ agree well with the latest experimental values from electric field-induced second harmonic generation. In addition, the dispersion of the perpendicular component γ⊥ and the hyperpolarizability ratios γ‖∕γ⊥ has been studied for the first time on a consistently correlated ab initio level. The analysis of the results indicates that, in particular for neon and krypton, the presently available experimental values are flawed.
Effective polarization interaction potential “charge-atom” for partially ionized dense plasma
Authors: Ramazanov T. S. , Dzhumagulova K. N. , Omarbakiyeva Y. A.
Published: September 2005

The pseudopotentials of particle interaction, taking into account quantum-mechanical effects of diffraction at short distances and also screening effects at large distances, are obtained for a partially ionized plasma. The dielectric function method was used.
Dynamics of a suspension confined in a thin cell
Authors: Alvarez Alejandra , Soto Rodrigo
Published: September 2005

A suspension confined between two close parallel plates is studied in the Stokesian regime. The use of boundary integral equations and the lubrication approximation allows computation of the hydrodynamic forces acting on the particles. The forces are long ranged (decaying as R−2) and depend on the orientation of the relative position and velocity of particles. This tensorial character predicts an “antidrag” that is observed in experiments. Also, the far-field forces vanish when a particle is surrounded by an homogeneous suspension, but net forces appear in the presence of abrupt discontinuities of the suspension. The effect of the computed hydrodynamic forces is studied in the dynamics of a cluster of particles falling in a gravitational field, where the different features of the hydrodynamic forces are present. The cluster spreads and deforms from the initial circular shape due to the action of the hydrodynamic forces in the presence of the cluster boundary. The expression for the hydrodynamic forces at long distances allows application of a mean-field approximation, where the forces on a particle can be computed in terms of the particle current field. This approximation gives an excellent numerical agreement with the direct computation of all the hydrodynamic forces, being numerically much faster, yet preserving the accuracy.
GeV Wakefield acceleration of low energy electron bunches using Petawatt lasers
Authors: Lifschitz A. F. , Faure J. , Malka V. , Mora P.
Published: September 2005

The possibility of accelerating electrons to the GeV level using a Petawatt laser focused in a uniform plasma is investigated. The proposed scheme relies on the wakefield acceleration of an electron bunch from a state-of-the-art radio-frequency accelerator. Using an analytical model as well as numerical simulations performed with WAKE [P. Mora and T. M. Antonsen, Phys. Plasmas 4, 217 (1997)], a systematical study of the injector parameters is carried out. In particular, it is found that the quality of the accelerated electron bunch—in terms of bunch length and energy spread—depends crucially on the injection energy. Injection energies of a few MeV lead to a GeV electron beam with sub-100fs bunches and 10% energy spreads. Most of the features of the acceleration process can be explained within the linear response framework, including both the reduction of energy spread and bunch length at low injection energies. The role of nonlinear effects is discussed.
Towards accurate all-electron quantum Monte Carlo calculations of transition-metal systems: Spectroscopy of the copper atom
Authors: Caffarel Michel , Daudey Jean-Pierre , Heully Jean-Louis , Ramírez-Solís Alejandro
Published: September 2005

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the S2, D2, and P2 electronic states of Cu and the S1 ground state of Cu+. We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600cm−1 and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources.
Brillouin scattering study on the single-crystal elastic properties of natrolite and analcime zeolites
Authors: Sanchez-Valle Carmen , Sinogeikin Stanislav V. , Lethbridge Zoe A. D. , Walton Richard I. , Smith Christopher W. , Evans Kenneth E. , Bass Jay D.
Published: September 2005

The Brillouin light-scattering technique was used to investigate the single-crystal elastic properties of two aluminosilicate zeolites, natrolite (NAT) and analcime (ANA), at ambient conditions. An inversion of the acoustic velocity data results in the full set of elastic stiffness moduli (Cij’s) for both materials. From the single-crystal moduli the aggregate adiabatic bulk moduli (Ks), shear moduli (G), and Poisson’s ratios (ν) were found to be Ks=48.5(1.0)GPa, G=31.6(1.0)GPa, and ν=0.232(5) for NAT, and Ks=59.8(1.2)GPa, G=32.1(1.0)GPa, and ν=0.272(5) for ANA (Voigt-Reuss-Hill averages). The bulk and shear moduli of both zeolites are relatively low compared with those of densely packed aluminosilicates, reflecting an open framework structure of (Al,SiO4) tetrahedra which is easily deformed by bending the Si–O–Al angles. As expected for a less dense crystal, NAT is softer and more compressible than ANA. An evaluation of the directional Young’s moduli shows that the compressibility of NAT is nearly uniform along the [100] and [010] axes, while [001] is stiffer, in agreement with previous compression studies. We do not find experimental evidence of negative Poisson’s ratios for NAT zeolites as predicted by recent theoretical calculations.
Electromechanical response of 1-3 piezoelectric composites: Effect of poling characteristics
Authors: Kar-Gupta Ronit , Venkatesh T. A.
Published: September 2005

The present study characterizes the effects of variations in the poling characteristics of the matrix and the fiber phase on the overall electromechanical behavior of a 1-3 piezocomposite. Upon identifying 15 types of 1-3 piezocomposites based on two factors: the spatial arrangement of the fibers and the relative orientation of the poling direction of one phase with respect to the second phase, a comprehensive finite-element-based numerical model that can fully capture the electromechanical response of all the 15 types of 1-3 piezocomposites is developed and the following principal results are obtained: (i) a judicious selection of the combination of poling characteristics of the individual constituents, that each exhibit unidirectional sensitivity, could lead to the development of a piezocomposite with significant bidirectional sensitivity; (ii) variations in the elastic, piezoelectric, and dielectric material constants with fiber volume fraction are in general nonlinear, while converging to the expected monolithic material limits at very low and very high volume fractions of the fiber phase in the piezocomposite; (iii) concentrations in mechanical stresses typically occur at the fiber-matrix interface and thus the location and conditions for the onset of mechanical failure in the piezocomposites could be predicted; (iv) highest efficiency in energy transfer from the mechanical to the electrical domain and vice versa is obtained in class I piezocomposites where the fiber phase and matrix phase are poled in a direction parallel to the fiber axis. However, a wide range of specific acoustic impedances are generated in a composite by a variation in the poling characteristics, thus enhancing the tunability of piezocomposites for specific applications.
Lock in of magnetic stripe domains to pinning lattices produced by focused ion-beam patterning
Authors: Konings Stan , Miguel Jorge , Luigjes Jeroen , Schlatter Hugo , Luigjes Huib , Goedkoop Jeroen , Gadgil Vishwas
Published: September 2005

With focused ion-beam irradiation it is possible to engineer the anisotropy of magnetic films on nanometer length scales. We used this technique to write square lattices of artificial domain-wall pinning centers in a perpendicular anisotropy GdFe film displaying a well-defined stripe domain pattern. We observe a clear lock in of the intrinsic meandering stripe pattern to the pinning lattices, resulting in highly ordered domain patterns. We find that at remanence the dots pin the domain walls, while in perpendicular applied magnetic fields they host the down domains.
Formation of ice nanotube with hydrophobic guests inside carbon nanotube
Authors: Tanaka Hideki , Koga Kenichiro
Published: September 2005

A composite ice nanotube inside a carbon nanotube has been explored by molecular-dynamics and grand canonical Monte Carlo simulations. It is made from an octagonal ice nanotube whose hollow space contains hydrophobic guest molecules such as neon, argon, and methane. It is shown that the attractive interaction of the guest molecules stabilizes the ice nanotube. The guest occupancy of the hollow space is calculated by the same method as applied to clathrate hydrates.
Broad-spectrum light emission at microscopic breakdown sites in metal-insulator-silicon tunnel diodes
Authors: Mihaychuk J. G. , Denhoff M. W. , McAlister S. P. , McKinnon W. R. , Chin A.
Published: September 2005

In addition to Si band-edge electroluminescence (EL) near 1.1 eV, we observe hot-electron EL in metal-insulator-silicon tunnel diodes that can span a detector-limited range from 0.7 to 2.6 eV (1780–480 nm). The maximum photon energy increases with increasing forward bias. In one implementation, sub-micron-sized EL sites appear during the forward-bias stress. The number of sites grows linearly with the current, consistent with the dielectric breakdown of the insulator. We compare the poststress current-voltage data with the quantum-point-contact model. Results are presented for various p-type Si(100) devices having 2–8-nm-thick SiO2, Al2O3, and HfOxNy insulators. We also describe devices in which electron-beam lithography of an 18-nm-thick SiO2 is used to define EL sites.
Explicit secular equations for piezoacoustic surface waves: Rayleigh modes
Authors: Collet Bernard , Destrade Michel
Published: September 2005

The existence of a two-partial Rayleigh wave coupled to an electrical field in 2 mm piezoelectric crystals is known but has rarely been investigated analytically. It turns out that the Z cut X propagation problem can be fully solved, up to the derivation of the secular equation as a polynomial in the squared wave speed. For the metallized (unmetallized) boundary condition, the polynomial is of degree 10 (48). The relevant root is readily identified and the full description of the mechanical and electrical fields follows. The results are illustrated in the case of the superstrong piezoelectric crystal, potassium niobate, for which the effective piezoelectric coupling coefficient is calculated to be about 0.1.
Determination of relative internal quantum efficiency in InGaN∕GaN quantum wells
Authors: Martinez C. E. , Stanton N. M. , Kent A. J. , Graham D. M. , Dawson P. , Kappers M. J. , Humphreys C. J.
Published: September 2005

We have investigated the relative quantum efficiency in a series of InGaN∕GaN single quantum wells with differing indium concentration. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time-resolved photoluminescence studies. Using these complementary techniques we have extracted the low-temperature internal quantum efficiency of the recombination and observed the effect of free-carrier screening on the radiative and nonradiative processes in the quantum well samples. All the samples exhibit high quantum efficiency, with the maximum being observed in the 10% indium sample. In addition, we observe the appearance of a delayed phonon signal, which we correlate to the measured quantum efficiency of the samples.
Size-dependent change in interband absorption and broadening of optical plasma-resonance absorption of indium particles
Authors: Anno E. , Tanimoto M.
Published: September 2005

Optical absorption of indium-island films, consisting of indium particles smaller than about 500 Å in diameter, has been investigated experimentally. The interband absorption, the position of which is almost constant (at about 234 nm) irrespective of particle size, was found to disappear in particle sizes below about 200 Å in diameter. This disappearance is pointed out to be similar to that in lead particles, where the disappearance is due to the effect of surface atoms. Optical plasma-resonance absorption appeared at 276–335 nm. By simulating this absorption with a Maxwell-Garnett-type effective-medium theory, we investigated the relaxation time τ and the mean free path l of conduction electrons. τ and l for particle sizes of about 120 and 160 Å in diameter are estimated to be (3.33–3.36)×10−16s and 5.79–5.85 Å, respectively, which is smaller than the bulk values (0.38×10−14s and 66.1 Å). The small τ and l are attributed to the scattering of the conduction electrons at lattice defects internal to the particles.
Reduction of detonating liquid nitromethane’s chemical reaction-zone length by chemical sensitization
Authors: Engelke Ray , Sheffield Stephen A. , Stacy Howard L. , Quintana John P.
Published: September 2005

We examine the effect of the addition of small amounts of the organic base diethylenetriamine [NH2(CH2CH2)NH(CH2CH2)NH2] on the chemical reaction-zone length (CRZ) of detonating liquid nitromethane (CH3NO2). This is done by making accurate measurements of the detonating materials’ diameter-effect curves (i.e., detonation speed versus lateral charge size) as a function of the amount of chemical sensitizer added. Detonation speed experiments were performed with additions of the organic base in amounts between 0.00 and 0.25wt%. Reductions in the CRZ of as much as 25% were produced by base addition. Most of the reduction in length is produced by very small amounts of base addition—i.e., ca. 0.05wt% of base or less (i.e., 1 molecule of the base per 3300 nitromethane molecules or less). Measured detonation speeds are given for five compositions of nitromethane and base as a function of charge internal diameter. Absolute CRZs are estimated using a value of liquid nitromethane’s CRZ obtained by other means. Earlier work has determined a relationship between the failure diameter of sensitized nitromethane compositions and the amount of base addition. Here we have found a relationship between relative CRZ and the amount of base addition. By eliminating the base parameterization, we give a phenomenological relationship between the failure diameter of such materials and their steady one-dimensional CRZs.
The effect of discharge chamber geometry on the ignition of low-pressure rf capacitive discharges
Authors: Lisovskiy V. , Martins S. , Landry K. , Douai D. , Booth J.-P. , Cassagne V. , Yegorenkov V.
Published: September 2005

This paper reports measured and calculated breakdown curves in several gases of rf capacitive discharges excited at 13.56 MHz in chambers of three different geometries: parallel plates surrounded by a dielectric cylinder (“symmetric parallel plate”), parallel plates surrounded by a grounded metallic cylinder (“asymmetric parallel plate”), and parallel plates inside a much larger grounded metallic chamber (“large chamber”). The breakdown curves for the symmetric chamber have a multivalued section at low pressure. For the asymmetric chamber the breakdown curves are shifted to lower pressures and rf voltages, but the multivalued feature is still present. At higher pressures the breakdown voltages are much lower than for the symmetric geometry. For the large chamber geometry the multivalued behavior is not observed. The breakdown curves were also calculated using a numerical model based on fluid equations, giving results that are in satisfactory agreement with the measurements.
What controls the decay of passive scalars in smooth flows?
Authors: Haynes P. H. , Vanneste J.
Published: September 2005

The exponential decay of the variance of a passive scalar released in a homogeneous random two-dimensional flow is examined. Two classes of flows are considered: short-correlation-time (Kraichnan) flows, and renewing flows, with complete decorrelation after a finite time. For these two classes, a closed evolution equation can be derived for the concentration covariance, and the variance decay rate γ2 is found as the eigenvalue of a linear operator. By analyzing the eigenvalue problem asymptotically in the limit of small diffusivity κ, we establish that γ2 is either controlled (i) locally, by the stretching characteristics of the flow, or (ii) globally, by the large-scale transport properties of the flow and by the domain geometry. We relate the eigenvalue problem for γ2 to the Cramer function encoding the large-deviation statistics of the stretching rates; hence we show that the Lagrangian stretching theories developed by Antonsen et al. [Phys. Fluids 8, 3094 (1996)] and others provide a correct estimate for γ2 as κ→0 in regime (i). However, they fail in regime (ii), which is always the relevant one if the domain scale is significantly larger than the flow scale. Mathematically, the two types of controls are distinguished by the limiting behavior as κ→0 of the eigenvalue identified with γ2: in the local case (i) it coincides with the lower limit of a continuous spectrum, while in the global case (ii) it is an isolated discrete eigenvalue. The diffusive correction to γ2 differs between the two regimes, scaling like 1∕log2κ in regime (i), and like κσ for some 0<σ<1 in regime (ii). We confirm our theoretical results numerically both for Kraichnan and renewing flows.
Contributions to the dielectric losses of textured SrTiO3 thin films with Pt electrodes
Authors: Lu Jiwei , Schmidt Steffen , Ok Young-Woo , Keane Sean P. , Stemmer Susanne
Published: September 2005

The factors controlling low-frequency (1MHz) dielectric losses of textured SrTiO3 thin films deposited by radio-frequency magnetron sputtering on platinized sapphire substrates were investigated. In particular, the influence of film texture, phase transformations, applied bias field, temperature, and annealing atmospheres was studied. Films that were (111) textured showed a phase transformation at ∼150K, whereas films that were predominantly (110) oriented did not exhibit a phase transformation in the measured temperature range (100–300K). Two major contributions to the dielectric losses were identified: a low-temperature loss increase for the (111) oriented film, which could be suppressed by an applied bias field, and a loss peak at ∼250K (at 1MHz), which was strongly frequency-dependent and likely associated with a relaxing defect. The low-temperature loss mode was related to the appearance of a phase transformation and contributed to the dielectric losses even at temperatures that were more than 100K above the phase transformation. In contrast to the leakage properties of the films, which were strongly dependent on annealing atmospheres, annealing under reducing conditions had no significant influence on any of the observed loss modes. Possible origins of the different loss contributions were discussed.
Dielectric enhancement in polymer-nanoparticle composites through interphase polarizability
Authors: Murugaraj Pandiyan , Mainwaring David , Mora-Huertas Nelson
Published: September 2005

Dielectric measurements on polyimide-oxide nanoparticle composite thin films show a composite dielectric constant (εcomposite) that increased monotonically with increasing oxide content well above the value predicted by Maxwell’s rule for dielectric mixtures below the percolation threshold. Above certain volume fractions, the measured εcomposite values were found to exceed the corresponding nanoparticle ε such that εpolymer<εparticle<εcomposite contrasted to conventional composites where εpolymer<εcomposite<εparticle. The εcomposite was independent of frequency to 10MHz with dielectric loss of <0.005 throughout this range, indicating that the observed enhancement in ε does not originate from space-charge related contributions and hence should be due to dipolar contributions. The observed ε enhancement (εcomposite−εMaxwell) showed a correlation with the total surface area of the nanoparticles. The dielectric model of Vo and Shi [Microelectron. J. 33, 409 (2002), and references therein] showed that the enhanced dielectric behavior originates from significant interfacial nanoparticle-polymer interactions and the critical role of additional contributions to polarizability through specific physicochemical interactions within the interphase region. An interphase εint considerably higher than that of the nanoparticle and a high interface interaction constant of 3.24 for the nanocomposite suggest a strong interaction between the functional groups of the polymer and the nanoparticle surface. Although modeling suggests a maximum of ε∼65vol%, loss in micromechanical stability occurred above 20% due to incomplete polymer wetting films arising from the high nanoparticle surface areas.
Crystal morphology and nucleation in thin films of amorphous Te alloys used for phase change recording
Authors: Kalb J. A. , Wen C. Y. , Spaepen Frans , Dieker H. , Wuttig M.
Published: September 2005

Ex situ transmission electron microscopy (TEM) was used to study the crystal morphology in sputtered amorphous Ge4Sb1Te5, Ge2Sb2Te5, and Ag0.055In0.065Sb0.59Te0.29 thin films used for phase change recording. Tilting of plan view samples revealed that each crystallized growth formation is a bent single crystal. Cross-sectional TEM showed that crystals only nucleate heterogeneously at the (naturally oxidized) film surface. These findings allow the determination of nucleation parameters around 150°C from earlier experiments [J. Kalb, F. Spaepen, and M. Wuttig, Appl. Phys. Lett. 84, 5240 (2004)]. The time lag for nucleation has an activation energy of (2.74±0.13)eV for Ge2Sb2Te5 and (2.33±0.18)eV for Ag0.055In0.065Sb0.59Te0.29. The activation energies for the steady-state nucleation rate were (4.09±0.20)eV for Ge4Sb1Te5 and (3.50±0.17)eV for Ge2Sb2Te5. With the activation energy for the crystal-growth velocity found in the earlier article the critical work for formation of the nucleus was found to be (1.35±0.23)eV for Ge4Sb1Te5 and (1.15±0.22)eV for Ge2Sb2Te5. These values are lower limits for homogeneous nucleation.
The effect of octahedral tilting on proton binding sites and transition states in pseudo-cubic perovskite oxides
Authors: Gomez Maria A. , Griffin Mary A. , Jindal Saryu , Rule Kristin D. , Cooper Valentino R.
Published: September 2005

Proton conducting oxide ceramics have shown potential for use in fuel cell technologies. Understanding the energy pathways for proton conduction could help us design more efficient fuel cell materials. This paper describes how octahedral tilting affects the relative energies of proton binding sites, transition states, and conduction pathways in cubic and pseudo-cubic perovskites. First, the structure for cubic and pseudo-cubic forms of BaTiO3, BaZrO3, CaTiO3, and CaZrO3, is found. Even when cubic symmetry is enforced, CaTiO3, and CaZrO3 exhibit octahedral tilting distortions characteristic of orthorhombic phases while BaTiO3 and BaZrO3 remain undistorted. Octahedral tilting gives rise to proton binding sites facilitating inter- and intra-octahedral proton transfer while the proton binding sites of undistorted perovskites facilitate only intra-octahedral proton transfer. The nudged elastic band method is used to find minimum energy paths between the proton binding sites. As distortions increase, inter-octahedral proton transfer barriers decrease while intra-octahedral proton transfer barriers increase. Concurrently, rotational barriers from oxygens facilitating inter-octahedral proton transfer increase while rotational barriers from oxygens facilitating intra-octahedral proton transfer decrease. Intra-octahedral transfer is the rate-limiting step to the lowest energy extended proton conduction pathway in all the perovskites considered.
Comprehensive study of Ohmic electrical characteristics and optimization of Ti∕Al∕Mo∕Au multilayer Ohmics on undoped AlGaN∕GaN heterostructure
Authors: Sun Yunju , Chen Xiaodong , Eastman L. F.
Published: September 2005

The performance of Ohmic contacts is highly dependent on the metal stack contents and layer thicknesses evaporated, annealing temperature, and annealing time. A four-layer-based metal stack was used to form Ohmic contacts on an undoped AlGaN∕GaN material structure, grown by molecular-beam epitaxy technique on a sapphire substrate. The thicknesses of the overlayers Ti∕Au, or Mo∕Au, investigated were fixed. The rapid thermal annealing time and temperature in a N2 ambient are optimized for Ti∕Al∕Mo∕Au (from bottom to top) metal stack system, with bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The lowest contact transfer resistance rt of 0.4Ωmm was obtained for Ti∕Al metal at thickness ratio of 1:5. The sheet resistance Rsh associated with it was 600Ω∕sq. The edge acuity of Ti∕Al∕Mo∕Au Ohmic contacts after annealing has been discussed and examined under scanning electron microscopy for the optimized Ohmic contact transfer resistance associated with the bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The straightness of the edge of the metal contact has also been compared with the one using Ti∕Al∕Ti∕Au as the Ohmic metal stack under the condition of same annealing environment and metal thickness.
Analysis of radially resolved spectra and potential for lasing in Mo wire array Z pinches
Authors: Hansen S. B. , Safronova A. S. , Apruzese J. P. , LePell P. D. , Coverdale C. , Deeney C. , Fournier K. B. , Safronova U. I.
Published: September 2005

Measurements of radially resolved L-shell Mo spectra from wire array pinches on Sandia's 20 MA, 100 NS Z generator [Phys. Plasmas 5, 2105 (1998)] are presented and analyzed using a collisional-radiative model. The spectra indicate large radial gradients in density over the ∼8-mm-diameter plasma column, but only the emission from the ∼2mm central region of the pinch appears to be influenced by opacity. Population inversions and significant gain factors for 100–200Å transitions in Ne-like Mo are predicted to exist at the diagnosed plasma conditions.
A two-dimensional gas detector with individual readouts for neutron detection with a high spatial resolution and fast temporal response
Authors: Tanaka Hiroki , Nakamura Tatsuya , Yamagishi Hideshi , Soyama Kazuhiko , Aizawa Kazuya
Published: September 2005

A gaseous neutron-imaging detector with individual signal readouts was developed for a high spatial resolution and a fast temporal response. The system comprises a multiwire head, a gas chamber with 541 channels, fast amplifier-shaper-discriminator boards, a logic circuit for position calculation and encoding, and a fast data-acquisition system. The developed prototype detector incorporating 40×40 channels exhibited a temporal response of ∼100ns (pulse width of the amplified signal) and a spatial resolution of 0.9 and 1.4mm (full width at half maximum) in the x- and y-directions, respectively, with a gas pressure of 4.2-atm helium with 1.8-atm CF4.
Comparison of four conductive needle probe designs for determination of bubble velocity and local gas holdup
Authors: Munholand Luke , Soucy Gervais
Published: September 2005

Four conductive needle probe designs were compared in order to determine which probe was most effective at measuring local gas fraction holdup and bubble velocity. Test conditions were identical for all probes with global gas fraction holdup εg=0.018±0.002Lgas∕Ltotal and mean bubble velocities at 0.296±0.141m∕s. Video was used to validate the probe measurements. Fluid mechanics theory suggests the smallest probe should give the most accurate results. However, in tests the smallest probe performed poorly due to a low signal-to-noise ratio (SNR). The largest probe gave the best results due to its excellent SNR and appropriately spaced sensors.
Self-contained high-pressure cell, apparatus, and procedure for the preparation of encapsulated proteins dissolved in low viscosity fluids for nuclear magnetic resonance spectroscopy
Authors: Peterson Ronald W. , Wand A. Joshua
Published: September 2005

The design of a sample cell for high-performance nuclear magnetic resonance (NMR) at elevated pressure is described. The cell has been optimized for the study of encapsulated proteins dissolved in low viscosity fluids but is suitable for more general nuclear magnetic resonance (NMR) spectroscopy of biomolecules at elevated pressure. The NMR cell is comprised of an alumina-toughened zirconia tube mounted on a self-sealing nonmagnetic metallic valve. The cell has several advantages, including relatively low cost, excellent NMR performance, high-pressure tolerance, chemical inertness, and a relatively large active volume. Also described is a low volume sample preparation device that allows for the preparation of samples under high hydrostatic pressure and their subsequent transfer to the NMR cell.
Fast switching of alkali atom dispensers using laser-induced heating
Authors: Griffin P. F. , Weatherill K. J. , Adams C. S.
Published: September 2005

We show that by using an intense laser source to locally heat an alkali atom dispenser, one can generate a high flux of atoms followed by fast recovery (<100ms) of the background pressure when the laser is extinguished. For repeated heating pulses a switch-on time for the atomic flux of 200ms is readily attainable. This technique is suited to ultracold atom experiments using simple ultrahigh vacuum (UHV) chambers. Laser-induced heating provides a fast repetition of the experimental cycle, which, combined with low atom loss due to background gas collisions, is particularly useful for experiments involving far-off resonance optical traps, where sufficient laser power (0.5–4W) is readily available.
A coil shape in superconducting quadrupole magnets for obtaining the perfect field
Authors: Sarma P. R. , Pattanayak S. K.
Published: September 2005

A cos(2θ) current distribution over a cylindrical surface gives an ideal quadrupolar field in so-called cos(2θ) quadrupoles. A constant current in a coil configuration generated by two perpendicular intersecting ellipses also produces the perfect quadrupole field. We have found that an ideal quadrupole field can be obtained with a coil shape, different from the intersecting ellipses. Block-coil quadrupoles have been designed based on the coil shape as well as the intersecting ellipse shape.
Establishment of a computer-controlled retroreflection measurement system at the National Metrology Institute of Turkey (UME)
Authors: Samedov Farhad , Celikel Oguz , Bazkir Ozcan
Published: September 2005

In order to characterize photometric properties of retroreflectors, a fully automated retroreflector measurement system is designed in National Metrology Institute of Turkey (UME). The system is composed of a lighting projector, a goniometer, filter radiometers, 100dB transimpedance amplifiers, and 24-bit resolution analog-digital converter card with a special software. The established system provides a new calibration capability to determine the luminous intensity and retroreflection coefficients of the retroreflective materials with the expanded uncertainties of 1.07% and 1.13% (k=2), respectively. The traceability in retroreflection measurements was linked to the detector-based photometric scale of UME.
Development of erbium-doped silica sensor probe for fiber-optic fluorescence thermometer
Authors: Aizawa H. , Takei K. , Katsumata T. , Komuro S. , Morikawa T. , Ishizawa H. , Toba E.
Published: September 2005

A fabrication process of the erbium (Er)-doped silica sensor probe, in which the sensor head is directly coupled with silica glass fiber, has been developed for the fiber-optic thermometer application. In this fabrication process, a droplet of slurry of Er-doped silica powders are formed on the end of the silica glass fiber, and are dried, sintered, and then melted and solidified in a LPG-O2 gas furnace. The temperature dependence of the photoluminescence (PL) lifetime from the Er-doped silica senor probe with various dopant concentrations has been evaluated for the fiber-optic thermometer application. An Er-doped silica sensor probe with an Er density above 10000ppm and aluminum (Al) content about Al∕Er=20 is considered to be suitable for a fiber-optic thermometer because of the strong PL intensity and long PL lifetime. The PL lifetimes of the Er sensor head decreases from 9.9msto8.1ms at temperatures from 273Kto473K. An Er-doped silica sensor probe, which is fabricated by a modified process, is considered to be potentially useful for a fiber-optic fluorescence thermometer.
A frequency multiplexed near-infrared topography system for imaging functional activation in the brain
Authors: Everdell N. L. , Gibson A. P. , Tullis I. D. C. , Vaithianathan T. , Hebden J. C. , Delpy D. T.
Published: September 2005

We have developed a novel near-infrared optical topography system that can acquire images of functional activation in the human brain at 10 frames per second using 32 detectors. The image acquisition rate is inversely proportional to the number of detectors, so the maximum acquisition rate using four detectors is 80Hz. 16 laser diode sources (8 at 785 and 8 at 850nm) are illuminated simultaneously, and each of 8 avalanche photodiode detectors records light from several sources at the same time. The contribution from each source is demultiplexed in software using fast Fourier transforms. This allows for a more flexible, smaller, and less complex system than is achievable using traditional hardware demodulation techniques, such as lock-in amplifiers. The system will eventually incorporate a total of 64 sources and 32 detectors, enabling the entire adult cortex to be imaged. The system is designed to be as flexible as possible, and to be applicable to a wide variety of experimental and clinical needs. To this end, it can operate in two distinct modes: As a frequency multiplexed system and as a time multiplexed system. We describe phantom and in vivo investigations that have been undertaken using the new instrument in its frequency multiplexed operating mode.
Super-resolution variable fiber optic attenuator instrument using digital micromirror device (DMD™)
Authors: Riza Nabeel A. , Ghauri Farzan N.
Published: September 2005

Demonstrated is a super resolution, low loss, 100% repeatable and fast response digital variable fiber-optic attenuator instrument using a digital micromirror device. The attenuator design exploits beam expansion optics and a retroreflective architecture to achieve super-resolution with digital repeatability. The proof-of-concept attenuator at 1550nm demonstrates a 41.5dB optical attenuation range, optical insertion loss of 3.8dB, a 1ms maximum attenuation reset time, an optical polarization dependant loss of less than 0.07dB, and an optical resolution range of 2.9–0.0517mdB. The demonstrated attenuator is appropriate for fiber-optic test and instrumentation applications.
Supersonic-jet cryogenic-resonator coaxially oriented beam-resonator arrangement Fourier transform microwave spectrometer
Authors: Grabow Jens-Uwe , Palmer E. Samuel , McCarthy Michael C. , Thaddeus Patrick
Published: September 2005

The design and performance of a cryogenic (T=77K) microwave spectrometer are reported. This instrument, being a time domain experiment in microwave molecular spectroscopy, consists of three basic components: a near-confocal Fabry-Perot-type resonator, a pulsed microwave excitation Fourier transform system, and a pulsed molecular-beam source in a coaxially oriented beam-resonator arrangement. With the recently developed spectrometer, which employs confocal reflectors cooled to liquid-nitrogen temperature, we were able to reduce the system noise of the instrument significantly. The high sensitivity of this next generation spectrometer allows the detection of transient species produced in a dc-discharge nozzle, other short-lived molecules and complexes at low abundance, or nearly nonpolar molecules. We will discuss the spectrometer design with respect to critical considerations of the resonator theory in the context of cryogenic operation, as well as important aspects of classical circuit theory applied to excitation and detection of molecular systems in a resonator. Following the description of the dc-discharge source, the performance of the instrument is demonstrated.
Cherenkov radiation generated by a beam of electrons revisited
Authors: Zheng Jian , Yu C. X. , Zheng Z. J. , Tanaka K. A.
Published: September 2005

Cherenkov radiation generated by a beam of electrons is theoretically investigated. In the case that the boundary effect is negligible, coherent Cherenkov radiation does not depend on the longitudinal bunch form of the electron beam, which is remarkably different from other kinds of coherent radiation like coherent transition radiation and coherent synchrotron radiation. The reason for this result is ascribed to the criterion of the emission of Cherenkov radiation. The angular distribution of coherent Cherenkov radiation is mainly determined by the transverse bunch form of the beam. The spectral intensity of incoherent Cherenkov radiation is proportional to the velocity distribution function of the electrons in the beams. Based on these results, some methods are suggested to study hot electrons with the measurement of Cherenkov radiation.
A microfluidic setup for studies of solid-liquid interfaces using x-ray reflectivity and fluorescence microscopy
Authors: Reich Christian , Hochrein Marion B. , Krause Bärbel , Nickel Bert
Published: September 2005

In this paper we present a concept for a microfluidic chamber optimized for x-ray reflectivity studies at solid-liquid interfaces. Experiments of this kind are usually considerably limited by strong beam attenuation due to interactions with the aqueous environment. First experiments at synchrotron sources using supported model membranes showed that the microfluidic setup yields a very effective solution for minimizing background scattering and beam absorption, which are often accompanied by radiation damage of biological samples. Additionally, the setup is also well suited for the application of fluorescence microscopy. The application of these two different techniques on the same sample offers unique possibilities for complementary studies.
Field-induced self-assembled ferrofluid aggregation in pulsatile flow
Authors: Ganguly Ranjan , Zellmer Brian , Puri Ishwar K.
Published: September 2005

Ferrofluid aggregation and dispersion occurs at several length scales in pulsatile flow applications, e.g., in ferrofluidic pumps, valves, and biomedical applications such as magnetic drug targeting. Because of a yet limited understanding, ferrohydrodynamic investigations involving laboratory-scale studies in idealized geometries are of considerable use. We have injected a ferrofluid into a pulsatile host flow and produced field-induced dissolution (aggregation) using external magnets. A comparison is made with ferrofluid aggregation in a steady flow. Subsequently, the accumulation and dispersion of the ferrofluid aggregates in pulsatile flow are characterized by analyzing their size, mean position, and the flow frequency spectrum. The maximum aggregate size Amax, time to form it tmax, and the aggregate half-life thalf are found to scale according to the relations Amax∝Re−0.71, tmax∝Re−2.1, and thalf∝Re−2.2. While the experiments are conducted at a macroscopic length scale for useful experimental resolution, the results also enable an understanding of the micro- and mesoscale field-assisted self-assembly of magnetic nanoparticles.