bluuebunny/crossref_metadata_2025 huggingface dataset:
The electron-beam-plasma ion source as source of negative fluorine
Authors: Alton G. D. , Welton R. F. , Murray S. N. , Cui B.
Published: July 2002

Radioactive ion beams (RIBs) of short-lived isotopes of fluorine are in demand for investigating astrophysical phenomena related to the hot CNO cycle and rp processes responsible for stellar nucleosynthesis. Since negative ion beams are required for injection into tandem electrostatic accelerators, such as the 25 MV tandem accelerator used for post acceleration of RIBs for the Holifield radioactive ion beam facility research program at the Oak Ridge National Laboratory, efficient, direct-formation F− ion sources are highly desirable. We have conceived and evaluated a direct extraction F− source for potential RIB applications which is predicated on the reverse polarity operation of a positive electron-beam-plasma target/ion source (EBPTIS) while simultaneously feeding fluorine-rich compounds and Cs vapor into the source. The source is found to operate in two separately distinct temperature regimes for the generation of F−: (1) a high cathode temperature regime or plasma mode and (2) a lower cathode temperature regime or surface ionization mode. For the latter mode of operation, net efficiencies of η=0.2% were attained for the EBPTIS; delay times, τ, attributable to the transport of F and fluoride compounds from the target to the ionization chamber of the source, typically, were found to be τ∼60 s. Brief descriptions of the EBPTIS and experimental techniques used in the studies, as well as net efficiency and effusive flow data for the negative EBPTIS, are presented in this article.
Wide bandwidth transimpedance preamplifier for a scanning tunneling microscope
Authors: Demming F. , Dickmann K. , Jersch J.
Published: July 2002

A new two-stage transimpedance preamplifier in feedback picoammeter setup for use in scanning tunneling microscopy is represented. The first stage consisting of a wide band field effect transistor input operational amplifier with resistive feedback has a transimpedance of 20 kΩ and a 3 dB bandwidth of 35 MHz. This setup is built to investigate sub μs fluctuations in the tunneling current.
Development of imaging bolometers for long-pulse MFE experiments (invited)
Authors: Wurden G. A. , Peterson B. J.
Published: July 2002

We have developed the concept of an imaging bolometer, capable of operation with 100’s of individual channels, while relying on optical (infrared) readout of the temperature rise in a thin foil. A thin gold foil (0.5–5 μm thick) is sandwiched between pieces of copper. The copper mask (a large thermal mass) has a hole pattern drilled into it to form many “individual pixels,” effectively forming many separate sensors. This segmented foil/mask combination is exposed on its front side to plasma radiation through a cooled pinhole camera geometry. Simultaneously, a high-resolution infrared camera monitors any temperature change on the backside of the thin foil. A sensitive infrared (IR) camera views the foil through an IR telescope/periscope system, and is shielded from the magnetic and nuclear radiation fields, either by distance and/or material shielding. A simple time-dependent design algorithm, using 1D heat transport to a cold boundary, has been written in MathCad, which allows us to select optimal material and geometries to match the expected plasma conditions. We have built a compact prototype with 149 channels, and tested it successfully both in a vacuum test stand in the laboratory, and on a plasma in the CHS at the National Institute for Fusion Science, subjecting it to electron cyclotron heated and neutral beam injection heated conditions. A water-cooled version has been built for the new LHD. Since the IR imaging bolometer uses only metal parts near the plasma, and has no need for wiring or wiring feedthrus, it is intrinsically radiation hard, and has direct application to ignition device to test engineering concepts (ITER), or ITER-class experiments.
Ortho/para hydrogen converter for rapid deposition matrix isolation spectroscopy
Authors: Tam Simon , Fajardo Mario E.
Published: July 2002

We describe the construction and operation of a catalyst-bed type device for precooling and equilibrating the ortho/para composition of a hydrogen gas flow. We use this device to vapor deposit millimeters thick cryogenic parahydrogen (pH2) solids which are remarkably transparent [M. E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237 (1998)]. Infrared absorption spectra of solids deposited at pH2 flow rates up to 290 mmol/h (solid thickness growth rates up to 75 μm/min indicate a residual orthohydrogen (oH2) content of less than 0.01%. Gas phase thermal conductivity measurements indicate a residual oH2 content of 0(−0+5)% for flow rates up to 1.9 mol/h. These pH2 solids can be doped readily by simple codeposition of various impurities produced by any of the numerous dopant sources developed in previous matrix isolation spectroscopy (MIS) studies. The long achievable path lengths, and the desirable properties of pH2 as a matrix host, will enable significant new fundamental and practical applications for MIS.
A spectroscopic method to measure the spectral emissivity of semi-transparent materials up to high temperature
Authors: Rozenbaum O. , Meneses D. De Sousa , Auger Y. , Chermanne S. , Echegut P.
Published: July 2002

A spectroscopic method to measure directional spectral emissivity for homogeneous and heterogeneous semi-transparent materials, giving access to a large spectral 10–12 000 cm−1 range and to temperatures lying between 600 and 3000 K is reported. Sample heating is supplied by a carbon dioxide laser and the blackbody flux reference is obtained with a lanthanum chromite furnace. Experimental results obtained with this setup on several dielectric oxides such as silica, alumina, and magnesia are in good agreement with the results obtained by an indirect method based on the Kirchhoff’s laws. A brief overview of the setup abilities and a detailed discussion of the emissivity spectra are also proposed.
Coherent tunable THz-wave generation from LiNbO3 with monolithic grating coupler
Authors: Kawase Kodo , Sato Manabu , Taniuchi Tetsuo , Ito Hiromasa
Published: July 2002

Coherent THz-wave generation is demonstrated utilizing a grating coupler fabricated on the surface of a LiNbO3 crystal pumped by a Q-switched Nd:YAG laser. Wide tunability, sharp directivity, and high efficiency were achieved.
Optically pumped lasing of ZnO at room temperature
Authors: Bagnall D. M. , Chen Y. F. , Zhu Z. , Yao T. , Koyama S. , Shen M. Y. , Goto T.
Published: July 2002

We report the observation of optically pumped lasing in ZnO at room temperature. Thin films of ZnO were grown by plasma-enhanced molecular beam epitaxy on (0001) sapphire substrates. Laser cavities formed by cleaving were found to lase at a threshold excitation intensity of 240 kW cm−2. We believe these results demonstrate the high quality of ZnO epilayers grown by molecular beam epitaxy while clearly demonstrating the viability of ZnO based light emitting devices.
Photonic crystal properties of packed submicrometric SiO2 spheres
Authors: Mı́guez H. , López C. , Meseguer F. , Blanco A. , Vázquez L. , Mayoral R. , Ocaña M. , Fornés V. , Mifsud A.
Published: July 2002

In this letter, we investigate the optical properties of packed monodisperse silica submicron spheres by means of optical transmission measurements. The results are compatible with a three dimensional face centered cubic order in these solid structures. The lattice parameter of these structures, and therefore their optical properties, can be easily tuned through the sphere size (between 200 and 700 nm) thus covering the whole visible and near infrared spectrum.
Large positive magnetoresistance at low temperatures in a ferromagnetic natural multilayer, LaMn2Ge2
Authors: Mallik R. , Sampathkumaran E. V. , Paulose P. L.
Published: July 2002

The results of magnetoresistance measurements on a naturally occurring multilayer LaMn2Ge2, which is ferromagnetic below 326 K, are reported. The magnitude of magnetoresistance is found to be positive below 70 K gradually increasing to an unusually large value (nearly 100%) at 4.2 K in the presence of a field of 70 kOe as the temperature is lowered, similar to the recent observations by Verbanck, Temst, Mae, Schad, Van Bael, Moshchalkov, and Bruynseraede [Appl. Phys. Lett. 70, 1477 (1997)] in Cr/Ag/Cr trilayers. The positive sign of magnetoresistance for a ferromagnet is unexpected and possible explanations are offered.
Properties of ion exchanged planar and channel optical waveguides fabricated in Cu doped KTiOPO4 substrates
Authors: Shi L. P. , Chong T. C. , Zhuo Z. , Hou W. X. , Hu P. F.
Published: July 2002

Planar and channel optical waveguides were fabricated in Cu doped KTiOPO4 (KTP) substrates using Rb+:K+ ion exchange process and annealing process. The effective refractive indices were measured using the m-line method and the refractive index profiles of these waveguides were calculated using the inverse WKB method. For waveguides in Cu:KTP substrates the difference of diffusion depth d and index change Δn at the z− and z+ surfaces can hardly be observed. This observation is different from KTP. Compared with undoped KTP substrates the diffusion depth d at the z− surfaces is larger, whereas the index change Δn is smaller. At the z+ surfaces both d and Δn are larger. The light induced effective index changes in Cu doped KTP channel waveguides were measured at 0.476, 0.488, 0.514, 0.6328, and 0.83 μm wavelengths. No changes were observed at the power flow of 1×108 W/m2, revealing that Cu ion does not play an important role in photorefractive effect of KTP crystals.
On the initial unloading slope in indentation of elastic-plastic solids by an indenter with an axisymmetric smooth profile
Authors: Cheng (Zheng Zhemin) Che-Min , Cheng Yang-Tse
Published: July 2002

A simple relationship between the initial unloading slope, the contact area, and the elastic modulus is derived for indentation in elastic-plastic solids by an indenter with an arbitrary axisymmetric smooth profile. Although the same expression was known to hold for elastic solids, the new derivation shows that it is also true for elastic-plastic solids with or without work hardening and residual stress. These results should provide a sound basis for the use of the relationship for mechanical property determination using indentation techniques.
Phase-matched second harmonic generation in asymmetric double quantum wells
Authors: Vodopyanov K. L. , O’Neill K. , Serapiglia G. B. , Phillips C. C. , Hopkinson M. , Vurgaftman I. , Meyer J. R.
Published: July 2002

Efficient (∼1%) second harmonic generation, resonantly enhanced near λ=8.6 μm, has been observed in asymmetric double multi-quantum well structures. We used (i) edge-emitting waveguide geometry where the phase matching was achieved by incorporating a separate multiple quantum well region which modifies (via the Kramers–Kronig relation) the dispersion of light and (ii) 45° wedge multi-bounce geometry where the phases of second harmonic waves generated at sequential bounces were synchronized by changing the angle of incidence.
Disorder effects in epitaxial thin films of (La,Ca)MnO3
Authors: Aarts J. , Freisem S. , Hendrikx R. , Zandbergen H. W.
Published: July 2002

We have investigated as-grown sputtered films of La0.7Ca0.3MnO3 in a thickness range between 5 and 200 nm on SrTiO3 substrates. The films are epitaxial, strained, and smooth. All films order magnetically around 175 K. Very thin films show full magnetization at low temperatures, but the temperature of the metal–insulator transition is appreciably lower than the magnetic ordering temperature. In thick films, the magnetization is much lower than expected. Both effects are probably related to structural disorder as found by transmission electron microscopy.
Analysis of terahertz pulse measurement with a chirped probe beam
Authors: Sun F. G. , Jiang Zhiping , Zhang X.-C.
Published: July 2002

In this letter, terahertz (THz) pulse measurements with a chirped probe pulse are analyzed. The method of stationary phase is used to explore the relation between the temporal waveform of an input THz pulse and spectral distribution of an output probe pulse on a detector array. The dependence of the temporal resolution on the chirp rate, the spectrum of the chirped probe beam, and the spectral resolution of the spectrometer is discussed. We confirm that the temporal resolution of the chirped pulse technique is equal to the square root of the product of the original unchirped probe pulse duration and the chirped pulse duration.
Relationship between phase shift and energy dissipation in tapping-mode scanning force microscopy
Authors: Tamayo Javier , Garcı́a Ricardo
Published: July 2002

Force curves taken during a load–unload cycle show the presence of a hysteresis loop. The area enclosed by the loop is used to measure the energy dissipated by the tip-sample interaction in tapping-mode scanning force microscopy. The values of the energy loss obtained from force curves are compared with the results derived from a model based on phase shift measurements. The agreement obtained between both methods demonstrates that for the same operating conditions, the higher the phase shift the larger the amount of energy dissipated by the tip-sample interaction. It also confirms the prediction that phase-contrast images can only arise if there are tip-sample inelastic interactions.
Size and strain effects in the E1-like optical transitions of InAs/InP self-assembled quantum dot structures
Authors: Prieto J. A. , Armelles G. , Groenen J. , Carles R.
Published: July 2002

The optical transitions of uncapped and capped InAs/InP self-assembled quantum dot structures in the energy range of the bulk InAs E1 transition are studied using modulation spectroscopy and Raman scattering. Islands and wetting layer exhibit one and two features, respectively. The deformation potential theory and the single band effective mass approximation account for the island-related feature successfully in a wide island-height range (3–7 nm). These models also explain the existence of the highest energy wetting layer related feature, but not of the lowest energy one.
Influence of optical interference on quantum well infrared photodetectors in a 45° waveguide geometry
Authors: Schneider H. , Schönbein C. , Walther M. , Koidl P. , Weimann G.
Published: July 2002

Standing light waves caused by the interference of incident and reflected beams lead to a spatial modulation of the electromagnetic field within the active region of a quantum well infrared photodetector (QWIP). The optical excitation—determined by the electric field component perpendicular to the quantum well plane—is thus inhomogeneous, influencing the QWIP responsivity and its voltage dependence. The inhomogeneity can be suppressed by using mesa photodetectors where only half of the top contact is covered with metal.
Light invariant, efficient, multiple band gap AlGaAs/Si/metal hydride solar cell
Authors: Licht S. , Wang B. , Soga T. , Umeno M.
Published: July 2002

Electronic and ionic charge transfer provides a basis for composite semiconductor/electrolyte systems featuring simultaneous solar/electrical conversion and solar energy storage. This cell contains both multiple band gap and electrochemical storage, and provides a nearly constant energetic output in illuminated or dark conditions. Multiple semiconductor band gaps can enhance the energetics of this interaction. The cell combines bipolar AlGaAs (Eg=1.6 eV) and Si (Eg=1.1 eV) and AB5 metal hydride/NiOOH storage, and generates a light variation insensitive potential of 1.2–1.3 V at total (including storage losses) solar/electrical energy conversion efficiency of 18.1%.
Characteristics of homoepitaxial 4H-SiC films grown on
c
-axis substrates offcut towards 〈11̄00〉 or 〈112̄0〉
Authors: Landini B. E. , Brandes G. R.
Published: July 2002

Substrate orientation plays a critical role in determining SiC epilayer quality, but limited work has been done to examine the effect of offcut directions on 4H-SiC epilayers. 4H-SiC epilayers were grown on substrates offcut 8° towards either the 〈11̄00〉 or the 〈112̄0〉 crystalline direction. Epilayers up to 15.5 μm thick, possessing smooth morphologies free of large-scale step bunching, were grown on both substrate orientations. Triangular defects were observed primarily on the wafer periphery, and the triangular defect density depended only on offcut angle. Atomic force microscopy analysis confirmed smooth surfaces for epilayers up to 15.5 μm thick grown on 〈11̄00〉 offcut substrates. Low-energy electron diffraction analysis of epilayers grown on substrates offcut towards 〈11̄00〉 showed an ordered surface. Nitrogen and aluminum doping were successfully obtained for epilayers grown on substrates offcut towards 〈11̄00〉, and n-type and p-type doping control were demonstrated.
Growth of Si/3C–SiC/Si(100) heterostructures by pulsed supersonic free jets
Authors: Ikoma Y. , Endo T. , Watanabe F. , Motooka T.
Published: July 2002

We have investigated the epitaxial growth of multilayer structures of Si/3C–SiC/Si(100) by pulsed supersonic free jets of methylsilane (CH3SiH3) for SiC growth and trisilane (Si3H8) for Si growth. Epitaxial Si layers were obtained only on very thin (≈3 nm) 3C–SiC epitaxial layers, while polycrystalline Si was grown on thicker 3C–SiC layers. It was also found that the transition regions with a thickness of ≈1 nm existed at the interface between epitaxial 3C–SiC and Si layers by high-resolution transmission electron microscopy observation. These results suggest that the surface roughness and thickness of the 3C–SiC layer play an important role for epitaxial growth of Si.
Properties of BaFe12O19 films prepared by laser deposition with in situ heating and post annealing
Authors: Lu Y. F. , Song W. D.
Published: July 2002

BaFe 12 O 19 films on (001) sapphire substrates are prepared by laser deposition with in situ heating and postannealing. The properties of the films are analyzed by x-ray diffractometry, vibrating sample magnetometer, atomic force microscopy, and Raman spectroscopy. The relationship among the coercivity, crystalline orientation, and grain shape and size is discussed. The film with coercivity of 5.1 kOe has been obtained by laser deposition with postannealing. The film with a preferential c-axis orientation normal to the film plane and the grains having good crystallinity with hexagonal symmetry have been obtained by laser deposition with in situ heating.
Lattice dynamics of hexagonal and cubic InN: Raman-scattering experiments and calculations
Authors: Kaczmarczyk G. , Kaschner A. , Reich S. , Hoffmann A. , Thomsen C. , As D. J. , Lima A. P. , Schikora D. , Lischka K. , Averbeck R. , Riechert H.
Published: July 2002

We present results of first- and second-order Raman-scattering experiments on hexagonal and cubic InN covering the acoustic and optical phonon and overtone region. Using a modified valence-force model, we calculated the phonon dispersion curves and the density of states in both InN modifications. The observed Raman shifts agree well the calculated Γ-point frequencies and the corresponding overtone density of states. A tentative assignment to particular phonon branches is given.
Effect of the Coulomb interaction on the response time and impedance of the resonant-tunneling diodes
Authors: Feiginov Michael N.
Published: July 2002

We demonstrated that the response time of the resonant-tunneling structures (τresp) can be much smaller as well as much larger than the quasibound-state lifetime. A simple analytical expression for the impedance of the resonant-tunneling diode has been derived, it takes into account the Coulomb interaction and the quasibound-state lifetime. A simple equation relating τresp to the static differential conductance has also been obtained; it allows one to get τresp in the static measurements of the current–voltage curve.
Luminescence energy shift and carrier lifetime change dependence on carrier density in In0.12Ga0.88N/In0.03Ga0.97N quantum wells
Authors: Kuroda Takamasa , Tackeuchi Atsushi , Sota Takayuki
Published: July 2002

To clarify the carrier density dependence of carrier lifetime and luminescence energy, we have performed a systematic study of time-resolved photoluminescence (PL) measurements of In0.12Ga0.88N/In0.03Ga0.97N multiple quantum wells for various carrier density. The carrier recombination rate and the PL energy, for the carrier density below 1.5×1018cm−3, are found to decrease nonlinearly with the decrease of the carrier density, although the carrier lifetime is constant and the PL energy shifts linearly for carrier density above this value. We show that the energy shift for the small carrier density and the change in the carrier lifetime are well explained by the free carrier screening effect which compensates the internal electric field. The linear energy shift for the high carrier density is attributed to the band-filling effect.
Mg-doped Ba0.6Sr0.4TiO3 thin films for tunable microwave applications
Authors: Joshi P. C. , Cole M. W.
Published: July 2002

We report on the microstructural and electrical properties of Mg-doped Ba0.6Sr0.4TiO3 thin films prepared by the metalorganic solution deposition technique using carboxylate-alkoxide precursors at a postdeposition annealing temperature of 750 °C. The structure and morphology of the films were analyzed by x-ray diffraction and atomic force microscopy studies. The electrical measurements were conducted on metal-ferroelectric-metal capacitors using Pt as the top and bottom electrode. The typical measured small signal dielectric constant and dissipation factor of undoped Ba0.6Sr0.4TiO3 thin films at a frequency of 100 kHz were 450 and 0.013, respectively. The undoped Ba0.6Sr0.4TiO3 thin films exhibited a high tunability of 28.1% and resistivity of 0.4×1012 Ω cm at an applied electric field of 200 kV/cm. The Mg-doped Ba0.6Sr0.4TiO3 thin films exhibited significantly improved dielectric loss and insulating characteristics compared to undoped Ba0.6Sr0.4TiO3 thin films. The effects of Mg doping on the microstructural, dielectric, and insulating properties of Ba0.6Sr0.4TiO3 thin films were analyzed. The high dielectric constant, low dielectric loss, high tunability, and low leakage current show the potential of pure and Mg-doped Ba0.6Sr0.4TiO3 thin films for integrated capacitor and microwave communication devices.
Ge composition in Si1−xGex films grown from SiH2Cl2/GeH4 precursors
Authors: Suh K. Y. , Lee Hong H.
Published: July 2002

A simple model for the Ge composition in Si1−xGex films grown from SiH2Cl2/GeH4 precursors is developed on the basis of adsorption and desorption kinetics for the intermediate temperature range (600 °C<T<900 °C). For this system, the solid phase composition of Ge, x, is related to the gas phase composition ratio of the two source gases, G, by x2/(1−x)=constant×G, which contrasts with the conventional relationship, x/(1−x)=constant×G, that is known for SiH4/GeH4 chemical vapor deposition. The proportionality constant depends not only on temperature but also on pressure. The model compares well with the experimental data in the literature.
Electronic distribution in superlattice quantum cascade lasers
Authors: Troccoli Mariano , Scamarcio Gaetano , Spagnolo Vincenzo , Tredicucci Alessandro , Gmachl Claire , Capasso Federico , Sivco Deborah L. , Cho Alfred Y. , Striccoli Marinella
Published: July 2002

The electron population in the excited miniband of quantum cascade structures with intrinsic superlattice active regions is extracted from the fine structure analysis of spontaneous interminiband electroluminescence spectra. At current densities typical of laser thresholds, the electrons injected into the excited miniband of a (GaInAs)6 nm/(AlInAs)1.8 nm superlattice are described by a nonequilibrium thermal distribution characterized by temperatures Te>200 K, much higher than the lattice temperature TL=15 K.
High-carbon concentrations at the silicon dioxide–silicon carbide interface identified by electron energy loss spectroscopy
Authors: Chang K. C. , Nuhfer N. T. , Porter L. M. , Wahab Q.
Published: July 2002

High carbon concentrations at distinct regions at thermally-grown SiO2/6H–SiC(0001) interfaces have been detected by electron energy loss spectroscopy (EELS). The thickness of these C-rich regions is estimated to be 10–15 Å. The oxides were grown on n-type 6H–SiC at 1100 °C in a wet O2 ambient for 4 h immediately after cleaning the substrates with the complete RCA process. In contrast, C-rich regions were not detected from EELS analyses of thermally grown SiO2/Si interfaces nor of chemical vapor deposition deposited SiO2/SiC interfaces. Silicon-rich layers within the SiC substrate adjacent to the thermally grown SiO2/SiC interface were also evident. The interface state density Dit in metal–oxide–SiC diodes (with thermally grown SiO2) was approximately 9×1011 cm−2 eV−1 at E−Ev=2.0 eV, which compares well with reported values for SiC metal–oxide–semiconductor (MOS) diodes that have not received a postoxidation anneal. The C-rich regions and the change in SiC stoichiometry may be associated with the higher than desirable Dit’s and the low channel mobilities in SiC-based MOS field effect transistors.
Influence of annealing temperature on ferroelectric properties of SrBi2Ta2O9 thin films prepared by off-axis radio frequency magnetron sputtering
Authors: Tay S. T. , Jiang X. H. , Huan C. H. A. , Wee A. T. S. , Liu R.
Published: July 2002

Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited on Pt/TiOx/SiO2/Si substrates using the off-axis radio frequency magnetron sputtering technique. X-ray diffraction and atomic force microscopy experiments showed that the crystallization of SBT thin films at >700 °C correlated with the formation of rod-like grains. Cross-sectional field emission scanning electron microscopy images revealed that the apparent thickness of SBT decreased while the thickness of Pt increased as the annealing temperature was increased. The apparent decrease in the thickness of SBT was attributed to crystallization and densification in the film whereas the apparent increase in Pt thickness was due to diffusion of Ti and Bi into the Pt layer. This diffusion at high annealing temperatures (800 °C and above) alters the Pt purity and degrades the Pt as the bottom electrode for the ferroelectric capacitor. Good insulating properties were obtained when the SBT film was annealed at 700 and 750 °C whereas higher leakage currents were observed at annealing temperatures >800 °C. A remnant polarization (Pr) of 4.35 μC/cm2 and coercive field (Ec) of 31.5 kV/cm were obtained for the SBT thin film annealed at 750 °C with a leakage current density of <10−7 A/cm2.
Measuring multipole moments of Weyl metrics by means of gyroscopes
Authors: Herrera L. , Pastora J. L. Hernández
Published: July 2002

Using the technique of Rindler and Perlick we calculate the total precession per revolution of a gyroscope circumventing the source of Weyl metrics. We establish thereby a link between the multipole moments of the source and an “observable” quantity. Special attention deserves the case of the γ-metric. As an extension of this result we also present the corresponding expressions for some stationary space–times.
What can x-ray scattering tell us about the radial distribution functions of water?
Authors: Sorenson Jon M. , Hura Greg , Glaeser Robert M. , Head-Gordon Teresa
Published: July 2002

We present an analysis of the Advanced Light Source (ALS) x-ray scattering experiment on pure liquid water at ambient temperature and pressure described in the preceding article. The present study discusses the extraction of radial distribution functions from the x-ray scattering of molecular fluids. It is proposed that the atomic scattering factors used to model water be modified to include the changes in the intramolecular electron distribution caused by chemical bonding effects. Based on this analysis we present a gOO(r) for water consistent with our recent experimental data gathered at the ALS, which differs in some aspects from the gOO(r) reported by other x-ray and neutron scattering experiments. Our gOO(r) exhibits a taller and sharper first peak, and systematic shifts in all peak positions to smaller r. Based on experimental uncertainties, we discuss what features of gOO(r) should be reproduced by classical simulations of nonpolarizable and polarizable water models, as well as ab initio simulations of water, at ambient conditions. We directly compare many water models and simulations to the present data, and discuss possible improvements in both classical and ab initio simulation approaches in the future.
A density functional study of crystalline acetic acid and its proton transfer polymorphic forms
Authors: Rovira Carme , Novoa Juan J.
Published: July 2002

We present a density functional study of the structure and dynamics of solid acetic acid. Our calculations are based on density functional theory combined with molecular dynamics, within the Car–Parrinello scheme. The computed structure of the acetic acid crystal, optimized without symmetry constraints, is in very good agreement with the experiment and reproduces the changes in the intramolecular structure when going from the gas phase to the solid. The cell parameters of the experimental structure are also well reproduced. Cooperative effects along the molecular chains building the crystal are found to be small (1.2 kcal/mol), although larger than it had been previously estimated. The anti conformation of the COOH fragment leads to a stable structure up to 250 K, with an energy of only 3 kcal/mol above that of the known syn form. The energy barrier associated with the most likely pathway for the syn to anti conformations, involving proton transfer along the OH⋯O units (<5.8 kcal/mol) is much smaller than the experimental barrier for the syn/anti isomerism in gas phase and in solution. Intramolecular and intermolecular reorganizations upon change to the anti structure are analyzed. Overall, our results pinpoint the anti form as a good candidate for a possible acetic acid polymorph.
Charged-particle acceleration and energy loss in laser-produced plasmas
Authors: Hicks D. G. , Li C. K. , Séguin F. H. , Ram A. K. , Frenje J. A. , Petrasso R. D. , Soures J. M. , Glebov V. Yu. , Meyerhofer D. D. , Roberts S. , Sorce C. , Stöckl C. , Sangster T. C. , Phillips T. W.
Published: July 2002

Spectral measurements have been made of charged fusion products produced in deuterium + helium-3 filled targets irradiated by the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Comparing the energy shifts of four particle types has allowed two distinct physical processes to be probed: Electrostatic acceleration in the low-density corona and energy loss in the high-density target. When the fusion burn occurred during the laser pulse, particle energy shifts were dominated by acceleration effects. Using a simple model for the accelerating field region, the time history of the target electrostatic potential was found and shown to decay to zero soon after laser irradiation was complete. When the fusion burn occurred after the pulse, particle energy shifts were dominated by energy losses in the target, allowing fundamental charged-particle stopping-power predictions to be tested. The results provide the first experimental verification of the general form of stopping power theories over a wide velocity range.
Inverse scattering for near-field microscopy
Authors: Carney P. Scott , Schotland John C.
Published: July 2002

We derive the analytic singular value decomposition of the linearized scattering operator for scalar waves. This representation leads to a robust inversion formula for the inverse scattering problem in the near zone. Applications to near-field optics are described.
Relaxor features in ferroelectric superlattices: A Maxwell–Wagner approach
Authors: Catalan G. , O’Neill D. , Bowman R. M. , Gregg J. M.
Published: July 2002

A Maxwell–Wagner series capacitor model is proposed to explain anomalous dielectric properties of ferroelectric superlattices. The results of the model show that a superlattice consisting of normal ferroelectric layers separated by low-resistivity interfacial regions can account for most experimental results reported to date, namely: dielectric enhancement for certain stacking periodicities, giant permittivities, and temperature migration of dielectric maxima as a function of frequency. The predictions of the model are discussed and compared to our own experimental results from thin film superlattice capacitors made by pulsed-laser deposition.
The formation of laser beams with pure azimuthal or radial polarization
Authors: Oron Ram , Blit Shmuel , Davidson Nir , Friesem Asher A. , Bomzon Zeev , Hasman Erez
Published: July 2002

Laser resonator configurations for obtaining pure azimuthal and radial polarized beams are presented. They involve the coherent summation, inside the laser resonator, of two orthogonally polarized TEM01 modes. Basic principles and experimental results with a Nd:YAG laser are presented. The results include a full space variant polarization measurement and show efficient formation of high-quality azimuthal and radial polarized beams.
Vibrational effects on electric properties of cyclopropenone and cyclopropenethione
Authors: Eckart Udo , Ingamells Victoria E. , Papadopoulos Manthos G. , Sadlej Andrzej J.
Published: July 2002

Vibrational contributions to the dipole polarizabilities and hyperpolarizabilities of cyclopropenone and cyclopropenethione are reported. Simultaneous calculations have been carried out for the respective saturated and noncyclic counterparts of these molecules. The results have been analyzed and interpreted in terms of different structural and electronic features of the studied molecules: i.e., the oxygen–sulfur replacement, the saturation of the carbon ring, and the removal of the intramolecular strain. For static dipole polarizabilities the vibrational contribution computed at the molecular equilibrium geometry can account for more than 10% of the total value for a given component of the tensor. Both the zero-point vibrational average and pure vibrational terms are important for most of the studied molecules. In the case of the first dipole hyperpolarizability the total vibrational correction is dominated by the pure vibrational term. The vibrational corrections to dipole polarizabilities show certain regularities which can be interpreted in terms of the electronic structure of the investigated molecules. Much less clear is the pattern of behavior for the first dipole hyperpolarizability. It has been found, however, that the analysis of these data is greatly facilitated by the separate consideration of the nuclear relaxation and curvature terms. Limited study of the frequency dependence of the calculated vibrational corrections shows that in the case of the dipole polarizability, they are almost quenched to zero. In contrast, at nonzero frequencies, the vibrational contribution to the first dipole hyperpolarizability tensor β(−ω;ω,0), which determines the so-called electro-optic Pockels effect, can be as important as the pure electronic term.
Fine structure of the plasmon resonance absorption peak of Ag nanoparticles embedded in partially oxidized Si matrix
Authors: Yang L. , Li G. H. , Zhang J. G. , Zhang L. D. , Liu Y. L. , Wang Q. M.
Published: July 2002

Ag/Si nanocomposite films were prepared by the radio-frequency magnetron cosputtering method. The fine structure of the plasmon resonance absorption peak was found in film samples. X-ray photoelectron spectroscopy analysis indicated that the samples were composed of a two-layer structure, which accounted for the structure of the optical absorption spectra. The peak located near 445 nm is the plasmon resonance absorption peak of Ag nanoparticles embedded in a partially oxidized Si matrix. Its intensity decreases with decreasing film thickness and disappears in a very thin sample. The peak located near 380 nm originates from the plasmon resonance absorption of the thoroughly oxidized surface layer of the sample. Its intensity does not change with increasing thickness, but it cannot be observed in the very thick sample.
The Jacobi–Wilson method: A new approach to the description of polyatomic molecules
Authors: Leforestier C. , Viel A. , Gatti F. , Muñoz C. , Iung C.
Published: July 2002

We present a new method adapted to the calculation of excited rovibrational states of semirigid molecules. It first relies on a description of the molecule in terms of polyspherical coordinates of Jacobi vectors, in order to obtain a compact expression for the kinetic energy operator T̂(q). This general description is then adapted to the molecule considered by defining curvilinear normal modes from the corresponding zero order harmonic Hamiltonian Ĥ0=T̂(qeq)+Vharm(q), the solutions of which are being used as the working basis set. The residual kinetic term ΔT̂ is treated mainly analytically in this basis, and displays no radial contribution. Anharmonic coupling ΔV(q) is handled by means of a pseudospectral scheme based on Gauss Hermite quadratures. This method is particularly adapted to direct iterative approaches which only require the action of Ĥ on a vector, without the need of the associated matrix, thus allowing ultralarge bases to be considered. An application to the excited vibrational states of the HFCO molecule is presented. It is shown in this example that energy levels can be trivially assigned from the leading expansion coefficient of the associated eigenvector.
A generalized Weyl relation approach to the time operator and its connection to the survival probability
Authors: Miyamoto Manabu
Published: July 2002

The time operator, an operator which satisfies the canonical commutation relation with the Hamiltonian, is investigated, on the basis of a certain algebraic relation for a pair of operators T and H, where T is symmetric and H self-adjoint. This relation is equivalent to the Weyl relation, in the case of self-adjoint T, and is satisfied by the Aharonov–Bohm time operator T0 and the free Hamiltonian H0 for the one-dimensional free-particle system. In order to see the qualitative properties of T0, the operators T and H satisfying this algebraic relation are examined. In particular, it is shown that the standard deviation of T is directly connected to the survival probability, and H is absolutely continuous. Hence, it is concluded that the existence of the operator T implies the existence of scattering states. It is also shown that the minimum uncertainty states do not exist. Other examples of these operators T and H, than the one-dimensional free-particle system, are demonstrated.
Dissociative recombination and excitation of O2+: Cross sections, product yields and implications for studies of ionospheric airglows
Authors: Peverall Robert , Rosén Stefan , Peterson James R. , Larsson Mats , Al-Khalili Ahmed , Vikor Ljiljana , Semaniak Jacek , Bobbenkamp Rolf , Le Padellec Arnaud , Maurellis A. N. , van der Zande Wim J.
Published: July 2002

We present experimental data on the dissociative recombination (DR) and the dissociative excitation (DE) of O2+ in its electronic and vibrational ground state using a heavy ion storage ring. The absolute DR cross section has been determined over an electron collision energy range from 1 meV to 3 eV. The thermal DR rate coefficient is derived; α(Te)=2.4×10−7(300/Te)0.70±0.01 cm3 s−1, for T>200 K. The threshold for DE was observed near its energetic threshold of 6.7 eV. The DE cross section curve has a maximum of 3×10−16 cm2 near 15 eV. We have determined the branching fractions to the different dissociation limits and present atomic quantum yields for the DR process between 0 to 300 meV collision energy. The quantum yield of O(1D) is found to be 1.17±0.05, largely independent of the electron energy. Arguments are presented that the branching fraction to O(3P)+O(1S) is negligible. The branching fraction to the O(1S)+O(1D) is smaller than 0.06 and varies strongly as a function of collision energy. The O(1S) quantum yield is a strong function of electron temperature. Hence, the relative strength of the green, O(1S), and the red, O(1D), airglows may be used as a measure of the electron temperature of the upper atmosphere. A qualitative explanation is given of the consequences of nonadiabatic interactions in the dissociation step of the DR process.
Oxygen-vacancy-related dielectric relaxation in SrBi2Ta1.8V0.2O9 ferroelectrics
Authors: Wu Yun , Forbess Mike J. , Seraji Seana , Limmer Steven J. , Chou Tammy P. , Cao Guozhong
Published: July 2002

The strontium bismuth tantalate vanadate, SrBi2Ta1.8V0.2O9, (SBTV) layered perovskite ferroelectrics were made by solid state powder sintering. It was found that the SBTV ferroelectrics had the same crystal structure as that of strontium bismuth tantalate, SrBi2Ta2O9 (SBT), but an increased paraferroelectric transition temperature at ∼360 °C as compared to 305 °C for SBT. In addition, SBTV ferroelectrics showed a frequency dispersion at low frequencies and broadened dielectric peaks at the paraferroelectric transition temperature that shifted to a higher temperature with a reduced frequency. However, after a postsintering annealing at 850 °C in air for 60 h, SBTV ferroelectrics showed reduced dielectric constants and tangent loss, particularly at high temperatures. In addition, no frequency dependence of paraferroelectric transition was found in the annealed SBTV ferroelectrics. Furthermore, there was a significant reduction in dc conductivity with annealing. The prior results implied that the dielectric relaxation in the as-sintered SBTV ferroelectrics was most likely due to the oxygen-vacancy-related dielectric relaxation instead of relaxor ferroelectric behavior.
Electronic structure and bonding in metal phthalocyanines, Metal=Fe, Co, Ni, Cu, Zn, Mg
Authors: Liao Meng-Sheng , Scheiner Steve
Published: July 2002

Electronic structure and bonding in metal phthalocyanines (Metal=Fe, Co, Ni, Cu, Zn, Mg) is investigated in detail using a density functional method. The metal atoms are strongly bound to the phthalocyanine ring in each case, by as much as 10 eV. The calculated orbital energy levels and relative total energies of these D4h structures indicate that Fe and Co phthalocyanines have A2g3 and Eg2 ground states, respectively, but that these states are changed upon interaction with strong-field axial ligands. The valence electronic structures of Fe and Co phthalocyanines differ significantly from those of the others. The HOMOs in Fe, Co, and Cu phthalocyanine are metal 3d-like, whereas in Ni and Zn phthalocyanines, the HOMO is localized on the phthalocyanine ring. The first ionization removes an electron from the phthalocyanine a1u orbital in all cases, with very little sensitivity of the ionization energy to the identity of the metal. Whereas the first reduction in Fe and Co phthalocyanine occurs at the metal, it is the phthalocyanine that is reduced upon addition of an electron to the other systems. Fe, Ni, and Cu phthalocyanines have smaller HOMO–LUMO separations than do Zn and Co phthalocyanine. There is very little variation in atomic charges within the phthalocyanine from one metal to the next.
Wide bandwidth millimeter wave mixer using a diffusion cooled two-dimensional electron gas
Authors: Lee Mark , Pfeiffer L. N. , West K. W. , Baldwin K. W.
Published: July 2002

Very fast millimeter wave mixers have been fabricated from a high mobility two-dimensional electron gas (2DEG) in a GaAs–AlGaAs heterostructure. Using a 115 GHz local oscillator, intermediate frequency (IF) bandwidths exceeding 20 GHz have been obtained for channel lengths L of a few microns. Operating at 77 K, the IF bandwidth scales as L−2, indicating that the response speed is determined by the diffusion transit time of hot electrons to the leads.
Displacement currents and the real part of high-frequency conductance of the resonant-tunneling diode
Authors: Feiginov Michael N.
Published: July 2002

I have shown that weak variation of the tunnel transparency of the collector barrier with bias has substantial (and frequently crucial) effect on the high-frequency properties of the resonant-tunneling diodes (RTDs). Also it has been shown that the real part of the RTD conductance can be negative and large at the frequencies much higher than the reciprocal quasibound-state lifetime in the quantum well between the barriers of RTD, if (as opposed to common practice) the RTD collector is heavily doped and does not have thick spacer layers. The displacement currents are responsible for the effects. A simple equivalent circuit of RTD is proposed, and it fairly well describes the published experimental data.
Improvement of the Grad 13 moment method for strong shock waves
Authors: Ohr Young Gie
Published: July 2002

The classical problem of the mathematical limitation of the 13 moment theory for strong shock waves has been reexamined by introducing the Mott-Smith bimodal function as a reference function in the Grad moment method. The shock wave of monatomic Maxwell molecules has been formulated in time-dependent equations by using the Boltzmann equation. In the steady state, the proposed theory improves the original Grad theory. The obtained 13 moment equation gives a solution curve which connects a critical saddle to a critical node in the phase space when M<4.14, while the original Grad theory gives the solution curve only when M<1.65. Numerical calculations show that the present theory also gives a solution curve when M⩾4.14 by connecting two critical saddles. The saddle-saddle connection in the phase space is possible due to the existence of the regular singular point located between the saddles. The predicted shock profile for monatomic Maxwell molecules shows a reasonable agreement between the theory and the Monte Carlo direct simulation. Also, the theory predicts a small overshoot of the kinetic temperature profile at the downstream wing when M>3.3.
Two- and three-dimensional Hamiltonians with shape invariance symmetry
Authors: Jafarizadeh M. A. , Panahi-Talemi H. , Faizi E.
Published: July 2002

Via a special dimensional reduction, that is, Fourier transforming over one of the coordinates of Casimir operator of su(2) Lie algebra and 4-oscillator Hamiltonian, we have obtained two- and three-dimensional Hamiltonian with shape invariance symmetry. Using this symmetry we have obtained their eigenspectrum. In the mean time we show equivalence of shape invariance symmetry and Lie algebraic symmetry of these Hamiltonians.
Detection of picosecond electrical pulses using the intrinsic Franz–Keldysh effect
Authors: Lampin J. F. , Desplanque L. , Mollot F.
Published: July 2002

We report time-resolved measurements of ultrafast electrical pulses propagating on a coplanar transmission line using the intrinsic Franz–Keldysh effect. A low-temperature-grown GaAs layer deposited on a GaAs substrate allows generation and also detection of ps pulses via electroabsorption sampling (EAS). This all-optical method does not require any external sampling probe. A typical rise time of 1.1 ps has been measured. EAS is a good candidate for use in THz characterization of ultrafast devices.
Physical properties of N2O and NO-nitrided gate oxides grown on 4H SiC
Authors: Jamet P. , Dimitrijev S.
Published: July 2002

N 2 O and NO nitridation by either annealing or direct growth of gate oxides on 4H SiC is analyzed in this letter. The analysis is based on x-ray photoelectron spectroscopy binding energies and secondary ion mass spectroscopy depth profiles of nitrogen at the SiO2–SiC interface. A clean SiO2–SiC interface is found in both NO and N2O annealed/grown samples, as opposed to the interface annealed in Ar which exhibits complex suboxides and oxide–carbon compounds. The results demonstrate that nitridation in the industry-preferred N2O ambient could be as effective as nitridation in NO, provided appropriate process optimization is performed.
Quantum chemistry using the density matrix renormalization group
Authors: Mitrushenkov Alexander O. , Fano Guido , Ortolani Fabio , Linguerri Roberto , Palmieri Paolo
Published: July 2002

A new implementation of the density matrix renormalization group is presented for ab initio quantum chemistry. Test computations have been performed of the dissociation energies of the diatomics Be2, N2, HF. A preliminary calculation on the Cr2 molecule provides a new variational upper bound to the ground state energy.