bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the photoluminescence intensity of ordered and disordered In0.48Ga0.52P
Authors: Lambkin J. D. , Considine L. , Walsh S. , O’Connor G. M. , McDonagh C. J. , Glynn T. J.
Published: July 2002

The integrated photoluminescence (PL) intensities of both ordered and disordered epilayers of InGaP grown on GaAs have been measured as a function of temperature. The highest PL efficiency occurs in the most disordered sample. We find that the PL intensities can drop from 2 to almost 4 orders of magnitude between 12 and 280 K. The samples show an Arrhenius behavior characterized by two activation energies. Below 100 K the activation energies lie in the region of 10–20 meV. Above 100 K the activation energy is approximately 50 meV except in the most disordered sample where it increases to 260 meV. We conclude that the low-temperature PL efficiency is most likely controlled by carrier thermalization from spatial fluctuations of the band edges followed by nonradiative recombination. At higher temperatures the PL efficiency is dominated by a nonradiative path whose characteristic activation energy and transition probability depend upon the degree of sublattice ordering.
Compact, inexpensive target design for steady-state heat removal in high-heat-flux fusion applications
Authors: Combs S. K. , Milora S. L. , Foster C. A. , Haselton H. H. , Menon M. M. , Tsai C. C.
Published: July 2002

A high-heat-flux target has been developed for intercepting multimegawatt, multisecond neutral beams at Oak Ridge National Laboratory (ORNL). Water-cooled copper swirl tubes are used for the heat transfer medium; these tubes exhibit an enhancement in burnout heat flux over conventional axial flow tubes. The target consists of 126 swirl tubes [each 0.95 cm in outside diameter (o.d.) with 0.16-cm-thick walls and ≊1 m long] arranged in a V shape and inclined with respect to the beam axis. In tests with the ORNL long-pulse ion source (13×43-cm grid), the target has handled up to 3-MW, 30-s beam pulses with no deleterious effects. The peak power density was estimated at ≊15 kW/cm2 normal to the beam axis (>5 kW/cm2 maximum on tube surfaces). The water flow rate through the tubes was 0.33 l/s (5.2 gal/min) per tube (axial flow velocity=11.6 m/s) with a corresponding pressure drop of 1.14 MPa (165 psi). To date, the target has absorbed an estimated 25 000 full-power (≊3 MW) pulses for a cumulative time of ≊100 000 beam seconds without failure. Other potential applications for swirl tube technology in the fusion area include plasma limiters, divertor collector plates, rf launchers, and Faraday shields.
Quasi-steady-state multimegawatt ion source for neutral beam injection
Authors: Menon M. M. , Tsai C. C. , Whealton J. H. , Schechter D. E. , Barber G. C. , Combs S. K. , Dagenhart W. K. , Gardner W. L. , Haselton H. H. , Ponte N. S. , Ryan P. M. , Stirling W. L. , Wright R. E.
Published: July 2002

A quasi-steady-state (pulse duration of 30 s) ion source of the duoPIGatron type has been developed for fusion applications. It was designed to deliver an 80-keV hydrogen ion beam of low beamlet divergence (Θrms= 0.26°) at a current density of 0.19 A cm−2. Hydrogen ion beams of 40 to 48 A were extracted at beam energies of 77 to 80 keV for 30-s-long pulses. The reliability and stability of the ion source operation were demonstrated by extracting about 600 beam pulses at full power and full pulse length. The ion source was also operated with deuterium as the working gas, and the optimum current at 80 keV was found to be about 33 A, in agreement with the expected inverse square-root scaling of current density with atomic mass.
Performance of a cooled sapphire and beryllium assembly for filtering of thermal neutrons
Authors: Tennant D. C.
Published: July 2002

Transmission measurements have been made at 80 and 300 K for sapphire and beryllium filters in service on a triple-axis neutron spectrometer. The measurements agree with earlier work and confirm that sapphire is a more effective neutron filter than either quartz or silicon. Curves of transmission as a function of neutron wavelength are presented in a form useful for planning thermal neutron experiments in which higher-order thermal neutrons and fast neutrons must be removed from the incident beam.
A fast UV/visible pyrometer for shock temperature measurements to 20 000 K
Authors: Radousky H. B. , Mitchell A. C.
Published: July 2002

An ultraviolet/visible pyrometer is described which can measure shock temperatures from 3000 to 20 000 K. The system is modular, and in general consists of six photomultiplier tubes and two linear intensified diode array/spectrograph systems which can cover the range from 250 to 800 nm. Extension of the pyrometer’s capabilities into the ultraviolet is necessary for accurate measurements above 8000 K. The nature of the shock environment requires the photomultiplier tubes to have rise times on the order of 2 ns, with a typical experiment lasting between 20 and 500 ns. The system measures absolute intensity, and is calibrated against a known tungsten lamp prior to each experiment. The highest temperature measured was 18 300 K for fluid Xe. The targets needed to contain this type of cryogenic sample are described as well.
High time resolution x-ray measurements with an avalanche photodiode detector
Authors: Kishimoto Shunji
Published: July 2002

An avalanche photodiode (APD) detector has been developed for x-ray timing measurements with a subnanosecond time resolution. The performance of the APD detector was investigated with synchrotron x-ray beams in multi- and single-bunch runs at the Photon Factory (PF) ring. A time resolution of 0.28 ns (FWHM) and a peak-to-background ratio of more than 107 were obtained from the main peak of 14.4-keV x rays in a single-bunch time spectrum. The intrinsic efficiency and the counting-rate capability were also measured. As an application with a high time resolution, time spectra of quantum beats in nuclear resonance of 57Fe were observed with the APD detector at the undulator beam line BL-NE3 of the TRISTAN accumulation ring; the first peak of the beats was completely separated from the prompt peak of the non-nuclear components with a good time resolution and the beat structure was sharply seen to the range of ≳100 ns later with a very low background.
Dark-field imaging with the scanning transmission x-ray microscope
Authors: Morrison G. R. , Browne M. T.
Published: July 2002

The King’s College London scanning transmission x-ray microscope in use on beam line 5U2 at the SRS, SERC Daresbury Laboratory, has been modified to allow dark-field images to be formed using only the x rays scattered by the specimen. Experiments have been performed with a number of different detector geometries, and this has confirmed that the strongest scattering arises from edges or thickness gradients in the specimen. Although the dark-field signal is only a small fraction of the normal transmitted bright-field signal, features can be revealed with high contrast, and it has proved possible to detect the presence of features that are below the resolution limit of the microscope.
Surface negative ion production in ion sources
Authors: Belchenko Yu.
Published: July 2002

Negative ion sources and the mechanisms for negative ion production are reviewed. Several classes of sources with surface origin of negative ions are examined in detail: surface-plasma sources where ion production occurs on the electrode in contact with the plasma, and ‘‘pure surface’’ sources where ion production occurs due to conversion or desorption processes. Negative ion production by backscattering, impact desorption, and electron- and photo-stimulated desorption are discussed. The experimental efficiencies of intense surface negative ion production realized on electrodes contacted with hydrogen-cesium or pure hydrogen gas-discharge plasma are compared. Recent modifications of surface-plasma sources developed for accelerator and fusion applications are reviewed in detail.
Feedback-stabilized focal plane control for light microscopes
Authors: Lanni Frederick
Published: July 2002

The study of the three-dimensional structure of biological specimens by light microscopy requires accurate and precise control of focal plane setting. For imaging of live cells, speed is also an important constraint, due to Brownian and active motions within the specimen. We have used a linear capacitance gauge and piezoelectric elements in a closed-loop configuration to stabilize the specimen-to-objective lens distance in a microscope with a precision of 0.012 μm. The circuit also allows for rapid, arbitrary, electronic control of focal plane setting independent of the mechanical focusing drive of the microscope.
Digital signal processor control of scanned probe microscopes
Authors: Baselt David R. , Clark Steven M. , Youngquist Michael G. , Spence Charles F. , Baldeschwieler John D.
Published: July 2002

Digital signal processors have made it possible to control scanned probe microscopes using straightforward software emulations of analog circuits. Using a system consisting of a commercially available digital signal processor board interfaced to analog I/O, we have developed algorithms for self-optimizing feedback, raster generation (with hysteresis correction, sample tilt compensation, and scan rotation), lock-in detection, and automatic tip-sample approach. We also discuss an instruction parser that takes advantage of the digital architecture to allow automatic operation for extended periods.
Modified Fowler–Milne method for the spectroscopic measurement of temperature and composition of multielement thermal plasmas
Authors: Murphy A. B.
Published: July 2002

A technique is presented by which the Fowler–Milne spectroscopic method can be applied to thermal plasmas containing two or more atomic elements. The technique requires the measurement of the emission coefficient for an atomic transition of each element. These values are normalized to the maximum emission coefficient for each transition in the respective single-element plasmas. Neither calibration of the sensitivity of the apparatus nor knowledge of atomic transition probabilities is required. The technique allows the relative concentration of the elements to be derived significantly more precisely than other spectroscopic methods. The technique is applied to the measurement of the radial profiles of temperature and composition of free-burning arcs in mixtures of argon and nitrogen. Significant demixing is observed, with the direction of the demixing depending on the relative concentrations of argon and nitrogen.
Diode laser autodetachment spectroscopy
Authors: Marawar R. W. , Cowles Daniel C. , Keeler Raymond E. , White Andrew P. , Farley John W.
Published: July 2002

Autodetachment spectroscopy of a molecular anion has been demonstrated for the first time using a weak infrared diode laser. A low-power (5 μW) laser beam overlaps a fast ion beam, driving an infrared vibrational-rotational transition in a molecular anion. The vibrationally excited anion autodetaches, and the resulting fast neutral is detected. In the first demonstration of the new technique, a previously unmeasured transition in the fundamental (1←0) band of NH− was observed near 2806 cm−1 with a signal-to-noise ratio of 15:1. There is reasonable agreement between calculated and observed signal strengths. Future instrumental improvements are expected to yield substantial improvement in sensitivity. The present experiment extends the range of applicability of autodetachment spectroscopy beyond the range of color-center lasers. With straightforward improvements, the technique is applicable to a number of larger molecular anions having low electron affinity.
Electric-field-induced exciton transport in coupled quantum well structures
Authors: Hagn M. , Zrenner A. , Böhm G. , Weimann G.
Published: July 2002

We report a conceptionally new approach to achieve electrostatically induced transport and confinement for spatially indirect excitons. Experimentally, exciton transport is demonstrated in an electric-field-tunable GaAs/AlAs coupled quantum well structure, which is configured as a three-terminal device. In spatially resolved photoluminescence experiments, it is shown that indirect excitons experience a drift field, which is given by an electrostatically induced band-gap gradient in the plane of the coupled quantum well structure.
1 nm deep mechanical processing of muscovite mica by atomic force microscopy
Authors: Miyake Shojiro
Published: July 2002

Atomic-scale mechanical processing of layered materials such as muscovite mica was performed using an atomic force microscope (AFM). Processing began at a certain critical load above 130 nN, and the processing depth increased discretely with load. Fracture easily occurred at the two cleavage planes of SiO4–K and K–SiO4 interfaces. With a load slightly larger than the critical load, several repetitions of mechanical sliding of the tip generated a 1 nm deep groove which corresponds to the distance from the top surface of SiO4 to the top surface of the next SiO4 layer beneath it with the removal of residual potassium on the surface. For example, a groove with four steps of 1 nm depth was processed by step-by-step mechanical sliding.
Multibeam honeycomb surface-plasma source with plasma injection from hollow cathodes
Authors: Belchenko Yu. I. , Kupriyanov A. S.
Published: July 2002

The small model of scaled honeycomb surface-plasma source for intense multibeam H−(D−) ion production was studied. The increased sizes of the honeycomb cells (to 3×3 cm2) and of discharge layer thickness (to 1.5 cm) were tested. The plasma injection from the hollow hydrogen–cesium cathodes was used for a high-current semiplanotron discharge uniform operation at a low gas pressure. An optimal hydrogen pressure was about 1 Pa at maximal negative ion production. Extraction of H− ions from four emission apertures of 4×10 mm size each produces four elementary H− beams with intensity up to 100 mA each. No essential isotopic effect was found—the intensity of D− beams had about the same value (when discharge was operated in deuterium).
Design and performance of graded multilayers as focusing elements for x-ray optics
Authors: Morawe Ch. , Pecci P. , Peffen J. Ch. , Ziegler E.
Published: July 2002

We have developed a design method to produce laterally graded multilayer x-ray mirrors based on sputter deposition techniques. The optimization of all relevant parameters yields an ab initio estimation of any layer gradient suitable to achieve precise x-ray focusing devices. The performance and the accuracy of this method are demonstrated. A graded W/B4C multilayer was deposited on a flat substrate that was bent to a parabola. The obtained nonlinear lateral gradient differed from the theoretical calculations by less than 1%. Focusing experiments performed at an x-ray energy of 8 keV on the ESRF optics beamline revealed an excellent performance with a focal spot size of about 7 μm.
An experimental station for advanced research on condensed matter under extreme conditions at the European Synchrotron Radiation Facility - BM29 beamline
Authors: Filipponi Adriano , Borowski Michael , Bowron Daniel T. , Ansell Stuart , Di Cicco Andrea , De Panfilis Simone , Itiè Jean-Paul
Published: July 2002

We describe state-of-the-art experimental techniques using the beamline BM29 of the European Synchrotron Radiation Facility (ESRF). This station exploits the unique characteristics of an ESRF bending magnet source to provide a tunable, collimated, x-ray beam to perform high quality x-ray absorption spectroscopy within the energy range of E=5–75 keV using Si(111), Si(311), and Si(511) crystal pairs. Energy scans can be performed over this wide energy range with excellent reproducibility, stability and resolution, usually better than ΔE/E≃5×10−5. The experimental setup has been exploited to study condensed matter under extreme conditions. We describe here two sample environment devices; the L’ Aquila–Camerino oven for high-temperature studies up to 3000 K in high vacuum and the Paris–Edinburgh press suitable for high-pressure high-temperature studies in the range 0.1–7 GPa and temperatures up to 1500 K. These devices can be integrated in an experimental setup which combines various control and detection systems suitable to perform x-ray absorption spectroscopy, x-ray absorption temperature scans, and energy scanning x-ray diffraction (ESXD). The ESXD setup is based on a scintillator detector behind a fixed angle collimator aligned to the sample. The combination of these three measurements, which can be performed in rapid sequence on the sample during the experiment, provides an essential tool for structural investigations and in situ sample characterization.
Far field characterization of diffracting circular apertures
Authors: Obermüller Christian , Karrai Khaled
Published: July 2002

The far field angular intensity distribution I(θ) of the λ=633 nm radiation transmitted through diffracting circular apertures is measured for diameters ranging between 60 and 500 nm. The circular apertures are located at the apex of aluminum coated tapered optical fiber tips. I(θ) depends sensitively on the aperture diameters down to λ/6. This property is used to determine the optical aperture size of metal coated tapered optical fiber tips used for near field scanning optical microscopy.
Phenomenology of Zn diffusion and incorporation in InP grown by organometallic vapor-phase epitaxy (OMVPE)
Authors: Schubert E. F. , Pinzone C. J. , Geva M.
Published: July 2002

The diffusion characteristics and incorporation characteristics of Zn dopants in OMVPE-grown InP are studied. The Zn diffusion constant depends strongly on concentration and increases by four orders of magnitude in the Zn concentration range 2×1018–8×1018 cm−3. This drastic concentration dependence of the Zn diffusion constant is shown to determine the Zn incorporation characteristics during OMVPE growth. A spread of Zn dopants into intentionally undoped regions may result at high Zn doping concentrations in InP.
Scattering spectroscopy of molecules at nanometer resolution
Authors: Martin Y. , Zenhausern F. , Wickramasinghe H. K.
Published: July 2002

A new form of optical spectroscopy is demonstrated whereby the local scattering from the interaction between a nanometer size probe tip and a sample is measured as a function of wavelength. We show images of viruses and molecules at molecular spatial resolution and demonstrate how the image contrast varies with wavelength. To first approximation, the contrast as a function of wavelength varies in the same way as the local polarizability of the sample
Selective solid phase crystallization for control of grain size and location in Ge thin films on silicon dioxide
Authors: Yang C. M. , Atwater Harry A.
Published: July 2002

Selective solid phase crystallization for control of grain size and location in polycrystalline thin Ge films on amorphous silicon dioxide substrates is described. The approach consists of solid phase crystal nucleation at predefined nucleation sites, which consist of metal islands deposited on top of the amorphous Ge film, followed by lateral solid phase epitaxial growth. Grain sizes as large as 30 μm have been achieved in 50-nm-thick Ge films at temperatures less than 475 °C.
Reduction of threading dislocation density in InP-on-Si heteroepitaxy with strained short-period superlattices
Authors: Samonji K. , Yonezu H. , Takagi Y. , Iwaki K. , Ohshima N. , Shin J. K. , Pak K.
Published: July 2002

We applied multistrained short-period superlattices (SSPSs) and GaP buffer layers to the InP-on-Si heteroepitaxy, in order to suppress the generation of threading dislocations. As a result, it was found that the density of threading dislocations in an InP/SSPSs/GaAs/SSPSs/GaP/Si structure including (InAs)m(GaAs)n SSPSs and (GaAs)i(GaP)j SSPSs was remarkably reduced, compared with that in the InP/GaP/Si structure. Misfit dislocations lying along the 〈011〉 directions were observed at heterointerfaces in the InP/SSPSs/GaAs/SSPSs/GaP/Si structure. Therefore, the lattice mismatch strain was stepwise accommodated by the generation of misfit dislocations at the heterointerfaces. From these results, it was clarified that multi-SSPSs are effective for reducing the density of threading dislocations in heteroepitaxy with a large lattice mismatch.
Magnetoresistance in thin films and bulks of layered-perovskite La2−2xCa1+2xMn2O7
Authors: Asano H. , Hayakawa J. , Matsui M.
Published: July 2002

Epitaxial thin-film and polycrystalline bulk samples of ferromagnetic La2−2xCa1+2xMn2O7 (x=0.3) with a layered perovskite structure have been examined with respect to their magnetotransport properties. In addition to a large magnetoresistance (MR) effect at temperatures around the metal–insulator transition, a-axis thin films exhibit unusual low-temperature MR behavior with apparent hysteresis in applied magnetic fields. The features in the bulk samples are a low-temperature MR effect without such hysteresis, and also an MR effect at high temperatures well above the metal–insulator transition temperature. The unique MR behavior depending on the sample form would reflect an anisotropic nature in the layered-perovskite ferromagnet with two-dimensional Mn-O networks.
Large band gap bowing of InxGa1−xN alloys
Authors: McCluskey M. D. , Van de Walle C. G. , Master C. P. , Romano L. T. , Johnson N. M.
Published: July 2002

Band gap measurements have been performed on strained InxGa1−xN epilayers with x⩽0.12. The experimental data indicate that the bowing of the band gap is much larger than commonly assumed. We have performed first-principles calculations for the band gap as a function of alloy composition and find that the bowing is strongly composition dependent. At x=0.125 the calculated bowing parameter is b=3.5 eV, in good agreement with the experimental values.
Quantitative topographic imaging using a near-field scanning microwave microscope
Authors: Vlahacos C. P. , Steinhauer D. E. , Dutta S. K. , Feenstra B. J. , Anlage Steven M. , Wellstood F. C.
Published: July 2002

We describe a technique for extracting topographic information using a scanning near-field microwave microscope. By monitoring the shift of the system’s resonant frequency, we obtain quantitative topographic images of uniformly conducting metal surfaces. At a frequency of 9.572 GHz, our technique allows a height discrimination of about 55 nm at a separation of 30 μm. We present topographic images of uneven, conducting samples and compare the height response and sensitivity of the system with theoretical expectations.
Evidence of ferromagnetic behavior of small liquid droplets produced from amorphous alloys by laser ablation
Authors: Grigorenko A. N. , Nikitin P. I. , Toporov A. Yu. , Ghorbanzadeh A. M. , Perrone A. , Zocco A. , De Giorgi M.-L.
Published: July 2002

Experimental evidence of ferromagnetic behavior of liquid droplets produced by laser ablation from amorphous alloys is presented. Thin films of amorphous magnetic materials were fabricated by laser ablation deposition in the presence and in the absence of a magnetic field. The difference in parameters of deposited films is attributed to ferromagnetic properties of small size liquid droplets.
Optical modulation in a resonant tunneling relaxation oscillator
Authors: Figueiredo J. M. L. , Stanley C. R. , Boyd A. R. , Ironside C. N. , McMeekin S. G. , Leite A. M. P.
Published: July 2002

We report high-speed optical modulation in a resonant tunneling relaxation oscillator consisting of a resonant tunneling diode (RTD) integrated with a unipolar optical waveguide and incorporated in a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can provide wide-bandwidth electrical gain. For wavelengths near the material band edge, small changes of the applied voltage give rise to large, high-speed electroabsorption modulation of the light. We have observed optical modulation at frequencies up to 14 GHz, associated with subharmonic injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line, as well as generation of 33 ps optical pulses due to relaxation oscillation.
Isotropic polarization modulational instability and domain walls in spun fibers
Authors: Kockaert P. , Haelterman M. , Pitois S. , Millot G.
Published: July 2002

By means of a very low birefringence spun fiber, we investigate experimentally the phenomenon of polarization modulational instability in the conditions of effective isotropy of the Kerr medium. By showing that birefringence in spun fibers can be negligibly small as compared to nonlinear effects, our results open up new prospects in the development of applied nonlinear fiber optics. This aspect is illustrated through the generation of polarization domain walls whose interest for optical fiber data transmissions has been recently discussed.
Arsenic impurities in GaN
Authors: Van de Walle Chris G. , Neugebauer J.
Published: July 2002

We present a comprehensive first-principles investigation of arsenic incorporation in GaN. Incorporation of As on the N site, which has previously been implicitly assumed, is favorable only under n-type conditions in a Ga-rich environment. Less Ga-rich conditions, and particularly p-type doping, strongly favor incorporation of As on the Ga site, where it behaves as a deep double donor. Arsenic thus acts as a compensating center, forming a real threat to acceptor doping of GaN and making p-type doping of GaAsN alloys impossible. The calculated donor levels for AsGa are consistent with luminescence lines around 2.6–2.7 eV in GaN intentionally doped with As.
Lattice and energy band engineering in AlInGaN/GaN heterostructures
Authors: Khan M. Asif , Yang J. W. , Simin G. , Gaska R. , Shur M. S. , zur Loye Hans-Conrad , Tamulaitis G. , Zukauskas A. , Smith David J. , Chandrasekhar D. , Bicknell-Tassius R.
Published: July 2002

We report on structural, optical, and electrical properties of AlxInyGa1−x−yNGaN heterostructures grown on sapphire and 6H–SiC substrates. Our results demonstrate that incorporation of In reduces the lattice mismatch, Δa, between AlInGaN and GaN, and that an In to Al ratio of close to 1:5 results in nearly strain-free heterostructures. The observed reduction in band gap, ΔEg, determined from photoluminescence measurements, is more than 1.5 times higher than estimated from the linear dependencies of Δa and ΔEg on the In molar fraction. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional electron gas at the heterointerface. The obtained results demonstrate the potential of strain energy band engineering for GaN-based electronic applications.
Diffusion kinetics of formation of holographic polymer-dispersed liquid crystal display materials
Authors: Bowley C. C. , Crawford G. P.
Published: July 2002

The underlying physical phenomena behind the increased reflection/diffraction efficiency of holographically formed polymer-dispersed liquid crystal gratings formed from blended oligomers are investigated. Using parameters determined by in situ diffraction measurements during grating formation, a phenomenological diffusion model shows that a composition modulation in the polymer network results in blended oligomer systems. Scanning electron microscope studies reveal polymer morphologies consistent with this hypothesis.
Dependencies of secondary electron yields on work function for metals by electron and ion bombardment
Authors: Kudo M. , Sakai Y. , Ichinokawa T.
Published: July 2002

Secondary electron yields depending on work function were measured for 30 species of metal in ultrahigh vacuum by electron and ion bombardment. Secondary electron yields induced by electrons at 10 keV increase with work function, while those by Ar+ ions at 3 keV decrease with increasing work function. The opposite dependencies of secondary electron yields on work function between electron and ion bombardment are discussed on the basis of the different mechanisms of secondary electron emission, i.e., kinetic and potential emission for electron and ion bombardment, respectively.
Computing forces with quantum Monte Carlo
Authors: Assaraf Roland , Caffarel Michel
Published: July 2002

We present a simple and stable quantum Monte Carlo approach for computing forces between atoms in a molecule. In this approach we propose to use as Monte Carlo estimator of the force the standard Hellmann–Feynman expression (local force expressed as the derivative of the total potential energy with respect to the internuclear coordinates). Invoking a recently introduced zero-variance principle it is shown how the infinite variance associated with the Hellmann–Feynman estimator can be made finite by introducing some suitably renormalized expression for the force. Practical calculations for the molecules H2, Li2, LiH, and C2 illustrate the efficiency of the method.
Glass transition relaxation and fragility in the molecular glass forming m-toluidine: A study by thermally stimulated depolarization currents
Authors: Correia Natália T. , Alvarez Cristina , Ramos Joaquim J. Moura , Descamps Marc
Published: July 2002

The glass transition relaxation in m-toluidine was studied by thermally stimulated depolarization currents (TSDC). The departure from the zero entropy prediction, also called compensation behavior, was analyzed and the fragility index of the glass-former m-toluidine was calculated from the TSDC data according to a recently proposed procedure. The obtained result was compared with literature values. A particular attention was focused on the physical aging process of the nonequilibrium glass, which was followed easily because of its slow evolution. The influence of the aging temperature on the aging rate was analyzed. The significance of the so-called compensation point was discussed on the basis of the results obtained in this study of structural aging.
Direct numerical simulation of flow separation behind a swept, rearward-facing step at ReH=3000
Authors: Kaltenbach Hans-Jakob , Janke Gerd
Published: July 2002

The effect of sweep on a transitional separation bubble behind a backward-facing step is investigated using direct numerical simulation (DNS). The Reynolds number based on step height and free stream component normal to the step, Reα=(HC∞ cos α)/ν, is kept constant at 3000 for sweep angles α between 0° and 60°. Results agree well with two experimental investigations. Up to α=40°, the mean flow follows the sweep-independence principle. For higher sweep angles, the size of the separated flow region is shortened considerably which is mainly due to an upstream shift of transition in the laminar shear layer emanating from the step and—to a lesser degree—to an increase in turbulent momentum flux uv¯. Fluctuations of wall-stress and wall pressure are largest near the reattachment location and their magnitude scales with (C∞ cos α)2 for all sweep angles. Transition is dominated by a Kelvin–Helmholtz-type instability of the free shear layer. The influence of skewing on the growth rate of instability waves is weak in accordance with linear stability theory predictions of Lu and Lele [J. Fluid Mech. 249, 441 (1993)]. The upstream shift of transition for α⩾50° is related to the way how disturbances spread sidewise in a wedge-shaped region inside of the shear layer.
Theoretical studies of the X̃ 2Π and Ã 2Σ+ states of the He⋅OH and Ne⋅OH complexes
Authors: Lee Hee-Seung , McCoy Anne B. , Toczyłowski Rafał R. , Cybulski Sławomir M.
Published: July 2002

Potential energy surfaces for the X̃ 2Π and Ã 2Σ+ states of the He⋅OH and Ne⋅OH complexes have been developed, using the restricted open-shell coupled cluster [RCCSD(T)] method. These potentials have been used to calculate rotation-vibration energies for both electronic states of these complexes and their deuterated analogs. Good agreement with the available experimental vibrational energies and rotational constants is obtained. In spite of being relatively weakly bound, the vibrational eigenstates for both electronic states of these species display significant deviations from the simple zero-order free-rotor/anharmonic oscillator limit, leading to potentially rich spectra. © 2000 American Institute of Physics.
Renormalized CCSD(T) and CCSD(TQ) approaches: Dissociation of the N2 triple bond
Authors: Kowalski Karol , Piecuch Piotr
Published: July 2002

The recently proposed renormalized and completely renormalized CCSD(T) and CCSD(TQ) methods, which can be viewed as generalizations of the noniterative perturbative CCSD(T) and CCSD(TQf) schemes and which result from the more general method of moments of coupled-cluster equations, are applied to the dissociation of the ground-state N2 molecule. It is shown that the renormalized and completely renormalized CCSD(T) and CCSD(TQ) methods provide significantly better results for large N–N separations than their unrenormalized CCSD(T) and CCSD(TQf) counterparts.
Porous-silicon vapor sensor based on laser interferometry
Authors: Gao Jun , Gao Ting , Sailor Michael J.
Published: July 2002

Certain porous-silicon (PS) films exhibit well-resolved Fabry–Pérot fringes in their optical reflection spectra due to thin-film interference. The fringes shift to higher wavelengths when the PS is exposed to vapors from organic solvents, as a result of an increase in the average refractive index of the PS layer. If a small diode laser is used as the light source, the shift of the Fabry–Pérot fringes upon analyte adsorption results in a change in the reflected light intensity, which correlates with the concentration of the analyte (ethanol) in an air stream. Based on this principle, a PS vapor sensor has been demonstrated with a detection limit of 500 ppb and a dynamic range of nearly five orders of magnitude.
Theoretical studies of the X̃ 2Π and Ã 2Σ+ states of He⋅SH and Ne⋅SH complexes
Authors: Cybulski Sławomir M. , Toczyłowski Rafał R. , Lee Hee-Seung , McCoy Anne B.
Published: July 2002

The two-dimensional potential energy surfaces for the X̃ 2Π and Ã 2Σ+ states of the He⋅SH and Ne⋅SH complexes have been calculated using the restricted open-shell coupled cluster theory [RCCSD(T)] and the triple-zeta augmented correlation consistent polarized basis sets with an additional (3s3p2d2f1g) set of bond functions. In the case of the Ã 2Σ+ state of Ne⋅SH the entire surface has also been developed using the quadruple-zeta basis set with bond functions as exploratory calculations demonstrated significant differences between the RCCSD(T) results obtained with the triple- and quadruple-zeta basis sets. These potentials are somewhat shallower and less anisotropic in comparison to the surfaces for the related He⋅OH and Ne⋅OH complexes. In contrast to He⋅OH and Ne⋅OH, we find that the linear Rg–SH (Rg=He, Ne) configurations are in all but one case lower in energy than the Rg–HS geometries. Variational calculations of the bound rotation-vibration states have been performed using Hamiltonians that included the RCCSD(T) potentials. The calculated ground-vibrational-state dissociation energy, D0, the frequency of the intermolecular stretching vibration, and the rotational constant are in very good agreement with the available experimental results for the X̃ 2Π state of both Ne⋅SH and Ne⋅SD. The energies of rotation-vibration levels for the Ne⋅SH and Ne⋅SD complexes in the Ã 2Σ+ state calculated using the triple- or quadruple-zeta potentials differ significantly, but agreement with the experimental rovibrational transition frequencies and rotational constants is very good regardless of which potential is used.
Electronic structure of cubic silicon–carbide doped by 3d magnetic ions
Authors: Gubanov V. A. , Boekema C. , Fong C. Y.
Published: July 2002

We have studied the electronic properties of cubic silicon–carbide (3C-SiC) doped with Cr, Mn, Fe, and Co magnetic atoms using the tight-binding linear combination of muffin-tin orbitals with atomic sphere approximation method. By directly comparing the difference of the total energy between a vacancy and a dopant filling the vacant site, we found that the Mn doped at C site gains the least energy as compared to the other cases. Heavier Fe and Co atoms appear to be nonmagnetic. For lighter Cr and Mn atoms at the Si site, the dopings result in 1.6 μB (Bohr magneton) for Cr and 0.7 μB for Mn, respectively. The magnetic moment for Cr atom substituting a C atom is 0.907 μB. 3d down spin hole states exist, but the mobility associated with these states is not expected to be large. Photoluminescence measurements are suggested to probe the narrow 3d structures in the gap.
High-power high-frequency-induced Langmuir turbulence in the smooth ionosphere at Arecibo. I. Theoretical predictions for altitude-resolved plasma line radar spectra
Authors: DuBois D. F. , Russell D. A. , Cheung P. Y. , Sulzer M. P.
Published: July 2002

This is the first of two papers comprising a theoretical and observational study of new, altitude-resolved, observations at Arecibo of Langmuir turbulence induced in the ionosphere by a powerful high-frequency (hf) heater operated at very low duty cycles. As shown in paper II [Cheung et al., Phys. Plasmas 8, 802 (2001)], higher power enabled the first observation at Arecibo of the well-developed decay-cascade features in the Thomson scatter radar power spectrum at the unmodified matching altitudes. New theoretical predictions are presented here for the parameters of these observations emphasizing the altitude and pump power dependence of the radar spectra and the time dependence of the spectra from the decaying spectra following heater switch-off. Further details of the strong turbulence signatures from higher altitudes are also presented. At the lower matching altitudes the increase, with hf power, of the angular width of the well-developed decay-cascade spectrum allows these spectral features to come into the view of the Arecibo radar. The favorable comparison of the simulation predictions and observations is discussed in the second paper.
The generalized active space concept for the relativistic treatment of electron correlation. I. Kramers-restricted two-component configuration interaction
Authors: Fleig Timo , Olsen Jeppe , Marian Christel M.
Published: July 2002

As a prelude to a series of presentations dealing with the treatment of electron correlation and special relativity, we present the theoretical background and the implementation of a new two-component relativistic configuration interaction program. It is based on the method of generalized active spaces which has been extended from a nonrelativistic implementation to make use of two-component Hamiltonians and time reversal and double point group symmetry at both the spinor and Slater determinant level. We demonstrate how the great computational effort arising from such a general approach—the treatment of spin–orbit interaction and electron correlation in a fully variational framework—can be markedly reduced by the use of the aforementioned symmetries. Evidence for the performance of the program is given through a number of calculations on light systems with a significant spin–orbit splitting in low-lying electronic states and the well-known problem case thallium, which often serves as a rigorous test system in relativistic electronic structure calculations.
Magic numbers in copper-doped aluminum cluster anions
Authors: Thomas Owen C. , Zheng Weijun , Bowen Kit H.
Published: July 2002

Copper-doped aluminum cluster anions, CuAln− were generated in a laser vaporization source and examined via mass spectrometry (n=2–30) and anion photoelectron spectroscopy (n=2–15). The mass spectrum of the CuAln− series is dominated by CuAl13− with other magic numbers also appearing at n=6, 19, and 23. The electron affinity versus cluster size trend shows a peak at n=6 and a dip at n=13. These results are discussed in terms of the reordering of shell model energy levels and the enhanced stability of neutral CuAl13. Reordering, which is a consequence of the copper atom residing in the central region of these clusters, provides an anion-oriented electronic rationale for the observed magic numbers.
Study of in-plane magnetic domains with magnetic transmission x-ray microscopy
Authors: Fischer P. , Eimüller T. , Schütz G. , Köhler M. , Bayreuther G. , Denbeaux G. , Attwood D.
Published: July 2002

Magnetic transmission x-ray microscopy is a novel technique to image element specifically magnetic domain structures. A lateral resolution down to 25 nm is provided by the Fresnel zone plates used as optical elements in soft x-ray microscopy. The magnetic contrast is given by x-ray magnetic circular dichroism, i.e., large magnetic contributions up to 25% to the absorption cross section of circularly polarized x rays that occur in the vicinity of, e.g., the Fe L3,2 edges (706 and 719 eV) and depend on the relative orientation of the projection of the magnetization of the sample onto the photon propagation direction. Thus, both in-plane and out-of-plane contributions to the magnetization are accessible. Here we present images of the magnetic domain structure of a (3 nm Cr/50 nm Fe/6 nm Cr) thin film system with a preferentially in-plane magnetization recorded at the Fe L edges. The samples have been prepared by thermal evaporation onto a 100 nm thin Si3N4 membrane and were mounted under a tilt of 30° with respect to the transmission direction of the photons in the full-field microscope. Corresponding images taken under a tilt of 0° ruled out out-of-plane contributions. Images recorded in applied varying external magnetic fields allowed to study the switching behavior. These trial results have a large impact on further investigations of nanostructured magnetic systems, e.g., spintronic devices and magnetic sensors with magnetic soft x-ray microscopy.
Magnetostriction and magnetism of rare earth intermetallic compounds: First principle study
Authors: Gavrilenko V. I. , Wu R. Q.
Published: July 2002

Magnetism and magnetostriction of rare earth intermetallic compounds, GdCo2, GdFe2, NdCo2, SmCo2, and ErCo2, have been studied by using the first principles full-potential linearized augmented plane-wave method with the generalized gradient approximation. The calculated magnetostriction coefficients agree well with experiment. The itinerant electrons of transition metal elements are found to play a significant role in magnetoelastic coupling. The strong anisotropy of magnetostriction in GdCo2 is explained. Contributions due to spatial anisotropic charge distribution of the incomplete 4f shells are calculated and discussed.
Time-of-flight and emission spectroscopy study of femtosecond laser ablation of titanium
Authors: Ye Mengqi , Grigoropoulos Costas P.
Published: July 2002

Femtosecond laser ablation of titanium has been studied via time-of-flight (TOF) and emission spectroscopy measurement. Laser pulses of 80 fs full width at half maximum at λ=800 nm were delivered by a Ti:sapphire femtosecond laser system. A vacuum chamber with a base pressure of 10−7 Torr was built for ion TOF measurement. These ion TOF spectra were utilized to determine the velocity distribution of the ejected ions. While nanosecond laser ablation typically generates ions of tens of eV, femtosecond laser irradiation even at moderate energy densities were found to produce energetic ions with energies in the range of a few keV. Two ablation regimes, exhibiting different laser fluence dependence of the total ion yields, and the corresponding percentage of energetic ions and the crater depth, were identified and explained on the basis of the two-temperature model. Clean craters were observed by interferometric microscope measurements, indicating the advantages of and potential for applying femtosecond lasers to micromachining and advanced materials treatment. Emission spectroscopy and imaging have been carried out via a monochromator and an intensified CCD camera. Femtosecond laser-induced plumes were found to be much smaller and weaker in intensity than those induced by nanosecond laser pulses.
Soft elasticity and mechanical damping in liquid crystalline elastomers
Authors: Clarke S. M. , Tajbakhsh A. R. , Terentjev E. M. , Remillat C. , Tomlinson G. R. , House J. R.
Published: July 2002

The dynamic soft response of polydomain liquid crystalline elastomers to simple shear is reported. Significantly, these materials also show extremely large loss behavior with tan δ exceeding 1 or even 1.5 over very wide temperature ranges, with clear implications for damping applications. By comparing materials that exhibit different types of liquid crystalline phases, we identify the nematic state as a better damping phase than that in materials with smectic phases. Additionally, we provide experimental evidence for directions which should be explored for further improvements in the damping behavior of liquid crystalline elastomers.
LiNbO
3
thick films grown on sapphire by using a multistep sputtering process
Authors: Lansiaux X. , Dogheche E. , Remiens D. , Guilloux-viry M. , Perrin A. , Ruterana P.
Published: July 2002

We present the results of investigations of lithium niobate (LiNbO3) films grown on (0001) sapphire substrate by radio-frequency magnetron sputtering. With the aim of producing thick films we have developed a multistep process. To assess the structural quality of our films, we performed x-ray diffraction, transmission electron microscopy, and atomic force microscopy of the films. The results revealed that the epitaxial structure is verified on sapphire whatever the film thickness. Based on our studies we concluded that the proposed multistep process leads to a good controlled epitaxy of thick LiNbO3 samples on sapphire. Using the prism coupling technique we have characterized the optical properties of the deposited layers. The ordinary (no) and extraordinary (ne) refractive indices are, respectively, 2.289±0.001 and 2.206±0.001 at 632.8 nm, and waveguide losses of 1.2 dB/cm have been demonstrated. In addition, we have qualified the film properties between each growth step and between the substrate to layer interface, directly from the measured optical data.
Decomposition and nanocrystallization in reactively sputtered amorphous Ta–Si–N thin films
Authors: Pinnow C. U. , Bicker M. , Geyer U. , Schneider S. , Goerigk G.
Published: July 2002

The nanocrystallization process of reactively sputtered thin amorphous Ta–Si–N films is investigated by anomalous small angle x-ray scattering (ASAXS) and x-ray diffraction (XRD). Changes in the microstructure in Ta40Si14N46 films, density variations in the amorphous matrix, decomposition, formation, and growth of nanocrystals after vacuum anneals at different temperatures in the range between 800 and 1000 °C are observed and the results of the different techniques are compared. From a Fourier analysis of ASAXS intensities the nanostructure of the investigated ternary system is derived using a model of hard spheres according to Guinier and Fournet. ASAXS investigations indicate that the noncrystalline samples can be described by a monophase fit and the crystallized samples by a bimodal-phase fit, the latter results being consistent with XRD which identifies TaN and Ta5Si3 phases. Detailed analysis shows that TaN nanograins of approximately 2 nm size develop after a decomposition process. Larger grains of Ta5Si3 are observed in addition to the TaN grains if annealing is performed at temperatures higher than 950 °C. The aim of these investigations is to give a generally applicable explanation of the barrier failure mechanism for Ta–Si–N diffusion barriers, which is actually observed at temperatures below the crystallization temperature if the films are used in contact with Cu or Al.