bluuebunny/crossref_metadata_2025 huggingface dataset:
An improved density matrix functional by physically motivated repulsive corrections
Authors: Gritsenko Oleg , Pernal Katarzyna , Baerends Evert Jan
Published: May 2005

An improved density matrix functional [correction to Buijse and Baerends functional (BBC)] is proposed, in which a hierarchy of physically motivated repulsive corrections is employed to the strongly overbinding functional of Buijse and Baerends (BB). The first correction C1 restores the repulsive exchange-correlation (xc) interaction between electrons in weakly occupied natural orbitals (NOs) as it appears in the exact electron pair density ρ2 for the limiting two-electron case. The second correction C2 reduces the xc interaction of the BB functional between electrons in strongly occupied NOs to an exchange-type interaction. The third correction C3 employs a similar reduction for the interaction of the antibonding orbital of a dissociating molecular bond. In addition, C3 applies a selective cancellation of diagonal terms in the Coulomb and xc energies (not for the frontier orbitals). With these corrections, BBC still retains a correct description of strong nondynamical correlation for the dissociating electron pair bond. BBC greatly improves the quality of the BB potential energy curves for the prototype few-electron molecules and in several cases BBC reproduces very well the benchmark ab initio potential curves. The average error of the self-consistent correlation energies obtained with BBC3 for prototype atomic systems and molecular systems at the equilibrium geometry is only ca. 6%.
Redshift and blueshift of the Ar–H vibrational stretching frequency in complexes of FArH and acetylene
Authors: McDowell Sean A. C.
Published: May 2005

Two planar hydrogen-bonded complexes of FArH and acetylene were found to be stable using second-order Møller–Plesset perturbation theory (MP2) with 6-311++G(2d,2p) basis sets. The more stable complex involves bonding between the F atom of FArH and a H atom of acetylene, while the other isomer is a T-shaped complex with the H atom of FArH bonded to the center of the CC bond of acetylene; the zero-point energy corrected dissociation energies are 29 and 27kJmol−1, respectively. Interestingly, the Ar–H harmonic vibrational stretching frequency is blueshifted in the more stable isomer and redshifted in the less stable form. The electron density rearrangement of FArH on complexation was investigated and used to explain these unusual findings.
Rosenbluth-sampled nonequilibrium work method for calculation of free energies in molecular simulation
Authors: Wu Di , Kofke David A.
Published: May 2005

We present methods that introduce concepts from Rosenbluth sampling [M. N. Rosenbluth and A. W. Rosenbluth, J. Chem. Phys. 23, 356 (1955)] into the Jarzynski nonequilibrium work (NEW) free-energy calculation technique [C. Jarzynski, Phys. Rev. Lett. 78, 2690 (1997)]. The proposed hybrid modifies the way steps are taken in the NEW process. With it, each step is selected from a range of alternatives, with bias given to steps that contribute the least work. The definition of the work average is modified to account for the bias. We introduce two variants of this method, λ-bias sampling and configuration-bias sampling, respectively; a combined λ- and configuration-bias method is also considered. By reducing the likelihood that large nonequilibrated work values enter the ensemble average, the Rosenbluth sampling aids in remedying problems of inaccuracy of the calculation. We demonstrate the performance of the proposed methods through a model system of N independent harmonic oscillators. This model captures the difficulties involved in calculating free energies in real systems while retaining many tractable features that are helpful to the study. We examine four variants of this model that differ qualitatively in the nature of their phase-space overlap. Results indicate that the λ-bias sampling method is most useful for systems with entropic sampling barriers, while the configuration-bias methods are best for systems with energetic sampling barriers. The Rosenbluth-sampling schemes yield much more accurate results than the unbiased nonequilibrium work method. Typically the accuracy can be improved by about an order of magnitude for a given amount of sampling; this improvement translates into two or more orders of magnitude less sampling required to obtain a given level of accuracy, owing to the generally slow convergence of the NEW calculation when the inaccuracy is large.
Dynamics of fundamental electromagnetic emission via beam-driven Langmuir waves
Authors: Li B. , Willes A. J. , Robinson P. A. , Cairns I. H.
Published: May 2005

The nonlinear process of electromagnetic Langmuir decay, which leads to radio emission near the plasma frequency, is studied for situations in which Langmuir waves are directly driven by an electron beam and indirectly generated via electrostatic Langmuir decays. The electromagnetic Langmuir decay is stimulated by the presence of ion-acoustic waves. An approximate method is devised for studying this emission process with axial symmetry (along the direction of beam propagation) in three spatial dimensions, based upon the Langmuir and ion-acoustic wave dynamics in one spatial dimension. Numerical studies of the fundamental electromagnetic emission starting from electron dynamics are then carried out via quasilinear theory, and the results are explored for illustrative parameters. The evolution of the fundamental transverse waves shows the combined effects of local emission and propagation away from the source. At a given location, the emission rate shows a series of peaks associated with successive electromagnetic decays of the Langmuir waves, which are either driven by the beam or produced by successive electrostatic decays. The emission rate for a given electromagnetic decay decreases with time, following an initial increase. In addition, the emission rate for a specific electromagnetic decay shows approximate dipolar form, consistent with previous analytical work. Consequently, the fundamental transverse waves emitted locally propagate approximately symmetrically in both the forward and the backward directions. Variation of the background electron to ion temperature ratio, beam injection parameters, and angular widths of the Langmuir and ion-acoustic spectra are found to affect the emission rate and, hence, the fundamental transverse wave levels. Furthermore detailed studies show that the wave numbers of the maximum emission rates are also in good agreement with an approximate prediction for simple model Langmuir and ion-acoustic spectra.
Finite element simulations of the laser-heated diamond-anvil cell
Authors: Kiefer Boris , Duffy Thomas S.
Published: May 2005

Axial and radial temperature gradients in the laser-heated diamond-anvil cell are examined using finite element simulations. Calculations are carried out for an optically thin silicate or oxide sample separated from the diamonds by an insulation medium and heated by a TEM00 mode from an infrared laser. The peak temperature of the simulations was chosen to be a representative value (2200K) and sample dimensions are typical for experiments in the 20–50-GPa range. The distance between the anvils is 30μm. The total temperature drop across the sample in the axial direction is controlled by two parameters: the filling fraction (thickness of sample∕distance between anvils) and the ratio of thermal conductivity between the sample and insulator (kS∕kI). The results of the numerical calculations agree well with a one-dimensional numerical model. For a sample filling fraction of 0.5, the axial temperature drop will range from about 1000K (>45%) for a thermal conductivity ratio of 1 to about 200K (<10%) for a conductivity ratio of 10. If the conductivity ratio between sample and insulator is reduced to 1, then a sample filling fraction of less than 0.1 is required to keep the axial temperature decrease to be less than 10%. The effects of asymmetric samples and variations in absorption length are also examined. For a given gasket thickness and conductivity ratio, we find that radial gradients are minimal at a filling fraction of about 50% and then increase at higher and lower filling ratios. The anvil surface remains close to room temperature in all calculations. Our results demonstrate that reduction of axial temperature variations in optically thin laser-heated samples requires the use of thick, low thermal conductivity insulation media.
Structural analysis of the reconstructed Si(001)-C surface
Authors: Park J. Y. , Seo J. H. , Whang C. N. , Kim S. S. , Choi D. S. , Chae K. H.
Published: May 2005

The atomic structure of reconstructed Si(001)c(4×4)-C surface has been studied by coaxial impact collision ion scattering spectroscopy. When the 100L of ethylene (C2H4) molecules have been exposed on Si(001)-(2×1) surface at 700°C, it is found that C atoms cause the ordering of missing Si dimer defects and occupy the fourth layer of Si(001) directly below the bridge site. Our results provide the support for the previous model in which a missing dimer structure is accompanied by C incorporation into the subsurface.
Long-range many-body polyelectrolyte bridging interactions
Authors: Podgornik Rudi , Saslow Wayne M.
Published: May 2005

We investigate polyelectrolyte bridging interactions mediated by charged, flexible, polyelectrolyte chains between fixed cylindrical macroions of opposite charge in a two-dimensional hexagonal crystalline array. We show that in the asymptotic regime of small macroion density, the polyelectrolyte-mediated attraction is long range, falling off approximately linearly with the macroion array density. We investigate the polyelectrolyte free energy as a function of the macroion density and derive several analytic limiting laws valid in different regimes of the parameter space.
Controlled subnanosecond isomerization of HCN to CNH in solution
Authors: Gong Jiangbin , Ma Ao , Rice Stuart A.
Published: May 2005

We report a study of control of the HCN→CNH isomerization in a liquid Ar solution. We show, using molecular dynamics simulations, nearly complete conversion from HCN to CNH can be achieved in solution on the subnanosecond time scale without requiring laser pulse shaping or molecular alignment. The mechanism of the isomerization reaction involves multiphoton rovibrational excitation on the ground electronic state potential energy surface coupled with rapid rovibrational relaxation in solution. The results demonstrate the important role of rotation-vibration coupling in multiphoton excitation of small molecules and constitute the first realistic computational demonstration of fast, robust, and high-yield laser field manipulation of solution-phase molecular processes.
Ambipolar injection in a submicron-channel light-emitting tetracene transistor with distinct source and drain contacts
Authors: Reynaert J. , Cheyns D. , Janssen D. , Müller R. , Arkhipov V. I. , Genoe J. , Borghs G. , Heremans P.
Published: May 2005

We report on organic light-emitting transistors with a submicron-channel length, gold source, and calcium drain contacts. The respective contact metals allow efficient injection of holes and electrons in the tetracene channel material. Transistor characteristics were measured in parallel with electroluminescence being recorded by a digital camera focused on the transistor channel. In the case of submicron-channel lengths, the transistor source-drain current at higher gate voltages was determined by the source-drain voltage. At larger channel lengths, the source-drain current was limited by the injection of electrons from the calcium contact, as hole ejection to this contact was fully blocked. The hole blocking is explained in terms of a chemical reaction occurring at the Ca/tetracene interface.
Valley splitting in V-shaped quantum wells
Authors: Boykin Timothy B. , Klimeck Gerhard , von Allmen Paul , Lee Seungwon , Oyafuso Fabiano
Published: May 2005

The valley splitting (energy difference between the states of the lowest doublet) in strained silicon quantum wells with a V-shaped potential is calculated variationally using a two-band tight-binding model. The approximation is valid for a moderately long (approximately 5.5–13.5nm) quantum well with a V-shaped potential which can be produced by a realistic delta-doping on the order of nd≈1012cm−2. The splitting versus applied field (steepness of the V-shaped potential) curves show interesting behavior: a single minimum and for some doublets, a parity reversal as the field is increased. These characteristics are explained through an analysis of the variational wave function and energy functional.
Reduction of acoustic-phonon deformation potential in one-dimensional array of Si quantum dot interconnected with tunnel oxides
Authors: Uno Shigeyasu , Mori Nobuya , Nakazato Kazuo , Koshida Nobuyoshi , Mizuta Hiroshi
Published: May 2005

The scattering potential for the acoustic deformation potential scattering in a one-dimensional silicon quantum dot array interconnected by thin oxide layers is theoretically investigated. One-dimensional phonon normal modes are numerically obtained using the linear atomic chain model. The strain caused by an acoustic-phonon vibration is absorbed by the oxide layers, resulting in the reduction of the strain in the Si dots. This effect eventually leads to ∼40% reduction of the scattering potential all over the structure. The amount of the reduction does not depend on the phonon energy, but rather on the ratio of the Si dot size to the oxide thickness.
Electron traps in organic light-emitting diodes
Authors: Tsai Min-Jan , Meng Hsin-Fei
Published: May 2005

This work presents the effects of electron traps in organic light-emitting diodes using a model which includes charge injection, transport, and recombination. For electron-only devices, the electron current is reduced by the traps for several orders of magnitude at fixed voltage, and the traps strongly increase the transient time. For bipolar devices, due to negative trapped charges, traps enhance the hole current and the total current, opposite to the electron-only devices. The traps also make the recombination region close to the cathode. There is a voltage-dependent critical trap density beyond which the quantum efficiency decreases and transient time rises dramatically. The quantum efficiency is doubled if the hole traps are added to balance the electron and hole injections. Finally, the trap effect can be used in a bilayer light-emitting diode to make the emission color-tunable.
The effects of fill tubes on the hydrodynamics of ignition targets and prospects for ignition
Authors: Edwards John , Marinak Marty , Dittrich Tom , Haan Steve , Sanchez Jorge , Klingmann Jeff , Moody John
Published: May 2005

The notion of using a narrow bore fill tube to charge an ignition capsule in situ with deuterium-tritium (DT) fuel is very attractive because it eliminates the need for cryogenic transport of the target from the filling station to the target chamber, and in principle is one way of allowing any material to be considered as an ablator. We are using the radiation hydrocode HYDRA [M. M. Marinak et al., Phys. Plasmas 8, 2275 (2001)] in two dimensions to study the effect of fill tubes on graded copper-doped Be ignition capsule implosions. The capsule is ∼1.1-mm radius and driven at ∼300eV. Fill tubes are made of glass and range in diameter from 10–20μm. These are inserted between 5 and 40μm into the ablator surface, and a glue layer around the capsule ∼2-μm thick is included. The calculations are unusually demanding in that the flow is highly nonlinear from the outset, and very high angular resolution is necessary to capture the initial evolution of the tube, which is complex. Despite this complexity, the net result is that by the time the capsule implosion takes off, a preferred, simple Bessel-like mode is set up that is almost independent of, and much larger than, the initial tube size, and close to the fastest growing mode for the capsule. The perturbation continues to grow during the unstable acceleration phase, and inverts as the capsule begins to stagnate, sending a spike of cold DT into the forming hot spot. In all cases studied the capsule ignites and gives close to clean one-dimensional yield. The principal seed of the perturbation appears to be shielding of the ablator in the close vicinity of the fill tube, and the growth is found to vary linearly with the diameter of the tube. The simulations and results are discussed.
Thermoelectric performance of films in the bismuth-tellurium and antimony-tellurium systems
Authors: da Silva Luciana W. , Kaviany Massoud , Uher Ctirad
Published: May 2005

Coevaporated bismuth-tellurium and antimony-tellurium films were fabricated under various deposition conditions (controlled evaporation rates of individual species, substrate temperature, and substrate material), and their thermoelectric (TE) properties (Seebeck coefficient, electrical resistivity, and carrier concentration) were measured in search of optimal TE performance. The tellurium atomic concentration was varied from 48% to 74%, the substrate temperature ranged from 130 to 300 °C, and glass, mica, magnesium oxide, and sapphire substrates were used. The chemical composition and crystal structure of the films were recorded (using microprobe and x-ray diffractometer, respectively), analyzed, and compared with available standard Bi2Te3 and Sb2Te3 single-crystal samples. High-performance TE films had tellurium atomic concentration around 60% and were deposited at a substrate temperature between 260 and 270 °C.
Characterization of AlGaN∕GaNp-n diodes with selectively regrown n-AlGaN by metal-organic chemical-vapor deposition and its application to GaN-based bipolar transistors
Authors: Xing H. , DenBaars S. P. , Mishra U. K.
Published: May 2005

p - n junctions formed by selective regrowth of n-(Al)GaN on p-GaN were characterized by I–V measurements and secondary-ion-mass spectrometry (SIMS). Regrown p-n diodes of high quality with low leakage currents were achieved, comparable to as-grown GaN p-n diodes, by optimizing the regrowth conditions and structures. SIMS revealed a sharp Mg profile at the regrowth interface. It confirms that these regrown structures are suitable to serve as emitter-base junctions for GaN-based bipolar transistors. This study offers us insight into analyzing n-p-n GaN homojunction bipolar transistors with a current gain of 10 and Al0.05GaN∕GaN heterojunction bipolar transistors with a current gain of 20, demonstrated using emitter regrowth technique.
General expressions for quantum transport in arbitrary potential profile: L-electron effect on AlAs–GaAs–AlAs double-barrier structure
Authors: Chen Chun-Nan
Published: May 2005

A theoretical method for the calculation of quantum transport in an ellipsoidal valley is presented. This method is developed using an effective-mass equation including off-diagonal effective-mass tensor elements. The L-electron effect on AlAs–GaAs–AlAs double-barrier structures oriented in the [001], [111], and [110] growth directions is explored using the developed method.
Atomic structure of Al88Y7Fe5 metallic glass
Authors: Saksl K. , Jóvári P. , Franz H. , Jiang J. Z.
Published: May 2005

The structure of Al88Y7Fe5 metallic glass has been investigated by differential scanning calorimetry, x-ray powder diffraction, and x-ray absorption fine structure (XAFS) techniques. The amorphous alloy crystallizes according to the following scheme: amorphous→α-Al+residual amorphous→α-Al+Al3Y+AlFeY. The atomic structure of the amorphous alloy was modeled by reverse Monte Carlo methods. The resulting structure indicates that the prepeak (at 1.42Å−1) in the static structure factor S(Q) is caused by contributions of distinct Y-Y, Y-Fe pairs. From the analysis of our XAFS spectra at the Fe and Y K edge the following characteristics of the local structure are found: The interatomic distance between Fe and coordinated Al atoms in the amorphous state is significantly (8.9%) shorter than the sum of the nominal metallic-state radii. The average coordination number is anomalously 36% reduced compared to the value derived from the dense-random-packing (DRP) model, using again the nominal metallic-state radii. On the other hand, the Y-Al distance as well as the number of Al atoms coordinating Y is close to the values predicted by the DRP model. These anomalous changes around the Fe atoms indicate a strong interaction between Fe and Al, which corroborate a covalent bonding. In connection with the values obtained from the XAFS data analysis, the effective atomic radii in the amorphous phase have been calculated. Consequently, applying them to the calculation of the atomic size factor (λ=0.1076) confirms that Al88Y7Fe5 follows the rule of atomic size ratio for glass formation.
Structural defects and luminescence features in heteroepitaxial GaN grown on on-axis and misoriented substrates
Authors: Bai J. , Dudley M. , Chen L. , Skromme B. J. , Wagner B. , Davis R. F. , Chowdhury U. , Dupuis R. D.
Published: May 2005

The microstructure and luminescence properties of a series of GaN epilayers grown on sapphire and SiC substrates with various misorientations have been correlated to assess the origins of the luminescence features in the misoriented samples. Samples grown on 3.5° offcut SiC and 5° and 9° offcut sapphire substrates both exhibit photoluminescence peaks near ∼3.2 and ∼3.4eV, which are absent in the on-axis SiC and sapphire cases. Transmission electron microscopy shows that the misoriented samples have configurations of I1 basal plane stacking faults, which fold into prismatic stacking faults with stair-rod dislocations at their intersections. The luminescence features are proposed to possibly arise from transitions involving the prismatic stacking faults and∕or the stair-rod dislocations associated with their mutual intersections and their intersections with the basal plane stacking faults.
Effect of nanoscale surface morphology on the phase stability of 3C-AlN films on Si(111)
Authors: Lebedev V. , Cimalla V. , Kaiser U. , Foerster Ch. , Pezoldt J. , Biskupek J. , Ambacher O.
Published: May 2005

In this work, we report on the stabilization of 3C-AlN polytype by molecular-beam epitaxy (MBE) on 3C-SiC∕Si(111) pseudosubstrates. The main purpose of the present studies is to analyze the mechanisms forcing the epitaxy of the 3C-AlN at typical MBE conditions. The forces driving the cubic polytype formation have been considered including supersaturation, macroscopic stress, interfacial energy, and interface morphology. We conclude that the growth of 3C-AlN phase can be stabilized on the “rough” 3C-SiC(111) surface by the polytype replication due to the lateral character of the nucleation and the reduction in the total energy of the film provided by periodical undulations of the template surface.
Effects of physical properties of components on reactive nanolayer joining
Authors: Wang J. , Besnoin E. , Knio O. M. , Weihs T. P.
Published: May 2005

We studied the effects of the physical properties of components on a reactive joining process that uses freestanding nanostructured Al∕Ni multilayer foils as local heat sources to melt AuSn solder layers and thereby bond the components. Stainless-steel reactive joints were compared with Al reactive joints. The strengths of both the stainless-steel and the Al joints increase as the foil thickness and thus the total heat of reaction increases until the foil thickness reaches a critical value. For foils thicker than the critical value, the shear strengths are constant at approximately 48 and 32MPa for the stainless-steel joints and Al joints, respectively. The critical foil thickness for stainless-steel joining is 40μm, compared with 80μm for the joining of Al. Numerical studies of heat transfer during reactive joining and the experimental results suggest that the duration of melting of the AuSn solder is shorter when Al specimens are joined. Thus, a thicker foil is required to enable a sufficient duration (0.5ms) of melting of the AuSn solder and full wetting of the metallic samples in order to form a strong joint. In general, when components with higher thermal conductivity, higher heat capacity, and higher density are joined, the duration of melting of the solder or braze layer is shorter and therefore a thicker foil is required to ensure the formation of a strong joint.
Multi-instantons in seven dimensions
Authors: Loginov E. K.
Published: May 2005

We consider the self-dual Yang–Mills equations in seven dimensions. Modifying the t’Hooft construction of instantons in d=4, we find N-instanton 7d solutions which depend on 8N effective parameters and are E6 invariant.
Wyman’s solution, self-similarity, and critical behavior
Authors: Oliveira-Neto G. , Takakura F. I.
Published: May 2005

The Wyman solution depends on two parameters, the mass M and the scalar charge Σ. If one fixes M to a positive value, say M0, and lets Σ2 take values along the real line, we show that this solution exhibits a type of critical behavior, in analogy with the nonstatic massless scalar field solutions. For Σ2>0 the space-times have naked singularities, for Σ2=0 one has a Schwarzschild black hole of mass M0 and finally for −M02⩽Σ2<0 one has “wormhole-like” solutions. We also show that the Wyman solution is not self-similar, i.e., it does not admit a homothetic Killing vector.
Precision instrument for characterizing shape memory alloy wires in bias spring actuation
Authors: Chikkamaranahalli Sumanth , Vallance R. Ryan , Khan Afzal , Marsh Eric R. , Rawashdeh Osamah A. , Lumpp J. E. , Walcott Bruce L.
Published: May 2005

Some metallic alloys such as Nitinol (NiTi) exhibit the shape memory effect, which is suitable for generating force and displacement when the alloy changes phase during a heating and cooling cycle. These shape memory alloys are often formed into one-dimensional wires, tubes, and ribbons that are preloaded by bias springs to create inexpensive actuators for electromechanical devices. This article describes a new instrument for measuring the quasistatic characteristics of the alloy and the transient performance of bias-spring actuators when resistively heated and convectively cooled. The instrument achieves more accurate measurements by eliminating rolling friction and by sensing force and displacement in line with the bias spring and shape memory alloy wire. Data from the instrument enables calculation of stress and strain at constant temperatures and during actuation cycles.
White organic light-emitting devices with CdSe/ZnS quantum dots as a red emitter
Authors: Li Yanqin , Rizzo Aurora , Mazzeo Marco , Carbone Luigi , Manna Liberato , Cingolani Roberto , Gigli Giuseppe
Published: May 2005

White hybrid organic/inorganic light-emitting devices (LEDs) have been fabricated by using stable red-emitting CdSe/ZnS core-shell quantum dots (QDs) covered with a trioctylphosphine oxide organic ligand. The device-active structure consists in a host/guest system with a blue-emitting poly[(9,9-dihexyloxyfluoren-2,7-diyl)-alt-co-(2-methoxy-5-{2-ethylhexyloxy}phenylen-1,4-diyl)] (PFH-MEH) polymer doped with red-emitting QDs and a green emitting metal chelate complex Alq3, with improved electron injection and transfer properties. A fairly pure white OLED with Commission Internationale de l’Eclairage coordinates of (0.30,0.33) is fabricated by accurate control of the Förster energy and charge-transfer mechanisms between the different device constituents obtained by tuning the concentration ratio of the QDs/PFH-MEH blend. In particular, charge-transfer processes to CdSe/ZnS core-shell quantum dots are found to be the key element for well-balanced white emission. Maximum external quantum efficiency up to 0.24% at 1mAcm−2 and 11 V in air atmosphere are reported, showing that hybrid LEDs can be a promising route towards more stable and efficient light-emitting devices for lighting applications.
Implementation of a Rogowski coil for the measurement of partial discharges
Authors: Argüeso Marta , Robles Guillermo , Sanz Javier
Published: May 2005

In order to obtain partial discharge (PD) measurements a PD measuring probe based on a Rogowski coil was constructed. Up to now, Rogowski coils had been used to measure high level currents. In this article, the design of a Rogowski coil to measure very low currents, but with a very high frequency, such as partial dischages, is exposed. The Rogowski coil used as a PD transducer is very advantageous because it is inexpensive and easy to use. Besides, it provides the needed bandwidth for this application.
High-voltage pulse generator for THz field generation
Authors: Giraud G. , Rittener J. , Chergui M. , van Mourik F.
Published: May 2005

We describe a simple and low cost design for generating high voltage pulses using commercially available Ruhmkorff’s coils. Our application is the biasing of large area photoconductive switches, used for THz generation, operating at kHz repetition rates. The described circuit produces 100μs pulses of up to 60 kV.
Influence of vignetting on signal analysis of photocarrier radiometry of semiconductor wafers
Authors: Li Bincheng , Shaughnessy Derrick , Mandelis Andreas
Published: May 2005

The influence of vignetting on the photocarrier radiometry (PCR) measurements of semiconductor wafers has been investigated both theoretically and experimentally by analyzing the vignetting effect on the PCR amplitude and on the frequency dependence of the PCR amplitude and phase. The vignetting effect significantly reduces the PCR amplitude and modifies the frequency dependencies that are widely used to extract simultaneously the electronic transport properties (that is, the carrier lifetime, the carrier diffusion coefficient, and the front and rear surface recombination velocities) of semiconductor wafers. When using the frequency dependence of the PCR signal to determine the transport properties, the effect of vignetting can be accounted for by an “effective detector size”—a reduced detector size determined by the actual detector size and the vignetting effect.
Development of a microwave thermogravimetric analyser based on a multimode oven
Authors: Parkes Gareth M. B. , Williams Howard M.
Published: May 2005

This article describes the development of a thermogravimeter (thermobalance) based on a multimode microwave cavity utilizing pulse-width modulation control of the power and five figure (0.01mg resolution) balance. The operation and capabilities of the instrument for the study of thermally induced processes involving mass changes are demonstrated with examples of decomposition, dehydration, and oxidation reactions. The potential for combining microwave thermogravimetry with “sample controlled” techniques is also discussed.
A portable, low coherence interferometry based instrument for fine needle aspiration biopsy guidance
Authors: Iftimia Nicusor V. , Bouma Brett E. , Pitman Martha B. , Goldberg Brian , Bressner Jason , Tearney Guillermo J.
Published: May 2005

A portable, low coherence interferometry (LCI) based instrument for fine-needle aspiration biopsy guidance is presented. The instrument consists of a fiber-based low coherence interferometer, a data acquisition, processing and display unit, and a probe. The probe, consisting of a 250μm diameter single-mode optical fiber inserted within the bore of a fine needle, is used to illuminate tissue and collect light from tissue at the tip of the needle. Light returning out of the probe is detected by the LCI system, which is capable of measuring depth-resolved information (reflectivity, spectra, birefringence) with a spatial resolution of 10μm over a depth range of approximately 1.4mm. The LCI based instrument can be used to guide the fine needle during biopsy procedures to potentially diagnose neoplasms, infections, inflammations, or infiltrations. The design and performance of the instrument, as well as preliminary measurements on excised breast tissue specimens, are presented in detail.
Ultrahigh vacuum cluster deposition source for spectroscopy with synchrotron radiation
Authors: Lau J. T. , Achleitner A. , Ehrke H.-U. , Langenbuch U. , Reif M. , Wurth W.
Published: May 2005

A versatile cluster source has been developed for the deposition and investigation of mass selected metal clusters on single crystal substrates under ultrahigh vacuum conditions. The cluster deposition experiment is designed for spectroscopy with synchrotron radiation to probe the properties of mass selected clusters in x-ray absorption, x-ray magnetic circular dichroism, and x-ray photoelectron spectroscopy. The experimental setup consists of three stages, and is based on a sputter source for cluster production, a magnetic dipole field for mass selection, and an ultrahigh vacuum chamber for cluster deposition. With this cluster source, metal clusters of up to 40 atoms per cluster can be produced, mass separated and deposited onto a substrate. In this size range, cluster current densities of 20pAmm−2–10nAmm−2 have been determined experimentally, depending on cluster material and size. For substrate preparation, the experimental chamber is fully equipped with standard surface science tools. Cluster yields are presented for a variety of sputter targets. The capability to produce truly size-selected clusters is demonstrated with mass spectra.
On the diffusion-controlled growth of multiwalled carbon nanotubes
Authors: Bartsch K. , Biedermann K. , Gemming T. , Leonhardt A.
Published: May 2005

Multiwalled carbon nanotubes were deposited by microwave chemical-vapor deposition on Fe, Co, and Ni catalyst layers. By adjusting the deposition temperature and time predominantly either a tubular structure or a bamboolike structure was obtained. The corresponding growth rates of these forms were determined. The activation energy of diffusion and the diffusion coefficient of carbon in the catalyst particles were evaluated to be as high as 0.3–0.6eV and (0.4–1)×10−10m2∕s for the tubular structure and 1.4–1.6eV and (0.6–1.8)×10−11m2∕s for the bamboolike structure. These data prove the occurrence of a liquid catalyst state during tubular growth and of a solid state in the case of bamboolike growth. It is likely that surface melting of the catalyst particles can also occur during the growth process. The tube growth is limited by time due to the metastable state of the catalyst particles.
A physically based lifetime model for stress-induced voiding in interconnects
Authors: Zhai Charlie Jun , Blish Richard Clark
Published: May 2005

We use a simple closed-form lifetime model for stress-induced voiding (SIV) or stress migration for Al- or Cu-based interconnects. Stress-induced voiding is treated as a process of void nucleation/growth and stress relaxation through atomic diffusion driven by a stress gradient. We developed a physically based method of modeling atomic diffusion and void growth, which explicitly accounts for the dependence of void growth on several factors including stress, temperature, diffusivity, and effective modulus. Based on basic physics associated with void growth, we define zones as SIV plate, SIV long line, and SIV short line, respectively, i.e., the three sequential stages for the process of void growth, depending on a nondimensional time-dependent parameter ψ and the aspect ratio of the interconnect. Simple form solutions to the governing equations that describe void growth are then sought for each scenario separately. Time to failure (TTF) is calculated as a function of stressing temperature, mechanical stress, interlayer dielectrics property, and line dimension based on an explicit formulation. The model provides insight into the separate impact of each input factor upon the SIV rate. We find that TTF is invariant with line width in the early zone (SIV plate), while void generation rate is linear with time. TTF is a strong function of line width for both the later zones. The void generation rate varies as the square root of time in the SIV long-line realm, but is constant with time in the SIV short-line zone.
Magnetoresistive magnetometer with improved bandwidth and response characteristics
Authors: Bertoldi Andrea , Bassi Davide , Ricci Leonardo , Covi Daniele , Varas Stefano
Published: May 2005

We report on the realization and characterization of a high performance, compact magnetometer based on a magnetoresistive sensor and working in the range ±600μT. The output is provided both numerically and by means of a field-proportional voltage. Spurious offset effects are suppressed by flipping the film magnetization at a frequency of 100kHz, 2∕3 orders of magnitude higher than in conventional applications. The design allows for a field resolution of 20nT, low output noise density, and high precision and accuracy (relative full-scale uncertainty of about 500ppm).
Reflection scanning near-field optical microscopy in ultrahigh vacuum
Authors: Hoppe Stefan , Ctistis Georgios , Paggel Jens J. , Fumagalli Paul
Published: May 2005

A reflection scanning near-field optical microscope with polarization-sensitive light detection for operation in ultrahigh vacuum is presented. All necessary stages to reach the final goal of subwavelength resolution in magneto-optics are considered step by step, validating our approach and demonstrating the usefulness of the final instrument. A number of problems are attacked and discussed, but some are only treated to an extent necessary to bring the instrument to operation. Sub-λ resolution of a nonmagnetic polarization signal is demonstrated.
Detection of O(PJ3) atoms formed by reaction, Al+O2→ AlO+O under crossed-beam condition
Authors: Ishida Masayuki , Higashiyama Tomohiko , Matsumoto Yoshiteru , Honma Kenji
Published: May 2005

The vacuum ultraviolet laser-induced fluorescence technique was employed to detect the oxygen atoms formed by the reaction, Al+O2→ AlO+O. The measurements were carried out under the crossed-beam condition at 12.2kJ∕mol of collision energy. The relative populations of three spin-orbit states of O(PJ3) were determined to be 3.8, 1.0, and 0.2 for J=2, 1, and 0, respectively. They show nonstatistical populations, i.e., more population in O(P23) and less population in O(P03) than the statistical expectation. These populations were almost identical for two Al beam conditions where the relative concentrations of two spin-orbit states of Al, P1∕22, and P3∕22, are different. These results suggest that the reaction of Al with O2 proceeds via an intermediate complex where the memory of the initial spin-orbit state is lost. Deviation from the statistical population of O(PJ3) implies the occurrence of the interaction among potential surfaces in the exit channel.
Interface characterization in Pd–Fe–Al(oxide)–Fe systems using conversion electron mössbauer spectroscopy
Authors: Schmitt H. , Stahl B. , Ghafari M. , Hahn H.
Published: May 2005

A Fe–Al(oxide)-bilayer was deposited on a thermally oxidized and Pd-buffered Si substrate. In order to elucidate the structural and magnetic properties of the insulators lower interface by Mössbauer spectroscopy, a thin Fe57 tracer was placed at the Al(oxide) interface. Conversion electron Mössbauer spectroscopy at room temperature and 30K revealed the changes at the interface after Al deposition and its oxidation with molecular oxygen. This nuclear probe technique can resolve the different phases that appear during the different preparation steps. The magnetic hyperfine splitting and the isomer shift give information about the chemical, structural, and magnetic orders. This is important for an understanding of magnetoresistance in likewise composed tunneling magnetoresistance samples. A transmission electron microscopy image of a layered reference system Pd–Fe–Al2O3–Fe confirms the layered structure of the sample and the lower interface between Fe and Al (Al2O3).
Inversible Max-Plus algebras and integrable systems
Authors: Ochiai Tomoshiro , Nacher Jose C.
Published: May 2005

We present an extended version of max-plus algebra which includes the inverse operator of “max.” This algebra enables us to ultradiscretize the system including subtractions and obtain new ultradiscrete equations. The known ultradiscrete equations can also be recovered by this construction.
High throughput oxide lattice engineering by parallel laser molecular-beam epitaxy and concurrent x-ray diffraction
Authors: Ohtani M. , Lippmaa M. , Ohnishi T. , Kawasaki M.
Published: May 2005

A laser molecular-beam epitaxy (LMBE) system for the fabrication of atomically controlled oxides superlattices and an x-ray diffractometer that measures spatially resolved x-ray diffraction spectra have been developed based on the concept of combinatorial methodology. The LMBE chamber has two moving masks, an automated target stage, a substrate heating laser, and an in situ scanning reflection high-energy electron diffraction system. The x-ray diffractometer with a curved monochromator and two-dimensional detector is used for rapid concurrent x-ray diffraction intensity mapping with the two axes of the detector corresponding to the diffraction angle and a position in the sample.
High-throughput mechanical characterization of free-standing polymer films
Authors: Sormana Joe-Lahai , Chattopadhyay Santanu , Carson Meredith J.
Published: May 2005

In this article we review the design, construction, and operation of an instrument for high-throughput mechanical characterization (HTMECH) of polymer films. In particular, the HTMECH is applied here to free-standing polymer libraries with gradients in properties such as temperature and composition. Here, the goal is discovery of mechanical property relationships with polymer formulation and processing. However, HTMECH can also measure strain-rate libraries for uniform samples from near-static to meters-per-second (impact velocities). HTMECH measures force and deformation of film regions that undergo transverse biaxial loading by an instrumented thin contact tip. We demonstrate the salient features of mechanical models of the HTMECH assay and how they are employed to extract meaningful parameters from the raw data. The HTMECH instrument is compared and contrasted with conventional implementations of the dart impact and blister techniques.
Highly reliable 64-channel sequential and parallel tubular reactor system for high-throughput screening of heterogeneous catalysts
Authors: Oh Kwang Seok , Park Yong Ki , Woo Seong Ihl
Published: May 2005

Highly reliable 64-channel sequential and parallel tubular reactor for high-throughput screening of heterogeneous catalysts is constructed with stainless steel. In order to have a uniform flow rate at each channel, 64 capillaries are placed between the outlet of multiport valve and the inlet of each reactor. Flow rate can be controlled within ±1.5%. Flow distribution can be easily adjusted for sequential and parallel mode of operation. The reactor diameter is too big to have a uniform temperature distribution. Hence, the reactor body is separated into three radial zones and controlled independently with nine thermocouples. Temperature accuracy is ±0.5°C at 300°C and ±1°C at 500°C in sequential mode, while it is ±2.5°C in the range of 250–500°C in parallel mode. The temperature, flow rate, reaction sequence, and product analysis are controlled by LABVIEW™ software and monitored simultaneously with displaying a live graph. The accuracy in the conversion is ±2% at the level of 73% conversion when all reactors are loaded with same amount of catalyst. A quaternary catalyst library of 56 samples composed of Pt, Cu, Fe, and Co supported on AlSBA-15 (SBA-15 substituted with Al) are evaluated in the selective catalytic reduction of NO at various temperatures with our system. The most active compositions are rapidly screened at various temperatures.
Heterodyne Fourier transform spectroscopy based on double-grating frequency shifter
Authors: Wang Kaiwei , Zeng Lijiang
Published: May 2005

A heterodyne Fourier transform spectroscopy is proposed in which a double-grating frequency shifter was used to provide the stable achromatic frequency shift. Compared with conventional Fourier transform spectroscopy, our method can obtain spectra with high sensitivity due to optical frequency modulation and high wavelength resolution due to there being no limit for its scanning range. A wavelength resolution of 0.16nm was achieved in the spectra measurements.
Slip-stick step-scanner for scanning probe microscopy
Authors: Meyer Christine , Sqalli Omar , Lorenz Heribert , Karrai Khaled
Published: May 2005

A slip-stick positioning system is shown to work as a step-by-step scanning device. Scanning confocal optical images with sizes up to 100μm by 100μm were taken in reflectivity using a 635nm wavelength laser and an objective of numerical aperture=0.8. The images were taken under ambient and cryogenic conditions on samples with periodic patterns as well as with nanomechanical systems. They show exceptional low distortion and high linearity. The use of the slip-stick step motion for image scanning simplifies the scanning confocal microscope since the long-range positioning unit and the scanning unit merge into only one unit that can do both.
Femtosecond synchronism of x rays to visible light in an x-ray free-electron laser
Authors: Adams Bernhard W.
Published: May 2005

A way is proposed to obtain intense infrared/visible light from an electron bunch in an x-ray free-electron laser in femtosecond synchronism with the x-rays themselves. It combines the recently proposed technique of emittance slicing in a free-electron laser with transition undulator radiation (TUR). The part of the electron bunch that is left unspoiled in the emittance slicing process is the source of both coherent x rays and of coherent TUR at near-infrared wavelengths. An extension of the concept also exploits the fact that the electrons that participate in the free-electron lasing process lose a significant part of their energy.
Piezoresistive mechanical detector for magnetic resonance force microscopy
Authors: Volodin A. , Buntinx D. , Brems S. , Van Haesendonck C.
Published: May 2005

We describe the design and performance of a detection unit for magnetic resonance force microscopy (MRFM) based on commercially available piezoresistive cantilevers. The magnetic tip attached to the cantilever acts as a field gradient source. The adjustable microstrip-line radio-frequency (rf) circuit, which is placed above the cantilever, enables to use a wide range of frequencies between 2.3 and 4.3GHz, and to fully benefit from a frequency sweep of the rf magnetic field. The integration of the necessary field gradient source and broadband microstrip-line rf excitation circuit into the MRFM detector allows us to investigate samples of arbitrary size and shape. The integrated system can also be operated as a conventional magnetic force microscope (MFM) detection unit. Performance of our mechanical detector is demonstrated with ferromagnetic resonance and MFM data obtained on CoO∕Co exchange biased microstructures. Spatial variations of the MRFM signal, which are induced by a spatially modulated exchange bias, are monitored with a lateral resolution of about 5μm.
Vacuum ultraviolet photon detector with improved resolution for inverse photoemission spectroscopy
Authors: Stiepel R. , Ostendorf R. , Benesch C. , Zacharias H.
Published: May 2005

We have significantly improved the energy resolution of a vacuum ultraviolet isochromat spectrometer for inverse photoemission spectroscopy. The detector is based on a Geiger–Müller counting tube with acetone as filling gas and a CaF2 entrance window working as an energy selective optical bandpass at a mean energy of 9.84eV. Under this operating conditions the detector achieves an optical energy resolution of 320meV (FWHM). By adding a gas chamber with two CaF2 windows at the entrance of the counting tube we are able to use absorption lines of oxygen and krypton to further enhance the optical resolution of the detector system. In the case of krypton the mean energy shifts to a somewhat lower value of 9.72eV. The energy resolution improves to about 115meV (FWHM), while the integrated sensitivity decreases to approximately 30%. Due to the ability to easily switch between high sensitivity and high resolution this new counting tube differs from other resolution-enhancing suggestions. Measurements on the prominent (n=1) image potential state on the Cu(111) surface indicate that the photon detector is no longer the major resolution limiting component in the spectrometer.