bluuebunny/crossref_metadata_2025 huggingface dataset:
Tomographic imaging of electron distributions: Leveraging computing power advances to produce inexpensive, low-power, lightweight, and robust instrumentation
Authors: Collier Michael R. , Chornay Dennis J. , Keller John W. , Coplan Michael , Dionne Norman J. , Marji Brahim El , Herrero Federico , Hirman Joe , Houser Jeffrey , Moore John , Slavin James A.
Published: January 2003

We describe a prototype of a novel electron spectrometer that employs a two-dimensional tomographic method to obtain the electron velocity distribution in a space plasma. The spectrometer consists of a magnetic field cavity with a single entrance aperture and a one-dimensional position sensitive microchannel plate/anode assembly electron detector. Electrons entering the instrument through the aperture which have the same component of velocity normal to the aperture plane will all strike the electron detector at the same location regardless of the magnitude of the other velocity component. Thus, the instrument performs line integrals in velocity space. By placing the instrument on a spinning spacecraft, it will provide a complete set of these integrals that may be deconvoluted using standard tomographic techniques. In many cases, basic plasma parameters can be inferred directly from the raw data without deconvolution. This instrument has advantages over conventional electrostatic analyzers: it does not require power consuming voltage stepping, makes efficient use of volume, and operates with a high duty cycle. A modified version of the instrument that is insensitive to photons and can determine three-dimensional distribution functions is also described.
A computational study of DNA separations in sparse disordered and periodic arrays of posts
Authors: Patel Prateek D. , Shaqfeh Eric S. G.
Published: January 2003

We use Brownian dynamics simulation of bead-rod chains to model the electrophoresis of double-stranded DNA molecules through dilute post arrays, in which post spacings are large relative to the Kuhn step. We first consider hairpin collisions with a single post in strong electric fields and generalize these results to describe electrophoresis through post arrays in which chains completely relax between collisions. We develop expressions relating chain velocity (or mobility) and dispersion to chain length and post density and then evaluate these predictions from the single-post model by simulating chain migration through dilute arrays of randomly positioned posts. We find that the single-post model is limited to very dilute arrays in which only weak separations are generated. During electrophoresis through random arrays, the formation of hairpins is found to be most frequent at moderate electric field strengths where both hairpin formation and chain relaxation are important. By determining streamwise dispersion coefficients, we evaluate the performance of dilute random arrays as separation devices and make comparisons with other techniques. Finally, after simulating chain migration through ordered arrays (i.e., square and hexagonal arrangements), we find that disordered post arrangements are essential for separations in strong electric fields.
A thermoluminescence detector-based few-channel spectrometer for simultaneous detection of electrons and photons from relativistic laser-produced plasmas
Authors: Behrens R. , Schwoerer H. , Düsterer S. , Ambrosi P. , Pretzler G. , Karsch S. , Sauerbrey R.
Published: January 2003

A new method was applied to simultaneously measure the absolute energy- and angle-dependent emission of electrons (500 keV to 20 MeV) and photons (50 keV to 2 MeV) emitted by laser-produced plasmas. For this purpose, a newly developed few-channel spectrometer based on thermoluminescence detectors was used. The device measures the curve of depth dose values in a stack of different materials. The deconvolution of electron and photon spectra from the depth dose curve was performed using a computing algorithm based on a Bayesian inference using Gibbs sampling. Several characteristics of the measured particle spectra were investigated: The electron distribution function of the electrons was found to be describable by Maxwellian distributions in energy. The hot electron temperatures obtained (between 1.1 and 1.7 MeV depending on target material and thickness) are in accordance with well-known scaling laws. The angular emission of the electrons was found to be highly anisotropic with a maximum in the direction of the laser reflection (region of the target normal and parallel to the target surface) for a thick target and an additional maximum in the forward direction of the laser for a thin target. Conversion efficiencies depending on the material and thickness of the target for the conversion of laser light energy to relativistic electrons and of electrons to photons were determined to be up to 10% and 1%, respectively.
Elemental analysis of multilayer samples by means of combined laser ablation and x-ray fluorescence spectrometry
Authors: Fiorini C. , Gianoncelli A. , Longoni A. , Comelli D. , D’Andrea C. , Valentini G.
Published: January 2003

Even though absolutely noninvasive techniques are ideally required for the analysis of valuable works of art, in some cases, the historical study or the restoration would benefit from deep investigations that require some kind of alterations on the sample. This is the case when a stratigraphic analysis of a paint has to be carried out to determine pigments and ligants used by the painter. To perform this task, a fully automated device working in situ without physical contact with the sample is certainly the best choice in terms of minimal damage, ease of operation, and quickness of analysis. Such a device could result from the combination of the state of the art portable x-ray fluorescence (XRF) analyzer with a digging tool capable of micrometric control of positioning and depth of operation. In this article, we propose a new method for the determination of chemical element distribution in multilayer samples, based on the use of an alternate sequence of laser photoablations and XRF analysis. The laser source used in this work is a flash lamp pumped Er:YAG laser (2.94 μm), assembled in our laboratories. It operates at a repetition rate of 8 Hz in a Q-switching regime and generates an average power of 60 mW. The XRF spectrometer is a portable instrument based on a Peltier-cooled silicon drift detector (operated at about −10 °C) which is characterized by an energy resolution of about 155 eV full width at half maximum at 6 keV. An x-ray tube with a tungsten anode is used as excitation source. In this article, the measurement technique is described and the first experimental results obtained on samples composed by layers of different thickness and composition are presented.
A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors
Authors: Kulawik M. , Nowicki M. , Thielsch G. , Cramer L. , Rust H.-P. , Freund H.-J. , Pearl T. P. , Weiss P. S.
Published: January 2003

We present an electrochemical etching scheme for producing sharp tungsten tips for use in scanning probe microscopes. The motivation behind the development of this particular method comes from the need to have an etched probe attached to a quartz tuning fork. Comparisons with existing etching methods are made. This rather simple scheme incorporates the key advantages of previously established techniques to give reproducible and controlled etching cycles.
Continuous pulse duration variation in quenched cavity dye laser
Authors: Lee Young-Woo , Kim Yong-Pyung
Published: January 2003

We have suggested a method of generating short pulses with a continuous pulse duration variation from the quenched cavity dye laser (QCDL) system theoretically and experimentally. The XeCL excimer laser pumps a QCDL system to give a continuously varying pulse of duration from 86 to 201 ps as a function of focused pumping thickness in dye cell from 273 to 845 μm. These measurements are in good agreement with the results of numerical analysis. From these results, it is demonstrated that the linear pulse duration variation can be obtained by just changing the focused pumping volume.
Electric field measurement using a hybrid optical instrument
Authors: Sung Youl-Moon , Kuroki Ryusuke , Otsubo Masahisa , Honda Chikahisa
Published: January 2003

A hybrid optical system using an optical field sensor (OFS) and a laser-streamer method (LSM) for measurement of the electric field due to a corona discharge was proposed. The upper limit of the measurable distance from a high-voltage electrode was given by the OFS, and the lower limit was given by the LSM. In general, the measured values agreed well with the calculated values, and therefore it was concluded that the hybrid sensor system was useful to measure the electric field over the whole electrode gap.
Accuracy of spectroscopic constants of diatomic molecules from ab initio calculations
Authors: Pawłowski Filip , Halkier Asger , Jørgensen Poul , Bak Keld L. , Helgaker Trygve , Klopper Wim
Published: January 2003

The basis-set convergence of cc-pVXZ basis sets is investigated for the MP2 and CCSD equilibrium bond distances and harmonic frequencies of BH, HF, CO, N2, and F2 by comparing with explicitly correlated R12 results. The convergence is, in general, smooth but slow—for example, for harmonic frequencies at the quadruple-zeta level, the basis-set error is typically 7 cm−1; at the sixtuple-zeta level, it is about 2 cm−1. For most constants, the convergence can be accelerated by using a two-point linear extrapolation procedure. Equilibrium bond distances, harmonic frequencies, anharmonic contributions, vibration-rotation interaction constants, and rotational constants for the vibrational ground state have been calculated for the same set of molecules using standard wave function and basis-set levels of ab initio theory. The accuracy of the calculated constants has been established by carrying out a statistical analysis of the deviations with respect to experiment. The largest errors for bond distances and harmonic frequencies calculated at the core-corrected CCSD(T)/cc-pV6Z level are 0.4 pm and 13.4 cm−1, respectively. Much smaller errors occur for the anharmonic contributions.
Probes for high-frequency measurements in a plasma gun
Authors: Hurtig Tomas , Wistedt Jan
Published: January 2003

High-frequency correlation measurements of plasma density, and electric and magnetic fields in a plasma gun have been performed. In this article, we give a description of the probes developed for this purpose. The probes described are characterized by a large bandwidth (>15 MHz) and very low susceptibility to electrostatic noise.
Determination of accommodation coefficients of translational and internal energy using a thin wire in a free-molecular flow
Authors: Rebrov A. K. , Morozov A. A. , Plotnikov M. Yu. , Timoshenko N. I. , Maltsev V. A.
Published: January 2003

A new experimental–computational method for determination of translational and internal energy accommodation coefficients on a thin wire in a free-molecular flow is elaborated. The method is based on numerical solution of the heat balance equation of the wire. Measurements were performed in a low-density wind tunnel with Ar, He, N2, CH4, and CO2. In addition, the proposed method provides determination of thermophysical properties of the wire.
Approach for investigating the astigmatism of a magnetic prism in low-energy electron microscopy
Authors: Kan H.-C. , Auerbach Daniel , Phaneuf R. J.
Published: January 2003

We report an approach for investigating the electron optical properties of a magnetic prism, which includes experimental measurement of the focal point positions and the focal lengths of a magnetic prism and a direct comparison between the measurement and the electron optical simulation. We applied this approach to the magnetic prism we constructed as the beam separator for our low-energy electron microscope (LEEM). The magnetic prism consists of two sets of concentric round pole pieces and a rectangular housing as the return circuit of the magnetic flux. The experimental measurements were obtained from images of a square array of submicron size silver dots patterned on a Si(100) substrate recorded with our LEEM operated in photoemission electron microscopy mode. The measurements were compared to results of numerical simulations done with two different approaches. Both calculations agree with the measurements quantitatively.
Balloon borne optical particle counter for stratospheric observation
Authors: Kasai Takeshi , Tsuchiya Masayoshi , Takami Katsumi , Hayashi Masahiko , Iwasaka Yasunobu
Published: January 2003

This article presents the ambient air pressure effects on a balloon borne optical particle counter (an aerosol sonde: AS) equipped with a laser as the light source, and the relevant measures to overcome these effects. To investigate the effects of ambient air pressure varying from 1013 to 10 hPa (from ground level to an altitude of about 30 km) and estimate the general performance of the AS, a novel versatile pressure-variable test chamber was constructed equipped with a built-in nebulizer system. To overcome the direct effect of ambient air pressure on the sensing zone occurring when an open cavity laser (an external mirror-type laser) is used, a flat parallel window was adopted in place of the Brewster window, and in addition, only the laser tube was sealed in an aluminum tube under normal atmospheric pressure. Consequently, the laser power change was suppressed to within ±0.5% for the pressure variation range. To overcome the large dependence in the aerosol sampling flow rate on the ambient air pressure, a new flow rate ratio (flow rate at low pressure divided by that at 1013 hPa) was defined to fall within the ±0.5% variation, as measured using a newly developed technique for measuring the flow rate ratio, owing to an incorporated gear pump system devised to be speed controlled through a pressure sensor. The nonlinear increase of the noise component with decreasing ambient air pressure is discussed, and shown to be overcome electrically, confirming the presumption that this increase is ascribable to the corona discharge caused by high voltage. Thus, for polystyrene latex spheres 0.1 μm in diameter, the developed AS maintained signal-to-noise ratio larger than 2–3 for the pressure variation, as revealed from analysis of the histograms obtained with a multichannel analyzer. Finally, actual field measurements were performed at Bandong, Indonesia, and the results were subjected to a cross-check with those obtained almost simultaneously at the same location using an established conventional sonde, confirming the satisfactory accuracy and reliability of the developed AS.
Chemomechanical surface patterning and functionalization of silicon surfaces using an atomic force microscope
Authors: Wacaser Brent A. , Maughan Michael J. , Mowat Ian A. , Niederhauser Travis L. , Linford Matthew R. , Davis Robert C.
Published: January 2003

Surface modification and patterning at the nanoscale is a frontier in science with significant possible applications in biomedical technology and nanoelectronics. Here we show that an atomic force microscope (AFM) can be employed to simultaneously pattern and functionalize hydrogen-terminated silicon (111) surfaces. The AFM probe was used to break Si–H and Si–Si bonds in the presence of reactive molecules, which covalently bonded to the scribed Si surface. Functionalized patches and patterned lines of molecules were produced. Linewidths down to 30 nm were made by varying the force at the tip.
Torsional selection rules, Raman tensors, and cross sections for degenerate modes of C2H6
Authors: Fernández J. M. , Montero S.
Published: January 2003

We analyze the peculiarities induced by the torsional motion on the Raman spectra of the degenerate vibrational bands of ethane. Selection rules for the Raman transitions between the torsionally split energy levels are derived first in terms of symmetry arguments. Then, their associated intensities are calculated with a model for the torsional dependence of the molecular polarizability. New experimental spectra of the Raman degenerate bands of C2H6, some recorded in jet expansions, are also included and analyzed to show the current state of the problem.
Photodissociation dynamics of enolic-acetylacetone at 266, 248, and 193 nm: Mechanism and nascent state product distribution of OH
Authors: Upadhyaya Hari P. , Kumar Awadhesh , Naik Prakash D.
Published: January 2003

The photodissociation dynamics of acetylacetone (H3C–CO–CH2–CO–CH3), which exists predominantly as an enolic form [H3C–COCH=C(OH)–CH3] in gas phase, is studied using pulsed laser photolysis laser induced fluorescence (LIF) “pump-and-probe” technique at room temperature. Although two pathways for OH formation have been observed, we have focused on the nascent state of the primary OH radical, formed after photo-excitation of the molecule to its (π,π*) and Rydberg states. The (π,π*) and Rydberg transitions are prepared by excitation with fourth harmonic of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in vibrationally cold state at all the above wavelengths of excitation, but differ in rotational state distributions. The rotational distribution is Boltzmann-like, and characterized by rotational temperatures of 950±50, 1130±60, and 1010±80 K at 266, 248, and 193 nm photodissociation, respectively. The spin–orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 16.0−4.0+1.0, 17.3±4.2, and 19.2±4.7 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH production channel, detailed ab initio calculations are performed using configuration interaction-singles method. Although acetylacetone is initially prepared in the (1ππ*) state at 266 and 248 nm excitation, it is concluded that the OH fragment is formed from the lowest (3ππ*) state. However, upon excitation at 193 nm, the initially prepared Rydberg state of acetylacetone crosses over fast to the nearby σ* repulsive state along the C–OH bond, and dissociates to give the OH radical.
An ab initio study of trans-1-chloro-2-fluoroethylene: Equilibrium structure and molecular properties
Authors: Puzzarini Cristina , Cazzoli Gabriele , Gambi Alberto
Published: January 2003

The equilibrium structure and molecular properties of trans-1-chloro-2-fluoroethylene have been investigated theoretically at high level of theory. Very accurate results are presented. As far as the equilibrium structure is concerned, coupled-cluster approach with perturbative inclusion of triples and basis sets of triple and quadruple zeta quality have been employed. Core correlation effects and basis set limit extrapolation have been taken into account in order to obtain best estimates of equilibrium geometry. The molecular dipole moment has been calculated at coupled-cluster level using bases of different quality including diffuse functions and performing the extrapolation to the infinite basis set limit. In addition, the complete inertial nuclear quadrupole tensor, evaluated from the electric field gradient at the chlorine nucleus, has been computed at different level of theory: The multiconfiguration self-consistent field, the Møller–Plesset many-body perturbation to second order and the coupled-cluster methods have been employed. Finally, the harmonic force field has been calculated by means of coupled-cluster approach with a basis of triple zeta quality.
Ab initio quantum dynamics study of rotationally inelastic scattering of glycine by hydrogen atom
Authors: Zhang Da W. , Wang Ming L. , Zhang J. Z. H.
Published: January 2003

Quantum mechanical study has been carried out to investigate rotationally inelastic scattering of glycine by a hydrogen atom. In this study, the glycine is treated as a rigid body fixed in an equilibrium geometry, which results in a three-dimensional quantum scattering problem. In our study, ab initio calculations are first performed to generate potential energies in a three-dimensional (3-D) grid. These discrete energy points are used to fit into a 3-D potential energy surface using a local fitting method to generate potential energies at any given point in 3-D space. Time-dependent quantum dynamics calculation is performed to obtain state-to-state rotationally inelastic transition probabilities of glycine. The individual transition probabilities exhibit oscillatory behavior and have similar magnitude on the order of a few percent. Overall, the energy transfer between translation and rotation is generally small due to the smallness of the rotation constant of glycine.
Volume transition of nematic gels in nematogenic solvents
Authors: Urayama Kenji , Okuno Yuko , Nakao Toshio , Kohjiya Shinzo
Published: January 2003

Equilibrium swelling and phase behavior of liquid crystalline (LC) networks swollen in miscible nematogenic solvents has been investigated by polarizing microscopy as a function of temperature. Four systems, i.e., each of two different LC networks comprising dissimilar mesogens in two different nematic solvents, exhibit essentially the same swelling and phase characteristics. The swelling characteristics strongly correlate with the phases of the LC molecules inside and outside the gel. The two independent nematic-isotropic transition temperatures for the gel (TNIG) and the surrounding pure solvent (TNIS; TNIG>TNIS for all the systems examined) yield three characteristic temperature regions. In the totally isotropic and nematic phases (T>TNIG and T<TNIS, respectively), the degree of equilibrium swelling (Q) is almost independent of T, and the magnitudes of Q in these phases are comparable. Meanwhile, Q strongly depends on T in the region TNIS<T<TNIG where the LC phases inside and outside the gel are different, i.e., nematic and isotropic, respectively. Upon cooling down to TNIG from the totally isotropic phase, the nematic network and the miscible nematic solvent inside the gel form a single nematic phase, which causes a discontinuous decrease in gel volume: The swollen isotropic gel is discontinuously transformed into the shrunken nematic gel. As temperature decreases further, Q of the shrunken nematic gel surrounded by the isotropic LC solvent increases again, and Q at T≈TNIS reaches almost the same magnitude as that in the totally isotropic phase. The nematic ordering of the surrounding pure solvent takes place at TNIS, which yields an inflection of the Q-T curve without discontinuity. A mean field theory for nematic gel successfully describes the swelling and phase behavior observed. The theory also demonstrates that the nematic-isotropic transition of gel drives a volume transition; an increase in nematic order inside gel induces a further swelling of nematic gel surrounded by isotropic LC solvent.
Polarization consistent basis sets. IV. The basis set convergence of equilibrium geometries, harmonic vibrational frequencies, and intensities
Authors: Jensen Frank
Published: January 2003

The basis set convergence of geometries, vibrational frequencies, and associated infrared intensities is investigated for density-functional methods. The recently proposed polarization consistent basis sets give an exponential convergence to the basis set limit, with each step up in quality providing an improvement of almost an order of magnitude. It is shown that the polarization consistent basis sets provide results closer to the basis set limit than alternative basis sets of similar quality.
Hydride stretch infrared spectra in the excited electronic states of indole and its derivatives: Direct evidence for the 1πσ* state
Authors: Dian Brian C. , Longarte Asier , Zwier Timothy S.
Published: January 2003

The hydride stretch infrared spectra of indole, indole-H2O, 3-methyl indole, 3-methyl indole-H2O, the main conformer of tryptamine (TRA), two conformers of N-acetyl tryptophan amide (NATA), and three conformers of N-acetyl tryptophan methyl amide (NATMA), have been recorded in the electronically excited singlet states using excited-state fluorescence-dip infrared spectroscopy. NATA and NATMA are methyl-capped dipeptides of tryptophan that have conformational flexibility and exhibit sensitivity in their electronic spectra to the conformation of the dipeptide backbone. In the indole monomer, the indole NH stretch fundamental at the S1 origin is shifted from its ground-state value (3525 cm−1) to 3478 cm−1. The corresponding band in the indole-H2O complex appears at 3387 cm−1, shifted by a similar amount from its ground-state position (3436 cm−1). Higher vibronic levels within 1500 cm−1 of the S1 origin, which have been identified previously [B. J. Fender et al., Chem. Phys. Lett. 239, 31 (1995)] as being 1Lb or 1La in character, all show similar excited state indole NH stretch absorptions. The corresponding spectra in 3-methyl indole, 3-methyl indole-H2O, TRA, and in the C5 conformers of NATA and NATMA all are missing the indole NH stretch absorption. In its place, a broad background absorption appears, spread over the entire 2800-3800 cm−1 region. In these molecules, other CH stretch or amide NH stretch absorptions remain sharp, appearing in their expected frequency ranges. Finally, the C7 conformations of NATA and NATMA, which possess an intramolecular hydrogen bond in the dipeptide backbone, have all infrared transitions washed out, replaced by a stronger broad background absorption. The entire data set can be explained by the presence of an excited 1πσ* state which is dissociative along the indole NH stretch coordinate, as recently predicted by Sobolewski and Domcke [Chem. Phys. Lett. 315, 293 (1999)]. In the weak coupling case (indole, indole-H2O), the gap between the 1πσ* state and the S1 origin is too large to be reached by infrared excitation. The selective loss of the indole NH stretch in the intermediate coupling case reflects the strong coupling of the 1Lb state NH stretch (v=1) level to the 1πσ* state, which is dissociative along the NH stretch coordinate. In the NATA and NATMA C7 conformers, an inversion of ordering of the electronic states occurs, pushing the 1La state below the 1Lb origin, and strengthening the coupling of all hydride stretch vibrational levels to the 1πσ* dissociative continuum. These results highlight the important influence of the conformation of the polypeptide backbone on the photophysics of tryptophan in polypeptides.
Simulated carrier transport in smectic mesophase and its comparison with experimental result
Authors: Ohno A. , Hanna J.
Published: January 2003

We have investigated charge carrier transport in the smectic mesophases of a liquid crystal, i.e., a 2-phenylnaphthalene derivative, by computational Monte Carlo simulation and time-of-flight experiments. The unique carrier transport properties characterized by field-independent high mobility over 10−3 cm2/V s observed is well explained by a two-dimensional hopping model with a Gaussian distribution of density of states. The deviation of Gaussian distribution, small σ, is determined to be 60 meV in the smectic B phase. This model is proven to be valid by demonstrating a predicted Poole–Frenkel-type behavior at the high electric-field region.
Enhanced bulk oxygen outdiffusion from LiNbO3 induced by preferential sputtering of oxygen at elevated temperatures
Authors: Lu M. , Makarenko B. N. , Hu Y.-Z. , Rabalais J. W.
Published: January 2003

The optical absorption spectra of 5 keV Ar+ and Ti+ irradiated LiNbO3 Z-cut single crystals at various temperatures have been investigated. After Ar+ irradiation to a dose of 1017 ions/cm2, an optical absorption band develops in the range 326–800 nm at elevated temperatures. This band is ascribed to the formation of bulk oxygen vacancy related polarons, its intensity increasing with temperature. By using various sequences of irradiation and annealing, a synergetic effect between ion irradiation and substrate annealing on the oxygen vacancy generation in LiNbO3 is revealed. Irradiation with Ti+, whose ballistic behavior is similar to Ar, produced an absorption band at room temperature, while for Ar+ no such absorption was observed. This band is ascribed to Ti3+ transitions. However, at 660 °C, the Ti+ irradiation induced absorption band had a significantly lower intensity than that induced by Ar+. It is proposed that this “chemical effect” of subplanted Ti in the surface region is a result of Ti reaction with oxygen, thereby trapping some of the outdiffusing oxygen as TiOx and reducing the oxygen deficiency in the surface region. A bulk oxygen outdiffusion mechanism is developed based on these observations, which involves preferential sputtering of oxygen during irradiation, rendering an oxygen deficiency in the surface region. This deficiency of surface oxygen enhances the outdiffusion of highly mobile oxygen in the bulk towards the surface at elevated temperatures.
Dot size dependence of vertical and lateral ordering in self-organized PbSe/Pb1−xEuxTe quantum-dot superlattices
Authors: Springholz G. , Raab A. , Lechner R. T. , Holy V.
Published: January 2003

Self-organized vertical and lateral ordering in PbSe/Pb1−xEuxTe quantum-dot superlattices is investigated as a function of PbSe dot layer thickness. An efficient lateral ordering and face centered cubic-like dot stacking occurs only for PbSe thicknesses between 4–6 monolayers. For smaller thicknesses, no correlations are formed, whereas for larger thicknesses the dots are vertically aligned along the growth direction. These transitions are explained by changes in interlayer dot interactions as a function of the dot size.
Elastic instability of nanomechanical beams
Authors: Carr S. M. , Wybourne M. N.
Published: January 2003

We report on elastic instability of nanomechanical SiO2 beams with widths 20 nm<d<110 nm and lengths 5 μm<L<10 μm. The beams are fabricated from a silicon substrate with a 500 nm thermal oxide layer. After release from the silicon substrate by reactive ion etching the beams buckle due to the residual Si/SiO2 strain. The measured buckling displacements of the beams are compared with the predictions of nonlinear continuum elasticity theory. We observe a continuous buckling transition, qualitatively different than the critical transition predicted by Euler buckling theory, which we attribute to system asymmetry. Finally, we determine the effective potential energy of the fundamental buckling mode.
Ink-jet printing of nanoparticle catalyst for site-selective carbon nanotube growth
Authors: Ago Hiroki , Murata Kazuhiro , Yumura Motoo , Yotani Junko , Uemura Sashiro
Published: January 2003

We report on site-selective growth of multiwalled carbon nanotubes (MWNTs) from a Co nanoparticle catalyst patterned by an ink-jet printing (IJP) technique. The dispersion of the Co nanoparticles was employed as “catalyst ink” for the IJP, and the catalyst pattern was subjected to chemical vapor deposition of acetylene gas. The patterned array of MWNTs was obtained with a dot size around 5–30 μm and showed field emission of electrons corresponding to the printed pattern. The present method offers a simple and powerful means to pattern carbon nanotubes at desired positions with any patterns.
Temperature dependence of electron transfer in coupled quantum wells
Authors: Majumdar Amlan , Choi K. K. , Reno J. L. , Rokhinson L. P. , Tsui D. C.
Published: January 2003

We report on the temperature dependence of electron transfer between coupled quantum wells in a voltage tunable two-color quantum-well infrared photodetector (QWIP). The detection peak of this QWIP switches from 7.1 μm under positive bias to 8.6 μm under negative bias for temperatures T⩽40 K. For T⩾40 K, the 7.1 μm peak is present under both bias polarities and increases significantly with T while the 8.6 μm peak decreases correspondingly. We determine the temperature dependence of electron densities in the two QWs from the detector absorption spectra that are deduced using corrugated QWIPs and find that electron transfer is efficient only when thermionic emission is not significant.
Effective mass in remotely doped Ge quantum wells
Authors: Rößner Benjamin , Isella Giovanni , Känel Hans von
Published: January 2003

We report on the dependence of the effective masses on hole density in remotely doped strained Ge layers on relaxed Si0.3Ge0.7 buffers with sheet densities from 2.9×1011 cm−2 to 1.9×1012 cm−2. The masses have been determined using temperature dependent Shubnikov–de Haas oscillations. No noticeable dependence of the mass on the magnetic field has been found. The extrapolated Γ point effective mass has been found to be 0.080 times the free electron mass. From the measured data the variation of the mass with kinetic energy and the shape of the topmost heavy hole subband have been calculated. The results are in good agreement with theoretical predictions.
Time-division single-photon Sagnac interferometer for quantum key distribution
Authors: Zhou Chunyuan , Zeng Heping
Published: January 2003

We introduce a single-photon Sagnac interferometer for quantum cryptography using time-division phase modulation. The interferometer was arranged to allow coherent laser pulses to travel clockwise and counter-clockwise along the same fiber loop with the corresponding phases modulated in separated periods, interfering at the exit coupler. High visibility of the single-photon interference was observed over a long-distance Sagnac loop. Stable performance was realized by passive compensation of stress and temperature-dependent drifts of the fiber-optic path. Time-division phase encoding and decoding can be realized by controlling the applied electric pulses on integrated phase modulators in the Sagnac loop, which makes this system suitable for a practical quantum cryptography system.
Reversible barrier height changes in hydrogen-sensitive Pd/GaN and Pt/GaN diodes
Authors: Kim Jihyun , Ren F. , Gila B. P. , Abernathy C. R. , Pearton S. J.
Published: January 2003

The forward current of Pd/GaN and Pt/GaN Schottky diodes is found to increase significantly upon introduction of H2 into a N2 ambient. Analysis of the current–voltage characteristics as a function of temperature showed that the current increase is due to a decrease in effective barrier height through a decrease in metal work function upon absorption of hydrogen. The introduction of 10% H2 into a N2 ambient was found to lower the effective barrier height of Pd on GaN by 50∼70 meV over the temperature range of 298 to ∼423 K and of Pt on GaN by 30∼60 meV over the range of 443 to ∼473 K. The magnitude of the changes increased with temperature due to the effective cracking of the H2. The changes in barrier height were completely reversible upon restoration of N2 ambient.
Imaging femtosecond laser-induced electronic excitation in glass
Authors: Mao Xianglei , Mao Samuel S. , Russo Richard E.
Published: January 2003

While substantial progress has been achieved in understanding laser ablation on the nanosecond and picosecond time scales, it remains a considerable challenge to elucidate the underlying mechanisms during femtosecond laser material interactions. We present experimental observations of electronic excitation inside a wide band gap glass during single femtosecond laser pulse (100 fs, 800 nm) irradiation. Using a femtosecond time-resolved imaging technique, we measured the evolution of a laser-induced electronic plasma inside the glass and calculated the electron number density to be on the order of 1019 cm−3.
Scanning probe with an integrated diamond heater element for nanolithography
Authors: Bae Joon Hyung , Ono Takahito , Esashi Masayoshi
Published: January 2003

This letter reports the microfabrication, evaluation, and application of a boron-doped diamond microprobe with an integrated resistive heater element. The diamond heater with a pyramidal tip, which is formed at the end of two diamond beams, can be electrically heated by a flowing current. The high thermal conductivity of the diamond base supporting the heater element allows very quick thermal response of 0.45 μs. A hard-wearing sharp diamond tip formed by the silicon-lost mold technique shows excellent durability in contact operation with a sample. Diamond is well suited to use as a nanolithography tool for modification of a polymer, because polymer is hard to deposit on the tip during scanning due to the chemical inertness of the diamond surface. Demonstration of thermomechanical nanolithography with this heated probe exhibits line patterns with the feature size of 40 nm on a poly(methylmethacrylate) film.
Mode splitting in photoluminescence spectra of a quantum-dot-embedded microcavity
Authors: Hu C. Y. , Zheng H. Z. , Zhang J. D. , Zhang H. , Yang F. H. , Zeng Y. P.
Published: January 2003

A microcavity structure, containing self-assembled InGaAs quantum dots, is studied by angle-resolved photoluminescence (PL) spectroscopy. A doublet with the splitting energy of 0.5–1.5 nm appears when the detection angle is larger than 35°. This doublet is identified as mode splitting (not the Rabi splitting) by polarization measurements. We find that it is the considerable deviation of the cavity-mode frequency from the central frequency of the stop band that makes the TE and TM cavity modes split more discernibly. The inhomogeneous broadening of quantum dots gives the TE and TM cavity modes a chance to show up simultaneously in the PL spectra.
Carbon nanotube networks by chemical vapor deposition
Authors: Cassell Alan M. , McCool Geoff C. , Ng Hou Tee , Koehne Jessica E. , Chen Bin , Li Jun , Han Jie , Meyyappan M.
Published: January 2003

We have demonstrated assembly of two- and three-dimensional networks of single-walled carbon nanotubes (SWNTs) using a microsphere assembly approach. The catalyst microcapsules are made from the solution based impregnation of uniform diameter, porous polystyrene microspheres. Chemical vapor deposition on the microcapsule arrays produces highly interconnected SWNT networks. Varying the microsphere diameter and catalyst solution composition allows varying the pattern spacing, catalyst yield, and network interconnectivity.
Random anisotropy and domain-wall pinning process in the magnetic properties of rapidly quenched Nd60Fe30Al10
Authors: Bracchi A. , Samwer K. , Schneider S. , Löffler J. F.
Published: January 2003

The magnetic properties and microstructure of glassy Nd60Fe30Al10 splat-quenched samples have been investigated by small-angle neutron scattering, Mössbauer spectroscopy, and vibrating sample magnetometry. The presence of chemical inhomogeneities in the amorphous sample is evidenced by small-angle neutron scattering which indicates a complex network of atom clusters with an average diameter of 15 nm. These results are confirmed by the magnetization measurements which show the coexistence of two magnetic phases with different ordering temperatures and indicate phases separation in rapidly quenched Nd60Fe30Al10 samples. Magnetic neutron scattering analysis reveals magnetic correlation volumes in the ferromagnetic phase of about 34 nm in diameter. Random anisotropy and a domain-wall pinning process within the two phases are discussed to explain the magnetic properties of amorphous Nd60Fe30Al10.
Reflections on the meritocracy debate in Britain: a response to Richard Breen and John Goldthorpe1
Authors: Saunders Peter
Published: January 2003

A paper by Breen and Goldthorpe recently claimed to have exposed ‘fatal flaws’ in my work on meritocracy in Britain. This paper responds to their criticisms. The results of their re‐analysis of the NCDS data set are shown to be consistent with my earlier findings and arguments. Furthermore, re‐running some of my earlier models using measures that they favour and a method designed to privilege their position, the results once again demonstrate that, while class origins have some effect on class destinations (in particular, for those born into the middle class), ability and effort exert a much greater effect. Based on these results, the paper identifies three core propositions about meritocracy in Britain on which all parties to this debate should now be able to agree.
Ideal types, stereotypes and classes
Authors: Prandy Kenneth
Published: January 2003

Several recent articles seeking to validate the Goldthorpe class schema have implicitly raised some interesting questions about the relation between analytical techniques and theory. The favoured technique of latent class analysis actually brings this variety of class analysis close to the much older tradition of ideal‐type analysis. In doing so, it helps to illustrate a much wider problem that arises in any conceptualization in terms of classes. The paper seeks to show how, just as in popular discourse around ‘class’, stereotypes are used to simplify and summarize a complex set of related characteristics, so similarly ideal types, as used in class analysis, are an over‐simplification. As elsewhere in social science, the danger of using them is that they serve to protect a theoretical concept from empirical refutation. An alternative ‘research programme’ to that of class analysis is presented, based on measures of stratification arrangements that are more closely linked to the reproducing structure of inequalities.
Rat Kidney Pathology Induced by Chronic Exposure to Fumonisin B1 Includes Rare Variants of Renal Tubule Tumor
Authors: Hard Gordon C. , Howard Paul C. , Kovatch Robert M. , Bucci Thomas J.
Published: January 2003

The carcinogenicity of fumonisin B1 (FB1), a worldwide contaminant of corn produced by Fusaria species of fungi, has been tested recently in 2-year feeding studies in Fischer F344 rats and B6C3F1 mice. Inclusion of FB1 at 50 and 80 ppm in the diet induced liver tumors in female mice, and at 50 and 150 ppm induced renal tumors in male rats (22). In the present study, the kidneys from the rat bioassay were examined to characterize the various histopathological changes associated with renal tumor induction. In all high-dose (150 ppm) and mid-dose (50 ppm) male rats, and to a lesser extent in high-dose (100 ppm) female rats, there was evidence of sustained nephrotoxicity manifested as basophilia, apoptosis, cell regeneration, and simple tubule hyperplasia, affecting proximal convoluted tubules in the deep cortex, extending into the outer region of the outer stripe of outer medulla. A further alteration consisted of sporadic areas of interstitial hyalinization in deep cortex, suggestive of expanded basement membrane, coupled with tubule atrophy. The continued presence of nephrotoxicity throughout chronic exposure to FB1 suggested that renal tumor development may have been an outcome of sustained cell loss and compensatory regeneration. In some cases, preneoplastic tubules or incipient renal tumors presented an immature or fetal form in association with interstitial hyalinization. The renal tubule tumors induced by FB1 were typifi ed by a rare, highly malignant, anaplastic variant capable of growth by infi ltration. Of the 10 renal tubule tumors diagnosed in the mid-dose males, and the 16 in the high-dose males, 8 and 10, respectively, were graded as carcinomas. Anaplastic variants represented 50% of the mid-dose carcinomas and 80% of the high-dose carcinomas. One of the anaplastic carcinomas in a mid-dose male was a true sarcomatoid phenotype not previously recorded in the rodent. Metastatic invasion of the lung occurred with 25% of the mid-dose carcinomas and 50% of the high-dose carcinomas. It was speculated that FB1 may have been influencing the growth characteristics of the induced renal tumors via its inhibitory action on the synthesis of sphingolipids, which in turn, participate in regulating cell contact, growth, and differentiation, or alternatively by affecting cell adhesion molecules.
An In Vivo Method for Using 5-Bromo-2'-deoxyuridine (BrdU) as a Marker of Chemically-Induced Hepatocellular Proliferation in the Japanese Medaka (Oryzias latipes)
Authors: Brennan Linda M. , Boncavage-Hennessey E. Maxine , Wolfe Marilyn J. , Toussaint Margaret W. , Dennis William E. , Rosencrance Alan B. , Gardner Henry S.
Published: January 2003

Japanese medaka fish ( Oryzias latipes) were used to develop an in vivo method to assess hepatocellular proliferation in a nonmammalian model. Proliferative responses were assessed in medaka at 7, 17, 24, and 94 days after a 48-hour exposure to 10 or 100 mg/L diethylnitrosamine (DEN). Subgroups of medaka were exposed to 50 or 75 mg/L of 5-bromo-2'-deoxyuridine (BrdU) in water for 72 hours, sacrificed, and then processed for immunohistochemical staining. Proliferative indices of BrdU-labeled hepatocytes were quantified and compared using both count and area measurements. There was a significant increase ( p < 0.05) in hepatocellular proliferation in the 100 mg/L DEN-treated fish as compared to controls and 10 mg/L DEN-treated fish for the first 3 time points. Hepatocarcinogenicit y was evaluated 26 weeks post-DEN exposure. There was a significant increase ( p < 0.0001) in hepatocellular neoplasms in 100 mg/L DEN-treated fish compared to other fish. Effective BrdU-labeling of S-phase hepatocytes in medaka was achieved by adding BrdU to the aquarium water, and an increase in hepatocellular proliferation using this method was detected 7 days after exposure to a carcinogenic concentration of DEN. Additionally, the new method of area measurement indices of proliferation were as precise as count indices (R2≥0.92).
The Development and Regression of Deciduosarcomas and Other Lesions Caused by Estrogens and Progestins in Rabbits
Authors: Zook Bernard C. , Jänne Olli A. , Abraham Andrew A. , Nash Harold A.
Published: January 2003

A series of experiments were conducted to study the histopathological effects of a combination of exogenous estrogens and progestins in mature rabbits. Estradiol (14—45 μ g/day) and levonorgestrel (30—233 μg/day) were administered by intravaginal or subdermal Silastic devices for various time intervals to study the development of lesions with time and to determine if lesions regressed following withdrawal of the steroids. The origin of splenic decidual tumors (primary or metastasis from the uterus) was determined by administering the same steroid combination to castrated male rabbits. It was determined that uterine decidualization is present after 7 days of steroid treatment and that neoplasms of decidual cells may appear in the uterus after only 30 days of steroid administration. Decidual changes were observed frequently in uterine arteries, often concurrent with infarct-like areas of necrosis of the uterine wall. Withdrawal of contraceptive steroids for 14—120 days after 60 days' administration resulted in atrophy and disappearance of decidual cells and decidual tumors. Decidual neoplasms developed in the spleen of all castrated male rabbits given subdermal steroids, demonstrating that these tumors can arise as primary neoplasms of the spleen. The foregoing lesions appear to be peculiar to the rabbit and, together with previous data, suggest the rabbit to be a poor model for evaluating the effects of contraceptive steroids in other species.