bluuebunny/crossref_metadata_2025 huggingface dataset:
Bragg transmission phase plates for the production of circularly polarized x rays
Authors: Lang J. C. , Srajer George
Published: July 2002

The x-ray optics for a thin-crystal Si (400) Bragg transmission phase plate have been constructed for the production of 5 to 12 keV circularly polarized x rays. Using multiple beam diffraction from a GaAs crystal, a direct measurement of the degree of circular polarization as a function of off-Bragg position was made. These measurements indicated nearly complete circular polarization (‖Pc‖≥0.95) and full helicity reversal on opposite sides of the rocking curve.
Three-dimensional photothermal radiometry for the determination of the thermal diffusivity of solids
Authors: Fabbri L. , Fenici P.
Published: July 2002

A radiometric method for measuring the thermal diffusivity of solids is presented. This method is based on the dynamic detection of an ac-temperature three-dimensional-distribution induced by the application of a localized and periodic optical excitation. Specifically the signal phase profile of the ac-temperature was detected along the sample surface. This procedure gives direct information on the material thermal diffusion length and, therefore, on its thermal diffusivity. Measurements on different bulk materials (i.e., Poco Graphite AXM-5Q, Armco Iron, Al2O3, and ZrO2) were performed and compared to the data obtained by the laser-flash method on the same set of samples. A further investigation on a very thin Si specimen (292 μm) showed the effect of thermal wave reflections at the sample backsurface on the measured thermal diffusivity. All the data obtained with this technique agreed to within 5% with those obtained using the laser-flash method and with the values taken from the literature for similar materials. Values in the range (8×10−7–0.9×10−4 m2/s) were obtained.
A three-axis micropositioner for ultrahigh vacuum use based on the inertial slider principle
Authors: Erlandsson R. , Olsson L.
Published: July 2002

We present a three-dimensional micropositioner using the inertial slider principle which is used to position an optical fiber in an atomic force microscope. It uses only two moving parts as the sideways and vertical motions are realized by either moving a cylinder along its axis or rotating it around its axis and translating the rotation into an approximately vertical motion. The device operates reliably in a baked ultrahigh vacuum system, allows positioning with sub-μm accuracy, and has a forward range of 11.3 mm, a sideways range of 5 mm, and a vertical range of approximately 5 mm. The measured speeds without extra load fall in the range between 1.6 and 3.3 mm/min, in good agreement with the amplitude and curve shape of the applied drive signal. The minimal step size allowing consistent motion is below 25 nm.
Tunable Q-band resonator for low temperature electron paramagnetic resonance/electron nuclear double resonance measurements
Authors: Sienkiewicz Andrzej , Smith Brian G. , Veselov Andrei , Scholes Charles P.
Published: July 2002

We present a tunable Q-band cavity for performing electron paramagnetic resonance and electron nuclear double resonance experiments at cryogenic temperatures. The resonator is a TE011 brass cavity with radio frequency and magnetic field modulation (100 kHz) posts inside the microwave resonator. These posts are located close to the central sample position and thereby enhance the effective modulation and radio frequency fields. The crucial novel design aspect is the provision for continuously tuning the cavity resonant frequency, even under pumped liquid helium conditions over a broad frequency range (≳2.0 GHz). Such a range will compensate for cooling-induced cavity contraction, presence of cryogenic in the cavity, and insertion of high dielectric frozen aqueous samples. The tuning range is indispensable for use with present commercial Q-band bridges whose low noise Gunn diode oscillators provide a small ∼50 MHz tuning range.
An antenna electrometer system for atmospheric electrical measurements
Authors: Harrison R. G.
Published: July 2002

A ±300 V dynamic range, battery-powered, and inexpensive electrometer system is described that has been used with passive horizontal wire antennas to measure the atmospheric electric potential at 1 and 2 m above the earth’s surface. Input currents drawn by the electrometer circuit are of order femtoamperes, and a guard drive output is provided to minimize leakage. The implementation of the passive wire antenna is described, using rigorous insulation and signal guarding, together with results obtained in fair weather conditions. Linearity of the portable electrometer over the input range is demonstrated to be comparable with a commercial laboratory electrometer, and the antenna potential can be determined to within a few volts in the fair weather electric field. Several such portable systems can be cheaply combined, sharing a common high tension supply, to measure the profile of the atmospheric electrical potential above the surface, at a remote site. Damage to the electrometer sustained in electrical storms can be cheaply repaired.
A two-dimensional capacitive position transducer with rotation output
Authors: Barker M. J. , Colclough M. S.
Published: July 2002

A two-dimensional capacitive position transducer is described which has x, y and angular outputs and a fixed zero. The prototype has submicron and arcsecond sensitivity, 10 mm range, and a nonlinearity of 0.1%, which is easily corrected to 0.01%. The transducer employs a synthetic dual full-bridge technique and is compact, easily fabricated, and suitable for use in cryogenic environments.
Advances in the use of laser-flash techniques for thermal diffusivity measurement
Authors: Sheindlin M. , Halton D. , Musella M. , Ronchi C.
Published: July 2002

A laser-flash apparatus has been constructed for the measurement of thermal diffusivity. The apparatus is specially designed to operate under conditions imposed by the requirement to measure the thermal diffusivity of highly radioactive reactor-irradiated nuclear fuels. Among the various requirements, relating to the measurement of irradiated samples, were the ability to characterize sample platelets of irregular contours and different sizes, to make these measurements in a sufficiently short experimental time frame, and to maintain good experimental accuracy while keeping pulse laser energies at low levels. This article shows that improvement of key components above the current standards—in particular of the laser-beam homogeneity and of the transient temperature detector—makes it possible to create more flexible and controllable experimental conditions, enabling reliable measurements to be carried out in a broad range of modes. The method used to analyze the collected temperature pulse data is based on a least-squares fitting procedure of integrals of the temperature diffusion equation with realistic boundary conditions. Numerical analysis techniques have been employed to interpret the experimental data, measured in different modes, as imposed by the above-mentioned conditions. The testing and characterization of the machine using POCO graphite and UO2 are presented.
Michelson fiberoptic accelerometer
Authors: Chen Caihe , Ding Guilan , Zhang Delong , Cui Yuming , Li Shuqing
Published: July 2002

An all fiberoptic accelerometer, based on Michelson interferometry, has been developed theoretically and experimentally. It makes use of an oscillating mass suspended by a transduction fiber in a mass support. The interferometric configuration is formed by a 3 dB single-mode fiberoptic beam splitter, on two distal endfaces of which high-reflectance aluminum films were directly coated and served as reflecting mirrors. An ac phase tracking modulation–demodulation scheme has been employed to process the signal. The performance of a prototype of the accelerometer has been investigated, and the results indicated that the measured frequency spectrum is qualitatively consistent with the theoretical prediction, and the detected output wave forms are also in good agreement with that of the applied simulation signals. In addition, the constraint on the lateral movement of the acceleration-sensitive mass with two specific diaphragms has been experimentally verified, nevertheless, it needs to be optimized further.
Characterization of individual interface traps with charge pumping
Authors: Saks N. S. , Groeseneken G. , DeWolf I.
Published: July 2002

Charge pumping (CP) of individual interface traps has been studied in small field effect transistors. The amplitude of the CP current is found to be quantized in units of qf, where f is the frequency of the applied CP wave form, and each unit represents the response of a single trap. The time dependence for emission of a trapped electron from a single interface trap has also been measured with CP. The emission probability is found to increase exponentially with time, consistent with Shockley–Read–Hall statistics. We also compare the relative merits of random telegraph noise experiments and CP for characterizing individual interface traps.
InP/GaAsSb/InP and InP/GaAsSb/InGaAsP double heterojunction bipolar transistors with a carbon-doped base grown by organometallic chemical vapor deposition
Authors: Bhat R. , Hong W-P. , Caneau C. , Koza M. A. , Nguyen C-K. , Goswami S.
Published: July 2002

InP/GaAsSb double heterojunction bipolar transistors (DHBTs) may be an attractive alternative to InP/InGaAs DHBTs, since estimates of the band alignment indicate that it is ideal for fabricating n-p-n DHBTs. We have demonstrated the first organometallic chemical vapor deposition grown InP/GaAsSb DHBTs, with carbon-doped bases having an ft and fmax of 30 and 45 GHz, respectively.
Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy
Authors: Park Y. , Choong V. , Gao Y. , Hsieh B. R. , Tang C. W.
Published: July 2002

We used ultraviolet and x-ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.
Enhanced quantum well infrared photodetector with novel multiple quantum well grating structure
Authors: Schimert T. R. , Barnes S. L. , Brouns A. J. , Case F. C. , Mitra P. , Claiborne L. T.
Published: July 2002

An enhanced quantum well infrared photodetector (EQWIP) with lower dark current and improved performance relative to a conventional QWIP is described. Dark current reduction and external quantum efficiency improvements are achieved by novel structural enhancements that involve patterning the GaAs/AlGaAs multiple quantum well into a diffraction grating and reducing the number of wells. A 64×64 long wave infrared EQWIP array with 60 μm pixel pitch and peak D*∼8×1010 cm Hz1/2/W was demonstrated at 77 K. The low bias current permits hybridization to conventional readout circuits. Test results for pixel pitches down to 30 μm show that high EQWIP performance is achievable in the small pixels required for large focal plane array formats.
Spiral growth and threading dislocations for molecular beam epitaxy of PbTe on BaF2 (111) studied by scanning tunneling microscopy
Authors: Springholz G. , Ueta A. Y. , Frank N. , Bauer G.
Published: July 2002

Molecular beam epitaxy of PbTe on BaF2 (111) is studied using UHV–scanning tunneling microscopy and atomic force microscopy. It is shown that PbTe growth is totally dominated by growth spirals formed around threading dislocations (TD) that originate from the growth on the 4.2% lattice-mismatched substrate. Due to dislocation annihilation, the TD density rapidly decreases with layer thickness, which results in a dramatic increase of the electron mobilities in the layers.
Large magnetic-field-induced strains in Ni2MnGa single crystals
Authors: Ullakko K. , Huang J. K. , Kantner C. , O’Handley R. C. , Kokorin V. V.
Published: July 2002

Strains of nearly 0.2% have been induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K. These stains are associated with the superelastic motion of twin boundaries in the martensitic phase that is stable below about 274 K.
Time-dependent and trap-related current conduction mechanism in ferroelectric Pb(ZrxTi1−x)O3 films
Authors: Chen Hong-Ming , Lan Jian-Ming , Chen Jia-Lin , Ya-min Lee Joseph
Published: July 2002

The correlation between trap density and leakage current of Pb(Zr53Ti47)O3 (PZT) capacitors is studied by examining the current–time (I–t) and the current–voltage (I–V) characteristics. The increase of leakage current after dc electrical field stress is correlated with the number of charges trapped inside the films. The spatial density distribution of trapped charges is calculated by analyzing the decay of discharging current after the application of dc stress. The discharging current is well fitted by a 1/t relationship where t is discharging time. This behavior can be explained by using the tunneling front model. A discharging process is proposed based on this consideration.
Hydrogen desorption in SiGe films: A diffusion limited process
Authors: Vizoso J. , Martín F. , Suñé J. , Nafría M.
Published: July 2002

A model to explain the hydrogen desorption kinetics in SiGe alloys is presented. This is an extension of a previous desorption model of hydrogen from Si, that considers the presence of three dimer types in the surface in which hydrogen atoms tend to pair before the desorption reaction. Surface diffusion is included in the model. The comparison with experimental results shows that desorption is a diffusion limited process.
Mechanism for thermal quenching of luminescence in SiGe/Si structures grown by molecular beam epitaxy: Role of nonradiative defects
Authors: Buyanova I. A. , Chen W. M. , Pozina G. , Monemar B. , Ni W.-X. , Hansson G. V.
Published: July 2002

Thermal quenching of photoluminescence from SiGe/Si quantum wells (QWs) grown by low-temperature molecular beam epitaxy is shown to be significantly improved by postgrowth thermal annealing. The dominant mechanism responsible for this improvement is shown to be a reduction of grown-in nonradiative defects, such as vacancy-related complexes. Postgrowth hydrogenation is demonstrated to be less effective as compared to thermal annealing in removing the nonradiative defects. Selective optical excitation has been used to determine the relative contributions of nonradiative recombination channels present in the SiGe QWs and the Si barriers.
Ultraviolet and violet GaN light emitting diodes on silicon
Authors: Guha Supratik , Bojarczuk Nestor A.
Published: July 2002

We report the fabrication and characterization of GaN-based double heterostructure light emitting diodes grown by molecular beam epitaxy on Si(111) substrates. Light emitting diode operation is achieved by using the conducting Si(111) substrate as a backside n contact and a standard transparent Ni/Au p contact. We observe electroluminescence peaked in the ultraviolet ∼360 nm, with a full width at half maximum of ∼17 nm and in the violet at ∼420 nm. Electron microscopy studies indicate a high density of threading and planar defects. Consequences of these are discussed.
Schottky barrier photodetectors based on AlGaN
Authors: Osinsky A. , Gangopadhyay S. , Lim B. W. , Anwar M. Z. , Khan M. A. , Kuksenkov D. V. , Temkin H.
Published: July 2002

We report solar-blind AlxGa1−xN photovoltaic detectors with cutoff wavelengths as short as 290 nm. Mesa geometry devices of different active areas are fabricated and characterized for spectral responsitivity, speed, and noise performance. The responsivity of the devices near the cutoff wavelength is 0.07 A/W. The detector noise is found to be 1/f limited, with a noise equivalent power of 6.6×10−9 W over the total response bandwidth of 100 kHz.
High definition aperture probes for near-field optical microscopy fabricated by focused ion beam milling
Authors: Veerman J. A. , Otter A. M. , Kuipers L. , van Hulst N. F.
Published: July 2002

We have improved the optical characteristics of aluminum-coated fiber probes used in near-field scanning optical microscopy by milling with a focused ion beam. This treatment produces a flat-end face free of aluminum grains, containing a well-defined circularly-symmetric aperture with controllable diameter down to 20 nm. The polarization behavior of the tips is circularly symmetric with a polarization ratio exceeding 1:100. The improved imaging characteristics are demonstrated by measuring single molecule fluorescence. Count rates increase more than one order of magnitude over unmodified probes, and the molecule images map a spatial electric field distribution of the aperture in agreement with calculations.
Raman imaging of patterned silicon using a solid immersion lens
Authors: Poweleit C. D. , Gunther A. , Goodnick S. , Menéndez José
Published: July 2002

We show an enhanced spatial resolution using a solid immersion lens by directly imaging the Raman scattered light from silicon masked by periodic metal lines. A glass hemisphere solid immersion lens with an index of refraction n=1.868 at 488 nm is used in conjunction with a 100×0.8 numerical aperture objective to obtain the enhanced spatial resolution. The increased numerical aperture is demonstrated by a direct line scan over the periodic metal lines. Compared with near-field optical microscopy, the solid immersion lens technique overcomes the difficulty of limited excitation power obtainable with tapered fibers, while providing excellent spatial resolution which in principle could be extended to the 0.1 μm range.
Giant Rabi splitting in a microcavity containing distributed quantum wells
Authors: Bloch J. , Freixanet T. , Marzin J. Y. , Thierry-Mieg V. , Planel R.
Published: July 2002

We present a microcavity design where the overlap between quantum well excitons and the electromagnetic field is much larger than in previously reported structures: quantum wells are located not only in the central cavity layer but also in the Bragg mirrors. We obtain a Rabi splitting of 19 meV. Since the Rabi splitting is larger than the exciton binding energy, exciton excited states have to be taken into account in the reflectivity calculation to describe the spectral shape of the two polariton lines.
Polytypoid structures in annealed In2O3–ZnO films
Authors: Yan Y. , Pennycook S. J. , Dai J. , Chang R. P. H. , Wang A. , Marks T. J.
Published: July 2002

Atomic-resolution Z-contrast images demonstrate unambiguously that the annealed, metalorganic chemical vapor deposition derived transparent In2O3–ZnO films have a polytypoid microstructure, consisting of ZnO slabs of variable width separated by single In–O octahedral layers. These In–O layers induce a polarity inversion in the two adjacent ZnO layers, which is reversed again by a mirror domain boundary inside each ZnO slab.
Examination of deuterium transport through device structures
Authors: Chen P. J. , Wallace R. M.
Published: July 2002

We use secondary ion mass spectrometry to characterize the hydrogen/deuterium distribution and concentration on 0.18 μm “metal” oxide silicon test structures subjected to deuterium anneals. We examine the temperature dependence and the influence of doping on the transport of deuterium to the gate oxide interfaces resulting in interface passivation. We find that undoped polycrystalline silicon appears to be an efficient barrier for deuterium transport at typical postmetallization sintering temperatures.
Transient grating measurements of picosecond acoustic pulses in metal films
Authors: Crimmins Timothy F. , Maznev A. A. , Nelson Keith A.
Published: July 2002

A transient grating technique is used to detect picosecond acoustic pulses in supported metal films. Crossed femtosecond laser pulses generate acoustic responses with longitudinal components propagating normal to the film plane and surface acoustic wave components propagating in the film plane. Surface “ripple” associated with both components is detected through the diffraction of a probe beam. The measurements yield enhanced information content for characterization of film thickness and mechanical properties.
Red light emission by photoluminescence and electroluminescence from Eu-doped GaN
Authors: Heikenfeld J. , Garter M. , Lee D. S. , Birkhahn R. , Steckl A. J.
Published: July 2002

Visible light emission has been obtained at room temperature by photoluminescence (PL) and electroluminescence (EL) from Eu-doped GaN thin films. The GaN was grown by molecular beam epitaxy on Si substrates using solid sources (for Ga and Eu) and a plasma source for N2. X-ray diffraction shows the GaN:Eu to be a wurtzitic single crystal film. Above GaN band gap photoexcitation with a He–Cd laser at 325 nm resulted in strong red emission. Observed Eu3+ PL transitions consist of a dominant narrow red line at 621 nm and several weaker emission lines were found within the green through red (543 to 663 nm) range. Below band gap PL by Ar laser pumping at 488 nm also resulted in red emission, but with an order of magnitude lower intensity. EL was obtained through use of transparent indium–tin–oxide contacts to the GaN:Eu film. Intense red emission is observed in EL operation, with a spectrum similar to that seen in PL. The dominant red line observed in PL and EL has been identified as the Eu3+ 4f shell transition from the D05 to the F27 state.
Eu
3+
emission in an anisotropic photonic band gap environment
Authors: Romanov S. G. , Fokin A. V. , De La Rue R. M.
Published: July 2002

Eu 3+ ions have been incorporated into the silica skeleton of synthetic opal. The effect of the anisotropic photonic band gap structure upon the emission characteristics has been studied in the case where the emission bandwidth is narrower than the stop-band. Either suppression or enhancement of the spontaneous emission at the wavelength of the radiative transition has been observed, depending on the relative position of the emission band and the stop-band.
High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy
Authors: Ng H. M. , Moustakas T. D. , Chu S. N. G.
Published: July 2002

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free.
An alternative way of thinking about electron transfer in proteins: Proton assisted electron transfer between the primary and the secondary quinones in photosynthetic reaction centers
Authors: Peluso Andrea , Di Donato Mariangela , Saracino Gloria Anna Ada
Published: July 2002

The mechanism of electron transfer between the primary and the secondary quinone of the photosynthetic reaction center of the Rhodobacter Sphaeroides has been studied theoretically, on the basis of the x-ray structure obtained under illumination. It is shown that in such a supramolecular aggregate where the different molecular blocks form a chain of H bonds, a mobile electron injected on one end of the chain can be carried to the opposite end by switching the positions of the H-bonded hydrogens, in a Bjerrium-type mechanism in which not only protons but also hydrogen atoms are involved. Energy estimates predict that the various steps of the mechanism are all slightly exergonic, so that the proposed mechanism appears to be at least plausible and worthy of further experimental investigations.
Using the speed of sound for in situ calibration of mass flow controllers
Authors: Wajid Abdul
Published: July 2002

Mass flow controllers (MFCs) are widely used for precise delivery of process gases, particularly in semiconductor industry. Often a specific MFC is factory calibrated with an inert gas. In order to estimate the actual delivery of process gases, which may be corrosive and hazardous, a multiplier called the gas calibration factor is used. Moreover, a single MFC may be used to deliver multiple gases and its thermal characteristics do change over time. We describe here a very simple method of calibrating thermal mass flow controllers, in situ, with actual process gases, utilizing the speed of sound. We have used a commercially available instrument that uses acoustic resonance technique for determining the composition of a binary gas mixture.
Anharmonic treatment of the lowest-energy conformers of glycine: A theoretical study
Authors: Bludský Ota , Chocholoušová Jana , Vacek Jaroslav , Huisken Friedrich , Hobza Pavel
Published: July 2002

The structure and energetics of the four lowest-energy conformers of glycine were determined at the MP2/aug-cc-pVDZ level of theory. The optimized structural parameters for these conformers agree with previous theoretical results obtained by highly correlated ab initio methods and with available experimental data. The only structure with planar heavy atom arrangement is conformer I (global minimum), the other conformers have nonplanar heavy atom arrangements. In accordance with temperature dependence studies of the vibrational spectra in various rare gas environments, conformers III and IV have small interconversion barriers to conformer I (940 and 740 cm−1). Our calculations have shown that full-dimensional anharmonic treatment is required for an accurate description of the vibrational modes in various glycine conformers. The most pronounced effect has been observed for conformer II with the intramolecular O–H⋅⋅⋅N bond. The theoretical results obtained at the MP2/aug-cc-pVDZ level reproduce quantitatively the argon matrix experiments. The calculation uses the quartic force field approximation in the framework of second-order perturbation theory. An estimate of the higher-order correction is also given.
A method whereby Onsager coefficients may be evaluated
Authors: Elliott Janet A. W. , Elmoazzen Heidi Y. , McGann Locksley E.
Published: July 2002

Much of our understanding of transport phenomena is based on linear irreversible thermodynamics formalized most notably by Onsager in 1931. As with any theory based on an a priori linear assumption, coefficients appear that may not be calculated from within the theory. In the case of transport these coefficients are usually found empirically. To correctly apply a linear theory the criteria for linearity should be evaluated. In this paper, a general theory of nonequilibrium thermodynamics, statistical rate theory, is used to outline a method whereby Onsager coefficients may be evaluated for a given circumstance. For a particular osmotic transport example, expressions for the Onsager coefficients are found which are shown to satisfy Onsager’s reciprocity hypothesis. Most importantly, criteria for linearity are explicitly formulated.
Inviscid damping of asymmetries on a two-dimensional vortex
Authors: Schecter D. A. , Dubin D. H. E. , Cass A. C. , Driscoll C. F. , Lansky I. M. , O’Neil T. M.
Published: July 2002

The inviscid damping of an asymmetric perturbation on a two-dimensional circular vortex is examined theoretically, and with an electron plasma experiment. In the experiment, an elliptical perturbation is created by an external impulse. After the impulse, the ellipticity (quadrupole moment) of the vortex exhibits an early stage of exponential decay. The measured decay rate is in good agreement with theory, in which the perturbation is governed by the linearized Euler equations. Often, the exponential decay of ellipticity is slow compared to a vortex rotation period, due to the excitation of a quasimode. A quasimode is a vorticity perturbation that behaves like a single azimuthally propagating wave, which is weakly damped by a resonant interaction with corotating fluid. Analytically, the quasimode appears as a wave packet of undamped continuum modes, with a sharply peaked frequency spectrum, and it decays through interference as the modes disperse. When the exponential decay rate of ellipticity is comparable to the vortex rotation frequency, the vorticity perturbation does not resemble a quasimode; rather, it is rapidly dominated by spiral filaments. Over longer times, linear theory predicts algebraic decay of ellipticity; however, nonlinear oscillations of ellipticity emerge in the experiment before a transition to algebraic decay would occur.
Laser vector measurement technique for the determination and compensation of volumetric positioning errors. Part I: Basic theory
Authors: Wang Charles
Published: July 2002

A new laser vector method for the measurement of the volumetric positioning errors of a computer numerically controlled machine or a precision instrument is described here. Compared with conventional laser interferometer measurement, the laser vector method measures the vector errors, namely, the displacement error, vertical straightness error, and horizontal straightness error, rather than the displacement error only. The key to the laser vector method is that the measurement direction or the laser beam direction is not parallel to the displacement direction. With four setups, all three displacement, six straightness, and three squareness errors can be determined. Once the volumetric positioning errors are measured, they can be used to compensate for the repeatable positioning errors of the machine or instrument. The basic concept, theory, and measurement errors are described. Experimental verification of the vector method is in the Part II of this article.
On the condensed Fukui function
Authors: Fuentealba P. , Pérez P. , Contreras R.
Published: July 2002

A critical comparison among recently proposed methods for evaluating the condensed Fukui function neglecting relaxation effects is presented. The sign of the condensed Fukui function is discussed and arguments for a positive definite condensed Fukui function are given. Our numerical calculations in two series of molecules show that: (i) the condensed Fukui function can give, in general, valuable information about the site selectivity in chemical reactions and systematization in a family of molecules. In particular, it has been shown that the selectivity towards protonation in anilines and derivatives molecules can be correctly assessed by the electrophilic Fukui function described in this paper. Within this approach non-negative values for the condensed Fukui function are obtained for the relevant protonation sites in these polyfunctional systems; and (ii) the solvent effects on the condensed Fukui function are negligible, confirming a recently presented theoretical prediction.
Electroluminescence of Ge/Si self-assembled quantum dots grown by chemical vapor deposition
Authors: Brunhes T. , Boucaud P. , Sauvage S. , Aniel F. , Lourtioz J.-M. , Hernandez C. , Campidelli Y. , Kermarrec O. , Bensahel D. , Faini G. , Sagnes I.
Published: July 2002

We have fabricated light-emitting diodes on Si operating in the near-infrared. The active region of the p–i–n diodes consists of Ge/Si self-assembled quantum dots. The Ge islands were grown in an industrial 200 mm single-wafer chemical vapor deposition reactor. The photoluminescence and the electroluminescence of the islands are resonant in the spectral range around 1.4–1.5 μm wavelength. The electroluminescence is observed up to room temperature.
Sequencing of folding events in Go-type proteins
Authors: Hoang Trinh Xuan , Cieplak Marek
Published: July 2002

We have studied folding mechanisms of three small globular proteins: crambin, chymotrypsin inhibitor 2 (CI2), and the fyn Src Homology 3 domain (SH3) which are modeled by a Go-type Hamiltonian with the Lennard-Jones interactions. It is shown that folding is dominated by a well-defined sequencing of events as determined by establishment of particular contacts. The order of events depends primarily on the geometry of the native state. Variations in temperature, coupling strengths, and viscosity affect the sequencing scenarios to a rather small extent. The sequencing is strongly correlated with the distance of the contacting amino acids along the sequence. Thus α helices get established first. Crambin is found to behave like a single-route folder, whereas in CI2 and SH3 the folding trajectories are more diversified. The folding scenarios for CI2 and SH3 are consistent with experimental studies of their transition states.
Setup for observing living cells using a commercial atomic force microscope
Authors: Kasas Sandor , Wang Xin , Hirling Harald , Catsicas Stefan , Haeberli Christian , Dietler Giovanni , Thomson Neil
Published: July 2002

In this note, we present a setup which can be adapted to commercially available atomic force microscopes (Nanoscope II and III) to permit the observation of living cells in nearly physiological conditions. The setup permits one to heat the sample up to 40 °C, to exchange (without the use of the “O” ring) the imaging buffer while measuring, and to distinguish fluorescently labeled cell subpopulations.
Theoretical formulation of the vacuum ultraviolet production efficiency in a plasma display panel
Authors: Suzuki Keizo , Kawanami Yoshimi , Ho Shirun , Uemura Norihiro , Yajima Yusuke , Kouchi Noriyuki , Hatano Yoshihiko
Published: July 2002

The theoretical formulation given in this article allows the vacuum ultraviolet (VUV) production efficiency to be calculated from the electron temperature of the plasma and the gas parameters including gas mixing ratio, excitation energies, and excitation cross sections using the separately determined conversion efficiency of the plasma input power into the electron heating power. The VUV production efficiencies calculated for (Ne+Xe) mixture (neon (Ne) and xenon (Xe) mixture) discharge gases using the formulation show that the efficiency can be increased by decreasing the electron temperature and by increasing the amount of Xe in the gas mixture. A method for determining the electron temperature of the plasma display panel (PDP) plasma from emission intensity measurements was also given, and was used to show that the electron temperature in the ordinary PDP plasma is 3 eV.
Interfacial tension of electrolyte solutions
Authors: Levin Yan
Published: July 2002

A theory is presented to account for the increase in surface tension of water in the presence of electrolyte. Unlike the original “grand-canonical” calculation of Onsager and Samaras, which relied on the Gibbs adsorption isotherm and led to a result which could only be expressed as an infinite series, our approach is “canonical” and produces an analytic formula for the excess surface tension. For small concentrations of electrolyte, our result reduces to the Onsager–Samaras limiting law.
Structural and electronic properties of CdS and CdSe clusters
Authors: Troparevsky M. Claudia , Chelikowsky James R.
Published: July 2002

The structural and electronic properties of CdnXn (X=S,Se; n=2–8) are calculated using the finite-difference pseudopotential method in real space. The computed ground state atomic configurations for the CdnXn clusters deviate substantially from those for Si2n and GanAsn. The increased ionic character of the bonding in CdnXn clusters accounts for the observed structural variations. We calculate the highest occupied molecular orbital–lowest unoccupied molecular orbital gaps, binding energies, and polarizabilities of the clusters. We find a strong correlation between the binding energies and gaps. The computed polarizabilities for the (CdS, CdSe) clusters follow the same trend with size as for Si and GaAs. The polarizabilities rapidly approach the bulk limit from above for clusters with more than ∼10 atoms.
Amino acid classes and the protein folding problem
Authors: Cieplak Marek , Holter Neal S. , Maritan Amos , Banavar Jayanth R.
Published: July 2002

We present and implement a distance-based clustering of amino acids within the framework of a statistically derived interaction matrix and show that the resulting groups faithfully reproduce, for well-designed sequences, thermodynamic stability in and kinetic accessibility to the native state. A simple interpretation of the groups is obtained by eigenanalysis of the interaction matrix.
Microscopic theory of protein folding rates. II. Local reaction coordinates and chain dynamics
Authors: Portman John J. , Takada Shoji , Wolynes Peter G.
Published: July 2002

The motions involved in barrier crossing for protein folding are investigated in terms of the chain dynamics of the polymer backbone, completing the microscopic description of protein folding presented in the preceding paper. Local reaction coordinates are identified as collective growth modes of the unstable fluctuations about the saddle points in the free energy surface. The description of the chain dynamics incorporates internal friction (independent of the solvent viscosity) arising from the elementary isomerization of the backbone dihedral angles. We find that the folding rate depends linearly on the solvent friction for high viscosity, but saturates at low viscosity because of internal friction. For λ-repressor, the calculated folding rate prefactor, along with the free energy barrier from the variational theory, gives a folding rate that agrees well with the experimentally determined rate under highly stabilizing conditions, but the theory predicts too large a folding rate at the transition midpoint. This discrepancy obtained using a fairly complete quantitative theory inspires a new set of questions about chain dynamics, specifically detailed motions in individual contact formation.
Nöther charges, Brown–York quasilocal energy, and related topics
Authors: Fatibene L. , Ferraris M. , Francaviglia M. , Raiteri M.
Published: July 2002

The Lagrangian proposed by York et al. and the covariant first-order Lagrangian for general relativity are reviewed. They both deal with the (vacuum) gravitational field on a reference background and were conjectured to be equivalent. The two corresponding actions are compared and we show that the first one can in fact be obtained from the latter under suitable hypotheses. A conditioned correspondence among Nöther conserved quantities of covariant first-order Lagrangian, Brown–York quasilocal energy and the standard ADM Hamiltonian is also established.
High resolution eddy current microscopy
Authors: Lantz M. A. , Jarvis S. P. , Tokumoto H.
Published: July 2002

We describe a sensitive scanning force microscope based technique for measuring local variations in resistivity by monitoring changes in the eddy current induced damping of a cantilever with a magnetic tip oscillating above a conducting sample. To achieve a high sensitivity, we used a cantilever with an FeNdBLa particle mounted on the tip. Resistivity measurements are demonstrated on a silicon test structure with a staircase doping profile. Regions with resistivities of 0.0013, 0.0041, and 0.022 Ω cm are clearly resolved with a lateral resolution of approximately 180 nm. For this range of resistivities, the eddy current induced damping is found to depend linearly on the sample resistivity.
The Jahn–Teller and related effects in the cyclopentadienyl radical. I. The ab initio calculation of spectroscopically observable parameters
Authors: Applegate Brian E. , Miller Terry A. , Barckholtz Timothy A.
Published: July 2002

Ab initio calculations are performed for the X̃ 2E1″ and Ã 2A2″ states of the cyclopentadienyl radical. An important goal of these calculations is to guide the analysis of the experimentally observed Ã 2A2″– X̃ 2E1″ electronic spectrum. Vibrational frequencies for both the X̃ and Ã state are reported. Large changes in frequency between the states for out-of-plane vibrations are found, leading to the expectation that overtones of these modes will appear strongly in the spectrum. Additionally, spectroscopically obtainable parameters describing the Jahn–Teller effect are calculated for the X̃ state. Using all this information the X̃−Ã electronic spectrum is predicted for both C5H5 and C5D5.
Bistability in cell signaling: How to make continuous processes discontinuous, and reversible processes irreversible
Authors: Ferrell James E. , Xiong Wen
Published: July 2002

Xenopus oocyte maturation is an example of an all-or-none, irreversible cell fate induction process. In response to a submaximal concentration of the steroid hormone progesterone, a given oocyte may either mature or not mature, but it can exist in intermediate states only transiently. Moreover, once an oocyte has matured, it will remain arrested in the mature state even after the progesterone is removed. It has been hypothesized that the all-or-none character of oocyte maturation, and some aspects of the irreversibility of maturation, arise out of the bistability of the signal transduction system that triggers maturation. The bistability, in turn, is hypothesized to arise from the way the signal transducers are organized into a signaling circuit that includes positive feedback (which makes it so that the system cannot rest in intermediate states) and ultrasensitivity (which filters small stimuli out of the feedback loop, allowing the system to have a stable off-state). Here we review two simple graphical methods that are commonly used to analyze bistable systems, discuss the experimental evidence for bistability in oocyte maturation, and suggest that bistability may be a common means of producing all-or-none responses and a type of biochemical memory.
Diffusion-controlled reactions: A revisit of Noyes’ theory
Authors: Van Beijeren H. , Dong W. , Bocquet L.
Published: July 2002

Noyes’ theory of diffusion-controlled reactions is revisited in great details. First, it is shown that this theory provides an interesting alternative approach to perform molecular dynamics simulations for diffusion-controlled reactions. With this approach, reaction rate can be determined from simulations of nonreactive equilibrium systems. No annihilation procedure is needed to simulate the reaction event. Provided that encounters with different reactants are strictly uncorrelated, the reaction rate can be determined more directly and accurately than by the methods that compute the survival probability. We describe in detail the method for accurately determining the key quantity in Noyes’ theory, i.e., the first recollision probability, from molecular dynamics simulations. It will also be shown that arguments similar to those in Noyes’ theory allow us to establish an exact relation (under the same assumptions of absence of correlations) between the distribution function of a reacting system at the encounter distance and that of a nonreactive equilibrium system. This relation can be used to fix the boundary condition at the reaction distance in the approaches based on a diffusion equation. New insights have been gained into the usefulness of the recollision probability. The recollision probability also provides a very useful tool for characterizing quantitatively some dynamic features of the cage effect for reactions in dense liquids. Finally, the method presented here may also be used to calculate reaction rates for diffusion-controlled reactions in systems where the dynamics cannot be described by a diffusion equation.
Thermoelectric and transport properties of CeBiPt and LaBiPt
Authors: Jung M. H. , Yoshino T. , Kawasaki S. , Pietrus T. , Bando Y. , Suemitsu T. , Sera M. , Takabatake T.
Published: July 2002

The electrical resistivity ρ, Hall coefficient RH, thermoelectric power S, and thermal conductivity κ have been measured on an antiferromagnet CeBiPt (TN=1 K) and the nonmagnetic analogue LaBiPt, which crystallize in the cubic MgAgAs-type structure. The large values |RH|∼1 cm3/C and ρ∼1.2 mΩ cm at 300 K indicate that both compounds are low-carrier concentration semimetals. The strong temperature dependences of ρ, S, and RH for CeBiPt contrast with the weak temperature dependences for LaBiPt. For CeBiPt, S(T) has a positive maximum of 120 μV/K at 100 K. However, the sign of RH(T) changes from positive to negative with increasing temperature at 170 K, above which S(T) of CeBiPt becomes smaller than LaBiPt. These observations suggest strong dependence of mobility for hole and electron carriers in CeBiPt. In this system, κ(T) is dominated by the phonon contribution, which is consistent with a small carrier concentration. Furthermore, the combination of S, ρ, and κ allows us to estimate the figure of merit for thermoelectric application, Z=1.7×10−4 K−1 at 75 K for CeBiPt.
Does fragility depend on pressure? A dynamic light scattering study of a fragile glass-former
Authors: Paluch Marian , Gapinski Jacek , Patkowski Adam , Fischer Erhard W.
Published: July 2002

Relaxation times of the α-process in the fragile glass-forming liquid diglycidyl ether of bisphenol-A (EPON 828) were measured in a broad pressure (1–1500 bar) and temperature (264–293 K) ranges by means of the depolarized dynamic light scattering—photon correlation spectroscopy. Based on this experimental data the fragility of the supercooled liquid was calculated in two ways: as a steepness index m of the “Angell plot” and as the DT-parameter from the Vogel–Fulcher–Tammann Law, and was studied as a function of pressure. It was found, that while the steepness index depends on pressure, the DT parameter is pressure independent. The pressure dependence of the glass transition temperature Tg in EPON 828 was found to be nonlinear. Additionally, we established a relationship between the steepness index mT, the activation volume ΔV#, and the coefficient ∂Tg/∂Pg. In this pressure dependent study we found that also for EPON 828 the nonexponentiality of the correlation function of the α-process correlates well with the non-Arrhenius behavior (steepness index) of the relaxation times. An equation of state describing the temperature and pressure dependence of the structural relaxation time was proposed and verified using experimental data.
The Korteweg–de Vries–Zakharov–Kuznetsov equation for electron-acoustic waves
Authors: Mace R. L. , Hellberg M. A.
Published: July 2002

Motivated by a recent paper [Phys. Plasmas 7, 2987 (2000)] highlighting the potential importance of the electron-acoustic wave in interpreting the solitary waves observed by high time resolution measurements of the electric field in the auroral region, the effect of a magnetic field on weakly nonlinear electron-acoustic waves is investigated. A Korteweg–de Vries–Zakharov–Kuznetsov (KdV-ZK) equation is derived for a plasma comprised of cool and hot electrons and a species of fluid ions. Two models are employed for the ions: magnetized and unmagnetized. When the ions are magnetized the frequency constraints imposed upon the electron-acoustic wave packet prove to be too limiting to be of general use. The second model, which treats the ions as a stationary neutralizing background, overcomes the restrictions imposed by the former and is more fitting for the frequency domain of the electron-acoustic wave. Plane and ellipsoidal soliton solutions are admitted by the KdV-ZK equation, the latter perhaps able to explain some of the two dimensional features of the solitary waves observed in the Earth’s high altitude auroral region. Both models for the ions predict only negative potential solitons. It is discussed how the plasma model might be adapted to produce positive potential solitons.