bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-similar pinch-off of power law fluids
Authors: Doshi Pankaj , Basaran Osman A.
Published: January 2004

Pinch-off dynamics of slender liquid threads of power law fluids without inertia are studied by asymptotic analysis. Because the threads are slender, their dynamics are governed by a pair of spatially one-dimensional, nonlinear evolution equations for the thread shape and axial velocity that results from a long-wave asymptotic expansion of the creeping flow equations. By means of an approach that differs from those used previously in analyses of capillary pinching of threads of Newtonian fluids, a similarity transformation is derived that reduces the evolution equations to two coupled similarity equations. As in the Newtonian case, it is shown that for each value of the power law exponent n where 0⩽n⩽1, there is a family of similarity solutions for capillary pinching of threads of power law fluids. For a given family of solutions, the radial and axial scales vary with time τ to pinch-off as τn and τδ, respectively, where δ is the axial scaling exponent. It is shown that for a given family of solutions characterized by a fixed value of n, each member of the family has a different scaling exponent δ. Since the viscosity of a power law fluid varies as |γ̇|(n−1), where γ̇ is the deformation rate, for each value of n a numerical method based on domain splicing is used to compute the values of the axial scaling exponent δ and the similarity solutions.
Finite element model of laser-generated surface acoustic waves in coating-substrate system
Authors: Xu Baiqiang , Shen Zhonghua , Ni Xiaowu , Lu Jian , Wang Yawei
Published: January 2004

The generation of ultrasound in coating-substrate systems subjected to laser beam illumination has been studied quantitatively by using the finite element method. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field can be obtained in different coating-substrate systems. According to the thermoelastic theory, these temperature gradient fields are taken as bulk sources to generate ultrasound in coating-substrate systems. The typical surface acoustic waves (SAWs) in systems, a slow coating on a fast substrate and a fast coating on a slow substrate, are obtained. The influence of the propagation distance and the coating thickness on the SAWs is analyzed.
Dynamical nonlinearity in strained InGaAs (311)A sidewall quantum wires
Authors: Alderighi Daniele , Zamfirescu Marian , Vinattieri Anna , Gurioli Massimo , Sanguinetti Stefano , Povolotskyi Michael , Gleize Jerome , Di Carlo Aldo , Lugli Paolo , Nötzel Richard
Published: January 2004

Significant optical nonlinearity has been found in InGaAs (311)A sidewall quantum wires by means of time resolved photoluminescence measurements. A strong reverse quantum confined Stark effect has been observed and attributed to the dynamical screening of both the internal piezoelectric field and the Coulomb interaction between carriers. The time evolution of the quantum wire emission has been reproduced by means of self-consistent calculations that take into account excitonic effects, strain, and induced piezoelectric charges.
Optical properties of Cd1−xMgxTe (x=0.00, 0.23, 0.31, and 0.43) alloy films
Authors: Ihn Y. S. , Kim T. J. , Kim Y. D. , Aspnes D. E. , Kossut J.
Published: January 2004

Pseudodielectric functions 〈ε〉 of Cd1−xMgxTe ternary alloy films of compositions x=0.00, 0.23, 0.31, and 0.43 have been determined from 1.5 to 6.0 eV by spectroscopic ellipsometry. We obtain approximations to the bulk dielectric functions ε by performing wet-chemical etching to remove overlayers and using parametric modeling to remove interference oscillations below the fundamental band gap. The values of the E0, E0+Δ0, E1, E1+Δ1, E2, and E0′ critical point energies and their x dependences at room temperature were determined from numerically calculated second energy derivatives of these data.
Motion of a solid object through a pasty (thixotropic) fluid
Authors: Ferroir T. , Huynh H. T. , Chateau X. , Coussot P.
Published: January 2004

For materials assumed to be simple yield stress fluids the velocity of an object should continuously increase from zero as the applied force increases from the critical value for incipient motion. We carried out experiments of fall of a sphere in a typical, thixotropic, pasty material (a laponite suspension). We either left a sphere falling in the fluid in different initial states of structure or vibrated the fluid in a given state of structure at different frequencies. In each case three analogous regimes appear either for increasing restructuring states of the fluid or decreasing frequencies: A rapid fall at an almost constant rate; a slower fall at a progressively decreasing velocity; a slow fall at a rapidly decreasing rate finally leading to apparent stoppage. These results show that the motion of an object, due to gravity in a pasty material, is a more complex dynamical process than generally assumed for simple yield stress fluids. A simple model using the basic features of the (thixotropic) rheological behavior of these pasty materials makes it possible to explain these experimental trends. The fall of an object in such a fluid thus appears to basically follow a bifurcation process: For a sufficiently large force applied onto the object its rapid motion tends to sufficiently liquify the fluid around it so that its subsequent motion is more rapid and so on until reaching a constant velocity; on the contrary if the force applied onto the object is not sufficiently large the fluid around has enough time to restructure, which slows down the motion and so on until the complete stoppage of the object.
Miscible displacements in capillary tubes: Effect of a preexisting wall film
Authors: Chen Ching-Yao , Meiburg Eckart
Published: January 2004

For miscible displacements in capillary tubes, the impact of a preexisting wall film on the tip velocity of the displacing fluid finger is analyzed by means of axisymmetric Stokes simulations. The wall film is assumed to have the same viscosity as the displacing fluid, which is less viscous than the displaced fluid. The finger of the displacing fluid is seen to move in a quasisteady fashion, with a tip velocity below the centerline velocity of an equivalent Poiseuille flow. The explanation for this behavior, which is in contrast to our earlier findings for miscible displacements without wall films, lies in the lubricating effect of the wall film. The condition is established for which the displaced fluid moves in a nearly solid body-like motion. In this limit, a closed expression is derived for the finger tip velocity. A comparison between the simulation data and the closed form results shows reasonable agreement, provided that the criterion for solid body-like motion is satisfied. Furthermore, results are presented for the practically relevant limit of large viscosity ratios.
Investigation of the multiphoton excitation process of the 4f 25d configuration in LiYF4 and LiLuF4 crystals doped with trivalent neodymium ion
Authors: Librantz A. F. H. , Gomes L. , Tarelho L. V. G. , Ranieri I. M.
Published: January 2004

Ultraviolet (UV) fluorescence of Nd3+ ions induced by multistep laser excitation was investigated in Nd-doped LiYF4 and LiLuF4 crystals using a technique of time-resolved spectroscopy. The observed UV luminescence was due to transitions between the bottom of 4f25d configuration and the 4f3 states of Nd3+ ions. The first absorption band of 4f25d configuration, which starts around 56 700 cm−1, was excited by three stepwise absorptions of photons in the green (500–535 nm) from a short pulse laser excitation leading to broad emission bands in UV range (180–280 nm). An excitation band in the blue (468–486 nm) was observed due to the excitation of the second absorption band of 4f25d configuration around 63 000 cm−1 according to the photon absorption sequence: I9/24+hν(480 nm)→2G(1)9/2+hν(480 nm)→2F(2)7/2+hν(480 nm)→4f25d(second). The observed UV emissions (180–280 nm) from the bottom of the 4f25d configuration (first state) have a lifetime of 35 ns (parity allowed) and are broadband in contrast to UV emissions from 4f3 configuration, which are also present in the luminescence investigation but having a longer lifetime (8.5 μs) and structures composed of narrow lines. The excitation spectrum of fast UV luminescence exhibited different structures depending on the excitation geometry (σ or π) with respect to the c axis of the crystal. We observed two emissions from the first state of 4f25d configuration with peaks at 535 and 595 nm modifying the luminescence branching ratio of the bottom of the 4f25d configuration around 55 500 cm−1. The equivalent cross section of three and two excitation processes was estimated at 510 nm by solving the rate equations of the system under short laser excitation, which shows that is possible to have laser action under pulsed laser pumping with intensity below the crystal damage threshold.
Excimer laser inscribed submicron period relief gratings in InOx films and overlaid waveguides
Authors: Pissadakis S. , Reekie L. , Zervas M. N. , Wilkinson J. S.
Published: January 2004

Relief gratings of submicron periodicity are patterned in InOx thin oxide films using 248 nm interferometric excimer laser ablation. The ablation process is studied in terms of grating depth versus exposure conditions, using optical diffraction efficiency measurements. Real time monitoring of grating growth and film resistivity during grating writing are also presented. To study the exact grating morphology, scanning electron microscopy and atomic force microscopy microscans of the machined structures are performed. A discussion on the ablation behavior of InOx thin films is given in accordance with the experimental data obtained. Relief gratings are patterned in InOx thin films overlaid on ion-exchanged channel waveguides, and reflection spectra are also reported.
Wurtzite to zinc-blende phase transition in gallium nitride thin films
Authors: Kim Joo Han , Holloway Paul H.
Published: January 2004

The phase transition in gallium nitride (GaN) thin films at room temperature from the thermodynamically stable wurtzite to the metastable zinc-blende structure is studied. The GaN films were grown at room temperature on (100)-oriented silicon substrates by radio-frequency planar magnetron sputtering of a GaN target in pure nitrogen atmosphere. As the nitrogen gas pressure during the film growth is decreased from 30 to 10 mTorr, the crystalline phase of GaN films changed from a hexagonal wurtzite to a cubic zinc-blende structure with (111) crystallographic planes oriented preferentially parallel to the film surface. The mechanisms responsible for the formation of the nonequilibrium cubic zinc-blende GaN phase at room temperature are suggested, in which the impact of the hyperthermal species impinging on the surface of growing films plays a key role in stabilizing the zinc-blende phase of GaN.
Cell writing selection when using precessional switching in a magnetic random access memory
Authors: Devolder T. , Chappert C.
Published: January 2004

Precessional switching can be used to selectively write a cell in a matrix of soft magnetic elements organized following a magnetic random access memory (MRAM) architecture. We model the required addressing strategy using a fully analytical formalism. We describe the magnetization trajectories for field combinations leading to nonswitching, switching, and switching with bounce occurrence, assuming a cell being a lossless macrospin with uniaxial in-plane anisotropy. We find quite simple and rather accurate (±0.7%) analytical equivalents of the so-called dynamical astroïd. The latter had been so far determined solely by numerical integrations of the Landau–Lifchitz equation and subsequent dichotomy. Additional heuristic arguments are used to derive the characteristic time scales of the reversal process, which unravels the physics of the magnetization reversal rate along the magnetization vector trajectory. Our analytical study is a useful guideline to assess which field magnitudes and timings lead to reliable precessional switching in MRAM, where complicated other phenomena render cumbersome the purely numerical calculations. To illustrate this, we show how the addressing window is quantitatively reduced in the presence of random intercell dipolar coupling. A necessary condition for an addressing strategy to be reliable is that the intercell dipolar coupling along the easy axis be lower than 18% of the anisotropy field.
Calibration scheme for three-dimensional particle tracking with a prismatic light
Authors: Matsushita Hirohiko , Mochizuki Takaaki , Kaji Nobufuji
Published: January 2004

A scheme is described for determining the three-dimensional translational motions of rigid particles in a liquid medium using only one color video camera. It is based on the principle that when rigid particles pass through a light sheet whose spectrum varies with its thickness, they reflect rays of varying color. If the particles are observed using a camera whose axis is set normal to the light sheet, their motions can be determined from their loci and colors, as observed on a video monitor. The rays reflected from the particles are matched to their loci through color analysis. A calibration curve obtained using this scheme was used to estimate the loci of particles with a diameter of about 100 μm; the maximum error was 0.51 mm, and the standard deviation of the calibration curve was 0.21 mm when the hue angle of the reflected rays was between 0.9° and 110° (this hue angle range corresponds to a particle displacement of about 3.8 mm in the spectrum-change direction).
Three-stage light-gas gun with a preheating stage
Authors: Moritoh Tatsumi , Kawai Nobuaki , Nakamura Kazutaka G. , Kondo Ken-ichi
Published: January 2004

The performance of a three-stage light-gas gun with a preheating stage that uses hydrogen gas is described. In the preheating tests, in which the gun is not fired, the hydrogen gas was preheated up to 500 K and the temperature returned to room temperature in 250 ms. The preheated working gas was sealed in a pump tube constantly by using an improved big piston. In the firing tests, a projectile speed of 7.2 km/s, which is higher than the maximum speed without the preheating stage, was achieved with a 1.0 g projectile. It was shown that the light-gas gun is promising for a gas-dynamics gun in a wide range of operating conditions.
Unsteady force measurement technique in shock tubes
Authors: Tanno H. , Komuro T. , Takahashi M. , Takayama K. , Ojima H. , Onaya S.
Published: January 2004

An aerodynamic force measurement technique with an extremely short test time was developed. A signal recovery method based on a frequency domain deconvolution technique was applied on a direct acceleration measurement with a weakly restrained test model. With the technique, an unsteady drag force of an 80 mm diameter sphere model was measured when a planar shock wave of Ms=1.22 passed the sphere in a vertical shock tube of 300×300 mm square cross section. The measured drag force was evaluated with a numerical simulation, the reliability of which was verified with pressure and optical measurements. The evaluation revealed that the present force measurement technique has enough accuracy and time resolution for phenomenon with a duration of a few hundred μs. Additionally, the unsteady force measurement of a sphere revealed that drag force had been negative for approximately 200 μs after the shock wave passed over the sphere. Numerical analysis showed that this negative drag was caused by the high pressure produced by shock waves converging at the lower part of the sphere.
Characterization of a nonequilibrium XeCl laser-plasma by a movable Faraday cup
Authors: Doria D. , Lorusso A. , Belloni F. , Nassisi V.
Published: January 2004

In this work the experimental results of a nonequilibrium laser-plasma induced by an ultraviolet 308 nm excimer laser are reported. All measurements were performed fixing the laser energy at 70 mJ. It was concentrated on a 0.0099 cm2 spot by a convergent focal lens of 15 cm focal length. The utilized target was a 99.99% pure Cu disk. An 8 cm in diameter movable Faraday cup was developed in order to detect the plasma flow pulse at different positions along a drift tube. Analyzing the time-of-flight pulse under different cup bias voltage, we were able to distinguish the electron pulse, the suprathermal ions, and the thermal evolution of the plasma. In addition, by applying a breakdown voltage as polarizing cup voltage, we characterized the duration of the neutral component. To determine the system particle production efficiency, the total etched material per pulse, 0.235 μg, and the fractional ionization were measured. The expelled particle flux distribution was measured by an optical transmission analysis of a Cu deposited film on a glass substrate. The plasma flow was detected along its propagation axis, between 6 and 40 cm far from the target. The ablation process expelled particles with an initial velocity of 34 km/s, while the maximum ion concentration was 1 μs after the laser pulse. The plasma created propagates with a mean velocity of about 20 km/s. During the propagation, the longitudinal plasma dimension changed from 2.8 cm, near the target, to 31 cm at the maximum cup distance analyzed. At lowest distances, the cup signal wave forms presented a plateau due to the high dense plasma undergone to the space charge regime governed by the Child–Langmuir law.
Design and characterization of a late-mixing pulsed nozzle
Authors: Camden Jon P. , Bechtel Hans A. , Zare Richard N.
Published: January 2004

A pulsed source that allows mixing of two gases without appreciable reaction prior to expansion is constructed for the study of photoinitiated reactions. The source is characterized by the rotational temperature (80±10 K) and translational temperature (<10 K) of HCl in the expansion. The photoinitiated reaction Cl+CH3OH is studied to illustrate the usefulness of this source. The design is easy to implement and should be effective for a wide range of reaction dynamics experiments requiring the coexpansion of reactive gases.
Development of an ultralow current amplifier for scanning tunneling microscopy
Authors: Carlà M. , Lanzi L. , Pallecchi E. , Aloisi G.
Published: January 2004

A transimpedance amplifier for ultralow current scanning tunneling microscopy has been developed. Conditions for maximum signal-to-noise ratio have been explored, showing that best results can be obtained with a simple circuital arrangement. The amplifier associates a very high amplification factor (0.5 V/pA) to a sufficiently wide bandwith (1.6 kHz) and very low noise current (49 fA). Those features enable microscopy studies on an almost insulating surface, such as a freshly cleaved mica surface.
Optical single-angle plane-wave transmittances/reflectances from Schwarzschild objective variable-angle measurements
Authors: Gaylord Thomas K. , Kilby Gregory R.
Published: January 2004

Photonic crystal structures and other nanoscale and microscale optical structures are centrally important to future device technology. The fundamental infrared single-angle plane-wave experimental characterization of these structures is needed to evaluate the analysis, design, and fabrication progress on these devices. The very small sizes of these devices necessitates focusing the infrared probe light typically with a Schwarzschild reflecting objective. The small spot size inherently requires the large range of incident angles associated with the objective. In this work, a variable-angle measurement method is presented for obtaining the optical single-angle plane-wave transmittances/reflectances. The primary steps in this method are (1) calculating the reference sample single-angle plane-wave transmittance/reflectance, (2) measuring the composite transmittances/reflectances of a reference sample over a range of objective angles of incidence, (3) calculating the intensity-angular-weighting coefficients for the objective using the Moore–Penrose (overdetermined linear equations) matrix inversion technique, (4) measuring the composite transmittances/reflectances of a sample-under-test over a range of objective angles of incidence, and (5) calculating the single-angle plane-wave transmittances/reflectances using the Moore–Penrose matrix inversion technique.
Sub-10 nm lateral spatial resolution in scanning capacitance microscopy achieved with solid platinum probes
Authors: Bussmann E. , Williams C. C.
Published: January 2004

Sub-10 nm resolution can be obtained in scanning capacitance microscopy (SCM) if the probe tip is approximately of the same size. Such resolution is observed, although infrequently, with present commercially available probes. To acquire routine sub-10 nm resolution, a solid Pt metal probe has been developed with a sub-10 nm tip radius. The probe is demonstrated by SCM imaging on a cross-sectioned 70 nm gatelength field-effect transistor (FET), a shallow implant (n+/p, 24 nm junction depth), and an epitaxial staircase (p, ∼75 nm steps). Sub-10 nm resolution is demonstrated on the FET device over the abrupt meeting between a silicon-on-insulator oxide layer and a neighboring Si region. Comparable resolution is observed on the implant structure, and quantitative SCM dopant profiling is performed on it with sub-10 nm accuracy. Finally, the epitaxial staircase structure is quantitatively profiled demonstrating the accuracy obtained in quantitative profiling with the tips.
Laser-diffraction slit rheometer to measure red blood cell deformability
Authors: Shin S. , Ku Y. H. , Park M. S. , Moon S. Y. , Jang J. H. , Suh J. S.
Published: January 2004

The laser-diffraction technique has been applied to design a slit rheometer for measuring red blood cell deformability over a range of shear stress. Flow-rate and pressure-drop measurements are replaced with a measurement of pressure variation with time. Using a precision pressure transducer, one can measure the variation of pressure in the vacuum chamber, p(t), from which the shear stress and shear rate are mathematically calculated. In addition, a laser beam traverses a diluted blood suspension and is diffracted by red blood cells (RBCs) in the volume. The diffraction patterns are captured by a charge coupled device-video camera, linked to a frame grabber integrated with a computer. When deforming under decreasing shear stress, RBCs change gradually from the prolate ellipsoid towards a circular biconcave morphology. The elongation index as a measure of RBC deformability is determined from an isointensity curve in the diffraction pattern using an ellipse-fitting program. The advantages of this design are simplicity, i.e., ease of operation and no moving parts, low cost, short operating time, and the disposable kit which is in contact with the blood sample.
Ultraviolet diffraction limited nanosurgery of live biological tissues
Authors: Colombelli Julien , Grill Stephan W. , Stelzer Ernst H. K.
Published: January 2004

A laser nanodissection system for in vivo and in situ biological tissues is presented. A pulsed laser beam operating at a wavelength of 355 nm enables diffraction limited dissection, providing an optimal tool for intracellular nanosurgery. Coupled into a conventional inverted microscope and scanned across a field of up to 100×100 μm2, this optical nanoscalpel performs in vivo photoablation and plasma-induced ablation inside organisms ranging from intracellular organelles to embryos. The system allows the use of conventional microscopy contrasts and methods, fast dissection with up to 1000 shots per second, and simultaneous dissection and imaging. This article outlines an efficient implementation with a small number of components and reports an improvement of this state of the art of plasma-induced ablation technique over previous studies, with a ratio of plasma volume to beam focal volume of 5.2. This offers, e.g., the possibility of writing information directly at the sample location by plasma glass nanopatterning.
Wide-field coherent anti-Stokes Raman scattering microscopy
Authors: Heinrich Christoph , Bernet Stefan , Ritsch-Marte Monika
Published: January 2004

We suggest a variant of coherent anti-Stokes Raman scattering (CARS) microscopy, which is a nonlinear microscopy method that has recently proven very successful in functional imaging of living cells with vibrational contrast. Whereas existing CARS microscopy techniques involve confocal scanning, in our setup excitation and detection are performed in a wide-field (nonconfocal) geometry, similar to a combination of dark-field and epifluorescence microscopy. The use of a nanosecond laser system rather than the typically used pico- or femtosecond systems furthermore implies a high spectroscopic resolution for various Raman-active substances. We demonstrate the feasibility of the wide-field approach on test samples and discuss its prospects to become a complemental method to confocal scanning CARS microscopy.
Influence of three-dimensional transition elements on magnetic and structural phase transitions of Ni-Mn-Ga alloys
Authors: Cherechukin A. A. , Takagi T. , Miki H. , Matsumoto M. , Ohtsuka M.
Published: January 2004

Differential scanning calorimetry and magnetic measurements were performed to study the influence of the ferromagnetic three-dimensional transition elements Fe and Co on the structural and magnetic properties of the ferromagnetic shape memory alloys Ni-Mn-Ga. Substitution of Fe or Co for Ni decreases the temperature of martensitic phase transition Tm, whereas substitution of Co for Mn results in a considerable increase of Tm. Magnetic measurement revealed that Curie temperature TC increased upon substitution of Fe or Co for Ni. This may be useful for design of high-temperature ferromagnetic shape memory alloys.
Current-induced phase control in charged-ordered Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 crystals
Authors: Parashar Sachin , Sudheendra L. , Raju A. R. , Rao C. N. R.
Published: January 2004

Single crystals of Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 show current-induced insulator–metal (IM) transitions at low temperatures. In addition, the charge-ordering transition temperature, Tco, decreases with increasing current. The electroresistive ratio, defined as ρ0.5/ρI where ρ0.5 is the resistivity at a current of 0.5 mA and ρI the resistivity at a given applied current, I, varies markedly with temperature and the value of I. Thermal hysteresis observed in Nd0.5Ca0.5MnO3 and Pr0.6Ca0.4MnO3 at the IM transition indicates that the transition is first order. The current-induced changes are comparable to those induced by magnetic fields, and accordingly the IM transition in Pr0.6Ca0.4MnO3 is associated with a larger drop in resistivity.
Charge transfer in carbon nanotube actuators investigated using in situ Raman spectroscopy
Authors: Gupta S. , Hughes M. , Windle A. H. , Robertson J.
Published: January 2004

Charge transfer dynamics on the surface of single-wall carbon nanotube sheets is investigated using in situ Raman spectroscopy in order to understand the actuation mechanism of an electrochemical actuator and to determine associated parameters. We built an actuator from single-wall carbon nanotube mat and studied its actuation in several alkali metal (Li, Na, and K) and alkaline earth (Ca) halide and sulfate solutions in order to clarify the role of counterion as mobile ions in the film. The variation of bonding with applied potential was monitored using in situ Raman spectroscopy. This is because Raman can detect changes in C–C bond length: the radial breathing mode at ∼190 cm−1 varies inversely with the nanotube diameter, and the G band at ∼1590 cm−1 varies with the axial bond length. In addition, the intensities of both the modes vary with the emptying/depleting or filling of the bonding and antibonding states due to electrochemical charge injection. We discussed the variation of peak height and wave numbers of these modes providing valuable information concerning electrochemical charge injection on the carbon nanotube mat surface. We found in-plane microscopic compressive strain (∼−0.25%) and the equivalent charge transfer per carbon atom (fc∼−0.005) as an upper bound for the actuators studied hereby. It is demonstrated that though the present analysis does comply with the proposition for the actuation principle made earlier, the quantitative estimates are significantly lower if compared with those of reported values. Furthermore, the extent of variation, i.e., coupled electro-chemo-mechanical response of single-wall carbon nanotubes (SWNT) mat depended upon the type of counterion used (Group I versus Group II). The cyclic voltammetry and ac electrochemical impedance spectroscopy results were described briefly, which help to demonstrate well-developed capacitive behavior of SWNT mat and to estimate the specific capacitances as well. Summarizing, the impact of these findings on the suitability of such material for use in electrochemical devices such as actuators is emphasized.
Quantum-mechanically corrected variational principle for metal–oxide–semiconductor devices, leading to a deep sub-0.1 micron capacitor model
Authors: Gunther Norman G. , Mutlu Ayhan A. , Rahman Mahmud
Published: January 2004

We treat an integral expression for electrostatic energy as a variational principle, with electric potential as the sole argument. We modify this principle to incorporate the quantum-mechanical (QM) effect of metal–oxide–semiconductor (MOS) interface charge confinement. The result is a QM-corrected variational principle for application to MOS devices. We apply this principle to develop a model of the sub-0.1 μm MOS capacitor. This variational-quantum-mechanical (VQM) model gives closed-form expressions for the behavior of threshold voltage, Vth, oxide capacitance, Cox, and depletion capacitance, Cj, as functions of the perimeter and area of the gate, thickness of the oxide, doping level, and temperature. Using this model, we further obtain closed-form expressions for QM-corrected dopant fluctuation-induced statistical deviation of Vth and Ctotal, as functions of gate dimensions, oxide thickness, doping level, and temperature. Excellent agreement is obtained when comparison with published detailed three-dimensional Poisson–Schroedinger numerical simulations are made for the behavior of Vth and deviation of Vth, σ (Vth), as functions of gate dimensions and doping levels. Our model further shows that the oxide and depletion capacitance both consist of one term which is proportional to the gate area plus another term proportional to the perimeter. For very large devices, only the area term is significant, while, for very small devices, the perimeter term dominates. The perimeter term is clearly to be associated with the contribution of the fringe field at the edge of the capacitor. The decomposition of capacitance into four parts clearly indicates that the standard equivalent circuit model for MOS capacitance, i.e., two capacitors in series, should be modified to account for the contributions of the fringe field. Interestingly, our model reveals that there exists a gate perimeter-to-area ratio for which the capacitance is wholly insensitive to oxide thickness, and another for which the standard deviation of the depletion capacitance due to dopant fluctuations, σ(Cj), has a minimum, when the doping level is high.
Low-frequency spectral enhancement of THz electromagnetic waves emitted from InAs surface with increased excitation intensity
Authors: Yano R. , Gotoh H. , Hirayama Y. , Miyashita S. , Kadoya Y. , Kusuda K. , Yamanishi M.
Published: January 2004

We studied the excitation intensity dependence of the spectrum of the THz electromagnetic waves emitted from the surface of InAs excited by a femtosecond laser pulse. An enhancement of the spectral amplitude in the low-frequency regime (<0.6 THz) of the THz electromagnetic waves was observed when the excitation intensity was increased. To clarify the mechanism of the above phenomenon, we also performed THz wave emission experiments using a semi-insulating-GaAs sample. Due to the difference of the band-gap energies between InAs and GaAs, carriers excited to the conduction band experience different relaxation processes when they are excited by a laser pulse with a photon energy of 1.5 eV. Comparison of the experimental results for InAs and GaAs showed that the suppression of the intra-band relaxation induces the enhancement of the amplitude of the THz spectrum in the low-frequency regime for InAs.
Astrophysical neutrino telescopes
Authors: McDonald A. B. , Spiering C. , Schönert S. , Kearns E. T. , Kajita T.
Published: January 2004

This review describes telescopes designed to study neutrinos from astrophysical sources. These sources include the sun and supernovae emitting neutrino energies up to tens of MeV, atmospheric neutrino sources caused by cosmic ray interactions, and other sources generating neutrino energies ranging up to 1×1020 eV. Measurements with these telescopes also provide information on neutrino properties, including clear evidence for neutrino flavor change. Telescopes in operation in the past and present are described, along with plans for future instruments to expand this rapidly growing field of particle astrophysics.
Triggerable general-purpose interface bus controller
Authors: Hall David S.
Published: January 2004

We have developed the hardware and software for a simple, inexpensive control system that executes one general-purpose interface bus (GPIB) command each time a digital logic trigger is received. The system permits accurate and reproducible timing of complicated GPIB sequences, independent of the microprocessor of the computer controlling the experiment.
High-power 47.6 and 57.2 μm CH3OD lasers pumped by continuous-wave 9R(8) CO2 laser
Authors: Nakayama K. , Tazawa H. , Okajima S. , Kawahata K. , Tanaka K. , Tokuzawa T. , Ito Y.
Published: January 2004

Powerful lasers in the far-infrared wavelength range (47.6 and 57.2 μm) have been developed to measure the plasma density in the Large Helical Device at National Institute for Fusion Science and future plasma devices such as the International Thermonuclear Experimental Reactor. The intensification of these lasers has been done by cooling the laser tube wall, adding He as the buffer gas, and using a chemical-vapor-deposited diamond output window. The output powers for the 57.2 and 47.6 μm lasers have been found to be 1.6 and 0.8 W, respectively.
Field ionization detection of supersonic molecular beams
Authors: Doak R. B. , Ekinci Y. , Holst B. , Toennies J. P. , Al-Kassab T. , Heinrich A.
Published: January 2004

Seminal field ionization detection of a supersonic helium beam is reported, including intensity measurements on a continuous beam and time-of-flight measurements on a chopped beam. Also unique to these experiments is the care with which the field ionization tip was prepared and characterized prior to its use. Comparative measurements on a static helium gas are presented and the effective detection area of the field ionization tip is extracted for both beam and static gas. These areas are much smaller than those reported in previous experiments, suggesting that detection efficiency depends strongly on energy and momentum transfer during the initial atom–tip collision.
Multicusp ion source with external rf antenna for production of protons
Authors: Hahto S. K. , Hahto S. T. , Ji Q. , Leung K. N. , Wilde S. , Foley E. L. , Grisham L. R. , Levinton F. M.
Published: January 2004

Proton beams are needed in neutral-beam injection for diagnostic development of an internal magnetic field measurement. High proton fraction, low axial energy spread, current density in excess of 30 mA/cm2, and a parallel ion beam with cw operation are the requirements for the ion source/extraction system. A multicusp-type ion source with an external rf antenna was constructed at Lawrence Berkeley National Laboratory. A proton fraction of 85% and proton current density of 32 mA/cm2 were achieved at 1.8 kW of rf power. Plasma parameters were measured with a rf compensated Langmuir probe.
Time resolved radiated power during tokamak disruptions and spectral averaging of AXUV photodiode response in DIII-D
Authors: Gray D. S. , Luckhardt S. C. , Chousal L. , Gunner G. , Kellman A. G. , Whyte D. G.
Published: January 2004

Silicon absolute extreme ultraviolet (AXUV) photodiodes have been employed in a disruption radiometer diagnostic for measurement of radiant power in the DIII-D tokamak with a 170 kHz bandwidth. This is motivated by a need to improve the understanding of radiative processes in tokamak disruptions. The diagnostic described in this article has a single line of sight though the central plasma. Accounting for the photon energy dependence of the AXUV photodiode responsivity is made possible by optical filtering, with the aid of spectra from an extreme ultraviolet survey spectrometer. The appropriate effective responsivity for interpretation of the data is lower than the nominal value typically used for the detector. In the current quench phase of disruptions, it is less than half the nominal value. Comparisons with results from a foil bolometer find good agreement.
Data analysis process of a toroidal analyzer for electron–ion collision experiments
Authors: Matsumoto Jun , Kobayashi Nobuo , Danjo Atsunori , Yoshino Masuhiro
Published: January 2004

A toroidal analyzer has been developed to measure angular differential cross sections for electron scattering in electron–ion collisions. Energy and angular dispersed electron images are recorded by using a two-dimensional position sensitive detector. Operation and calibration of the analyzer are described with particular emphasis on a procedure to deduce the differential cross sections from the acquired two-dimensional data. Angular differential cross sections for elastic scattering of electrons from Ar7+ and Ar8+ ions are presented in the angular range from 34° to 85° at a collision energy of 100 eV in the center of mass frame.
Cryogenic neutron detector comprising an InSb semiconductor detector and a supercritical helium-3 gas converter
Authors: Nakamura Tatsuya , Katagiri Masaki , Aratono Yasuyuki , Kanno Ikuo , Hishiki Shigeomi , Sugiura Osamu , Murase Yasuhiro
Published: January 2004

We evaluated the neutron-detection characteristics of a proposed cryogenic neutron detector comprising an InSb semiconductor detector and a helium-3 gas converter. The neutron detector was operated at 4.2 K with helium-3 gas filling up to 1.5 atm, at which the density of the helium-3 nucleus corresponds to that at 160 atm at room temperature. The secondary particles generated by the He3(n,p)T reaction were successfully detected by the InSb detector with a time response of ∼80 ns at all tested gas pressures.
Procedure proposal for the correction of nonadiabatic thermoelastic stress analysis results
Authors: Salerno A. , Desiderati S.
Published: January 2004

Design of many aeronautic components takes into account fatigue tests intended to study the component behavior under load and eventually to verify their safety before they are put into production. Components under cyclic loads undergo a periodic temperature fluctuation proportional to the local principal stress sum (thermoelastic effect). In case of adiabatic conditions, which, for a material with a given thermal diffusivity, are achieved for frequencies above a certain level, the measured amplitude of the oscillating temperature can be used to determine the local stress in the sample. In many cases, fatigue tests are performed at low frequencies and they are unable to achieve the adiabatic conditions for a given material. For nonadiabatic conditions, the heat flow, that takes place from points with higher stress concentration to points with lower stress concentration, modifies the temperature distribution, preventing the point by point measurement of the correct stress value. The present article describes a procedure which has been elaborated and successfully used to recover the adiabatic surface temperature and measure the stress distribution for metal samples subjected to fatigue test in nonadiabatic conditions.
Velocity-map imaging electron spectrometer with time resolution
Authors: Baguenard B. , Wills J. B. , Pagliarulo F. , Lépine F. , Climen B. , Barbaire M. , Clavier C. , Lebeault M. A. , Bordas C.
Published: January 2004

A velocity-map imaging system for time-resolved photoelectron spectroscopy is presented and discussed. Nanosecond gating of microchannel plates synchronized with the excitation laser in a velocity-map imaging spectrometer has been used to generate time-resolved images of electrons emitted from photoexcited xenon atoms and small carbon cluster anions. These images represent a time slice in the kinetic energy spectrum and reveal the differing time scales of competing electron emission processes.
Method to determine the spring constant of atomic force microscope cantilevers
Authors: Gibson Christopher T. , Johnson Daniel J. , Anderson Christopher , Abell Chris , Rayment Trevor
Published: January 2004

We present a method for determining the spring constant of atomic force microscope cantilevers that involves a simple expression that relates the spring constant of any two cantilevers on the same chip. The method requires only the measurement of the resonant frequency and the plan view surface area of the cantilevers. One limitation, however, is that one cantilever on the chip must have its spring constant determined accurately using another technique. The method itself is simple, easy to use and can potentially provide accuracy on the order of 10% or better.
Thermal conductivity of periodic microporous silicon films
Authors: Song David , Chen Gang
Published: January 2004

This letter reports the experimental in-plane thermal conductivity of microfabricated, free-standing, single-crystal silicon thin films with periodically arranged through-film micropores. The experimental data suggest that strong size effects exist even in micro-sized porous silicon structures, particularly at low temperatures. Even at room temperature, all porous membranes show thermal conductivity values lower than expected by porosity and bulk phonon mean free path of silicon, and small-pore membranes have smaller thermal conductivity compared to the large-pore ones despite that they have close porosity values.
Enhanced weak Anderson localization phenomena in the magnetoresistance of n-type (Ga,In)(N,As)
Authors: Teubert J. , Klar P. J. , Heimbrodt W. , Volz K. , Stolz W. , Thomas P. , Leibiger G. , Gottschalch V.
Published: January 2004

Ga 1−y In y N x As 1−x with doping densities between 1017 and 1019 cm−3 was grown lattice matched on (100) GaAs by metalorganic vapor-phase epitaxy. Si or Te and Zn served as donors and acceptors, respectively. The magnetoresistance (MR) was measured between 1.6 and 280 K in magnetic fields up to 10 T. The MR of p-type Ga1−yInyNxAs1−x is typical for highly doped III–V semiconductors showing parabolic behavior at all temperatures with a small negative contribution due to weak localization at low fields and low temperatures. In contrast, n-type Ga1−yInyNxAs1−x exhibits a much stronger negative contribution to the MR. For some samples this negative contribution persists up to 280 K and Hmin>10 T. The N-induced conduction band structure changes lead to a strong enhancement of weak localization effects in the electron transport of Ga1−yInyNxAs1−x.
Analysis of electroluminescence spectra of silicon and gallium arsenide p–n junctions in avalanche breakdown
Authors: Lahbabi M. , Ahaitouf A. , Fliyou M. , Abarkan E. , Charles J.-P. , Bath A. , Hoffmann A. , Kerns S. E. , Kerns D. V.
Published: January 2004

We present a generalized study of light emission from reverse biased p–n junctions under avalanche breakdown conditions. A model is developed based on direct and indirect interband processes including self-absorption to describe measured electroluminescence spectra. This model was used to analyze experimental data for silicon (Si) and gallium arsenide p–n junctions and can be extended to several types of semiconductors regardless of their band gaps. This model can be used as a noninvasive technique for the determination of the junction depth. It has also been used to explain the observed changes of the Si p–n junction electroluminescence spectra after fast neutron irradiation. In particular, it is demonstrated that the neutron irradiation affects both the semiconductor and the overlying passivation oxide layer.
Single photon emission from site-controlled pyramidal quantum dots
Authors: Baier M. H. , Pelucchi E. , Kapon E. , Varoutsis S. , Gallart M. , Robert-Philip I. , Abram I.
Published: January 2004

We demonstrate that a single photoexcited InGaAs semiconductor quantum dot (QD) grown by organo-metallic chemical vapor deposition on prepatterned substrates emits one photon at a time, with no uncontrolled background photon emission, making it an excellent single photon emitter. Moreover, our fabrication technique offers complete site control and small inhomogeneous broadening of QD arrays, which is essential for the practical implementation of QDs in efficient solid-state single photon emitting devices.
Role of osmium in the electrical transport mechanism of polycrystalline tin oxide thin films
Authors: Forleo A. , Capone S. , Epifani M. , Siciliano P. , Rella R.
Published: January 2004

Undoped and Os-doped SnO2 thin films have been deposited by the sol–gel method. Their electrical properties have been investigated by resistivity and Hall effect measurements in order to understand the role of Os in the electrical transport mechanism. The measurements have been carried out in 100–500 K temperature range both in a vacuum and in air. The experimental results have been analyzed according to the grain-boundary scattering mechanism and they have been used to explain the response toward CH4 of the Os-doped SnO2 thin-film-based gas sensors.
Geometric constraints in the growth of nanotube-templated polymer monolayers
Authors: Coleman J. N. , Ferreira M. S.
Published: January 2004

We present a simple and general model that describes the ordered assembly of polymer strands on nanotube surfaces. Energetically favorable coiling angles are identified based on geometric constraints that limit the maximum coverage of polymers on the walls of the nanotubes. The coiling angles can be controlled by selecting the appropriate nanotube diameters, opening the possibility of engineering the strength of composite fibers. The model is applied to two different polymeric molecules; namely, polyacetylene and a polyphenylenevinylene derivative.
Wall roughness effects on an electron bunch
Authors: Banna S. , Schieber D. , Schächter L.
Published: January 2004

The effect of the surface roughness on a moving electron bunch is considered by resorting to a model of a metallic structure with random perturbations on its surface. Based upon this model, analytic expressions have been derived for the average energy emitted per groove (EPG) and for its standard deviation. For a relativistic bunch, both quantities are shown to be virtually independent of the momentum. Moreover, it has been found that the standard deviation of the EPG is proportional to that of the roughness parameter to the power of 1/4.