bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhancement of the low-field magnetoresistance of La1−x−yCaxSryMnO3+d at room temperature
Authors: Kolesnik S. , Dabrowski B. , Bukowski Z. , Mais J.
Published: July 2002

We have studied magnetoresistance of a series of La1−x−yCaxSryMnO3+d samples with the Curie temperature (TC) close to room temperature. For these samples, the rhombohedral–orthorombic structural phase transformation temperature (Ts) is slightly higher than TC. On cooling in zero magnetic field, we observe a rapid increase of resistivity at Ts. This increase is followed by a rapid decrease due to the ferromagnetic transition. The applied magnetic field significantly shifts the ferromagnetic transition to higher temperatures and suppresses the structure-related resistivity increase. We show that a combination of these two effects leads to an enhancement of the negative magnetoresistance due to strong spin-lattice coupling. Furthermore, by annealing the samples at appropriate temperatures in air, the enhancement can be optimized to appear in relatively low magnetic fields and at room temperature.
Formation of quasicrystals in Zr46.8Ti8.2Cu7.5Ni10Be27.5 bulk glass
Authors: Wanderka N. , Macht M.-P. , Seidel M. , Mechler S. , Ståhl K. , Jiang J. Z.
Published: July 2002

The formation of the quasicrystalline phase is observed as a first step of crystallization during isothermal annealing of the Zr46.7Ti8.3Cu7.5Ni10Be27.5 bulk glass. The structure of the quasicrystals and the sequence of phase formation have been investigated by x-ray powder diffraction and transmission electron microscopy. The structure of the quasicrystals is determined to be primitive icosahedral with a quasilattice constant of 4.779 Å. The quasicrystals decompose into several intermetallic compounds after prolonged annealing at lower temperatures or in a short time period (less than 5 min) at high temperatures above 683 K.
Scanning-tunneling-microscopy-induced optical spectroscopy of a single GaAs quantum well
Authors: Dumas Ph. , Derycke V. , Makarenko I. V. , Houdré R. , Guaino P. , Downes A. , Salvan F.
Published: July 2002

We have investigated the scanning-tunneling-microscopy-induced light emission originating from a single GaAs quantum well. The 5-nm-thick quantum well was confined between a 30-nm-thick AlAs barrier (grown onto a GaAs substrate) and the vacuum tunneling gap. Low currents ensured a nonintrusive investigation of the surface. Optical spectroscopy of the light emitted while injecting electrons from the tip revealed two peaks associated with the band-to-band recombination in the bulk GaAs (at 1.43 eV), and with the electronic transition in the surface quantum well (at 1.52 eV). The surface sensitivity of the technique is evidenced and the quantum efficiencies of both processes are estimated.
Modeling laser texturing of silicate glass
Authors: Bennett Ted D. , Li Lei
Published: July 2002

A refined model of a glass laser texturing process used on computer disk substrates is presented. Field equations for fictive temperature and elasto-viscoplastic strain in the glass are numerically integrated over the thermal cycle created with a microsecond CO2 laser pulse. Calculating the fictive temperature change as part of the solution provides for a consistent treatment of glass properties that depend on fictive temperature. The short time scale of the thermal cycle causes the final altered state of fictive temperature in the heat affected zone to be relatively constant over the depth of change, and higher than the initial value by more than 300 K. Plastic strain resulting from thermomechanical stresses and the fictive temperature rise are considered in this description. The model illustrates the rise in the compressive stress caused by initial heating, the relaxation process that occurs in the molten region above the transition temperature, and the subsequent introduction of tensile stress during cooling. At the end of the thermal cycle, the region of glass with altered fictive temperature is left in a state of high tensile stress. The time evolution of surface topography is studied with the model, and shows good agreement with measured dimensions of the final bump geometry over a range of laser pulse energies.
Systems of reaction diffusion equations and their symmetry properties
Authors: Nikitin A. G. , Wiltshire R. J.
Published: July 2002

A constructive algorithm is proposed for the investigation of symmetries of partial differential equations. The algorithm is used to present classical Lie symmetries of systems of two nonlinear reaction diffusion equations.
Interdiffusion in amorphous Si/Ge multilayers by Auger depth profiling technique
Authors: Csik A. , Langer G. A. , Beke D. L. , Erdélyi Z. , Menyhard M. , Sulyok A.
Published: July 2002

It has been shown by the Auger depth profiling technique that the concentration profile at the initially sharp Si/Ge interface in amorphous Si/Ge multilayers shifted but remained still sharp after a heat treatment at 680 K for 100 h. At the same time the fast diffusion of Si resulted in the formation of an almost homogeneous Ge(Si) amorphous solid solution, while there was practically no diffusion of Ge into the Si layer. This is direct evidence on the strong concentration dependence of the interdiffusion coefficient in amorphous Si/Ge system, and it is in accordance with the previous indirect result obtained from the measurements of the decay of the small angle Bragg peaks, as well as with finite difference simulations.
Monte Carlo study of secondary electron emission
Authors: Ding Z. J. , Tang X. D. , Shimizu R.
Published: July 2002

Based on our previous Monte Carlo simulation model of electron interactions with solids, including cascade secondary electron production, in which an optical dielectric function was used to describe electron energy loss and the associated secondary electron excitation, we have systematically investigated secondary electron generation and emission for 19 metals. The calculated secondary yield curve for primary beam energy ranging from 100 eV to 2 keV was found to correspond with the experimental universal curve. The dependence of the secondary yield on the work function was studied numerically, leading to a remarkable scattered deviation from Baroody’s relationship. This deviation shows that the secondary yield relates to different aspects of behavior by electrons in a metal, such as the cascade production process, the stopping power and specific energy loss mechanism for a sample, and the dependence on the electron density of states. The results provide an explanation for the scattered data on the experimental yield versus the work function. The calculations indicate that the characteristic energy loss of primaries may result in a corresponding feature in the energy distribution of secondaries.
Diffusion length in CdTe by measurement of photovoltage spectra in CdS/CdTe solar cells
Authors: Toušek J. , Kindl D. , Toušková J. , Dolhov S. , Poruba A.
Published: July 2002

Modified method of surface photovoltage (SPV), spectral response measurement, and constant photocurrent method (CPM) were applied to thin film CdS/CdTe solar cells with the aim of finding diffusion length of minority carriers (L) in the CdTe material. The SPV signal was theoretically calculated without constraints of absorption coefficients for the incident radiation and thickness of the sample assuming one space charge region (SCR) located on the CdS/CdTe interface. In addition to the diffusion length, the SPV is a function of the surface recombination velocity and the parameters of the SCR, which complicates the evaluation. Illuminating the back side of the solar cell (without ohmic contact) we obtain a photovoltage spectrum predominantly influenced by the diffusion length. On the other hand, the standard measurement using light penetrating from the CdS side strongly depends on the thickness of the SCR. The small signal approximation model presented here successfully explains both measured spectra and permits extraction of the diffusion length of minority carriers and thickness of the SCR in CdTe absorber. The CPM is used for determination of absorption coefficients in the CdTe layer. The absorption of this material depends on its preparation and must be known for correct evaluation of experimental data.
Hot carrier recombination model of visible electroluminescence from metal–oxide–silicon tunneling diodes
Authors: Liu C. W. , Chang S. T. , Liu W. T. , Chen Miin-Jang , Lin Ching-Fuh
Published: July 2002

We report the visible electroluminescence at room temperature from metal–oxide–silicon tunneling diodes. As biased in the Fowler–Nordheim regime, the electrons tunnel from the gate electrode through the ultrathin oxide and reach the Si anode with sufficiently high energy. The hot electrons cause the impact ionization, and generate the secondary hot electrons and hot holes in Si substrates. The visible light comes from the radiative recombination between the secondary hot electrons and hot holes, and the hot carrier recombination model can fit the visible electroluminescence spectra.
The nuclear spin–spin coupling constant in He2
Authors: Pecul Magdalena
Published: July 2002

The scalar nuclear spin–spin coupling constant was calculated for the helium dimer using full configuration interaction (FCI) and EOM-CCSD methods. The Fermi-contact was found to have nonnegligible value of 1.3 Hz at R=5.6 a.u., while the other contributions are zero. This suggests that the nuclear spin–spin coupling constants transmitted through van der Waals interactions are a much more common phenomenon than previously thought. All contributions exhibit sharp exponential decay with the internuclear distance.
Resonant energy transfer from argon dimers to atomic oxygen in microhollow cathode discharges
Authors: Moselhy M. , Stark R. H. , Schoenbach K. H. , Kogelschatz U.
Published: July 2002

The emission of atomic oxygen lines at 130.2 and 130.5 nm from a microhollow cathode discharge in argon with oxygen added indicates resonant energy transfer from argon dimers to oxygen atoms. The internal efficiency of the vacuum-ultraviolet (VUV) radiation was measured as 0.7% for a discharge in 1100 Torr argon with 0.1% oxygen added. The direct current VUV point source operates at voltages below 300 V and at current levels of milliamperes.
Characteristics of Co2+:ZnS saturable absorber Q-switched neodymium lasers at 1.3 μm
Authors: Tsai Tzong-Yow , Birnbaum Milton
Published: July 2002

Passively Q-switched neodymium lasers, Nd:YAG, Nd:YALO, and Nd:BEL, at 1.3 μm using the saturable absorber Co:ZnS were characterized. The best performance, about ∼0.35 mJ pulse energy and 300 ns pulse duration, was obtained using the Nd:BEL laser. In a Co:ZnS Q-switched Nd:YAG laser, sequential dual-wavelength Q-switched pulses at 1319 and 1338 nm were observed. The time separation between the output pulses was about 2–10 μs. Theoretical analysis is presented including analysis of the dual-mode Q-switched laser operation.
Energy coupling efficiency of a hydrogen microwave plasma reactor
Authors: Gordon M. H. , Duten X. , Hassouni K. , Gicquel A.
Published: July 2002

Zero-dimensional and two-dimensional plasma models and optical emission spectroscopy are used in tandem to investigate the power coupling efficiency for a pure hydrogen microwave plasma. The zero-dimensional model accounts for the vibrational kinetics of H2, the chemistry of H2 and H excited states, and the kinetics of ground-state species. The set of species conservation equations are then coupled to the electron Boltzmann equation (to account for the non-Maxwellian electron energy distribution function) and the total energy equation for solution. The two-dimensional model makes use of a simpler thermochemical description of the plasma. The chemistry is described with nine species and thirty chemical reactions. Three energy modes are considered to describe the plasma’s thermal nonequilibrium, and Maxwellian distribution functions for kinetic and vibrational modes are assumed. The non-Maxwellian nature of the electron energy distribution function is separately accounted for. Experimentally, the absolute line emission intensity is utilized to obtain number densities of up to five hydrogen excited states using the following transitions: Hα (6563 Å), Hβ (4861 Å), Hγ (4340 Å), Hδ (4102 Å), and Hε (3970 Å). The first three transitions were used for a 38 Torr, 1000 W hydrogen discharge, and all five transitions were used for a 121 Torr, 4000 W hydrogen discharge. The absolute continuum emission from the plasma was compared to numerical predictions. The comparison of the numerical and experimental data indicates that 90%–100% of the input power is deposited in the plasma and that both the line and continuum emission match within a factor of 3, with the exception of the high energy excited states for the 4000 W plasma. A control volume heat transfer analysis validates the energy coupling.
The harmony in the Kepler and related problems
Authors: Nucci M. C. , Leach P. G. L.
Published: July 2002

The technique of reduction of order developed by Nucci [J. Math. Phys. 37, 1772–1775 (1996)] is used to produce nonlocal symmetries in addition to those reported by Krause [J. Math. Phys. 35, 5734–5748 (1994)] in his study of the complete symmetry group of the Kepler problem. The technique is shown to be applicable to related problems containing a drag term which have been used to model the motion of low altitude satellites in the Earth’s atmosphere and further generalizations. A consequence of the application of this technique is the demonstration of the group theoretical relationship between the simple harmonic oscillator and the Kepler and related problems.
Effects of biaxial strain and chemical ordering on the band gap of InGaN
Authors: Wright A. F. , Leung K. , van Schilfgaarde M.
Published: July 2002

We have performed first-principles calculations to examine the effects of biaxial strain and chemical ordering on the band gap of wurtzite InxGa1−xN in the range 0⩽x⩽0.5. Our results for random unstrained alloys are in good agreement with theoretical estimates and measurements on unstrained zinc-blende alloys, but are in poor agreement with recent measurements on strained wurtzite alloys which display significantly lower band gaps. Biaxial strain is found to have a nonlinear effect on alloy band gaps, increasing them for x<0.25 and decreasing them for x>0.25. However, the overall agreement with measurements on wurtzite alloys remains poor. Chemical ordering along the [0001] direction in strained alloys is found to decrease their band gaps considerably, reducing the discrepancy with measurements. We discuss our results with regard to the current understanding of InGaN alloys.
Phase behavior of a simple model for membrane proteins
Authors: Noro Massimo G. , Frenkel Daan
Published: July 2002

We report a numerical simulation of the phase diagram of a simple model for membrane proteins constrained to move in a plane. In analogy with the corresponding three-dimensional models, the liquid–gas transition becomes metastable as the range of attraction decreases. Spontaneous crystallization happens much more readily in the two-dimensional models rather than in their three-dimensional counterparts.
Performance of cryogenic microbolometers and calorimeters with on-chip coolers
Authors: Anghel D. V. , Luukanen A. , Pekola J. P.
Published: July 2002

Astronomical observations of cosmic sources in the far-infrared and x-ray bands require extreme sensitivity. The most sensitive detectors are cryogenic bolometers and calorimeters operating typically at about 100 mK. The last stage of cooling (from 300 to 100 mK) often poses significant difficulties in space-borne experiments, both in system complexity and in reliability. We address the possibility of using refrigeration based on normal metal/insulator/superconductor (NIS) tunnel junctions as the last stage cooler for cryogenic thermal detectors. We compare two possible schemes: direct cooling of the electron gas of the detector with the aid of NIS tunnel junctions and the indirect cooling method, when the detector lattice is cooled by the refrigerating system, while the electron gas temperature is decreased by electron–phonon interaction. The latter method is found to allow at least an order of magnitude improvement in detector noise equivalent power compared to direct electron cooling.
High-resolution laser spectroscopy of LiAr: Spectroscopic parameters and interaction potentials of the A 2∏ and the B 2∑ states
Authors: Brühl Rüdiger , Zimmermann Dieter
Published: July 2002

The absorption spectrum of the LiAr molecule has been recorded with high resolution using a supersonic beam and applying the method of laser-induced fluorescence. Altogether, about 1400 absorption lines due to the transitions A 2∏←X 2∑ and B 2∑←X 2∑ of the two isotopomers 7Li40Ar and 6Li40Ar could successfully be assigned. The spectroscopic parameters of vibration and rotation, of spin-orbit splitting, and of λ-type doubling have been obtained for vibrational levels 5–10 of A 2∏ and 0–3 of B 2∑. Using the observed rovibrational energies as input data the interatomic potentials of both electronic states have been determined in form of analytical functions by means of a fully quantum-mechanical method of approach. For the equilibrium values we get Re=2.50(8) Å, De=957(30) cm−1 for A 2∏ and Re=6.24(5) Å, De=33.8(1.0) cm−1 for B 2∑.
Ultralow biased field emitter using single-wall carbon nanotube directly grown onto silicon tip by thermal chemical vapor deposition
Authors: Matsumoto Kazuhiko , Kinosita Seizo , Gotoh Yoshitaka , Uchiyama Tetsuo , Manalis Scott , Quate Calvin
Published: July 2002

A carbon-nanotube field emitter which has single-wall carbon nanotubes with a diameter of 1–2 nm grown directly onto the Si tips by thermal chemical vapor deposition was developed. Owing to the 10–20 times smaller diameter of the nanotube than the conventional silicon (Si) tip, the fabricated carbon-nanotube field emitter showed an ultralow threshold voltage of 10 V for the field emission of electrons, which is more than ten times smaller value than the conventional Si emitter.
On the role of large-scale structures in wall turbulence
Authors: Marusic Ivan
Published: July 2002

Recent experimental and computational studies by Adrian and co-workers, such as Adrian et al. [J. Fluid Mech. 422, 1 (2000)] and Zhou et al. [J. Fluid Mech. 387, 353 (1999)], have proposed that a dominant structure in wall turbulence is the organization of hairpin vortices in spatially correlated packets or trains of vortices. In this study this scenario is investigated using the attached eddy model of Perry and Marusic [J. Fluid Mech. 298, 361 (1995)] by calculating structure angles, two-point velocity correlations and autocorrelations and comparing them to experimental measurements across a zero-pressure-gradient turbulent boundary layer. The results support the conclusion that spatially coherent packets are a statistically significant structure for Reynolds stresses and transport processes in the logarithmic region of the flow.
Helium/deuterium coimplanted silicon: A thermal desorption spectrometry investigation
Authors: Corni F. , Nobili C. , Tonini R. , Ottaviani G. , Tonelli M.
Published: July 2002

Thermal desorption spectrometry has been applied to investigate the blistering and exfoliation phenomena which occur at the surface of a p-type (100) silicon wafer coimplanted with helium and deuterium. During the heat treatments in linear temperature ramp, an explosive emission of both gases occurs. The phenomenon is kinetically controlled with an effective activation energy of 1.3±0.2 eV. In addition, the desorption spectra present a second contribution, attributed to deuterium emission from buried cavities. Also in this case, the process is kinetically controlled with an effective activation energy of 1.9±0.3 eV. Thermal desorption spectrometry is a suitable technique to have information about various phenomena which occur during blistering and exfoliation.
Size dependence of Eu2+ fluorescence in ZnS:Eu2+ nanoparticles
Authors: Chen Wei , Malm Jan-Olle , Zwiller Valéry , Wallenberg Reine , Bovin Jan-Olov
Published: July 2002

The emission bands of the 4.2, 3.2 and 2.6 nm sized ZnS:Eu2+ nanoparticles are peaking at 670, 580 and 520 nm, respectively. The emission of the 4.2 nm sized nanoparticles originates from the recombination of the Eu2+-bound exciton, while the emission of the 3.2 and 2.6 nm sized nanoparticles is from the Eu2+ intra-ion transition of 4f65d1(t2g)–4f7. Possible mechanisms for the size dependence of the 4f65d1(t2g)–4f7 transition of Eu2+ in ZnS:Eu2+ nanoparticles were investigated, and it was concluded that the decreases in the electron–phonon coupling and in crystal field strength upon a decrease in size are the two major factors responsible for the shift.
Gratings in indium oxide film overlayers on ion-exchanged waveguides by excimer laser micromachining
Authors: Pissadakis S. , Reekie L. , Zervas M. N. , Wilkinson J. S. , Kiriakidis G.
Published: July 2002

Relief Bragg gratings were imprinted by 248 nm interferometric excimer laser ablation on potassium ion-exchanged channel waveguides in BK-7 glass overlaid with a thin high-index InOx film. Using five pulses of energy density 60 mJ/cm2, a spectral transmittance notch of depth 66% and ΔλFWHM<0.1 nm was obtained at 1547 nm in the TE polarization for a waveguide having a nominal width of 8 μm and a 135-nm-thick InOx overlayer. In waveguides coated with 100 nm InOx, with widths increasing from 3 to 8 μm, the reflection wavelength shifted by 0.12 nm/μm and the reflectivity increased monotonically.
Infrared optical properties of LaNiO3–platinized silicon and PbZrχTi1−χO3–LaNiO3–platinized silicon heterostructures
Authors: Yu J. , Huang Z. M. , Meng X. J. , Sun J. L. , Chu J. H. , Tang D. Y.
Published: July 2002

Using infrared spectroscopic ellipsometry, the optical constants of LaNiO3 thin films on Pt(111)–Ti–SiO2–Si substrates are obtained in the 2.5–12.6 μm range, in which the infrared optical constants decrease when the annealing temperature increases from 600 to 650 °C. At the same time, the infrared optical properties of PbZrχTi1−χO3(PZT) thin films with χ=0.3 and 0.5 on LaNiO3–Pt–Ti–SiO2–Si substrates are simultaneously studied with respect to annealing temperatures. The infrared optical properties are associated closely with the grain size and crystallographic orientation of the films induced by annealing temperature, combined by the particular substrate. For the Ni–PZT–LaNiO3–Pt multilayer heterostructures, the infrared absorptance better than 99% can be achieved for PZT pyroelectric thin film infrared microsensors.
Pressure-induced cubic-to-orthorhombic phase transformation in the negative thermal expansion material HfW2O8
Authors: Jorgensen J. D. , Hu Z. , Short S. , Sleight A. W. , Evans J. S. O.
Published: July 2002

The effect of pressure on the crystal structure of HfW2O8 has been investigated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa at room temperature the cubic material transforms, with a 5% reduction in volume, to the same orthorhombic phase that is seen in the isostructural compound ZrW2O8 above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastable orthorhombic phase transforms back to the cubic phase. The substantially higher pressure for the cubic-to-orthorhombic transition in HfW2O8, compared to ZrW2O8, may be important for the application of this material in composites with controlled thermal expansion because rather large local pressures can occur in such composites.
New water shock sensor
Authors: Mori Y. , Shimada K. , Nakahara M. , Nagayama K.
Published: July 2002

A new shock sensor for liquids is described in this article which is suitable for precise shock Hugoniot compression curve measurement of water in the relatively low pressure region of ⩽1 GPa. The present method is based on the fact that the refractive index of water has a very large pressure dependence. A laser beam incident on the liquid–glass boundary from the proper angle of incidence is totally reflected at the interface. The condition of total internal reflection is violated if the refractive index changes instantaneously by shock compression. The shock target assembly was designed for a gas gun experiment based on the above concept. Two examples of the shock Hugoniot measurement data of water were given.
High mobility in n-type GaN substrates
Authors: Saxler A. , Look D. C. , Elhamri S. , Sizelove J. , Mitchel W. C. , Sung C. M. , Park S. S. , Lee K. Y.
Published: July 2002

High peak electron mobilities were observed in freestanding c-plane GaN layers. Two well-defined electrical layers, a low mobility degenerate interface layer, and a high mobility nondegenerate bulk layer, were present in these samples. The carrier concentrations and mobilities for the layers were extracted using two methods: (1) magnetic field dependent Hall effect analysis; and (2) a simple two layer Hall model with the assumption that one of the layers is degenerate. The electron Hall mobility of the bulk layer is found to peak at nearly 8000 cm2/V s at low temperature using the magnetic field dependent Hall effect analysis.
Resolution and polarization in apertureless near-field microscopy
Authors: Larsen Ross E. , Metiu H.
Published: July 2002

We present numerical calculations of the electromagnetic fields produced around a sharp metal tip held above a dielectric surface and illuminated from above by visible light. This geometry is that of an apertureless near-field scanning optical microscope (A-NSOM). The electric fields produced by irradiation are calculated using a real-space Green’s function technique, also known as the frequency-domain method of moments. We investigate numerically the convergence properties of this method with respect to the grid size used to discretize the equations and find that the usual criterion invoked to ensure convergence does not apply inside small metallic scatterers; for this reason, the fields near the border of the tip keep changing as we make the grid smaller and smaller. Although the calculations do not show convergence (for the intensity of the fields near the borders of the probe), the intensity and the polarization of the field in other regions of space are converged; they do not change as the grid spacing is made smaller. When the tip is five nanometers above a flat dielectric surface the field under the tip is strongly enhanced in a region having a diameter of five nanometers, even though the end of the tip has lateral dimensions of order ten nm. The light intensity falls off rapidly as the tip–surface separation is increased and the region where the field is enhanced becomes larger. This implies that illuminating very small areas with this device requires very good control of the tip height. Most of the results presented here are for an Al tip, but we have also performed calculations for tips made of sapphire, gold, silver, and tungsten. For all probes, the field localization, and hence the NSOM resolution, depends only weakly on the tip composition, whereas the magnitude of light intensity enhancement is strongly dependent on the dielectric properties of the tip. When the probe is very close to the surface, giving the best lateral resolution, the spatial variation of the electric field near the surface is quite complicated, and is different for different components of the electric field vector; the polarization of the field under the tip is not the same as the polarization of the incident far field or that of the field in the absence of the tip. Regardless of the incident polarization, the tip tends to make the induced electric field perpendicular to the substrate. Moreover, different components of the field have different spatial distributions; knowing the orientation of the field creates the possibility of deducing the orientation of molecules adsorbed on the surface.
Transient response of a bilayer organic electroluminescent diode: Experimental and theoretical study of electroluminescence onset
Authors: Hassine L. , Bouchriha H. , Roussel J. , Fave J.-L.
Published: July 2002

The experimental transient electroluminescence response of a bilayer organic light-emitting diode reported here shows an initial overshoot when sufficient direct current bias is superimposed to pulse excitation. Using a theoretical model giving the kinetic equations which govern the time evolution of electrical fields, currents and charge densities in the device, we calculate transient response of light emission. The results show a good agreement with experiments and evidence the charge carriers accumulation occurring in the region of organic–organic interface due to unipolar injection. Application of periodic excitation indicates that a characteristic time of 300 ms is needed to recover the initial electrical equilibrium state.
Transparent conducting Zr-doped In2O3 thin films for organic light-emitting diodes
Authors: Kim H. , Horwitz J. S. , Kushto G. P. , Qadri S. B. , Kafafi Z. H. , Chrisey D. B.
Published: July 2002

Zirconium-doped indium oxide (ZIO) thin films (∼2000 Å thick) have been deposited by pulsed-laser deposition on glass substrates without a postdeposition anneal. The structural, electrical and optical properties of these films have been investigated as a function of substrate temperature and oxygen partial pressure during deposition. Films were deposited at substrate temperatures ranging from 25 °C to 400 °C in O2 partial pressures ranging from 0.1 to 50 mTorr. The films (∼2000 Å thick) deposited at 200 °C in 25 mTorr of oxygen show electrical resistivities as low as 2.5×10−4 Ω cm, an average visible transmittance of 89%, and an optical band gap of 4.1 eV. The ZIO films were used as a transparent anode contact in organic light emitting diodes and the device performance was studied. The external quantum efficiency measured from these devices was about 0.9% at a current density of 100 A/m2.
Apparatus-specific analysis of fluid adhesion measurements
Authors: Francis B. A. , Horn R. G.
Published: July 2002

A thin film of viscous liquid between two solids acts as an adhesive due to the large force resisting separation of the solids. This effect is exploited in pressure-sensitive adhesive bonds. One method of investigating such bonds is to use a probe tack test in which a rigid probe is indented into a thin adhesive layer coating a rigid flat base. These experiments are characterized by two quantities: the total work of separation, that is, the work done in extracting the rigid punch from the adhesive film, and the peak adhesive force, otherwise known as the adhesive strength. Little effort, however, has been spent on understanding the connection between these quantities and the apparatus used to measure them. In this article we shall study the simplest case of fluid adhesion where a spherical probe and flat are bound by a high viscosity Newtonian polymer melt (polydimethylsiloxane or polybutene) and examine the role of apparatus-specific parameters in determining the measured adhesive strength and work of separation. We shall show how a dimensionless master-curve can be derived to capture the dependence of the adhesive strength on the testing regime. Specific attention is paid to the effect of system compliance on the adhesion, as introduced by the presence of a compliant load cell used to measure the adhesive force. A relationship linking the adhesive strength of a Newtonian film and the work of separation is also presented.
Self-assembled metallic dots and antidots: Epitaxial Co on Ru(0001)
Authors: Yu Chengtao , Li Dongqi , Pearson J. , Bader S. D.
Published: July 2002

We have grown ∼1–420 nm thick epitaxial Co wedges on Ru(0001) with molecular-beam epitaxy at 350 °C and characterized them with atomic force microscopy. A metal-on-metal growth mode was observed where three-dimensional islands (dots) or a flat film network with holes (antidots) in truncated pyramidal shapes exist below or above ∼ 20 nm, respectively. The top of the islands and the rim of the holes are flat with a roughness of ∼0.3 nm, and the lateral sizes of these dots/antidots, ∼102 nm, tend to be uniform. We suggest that this self-assembled growth be mainly driven by strain.
X-ray absorption spectroscopy and imaging of heterogeneous hydrothermal mixtures using a diamond microreactor cell
Authors: Fulton John L. , Darab John G. , Hoffmann Markus M.
Published: July 2002

Hydrothermal synthesis is an important route to novel materials. Hydrothermal chemistry is also an important aspect of geochemistry and a variety of waste remediation technologies. There is a significant lack of information about the speciation of inorganic compounds under hydrothermal conditions. For these reasons we describe a high-temperature, high-pressure cell that allows one to acquire both x-ray absorption fine structure (XAFS) spectra and x-ray transmission and absorption images of heterogeneous hydrothermal mixtures. We demonstrate the utility of the method by measuring the Cu(I) speciation in a solution containing both solid and dissolved Cu phases at temperatures up to 325 °C. X-ray imaging of the various hydrothermal phases allows micro-XAFS to be collected from different phases within the heterogeneous mixture. The complete structural characterization of a soluble bichloro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coordination structure. The Cu–Cl distance was determined to be 2.12 Å for the CuCl2− species and the complete loss of tightly bound waters of hydration in the first shell was observed. The microreactor cell described here can be used to test thermodynamic models of solubility and redox chemistry of a variety of different hydrothermal mixtures.
Low-noise photodetectors based on heterojunctions of AlGaN–GaN
Authors: Kuryatkov V. V. , Temkin H. , Campbell J. C. , Dupuis R. D.
Published: July 2002

We describe detailed current–voltage and noise measurements carried out on AlGaN–GaN heterojunction photodetectors. Dark current densities below 1×10−10 A/cm2 are measured at a bias level of −5 V, at room temperature. In diodes with a diameter of 50 μm, low leakage currents result in the zero-bias noise spectral density as low as 3.6×10−32 A2/Hz. Based on the combined electrical and noise measurements, we calculate room-temperature thermally limited specific detectivity greater than 2.4×1014 cm Hz1/2 W−1. Background-limited specific detectivity exceeds 3.5×1013 cm Hz1/2 W−1.
Film thickness dependence of ferroelectric properties of c-axis-oriented epitaxial Bi4Ti3O12 thin films prepared by metalorganic chemical vapor deposition
Authors: Watanabe Takayuki , Saiki Atsushi , Saito Keisuke , Funakubo Hiroshi
Published: July 2002

The film thickness dependence of the ferroelectricity of c-axis-oriented epitaxially grown Bi4Ti3O12 thin films was investigated. The c-axis-oriented Bi4Ti3O12 films with thicknesses from 35 to 400 nm were epitaxially grown on (100)CaRuO3//(100)SrTiO3 substrates by metalorganic chemical vapor deposition. The dielectric constant and the coercive field of the films were almost independent of the film thickness. These values were about 151 and 18.8 kV/cm, respectively. The spontaneous polarization Ps was almost constant at 4.0 μC/cm2 and was independent of the film thickness above 140 nm. This value is almost the same as the reported value for the single crystal along the c axis. However, the Ps value decreased with decreasing film thickness below 100 nm. Moreover, the remanent polarization continuously decreased as the film thickness decreased below 400 nm. As a result, the ferroelectric property of the epitaxially grown Bi4Ti3O12 films also depended on the film thickness and was similar to that of simple perovskite-structured ferroelectric thin films, such as Pb(Zr, Ti)O3 and (Pb, La)(Zr, Ti)O3 thin films.
Conservation laws of semidiscrete Hamiltonian equations
Authors: Kozlov Roman
Published: July 2002

Many evolution partial differential equations (PDEs) can be cast into Hamiltonian form. Conservation laws of these equations are related to one-parameter Hamiltonian symmetries admitted by the PDEs [P. J. Olver, Applications of Lie Groups to Differential Equations (Springer, New York, 1986)]. In this paper we consider symmetries and Noether’s theorem for semidiscrete Hamiltonian equations which are obtained by space discretization of Hamiltonian PDEs. Using symmetries, one can find conservation laws of these equations. Several applications including a transfer equation and the Korteweg–de Vries equation are presented.
A double coil apparatus for Barkhausen noise measurements
Authors: Puppin E. , Zani M. , Vallaro D. , Venturi A.
Published: July 2002

A pickup coil wound around the sample is the standard method for Barkhausen noise (BN) measurements. Here we describe an apparatus where two coils are used instead of one. The relative position of the coils can be changed and this allows the experimental investigation of spatial correlation effects in BN. The signals induced in the coils are very similar when the coils are placed nearby whereas a progressive decrease of their correlation is observed by moving the coils apart. We tested our system on a ribbon of amorphous Fe63B14Si8Ni15 100 mm long. For a distance between the coils of 40 mm the signal correlation is nearly vanished.
Rotational-state and velocity-subgroup dependence of the rotational alignment of N2+ drifted in He
Authors: Anthony Eric B. , Bierbaum Veronica M. , Leone Stephen R.
Published: July 2002

Experimental results are presented for the rotational alignment of N2+(v″=0) as a function of both rotational state and component Doppler velocity. A single-frequency ring dye laser is used to probe the alignment of N2+ drifted in helium in a flow-drift apparatus by the technique of polarized laser-induced fluorescence. The collision-induced quadrupole rotational alignment parameter A0(2) is determined as a function of the field direction component of sub-Doppler laboratory velocity at a fixed field strength of 12 Td for five rotational states. A dramatic difference in velocity-selected alignment as a function of rotational state is observed, with the higher rotational states exhibiting a greater degree of alignment than the lower rotational states. Additionally, for sufficiently low rotational state (J=11.5), A0(2) changes sign across the Doppler profile, a behavior that has not previously been reported in the literature. A companion theoretical paper presents molecular dynamics calculations that are in excellent agreement with these experimental observations.
Molecular dynamics study of the collision-induced rotational alignment of N2+ drifting in helium
Authors: Baranowski R. , Wagner B. , Thachuk M.
Published: July 2002

The full velocity-angular momentum distribution function for gas-phase N2+ drifting in helium is calculated using a molecular dynamics method, and utilized to examine collision-induced rotational alignment in detail. These results are also compared with experimental measurements, most especially those of Anthony et al. [J. Chem. Phys. 112, 10269 (2000)] and those appearing in the preceding article [Anthony et al., J. Chem. Phys. 114, 6654 (2001)]. Both the calculations and experiments show a number of interesting features including, drift velocities which depend upon rotational state, and quadrupolar alignment parameters which change from negative at high velocities to positive at low velocities.
Effect of surface reconstruction on the structural prototypes of ultrasmall ultrabright Si29 nanoparticles
Authors: Mitas L. , Therrien J. , Twesten R. , Belomoin G. , Nayfeh M. H.
Published: July 2002

We propose, using density functional, configuration interaction, and quantum Monte Carlo calculations, structural prototypes of ultrasmall ultrabright particles prepared by dispersion from bulk. We constructed near spherical structures (Td point group symmetry) that contain 29 Si atoms, five of which constitute a tetrahedral core and the remaining 24 constitute a hydrogen terminated reconstructed Si surface. The surface is a highly wrinkled or puckered system of hexagons and pentagons (as in a filled fullerene). We calculated, for several surface reconstruction models, the coordinates of atoms, the absorption spectrum, the absorption edge, polarizability, and the electron diffraction pattern. The Si29H24 (six reconstructed surface dimers) gives a size of 0.9 nm, an absorption spectrum and bandgap (3.5±0.3 eV), in fair agreement with measurement. The structure yields a polarizability of 830 a.u. with an effective “dielectric” constant of ∼6.0. The calculated electron diffraction of single particles shows residual crystalline coherent scattering for large but not small scattering angles.
Slow transient photoresponse in Schottky-type junctions based on poly(4,4″-dipenthoxy-3′-octyl-2,2′:5′,2″-terthiophene)
Authors: Camaioni N. , Casalbore-Miceli G. , Martelli A.
Published: July 2002

We report a study of the slow transient phenomena observed, under continuous illumination, in the response of the sandwich-type structure indium–tin–oxide/poly(4,4″-dipenthoxy-3′-octyl2,2′:5′,2″-terthiophene)/Al. The analysis of the transient component of the open-circuit voltage revealed that recombination processes are greatly enhanced under illumination, limiting the performance of this kind of devices.
On the exchange bias in single and polycrystalline ferro/antiferromagnetic bilayers
Authors: Li Zhanjie , Zhang Shufeng
Published: July 2002

By incorporating a random interfacial exchange interaction into the Landau–Lifshitz–Gilbert equation, a unified picture of exchange bias for single crystals, textured crystals, twin structures, and polycrystals of antiferromagnets is presented. It is found that the lateral interaction in the antiferromagnet is a key element governing the exchange bias and magnetization reversal of the ferromagnet.
Fabrication of InAs/AlSb/GaSb heterojunction bipolar transistors on Al2O3 substrates by wafer bonding
Authors: Moran P. D. , Chow D. , Hunter A. , Kuech T. F.
Published: July 2002

High-frequency integrated circuit applications of GaSb-based materials are hampered by the lack of a suitable lattice-matched insulating substrate. Wafer bonding was used to fabricate InAs/AlSb/GaSb-based heterojunction bipolar transistors (HBTs) on an insulating sapphire substrate through a low temperature bonding process that results in a high bond strength and permitted the mechanical and chemomechanical removal of the initial GaSb substrate. The use of selective etches allows for the retention of the epitaxial device layers over virtually the entire wafer area. Minimal degradation of the transferred layers occurred in the bonding and substrate removal process. The resulting transferred structures were fabricated into functional HBTs exhibiting a dc current gain of ∼5.
Fabrication of single crystal GaAs(001) barriers for magnetic tunnel junctions
Authors: Kreuzer Stephan , Wegscheider Werner , Weiss Dieter
Published: July 2002

A preparation method for magnetic tunnel junctions with single crystal GaAs(001) barriers is demonstrated. The method is based on an epoxy bond and stop etch technique and does not require epitaxial growth of a semiconductor on top of a metal. Stable GaAs tunnel barriers down to 6 nm in thickness were prepared. The I–V measurements show a pronounced nonlinearity. Their variation with temperature depends strongly on the barrier thickness which is weak for the 6 nm barriers, a clear indication for quantum mechanical tunneling as the dominant transport channel.
Planar nematic anchoring due to a periodic surface potential
Authors: Wen Bing , Rosenblatt Charles
Published: July 2002

A polyimide-coated substrate is rubbed periodically with the stylus of an atomic-force microscope. The effective polar and azimuthal anchoring strength coefficients for planar anchoring are determined as a function of the spatial separation L of rub lines. Both anchoring coefficients are found to decrease monotonically with L, apparently leveling off at nonzero values for L≳2 μm. The observed behavior is discussed in terms of a combination of intrinsic anchoring strength in the rubbed regions and a surface memory effect.
Dissociation dynamics of I2(B)–Ar: Rotational population distributions of I2(B,v) fragments from the T-shaped and linear complexes
Authors: Burroughs Amy , Heaven Michael C.
Published: July 2002

Optical-optical double resonance techniques have been used to examine the dissociation dynamics of I2(B)–Ar. Rotational population distributions were characterized for the I2(B,v) fragments. Vibrational predissociation of the T-shaped complex yielded fragments with smooth rotational distributions. The high-energy limits of the distributions were consistent with events that channeled almost all of the available energy into product rotation. These data indicate a dissociation energy for I2(B)–Ar of D0(C2v)=220 cm−1. Most initial states of the complex produced bimodal rotational population distributions, but a few gave Boltzmann-type product distributions. The dependence of the character of the distribution on the initially excited state suggests that predissociation is mediated by intramolecular vibrational energy redistribution. Dissociation of linear I2(B)–Ar yielded fragments with Boltzmann type rotational population distributions. Excitation of the complex within the bound regions of the B–X transition gave rotationally cold I2(B,v) fragments, consistent with direct dissociation from a near-linear geometry. Excitation above the B state dissociation limit produced I2(B,v) fragments via caged recombination. The rotational distributions of these fragments were cold, supporting earlier studies that attribute the one atom cage recombination to the linear isomer.
Computer studies on the spontaneous fast reconnection evolution in various physical situations
Authors: Ugai M. , Kondoh K.
Published: July 2002

The spontaneous fast reconnection evolution is studied in a long current sheet system in various physical situations, where the threshold of current-driven anomalous resistivity is assumed to increase with the thermal velocity. If the initial threshold VC0 is sufficiently large in a low-β plasma, the fast reconnection mechanism can fully be set up; on the other hand, if VC0 is so small that the anomalous resistivity can easily occur in the usual circumstances, the resulting diffusion region notably lengthens so that the reconnection process becomes much less effective. Also, the fast reconnection evolution is strongly influenced by plasma β in the ambient magnetic field region, and an essential condition for the fast reconnection mechanism to evolve explosively is that the plasma β is sufficiently small. In fact, only in a low-β plasma does the magnetic tension force involved play the dominant role in the overall system dynamics and in the drastic magnetic energy release. It is also demonstrated that the fast reconnection evolution does not depend on the detailed functional form of the (current-driven) anomalous resistivity model. This is because the positive feedback between the anomalous resistivity and the reconnection flow effectively works so long as an anomalous resistivity is assumed to increase with the relative electron-ion drift velocity.
Identification of defects associated with second-order optical nonlinearity in thermally poled high-purity silica glasses
Authors: Kameyama Akihiro , Yokotani Atsushi , Kurosawa Kou
Published: July 2002

A large second-order optical nonlinearity has been found to be generated in various kinds of silica glasses in which an even-order optical nonlinearity is inherently prohibited. Thermal poling is a typical procedure to generate such a second-order optical nonlinearity, but a mechanism behind the generation is not elucidated completely. It should be pointed out, however, that the nonlinearity was not added to high-purity silica glasses by poling. In this article, we show that the nonlinearity is generated in high-purity silica glasses irradiated by a KrF excimer laser before poling. We also show that the laser pulses erase the nonlinearity induced in the glasses. In addition to the laser irradiation effects on the generation and erasure, optical absorption and luminescence spectra in the glasses show that point defects of ≡Si–O− play a key role for the nonlinearity.