bluuebunny/crossref_metadata_2025 huggingface dataset:
Adhesion and liquid–liquid phase separation in globular protein solutions
Authors: Braun F. N.
Published: July 2002

We derive a form of adhesive interaction between protein globules in solution, developing from the premise that amino-acid residues are able to rearrange around a globule–globule contact. We show in this approach that globules only become adhesive above a certain threshold temperature, which is sensitive to an osmotic modulus characterizing individual globule elasticity. Above the threshold, adhesive strength increases monotonically with temperature, driving a temperature-reversed liquid–liquid phase separation.
Details of below band-gap uniaxial dielectric function of SiC polytypes studied by spectroscopic ellipsometry and polarized light transmission spectroscopy
Authors: Kildemo M. , Hansteen F. , Hunderi O.
Published: July 2002

The optical properties of the below band-gap uniaxial dielectric function are studied as a function of the degree of hexagonality. The polytypes investigated were lightly doped 4H, 6H, 15R, and 21R silicon carbide. The ordinary component of the dielectric function is extracted from phase modulated spectroscopic ellipsometric measurements including mathematical removal of the overlayer. Accurate spectroscopic birefringence measurements are performed using a θ–2θ ellipsometric setup. The resulting birefringence varies as a third order polynomial as a function of the degree of hexagonality, with a turning point at 50% hexagonality. Furthermore, birefringence resulting from the intraconduction band transitions is studied both directly from the variation in the measured birefringence, and through Kramers–Kronig analysis of the extinction coefficient measured by p and s polarized transmission spectroscopy at 70° incidence. The second derivative of the dielectric function of the strongest below band-gap transition in the extraordinary component fit a one-dimensional critical point, while its energy position appears to be a linear function of the degree of hexagonality of the polytypes under investigation.
A complete look at the dissociation dynamics of vinylfluoride at 157 nm
Authors: Tu Jinying , Lin Jim J. , Lee Yuan T. , Yang Xueming
Published: July 2002

Photodissociation of vinylfluoride (H2CCFH) at 157 nm has been investigated using the photofragment translational spectroscopy technique. Five dissociation pathways have been clearly observed: H2CCF/HCCHF+H, HCCF+2H, HCCH+H+F, HCCH/H2CC:+HF, and HCCF/HFCC:+H2. Molecular HF elimination and the HCCH+H+F triple dissociation channel are found to be equally important, while other channels are all minor. The product kinetic energy distributions for all dissociation channels have been determined. Branching ratios for different channels have also been estimated.
Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)
Authors: Carini G. , D’Angelo G. , Tripodo G. , Bartolotta A. , Di Marco G. , Lanza M. , Privalko V. P. , Gorodilov B. Ya. , Rekhteta N. A. , Privalko E. G.
Published: July 2002

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below Tg, the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.
Forces between aqueous nonuniformly charged colloids from molecular simulation
Authors: Striolo A. , Bratko D. , Wu J. Z. , Elvassore N. , Blanch H. W. , Prausnitz J. M.
Published: July 2002

NVT Monte Carlo simulation results are presented for the forces between charged colloids within the primitive model for electrolytes. The calculations show that when charged colloids have a net dipole moment, a strong attraction can arise at short separations. The attractive force is not purely electrostatic; significant contributions follow from hard-sphere collisions between the electrolyte ions and the colloidal particles. In divalent electrolyte solutions, nonuniformly charged colloids show an oscillatory force profile as a function of separation, due to layering of electrolyte ions around the interacting colloids. Simulation results are compared to two analytical models derived from classical Debye–Hückel screened potentials. In the first model, contributions from charge–charge, dipole–dipole, and charge–dipole interactions are independently angle-averaged and then added to obtain the colloid–colloid potential. In the second model, the pair potential is obtained by simultaneously angle-averaging all interactions. Our results show that simultaneous angle-averaging of anisotropic interactions provides significant improvement over the commonly used additivity approximation.
Phonon replicas in ZnO/ZnMgO multiquantum wells
Authors: Sun H. D. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

The photoluminescence properties of ZnO/ZnMgO multiquantum wells have been investigated by continuous and time-resolved photoluminescence spectra at various temperatures. Strong first-order and second-order longitudinal optical (LO)-phonon replicas were observed in the emission spectra. Both the zero-phonon emission and its LO-phonon replicas can be resolved into two components with different temperature dependence. The observation of LO-phonon replicas provided an additional way to characterize the recombination mechanism in quantum wells. Two kinds of excitons are responsible for the resolved emission components and have different coupling strength with LO phonons: one is donor bound excitons which dominated at low temperatures, the other one is the localized excitons due to the well width fluctuation which took over the dominated emission at high temperatures.
Integral and fractional charge filling in a InAs/GaAs quantum dot p–i–n diode by capacitance–voltage measurement
Authors: Tang Shiang-Feng , Lin Shih-Yen , Lee Si-Chen , Cherng Ya-Tung
Published: July 2002

The temperature-dependent capacitance–voltage (C–V) characteristics of two stacked InAs/GaAs quantum dot p–i–n diode were investigated. The capacitance discontinuities observed are attributed to charge storage in the InAs quantum dots. The average storage electrons at each InAs quantum dot thus obtained are two and three electrons at room temperature and at temperature below 100 K, respectively. In the intermediate temperature range from 100 to 250 K, fractional charge occupation is observed in each dot. When the C–V measurement frequency is lowered from 800 to 80 kHz, the capacitance turns into a negative value under low biases which indicates the dominance of the inductance at lower frequency.
The effect of weak interactions on the ring puckering potential in the coumaran–argon van der Waals complex: Experimental and ab initio analysis of the intermolecular and low-frequency intramolecular vibrations
Authors: Watkins M. J. , Belcher D. , Cockett M. C. R.
Published: July 2002

The coumaran–argon van der Waals (vdW) complex has been investigated using a combination of (1+1′) REMPI and ZEKE spectroscopy, supported by ab initio molecular orbital calculations. Coumaran (2,3-dihydrobenzofuran) has a puckered, nonplanar equilibrium structure in the electronic ground state which allows for the formation of two energetically nonequivalent π-bound geometrical conformations. The experimental observation of bands attributable to two isomers in the REMPI spectrum is consistent with a significant barrier to planarity existing in both S0 and S1 states. The two isomers are related through the ring-puckering motion but the interaction of the argon atom with the monomer results in an induced asymmetry in the potential which transforms the ring-puckering tunnelling motion in the isolated monomer to a localized vibration near nonequivalent local minima. Both REMPI and ZEKE spectra show rich vibrational structure, characteristic of excitation of the long axis van der Waals bending mode and the stretching mode, as well as combination bands involving the vdW modes with low-frequency out of plane intramolecular vibrations. The vibrational structure is consistent with a shift in the position of the argon atom along the long axis upon excitation and subsequent ionization, as well as a modest reduction in the van der Waals bond length.
Highly excited states of gerade symmetry in molecular nitrogen
Authors: de Lange Arno , Lang Rüdiger , van der Zande Wim , Ubachs Wim
Published: July 2002

Highly excited states of gerade symmetry in molecular nitrogen have been investigated in a resonance-enhanced XUV+Vis (extreme ultraviolet+visible) transition scheme. Nineteen bands have been observed, of which only four involve known states [k 1Πg (v=1), x 1Σg− (v=2), and y 1Πg (v=1,2)], albeit in new systems. Three of the newly observed states have been assigned as y 1Πg (v=3) and k 1Πg (v=2,3). Level energies are determined with an accuracy of ≈0.20 cm−1.
Isotope effects on the rate of electron-beam dissociation of Mg–H complexes in GaN
Authors: Seager C. H. , Myers S. M. , Vaandrager B. , Nelson J. S.
Published: July 2002

The effect of low-energy electron-beam irradiation on the stability of acceptor–hydrogen complexes in Mg-doped GaN has been directly examined with infrared optical spectroscopy. Consistent with prior electrical transport data, we find that Mg–H pairs begin to break apart under 25 keV electron-beam exposure at doses of a few mC/cm2. However, we find that, even after long exposures, roughly 12 of the acceptor–hydrogen pairs remain unaffected by the electron exposure. Using Mg-doped samples that have been vacuum annealed and D2 gas exposed, we demonstrate that there is a large (∼×5) isotopic shift in the beam-induced debonding rate of these acceptor–hydrogen complexes. H and D remain in the material during these treatments, and Mg–H or Mg–D reforms during postirradiation annealing. The implications of these observations for understanding the nature of the debonding process and the subsequent reactions of the detached H/D in the GaN lattice are discussed.
Reflection–absorption infrared investigation of hydrogenated silicon oxide generated by the thermal decomposition of H8Si8O12 clusters
Authors: Nicholson K. T. , Zhang K. Z. , Banaszak Holl M. M. , McFeely F. R.
Published: July 2002

Reflection–absorption infrared spectroscopy has been employed to observe Si–H bonds within a model, ultrathin silicon oxide. Upon heating a monolayer of H8Si8O12/Si(100−2×1 to 700 °C, Si–H bonds as a part of HSiO3 entities are still detected within the oxide layer after cooling. These fragments appear to be stable to temperatures of at least 850 °C. Reversible hydrogen/deuterium exchange for these entities is also directly observed.
Midinfrared intersubband absorption on AlGaN/GaN-based high-electron-mobility transistors
Authors: Hofstetter Daniel , Diehl Laurent , Faist Jérôme , Schaff William J. , Hwang Jeff , Eastman Lester F. , Zellweger Christoph
Published: July 2002

Intersubband absorption measurements on two nominally undoped AlGaN/GaN-based high-electron-mobility transistors with different Al compositions in the barrier layer are presented. The first transistor with a barrier consisting of Al0.6Ga0.4N showed an absorption peak at 247 meV (1973 cm−1) with a full width at half maximum (FWHM) of 126 meV, while the second device utilizing an Al0.8Ga0.2N barrier had its peak at 306 meV (2447 cm−1) with a FWHM of 86 meV. Self-consistently computed potentials and intersubband transition energies showed good agreement with the experimental findings, and therefore confirmed previously published values for the internal piezoelectric field in such structures.
Dichroic behavior of multilayer structures based on anisotropically nanostructured silicon
Authors: Diener J. , Künzner N. , Kovalev D. , Gross E. , Koch F. , Fujii M.
Published: July 2002

Multilayer structures of anisotropically nanostructured (birefringent) silicon have been fabricated and studied by polarization-resolved reflection and transmission measurements. We demonstrate that stacks of birefringent porous silicon layers with alternating refractive indices and thicknesses act as dichroic Bragg reflectors or dichroic microcavities with a transmission/reflection dependent on the polarization direction of the incident light. The possibility of separate fine tuning of two orthogonally polarized transmission/reflection bands and their spectral splitting is demonstrated.
Influence of a space charge region on charge carrier kinetics in silicon wafers
Authors: von Aichberger S. , Abdallah O. , Wünsch F. , Kunst M.
Published: July 2002

p-type Si wafers provided with silicon nitride films at the surface are investigated by contactless transient photoconductivity measurements. Comparing wafers of different resistivities and wafers at one side and at two sides provided with nitride films shows that separation and storage of excess charge carriers in the space charge region is the main kinetic process at low injection level. Fitting of the experimental data with a simple model leads to the conclusion that the electron mobility in the interface is strongly reduced, a value 250(−+20%) cm2 V−1 s−1 is determined, independent of the wafer resistivity. This confirms the reduction of the electron mobility reported in the literature on the basis of completely different measurements.
Dielectric measurements on oxidized and hydrogenated chemical vapor deposited diamond films
Authors: Haenen K. , Rouleau J. F. , Nesládek M. , Goyette J. , Stals L. M. , Bose T. K.
Published: July 2002

The influence of oxidation, hydrogenation, and annealing on the dielectric properties of microwave plasma enhanced chemical vapor deposited diamond films was measured in the 45 MHz–20 GHz range using an open-ended coaxial probe. A dielectric response was detected around 108–109 Hz for the hydrogenated samples, in contrast to the oxidized films, which showed no response in that frequency range. The amplitude and position of the detected response were clearly dependent on the sample quality and the state of the surface. Fitting of the experimental data to the Cole–Cole expression for dielectric relaxation showed a near-Debye behavior of the dielectric permittivity. We propose that the dielectric response is a result of two-dimensional plasmons, corresponding with a hole concentration of ∼1013 cm−2 at the diamond surface, induced by the hydrogen termination. Analogous to the well-known hydrogen-induced surface conductivity, the dielectric signal disappeared after annealing the diamond films at 200 °C.
Magnetoresistance observed by decomposition of the magnetic moment in La1−xCaxMnO3 films
Authors: Kim Hyun-Tak , Kang Kwang-Yong , Lee Eun-Hee
Published: July 2002

A ferromagnetic phase, characterized by electron carriers and a high temperature colossal magnetoresistance (HTCMR) dependent on the magnetic moment, and a semiconducting phase, characterized by hole carriers and a low temperature CMR (LTCMR), are observed in La1−xCaxMnO3 thin films by the van der Pauw method. The LTCMR is much more sensitive to the magnetic field than the HTCMR. In the ferromagnetic phase for films with anisotropic moments in two dimensions, a remnant resistivity of the order of 10−8 Ω m is observed up to 100 K and increases exponentially with both a temperature up to Tc and a magnetic field above 1 T (a positive magnetoresistivity). We found that the ferromagnetic phase below Tc is in a polaronic state with a polaronic mobile conduction, and the carrier density dips near Tc. For resistances measured by the four-probe method with line electrodes, low temperature information of the HTCMR is not revealed. The van der Pauw method is more effective for the resistance measurement of a magnetic material than the four-probe method.
Relaxor and normal ferroelectric behavior in ordered thin films of PbSc0.5Nb0.5O3
Authors: Tyunina M. , Levoska J. , Leppävuori S.
Published: July 2002

Epitaxial heterostructures of ordered perovskite PbSc0.5Nb0.5O3 thin films were grown by pulsed laser deposition using MgO and LaAlO3 single-crystal substrates, and La0.5Sr0.5CoO3 bottom electrode layers. The small-signal dielectric response of the heterostructures was measured in a broad range of temperatures (290–625 K) and frequencies (102–106 Hz). The contribution of the film–electrode interfaces was evaluated, and the true properties of the films were analyzed. Both relaxor and normal ferroelectric types of behavior were identified in the films. The evolution of behavior and shift of dielectric maxima were found to correlate with the temperature of deposition and the type of the substrate. This was suggested to refer to the microstructural factors such as in-plane strain, grain size, and misfit dislocations.
Spectroscopic line shape broadening mechanisms in PbSe/PbSrSe quantum wells
Authors: Shen W. Z. , Jiang L. F. , Wang K. , Wu H. Z.
Published: July 2002

Temperature-dependent infrared transmission measurements on molecular-beam-epitaxy-grown PbSe/Pb0.934Sr0.066Se multiple-quantum-well structures (MQWs) have been carried out for the study of line shape broadening mechanisms. The longitudinal–optical phonon scattering strength is found to be much smaller in PbSe/Pb0.934Sr0.066Se MQWs (∼11–12 meV) than that in the corresponding IV–VI films (∼51 meV), which is in good agreement with the theoretical prediction. The low-temperature transmission line shapes of PbSe/Pb0.934Sr0.066Se MQWs are analyzed on the basis of the modeling of three broadening mechanisms. For the transmission spectra, the line shape broadening is mainly due to the well width fluctuation for quantum wells thinner than 15 nm and due to the interface roughness for quantum wells thicker than 15 nm. However, the line shape broadening due to the sheet carrier density in PbSe wells has been found to play a dominant role in the photoluminescence linewidth.
Photoluminescence study of interfaces between heavily doped Al0.48In0.52As:Si layers and InP (Fe) substrates
Authors: Poças L. C. , Duarte J. L. , Dias I. F. L. , Laureto E. , Lourenço S. A. , Filho D. O. Toginho , Meneses E. A. , Mazzaro I. , Harmand J. C.
Published: July 2002

Properties of the interface between the epitaxial layer of heavily doped Al0.48In0.52As:Si and the InP(Fe) substrate are investigated by photoluminescence in AlInAs:Si/InP(Fe) heteroestructures grown by molecular beam epitaxy. The effect on heterostructure optical properties of including a thin Al0.22Ga0.26In0.52As:Si layer at the interface is investigated as well. To explain the different interface emission energies observed, the results are analyzed by using the mixed-type I–II interface model, which considers in the type II interface a narrow InAs well, with variable width, between AlInAs and InP. The observation of the interface emission at energies as high as 1.36 eV, at low excitation intensity, is explained taking into account the high doping level of the samples. The observed interface transition luminescence thermal quenching is tentatively explained by analyzing the spatial distribution of electrons in the triangular quantum well formed at the type II interface (or at the mixed I–II interface) as a function of the temperature.
Crossover from in-plane to perpendicular anisotropy in Pt/CoFe/AlOx sandwiches as a function of Al oxidation: A very accurate control of the oxidation of tunnel barriers
Authors: Monso S. , Rodmacq B. , Auffret S. , Casali G. , Fettar F. , Gilles B. , Dieny B. , Boyer P.
Published: July 2002

By measuring the extraordinary Hall effect on a series of naturally oxidized Pt3 nm/Co90Fe10 0.6 nm/Al tAl samples with 0<tAl<1.2 nm, a first crossover from in-plane to perpendicular magnetic anisotropy is observed when tAl is varied from 0 to 0.2 nm. The CoFe magnetization remains out of plane for 0.2<tAl<0.5 nm. When the Al thickness is further increased, a second crossover back to in plane takes place. In a series of samples in which the Al thickness is kept constant, the same behavior is observed as the time of exposure to an oxygen plasma is varied. The results clearly indicate that the degree of oxidation of Al at the CoFe/AlOx interface has a dramatic effect on the magnetic anisotropy of the transition-metal layer. These sharp crossovers of anisotropy provide a very accurate and convenient way to monitor the oxidation of the tunnel barrier in magnetic tunnel junctions. This technique is also applied to characterize the oxidation kinetics of various ultrathin metallic layers as well as the aging effect in alumina barriers.
Rossby solitary vortices, on giant planets and in the laboratory
Authors: Nezlin Mikhail V.
Published: July 2002

This is a review of laboratory experiments with a layer of shallow water having a free surface and rotating together with a vessel of parabolic form. Such a (rather original) setup has allowed one to create Rossby solitary vortex for the first time. The latter is an anticyclonic Rossby vortex not subjected to dispersive spread owing to its compensation by the nonlinearity of KdV type. By its structural, collisional, and other properties, including clear-cut cyclonic–anticyclonic asymmetry, it may be considered as a physical prototype of the large-scale long-lived anticyclonic Rossby vortices like the Great Red Spot of Jupiter or the Great Dark Spot of Neptune (this remarkable vortex was discovered by the spacecraft Voyager-2 during its farewell to the Solar System) and other vortices dominating in the atmospheres of giant planets and created by the unstable zonal flows. It has been shown that the vortex under study is a long-lived entity provided it satisfies ‘‘antitwisting condition,’’ i.e., it has rather large amplitude (at which it rotates more quickly than it propagates and thereby carries the trapped fluid). In this case, it is not subjected to the ‘‘twisting’’ deformation and may be ascribed by the generalized Charney–Obukhov equation for Rossby vortices on shallow water with a free surface. The results of creating the vortex under consideration by the different methods have been compared with the results obtained by other authors in the experiments on shear-flow generation of Rossby vortices.
Effect of interfacial tension on propagating polymerization fronts
Authors: Texier-Picard R. , Pojman J. A. , Volpert Vit. A.
Published: July 2002

This paper is devoted to the investigation of polymerization fronts converting a liquid monomer into a liquid polymer. We assume that the monomer and the polymer are immiscible and study the influence of the interfacial tension on the front stability. The mathematical model consists of the reaction-diffusion equations coupled with the Navier–Stokes equations through the convection terms. The jump conditions at the interface take into account the interfacial tension. Simple physical arguments show that the same temperature distribution could not lead to Marangoni instability for a nonreacting system. We fulfill a linear stability analysis and show that interaction of the chemical reaction and of the interfacial tension can lead to an instability that has another mechanism: the heat produced by the reaction decreases the interfacial tension and initiates the liquid motion. It brings more monomer to the reaction zone and increases even more the heat production. This feedback mechanism can lead to the instability if the frontal Marangoni number exceeds a critical value.
Model of magnetic-field-induced ordering in dispersions of fine paramagnetic particles
Authors: Miles J. J. , Chantrell R. W. , Parker M. R.
Published: July 2002

A model of properties of colloidal dispersions of fine paramagnetic particles has been investigated. The model has been used to investigate the effects of large magnetic fields on the short-range order within the dispersions. The onset of significant short-range order at a well-defined critical field has been found, in agreement with previous experimental work.
Observation of current sheath transition from snowplow to deflagration
Authors: Woodall D. M. , Len L. K.
Published: July 2002

The measured temporal evolution of the current distribution in a capacitively-driven, coaxial plasma gun discharge is presented. The variation of the delay between admitting a gas puff at the breech of the gun and the discharge of the capacitor bank results in distinct modes of operation. Snowplow occurs for a long time delay or uniform prefill. Deflagration occurs for a short time delay, and is due to the domination of current sheath dynamics by the resistive magnetic field diffusion.
Transition-metal silicides lattice-matched to silicon
Authors: Zur A. , McGill T. C. , Nicolet M-A.
Published: July 2002

We have used a systematic search to determine all the possible transition-metal silicides that are geometrically lattice-matched to either the (100), (110), or the (111) face of silicon. A short table with the best possible matches is presented here, and a more comprehensive table including slightly worse matches is deposited with the editor.
Magnetic properties of mixed crystals with NiAs-type structure
Authors: Ido H.
Published: July 2002

Magnetic measurements have been carried out for the new mixed crystals such as Mn1−xTixAs (and Sb) and Cr1−xCoxSb with x ranging from 0 to 1. Mn1−xTixAs is ferromagnetic if x is less than 0.7, and the magnetic moment per Mn atom is found to be nearly constant (∼3.6 μB) for changing x. The Curie temperature TC and the asymptotic Curie temperature θp have a broad peak at around x=0.2. Mn1−xTixSb is also ferromagnetic if x is less than 0.7. The magnetic moment per Mn atom estimated from the saturation magnetization at 77 K is found to be about 10% larger than that estimated from the Curie–Weiss law above TC, but both remain nearly constant for changing x. The compounds Cr1−xCoxSb are antiferromagnetic but the detail is not clear at the present stage. The magnetic susceptibility does not obey the Curie–Weiss law for each specimen. The Néel temperature decreases on increasing x.
Surface discharges as intense photon sources in the extreme ultraviolet
Authors: Woodworth J. R. , McKay P. F.
Published: July 2002

We are investigating surface discharges as XUV photon sources for in situ cleaning and preionizing of anode surfaces in light-ion-fusion accelerators. In these experiments the surface discharge was formed by discharging a capacitor bank across a spark gap which consisted of two copper electrodes attached to an insulator and facing each other across a 5.5-mm gap. The surface discharge and its power feeds were constructed in a stripline configuration to minimize overall system inductance. When the surface discharge was driven by a 2.9-μF, 45-kV capacitor bank, the peak current thru the discharge was ∼250 kA. When driven at these levels, a single discharge radiated with a peak power of over 80 MW of 10–70 eV photons and had a total extreme ultraviolet output energy of 60 J per pulse.
Reactions of Pd on (100) and (110) GaAs surfaces
Authors: Kuan T. S. , Freeouf J. L. , Batson P. E. , Wilkie E. L.
Published: July 2002

The reactions of Pd on atomically clean or air-exposed (100) and (110) GaAs surfaces at temperatures between 20 to 500 °C in different ambients were investigated by transmission electron microscopy. Interfacial reactions quite different from previous x-ray results were observed and two new Pd-Ga-As ternary phases were identified for the first time. At lower temperatures (T≲250 °C) the formation of a ternary phase PdGa∼0.3As∼0.2, which has a hexagonal structure very similar to that of Pd2Ge or Pd2Si with a0=b0=0.672 nm and c0=0.340 nm, was observed. This ternary phase is epitaxially oriented with (12̄0)ternary∥(100)GaAs and [001]ternary∥[011]GaAs on (100) GaAs substrates, and with (11̄0)ternary∥(110)GaAs and [001]ternary∥[11̄0]GaAs on (110) GaAs substrates. At temperatures between 350 and 500 °C only one phase, PdGa, was observed to form in a high vacuum environment, whereas in a forming gas ambient, either a mixture of PdAs2 and another ternary phase PdGa∼0.6As∼0.4 (at 350 °C) or a mixture of PdAs2 and PdGa (at 500 °C) was observed. The ternary phase PdGa∼0.6As∼0.4 is also hexagonal in structure with a0=b0=0.947 nm and c0=0.374 nm. The PdGa phase formed at high temperature is epitaxially oriented on (100) substrates with (110) PdGa∥(100)GaAs and [1̄11]PdGa∥[011]GaAs, but is randomly oriented on (110) substrates. All these observations indicate that the Pd-GaAs reactions at T≳350 °C are very sensitive to the ambient conditions but not as sensitive to the GaAs surface cleanliness or substrate orientation. Correlation of these structural observations to ultraviolet and x-ray photoelectron spectroscopy data obtained from the same reacted interfaces are also discussed.
Measurements of the diffusion rate of lithium in aluminum at low temperature by elastic recoil detection analysis
Authors: Moreau C. , Allouche A. , Knystautas E. J.
Published: July 2002

The diffusion coefficient of lithium in aluminum has been investigated as a function of temperature between 150 and 240 °C by elastic recoil detection analysis. This dependence is found to be well described by the following expression: D =0.37(+1.30−0.28)exp{−[(126.1 ±5.2)kJ/ mol/RT]} cm2/s.
Liquid-junction cells with thin-film CuInSe2
Authors: Szot J. , Haneman D.
Published: July 2002

Thin films (n type) of CuInSe2 have been successfully adapted for use in liquid-junction photoelectrochemical (PEC) cells. The best films, of 4-μm thickness, showed 5.9% solar conversion efficiency and no degradation for over 15 kC cm−2, the most stable performance to date for thin-film PEC systems. Some 2-μm depth from the surface is affected by the iodine-based electrolyte, forming a p-type chemical film with a heterojunction interface and stabilized by saturating the electrolyte with CuI. The latter appears to be an important component of the chemical film.
An analytical model of thermionic discharges
Authors: Lawless J. L. , Lam S. H.
Published: July 2002

An analytical model is developed to describe a thermionic plasma discharge. With three easily interpreted algebraic equations, the behavior of the temperature, electron density, and arc drop are determined. Excellent agreement with experiment is found. The model considers a transverse discharge between two electrodes, one of which emits electrons thermionically. Transport of electrons and ions by electric field and density gradient is included. Ions are assumed to be created by volume ionization and are destroyed by recombination either on the walls or by three-body collision in the volume. Conservation of energy for the electrons requires that ohmic heating balance energy losses by collisions and by heat transfer to the walls. Thermionic discharges are also known as low-voltage arcs and occur in some types of thermionic energy converters. The role of the double-valued sheath and Schottky effects is discussed. A possible cause for discharge constriction is mentioned. Also, a ‘‘plasma ideal’’ thermionic discharge is defined and compared with the well-known ‘‘vacuum ideal’’ thermionic diode.
Monolithically integrated external feedback InGaAsP lasers using stop-cleaving technique
Authors: Antreasyan A. , Chen C. Y. , Napholtz S. G. , Wilt D. P. , Ostermayer F. W.
Published: July 2002

We have developed a monolithically integrated external feedback laser utilizing a pair of stop-cleaved double channel planar buried heterostructure lasers emitting at 1.3 μm. The external feedback is provided by the stop cleaved facet of the second cavity. A sidemode rejection ratio of 200 : 1 has been obtained with a laser having a 250-μm cavity length and an external resonator length of 200 μm. Since both lasers are fabricated in a self-aligned structure, this device may prove to be a powerful scheme for injection locking to reduce dynamic linewidth.
ArF short-pulse extraction studies
Authors: Mandl A.
Published: July 2002

A series of measurements characterizing an e-beam pumped ArF* laser was carried out using a 200-ns e-beam pulse having a rise time of 25 ns at currents up to 50 A/cm2. These pump conditions are relevant for inertial confinement fusion laser drivers. Intrinsic laser efficiencies up to 7.7% and power efficiencies up to 10% were observed. At high pressures (2 atm<P<5 atm) F2/Ar/Ne mixtures performed significantly better than F2/Ar mixtures. Absorption measurements were performed in Ne-buffered mixtures as a function of pressure, wavelength, and time. It appears that the background absorption peaks at a wavelength of 220 nm and is significantly smaller near 193 nm.
Transport coefficients of a chemically reacting plasma
Authors: Mittal M. L. , Gowda G. Paran
Published: July 2002

The analysis for the calculation of the transport coefficients of a chemically reacting partially ionized gas in the presence of a magnetic field is presented here. Taking the phenomenological kinetic equations and considering the ionization and recombination processes only due to electron impact parameter, the Chapman–Enskog method is used to obtain these transport coefficients. Sample numerical calculations are made for the transport coefficients of a potassium-seeded argon gas.
The nucleation and growth of glow-discharge hydrogenated amorphous silicon
Authors: Collins R. W. , Pawlowski A.
Published: July 2002

We present detailed analysis of in situ ellipsometry measurements taken during the deposition of hydrogenated amorphous silicon (a-Si:H) on crystalline silicon substrates in a conventional rf glow-discharge apparatus. This analysis concentrates on the initial growth data (<100 Å in thickness) which exhibit measurable deviation from a uniform growth model. Taking into account a surface roughness layer estimated from spectroscopic ellipsometry measurements, the deviation is found to be consistent with a nucleation process whereby the a-Si:H coalesces to bulk density at a thickness of about 55 Å. By stopping the glow discharge and restarting it again without changing the deposition parameters, the regrowth process and the nature of the regrowth interface have been studied. We have observed extremes in behavior from renucleation on a scale of 35 Å to the formation of an 80-Å-thick structurally defective layer at the regrowth interface. Such results provide a direct measure of the extent of structurally defective layers at amorphous semiconductor interfaces.
Mechanism of surface compound formations by cw CO2 laser irradiation of zirconium samples in air
Authors: Ursu I. , Mihăilescu I. N. , Nistor L. C. , Popescu M. , Teodorescu V. S. , Prokhorov A. M. , Ralchenko V. G. , Konov V. I.
Published: July 2002

The structure of hard and adherent surface layers formed by cw laser irradiation in air of zirconium disks is examined by electron microscopy and x-ray diffraction. A model is proposed for the evolution of the interaction process in this case.
Spark breakdown of liquid hydrocarbons. II. Temporal development of the electric spark resistance in n-pentane, n-hexane, 2,2 dimethylbutane, and n-decane
Authors: Fuhr Jitka , Schmidt Werner F.
Published: July 2002

The temporal variation of the spark resistance RFS of a tip-plane and a rod-plane electrode gap immersed in n-hexane, n-pentane, n-decane, and 2,2 dimethylbutane (neohexane) was measured by employing fast high-voltage and current measurement techniques. Three distinct regions were found: (1) rapid decrease of the resistance to 103 Ω, (2) development of the spark channel with RFS decreasing to 102 Ω, and (3) an exponential variation of RFS in time.
Identification of hydrogen platelets in proton-bombarded GaAs
Authors: Neethling J. H. , Snyman H. C.
Published: July 2002

The technique of cross-sectional transmission electron microscopy has been applied to study the nature of the radiation damage in GaAs bombarded with 300 keV protons to total doses of 1015 and 5×1015 protons cm−2. The results indicate that upon annealing at temperatures of 500 °C and above, the precipitated damage becomes visible in the form of hydrogen platelets (i.e., hydrogen-filled edge vacancy loops) on the {110} cleavage planes of GaAs.
Diffusion and desorption steps in the transport of Br and Br2 from a graphite effusion source at high temperatures, observed by molecular-beam mass spectrometry
Authors: Sigurdsson Arnar , Holmlid Leif
Published: July 2002

The transport of bromine out of a graphite source at 300–1350 K was studied with molecular-beam mass spectrometry. The mass-spectrometer signals from both Br and Br2 varied approximately linearly with the total pressure in the source. A theoretical comparison with the gas-phase density of Br in the source gave good fits. A pressure-dependent grain-boundary diffusion part and a pressure-independent, probably bulk diffusion related part of the signals can be separated in the analysis of the results. Apparent desorption energies for Br and Br2 from the graphite surface were found to be 0.7±0.1 and 0.3±0.1 eV, respectively, from the temperature dependence of the bulk diffusion signal.
Attenuation measurements of an ultrasonic Rayleigh wave propagating along rough surfaces
Authors: de Billy M. , Quentin G. , Baron E.
Published: July 2002

The attenuation of Rayleigh surface waves by one- and two-dimensional rough surfaces is investigated on duraluminum and titanium samples. The root-mean-square deviation of the surface from flatness h (8.5<h<90 μm) satisfies the condition of validity of the perturbation theory approach ε=h/λR≪1. The excitation frequencies of the transmitter vary from 2.2. to 10 MHz. Experimental results for one- and two-dimensional rough surfaces are presented and discussed. A comparison with the theoretical calculations of Eguiluz and Maradudin [Phys. Rev. B 28, 728 (1983)] confirms the F5 dependence of the attenuation α as aq∥→0.
Oxidation behavior of Nd-Fe-B magnets
Authors: Jacobson J. , Kim A.
Published: July 2002

This corrosion study was performed to determine the reaction kinetics of Nd-Fe-B type magnets in oxidizing environments. Thus the oxidation model developed provides basic information for the development of oxidation-resistant magnets. The oxidation of Nd-Fe-B magnets was studied at temperatures between 20 and 150 °C. The oxidation rates of the magnets were measured as functions of humidity in air, temperature, and alloy composition. Aerated salt and distilled water, at room temperature, were used as accelerating environments. The morphology of the reaction products was analyzed with scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) and used to aid the interpretation of the reaction kinetics. The kinetics for the oxidation of magnets generally exhibit a parabolic weight increase in humid air at room temperature. Minor alloying additions affect the corrosion rate. In aerated distilled water, Nd-Fe-B magnets generally exhibit a linear weight loss. The magnets in aerated salt water show an inverse parabolic weight loss. Magnetization of the magnets substantially reduced the oxidation rate in the salt water test. The oxidation mechanism will be discussed.
Magnetostriction ‘‘jumps’’ in twinned Tb0.3Dy0.7Fe1.9
Authors: Clark A. E. , Teter J. P. , McMasters O. D.
Published: July 2002

Large ‘‘jumps’’ in the magnetostriction have been observed in twinned single crystals of Tb0.3Dy0.7Fe1.9 (Terfenol-D) for magnetic fields parallel to the crystalline [112̄] direction. The interpretation of these large magnetostriction discontinuities is based upon a model of twinned dendritic Terfenol-D in which the magnetization of one twin jumps between two [111] directions while the magnetization of the remaining twin undergoes a continuous rotation of the magnetization. The field dependence of the magnetization and magnetostriction of cubic single crystals with λ111≫λ100 was calculated using an expression which included the anisotropy constants K1 and K2 and compressive loads along [112̄]. With K1=−0.6 J/m3 and K2=−2.0 J/m3 (values appropriate for Terfenol-D near room temperature), magnetization ‘‘jumping’’ is predicted. For the twinned crystal, the jump in the magnetostriction was calculated to be greater than 1000 ppm. Because of this large magnetostriction, it is possible to configure a device to perform a substantial amount of work by the application of only a triggering magnetic field centered about an optimum bias field.
Dependence on peak induction and grain size of power losses in nonoriented SiFe steels
Authors: Ban G. , Bertotti G.
Published: July 2002

We report loss separation measurements performed on 1.8% SiFe nonoriented 0.2-mm laminations of variable grain size 20 μm≤s≤130 μm, in the frequency range 1 Hz≤ f≤300 Hz and peak induction range 0.7 T≤B≤1.5 T. It is shown that the excess loss behavior is well reproduced by the equation Pe =k(Bf)3/2, in agreement with a theory recently proposed by one of the authors, and the microstructural interpretation of the parameter k is discussed. The results obtained suggest that a systematic investigation of the properties of k may represent a new, promising approach to the characterization of microstructure and texture properties of nonoriented electrical steels.
Laser chemical etching of metals in sodium nitrate solution
Authors: von Gutfeld R. J. , Vigliotti D. R. , Datta M.
Published: July 2002

A focused cw argon-ion laser has been used to etch several metals, including copper, molybdenum, nickel, niobium, and stainless steel in a 0.5M sodium nitrate neutral salt solution. Exceedingly smooth etch surfaces have been obtained for several of these metals with etch rates as high as 4 μm/s. Experiments were carried out as a function of laser intensity and laser dwell time to etch arrays of small holes, 100 μm or less in diameter on the different samples. From experiments performed in water under similar laser conditions we conclude that melt or near-melt temperatures are required to obtain etching in the salt solution. A comparison of the power required to produce incipient etching is made with calculated temperatures obtained for similar powers using a theory which approximates the experimental conditions. These calculated temperatures are reasonably close to the melt temperatures for most cases and offer supporting evidence for our etching model.
An atomic-resolution atomic-force microscope implemented using an optical lever
Authors: Alexander S. , Hellemans L. , Marti O. , Schneir J. , Elings V. , Hansma P. K. , Longmire Matt , Gurley John
Published: July 2002

We present the first atomic-resolution image of a surface obtained with an optical implementation of the atomic-force microscope (AFM). The native oxide on silicon was imaged with atomic resolution, and ≊5-nm resolution images of aluminum, mechanically ground iron, and corroded stainless steel were obtained. The relative merits of an optical implementation of the AFM as opposed to a tunneling implementation are discussed.
The cooling rate dependence of cation distributions in CoFe2O4
Authors: De Guire Mark R. , O’Handley Robert C. , Kalonji Gretchen
Published: July 2002

The room-temperature cation distributions in bulk CoFe2O4 samples, cooled at rates between <10−2 and ∼103 °C s−1, have been determined using Mössbauer spectroscopy in an 80-kOe magnetic field. With increasing cooling rate, the quenched structure departs increasingly from the mostly ordered cation distribution ordinarily observed at room temperature. However, the cation disorder appears to saturate just short of a random distribution at very high cooling rates. These results are interpreted in terms of a simple relaxation model of cation redistribution kinetics. The disordered cation distributions should lead to increased magnetization and decreased coercivity in CoFe2O4.
Theory of electronic transport in two-dimensional Ga0.85In0.15As/Al0.15Ga0.85As pseudomorphic structures
Authors: Park Duke H. , Brennan Kevin F.
Published: July 2002

We present calculations of the electron drift velocity and valley populations as a function of applied electric field in two different Ga0.85In0.15As/Al0.15Ga0.85 As heterostructure systems which show two-dimensional size quantization effects. Electronic transport in both a pseudomorphic, double heterostructure system formed by use of three separate semiconductor layers is examined and compared to that of a simpler two-layer, single heterostructure. The calculations are made based on an ensemble Monte Carlo simulation which includes the full physics of the two-dimensional system. The double heterostructure consists of a Ga0.85In0.15 As quantum well bordered on one side by a highly doped (1×1018 1/cm3 ) Al0.15Ga0.85 As layer and on the other by a nearly intrinsic GaAs layer consistent with the design of most pseudomorphic high electron mobility transistors. In order to assess the effect of the electronic confinement within the quantum well on the resulting carrier velocities, the results are compared to calculations of the electron drift velocity in a single heterostructure. The single-heterostructure system consists of two separate layers of doped Al0.15Ga0.85 As and nearly intrinsic Ga0.85In0.15 As. It is found that the electron drift velocity and mobility is much larger within the double heterostructure than within the single heterostructure over a wide range of applied electric field strengths. The presence of the surrounding GaAs layer in the double heterostructure acts to confine the electrons within the two-dimensional system resulting in a significant increase in the average steady-state electron drift velocity.
Theoretical comparison of electron real-space transfer in classical and quantum two-dimensional heterostructure systems
Authors: Brennan Kevin F. , Park Duke H.
Published: July 2002

We present a comparison of the effect of real-space transfer on the electron drift velocities in both classical heterostructure systems, those in which spatial quantization effects do not occur, and in two-dimensional heterostructure systems using an ensemble Monte Carlo simulation. The calculations for the two-dimensional system are based on a first-principles formulation of electron transport in a triangular quantum well system using an ensemble Monte Carlo code tailored to include the basic physics of two-dimensional systems. In addition, we present an analysis, again based on a complete ensemble Monte Carlo simulation, of real-space transfer from classical systems, ones in which no two-dimensional gas is formed at the heterointerface. Electron drift velocities within the classical system greater than that possible in the constitutive bulk materials are thwarted by either real-space transfer out of the high mobility material into the adjacent low mobility material or k-space transfer within the narrow gap material itself. In contrast, higher electron drift velocities than that achievable in the bulk occur in a system in which two-dimensional effects are present. In this case, when the electrons are confined within the two-dimensional gas, their corresponding drift velocities are somewhat larger than within the bulk three-dimensional system. We conclude that in electronic devices in which the electric field is applied parallel to the heterostructure layers, that the highest steady-state electron velocities are achieved for transport within the two-dimensional gas. In structures in which either a two-dimensional system is not present or the carriers all reside outside of the quantized states, the steady-state electron drift velocity is always less than or equal to the corresponding velocity in the bulk material due to the combined actions of real-space and k-space transfer.