bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase-energy approach to collision-induced vibrational relaxation
Authors: Kazansky A. K. , Ermoshin V. A. , Engel V.
Published: July 2002

A representation of the diatomic vibrational distribution function in energy and phase variables yields a transparent physical picture of collision-induced vibrational relaxation. In this way, the energy— and phase—relaxation processes can be separated. A simple parametrization of the phase distribution allows for the extraction of the dephasing time and the reconstruction of autocorrelation functions and femtosecond pump–probe signals. It is shown that the dephasing time can be derived from the Fourier spectrum of the pump–probe signal in favorable cases.
Stoichiometry changes by selective vacancy formation on (110) surfaces of III–V semiconductors: Influence of electronic effects
Authors: Semmler U. , Simon M. , Ebert Ph. , Urban K.
Published: July 2002

We investigated the kinetics of thermal formation of anion vacancies and the subsequent stoichiometry changes on (110) cleavage surfaces of III–V semiconductors by scanning tunneling microscopy. We found that the rate of spontaneous formation of monovacancies depends very sensitively on the doping of the underlying semiconductor and the concentration of surface vacancies. It is shown that the position of the Fermi energy at the surface is the major electronic influence on the energy barrier height for the vacancy formation. We found barrier heights in the range of 1.1–1.3 eV for GaAs and InP. The physical factors affecting the vacancy formation and the surface stoichiometry are discussed in detail.
Transport properties of GaAs1−xNx thin films grown by metalorganic chemical vapor deposition
Authors: Ahrenkiel R. K. , Johnston S. W. , Keyes B. M. , Friedman D. J. , Vernon S. M.
Published: July 2002

A series of devices with the structure GaAs/GaAs1−xNx/GaAs and 0.01<x<0.03 have been grown by metalorganic chemical vapor deposition. The transient photoconductive decay of these structures is measured as a function of excitation wavelength and injection level. The decay process is generally described by a stretched exponential function with anomously large decay times. The photoconductive excitation spectrum extends into the infrared, well beyond the bandgap of the given alloy. The processes here can be explained by nitrogen clusters that produce charge separation.
Narrow luminescence lines from self-assembled CdSe quantum dots at room temperature
Authors: Zheng Z. H. , Okamoto K. , Ko H. C. , Kawakami Y. , Fujita Sg.
Published: July 2002

Narrow luminescence lines from self-assembled CdSe quantum dots (QDs) are studied at room temperature by using time-resolved photoluminescence spectroscopy. The narrowest line of the luminescence has a full width at half maximum of 2.1 meV at room temperature. The narrow luminescence lines are mainly the contribution of CdSe QDs with a longer recombination lifetime to the luminescence.
Investigations of pure rotational transitions of H2 perturbed by He. II. High-temperature calculations and extrapolations
Authors: Dubernet M.-L. , Tuckey P. A. , Jolicard G. , Michaut X. , Berger H.
Published: July 2002

Close-coupling calculations of broadening and shifting coefficients of Raman S0(j) lines of H2 perturbed by He are performed up to 3000 K. They are used to test two approximate methods, the quantum coupled states theory and the RB semiclassical model. They are further applied to study the use of simple functional forms for fitting and extrapolating experimental data, and some new fits of experimental broadening coefficients are given.
Triple-point wetting of Ne on solid CO2
Authors: Bruschi L. , Paniz E. , Mistura G.
Published: July 2002

We have studied the wetting behavior of Ne adsorbed on solid CO2 by means of a quartz crystal microbalance technique. In contrast to recent density functional calculations that predict a prewetting transition near the bulk critical point of Ne, we have observed the more common triple-point wetting behavior.
Extraction of ions from the matrix sheath in ablation-plasma ion implantation
Authors: Qi Bo , Lau Y. Y. , Gilgenbach R. M.
Published: July 2002

A simple one-dimensional theory is presented to assess the implantation of ions from the ion matrix sheath (IMS) in an ablated plasma plume that is approaching a negatively biased substrate. Under the assumption that the plume geometry, the electron and ion density distributions, and the potential distribution are frozen during the IMS extraction, the implanted ion current is calculated as a function of time for various substrate-plume separations. This model accurately recovers Lieberman’s classic results when the plume front is initially in contact with the substrate.
Photothermal models for lock-in thermographic evaluation of plates with finite thickness under convection conditions
Authors: Bai W. , Wong B. S.
Published: July 2002

Lock-in thermography is a technique which is increasingly being used for the evaluation of relatively thin materials such as composite materials in aircraft structures. In such cases, the models available in the literature cannot accurately predict the difference in phase values between defective and nondefective regions. This is because the models do not consider the convection conditions and the fact that the thickness of the sample is finite. This article proposes two models (a single-layer model and a multilayer model) to rectify this situation. The single-layer model was used to analyze the thermal wave behavior in homogeneous media and the multilayer model was used to analyze the thermal wave behavior in inhomogeneous media. Based on the theoretical and experimental results, the influence of the combined heat transfer coefficient was discussed. The results predicted by the models were consistent with that obtained experimentally.
The interaction of the solar wind and stellar winds with the partially ionized interstellar medium
Authors: Zank G. P. , Müller H.-R. , Wood B. E.
Published: July 2002

Plasmas in the laboratory and interstellar and interplanetary space are frequently partially ionized. Thus, the solar wind and stellar winds often interact with an interplanetary medium that is an admixture of protons, electrons, other charged ions, and neutral atoms. For example, the very local interstellar medium surrounding our heliosphere may be less than 50% ionized, with the dominant constituent being neutral hydrogen (H). As a result, the composition of the solar wind in the outer heliosphere beyond some 10–15 AU is dominated by neutral interstellar H. Our understanding of the complex physics describing the interaction of the solar wind with the partially ionized local interstellar medium (ISM) has advanced significantly in the last 5 years with the development of very sophisticated models which treat the coupling of neutral atoms and plasma self-consistently. A number of major predictions have emerged from these models, such as the existence of a large wall of heated neutral hydrogen upstream of the heliosphere. Remarkably, in the ensuing years, this prediction has been confirmed by high-resolution Hubble Space Telescope Lyman-α spectroscopic data. Subsequent models now consider the interaction of various stellar winds with the ISM, and Hubble observations provide supporting data. An introductory review of the basic physics, and associated observations, of the interaction of the solar wind and stellar winds with the interstellar medium is presented for this exciting and rapidly developing field.
Induced magnetic anisotropy in metallic glasses irradiated by swift heavy ions
Authors: Juraszek J. , Fnidiki A. , Toulemonde M.
Published: July 2002

Effects of swift heavy ion irradiation on ferromagnetic amorphous Fe81B13.5Si3.5C2 ribbons were investigated by transmission and conversion electron Fe57 Mössbauer spectrometry. Irradiation was found to induce changes in the magnetic texture depending on the ion energy and the corresponding projected range. The magnetic anisotropy tends to become randomly distributed in the case of the highest ion energy as the result of the strong level of electronic excitations induced throughout the sample. A perpendicular magnetic anisotropy was evidenced in the case of implantation of the ion beam inside the amorphous alloys. The origin of the induced perpendicular magnetic anisotropy was interpreted in terms of magnetoelastic effects due to strong stresses generated by the reaction of the nonirradiated region to the anisotropic deformation of the irradiated amorphous material.
Scanning impedance microscopy of electroactive interfaces
Authors: Kalinin Sergei V. , Bonnell Dawn A.
Published: July 2002

A scanning probe technique based on the detection of the phase change of cantilever oscillations induced by a lateral bias applied to the sample is developed. This technique is used to investigate Σ5 grain boundary in Nb-doped SrTiO3 bicrystal. Tip bias, frequency, and driving amplitude dependence of cantilever response to sample ac bias were found to be in excellent agreement with the theoretical model. This technique, further referred to as scanning impedance microscopy, allows mapping of the local phase angle of complex microstructures. This technique is complemented by scanning surface potential microscopy (SSPM). Ramping the lateral dc bias during SSPM measurements allows the voltage characteristics of the grain boundary to be reconstructed and dc transport properties to be obtained by an equivalent circuit method. The combination of scanning impedance microscopy and scanning surface potential microscopy allows independent quantification of interface resistivity and capacitance, thus providing spatially resolved impedance spectra of complex microstructures.
High-density plasma production with potential confinement in the GAMMA 10 tandem mirror
Authors: Ichimura M. , Cho T. , Hirata M. , Hojo H. , Ishii K. , Itakura A. , Katanuma I. , Kohagura J. , Nakashima Y. , Saito T. , Tamano T. , Tanaka S. , Tatematsu Y. , Yatsu K. , Yoshikawa M.
Published: July 2002

The improvement of potential confinement was attained in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985); Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 2, p. 539] by axisymmetrization of heating systems for the plasma production, heating, and potential formation. A significant increase of the density and diamagnetism by the potential confinement was observed. In the previous experiment, it was difficult to increase the central cell density higher than 2.7×1018 m−3. One of the possible mechanisms is the density clamping due to the eigenmode formation of the ion–cyclotron-range of frequency (ICRF) waves in the axial direction. With high harmonic ICRF waves (RF3), the experiments to overcome this problem have been performed. In preliminary experiments with RF3 and NBI the maximum density of 4×1018 m−3 was attained.
How astrophysical mean field dynamos can circumvent existing quenching constraints
Authors: Blackman Eric G. , Field George B.
Published: July 2002

Mean field dynamo theory is a leading candidate to explain the large scale magnetic fields of galaxies and stars. However, controversy arises over the extent of quenching by the backreaction of the growing field. Here boundary conditions and magnetic helicity flow are shown to play a role in determining whether the mean field dynamo action is fast, as required by astrophysical systems, or resistively limited (slow). Existing work suggesting that mean field dynamos are resistively limited include restrictive approximations such as stationarity and periodic boundary conditions that suppress magnetic helicity flow. Thus even though the backreaction is present, such studies cannot unambiguously reveal whether real astrophysical mean field dynamos are dynamically suppressed when the helicity flow is allowed. If the dynamo is sustained by an outflow of helicity from the system, then a magnetically active corona is expected. Open boundaries alone may not be sufficient for rapid dynamo action and the additional physics of buoyancy and outflows may be required. Possible simulation approaches to test some of the principles are briefly discussed. Some limitations of the “Zeldovich relation” are also addressed.
Modeling of vacancy cluster formation in ion implanted silicon
Authors: Chakravarthi Srinivasan , Dunham Scott T.
Published: July 2002

Ion implantation of silicon introduces excess point defects that quickly recombine during annealing leaving net interstitial and vacancy populations. For higher energy implants, the separation between interstitials and vacancies is larger, leading to a vacancy rich region towards the surface and an interstitial rich region deeper in the bulk. The high supersaturation of vacancies in the near surface region can lead to their aggregation into vacancy clusters or voids. In this work we have developed a continuum model for vacancy clusters using discrete cluster sizes. Results from atomistic calculations [Bongiorno et al., Europhys. Lett. 43, 695 (1998)] are used for the energetics of the cluster growth/dissolution. The model is compared to data from Venezia et al. [Appl. Phys. Lett. 73, 2980 (1998)] for Au indiffusion subsequent to Si high energy implants. We found good agreement with experimental data using this model without any tuning of the parameters. However, this model is too complex and computationally expensive to be effectively incorporated into continuum process simulation tools. Hence we reduced this system of discrete rate equations into a two-moment model by carefully considering the behavior of the full model under a range of conditions. The parameters of the moment-based model follows from the full model, which in turn is based on atomistic calculations. The resulting simple and computationally efficient model is found to accurately reproduce the Au labeling experiments.
Oxygen-dislocation interactions in silicon at temperatures below 700 °C: Dislocation locking and oxygen diffusion
Authors: Senkader S. , Wilshaw P. R. , Falster R. J.
Published: July 2002

The locking of dislocations by oxygen atoms in Czochralski–silicon at temperatures between 350 and 700 °C has been studied. Both experimental and theoretical investigations were carried out for different oxygen concentrations, different annealing times (from 10 to 3×107 s), and different point defect concentrations. It was found that the unlocking stress of dislocations at low temperatures follows similar trends to those previously observed at higher temperatures and is determined by annealing temperature, time, and oxygen concentration. However, in the present temperature range, experimental results indicate an enhanced transport of oxygen to dislocations. Numerical simulations solving the diffusion equation for oxygen transport to the dislocations show that the effective diffusivity of oxygen at lower temperatures diverges from “normal” diffusivity of oxygen. We have shown that oxygen transport can be as much as three orders of magnitude higher than that which would be assumed by extrapolation of the “normal” data obtained at higher temperatures. In the low temperature regime the effective diffusivity is dependent on the oxygen concentration and has an activation energy of about 1.5 eV.
La doped Ba1−xSrxTiO3 thin films for tunable device applications
Authors: Cole M. W. , Joshi P. C. , Ervin M. H.
Published: July 2002

Pure and La doped Ba0.6Sr0.4TiO3 thin (BST) films were fabricated via the metalorganic solution deposition technique using carboxylate-alkoxide precursors on Pt–Si substrates. The La doping concentration, from 0 to 10 mol %, was found to have a strong influence on the 750 °C postdeposition annealed films material properties. All films possessed a nontextured polycrystalline microstructure with no evidence of secondary phase formation. The pure and 1 mol % La doped films exhibited a uniform microstructure suggestive of a fully developed film at this annealing temperature. Improved dielectric and insulating properties were achieved for the 1 mol % La doped BST thin films with respect to that of undoped BST films. The 1 mol % La doped BST film exhibited a lower dielectric constant, (283 vs 450) and enhanced resistivity (31.4×1013 Ω cm vs 0.04×1013 Ω cm) with respect to that of undoped BST films. The loss tangent and tunability (at 100 kHz) of the 1 mol % La doped BST films were 0.019% and 21% (at E=300 kV/cm), respectively. Films doped at concentrations between 5 and 10 mol % possessed under developed microstructures suggesting that higher annealing temperatures and/or longer annealing times are required. The single phase structure of the 5–10 mol % La doped BST films, combined with the beneficial influence of the 1 mol % La doping on the BST films dielectric and insulating properties, suggest potential for further enhancement of the films material properties after optimization of the thermal treatments for the 5–10 mol % La doped BST thin films.
Self-heating in small mesa structures
Authors: Krasnov V. M. , Yurgens A. , Winkler D. , Delsing P.
Published: July 2002

We study analytically and numerically a problem of self-heating in small mesa structures. Our results show that self-heating is proportional to a characteristic in-plane size of the mesa. Experimental data for small high-Tc superconductor Bi2212 mesas are in qualitative agreement with our calculations. We estimate self-heating in Bi2212 mesas with different sizes and demonstrate that self-heating can effectively be obviated in small mesa structures.
Branching ratios of C2 products in the photodissociation of C2H at 193 nm
Authors: Mebel A. M. , Hayashi M. , Jackson W. M. , Wrobel Jacek , Green Martina , Xu Dadong , Lin S. H.
Published: July 2002

Experimental and theoretical results are combined to show that vibrationally excited C2H radicals undergo photodissociation to produce C2 radicals mainly in the B 1Δg state. Infrared (IR) emissions from the photolysis of acetylene with a focused and unfocused 193 nm excimer laser have been investigated using step-scan Fourier transform infrared (FTIR) emission spectroscopy at both low and high resolution. With an unfocused laser, the low-resolution infrared emission spectra from the C2H radicals show a few new vibrational bands in addition to those previously reported. When the laser is focused, the only emissions observed in the 2800–5400 cm−1 region come from the electronic transitions of the C2 radicals. Most of the emissions are the result of the B 1Δg→A 1Πu transition of C2 although there are some contributions from the Ballik–Ramsay bands C2(b 3Σg−→a 3Πu). A ratio of [B 1Δg]/[b 3Σg−]=6.6 has been calculated from these results. High quality theoretical calculations have been carried out to determine what kind of ratio could be expected if the photodissociation products are formed solely by adiabatic dissociation from the excited states of C2H. To accomplish this, the geometries of different electronic states of C2H (X 2Σ+, A 2Π, 3–6 2A′, and 2–5 2A″) were optimized at the complete active space self consistent field [CASSCF(9,9)/6-311G**] level. The calculated normal modes and vibrational frequencies were then used to compute Franck–Condon factors for a variety of vibronic transitions. In order to estimate the oscillator strengths for transitions from different initial vibronic states of C2H, transition dipole moments were computed at different geometries. The overall Franck–Condon factor for a particular excited electronic state of C2H is defined as the sum of Franck–Condon factors originating from all the energetically accessible vibrational levels of C2H(X,A) states. The adiabatic excitation energies were calculated with the multi-reference configuration interaction/correlation-consistent polarized valence triple zeta [MRCI(9,9)/cc-PVTZ] method. The overall Franck–Condon factors were then multiplied by the corresponding oscillator strengths to obtain the total absorption intensities characterizing the probabilities for the formation of different excited states. Then, the excited states of C2H were adiabatically correlated to various electronic states of C2 (B 1Δg, A 1Πu, B′ 1Σg+, c 3Σu+, and b 3Σg−) to predict the photodissociation branching ratios from the different states of C2H, such as X(0,ν2,0), X(0,ν2,1), A(0,0,0), and A(0,1,0). For C2H produced by 193 nm photodissociation of acetylene, the calculations gave the following B:A:B′:b:c branching ratios of 38:32:10:14:6. This means that the theoretical branching ratio for the [B 1Δg]/[b 3Σg−] is 2.7, which is in excellent agreement with experiment.
Grazing incidence scattering of hyperthermal C60+, C76+, and C84+ from graphite: Electron transfer dynamics
Authors: Hillenkamp Matthias , Pfister Jörg , Kappes Manfred M.
Published: July 2002

Hyperthermal grazing incidence scattering (Ecoll=500–700 eV, θin=80°) of fullerenes C60+, C76+, and C84+ from highly oriented pyrolytic graphite surfaces is associated with efficient one and two electron transfer to yield parent neutrals and anions—in addition to parent cations. We report lower limits to nascent anion yields under conditions for which collisional heating is kept relatively small. Largest anion yields were observed for C84, which also has the largest electron affinity of the projectiles probed. Here >3% of the particles scattered upon 11 500 ms−1 collision survive detection as anions. This can be qualitatively understood in terms of a resonant electron transfer mechanism and a simple kinetic scheme.
Molecular dynamics study of n-alcohols adsorbed on an aqueous electrolyte solution
Authors: Daiguji Hirofumi
Published: July 2002

The distribution of normal alcohol (n-alcohol) on water and the effect of salt on the structural and dynamical properties of n-alcohol on aqueous electrolyte solutions were investigated using molecular dynamics simulation. The stability of the alcohol distribution was studied for three types of n-alcohol (n-propanol, C3H7OH; n-heptanol, C7H15OH; and n-undecanol, C11H23OH), four or five concentrations of alcohol, and three concentrations of salt. The simulation results reveal the following. The distribution of n-propanol on water is homogeneous at all n-alcohol concentrations studied here and the distribution of n-heptanol and n-undecanol on water is heterogeneous. The n-alcohol concentration at which fluctuations in the alcohol distribution begin to increase depends on the length of the hydrocarbon chain of the n-alcohol. Salt concentration affects the surface excess concentration of n-alcohol and the stability of the adsorbed layer of n-alcohol. The degree of each effect depends on the length of the hydrocarbon chain of the n-alcohol. For n-undecanol, the surface structure of n-alcohol is independent of salt concentration because interaction between the hydrocarbon chains is sufficiently strong. In absorption refrigeration technology, to enhance the absorption rate of water vapor into a highly concentrated aqueous electrolyte solution, a small amount of alcohols is added to the aqueous electrolyte solution, which induces cellular convection referred to as Marangoni instability. Among the three types of n-alcohol studied here, only n-heptanol induces strong cellular convection. The simulations reveal two required conditions for Marangoni instability: generation of fluctuations in the alcohol distribution on water, and strong correlation between the structural and dynamical properties and salt concentration. Among the three types of n-alcohol studied here, based on the simulations, only n-heptanol satisfies both conditions.
Transferability in alkyl monoethers. II. Methyl and methylene fragments
Authors: Vila Antonio , Mosquera Ricardo A.
Published: July 2002

The transferability of the atomic and bond properties of the methyl and methylene fragments in linear unbranched alkyl monoethers was studied using the Theory of Atoms in Molecules (AIM). This theory has been applied to the analysis of the HF/6-31++G**//HF/6-31G* electron charge distributions of a series of 33 dialkyl ethers, CH3(CH2)mO(CH2)nCH3, [n=0,1(n⩽m⩽9), n=2,3(n⩽m⩽8), n=4(n⩽m⩽5)]. The results obtained indicate that the methyl and methylene fragments situated in α, β, γ, or δ positions with respect to the oxygen atom are different to those of an n-alkane. Nevertheless, CH3 and CH2 at more distant positions can be considered as standard units, whose nonenergetic properties coincide with those of the corresponding fragment in an n-alkane. On the contrary, the energetic properties of the fragments maintain a differential value with respect to the n-alkane in all of the positions studied in the series. The properties of the methyl or methylene fragments in α to the oxygen depend on the size (methyl or larger) of the other alkyl chain bonded to the oxygen. The properties of methylenes are also different when they are α to the terminal CH3. Thus, the CH2 and CH3 fragments of dialkyl ethers can be classified into 9 CH3 and 12 CH2 groups. All of the groups proposed verify the transferability of bond properties, charge, and volume throughout all the fragments that it includes. Though the energy of the fragments depend on the size of the molecule, fragments included in the same group display a common dependence. This dependence does not impede the appearance of excellent linear relationships between the total molecular energy and the number of CH2 groups. Nevertheless, the AIM computed energies for the oxygen atom are always more negative than those obtained from the fittings of total electron energies to the number of CH2 groups in the molecule. This stabilization is produced at the cost of destabilizing the CH2 or CH3 groups in the α position. Whereas, if the CH2 groups bonded to a methyl group are excluded, the remaining CH2 and CH3 groups are slightly stabilized (in a magnitude that depends on the size of the molecule and which oscillates slightly in its position with respect to the oxygen atom).
Ab initio structures and polarizabilities of sodium clusters
Authors: Kronik Leeor , Vasiliev Igor , Jain Manish , Chelikowsky James R.
Published: July 2002

We present quantitative ab initio calculations for Na cluster structures and polarizabilities, for all cluster sizes up to 20 atoms. Our calculations are performed by combining an ab initio core-corrected pseudopotential and a gradient-corrected density functional within a real space approach. We find the cluster bonding to be very floppy and catalog a host of low-energy quasi-degenerate isomers for all second-decade clusters. The existence of these isomers results in a band of polarizability values for each cluster size even at zero temperature. This eliminates any finer structure in the polarizability curve. We further show that the experimental polarizability values are consistently underestimated by calculations at zero temperature. By computing the effects of structure expansion and distortion due to a finite temperature we arrive at a quantitative agreement between theory and experiment.
A fractional diffusion equation for a marker in porous media
Authors: Erochenkova G. , Lima R.
Published: July 2002

To study the properties of the flow of a marker through an irregular packed bed, porosity is described by stochastic processes with exponentially decaying correlation functions. We show that the ensemble average concentration of the marker satisfies a fractional diffusion equation.
Density depletion profile and solvation free energy of a colloidal particle in a polymer solution
Authors: Maassen R. , Eisenriegler E. , Bringer A.
Published: July 2002

The solvation free energy and polymer density depletion profile of a single mesoscopic colloidal particle in a solution of free nonadsorbing polymer chains are investigated theoretically. Keeping both the particle to polymer size ratio and the degree of inter-chain overlap arbitrary, we see how the qualitatively different behavior evolves in the limits of small and large size ratios and of dilute and semidilute solutions. While most of our results are obtained within a mean-field approach, we also use a “renormalized tree approximation” to estimate the surface tension and the coefficient of spontaneous curvature in a Helfrich expansion for large particle to polymer size ratio. There is a weak maximum in the polymer density profile for arbitrary size ratio. For small size ratio the maximum can be explained in terms of a minimum in the bulk polymer density correlation function.
Current crowding in GaN/InGaN light emitting diodes on insulating substrates
Authors: Guo X. , Schubert E. F.
Published: July 2002

GaN/InGaN light emitting diodes (LEDs) grown on sapphire substrates have current transport along the lateral direction due to the insulating nature of the substrate. The finite resistance of the n-type GaN buffer layer causes the pn junction current to be nonuniform and “crowd” near the edge of the contact. The current-crowding effect is analyzed both theoretically and experimentally for p-side-up mesa structure GaN/InGaN LEDs. The calculation yields an exponential decay of the current distribution under the p-type contact with a characteristic current spreading length, Ls. It is shown that GaN/InGaN LEDs with high p-type contact resistance and p-type confinement layer resistivity have a relatively uniform current distribution. However, as the p-type GaN conductivity and p-type ohmic contact conductivity is improved, significant current crowding near the contact edge will occur. The current crowding effect is analyzed experimentally in GaN/InGaN LEDs emitting in the blue spectral range. Experimental results show the light intensity decreasing with distance from the contact edge. A current spreading length of Ls=525 μm is found, in good agreement with theory.
Contact pair correlation functions and equation of state for additive hard disk fluid mixtures
Authors: Barrio C. , Solana J. R.
Published: July 2002

The contact pair correlation functions and the equation of state for a binary mixture of additive hard disks is obtained using a procedure similar to that leading to the Boublı́k–Mansoori–Carnahan–Starling–Leland equation of state for mixtures of additive hard spheres. The results from the derived equations are tested against new Monte Carlo data obtained for several diameter ratios and mole fractions. The overall agreement is excellent. The equation of state reproduces exactly the second virial coefficient of the mixture and the third with great accuracy. Predicted values of the fourth and fifth virial coefficients are also in very good agreement with numerical data.
AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy
Authors: Smorchkova I. P. , Chen L. , Mates T. , Shen L. , Heikman S. , Moran B. , Keller S. , DenBaars S. P. , Speck J. S. , Mishra U. K.
Published: July 2002

We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to 3.6×1013 cm−2. Room-temperature sheet resistance of 180 Ω/□ is demonstrated in the AlN/GaN structure with a 35 Å AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 Å) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 Å) AlN layer between AlxGa1−xN and GaN in the AlxGa1−xN/GaN (x∼0.2–0.45) 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mobility in comparison to standard AlGaN/GaN structures. At room temperature, a combination of the high 2DEG sheet density of 2.15×1013 cm−2 and high electron mobility of 1500 cm2/V s in Al0.37Ga0.63N/AlN/GaN yielded a low sheet resistance value of 194 Ω/□.
Transition strengths and potential curves for the valence transitions in Br2 from a reanalysis of the ultraviolet-visible absorption at low resolution
Authors: Tellinghuisen Joel
Published: July 2002

The absorption profile of gaseous Br2 in the 320–675 nm region is reanalyzed using a quantum mechanical nonlinear least-squares approach. For the first time the A(1u 3Π)←X(1Σg+) transition is included along with the two stronger transitions, B(0u+ 3Π)←X and C(1u 1Π)←X. The analyzed data include absorption spectra at temperatures between 23 °C and 440 °C along with specific estimates of B←X and (A+C)←X absorption. A new computational device facilitates inclusion of the discrete region of the spectrum (λ>510 nm) in the analysis: The A←X and B←X transitions are treated as entirely bound-free, which is accomplished computationally by just removing the attractive branches of the A and B potentials. The new analysis determines the molar absorptivities of all three transitions within ∼1 L mol−1 cm−1. Although the high precision is partly due to the inclusion of the specific B←X and (A+C)←X data, test computations show that the resolution can be achieved reliably by fitting just the T-dependent spectra. At 23 °C the component peaks are: 10.6 L mol−1 cm−1 at 525 nm (A←X), 82.5 L mol−1 cm−1 at 480 nm (B←X), and 158 L mol−1 cm−1 at 412 nm (C←X).
Properties of the Lenard–Balescu collision operator: A numerical study
Authors: Ricci Paolo , Lapenta Giovanni
Published: July 2002

The kinetic evolution of a one-dimensional electrostatic plasma is discussed, comparing the different predictions of three kinetic models: the Vlasov model, the Lenard–Balescu model, and the exact transport model. The attention is focused on the Lenard–Balescu approach whose properties are studied in detail. The kinetic parameters present in the Lenard–Balescu collision operator are studied analytically and the kinetic equation with the Lenard–Balescu collision operator is solved numerically. The numerical solutions are compared with the results of an exact microscopic model and found in remarkable agreement.
Conceptual design of a hybrid-type elliptically polarizing undulator
Authors: Sasaki Shigemi
Published: July 2002

A hybrid-type planar undulator was designed to generate circularly polarized radiation. It is an APPLE-type design consisting of four rows of hybrid structures that can be shifted with respect to each other. The magnetic field on axis can thus be adjusted so it can have linear or circular polarization including intermediate (elliptical polarization) positions. A short-period device of this kind can provide 100% circularly polarized radiation in a hard x-ray region when it is installed in a high-energy storage ring, such as the Advanced Photon Source.
EPICS on the RTEMS real-time executive for multiprocessor systems
Authors: Norum W. Eric
Published: July 2002

The Experimental Physics and Industrial Control System (EPICS) software is used in Synchrotron Radiation facilities around the world. An obstacle blocking small research groups such as beamline developers from using, or at least evaluating, EPICS has been the need to purchase the commercial operating system used by the EPICS base software. The goal of recent EPICS development has been to remove this requirement by allowing EPICS to run on a much wider range of operating systems. This article presents the results of this development for one particular operating system, the freely available, open-source, Real-Time Executive for Multiprocessor Systems (RTEMS). The capabilities of RTEMS are described and shown to provide an adequate basis for supporting EPICS. Performance comparisons between previous versions of EPICS and the RTEMS implementation are presented. Ongoing efforts to add support for more hardware are described.
Current crowding in InAsSb light-emitting diodes
Authors: Malyutenko V. K. , Malyutenko O. Yu. , Podoltsev A. D. , Kucheryavaya I. N. , Matveev B. A. , Remennyi M. A. , Stus’ N. M.
Published: July 2002

High-resolution two-dimensional infrared (IR) imaging of dynamic electronic processes in the surface-emitting p-InAsSb/n-InAsSbP light-emitting diodes (LEDs) (λ=4.3 μm, T>300 K) showed that forward current crowding drastically decreases efficiency of LEDs with point contacts. Current flows and IR emittance “forget” the emitting area size and geometry, whereas extended areas far off the point contacts become even “darker” with the current increase. Contrary to this, the reverse bias causes remarkable current spreading and uniform “negative emittance” distribution. Therefore the negative luminescence mode is more favorable for IR LEDs operating at higher temperatures.
Manipulation and detection of x rays on the femtosecond time scale (invited)
Authors: Adams B. W.
Published: July 2002

X-ray studies with femtosecond time resolution will become highly important within the next few years because of a confluence of scientific interest and availability of sources. This poses a challenge to instrumentation: Rapid switches and more elaborate devices for coherent control of x rays will have to be developed, and detectors will have to cope with high intensities and orders of magnitude better time resolution. Especially for detectors, a transition from classical electronics to optical control will become necessary to achieve few-femtosecond time resolution. The purpose of this article is to highlight a few of the challenges and to give a few examples of high-speed purely optical control and detection of x rays.
Energy-tunable x-ray diffraction: A tool for depth profiling in polycrystalline materials
Authors: Zolotoyabko E. , Quintana J. P.
Published: July 2002

We have developed a new variant of depth-sensitive x-ray diffraction technique to study structural parameters in inhomogeneous polycrystalline materials. In this method, diffraction patterns are measured at different x-ray energies which are varied by small steps, and then the depth-resolved structural characteristics are retrieved from the energy-dependent x-ray diffraction data. In the current articles, this approach is applied to extract preferred orientation with depth resolution. In the case of uniaxial preferred orientation, the analytical algorithm has been developed based on March functions. Application of this technique to seashells allowed us to characterize the microstructure evolution in the nacre layer. Near the inner surface, adjacent to the mollusk mantle, the nacre consists of well-defined lamellas which reveal a high degree of the (001)-preferred orientation. This preferred orientation deteriorates in depth due to the accumulation of cracks and other imperfections. The texture distribution is characterized quantitatively by depth-dependent March parameters, which allows us to compare samples taken from different shells. In a similar way, energy-variable x-ray diffraction can be used for nondestructive characterization of a very broad spectrum of laminated structures and composite materials and systems.
Setup for measurements of partial ion yields at the Synchrotron Radiation Center
Authors: Wehlitz R. , Lukić D. , Koncz C. , Sellin I. A.
Published: July 2002

A new setup for measuring partial photoion yields was developed and built at the Synchrotron Radiation Center. The vacuum chamber, which accommodates an ion time-of-flight spectrometer, a metal vapor oven, and a liquid nitrogen cooled trap, consists mainly of a standard conflat 6 in. six-way cross and a 6 in. tee. A differential pumping stage separates the vacuum chamber from the beam line. First experiments with this apparatus were performed using neon, lithium, and beryllium.
Dust diffusion across a magnetic field due to random charge fluctuations
Authors: Khrapak S. A. , Morfill G. E.
Published: July 2002

Dust particle diffusion across a magnetic field due to random charge fluctuations is reconsidered. Previously it was suggested that this mechanism could be one of the most effective processes which transport particles in the inner Jovian magnetosphere [G. E. Morfill, E. Grün, and T. V. Johnson, Planet Space Sci. 28, 1087 (1980)]. A general expression for the diffusion constant is derived using the stochastic equation of dust particle motion and known properties of random charge fluctuations. The dependence of the diffusion constant on magnetic-field strength and fluctuation amplitude is discussed. Numerical evaluation for conditions typical for the inner Jovian magnetosphere is presented.
A new X-band coaxial transit-time oscillator
Authors: Yang Wenyuan , Ding Wu
Published: July 2002

A new structure of a high power microwave oscillator based on the transit-time instability is proposed and studied numerically in this paper. Due to its coaxial structure with large radius, the oscillator allows larger input and output power and can work at higher frequency. With no external magnetic field being used for beam focusing, it keeps the simplicity of a conventional transit-time oscillator. Using an electron beam of 180 kV and 10 kA, a stable and monochromatic output with peak power of 700 MW is achieved by simulation at 8.35 GHz with an rms efficiency of 20%.
Kinetic regimes of high frequency magnetic reconnection in a neutral sheet configuration
Authors: Attico N. , Califano F. , Pegoraro F.
Published: July 2002

A collisionless plasma configuration with a neutral magnetic sheet allows a fast electron tearing mode to grow due to a resonant wave–particle interaction near the null line. Using a fully kinetic treatment, a set of dispersion equations that bridges the resonant tearing mode and the well-known cold fluid tearing mode driven by electron inertia is derived. The whistler frequency range where magnetic reconnection does not involve the ion dynamics is considered. This frequency range is of interest to the study of the laser–plasma interaction, where magnetic reconnection is expected to play an important role in the magnetic field dynamics in the wake of an ultra intense laser pulse.
X-ray optics fabricated by deep reactive ion etching (invited)
Authors: Finkelstein K. D. , Rosenblatt Sami , Cottle Paul
Published: July 2002

When hard x-ray focusing optics require segmented bending, the focal spot can be minimized by matching segment size to the synchrotron source size and focal ratio. At modern sources this implies features etched or cut into silicon on the 100 μm scale. We describe our experience using photolithography and high aspect ratio reactive ion etching (RIE) to produce sagittal focusing crystals. The work was carried out at the Cornell Nanofabrication Facility. RIE has been of great utility for the manufacture of microelectromechanical systems (MEMS) which contain features typically a few microns in size. We have learned that because of the very large dielectric constant of silicon, a number of important considerations must go into a successful design when features are scaled up for x-ray optics applications. Nevertheless, we have designed, built and tested x-ray optical devices produced using MEMS techniques, and in fact they are in regular use at CHESS. We describe our results from the point of view of mechanical robustness and focus quality, and offer general guidelines for future designers.
A novel crystal bender for x-ray synchrotron radiation monochromators
Authors: Zaeper Ralf , Richwin Matthias , Lützenkirchen-Hecht Dirk , Frahm Ronald
Published: July 2002

A new bending mechanism for an indirectly water cooled monochromator crystal has been developed. The main design goals were a lightweight construction, ease of manufacture, and control. The construction consists of a U-shaped first Si(111) crystal, which can be bent by compressed air to compensate the bowing induced by the heat load from the impinging white synchrotron radiation. The performance of the system was tested at the x-ray undulator beamline BW1 at HASYLAB (Hamburg, Germany). For heat loads between ∼100 and 400 W, rebending of the crystal significantly increases the intensity of the monochromatic beam while the rocking curve is narrowed accordingly, typical widths obtained at 8.9 keV photon energy amount to about 10 arc sec.
Monochromator crystal mounting stage
Authors: Schildkamp Wilfried
Published: July 2002

An insertion device beamline at the Advanced Photon Source had received a second insertion device “undulator A” and consequently needed an upgrade of the optics. The decision to use diamond optics required a mechanical modification of the double crystal monochromator and its crystal alignment stages. Specifications for the tilt alignment accuracy are developed. A crystal alignment stage with independent tilt and translation is described. A new crystal holder was developed, which reduces the temperature of the diamond crystal by over 100 °C and allows for strain-free mounting.
Magnetoresistance and interlayer coupling in spin valves employing very thin Cu spacer
Authors: Jo S. , Seigler M.
Published: July 2002

Magnetoresistance and interlayer coupling behavior near the first antiferromagnetic coupling maximum in underlayer 55 Å/NiFe 10 Å/CoFe/Cu/CoFe/Ru 4 Å/CoFe/IrMn 70 Å/overlayer 50 Å top spin valve structures are reported. As the Cu spacer thickness decreased, interlayer coupling turned to antiferromagnetic at 10 Å of Cu thickness and the coupling field reached maximum near 8 Å of Cu thickness, when the thickness of the CoFe layers was ∼20 Å. Magnetoresistance ratio reached a local minimum at 12 Å of Cu thickness where the ferromagnetic coupling is maximum, and at 10 Å where the coupling field is nearly zero. The sheet resistance change increased slightly at very thin Cu thickness for films having CoFe thickness of ∼20 Å, but increased significantly (by 0.5 Ω/□ from 2.2 to 2.7 Ω/□) for films having very thin (∼10 Å) CoFe layers due to considerable decrease of current shunting. These thin CoFe and Cu layers may be utilized for very small devices where the RKKY coupling of the layers is dominated by high demagnetizing fields.
Patterned heteroepitaxial processing applied to ZnSe and ZnS0.02Se0.98 on GaAs (001)
Authors: Zhang X. G. , Rodriguez A. , Li P. , Jain F. C. , Ayers J. E.
Published: July 2002

We have demonstrated the patterned heteroepitaxial processing (PHP) approach for the removal of threading dislocations (TDs) from ZnSe and ZnS0.02Se0.98 on GaAs (001). PHP involves the growth of a continuous heteroepitaxial layer followed by postgrowth patterning and annealing. We found that the basic mechanism of TD removal by PHP is thermally activated dislocation motion in the presence of sidewalls. By studying the temperature dependence we showed that the activation energy for the annealing process (∼0.7 eV in ZnSe on GaAs) is consistent with dislocation motion by glide. We showed that there is a minimum mesa thickness required for the complete removal of TDs by PHP (∼3000 Å for 70 μm×70 μm mesas of ZnSe on GaAs). This is because the lateral forces acting on TDs are proportional to the mesa thickness. We also conducted a preliminary study of the mismatch dependence of PHP. Our results suggest that PHP removes TDs more effectively in the higher lattice mismatch system ZnSe/GaAs (001) than in the lower lattice mismatch system ZnS0.02Se0.98/GaAs (001). This is expected based on the mismatch dependence of the line tension forces in the misfit segments of dislocations.