bluuebunny/crossref_metadata_2025 huggingface dataset:
A mixed quantum classical rate theory for the collinear H+H2 reaction
Authors: Zheng Yujun , Pollak Eli
Published: July 2002

A mixed quantum classical rate theory (MQCLT) is applied to the collinear hydrogen exchange reaction on the LSTH and PK II potential energy surfaces. Classical trajectories are combined with a numerically exact quantum Monte Carlo evaluation of the thermal flux operator to compute the thermal reaction rate. The MQCLT results are compared to quantum transition state theory (QTST) and centroid rate theory computations. The computed rates are found to bound the exact results from above for temperatures ranging from T=200 K to T=1000 K. As in previous studies, the mixed quantum classical theory gives better agreement with numerically exact computations, than the QTST computations, while the added numerical effort is not prohibitive. The MQCLT rate is almost exact at high temperature. At T=200 K it is a factor of 2.8 (2.0) greater than the exact rate on the LSTH (PK II) potential energy surface, a significant improvement over the QTST overestimate of 3.7 (3.4). The mixed quantum classical results are comparable in accuracy to the centroid theory computations, except that the centroid theory is always lower than the exact result while MQCLT is always higher.
Compton scattering and the character of the hydrogen bond in ice Ih
Authors: Romero Aldo H. , Silvestrelli Pier Luigi , Parrinello Michele
Published: July 2002

Compton scattering is an experimental technique which probes the electron momentum density. Here we present a method which allows the calculation of the Compton profile using the formalism of the maximally localized Wannier functions. We applied this method to analyze recent experimental measurements of the anisotropy component of the Compton profile which have been interpreted as the signature of the partial covalent character of the hydrogen bond in ice Ih. We are able to reproduce well the experimental results, however we do not find evidence of a covalent nature of the hydrogen bond. In fact our analysis shows that, although the wave functions in ice Ih have a quantum-mechanical, multicenter character, this gives rise to an antibonding, repulsive interaction between neighboring water molecules. Application to our technique to other, nonhydrogen bonded systems, and analysis of the corresponding Compton scattering profile anisotropy, gives further support to our interpretation.
Simple, accurate, and precise measurements of thermal diffusivity in liquids using a thermal-wave cavity
Authors: Balderas-López J. A. , Mandelis A.
Published: July 2002

A simple methodology for the direct measurement of the thermal wavelength using a thermal-wave cavity, and its application to the evaluation of the thermal diffusivity of liquids is described. The simplicity and robustness of this technique lie in its relative measurement features for both the thermal-wave phase and cavity length, thus eliminating the need for taking into account difficult-to-quantify and time-consuming instrumental phase shifts. Two liquid samples were used: distilled water and ethylene glycol. Excellent agreement was found with reported results in the literature. The accuracy of the thermal diffusivity measurements using the new methodology originates in the use of only difference measurements in the thermal-wave phase and cavity length. Measurement precision is directly related to the corresponding precision on the measurement of the thermal wavelength.
Negative differential photoconductivity in quantum-dot infrared photodetectors
Authors: Ryzhii V.
Published: July 2002

We present a simple model for quantum-dot infrared photodetectors (QDIPs) describing nontrivial (decreasing) dependences of the photocurrent on the QD density and the applied voltage. It is shown that recent experiments demonstrating a negative differential photoconductivity in QDIPs can be interpreted in terms of this model. The effects under consideration can be attributed to the repulsive potential of charged quantum dots and heating of mobile electrons influencing the rate of the electron capture. Qualitative distinctions between the QDIP photocurrent–voltage and dark current–voltage characteristics are explained as well.
Temperature dependence of hydrophobic hydration and entropy convergence in an isotropic model of water
Authors: Garde Shekhar , Ashbaugh Henry S.
Published: July 2002

We have investigated temperature dependence of hydrophobic hydration and molecular-scale density fluctuations in an isotropic single-site model of water originally devised by Head-Gordon and Stillinger [J. Chem. Phys. 98, 3313 (1993)] using Monte Carlo simulations. Our isotropic model of water, HGS water, has the same oxygen–oxygen radial distribution function as that of the simple point charge (SPC) water at room temperature and water density. For HGS water, we find that non-Gaussian occupancy fluctuations lead to cavity formation probabilities that are considerably lower than in SPC water. Wetting of a hard-sphere solute by HGS water is also found to be significantly greater than that by SPC water. These observations can be understood in terms of differences in Hamiltonians of the two water models. Despite these differences in the details of hydration, small hydrophobic solutes display many of the well-known thermodynamic finger prints of hydrophobic hydration once the variation of density with temperature, ρ(T), along the saturation curve of real liquid water is followed for HGS water. For the hydration of small solutes, the “entropy convergence” is observed at temperatures of ≈400 K. These observations emphasize that the phase behavior of liquid water contains crucial information regarding thermodynamics of solvation phenomena.
Suppression of structural and magnetotransport transitions in compressed Pr0.5Sr0.5MnO3 thin films resulting in colossal magnetoresistance effect
Authors: Mercey B. , Hervieu M. , Prellier W. , Wolfman J. , Simon C. , Raveau B.
Published: July 2002

Thin films of Pr0.5Sr0.5MnO3 have been deposited on [100]-LaAlO3 using laser ablation. In contrast to the bulk compounds, such films do not exhibit any structural and magnetotransport transitions versus temperature; more particularly the A-type antiferromagnetic phase with the Fmmm structure which exists in the bulk below TN=135 K is suppressed, and the film is an insulator in the absence of a magnetic field. However a colossal magnetoresistance effect is observed, with resistance ratios much larger than in the bulk. These differences with respect to the bulk, are explained by the presence of substrate-induced strains.
Dependence of GaN polarity on the parameters of the buffer layer grown by molecular beam epitaxy
Authors: Huang D. , Visconti P. , Jones K. M. , Reshchikov M. A. , Yun F. , Baski A. A. , King T. , Morkoç H.
Published: July 2002

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.
Why lanthanum-substituted bismuth titanate becomes fatigue free in a ferroelectric capacitor with platinum electrodes
Authors: Ding Y. , Liu J. S. , Qin H. X. , Zhu J. S. , Wang Y. N.
Published: July 2002

Bi 4 Ti 3 O 12 , (BTO) a Bi-layered perovskite oxide, shows fatigue after repeated ferroelectric polarization reversals. On the other hand, Bi3.25La0.75Ti3O12 (BLT) is fatigue free. From an extensive transmission electron microscopy study, it was found that there is a high density of antiphase boundaries (APBs) in BLT like in the fatigue-free SrBi2Ta2O9 but not in BTO. It is proposed that the existence of APBs possibly plays a key role in the fatigue-free behavior of Bi-layered perovskite oxides.
Bubble-induced agitation and microstructure in uniform bubbly flows at small to moderate particle Reynolds numbers
Authors: Cartellier Alain , Rivière Nicolas
Published: July 2002

The bubble-induced agitation has been quantified in dilute and homogeneous bubbly flows. To characterize the microstructure, experimental techniques have been developed that provide the average perturbed liquid velocity and the pair density. For particulate Reynolds numbers Rep about unity and contaminated bubbles, the liquid agitation is shown to increase as α/Rep where α is the local void fraction. Although in partial agreement with the scaling proposed by Koch (Phys. Fluids, 1993), the pair-density distribution does not exhibit any deficit. On the opposite, experiments at Rep=O(10) and clean bubbles reveal a strong deficit in the rear of test bubbles as well as a moderate accumulation both in a horizontal plane and all along the deficit zone. The extent of the deficit slowly decreases with the void fraction, roughly as α−0.3±0.1. It is shown that the agitation originates from this near wake region, the structure of which strongly differs from the one due to a single inclusion both in terms of its lateral extent and of the perturbed velocity decay rate. These features are related to the extra momentum sources induced by the microstructure surrounding the test bubble. Accordingly, the resulting velocity variance, that has been found independent of the vessel dimension and of the base flow velocity, is significantly higher than Koch’s predictions. Finally, a comparison is achieved with available data and models, and an explanation is suggested for the evolution of the bubble-induced agitation at moderate Rep.
Coherent control of stimulated Raman scattering using chirped laser pulses
Authors: Dodd Evan S. , Umstadter Donald
Published: July 2002

A novel method for the control of stimulated Raman scattering and hot electron production in short-pulse laser-plasma interactions is proposed. It relies on the use of a linear frequency chirp in nonbandwidth limited pulses. Theoretical calculations show that a 12% bandwidth will eliminate Raman forward scattering for a plasma density that is 1% of the critical density. The predicted changes to the growth rate are confirmed in two-dimensional particle-in-cell simulations. Relevance to areas of current research is also discussed.
A square-well model for the structural and thermodynamic properties of simple colloidal systems
Authors: Acedo L. , Santos A.
Published: July 2002

A model for the radial distribution function g(r) of a square-well fluid of variable width previously proposed [Yuste and Santos, J. Chem. Phys. 101, 2355 (1994)] is revisited and simplified. The model provides an explicit expression for the Laplace transform of rg(r), the coefficients being given as explicit functions of the density, the temperature, and the interaction range. In the limits corresponding to hard spheres and sticky hard spheres, the model reduces to the analytical solutions of the Percus–Yevick equation for those potentials. The results can be useful to describe in a fully analytical way the structural and thermodynamic behavior of colloidal suspensions modeled as hard-core particles with a short-range attraction. Comparison with computer simulation data shows a general good agreement, even for relatively wide wells.
Photodissociation of ICN at the liquid/vapor interface of chloroform
Authors: Vieceli John , Chorny Ilya , Benjamin Ilan
Published: July 2002

The photodissociation of ICN initially adsorbed at the liquid/vapor interface of chloroform is studied using classical molecular dynamics computer simulations. The photodissociation and subsequent geminate recombination on the ground state of ICN is compared with the same reaction in the bulk liquid. We find that the probability for cage escape at the interface is significantly enhanced due to the possibility that one or both of the photodissociation fragments desorb into the gas phase. The desorption probability is sensitive to the initial location and orientation of the ICN. An examination of the energy disposal into these fragments provides additional information about the competition between geminate recombination and cage escape at the interface.
Quantum limit of magnetic recording density
Authors: Held G. A. , Grinstein G.
Published: July 2002

Macroscopic quantum tunneling imposes an upper limit on the areal density of magnetic disk drives. We derive a general expression for this limit, and apply it to Co-alloy-based longitudinally recorded media, as well as to potential future media based on the high anisotropy material FePt. We estimate the temperature at which the switching rate due to quantum tunneling becomes comparable to the thermally activated switching rate. This is the lowest temperature to which a magnetic storage device can be cooled with the expectation of increasing the areal density by enhancing stability against spontaneous switching.
Piezoelectric field and its influence on the pressure behavior of the light emission from GaN/AlGaN strained quantum wells
Authors: Łepkowski S. P. , Teisseyre H. , Suski T. , Perlin P. , Grandjean N. , Massies J.
Published: July 2002

We have studied the influence of hydrostatic pressure on the light emission from a strained GaN/AlGaN multiquantum well system. We have found that the pressure coefficients of the photoluminescence peak energies are dramatically reduced with respect to that of GaN energy gap and this reduction is a function of the quantum well thickness. The decrease of the light emission pressure coefficient may be as large as 30% for a 32 monolayer (8 nm) thick quantum well. We explain this effect by the hydrostatic-pressure-induced increase of the piezoelectric field in quantum structures. Model calculations based on the k×p method and linear elasticity theory reproduce the experimental results well, demonstrating that this increase may be explained by small anisotropy of the wurtzite lattice of GaN and a specific interplay of elastic constants and values of the piezoelectric tensor.
IUPAC-NIST Solubility Data Series. 76. Solubility of Ethyne in Liquids
Authors: Fogg Peter G. T. , Bligh Sim-wan Annie , Derrick M. Elizabeth , Yampol’skii Yuri P. , Clever H. Lawrence , Skrzecz Adam , Young Colin L. , Fogg Peter G. T.
Published: July 2002

Ethyne was probably first made in the laboratory by Edmund Davy in 1836. It was rediscovered nearly a quarter of a century later by Berthelot who gave it the name acetylene. Since that time ethyne has become a cheap raw for the synthesis of and an important industrial fuel. A summary of the available data for ethyne was published by Miller in 1965 [S. A Miller, Acetylene—Its Properties, Manufacture, and Uses (Academic, New York, 1965), Vol. I]. Many more data are now available in a wide range of research papers and applications. These data vary in their reliability. In the current work the data for systems included in Miller’s book have been reassessed and complemented by data published more recently. Literature has been surveyed to 1999. Data for a system may be unreliable unless two or more groups of workers have published values in close agreement. Where possible values of the mole fraction at a partial of 101.3 kPa have been tabulated. Equations have been given for the variation of mole fraction with temperature in cases in which values over a temperature range are available. The greater the number of independent sources of the data the more the reliance which can be placed on the utility of the resulting equation. Extrapolation of such equations beyond the temperature range of experimental can lead to errors. In many of the systems it may be assumed that approximate values of the mole fraction x1, at a partial of 101.3 kPa may be obtained by linear extrapolation of values for lower partial p1, on the assumption that x1/p1 is approximately constant. However a similar linear extrapolation of at appreciably higher that 101.3 kPa to give mole fraction at 101.3 kPa can lead to gross errors. For the purpose of evaluation of data use has been made of the Krichevsky–Il’inskaya equation to obtain approximate values of at 101.3 kPa from at These values were then compared with made at or near to 101.3 kPa.
Combinatorial fabrication and studies of intense efficient ultraviolet–violet organic light-emitting device arrays
Authors: Zou L. , Savvate’ev V. , Booher J. , Kim C.-H. , Shinar J.
Published: July 2002

Arrays of ultraviolet–violet (indium tin oxide)/[copper phthalocyanine (CuPc)]/[4,4′-bis(9-carbazolyl)biphenyl (CBP)]/[2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4oxadiazole (Bu-PBD)]/CsF/Al organic light-emitting devices, fabricated combinatorially using a sliding shutter technique, are described. Comparison of the OLED electroluminescence and CBP photoluminescence spectra indicates that the emission originates from the bulk of that layer. In arrays of devices in which the thickness of the CuPc and Bu–PBD were varied, but that of CBP was fixed at 50 nm, the optimal radiance R was obtained at CuPc and Bu–PBD thicknesses of 15 and 18 nm, respectively. At 10 mA/cm2, R was 0.38 mW/cm2, i.e., the external quantum efficiency was 1.25%; R increased to ∼1.2 mW/cm2 at 100 mA/cm2.
Composition, volume, and aspect ratio dependence of the strain distribution, band lineups and electron effective masses in self-assembled pyramidal In1−xGaxAs/GaAs and SixGe1−x/Si quantum dots
Authors: Califano M. , Harrison P.
Published: July 2002

We present a systematic investigation of the strain distribution of self-assembled pyramidal In1−xGaxAs/GaAs and SixGe1−x/Si quantum dots for the case of growth on a (001) substrate. The dependence of the biaxial and hydrostatic components of the strain on the quantum dot volume, aspect ratio, composition, and percentage of alloying x is studied using a method based on a Green’s function technique. The dependence of the carriers’ confining potentials and the electronic effective mass on the same parameters is then calculated in the framework of eight-band k⋅p theory. The results for which comparable published data are available are in good agreement with the theoretical values for strain profiles, confining potentials, and electronic effective mass.
Site-selective laser spectroscopy of 4fn–4fn−15d transitions in CaF2:Pr3+ with F−, D−, H−, Li+, or Na+ charge compensation
Authors: van Pieterson L. , Dullens R. P. A. , Peijzel P. S. , Meijerink A. , Jones G. D.
Published: July 2002

Site-selective luminescence spectroscopy of the 4f2−4f5d(fd) transition is reported for six different Pr3+ sites in CaF2:Pr3+. In addition to the naturally occurring distantly charge-compensated cubic site and the locally charge-compensated Pr3+–F− site, charge compensating ions (H−, D−, Li+, and Na+) were added to create four other types of sites. In the fd luminescence spectra fine structure (zero-phonon lines and vibronic lines) is observed. The positions of zero-phonon lines and vibrational frequencies vary for these different Pr3+ sites. For the H− and D− charge-compensated site the lowest 4f5d level is shifted to some 3000 cm−1 lower energy compared to the cubic site. This is attributed to a higher degree of covalency and a larger crystal-field splitting. Contrary to previous results for Ce3+–H− and Ce3+–D−, only a small (close to zero) isotope shift is observed. In the fd emission spectra, the 4f5d–1I6, PJ3 transitions are used to reveal marked differences between sites which allows their identification.
Effect of neutral atoms on tokamak edge plasmas
Authors: Fülöp T. , Catto Peter J. , Helander P.
Published: July 2002

Neutral atoms can significantly influence the physics of tokamak edge plasmas, e.g., by affecting the radial electric field and plasma flow there, which may, in turn, be important for plasma confinement. Earlier work [Fülöp et al., Phys. Plasmas 5, 3969 (1998)], assuming short mean-free path neutrals and Pfirsch–Schlüter ions, has shown that the ion-neutral coupling through charge-exchange affects the neoclassical flow velocity significantly. However, the mean-free path of the neutrals is not always small in comparison with the radial scale length of densities and temperatures in the edge pedestal. It is therefore desirable to determine what happens in the limit when the neutral mean-free path is comparable with the scale length. In the present work a self-similar solution for the neutral distribution function allowing for strong temperature and density variation is used, following the analysis of Helander and Krasheninnikov [Phys. Plasmas 3, 226 (1995)]. The self-similar solution is possible if the ratio of the mean-free path to the temperature and density scale length is constant throughout the edge plasma. The resulting neutral distribution function is used to investigate the neutral effects on the ion flow and electrostatic potential as this ratio varies from much less than one to order unity.
Current self-oscillation and driving-frequency dependence of negative-effective-mass diodes
Authors: Cao J. C. , Li A. Z. , Lei X. L. , Feng S. L.
Published: July 2002

We have analyzed spatio-temporal current patterns and current–voltage characteristics of negative-effective-mass (NEM) p+pp+ diodes driven by dc bias and terahertz (THz) electromagnetic radiation. Interesting nonlinear dynamics are presented, including current synchronization, frequency doubling, and transition to chaos. Discussions of suppressing possible chaos in NEM semiconductor devices are included.
Ultrafast dynamics at a silicon surface detected with femtosecond transient reflecting grating spectroscopy
Authors: Katayama K. , Sugai K. , Inagaki Y. , Sawada T.
Published: July 2002

The recently developed spectroscopic transient reflecting grating technique with a time resolution of 200 fs was applied to a silicon surface under the pump intensity of more than 1 mJ/cm2. This method provides information on excited free carrier dynamics and subsequent heat generation and diffusion selectively based on appropriate choice of probe wavelength. With regard to the thermal component, the temperature at the surface increased within several picoseconds and then decayed after about 300 ps. As the pump intensity was increased, the maximum temperature rise showed a nonlinear dependence on it, and also the temperature rise time became faster. The results led to the conclusion that the carrier dynamics causing a temperature rise at a silicon surface is dominated mainly by Auger recombination, not by the decay to a band edge under the high carrier density conditions.
On harmonic oscillators on the two-dimensional sphere S2 and the hyperbolic plane H2
Authors: Rañada Manuel F. , Santander Mariano
Published: July 2002

Two Harmonic Oscillators (isotropic and nonisotropic 2:1) are studied on the two-dimensional sphere S2 and the hyperbolic plane H2. Both systems are integrable and super-integrable with constants of motion quadratic in the momenta. These properties are shown to derive from a complex factorization for the constants of motion, which holds for arbitrary values of the curvature κ, and the dynamics of the Euclidean harmonic 1:1 and 2:1 oscillators is directly recovered for κ=0. The harmonic oscillators on either the standard unit sphere (radius R=1) or the unit Lobachewski plane (“radius” R=1) appear as the particular values of the κ-dependent potentials for the values κ=1 and κ=−1. Finally a particular potential is proposed for representing the general spherical (hyperbolic) n:1 anisotropic harmonic oscillator on a two-dimensional manifold of constant curvature.
Control and mass selection of CnHm+ fragments in an inductively coupled pulsed plasma
Authors: Zeze D. A. , Joyce A. M. , Anderson C. A. , Brown N. M. D.
Published: July 2002

We report on a method of selecting CnHm+ fragments using a pulsed inductively coupled plasma (ICP) driven by a rf supply and a pulse control unit providing user-defined on/off regimes. Langmuir probe and mass-energy spectrometric data show that the characteristics of the plasma can be monitored so demonstrating that hydrocarbon clusters of different C:H ratios and abundances can be generated preferentially by using appropriately chosen on/off regimes. Postdeposition x-ray photoelectron spectroscopy (XPS) analysis underlines clearly the influence of the deposition regimes, on the structure and composition of CxNy:H films grown via CnHm+ clusters generated using adamantane (C10H16) vapor in an argon/nitrogen mixture.
Emission of electron Bernstein waves in plasmas
Authors: Ram A. K. , Bers A. , Lashmore-Davies C. N.
Published: July 2002

In previous publications [A. K. Ram and S. D. Schultz, Phys. Plasmas 7, 4084 (2000); A. Bers, A. K. Ram, and S. D. Schultz, in Proceedings of the Second Europhysics Topical Conference on RF Heating and Current Drive of Fusion Devices, edited by J. Jacquinot, G. Van Oost, and R. R. Weynants (European Physical Society, Petit-Lancy, 1998), Vol. 22A, pp. 237–240] it has been shown that, in overdense plasmas of the type encountered in spherical tori, electron Bernstein waves can be excited in a plasma by mode conversion of either an externally launched X mode or an O mode. The electron Bernstein waves are strongly absorbed by electrons in the region where the wave frequency matches the Doppler broadened electron cyclotron resonance frequency or its harmonics. The strong absorption also implies that electron Bernstein waves are emitted by a thermal plasma. These waves can then mode convert to the X mode and to the O mode and be observed external to the plasma. In this paper an approximate kinetic model describing the coupling between the X mode, the O mode, and the electron Bernstein waves is derived. This model is used to study the mode conversion properties of electron Bernstein wave emission from the plasma interior. It is shown, analytically and numerically, that the energy flow conversion efficiencies of the electron Bernstein wave to the X mode and to the O mode are the same as the energy flow conversion efficiency of the X mode to electron Bernstein waves and of the O mode to the electron Bernstein waves, respectively. This has important experimental consequences when designing experiments to heat overdense plasmas by electron Bernstein waves.
Accel–decel extraction system for PIG sources
Authors: Spädtke P. , Heymach F. , Hollinger R. , Leible K. D. , Mayr R. , Shi L. Q.
Published: July 2002

Increasing the brightness of the ion beam is a typical demand for any accelerator. In order to fill the heavy ion synchrotron (SIS) up to the space charge limit, higher pulse currents are necessary. The UNILAC is operated with a pulse length up to 6 ms and a repetition rate up to 50 s−1. For the injection into the SIS a pulse length of 300 μs at 0.3 s−1 is required. The transverse acceptance of the RFQ is 138 π mm mrad. According to Child’s law, the ion current density which can be extracted from a plasma source is for the space charge limited flow proportional to Φ1.5/d2. With Φ and d denoting the extraction voltage and the width of the extraction gap. One approach to generate higher ion currents is to increase the extraction voltage in the regularly used extraction system. However, the required beam velocity is fixed by the RFQ structure with 2.2 keV/u. The mass-to-charge ratio which is to be accelerated can be between 1 and 65, resulting in a total voltage drop from 2.2 up to 143 kV (extraction and postacceleration voltage). The other approach is to decrease the gap width in the extraction system, but there is an optimum in the aspect ratio. To generate an ion beam with a higher current at low energy, a triode extraction system operated in accel–decel (Uacc,Udec) mode is a possible solution. By applying a negative potential at the second electrode a higher extraction field strength can be achieved. The effects of extraction field, arc current, and ion energy on the extracted ion beam current and its emittance were investigated.
Ion source modeling with LORENTZ-2D
Authors: Asi Ali
Published: July 2002

LORENTZ-2D is a boundary element package developed and marketed by Integrated Engineering Software, which can be used for a variety of charged particle optical analyses. In this article, the program is used in the analysis of two types of Cs+ sputter ion sources. The results are presented and compared versus some earlier empirical and numerical simulations.
Anisotropy in detectivity of GaN Schottky ultraviolet detectors: Comparing lateral and vertical geometry
Authors: Katz O. , Garber V. , Meyler B. , Bahir G. , Salzman J.
Published: July 2002

Vertical and lateral geometry GaN-based Schottky barrier photodetectors have been implemented, using similar quality material and the same fabrication process. The vertical detector exhibits two orders of magnitude higher responsivity. This is attributed to improved ohmic backcontacts, due to the highly doped buried layer. The vertical detectors exhibits also lower 1/f noise level, which is attributed to the reduced effect of dislocations on the carrier transport, resulting in lower mobility fluctuations. The vertical detector normalized detectivity is four orders of magnitude higher.
Mechanical properties and stability of multimacroion domains in polyelectrolyte solutions
Authors: Sedlák Marián
Published: July 2002

Mechanical properties and stability of multimacroion domains in polyelectrolyte solutions were investigated by passing domains through micropores with dimensions smaller, comparable or larger than domain dimensions. This was achieved by pressure filtration of salt-free sodium poly(styrene sulfonate) solutions through membrane filters with various pore sizes and subsequent static and dynamic light scattering measurements. This process proved to be interesting to investigate since it provides an experimental tool for getting more information on these so far weakly understood structures. In addition, filtration in general is a routine procedure used in light scattering from polymers. Results from this work show that this procedure substantially influences experimental data on domains and it is necessary to know how. This is important when measuring and discussing the influence of various physicochemical factors, which may have a much weaker effect themselves. Some literature data should be accordingly accepted with caution. Natural size distribution of domains (size distribution unaffected by filtration procedure) was calculated from scattering data and subsequently verified by filtration through a set of filters with various pore sizes.
Paramagnetic to antiferromagnetic transition in AMnCl3 (A=Rb and Cs) single crystals as observed by Rb87 and Cs133 spin-lattice relaxation
Authors: Lim Ae Ran , Jung Jae Kap , Jeong Se-Young
Published: July 2002

RbMnCl 3 and CsMnCl3 order antiferromagnetically at Néel temperatures TN=94.6 K and TN=67 K, respectively. The Rb87 and Cs133 spin-lattice relaxation times in these crystals were measured in the temperature range of 270–60 K. The large change in the relaxation rate that is observed near TN indicates that the magnetic phase in these crystals undergoes a paramagnetic to antiferromagnetic transition. In both crystals, the temperature dependencies of the relaxation rates for TTN are in accordance with a single-phonon process.
Shear flow instabilities in magnetized partially ionized dense dusty plasmas
Authors: Birk Guido T. , Wiechen H.
Published: July 2002

Shear flow instabilities may play an important role in the dynamics of partially ionized dusty plasmas. Within a multifluid approach the onset criteria with and without electrical resistivity are derived. Long wave length modes can be stabilized by dust–neutral gas collisional momentum transport. Self-consistent numerical simulations of the multifluid plasma dynamics illustrate the nonlinear development of vortical structures in the dust, ion, and neutral velocity as well as in the magnetic field. The unstable modes lead to a significant local amplification of the magnetic field strength. Moreover, electric current sheets form that also show vortices.
Magnetic relaxation in metallic films: Single and multilayer structures
Authors: Heinrich B. , Urban R. , Woltersdorf G.
Published: July 2002

The intrinsic magnetic relaxations in metallic films will be discussed. It will be shown that the intrinsic damping mechanism in metals is caused by incoherent scattering of itinerant electron-hole pair excitations by phonons and magnons. Berger [L. Berger, Phys. Rev. B 54, 9353 (1996)] showed that the interaction between spin waves and itinerant electrons in multilayers can lead to interface Gilbert damping. Ferromagnetic resonance (FMR) studies were carried out using magnetic single and double layer films. The FMR linewidth of the Fe films in the double layer structures was found to always be larger than the FMR linewidth measured for the single Fe films having the same thickness. The increase in the FMR linewidth scaled inversely with the film thickness, and was found to be linearly dependent on the microwave frequency. These results are in agreement with Berger’s predictions.
Role of hydrogen for the elastic properties of alumina thin films
Authors: Schneider Jochen M. , Larsson Karin , Lu Jun , Olsson Eva , Hjörvarsson Björgvin
Published: July 2002

We describe the role of the presence of hydrogen on the elastic properties of AlxOyHz (0.32⩽x⩽0.4; 0.54⩽y⩽0.6; 3×10−4⩽z⩽0.14) films. The films were deposited by reactive magnetron sputtering in an Ar/O2/H2O discharge and were studied by Rutherford backscattering spectrometry, nuclear resonance analysis, selected area electron diffraction, as well as nanoindentation. As the hydrogen concentration is increased from 0.03% to 13.9% the measured elastic modulus value is reduced by approximately 53%. The measured elastic modulus is in excellent agreement with our electronic structure calculations. The large scattering in the reported values of the elastic properties of amorphous alumina thin films can readily be understood by hydrogen incorporation during synthesis.
On the use of the adiabatic molecular dynamics technique in the calculation of free energy profiles
Authors: Rosso Lula , Mináry Peter , Zhu Zhongwei , Tuckerman Mark E.
Published: July 2002

A new molecular dynamics method for calculating free energy profiles for rare events is presented. The new method is based on the creation of an adiabatic separation between a reaction coordinate subspace and the remaining degrees of freedom within a molecular dynamics run. This is achieved by associating with the reaction coordinate(s) a high temperature and large mass, thereby allowing the activated process to occur while permitting the remaining degrees of freedom to respond adiabatically. In this limit, by applying a formal multiple time scale Liouville operator factorization, it can be rigorously shown that the free energy profile is obtained directly from the probability distribution of the reaction coordinate subspace and, therefore, no unbiasing of the configuration space or postprocessing of the output data is required. The new method is applied to a variety of model problems including a two-dimensional free energy surface and its performance tested against free energy calculations using the “blue moon ensemble” approach. The comparison shows that free energy profiles can be calculated with greater ease and efficiency using the new method.
Band-edge emission of undoped and doped ZnO single crystals at room temperature
Authors: Ohashi Naoki , Sekiguchi Takashi , Aoyama Kouichiroh , Ohgaki Takeshi , Terada Yoshihiro , Sakaguchi Isao , Tsurumi Takaaki , Haneda Hajime
Published: July 2002

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.
Factors affecting surface roughness and coercivity of Ni80Fe20 thin films
Authors: Ng V. , Hu J. F. , Adeyeye A. O. , Wang J. P. , Chong T. C.
Published: July 2002

This article reports the effects of dc sputter power, film thickness and rf substrate bias on surface roughness and magnetic properties of Ni80Fe20 thin films sputtered on Si (100) substrates for their application in magnetic tunneling junctions. The surface roughness of Ni80Fe20 thin films varied from 0.197 nm to 0.376 nm when dc sputter power was changed from 50 to 250 W. The optimum value was achieved with 200 W dc sputter power. Film thickness dependence study shows that the surface roughness of Ni80Fe20 thin films changes from 0.164 nm to 0.264 nm, with a minimum at film thickness of 8 nm. The small magnitude of change was attributed to a 20 W rf substrate bias that was applied during the thickness studies. We varied the substrate rf bias from 0–20 W, with fixed 200 W dc power and 8 nm film thickness. A dramatic change of surface roughness of Ni80Fe20 film from 0.984 nm (without rf bias) to 0.197 nm (with 20 W rf bias) on Si was achieved. The coercivity of the Ni80Fe20 thin films changes in a similar way as surface roughness. We conclude that the surface roughness of Ni80Fe20 thin films can be well controlled by applying a rf substrate bias during the deposition.
Temperature and stress dependencies of the magnetic and magnetostrictive properties of Fe0.81Ga0.19
Authors: Kellogg R. A. , Flatau A. B. , Clark A. E. , Wun-Fogle M. , Lograsso T. A.
Published: July 2002

It was recently reported that the addition of nonmagnetic Ga increased the saturation magnetostriction (λ100) of Fe over tenfold while leaving the rhombohedral magnetostriction (λ111) almost unchanged. To determine the relationship between the magnetostriction and the magnetization we measured the temperature and stress dependence of both the magnetostriction and magnetization from −21 °C to +80 °C under compressive stresses ranging from 14.4 MPa to 87.1 MPa. For this study a single crystal rod of Fe0.81Ga0.19 was quenched from 800 °C into water to insure a nearly random distribution of Ga atoms. Constant temperature tests showed that compressive stresses greater than 14.4 MPa were needed to achieve the maximum magnetostriction. For the case of a 45.3 MPa compressive stress and applied field of 800 Oe, the maximum magnetostriction at 80 °C decreases from its value at −21 °C by 12.9%. This small magnetostrictive decrease is consistent with a correspondingly small 3.6% decrease in magnetization over the same temperature range. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuator, transducer, and active damping applications. The measured value of Young’s modulus is low (∼55±1 GPa) and almost temperature independent. The large magnetostriction over a wide temperature range combined with the nonbrittle nature of the alloy is rare.
Thermal relaxation in soft magnetic patterned elements
Authors: Yin H. Q. , Doyle W. D.
Published: July 2002

The thermal decay of Ni80Fe20 nominally rectangle bar arrays with different sizes (1×5, 1×10, 1×15, and 2×10 μm2) and thicknesses (7.9, 10.0, 18.3, and 43.5 nm) has been studied. Observation of the remanent domain structure using magnetic force microscopy showed that the bars were uniformly magnetized with simple end curling in the thinnest samples, with simple end curling and end vortices in the intermediate samples, and with only end vortices in the thickest samples. The 1 μm wide elements switched abruptly from the “up” polarized domain to the “down” polarized domain state during reversal. Time-dependent coercivity measurements were analyzed using Sharrock’s formula to extract the intrinsic switching fields and the thermal sensitivity factors. The latter increased from 113 to 243 in the 7.9-nm-thick samples to 167–628 in the 18.3 nm samples, but decreased precipitously to 66–94 for the 43.5 nm samples. This behavior is correlated to the element’s size and the end domain structure.
Ferromagnetism in II–VI diluted magnetic semiconductor Zn1−xCrxTe
Authors: Saito H. , Zaets W. , Yamagata S. , Suzuki Y. , Ando K.
Published: July 2002

Magnetic and transport properties of an epitaxial film of ferromagnetic II–VI diluted magnetic semiconductor (DMS) Zn1−xCrxTe (x=0.035) were investigated. The Curie temperature TC of the film was about 15 K, which is the highest among the reported ferromagnetic II–IV DMS. Hall effect measurements at room temperature showed a hole concentration p of about 1×1015 cm−3, which is several orders lower than that reported for carrier-induced ferromagnetic Zn1−xMnxTe. These results suggest that a ferromagnetic superexchange interaction between Cr ions is responsible for the observed ferromagnetism in Zn1−xCrxTe.
Tokamak edge plasma simulation including anomalous cross-field convective transport
Authors: Pigarov A. Yu. , Krasheninnikov S. I. , Rognlien T. D. , Schaffer M. J. , West W. P.
Published: July 2002

Multi-fluid two-dimensional transport models such as the UEDGE code model [T. D. Rognlien et al., J. Nucl. Mater. 196–198, 34 (1992)] are widely used in the simulation of tokamak edge plasmas. Usually these models are based on the assumption of anomalous plasma diffusion in the direction perpendicular to magnetic field lines. As will be shown, the pure diffusive cross-field transport model is inadequate and fails to match properly plasma parameters measured both in the scrape-off layer (SOL) and in the divertor of the DIII-D tokamak. Recently it has been suggested that specific nondiffusive transport occurs in the edge plasma [S. I. Krasheninnikov, Phys. Lett. A 283, 368 (2001)]. The nondiffusive transport is incorporated to the UEDGE model by adding the anomalous cross-field convective velocity for plasma species and by prescribing a specific two-dimensional profile to this velocity. A series of highly radiative discharges obtained on the DIII-D tokamak is analyzed using the UEDGE code with the hybrid, convective and diffusive, cross-field transport model. For these discharges, anomalous convective velocity profiles are adjusted until the simulated radial profiles agree with measurements in the SOL and in the divertor. It is found that in order to reproduce most of the extensive experimental data, anomalous plasma convection should play the dominant role in the outboard edge-plasma region.
Heat transport and electron distribution function in laser produced plasmas with hot spots
Authors: Batishchev O. V. , Bychenkov V. Yu. , Detering F. , Rozmus W. , Sydora R. , Capjack C. E. , Novikov V. N.
Published: July 2002

Using Fokker–Planck and particle-in-cell simulations, the evolution of a single hot spot and multiple hot spot systems have been studied in laser produced plasmas. A practical formula for nonlocal heat flux has been derived as a generalized expression of a nonlocal linear approach [Bychenkov et al., Phys. Rev. Lett. 75, 4405 (1995)] and is tested in simulations. The electron distribution function is studied at different spatial locations with respect to a localized heating source. The electron distribution function displays several non-Maxwellian features which depend on the interplay between the effects of inverse bremsstrahlung heating and nonlocal transport. In particular, significant high-energy tails are found. They may have impact on the behavior of parametric instabilities in nonuniformly heated laser plasma.
Fluctuation conductivity in YBa2Cu3O7−y films with different oxygen content. II. YBCO films with Tc≈80 K
Authors: Solovjov A. L. , Habermeier H.-U. , Haage T.
Published: July 2002

The fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with T≈80 K is investigated for the first time. Unlike the optimally doped samples, these films exhibit a transition from a Maki–Thompson (MT) mechanism for the scattering of fluctuational pairs to a Lawrence–Doniach (LD) mechanism and, as the temperature approaches Tc, to an Aslamazov–Larkin (AL) mechanism. It is shown that the coherence length ξc(0) along the c axis and the phase relaxation time τφ(100 K) of the fluctuational pairs are determined by the temperature of this second transition. The features observed on both the fluctuation conductivity and resistive behavior are sharply enhanced as Tc approaches 80 K, probably because of a significant increase in the intensity of the magnetic interaction in high-Tc superconductors at these temperatures. In spite of this, for two samples a value τφ(100 K)=(3.35±0.01)×10−13 s is measured, i.e., the same value as for optimally doped YBCO films. It is shown that the dependence of ξc(0) on Tc obeys the standard theory of superconductivity. The mechanisms for the scattering of charge carriers and the superconducting pairing in YBCO are analyzed.
Ideal magnetohydrodynamic equations for low-frequency waves in toroidal plasmas
Authors: Fesenyuk O. P. , Kolesnichenko Ya. I. , Wobig H. , Yakovenko Yu. V.
Published: July 2002

Reduced linear equations of magnetohydrodynamics in high-aspect-ratio toroidal devices are derived, which are intended, first of all, for studying the Alfvén eigenmodes in stellarators and tokamaks. The equations take into account the effects of the plasma pressure and compressibility, which are known to be of importance for toroidicity-induced Alfvén eigenmodes, and are applicable to perturbations with arbitrary perpendicular wavelength. The reduction consists in eliminating high-frequency fast magnetoacoustic waves from the system and is shown not to affect the continuous spectrum of Alfvén and slow magnetoacoustic waves, which, to a large extent, determines the behavior of the waves of interest.
Numerical study of a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base
Authors: Konistis Konstantinos , Hu Qing
Published: July 2002

We consider a GaAs-based heterojunction bipolar transistor with stepwise alloy-graded base. The step height is slightly greater than the longitudinal optical (LO) phonon energy ℏωLO in order to facilitate LO-phonon-enhanced forward diffusion. Impurity and LO phonon scattering are considered as the dominant scattering mechanisms. We solve the Boltzmann transport equation (BTE) numerically in one dimension across the base and at arbitrary frequencies, unifying and extending previous theoretical treatments. The base transport factor is analyzed in the frequency domain by varying device parameters. The solution of the BTE is incorporated into the small-signal parameters of the intrinsic part of the device. Using a simple circuit approach for the extrinsic elements, high-frequency figures of merit are presented, casting light on the potential of the device as a high-gain amplifier and more importantly as a transit-time oscillator for millimeter-wave applications.
Structure of the strongly associated liquid antimony pentafluoride: An ab initio molecular dynamics study
Authors: Raugei Simone , Klein Michael L.
Published: July 2002

Under ambient conditions, pure antimony pentafluoride (SbF5) is a strongly associated liquid. Early NMR works established that the molecules oligomerize via cis-fluorine chains. We have employed ab initio molecular dynamics to investigate the structure of liquid SbF5. The simulation confirms the strong tendency of molecules to oligomerize via a barrierless, diffusion-limited process. The nature of the SbF5 chemical bond and the experimentally observed propensity to form cis-fluorine chains has been analyzed in terms of Wannier localized orbital functions. The calculation indicated a very strong ionic character for the Sb–F bond.