bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of wall boundary condition on scalar transfer in a fully developed turbulent flume
Authors: Tiselj Iztok , Pogrebnyak Elena , Li Changfeng , Mosyak Albert , Hetsroni Gad
Published: July 2002

We performed direct numerical simulation of fully developed turbulent velocity and temperature fields in a flume, for Reynolds number, based on the wall shear velocity and the height of the flume, Re=171 and Prandtl numbers Pr=1.0 and Pr=5.4. To elucidate exactly the role of the wall boundary condition for passive scalar, the system considered was the flow at constant properties of the fluid. Two types of thermal wall boundary conditions (BCs) for the dimensionless temperature equation were studied: isothermal wall boundary condition—H1, and isoflux wall boundary condition—H2. The profile of the mean temperature was not affected by the type of BC. However, the type of BC has a profound effect on the statistics of the temperature fluctuations in the near-wall region y+<10. Comparison of near-wall statistics of temperature fluctuations shows that at Pr=1 the buffer part of the turbulent boundary layer significantly influences the scalar transfer in the conductive sublayer, whereas at Pr=5.4 the near-wall temperature field may be associated with predominant motion in the viscous sublayer.
Effective compliant substrate for low-dislocation relaxed SiGe growth
Authors: Luo Y. H. , Liu J. L. , Jin G. , Wan J. , Wang K. L. , Moore C. D. , Goorsky M. S. , Chih C. , Tu K. N.
Published: July 2002

An effective compliant substrate was fabricated for the growth of high-quality relaxed SiGe templates, by synthesizing a 20% B2O3 concentration borosilicate glass (BSG) in the silicon on insulator wafers. Substrates with 5%, 10%, and 20% B2O3 were used for 150 nm Si0.75Ge0.25 epitaxy. Double-axis x-ray diffraction measurements determined the relaxation and composition of the Si1−xGex layers. Cross-sectional transmission electron microscopy was used to observe the lattice of the SiGe epilayer and the Si substrate, dislocation density, and distribution. Raman spectroscopy was combined with step etch to study the samples. The strain sharing effect of the 20% BSG substrate was demonstrated. Thus, we concluded that this compliant substrate is a highly promising candidate for the growth of low-dislocation relaxed SiGe layers.
Effects of successive proton irradiation on the peak effect in YBa2Cu3O7−δ single crystals
Authors: Tobos V. , Paulius L. M. , Petrean A. M. , Ferguson S. , Snyder J. W. , Olsson R. J. , Kwok W.-K. , Crabtree G. W.
Published: July 2002

The effect of increasing the density of uncorrelated defects on the peak effect is studied in a detwinned YBa2Cu3O7−δ single crystal. The defects were introduced with 9 MeV proton irradiation where the total dose ranged from 0.25×1015 to 2×1015 p/cm2. With each successive irradiation, the peak effect shifts down in field and is eventually suppressed in both the electrical resistivity and the dc magnetization by a sufficiently high irradiation dose. The peak effect reappears after a high temperature anneal, suggesting that it is primarily the point defects introduced by the irradiation which interfere with the formation of the peak effect.
Strength of the electric field in apertureless near-field optical microscopy
Authors: Martin Yves C. , Hamann Hendrik F. , Wickramasinghe H. Kumar
Published: July 2002

Enhancement γ of the electrical field at the end of a tip relative to the incident field in a focused radiation beam is calculated by the finite-element time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions. The calculations on these test structures also reveal that phase retardation effects substantially modify γ when the size of the structure is larger than approximately λ/4, λ being the radiation wavelength. For plasmon resonance, in particular, phase retardation severely reduces the resonance and the expected field enhancement for a gold tip. The small value of γ=4 calculated by FETD is about an order of magnitude smaller than the value found in recent published work. Resonance effects can be recovered for special tips, which have a discontinuity or a different material composition at the end of the tip. Some tuning of the discontinuity dimension is needed to maximize the resonance. Under optimal conditions for plasmon resonance, an enhancement in the electric field of about 50 is calculated at the end of a small gold protrusion mounted on a wider silicon or glass tip.
Computer simulation of disordering and amorphization by Si and Au recoils in 3C–SiC
Authors: Gao F. , Weber W. J.
Published: July 2002

Molecular dynamics has been employed to study the disordering and amorphization processes in SiC irradiated with Si and Au ions. The large disordered domains, consisting of interstitials and antisite defects, are created in the cascades produced by Au primary knock-on atoms (PKAs); whereas Si PKAs generate only small interstitial clusters, with most defects being single interstitials and vacancies distributed over a large region. No evidence of amorphization is found at the end of the cascades created by Si recoils. However, the structure analysis indicates that the large disordered domains generated by Au recoils can be defined as an amorphous cluster lacking long-range order. The driving force for amorphization in this material is due to the local accumulation of Frenkel pairs and antisite defects. These results are in good agreement with experimental evidence, i.e., the observed higher disordering rate and the residual disorder after annealing for irradiation with Au2+ are associated with a higher probability for the in-cascade amorphization or formation of a large disordered cluster.
CO/Rh(111): Vibrational frequency shifts and lateral interactions in adsorbate layers
Authors: Linke R. , Curulla D. , Hopstaken M. J. P. , Niemantsverdriet J. W.
Published: July 2002

High resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) were used to study lateral interactions in the adsorbate layer of the CO/Rh(111) system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C–O vibrations is observed over the whole coverage range. The frequency of the metal CO (M–CO) vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M–CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of CO12 and CO13 were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C–O stretching vibration at total CO coverage of 0.33 ML in the (√3×√3)R30° structure arise purely from dipole–dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole–dipole coupling. Therefore, experiments with a small amount of CO13 as a tracer to monitor the frequency shifts in the CO12 adlayer were carried out over the entire coverage range of CO12. The results demonstrate that dipole–dipole coupling causes the frequency shifts at low coverage (<0.5 ML), whereas chemical effects set in at higher coverage (0.5–0.75 ML), connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules.
Spontaneous ridge-structure formation and large field emission of heavily Si-doped AlN
Authors: Kasu Makoto , Kobayashi Naoki
Published: July 2002

Sharp ridge structures with a 3 nm wide (0001) top facet and {11̄01} sidewall facets formed on the surface of a heavily Si-doped AlN layer on a 6H-SiC (0001) substrate during metalorganicvapor-phase-epitaxy growth. This is caused by {11̄01} facet growth induced by heavy Si doping. We obtained a large field emission (FE) current density of 11 mA/cm2 at 84 V/μm. One of the reasons for the large FE is that the ridge-structure formation decreases the energy barrier necessary for FE by about 2.4 eV.
Photoluminescence of Ge quantum dots prepared on porous silicon by ultrahigh vacuum chemical vapor deposition
Authors: Huang Jingyun , Ye Zhizhen , Zhao Binghui , Ma Xiangyang , Wang Yadong , Que Duanlin
Published: July 2002

This letter reports a way of preparing Ge quantum dots on anodized porous silicon layers by ultrahigh vacuum chemical vapor deposition at a low temperature of 720 °C. The porous silicon was formed by anodic conversion of p-type (100) oriented crystalline silicon in hydrofluoric acid diluted by alcohol. A clear phonon-resolved photoluminescence (PL), as a no-phonon (NP) and its transverse acoustic phonon replica, was observed from the Ge dots at the temperature of 10 K. The blueshift energy is as high as about 136 meV, but the full width at half maximum of the NP PL spectrum is only 1.23 meV. We attributed the very large blueshift in energy of the PL peak to quantum size confinement effect of the Ge quantum dots.
A miniature capacitive probe array for transient high voltage capillary discharges
Authors: Favre M. , Chuaqui H. , Leñero A. M. , Wyndham E. , Choi P.
Published: July 2002

The design and construction of a millimeter size noninvasive capacitive probe array to investigate ionization growth phenomena in pulsed capillary discharges are presented. The probes estimated to be characterized by a time response better than 0.5 ns, with very good electric noise rejection. The probes have identified a fast ionization wave in the prebreakdown phase of a hollow cathode initiated fast pulsed capillary discharge.
Differential interference contrast x-ray microscopy with submicron resolution
Authors: Wilhein Thomas , Kaulich Burkhard , Di Fabrizio Enzo , Romanato Fillipo , Cabrini Stefano , Susini Jean
Published: July 2002

Progress in lithography and nanofabrication [E. Di Fabrizio et al., Nature (London) 401, 895 (1999)] has made it possible to apply differential interference contrast (DIC) in x-ray microscopy using an original x-ray doublet lens based on two specially developed zone plates. Switching from bright-field imaging (absorption contrast) to x-ray DIC, we observe, similar to visible-light microscopy, a dramatic increase in image contrast for weak absorbing samples. We anticipate that this technique will have a significant impact on x-ray imaging and may play a role comparable to DIC imaging in visible-light microscopy.
Full-field optical measurement of curvatures in ultra-thin-film–substrate systems in the range of geometrically nonlinear deformations
Authors: Lee Hansuk , Rosakis Ares J. , Freund L. B.
Published: July 2002

This article describes coherent gradient sensing (CGS) as an optical, full-field, real-time, nonintrusive, and noncontact technique for the measurement of curvatures and nonuniform curvature changes in film–substrate systems. The technique is applied to the study of curvature fields in thin Al films (6 μm) deposited on thin circular silicon wafers (105 μm) of “large” in-plane dimensions (50.8 mm in diameter) subjected to thermal loading histories. The loading and geometry is such that the system experiences deformations that are clearly within the nonlinear range. The discussion is focused on investigating the limits of the range of the linear relationship between the thermally induced mismatch strain and the substrate curvature, on the degree to which the substrate curvature becomes spatially nonuniform in the range of geometrically nonlinear deformation, and finally, on the bifurcation of deformation mode from axial symmetry to asymmetry with increasing mismatch strain. Results obtained on the basis of both simple models and more-detailed finite-element simulations are compared with the full-field CGS measurements with the purpose of validating the analytical and numerical models.
Synchrotron hard x-ray microprobe: Fluorescence imaging of single cells
Authors: Bohic S. , Simionovici A. , Snigirev A. , Ortega R. , Devès G. , Heymann D. , Schroer C. G.
Published: July 2002

Imaging of trace elements in single cells was achieved by synchrotron-induced x-ray fluorescence (SXRF) in the hard x-ray range. Monochromatic and “pink” excitations at 14 keV were used with compound refractive lenses resulting in a 1×10 μm2 beam size. The experiment shows that SXRF is well suited for microanalysis of freeze-dried cells, and demonstrated high accuracy in quantitative imaging of trace element in cells treated with pharmacological doses of an iodine-labeled anticancer drug.
Colloidal crystal: bead–spring lattice immersed in viscous media
Authors: Ohshima Yuichi Nobi , Nishio Izumi
Published: July 2002

We present a report about a new approach that can be used to describe the single-particle dynamics of colloidal crystals. This approach regards the colloidal crystal as a classical bead–spring lattice immersed in viscous fluid. In this model, the mean square displacement of a particle (MSD) and the mean product of displacement of a particle and that of another particle (x-MSD) are obtained exactly using the Langevin treatmentlike method. In other words, MSD and x-MSD are, respectively, an autocorrelation function of a particle and a cross-correlation function of two particles. As the first-order approximation of hydrodynamic interaction, effective Stokes’ viscous drag coefficient γeff is introduced to the model that includes all of the hydrodynamic effects due to the presence of all other particles. As a result of the viscous media, traveling phonon modes are transformed into relaxation modes, and the motion of a particle is comprehended as a superposition of these relaxation modes. The predicted MSD for face-centered-cubic lattice type crystals is in good agreement with the MSD observed by Bongers et al. [J. Chem. Phys. 104, 1519 (1996)]. As no experimental study of x-MSD has been published to date, the validity of the predicted x-MSD remains to be evaluated. Moreover, it has been demonstrated that, in the case of d=1, d=2, and d⩾3 (where d is the dimension of the system), MSD and x-MSD diverge, logarithmically diverge and converge, respectively. The presented results show that bead–spring lattices immersed in viscous media are unstable, quasistable, and stable, in the case of d=1, d=2, and d⩾3, respectively. These properties of the model are in agreement with the widely believed notions regarding how the dimension of a system affects the stability of a crystal according to solid state physics, as well as statistical mechanics. The presented model may be utilized to account for the elastic properties of colloidal crystals, such as the bulk modulus; the single-particle dynamics of colloidal crystals are also accounted for. The presented model may therefore lead to a better understanding of various macroscopic phenomena in which the corrective motion of particles or the effects of fluctuations play key roles.
Measurement of the autodetachment lifetime of SF6−* as a function of electron energy in a free jet expansion
Authors: Le Garrec Jean-Luc , Steinhurst Daniel A. , Smith Mark A.
Published: July 2002

A pure SF6 free jet expansion has been used for the measurement of the autodetachment lifetime of the resonant excited anion SF6−* as a function of electron energy in the range 0–100 meV. In this low temperature environment, the lifetime was found to be 19.1±2.7 μs and independent of electron energy. This result is consistent with earlier statistical modeling of this process in terms of a coupling to the bath of SF6− states which is only a smoothly varying function across the energy range spanned by the electron affinity plus the small variation in electron kinetic energy. The temperature dependence of the autodetachment lifetime also suggests a strong coupling of vibrational motion in SF6 with the autodetachment process consistent with the picture of nuclear excited Feshbach resonances. In addition, the rate coefficient for the very inefficient secondary stabilization rate of the SF6−* states on collision with SF6 neutrals below 10 K is determined to be 3±1×10−12cm3 s−1.
Dynamics and steady-state properties of photorefractive poly(N-vinylcarbazole)-based composites sensitized with (2,4,7-trinitro-9-fluorenylidene)malononitrile in a 0–3 wt % range
Authors: Van Steenwinckel David , Hendrickx Eric , Persoons André
Published: July 2002

This paper reports on the characterization of six low-Tg poly(N-vinylcarbazole)-based photorefractive (PR) composites sensitized with (2,4,7-trinitro-9-fluorenylidene)-malononitrile (TNFM) in different concentrations, ranging from 0 to 3 wt %. At 780 nm, two-beam coupling gain coefficients, four-wave mixing diffraction efficiencies, and photoconductivities were measured versus electric field, writing beam intensity, and temperature. Dynamic measurements pointed out that chromophore reorientation is not rate-limiting in any of the six samples. In samples with sensitizer concentrations up to 1.24 wt %, increasing the sensitizer concentration leads to a faster grating buildup through a faster charge generation. The grating buildup in these samples is rate- limited by the photogeneration speed. We provide evidence that the TNFM− anions, formed by photoreduction of TNFM, can act as a trap, similar to what has been observed in C60-sensitized samples. As a result, above 1.49 wt % of TNFM, the larger amount of traps produced by photoreduction of the sensitizer reduces the mobility of the charges. Then, the grating buildup speed becomes mobility limited, and smaller buildup rates are observed. Except for the sample with 3 wt % TNFM, increased writing beam intensities or sensitizer concentrations give rise to a larger dynamic range. The different behavior of the sample with the largest sensitizer concentration is explained theoretically in terms of the trap density. The PR phase shifts were found to decrease with increasing writing beam intensity or sensitizer concentration. This provides evidence that the PR phase shifts are controlled by the charge mobility rather than by the photogeneration efficiency. PR measurements as a function of temperature and electric field evolve as predicted by theory.
Effect of growth termination conditions on the performance of AlGaN/GaN high electron mobility transistors
Authors: Keller S. , Vetury R. , Parish G. , DenBaars S. P. , Mishra U. K.
Published: July 2002

The effect of the growth conditions of the top 2.5 nm thick AlGaN cap layer and wafer cool down conditions on AlGaN/GaN high electron mobility transistor performance was investigated. The AlGaN/GaN heterostructures were deposited on sapphire by metalorganic chemical vapor deposition and consisted of a 3 μm thick semi-insulating GaN layer and an 18 nm thick Al0.33Ga0.67N layer, the top 2.5 nm of which was deposited under various conditions. The power performance of the devices severely degraded for all samples where the Al content of the top 2.5 nm of AlGaN was increased and/or the ammonia flow during growth of the top layer was decreased. A modest improvement in the output power density was observed when the growth conditions of the cap layer were identical of those of the rest of the AlGaN layer, but when the wafer was cooled down in pure nitrogen.
Photon-induced current of polycrystalline-silicon thin-film transistors
Authors: Ikeda H.
Published: July 2002

Photon-induced current (Iph) in polycrystalline-silicon thin-film transistors (poly-Si TFTs) has been investigated using a line-shaped light beam scanning method. In the off-state region, photon-excited carriers in the channel region diffuse to the drain with the diffusion length independent of the poly-Si grain size. The diffusion length is also independent whether poly-Si is hydrogenated or not. This phenomenon is observed in both n-channel and p-channel TFTs. In the subthreshold region, Iph follows the general transport equation in poly-Si. In the inversion region, Iph is proportional only to the total amount of excited carriers in the channel.
Separation of reversible domain-wall motion and magnetization rotation components in susceptibility spectra of amorphous magnetic materials
Authors: Yoon S. S. , Kim C. G.
Published: July 2002

The reversible susceptibility spectra are measured for rectangular Co66Fe4NiB14Si15 samples with various easy-axis angles, α, relative to the sample axis. A phenomenological method is proposed for the reversible spectra to separate the relaxation processes of domain-wall motion and magnetization rotation. The separation provides a method for measuring the static susceptibilities and the relaxation frequencies for the two reversible magnetization processes. The α and the longitudinal stress dependence show that the separated spectra with relaxation frequencies near 360 kHz and 1.6 MHz correspond to relaxations of domain-wall motion and to magnetization rotation, respectively.
Design of a semiconductor ferromagnet in a quantum-dot artificial crystal
Authors: Shiraishi Kenji , Tamura Hiroyuki , Takayanagi Hideaki
Published: July 2002

We present the theoretical design of quantum-dot (QD) artificial ferromagnetic crystals. The electronic structure calculations based on local spin density approximation show that our designed QD artificial crystal from a structure comprising the crossing 0.104 μm wide InAs quantum wires (an effective Kagome lattice) has flat band characteristics. Our examined QD artificial crystal has the ferromagnetic ground state when the flat band is half filled, even though it contains no magnetic elements. The ferromagnetic and paramagnetic states can be freely switched by changing the electron filling via a gate voltage.
Electron irradiation effects in single wall carbon nanotubes
Authors: Smith Brian W. , Luzzi David E.
Published: July 2002

We determine, with excellent agreement between theory and experiment, the behavior of single wall carbon nanotubes during uniform electron irradiation. Calculations utilizing known ejection threshold energies predict that an isolated nanotube will damage preferentially on surfaces that lie normal to the electron beam. A minimum incident electron energy of 86 keV is required to remove a carbon atom by a knock-on collision for this geometry. Higher electron energies are required for any other geometry, and at energies exceeding 139 keV every atom on a nanotube is susceptible to ballistic ejection. Transmission electron microscopy observations of nanotubes using 80–400 keV electrons corroborate these conclusions. Based upon empirical observations, we also explain damage processes in nonisolated nanotubes.
Ultrasensitive Si phototransistors with a punchthrough base
Authors: Luo Hailin , Chang Yuchun , Wong K. S. , Wang Y.
Published: July 2002

Si phototransistors with a punchthrough base were fabricated with regular planar technology. Optical conversion gains larger than 15 000 were observed. In addition to very high gain, the unique structure of the device also resulted in a fast transient response as well as low output noise. The measured full width at half maximum of the device transient response is 1.6 ns and a −3 dB bandwidth of 300 MHz. The measured output noises at different currents can be well fitted by the relation in2¯=2qIC.
Pulsed atomic layer epitaxy of quaternary AlInGaN layers
Authors: Zhang J. , Kuokstis E. , Fareed Q. , Wang H. , Yang J. , Simin G. , Khan M. Asif , Gaska R. , Shur M.
Published: July 2002

In this letter, we report on a material deposition scheme for quaternary AlxInyGa1−x–yN layers using a pulsed atomic layer epitaxy (PALE) technique. The PALE approach allows accurate control of the quaternary layer composition and thickness by simply changing the number of aluminum, indium, and gallium pulses in a unit cell and the number of unit cell repeats. Using PALE, AlInGaN layers with Al mole fractions in excess of 40% and strong room-temperature photoluminescence peaks at 280 nm can easily be grown even at temperatures lower than 800 °C.
Hydrostatic-pressure dependence of the photoconductivity of single-crystal pentacene and tetracene
Authors: Rang Zhenlin , Haraldsson Anders , Kim Dong M. , Ruden P. Paul , Nathan Marshall I. , Chesterfield Reid J. , Frisbie C. Daniel
Published: July 2002

Pentacene and tetracene show readily observable photoconductivity when illuminated with light in the blue part of the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of both materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a carrier-mobility increase under pressure is most likely to cause the increase in photoconductivity in the case of pentacene. For tetracene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results.
Strain relief by long line defects in tensile GaxIn1−xP layers grown on InP substrates
Authors: Kahn M. , Ritter D.
Published: July 2002

Fully strained layers of GaInP on InP significantly thicker than the theoretical critical thickness were grown by metalorganic molecular beam epitaxy. The excess strain energy of these layers is most probably accommodated by long line defects observed by atomic force microscopy. The thickness at which the long line defects appear is about four times the Matthews and Blakeslee critical thickness [J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974)]. Partial relaxation is measured by x-ray at about 14 times the theoretical critical thickness.
Spontaneous formation of ordered indium nanowire array on Si(001)
Authors: Li Jian-Long , Liang Xue-Jin , Jia Jin-Feng , Liu Xi , Wang Jun-Zhong , Wang En-Ge , Xue Qi-Kun
Published: July 2002

Growth of In on the Si(001)-2×n nanostructured surface is investigated by an in situ scanning tunneling microscope (STM). The deposited In atoms predominantly occupy the normal 2×1 dimer-row structure, and develop into a uniform array of In nanowires at a coverage of ∼0.2 ML. High-resolution STM images show that the In atoms form a stable local 2×2 reconstruction that removes surface Si dangling bonds states and saturates all In valency. Since the dimensions of the Si(001)-2×n vacancy line structure depend on impurity concentrations, this study demonstrates that the 2×n surface can be used for spontaneous fabrication of various metal nanowire arrays.
Dynamics and rupture of planar electrified liquid sheets
Authors: Tilley B. S. , Petropoulos P. G. , Papageorgiou D. T.
Published: July 2002

We investigate the stability of a thin two-dimensional liquid film when a uniform electric field is applied in a direction parallel to the initially flat bounding fluid interfaces. We consider the distinct physical effects of surface tension and electrically induced forces for an inviscid, incompressible nonconducting fluid. The film is assumed to be thin enough and the surface forces large enough that gravity can be ignored to leading order. Our aim is to analyze the nonlinear stability of the flow. We achieve this by deriving a set of nonlinear evolution equations for the local film thickness and local horizontal velocity. The equations are valid for waves which are long compared to the average film thickness and for symmetrical interfacial perturbations. The electric field effects enter nonlocally and the resulting system contains a combination of terms which are reminiscent of the Kortweg–de-Vries and the Benjamin–Ono equations. Periodic traveling waves are calculated and their behavior studied as the electric field increases. Classes of multimodal solutions of arbitrarily small period are constructed numerically and it is shown that these are unstable to long wave modulational instabilities. The instabilities are found to lead to film rupture. We present extensive simulations that show that the presence of the electric field causes a nonlinear stabilization of the flow in that it delays singularity (rupture) formation.
Field-emission characteristics and large current density of heavily Si-doped AlN and AlxGa1−xN (0.38⩽x<1)
Authors: Kasu Makoto , Kobayashi Naoki
Published: July 2002

From a linear relation between the applied bias and the anode–sample distance, the electric field necessary for field emission (FE) can be obtained reproducibly. For heavily Si-doped AlN and AlxGa1−xN (0.38⩽x<1), the band gap (Al-mole fraction, x), Si-dopant density (NSi), and thickness dependences of the field emission are investigated. After optimizing the sample structure, we obtained a FE current density of 0.22 A/cm2 with a 0.3-mm-diameter rod anode from 0.8-μm-thick Si-doped (NSi:1×1021 cm−3) AlN.
Influence of varying N-environments on the properties of (GaIn)(NAs) vertical-cavity surface-emitting lasers
Authors: Choulis S. A. , Hosea T. J. C , Klar P. J. , Hofmann M. , Stolz W.
Published: July 2002

We report electromodulated reflectance studies on the band structure of a dilute-N (∼1%) (GaIn)(NAs)/GaAs/AlAs vertical-cavity surface-emitting laser (VCSEL), as a function of temperature and incidence angle. The wide range of operating temperatures observed for this type of VCSEL (∼360 K here) is due to the reduced temperature variation of the effective band gap of the active (GaIn)(NAs) quantum wells, and broad gain. By comparing lasing properties and band structure we argue that the gain broadening is not simply due to alloy disorder but arises from a recently-proposed intrinsic property of (GaIn)(NAs): the existence of different band gaps for the five possible nearest-neighbor configurations of the N substitutional impurity.
Interlayer coupling and magnetic reversal of antiferromagnetically coupled media
Authors: Margulies D. T. , Schabes M. E. , McChesney W. , Fullerton E. E.
Published: July 2002

We characterize the interlayer coupling and magnetic reversal of antiferromagnetically coupled recording layers. We show that the granular structure and reduced thickness values of the lower layer result in its reversal being strongly thermally activated at room temperature. We also find that the granular structure of the layers produces large dipolar fields which favor antiparallel alignment of the layer magnetizations. These dipolar fields overcome the ferromagnetic regions in the oscillatory coupling as a function of Ru thickness tRu, causing the interlayer coupling to be only antiferromagnetic for tRu>5 Å.
Experimental observation of sheath and magnetic presheath over an oblique metallic plate in the presence of a magnetic field
Authors: Singha Bornali , Sarma A. , Chutia Joyanti
Published: July 2002

Experimental observation on sheath and magnetic presheath over an oblique metallic plate in the presence of a magnetic field has been performed. The plate has been deployed in the plasma environment, making some finite angle with the externally applied magnetic field. Studies have been carried out for various angles, magnetic field gradients and plate-biasing voltages as well. The analyses reveal that the magnetic presheath thickness increases while the angle is varied from minimum to maximum; whereas the reverse happens in case of the sheath thickness. Furthermore, measurements of the sheath widths with increasing magnetic field strengths and the plate biasing voltages have been done which imply that sheath width enhances with increasing both the parameters. The electron temperature in the bulk plasma region is also evaluated for all the plasma conditions so as to find out its influence on the characteristic behavior of the sheath formation.
Surface–plasma negative ion source for the medicine accelerator (abstract)
Authors: Belchenko Y. I. , Grigoryev E. V.
Published: July 2002

A neutron source for boron capture neutron therapy is under development at the Budker Institute of Nuclear Physics (BINP). It is based on a tandem vacuum-isolated accelerator using a dc negative hydrogen ion source. A compact dc ion source model having reliable operation and simplified maintenance was developed at BINP as a prototype of the high current source for the medicine accelerator. The compact model (CM) uses a glow discharge with modified Penning geometry of the electrodes and explores surface–plasma negative ion (NI) production on the cesiated anode surface. The CM optimal parameters are as follows: discharge voltage 60–80 V, discharge current up to 6 A, hydrogen pressure 4–5 Pa, cesium consumption <1 mg/h, transverse magnetic field 0.5–1 kG s, optimal anode temperature 250–350 °C. A built-in heater provides the CM a quick start and simplifies CM electrode conditioning. The CM delivers a NI beam with a current of 5 mA, beam energy of 17 keV, normalized emittance of about 0.3 π mm mrad and an emission current density of about 0.1 A/cm2. NI beam current is directly proportional to the discharge current and to the emission hole area. The CM quick start and stable dc operation for several hour runs were multiply tested. No essential erosion of electrodes was recorded.
Surface-induced photorefractive-like effect in pure liquid crystals
Authors: Pagliusi P. , Cipparrone G.
Published: July 2002

We report a study of diffraction gratings observed in planar cells of nematic mixture (cyanobiphenyl and cyanotriphenyl derivatives, E7) made with different aligning substrates. Under the combined application of very low intensity laser beams (1.8 mW/cm2) and low dc voltage (4 V), highly sensitive gratings have been observed in degenerate four-wave-mixing experiments. Beam coupling measurements confirm the photorefractive-like nature of the gratings. Interesting results are obtained using two different liquid crystals, depending on the aligning substrates. The observed behavior suggests the fundamental role of the interfaces in the photoinduced processes.
Correlation of charge transport to local atomic strain in Si-rich silicon nitride thin films
Authors: Habermehl S. , Carmignani C.
Published: July 2002

Charge transport in Si-rich silicon nitride thin films is investigated. Over a composition range from Si3N4 to SiN0.54 the field dependent current density is observed to increase by as much as seven orders of magnitude, the residual Si–N bond strain decreases from 0.006 to −0.0026 and the Poole–Frenkel barrier height, ΦB, decreases from 1.08 to 0.55 eV. A direct correlation between ΦB and the local atomic strain is observed. It is concluded that reductions in ΦB are a manifestation of compositionally induced strain relief at the molecular level. Reductions in ΦB result in increased Poole–Frenkel emission rates and consequently higher conductivities in Si-rich films.
Red electrophosphorescence from osmium complexes
Authors: Jiang Xuezhong , Jen Alex K.-Y. , Carlson Brenden , Dalton Larry R.
Published: July 2002

We report red electrophosphorescence from light-emitting diodes based on osmium (Os) complexes. Efficient red emission was achieved using an in situ polymerized tetraphenyldiaminobiphenyl-containing polymer as the hole-transporting layer and Os complexes doped blend of poly(N-vinylcarbazole) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole as the emitting layer. The emission peaks of the reported Os complexes, ranging from 620 to 650 nm, can be tuned by changing the structures of the ligands because the emission originates from triplet metal-to-ligand-charge-transfer excited state. The Os complexes trap both electrons and holes, which facilitates the direct recombination of holes and electrons on the complex sites. The peak external quantum efficiency and brightness achieved from the complexes were 0.82% and 970 cd/m2, respectively. The Commission Internationale de I’Eclairage chromaticity coordinates (x,y) for the best red emission from the complexes are (0.65, 0.33).
Ultrasensitive hot-electron kinetic-inductance detectors operating well below the superconducting transition
Authors: Sergeev A. V. , Mitin V. V. , Karasik B. S.
Published: July 2002

While most experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, kinetic-inductance detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance. We modeled the noise characteristics of the kinetic-inductance detectors made from disordered superconducting Nb, NbC, and MoRe films. Low-phonon transparency of the interface between the superconductor and the substrate causes substantial retrapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation–recombination noise can be as small as ∼10−19 W/Hz at He4 temperatures.
The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules
Authors: Andersen Jens Ulrik , Hvelplund Preben , Nielsen Steen Brøndsted , Tomita Shigeo , Wahlgreen Helge , Møller Søren Pape , Pedersen Ulrik Vindelev , Forster James S. , Jørgensen Thomas J. D.
Published: July 2002

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (∼106 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (∼102–104 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 103–104 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine relative destruction cross sections. The technique could be useful to probe conformers differing only in size. Furthermore, our setup can be used for spectroscopic measurements of the ion-photon interaction such as the excitation of [Cytochrome c+17H]17+ protein ions with 532 nm photons.
Improved thermal stability of wet-oxidized AlAs
Authors: Jia H. Q. , Chen H. , Wang W. C. , Wang W. X. , Li W. , Huang Q. , Zhou Junming , Xue Q. K.
Published: July 2002

Lateral thermal wet oxidization of the AlAs layer in a GaAs/AlAs/GaAs sandwiched structure is studied by Raman spectroscopy and Nomarski microscopy. A significant improvement in thermal stability of the oxidized AlAs layer has been achieved by optimizing the oxidation conditions, which can be used to fabricate reliable devices. We show that the thermal stability is strongly related to the removal of volatile products, such as As and As2O3, as evidenced by the Raman spectroscopy measurement.
PbTe based superlattice structures with high thermoelectric efficiency
Authors: Beyer H. , Nurnus J. , Böttner H. , Lambrecht A. , Roch T. , Bauer G.
Published: July 2002

We report on an enhanced thermoelectric figure of merit ZT=σS2T/λ (where σ is electrical conductivity, S is thermopower, T is absolute temperature, and λ is thermal conductivity) for PbTe/PbSe0.20Te0.80 superlattices (SLs) and PbTe doping SLs due to a reduction of the thermal conductivity λ parallel to the layer planes. Despite a small decrease of the power factors σS2 due to a reduction of σ in these superlattices, the figure of merit is higher as compared to the corresponding bulk materials and reaches maximum values in the temperature range between 400 and 570 K.
Micro-superconducting quantum interference device characteristics
Authors: Hasselbach K. , Mailly D. , Kirtley J. R.
Published: July 2002

We report on the dependence on field and temperature of the critical current of micro superconducting quantum interference devices (SQUIDs): SQUIDs with diameters as small as 1 μm using Dayem bridges as weak links. We model these SQUIDs by solving the Ginzburg–Landau equations with appropriate boundary conditions to obtain the supercurrent–phase relationships. These solutions show that the phase drops and depression of the order parameter produced by supercurrent flow are often distributed throughout the micro-SQUID structure, rather than being localized in the bridge area, for typical micro-SQUID geometries and coherence lengths. The resultant highly nonsinusoidal current–phase relationships Ic(φ) lead to reduced modulation depths and triangular dependences of the micro-SQUID critical currents on applied magnetic flux Ic(Φ). Our modeling agrees well with our measurements on both Al and Nb micro-SQUIDs.
Mark shapes in hybrid recording
Authors: Herget P. , Rausch T. , Shiela A. C. , Stancil D. D. , Schlesinger T. E. , Zhu J.-G. , Bain J. A.
Published: July 2002

We describe the theoretical and experimental investigation of mark shapes in thermally assisted or hybrid recording. The effect of alignment was studied by recording marks on perpendicular TbFeCo media using a commercial recording head and 840 nm laser. We found that mark shapes are primarily controlled through laser/head alignment, and that maximally straight transitions are obtained by placing the laser spot on the leading pole tip near the gap. Simulations of the recording process confirm the experimental findings and are used in conjunction with static write contours to show how transitions form.
The glass transition of thin film polymer/polymer blends: Interfacial interactions and confinement
Authors: Pham Joseph Q. , Green Peter F.
Published: July 2002

We examined the influence of film thickness and composition on the effective Tg of compatible thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on SiOx/Si substrates using spectroscopic ellipsometry. Our measurements reveal that while the Tg of TMPC films increased with decreasing film thickness, h, the effective Tg of thin film mixtures of PS and TMPC decreased with decreasing film thickness. In these mixtures, Tg was independent of film thickness at large h. We also found that while the Tg of bulk mixtures of TMPC/PS exhibited large negative deviations from additivity with composition, such deviations were negligible in the thin film mixtures. The thickness dependence of Tg is compared with theory.
Magnetic control of flow past a blunt body: Numerical validation and exploration
Authors: Poggie Jonathan , Gaitonde Datta V.
Published: July 2002

Computational and theoretical studies of a Mach 5 flow over a hemisphere were carried out to validate a new computer code for magnetogasdynamic simulation, and to examine the possibility of heat transfer mitigation through magnetic control. Sample calculations were made using a local solution developed by W. B. Bush [J. Aerosp. Sci. 25, 685 (1958); 28, 610 (1961)] for the stagnation point flow over an axisymmetric blunt body with an imposed dipole magnetic field. Numerical computations, which obviated many of the simplifications inherent in the Bush theory, were carried out employing the low magnetic Reynolds number approximation. Both models indicate that an imposed dipole field can slow the flow in the conductive shock layer and consequently reduce the wall heat flux in the vicinity of the stagnation point. The theoretical model predicts a slightly higher level of heat transfer than that obtained computationally, but there is good agreement between the two models in the fractional change in heat transfer with increasing strength of the applied magnetic field. For both models, nonuniform electrical conductivity was found to reduce the effectiveness of a given applied field. Magnetic flow control is seen to have a sound physical basis, and may prove to be a useful technology for heat transfer mitigation.
Enhanced tensile ductility and toughness in nanostructured Cu
Authors: Wang Y. M. , Ma E. , Chen M. W.
Published: July 2002

Pure copper with ultrafine grain sizes and nanoscale subgrain (dislocation) structures was prepared by using severe plastic deformation through cold rolling at subambient temperatures, with or without subsequent recovery annealing. We report coexisting high strength and tensile ductility (large elongation to failure and ductile fracture). Factors leading to the simultaneous strengthening and toughening with increasing cold deformation and microstructural refinement are discussed.
Influence of different types of threading dislocations on the carrier mobility and photoluminescence in epitaxial GaN
Authors: Shi J. Y. , Yu L. P. , Wang Y. Z. , Zhang G. Y. , Zhang H.
Published: July 2002

High-resolution x-ray diffraction has been used to analyze the type and density of threading dislocations in a series (0001)-oriented GaN epitaxial film. Photoluminescence (PL) and carrier mobility of the films are measured at room temperature. The intensities of both the band edge (3.42 eV) peak and yellow luminescence (YL) are strongly related to the threading dislocation density of the GaN films. But different types of dislocations show different relationship with the intensities of PL and YL. The fundamental correlation is found not only between the interaction of edge- and screw-type dislocations and the carrier mobility but also between the interaction and the intensities of both the band edge peak and the YL.
Criticality and phase behavior in the restricted-primitive model electrolyte: Description of ion association
Authors: Jiang Jianwen , Blum Lesser , Bernard Oliver , Prausnitz John M. , Sandler Stanley I.
Published: July 2002

Ion association is incorporated into the restricted-primitive model electrolyte to account for the strong attraction between unlike ions. Two methods are investigated within the McMillan–Mayer framework: first is the binding mean-spherical approximation (BIMSA) based on the Wertheim Ornstein–Zernike integral equation formalism; and the second is the combination of the BIMSA with a simple interpolation scheme based on the Wertheim thermodynamic perturbation theory. The latter gives a better description. Four different association constants are used to calculate the degree of dissociation, the critical point, and the vapor–liquid coexistence curve. An increase in the association constant leads to a lower critical temperature and a higher critical density, and better agreement with computer simulations. When unlike ions are fully paired, corresponding to a charged hard dumbbell system, we obtain the best agreement with the most recent computer simulations of the RPM electrolyte.
Magnetoviscosity and orientational order parameters of dilute ferrofluids
Authors: Ilg Patrick , Kröger Martin , Hess Siegfried
Published: July 2002

The linear and nonlinear rheological behavior of dilute ferrofluids is determined from an underlying kinetic model and the dependence of the viscosity coefficient on the scalar orientational order parameters is obtained. In case of uniaxial symmetry, the antisymmetric contribution to the hydrodynamic stress tensor is of the same form as in the classical Ericksen–Leslie theory of uniaxial nematic liquid crystals and the linear magnetoviscosity is found to coincide with earlier results obtained by the so-called effective field method. While the assumption of uniaxial symmetry is fulfilled exactly in the limit of strong vorticity and weak magnetic field, the exact result for the linear magnetoviscosity shows corrections due to contributions from biaxial symmetry. Measures for the deviations from uniaxial symmetry are introduced and the generalization of the stress tensor in case of biaxial symmetry is obtained. The investigations are accompanied by numerical simulation of the kinetic equation and reveal that the assumption of uniaxial symmetry seems to be a good approximation for most values of the magnetic field and vorticity.