bluuebunny/crossref_metadata_2025 huggingface dataset:
Increased confinement improvement in a reversed-field pinch using double-pulsed poloidal current drive
Authors: Yagi Y. , Koguchi H. , Hirano Y. , Shimada T. , Sakakita H. , Sekine S. , Chapman B. E. , Sarff J. S.
Published: June 2003

The pulsed poloidal current drive (PPCD) [J. S. Sarff et al., Phys. Rev. Lett. 72, 3670 (1994)] experiment is conducted in a reversed-field pinch device, the toroidal pinch experiment RX (TPE-RX) after providing an auxiliary power supply system with increased energy in the main power supply system for the PPCD. The PPCD system thus provides double-pulsed operation with higher current in the toroidal coil than that in single-pulsed PPCD operation in TPE-RX [Y. Yagi et al., Plasma Phys. Controlled Fusion 44, 335 (2002)]. The central electron temperature, ion temperature, and electron density increase during PPCD, and there is, on average, a fivefold improvement in energy confinement, τE, relative to standard discharges. Double-pulsed PPCD yields better performance than that of single-pulsed PPCD operation where twofold improvement in τE was obtained. It is shown that the enhancement factor of τE in the double-pulsed PPCD experiment in TPE-RX is consistent with the trends, observed previously, versus magnetic fluctuation amplitude and versus Δγ, where Δγ is the difference in γ [=(1−F)/Θ] between the start and the end of the PPCD period.
A “periodic table” for supersymmetric M-theory compactifications
Authors: Doran Charles F. , Faux Michael
Published: June 2003

We develop a systematic method for classifying supersymmetric orbifold compactifications of M-theory. By restricting our attention to Abelian orbifolds with low order, in the special cases where elements do not include coordinate shifts, we construct a “periodic table” of such compactifications, organized according to the orbifolding group (order ⩽12) and dimension (up to 7). An intriguing connection between supersymmetric orbifolds and G2 structures is explored.
Effect of charge reduction on shielding in dusty plasmas
Authors: Avinash K. , Bhattacharjee A. , Merlino R.
Published: June 2003

The shielding of dust particles by each other in a dusty plasma is studied taking into account the effects of dust charging. It is shown that the assumption of a Boltzmann response for dust species is not appropriate under these circumstances. Further, it is shown that in the close-packing limit, dust grains screen each other by charge reduction, and an expression is obtained for this shielding scale length.
Magnetic dissipation in a force-free plasma with a sheet-pinch configuration
Authors: Li H. , Nishimura K. , Barnes D. C. , Gary S. P. , Colgate S. A.
Published: June 2003

This paper describes the study of a force-free plasma with an initial sheet-pinch configuration ∇×B=−αB, where α is a scalar constant, using both linear theory and 2-1/2 dimensional particle-in-cell simulations with doubly periodic boundary conditions. Previous studies have shown that this configuration is unstable to the collisionless tearing instability. In this work, the linear growth rate in the long wavelength limit is found to have an upper bound at 1/τA, where τA is the Alfvén traversal time through the region with sheared magnetic fields. The simulations show that the initial force-free state evolves by seeking a more “relaxed,” lower energy state within the constraints of the simulation geometry. Under the periodic boundary conditions, the amount of magnetic energy available for dissipation is determined mostly by the geometry. The reconnection region shows a multiscale structure, separated by ion and electron inertial lengths. Particle and flow dynamics at the reconnection regions have been analyzed. The reconnection rate is shown to be high, with an ion inflow speed of ∼0.2 Alfvén speed, and the timescale for reconnection is several Ωi−1. Most of the dissipated magnetic energy goes into the thermal energy of the particles.
Ion swarm data for electrical discharge modeling in air and flue gas mixtures
Authors: Nelson D. , Benhenni M. , Eichwald O. , Yousfi M.
Published: June 2003

The first step of this work is the determination of the elastic and inelastic ion-molecule collision cross sections for the main ions (N2+, O2+, CO2+, H2O+ and O−) usually present either in the air or flue gas discharges. The obtained cross section sets, given for ion kinetic energies not exceeding 100 eV, correspond to the interactions of each ion with its parent molecule (symmetric case) or nonparent molecule (asymmetric case). Then by using these different cross section sets, it is possible to obtain the ion swarm data for the different gas mixtures involving N2, CO2, H2O and O2 molecules whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in gas mixtures. This also allows us to clearly show that the classical linear approximations usually applied for the ion swarm data in mixtures such as Blanc’s law are far to be valid. Then, the ion swarm data are given in three cases of gas mixtures: a dry air (80% N2, 20% O2), a ternary gas mixture (82% N2, 12% CO2, 6% O2) and a typical flue gas (76% N2, 12% CO2, 6% O2, 6% H2O). From these reliable ion swarm data, electrical discharge modeling for a wire to plane electrode configuration has been carried out in these three mixtures at the atmospheric pressure for different applied voltages. Under the same discharge conditions, large discrepancies in the streamer formation and propagation have been observed in these three mixture cases. They are due to the deviations existing not only between the different effective electron-molecule ionization rates but also between the ion transport properties mainly because of the presence of a highly polar molecule such as H2O. This emphasizes the necessity to properly consider the ion transport in the discharge modeling.
Transformation of watergel samples in liquid helium
Authors: Kokotin A. M. , Mezhov-Deglin L. P.
Published: June 2003

It is found that the shape and structure of watergel samples formed in the condensation of gaseous He4 containing a water vapor impurity on the surface of superfluid He II cooled to 1.4 K depend substantially not only on the conditions of admission of the mixture but also on the construction of the experimental cell. The results of observations of the evolution of samples prepared in a wide cell, 29 mm in diameter, at a low rate of admission of the mixture are presented.
EPR spectra and rotation of CH3, CH2D, CHD2, and CD3 radicals in solid H2
Authors: Dmitriev Yu. A. , Zhitnikov R. A.
Published: June 2003

The EPR spectra of CH3, CH2D, CHD2, and CD3 radicals in a H2 matrix have been observed in the temperature range 1.6–4.2 K. The radicals were obtained by condensation of two gas flows on a cold substrate: deuterium mixed with 2 mol % methane passed through a discharge and pure hydrogen bypassing the discharge. The CD3 and CHD2 spectra were found to be a superposition of two spectra: high- and low-temperature. A transformation of the shape of the CD3 and CHD2 spectra with decreasing sample temperature was observed. This was attributed to a change in the populations of the lowest rotational states of the radicals. Compared to the known results for deuterated methyl radicals in Ar, the present observations suggest the existence of a barrier hindering the rotation of radicals in solid H2.
Quench deposited Kr–H2 and Ar–H2 mixtures: in quest of impurity–hydrogen gels
Authors: Strzhemechny M. A. , Galtsov N. N. , Prokhvatilov A. I.
Published: June 2003

The structure and morphology of low-temperature quench condensed binary alloys of hydrogen with argon and krypton were studied by powder x-ray diffraction. The nominal hydrogen fraction c in both systems was varied from 0 to 50%; the condensation was performed at 5–6 K; both as-prepared and annealed samples were examined by x-ray diffraction. Few reflections (and often only one) can be unambiguously detected for the as-grown alloy samples. In Kr–H2 condensates with c<10% the x-ray patterns show fine-grain krypton-rich crystallites with a rather high actual hydrogen content as estimated from Vegard’s law. For high nominal hydrogen fractions (c⩾10%) no reflections attributable to the krypton lattice were recorded and the incoherent background showed no characteristic swelling around the position of the (111) reflection from pure Kr; instead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was steadily raised the hydrogen reflections disappeared first. Then at a certain temperature the samples underwent an abrupt transformation as a result of which heat was released and larger x-ray detectable textured krypton crystallites were formed. In the as-grown Ar–H2 samples only the (111) reflections from the argon-rich phase were recorded. Warmup led to the same consequences, i.e. hydrogen effusion followed by recrystallization. In both systems the recrystallization onset temperature depends substantially on the nominal hydrogen fraction in the gas. The shift of the lattice parameter in the as-grown argon-based phases suggests strong suppression of the quantum nature of hydrogen in the argon lattice environment. The experimental findings viewed as a whole can be construed as evidence of the fact that quench-condensed hydrogen-containing alloys morphologically resemble helium-impurity solids (gels) whose structure and morphology are currently being studied at Cornell University.
Thermal conductivity of solid parahydrogen with methane admixtures
Authors: Krivchikov A. I. , Korolyuk O. A. , Sumarokov V. V. , Mucha J. , Stachowiak P. , Jeżowski A.
Published: June 2003

The thermal conductivity of a solid parahydrogen crystal with methane admixtures was measured in the temperature range 1.5 to 8 K. Solid samples were grown from the gas mixtures at 13 K. The concentration of CH4 admixture molecules in the gas varied from 5 to 570 ppm. A very broad thermal conductivity peak with an absolute value of about 110 W/(m⋅K) was observed at 2.6 K. The data were interpreted using Callaway’s model with resonance scattering of phonons by quasilocal vibrations of CH4 molecules and phonon–grain boundary and phonon–phonon scattering. As grain boundary scattering increases, the broadening of the peak decreases. The analysis shows that a solid mixture of p-H2 and CH4 is a heterogeneous solution for CH4 concentration higher than 0.1 ppm.
Vibrational dynamic and thermodynamic characteristics of metal–hydrogen superlattices
Authors: Feodosyev S. B. , Gospodarev I. A. , Grishaev V. I. , Syrkin E. S.
Published: June 2003

The phonon spectra and vibrational thermodynamic characteristics of Pd–H superlattices are investigated. Our calculations show that the neutron scattering and calorimetric data are consistent with one another and make it possible to explain for such systems the atomic mean-square amplitudes and the correlations between them.
Thermal expansion of Kr–CH4 solid solutions at liquid-helium temperatures
Authors: Aleksandrovskii A. N. , Gavrilko V. G. , Dolbin A. V. , Esel’son V. B. , Manzhelii V. G. , Udovidchenko B. G.
Published: June 2003

A negative contribution of the CH4 impurity to the thermal expansion of a solution was observed in dilatometric studies of the solid solutions Kr+0.76%CH4, Kr+5.25%CH4, and Kr+10.5% CH4 at 1–23 K. It was shown that the negative contribution is due to changes in the occupancy of the ground state of the A modification of isolated CH4 molecules. The contributions  of the CH4 impurity singles and clusters to the thermal expansion were estimated assuming them to be independent of one another. The contribution of the singles to the thermal expansion of the solid solution was found to be negative. The energies of the first excited rotational states were determined for singles and two- and three-body clusters of CH4 molecules.
Ba
8
ZnTa
6
O
24
:
a high-Q microwave dielectric from a potentially diverse homologous series
Authors: Moussa S. M. , Claridge J. B. , Rosseinsky M. J. , Clarke S. , Ibberson R. M. , Price T. , Iddles D. M. , Sinclair D. C.
Published: June 2003

The ceramic Ba8ZnTa6O24 has been synthesized in isolation and its dielectric and crystallographic properties characterized. The material affords excellent dielectric properties, with a high unloaded quality factor Qu=20 800 at 3.28 GHz, high relative permittivity εr=29 and a temperature coefficient of resonant frequency τf=29.4 ppm/°C. The crystal structure adopted is complex, comprising mixed cubic and hexagonal perovskite subunits, and contains cation vacancies on the octahedral sites. A second phase with a closely related structure is identified, demonstrating the existence of a family of materials. This structural complexity offers diverse opportunities for substitutions calculated to enhance the figures of merit reported.
Conducting atomic-force-microscope electrical characterization of submicron magnetic tunnel junctions
Authors: Worledge D. C. , Abraham David W.
Published: June 2003

We have developed the ability to electrically characterize submicron magnetic tunnel junctions using a conducting atomic force microscope (CAFM). This technique can be applied after only a short processing route, thereby saving time and resources, and reducing the potential for damaging the junctions. The CAFM can be used to measure hysteresis loops, magnetoresistance, resistance–area product, switching astroids, current–voltage curves, and breakdown voltage of tunnel junctions. In this letter, we outline the sample requirements, detail the CAFM processing route, describe tip preparation, and report examples of data we have obtained with this technique over the last two years.
Synergistic mitigation of the Rayleigh–Taylor instability in Z-pinch implosions by sheared axial flow and finite Larmor radius effect
Authors: Qiu X. M. , Huang L. , Jian G. D.
Published: June 2003

The synergistic stabilizing effect of sheared axial flow (SAF) and finite Larmor radius (FLR) on the Rayleigh–Taylor instability in Z-pinch implosions is considered by means of the magnetohydrodynamic (MHD) equations. The SAF is introduced into the MHD equations in a conventional way and the FLR is introduced in the same way as used by Roberts and Taylor [Phys. Rev. Lett. 8, 197 (1962)]. Therefore, the linearized MHD equations include both SAF and FLR effects. The results indicate that in the whole wavenumber region the synergistic effect of FLR and SAF can mitigate the Rayleigh–Taylor instability; at low flow velocity the synergistic effect of FLR and the SAF is slightly (∼10%) stronger than the mitigation effect of FLR alone and remarkably stronger than the mitigation effect of the SAF alone; at higher flow velocities in the large wavenumber region (for normalized wavenumber κ>2.4) the synergistic effect of FLR and the SAF is remarkably stronger than the mitigation effect due to either one of the two, respectively, and in the small wavenumber region (κ<2.4) it is stronger than the mitigation effect due to either one of the two, respectively.
The superposition of rotating and stationary magnetic sources: Implications for the auroral region
Authors: Rothwell Paul L.
Published: June 2003

Magnetohydrodynamic theory has been widely and successfully applied to explain many aspects of magnetospheric behavior. In particular, it has been used to predict where and when field-aligned auroral currents of electrons and ions will flow between the magnetosphere and ionosphere. On the other hand, in the auroral region, precipitating particles with energies of many keV are observed and are widely presumed to have been accelerated by field-aligned electric fields, which have been observed directly. These observations imply that additional plasma and/or external processes are present. In this report, an expression for the field-aligned electric field due to the superposition of the Earth’s rotating magnetic dipole and a stationary magnetic source is derived and evaluated at various locations throughout the magnetosphere. The results show a striking agreement with both the location and the energies of precipitating ions observed over a two-year period by the Defense Meteorological Satellite Program and also agrees with the size of the auroral oval as measured by ultraviolet data from the Viking satellite. The degree and characteristics of this agreement suggest that the parallel electric field associated with the rotating magnetic dipole is indeed present, is not canceled by polarization effects, and is a prime cause of energetic-ion precipitation.
Free electron laser with linearly polarized wiggler and ion channel guiding
Authors: Lal Seema , Kumar Punit , Jha Pallavi
Published: June 2003

A free electron laser (FEL) configuration utilizing a linearly polarized wiggler and ion-channel guiding has been studied for axial injection of the electron beam. The interaction results in an imperfect electron trajectory which is a superposition of two simple harmonic oscillations, one at the wiggler frequency and the other at the ion-channel frequency. The possibility of obtaining gain at the upshifted wiggler or the upshifted ion-channel betatron frequency and their odd harmonics has been shown. As a result the FEL can be tuned either by changing the electron energy or the ion-channel density.
Thermal property of tunnel-regenerated multiactive-region light-emitting diodes
Authors: Guo Xia , Shen Guang-Di , Ji Yuan , Wang Xue-Zhong , Du Jin-Yu , Zou De-Shu , Wang Guo-Hong , Gao Guo , Balk Ludwig J. , Heiderhoff Ralph , Lee Teck Hock , Wang Kang L.
Published: June 2003

The thermal property of tunnel-regenerated multiactive-region (TRMAR) light-emitting diodes (LEDs) is studied in detail in this letter. These devices have the advantages of high quantum efficiency and high output optical power. To obtain the same output optical power, it has been shown that the thermal performance for TRMAR LEDs is much better than that of conventional ones. The heat generated from the reverse-biased tunneling junction in TRMAR LEDs is small and can be neglected as compared with heat produced from the active region as illustrated in scanning thermal microscopy result. An experimental comparison shows that the improved input power dependence on the luminescence intensity proves that TRMAR LEDs have better thermal properties than those of conventional ones.
Laser threshold reduction in a spintronic device
Authors: Rudolph J. , Hägele D. , Gibbs H. M. , Khitrova G. , Oestreich M.
Published: June 2003

We demonstrate a reduction of the threshold of a semiconductor laser by optically pumping spin-polarized electrons in the gain medium. Polarized electrons couple selectively to one of two possible lasing light modes which effectively reduces the threshold by up to 50% compared to conventional pumping with unpolarized electrons. We theoretically show that our concept can be generalized to an electrically pumped laser.
Role of electric field on formation of silicon nanowires
Authors: Cheng Sau-Wan , Cheung Ho-Fai
Published: June 2003

An electric-field mechanism has been developed to explain the oxide-assisted growth of silicon nanowires. This mechanism assumes a strong electric field at the tip of silicon nanowires. Most of the SiO molecules would be attracted by this electric field and then land on the nanowire tip. Thus, the silicon nanowire growth is restricted at the tip only. The maximum electric field that could possibly exist has been estimated from the field emission data. The probability of SiO vapor landing on the nanowire tip is calculated for a wide range of conditions. The result shows that for a sufficiently strong electric field, all SiO vapor would land on the nanowire tip. This attractive force to the nanowire tip is even more pronounced if the SiO vapor first condenses to form a larger cluster. All these calculation results suggest that this electric-field mechanism is possible.
Structure function analysis of long-range correlations in plasma turbulence
Authors: Yu C. X. , Gilmore M. , Peebles W. A. , Rhodes T. L.
Published: June 2003

Long-range correlations (temporal and spatial) have been predicted in a number of different turbulence models, both analytical and numerical. These long-range correlations are thought to significantly affect cross-field turbulent transport in magnetically confined plasmas. The Hurst exponent, H—one of a number of methods to identify the existence of long-range correlations in experimental data—can be used to quantify self-similarity scalings and correlations in the mesoscale temporal range. The Hurst exponent can be calculated by several different algorithms, each of which has particular advantages and disadvantages. One method for calculating H is via structure functions (SFs). The SF method is a robust technique for determining H with several inherent advantages that has not yet been widely used in plasma turbulence research. In this article, the SF method and its advantages are discussed in detail, using both simulated and measured fluctuation data from the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. In addition, it is shown that SFs used in conjunction with rescaled range analysis (another method for calculating H) can be used to mitigate the effects of coherent modes in some cases.
Temperature gradient driven short wavelength modes in sheared slab plasmas
Authors: Gao Zhe , Sanuki H. , Itoh K. , Dong J. Q.
Published: June 2003

The temperature gradient (ηi and ηe) driven instability in the short wavelength regime, |kyρi|>1, is studied in a sheared slab. An analytical analysis is performed first to clarify the physics mechanisms for the modes in a shearless slab. The correlation between the growth rate and the real frequency of the modes is discussed in detail. The electron temperature gradient is found to have strong influences on the modes in short wavelength regions. Several series of the short wavelength modes are then identified with a kinetic integral equation code in a sheared slab. The radial widths of the modes are found to be comparable with the conventional ηi modes and not short, although the poloidal wavelengths are short. The lowest odd mode usually dominates in the weak magnetic shear and low β regime. However, the fundamental mode seems to be important in tokamak plasmas because the higher order modes are easily stabilized by finite β and/or by magnetic shear. The fundamental short wavelength mode cannot be stabilized by β when the magnetic gradient drift effect is taken into account. The modes are excited by both finite ηi and ηe, and may be stabilized by magnetic shear.
Radiated power measurement during the thermal quench phase of a density limit disruption
Authors: Hollmann E. M. , Gray D. S. , Whyte D. G. , Pigarov A. Yu. , Krasheninnikov S. I. , Boedo J. A. , Humphreys D. A.
Published: June 2003

Radiated power measurements taken during the thermal quench of a density limit disruption in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] demonstrate that ten times more power is radiated from the main chamber than is deposited into the divertor floor. This contrasts with the standard picture of the thermal quench of disruptions, where energy is lost along open field lines into the divertor. The observed radiation appears to result from a sudden increase in deuterium outgassing and carbon impurity release from the main chamber walls. The impurities and the associated cold front are observed to mix into the plasma core at a rate about 100 times larger than standard (H-mode) particle diffusion rates. Comparison with other types of disruptions in DIII-D suggests that plasma–main wall interactions may play an important role in the thermal quench of many disruptions.
Nonuniform current distribution in metal/diamond/metal vertical structures
Authors: Conte G. , Rossi M. C. , Salvatori S. , Tersigni F. , Ascarelli P. , Cappelli E.
Published: June 2003

The role of grain boundaries (GBs) in undoped polycrystalline diamond films has been investigated by dc and ac electrical measurements in a wide temperature range. Hopping transport along GB percolating paths and field-assisted thermal ionization of trapped charges are observed at low and high electric field strength, respectively. The temperature dependence of the ac conductivity, which reduces to a universal curve according to a random free-energy barrier model, suggests that, at low field strengths, current flow is mainly confined into GB domains in a wide temperature range. Only in the high-temperature and high-field ranges, do crystalline diamond grains become involved in current transport.
Voltage-induced phase transition in arrays of metallic nanodots: Computed transport and surface potential structure
Authors: Remacle F. , Levine R. D.
Published: June 2003

Computed dc transport in compressed arrays of metallic quantum dots exhibits a voltage-induced phase transition at low temperatures. The transition is seen in the temperature dependence of the conductance at different voltages: from a variable hopping dependence at low voltage to an ohmic, activated behavior at higher voltages. The computations also exhibit the transition as a break in the current versus voltage plots at low temperatures where, at higher voltages, the plot is linear. At higher temperatures, the conductance is ohmic. A many-electron basis is used. The same transition is seen in the surface potential contours.
Pyronin B as a donor for n-type doping of organic thin films
Authors: Werner A. G. , Li F. , Harada K. , Pfeiffer M. , Fritz T. , Leo K.
Published: June 2003

We present an approach to stable n-type doping of organic matrices using organic dopands. To circumvent stability limitations inherent in strong organic donors, we produce the donor from a stable precursor compound in situ. As an example, pyronin B chloride is studied as a dopant in a 1,4,5,8-naphthalene tetracarboxylic dianhydride matrix. Conductivities up to 2×10−4 S/cm are obtained, which is two orders of magnitude higher than obtained previously using bis(ethylenedithio)-tetrathiafulvalene as a dopant [A. Nollau, M. Pfeiffer, T. Fritz, and K. Leo, J. Appl. Phys. 87, 4340 (2000)]. Field-effect measurements are used to prove n-type conduction. Other matrices which can be doped are N,N′-dimethyl-perylene-3,4,9,10-tetracarboxylic diimide and fullerene C60, frequently used in organic solar cells. Visible light and Fourier-transform infrared spectroscopy confirm the donor properties of pyronin B.
Triangular gallium nitride nanorods
Authors: Bae Seung Yong , Seo Hee Won , Park Jeunghee , Yang Hyunik , Kim Hyunsuk , Kim Sangsig
Published: June 2003

Gallium nitride nanorods were synthesized by a chemical vapor deposition using the reaction of gallium/gallium nitride with ammonia. All nanorods have, exclusively, a triangle cross section with an average diameter of 50 nm. They consist of single-crystalline wurtzite structure crystal grown with the [010] direction. X-ray diffraction and Raman spectroscopy data suggest no shift of the lattice constants from those of the bulk. Temperature-dependent photoluminescence exhibits the I2 and free-to-bound emission peaks. The present triangular gallium nitride nanorods would be free from the stress, having the band-gap energy of the bulk.
Relaxed plasma-vacuum systems with pressure
Authors: Spies G. O.
Published: June 2003

Plasma pressure is introduced in the variational principle for relaxed toroidal plasma-vacuum systems by imposing the additional constraint of mass conservation.
Sheath criterion for a collisional sheath
Authors: Liu Jin-Yuan , Wang Zheng-Xiong , Wang Xiaogang
Published: June 2003

The sheath criterion in a collisional plasma sheath is investigated with a two-fluid model. It is shown that if neutral-ion collisionality in the sheath is taken into account, upper and lower limits for the sheath criterion exist.
Inversion domains and pinholes in GaN grown over Si(111)
Authors: Sánchez A. M. , Ruterana P. , Benamara M. , Strunk H. P.
Published: June 2003

High-resolution transmission electron microscopy is used to analyze the connection between pinholes and inversion domains at the AlN/GaN interface. From some pinholes on top of the AlN buffer layer, the subsequent growth of GaN was observed to lead to the formation of inversion domains. In addition, the pinhole area which was originally the termination of defects from the buffer layer is highly strained and its boundary facets in {112̄2} and {112̄3} atomic planes. Inside the GaN the inversion domain boundary quickly settles to the usual {101̄0} planes.
Surface pattern evolution during thermal Cl2 etching of GaAs(001)
Authors: Schmid J. H. , Mar R. , Tiedje T.
Published: June 2003

The evolution of one- and two-dimensional surface gratings during maskless thermal Cl2 etching of GaAs is investigated using atomic force microscopy. It is found that the limiting factor for pattern transfer is the anisotropy of the etch rate with respect to crystal orientation. A simple numerical model based on an interpolation of measured etch rates is presented that can be used to calculate the evolution of surface patterns.
Effects due to disorder on photonic crystal-based waveguides
Authors: Kwan Kai-Chong , Zhang Xiangdong , Zhang Zhao-Qing , Chan C. T.
Published: June 2003

Using the multiple-scattering method, we have studied the effects of various types of disorders on the performance of two-dimensional photonic crystal-based waveguides with cylindrical inclusions. The following three types of disorders are considered: (a) all cylinders are disordered; (b) only those cylinders in the boundary layer of the waveguide are disordered; and (c) all cylinders except those in the boundary layer of the waveguide are disordered. Our results show that, for both the cases of position and size randomness, the quality of the waveguide is insensitive to the type-(c) disorder, but very sensitive to type-(a) and type-(b) disorder. We thus conclude that the uniformity of the microstructures in the boundary layer of the waveguide is vital in ensuring the functionality of the waveguide.
Designs of terahertz waveguides for efficient parametric terahertz generation
Authors: Shi Wei , Ding Yujie J.
Published: June 2003

We show that for a slab waveguide, while pump and idler waves are weakly affected by the waveguide, their parametrically generated terahertz (THz) wave propagates in the form of multimodes. These waves may destructively interfere with one another in terms of their phases, resulting in inefficient THz parametric conversion. To avoid this problem, we designed a THz slab or rib waveguide that supports a quasi-single or single mode. Using these waveguides, the threshold powers for achieving oscillations can be significantly reduced due to the waveguide confinement of the THz waves. In addition, modal indices of refraction can be tailored for achieving phase matching without complicated periodic poling.
Direct measurement of polarization resolved transition dipole moment in InGaAs/GaAs quantum dots
Authors: Silverman K. L. , Mirin R. P. , Cundiff S. T. , Norman A. G.
Published: June 2003

The propagation of optical pulses resonant with the ground-to-excited state transition of InGaAs quantum dots is investigated. An analysis of low intensity excitation yields a dipole moment of 8.8×10−29 to 10.9×10−29 C m, depending on the quantum dot growth conditions. We observe polarization of the dipole moment exclusively in the plane perpendicular to the growth direction.
Third-harmonic generation of a continuous-wave Ti:Sapphire laser in external resonant cavities
Authors: Mes J. , van Duijn E. J. , Zinkstok R. , Witte S. , Hogervorst W.
Published: June 2003

An all-solid-state tunable continuous-wave (cw) laser operating near 272 nm with a bandwidth Γ≈3 MHz has been developed. The third harmonic of light from a single-cw Ti:Sapphire laser has been generated using two external enhancement cavities. An output power of 175 mW has been produced, corresponding to an overall conversion efficiency of 8%.
Ultrafast nonresonant third-order optical nonlinearity of a conjugated 3,3′-bipyridine derivative from 1150 to 1600 nm
Authors: Chen Qiying , Sargent Edward H. , Leclerc Nicolas , Attias André-Jean
Published: June 2003

A conjugated 3,3′-bipyridine derivative, an apolar structure of donor–acceptor–acceptor–donor (D–A–A–D) type was designed and characterized for third-order optical nonlinearity using Z-scan technique in the wavelength range of 1150–1600 nm. Z-scan spectra reveal a positive Kerr coefficient across this wavelength range with n2=(4.6±0.7)×10−6 cm2/GW at 1550 nm accompanied by negligible nonlinear absorption for wavelength around 1550 nm, indicating ideal figures of merit for applications in ultrafast all-optical switching.
Very-long wave ternary antimonide superlattice photodiode with 21 μm cutoff
Authors: Aifer E. H. , Jackson E. M. , Boishin G. , Whitman L. J. , Vurgaftman I. , Meyer J. R. , Culbertson J. C. , Bennett B. R.
Published: June 2003

We describe a ternary antimonide superlattice photodiode with a 21 μm cutoff wavelength. The active region consists of 150 periods of 10 monolayers (MLs) of In0.07Ga0.93Sb and 19 MLs of InAs with InSb-like interfacial bonds. The device has a detectivity of 3×109 cm√Hz/W, dynamic impedance-area product of 0.18 Ω cm2, and peak external quantum efficiency of 3% at 40 K. X-ray diffraction and cross-sectional scanning tunneling microscopy show the structure to have a high degree of order with abrupt interfaces. A simulation of the absorption spectrum effectively reproduces the observed spectrum.
Microwave intermodulation distortion of MgB2 thin films
Authors: Lamura G. , Purnell A. J. , Cohen L. F. , Andreone A. , Chiarella F. , Gennaro E. Di , Vaglio R. , Hao L. , Gallop J.
Published: June 2003

The two-tone intermodulation arising in MgB2 thin films deposited in situ by planar magnetron sputtering on sapphire substrates is studied. Samples are characterized using an open-ended dielectric puck resonator operating at 8.8 GHz. The experimental results show that the third-order products increase with the two-tone input power with a slope ranging between 1.5 and 2.3. The behavior can be understood introducing a mechanism of vortex penetration in grain boundaries as the most plausible source of nonlinearities in these films. This assumption is confirmed by the analysis of the field dependence of the surface resistance, that show a linear behavior at all temperatures under test.
Surface morphology control of InAs nanostructures grown on InGaAs/InP
Authors: Zhang Z. H. , Pickrell G. W. , Chang K. L. , Lin H. C. , Hsieh K. C. , Cheng K. Y.
Published: June 2003

The evolution of the surface morphology of InAs nanostructures grown on InGaAs/InP by molecular-beam epitaxy was studied through atomic force microscopy imaging. Randomly distributed quantum dots and quantum wires were reproducibly achieved by adjusting proper growth parameters such as InAs deposition thickness, growth temperature, arsenic overpressure, and InAs growth rate. It is observed that a thick InAs layer, high growth temperature, high arsenic overpressure, and high growth rate promote the formation of quantum dots. We propose that when InAs is deposited, the interaction of the total strain in the InAs layer and the surface strain distribution in the underlying matrix layer might be the determinant factor of the nanostructure morphology. Thick InAs, which increases the total strain of the InAs layer, is preferred to form quantum dots. Surface diffusion of In adatoms is another important factor affecting the surface morphology. A high growth temperature promotes homogeneous diffusion, while a high arsenic overpressure and growth rate reduces the surface diffusion of the In adatoms. These factors induce the formation of quantum dots.
Ballistic electron emission luminescence
Authors: Appelbaum Ian , Russell K. J. , Narayanamurti V. , Monsma D. J. , Marcus C. M. , Hanson M. P. , Gossard A. C. , Temkin H. , Perry C. H.
Published: June 2003

We describe the design, fabrication, and operation of a GaAs-based heterostructure device which emits band gap luminescence from solid-state tunnel-junction ballistic injection of electrons with sub-bandgap energy. We find that, due to energy conservation requirements, a collector bias exceeding a threshold determined by the Schottky barrier height and sample band gap energy must be applied for luminescence emission. The consequences of these results for a hybrid scanning-probe microscopy and spectroscopy combining both ballistic electron emission microscopy and scanning tunneling luminescence are emphasized.
Kohn–Sham equations for nanowires with direct current
Authors: Kosov D. S.
Published: June 2003

The paper describes the derivation of the Kohn–Sham equations for a nanowire with direct current. A value of the electron current enters the problem as an input via a subsidiary condition imposed by pointwise Lagrange multiplier. Using the constrained minimization of the Hohenberg–Kohn energy functional, we derive a set of self-consistent equations for current carrying orbitals of the molecular wire.
Infrared-active vibron bands associated with substitutional impurities in solid parahydrogen
Authors: Hinde Robert J.
Published: June 2003

We present a model for the line shapes of infrared-active Q1(0) vibron bands observed in solid parahydrogen doped with low concentrations of spherical substitutional impurities. The line shapes are highly sensitive to the H2 vibrational dependence of the dopant–H2 interaction. When this vibrational dependence is strong, the dopant can trap the infrared-active vibron in its first solvation shell; in this case, the trapped vibron manifests itself in the absorption spectrum as a narrow feature to the red of the pure solid’s vibron band.
Comment on “Kinetic theory of surface waves in plasma jets” [Phys. Plasmas 9, 701 (2002)]
Authors: Shoucri M.
Published: June 2003

It is the purpose of the present Comment to point out that the dispersion relation for surface waves propagating along a semi-bounded plasma jet, derived in a recent work [Phys. Plasmas 9, 701 (2002)], is incorrect.
Response to “Comment on ‘Kinetic theory of surface waves in plasma jets’ ” [Phys. Plasmas 10, 3035 (2003)]
Authors: Shokri B.
Published: June 2003

It will be shown that when plasma jet velocity is much smaller than light speed the dispersion equation obtained in the earlier work [Phys. Plasmas 9, 701 (2002)] is changed to the dispersion equation obtained earlier.
Kinetic effects on the ideal pressure-driven modes in an L=2 heliotron
Authors: Yamagishi Osamu , Nakamura Yuji , Kondo Katsumi , Nakajima Noriyoshi
Published: June 2003

The kinetic effects on the ideal ballooning and interchange/Mercier modes are studied in model equilibria for an L=2 heliotron, large helical device (LHD) [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)]. It is shown that the ion finite Larmor radius (FLR) effect stabilizes the modes with high toroidal mode number, n. On the other hand, the finite electron compressibility plays a double role, and stabilizes the low-n modes as the ideal magnetohydrodynamic (MHD) modes, while it destabilizes the high-n modes. It is discussed that the inclusion of the compressibility impacts the stability, and this effect is stronger in LHD than in a comparable tokamak, which is due to the larger magnitude of the local curvature. As a result of the competition between the FLR and the compressibility, it is shown in LHD that the low-n instabilities can become much weaker than that expected by the ideal MHD, while the high-n instabilities are prone to remain unstable near the plasma core region.
Table-top X-pinch for x-ray radiography
Authors: Beg F. N. , Krushelnick K. , Lichtsteiner P. , Meakins A. , Kennedy A. , Kajumba N. , Burt G. , Dangor A. E.
Published: June 2003

Measurements of the optical and x-ray emission from a small 40-kA, 30-ns (10%–90%) rise-time X-pinch plasma discharge are reported. This device is truly “table-top,” having a laboratory footprint of less than 1 m2. With tungsten wires, the total energy (hν>800 eV) is 60 mJ in a pulse of less than 10 ns. The size of the emitting region is less than 10 μm. The emission is reproducible with a jitter relative to the current pulse less than 5 ns. Initial experiments demonstrating the use of this X-pinch for applications in x-ray radiography are presented.
Strongly coupled critical current density values achieved in Y1Ba2Cu3O7−δ coated conductors with near-single-crystal texture
Authors: Foltyn S. R. , Arendt P. N. , Jia Q. X. , Wang H. , MacManus-Driscoll J. L. , Kreiskott S. , DePaula R. F. , Stan L. , Groves J. R. , Dowden P. C.
Published: June 2003

One of the most intensely researched subjects in the development of YBa2Cu3O7−δ (YBCO)-based coated conductors is the methodology for achieving ever-sharper film texture on flexible metal substrates. This is a critical issue due to the intrinsic weak-link behavior that results in depressed critical current density (Jc) in polycrystalline YBCO. Using ion-beam-assisted deposition of magnesium oxide, we have achieved YBCO texture on superalloy substrates approaching that on single-crystal oxide substrates. This advance has allowed us to fabricate coated conductor samples with Jcs that are as high as for films on oxide crystals; for example, >2 MA/cm2 (75 K, self-field) at a YBCO thickness of ∼1.5 μm.
Effect of surface treatment on leakage current of GaAs/AlGaAs laser microcavities
Authors: Kim Jun-Youn , Lee Jawoong , Kim Jungyeon , Kang Bongkoo , Kwon O’Dae
Published: June 2003

The effect of surface treatment on leakage current is investigated for two structures of the electropumped GaAs/AlGaAs laser microcavities, one with mesa and the other with hyperboloid-drum structures. Using surface sheet resistivity as a figure-of-merit parameter, the results of different surface treatment methods are compared. The best result is obtained when the sample has been polished in a H2SO4:H2O2:H2O=1:8:1000 solution for 5 s, and treated subsequently in a 6% excess sulfur-containing (NH4)2Sx solution at 60 °C for 8 mins. The treatment in (NH4)2Sx solution increases the surface sheet resistivity by a factor of ∼14. The cross-sectional shape of the cavity and the vertical profile of distributed Bragg reflector mirrors have little correlation with the leakage current.
Vertical channel all-organic thin-film transistors
Authors: Parashkov R. , Becker E. , Hartmann S. , Ginev G. , Schneider D. , Krautwald H. , Dobbertin T. , Metzdorf D. , Brunetti F. , Schildknecht C. , Kammoun A. , Brandes M. , Riedl T. , Johannes H.-H. , Kowalsky W.
Published: June 2003

Technologically simple and cost-effective processes are essential for the fabrication of organic electronic devices. In this letter, we present a concept for making vertical channel all-organic thin-film transistors on glass substrate. This concept avoids the need for patterning processes with high lateral resolution by defining the channel length through the thickness of an insulating layer. Our devices are based on commercially available poly(ethylene dioxythiophene)/poly(styrene sulfonate) dispersion for source, drain, and gate electrodes, photoresist as the insulating layer and photosensitized poly(vinyl alcohol) as the gate insulator. Pentacene was used as the organic semiconductor. Functional devices with channel length of 2.4 μm and width of 1 mm have been realized, and we report electrical characteristics of these devices.
Magnetic property enhancement of melt-spun Pr2Fe23B3 ribbons with dilute Ti substitution
Authors: Chang H. W. , Chiu C. H. , Chang W. C.
Published: June 2003

The phase evolution and magnetic properties of Pr2Fe23B3 and Pr2(Fe0.975Ti0.025)23B3 melt-spun ribbons have been investigated. For Pr2Fe23B3 spun at 25 m/s, the magnetically hard Pr2Fe14B phase coexists with α-Fe, Pr2Fe23B3, and Fe3B when annealed at temperatures above 700 °C, while merely two phases, namely, Pr2Fe14B and α-Fe, coexist if annealed above 800 °C. Because of the excessive annealing, these ribbons exhibit large grain sizes that lead to low Br and Hci values. A slight substitution of Ti for Fe in Pr2(Fe0.975Ti0.025)23B3 suppresses the formation of metastable Pr2Fe23B3 phase and leads to the presence of large amount of Pr2Fe14B and α-Fe phases of fine grain sizes in the matrix even at low annealing temperature (i.e., TH=650 °C). As a result, the magnetic properties improve remarkably from Br=6.7 kG, Hci=1.5 kOe and (BH)max=1.7 MG Oe of the ternary Pr2Fe23B3 to Br=9.9 kG, Hci=5.7 kOe and (BH)max=15.0 MGOe of the Pr2(Fe0.975Ti0.025)23B3.
Magneto-optic effects in spin-injection devices
Authors: Ruggiero S. T. , Williams A. , Tanner C. E. , Potashnik S. , Moreland J. , Rippard W. H.
Published: June 2003

The magneto-optic properties of semi-transparent ferromagnetic films are presented in the context of ferromagnet/GaAs spin-injection devices. We have measured the polarization-dependent photoresponse and magneto-optic properties of Co/n-GaAs, Co/p-GaAs and NiFe/n-GaAs Schottky diodes and NiFe/SiO2 and Co/SiO2 structures as a function of ferromagnetic film thickness, ranging from 8 to 100 nm. Our results show that magneto-optic effects intrinsic to the ferromagnetic films (2%–3%) are sufficient to account for the majority of the polarization-dependent photoresponse of the ferromagnet/GaAs systems studied. These effects are well described by a simple thin-film transmission model, which gives an upper limit of 0.4% for spin-transmission effects.
Evolution of the electron localization in a nonconventional alloy system GaAs1−xNx probed by high-magnetic-field photoluminescence
Authors: Wang Y. J. , Wei X. , Zhang Y. , Mascarenhas A. , Xin H. P. , Hong Y. G. , Tu C. W.
Published: June 2003

We demonstrate that a high magnetic field can be used effectively not only to probe the nature of the photoluminescence (PL) in a semiconductor, but also to reveal emission peaks that are unobservable at zero field since the magnetic field can alter energy relaxation processes and the statistical distribution of the photocarriers. Our systematic magneto-PL study of GaAs1−xNx (0.1%⩽x<2.5%) in fields up to 30 T indicates that the character of the low-temperature PL in this system changes drastically with varying nitrogen composition x and exhibits transitions with applying strong magnetic fields. For x<0.7%, the PL spectrum shows many discrete features whose energies remain nearly stationary up to the highest applied field. However, the magnetic confinement gives rise to a feature emerging on the higher energy side of the zero-field spectrum. This feature does show a diamagnetic shift, but it is much slower that that of the GaAs band-edge transition. For x>1%, the PL spectrum evolves into a broad band, and its diamagnetic shift resembles the band-edge transition in a conventional semiconductor, and the rate of shift is comparable to that of GaAs. From the diamagnetic shift of the band, the reduced effective masses for different composition of nitrogen have been derived for this system using the standard theory for the magneto-exciton in a three dimensional semiconductor.
Radiation of sound from a charged dust particle moving at high velocity
Authors: Guio P. , Pécseli H. L.
Published: June 2003

Radiation of sound waves from a rapidly moving charged dust particle in dusty plasmas is considered. For the limiting case, where the dusty plasma can be described by a Vlasov-type equation, analytical expressions are derived for the fully three-dimensional radiation pattern, including the effects of the Landau damping of sound waves. The results are illustrated by simple cases, which can be expressed by known functions. The radiation pattern obtained can be used to determine the sound speed, but also the temperature ratio of dust and singly charged particles, electrons, and ions. The analysis applies equally well for a fast charged dust particle propagating through a thermal plasma. The analytical results are supported by numerical simulations, including also a study of magnetized plasmas which are not as easily tractable for analytical investigations.