bluuebunny/crossref_metadata_2025 huggingface dataset:
On the robustness of the localized spatiotemporal structures in electron–positron–ion plasmas
Authors: Mahajan S. M. , Berezhiani V. I. , Miklaszewski R.
Published: July 2002

It is shown that, in an electron–positron plasma with a small fraction of ions, large-amplitude localized spatiotemporal structures (light bullets) can be readily generated and sustained. These light bullets are found to be exceptionally robust: they can emerge from a large variety of initial field distributions and are remarkably stable.
Parametric decays of a circularly polarized electromagnetic wave in an electron–positron plasma
Authors: Muñoz V. , Gomberoff L.
Published: July 2002

Parametric decays of a circularly polarized electromagnetic wave in an electron–positron unmagnetized plasma are studied. Like in the case of a plane polarized wave, there are two distinctive situations. One in which vs/c<c/vp (vs is the electroacoustic speed, c the speed of light, and vp is the phase velocity of the electromagnetic wave), and the other situation when vs/c⩾c/vp. In the first case, there is an ordinary decay instability and two modulational instabilities. One of the modulational instabilities is a resonant instability, and the other is a nonresonant, essentially electromagnetic instability in which the pump wave decays into two sideband waves. In the second situation (vs/c⩾c/vp), there are two modulational instabilities similar to the previous situation, but as the intensity of the pump wave increases, the resonant modulational instability disappears and only the nonresonant electromagnetic instability remains. The effect of Landau damping on the electroacoustic modes is also studied. This effect is simulated through a collisional term in the fluid equations.
Resonant stimulated Brillouin interaction of opposed laser beams in a drifting plasma
Authors: Cohen B. I. , Lasinski B. F. , Langdon A. B. , Williams E. A. , Wharton K. B. , Kirkwood R. K. , Estabrook K. G.
Published: July 2002

Particle simulations and solutions of coupled mode equations are used to analyze the energy transfer between two equal-frequency, opposed laser beams resonantly interacting with ion acoustic waves in a plasma drifting at the sound speed. The simulations and analysis illustrate the dependence of the energy transfer and the ion wave dynamics on laser intensities and detuning, and the time dependence of the phenomena. The simulation results are in qualitative agreement with experimental observations in the NOVA laser facility [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] at the Lawrence Livermore National Laboratory. This work is part of a continuing examination of possible resonant crossed-beam interactions in flowing plasmas and their potential effects on the fusion performance of current and future laser-fusion experiments with multiple crossing beams, e.g., proposed experiments in the National Ignition Facility [National Tech. Info. Service Document Nos. DE95017671-DE95017673 and DE95017676-DE95017700 (J. A. Paisner, E. M. Campbell, and W. J. Hogan, The Natl. Ignition Facility Project, UCRL-JC-117397 and UCRL-PROP-117093, May, 1994)].
On transport, in particular of impurities, in a stochastic magnetic field
Authors: Tokar’ M. Z.
Published: July 2002

The transport coefficients in a stochastic magnetic field are computed by taking into account analytical relationships for the distribution of plasma parameters along divergent lines of force. In the case of impurities, in contrast to what was believed before, stochastization primarily influences their convection but not their diffusive motion. At the plasma periphery, this convection is mainly caused by friction with the flow of background particles sustained by recycling and is directed toward the plasma boundary. This leads to an enhancement of the impurity exhaust, which provides a plausible explanation for the plasma decontamination observed in tokamaks with peripheral stochastic layers.
Evolution of the structure of the dense plasma near the cross point in exploding wire X pinches
Authors: Shelkovenko T. A. , Pikuz S. A. , Hammer D. A. , Dimant Y. S. , Mingaleev A. R.
Published: July 2002

The dynamics of the dense plasma near the cross point of an X pinch has been investigated using 1 ns x-ray backlighting images at different moments relative to the start of 100 ns [full width at half maximum (FWHM)] 200 kA current pulses. If the two metal wires are fine enough (e.g., 10 μm W or 17.5 μm Mo) to form a pinch at the cross point, accompanied by an x-ray burst, with the available current pulse, then the images show three stages of development: a radial explosion/expansion phase; an implosion during which a dense Z pinch of 200–300 μm length forms at the cross point together with plasma jets which move axially away from that point; and a breaking up of the Z pinch, coincident in time with one or two x-ray bursts, after which a 300 μm gap opens up. For W, the backlighter minimum sensitivity is 1017/cm2 areal density, and the dense Z pinch is estimated to have a volume density close to 1021/cm3. Shock waves appear to be expanding at about 50 μm/ns from the end points of the collapsing Z pinch, where the plasma was the most dense.
The coupling mode between Kelvin–Helmholtz and resistive instabilities in compressible plasmas
Authors: Shen C. , Liu Z. X.
Published: July 2002

Using a compressible magnetohydrodynamics (MHD) simulation method, the coupling mode between Kelvin–Helmholtz (K–H) and resistive instabilities has been investigated. It is found that, in compressible plasmas, the coupling mode can take place in a limited range of ambient shear flow velocity, beyond which the coupling mode is stabilized. The flow shear can significantly enhance the growth of the coupling mode. Only in a narrow range of plasma beta, could the coupling mode occur. It is deduced that this kind of coupling mode may appear at Earth’s magnetopause.
Solitons and shock fronts in the scattering of neutrinos by plasmas
Authors: Serbeto A.
Published: July 2002

The Weyl equation has been used to study the interaction of a neutrino flux with a plasma in a stellar medium. The derived basic nonlinear equations are solved numerically and analytically. Depending on the initial and boundary conditions, soliton, shock front, and stabilized periodic explosive solutions, for free dissipation plasma, are obtained.
Beam-plasma instability in inhomogeneous magnetic field and second order cyclotron resonance effects
Authors: Trakhtengerts V. Y. , Hobara Y. , Demekhov A. G. , Hayakawa M.
Published: July 2002

A new analytical approach to cyclotron instability of electron beams with sharp gradients in velocity space (step-like distribution function) is developed taking into account magnetic field inhomogeneity and nonstationary behavior of the electron beam velocity. Under these conditions, the conventional hydrodynamic instability of such beams is drastically modified and second order resonance effects become important. It is shown that the optimal conditions for the instability occur for nonstationary quasimonochromatic wavelets whose frequency changes in time. The theory developed permits one to estimate the wave amplification and spatio-temporal characteristics of these wavelets.
Nonlinear plasma maser driven by electron beam instability
Authors: Nambu M. , Saikia B. J. , Gyobu D. , Sakai J. I.
Published: July 2002

The detailed analysis of plasma maser interaction between high-frequency electromagnetic waves with right-handed polarization (R mode) and electrostatic Langmuir waves excited by an electron beam is presented. The analysis shows that generation of R-mode radiation near 2ωpe is possible only for magnetized plasmas; here ωpe is the electron plasma frequency. Simulations using a two-dimensional electromagnetic and relativistic particle code show that high-frequency electromagnetic waves with right-handed polarization (R mode) can be generated by a plasma maser mechanism from electrostatic Langmuir waves excited by an electron beam.
Global ship waves in poloidally rotating tokamak plasma
Authors: Pavlenko Vladimir P. , Revenchuk Sergei M.
Published: July 2002

Plasma rotation can lead to excitation of ship waves that have zero frequency in the laboratory frame. Ship waves of a drift type that have a global structure of a quasimode in a poloidally rotating toroidal plasma are considered for an axisymmetric, large aspect-ratio tokamak with concentric, circular magnetic surfaces. The analysis performed is based on rigorously derived eigenmode equations, coupled in poloidal mode numbers through the toroidal effects. Analytical dispersion relations for global drift ship waves are obtained and their simple solutions are found for some limiting cases. These global ship waves, resonantly excited due to a plasma rotation near the plasma edge, may give a simple explanation for the observed phenomenon of the fast response of the electron transport over a wide radial region in transitions from the low (L) to high (H) confinement regime (L–H transitions).
Role of Alfvén instabilities in energetic ion transport
Authors: Bernabei S. , Bell M. G. , Budny R. , Darrow D. , Fredrickson E. D. , Gorelenkov N. , Hosea J. C. , Majeski R. , Mazzucato E. , Nazikian R. , Phillips C. K. , Rogers J. H. , Schilling G. , White R. , Wilson J. R. , Zonca F. , Zweben S.
Published: July 2002

Experiments with plasma heating by waves at the ion cyclotron resonance of a minority species have shown that the heating efficiency degrades above a certain power threshold. It is found that this threshold is due to the destabilization of a branch of shear Alfvén waves, the Energetic Particle Modes, which causes a diffusive loss of fast ions. These modes not only play a fundamental role in the transport of the fast ions, but appear closely related to the formation of giant sawteeth.
Unusual radiation effects from atoms in gases and plasmas
Authors: Savchenko V. I. , Fisch N. J. , Starostin A. N. , Panteleev A. A.
Published: July 2002

New interesting effects arise, when three-level atoms interact with the plasma, laser field, or a heat bath. If the atoms inside the plasma are excited by the polarized laser pulse or an electron beam, the line wings of the radiation are completely polarized, while the line core is only partially polarized. The Einstein coefficients are generalized to the case, when the atomic decay channels are subject to quantum interference. Such atoms may develop nonzero quantum coherence between upper levels, if they are in contact with the heat bath in thermal equilibrium. This coherency changes the emissivity of the medium and hence can be experimentally observed. In the case when three-level atoms interact with the laser field, spontaneous emission can be suppressed over the whole frequency spectrum.
Compressional effects in nonneutral plasmas, a shallow water analogy and m=1 instability
Authors: Finn John M. , del-Castillo-Negrete Diego , Barnes Daniel C.
Published: July 2002

Diocotron instabilities form an important class of E×B shear flow instabilities which occur in nonneutral plasmas. The case of a single-species plasma confined in a cylindrical Penning trap, with an axisymmetric, hollow (nonmonotonic) density profile is studied. According to the standard linear theory, the m=1, kz=0 diocotron mode is always stable. On the other hand, experiments by Driscoll [Phys. Rev. Lett. 64, 645 (1990)] show a robust exponential growth of m=1 diocotron perturbations in hollow density profiles. The apparent contradiction between these experimental results and linear theory has been an outstanding problem in the theory of nonneutral plasmas. A new instability mechanism due to the radial variation of the equilibrium plasma length is proposed in this paper. This mechanism involves the compression of the plasma parallel to the magnetic field and implies the conservation of the line integrated density. The predicted growth rate, frequency, and mode structure are in reasonable agreement with the experiment. The effect of a linear perturbation of the plasma length is also shown to give instability with a comparable growth rate. The conservation of the line integrated density in the plasma is analogous to the conservation of the potential vorticity in the shallow water equations used in geophysical fluid dynamics. In particular, there is an analog of Rossby waves in nonneutral plasmas.
Plasma density cavitation due to inertial Alfvén wave heating
Authors: Shukla P. K. , Stenflo L.
Published: July 2002

It is found that the differential Joule heating of the electrons in the inertial Alfvén wave fields can be responsible for the fine scale density cavitations that are observed by the FREJA and FAST (Fast Auroral SnapshoT) spacecrafts in the Earth’s auroral plasma.
Electron beam acceleration by Compton scattering of extraordinary waves
Authors: Sugaya R.
Published: July 2002

Acceleration and heating of a high-energy or relativistic electron beam due to Compton scattering induced by nonlinear Landau damping of almost perpendicularly propagating extraordinary waves are investigated theoretically based on kinetic wave equations and transport equations derived from Vlasov–Maxwell equations. The numerical analysis of nonlinear wave–particle coupling coefficients in these equations has shown that the electron beam can be accelerated efficiently to the phase velocity of the beat wave near the speed of light by Compton scattering of two extraordinary waves with almost the same frequencies. The acceleration or deceleration of the electron beam occurs in accordance with whether the phase velocity of the beat wave is slightly larger or smaller than the velocity of the electron beam, respectively. For the frequencies of two waves lower than the upper-hybrid frequency (ω≲ωh), or for those exceeding the right-hand cutoff frequency (ω≳ωR), the acceleration and deceleration of the electron beam become significantly strong.
Fluid description of the longitudinal instability in high current coasting beams
Authors: Rumolo G. , Miano G. , Serpico C. , Hofmann I.
Published: July 2002

The longitudinal instability of an intense coasting beam in the ESR (Experimentier–Speicherring) [G. Rumolo et al., Nucl. Instrum. Methods A415, 411 (1998)] has been investigated far away from the stability boundary for several values of the machine impedance. Experiments have clearly shown the growth of the slow wave, generation of higher harmonics, asymmetric wave steepening, and saturation. A one-dimensional fluid model has been successfully employed to explain the growth of the slow wave as well as the nonlinear steepening and the harmonic generation. Taking into account the effects of the initial momentum spread of the beam through a modified space charge impedance, the model predicts with high accuracy the rise time and the frequency shift of the unstable wave. Subsequently, nonlinear convective effects are shown to give rise to wave steepening and harmonic generation. Predictions of the fluid model are compared to experimental data and with those obtained under a full kinetic model as well as from the particle-in-cell code PATRIC [I. Hofmann, Z. Naturforsch. 87a, 939 (1982)].
Stability and transport of parallel velocity shear driven mode with negative magnetic shear
Authors: Sen S. , Cairns R. A. , Storer R. G. , McCarthy D. R.
Published: July 2002

The linear and quasilinear behavior of the drift-like perturbation with a parallel velocity shear is studied in a sheared slab geometry. Full analytic studies show that when the magnetic shear has the same sign as the second derivative of the parallel velocity with respect to the radial coordinate, the linear mode may become unstable and turbulent momentum transport increases. On the other hand, when the magnetic shear has opposite sign to the second derivative of the parallel velocity, the linear mode is completely stabilized and turbulent momentum transport reduces.
Ion temperature anisotropy limitation in high beta plasmas
Authors: Scime Earl E. , Keiter Paul A. , Balkey Matthew M. , Boivin Robert F. , Kline John L. , Blackburn Melanie , Gary S. Peter
Published: July 2002

Measurements of parallel and perpendicular ion temperatures in the Large Experiment on Instabilities and Anisotropies (LEIA) space simulation chamber display an inverse correlation between the upper bound on the ion temperature anisotropy and the parallel ion beta (β=8πnkT/B2). Fluctuation measurements indicate the presence of low frequency, transverse, electromagnetic waves with wave numbers and frequencies that are consistent with predictions for Alfvén Ion Cyclotron instabilities. These observations are also consistent with in situ spacecraft measurements in the Earth’s magnetosheath and with a theoretical/computational model that predicts that such an upper bound on the ion temperature anisotropy is imposed by scattering from enhanced fluctuations due to growth of the Alfvén ion cyclotron instability.
Anderson-localized ballooning modes in general toroidal plasmas
Authors: Cuthbert P. , Dewar R. L.
Published: July 2002

Ballooning instabilities are investigated in three-dimensional magnetic toroidal plasma confinement systems with low global magnetic shear. The lack of any continuous symmetry in the plasma equilibrium can lead to these modes being localized along the field lines by a process similar to Anderson localization. This produces a multibranched local eigenvalue dependence, where each branch corresponds to a different unit cell of the extended covering space in which the eigenfunction peak resides. These phenomena are illustrated numerically for the three-field-period heliac H-1 [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)], and contrasted with an axisymmetric s-α tokamak model. The localization allows a perturbative expansion about zero shear, enabling the effects of shear to be investigated.
Local effects of upstream divertor components on divertor plasma parameters and impurity retention forces
Authors: Meo F. , Stansfield B. L. , Abel G. , Haddad E. , Marchand R. , Ratel G. , Elder J. D. , Bonnin X.
Published: July 2002

The toroidally-viewing spectroscopic imaging system on Tokamak de Varennes (TdeV) [F. Meo et al., Rev. Sci. Instrum. 68, (1997)] has given new insight into the physics of a closed divertor, where the particles interact with a vertically oriented inclined plate situated upstream from the target plate. The system has enabled two-dimensional (2D) measurements of the absolute emissivities of Hydrogen Balmer lines, as well as Carbon lines. Using the Balmer ratios, the poloidal profile of the electron density has been calculated using a Collisional-Radiative model [K. Sawada and T. Fujimoto, J. Appl. Phys. 78, 2913 (1994)]. Results show that the plasma parameters are locally affected by the divertor components particularly the inclined plate. This inclined plate constitutes an extra recycling surface which causes an increase in the atomic density and a decrease in the electron density in this region and affecting the plasma parameters at the target plate. Finally, the calculated poloidal plasma parameters are projected onto the field lines and are used in one-dimensional simplified (1D) particle and momentum balance equations. The simplified particle balance equation has shown that the extra neutrals from the inclined plate increase the parallel velocity locally. According to the simplified 1D fluid model, the analysis of the momentum balance equation has allowed the calculation of the poloidal profile of the ion temperature and hence the ion thermal force. The ∇Ti force on the impurity ions increases in the region of the inclined plate, which may seriously affect the impurity retention in the divertor.
Maintaining the closed magnetic-field-line topology of a field-reversed configuration with the addition of static transverse magnetic fields
Authors: Cohen S. A. , Milroy R. D.
Published: July 2002

The effects on magnetic-field-line structure of adding various static transverse magnetic fields to a Solov’ev-equilibrium field-reversed configuration are examined. It is shown that adding fields that are antisymmetric about the axial midplane maintains the closed field-line structure, while adding fields with planar or helical symmetry opens the field structure. Antisymmetric modes also introduce pronounced shear.
Collapses of density and vorticity by convection in simple two-dimensional flows
Authors: Schamel H. , Karimov A. R.
Published: July 2002

The initial value problem of an ideal, compressible medium is studied in two-dimensional geometry by the Lagrangian method. It is shown that the nonlinearity of a convection-driven flow can lead to singularities at finite time in density and vorticity. Conditions for the appearance of such a behavior are presented.
Mean spherical model for strongly coupled dusty plasmas
Authors: de Angelis U. , Forlani A. , Masiello G.
Published: July 2002

A solution of the mean spherical model is presented for particles interacting with a pair potential which is repulsive at short-range and attractive at long-range. The solution can be used for the description of the structure of strongly-coupled dusty plasmas, represented as one component systems of dust particles with an interaction potential determined by the plasma dynamics; a screened Coulomb repulsive interaction at short distances and a long-range attraction due to a screened “shadow effect” on the plasma fluxes to the dust particles. A comparison with recent experimental results shows a better agreement when the attractive potential is present.
High efficiency microwave discharge XeCl laser
Authors: Christensen C. P. , Waynant R. W. , Feldman B. J.
Published: July 2002

Efficiency of a microwave discharge XeCl laser exceeding 1% is demonstrated, an increase of more than one order of magnitude over previously reported values. Laser pulse durations of 320 ns are also reported.
Radiation resistance of InP solar cells under light illumination
Authors: Ando Koushi , Yamaguchi Masafumi
Published: July 2002

The demonstration of superior radiation resistance of InP solar cells during solar cell operation is reported. High-energy electron irradiation effects on InP n+-p junction solar cells under light illumination are compared with those under dark conditions. The InP solar cells under light illumination are found to have less degradation in solar cell properties. This is explained by minority-carrier injection enhanced annealing of radiation-induced defects, analyzed using deep level transient spectroscopy, in the p-InP layer.
Lattice thermal conductivity of small grain size PbSnTe and PbGeTe thermoelectric material
Authors: Rowe D. M. , Bhandari C. M.
Published: July 2002

The effect of phonon grain-boundary scattering on the lattice thermal conductivity λL of PbSnTe and PbGeTe has been investigated theoretically as a function of grain size and doping. It is concluded that for optimally doped thermoelectric material with a mean grain size of 0.5 μm, the reductions in λL of PbSnTe and PbGeTe are 11% and 14%, respectively, compared to corresponding ‘‘single crystal’’ or large grain-size material. This reduction in λL, which accompanies the use of very small grain-size material, would be reflected in a significant increase in the thermoelectric figure of merit and, hence, in the conversion efficiency of generators based on lead telluride technology.
Amorphous transition phase of NiSi2
Authors: Vanderwalker D. M.
Published: July 2002

A glass transition phase forms at the Ni-Si interface as a precursor to NiSi2. The crystalline to amorphous transformation occurs when a sufficient concentration of Ni atoms is present to raise the elastic energy of crystalline Si and lower the energy barrier to the reaction. NiSi2 crystals nucleate in the glass at the interface. The barrier height can depend on properties of the amorphous layer.
Polydiacetylene single crystal thin films
Authors: Berrehar J. , Lapersonne-Meyer C. , Schott M.
Published: July 2002

A method for obtaining single crystal thin films of polydiacetylenes using electron irradiation is described. Film thickness can be controlled by adjusting the incident electron energy. Typical thicknesses between 100 and 2000 Å for a sample surface of a few tenths of cm2 are easily obtained. Such films may find application in nonlinear optics.
GaAs p
n junction studied by scanning tunneling potentiometry
Authors: Muralt P.
Published: July 2002

The potential distribution across the cleaved end face of a forward-biased GaAs pn junction was simultaneously mapped with its surface topography. The space-charge region along the interface is clearly visible at zero bias or small forward bias voltages.
Nonlinear optical processes in a polydiacetylene measured with femtosecond duration laser pulses
Authors: Carter G. M. , Hryniewicz J. V. , Thakur M. K. , Chen Y. J. , Meyler S. E.
Published: July 2002

The temporal response of the third-order nonlinear optical susceptibility in poly bis( p-toluene sulfonate) of 2,4-hexadiyne-1,6 diol was determined by time-resolved degenerate four-wave mixing using tunable dye laser pulses with 300 fs duration. In the material’s absorption region these experiments were combined with saturated absorption techniques yielding a measurement for the excited state lifetime of 1.8 ps. Off resonance the nonlinear response was determined to be optical pulse width limited.
68.4-T-long pulse magnet: Test of high strength microcomposite Cu/Nb conductor
Authors: Foner S.
Published: July 2002

A multilayer wire-wound pulsed field magnet was fabricated using a new metal-matrix microcomposite Cu/Nb conductor. The 0.5-in.-i.d. magnet used square cross-section Cu/Nb wire, precompressed in a hardened steel container and cooled to 77 K, to generate 68.4±1 T for a 5.6-ms half-period. The maximum field was generated using a 100-kJ, 4-kV capacitor bank of the Francis Bitter National Magnet Laboratory pulsed field facility. The Cu/Nb composite had an ultimate tensile strength of 180 ksi at 77 K and 140 ksi at 293 K. These new microcomposites have exceptional strength, electrical, and thermomechanical properties for very high stress applications.
Scanning tunneling microscopy and potentiometry on a semiconductor heterojunction
Authors: Muralt P. , Meier H. , Pohl D. W. , Salemink H. W. M.
Published: July 2002

The potential distribution across the cleaved end face of a forward-biased GaAs double heterojunction laser diode was mapped using scanning tunneling potentiometry. Space-charge regions next to the heterojunction interface as well as the electron-hole recombination region within the active layer are outlined with nanometer resolution. The carrier injection zone in the active layer is observed as a function of junction voltage.
Observation of Si(111) surface topography changes during Si molecular beam epitaxial growth using microprobe reflection high-energy electron diffraction
Authors: Ichikawa M. , Doi T.
Published: July 2002

Si(111) surface topography changes during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (350 °C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500 °C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer-by-layer two-dimensional nucleation growth.
Growth and characterization of a delta-function doping layer in Si
Authors: Zeindl H. P. , Wegehaupt T. , Eisele I. , Oppolzer H. , Reisinger H. , Tempel G. , Koch F.
Published: July 2002

We present a procedure whereby a sheet of donor atoms is incorporated in (100) Si during molecular beam epitaxial growth. Analysis by secondary ion mass spectroscopy and transmission electron microscopy shows that the width of such δ-function doping layers is only a few lattice planes. Tunneling spectroscopy and transport measurements give evidence for quantum confinement of the electronic charge in the layer and thus confirm the narrow width.
Effect of i
n s
i
t
u and e
x s
i
t
u annealing on dislocations in GaAs on Si substrates
Authors: Choi C. , Otsuka N. , Munns G. , Houdre R. , Morkoç H. , Zhang S. L. , Levi D. , Klein M. V.
Published: July 2002

Gallium arsenide layers grown by molecular beam epitaxy on (100) Si substrates were subjected to annealing under As overpressure at 650, 750, and 850 °C for 1/2 h. A substantial reduction in the dislocation density near the interface and in the bulk of the epitaxial layers was observed for the 850 °C anneal. In situ annealing at 700 °C for 1/2 h after 1/2 h of growth followed by a deposition of InGaAs/GaAs strained-layer superlattices and bulk layers also resulted in reduced dislocation densities.
Low-temperature molecular beam epitaxy of gallium arsenide
Authors: Missous M. , Singer K. E.
Published: July 2002

Device quality GaAs was grown at substrate temperatures as low as 430 °C, by decreasing the growth rate to 0.2 μm/h and by using dimeric arsenic. Precise doping control down to 1×1016 cm−3 has been easily achieved and Hall mobilities of up to 66% of the best molecular beam epitaxy values have been measured. By contrast, the use of As4 under the same growth conditions led to films with poor electrical and optical properties. These results add further evidence to the superiority of As2 in reducing the concentration of compensation deep levels, and provide the first conclusive evidence of its significant role in the low-temperature growth of high quality GaAs.
Measurement of heterojunction band offsets by admittance spectroscopy: InP/Ga0.47In0.53As
Authors: Lang D. V. , Panish M. B. , Capasso F. , Allam J. , Hamm R. A. , Sergent A. M. , Tsang W. T.
Published: July 2002

We discuss the use of admittance spectroscopy to measure the band offsets of semiconductor heterojunctions. By using this method to analyze the dynamic response of p-n junctions containing lattice-matched InP/Ga0.47In0.53As superlattices we can independently determine both the conduction- and valence-band offsets for this materials system. We find that the sum of these offsets equals the known band-gap difference between InP and Ga0.47In0.53As and that the ratio of the conduction-band offset to the valence-band offset is 42:58.
Effects of passivating ionic films on the photoluminescence properties of GaAs
Authors: Skromme B. J. , Sandroff C. J. , Yablonovitch E. , Gmitter T.
Published: July 2002

The passivating effects of spin-coated films of Na2S⋅9H2O on GaAs surfaces have been studied using room-temperature photoluminescence (PL) and low-temperature PL spectroscopy. After passivation, the 300 K PL efficiency is increased on both n- and p-type material; improvements of up to 2800× are observed. The surface field and surface recombination-related notch features in the free and bound exciton emission spectra at low temperature are eliminated, implying that the residual band bending under illumination is less than 0.15 V.
Epitaxial yttrium silicide on (111) silicon by vacuum annealing
Authors: Gurvitch M. , Levi A. F. J. , Tung R. T. , Nakahara S.
Published: July 2002

Epitaxial YSi2−x films have been fabricated. The smooth 430-Å-thick silicide films on Si (111) substrates were characterized by a Rutherford backscattering minimum channeling yield χmin =8%, establishing YSi2−x as one of the best known epitaxial silicides. Results of electrical measurements are also presented.
Interstitial defect reactions in silicon
Authors: Asom M. T. , Benton J. L. , Sauer R. , Kimerling L. C.
Published: July 2002

Deep level transient spectroscopy has been employed in a study of impurity-interstitial defect reactions in silicon following room-temperature electron irradiation. Three defects have been isolated and identified from their reactions and electrical properties as Cs-Ci, Ci-Oi, and Ps-Ci. The Cs-Ci, ME[(0.10), (0.17)] and Ps-Ci, ME[(0.21), (0.23), (0.27), (0.30)] defects exhibit metastable structural transformations. Our results reveal the multistructural nature and chemical reactivity of the silicon self-interstitial.
Conductivity oscillation due to quantum interference in a proposed wash-board transistor
Authors: Tokura Yasuhiro , Tsubaki Kotaro
Published: July 2002

Conductivity oscillation due to quantum interference is theoretically studied in a proposed wash-board transistor. The transistor has a two-dimensional electron gas modulated by a weak periodic potential, and its gate voltage can change the Fermi energy. The periodic potential is approximated by a square well considering the screening effect. Conductivity oscillation is found to originate in the electron velocity modulation caused by a quantum interference effect, not in the density of states modulation. This result is confirmed by comparison with experimental results, which give some clues to the real potential formed on a two-dimensional interface.
Linear polarization effects in anisotropic photoemission from GaAs/AlAs short-period superlattices
Authors: Fujiwara K. , Tsukada N. , Nakayama T. , Nishino T.
Published: July 2002

By investigating the linear polarization dependence of intrinsic emissions from undoped GaAs/AlAs short-period superlattices at room temperature, it is shown that a dominating peak observed in the normal emission spectra is due to the free-carrier recombination associated with the lowest electron to mixed-hole subbands with an unexpectedly large light-hole character instead of with the lowest electron to pure heavy-hole subbands expected in an ideal superlattice. Origins of the extensive valence-band mixing in the synthesized superlattices are discussed in relation to microscopic fluctuations in the layer thicknesses observed in the transmission electron microscope lattice image.
Effectiveness of strained-layer superlattices in reducing defects in GaAs epilayers grown on silicon substrates
Authors: El-Masry N. , Tarn J. C. L. , Humphreys T. P. , Hamaguchi N. , Karam N. H. , Bedair S. M.
Published: July 2002

In GaAs-GaAsP strained-layer superlattices grown lattice matched to GaAs are effective buffer layers in reducing dislocations in epitaxial GaAs films grown on Si substrates. The strained-layer superlattice structure permits high values of strain to be employed without the strained-layer superlattice generating dislocations of its own. We find that the strained-layer superlattice buffer is extremely effective in blocking threading dislocations of low density and is less effective when the dislocation is high. It appears that for a given strained-layer superlattice there is a finite capacity for blocking dislocations. Transmission electron microscopy has been used to investigate the role of the superlattice buffer layer.
Detection of Cl and chlorine-containing negative ions in rf plasmas by two-photon laser-induced fluorescence
Authors: Selwyn Gary S. , Baston L. D. , Sawin H. H.
Published: July 2002

Chlorine atoms have been detected in rf etching plasmas of CClF3 and CCl2F2 with three-dimensional spatial resolution using a two-photon laser-induced fluorescence technique. The spin-forbidden 4p(4S0)–3p(2P0) transition is pumped by absorption of two 233.3-nm laser photons. Decay of the excited state results in 725–775 nm emission. Spatially resolved plasma concentration measurements under certain etching conditions indicate an anomalously large signal spike at the plasma/sheath boundary, an effect attributed to an aggregation of chlorine-containing negative ions which are also detected by this technique. The negative ions are detected by laser-induced photodetachment followed by two-photon excitation of atomic Cl.
Microstructure and the light-induced metastability in hydrogenated amorphous silicon
Authors: Bhattacharya Enakshi , Mahan A. H.
Published: July 2002

Using a parameter obtained from infrared measurements of the silicon-hydrogen stretch mode, the amout of light-induced degradation in hydrogenated amorphous silicon (a-Si:H) has been explored as a function of the amount of microstructure present in our samples. We find that samples with more microstructure, and also more bonded hydrogen, show an increased light-induced effect. At the same time, the volume density of states in the initial (annealed) state remains virtually unchanged. We discuss how the present results relate to existing models proposed to describe the light-induced effect.
Mechanisms of doping-enhanced superlattice disordering and of gallium self-diffusion in GaAs
Authors: Tan T. Y. , Gösele U.
Published: July 2002

Recently available Ga-Al interdiffusion results in GaAs/AlAs superlattices allow us to conclude that Ga self-diffusion in GaAs is carried by triply negatively charged Ga vacancies under intrinsic and n-doping conditions. The mechanism of the Si-enhanced superlattice disordering is the Fermi-level effect which increases the concentrations of the charged point defect species. For the effect of the p dopants Be and Zn, the Fermi-level effect has to be considered together with dopant diffusion induced Ga self-interstitial supersaturation or undersaturation. Self-diffusion of Ga in GaAs under heavy p-doping conditions is governed by positively charged Ga self-interstitials.
Surface-superlattice effects in a grating-gate GaAs/GaAlAs modulation doped field-effect transistor
Authors: Ismail K. , Chu W. , Antoniadis D. A. , Smith Henry I.
Published: July 2002

We report transport phenomena exhibited by a two-dimensional electron gas at the interface of a modulation doped GaAs/GaAlAs heterostructure in the presence of a field-effect-controlled periodic potential modulation. By means of x-ray lithography and lift-off, a 0.2-μm-period Schottky barrier grating gate was fabricated in lieu of the common continuous gate in a field-effect transistor configuration. Conductance measurements at 4.2 K provide evidence of a superlattice effect.
Magnetic properties of ion-beam-sputtered Fe/Co and Fe/CoFe multilayer films
Authors: Senda Masakatsu , Nagai Yasuhiro
Published: July 2002

This letter reports on the magnetic properties of Fe/Co and Fe/CoFe multilayer films. A zero magnetostriction is realized by changing the thickness of each layer. In addition, Fe/Co films with high magnetization of more than 2 T and good soft-magnetic properties of 1.5 Oe coercivity are confirmed. By 300 °C annealing, the hard-axis coercivity in Fe/Co films decreases to 0.8 Oe. Furthermore, a suitable magnetic domain structure for a thin-film head is confirmed, and the structure is found to be determined by the total magnetostriction.
Direct evidence of the D
X center link to the L-conduction-band minimum in GaAlAs
Authors: Calleja E. , Gomez A. , Muñoz E.
Published: July 2002

Hydrostatic pressure techniques together with deep level transient spectroscopy (DLTS) measurements have shown that the Si-DX center in GaAlAs is linked to the L-conduction-band minimum. When hydrostatic pressure is applied to a 74% Al content sample, an exponential reduction of the DLTS signal is observed. This exponential dependence with pressure arises from the reduction of the DX filling factor (electron occupancy) due to the increasing X-L energy difference with pressure. Our results, together with previous data, also show that the capture barrier height originating from the lattice relaxation is an intrinsic parameter of both the material and the donor species, that does not depend on Al content or conduction-band structure.
High-resolution magnetic imaging of domains in TbFe by force microscopy
Authors: Martin Y. , Rugar D. , Wickramasinghe H. K.
Published: July 2002

High-resolution images of domains written in a magnetic thin film have been obtained for the first time using force microscopy. The sample consisted of 500-Å-thick Tb19Fe81 with magnetization of 109 emu/cm3. Micron-sized magnetic domains were thermomagnetically written in the sample using a focused laser beam. Domain images were obtained by observing the magnetic interaction of the sample with a small vibrating magnetized iron tip. Typical observed force gradients were in the range 0.8×10−4–6×10−4 N/m and the forces were in the range 10−12–10−11 N. The spatial resolution of the technique was on the order of 1000 Å. This was sufficient resolution to see irregularities in those laser written marks which were recorded using low bias field.
Measurement of the pressure dependence of the direct band gap of In0.53Ga0.47As using stimulated emission
Authors: People R. , Jayaraman A. , Wecht K. W. , Sivco D. L. , Cho A. Y.
Published: July 2002

Stimulated emission, obtainable at high optical pumping levels, has been used to follow the pressure dependence of the Γ-band gap of molecular beam epitaxial In0.53Ga0.47As on (001)InP. Hydrostatic pressure was generated using a diamond anvil cell, and all measurements were made at room temperature. The gap varies sublinearly with pressure for P≳10 kbar, having an initial slope of 12.44 meV/kbar. The deviation from a linear behavior is largely due to nonlinearities in the equation of state at higher pressures. The deformation potential (Ξd+ (1)/(3) Ξu−a) =−(7.79±0.4)eV, for the Γ-band gap.
Low-temperature amorphous-to-crystalline transformation of CoSi2 films
Authors: Cros A. , Tu K. N. , Smith D. A. , Weiss B. Z.
Published: July 2002

Amorphous-to-crystalline transformation of e-gun co-deposited CoSi2 films occurs from 150 to 200 °C, as observed by in situ resistivity measurement and transmission electron microscopy. Resistivity changes abruptly from 1500 to 110 μΩ cm as the amorphous film transforms into circular crystallites that are circular before impingement occurs. A room-temperature resistivity of 30 μΩ cm was obtained by annealing the film to 500 °C.
High-finesse (Al,Ga)As interference filters grown by molecular beam epitaxy
Authors: Jewell J. L. , Lee Y. H. , McCall S. L. , Harbison J. P. , Florez L. T.
Published: July 2002

We have measured finesse values of at least 160 in (Al,Ga)As Fabry–Perot interference filters grown by molecular beam epitaxy. Losses are low and the finesses are close to predicted values, suggesting that with additional mirror layers much higher finesse should be achievable. This has important implications in the development of resonator-based low-energy photonic logic devices and lasers for information processing and communication. Formulas for calculating the finesse and other parameters such as the effective optical thickness of the cavity in such structures are also given.
Measurement of stable species present during filament-assisted diamond growth
Authors: Harris Stephen J. , Weiner Anita M. , Perry Thomas A.
Published: July 2002

We have measured mole fractions of two of the major stable species at the surface of a silicon substrate during filament-assisted diamond growth as a function of the filament-to-substrate distance. Input gases were methane and hydrogen. A quartz probe withdrew gases at the growing surface, and the gases were sampled with an on-line mass spectrometer. Close to the filament the methane is largely consumed, with most of the remaining gas phase carbon in the form of acetylene. Mass spectral results are compared to compositions calculated with a detailed chemical kinetics model. Our initial analysis suggests that diamond growth comes mainly from reaction of acetylene, ethylene, methane, or methyl radical.
Theory of magnetic imaging by force microscopy
Authors: Sáenz J. J. , García N. , Slonczewski J. C.
Published: July 2002

A new method to obtain information about local surface magnetic properties has been proposed recently. This new technique is based on the idea of measuring magnetic forces with a scanning tunneling force microscope. In this work we present a theoretical analysis of the relevant forces involved in magnetic force microscopy. Recent experiments with high-resolution images of laser-written domains in a magnetic thin film are discussed.
Improved recognition of native-like protein structures using a family of designed sequences
Authors: Koehl Patrice , Levitt Michael
Published: July 2002

The goal of the inverse protein folding problem is to identify amino acid sequences that stabilize a given target protein conformation. Methods that attempt to solve this problem have proven useful for protein sequence design. Here we show that the same methods can provide valuable information for protein fold recognition and for
ab initio
protein structure prediction. We present a measure of the compatibility of a test sequence with a target model structure, based on computational protein design. The model structure is used as input to design a family of low free energy sequences, and these sequences are compared with the test sequence by using a metric in sequence space based on nearest-neighbor connectivity. We find that this measure is able to recognize the native fold of a myoglobin sequence among different globin folds. It is also powerful enough to recognize near-native protein structures among nonnative models.
Roles for genomic imprinting and the zygotic genome in placental development
Authors: Georgiades Pantelis , Watkins Marie , Burton Graham J. , Ferguson-Smith Anne C.
Published: July 2002

The placenta contains several types of feto-maternal interfaces where zygote-derived cells interact with maternal cells or maternal blood for the promotion of fetal growth and viability. The genetic factors regulating the interactions between different cell types within feto-maternal interfaces and the relative contributions of the maternal and zygotic genomes are poorly understood. Genomic imprinting, the epigenetic process responsible for parental origin-dependent functional differences between homologous chromosomes, has been proposed to contribute to these events. Previous studies showed that mouse conceptuses with an absence of imprinted differences between the two copies of chromosome 12 (upon paternal inheritance of both copies) die late in gestation and have a variety of defects, including placentomegaly. Here we examined the role of chromosome 12 imprinting in these placentae in more detail. We show that the spatial interactions between different cell types within feto-maternal interfaces are defective and identify abnormal behaviors in both zygote-derived and maternal cells that are attributed to the genome of the zygote but not the mother. These include compromised invasion of the maternal decidualized endometrium and the central maternal artery situated within it by zygote-derived trophoblast, abnormalities in the wall of the central maternal artery, and defects within the zygote-derived cellular layer of the labyrinth, which is in direct contact with maternal blood. These findings demonstrate multiple roles for chromosome 12 imprinting in the placenta that have not previously been associated with imprinting effects. They provide insights into the function of imprinting in placental development and have evolutionary and clinical implications.
Genes on chromosomes 1 and 13 have significant effects on
Ascaris
infection
Authors: Williams-Blangero Sarah , VandeBerg John L. , Subedi Janardan , Aivaliotis Mary Jo , Rai Dev Raj , Upadhayay Ram Prasad , Jha Bharat , Blangero John
Published: July 2002

Nematode parasites show a characteristic aggregated distribution among hosts. This observation has important implications for pathogenesis, immunology, and control of these infections, but the relative roles of environment and genetics in determining these patterns have remained uncertain. This paper presents the results of the first genome scan for susceptibility to infection with roundworm (
Ascaris lumbricoides
). Data on 375 genetic markers were generated for each of 444 members of a genetically isolated Nepalese population, the Jirels.
Ascaris
worm burden as assessed by egg counts was measured in these same individuals by using the Kato Katz thick smear method. The extensive genealogical data available for the population allowed assignment of all 444 individuals to a single pedigree that contained 6,209 pairs of relatives that were informative for genetic analysis. A variance components linkage analysis resulted in the unequivocal localization of two genes (one on chromosome 1 and another on chromosome 13) with clear, significant effects on susceptibility to
Ascaris
infection. This is the first evidence that individual quantitative trait loci influence variation in
Ascaris
burden in humans.