bluuebunny/crossref_metadata_2025 huggingface dataset:
Analytical model for the disruption of a metal jet penetrating detonating high explosive
Authors: Chapyak E. J. , Kerrisk J. F. , Godwin R. P.
Published: July 2002

We investigate the interaction of a constant-velocity, long-rod penetrator with detonating high explosive. An analytical flow-field model extending the Birkhoff model of inert target penetration is developed and compared with numerical simulations. We propose a rod-disruption model, discuss the effect of enhanced “anomalous” rod erosion on penetration history, and estimate the maximum coherent penetration depth. The model is extended to idealized stretching jet penetrators and its predictions are compared with jet-high-explosive (HE) interaction experiments. We predict a strong dependence of the high-explosive penetration on penetrator diameter and a very high “virtual” HE density in agreement with experimental observations.
Internal transport barriers on Alcator C-Mod
Authors: Fiore C. L. , Rice J. E. , Bonoli P. T. , Boivin R. L. , Goetz J. A. , Hubbard A. E. , Hutchinson I. H. , Granetz R. S. , Greenwald M. J. , Marmar E. S. , Mossessian D. , Porkolab M. , Taylor G. , Snipes J. , Wolfe S. M. , Wukitch S. J.
Published: July 2002

The formation of internal transport barriers (ITBs) has been observed in the core region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under a variety of conditions. The improvement in core confinement following pellet injection (pellet enhanced performance or PEP mode) has been well documented on Alcator C-Mod in the past. Recently three new ITB phenomena have been observed which require no externally applied particle or momentum input. Short lived ITBs form spontaneously following the high confinement to low confinement mode transition and are characterized by a large increase in the global neutron production (enhanced neutron or EN modes). Experiments with ion cyclotron range of frequencies power injection to the plasma off-axis on the high field side results in the central density rising abruptly and becoming peaked. The ITB formed at this time lasts for ten energy confinement times. The central toroidal rotation velocity decreases and changes sign as the density rises. Similar spontaneous ITBs have been observed in ohmically heated H-mode plasmas. All of these ITB events have strongly peaked density profiles with a minimum in the density scale length occurring near r/a=0.5 and have improved confinement parameters in the core region of the plasma.
Stability criterion for edge-localized, high-n external modes in tokamaks
Authors: Ramos J. J. , Verástegui R. , Hastie R. J.
Published: July 2002

The ideal-magnetohydrodynamics (MHD) energy principle is used to derive a necessary stability criterion for high-toroidal-number (n) external modes in axisymmetric equilibria. The corresponding trial functions are expressed in the ballooning representation, but have a finite amplitude at the plasma boundary and can apply to equilibria where the conventional, high-n internal ballooning criterion predicts stability. These trial functions are constructed by solving the standard local ballooning equation at the plasma boundary flux surface with the radial wave number parameter as a complex eigenvalue, such that the radial envelope of the mode is an exponential decaying into the plasma. The resulting stability criterion includes the surface and vacuum contributions to the MHD potential energy associated with the mode finite edge amplitude, and provides a framework for analyzing free-boundary ballooning and peeling modes.
Phase modulation at 125 kHz in a Michelson interferometer using an inexpensive piezoelectric stack driven at resonance
Authors: Hoeling Barbara M. , Fernandez Andrew D. , Haskell Richard C. , Petersen Daniel C.
Published: July 2002

Fast phase modulation has been achieved in a Michelson interferometer by attaching a lightweight reference mirror to a piezoelectric stack and driving the stack at a resonance frequency of about 125 kHz. The electrical behavior of the piezo stack and the mechanical properties of the piezo-mirror arrangement are described. A displacement amplitude at resonance of about 350 nm was achieved using a standard function generator. Phase drift in the interferometer and piezo wobble were readily circumvented. This approach to phase modulation is less expensive by a factor of roughly 50 than one based on an electro-optic effect.
Three-dimensional subsurface microprocessing of collagen by ultrashort laser pulses
Authors: Smith Nicholas I. , Fujita Katsumasa , Nakamura Osamu , Kawata Satoshi
Published: July 2002

Submicron sized damage spots were created by multiphoton absorption of ultrashort 775 nm laser pulses in collagen gel stained with fluorescent dye. Submicron nodules were formed in the collagen both near the surface and at a depth of up to 30 μm. We have shown that large numbers of pulses at low pulse energy provide well-controlled sample damage restricted to a small focal volume. The damage threshold was dependent on dye concentration but not significantly dependent on the depth. This will be useful for experiments that study laser-induced damage in living biological samples on a cellular scale and deep within the specimen.
Effects of ultraviolet and thermal pretreatment on the formation of self-written χ(2) gratings and optical damage
Authors: Blazkiewicz Paul , Town Graham
Published: July 2002

In this article we report the effects of thermal and ultraviolet light pretreatments of a variety of optical fibers with different core dopants. Except for fibers codoped with phosphorus, a negative correlation was found between the onset of optical damage to the fibers and the formation of seeded second-harmonic generation (SHG). From our results and from those of other workers, we conclude that there is a link between susceptibility to optical damage and the formation of seeded SHG in germanosilicate optical fibers.
Kelvin probe force microscopy on InAs thin films grown on GaAs giant step structures formed on (110) GaAs vicinal substrates
Authors: Ono S. , Takeuchi M. , Takahashi T.
Published: July 2002

Surface potential measurements on InAs thin films grown on GaAs giant steps were performed by Kelvin probe force microscopy. We found that the removal of the water-related layer from both surfaces on a sample and a tip was very effective to improve the reliability of the surface potential measurements. The measured potential distribution corresponds to the surface corrugation of the InAs thin films. In addition, the InAs layer thickness dependence of the surface InAs Fermi levels is investigated, indicating that the surface Fermi level shifts toward the vacuum level as the increase of the InAs layer thickness.
Experimental study of energy coupling and plasma breakdown in a pulsed high frequency resonant cavity
Authors: Lacoste A. , Dias F. M. , Boisse-Laporte C. , Leprince P.
Published: July 2002

An experimental study of the electromagnetic behavior of a pulsed high frequency resonant cavity, operating in its fundamental TE111 mode, is presented. Typical pulse characteristics are a pulse width of 10 μs, microwave frequency of 1 GHz, and microwave power of a few kW. The various parameters (characteristic time τ, coupling factor β, incident power Pi) that determine the temporal variation of the electrical characteristics (power entering in the cavity, electric field) during the pulse are defined. In particular, the influence of the coupling factor on the transition stage to a steady state is discussed for the cavity without plasma. The temporal behavior of the cavity in the presence of a gas (argon and hydrogen) is also studied. For pressures ranging from 0.5 to 100 Torr, the discharge breakdown parameters (time τb, electric field Eb) in hydrogen are deduced from the experiments (tb∼0.1–1 μs, Eb∼300–3000 V/cm). The conditions to reach a quasisteady state plasma are derived from the qualitative analysis of the cavity behavior.
Multistage, monolithic ceramic microdischarge device having an active length of ∼0.27 mm
Authors: Vojak B. A. , Park S.-J. , Wagner C. J. , Eden J. G. , Koripella R. , Burdon J. , Zenhausern F. , Wilcox D. L.
Published: July 2002

A three-stage, multilayer ceramic microdischarge device, having an active length of ∼267 μm and a cylindrical discharge channel 140–150 μm in diameter, has been developed and operated continuously in Ne gas. Stable glow discharges are produced for pressures above 1 atm, operating voltages as low as 137 V (at 800 Torr), and specific power loadings of ∼40 kW cm−3. The V–I characteristics for a fired ceramic structure exhibit a negative resistance, whereas the resistance is positive prior to firing. The manufacturability of the fabrication process as well as the “flow-through” and multistage design of this device make it well suited for the excitation of gas microlasers or the dissociation of toxic or environmentally hazardous gases and vapors.
Room-temperature midinfrared electroluminescence from asymmetric AlSbAs/InAs/CdMgSe heterostructures grown by molecular beam epitaxy
Authors: Ivanov S. V. , Solov’ev V. A. , Moiseev K. D. , Sedova I. V. , Terent’ev Ya. V. , Toropov A. A. , Meltzer B. Ya. , Mikhailova M. P. , Yakovlev Yu. P. , Kop’ev P. S.
Published: July 2002

A hybrid double heterostructure with large asymmetric band offsets, combining AlAsSb/InAs (as a III–V part) and CdMgSe/CdSe (as a II–VI part), has been proposed as a basic element of a midinfrared laser structure design. The p-i-n diode structure has been successfully grown by molecular beam epitaxy and has exhibited an intense long-wavelength electroluminescence at 3.12 μm (300 K). A less than 10 times reduction of electroluminescence intensity from 77 to 300 K indicates an efficient carrier confinement in the InAs active layer due to high potential barriers in conduction and valence bands, estimated as ΔEC=1.28 eV and ΔEV∼1.6 eV. The type of band lineups at a coherent InAs/Cd1−xMgxSe interface is discussed for 0⩽x⩽0.15.
Large magnetoresistance and finite-size effect in electrodeposited bismuth lines
Authors: Yang F. Y. , Strijkers G. J. , Hong K. , Reich D. H. , Searson P. C. , Chien C. L.
Published: July 2002

Finite-size effects in Bi lines of cross section 3 μm×w, where w=8–120 μm, have been studied. The lines were made by optical lithography followed by electrodeposition. At T=5 K, the magnetoresistance (MR) decreases monotonically with decreasing line width of the Bi films, from 44 000% at w=120 μm to 4000% at w=8 μm in a field of 50 kOe. The decrease of the MR with decreasing linewidth is due to a decrease of the size-limited effective-mean-free path. At low temperatures, both the resistivity and the magnetoresistivity show a strong dependence on w, whereas at room temperature, they are independent of w. The MR at room temperature remains at about 230% for linewidths w=8–120 μm at 50 kOe. This demonstrates that microstructured Bi lines can be made for field-sensing applications without compromising their MR characteristics.
Magnetization and magnetoresistance in melt-spun Cu80Fe5Ni15
Authors: Martins C. S. , Missell F. P.
Published: July 2002

Magnetization and magnetoresistance (MR) were studied (0<H<70 kOe, 4.2 K<T<300 K) in as-cast and annealed (Tan=400, 450, and 500 °C) samples of melt-spun Cu80Fe5Ni15. Field-cooled magnetization curves, when compared with the Curie–Weiss law, suggest the presence of antiferromagnetic interactions (Θ=−57 K) between nanoparticles for Tan=450 and 500 °C. Antiferromagnetic interactions have been predicted when dipolar interactions dominate Ruderman–Kittel–Kasuya–Yosida interactions in large particles. Here antiferromagnetic interactions are attributed to particles roughly 4 to 5 nm in size. The largest MR value (MR∼17% at H=70 kOe and T=10 K) is found for the as-cast material. For samples annealed at 400 °C, MR curves are linear in H above 10 kOe and are in qualitative agreement with a model which considers short-range magnetic scattering by particles of different sizes.
Quantum critical points of an anisotropic multichannel Kondo impurity
Authors: Schlottmann P.
Published: July 2002

The low-temperature behavior of a magnetic impurity of spin S interacting with an electron gas via an anisotropic spin exchange is studied via Bethe’s ansatz. The multichannel Kondo model with U(1) invariance is integrable as a function of two continuous (the exchange and the anisotropy) and two discrete parameters, namely the impurity spin S and the number of channels n. As a function of S and n we distinguish: (i) the compensated case with n=2S, (ii) the overcompensated case if n>2S, and (iii) the undercompensated case (n<2S). While in case (i) the ground state is a singlet, the cases (ii), and (iii) yield quantum critical points. The undercompensated one is of a new type with the critical exponents depending on the anisotropy.
Electronic properties of UIrGe in high magnetic fields
Authors: Chang S. , Jung M. H. , Nakotte H. , Brück E. , Klaasse J. C. P. , Mihalik M. , Lacerda A. H. , Prokeš K. , Torikachvili M. S. , Schultz A. J.
Published: July 2002

UIrGe crystallizes in the orthorhombic TiNiSi structure and exhibits a large magnetic anisotropy. Here, we present the results of magnetoresistance and specific-heat studies of single-crystalline UIrGe in magnetic fields up to 18 T applied along the principal axes. The phase boundaries of the zero-field antiferromagnetic phase (TN≈14.1 K) are established, and we find evidence of a field-induced phase above 12.5 T at 2.1 K and 14 T at 2.5 K for magnetic fields applied along the b and c axis, respectively.
Particle transport in DIII-D discharges with internal regions of enhanced confinement and counter injected neutral beams
Authors: Baker D. R. , Greenfield C. M. , DeBoo J. C. , Wade M. R. , Stallard B. W.
Published: July 2002

An analysis of experimentally measured particle transport in tokamak plasmas with negative central magnetic shear is presented. The analysis is presented in terms of a simple model for turbulent transport which allows the separation of diagonal and off diagonal terms and allows the direct comparison of particle and energy transport. Comparing the measured fluxes to the fluxes predicted by a simple quasi analytical model which specifies a relation between the diagonal and off diagonal terms allows an understanding of the reason for the difference between energy and particle fluxes. In the center of discharges with a region of enhanced confinement (or internal transport barrier), the ion thermal diffusivity becomes small and comparable to neoclassical values and the particle diffusivity also becomes small and approaches the neoclassical values.
Si single-electron transistors with in-plane point-contact metal gates
Authors: Wang T. H. , Li H. W. , Zhou J. M.
Published: July 2002

We report on the operation of Si single-electron transistors with in-plane point-contact metal gates. These in-plane gates are fabricated by a self-aligned process, which are used to squeeze the channel and to form a single dot at the constriction of the channel. The characteristics of such single-electron transistors strongly depend on the channel width and the voltage of the in-plane gates. A few dips are observed at the less positive gate voltages for a device with a 70 nm wide channel. Applying negative voltages to the in-plane gates leads to the formation of a single dot in the conducting channel. These in-plane gates facilitate fabricating Si single-electron transistors with single dot structures.
Anisotropic magnetic domain structure of layered manganite La1.4Sr1.6Mn2O7
Authors: Fukumura T. , Hasegawa T. , Sugawara H. , Tanaka K. , Sakaki H. , Kimura T. , Tokura Y.
Published: July 2002

The anisotropic magnetic domain structure of a layered manganite La1.4Sr1.6Mn2O7 has been analyzed quantitatively. The evaluated dimensions of the domain indicate the low dimensionality and reproduce spontaneous formation of the bubble domain taking stability of the bubble domain into account. The ferromagnetic regions observed below the Neel temperature might be responsible for the limited tunneling magnetoresistance ratio reported in this compound.
Magnetic imaging of a buried SmCo layer in a spring magnet
Authors: Pollmann J. , Srajer G. , Haskel D. , Lang J. C. , Maser J. , Jiang J. S. , Bader S. D.
Published: July 2002

Images of the magnetic domain structure in a 1600 Å thick buried SmCo layer of a Fe/SmCo spring magnet were obtained using a newly developed x-ray microprobe. This probe combines circularly polarizing optics with a microfocusing Fresnel zone plate to obtain a highly polarized, small cross-section x-ray beam in the energy range between 5 and 10 keV. X-ray magnetic circular dichroism was used to provide contrast between different magnetic domains as a function of the externally applied magnetic field. The obtained images show domain walls not oriented parallel to the external field but correlated with structural features in the sample. A plausible explanation is pinning of domain walls at stacking faults in the layer. The range of external fields for which the magnetic reorientation of a particular microscopic domain occurred was much smaller than the range measured for a macroscopic hysteresis loop, indicating that the reorientation is due to growth of domains developed at local nucleation points.
Time dependence of the thermal switching field of long ferromagnetic particles
Authors: Khapikov A. F.
Published: July 2002

A simple one-dimensional micromagnetic model is used to find the nucleation field and the energy barrier for the magnetization reversal at the end of a semi-infinite thin ferromagnetic cylinder. Time dependence of the thermal switching field is calculated. The intrinsic coercivity and thermal stability factor predicted by the model are in a good agreement with recent experiments on CrO2 tapes and on individual Ni long particles.
High β relaxed states with internal conductor plasma configuration
Authors: Iqbal M. , Mirza A. M. , Murtaza G. , Yoshida Z.
Published: July 2002

In a two-fluid dissipationless plasma, strong coupling of magnetic field and flow velocity introduced by the Hall term leads to the equilibrium solution characterized by the “double curl Beltrami equation.” Internal conductor plasma configuration is investigated employing the double curl Beltrami formulation. Numerical results reveal that relaxed states of plasma exhibiting ultrahigh β associated with minimum values of field-aligned current and flow can be created which are suitable for the advanced fusion concept.
Observation of negative differential resistance of a trench-type narrow InGaAs quantum-wire field-effect transistor on a (311)A InP substrate
Authors: Sugaya Takeyoshi , Ogura Mutsuo , Sugiyama Yoshinobu , Matsumoto Kazuyuki , Yonei Kenji , Jang Kee-Youn
Published: July 2002

A trench-type narrow InGaAs quantum-wire field-effect transistor (QWR–FET) with a cross section of 8×25 nm has been fabricated on a (311)A InP V-grooved substrate by molecular-beam epitaxy. The trench-type InGaAs QWR–FET has normal static characteristics at room temperature, and demonstrates clear negative differential resistance characteristics at 40 K with a high peak-to-valley current ratio (PVR=4.3) and a low onset voltage of 0.12 V.
Scanning tunneling microscope-induced modification of Ag films and nanowire arrays on Cu(100) surfaces
Authors: York S. M. , Leibsle F. M.
Published: July 2002

We demonstrate the ability to use scanning tunneling microscopy to create nanometer-scale pits on Ag films grown on Cu(100) substrates. Atomic resolution images show that the Ag structures are intact within these pits. We also demonstrate how we can selectively modify segments of Ag nanowire arrays grown on atomic nitrogen modified Cu(100) surfaces.
Observation of sub-100-fs optical response from spin-coated films of squarylium dye J aggregates
Authors: Furuki Makoto , Tian Minquan , Sato Yasuhiro , Pu Lyong Sun , Kawashima Hitoshi , Tatsuura Satoshi , Wada Osamu
Published: July 2002

For spin-coated films of squarylium dye J aggregates, ultrafast nonlinear optical responses were investigated by pump–probe measurements. By using a broadband mode-locked titanium:sapphire laser, we succeeded in observing the optical response with a time resolution of better than 60 fs. Time-resolved transmission data are shown for different excitation wavelengths, resonant to the excitonic absorption band and off-resonant. Relaxation times of the absorption saturation were evaluated to be 140 fs (fast component) and 950 fs (slow component) in the case of resonant excitation and 98 fs in the case of off-resonant excitation.
The microwave spectrum of cobalt monoxide: Hyperfine interactions in the X 4Δ state
Authors: Namiki Kei-ichi C. , Saito Shuji
Published: July 2002

The rotational spectrum of CoO in the Δ4 ground state was observed using a source-modulated submillimeter-wave spectrometer. A direct current sputtering method using cobalt powder placed in the hollow cathode electrode was used to generate the CoO radical in an atmosphere of oxygen and helium. In total 84 spectral lines were precisely measured in the frequency region of 255–465 GHz for eight rotational transitions of the two lowest spin substates. Each rotational transition consists of eight components due to the Co59 (I=7/2) hyperfine interaction. A least squares analysis of the measured frequencies gave a new set of molecular parameters: rotational constant, hyperfine coupling constants, a, bF, c, and eQq, centrifugal distortion corrections to rotation and spin–orbit interaction, and the less well known higher-order spin–orbit distortion term to the Fermi contact interaction, bS. A comparison of the determined molecular hyperfine coupling constants was made with those of atomic/ionic Co in order to assess the inherent characteristics of the molecular orbitals around the Co nucleus.
Formation of GaAs nanocrystals by laser ablation
Authors: Perrière J. , Millon E. , Chamarro M. , Morcrette M. , Andreazza C.
Published: July 2002

The pulsed laser ablation method has been used to form GaAs nanocrystals. A quadrupled frequency Nd:yttrium–aluminum–garnet laser beam is focused onto a GaAs single crystal target, and a nitrogen flowing gas is sent at the neighborhood of the target in order to transfer in an ethanol bath, the nanoparticles grown in gas phase. The composition of the particles is close to stoichiometry and transmission electron microscopy analyses highlight zinc-blende GaAs nanocrystals with a rather well defined size: 5–8 nm diameter. The low temperature photoluminescence and photoluminescence excitation spectra show quantum confinement of about 870 meV via an emission band in the visible range (500–560 nm) for the GaAs nanocrystals produced by laser ablation without any postannealing treatment.
Relationship between surface reconstruction and morphology of strained Ga1−xInxP layers grown on GaP (001) by gas-source molecular-beam epitaxy
Authors: Wallart X. , Deresmes D. , Mollot F.
Published: July 2002

We study the growth of strained Ga1−xInxP layers on GaP (001) by gas-source molecular-beam epitaxy for x varying from 0.25 to 1. At a growth temperature of 520 °C, we find two main differences with respect to the well known GaInAs/GaAs system. First, for 0.25⩽x⩽0.5, we observe the development of wire-like structures oriented along the [110] direction and on the other hand, the growth of InP on GaP leads to the formation of huge dots in small density. The influence of the growth parameters such as the growth temperature or the phosphine flow rate is presented. The whole set of results is discussed in light of recent work on the phosphide surface reconstructions with a particular emphasis on the role of the cation-rich one.
Large reduction of leakage current by graded-layer La doping in (Ba0.5, Sr0.5)TiO3 thin films
Authors: Saha S. , Krupanidhi S. B.
Published: July 2002

A large reduction in the leakage current behavior in (Ba, Sr)TiO3 (BST) thin films was observed by graded-layer donor doping. The graded doping was achieved by introducing La-doped BST layers in the grown BST films. The films showed a large decrease (about six orders of magnitude) in the leakage current in comparison to undoped films at an electric field of 100 kV/cm. The large decrease in leakage current was attributed to the formation of highly resistive layers, originating from compensating defect chemistry involved for La-doped films grown in oxidizing environment. Temperature-dependent leakage-current behavior was studied to investigate the conduction mechanism and explanations of the results were sought from Poole–Frenkel conduction mechanism.
Picosecond Raman scattering studies of nonequilibrium electron distributions and energy-loss rate in InxGa1−xAs1−yNy
Authors: Chen Y. , Tsen K. T.
Published: July 2002

Nonequilibrium electron distributions and energy-loss rate in a metal–organic chemical-vapordeposition-grown InxGa1−xAs1−yNy (x=0.03 and y=0.01) epilayer on a GaAs substrate have been studied by picosecond Raman spectroscopy. It is demonstrated that for electron density n≅1018 cm−3, electron distributions can be described very well by Fermi–Dirac distributions with electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of the excitation laser, the energy-loss rate in InxGa1−xAs1−yNy is estimated to be 64 meV/ps. These experimental results are compared with those of GaAs.
Terahertz repetition frequencies from harmonic mode-locked monolithic compound-cavity laser diodes
Authors: Yanson D. A. , Street M. W. , McDougall S. D. , Thayne I. G. , Marsh J. H. , Avrutin E. A.
Published: July 2002

Compound-cavity laser diodes are mode locked at a harmonic of the fundamental round-trip frequency to achieve repetition rates of up to 2.1 THz. The devices are fabricated from GaAs/AlGaAs material at a wavelength of 860 nm and incorporate two gain sections with an etched slot reflector between them, and a saturable absorber section. Autocorrelation studies are used to investigate device behavior for different reflector types and reflectivity. These lasers may find applications in terahertz imaging, medicine, ultrafast optical links, and atmospheric sensing.
Evidence for the importance of radial transport in plasma detachment in the Nagoya University Divertor Simulator (NAGDIS-II)
Authors: Hollmann E. M. , Whyte D. G. , Nishijima D. , Ohno N. , Uesugi Y. , Ezumi N.
Published: July 2002

Measurements of ion and electron temperatures have been performed in detaching helium–hydrogen plasmas in a linear divertor simulator experiment using spectroscopy, a Langmuir probe, and an omegatron mass spectrometer. Detachment in these plasmas is characterized by a significant (≈20×) reduction in the central plasma flux at the target plate as the target region neutral pressure is increased from 2 to 12 mTorr. The data indicate that partially detached gas-target plasmas consist of a hot (Te≈5 eV) core region along the axis of the plasma column, surrounded by a cold (Te≈0.1 eV) halo region of recombining plasma. At Te=5 eV, plasma recombination is negligible compared with ionization; these experiments therefore provide evidence that detachment is primarily caused by radial transport and by a gradual drop in the ionization source as the temperature of the core region drops below 5 eV.
Two-dimensional photonic crystals: Candidate for wave plates
Authors: Li Lie-Ming
Published: July 2002

We have numerically investigated the optical paths of S and P waves propagating through two-dimensional photonic crystals consisting of dielectric cylinders parallel to each other, where S waves and P waves specify electromagnetic waves with the electric field parallel to the cylinders and electromagnetic waves with the electric field perpendicular to the cylinders, respectively. It is found that the difference between the optical paths of S and P waves can be many times the path difference in the case of a conventional wave plate with the same thickness. Therefore, we propose that two-dimensional photonic crystals can be employed as thin wave plates that can be used as integrated optical and microwave components.
Phonon-pumped terahertz gain in n-type GaAs/AlGaAs superlattices
Authors: Sun Gregory , Soref Richard A.
Published: July 2002

Local population inversion and far-IR gain are proposed and theoretically analyzed for an unbiased n-doped GaAs/Al0.15Ga0.85As superlattice pumped solely by phonons. The lasing transition occurs at the Brillouin zone boundary of the superlattice wave vector kz between the two conduction minibands CB1 and CB2 of the opposite curvature in kz space. The proposed waveguided structure is contacted above and below by heat sinks at 300 K and 77 K, respectively. Atop the superlattice, a heat buffer layer confines longitudinal optical phonons for enhanced optical-phonon pumping of CB1 electrons. A gain of 345 cm−1 at 4.5 THz is predicted for a doping density of 2.8×1016 cm−3.
Passivation of dislocations in GaAs grown on Si substrates by phosphine (PH3) plasma exposure
Authors: Wang G. , Ogawa T. , Soga T. , Jimbo T. , Umeno M.
Published: July 2002

The phosphidization effect on dislocations in GaAs grown on Si substrate (GaAs/Si) has been investigated. It was found that the high density of dislocations in GaAs/Si heteroepitaxial layers largely enhanced the diffusion of phosphorus (P) atoms during the phosphine (PH3) plasma exposure. The incorporated P atoms strongly passivated the electrical states of residual dislocations in GaAs/Si solar cell. As a result, the PH3 plasma exposure largely increased the open circuit voltage (Voc) and the efficiency of GaAs/Si solar cell.
On equilibria of finite pressure plasma in magnetic multipoles
Authors: Krasheninnikov S. I. , Soboleva T. K.
Published: July 2002

Separable solutions of finite plasma pressure equilibrium in a point multipole are found. Using the energy principle it is demonstrated that these solutions are interchange stable for quadripole and sextipole configurations. The low and high pressure forms of the solution in sextipole configuration are explicitly displayed.
Linear and nonlinear resistive magnetohydrodynamic stability of tokamak discharges with negative central shear
Authors: Leboeuf J. N. , Lynch V. E. , Carreras B. A.
Published: July 2002

Linear and nonlinear magnetohydrodynamic (MHD) calculations have been performed to study the stability of tokamak discharges with reversed or negative central magnetic shear using the finite aspect ratio (FAR) suite of MHD codes [Charlton et al., J. Comput. Phys. 86, 270 (1990)]. The linear calculations confirm anew that radially localized resistive interchange modes can be unstable in the reversed or negative central magnetic shear region. The calculations further show that these resistive interchange modes are more unstable for toroidal mode numbers n higher than n=1. The nonlinear calculations do however demonstrate that toroidal mode number n=1, enhanced by the nonlinear couplings of the linearly more unstable higher n toroidal harmonics, dominates the nonlinearly saturated steady state. While the resistive interchange modes may account for the precursors detected experimentally, the saturated levels of magnetic fluctuations obtained in the nonlinear calculations do not appear large enough to cause the experimentally observed global collapse of such discharges.
Surface hydrogenation as a method to purify and flatten a silicon surface
Authors: Higai Shin’ichi , Ohno Takahisa
Published: July 2002

We performed first-principles theoretical calculations and found that the hydrogenation of a Si surface causes the segregation of impurity metal atoms from the subsurface onto the surface top. Since the contamination by metallic impurities is a principal cause of surface defects, it is expected that, by the surface hydrogenation, a highly pure and atomically flat Si surface is realized.
The toroidicity-induced Alfvén eigenmode structure in DIII-D: Implications of soft x-ray and beam-ion loss data
Authors: Carolipio E. M. , Heidbrink W. W. , Cheng C. Z. , Chu M. S. , Fu G. Y. , Jaun A. , Spong D. A. , Turnbull A. D. , White R. B.
Published: July 2002

The internal structure of the toroidicity-induced Alfvén eigenmode (TAE) is studied by comparing soft x-ray profile and beam ion loss data taken during TAE activity in the DIII-D tokamak [W. W. Heidbrink et al., Nucl. Fusion 37, 1411 (1997)] with predictions from theories based on ideal magnetohydrodynamic (MHD), gyrofluid, and gyrokinetic models. The soft x-ray measurements indicate a centrally peaked eigenfunction, a feature which is closest to the gyrokinetic model’s prediction. The beam ion losses are simulated using a guiding center code. In the simulations, the TAE eigenfunction calculated using the ideal MHD model acts as a perturbation to the equilibrium field. The predicted beam ion losses are an order of magnitude less than the observed ∼6%–8% losses at the peak experimental amplitude of δBr/B0≃2–5×10−4.
Dynamic stability of field emission from molybdenum microtips exposed to oxygen
Authors: Sosa Edward D. , Abbott Patrick , Golden David E.
Published: July 2002

The emission current in a molybdenum field emission array can decrease by 50% in 1000 s at an oxygen pressure of 10−6 Torr. To overcome this disadvantage of molybdenum microtips, the effectiveness of dynamic surface cleaning has been investigated in a single-aperture gated-diode configuration. For dynamic surface cleaning, tip surface oxide buildup is balanced by tip oxide removal due to sputtering by ions created in ionizing collisions with field-emitted electrons. The present results demonstrate stable dynamic cleaning with clean and partially oxidized molybdenum tips for currents ranging from 10−11 to 10−9 A with oxygen exposures of up to 1000 L. For currents above 10−9 A, ion bombardment causes the tip shape to become unstable leading to failure with increasing oxygen exposure.
Ion dynamics in nonlinear electrostatic structures
Authors: Skiff F. , Bachet G. , Doveil F.
Published: July 2002

Direct experimental determination of terms in the ion kinetic equation is used to address the question of ion dynamics in low-frequency electrostatic structures. Problems in quantitative laser-induced florescence in plasmas are addressed, especially the problem of the relative metastable fraction. In addition to determining terms in the ion kinetic equation and the plasma electric field, the measurements determine the energy-dependent cross section for metastable quenching.
Measurements of the trapped particle sideband instability compared to the macroparticle model
Authors: Hartmann D. A. , Driscoll C. F.
Published: July 2002

The upper and lower sidebands are measured on a traveling wave tube where a cold electron beam is trapped by a large amplitude wave. The two strongly coupled sidebands form a normal mode that is characterized by the sideband growth rates, wave number shifts, amplitude ratio, and phase relationship. The measured values agree only qualitatively with the macroparticle model of Kruer, Dawson, and Sudan [Phys. Rev. Lett. 23, 838 (1969)]. Also, the macroparticle model prediction for a nonlinear product wave does not agree with the experiment. Quantitative agreement is found between the experiment and computer simulations that follow the electron orbits, suggesting that the trapped particle model is too simple for quantitative predictions.
Microwave properties of superconducting MgB2
Authors: Hakim N. , Parimi P. V. , Kusko C. , Sridhar S. , Canfield P. C. , Bud’ko S. L. , Finnemore D. K.
Published: July 2002

Measurements of 10 GHz microwave surface resistance, Rs, of dense MgB2 wire and pellet are reported. Significant improvements are observed in the wire with reduction of porosity. The data lie substantially above the theoretical estimates for a pure Bardeen–Cooper–Schrieffer s-wave superconductor. However, the Rs (20 K) of the wire is an order of magnitude lower than that of polycrystal YBa2Cu3O6.95 and matches with single-crystal YBa2Cu3O6.95. The results show promise for the use of MgB2 in microwave applications.
Spatio-temporal interfacial potential patterns during the electrocatalyzed oxidation of formic acid on Bi-modified Pt
Authors: Lee Jaeyoung , Christoph Johannes , Strasser Peter , Eiswirth Markus , Ertl Gerhard
Published: July 2002

We report experimental observations of the spatio-temporal dynamics in the electro-oxidation of formic acid on a Pt ring electrode modified by bismuth adatoms. Bismuth modification significantly enhanced the current density and it was found to considerably increase the existence range of oscillations and spatio-temporal self-organization. Hidden negative differential resistance and the existence of a Hopf bifurcation were deduced from the electrochemical impedance spectra and the occurrence of galvanostatic oscillations. The pattern formation resulted from hybrid effects of the nonlinear chemistry during formic acid oxidation and the long-range coupling of the interfacial potential induced by the chosen geometry (ring type) of the working electrode. Reversible transitions between traveling pulses and oscillating standing waves were observed when the outer potential or the formic acid concentration near the electrode were used as control parameters. Experimental results were compared with computer simulations of a reaction-migration system. The role of electrode inhomogeneities in pattern formation and the transform between patterns were discussed.
Region of validity for potato-plateau transport fluxes
Authors: Shaing K. C.
Published: July 2002

The region of validity for the potato-plateau transport fluxes in the near (magnetic) axis region is reexamined by considering the linear drift kinetic equation in the plateau regime. It is shown that the potato-plateau transport fluxes are valid in a region of the order of a potato width around the axis as stated in Shaing and Hazeltine [Phys. Plasmas 4, 4331 (1997); 5, 953 (1998)].
Some characteristics of the predisruption phase in the Damavand tokamak
Authors: Farshi E. , Brevnov N. , Bortnikov A. , Gott Y. , Shurygin V.
Published: July 2002

The time evolution of magnetohydrodynamic activity, charge exchange neutral particles spectra, the line emission of the impurities in the visible spectral range and soft x-ray radiation were investigated in the predisruption phase of the small Damavand tokamak [Amrollahi et al., Plasma Phys. Rep. 23, 561 (1997)]. The experimental data show that the main disruptive processes, particularly the kinetic, take place near the plasma periphery. The experimental results also show that the interaction between high energy ions of bulk plasma and impurity ions may play an important role in the plasma radiation processes. A new mechanism for the plasma radiation processes in the predisruption phase may be obtained that can be considered for the investigation of the disruption instability.
Comparative dielectric response in CaTiO3 and CaAl1/2Nb1/2O3 from first principles
Authors: Cockayne Eric
Published: July 2002

CaTiO 3 (CT) and CaAl1/2Nb1/2O3 (CAN) have similar structures but very different dielectric properties. For CT at room temperature, the relative permittivity κ≈170 and the temperature coefficient of resonance frequency τf≈900×10−6 K−1, but for CAN, κ≈27 and τf≈−88×10−6 K−1. I use first-principles density functional theory calculations to investigate the origin of the contrasting behaviors. I find important differences between the two systems in both the frequencies and the eigenvectors of the low-frequency polar phonons. In CT, the frequencies are lower and the perovskite B site cations move against the surrounding oxygen octahedra; whereas in CAN, the frequencies are higher and the B site cations move with the oxygen octahedra. These two factors are equally important in explaining the differences in κ. I introduce and solve a decoupled quantum oscillator model for the temperature-dependent permittivity. This model predicts a large positive τf for CT and a small τf for CAN, in qualitative agreement with experiment. I relate the different dielectric behaviors to differences in the electronic structures.
Resonant tunneling and bistability in a double barrier structure under an intense terahertz laser
Authors: Zhang C.
Published: July 2002

By using exact wave functions of an electron in a terahertz laser field, we calculated the electron resonant tunneling through a double barrier structure. It is found that the laser field has two effects on the current voltage characteristics. First, it introduces additional tunneling states through the structure due to multiple photon processes (emission, absorption, scattering). Second, it reduces the width of the bistable region. At high field strength and low frequencies, the bistability can be completely removed. This can provide a method by which to tune the bistable region in a double barrier structure.
Microstructural studies of top and bottom magnetic tunnel junctions
Authors: Portier X. , Petford-Long A. K. , Nickel J. H. , Anthony T. C. , Brug J. A.
Published: July 2002

High-resolution electron microscopy has been used to study the microstructural properties of top and bottom magnetic tunnel junctions. Different physical properties have been found for the two configurations, and they have been interpreted in terms of microstructural features. All physical parameters favor the bottom configuration except for the coupling field, which is higher for the bottom structure. This latter result has been explained by the more pronounced roughness at the interfaces between the electrodes and the insulator film.
Aperiodic optical superlattices engineered for optical frequency conversion
Authors: Liu H. , Zhu Y. Y. , Zhu S. N. , Zhang C. , Ming N. B.
Published: July 2002

An aperiodic optical superlattice is designed. The designing method is universal and can be applied to all frequency conversion processes by using the coupling of quasiphase matching, without any limitations to special materials and to given fundamental wavelengths.
Coherent all-optical polarization switching based on exciton–exciton interactions in quantum wells
Authors: Gansen Eric J. , Jarasiunas K. , Hawkins Scot A. , Stevens Martin J. , Smirl Arthur L.
Published: July 2002

A coherent all-optical nonlinear polarization switch based on exciton–exciton correlations is demonstrated in a multiple-quantum-well semiconductor structure. A contrast ratio of 8:1 and a relaxation time of less than a picosecond are reported at 80 K using only ten wells. The results are compared to a simple phenomenological model to demonstrate that many-body effects are solely responsible for the switching action and that the turn-on and turn-off times are determined by the dephasing time.
Simplified treatment of scattering processes in quantum well structures
Authors: Hagston W. E. , Stirner T. , Rasul F.
Published: July 2002

Arguments are developed which show that, to a good approximation, the essential physics and many of the quantitative details pertaining to the relative rates of carrier–carrier, carrier–longitudinal optical phonon, and carrier–photon scattering in different quantum well structures can be understood in terms of a simplified approach. The latter is based on fundamental concepts of quantum theory and involves the separation of the matrix element in the scattering rate calculation into two simpler parts: an “energy effect” term and a “wave-function effect” term. As an application of the method, the question of attaining lasing action in the far-infrared (terahertz) region of the spectrum is discussed briefly.
Large magnetoresistance in Ni1−xFexS near room temperature
Authors: Chen P. , Du Y. W.
Published: July 2002

Magnetic field-induced magnetic and electrical transition from the antiferromagnetic nonmetal phase to the paramagnetic metal phase takes place in Ni1−xFexS (x=0, 0.12, 0.2, and 0.3). Large magnetoresistance (MR) was observed near room temperature, MR=730% for x=0.12 at 299 K in a magnetic field of 4 T. The large MR is related to the field-induced nonmetal–metal transition.
Stability of miscible displacements across stratified porous media
Authors: Shariati Maryam , Yortsos Yanis C.
Published: July 2002

We consider the stability of miscible displacements across stratified porous media, where the heterogeneity is along the direction of displacement. Asymptotic results for long and short wavelengths are derived. It is found that heterogeneity has a long-wave effect on the instability, which, in the absence of gravity, becomes nontrivial when the viscosity profiles are nonmonotonic. In the latter case, profiles with end-point viscosities, predicted to be stable using the Saffman–Taylor criterion, can become unstable, if the permeability contrast in the direction of displacement is sufficiently large. Conversely, profiles with end-point viscosities predicted to be unstable, can become stable, if the permeability decrease in the direction of displacement is sufficiently large. Analogous results are found in the presence of gravity, but without the nonmonotonic restriction on the viscosity profile. The increase or decrease in the propensity for instability as the permeability increases or decreases, respectively, reflects the variation of the two different components of the effective fluid mobility. While permeability remains frozen in space, viscosity varies following the concentration field. Thus, the condition for instability does not solely depend on the overall fluid mobility, as in the case of displacements in homogeneous media, but it is additionally dependent on the permeability variation.
Structural and ferromagnetic resonance characteristics of BaFe12O19 films with minimal linewidths
Authors: Saraf L. V. , Lofland S. E. , Cresce Arthur V. , Bhagat S. M. , Ramesh R.
Published: July 2002

Hexagonal barium ferrite films with thickness between 0.3 and 8 μm were deposited on (0001) oriented Al2O3 using pulsed laser deposition and optimized growth and annealing conditions. They were characterized by measurements of x-ray diffraction, Rutherford backscattering, magnetization, atomic force microscopy and ferromagnetic resonance (FMR) at 58 GHz. The narrowest FMR lines and largest number of spin wave resonances are obtained when the field is perpendicular to the film plane of 0.3 and 0.46 μm films. To quantitatively account for variations with field angle (θ) we assume (i) a strain-induced anisotropy energy with cos4 θ dependence and (ii) that the film should be pictured as a mosaic whose tiles are tilted out of the film plane.
Classical limit of fermions in phase space
Authors: Bolivar A. O.
Published: July 2002

Using the mathematical structure of the Grassmann algebra, studied by Schönberg, we write down the Pauli equation and the Dirac equation in phase space. In addition, in order to investigate the physical nature of the spin degree of freedom inherent in these equations we set up a novel classical limiting process ℏ→0. Thus we are able to derive relativistic and nonrelativistic classical statistical mechanics, for particle with spin 1/2, within a geometric algebra framework.
Temperature dependence of the ohmic conductivity and activation energy of Pb1+y(Zr0.3Ti0.7)O3 thin films
Authors: Chu D. P. , McGregor B. M. , Migliorato P. , Durkan C. , Welland M. E. , Hasegawa K. , Shimoda T.
Published: July 2002

The ohmic conductivity of the sol–gel derived Pb1+y(Zr0.3Ti0.7)O3 thin films (with the excess lead y=0.0 to 0.4) are investigated using low frequency small signal ac and dc methods. Its temperature dependence shows two activation energies of 0.26 and 0.12 eV depending on temperature range and excess Pb levels. The former is associated with Pb3+ acceptor centers, while the latter could be due to a different defect level yet to be identified.
Hybrid acoustic waves in thin potassium niobate plates
Authors: Zaitsev B. D. , Kuznetsova I. E. , Joshi S. G.
Published: July 2002

The present article seeks to theoretically predict an effect and namely, the resonant interaction between fundamental acoustic modes propagating in a thin piezoelectric plate. It has been theoretically shown that for certain crystallographic cuts and propagation directions the electrical shorting of one of the strong piezoelectric plate sides may result in a coupling between fundamental acoustic modes. For example, the hybrid mode is generated for a frequency of about 10 MHz by an antisymmetric wave A0 and a shear-horizontal wave SH0, which propagate along the X+15° direction in a metallized Y-cut 100-μm-thick potassium niobate plate.