bluuebunny/crossref_metadata_2025 huggingface dataset:
The consistent application of Maxwell–Garnett effective medium theory to anisotropic composites
Authors: Skryabin I. L. , Radchik A. V. , Moses P. , Smith G. B.
Published: July 2002

The Maxwell–Garnett class of effective medium model applies if a representative cell can be found whose polarization vanishes upon insertion in the effective medium. For an anisotropic composite with randomly distributed ellipsoidal particles aligned along the principal axis, this leads to electrostatic restraints on the shape of such cells. It is shown that the cell boundary must have different depolarization factors to the inclusion within the cell. Practice is to equate them. A new physically correct ellipsometric modeling routine still with only two fitting parameters is demonstrated.
Controlling polarization of quantum-dot surface-emitting lasers by using structurally anisotropic self-assembled quantum dots
Authors: Saito Hideaki , Nishi Kenichi , Sugou Shigeo , Sugimoto Yoshimasa
Published: July 2002

Polarization control is achieved in vertical-cavity surface-emitting lasers (VCSELs) by using structurally anisotropic self-assembled quantum dots (QDs) as active layers. The dot shape is long in the [01̄1] direction on the (100) surface. The photoluminescence from the dots has a polarization dependence whose intensity along the [01̄1] direction is 1.37 times stronger than that along the orthogonal [011] direction. The QD VCSEL operates at the wavelength of the QD ground state transition. Lasing emission shows polarization along the [01̄1] direction and an orthogonal polarization suppression ratio of 18 dB.
Enhanced luminance in polymer composite light emitting devices
Authors: Carter S. A. , Scott J. C. , Brock P. J.
Published: July 2002

We demonstrate that mixing insulating oxide nanoparticles into electroluminescent polymer materials results in increased current densities, radiances, and power efficiencies in polymer light emitting diode devices. For low driving voltages, an order of magnitude increase in current density and light output is achieved with minimal loss in device lifetime. At 5 V, we achieve radiances of 10 000 cd/m2 with external quantum efficiencies ∼1% for nanoparticle/MEH–PPV composite films.
High-power, continuous-wave, singly resonant, intracavity optical parametric oscillator
Authors: Edwards T. J. , Turnbull G. A. , Dunn M. H. , Ebrahimzadeh M. , Colville F. G.
Published: July 2002

A high-power continuous-wave optical parametric oscillator based on the nonlinear material KTiOAsO4 and pumped internal to a tunable Ti:sapphire laser is described. The use of the intracavity pumping approach has enabled operation of a singly resonant oscillator (SRO), resulting in the generation of as much as 1.46 W of infrared power in a 11.5-mm-long crystal. Amplitude-stable signal and idler outputs, each in excess of 500 mW, over the respective wavelength ranges of 1.11–1.20 and 2.44–2.86 μm have been extracted from the SRO. We demonstrate up to 90% down-conversion of the optimum Ti:sapphire output power to the SRO, confirming our recent theoretical predictions. The performance characteristics of the device demonstrate that practical, stable, and efficient operation of continuous-wave SROs at watt-level output power can be readily achieved in conventional birefringent materials by exploiting the intracavity pumping approach.
Local determination of the stacking sequence of layered materials
Authors: Fompeyrine J. , Berger R. , Lang H. P. , Perret J. , Mächler E. , Gerber Ch. , Locquet J.-P.
Published: July 2002

The ability to modify the stacking sequence of ultrathin films offers a unique way to change either the interaction strength or the doping, but demands a careful control of each atomic monolayer. Progress is hampered by the lack of a direct method that allows differentiation on a local scale between the various terminating layers of a crystal. Here, the combination of a vacuum annealing process and friction force microscopy reveals this local distinction on a SrTiO3 surface. Using the friction contrast, we find how the terminating layer of a single crystal profoundly influences the terrace edge structure.
Modification of InAs quantum dot structure by the growth of the capping layer
Authors: Lian G. D. , Yuan J. , Brown L. M. , Kim G. H. , Ritchie D. A.
Published: July 2002

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications.
Nitrogenated tetrahedral amorphous carbon films prepared by ion-beam-assisted filtered cathodic vacuum arc technique for solar cells application
Authors: Cheah L. K. , Shi X. , Liu E. , Shi J. R.
Published: July 2002

Fabrication and characterization of nitrogenated tetrahedral amorphous carbon (ta-C:N) semiconductor/crystalline p-type silicon (p-Si) heterojunction structures are reported. The electron-hole pairs generated from both ta-C:N and Si depletion regions were observed from photoresponse measurements. The peaks are centered at about 540 and 1020 nm, which correspond to the optical absorption edge of ta-C:N and p-Si, respectively. The reverse current increased by three orders of magnitude when the structures were exposed to AM1 light. A photovoltaic effect was observed from ta-C:N and the values of short circuit current, open circuit voltage, and field factor obtained are 5.05 mA cm−2, 270 mV, and 0.2631, respectively.
Non-Gaussian behavior and anticorrelations in ultrathin gate oxides after soft breakdown
Authors: Vandewalle N. , Ausloos M. , Houssa M. , Mertens P. W. , Heyns M. M.
Published: July 2002

The time dependence of the gate voltage VG(t) after soft breakdown of metal-oxide-semiconductor capacitors with a 2.4 nm SiO2 layer has been measured. It is found that the VG(t) fluctuation distributions are non-Gaussian, but can be described by a Lévy stable distribution. The long-range correlations in VG(t) are investigated within the detrended fluctuation analysis. The Hurst exponent is found to be H=0.25±0.04 independent of the value of the stress current density J. It is argued that these are universal features of soft breakdown and are due to trapping–detrapping of electrons in and away from the primary percolation path.
Mechanism of texture development in ion-beam-assisted deposition
Authors: Dong L. , Srolovitz D. J.
Published: July 2002

We perform a series of atomistic simulations of ion-beam-assisted deposition (IBAD) to identify the mechanism by which ion beams select crystallographic texture. Simulations were devised to distinguish between two previously proposed mechanisms: (1) preferential sputtering of differently oriented grains and (2) preferential damage of differently oriented grains followed by grain-boundary migration. We show that while preferential sputtering is capable of producing texture change, it does so much more slowly than observed in IBAD. Simulations that include only differential damage produce structures which are nearly indistinguishable from those seen in IBAD. These results conclusively demonstrate that texture evolution during ion-beam-assisted deposition is dominated by differential damage and subsequent recrystallization.
Single pulse nm-size grating formation in polymers using laser ablation with an irradiation wavelength of 355 nm
Authors: Lippert T. , Gerber T. , Wokaun A. , Funk D. J. , Fukumura H. , Goto M.
Published: July 2002

Laser ablation at 355 nm of a specially designed polymer was used as a true single step dry-etching process to create a two-beam interference grating. Gratings with groove spacings of 180 and 1090 nm were created with single laser pulses. Moreover, by varying the laser fluence and/or the angle between the two beams, variable modulation frequencies (depth/spacing) could be obtained. Additional pulses deteriorated the grating quality, demonstrating the importance of the single pulse approach.
Increase in response time of low-temperature-grown GaAs photoconductive switches at high voltage bias
Authors: Zamdmer N. , Hu Qing , McIntosh K. A. , Verghese S.
Published: July 2002

The response time of photoconductive submillimeter-wave emitters based on low-temperature-grown (LTG) GaAs is known to increase at high applied bias, which limits the output power of these devices at frequencies near 1 THz. We performed measurements of an LTG GaAs photoconductor embedded in a coplanar waveguide with both static and dynamic illumination to investigate the increase in response time and an increase in direct-current photoconductance that occurs at the same bias voltages. We attribute both phenomena to a reduction of the electron capture cross section of donor states due to electron heating and Coulomb-barrier lowering. We discuss why the phenomena cannot be explained by space-charge-limited current or other injection-limited currents, or by impact ionization.
Failure resistance of amorphous silicon transistors under extreme in-plane strain
Authors: Gleskova H. , Wagner S. , Suo Z.
Published: July 2002

We have applied strain on thin-film transistors (TFTs) made of hydrogenated amorphous silicon on polyimide foil. In tension, the amorphous layers of the TFT fail by periodic cracks at a strain of ∼0.5%. In compression, the TFTs do not fail when strained by up to 2%, which is the highest value we can set controllably. The amorphous transistor materials can support such large strains because they lack a mechanism for dislocation motion. While the tensile driving force is sufficient to overcome the resistance to crack formation, the compressive failure mechanism of delamination is not activated because of the large delamination length required between transistor layers and polymer substrate.
Exchange-bias systems with compensated interfaces
Authors: Kiwi Miguel , Mejı́a-López José , Portugal Ruben D. , Ramı́rez Ricardo
Published: July 2002

When a ferromagnetic metal (F) is in contact with an antiferromagnet (AF), often a shift of the hysteresis loop away from its normal, symmetric position around H=0, to HE≠0 does occur. This phenomenon is known as exchange bias (EB). We put forward an analytic model, for compensated AF interfaces, based on the AF interface freezing into a metastable canted spin configuration. The EB energy is reversibly stored in a spring-like magnet, or incomplete domain wall, in the F slab. Our theory yields the right values of HE and its F thickness dependence HE∝tF−1. It also predicts the F layer by layer magnetization profile.
Nonlinearity and electrothermal feedback of high Tc transition edge bolometers
Authors: Neff H. , Lima A. M. N. , Deep G. S. , Freire R. C. S. , Melcher E. , Khrebtov I. A. , Tkachenko A. D.
Published: July 2002

The effects of electrothermal feedback on the performance of a micromachined superconducting high Tc transition edge bolometer, over a temperature range 85–95 K, have been investigated. The system behaves nonlinearly, due to the variation of the resistance-temperature coefficient β with temperature. Optimum operating points of constant current mode (CCM) and constant voltage mode (CVM) modes vary with temperature and biasing conditions. In CCM, effective response time τeff varies little with temperature. The optimum output signal of CVM occurs at approximately 3–5 K lower temperature within the tail region, where β is maximum. The biasing voltage also displays a maximum, above which the bolometric performance degrades. The CCM exhibits a comparatively limited linear dynamic range. Negative thermal feedback causes a decrease of τeff of up to 2 orders of magnitude in CVM. CCM requires precise thermal stabilization at midtransition. Noise in both modes is limited by the 1/f contribution. Maximum D* values are slightly higher for CVM at higher frequencies. Bolometer operation in the CVM mode requires cooling the heatsink below 77 K, but does not need accurate temperature control.
A molecular based derivation of the nucleation theorem
Authors: Bowles R. K. , McGraw R. , Schaaf P. , Senger B. , Voegel J.-C. , Reiss H.
Published: July 2002

We show that, for condensation in an almost ideal vapor, the nucleation theorem is essentially a consequence of the law of mass action. The usual form of the theorem, with the effects of the translational degrees of freedom of the cluster included, is then derived using statistical mechanics and molecular theory, but only under the assumptions that the cluster excludes a volume to the surrounding vapor and that the vapor is ideal. The form of the result obtained via molecular theory is such that it appears unlikely (but not impossible) that the theorem remains valid for cases when the vapor is nonideal. This suggests that further work is necessary before the theorem can be regarded as established. We also consider the effects of the presence of a carrier gas.
X-ray scattering study and molecular simulation of glass forming liquids: Propylene carbonate and salol
Authors: Eckstein E. , Qian J. , Hentschke R. , Thurn-Albrecht T. , Steffen W. , Fischer E. W.
Published: July 2002

Wide angle x-ray scattering and classical molecular-dynamics simulations were employed to study structural and dynamic properties of two low-molecular weight glass-forming liquids, propylene carbonate, and salol. We observe pronounced changes in the liquid structure as a function of temperature in a wide temperature range bracketing the glass transition temperature. The experimental and simulation results compare the x-ray scattering from the liquid slightly and far above the experimental glass transition temperature, Tgexp. The simulations largely reproduce the characteristic behavior of the experimental x-ray intensities, which is interpreted in terms of clusterlike heterogeneities forming in the liquid as Tgexp is approached.
Translational and rotational excitation of the CO2(000) vibrationless state in the collisional quenching of highly vibrationally excited 2-methylpyrazine: Kinetics and dynamics of large energy transfers
Authors: Sevy Eric T. , Rubin Seth M. , Lin Zhen , Flynn George W.
Published: July 2002

The relaxation of highly vibrationally excited methylpyrazine (C5N2H6) by collisions with CO2 molecules has been investigated over the temperature range 243–364 K using diode laser transient absorption spectroscopy. Particular focus is placed on understanding both the dynamical features and the kinetics of collisions which are accompanied by large energy transfers into the CO2 rotational and translational degrees of freedom. Vibrationally hot methylpyrazine (E′=40 987 cm−1) was prepared by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions (J=58–80) of the 0000 ground state of CO2 resulting from collisions with hot methylpyrazine were probed at short times following the excimer laser pulse. Doppler spectroscopy was used to measure the distributions of CO2 recoil velocities for individual rotational levels of the 0000 state. In addition, the temperature dependence of the state resolved, absolute rate constants for collisions populating high J states of CO2 was determined. The rotational population distributions, distributions of recoil velocities, and quenching rates for production of CO2 high J states (J=58–80) exhibit a very weak temperature dependence. The slight temperature dependence indicates that CO2 molecules which scatter into high J states of the ground vibrationless level originate from rotational levels near the mean of the precollision thermal rotational distribution. A gap law model is used to estimate the average initial rotational state and velocity of the CO2 bath, which allows for the calculation of the energy transfer magnitudes, ΔE. The measured energy transfer probabilities which are indexed by final bath state are resorted as a function of ΔE to create the energy transfer distribution function, P(E,E′) from E′−E∼1500–6000 cm−1. P(E,E′) is fit to both single exponential and biexponential functions to extract a value for the average energy transferred in a single collision of methylpyrazine and CO2. This average energy transfer value is compared to donor loss energy transfer studies as well as previous bath energy gain studies on the pyrazine/CO2 and C6F6/CO2 systems. On average, methylpyrazine donates more energy per collision to CO2 than pyrazine but not as much as C6F6; however, methylpyrazine has the lowest probability for single collision energy transfers larger than 2000 cm−1 of the three molecules studied using this technique.
Equilibrium behavior of asymmetric ABA triblock copolymer melts
Authors: Matsen M. W.
Published: July 2002

Melts of ABA triblock copolymers with two different sized end blocks are examined using self-consistent field theory. At small asymmetries, the A block bidispersity reduces the stretching energy of the A domains. This effect causes a slight increase in the domain spacing and shifts the order–order transitions (OOTs) toward higher A volume fractions. At large asymmetries, a significant number of the short A blocks pull free of their domains allowing their B blocks to relax. This effect produces a much greater increase in domain spacing and shifts the OOTs toward lower A volume fractions. It also leads to a sizable region of close-packed spheres. At the crossover between these two effects, the ordered morphologies are shifted from their normal compositions by about 10%. The displacement of the complex gyroid region is found to have a strong influence on the metastability of the perforated-lamellar phase.
Determination of vibrational polarizabilities and hyperpolarizabilities using field-induced coordinates
Authors: Luis Josep M. , Duran Miquel , Champagne Benoı̂t , Kirtman Bernard
Published: July 2002

An analytical set of field-induced coordinates (FICs) is defined. It is shown that, instead of 3N−6 normal coordinates, a relatively small number of FICs is sufficient to describe the vibrational polarizability and hyperpolarizabilities due to nuclear relaxation. The fact that the number of FICs does not depend upon the size of the molecule leads to computational advantages. A method is provided for separating anharmonic contributions from harmonic contributions as well as effective mechanical from electrical anharmonicity. Hartree–Fock calculations on a dozen representative conjugated molecules illustrate the procedures and indicate that anharmonicity can be very important. Other potential applications including the determination of zero-point vibrational averaging corrections are noted.
Direct coupling of high-speed optical detector preamplifiers
Authors: Green Samuel I.
Published: July 2002

I describe simple techniques to couple commercially available wide bandwidth optical detector-preamplifier modules directly to low impedance loads. Manufacturers intend such devices to operate through a capacitor or into high impedance loads, but these devices will drive a dc coupled low impedance load when the designer forces the quiescent output voltage to zero. I show how to build an inexpensive device that operates well at 2 Gbit/s.
Electronic structure and optoelectronic properties of transparent p-type conducting CuAlO2
Authors: Yanagi Hiroshi , Inoue Shin-ichiro , Ueda Kazushige , Kawazoe Hiroshi , Hosono Hideo , Hamada Noriaki
Published: July 2002

Electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ∼3×10−1 S cm−1 and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type (log σ∝T−1/4) at <220 K. It was inferred that an admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transparent oxides.
Low-order scaling local electron correlation methods. IV. Linear scaling local coupled-cluster (LCCSD)
Authors: Schütz Martin , Werner Hans-Joachim
Published: July 2002

A new implementation of local coupled-cluster theory with single and double excitations (LCCSD) is presented for which asymptotically all computational resources (CPU, memory, and disk) scale only linearly with the molecular size. This is achieved by: (i) restricting the correlation space for each electron pair to domains that are independent of molecular size; (ii) classifying the pairs according to a distance criterion and treating only strong pairs at the highest level; (iii) using efficient prescreening algorithms in the integral transformation and other integral-direct procedures; and (iv) neglect of small couplings of electron pairs that are far apart from each other. The errors caused by the various approximations are negligible. LCCSD calculations on molecules including up to 300 correlated electrons and over 1000 basis functions in C1 symmetry are reported, all carried out on a workstation.
Anode modification in organic light-emitting diodes by low-frequency plasma polymerization of CHF3
Authors: Hung L. S. , Zheng L. R. , Mason M. G.
Published: July 2002

Plasma polymerization of CHF3 at low frequencies was utilized for anode modification in organic light-emitting diodes. The polymerized fluorocarbon films have a high ionization potential and a relatively low resistivity. The devices with a polymer-coated anode of indium–tin–oxide exhibited enhanced hole injection and superior operational stability.
Quantum-cascade lasers based on a bound-to-continuum transition
Authors: Faist Jérôme , Beck Mattias , Aellen Thierry , Gini Emilio
Published: July 2002

A quantum-cascade structure combining the advantages of the three-quantum well and superlattice active regions is demonstrated. In these devices, the emission occurs between a state localized close to the injection barrier and a miniband. A low threshold current density (3.6 kA/cm2), large slope efficiency (200 mW/A for 35 periods), and peak power (700 mW) are achieved at 30 °C while a peak power of 90 mW is obtained at temperatures as high as 150 °C.
Phonon heat conduction in a semiconductor nanowire
Authors: Zou Jie , Balandin Alexander
Published: July 2002

A model for phonon heat conduction in a semiconductor nanowire with dimensions comparable to the phonon mean free path is developed. It is based on the solution of Boltzmann’s equation, which takes into account (i) modification of the acoustic phonon dispersion due to spatial confinement, and (ii) change in the nonequilibrium phonon distribution due to partially diffuse boundary scattering. Numerical simulation is performed for a silicon nanowire with boundaries characterized by different interface roughness. Phonon confinement and boundary scattering lead to a significant decrease of the lattice thermal conductivity. The value of this decrease and its interface roughness and temperature dependence are different from the predictions of the early models. The observed change in thermal resistance has to be taken into account in simulation of deep-submicron and nanometer-scale devices.
Statics and dynamics of a single polymer chain confined in a tube
Authors: Sheng Yu-Jane , Wang Meng-Chieh
Published: July 2002

The static and nonequilibrium dynamic properties of a single linear chain confined in a cylindrical tube are studied by Monte Carlo simulations using a continuous model and by scaling calculations. Chain lengths from N=30 to 150 are considered. For the static results, our simulation data are consistent with the scaling law proposed by de Gennes. That is, for thin tube, the length of tube occupied by the chain scales as R∥∼ND−2/3 where D is the diameter of the tube. The nonequilibrium relaxation behavior of the chain, after the constraint is removed, is also studied. The relaxation time τ∥ is derived by scaling analysis as τ∥∼N2D1/3. This result is compared with our Monte Carlo data and is discussed in the light of scaling theories.
The enthalpy state of amorphous alloys in an immiscible system
Authors: He J. H. , Sheng H. W. , Ma E.
Published: July 2002

We performed calorimetric measurements of the enthalpy of formation of amorphous alloys created using vapor quenching in a highly immiscible system, Ag–Ni. The low-enthalpy states observed, as compared with extrapolated liquids and crystalline solid solutions, are in agreement with the findings from molecular dynamics (MD) simulations. The MD results support the proposition that the enthalpy reduction is associated with the ultra-fine-scale phase separation in the liquid/amorphous state.
E×B circulation at the tokamak divertor X point
Authors: Schaffer M. J. , Bray B. D. , Boedo J. A. , Carlstrom T. N. , Colchin R. J. , Hsieh C.-L. , Moyer R. A. , Porter G. D. , Rognlien T. D. , Watkins J. G. ,   DIII-D Team
Published: July 2002

Detailed measurements in two dimensions by probes and Thomson scattering reveal unexpected local electric potential and electron pressure (pe) maxima near the divertor X point in L-mode plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. The potential drives E×B circulation about the X point, thereby exchanging plasma between closed and open magnetic surfaces at rates that can be comparable to the total cross-separatrix transport. The potential is consistent with the classical parallel Ohm’s law. A simple model is proposed to explain the pressure and potential hills in low power, nearly detached plasmas. Recent two-dimensional edge transport modeling with plasma drifts also shows X-point pressure and potential hills but by a different mechanism. These experimental and theoretical results demonstrate that low power tokamak plasmas can be far from poloidal uniformity in a boundary layer just inside the separatrix. Additional data, although preliminary and incomplete, suggest that E×B circulation across the separatrix might be a common feature of low confinement behavior.
Gamma-neutron activation experiments using laser wakefield accelerators
Authors: Leemans W. P. , Rodgers D. , Catravas P. E. , Geddes C. G. R. , Fubiani G. , Esarey E. , Shadwick B. A. , Donahue R. , Smith A.
Published: July 2002

Gamma-neutron activation experiments have been performed with relativistic electron beams produced by a laser wakefield accelerator. The electron beams were produced by tightly focusing (spot diameter ≈6 μm) a high power (up to 10 TW), ultra-short (⩾50 fs) laser beam from a high repetition rate (10 Hz) Ti:sapphire (0.8 μm) laser system, onto a high density (>1019 cm−3) pulsed gasjet of length ≈1.5 mm. Nuclear activation measurements in lead and copper targets indicate the production of electrons with energy in excess of 25 MeV. This result was confirmed by electron distribution measurements using a bending magnet spectrometer. Measured γ-ray and neutron yields are also found to be in reasonable agreement with simulations using a Monte Carlo transport code.
Plasma oscillations in Hall thrusters
Authors: Choueiri E. Y.
Published: July 2002

The nature of oscillations in the 1 kHz–60 MHz frequency range that have been observed during operation of Hall thrusters is quantitatively discussed. Contours of various plasma parameters measured inside the accelerating channel of a typical Hall thruster are used to evaluate the various stability criteria and dispersion relations of oscillations that are suspected to occur. A band by band up-to-date overview of the oscillations is carried out with a description of their observed behavior and a discussion of their nature and dependencies through comparison of the calculated contours to reported observations. The discussion encompasses the excitation of low frequency azimuthal drift waves that can form a rotating spoke, axially propagating “transit-time” oscillations, azimuthal drift waves, ionization instability-type waves, and wave emission peculiar to weakly ionized inhomogeneous plasmas in crossed electric and magnetic fields.
Quiescent double barrier high-confinement mode plasmas in the DIII-D tokamak
Authors: Burrell K. H. , Austin M. E. , Brennan D. P. , DeBoo J. C. , Doyle E. J. , Fenzi C. , Fuchs C. , Gohil P. , Greenfield C. M. , Groebner R. J. , Lao L. L. , Luce T. C. , Makowski M. A. , McKee G. R. , Moyer R. A. , Petty C. C. , Porkolab M. , Rettig C. L. , Rhodes T. L. , Rost J. C. , Stallard B. W. , Strait E. J. , Synakowski E. J. , Wade M. R. , Watkins J. G. , West W. P.
Published: July 2002

High-confinement (H-mode) operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the beta limit and reduced core transport regions needed for advanced tokamak operation. Experimental results from DIII-D [J. L. Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] this year have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation that is ELM-free and yet has good density and impurity control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τE), yielding a quiescent double barrier regime. By slowly ramping the input power, we have achieved βNH89=7 for up to 5 times the τE of 150 ms. The βNH89 values of 7 substantially exceed the value of 4 routinely achieved in the standard ELMing H mode. The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density and impurity control in the quiescent H mode is possible because of the presence of an edge magnetohydrodynamic (MHD) oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.
Domain state model for exchange bias
Authors: Nowak U. , Misra A. , Usadel K. D.
Published: July 2002

Monte Carlo simulations of a system consisting of a ferromagnetic layer exchange coupled to a diluted antiferromagnetic layer described by a classical spin model show a strong dependence of the exchange bias on the degree of dilution in agreement with recent experimental observations on Co/CoO bilayers. These simulations reveal that diluting the antiferromagnet leads to the formation of domains in the volume of the antiferromagnet carrying a remanent surplus magnetization which causes and controls exchange bias. To further support this domain state model for exchange bias we study, in the present article, the dependence of the bias field on the thickness of the antiferromagnetic layer. It is shown that the bias field strongly increases with increasing film thickness and eventually goes over a maximum before it levels out for large thicknesses. These findings are in full agreement with experiments.
Reaction pathway, energy barrier, and rotational state distribution for Li (2 2PJ)+H2→LiH (X 1Σ+)+H
Authors: Chen Jye-Jong , Hung Yu-Ming , Liu Dean-Kuo , Fung Hok-Sum , Lin King-Chuen
Published: July 2002

By using a pump-probe technique, we have observed the nascent rotational population distribution of LiH (v=0) in the Li (2 2PJ) with a H2 reaction, which is endothermic by 1680 cm−1. The LiH (v=0) distribution yields a single rotational temperature at ∼770 K, but the population in the v=1 level is not detectable. According to the potential energy surface (PES) calculations, the insertion mechanism in (near) C2v collision geometry is favored. The Li (2 2PJ)–H2 collision is initially along the 2A′ surface in the entrance channel and then diabatically couples to the ground 1A′ surface, from which the products are formed. From the temperature dependence measurement, the activation energy is evaluated to be 1280±46 cm−1, indicating that the energy required for the occurrence of the reaction is approximately the endothermicity. As Li is excited to higher states (3 2S or 3 2P), we cannot detect any LiH product. From a theoretical point of view, the 4A′ surface, correlating with the Li 3 2S state, may feasibly couple to a repulsive 3A′ surface, from which the collision complex will rapidly break apart into Li (2 2PJ) and H2. The probability for further surface hopping to the 2A′ or 1A′ surfaces is negligible, since the 3A′ and 2A′ surfaces are too far separated to allow for an efficient coupling. The Li (3 2P) state is expected to behave similarly. The observation also provides indirect evidence that the harpoon mechanism is not applicable to this system.
Direct three-dimensional patterning using nanoimprint lithography
Authors: Li Mingtao , Chen Lei , Chou Stephen Y.
Published: July 2002

We demonstrated that nanoimprint lithography (NIL) can create three-dimensional patterns, sub-40 nm T-gates, and air-bridge structures, in a single step imprint in polymer and metal by lift-off. A method based on electron beam lithography and reactive ion etching was developed to fabricate NIL molds with three-dimensional protrusions. The low-cost and high-throughput nanoimprint lithography for three-dimensional nanostructures has many significant applications such as monolithic microwave integrated circuits and nanoelectromechanical system.
Sigma model approach to string theory effective actions with tachyons
Authors: Tseytlin A. A.
Published: July 2002

Motivated by recent discussions of actions for tachyon and vector fields related to tachyon condensation in open string theory we review and clarify some aspects of their derivation within the sigma model approach. In particular, we demonstrate that the renormalized partition function Z(T,A) of the boundary sigma model gives the effective action for massless vectors which is consistent with the string S-matrix and beta function, resolving an old problem with this suggestion in the bosonic string case at the level of the leading F2(dF)2 derivative corrections to Born–Infeld action. We give a manifestly gauge invariant definition of Z(T,A) in non-Abelian NSR open string theory and check that its derivative reproduces the tachyon beta function in a particular scheme. We also discuss the derivation of similar actions for tachyon and massless modes in closed bosonic and NSR (type 0) string theories. In the bosonic case the tachyon potential has the structure −T2e−T, but it vanishes in the NSR string case.
High density gas jet nozzle design for laser target production
Authors: Semushin S. , Malka V.
Published: July 2002

The optimization of a cylindrical nozzle design to generate a uniform density profile for laser plasmas studies has been investigated using numerical simulations. In addition gas jet flows have been characterized using a Mach–Zehnder interferometer. The experimental results are in very good agreement with the simulation.
Ionic contribution to the dielectric properties of a nematic liquid crystal in thin cells
Authors: Dhara Surajit , Madhusudana N. V.
Published: July 2002

We report measurements on dielectric properties of a nematic liquid crystal in planar aligned cells with two different thicknesses (1.2 and 6.7 μm). The nematic has a wide thermal range and the temperature variation of the effective dielectric constant (ε⊥) of the thick cell reflects that of the orientational order parameter. On the other hand, the thin cell exhibits a higher value of ε⊥ which is essentially temperature independent. We have conducted detailed dielectric measurements in the frequency range of 10 Hz–1.1 kHz on both the cells and analyzed the data using a simplified model which takes into account the ionic contribution. It leads to an estimation of the temperature dependencies of the number density and diffusion coefficient of the impurity ions.
The “injection problem” for quasiparallel shocks
Authors: Zank G. P. , Rice W. K. M. , le Roux J. A. , Cairns I. H. , Webb G. M.
Published: July 2002

For a particle to be accelerated diffusively at a shock by the first-order Fermi acceleration mechanism, the particle must be sufficiently energetic that it can scatter across all the micro- and macrostructure of the shock, experiencing compression between the converging upstream and downstream states. This is the well-known “injection problem.” Here the interaction of ions with the ramp of a quasiparallel shock is investigated. Some ions incident on the shock experience specular reflection, caused either by the cross-shock electrostatic potential or by mirroring as the magnetic field is bent and compressed through the ramp. Scattering of reflected ions by self-generated and pre-existing turbulence in the region upstream of the shock then acts to trap backstreaming ions and return them to the ramp, where some experience further reflections. Such repeated reflections and scattering energize a subpopulation of ions up to energies sufficiently large that they can be diffusively shock accelerated. Two ion distributions are considered: pickup ions which are assumed to be described by a shell distribution, are thermal solar wind ions which may be described by a kappa distribution. Injection efficiencies are found analytically to be very high for pickup ions and much lower for thermal solar wind ions, suggesting that this injection mechanism, stochastic reflected ion or SRI acceleration, is a natural precursor for the acceleration of the anomalous cosmic ray component at a quasiparallel shock. While significantly less efficient, SRI acceleration is also viable for thermal solar wind ions described by a kappa distribution.
Ferroelectricity in systems of induced and permanent dipoles: A Monte Carlo study
Authors: Farag N. , Kliem H.
Published: July 2002

Dielectric and ferroelectric properties in systems of permanent and induced dipoles are calculated with a Monte Carlo method, the importance-sampling method. The systems are dipole chains and dipole planes, which can be looked at as subsystems of a perovskite structure. Depending on the system and the parameters we find spontaneous polarizations, ferroelectric domains, temperature-dependent hysteresis loops of the polarization, and a Curie–Weiss law for the susceptibility. These properties are mainly due to the cooperation of induced and permanent dipoles. The most difficult problem is the calculation of the spontaneous polarization, i.e., the calculation of the polarization without an applied field. In this case, a high iteration number is required.
On the isothermal-isobaric ensemble partition function
Authors: Han Kyu-Kwang , Son Hyeon S.
Published: July 2002

We briefly review the isothermal-isobaric ensemble partition function correctly derived by Corti and Soto-Campos [J. Chem. Phys. 108, 7959 (1998)] from the view point of usual simulations in which the external bath of a system is appropriately approximated. We show that, for the homogeneous system, the partition function can be reduced to the result of Attard [J. Chem. Phys. 103, 9884 (1995)] which was criticized by Corti and Soto-Campos. We conclude that the volume scale cannot be defined in general but in some particular cases and the recently controverted volume scales are those of the cases. We also confirm the validity of Brown’s [Mol. Phys. 1, 68 (1958)] partition function which has never been used in simulations before, by demonstrating that it is equivalent to the one of Corti and Soto-Campos.
Comparison of light- and heavy-ion-irradiated quantum-wells for use as ultrafast saturable absorbers
Authors: Mangeney J. , Choumane H. , Patriarche G. , Leroux G. , Aubin G. , Harmand J. C. , Oudar J. L. , Bernas H.
Published: July 2002

We have compared light- and heavy-ion irradiation of InGaAs/InAlAs multiple-quantum wells for ultrafast saturable absorption applications. Under heavy-ion impacts, defect clusters were produced, as observed via transmission electronic microscopy. By contrast, in proton-irradiated samples, only point defects were formed. Nonlinear absorption measurements were performed with excitonic resonance pumping. The relaxation time of absorption saturation (minimum value 2 ps) did not depend on the irradiating ion, and was practically independent of the pulse repetition rate (up to 10 GHz) and optical excitation fluence (0.1 mJ/cm2). We conclude that irradiating multiple-quantum wells with light ions is as effective as using heavy ions, when fabricating ultrafast saturable absorber devices operating at high bit rate and near bandedge wavelength.
The pair density functional of the kinetic energy and its simple scaling property
Authors: Levy Mel , Ziesche Paul
Published: July 2002

For electronic systems, a simple property of the recently introduced kinetic energy T as a functional of the pair density n(r1,r2)is derived. Approximate explicit expressions for T[n] are presented.
Structure and Mössbauer spectra for the Fe–Cr system: From bulk alloy to nanoparticles
Authors: Petrov Yu. I. , Shafranovsky E. A. , Krupyanskii Yu. F. , Essine S. V.
Published: July 2002

The structure and hyperfine fields of Fe1−xCrx (x=0.0236–0.803) nanoparticles (average size of 27±2 nm) are studied at room temperature by combined x-ray diffraction and Mössbauer spectroscopy techniques. They are produced by fast evaporation of bulk alloys at 3 Torr Ar pressure. The bulk alloys of any composition are shown to exhibit a bcc structure, whereas the nanoparticles demonstrate a mixture of bcc and tetragonal σ phases in the Cr range from 24.4 to 83.03 at. %. At the Cr content of 2.36 at. % the lattice constant for nanoparticles is larger than that of the bulk alloy, though the values of hyperfine fields on Fe nuclei do not differ. The Mössbauer spectrum of nanoparticles contains an oxide doublet in addition to the sextet specific to that of the bulk alloy. In both cases the width of the sextet lines is rather narrow. However, even at ∼8 at. % Cr the lines of the sextet are broadened so much that it can be decomposed by two-three components. This is explained by freezing the high-temperature ferromagnetic fcc phase regions in the bcc lattice. As the Cr content increases, the Mössbauer spectra become more complex, transforming finally into a paramagnetic singlet. A complete ferromagnetic→paramagnetic transition is observed for the bulk alloy at 68 at. % Cr and for nanoparticles at 35 at. % Cr. The results are discussed under the assumption that at high temperatures the alloys are not homogeneous and exhibit fluctuations of the composition. With decrease of temperature these fluctuations result in decomposition of the alloy into two phases for nanoparticles whereas they are frozen at the cluster level in the bulk alloys holding a macroscopic homogeneity.
Frequency stabilization of an internal mirror He–Ne laser by digital control
Authors: Eom TaeBong , Choi HyunSeung , Lee SunKyu
Published: July 2002

A simple digital control system has been developed for the frequency stabilization of an internal mirror He–Ne laser. The system is based on a microcontroller with embedded BASIC interpreter and various functions such as analog and digital input/output. The new control system is very compact and totally digital. The output frequencies of two lasers have been stabilized using two-mode power-balance and beat frequency offset locking techniques in a transverse magnetic field. Their frequency stabilities for 1 s average are 7.2×10−11 and 4.0×10−10, respectively.
New criteria for cluster identification in continuum systems
Authors: Pugnaloni Luis A. , Vericat Fernando
Published: July 2002

Two new criteria, that involve the microscopic dynamics of the system, are proposed for the identification of clusters in continuum systems. The first one considers a residence time in the definition of the bond between pairs of particles, whereas the second one uses a life time in the definition of an aggregate. Because of the qualitative features of the clusters yielded by the criteria we call them chemical and physical clusters, respectively. Molecular dynamics results for a Lennard-Jones system and general connectivity theories are presented.
Influence of Co60 γ-rays on dc performance of AlGaN/GaN high electron mobility transistors
Authors: Luo B. , Johnson J. W. , Ren F. , Allums K. K. , Abernathy C. R. , Pearton S. J. , Dabiran A. M. , Wowchack A. M. , Polley C. J. , Chow P. P. , Schoenfeld D. , Baca A. G.
Published: July 2002

AlGaN/GaN high electron mobility transistors (HEMTs) with different gate length and widths were irradiated with Co60 γ-rays to doses up to 600 Mrad. Little measurable change in dc performance of the devices was observed for doses lower than 300 Mrad. At the maximum dose employed, the forward gate current was significantly decreased, with an accompanying increase in reverse breakdown voltage. This is consistent with a decrease in effective carrier density in the channel as a result of the introduction of deep electron trapping states. The threshold voltage shifted to more negative voltages as a result of the irradiation, while the magnitude of the drain–source current was relatively unaffected. This is consistent with a strong increase of trap density in the material. The magnitude of the decrease in transconductance of the AlGaN/GaN HEMTs is roughly comparable to the decrease in dc current gain observed in InGaP/GaAs heterojunction bipolar transistors irradiated under similar conditions.
Effect of interface structure on the optical properties of InAs/GaSb laser active regions
Authors: Lau Wayne H. , Flatté Michael E.
Published: July 2002

We present calculations of the fundamental band gap and intervalence absorption in InAs/GaSb materials incorporating both the intrinsic atomistic symmetry of interface bonding and typical compositional gradients near the interfaces. Including these effects quantitatively explains experimentally observed systematic trends in the band gaps of InAs/GaSb superlattices. Calculations of intervalence absorption indicate that the internal loss in laser active regions based on these materials can not be predicted quantitatively without including these effects.
Photoinduced stress in hydrogenated amorphous silicon films
Authors: Stratakis E. , Spanakis E. , Tzanetakis P. , Fritzsche H. , Guha S. , Yang J.
Published: July 2002

Photo-induced compressive stress ΔS in hydrogenated amorphous silicon (a-Si:H) has been studied using films deposited by plasma-enhanced or hot-wire chemical vapor deposition on crystalline silicon microcantilevers. The kinetics of ΔS(t) first rises with exposure time as t0.5 and follows a stretched exponential. The saturation values ΔSsat correspond to volume changes of about 10−3, which excludes the possibility that ΔS is a consequence of the light-induced creation of coordination defects. The highest-quality films have large initial stress, small values of the Young’s modulus, and a rapid approach of ΔS(t) towards saturation.