bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron mobility characteristics of n-channel metal-oxide-semiconductor field-effect transistors fabricated on Ge-rich single- and dual-channel SiGe heterostructures
Authors: Lee Minjoo L. , Fitzgerald Eugene A.
Published: January 2004

Strained Si (ε-Si) grown on Si-rich relaxed Si1−xGex buffers (single-channel heterostructures) can be used to fabricate n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) with enhanced performance over bulk Si. However, single-channel heterostructures grown on Ge-rich Si1−xGex buffers (i.e., x⩾0.5) exhibit much larger hole mobility enhancements than those on Si-rich buffers, and the highest hole mobilities have been attained in heterostructures where a compressively strained Ge (ε-Ge) layer is grown beneath the ε-Si cap (ε-Si/ε-Ge dual-channel heterostructures). In this article, we report on n-MOSFET mobility characteristics in single- and dual-channel heterostructures grown on Ge-rich Si1−xGex buffers. Single-channel n-MOSFETs were fabricated on virtual substrates with Ge contents as high as 70%, and electron mobility enhancements of 1.4–1.6 were observed. For dual-channel heterostructures, electron mobility enhancements of 1.7–1.9 were attained when the ε-Si cap was thick enough to confine electrons. Despite the high intrinsic electron mobility of bulk Ge, dual-channel n-MOSFETs with extremely thin Si caps (∼3 nm) exhibited mobility significantly below that of bulk Si. We speculate that the low extracted mobility in such heterostructures results from the difference in conduction band minima between Ge and Si.
Microstructure, surface morphology, and mechanical properties of nanocrystalline TiN/amorphous Si3N4 composite films synthesized by ion beam assisted deposition
Authors: Zhang C. H. , Liu Z.-J. , Li K. Y. , Shen Y. G. , Luo J. B.
Published: January 2004

Two-phase nanocrystalline TiN/amorphous Si3N4 composite films were synthesized at room temperature by ion beam assisted deposition. Film bonding states and microstructure were investigated by x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy. Surface morphology was measured by atomic force microscopy and quantitatively analyzed using height–height correlation function. The mechanical properties were evaluated from nanoindentation measurements. The films consisted of nanometer-sized TiN crystallites embedded in an amorphous Si3N4 matrix. A maximum hardness of about 42 GPa was observed in a film with silicon content of 11.4 at. %. The improved mechanical properties of Ti–Si–N films with the addition of Si into TiN were attributed to their densified microstructure with development of fine grain size and reduced surface roughness. The combination of hardness and grain size was correlated with film phase composition and structural analyses. The results have been interpreted with the help of a Monte Carlo Potts model simulation.
Ground-state interband transition of individual self-assembled InAs/Al0.6Ga0.4As quantum dots observed by scanning-tunneling-microscope light-emission spectroscopy
Authors: Tsuruoka T. , Ohizumi Y. , Ushioda S.
Published: January 2004

We have investigated the optical transitions in individual self-assembled InAs/Al0.6Ga0.4As quantum dots (QDs) by means of scanning-tunneling-microscope (STM) light-emission spectroscopy. Localized bright features were observed in the spectrally resolved light intensity images measured by injecting electrons from the STM tip. The light emission spectra measured over the bright features showed single emission peaks having different peak energies with linewidths of 30–45 meV. By comparing these results with atomic-force-microscope images and photoluminescence (PL) spectra, we have identified the bright features with the ground-state interband transition from individual InAs QDs. The emission peak energies were compared with the transition energies calculated for pyramidal-shaped QD structures, based on a single-band and constant-confining-potential model. A reasonable agreement was obtained between the experimental and calculated results. The emission linewidth of individual dots is much narrower than the linewidth (∼200 meV) measured by PL from an ensemble of dots. This fact demonstrates the importance of measuring the linewidth for individual dots rather than for an ensemble of dots with size distribution.
Hybrid nanocrystalline TiO2 solar cells with a fluorene–thiophene copolymer as a sensitizer and hole conductor
Authors: Ravirajan P. , Haque S. A. , Durrant J. R. , Poplavskyy D. , Bradley D. D. C. , Nelson J.
Published: January 2004

We report the effects of layer thickness, interface morphology, top contact, and polymer–metal combination on the performance of photovoltaic devices consisting of a fluorene–bithiophene copolymer and nanocrystalline TiO2. Efficient photoinduced charge transfer is observed in this system, while charge recombination is relatively slow (∼100 μs–10 ms). External quantum efficiencies of 13% and monochromatic power conversion efficiencies of 1.4% at a wavelength of 440 nm are achieved in the best device reported here. The device produced an open-circuit voltage of 0.92 V, short-circuit current density of about 400 μA cm−2, and a fill factor of 0.44 under simulated air mass 1.5 illumination. We find that the short-circuit current density and the fill factor increase with decreasing polymer thickness. We propose that the performance of the indium tin oxide/TiO2/polymer/metal devices is limited by the energy step at the polymer/metal interface and we investigate this situation using an alternative fluorene-based polymer and different top contact metals.
Improvement of dielectric loss tangent of Al2O3 doped Ba0.5Sr0.5TiO3 thin films for tunable microwave devices
Authors: Chong K. B. , Kong L. B. , Chen Linfeng , Yan L. , Tan C. Y. , Yang T. , Ong C. K. , Osipowicz T.
Published: January 2004

Al 2 O 3 doped Ba0.5Sr0.5TiO3 (BST) thin films, with different Al2O3 contents, were deposited on (100) LaAlO3 substrates by the pulsed laser deposition technique to develop agile thin films for tunable microwave device applications. The dielectric properties of Al2O3 doped BST films were determined with a nondestructive dual resonator near 7.7 GHz. We demonstrated that the Al2O3 doping plays a significant role in improving the dielectric properties of BST thin films. The Al2O3 doping successfully reduced the dielectric loss tangent (tan δ) from 0.03 (pure BST) to 0.011 (Al2O3 doped BST). Reduction in the loss tangent also leads to reduction in the dielectric constant and dielectric tunability. Our results showed that the BSTA4 film remains tunability=15.9%, which is sufficient for tunable microwave devices applications. Consequently, the Al2O3 doping improved the figure of merit (K) for the BST films from K=7.33 (pure BST) to K=14.45 (Al2O3 doped BST).
Optical thin-film interference effects in microcantilevers
Authors: Wig A. , Passian A. , Arakawa E. , Ferrell T. L. , Thundat T.
Published: January 2004

We report direct observation of thin-film interference effects in microcantilevers, an effect that can impact the optical monitoring of the microcantilever motion. When microcantilevers are illuminated with different wavelengths of light the amount of absorption and the wavelengths of maxima in the absorption depend upon the thickness of the layers, the materials used in the layers, and the direction of illumination. Wavelengths of maximum absorption are observed as microcantilever deflections due to heat-induced bending of the bimaterial structure of the microcantilever. Results are presented for different multilayer configurations and illumination directions. These results are then compared with theoretical calculations based on multilayer thin-film analysis.
Enhancement factor of open thick-wall carbon nanotubes
Authors: Kokkorakis G. C. , Roumeliotis J. A. , Xanthakis J. P.
Published: January 2004

We have calculated the electric field around and on the surface of an open thick-wall carbon nanotube (CNT) of height h, external radius R, and wall thickness w. To accomplish that we simulate the CNT as a vertical array of touching toroids, each of external radius R and cross section radius w/2, and then we express the problem in toroidal coordinates. From our calculations we obtain the enhancement factor γ as a function of h, R, and w. By fitting to our numerical results we obtain an empirical but simple formula for γ, which extrapolates to that of a closed CNT in the limiting case of w=R.
Capacitance variation of an assembly of clusters in the Coulomb blockade regime
Authors: Carrey J. , Seneor P. , Lidgi N. , Jaffrès H. , Nguyen Van Dau F. , Fert A. , Friederich A. , Montaigne F. , Vaurès A.
Published: January 2004

We fabricated tunnel junctions containing clusters embedded in a thin insulating layer. Low-temperature dc transport measurements reveal typical Coulomb blockade features. We measured the differential capacitance at different dc voltages and frequencies of these samples in the Coulomb blockade regime. The differential capacitance is constant at low dc bias and increases by 35% above a threshold voltage. This behavior can be well understood using a simple model which takes into account the Coulomb blockade effect. Such measurements could be used as a probe of Coulomb blockade properties of clusters assemblies. This effect is also promising for the fabrication of capacitors whose capacitance can be changed by varying the applied voltage.
Effect of near-surface band bending on dopant profiles in ion-implanted silicon
Authors: Jung Michael Y. L. , Gunawan Rudiyanto , Braatz Richard D. , Seebauer E. G.
Published: January 2004

Recent experimental work has demonstrated the existence of band bending at the Si–SiO2 interface after ion implantation. The present work employs FLOOPS-based numerical simulations to investigate the effects this bending can have upon dopant profiles that evolve during transient enhanced diffusion in post-implant annealing. In the case of boron, band bending induces significant junction deepening because the near-interface electric field repels charged interstitials from the interface. Band bending also provides a mechanism to explain the pile-up of electrically active boron within ∼1 nm of the interface. The results suggest that conflicting literature regarding the capacity of the interface to absorb interstitials can be rationalized by a modest inherent absorbing capability coupled with band bending.
Global eigenmodes of low frequency waves in field-reversed configuration plasmas
Authors: Iwasawa Naotaka , Okada Shigefumi , Goto Seiichi
Published: January 2004

Global eigenmodes of low frequency waves in field reversed configuration (FRC) plasmas have been obtained using the magnetohydrodynamic (MHD) model and one-dimensional equilibrium model. Dispersion relation and radial structure of the global wave fields are shown for the azimuthal mode number m=0. The results are compared with the results of a low frequency wave heating experiment. Possibilities of ion heating by the ion cyclotron damping, the transit-time magnetic damping, and Landau damping are discussed.
Envelope equations and conservation laws describing wakefield generation and electron acceleration
Authors: Cairns R. A. , Reitsma A. , Bingham R.
Published: January 2004

Previous authors have proposed various envelope equations to describe the behavior of an electromagnetic pulse generating a wakefield. In general these retain second-order derivatives, the reason being that the eikonal contains the initial wave frequency. Here it is shown that if the evolution of the wave frequency is followed using ray-tracing equations, a first-order evolution equation is obtained. It can be shown with this formalism that wave action is conserved and the energy lost from the electromagnetic wave can be explicitly accounted for in terms of energy gained by the plasma. The energy balance equations suggest that an electron bunch which will extract energy efficiently from a wakefield can be at least as efficiently accelerated by direct interaction with the electromagnetic pulse.
Low-glancing-angle x-ray diffraction study on the relationship between crystallinity and properties of C60 field effect transistor
Authors: Ohashi Hirotaka , Tanigaki Katsumi , Kumashiro Ryotaro , Sugihara Syuji , Hiroshiba Shinya , Kimura Shigeru , Kato Kenichi , Takata Masaki
Published: January 2004

The structures of C60 thin films grown on Si, SiO2, and Al2O3 are studied by a low-glancing-angle x-ray diffraction method and the properties of C60 field effect transistors (FETs) are understood on a basis of this technique. Analyses using the two-dimensional Ewald circle and the Laue function give the detailed interpretation of the diffraction peaks together with information of the preferred orientation and the grain size of polycrystalline thin films. The grain size varies from 100 to 500 Å upon the optimal conditions of the thin film growth and the preferred orientation of the C60 〈111〉 crystal axis with random on a substrate surface is obtained. It is clearly shown that these variations in quality of the C60 thin films exert unexpectedly little influence on the FET characteristics.
Well-width dependence of photoluminescence emission from a-plane GaN/AlGaN multiple quantum wells
Authors: Craven M. D. , Waltereit P. , Speck J. S. , DenBaars S. P.
Published: January 2004

This work investigates the room-temperature photoluminescence (PL) characteristics of nonpolar GaN/(∼100 Å Al0.16Ga0.84N) multiple quantum wells (MQWs) in comparison to c-plane structures as a function of GaN quantum well width. 10-period a-plane and c-plane MQW structures were simultaneously regrown on the appropriate GaN/sapphire template layers via metalorganic chemical vapor deposition with well widths ranging from 20 to 70 Å. The PL emission energy from a-plane MQWs followed a square well trend modeled using self-consistent Poisson–Schrödinger calculations while the c-plane MQW emission showed a significant redshift with increasing well width which is attributed to the quantum-confined Stark effect. Despite a higher dislocation density, the a-plane MQWs exhibit enhanced recombination efficiency as compared to the c-plane wells since well emission is no longer observed for c-plane wells wider than 50 Å. Optimal PL emission intensity was obtained for 52 Å a-plane wells compared to 28 Å c-plane wells, revealing the effects of internal fields on quantum well emission.
Structure of domains in an exchange-bias model
Authors: Misra A. , Nowak U. , Usadel K. D.
Published: January 2004

The structure of domains in the interface monolayer of the antiferromagnet in an exchange-bias system is investigated in the framework of the domain state model. These interface domains carrying remanent magnetization provide the bias field and are strongly influenced by the bulk. The stable part of the spin configurations at the interface, which is responsible for exchange bias, is identified. The stability analysis of the interface domains leads to an explanation of the nontrivial dependence of the bias field on thickness and anisotropy of the antiferromagnet.
Experimental and theoretical investigation of carrier confinement in InAs quantum dashes grown on InP(001)
Authors: Miska P. , Even J. , Platz C. , Salem B. , Benyattou T. , Bru-Chevalier C. , Guillot G. , Bremond G. , Moumanis Kh. , Julien F. H. , Marty O. , Monat C. , Gendry M.
Published: January 2004

Carrier confinement in InAs quantum dashes (QDas) grown on InP(001) is investigated both experimentally and theoretically. The aim of these studies is to reconstruct the electronic structure of the QDas. QDas with low size dispersion are achieved by improving growth conditions. Optical transitions between ground and excited states are studied by continuous-wave-photoluminescence and photoluminescence-excitation experiments at low temperature. We also report on infrared spectroscopy of conduction-band intersubband transitions. A simplified theoretical model is developed, yielding results consistent with the experimental data. Combining experimental and theoretical results, we propose an interpretation of the optical transitions occurring in these QDas, and we give a first theoretical absorption spectrum of these structures.
On the negative Hubbard correlation energy of the DX center in In-doped CdMnTe
Authors: Placzek-Popko E. , Szatkowski J. , Becla P.
Published: January 2004

Persistent photoeffects have been investigated in indium doped Cd1−xMnxTe of manganese content x=0.1, by means of photocapacitance and photoconductivity transient measurements run at 77 K. The transients are superpositions of two exponents with short and long time constants. The two exponents have been attributed to the two-stage ionization of two energy states of the DX centers present in the material. A detailed analysis of photoionization kinetics leads to the conclusion that the DX center which is responsible for the observed persistent photoeffects possesses negative effective Hubbard correlation energy. Thus the “fast” component of the phototransients corresponds to the ionization of the two-electron ground state of the DX center to an intermediate one-electron state. The “slow” component is a result of the photoionization of the electron from this state into the conduction band. The value of the optical ionization energy for the first transition is equal to E20=0.85 eV. Photoionization of an electron from the intermediate state to the conduction band requires energy E10=0.77 eV. It was also found that both states are localized and strongly coupled to the lattice as large numbers of phonons take part in the photoionization in both cases.
Path integral hybrid Monte Carlo algorithm for correlated Bose fluids
Authors: Miura Shinichi , Tanaka Junji
Published: January 2004

Path integral hybrid Monte Carlo (PIHMC) algorithm for strongly correlated Bose fluids has been developed. This is an extended version of our previous method [S. Miura and S. Okazaki, Chem. Phys. Lett. 308, 115 (1999)] applied to a model system consisting of noninteracting bosons. Our PIHMC method for the correlated Bose fluids is constituted of two trial moves to sample path-variables describing system coordinates along imaginary time and a permutation of particle labels giving a boundary condition with respect to imaginary time. The path-variables for a given permutation are generated by a hybrid Monte Carlo method based on path integral molecular dynamics techniques. Equations of motion for the path-variables are formulated on the basis of a collective coordinate representation of the path, staging variables, to enhance the sampling efficiency. The permutation sampling to satisfy Bose–Einstein statistics is performed using the multilevel Metropolis method developed by Ceperley and Pollock [Phys. Rev. Lett. 56, 351 (1986)]. Our PIHMC method has successfully been applied to liquid helium-4 at a state point where the system is in a superfluid phase. Parameters determining the sampling efficiency are optimized in such a way that correlation among successive PIHMC steps is minimized.
Mode- and plasma rotation in a resistive shell reversed-field pinch
Authors: Malmberg J.-A. , Brzozowski J. , Brunsell P. R. , Cecconello M. , Drake J. R.
Published: January 2004

Mode rotation studies in a resistive shell reversed-field pinch, EXTRAP T2R [P. R. Brunsell et al., Plasma Phys. Control. Fusion 43, 1 (2001)] are presented. The phase relations and nonlinear coupling of the resonant modes are characterized and compared with that expected from modeling based on the hypothesis that mode dynamics can be described by a quasi stationary force balance including electromagnetic and viscous forces. Both m=0 and m=1 resonant modes are studied. The m=1 modes have rotation velocities corresponding to the plasma flow velocity (20–60 km/s) in the core region. The rotation velocity decreases towards the end of the discharge, although the plasma flow velocity does not decrease. A rotating phase locked m=1 structure is observed with a velocity of about 60 km/s. The m=0 modes accelerate throughout the discharges and reach velocities as high as 150–250 km/s. The observed m=0 phase locking is consistent with theory for certain conditions, but there are several conditions when the dynamics are not described. This is not unexpected because the assumption of quasi stationarity for the mode spectra is not fulfilled for many conditions. Localized m=0 perturbations are formed in correlation with highly transient discrete dynamo events. These perturbations form at the location of the m=1 phase locked structure, but rotate with a different velocity as they spread out in the toroidal direction.
Off-axis electron cyclotron heating and the sandpile paradigm for transport in tokamak plasmas
Authors: March T. K. , Chapman S. C. , Dendy R. O. , Merrifield J. A.
Published: January 2004

Previous observations that suggest a substantial role for nondiffusive energy transport in tokamaks subjected to off-axis electron cyclotron heating (ECH) are compared to the output from a sandpile model. The observations considered include local and global aspects of temperature profile evolution in the DIII-D [for example, C. C. Petty and T. C. Luce, Nucl. Fusion 34, 121 (1994)] and RTP (Rijnhuizen Tokamak Project) [for example, M. R. de Baar, M. N. A. Beurskens, G. M. D. Hogeweij, and N. J. Lopes Cardozo, Phys. Plasmas 6, 4645 (1999)] tokamaks. The sandpile model employed is an extension, to incorporate noncentral fueling, of one used previously to address tokamak physics issues [S. C. Chapman, R. O. Dendy, and B. Hnat, Phys. Rev. Lett. 86, 2814 (2001)]. It is found that there are significant points of resemblance between the phenomenology of the noncentrally fueled sandpile and of the tokamaks with off-axis ECH. This suggests that the essential ingredient of the sandpile model, namely avalanching conditioned by a local critical gradient, may be one of the key transport effects generated by the tokamak plasma physics.
Comment on “Ion–acoustic waves in dusty plasma with charge fluctuations” [Phys. Plasmas 9, 1464 (2002)]
Authors: Lee Hee J.
Published: January 2004

It is shown that the damping rate of the ion-acoustic wave in a dusty plasma due to the grain charge fluctuations obtained by the authors of a recent work [Vranjes et al., Phys. Plasmas 9, 1464 (2002)] is based upon deficient equations. The result appears to be incorrect.
Improvement of pin-type amorphous silicon solar cell performance by employing double silicon-carbide p -layer structure
Authors: Myong Seung Yeop , Kim Sang Soo , Lim Koeng Su
Published: January 2004

We investigated a double silicon-carbide p-layer structure consisting of a undiluted p-type amorphous silicon-carbide (p-a-SiC:H) window layer and a hydrogen diluted p-a-SiC:H buffer layer to improve a pin-type amorphous silicon based solar cell. Solar cells using a lightly boron-doped (1000 ppm) buffer layer with a high conductivity, low absorption, well-ordered film structure, and slow deposition rate improves the open-circuit voltage (Voc), short-circuit current density, and fill factor by reducing recombination in the buffer layer and at p/buffer and buffer/i interfaces. It is found that a natural hydrogen treatment generated throughout the buffer layer deposition onto the p-a-SiC:H window layer is an advantage of this double p-layer structure. We achieved a considerable initial conversion efficiency of 11.2% without any back reflector.
Large cyclic magnetic-field-induced deformation in orthorhombic (14M) Ni–Mn–Ga martensite
Authors: Müllner P. , Chernenko V. A. , Kostorz G.
Published: January 2004

Magnetomechanical experiments were performed with a ferromagnetic Ni–Mn–Ga single crystal consisting of thermoelastic orthorhombic martensitic phase at room temperature. The crystal was deformed in uniaxial compression along 〈100〉 with an orthogonal magnetic field and without a magnetic field. The sample deforms due to motion of twin boundaries. When compressed with a magnetic field, twinning occurs at higher stress than without a magnetic field, and the twinning is reversible upon unloading. The stress–strain curves exhibit two plateaus which are related to two different twinning systems, namely (110)[11̄0] and (011)[011̄]. During cyclic experiments in a rotating magnetic field, the magnetic-field-induced strain increases from initially 6% to 9.7%. The latter value was repeatedly measured upon more than 1000 rotations of the field. The increase of magnetic-field-induced strain during magnetomechanical cycling is related to a transition from combined partial (110)[11̄0] and (011)[011̄] twinning to complete (101)[101̄] twinning. The magnetic anisotropy constants are obtained from an analysis of the magnetomechanical properties.
Micromagnetic simulation of the coercivity mechanism in Sm(Co,Fe,Cu,Zr)z magnets
Authors: Rong Chuan-bing , Zhang Hong-wei , Zhang Jian , Du Xiao-bo , Zhang Shao-ying , Shen Bao-gen
Published: January 2004

We developed a finite-element micromagnetic algorithm to study the magnetic properties of a Sm(Co,Fe,Cu,Zr)z magnet. The cellular microstructure, consisting of a 2:17-type matrix and 1:5-type cell boundaries, is built according to the experimental results. Calculated results show a strong influence of microstructural parameters on the magnetic properties. The coercivity increases with an increase in the 2:17-type cell size. A thick cell boundary gives rise to small coercivity. The dependence of the coercivity on the magnetic parameters of the boundary phase is also discussed. The coercivity mechanism is analyzed by comparing the calculated results with the experimental ones.
Stimulated Raman scattering of the beat wave of counter-propagating lasers
Authors: Salih H. A. , Tripathi V. K. , Rafat M.
Published: January 2004

Two counter-propagating lasers, with frequencies ω1,ω2, and wave numbers k1,k2 drive, in an underdense plasma of plasma frequency ωp, a nonresonant space charge wave at frequency difference ω0⩾2ωp and wave number k0≈k1+k2. The driven wave undergoes stimulated Raman scattering producing a plasma wave and a scattered electromagnetic sideband wave (ωs,k⃑s). The growth rate of the process, γ, is comparable to two plasmon decay and maximizes for side scattering at θs=π/2, i.e., when k⃗s⊥k⃑0.
Observation of ion wave streamers and low frequency sheath instability by the resonant absorption due to nonlinear interaction of microwave-plasma
Authors: Kamal-Al-Hassan Md. , Starodubtsev Mikhail , Ito Hiroaki , Yugami Noboru , Nishida Yasushi
Published: January 2004

Unmagnetized, inhomogeneous laboratory plasma irradiated by an oblique p-polarized microwave with pulse length 0.2–1.5 μs and power P=1–2 kW is studied. The incident electromagnetic wave is linearly converted into an electrostatic plasma wave when the incident wave frequency ω0 is equal to the local plasma frequency ωp. The localized linear enhancement of the driven oscillating field can lead to nonlinear phenomena driven by the ponderomotive force, which expels electrons from the resonance region, and the resulting ambipolar electrostatic fields also expel the ions, creating density cavities at the resonance region. Expelled ions tend to form an ion bunch and accelerate up to energies greater than 10 kTe. After all these processes are achieved, it has been observed in the experiment that the density cavity develops as ion wave streamers and propagate both up and down the density gradient from the resonant layer. It is observed that the downward streamer velocity Vdown and upward streamer velocity Vup have the relation as Vdown>Cs>Vup. Another physical phenomenon, called the low frequency sheath instability, in the plasma sheath area created by the accelerated ion bunch near the resonant region, is also observed in the experiment.
Plasma flow and confinement in the vicinity of a rotating island in collisional tokamak plasmas
Authors: Shaing K. C.
Published: January 2004

The theory for the electric field, plasma flows, and plasma confinement in the vicinity of a rotating magnetic island in tokamaks [Phys. Plasmas 9, 3470 (2002)] is extended to the collisional plasmas, i.e., the plateau-Pfirsch–Schluter regime. The electric field that is parallel to the magnetic field B, E∥, is assumed to vanish. It is found that plasmas flow in the toroidal direction at the same rate as the island rotation frequency. Island rotation frequency is calculated using an island-induced symmetry-breaking viscosity. The radial electric field in the vicinity of the island is also determined from the toroidal momentum balance equation that includes island-induced toroidal viscosity.
Process-dependent band structure changes of transition-metal (Ti,Zr,Hf) oxides on Si (100)
Authors: Fulton C. C. , Lucovsky G. , Nemanich R. J.
Published: January 2004

In this study, we have deposited Ti, Zr, and Hf oxides on ultrathin (∼0.5 nm) SiO2 buffer layers and have identified metastable states which give rise to large changes in their band alignments with respect to the Si substrate. This results in a potential across the interfacial SiO2 layer, significant band bending, and large shifts of the high-k valence band. The magnitude of the shift differs for the three materials and is dependant on both the SiO2 buffer layer thickness and annealing temperature. We propose a model where excess oxygen accumulates near the high-k-SiO2 interface providing electronic states, which are available to electrons that tunnel from the substrate.
Defect reduction mechanisms in the nanoheteroepitaxy of GaN on SiC
Authors: Sun X. Y. , Bommena R. , Burckel D. , Frauenglass A. , Fairchild M. N. , Brueck S. R. J. , Garrett G. A. , Wraback M. , Hersee S. D.
Published: January 2004

This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.
Microislands in tokamaks
Authors: Mikhailovskii A. B. , Kovalishen E. A. , Shirokov M. S. , Konovalov S. V. , Tsypin V. S. , Kamenets F. F. , Ozeki T. , Takizuka T.
Published: January 2004

Analytical treatment of magnetic islands with high poloidal mode numbers and widths smaller than ion Larmor radius (microislands) is presented. The profile functions and rotation frequencies of microislands in the scope of the standard two-fluid magnetohydrodynamics are investigated. The effects of anomalous perpendicular resistivity and anomalous perpendicular electron heat conductivity are included. It is shown that, in such a problem statement, the microislands are described in terms of two profile functions, one of which characterizes the electric field and perturbed plasma density (the microisland profile function) and second—perturbed electron temperature (the electron temperature profile function). Analytical expressions for these profile functions in the case of stationary microislands are derived. Contribution of the oscillatory parallel electric current (the polarization current) into the generalized Rutherford equation for the stationary island width is calculated. The problem of rotation frequencies of microislands is discussed. It is shown that, as in the case of large-scale magnetic islands, for calculation of these frequencies, it is necessary to take into account nonstationarities of profile functions in the near-separatrix region. These nonstationarities are due to discontinuities of the derivatives of profile functions with respect to island magnetic flux. It is shown that the contribution of these nonstationarities into the equation for the rotation frequency does not depend on this frequency. This leads to the conclusion that taking into account only the electron dissipation is insufficient for calculation of the rotation frequency. It is suggested that for such a calculation the ion dissipation should also be involved.
Destruction of invariant surfaces and magnetic coordinates for perturbed magnetic fields
Authors: Hudson S. R.
Published: January 2004

Straight-field-line coordinates are constructed for nearly integrable magnetic fields. The coordinates are based on the robust, noble-irrational rotational-transform surfaces, whose existence is determined by an application of Greene’s residue criterion. A simple method to locate these surfaces is described. Sequences of surfaces with rotational-transform converging to low order rationals maximize the region of straight-field-line coordinates.
Collisional sheath dynamics in the intermediate radio-frequency regime
Authors: Xiang N. , Waelbroeck F. L.
Published: January 2004

A sheath model is proposed for the case when the radio-frequency ω is comparable to or larger than the ion plasma frequency of the bulk plasma ωpi and the ion collisionality in the sheath is significant. In this case, the ion momentum equation can be solved easily. We find that the ion velocity in the sheath varies with time and the resulting ion energy distribution is bimodal even though the radio frequency is much larger than the ion plasma frequency in the sheath. The results of the model are compared with the numerical solutions of the fluid equations. Both are in very good agreement.
Raman scattering study of Tb(V1−xPx)O4 single crystals
Authors: Moura M. R. , Ayala A. P. , Guedes I. , Grimsditch M. , Loong C.-K. , Boatner L. A.
Published: January 2004

The polarized Raman spectra of a series of isostructural vanadate/phosphate compounds of the form: Tb(V1−xPx)O4 with 0⩽x⩽0.75 have been investigated at room temperature. While the observed Raman spectra are generally consistent with group theory predictions for a body-centered tetragonal crystal structure (D4h19), due to the replacement of P for V, additional spectral features induced by the disorder of the mixed vanadate/phosphate system are also observed. In particular, all of the external lattice modes are characterized by a one-phonon-like behavior, while the behavior of the internal modes of the (V,P)O4 tetrahedron is two-phonon-like.
Differences in deformation processes in nanocrystalline nickel with low- and high-angle boundaries from atomistic simulations
Authors: Caturla M.-J. , Nieh T. G. , Stolken J. S.
Published: January 2004

Molecular dynamics simulations show significant differences in the deformation mechanisms of nanocrystalline nickel with low- and high-angle boundaries. For the case studied with average grain size of 12 nm, low-angle boundaries present enhanced dislocation activity and reduced strength with respect to high-angle boundaries for low strains. In the latter, most of the deformation is accommodated at the grain boundaries with limited dislocation activity, while in the case of low-angle boundaries, most of the displacements observed are associated with the motion of partial dislocations nucleated at the grain boundaries.
Local structure around Mn atoms in room-temperature ferromagnetic (In,Mn)As thin films probed by extended x-ray absorption fine structure
Authors: Soo Y. L. , Kim S. , Kao Y. H. , Blattner A. J. , Wessels B. W. , Khalid S. , Sanchez Hanke C. , Kao C.-C.
Published: January 2004

An extended x-ray absorption fine structure technique has been employed to probe the average local structure around Mn impurity atoms in (In,Mn)As thin films prepared by organometallic vapor phase epitaxy. These films show ferromagnetism above room temperature. As the concentration of Mn increases, the average local environment surrounding Mn changes from that of a Mn–Mn interstitial pair to a dimer, trimer, or cubic MnAs structure and then to the hexagonal MnAs structure/interstitial pair. In contrast to random substitution, the Mn impurity atoms in these dimer, trimer, or cubic MnAs structure occupy adjacent sites in the In sublattice. Ferromagnetism above room temperature in these ordered substitutional samples provides an excellent example for ferromagnetic exchange interaction in (In,Mn)As diluted magnetic semiconductors without the formation of hexagonal MnAs clusters, this is potentially very important for spintronic applications.
Enhanced surface diffusion through termination conversion during epitaxial SrRuO3 growth
Authors: Rijnders Guus , Blank Dave H. A. , Choi Junghoon , Eom Chang-Beom
Published: January 2004

During the initial growth of the ferromagnetic oxide SrRuO3 on TiO2-terminated SrTiO3, we observe a self-organized conversion of the terminating atomic layer from RuO2 to SrO. This conversion induces an abrupt change in growth mode from layer by layer to growth by step advancement, indicating a large enhancement of the surface diffusivity. This growth mode enables the growth of single-crystalline and single-domain thin films. Both conversion and the resulting growth mode enable the control of the interface properties in heteroepitaxial multilayer structures on an atomic level.
Magnetic-curvature-driven interchange modes in dusty plasmas
Authors: Sandberg I. , Shukla P. K.
Published: January 2004

The magnetic-curvature-driven interchange mode instability of a weakly inhomogeneous dusty plasma is rigorously investigated. It is shown that the electric drift convection of the equilibrium dust charge density is a stabilizing factor for long wavelength interchange modes. In a fully nonlinear regime, the finite amplitude interchange modes may self-organize in the form of a dipolar vortex. The present results should be useful in the understanding of the properties of the interchange mode turbulence in nonuniform magnetized plasmas containing charged dust particles.
Aspect ratio scaling of ideal no-wall stability limits in high bootstrap fraction tokamak plasmas
Authors: Menard J. E. , Bell M. G. , Bell R. E. , Gates D. A. , Kaye S. M. , LeBlanc B. P. , Maingi R. , Sabbagh S. A. , Soukhanovskii V. , Stutman D. ,   NSTX National Research Team
Published: January 2004

Recent experiments in the low aspect ratio National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] have achieved normalized beta values twice the conventional tokamak limit at low internal inductance and with significant bootstrap current. These experimental results have motivated a computational re-examination of the plasma aspect ratio dependence of ideal no-wall magnetohydrodynamic stability limits. These calculations find that the profileoptimized no-wall stability limit in high bootstrap fraction regimes is well described by a nearly aspect ratio invariant normalized beta parameter utilizing the total magnetic field energy density inside the plasma. However, the scaling of normalized beta with internal inductance is found to be strongly aspect ratio dependent at sufficiently low aspect ratio. These calculations and detailed stability analyses of experimental equilibria indicate that the nonrotating plasma no-wall stability limit has been exceeded by as much as 30% in NSTX in a high bootstrap fraction regime.
Effect of the Fe substitution on the magnetic transition of NdCo9.5V2.5
Authors: Liu W. F. , Rao G. H. , Ouyang Z. W. , Yang H. F. , Liu G. Y. , Feng X. M. , Ma H. W. , Wang S. Y. , Liang J. K.
Published: January 2004

Crystal structure and magnetic properties of the NdCo9.5−xFexV2.5 compounds with x=0–3.5 have been investigated. The lattice parameters increase linearly with the Fe content and the Curie temperature increases substantially at small amount of Fe content and then gradually with the further increase of x. A domain-wall pinning phenomenon was observed in all of the compounds and becomes less obvious as x increases. At a higher field, the compounds exhibit a field-induced metamagnetic transition. The critical field for the metamagnetic transition increases first then decreases with the Fe content, whereas the difference of the magnetization between 14 and 5 T increases with x. The observation can be understood by considering an interplay of the volume expansion, enhanced exchange coupling parameter JRT and the T sublattice moment μT due to the Fe substitution for Co.
Complete theory for Langmuir envelope solitons in dusty plasmas
Authors: Shukla P. K. , Eliasson Bengt
Published: January 2004

A complete theory for Langmuir envelope solitons in an unmagnetized dusty plasma is presented, taking into account interactions between finite amplitude Langmuir waves and fully nonlinear dust ion-acoustic (DIA), dust acoustic (DA), and ion hole (IH) perturbations. For this purpose, a nonlinear Schrödinger equation is employed for the Langmuir wave envelope and expressions for plasma slow responses are derived, including a modified (by the Langmuir wave ponderomotive force) Boltzmann electron distribution and appropriate ion and dust density distributions for fully nonlinear dispersive DIA, DA, and IH perturbations, which include departure from the quasi-neutrality condition. In the stationary frame, the governing equations can be cast in the form of a Hamiltonian which is used to check the accuracy of the numerical scheme predicting stationary localized solutions of our governing nonlinear equations. Numerical results reveal different classes of Langmuir envelope solitons (cavitons) whose features differ from those in an electron-ion plasma without dust. Ion and dust thermal effects for the DIA and DA waves, respectively, have been included. It is suggested that new beam-plasma experiments in laboratory dust plasmas should be conducted to verify our theoretical predictions of cavitons.
Terahertz range quantum well infrared photodetector
Authors: Graf Marcel , Scalari Giacomo , Hofstetter Daniel , Faist Jérôme , Beere Harvey , Linfield Edmund , Ritchie David , Davies Giles
Published: January 2004

We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of λ=84 μm. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 5×107 cm √Hz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.
Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotube composites containing Fe catalyst
Authors: Kim H. M. , Kim K. , Lee C. Y. , Joo J. , Cho S. J. , Yoon H. S. , Pejaković D. A. , Yoo J. W. , Epstein A. J.
Published: January 2004

Thin and flexible composite films of raw or purified multiwalled carbon nanotube (MWCNT) with various mass fractions and poly(methylmethacrylate) (PMMA) were synthesized for electromagnetic interference (EMI) shielding material. From scanning electron microscopy and high-resolution transmission electron microscopy photographs, we observed the formation of a conducting network through MWCNTs in an insulating PMMA matrix and the existence of an Fe catalyst in MWCNTs. The dc conductivity (σdc) of the systems increased with increasing MWCNT mass fraction, showing typical percolation behavior. The measured EMI shielding efficiency (SE) of MWCNT–PMMA composites by using the extended ASTM D4935-99 method (50 MHz–13.5 GHz) increased with increasing MWCNT mass fraction as σdc. The highest EMI SE for raw MWCNT–PMMA composites was ∼27 dB, indicating commercial use for far-field EMI shielding. The contribution of absorption to total EMI SE of the systems is larger than that of reflection. Based on magnetic permeability, we suggest raw MWCNTs and their composites can be used for near-field EMI shielding.
Determination of In concentration in pseudomorphic InxGa1−xN quantum wells based on convergent-beam electron diffraction
Authors: Stirman J. N. , Takeguchi M. , McCartney M. R. , Smith David J.
Published: January 2004

An approach based on the technique of convergent-beam electron diffraction (CBED) has been used to determine the elemental composition of a pseudomorphic GaN/InxGa1−xN/GaN quantum-well structure on the nanometer scale. Areas of triangles formed by intersecting higher order Laue-zone (HOLZ) lines are highly sensitive to lattice-parameter variations. By calculating the ratio of triangle areas, the local In concentration is estimated to within ±0.5% once lattice expansion in the growth direction has been taken into account. Moreover, shifts in the HOLZ-line positions as the CBED probe is moved along the quantum well provide a simple and rapid indication of local fluctuations in In content. The technique is applicable to other pseudomorphically strained epitaxial layers.
The Urbach focus and hydrogenated amorphous silicon
Authors: Orapunt Farida , O’Leary Stephen K.
Published: January 2004

We critically examine the validity of the Urbach “focus” concept for the case of hydrogenated amorphous silicon. Our approach involves an analysis of a broad assortment of hydrogenated amorphous silicon experimental optical absorption data from many different sources. We find that these experimental data are consistent with the notion of the Urbach focus concept. The implications of this conclusion are explored.
Elastic and electronic properties of select M2AX phases
Authors: Lofland S. E. , Hettinger J. D. , Harrell K. , Finkel P. , Gupta S. , Barsoum M. W. , Hug G.
Published: January 2004

In this letter we report on the low-temperature specific heat of several M2AX phases: Ti2AlC, V2AlC, V2AsC, Nb2SnC, Ti2AlN, Hf2InC, Nb2AlC, and Cr2AlC. The Debye temperatures are quite high. The density of states at the Fermi level, N(EF) varies from ≈1.4 (eV formula unit)−1 to 6 (eV formula unit)−1. Ab initio calculations show that N(EF) is dictated by the transition metal d–d bands; the A-group element has little effect. We also measured the velocity of sound in V2AlC, V2AsC, Ti2AlC, and Ti2AlN. The average bulk modulus of these materials is over 100 GPa, with a high of ≈140 GPa for Ti2AlN. Our theoretical calculations correctly predict the trend in both the density of states and the bulk modulus, although there is some disagreement in the actual values.
Glass transition-assisted microstructuring in polystyrene
Authors: Juodkazis Saulius , Yamasaki Kazuhiko , Matsuo Shigeki , Misawa Hiroaki
Published: January 2004

We report on the use of a shape transition process to resize patterns prerecorded inside polystyrene film. The shape transition, which is shrinkage in two dimensions and expansion in the third (the volume is conserved), was brought about by annealing polystyrene above the glass transition temperature. This caused approximately twofold lateral shrinkage and fourfold axial stretching of the film, inside which micrometer-sized patterns had been recorded by femtosecond pulses. The transformation of these patterns corresponded to the macroscopic shape transformation of the film. The shape transition was also used to transform a diffraction grating. This allowed the transformation to be confirmed by the corresponding change in diffraction efficiency. The applicability of the shape transition process to nanofabrication is discussed.