bluuebunny/crossref_metadata_2025 huggingface dataset:
The near band edge photoluminescence of cubic GaN epilayers
Authors: As D. J. , Schmilgus F. , Wang C. , Schöttker B. , Schikora D. , Lischka K.
Published: July 2002

The near band edge photoluminescence (PL) of cubic GaN epilayers grown by radio frequency (rf) plasma-assisted molecular beam epitaxy on (100) GaAs is measured. Since the PL is excited with an unfocused laser beam it resembles the layer properties rather than the properties of micron-size inclusions or micro crystals. The low temperature PL spectra show well separated lines at 3.26 and 3.15 eV which are due to excitonic and donor-acceptor pair transitions (donor binding energy 25 meV, acceptor binding energy 130 meV). No emission above the band gap of the cubic phase is detected. PL results are confirmed by x-ray diffraction and atomic force microscopy which reveal only negligible contributions from hexagonal inclusions and micron size single crystals. The room temperature PL consists of an emission band at about 3.21 eV with a full width at half maximum of 117 meV.
Doping gas effects on plasma enhanced chemical vapor deposition on heavily phosphorus-doped n+silicon film
Authors: Kuo Yue
Published: July 2002

Unusual doping gas effects on the plasma enhanced chemical vapor deposited heavily phosphorus-doped silicon film have been observed. The deposition rate increases linearly with the silane (1% phosphine) feed gas flow rate. However, the film’s resistivity increased abruptly, i.e., more than 2 orders of magnitude, in a narrow range of flow rate. This was inconsistent with the change of the film’s dopant or hydrogen concentration. It was consistent with the variation of the film’s morphology. For example, below the transition flow rate, the film had a large volume fraction of microcrystal; above the transition flow rate, the film was amorphous. The enhancement of the doping efficiency is probably due to the segregation of the dopant at grain boundaries. In addition, the deposition rate was facilitated by the existence of the doping gas. It is possible that the doping gas influences the surface electrical characteristics during the film growing process, which accounts for the increase of the deposition rate and the mechanism of the crystal formation.
Auger carrier capture kinetics in self-assembled quantum dot structures
Authors: Uskov A. V. , McInerney J. , Adler F. , Schweizer H. , Pilkuhn M. H.
Published: July 2002

We establish rate equations to describe Auger carrier capture kinetics in quantum dot structures, calculate Auger capture coefficients for self-assembled quantum dots, and analyze Auger capture kinetics using these equations. We show that Auger capture times can be of the order of 1–100 ps depending on barrier carrier and dot densities. Auger capture rates depend strongly on dot diameters and are greatest at dot diameters of about 10–20 nm.
Control of epitaxial growth for SrBi2Ta2O9 thin films
Authors: Cho J. H. , Bang S. H. , Son J. Y. , Jia Q. X.
Published: July 2002

We report the (a00) oriented as well as the (00l) oriented epitaxial SrBi2Ta2O9 thin film grown on (100) LaAlO3 and (200) yttria-stabilized zirconia single crystal substrates. On (100) SrTiO3 single crystal substrate, we only observe the (00l) oriented epitaxial growth of SrBi2Ta2O9 thin film. We infer that the (a00) oriented epitaxial growth results from the strain by the lattice mismatch. The band gaps of the (00l) and the (a00) oriented SrBi2Ta2O9 films are ∼3.9 and ∼3.5 eV, respectively. The two epitaxial orientations of ferroelectric SrBi2Ta2O9 thin film can provide an opportunity to study the effects on microwave and optical waveguides as well as underlying anisotropic physical properties.
Comparison study of photoluminescence from InGaN/GaN multiple quantum wells and InGaN epitaxial layers under large hydrostatic pressure
Authors: Shan W. , Perlin P. , Ager J. W. , Walukiewicz W. , Haller E. E. , McCluskey M. D. , Johnson N. M. , Bour D. P.
Published: July 2002

We report the results of a comparison study of photoluminescence (PL) from an In0.15Ga0.85N/GaN multiple-quantum-well (MQW) sample and an In0.11Ga0.89N thick epitaxial-layer sample, which have very similar band-gap energies. Large hydrostatic pressures were used for our investigations. The PL emissions in both samples were found to shift linearly to higher energy with applied pressure. In the MQW sample, the pressure response of the InGaN is dominated by the GaN layers, which leads to a significantly weaker pressure dependence as compared to the epilayer sample. Our results yield a pressure coefficient of 2.8×10−3 eV/kbar for the In0.15Ga0.85N/GaN MQW sample and 4.0×10−3 eV/kbar for the In0.11Ga0.89N epilayer. An abrupt decrease of PL intensity in both samples was observed at pressures above 100 kbar, indicating the carriers involved in the radiative recombination processes in the samples originate primarily from the adjacent GaN layers.
Fabrication of highly efficient organic electroluminescent devices
Authors: Kido Junji , Iizumi Yasuhiro
Published: July 2002

Efficient organic electroluminescent (EL) devices were fabricated by using a highly fluorescent aluminum complex, tris(4-methyl-8-quinolinolato)aluminum(III) (Almq3), as an emitter layer. In addition to using this complex, a multilayer device structure, consisting of a hole-injecting layer, a hole transport layer, a dye-doped Almq3 emitting layer, and an electron transport layer, was employed in order to reduce the driving voltage as well as to maximize carrier recombination efficiency. From this device, a maximum luminance of over 140 000 cd/m2 and an external quantum efficiency of 7.1% was observed, which is the highest efficiency ever reported for organic EL devices.
Doping effects arising from Fe and Ge for Mn in La0.7Ca0.3MnO3
Authors: Sun J. R. , Rao G. H. , Shen B. G. , Wong H. K.
Published: July 2002

Structural, magnetic, and transport properties of polycrystalline La0.7Ca0.3Mn1−xFexO3 and La0.7Ca0.3Mn1−xGexO3 are experimentally studied. Single-phase samples are obtained in the range x=0–0.12 for Fe, and x=0–0.06 for Ge. There are no appreciable structure changes due to the introduction of Fe and Ge. The Mn-site doping favors a reduced magnetic/resistive transition, at rates of ∼22 K for 1% Fe and ∼28 K for 1% Ge, and an elevated resistivity. No metal–insulator transition occurs when the content of Fe exceeds ∼0.1. The enhanced doping effects in La0.7Ca0.3Mn1−xGexO3 can be ascribed to the reduced hole concentration noting that the presence of Fe and Ge influence the contents of mobile eg electrons and holes in the compounds, respectively. Equivalence of the effects from Fe and Ge doping, respectively, to those due to eg electron and hole trapping and the relation between Mn- and O-site doping are discussed.
Transmission electron microscopy investigation and first-principles calculation of the phase stability in epitaxial CuInS2 and CuGaSe2 films
Authors: Su D. S. , Wei Su-Huai
Published: July 2002

Ordering of metal atoms in CuInS2 and CuGaSe2 thin films grown by vapor-phase epitaxy on Si (001) and GaAs (001) substrates were studied using transmission electron microscopy and first-principles total energy calculations. Chalcopyrite and CuAu-like orderings of the metal atoms were observed coexisting in CuInS2 films, while only chalcopyrite ordering was found in CuGaSe2 films. First-principles total energy calculations find that the formation enthalpy difference between chalcopyrite and CuAu-like phases of CuInS2 is very small (2 meV/atom), while it is large for CuGaSe2 (9 meV/atom), indicating that the existence of CuAu-like phase in the nominally chalcopyrite semiconductors is controlled by bulk thermodynamics.
Vertical self-alignment of quantum dots in superlattice
Authors: Zhang Y. W. , Xu S. J. , Chiu C.-h.
Published: July 2002

Self-alignment of quantum dots separated by spacer layers is investigated by using a three-dimensional finite element method. We find that the morphology of the top islands is not just simply a reproduction of the buried islands. It is dependent on the arrangement of the buried islands, the interruption time, and the spacer layer thickness. If the buried islands are uniform and regular, with appropriate choice of their spacing and aspect ratio and with a thin spacer layer, there exists a regime in which the steady-state top islands are always uniform and regular, vertically aligned, and stable against small perturbations. For a thicker spacer layer, due to the change of the strain energy density distribution on top of the spacer layer, the top islands are misaligned with the buried islands. If the buried islands are not uniform and regular, our simulations demonstrate the top islands may self-assemble into more uniform and regular arrangements.
Improvement in spin-wave resonance characteristics of epitaxial barium-ferrite thin films by using an aluminum-doped strontium-ferrite buffer layer
Authors: Shinde S. R. , Lofland S. E. , Ganpule C. S. , Bhagat S. M. , Ogale S. B. , Ramesh R. , Venkatesan T.
Published: July 2002

We report on the effects of using SrFe7Al5O19 as a buffer layer for growth of high-quality epitaxial barium-ferrite thin films on sapphire substrates. X-ray diffraction studies reveal that the buffer layer causes the interfacial strains in the barium-ferrite films to relax. As a result, the ferromagnetic resonance linewidth decreases even in the as-deposited case. However, the more striking result is the drastic reduction in the linewidth that occurs when the barium-ferrite film is deposited on the buffer layer and subsequently annealed at 1000 °C for 2 h, allowing the observation of a large number of spin-wave resonances (up to the 15th mode), indicating an improvement in both the surface and interface characteristics.
1.4× efficiency improvement in transparent-substrate (AlxGa1−x)0.5In0.5P light-emitting diodes with thin (⩽2000 Å) active regions
Authors: Gardner N. F. , Chui H. C. , Chen E. I. , Krames M. R. , Huang J-W. , Kish F. A. , Stockman S. A. , Kocot C. P. , Tan T. S. , Moll N.
Published: July 2002

Improvement of 1.4× in the external quantum efficiency and luminous efficiency (lm/W) of transparent-substrate (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes is demonstrated. The improvement is accomplished by reducing the thickness of the active layer to ⩽2000 Å and increasing the internal quantum efficiency by using multiple thin (⩽500 Å) active layers. The maximum luminous efficiency achieved is 73.7 lm/W at λp∼615 nm and the maximum external quantum efficiency is 32.0% at λp∼632 nm.
Vacuum-ultraviolet laser absorption spectroscopy for absolute measurement of fluorine atom density in fluorocarbon plasmas
Authors: Tachibana Kunihide , Kamisugi Hideaki
Published: July 2002

Absolute density measurement of fluorine atoms has been performed by a vacuum-ultraviolet (VUV) laser absorption technique in fluorocarbon plasmas. A VUV laser tunable around 95 nm covering the resonance lines of F atoms has been produced in Xe gas by a two-photon resonance four-wave-mixing technique. In this method, the background absorption by the parent gases and species produced in the plasma can be eliminated by scanning the wavelength, and the absolute density of F atoms can be derived accurately from the integrated absorption line profile. The measured values of the density varied from 1×1011 to 4×1012 cm−3, depending on the source gas species and the operating conditions of an inductively coupled radio-frequency (400 kHz) discharge.
The influence of atomic structure on the formation of electrical barriers at grain boundaries in SrTiO3
Authors: Browning N. D. , Buban J. P. , Moltaji H. O. , Pennycook S. J. , Duscher G. , Johnson K. D. , Rodrigues R. P. , Dravid V. P.
Published: July 2002

An experimental atomic resolution analysis of an undoped Σ5 36° [001] tilt grain boundary in SrTiO3 shows that the structure contains incomplete oxygen octahedra. These incomplete octahedra act as effective oxygen vacancies and lead to a fixed, positive boundary charge. Annealing the boundary in the presence of MnO2 does not change the atomic structure of the boundary plane, and results in a high concentration of Mn3+ (acceptor) enrichment at the specific Ti4+ locations in closest proximity to the effective oxygen vacancies. This result can be explained in terms of standard charge compensation models and indicates that the formation of electrical barriers at oxide grain boundaries may be influenced by the atomic structure of the boundary plane.
1.54 μm electroluminescence from erbium (III) tris(8-hydroxyquinoline) (ErQ)-based organic light-emitting diodes
Authors: Curry R. J. , Gillin W. P.
Published: July 2002

Organic light-emitting diodes have been fabricated using erbium tris(8-hydroxyquinoline) as the emitting layer and N, N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine as the hole-transporting layer. Room-temperature electroluminescence was observed at 1.54 μm due to intra-atomic transitions between the I13/24 and I15/24 levels in the Er3+ ion. These results suggest a possible route to producing a silicon-compatible 1.54 μm source technology.
Breakdown of the high-voltage sheath in metal plasma immersion ion implantation
Authors: Anders André
Published: July 2002

It is suggested that breakdown of a space-charge sheath obeys similar breakdown laws as known for vacuum breakdown. When metal plasmas of vacuum arcs are used, the sheath between a biased substrate and plasma is very thin and the electric-field strength is very high. Field enhancement (e.g., at sharp edges of the substrate) leads to thermal instability of electron emission centers, followed by dense plasma formation which, in turn, electrically short circuits the sheath (breakdown). Theoretical and experimental evidence for this mechanism is presented.
Experimental studies of the advanced fast ignitor scheme
Authors: Norreys P. A. , Allott R. , Clarke R. J. , Collier J. , Neely D. , Rose S. J. , Zepf M. , Santala M. , Bell A. R. , Krushelnick K. , Dangor A. E. , Woolsey N. C. , Evans R. G. , Habara H. , Norimatsu T. , Kodama R.
Published: July 2002

Guided compression offers an attractive route to explore some of the physics issues of hot electron heating and transport in the fast ignition route to inertial confinement fusion, whilst avoiding the difficulties associated with establishing the stability of the channel formation pulse. X-ray images are presented that show that the guided foil remains hydrodynamically stable during the acceleration phase, which is confirmed by two-dimensional simulations. An integrated conical compression/fast electron heating experiment is presented that confirms that this approach deserves detailed study.
Raman spectra of CuInTe2, CuIn3Te5, and CuIn5Te8 ternary compounds
Authors: Rincón C. , Wasim S. M. , Marı́n G. , Hernández E. , Delgado J. M. , Galibert J.
Published: July 2002

From the analysis of Raman spectra, a comparative study of the lattice vibrational modes in CuInTe2, and of the ordered defect compounds (ODC) CuIn3Te5 and CuIn5Te8 of the Cu–In–Te system is reported. It is found that Raman lines in the ODCs appear at very nearly the same frequency as in CuInTe2. This is explained on the basis that the cation vacancy should not be considered merely as an empty site in the lattice but a localized region of positive Coulomb potential that attracts the valence electrons of the surrounding anions. It is proposed that the vacancy-Te force constant caused by the positive potential is of the same magnitude as in the Cu–Te stretching bond. The symmetry assignment of the observed lines is tentatively made by a comparative study of the vibrational modes of its binary analog Zn0.5Cd0.5Te, with phonon modes observed in CuInTe2 from polarized infrared optical measurements, and by estimating the modes frequency using several models reported in the literature for the chalcopyrite compounds. These are A1 mode at 127 cm−1; B1 at 58, 114, and 146 cm−1, E modes at around 48, 72, 137, 160, 192, and 218 cm−1; B2 at 64 and 104 cm−1; and E and/or B2 at 170 and 184 cm−1. The highest phonon frequencies observed at 237 and 270 cm−1 are attributed to the combination of E and B2 modes.
Two-dimensional numerical simulation of the resistive reconnection layer
Authors: Uzdensky D. A. , Kulsrud R. M.
Published: July 2002

In this paper a two-dimensional, time dependent, numerical simulation of a reconnection current layer in incompressible resistive magnetohydrodynamics with uniform resistivity in the limit of very large Lundquist numbers is presented. Realistic boundary conditions derived consistently from the outside magnetic field are used, and the effect of the back pressure from flow into the separatrix region is also taken into account. It is found that within a few Alfvén times the system evolves from an arbitrary initial state to a steady state consistent with the Sweet–Parker model, even if the initial state is Petschek-like.
Room-temperature low-threshold low-loss continuous-wave operation of 2.26 μm GaInAsSb/AlGaAsSb quantum-well laser diodes
Authors: Mermelstein C. , Simanowski S. , Mayer M. , Kiefer R. , Schmitz J. , Walther M. , Wagner J.
Published: July 2002

Strained single- and triple-quantum-well (SQW and TQW), large optical cavity GaInAsSb/AlGaAsSb/GaSb laser diodes emitting at 2.26 μm are investigated. Internal loss coefficients as low as 5 and 7.7 cm−1 for the SQW and TQW, respectively, and relatively high internal quantum efficiencies of 65% (SQW) and 69% (TQW) were obtained. Extrapolated threshold current densities for infinite cavity lengths of 55 and 150 A/cm2 have been deduced for the SQW and TQW, respectively. These values scale very well with the number of QWs and are among the lowest reported for diode lasers in this wavelength range. A differential quantum efficiency as high as 50% and a total power efficiency of 23% were achieved at 280 K. The temperature dependence of the threshold current density revealed a high characteristic temperature of 110 K. Single-ended output powers of 240 mW in continuous-wave mode and exceeding 0.5 W in pulsed operation were obtained for a TQW laser with high-reflection/antireflection coated facets at 280 K, mounted substrate-side down.
Self-consistent electrical charging of insulating layers and metal-insulator-semiconductor structures
Authors: Glavatskikh I. A. , Kortov V. S. , Fitting H.-J.
Published: July 2002

By means of a computer simulation the self-consistent charge transport with the current densities j(x,t), the respective charges ρ(x,t), field strengths F(x,t), and potential distributions V(x,t) in SiO2 layers are obtained as a function of the insulator depth x and the injection time t. The SiO2 layers are considered as open layers on silicon substrate or they are embedded in metal-oxide-semiconductor (MOS) structures. The given currents of primary electrons, the field-dependent ballistic currents of secondary electrons and holes as well as the Fowler–Nordheim injection of electrons from the substrate into the dielectric layer are taken into account. This method allows a defined charge storage and the explanation of complicated emission, charging-up, and breakdown processes within insulating layers during electron bombardment and/or high-field charge injection from adjacent electrodes, e.g., in MOS structures.
Feasibility of 5 Gbit/s wavelength division multiplexing using quantum dot lasers
Authors: Grundmann Marius
Published: July 2002

The dynamics of single-mode quantum dot lasers is modeled theoretically. It is predicted that, assuming reasonable material properties, eye-patterns remain open for 5 Gbit/s large signal modulation within a finite spectral range (>50 nm), corresponding to 64 wavelength division multiplexing channels with 0.8 nm separation.
The lowest triplet state A′3 of H3+: Global potential energy surface and vibrational calculations
Authors: Sanz Cristina , Roncero Octavio , Tablero César , Aguado Alfredo , Paniagua Miguel
Published: July 2002

The adiabatic global potential energy surface of the H3+ system for the lowest triplet excited state of A′ symmetry was computed for an extensive grid of conformations around the minimum region at full configuration interaction ab initio level, using a much more extended basis set than in a previous paper from the same authors. An accurate global fit (rms error lower than 27 cm−1 for energies lower than dissociation into separated atoms and lower than 5 cm−1 for energies lower than the dissociation channel) to these ab initio points and also to part of the previous calculated points (for a total of 7689 energies in the data set) of the lowest triplet excited state of A′ symmetry is obtained using a diatomics-in-molecules approach corrected by one symmetrized three-body term with a total of 109 linear parameters and 1 nonlinear parameter. This produces an accurate global potential which represents all aspects of the bound triplet excited state of H3+ including the minima and dissociation limits, satisfying the correct symmetry properties of the system. The vibrational eigenstates have been calculated using hyperspherical coordinates with symmetry adapted basis functions with the proper regular behavior at the Eckart singularities. The accuracy of the vibrational levels thus obtained is expected to be better than 2 cm−1 with respect to unknown experimental values. Due to the presence of three equivalent minima at collinear geometries (D∞h) the lower vibrational levels are close to triple degenerate. Since the interconversion barrier between the three minima is about 2640 cm−1, these states split for the upper excited vibrational levels. Such splitting can provide a key feature to identifying the unassigned transitions amongst the many H3+ lines that have been observed in hydrogen plasmas.
Magnetically driven isentropic compression experiments on the Z accelerator
Authors: Reisman D. B. , Toor A. , Cauble R. C. , Hall C. A. , Asay J. R. , Knudson M. D. , Furnish M. D.
Published: July 2002

Isentropic compression experiments (ICE) have been performed on the Z accelerator facility at Sandia National Laboratory. We describe the experimental design that used large magnetic fields to slowly compress samples to pressures in excess of 400 kbar. Velocity wave profile measurements were analyzed to yield isentropic compression equations of state (EOS). The method can also yield material strength properties. We describe magnetohydronamic simulations and results of experiments that used the “square short” configuration to compress copper and discuss ICE EOS experiments that have been performed with this method on tantalum, molybdenum, and beryllium.
High-pressure vessel for elastic and inelastic x-ray diffraction experiments for liquids over a wide temperature range
Authors: Hosokawa Shinya , Pilgrim Wolf-Christian
Published: July 2002

A new x-ray scattering environment is presented which can be employed at high temperatures up to 2000 K and high pressures up to 150 bar. This device is especially designed for angle-dispersive scattering experiments. It consists of a thick closed-end aluminum cylinder equipped with a steel flange. Incident and scattered radiation passes through continuous Be windows which cover an angle range up to 55°. The high potential of this scattering setup is demonstrated by representative example experiments at high temperatures and high pressures, which could be carried out in combination with a single-crystal sapphire cell. Inelastic-, elastic-, and anomalous scattering experiments using synchrotron radiation and energy dispersive static measurements employing a conventional tube device are reported.
Multiphoton ionization photoelectron spectroscopy of acetaldehyde via the Ã 1A″, B̃, C̃, and D̃ states
Authors: Kim Ho-Tae , Anderson Scott L.
Published: July 2002

REMPI spectra are reported for the acetaldehyde Ã 1A″, B̃, C̃, and D̃ states. Photoelectron spectroscopy is used to probe the nature of the intermediate states, measure the cation vibrational frequencies, and to identify useful routes for preparing state-selected ions. Ab initio calculations of neutral and cation vibrational frequencies are also reported. The B̃ state is found to be a well-behaved Rydberg state, but with some distortion relative to the cation geometry along the ν10 and ν15 coordinates. There are B̃ state REMPI transitions that produce well state-selected cations, with vibrational energies of up to 0.4 eV, and several new cation frequencies are observed. The Ã 1A″ state gives structured, if somewhat broadened, REMPI transitions, but ionizes to produce a broad population of vibrationally hot ions. Only the origin band of the C̃ state is observed in REMPI, despite high intensity for this state in absorption. A few D̃ state transitions are sharp, and ionize to produce cold cations, as expected for a good Rydberg state. Most D̃ state levels are strongly mixed and broadened, however, and ionize to hot cations. Inconsistencies in the literature are discussed in light of the photoelectron spectra and ab initio results.
The dynamic competition between stress generation and relaxation mechanisms during coalescence of Volmer–Weber thin films
Authors: Floro J. A. , Hearne S. J. , Hunter J. A. , Kotula P. , Chason E. , Seel S. C. , Thompson C. V.
Published: July 2002

Real-time measurements of stress evolution during the deposition of Volmer–Weber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystalline Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior occurs even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the film/substrate interfacial strength. For Ag films, interfacial shear dominated the early relaxation behavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the Al/SiO2 interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures, consistent with surface-diffusion-based mechanisms. In all the films studied here, stress relaxation was suppressed after the films became continuous.
Electromigration in Cu interconnects with very different grain structures
Authors: Hau-Riege C. S. , Thompson C. V.
Published: July 2002

To determine the effects of grain structures on the rate of electromigration-induced failure of Cu interconnects, scanned laser annealing (SLA) has been used to produce Cu interconnects with very different grain structures. SLA, in which a moving hot-zone induces local grain growth, can be used to produce interconnects with fully bamboo grain structures that have bamboo grain lengths up to ten times the interconnect width. Electromigration experiments have been carried out on interconnects with very-long-grained bamboo structures, as well as on interconnects with polygranular structures in which the average grain size is less than the linewidth. Such differences are known to lead to orders of magnitude changes in lifetimes for Al-based interconnects. However, no significant differences in the failure rates were found for these Cu interconnects. This result supports earlier work that suggested that electromigration in Cu interconnects with now-standard liners and interlevel diffusion-barrier layers occurs by mechanisms that are faster than grain boundary self-diffusion.
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dipolar recoupling under very fast magic angle spinning in solid-state nuclear magnetic resonance: Applications to distance measurements, spectral assignments, and high-throughput secondary-structure determination
Authors: Ishii Yoshitaka
Published: July 2002

A technique is presented to recouple homonuclear dipolar couplings between dilute spin pairs such as C13–C13 systems under very fast magic angle spinning (MAS) in solid-state nuclear magnetic resonance (NMR) spectroscopy. The presented technique, finite pulse rf driven recoupling (fpRFDR), restores homonuclear dipolar interactions based on constructive usage of finite pulse-width effects in a phase- and symmetry-cycled π-pulse train in which a rotor-synchronous π pulse is applied every rotation period. The restored effective dipolar interaction has the form of a zero-quantum dipolar Hamiltonian for static solids, whose symmetry in spin space is different from that obtained by conventional rf driven recoupling (RFDR) techniques. It is demonstrated that the efficiency of recoupling by fpRFDR is not strongly dependent on chemical shift differences or resonance offsets in contrast to previous recoupling methods under very fast MAS. To realize distance measurements without effects of spin relaxation, a constant-time version of fpRFDR (CT-fpRFDR) is introduced, in which the effective evolution period is varied by refocusing dipolar evolution with a rotor-synchronized solid echo while the total recoupling period is kept constant. From CT-fpRFDR experiments at a spinning speed of 30.3 kHz in a field of 17.6 T, the C13–C13 distance of [1-13C]Ala–[1-13C]Gly–Gly was determined to be 3.27 Å, which agrees well with the value of 3.20 Å obtained by x-ray diffraction. Also, two-dimensional (2D) C13/C13 chemical-shift correlation NMR spectrum in a field of 9.4 T was obtained with fpRFDR for fibrils of the segmentally C13- and N15-labeled Alzheimer’s β-Amyloid fragments, Aβ16–22 (residues 16–22 taken from the 40-residue Aβ peptide) in which Leu-17 through Ala-21 are uniformly C13- and N15-labeled. Most C13 resonances for the main chain as well as for the side chains are assigned based on 2D C13/C13 chemical-shift correlation patterns specific to amino-acid types. Examination of the obtained C13 chemical shifts revealed the formation of β-strand across the entire molecule of Aβ16–22. Possibility of high throughput determination of global main-chain structures based on C13 shifts obtained from 2D C13/C13 chemical-shift correlation under very fast MAS is also discussed for uniformly/segmentally C13-labeled protein/peptide samples.
Adsorption state of dimethyl disulfide on Au(111): Evidence for adsorption as thiolate at the bridge site
Authors: Hayashi Tomohiro , Morikawa Yoshitada , Nozoye Hisakazu
Published: July 2002

We studied the adsorption state of dimethyl disulfide and methylthiolate on the Au(111) surface by means of the density functional theory (DFT) within a generalized gradient approximation and experimental high-resolution electron energy loss spectroscopy (HREELS) techniques. It turns out that the methylthiolate adsorption is more stable than the dimethyl disulfide adsorption and that the most stable adsorption site for the methylthiolate is the bridge site slightly off-centered towards the fcc-hollow site with its S–C bond tilted from the surface normal by 53°. HREELS results are in excellent agreement with the DFT results, providing very strong support to the depicted adsorption scenario.
Dimensionality dependence of the Rayleigh–Taylor and Richtmyer–Meshkov instability late-time scaling laws
Authors: Oron D. , Arazi L. , Kartoon D. , Rikanati A. , Alon U. , Shvarts D.
Published: July 2002

The late-time nonlinear evolution of the three-dimensional (3D) Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities for random initial perturbations is investigated. Using full 3D numerical simulations, a statistical mechanics bubble-competition model, and a Layzer-type drag-buoyancy model, it is shown that the RT scaling parameters, αB and αS, are similar in two and three dimensions, but the RM exponents, θB and θS are lower by a factor of 2 in three dimensions. The similarity parameter hB/〈λ〉 is higher by a factor of 3 in the 3D case compared to the 2D case, in very good agreement with recent Linear Electric Motor (LEM) experiments. A simple drag-buoyancy model, similar to that proposed by Youngs [see J. C. V. Hanson et al., Laser Part. Beams 8, 51 (1990)], but using the coefficients from the A=1 Layzer model, rather than phenomenological ones, is introduced.
Ignition scaling laws and their application to capsule design
Authors: Herrmann Mark C. , Tabak Max , Lindl John D.
Published: July 2002

This paper investigates the amount of energy required to ensure the ignition of an inertial confinement fusion capsule. First, a series of one-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Plasma Phys. Controlled Fusion 2, 51 (1975)] simulations are performed to create a database of barely ignited capsules that span the parameter regime of interest. This database is used to develop scaling laws for the ignition energy in terms of both the stagnated capsule parameters and the in-flight capsule parameters, and to explore the connection between these two parameter sets. We then examine how much extra energy is required to overcome the effect of the inevitable surface imperfections that are amplified during the implosion process. These perturbations can lead to break up of the capsule in flight or to mix of cold fuel into the hot spot, both of which can cause the capsule to fail. As an example, a family of capsules with fixed adiabat, drive pressure, and absorbed energy is studied; the capsule from this family that is maximally robust to these failure modes is found.
Piezoelectric properties of 〈001〉 textured Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics
Authors: Sabolsky E. M. , James A. R. , Kwon S. , Trolier-McKinstry S. , Messing G. L.
Published: July 2002

The piezoelectric properties of (1−x) Pb(Mg1/3Nb2/3)O3–xPbTiO3 (x=0.3–0.35), ceramics with a high degree of 〈001〉 fiber texture were investigated for possible actuator applications. Piezoelectric coefficients (d33) in excess of 1200 pC/N associated with strain levels up to >0.3% were observed in samples prepared by a reactive templated grain growth process. No excess PbO was used in the starting composition. A high degree of fiber texture was achieved using 〈001〉 oriented BaTiO3 template particles in a fine-grained precursor for the PMN–32PT matrix. High densities together with texture resulted in a significant increase in strain levels and d33 values compared to their polycrystalline counterparts. Peak dielectric constants on the order of 22 000 with losses of ∼2% and well-saturated hysteresis loops with a Pr∼27 μC/cm2 were recorded on the textured samples. These domain engineered, textured ceramics have tremendous potential for high-performance actuators.
Passive mode-locking in 1.3 μm two-section InAs quantum dot lasers
Authors: Huang Xiaodong , Stintz A. , Li Hua , Lester L. F. , Cheng Julian , Malloy K. J.
Published: July 2002

Passive mode locking was achieved at 1.3 μm in oxide-confined, two-section, bistable quantum dot (QD) lasers with an integrated intracavity QD saturable absorber. Fully mode-locked pulses at a repetition rate of 7.4 GHz with a duration of 17 ps were observed under appropriate bias conditions. No self-pulsation accompanied the mode locking. These results suggest that a carefully designed QD laser is a candidate for ultrashort pulse generation.
Surfactant effects on doping of GaAs grown by organometallic vapor phase epitaxy
Authors: Shurtleff J. K. , Jun S. W. , Stringfellow G. B.
Published: July 2002

Recently, the addition of the isoelectronic surfactant Sb during organometallic vapor phase epitaxy (OMVPE) of GaInP was shown to eliminate ordering, resulting in a significant change in the band gap energy. These results suggest that surfactants added during growth could have profound affects on other important properties of semiconductors, such as doping. This letter presents the results of a recent study on the effects of the isoelectronic surfactant Sb on doping in GaAs. The addition of a small amount of triethylantimony during OMVPE of GaAs is found, using secondary ion mass spectroscopy analysis, to increase the Zn doping concentration from <6×1018 atoms/cm3 to 9×1018 atoms/cm3, a factor of 1.6. The amount of antimony introduced into the solid is only 2–3×1017 atoms/cm3. The addition of Sb also increases the impurity concentration of In in GaAs, but does not affect the concentration of Te or P.
Preactivation in the nucleation of ice by Langmuir films of aliphatic alcohols
Authors: Seeley L. H. , Seidler G. T.
Published: July 2002

We report a study of preactivation in the heterogeneous nucleation of ice by Langmuir films adsorbed at the surface of water droplets. The Langmuir films consisted of aliphatic alcohols with 21, 23, or 25 carbon atoms per molecules. We find that the mean freezing temperature Tf* of the water droplets decreases abruptly as the preparation temperature Tprep is increased. Tprep is the highest temperature to which the droplets are heated since the immediately preceding freezing event. Analysis of the distributions of freezing temperatures yields the dependence of thermodynamic parameters on Tprep, allowing us to identify the role of thermal history separately in the energy barrier and the attempt frequency for nucleation. We find that the change in Tf* is exclusively due to a change in the energy barrier for nucleation and that all preactivation characteristics are a strong function of chain length. We find good agreement between our results and those expected from models for preactivation which rely on the expected properties of icelike ordered water near the nucleating agent. We also discuss the possibility that structural changes in the Langmuir film play a role in preactivation. We discuss future experimental directions for this system, now that preactivation behavior has been established.
Experimental and theoretical study of Λ-doublet resolved rotationally inelastic collisions of highly rotationally excited CH(A 2Δ,v=0) with Ar
Authors: Nizamov Boris , Dagdigian Paul J. , Tzeng Yi-Ren , Alexander Millard H.
Published: July 2002

A collaborative experimental and theoretical study of rotationally inelastic collisions of resolved CH(A 2Δ,v=0) Λ-doublet levels of high rotational angular momentum N with argon is presented. Experimental state-to-state rate constants were extracted from CH A 2Δ→X 2Π resolved fluorescence spectra upon laser excitation to individual levels in the A 2Δ state in the presence of Ar. Fluorescence detection of the collision-induced final-state populations permits resolution of the Λ-doublet levels at high N, but without fine-structure discrimination. The largest rate constants were for ΔN=0 A′↔A″ symmetry-changing transitions. For exothermic ΔN<0 transitions, the rate constants decreased monotonically with increasing magnitude of ΔN, and a slight preference for conservation of the Λ-doublet symmetry was found. These rate constants have been compared and interpreted with theoretical rate constants computed through quantum coupled-states calculations of cross sections based on ab initio CH(A 2Δ)–Ar potential-energy surfaces determined by G. Kerenskaya, A. Kaledin, and M. C. Heaven (to be published). The electronic symmetry of the rotational, fine-structure/Λ-doublet levels of a Δ2 electronic state has also been analyzed.
Conducting fluid dynamics experiments with vertically falling soap films
Authors: Rutgers M. A. , Wu X. L. , Daniel W. B.
Published: July 2002

This article gives a detailed description of an apparatus in which flowing soap films are used to perform two dimensional fluid dynamics experiments. We have previously reported scientific findings made with the apparatus, but never carefully described the technique, or its full potential. A brief introduction is given on the nature of soap films as fluids and then all the details necessary for creating robust flowing films are listed. Typical parameters for the system are: flow speeds from 0.5 to 4 m/s, film thickness between 1 and 10 μm, and typical film sizes are 3 m tall and 10 cm wide although films of 20 m tall and 4 m wide have also been made. A vacuum apparatus is also described in which the air drag on the film can be reduced by a factor of 5–10. Finally, a large number of techniques for measuring flow and thickness are outlined and referenced.
Energetics and electronic structure of carbon doped aluminum clusters
Authors: Rao B. K. , Jena P.
Published: July 2002

The energetics and the electronic structure of AlnC clusters (n=3, 4, 5; 11, 12, 13) have been studied by a global optimization of their geometry without any symmetry constraint. The total energies of these clusters both in neutral and charged states are calculated using an all-electron basis and the generalized gradient approximation to the density functional theory. While Al4C and Al12C clusters share some characteristic features of closed shell structures, namely enhanced stability and low electron affinity compared to their neighboring sizes, their ionization potentials exhibit different behavior. These decrease steadily from Al3C to Al5C while that of Al12C is higher than its neighbors. Carbon is found to form planar structures in small AlnC clusters (n=3, 4, 5) irrespective of their charge state although neutral Al4C possesses a nearly degenerate tetrahedral isomer lying slightly higher in energy from the planar configuration. The results agree well with experimental and previous theoretical data. In larger AlnC (n=11, 12, 13) clusters, carbon occupies an interior site. In Al12C, carbon occupies the center of an icosahedron while it is off-centered in Al11C and Al13C. As an electron is attached, the near degeneracies of the neutral Al4C is lifted whereas nondegenerate isomers of neutral Al12C yield nearly degenerate anions. Both these features produce complicated photoelectron spectra making identification of their adiabatic electron affinity a difficult problem. With the exception of neutral Al12C, the bonding of carbon to aluminum atoms is governed primarily by covalent interaction. The above calculations were also performed with a simplified basis by freezing the atomic cores of aluminum. In most cases, this simple basis yields results in good agreement with all electron calculations.
Direct observation of “dynamic” chirality by Coulomb explosion imaging
Authors: Kitamura T. , Nishide T. , Shiromaru H. , Achiba Y. , Kobayashi N.
Published: July 2002

Instantaneous chirality induced by zero-point vibrations was observed directly by using the Coulomb explosion imaging (CEI) technique. The present results suggest the CEI would be generally applicable to diagnosis of the chirality of the isolated molecules in gas phase.
Impact of gas heating in inductively coupled plasmas
Authors: Hash D. B. , Bose D. , Rao M. V. V. S. , Cruden B. A. , Meyyappan M. , Sharma S. P.
Published: July 2002

Recently it has been recognized that the neutral gas in inductively coupled plasma reactors heats up significantly during processing. The resulting gas density variations across the reactor affect reaction rates, radical densities, plasma characteristics, and uniformity within the reactor. A self-consistent model that couples the plasma generation and transport to the gas flow and heating has been developed and used to study CF4 discharges. A Langmuir probe has been used to measure radial profiles of electron density and temperature. The model predictions agree well with the experimental results. As a result of these comparisons along with the poorer performance of the model without the gas–plasma coupling, the importance of gas heating in plasma processing has been verified.
Equation of state of model branched alkanes: Theoretical predictions and configurational bias Monte Carlo simulations
Authors: MacDowell Luis G. , Vega Carlos , Sanz Eduardo
Published: July 2002

We develop a general configurational bias Monte Carlo algorithm for the simulation of branched alkanes and compare the results with predictions from theoretical equations of state. We consider results for all the hexane isomers, as well as for several heptane and octane isomers. The interaction sites of our united atom model alkanes are hard spheres of equal diameter, thus allowing us to study the effect of branching in the equation of state without the need of considering the effect of changes in the size of the sites. We find that, at roughly constant molecular volume, branching has a small but noticeable effect on the equation of state, somewhat reducing the pressure at which a given density may be attained. We find that equations of state previously used for linear chains yield very good agreement with simulation results.
Electrical and mechanical properties of piezoelectric ceramic/metal composites in the Pb(Zr, Ti)O3/Pt system
Authors: Li Jing-Feng , Takagi Kenta , Terakubo Noriaki , Watanabe Ryuzo
Published: July 2002

As a model for composite materials of piezoelectric ceramic and metal, lead zirconate titanate (PZT) and platinum (Pt) particulate composites were fabricated by power processing. The electrical and mechanical properties of the PZT–Pt composites were measured as a function of the Pt volume fraction. The relative dielectric constants of the PZT–Pt composites increased markedly, while the piezoelectric constants and electromechanical coupling coefficients decreased with increasing Pt content. When the Pt volume fraction exceeded 30%, the PZT–Pt composite became electrically conductive because of percolation of the Pt particles. The Pt-dispersed PZT composites enhanced the mechanical properties, particularly the high fracture resistance, compared to the monolithic PZT ceramics.
A large volume plasma device
Authors: Mattoo S. K. , Anitha V. P. , Awasthi L. M. , Ravi G. ,   LVPD Team
Published: July 2002

Investigation of highly complex electromagnetic phenomena responsible for fundamental processes in geospace plasma calls for a laboratory device, large enough to provide minimum boundary effects on the electromagnetic waves under concern. The production of large volume, uniform, and quiescent plasma of density ∼1012 cm−3 requires a suitable large area plasma source. The influence of energetic electrons on the phenomena under study can be eliminated using pulsed, highly reproducible discharges so that the afterglow, devoid of energetic electrons, becomes the experimental plasma. Measurement over a large volume with high spatial and temporal resolution, acquisition, and storage of voluminous data form other difficult tasks. The requirement to have tasks of plasma generation, wave excitation, and data acquisition automatically sequenced, also demands a sophisticated control system. A large volume plasma device (LVPD), possessing the following unique features, has been developed in our laboratory: (i) a large area multifilamentary source, (ii) a pulsed power system (width ⩽15 ms and turn off time ≈10 μs), (iii) a three-axes probe drive, (iv) data acquisition and control system with 1 GS/s sampling rate and ≈20 GB memory, and (v) an extended magnet coil system giving desired field profile. The achieved plasma parameters include, ne∼1012 cm−3, Te≈8 eV in the main glow, and ne≈3×1011 cm−3 and Te≈2 eV in the after glow, and plasma uniformity within 5% over 1 m in axial and radial extents. The present article describes the LVPD device.
Local spin
Authors: Clark Aurora E. , Davidson Ernest R.
Published: July 2002

Equations are proposed for computing from ab initio wave functions quantities like 〈SA⋅SB〉, which appear in the Heisenberg model Hamiltonian of magnetism. These equations are based on projection operators derived from Löwdin orthogonalization. They result in local spin operators SA which obey the definition of angular momentum operators and commute with each other. These equations are evaluated for several typical closed and open shell molecules. For closed shells in the single Slater determinant approximation, 〈SA⋅SB〉 is −38 of the bond-order and 〈SA2〉 is +38 of the total number of bonds to center A. For open shells there are additional contributions from the unpaired electrons. In favorable cases, these additional terms have the value assumed as the whole answer in the usual applications of the Heisenberg Hamiltonian.