bluuebunny/crossref_metadata_2025 huggingface dataset:
Aharonov–Bohm phase effects and inelastic scattering in transport through a parallel tunnel-coupled symmetric double-dot device
Authors: Smirnov Anatoly Yu. , Horing Norman J. M. , Mourokh Lev G.
Published: July 2002

We have examined the effects of inelastic phonon scattering on the lead-to-lead current through a parallel-quantum-double-dot device to determine the persistence of coherence within the device. The tunnel-coupled double-dot system is taken to be symmetric and is threaded by Aharonov–Bohm magnetic flux. We show that at resonant values of the magnetic flux, when only the bonding or antibonding state is involved in transport through the double dot, the current–voltage characteristic changes drastically and the electron–phonon interaction has a pronounced effect on the level populations. However, it does not destroy coherence during the tunneling process through the system.
Electron field emission for ultrananocrystalline diamond films
Authors: Krauss A. R. , Auciello O. , Ding M. Q. , Gruen D. M. , Huang Y. , Zhirnov V. V. , Givargizov E. I. , Breskin A. , Chechen R. , Shefer E. , Konov V. , Pimenov S. , Karabutov A. , Rakhimov A. , Suetin N.
Published: July 2002

Ultrananocrystalline diamond (UNCD) films 0.1–2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4–Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60–100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In addition, scanning tunneling microscopy studies indicate that the emission sites from UNCD films are related to minima or inflection points in the surface topography, and not to surface asperities. These data, in conjunction with tight binding pseudopotential calculations, indicate that grain boundaries play a critical role in the electron emission properties of UNCD films, such that these boundaries: (a) provide a conducting path from the substrate to the diamond–vacuum interface, (b) produce a geometric enhancement in the local electric field via internal structures, rather than surface topography, and (c) produce an enhancement in the local density of states within the bulk diamond band gap.
On the magnetic anisotropy of lanthanide-containing metallomesogens
Authors: Mironov Vladimir S. , Galyametdinov Yury G. , Ceulemans Arnout , Binnemans Koen
Published: July 2002

A general theoretical model of the origin of the magnetic anisotropy in paramagnetic metal-containing liquid crystals is developed. General relations between the molecular magnetic anisotropy of mesogenic lanthanide complexes and the macroscopic magnetic anisotropy of these liquid crystals in the mesophase are obtained. The net magnetic anisotropy of a real metallomesogen is shown to be the result of a complex interplay between the molecular magnetic anisotropy, orientation of the long molecular axis, and disorder effects. The sign of the magnetic anisotropy Δχ depends not only on the anisotropy of the tensor of molecular magnetic susceptibility, but also on the orientation of the long molecular axis of rodlike lanthanide complexes with respect to the principal magnetic axes of the molecular tensor of magnetic susceptibility. The influence of micro- and macroscopic disorder in real liquid crystals is discussed. Numerical parametric calculations were used to rationalize the variation of the magnitude and sign of the magnetic anisotropy in a series of isostructural lanthanide-containing metallomesogens. Experimental magnetic susceptibility and magnetic anisotropy of a series of [Ln(LH)3(NO3)3] compounds (LH is a Schiff base) are well reproduced by calculations based on the present model. Limitations of the Bleaney theory of magnetic anisotropy are analyzed.
Composition gradient optimization and electrical characterization of (Pb, Ca)TiO3 thin films
Authors: Bao Dinghua , Mizutani Nobuyasu , Zhang Liangying , Yao Xi
Published: July 2002

Compositionally graded (Pb, Ca)TiO3 thin films were prepared by a monoethanolamine-modified sol–gel technique on platinum-coated silicon substrates at the annealing temperature of 600 °C. The composition gradient of the films was greatly improved by a modified annealing method. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 469 and 355, respectively. Both were larger than those reported for conventional (Pb, Ca)TiO3 thin films. The compositionally graded films had large polarization offsets in hysteresis loops when excited by an alternating electric field. The more smooth the composition gradient of the graded film, the larger the polarization offset. This was consistent with a theoretical model reported previously by Mantese and coworkers [Appl. Phys. Lett. 71, 2047 (1997)]. The magnitude of polarization offset displayed a power-law dependence on the electric field, and the direction of the offset depended on the direction of the composition gradient with respect to the substrate. Both up-graded and down-graded films had good leakage current characteristics.
Inflight electron impact excitation in ionized metal physical vapor deposition
Authors: Lu Junqing , Kushner Mark J.
Published: July 2002

Ionized metal physical vapor deposition (IMPVD) is a process in which sputtered metal atoms from a magnetron target are ionized by a secondary plasma before depositing onto the substrate. The sputtered metal atoms and neutralized ions reflected from the target have higher kinetic energies than the buffer gas and so are not in thermal equilibrium. These nonthermal (inflight) species can dominate the total metal species density at low pressures (<5 mTorr). As a result, electron impact of the inflight species may significantly contribute to excitation and ionization. To investigate these processes, a model was developed to include the inflight electron impact excitation (IEIE) of sputtered species during IMPVD. Results for Cu IMPVD indicate that the predicted Cu+ density at low pressure (2 mTorr) significantly increased when IEIE was taken into account. As the pressure increases, the rate of thermalization increases and the importance of IEIE decreases.
Raman spectroscopy of In(Ga)As/GaAs quantum dots
Authors: Chu L. , Zrenner A. , Bichler M. , Böhm G. , Abstreiter G.
Published: July 2002

We report on inelastic light scattering on self-assembled In(Ga)As/GaAs quantum dots. Phonon modes are investigated in backscattering from (001) surface and (110) cleaved edge both in polarized and depolarized geometry. One relatively broad phonon signal is observed which is attributed to optical phonon modes in and around the In(Ga)As quantum dots. The Raman selection rules are similar to the bulk material. Additionally, a broad peak is observed around 50 meV under resonance conditions which is attributed to electronic interlevel transitions in the n-doped In(Ga)As quantum dots.
Submilliwatt operation of AlGaN-based ultraviolet light-emitting diode using short-period alloy superlattice
Authors: Nishida T. , Saito H. , Kobayashi N.
Published: July 2002

Over 0.1 mW ultraviolet output was achieved by an AlGaN-based light-emitting diode. To realize a highly conductive and ultraviolet-transparent layer, a short-period alloy superlattice was introduced. The device was fabricated on SiC substrate. Low electric resistivity due to the short-period alloy superlattice and the high thermal conductivity of the SiC substrate enabled large current injection of up to 1.7 kA/cm2. The emission was monochromatic band-edge emission about 350 nm in wavelength without significant D–A and/or deep emissions.
Bunch stability during high-gradient wakefield generation in a dielectric-lined waveguide
Authors: Park S. Y. , Hirshfield J. L.
Published: July 2002

A recently developed analytic theory for wakefields generated when a charge bunch, or train of bunches, passes along a dielectric-lined waveguide is applied to examine stability issues for this system. Such a configuration could be the key element in a two-beam accelerator to create colliding TeV-range electron and positron beams. It is shown that a 587 MV/m peak acceleration gradient can be achieved when a train of five 2 nC, 1.0 GeV, 0.20 mm long drive bunches passes along the axis of an alumina-lined waveguide (ε=9.5) with inner and outer radii 0.50 and 5.0 mm. The energy gain of a 0.2 nC test bunch in a 4 m long stage under these conditions is shown to be 1.48 GeV, for an average acceleration gradient of 370 MV/m. However, when a drive bunch is injected parallel to, but displaced from, the waveguide axis, transverse (mainly dipole) forces cause the tail of the bunch to swerve sharply toward the waveguide wall (head-to-tail instability), and to intersect the wall after traversing a relatively short distance. These results suggest that successful exploitation of the appealing strong wakefields generated by a train of drive bunches will require some means of transverse stabilization, such as an array of externally applied focusing and defocusing quadrupoles.
State-to-state scattering in a reactive system: H2(v=1,J=1) from Cu(100)
Authors: Watts Elizabeth , Sitz Greg O.
Published: July 2002

We have measured state-to-state scattering in the H2–Cu(100) and H2–Cu(110) systems for H2 initially prepared in the v=1,J=1 state, for several final (v,J) states. We observe survival in the initial state (v=1,J=1), rotational excitation within v=1 (into v=1,J=3), and rotational excitation plus vibrational relaxation into final states (v=0,J=5) and (v=0,J=7). For each of these final states, we measure the branching fraction (relative to the incident v=1,J=1 flux) and the mean final translational energy. In addition, for rotational excitation from J=1 to 3, the dependence on initial translational energy and surface temperature is measured within v=1 and compared with results within v=0. Our results are compared with published state-resolved associative desorption results, similar scattering experiments, and recent theoretical calculations.
Chaos control and taming of turbulence in plasma devices
Authors: Klinger Thomas , Schröder Christiane , Block Dietmar , Greiner Franko , Piel Alexander , Bonhomme Gerard , Naulin Volker
Published: July 2002

Chaos and turbulence are often considered as troublesome features of plasma devices. In the general framework of nonlinear dynamical systems, a number of strategies have been developed to achieve active control over complex temporal or spatio-temporal behavior. Many of these techniques apply to plasma instabilities. In the present paper we discuss recent progress in chaos control and taming of turbulence in three different plasma “model” experiments: (1) Chaotic oscillations in simple plasma diodes, (2) ionization wave turbulence in the positive column of glow discharges, and (3) drift wave turbulence in a magnetized plasma column. Depending on the physical mechanism of the specific instability in each case, an appropriate control strategy is chosen out of a variety of different approaches; in particular discrete feedback, continuous feedback, or spatio-temporal open-loop synchronization. Electric control fields are externally applied to the plasma device and the chaotic or turbulent state is stabilized by only weak perturbations of the plasma equilibrium. The success of this approach is demonstrated in both experiment and numerical simulation and the actual effect of the applied control fields is investigated.
Band discontinuities in ZnMgSe/ZnCdSe(001) lattice-matched heterostructures
Authors: Pelucchi E. , Rubini S. , Bonanni B. , Franciosi A. , Peressi M.
Published: July 2002

We present experimental and theoretical studies of lattice-matched interfaces between Zn1−yMgySe and Zn1−xCdxSe wide-band gap semiconductors. Ab initio pseudopotential calculations predict lattice matching for y/x∼1.6 and a valence band contribution to the band gap difference Qv decreasing gradually from 0.34 to 0.27 with increasing x and y. Experimental x-ray diffraction and photoemission spectroscopy results are mostly consistent with such predictions, although non-negligible deviations of the band offsets from commutativity were observed for selected alloy compositions.
Spatiotemporal signatures of periodic pulling during ionization-wave-mode transitions
Authors: Koepke M. E. , Dinklage A. , Klinger T. , Wilke C.
Published: July 2002

Using measurements of spatiotemporal patterns in the light-emission fluctuations of the positive column of a neon glow discharge, the spatiotemporal nature of the nonlinear interaction, known as periodic pulling, which occurs between pairs of self-excited, propagating, ionization waves, is characterized. Transitions occur between discrete longitudinal ionization-wave modes if the discharge current is smoothly ramped. At a given discharge current, multiple modes coexist along the entire plasma column with one mode dominating over all others. As the current is incremented towards the transition from one dominant mode to another, the prospective dominant mode grows and interacts with the dominant mode in space and time. Spatiotemporal periodic pulling is experimentally demonstrated. Characteristic modulation in both time and space of both frequency and wave-number spectra are presented. This phase modulation is low frequency and long wavelength. It maximizes at the instant of transition when both modes are of comparable amplitudes. The transition involves an Eckhaus instability that triggers the spatiotemporal dislocation, which represents the creation or annihilation of a mode number.
Investigation of the cut location in hydrogen implantation induced silicon surface layer exfoliation
Authors: Höchbauer T. , Misra A. , Nastasi M. , Mayer J. W.
Published: July 2002

The physical mechanisms of hydrogen induced silicon surface layer exfoliation were investigated using the combination of ion beam analysis, secondary ion mass spectroscopy (SIMS), scanning electron microscopy (SEM), and cross section transmission electron microscopy (XTEM). A 〈100〉 oriented silicon wafer was implanted with 175 keV protons to a dose of 5×1016 cm−2. The implanted wafer was bonded to a silicon oxide capped 〈100〉 silicon wafer and then heated to an elevated temperature of 600 °C to produce exfoliation. The hydrogen-implanted sample was analyzed in the as-implanted state as well as after the cleavage of the silicon wafer. The depth distribution of the implantation damage was monitored by Rutherford backscattering spectrometry (RBS) in channeling condition and XTEM imaging. Elastic recoil detection analysis and SIMS was performed to examine the hydrogen depth distribution. Cross section SEM and RBS channeling was used to measure the thickness of the exfoliated layer after cleavage. A comparison of the results deduced from the methods listed shows conclusively that the cleavage of the silicon wafer takes place above the hydrogen concentration peak near the implantation damage peak, revealing the crucial role of the implantation damage in the crystal in terms of hydrogen induced cleavage of the silicon crystal. The stress and strain field in the proton-implantation induced damage region of the silicon crystal is proposed to explain the observed results.
Exchange enhancement and thermal anneal in Mn76Ir24 bottom-pinned spin valves
Authors: Li Haohua , Freitas P. P. , Wang Zhenjun , Sousa J. B. , Gogol P. , Chapman J.
Published: July 2002

Exchange enhancement through thermal anneal in bottom-pinned Mn76Ir24 spin valves is investigated. Samples were fabricated by ion beam deposition (IBD), post-annealed in vacuum (10−6 Torr) at 270 °C for 10 min, then cooled in a 3 kOe applied field. For a bilayer structure, glass/Ta 40 Å/NiFe 30 Å/MnIr 60 Å/CoFe 25 Å/Ta 40 Å, the exchange field (Hex) reaches 1148 Oe (Jex=0.4 erg/cm2) after anneal. X-ray diffraction (XRD) analysis shows strong enhancement of 〈111〉 texture upon anneal, while grain size obtained from XRD and transmission electron microscopy for as-deposited and annealed states shows no major change. With increasing MnIr thickness, the exchange field decreases, and blocking temperature (Tb) increases, reaching 295 °C for tMnIr=180 Å. Spin valves built with the same exchange bilayer (Ta 20 Å/NiFe 30 Å/MnIr 60 Å/CoFe 25 Å/Cu 22 Å/CoFe 20 Å/NiFe 40 Å/Ta 40 Å) show Hex=855 Oe (Jex=0.3 erg/cm2) and magnetoresistance (MR)=7.1%. The incorporation of nano-oxide layers in spin valves increases the MR signal to 11%. No signal degradation is found in these specular structures for anneals up to 310 °C.
Micro-Raman and dielectric phase transition studies in antiferroelectric PbZrO3 thin films
Authors: Dobal P. S. , Katiyar R. S. , Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

Antiferroelectric materials are found to be good alternative material compositions for high-charge-storage devices and transducer applications. Lead zirconate (PZ) is a room-temperature antiferroelectric material. The antiferroelectric nature of PZ thin films was studied over a temperature range of 24–300 °C, in terms of Raman scattering, dielectric constant, and polarization. Temperature-dependent dielectric and polarization studies indicated a nonabrupt phase transition. To alleviate the extrinsic effects influencing the phase transition behavior, Raman scattering studies were done on laser-ablated PZ thin films as a function of temperature and clear phase transformations were observed.
An experimental study of drop deformation and breakup in extensional flow at high capillary number
Authors: Ha Jong-Wook , Leal L. Gary
Published: July 2002

An experimental study is reported of the flow-induced stretching of drops in a four-roll mill at low Reynolds number, but for capillary numbers that are large compared to the critical capillary number for onset of stretching. We mainly consider Newtonian drops in a Newtonian suspending fluid, but also present a brief study of Newtonian drops in a viscoelastic (Boger fluid) suspending fluid. The stability of the drops following cessation of flow is determined, in either case, by the ratio of their extended length to the undeformed radius. If this ratio is large enough, the drops will break into two or more parts via the capillary flow process known as end-pinching. However, for Newtonian drops in a Newtonian suspending fluid, it is shown that the critical degree of stretch for breakup increases sharply with increase of the capillary number that characterizes the stretching process. Furthermore, it is shown, in this case, that the critical stretch ratio is not unique, but that there can be a discrete range of stretch ratios above the first (or smallest) critical value where the drop is again stable before it encounters a second larger “critical” stretch ratio. This “restabilization” is associated with the transition from two to three drops in the breakup process. Newtonian drops in the viscoelastic Boger fluid are found to be slightly more stable than the same drops in a Newtonian fluid when stretched at strain rates just exceeding the critical value. By this we mean that the critical elongation ratio necessary for the drops to break upon cessation of flow is increased by about 20%. When stretched at a higher strain rate, approximately 2.15 times the critical value, large drops in the viscoelastic fluid (above 100 microns in radius for this particular suspending fluid) are destabilized relative to their counterpart in a Newtonian suspending fluid, while smaller drops are strongly stabilized.
Granular growth of Fe3O4 thin films and its antiphase boundaries prepared by pulsed laser deposition
Authors: Zhou W. L. , Wang K.-Y. , O’Connor C. J. , Tang J.
Published: July 2002

Fe 3 O 4 thin film prepared by pulsed laser deposition on Si (100) substrate has been investigated by transmission electron microscopy from plane and cross-sectional views. The thin film, which shows a spinel structure as characterized by selected-area diffraction, is about 180 nm thick with granular growth of particle size about 50 nm. High resolution electron microscopy observations indicate reduced thickness of the native SiO2 layer between the thin film and Si due to rastering of the substrate surface by an unfocused laser beam before the deposition. By using a 220 diffraction spot in two beam condition near the 〈001〉 axis of a single Fe3O4 grain, antiphase boundaries (APBs) inside the grains could be clearly resolved. The existence of APBs in the thin film is consistent with the magnetic experiments including an open hysteresis loop and unsaturated magnetization in relatively high magnetic fields. This study suggests that APBs are not unique to Fe3O4 films grown on MgO substrates.
Experimental evidence for spatial damping of left-hand circularly polarized waves in an electron cyclotron resonance region
Authors: Kaneko T. , Murai H. , Hatakeyama R. , Sato N.
Published: July 2002

Selective launch, propagation, and absorption of right-hand (R) and left-hand (L) circularly polarized waves are investigated using an inhomogeneously magnetized plasma. When the R wave gets closer to the electron cyclotron resonance (ECR) point, the wave vanishes in the strong magnetic-field region before reaching the ECR point. On the other hand, the L wave, which has been considered not to be related to ECR, is clearly observed to be also absorbed near the ECR point. The former and latter phenomena are discussed in terms of the contribution of high-energy tail electrons and polarization reversal of the L wave to the R wave within the plasma, respectively.
Modeling of active control of external magnetohydrodynamic instabilities
Authors: Bialek James , Boozer Allen H. , Mauel M. E. , Navratil G. A.
Published: July 2002

A general circuit formulation of resistive wall mode (RWM) feedback stabilization developed by Boozer [Phys. Plasmas 5, 3350 (1998)] has been used as the basis for the VALEN computer code that calculates the performance of an active control system in arbitrary geometry. The code uses a finite element representation of a thin shell structure in an integral formulation to model arbitrary conducting walls. This is combined with a circuit representation of stable and unstable plasma modes. Benchmark comparisons of VALEN results with large aspect ratio analytic model of the current driven kink mode are in very good agreement. VALEN also models arbitrary sensors, control coils, and the feedback logic connecting these sensors and control coils to provide a complete simulation capability for feedback control of plasma instabilities. VALEN modeling is in good agreement with experimental results on DIII-D [Garofalo et al., Nucl. Fusion 40, 1491 (2000)] and HBT-EP [Cates et al., Phys. Plasmas 7, 3133 (2000)]. VALEN feedback simulations have also been used to evaluate and optimize the sensor/coil configurations for present and planned RWM experiments on DIII-D. These studies have shown a clear advantage for the use of local poloidal field sensors driving a “mode control” feedback logic control loop and configurations which minimize the control coil coupling to the stabilizing resistive wall.
Modeling of discharges generated by electron beams in dense gases: Fountain and thunderstorm regimes
Authors: Macheret S. O. , Shneider M. N. , Miles R. B.
Published: July 2002

In this paper we present an analysis of the predicted dynamics of plasmas generated in air and other gases by injecting beams of high-energy electrons. Two distinct regimes are found, differing in the way that the excess negative charge brought in by the ionizing electron beam is removed. In the first regime, called a fountain, the charge is removed by the back current of plasma electrons toward the injection foil. In the second, called a thunderstorm, a substantial cloud of negative charge accumulates, and the increased electric field near the cloud causes a streamer to strike between the cloud and a positive or grounded electrode, or between two clouds created by two different beams. A quantitative analysis, including electron beam propagation, electrodynamics, charge particle kinetics, and a simplified heat balance, is performed in a one-dimensional approximation.
Analysis of the water bound to a polymer matrix by infrared spectroscopy
Authors: Herrera-Gómez A. , Velázquez-Cruz G. , Martín-Polo M. O.
Published: July 2002

This article focuses on the interaction of water with solid polymers. A portion of the water adsorbed in hydrophilic materials is strongly bound to individual sites (bound water), while an additional amount is bound less firmly to the polymeric chain (free water). Both free and bound water have been the subject of many studies that have uncovered, in some cases, qualitative evidence of the presence of water directly bound. In this article, we report a method to determine the different fractions of water adsorbed in hydrophilic materials. One fraction corresponds to the first layer in contact with the adsorbing solid and the other corresponds to the remaining water absorbed layers. We were able to determine said quantification by means of measuring the change in the natural vibration frequency brought about by the interaction of water molecules with the solid. The measurement was done on films of methylcellulose exposed to humid environments using infrared spectroscopy and gravimetric static microclime sorption methods. Theoretical predictions for bound water were developed using multilayer adsorption models. The experimental concentration of bound water as a function of relative humidity was compared with these predictions, allowing us to judge the applicability of different adsorption models employed in the study of gas condensation.
Peculiarities of the volume magnetostriction of La1−xSrxMnO3 in the Curie point region
Authors: Koroleva L. I. , Abramovich A. I. , Demin R. V. , Michurin A. V.
Published: July 2002

The parallel λ∥ and perpendicular λ⊥ magnetostriction (with respect to the applied magnetic field) and the thermal expansion Δl/l are studied on La1−xSrxMnO3 single crystals with x=0.1, 0.15, and 0.3. For the conducting sample with x=0.3 and the semiconducting sample with x=0.15 the volume magnetostriction (ω=λ∥+2λ⊥) is negative and the |ω|(T) curves go through a maximum at the Curie point TC. At T>TC its Δl/l temperature dependence is stronger than linear. For the semiconducting sample with x=0.1, ω is negative at T<TC and |ω|→0 at T∼TC. Its Δl/l is linear at T⩾TC. The behavior of ω and Δl/l are explained by a magnetic two-phase state, due to strong s−d exchange.
Thermal-annealing dependence of C-related atomic configuration in Si1−x−yGexCy crystals grown by ultra-high-vacuum chemical-vapor deposition
Authors: Kanzawa Y. , Saitoh T. , Kubo M.
Published: July 2002

Si 1−x−y Ge x C y crystals were grown by ultra-high-vacuum chemical-vapor deposition and their thermal-annealing behaviors were studied by infrared (IR) absorption measurements. The present samples exhibited absorption peaks originating from local vibrational modes of substitutional C atoms with two different atomic configurations; one is a C atom with four Si neighbors (Si4 configuration) and the other is with one Ge and three Si neighbors (Si3Ge1 configuration). As the annealing temperature was increased from 700 to 950 °C, the absorption peak of the C atoms with Si3Ge1 configuration increased in intensity. This clearly demonstrated that thermal annealing leads to the formation of Ge–C bonds. Moreover, the composition dependence of the IR spectra suggested that Ge–C bonds are formed to reduce the strain energy of the Si1−x−yGexCy crystals.
Two-phase high-performance Nd–Fe–B powders prepared by mechanical milling
Authors: Neu V. , Schultz L.
Published: July 2002

Nanocrystalline two-phase NdFeB+α-Fe material was prepared by milling techniques and subsequent heat treatment. These isotropic exchange-coupled powders show excellent hard magnetic properties with enhanced remanences up to 1.20 T and energy densities (BH)max around 150 kJ/m3. The performance is based on (i) the use of grain refining additions, (ii) partial substitution of Fe by Co and (iii) the use of pre-melted master alloys. The distribution of Co among the hard and soft magnetic phases upon milling and annealing is studied via Curie temperature measurements up to 10 at. % Co substitution. The effect of Co on the intrinsic properties is estimated and clarifies the extrinsic magnetic behavior. An overview of samples prepared with different milling techniques and compositions is presented. It summarizes the above listed, three different approaches to improve the magnetic performance together with the influence of the phase ratio of hard and soft magnetic phases.
New type of noniterative energy corrections for excited electronic states: Extension of the method of moments of coupled-cluster equations to the equation-of-motion coupled-cluster formalism
Authors: Kowalski Karol , Piecuch Piotr
Published: July 2002

The recently proposed method of moments of coupled-cluster equations (MMCC) is extended to excited states via the equation-of-motion coupled-cluster (EOMCC) formalism. The main idea of the new MMCC theory is that of the noniterative energy corrections which, when added to the excited-state energies obtained in standard approximate EOMCC calculations, recover the exact energies. The MMCC corrections are expressed in terms of the generalized moments of the EOMCC equations. Approximate variants of the excited-state MMCC formalism, including the MMCC(2,3) approach, are introduced. In the MMCC(2,3) method, very simple energy corrections, expressed in terms of matrix elements of the triples-reference, triples-singles, and triples-doubles blocks of the EOMCCSD (EOMCC singles and doubles) similarity-transformed Hamiltonian, are added to the excited-state energies obtained in EOMCCSD calculations. The performance of the MMCC(2,3) approach is illustrated by the results of pilot calculations for the potential energy curves of ground and excited states of CH+.
Optical limiting behavior of a glass–ceramic containing sodium niobate crystallites
Authors: Maciel G. S. , Rakov N. , de Araújo Cid B. , Lipovskii A. A. , Tagantsev D. K.
Published: July 2002

The nonlinear absorption properties of a glass–ceramic containing ferroelectric sodium niobate (NaNbO3) crystallites were investigated. Two- and three-photon absorption coefficients were determined using nanosecond pulsed lasers. The optical power-limiting performance for light beams at 1064 and 532 nm was dependent on the volume fraction of NaNbO3 crystallites formed during the heat-treatment process.
The permanent electric dipole moment of MgNC
Authors: Steimle Timothy C. , Bousquet Robert R.
Published: July 2002

The Ã 2Π1/2–X 2Σ+ band system of a molecular beam sample of magnesium isocyanide, MgNC24, was recorded via laser induced fluorescence in the presence of a static electric field. The molecules were produced by ablating a solid magnesium rod in the presence of an acetonitrile/argon supersonic expansion. The magnitude of the permanent electric dipole moments, |μ|, for MgNC24 were determined to be 5.308(75) D and 6.138(46) D for the (0,0,0)X̃ 2Σ+ and (0,0,0)Ã 2Π1/2 states, respectively.
Split operator method for the nonadiabatic (J=0) bound states and (A←X) absorption spectrum of NO2
Authors: Serov V. N. , Sovkov V. B. , Ivanov V. S. , Atabek O.
Published: July 2002

A split operator three-dimensional wave packet propagation method is adapted for the determination of the bound states and absorption band shape of NO2 molecule presenting a conical intersection between its ground X 2A1 and first excited A 2B2 electronic states. The numerical task, basically resting on a Fourier transform methodology, may present interesting advantages over matrix diagonalization techniques. The calculations of bound levels over a wide energy range and the absorption (A 2B2←X 2A1) band shape, extending up to 40 000 cm−1, are put on an equal footing by a nonadiabatic three-dimensional wave packet propagation using available ab initio potential energy surfaces. Good agreement is obtained when comparing the calculated absorption spectrum to experimental data in a low resolution limit. The position and amplitude of the band shape are determined within only 2 and 3% of relative error, respectively, the total width being still overestimated by about 15%. An analysis of the causes of errors is presented stressing the need for more accurate transition dipole moment determinations.
Antireflection coating for miniband transport and Fabry–Pérot resonances in GaAs/AlGaAs superlattices
Authors: Pacher C. , Rauch C. , Strasser G. , Gornik E. , Elsholz F. , Wacker A. , Kießlich G. , Schöll E.
Published: July 2002

It is demonstrated that standard concepts for optics like Fabry–Pérot resonances and antireflection coatings can be extended to ballistic electron transport in semiconductor superlattices. The transmission through a superlattice miniband is increased by a factor of 2.4 if two further barriers are added at both sides of the structure. The condition for Fabry–Pérot resonances for Bloch states in finite periodic potentials is derived and experimental evidence for their existence is obtained in transport experiments.
Effect of N/Ge co-implantation on the Ge activation in GaN
Authors: Nakano Yoshitaka , Kachi Tetsu
Published: July 2002

N-type regions have been produced in undoped GaN films by Ge and N/Ge implantation, sequentially, and subsequent annealing with a SiO2 encapsulation layer at 1300 °C. Improved Ge-doping characteristics have been achieved for GaN by N/Ge co-implantation, attaining activation efficiencies above 95%, whereas in the case of conventional Ge implantation, the activation efficiency is low owing to the generation of N vacancies. In particular, overlapping of the N-implanted region with the Ge one can make the Ge activation higher at a N/Ge ratio of ∼1. Therefore, the co-implantation of additional N atoms drastically enhances the Ge activation based on a site-competition effect.
Structural effects of the thermal treatment on a GaInNAs/GaAs superlattice
Authors: Largeau L. , Bondoux C. , Patriarche G. , Asplund C. , Fujioka A. , Salomonsson F. , Hammar M.
Published: July 2002

We have studied structural changes that occur during annealing of GaInNAs/GaAs multiple quantum wells grown by metalorganic vapor-phase epitaxy (MOVPE). Different thermal treatments led to an improved room-temperature photoluminescence (PL) intensity, but also to room-temperature PL peak splitting. This splitting is related to the appearance of compositional clustering as displayed by transmission electron microscopy (TEM). In addition to this, interfacial layers on each side of the wells have also been observed by TEM and their composition is discussed on the basis of high resolution x-ray diffraction studies. It is suggested that the interface layers are indium deficient, but enriched in nitrogen, degrading the optical quantum well performance and indicating a need for improved switching sequences in the MOVPE growth.
Noise reduction mechanisms in laminated antiferromagnetically coupled recording media
Authors: Piramanayagam S. N. , Wang J. P. , Hee C. H. , Pang S. I. , Chong T. C. , Shan Z. S. , Huang L.
Published: July 2002

Experimental methods to obtain remnant moment-thickness product (Mrδ) reduction in the laminated antiferromagnetically coupled (LAC) recording media is described. The results indicate that the reduction in Mrδ (or, indirectly, a reduction in noise) can be obtained by altering the antiferromagnetic coupling constant J, or by the parameters of the underlayer such as anisotropy constant (Ku2), thickness (t2), etc. A method to enhance J by intrinsic means is proposed by which a high exchange field of about 1900 Oe can be obtained. Thermal energy has been found to help in obtaining a Mrδ reduction in LAC media. Substrates with lower surface roughness also enhance the Mrδ reduction.
Effect of initial states on the phase diagram of the structural pattern in magnetic fluid films under perpendicular magnetic field
Authors: Hong Chin-Yih , Chen C. A. , Chen Chun-Hui , Horng H. E. , Yang S. Y. , Yang H. C.
Published: July 2002

When a magnetic fluid thin film is subjected to a magnetic field perpendicular to the plane of the film, a portion of magnetic particles is condensed out from the initially monodispersed magnetic fluid and forms magnetic columns. These columns evolve from a disordered column phase to a two-dimensional ordered structural pattern as the field strength reaches some critical value, Hh. The structural pattern formative process of the ordered structures that follows depends on the condition of the disordered column phase. In the letter, the effect of the initial disordered column phase, which can be altered by changing the initial field strength, Hi, on the formative process of the ordered structures is studied, and a phase diagram on the H–Hi plane is constructed. With an initial field strength that exceeds Hh, the structural pattern formative process goes through a plateau, a transition phase, and another plateau. On the other hand, if the initial field strength is less than Hh, the columns in the disordered column phase split and new columns emerge in the magnetic fluid film as the field strength increases. Finally, the structural pattern reaches a plateau. In both processes, the distance between columns in the structural patterns converges to that of a pseudostatic case.
Growth and characterization of ultrathin zirconia dielectrics grown by ultraviolet ozone oxidation
Authors: Ramanathan Shriram , Wilk Glen D. , Muller David A. , Park Chang-Man , McIntyre Paul C.
Published: July 2002

In this letter, we report on the growth of ultrathin films of zirconia on silicon oxide and silicon oxynitride passivated (001) Si by ultraviolet ozone oxidation of Zr metal precursor layers. The oxidation kinetics has been measured using an accelerator-based nuclear reaction analysis. It was found that oxide films up to 55 Å could be grown at room temperature by oxidation at 600 Torr while oxidation at 80 mTorr is self-limiting at 20 Å. The interfaces between the dielectric and the substrate have been characterized by scanning transmission electron microscopy. The ZrO2 films were found to be crystalline as grown. Electrical measurements on capacitor structures with 30-Å-thick ZrO2 films grown on native oxide on silicon show a capacitance–voltage hysteresis of 15 mV and a capacitance-based equivalent oxide thickness of 17 Å at 100 kHz.
S
1
∼>T
1
intersystem crossing in π-conjugated organic polymers
Authors: Burrows Hugh D. , Seixas de Melo J. , Serpa C. , Arnaut L. G. , Monkman Andrew P. , Hamblett I. , Navaratnam S.
Published: July 2002

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.
Photoemission from the Sr/Si(001) interface
Authors: Herrera-Gómez A. , Aguirre-Tostado F. S. , Sun Y. , Pianetta P. , Yu Z. , Marshall D. , Droopad R. , Spicer W. E.
Published: July 2002

The growth of Sr on n-type Si(001) was studied in detail for coverages between 0 and 1 monolayer (ML) using core level photoemission spectroscopy. In a similar manner, the Sr saturation coverage was studied in the 600–925 °C temperature range. Data analysis was carried out by a method that allows accurate determination of the band-bending shifts. Using this method it is possible to pinpoint the formation and destruction of chemical species from bungled core level photoemission data without needing to know details of the chemical composition of the spectra. Through this analysis it was established that the interaction between Sr and Si breaks down the binding energy difference between upward and downward Si dimer atoms. In addition, it was found that the saturation coverage exhibits a clear plateau at 1 ML around 650 °C, and a slope change at 1/3 ML around 850 °C. The surface band bending suffers a discontinuous increase as the Sr coverage surpasses 12 ML and as the low energy electron diffraction symmetry changes from [2×3] to [2×1].
Gettering of copper in silicon at half of the projected ion range induced by helium implantation
Authors: Peeva A. , Fichtner P. F. P. , da Silva D. L. , Behar M. , Koegler R. , Skorupa W.
Published: July 2002

Secondary ion mass spectroscopy, transmission electron microscopy, Rutherford backscattering/channeling spectrometry, and elastic recoil detection analysis measurements were used to determine the Cu gettering behavior induced by He implanted into Si samples. This study was done in an iterative way by changing the implanted He fluence (5×1015–3×1016 cm−2), implantation temperature (room temperature or 350 °C), and implantation conditions (random or channel implants). Upon postimplantation annealing at 800 °C for 600 s, in addition to the gettering at the projected range (Rp) region, the room temperature implanted samples also present Cu gettering in a region corresponding to the half of the projected range (Rp/2) depth. Also a threshold fluence (Φ≈7×1015 at/cm2) was determined for the appearance of the Rp/2 effect. In contrast, for the 350 °C implants, the Cu impurities are detected only close to the Rp region where the He induced cavities are formed. The gettering effect at Rp/2 region is discussed in terms of the cavity formation mechanisms and their influence on the point defect fluxes taking place during the thermal annealing.
In as a surfactant for the growth of GaN (0001) by plasma-assisted molecular-beam epitaxy
Authors: Kruse C. , Einfeldt S. , Böttcher T. , Hommel D.
Published: July 2002

The influence of indium on the surface morphology of GaN (0001) grown by plasma-assisted molecular-beam epitaxy (MBE) has been investigated. The rough and grain-like surface under nitrogen-rich growth conditions becomes smoother and similar to surfaces grown under gallium-rich conditions when a sufficiently high indium flux is used. However, the use of indium instead of gallium-rich conditions prevents the formation of gallium droplets on the surface which are associated with voids at their edges. Since indium is not incorporated into GaN for growth temperatures above 700 °C, it can be used as a surfactant in MBE growth of GaN.
Soft-breakdown-suppressed ultrathin atomic-layer-deposited silicon–nitride/SiO2 stack gate dielectrics for advanced complementary metal–oxide–semiconductor technology
Authors: Khosru Quazi Deen Mohd , Nakajima Anri , Yoshimoto Takashi , Yokoyama Shin
Published: July 2002

We report a high-quality, ultrathin atomic-layer-deposited silicon–nitride/SiO2 stack gate dielectric. p+-polycrystalline silicon gate metal–oxide–semiconductor (MOS) capacitors with the proposed dielectrics showed enhanced reliability with respect to conventional SiO2. An exciting feature of suppressed soft-breakdown (SBD) events is observed in ramped voltage stressing which has been reconfirmed during time-dependent-dielectric breakdown measurements under constant field stressing. Introducing the idea of injected-carrier-induced localized physical damages resulting in the formation of conductive filaments near both Si/SiO2 and poly-Si/SiO2 interfaces, a model has been proposed to explain the SBD phenomena observed in the conventional SiO2 dielectrics. It is then consistently extended to explain the suppressed SBD in the proposed dielectrics. The reported dielectric can be a good choice to meet the urgent need for highly reliable ultrathin gate dielectrics in nanoscale complementary-MOS technology.
The construction of exact multipolar equilibria of the two-dimensional Euler equations
Authors: Crowdy Darren
Published: July 2002

Using ideas involving the Schwarz function of analytic curves, a new class of exact multipolar equilibria of the two-dimensional Euler equations characterized by an annular region of vorticity enclosing a region of irrotational fluid is constructed. The results generalize a recently derived class of exact solutions for multipolar vortex equilibria [Crowdy, Phys. Fluids 11, 2556 (1999)]. The solutions have many qualitative similarities to the multiple-vortex nonlinear saturation states of an unstable annulus of uniform vorticity. More generally, the results suggest the possibility of constructing multipolar equilibria of the steady Euler equations having distributed vortical regions of more or less arbitrary geometrical complexity.
Photoluminescence of GaN grown by molecular-beam epitaxy on a freestanding GaN template
Authors: Reshchikov M. A. , Huang D. , Yun F. , He L. , Morkoç H. , Reynolds D. C. , Park S. S. , Lee K. Y.
Published: July 2002

Photoluminescence (PL) studies were performed on a 1.5-μm-thick GaN layer grown by molecular-beam epitaxy on a freestanding GaN template that in turn was grown by hydride vapor-phase epitaxy. PL spectra from both the epilayer and the substrate contain a plethora of sharp peaks related to excitonic transitions. We identified the main peaks in the PL spectrum. Taking advantage of the observation of donor bound exciton peaks and their associated two-electron satellites, we have determined the binding energies of two distinct shallow donors (28.8 and 32.6 meV), which are attributed to Si and O, respectively.
Local structure and chemical valency of Mn impurities in wide-band-gap III–V magnetic alloy semiconductors Ga1−xMnxN
Authors: Soo Y. L. , Kioseoglou G. , Kim S. , Huang S. , Kao Y. H. , Kuwabara S. , Owa S. , Kondo T. , Munekata H.
Published: July 2002

Local structure and effective chemical valency of Mn impurity atoms incorporated in wide-band-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques. The x-ray results provide direct evidence for the substitution of majority Mn atoms for the Ga sites in GaN, with an effective valency close to Mn(II), up to a rather high Mn concentration about 2 at. %. A small fraction of the impurity atoms could also form Mn clusters.
High absorption (>90%) quantum-well infrared photodetectors
Authors: Liu H. C. , Dudek R. , Shen A. , Dupont E. , Song C. Y. , Wasilewski Z. R. , Buchanan M.
Published: July 2002

The intrinsic short carrier lifetime (∼5 ps) makes the quantum-well infrared photodetector (QWIP) well suited for high speed and high frequency applications. In such cases, since lasers are commonly used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature operation is desirable. Using GaAs/AlGaAs QWIPs, high absorption (∼100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.