bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of annealing a silicon wafer in argon with a very low oxygen partial pressure
Authors: Suzuki Toshiharu
Published: July 2002

The surface roughening of a silicon wafer during annealing in argon at an extremely low oxygen partial pressure, which restricts the conditions for the suppression of oxidation-induced stacking faults, was investigated. It was found that surface roughening occurs when the oxygen partial pressure is lower than 3×10−5 atm. The roughening is brought about by the generation of square pits and the anomalous growth of film. Fiber-like material is, sometimes, formed in the periphery of the wafer. It was found that grown film and the fiber-like material are SiO2 and that the pits have the shape of an inverted pyramid. The mechanism of this roughening is discussed with considering a boundary layer. As a result, the formation of the inverted-pyramidal pits is attributed to the occurrence of SiO evaporation in the boundary layer, where the gas flow is stagnant and the oxygen partial pressure is extremely low. The four exposed surfaces at the bottom of a pit have a {111} surface orientation. The anomalous growth of the oxide and the formation of the fiber-like material were interpreted to be due to the reoxidation of the evaporating SiO at the edge of the boundary layer and its redeposition in the form of SiO2 on the wafer. It was concluded that the inclusion of oxygen at a partial pressure higher than 4×10−5 atm is important to prevent surface roughening during annealing even in an inert gas like argon.
The resistive wall instability and critical flow velocity
Authors: Lashmore-Davies C. N.
Published: July 2002

A cylindrical model with an equilibrium surface current and a uniform equilibrium plasma flow velocity parallel to the axis of the cylinder is used to investigate resistive wall instability. This system can be unstable to the ideal, external kink mode, which can be stabilized by the presence of a perfectly conducting wall. This is the classic condition for the resistive wall instability and the effect of a plasma flow velocity on this mode is explored. It is noted that a stable kink mode, Doppler shifted by the flow velocity, can pass through zero frequency for a velocity which depends on the marginal condition for the external kink instability. The passage through zero frequency is the condition for the kink mode to carry negative energy. It is shown how this mode implies a critical flow speed at which the resistive wall mode is further destabilized, with a growth rate inversely proportional to the square root of the wall time. Under these circumstances, the resistive wall mode behaves more like an ideal instability. All flow velocities are shown to be potentially destabilizing and the flow velocity can produce a resistive wall instability even when the plasma is stable to the external kink mode in the absence of a wall. At velocities well above the critical flow speed, the resistive wall growth rate is much reduced (inversely proportional to the wall time and to the flow speed).
Growth of heteroepitaxial ZnO thin films by femtosecond pulsed-laser deposition
Authors: Millon E. , Albert O. , Loulergue J. C. , Etchepare J. , Hulin D. , Seiler W. , Perrière J.
Published: July 2002

ZnO thin films have been grown on various substrates by femtosecond pulsed-laser deposition. According to optical microscopy and atomic force microscopy analyses, the production of droplets is not significant using femtosecond pulses. Smooth, dense, stoichiometric, crystalline, and textured hexagonal ZnO films are epitaxially grown on (0001) sapphire at 700 °C with an in-plane epitaxial relationship corresponding to a 30° rotation of the ZnO basal plane with respect to the sapphire. Nevertheless, channeling experiments and rocking curve measurements show that the crystalline quality is not as good as that obtained with nanosecond pulses.
Characterization methodology for pseudomorphic high electron mobility transistors using surface photovoltage spectroscopy
Authors: Solodky S. , Leibovitch M. , Ashkenasy N. , Hallakoun I. , Rosenwaks Y. , Shapira Yoram
Published: July 2002

Pseudomorphic high electron mobility transistor structures have been characterized using surface photovoltage spectroscopy and numerical simulations. According to the effect of the electric fields in different regions of the device on the surface photovoltage spectra, a simple empirical model that correlates the spectral parameters and electrical parameters of the structure has been developed. The spectra and their analysis are shown to provide values for the electrical parameters of the structure. The sensitivity of the technique to the device electrical parameters is shown by three different examples. In these examples, the differences in doping level and surface charge have been monitored as well as the nonuniformity of doping level across the wafer.
Effects of temperature and other experimental variables on single molecule vibrational spectroscopy with the scanning tunneling microscope
Authors: Lauhon L. J. , Ho W.
Published: July 2002

Inelastic electron tunneling spectroscopy (IETS) was performed on single molecules with a variable temperature scanning tunneling microscope. The peak intensity, width, position, and line shape of single molecule vibrational spectra were studied as a function of temperature, modulation bias, bias polarity, and tip position for the (C–H,C–D) stretching vibration of acetylene (C2H2,C2D2) on Cu(001). The temperature broadening of vibrational peaks was found to be a consequence of Fermi smearing as in macroscopic IETS. The modulation broadening of vibrational peaks assumed the expected form for IETS. Extrapolation of the peak width to zero temperature and modulation suggested an intrinsic width of ∼4 meV due primarily to instrumental broadening. The inelastic tunneling cross section at negative bias was reduced by a factor of 1.7 for the C–H stretch mode. Low energy modes of other molecules did not show such a reduction. There was no evidence of a tip-induced Stark shift in the peak positions. The spatial variation of the inelastic signal was measured to determine the junction stability necessary for the acquisition of single molecule vibrational spectra.
Slow-time-scale magnetic fields driven by fast-time-scale waves in an underdense relativistic Vlasov plasma
Authors: Zhu Shao-ping , He X. T. , Zheng C. Y.
Published: July 2002

Slow-time-scale magnetic fields driven by fast-time-scale electromagnetic waves or plasma waves are examined from the perspective of the Vlasov–Maxwell equations for a relativistic Vlasov plasma. An equation for slow-time-scale magnetic field is obtained. The field proposed in the present paper is a result of wave–wave beating which drives a solenoidal current. The magnitude of the slow-time-scale magnetic field proposed here can be as high as 20 MG at the critical surface for a laser intensity I=1018 W/cm2 at wavelength λ0=1.05 μm. The predicted magnetic field is observed in two-dimensional particle simulations presented here.
Lagrangian invariants of trapped electron dynamics in tokamaks
Authors: Peng X. D. , Qiu X. M.
Published: July 2002

The trapped electron dynamics is considered in general tokamak magnetic field with positive or reversed shear. Starting from the continuity, energy-evolution, and motion equations of the trapped electron fluid and the definition of Lagrangian invariant, the Lagrangian invariants hidden in the dynamics are strictly found: L=ln[(n/B)c1(T/B2/3)c2], where c1 and c2 are dimensionless changeable parameters and c1∝c2. It yields n/B=const and T3/2/B=const. Further, based on them it is shown that 〈n〉ψq(ψ)=const and 〈T3/2〉ψq(ψ)=const. The former invariant qualitatively fits the experimental data in many tokamaks; the latter may be used to explain the steady-state hollow T-profile (the corresponding hollow j-profile) observed in reversed shear tokamak plasmas.
Calculations of Alfvén wave driving forces, plasma flow, and current drive in the Tokamak Chauffage Alfvén wave experiment in Brazil (TCABR)
Authors: Amarante-Segundo G. , Elfimov A. G. , Galvão R. M. O. , Ross D. W. , Nascimento I. C.
Published: July 2002

The current and plasma flows driven by ponderomotive forces are calculated for tokamak plasmas, using a kinetic code in the Alfvén range of frequencies. The rf (radio frequency) ponderomotive force is expressed as a sum of a gradient part and of a wave momentum transfer force, which is proportional to wave dissipation (electron Landau damping and transit time magnetic pumping). Finally, the rf force is balanced by the viscous force in the fluid momentum response to the rf fields in the plasma. The relative magnitudes of the different forces for kinetic and global Alfvén waves with low phase velocities are explicitly calculated. It is shown that, dissipating in electrons, Alfvén waves can drive ion flow via the gradient force, which is dominated in m=0-sideband harmonic resonance induced by toroidal mode coupling. Estimates of power requirements to drive substantial poloidal flow in the Tokamak Chauffage Alfvén wave heating experiment in Brazil (TCABR) [L. Ruchko, M. C. Andrade, R. M. O. Galvão, Nucl. Fusion 30, 503 (1996)] are made.
Resistive instabilities in Hall current plasma discharge
Authors: Litvak Andrei A. , Fisch Nathaniel J.
Published: July 2002

Plasma perturbations in the acceleration channel of a Hall thruster are found to be unstable in the presence of collisions. Both electrostatic lower-hybrid waves and electromagnetic Alfvén waves transverse to the applied electric and magnetic field are found to be unstable due to collisions in the E×B electron flow. These results are obtained assuming a two-fluid hydrodynamic model in slab geometry. The characterisitic frequencies of these modes are consistent with experimental observations in Hall current plasma thrusters.
Molecular dynamics analog of the reverse Monte Carlo method
Authors: Tóth Gergely , Baranyai András
Published: July 2002

A new molecular dynamics algorithm is reported to evaluate data of diffraction experiments on liquids or amorphous materials. The atoms, in addition to the excluding short-range forces, are moved in a fictitious potential field defined by the chi-square difference of the experimental structure factors. The results provided by the new reverse molecular dynamics algorithm are equivalent to those obtained using the reverse Monte Carlo method for the test model systems of the Lennard-Jones liquid and water. We exploited the test calculations of the water system to discuss the separation capabilities of these modeling techniques. The results were encouraging, if the number of different (good quality) experimental functions as input information is larger or equivalent with the number of particle pair-correlation functions necessary to characterize the structure of the system. However, the results indicated dubious outcomes, if there were more partials estimated than different experimental data sets. This questions the conclusions of some previous studies.
Instabilities of thermocapillary convection in a half-zone at intermediate Prandtl numbers
Authors: Levenstam Mårten , Amberg Gustav , Winkler Christian
Published: July 2002

The stability of thermocapillary convection inside a cylindrical liquid bridge is studied using both a direct numerical simulation of the three-dimensional problem and linear stability analysis of the axisymmetric basic state. Previously this has been studied extensively for low and high Prandtl numbers. However, the intermediate range of Prandtl numbers between approximately 0.07 and 0.8 which joins the low and high ranges is quite complicated and has not been studied to the same extent. One striking feature is that the axisymmetric base state is much more stable in this intermediate range than at high or low Prandtl numbers. We identify four different oscillatory modes in this range, which have different qualitative features. Direct numerical simulations have been carried out for representative parameter values, and show that the bifurcations are supercritical.
Ground state of the quantum anisotropic planar rotor model: A finite size scaling study of the orientational order–disorder phase transition
Authors: Hetényi Balázs , Berne Bruce J.
Published: July 2002

The ground state properties of the quantum anisotropic planar rotor (QAPR) model, which was constructed to describe the orientational ordering of homonuclear diatomic molecules on inert surfaces, are investigated theoretically using diffusion Monte Carlo. The implementation of the descendant weighing (DW) technique due to Casulleras and Boronat [Phys. Rev. B 52, 3654 (1995)] is used, for which an alternate derivation is presented, based on the path-integral representation of the imaginary time propagator. We calculate the order parameter and then perform finite size scaling in order to search for a critical reduced rotational constant Bc* at zero temperature. Our simulation results indicate that a critical rotational constant is at Bc*≈0.25. The behavior of the kinetic and potential energies show strong evidence for local, single-rotor tunneling as the driving mechanism for the phase transition. A Gaussian mean-field treatment is also presented, in which the most important mechanism is local, single-rotor tunneling. While quantitatively the mean-field phase transition is not in agreement with the simulation results, the energy curves show qualitative similarities. In both cases, the phase transition occurs at the point where the kinetic energy reaches a maximum as a function of the reduced rotational constant B*.
Effect of hydrogen on the photoluminescence of Si nanocrystals embedded in a SiO2 matrix
Authors: Cheylan S. , Elliman R. G.
Published: July 2002

Hydrogen passivation of Si nanocrystals is shown to result in a redshift of photoluminescence (PL) emission spectra, as well as the more commonly observed intensity increase. The shift is reversible, with spectra returning to their unpassivated values as hydrogen is removed from the samples by annealing. The magnitude of the redshift also depends on the implant fluence employed for nanocrystal synthesis, increasing with increasing fluence or particle size. These data are shown to be consistent with a model in which larger crystallites are assumed to contain a greater number of nonradiative defects, i.e., the number of nonradiative defects is assumed to scale with the surface area or volume of a nanocrystal. Hydrogen passivation then results in a disproportionate increase in emission from larger crystallites, giving rise to an apparent redshift in the composite PL emission spectrum.
Modeling of the competition of stimulated Raman and Brillouin scatter in multiple beam experiments
Authors: Cohen Bruce I. , Baldis Hector A. , Berger Richard L. , Estabrook Kent G. , Williams Edward A. , Labaune Christine
Published: July 2002

Multiple laser beam experiments with plastic target foils at the Laboratoire pour L’Utilisation des Lasers Intenses (LULI) facility [Baldis et al., Phys. Rev. Lett. 77, 2957 (1996)] demonstrated anticorrelation of stimulated Brillouin and Raman backscatter (SBS and SRS). Detailed Thomson scattering diagnostics showed that SBS always precedes SRS, that secondary electron plasma waves sometimes accompanied SRS appropriate to the Langmuir Decay Instability (LDI), and that, with multiple interaction laser beams, the SBS direct backscatter signal in the primary laser beam was reduced while the SRS backscatter signal was enhanced and occurred earlier in time. Analysis and numerical calculations are presented here that evaluate the influences on the competition of SBS and SRS, of local pump depletion in laser hot spots due to SBS, of mode coupling of SBS and LDI ion waves, and of optical mixing of secondary and primary laser beams. These influences can be significant. The calculations take into account simple models of the laser beam hot-spot intensity probability distributions and assess whether ponderomotive and thermal self-focusing are significant. Within the limits of the model, which omits several other potentially important nonlinearities, the calculations suggest the effectiveness of local pump depletion, ion wave mode coupling, and optical mixing in affecting the LULI observations.
An in situ x-ray photoelectron spectroscopy study of AlOx spin tunnel barrier formation
Authors: Kottler V. , Gillies M. F. , Kuiper A. E. T.
Published: July 2002

Thin aluminum oxide barrier layers have been studied by in situ x-ray photoelectron spectroscopy to determine their oxidation degree dependence on oxidation time and method (thermal or plasma oxidation). A 1.5 nm thick Al layer is found to completely oxidize by exposure to an oxygen plasma for a time in the interval 30–60 s, i.e., using the conventional plasma oxidation method. For times less than 30 s, however, we observed not only a metallic-Al peak but the formed oxide was substoichiometric. The composition of the formed oxide increased towards Al2O3 as the oxidation continued. It was also found possible to oxidize up to 1 nm of Al, at room temperature, upon deposition on Co previously exposed to 9.3 Pa (70 mTorr) oxygen for 10 s. Annealing junctions with the idealized structure Co/Al2O3/Co at up to 275 °C was found to increase their magnetoresistance (up to 35%) and resistance (up to a decade), if the Al was deposited on an oxidized Co bottom electrode.
Nonlocal theory and turbulence of the sheath-driven electron temperature gradient instability
Authors: McCarthy D. R. , Catto Peter J. , Krasheninnikov S. I.
Published: July 2002

A nonlocal linear theory of the sheath-driven ∇Te mode for a uniformly increasing temperature profile that properly includes effects of a varying Larmor radius has been developed. This theory was used to verify nonlinear fluid simulations of this mode in a two-dimensional slab. The nonlinear evolution of the mode was found to depend not only upon the growth rate, but also upon the variation of the E×B drift over the extent of the radial eigenmode. When the difference in the E×B drift frequency exceeded the growth rate, the linear mode structure was distorted at early times leading to a mixing of the vortices, which allowed a rapid nonlinear growth of the longer wavelengths. When the mode grew faster than any tilting of the vortices, the fluctuations remained coherent and localized for many growth times. Not until the fluctuations grew to very large amplitude (∼50%) did the longer wavelength modes grow and contribute to the turbulence. By comparing the analytical growth rate and the variation of the equilibrium drift frequency, a condition that predicts which state the mode will evolve to was derived in terms of physical parameters of the system.
Plasma kinetics in dusty plasmas
Authors: Ricci Paolo , Lapenta Giovanni , de Angelis U. , Tsytovich V. N.
Published: July 2002

The time evolution of the plasma distribution functions (electrons and ions) in dusty plasmas is found solving numerically the equations of the kinetic theory of dusty plasmas [Tsytovich and de Angelis, Phys. Plasmas 6, 1093 (1999); 7, 554 (2000)]. The role and importance of the plasma source and particle diffusion are investigated. The “equilibrium” distributions (asymptotic solutions) are used to calculate the static screening of a dust particle, and the difference with the usual result (Debye screening) of a three-component plasma is shown for various parameter regimes.
Dislocation reduction in GaN grown by hydride vapor phase epitaxy via growth interruption modulation
Authors: Zhang Wei , Roesel Stephan , Alves Helder R. , Meister Dirk , Kriegseis Wilhelm , Hofmann Detlev M. , Meyer Bruno K. , Riemann Till , Veit Peter , Blaesing Juergen , Krost Alois , Christen Juergen
Published: July 2002

Thick GaN layers have been deposited on c-Al2O3 (0001) substrates using hydride vapor phase epitaxy by modulating the growth process via switching on/off GaN growth. Cathodoluminescence and transmission electron microscopy images of the cross-sectional structure show that there are separated multilayers structure in the GaN films. The dislocations density decreases from 1010 cm−2 in the initial layer to 109–108 cm−2 in the subsequently grown layer and then to 107 cm−2 in the top layer. This shows that the interruption of the growth process is helpful for suppression of structural defects. X-ray diffraction and photoluminescence measurements also demonstrate the high quality of the GaN films.
Structure of the ambient temperature alkali metal molten salt AlCl3/LiSCN
Authors: Lee Yi-Chia , Price David L. , Curtiss Larry A. , Ratner Mark A. , Shriver Duward F.
Published: July 2002

The structure of the ambient temperature alkali metal molten salt system LiSCN/AlCl3 1:1 adduct was investigated by neutron diffraction, which demonstrates that the aluminum atom is surrounded by three chlorine atoms and one nitrogen atom, indicating the existence of the AlCl3NCS− anion, in which the NCS− coordinates to the Al center through nitrogen. Molecular orbital calculations using ab initio methods are also performed to study the optimized structures of the AlCl3NCS− and its isomer, AlCl3SCN−. The results are consistent with the neutron diffraction data and indicate that AlCl3NCS− is the major anionic complex in the 1:1 LiSCN/AlCl3 adduct.
Effects of induced steric hindrance on the dielectric behavior and H bonding in the supercooled liquid and vitreous alcohol
Authors: Johari G. P. , Kalinovskaya O. E. , Vij J. K.
Published: July 2002

The extent of H bonding in alcohols may be reduced by sterically hindering its OH group. This technique is used here for investigating the reasons for the prominent Debye-type dielectric relaxation observed in monohydroxy alcohols [Kudlik et al., Europhys. Lett. 40, 549 (1997); Hansen et al., J. Chem. Phys. 107, 1086 (1997); Kalinovskaya and Vij, ibid. 112, 3262 (2000)], and broadband dielectric spectroscopy of supercooled liquid and glassy states of 1-phenyl-1-propanol is performed over the 165–238 K range. In its molecule, the steric hindrance from the phenyl group and the existence of optical isomers reduce the extent of intermolecular H bonding. The equilibrium permittivity data show that H-bonded chains do not form in the supercooled liquid, and the total polarization decays by three discrete relaxation processes, of which only the slower two could be resolved. The first is described by the Cole–Davidson-type distribution of relaxation times and a Vogel–Fulcher–Tammann-type temperature dependence of its average rate, which are characteristics of the α-relaxation process as in molecular liquids. The second is described by a Havriliak–Negami-type equation, and an Arrhenius temperature dependence, which are the characteristics of the Johari–Goldstein process of localized molecular motions. The relaxation rate’s non-Arrhenius temperature dependence has been examined qualitatively in terms of the Dyre theory, which considers that the apparent Arrhenius energy itself is temperature dependent, as in the classical interpretations, and quantitatively in terms of the cooperatively rearranging region’s size, without implying that there is an underlying thermodynamic transition in its equilibrium liquid. The relaxation rate also fits the power law with the critical exponent of 13.4, instead of 2–4, required by the mode-coupling theory, thereby indicating the ambiguity of the power-law equations.
Monoclinic carbon nitride phase coherently grown on Si (001) substrates
Authors: Barucca G. , Majni G. , Mengucci P. , Leggieri G. , Martino M.
Published: July 2002

Carbon nitride films deposited on Si (001) substrates at room temperature by XeCl laser ablation of graphite targets in low pressure (1, 5, 10 and 50 Pa) N2 atmosphere at fluence of 12 J/cm2 (∼0.4 GW/cm2) have been investigated by x-ray diffraction and transmission electron microscopy in order to study the structure of the films. The results showed that the samples are constituted of a continuous amorphous film inside which microcrystals of a coherently grown CNx phase are dispersed. This phase is monoclinic with the following lattice parameters: a=b=0.384 nm, c=0.302 nm, α=γ=90°, and β=96.5°. The CNx phase grows with the (001) plane coincident with the (001) plane of the Si substrate and with the [010]CNx direction parallel to the [110]Si one.
The NO(X2Π)–Ne complex. I. IR-REMPI double resonance spectroscopy
Authors: Kim Y. , Fleniken J. , Meyer H.
Published: July 2002

The rotationally resolved infrared spectrum of the NO–Ne complex associated with the first overtone transition in NO(X2Π) is measured in an IR-UV double resonance experiment. (2+1) resonance enhanced multiphoton ionization (REMPI) involving the Rydberg states E2Σ, F2Δ, and H2Σ,H′2Π is used to detect the infrared absorption. In the complex, the NO overtone transition is located at 3724.02 cm−1, i.e., it is blueshifted by 0.17 cm−1 from the transition in the monomer. Three other bands detected at frequencies 3727.85, 3732.56, and 3739.20 cm−1 are assigned to combination bands involving different intermolecular bend-stretch vibrations. The rotational structure of the observed bands is consistent with a near T-shaped complex. Details of the rotational structure resemble the spectrum of a symmetric top subject to weak electronic interactions causing a Stark-type and P-type doubling of the rotational levels.
The HF stretch red shift as a function of internal energy in Arn–HF (n=12,54): Comparisons in the microcanonical ensemble
Authors: Curotto E.
Published: July 2002

A method for the evaluation of the HF stretch red shift as a function of energy for Arn–HF clusters is implemented. The estimator is obtained by extending the canonical ensemble expression to microcanonical simulations. At very low energies the red shift increases linearly with a slope of 0.0452±1×10−4 cm−1 K−1 for Ar12–HF and 0.0325±1×10−4 cm−1 K−1 for Ar54–HF. The difference in behavior between the two clusters is attributed to differences in isomerizations. These are investigated using a structural comparison algorithm. The core to surface isomerization is evident in the small cluster before melting, though its effects on the physical properties are more subtle than in the canonical ensemble. The HF–Ar exchange isomerizations are practically absent in the larger cluster prior to melting. These findings are in agreement with previous canonical simulations.
Plural voltage minima in an arc-heated channel flow
Authors: Sasoh A.
Published: July 2002

In flows through a channel with varying cross-sectional area, the impulse and total enthalpy can be increased by superimposing an electrical discharge. The flow field is determined from the inlet flow condition, channel geometry, and discharge specifications. In this study, steady-state, quasi-one-dimensional flows interacting with an arc discharge are computed numerically. Once the arc column configuration is given, the discharge voltage is computed from the solution of flow field variables. For a constant discharge current, there exist plural column configurations which yield a minimum discharge voltage. This result explains the fluid-dynamic mechanisms of the existence of plural voltage modes in an arcjet operation.
In situ determination of absolute number densities of nitrogen molecule triplet states in an rf-plasma sheath
Authors: Krames B. , Glenewinkel-Meyer Th. , Meichsner J.
Published: July 2002

A laser induced fluorescence technique (LIF) in combination with optical emission spectroscopy (OES) and Rayleigh scattering (RS) was applied to investigate absolute number densities of the population of the three nitrogen triplet states C 3Πu, B 3Πg, and the metastable A 3Σu+ in an asymmetric low pressure rf discharge. Primary targets of this investigation were the three lowest vibrational levels (v=0,1,2) of each triplet state and additionally v=8 of A 3Σu+ which can be populated very efficiently in the plasma sheath. Calibration of LIF intensities to absolute densities of A 3Σu+ and B 3Πg has been realized by comparison with the signal of the RS experiment done in pure nitrogen gas. Calibration of C 3Πu which we could not detect by LIF but rather only by OES was achieved after comparing OES and LIF signals of the B 3Πg state. Excitation energies of the analyzed states range from 6 up to 11.5 eV and the measured number densities differ by about seven orders of magnitude from as much as 1012 cm−3 down to almost 105 cm−3. In addition to the interpretation of the results this article describes the used calibration methods which are partly based on a suggestion made by P. Bogen (Proceedings of the XVI International Conference on Phenomena in Ionized Gases, Invited Papers, edited by W. Bötticher, H. Wenk, and E. Schultz-Gulde, Düsseldorf, 1983, pp. 164–173).
Interaction of hydrogen atoms with Si(111) surfaces: Adsorption, abstraction, and etching
Authors: Dinger A. , Lutterloh C. , Küppers J.
Published: July 2002

The interaction of H atoms with Si(111) surfaces with respect to adsorption, abstraction, and etching was investigated using thermal desorption and product detection techniques. The study covers a wide range of coverages and the temperature range 100–1000 K. After H admission to Si(111) at 100 K in H2 desorption spectra decomposition of trihydride (t), dihydride (d), and monohydride (m) was observed around 455, 700, and 820 K, respectively. Adsorption of H at 380 K leads to desorption from d and m, and after admission of H at 680 K desorption from m was observed. The kinetics of m, d, and t desorption is according to first-order kinetics, only the m peak exhibits at small coverages second-order phenomenology. H exposure above 400 K leads to desorption of subsurface α-hydrogen at 920 K in thermal desorption spectra. Nonstationary etching via silane formation was monitored around 630 K. The nonstationary silane etch peak occurs through a quasi-first-order process in the admission temperature range 100–500 K and assumes a second-order phenomenology at admission temperatures between 500 and 600 K. This silane is formed through the recombination of surface silyl (t) and H in silylene (d) groups. Its yield decreases with the temperature at which H was admitted and is negligible after admission above 620 K since silyl groups are no longer available on the surface. Stationary etching during subjecting the surface with a continuous H flux occurs via a direct reaction step between the incoming H and surface silyl groups. The stationary etch yield decreases from 200 to 600 K due to depletion of surface silyl groups. In parallel to stationary etching, H abstraction proceeds with much higher probability. The kinetics of D abstraction by H from the monodeuteride phase at 680 K, measured through the HD product rate, as well as the formation of homonuclear D2 products contradict the operation of an Eley–Rideal (ER) mechanism, but are in excellent agreement with the solutions of a hot-atom (HA) reaction kinetic model which was recently successfully applied to abstraction on metal surfaces. This model is based solely on hot-atom processes and includes competition of reaction and sticking of hot atoms. Four parameters are needed to reproduce the measured HD rate data. At 680 K the abstraction cross section is 3.2 Å2 and about 5% of the adsorbed D occurs in D2 products. Subsurface α-D is abstracted at 680 K or higher temperatures with a cross section of 1.2 Å2. Abstraction at lower temperatures, either from monodeuteride surfaces or from surfaces saturated with di- and trideuteride proceeds with a smaller cross section and a reduced D2 product yield. At 100 K the HD cross section is only 2.2 Å2 (monodeuteride) or 1.4 Å2 (saturated surface), the HD kinetics is phenomenologically like that required by the ER mechanism, and a negligible quantity of D2 is formed. The HA reaction model allows one to reproduce these features by adjusting the model parameters accordingly.
Selective growth of InAs self-assembled quantum dots on nanopatterned SiO2/Si substrate
Authors: Choi B. H. , Park C. M. , Song S.-H. , Son M. H. , Hwang S. W. , Ahn D. , Kim E. K.
Published: July 2002

We report the selective growth of InAs self-assembled quantum dots (SAQDs) on silicon-dioxide/silicon (SiO2/Si) substrates patterned in nanometer scale. The SiO2 thin film is found to be an efficient mask material for prohibiting the growth of InAs SAQDs, while the formation of stable SAQDs is observed on the exposed surface of Si. We have utilized this selectivity to demonstrate almost one-dimensional alignment of InAs SAQDs on Si stripes. The crystallinity of SAQDs is also identified by high-resolution transmission electron microscope observation. Our study opens up a possibility of reliably integrating III–V quantum dot devices with conventional Si circuits.
Influence of initial velocity field on the formation of coherent structures in simple hydrodynamic flows
Authors: Karimov A. R. , Schamel H.
Published: July 2002

The formation of essentially nonlinear, time-dependent structures in the compressible advection problem is examined. Based on the Lagrangian description, the influence of initial conditions on the emergence of new nonlinear structures is studied. Conditions leading to the formation of new regular, as well as singular, collapse-type structures and wavelet structures in the density profile are presented.
Manipulation of phase and amplitude modulation of spin magnetization in magic angle spinning nuclear magnetic resonance in the presence of molecular diffusion
Authors: Liu Yun , Leu Gabriela , Singer S. , Cory D. G. , Sen Pabitra N.
Published: July 2002

Nuclear magnetic resonance (NMR) experiments with a spinning sample [magic angle sample spinning (MASS)] are used to remove the line broadening in composite systems, where the susceptibility contrast of its constituents gives rise to an inhomogeneous field that causes a line broadening and obscures chemical information. The NMR signal in these experiments has a phase and an amplitude part. In the absence of diffusion, i.e., in the MASS spectra of solids, the amplitude of the signal from an isochromat is a constant independent of position and time and the phase is a periodic function of the rotor frequency νr. In fluids, the amplitude of a spin packet is a function of its position and time. The amplitude modulation and relaxation in diffusive MASS encodes the dynamics of motion and the landscape (geometry of pores and field gradients) probed by the motion. Here we use spin manipulation—total suppression of sidebands (TOSS)—to suppress the effects of phase with the goal of isolating the amplitude term. By the TOSS sequence the phase factor at time t for a spin packet at an azimuthal angle φ is made to depend on φ only as a function of ωrt−φ, which suppresses the sidebands in solids upon an integration over φ. Due to molecular diffusion, the amplitude part depends on φ, and, thus, diffusive TOSS cannot suppress the sidebands. The residual sidebands carry the information of dynamics and pore and magnetic field geometry, in addition, by reducing the size of the sidebands, TOSS is of course, also useful in identifying various fluid components in situ. The diffusive MASS gives a measure of the spread in local fields and diffusive TOSS gives a measure of the spread in local gradients.
Multifractality in edge localized modes in Japan Atomic Energy Research Institute Tokamak-60 Upgrade
Authors: Bak P. E. , Asakura N. , Miura Y. , Nakano T. , Yoshino R.
Published: July 2002

The temporal losses of confinement during edge localized modes in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) show multifractal scaling and the spectra are generally smooth, but in some cases there are signs of discontinuous derivatives. Dynamics of the Sugama–Horton model, interpreted as edge localized modes, also display multifractal scaling. The spectra display singularities in the derivative, which can be interpreted as a phase transition. It is argued that the multifractal spectra of edge localized modes can be used to discriminate between different experimental discharges and validate edge localized mode models.
Theory for the electron precipitation and current system that produces a magnetospheric substorm
Authors: Calvert W.
Published: July 2002

It is found that the electron precipitation during a magnetospheric substorm can be attributed to particle scattering into the loss cone by the cyclotron maser instability, where the resulting current system produces the accompanying dipolarization of the Earth’s magnetic field. This model for a substorm thereby explains substorm expansion, and predicts that the incoming electron precipitation during a substorm has to be the actual cause of a magnetospheric substorm.
Ground-based observations and plasma instabilities in auroral substorms
Authors: Friedrich E. , Samson J. C. , Voronkov I.
Published: July 2002

One of the “grand challenge” problems in space science today involves determining what mechanism triggers the intensification and expansive phase of the magnetospheric substorm. Remote sensing of the ionosphere has provided some clues to the process, leading researchers to the conclusion that the physics of the intensification is inherently fast and explosive. In particular, ground-based observations show the brightening of the auroral arc and the formation of large scale vortex structures in the ionosphere to take place on time scales of tens of seconds. High earthward pressure gradients, enhanced field line curvature, and strong convective flows with large-amplitude westward components, point to the shear flow ballooning instability (SFBI) as the mechanism leading to the intensification. New results from the meridian scanning photometers and the all sky imager of the Canadian Auroral Network for the OPEN Program Unified Study array show the time delay between the formation of these vortex structures and the beginning of enhanced reconnection in the midtail region of the magnetotail to be of the order of 1–5 min. This timing presents a serious constraint for many substorm models. In this paper we shall discuss the observational support for the SFBI model and highlight some of the other plasma instabilities thought to lead to expansive phase onset.
Effect of thin oxide layers incorporated in spin valve structures
Authors: Gillies M. F. , Kuiper A. E. T. , Leibbrandt G. W. R.
Published: July 2002

The enhancement of the magnetoresistance effect, induced by incorporating nano-oxide layers (NOLs) in a bottom-type spin valve, was studied for various preparation conditions. The effect of a NOL in the Co90Fe10 pinned layer was found to depend critically on the oxygen pressure applied to form the thin oxide film. Pressures over 10−3 Torr O2 yield oxides thicker than about 0.7 nm, which apparently deteriorate the biasing field which exists over the oxide. The magnetoresistance values can further be raised by forming a specular reflecting oxide on top of the sense layer. Promising results were obtained with an Al2O3 capping layer formed in a solid-state oxidation reaction that occurs spontaneously when a thin Al layer is deposited on the oxidized surface of the Co90Fe10 sense layer.
Comparison of semiclassical calculations to experiment for spin valve films grown on oxide surfaces
Authors: Gibbons Matthew R. , Mao Ming , Chien Chester
Published: July 2002

The physical properties of spin valve films grown on the surface of nano-oxide layers were studied as a function of Cu spacer layer thickness. In comparison to identical structures without the oxide surface, the films exhibited an increase in ΔR/R of 30% accompanied by a reduction of only 5% in resistance. Semiclassical calculations were preformed on these films with a close match to experiment. Assuming the oxide layer did not cause drastic changes in the properties of the other film layers, the specular reflection was changed to obtain a match with experiment. The increase in giant magnetoresistance response was achieved by increasing the specular reflection at the metal/oxide interfaces from 15% to 85%, indicating high efficiency for specular reflection at the nano-oxide interface.
Lowering of ordering temperature for fct Fe–Pt in Fe/Pt multilayers
Authors: Endo Yasushi , Kikuchi Nobuaki , Kitakami Osamu , Shimada Yutaka
Published: July 2002

We have explored magnetic properties and structural characteristics of Fe/Pt multilayers after annealing at various temperatures in order to clarify the growth process of the ordered Fe–Pt phase in the Fe/Pt multilayer structures. It is found that rapid diffusion at Fe/Pt interface occurs at temperatures 275–325 °C and the multilayer structure directly transforms to the ordered (fct) phase when Fe and Pt layer thickness is almost equal. The ordering parameter S is evaluated to be 0.50–0.65 after annealing at 300–325 °C, and then is significantly enhanced to ∼1.00 with increasing annealing temperatures. By comparing the thermal processes of these different multilayer structures, it is found that the rapid formation of the fct phase in the multilayers with Fe/Pt≅1 is due to relatively rapid diffusion at the interface.
Interdiffusion in direct ion beam deposited isotopic Fe/Si trilayers
Authors: Telling N. D. , Faunce C. A. , Bonder M. J. , Grundy P. J. , Lord D. G. , Van den Berg J. A. , Langridge S.
Published: July 2002

Interdiffusion and intermixing have been examined in Fe/Si trilayers prepared with ions of different energies, using the direct ion beam deposition technique. Isotope-pure layers were deposited and spin-polarized neutron reflectivity used to determine the role of Fe diffusion in iron silicide formation. It was found that a nonmagnetic iron silicide was formed that contained Fe from both the top and bottom layers in the trilayer, suggesting complete diffusion across the spacer region. Electron microscopy observations revealed the presence of an iron silicide phase and crystalline interface layers in films prepared with low energy ions (30 eV), while mixed and amorphous-like regions were found at higher ion energies (100 eV).
Brillouin scattering cross section in Fe(110)/Cu(111)/Fe(110) asymmetric bilayers
Authors: Zivieri R. , Giovannini L. , Nizzoli F. , Carlotti G. , Gubbiotti G. , De Crescenzi M.
Published: July 2002

A detailed study of the Brillouin scattering cross section from spin waves has been carried out for Fe(110)/Cu(111)/Fe(110)/Si(111) heterostructures, with thickness of the two Fe layers of 20 Å and 60 Å, and interlayer Cu thickness variable in the range 0–33 Å. We found that for thicknesses of the Cu spacer larger than 6 Å, the Brillouin spectrum consists of the acoustical and optical spin wave modes, whose frequency separation depends on the Cu layer thickness. A good agreement between the calculated and the measured cross sections has been obtained. An explanation of the Stokes/anti-Stokes asymmetry of the acoustical mode is given in terms of interference effects between the contributions due to the two dynamic magnetization components.
Tunneling magnetoresistance and magnetic properties of Fe–Al2O3 nanogranular films
Authors: Zhu T. , Wang Y. J. , Zhao H. W. , Zhao J. G. , Zhan W. S.
Published: July 2002

Tunneling giant magnetoresistance (MR) of the Fe–Al2O3 nanogranular films has been observed over a wide range of Fe volume fraction x and it took a maximum of 4.4% at room temperature for the film with x=0.45 at H=10 kOe. Furthermore, the field dependence of MR of the samples is well described by the form proportional to the square of the magnetization. Moreover, an estimate of the magnetic anisotropy energy density Ku increases with the decrease of x, yielding a value 2 orders of magnitude greater than the value for bulk Fe when x=0.23. The Bloch’s law, MS(T)=MS0(1−BTb), can also hold for all the samples but with nonbulk parameters dependent on the Fe volume fraction. These results reveal a percolation effect on the magnetic properties, as well as the conductance, in such nanogranular films.
Stabilization of itinerant (band) magnetism in FeAl by Ga substitution for Al
Authors: Papaconstantopoulos D. A. , Mazin I. I. , Hathaway K. B.
Published: July 2002

Band structure calculations of FeGa1−xAlx have been performed, to further investigate the stability of ferromagnetism in FeAl. The Stoner parameter increases by about 20% at the FeGa end. This is also confirmed by our spin-polarized calculations. We conclude that Ga substitution for Al is likely to stabilize the elusive (or illusive) ferromagnetic state in FeAl.
Solution of the radial Schrödinger equation in cylindrical and spherical coordinates by mapped Fourier transform algorithms
Authors: Borisov A. G.
Published: July 2002

When used in the on-the-grid solvers of the stationary or time-dependent Schrödinger equation, coordinate mapping allows one to achieve a very accurate description of the wave function with an optimal number of the grid points. The efficiency of the mapped Fourier grid methods has been recently demonstrated by V. Kokoouline, O. Dulieu, R. Kosloff, and F. Masnou-Seeuws [J. Chem. Phys. 110, 9865 (1999)] and by D. Lemoine [Chem. Phys. Lett. 320, 492 (2000)]. In this paper we propose a discrete coordinate representation based on a numerical mapping in cylindrical and spherical coordinates. Within proposed approach, the Hamiltonian matrix is Hermitian, and the use of the fast cosine and sine Fourier transforms provides a very efficient way of calculating the Laplacian operator.
A gas gun for plane and shear loading of inert and explosive targets
Authors: Bourne N. K. , Stevens G. S.
Published: July 2002

The design and construction of a 75 mm bore laboratory gas gun capable of velocities up to 500 m s−1 is described. The performance of the gun is compared with the analytical interior ballistics model of Pidduck and Kent [A. E. Seigel, Report No. AGARDograph 91 (1965)]. The gun is constructed for two idealized loading geometries. One is plate impact, in which the loading is in one-dimensional strain, accomplished by impacting plane impactors onto targets aligned to micron tolerances, precisely normal to the impact axis. Another is pressure-shear in which the target and impacting plate are angled, but still aligned to the same tolerances. The system requires flexibility for addressing the problem of reactive targets when design must allow complete detonation of the target (up to 250 g). This has been accomplished and the system approved for use by the appropriate authorities.
Thermally activated deterioration processes in Co/Cu GMR multilayers
Authors: Hecker M. , Tietjen D. , Elefant D. , Schneider C. M. , Qiu An , Cramer N. , Camley R. E. , Celinski Z.
Published: July 2002

The magnitude of the giant magnetoresistance (GMR) observed in multilayers is known to change irreversibly at elevated temperatures. To improve the thermal stability of devices, a fundamental understanding of the GMR and its correlation to the temperature-induced structural and morphological changes in a given system is mandatory. We therefore investigated the structural and magnetic properties of sputtered Co/Cu multilayers in the pre- and postannealed states (temperature regime for annealing up to 750 °C) by in situ x-ray diffraction, transport measurements, ferromagnetic resonance (FMR), and magneto-optical Kerr effect (MOKE). We were able to identify a sequence of distinct structural changes each of which sets in above a characteristic critical temperature. These critical temperatures depend strongly on the thickness of the individual layers. The structural alterations observed range from interfacial sharpening through texture reorientations up to the formation of a granular state, and are associated with distinct in/decreases of the GMR signal. Using FMR and MOKE we determined in-plane magnetic anisotropies and interlayer coupling for as-grown and annealed samples.