bluuebunny/crossref_metadata_2025 huggingface dataset:
High resolution inelastic x-ray scattering spectrometer at the advanced photon source
Authors: Schwoerer-Böhning M. , Macrander A. T. , Abbamonte P. M. , Arms D. A.
Published: July 2002

We have commissioned a new instrument for high resolution inelastic x-ray scattering on the inelastic scattering beamline of the Synchrotron Radiation Instrumentation Collaborative Access Team on sector 3 of the Advanced Photon Source. So far, the instrument is set up at 13.84 keV with a total energy resolution of 7.5 meV and a momentum resolution of ⩽0.1 Å−1. We present technical details of the instrument, which includes an in-line monochromator, a focusing mirror, and a focusing analyzer. The performance of the instrument was demonstrated in studies of phonons in diamond and chromium.
First investigations of a warm electron beam ion trap for the production of highly charged ions
Authors: Ovsyannikov V. P. , Zschornack G.
Published: July 2002

A compact warm electron beam ion trap (WEBIT) is described and investigated experimentally. The trap design is based on permanent magnets, an electron gun with a cathode emissivity of 25 A/cm2 and a gun perveance of 0.1 μA/V3/2 as well as a vacuum vessel allowing trap operation at pressures of 10−10 Torr. The electron beam diameter measured with a pinhole camera was 160 μm corresponding to an electron beam density of 248 A cm−2. X-ray spectra measured with a Si(Li) semiconductor detector indicate the production of Xe44+ and Ir59+.
Time-dependent study of low energy electron beam irradiation of Mg-doped GaN grown by metalorganic chemical vapor deposition
Authors: Li X. , Coleman J. J.
Published: July 2002

We have investigated the low energy electron beam irradiation (LEEBI) annealing kinetics of Mg-doped GaN films grown by metalorganic chemical vapor deposition. Our results show that LEEBI annealing at room temperature, monitored by cathodoluminescence spectroscopy as a function of irradiation time, occurs rapidly initially and then proceeds gradually until saturation. We have also demonstrated that LEEBI annealing is effective even at liquid helium temperature, indicating its athermal mechanism. Our observations support the dynamic model involving electronically stimulated dissociation of Mg–H bonds and the escaping and retrapping of atomic hydrogens.
Impact of mixing of disturbed bonding states on time-dependent dielectric breakdown in SiO2 thin films
Authors: McPherson J. W. , Mogul H. C.
Published: July 2002

A temperature-independent field acceleration parameter γ and a field-independent activation energy ΔH0 can be produced when different types of disturbed bonding states are mixed during time-dependent breakdown testing of SiO2 thin films. While γ for each defect type alone has the expected 1/T dependence and ΔH0 shows a linear decrease with electric field, a nearly temperature-independent γ and a field-independent ΔH0 can result when two or more states are mixed.
AlGaN/GaN quantum well ultraviolet light emitting diodes
Authors: Han J. , Crawford M. H. , Shul R. J. , Figiel J. J. , Banas M. , Zhang L. , Song Y. K. , Zhou H. , Nurmikko A. V.
Published: July 2002

We report on the growth and characterization of ultraviolet GaN quantum well light emitting diodes. The room-temperature electroluminescence emission was peaked at 353.6 nm with a narrow linewidth of 5.8 nm. In the simple planar devices, without any efforts to improve light extraction efficiency, an output power of 13 μW at 20 mA was measured, limited in the present design by absorption in the GaN cap layer and buffer layer. Pulsed electroluminescence data demonstrate that the output power does not saturate up to current densities approaching 9 kA/cm2.
Role of buffer surface morphology and alloying effects on the properties of InAs nanostructures grown on InP(001)
Authors: Brault J. , Gendry M. , Grenet G. , Hollinger G. , Desières Y. , Benyattou T.
Published: July 2002

We show the role played by the buffer surface morphology and by alloying effects on the size, shape and lateral distribution of InAs nanostructures grown on InP(001) substrates by molecular beam epitaxy. Three buffers, viz., In0.53Ga0.47As, In0.52Al0.48As, and InP lattice matched on InP have been studied. Differences in nanostructure morphology and in carrier confinement have been evaluated by atomic force microscopy and by low-temperature photoluminescence measurements, respectively. Alongside the classical relaxation mode through two-dimensional/three-dimensional surface morphology change, a chemical relaxation mode has to be introduced as a competitive mode of relaxation of strained layers. This chemical relaxation mode, due to alloying between the InAs deposit and the buffer, is thought to be responsible for most of the observed differences in the InAs nanostructure properties.
Effects of annealing and strain on La1−xCaxMnO3 thin films: A phase diagram in the ferromagnetic region
Authors: Prellier W. , Rajeswari M. , Venkatesan T. , Greene R. L.
Published: July 2002

Oriented, single phase thin films of La1−xCaxMnO3 have been deposited onto (100)-oriented LaAlO3 (0.1<x<0.5) substrates using the pulsed laser deposition technique. While for some compositions, the physical properties (transport and magnetization) of the as-grown films are higher than the bulk values, for other calcium contents the optimized properties are obtained only after annealing under oxygen. These data can be partly explained by changes in oxygen content resulting in cationic vacancies, and thus self-doping effects, accompanying structural changes, may be the cause of properties beyond the phase diagram. We propose a phase diagram for (La1−xCax)1−y□yMnO3 (0.1<x<0.5) thin films.
Effect of substrate temperature and V/III flux ratio on In incorporation for InGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy
Authors: O’Steen M. L. , Fedler F. , Hauenstein R. J.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) and laterally spatially resolved high resolution x-ray diffraction (HRXRD) have been used to identify and characterize rf plasma-assisted molecular-beam epitaxial growth factors which strongly affect the efficiency of In incorporation into InxGa1−xN epitaxial materials. HRXRD results for InxGa1−xN/GaN superlattices reveal a particularly strong dependence of average alloy composition x̄ upon both substrate growth temperature and incident V/III flux ratio. For fixed flux ratio, results reveal a strong thermally activated behavior, with over an order-of-magnitude decrease in x̄ with increasing growth temperature within the narrow range 590–670 °C. Within this same range, a further strong dependence upon V/III flux ratio is observed. The decreased In incorporation at elevated substrate temperatures is tentatively attributed to In surface-segregation and desorption processes. RHEED observations support this segregation/desorption interpretation to account for In loss.
High-velocity surface acoustic waves in diamond and sapphire with zinc oxide film
Authors: Naumenko N. F. , Didenko I. S.
Published: July 2002

We report two numerical examples of high-velocity surface acoustic waves, a type of surface waves which was recently shown to exist if a thin film is present on a surface. The nonattenuated high-velocity surface waves have been found in diamond orientation with Euler angles (0°,0°,45°) and sapphire orientation with Euler angles (0°,−20.3°,0°), with zinc oxide film. In diamond, the wave has “symmetric” structure with displacement in the symmetry plane while, in sapphire, the example of a “nonsymmetric” solution is presented. The numerical analysis has confirmed that, in both examples, the wave has a true surface nature, with one-partial structure in a substrate.
Optical modulation at around 1550 nm in an InGaAlAs optical waveguide containing an InGaAs/AlAs resonant tunneling diode
Authors: Figueiredo J. M. L. , Boyd A. R. , Stanley C. R. , Ironside C. N. , McMeekin S. G. , Leite A. M. P.
Published: July 2002

We report electroabsorption modulation of light at around 1550 nm in a unipolar InGaAlAs optical waveguide containing an InGaAs/AlAs double-barrier resonant tunneling diode (RTD). The RTD peak-to-valley transition increases the electric field across the waveguide, which shifts the core material absorption band edge to longer wavelengths via the Franz–Keldysh effect, thus changing the light-guiding characteristics of the waveguide. Low-frequency characterization of a device shows modulation up to 28 dB at 1565 nm. When dc biased close to the negative differential conductance region, the RTD optical waveguide behaves as an electroabsorption modulator integrated with a wide bandwidth electrical amplifier, offering a potential advantage over conventional pn modulators.
Enhanced dynamic response of the in-plane switching liquid crystal display mode through polymer stabilization
Authors: Escuti Michael J. , Bowley Chris C. , Crawford Gregory P. , Žumer Slobodan
Published: July 2002

A significant improvement in the dynamic response time of the in-plane switching nematic liquid crystal mode, useful in flat-panel display applications, is achieved through polymer stabilization. This improvement is achieved by introducing a low-density, stabilizing polymer network that causes the nematic director to favor the zero-field orientation at the expense of transmission and slightly higher drive voltages. We present a simple model that treats the polymer network as an effective field in the general framework of elastic continuum theory.
Acoustic band-gap engineering using finite-size layered structures of multiple periodicity
Authors: Shen Mingrong , Cao Wenwu
Published: July 2002

The transmission coefficient of a layered structure made of glass and water was calculated using transfer matrix method and also measured as a function of frequency. It was found that acoustic band gaps can be created using only 3–4 cells of a two-phase layered structure. By introducing two or more periods into the layered structure, very sharp passbands and very broad stopbands can be engineered for acoustic waves. Such acoustic band-gap materials could be used for making high-quality acoustic filters, acoustic mirrors and vibration insulation devices in selective frequency range.
Measurement of the linear electro-optic coefficients of sol-gel derived strontium barium niobate thin films using a two-beam polarization interferometer
Authors: Koo Junmo , Lee Changho , Jang Jae Hyeok , No Kwangsoo , Bae Byeong-Soo
Published: July 2002

A two-beam polarization interferometer in a reflection configuration is used to measure the electro-optic coefficients of highly oriented strontium barium niobate thin films prepared by a sol-gel method. The technique enables the determination of the electro-optic coefficients of films using a strong Fabry–Perot effect with automatic adjustment and maintenance of the operation point of the interferometer. The linear electro-optic coefficients increase with increasing Sr content in the films.
Dielectric function of V2O5 nanocrystalline films by spectroscopic ellipsometry: Characterization of microstructure
Authors: Losurdo Maria , Bruno Giovanni , Barreca Davide , Tondello Eugenio
Published: July 2002

Spectroscopic ellipsometry over the photon energy 1.5–5.0 eV is used to derive the dielectric function of V2O5 nanocrystalline films deposited by plasma-enhanced chemical vapor deposition. The dispersion in the optical response is described by a combination of Lorentzian oscillators. The results are obtained from a microstructure-dependent model, which considers the anisotropy of the V2O5 crystallites into the bulk film, as well as the presence of interface and surface roughness layers. The variation of the V2O5 thin-film dielectric function upon film crystallinity, going from pure nanocrystalline to amorphous material, is also investigated.
Early stages of spinodal decomposition in binary liquid crystal mixtures
Authors: Matsuyama Akihiko , Kato Tadaya
Published: July 2002

We study the early stages of spinodal decomposition in binary liquid crystal mixtures by solving linearized time-dependent Landau–Ginzburg equations for concentration (conserved order parameter) and two orientation (nonconserved) order parameters, one for each component. The theory takes into account a cross term between concentration and orientation gradients, which becomes an important factor for phase separation kinetics. We calculate growth rates and structure factors for the order parameters, following thermal quenches from a stable isotropic state into an unstable region. Depending on the quenched temperature and concentration, we find five unstable regions which are characterized by the time evolutions of the structure factors.
Impact ionization enhancements in AlxGa1−xSb avalanche photodiodes
Authors: Grein C. H. , Ehrenreich H.
Published: July 2002

The apparent inconsistency among several experiments examining the presence of an enhanced hole to electron impact ionization coefficient ratio in AlxGa1−xSb is shown to be associated with the failure to recognize that measurements at high and low electric fields lead to physically different results. The ratio exhibits an enhancement only for low fields because of “resonant” band structure effects associated with the equality of the energy gap and the spin orbit splitting. This effect is shown to be unimportant at high fields. The impact ionization calculations utilize realistic pseudopotential band structure Coulomb scattering matrix elements. Effects associated with alloy scattering are found to be unimportant.
Magnetic and optical properties of GaMnN magnetic semiconductor
Authors: Zaja̧c M. , Doradziński R. , Gosk J. , Szczytko J. , Lefeld-Sosnowska M. , Kamińska M. , Twardowski A. , Palczewska M. , Grzanka E. , Gȩbicki W.
Published: July 2002

Microcrystalline Ga1−xMnxN samples with Mn content up to x=0.005 were grown by an ammonothermal method and were studied using various techniques. X-ray diffraction showed characteristic diffraction lines for hexagonal GaN phase mixed with a small contribution (<5%) from the Mn3N2 phase. Raman spectra exhibited characteristic peaks of pure GaN and modes that could be associated with Mn-induced lattice disorder. Electron spin resonance and magnetization measurements were consistent with the dominant Mn2+(d5) configuration of spin S=5/2 which is responsible for the observed paramagnetic behavior of the GaMnN material.
Alloys by precision electrodeposition
Authors: Kazeminezhad I. , Blythe H. J. , Schwarzacher W.
Published: July 2002

We show that a Cu–Ni alloy film of arbitrary composition may be grown by electrodepositing well-defined submonolayer quantities of Cu and Ni in alternation. Active computer control of the deposition process is used to compensate for undesired electrochemical processes, such as partial redissolution of the Ni. Magnetic measurements were used to characterize alloy homogeneity. With this electrodeposition method it is possible to tailor the composition profile of a film with subnanometer precision. As an example, Cu0.19Ni0.81/Cu0.79Ni0.21 alloy/alloy multilayers were fabricated which gave prominent satellite peaks in high angle x-ray diffraction patterns.
First principles determination of the effects of a grain boundary on the anisotropy and exchange energies in Co1Pt5 superlattices
Authors: MacLaren J. M. , Willoughby S. D.
Published: July 2002

Self-consistent electronic structure calculations are used to study the changes in magnetic properties (specifically the magnetocrystalline anisotropy and exchange coupling) of a Co1Pt5 superlattice induced by a grain boundary. Guided by experiment, several possible grain boundaries have been studied, and we find as a result of calculations that the intergrain exchange coupling can be reduced by up to an order of magnitude. The local anisotropy is also reduced in the neighborhood of the boundary. These calculations are aimed at developing a more realistic description of magnetic thin films that takes into account details of the microstructure and thereby provide input to micromagnetic simulations.
Molecular-dynamics simulations of ion size effects on the fluid structure of aqueous electrolyte systems between charged model electrodes
Authors: Crozier Paul S. , Rowley Richard L. , Henderson Douglas
Published: July 2002

The effect of ion size on the structure of aqueous electrolyte solutions between charged nonpolarizable surfaces or electrodes is investigated using molecular-dynamics simulations of discrete water molecules and ions confined to a slab geometry. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution and potential profiles are reported for 1 M aqueous electrolyte solutions with ±0.1 C/m2 electrode surface charge at the electrode surfaces. Five different models for the ions are studied. The models can be characterized as (1) ions of equal size, (2) smaller cations, (3) larger anions, (4) smaller cations and larger anions, and (5) ions representing aqueous NaCl. Compared to the equal-size ion reference case, smaller cation size decreases the contact adsorption at the cathode, but interestingly anion size tends to moderate this effect somewhat. Whereas there is no contact adsorption of anions when the ions are the same size, an increase in anion size causes significant replacement of adsorbed water molecules at the anode with anions. In the case studied here, the larger anion size produced a 20 fold increase in localized density at the electrode surface. The electrostatic potential profile tends to be relatively insensitive to the changes in ion size.
Field evolution of magnetic vortex state in ferromagnetic disks
Authors: Guslienko K. Yu. , Novosad V. , Otani Y. , Shima H. , Fukamichi K.
Published: July 2002

The evolution of a magnetic “vortex” state in submicron ferromagnetic disks has been studied as functions of disk diameter and thickness. The vortex core displacement in the applied magnetic field was calculated by minimizing the total magnetic energy consisting of the magnetostatic, exchange, and Zeeman energies. A simple analytical expression for the initial magnetic susceptibility is deduced. The initial susceptibility increases with increasing disk diameter and decreasing thickness. The calculations agree well with the experimental data obtained for the 60 nm thick permalloy disk arrays with a variable diameter from 0.2 to 0.8 μm.
Femtosecond dissociation of ozone studied by the Auger Doppler effect
Authors: Rosenqvist L. , Wiesner K. , Naves de Brito A. , Bässler M. , Feifel R. , Hjelte I. , Miron C. , Wang H. , Piancastelli M. N. , Svensson S. , Björneholm O. , Sorensen S. L.
Published: July 2002

A Doppler-type shift in the kinetic energy of atomic Auger electrons emitted after fast dissociation of O3 molecules is observed. The resonant Auger spectrum from the decay of repulsive core-excited states reflects both the early molecular ozone decay and that from excited dissociation fragments. The kinetic energy of the fragment is manifested as an energy shift of the atomic Auger lines when the measurement is made under certain conditions. We report measurements of the energy-split atomic fragment emission lines arising from dissociation on a time scale comparable to the core-hole lifetime. For the O 1s–σ* states the kinetic energy release amounts to several electron volts. We report measurements for excitation of both the terminal and central oxygen 1s electrons. A simple kinematic model for extracting a lower limit for the kinetic-energy release is presented and is compared with the result of a Born–Haber cycle, which may be seen as an estimate of the maximum energy release.
Curvature effects on the dynamics of tearing modes in tokamaks
Authors: Lütjens Hinrich , Luciani Jean-François , Garbet Xavier
Published: July 2002

The curvature effects on the dynamics of magnetic island evolution in tokamaks are investigated both theoretically and numerically. By taking into account perpendicular and parallel heat diffusion, a new dispersion relation is derived for tearing modes that match the linear and nonlinear results. This evolution equation allows a quantitative description over the whole range of island sizes. It predicts a nonlinear instability, i.e., growing magnetic islands in linearly stable magnetic configurations. All these predictions are in excellent agreement with full tridimensional linear and nonlinear magnetohydrodynamic (MHD) computations with the latest version of XTOR [K. Lerbinger and J. F. Luciani, J. Comput. Phys. 97, 444 (1991)]. These results have important consequences on the onset of neoclassical tearing modes because they predict a resistive MHD threshold.
Magnetic properties and x-ray photoelectron spectroscopy study of NiO/NiFe films prepared by magnetron sputtering
Authors: Yu G. H. , Zeng L. R. , Zhu F. W. , Chai C. L. , Lai W. Y.
Published: July 2002

Ta/NiO x / Ni 81 Fe 19 /Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange-coupling field (Hex) and the coercivity (Hc) of NiOx/Ni81Fe19 as a function of the ratio of Ar to O2 during the deposition process were studied. The composition and chemical states at the interface region of NiOx/NiFe were also investigated using the x-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that the ratio of Ar to O2 has a great effect on the nickel chemical states in NiOx film. When the ratio of Ar to O2 is equal to 7 and the argon sputtering pressure is 0.57 Pa, the x value is approximately 1 and the valence of nickel is +2. At this point, NiOx is antiferromagnetic NiO and the corresponding Hex is the largest. As the ratio of Ar/O2 deviates from 7, the exchange-coupling field (Hex) will decrease due to the presence of magnetic defects such as Ni+3 or metallic Ni at the interface region of NiOx/NiFe, while the coercivity (Hc) will increase due to the metallic Ni. XPS studies also show that there are two thermodynamically favorable reactions at the NiO/NiFe interface: NiO+Fe=Ni+FeO and 3NiO+2Fe=3Ni+Fe2O3. These interface reaction products are magnetic defects at the interface region of NiO/NiFe, it is believed that these magnetic defects would have an effect on the exchange-coupling field (Hex) and the coercivity (Hc) of NiO/NiFe.
An interpretation of fluctuation induced transport derived from electrostatic probe measurements
Authors: LaBombard B.
Published: July 2002

Fluctuation-induced particle fluxes (Γñφ̃) in the edge of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are inferred from a fast-scanning probe using standard analysis techniques. The magnitude and profile shape of Γñφ̃ is clearly inconsistent with fluxes inferred from global particle and power balance. These differences are difficult to reconcile if Γñφ̃ is interpreted as a measure of the particle flux in the unperturbed plasma. However, if Γñφ̃ is reinterpreted as the particle flux which must “fill-in” the presheath zone formed by the probe, these inconsistencies are eliminated. In this case, an effective diffusivity in the presheath zone (Dps) can be estimated from Γñφ̃. Dps is found to be in the range of diffusivities inferred from global particle balance (DSOL), indirectly supporting the hypothesis. However, the profile of Dps and its dependency on discharge conditions are markedly different than DSOL, implying that Dps is also not simply related to transport in the unperturbed plasma.
Time-resolved luminescence and photoconductivity of polycrystalline ZnO films
Authors: Studenikin S. A. , Cocivera Michael
Published: July 2002

The relative intensities of the green and blue luminescence of a ZnO film was shown to depend on the excitation regime. Time-resolved and steady-state luminescence were studied along with photoconductivity transients. Under continuous excitation the film emitted green light, while under pulsed excitation the luminescence was either blue or green, depending on the intensity of the excitation pulse. The intensity of the blue component depended linearly on the pulse intensity while the green intensity followed a sublinear power law dependence with the exponent α=1/3. The transient luminescence exhibited fast (below nanosecond) and slow (microsecond) decay components at room temperature. The fast component was ascribed to interband exciton recombination, and the slow component was attributed to an electron-hole recombination involving a donor-acceptor complex, which most likely consisted of oxygen and zinc vacancies. In this model, the complex can emit light only when it is activated, i.e., oxygen vacancy is in its singly ionized state and the acceptor (zinc vacancy) captures a hole. The density of the activated complex depends on the Fermi level position, bend bending, and thickness of the depletion layer.
Electro-optic characteristics of de Vries tilted smectic liquid crystals: Analog behavior in the smectic A* and smectic C* phases
Authors: Clark N. A. , Bellini T. , Shao R.-F. , Coleman D. , Bardon S. , Link D. R. , Maclennan J. E. , Chen X.-H. , Wand M. D. , Walba D. M. , Rudquist P. , Lagerwall S. T.
Published: July 2002

Chiral smectic A liquid crystal materials of the de Vries type (with molecules tilted relative to the layer normal) exhibit analog field-induced (electroclinic) optic axis rotation accompanied by an increase in birefringence. We identify two such de Vries smectic A* materials and use them to develop and test models for these characteristic electro-optic effects. These materials also exhibit colossal analog field-induced optic axis rotation in the lower temperature smectic C* phase, a consequence of polarization charge stabilization, and of polarization screening of the applied field in the liquid crystal.
Invar-type effect induced by cold-rolling deformation in shape memory alloys
Authors: Kainuma R. , Wang J. J. , Omori T. , Sutou Y. , Ishida K.
Published: July 2002

Since the discovery of Fe–Ni alloys showing low thermal expansion (LTE) in 1896, many Invar alloys have been developed based on magnetovolume effect where negative thermal expansion is induced by magnetic transformation. Herein, we show that the control of stress-induced martensitic transformation due to cold working of the Cu–Zn–Al polycrystalline alloy results in the LTE. This type of LTE material is easily fabricated by conventional cold rolling, and the coefficient of thermal expansion in the range from about 0 to 32×10−6 K−1 can be obtained by controlling the reduction ratio. The LTE effect due to the present method can also be obtained for other shape memory (SM) alloys such as Ni–Ti, Cu–Mn–Al and Ni–Al base alloys, which have high potential for various practical applications.
Solution multistability in first-order nonlinear differential delay equations
Authors: Losson Jérôme , Mackey Michael C. , Longtin André
Published: July 2002

The dependence of solution behavior to perturbations of the initial function (IF) in a class of nonlinear differential delay equations (DDEs) is investigated. The structure of basins of attraction of multistable limit cycles is investigated. These basins can possess complex structure at all scales measurable numerically although this is not necessarily the case. Sensitive dependence of the asymptotic solution to perturbations in the initial function is also observed experimentally using a task specific electronic analog computer designed to investigate the dynamics of an integrable first-order DDE.
On noninvertible mappings of the plane: Eruptions
Authors: Billings Lora , Curry James H.
Published: July 2002

In this paper we are concerned with the dynamics of noninvertible transformations of the plane. Three examples are explored and possibly a new bifurcation, or ‘‘eruption,’’ is described. A fundamental role is played by the interactions of fixed points and singular curves. Other critical elements in the phase space include periodic points and an invariant line. The dynamics along the invariant line, in two of the examples, reduces to the one-dimensional Newton’s method which is conjugate to a degree two rational map. We also determine, computationally, the characteristic exponents for all of the systems. An unexpected coincidence is that the parameter range where the invariant line becomes neutrally stable, as measured by a zero Lyapunov exponent, coincides with the merging of a periodic point with a point on a singular curve.
Influence of the surface interaction on threshold values in the cholesteric-nematic phase transition
Authors: van Sprang H. A. , van de Venne J. L. M.
Published: July 2002

The threshold values for the field-induced cholesteric to nematic phase transition are described as crossover points of the free energy levels of the textures concerned. Known expressions for these energy levels as a function of an electric field are extended with a surface interaction term. By interpretation of experimental results in terms of the equations derived, the surface interaction energy is calculated both for planar and homeotropically aligned cholesteric liquid-crystal mixtures. Besides differences between planar and homeotropic wall alignment a difference between two types of planar wall alignment can now be understood, too. Additionally, the method appears very useful for calculating the twist elastic constant K22.
Magnetic properties of V/Fe superlattices
Authors: Wong H. K. , Yang H. Q. , Hilliard J. E. , Ketterson J. B.
Published: July 2002

We have measured the magnetic and superconducting properties of V/Fe superlattices. It is observed that ultrathin Fe layers behave in a 2-D fashion when isolated by thick V layers; however, on thinning the V layers a magnetic coupling between the Fe layers has been observed. No dead Fe layers were seen. When the V-layer thicknesses were on the order of the BCS coherence length, superconductivity was observed. A 2-D–3-D crossover was observed in the temperature dependence of the parallel critical field when the Fe layers were only a few atomic planes thick, implying the coexistence of ferromagnetism and superconductivity.
Upper critical fields in RE2Fe3Si5 (RE=Y, Sc, Lu) compounds
Authors: Umarji A. M. , Malik S. K. , Shenoy G. K.
Published: July 2002

The RE2Fe3Si5 compounds form with RE=Sm, Gd to Lu, Sc and Y and crystallize in the tetragonal Sc2Fe3Si5-type structure. The compounds with RE=Lu, Y, and Sc are superconducting while those with RE=Gd to Er are antiferromagnetically ordered. We have carried out upper critical field (Hc2) measurements on Y2Fe3Si5, Sc2Fe3Si5 and Lu2Fe3Si5. The Lu compound, with the zero field Tc of 6.0 K, has Hc2(0)=60 kOe, which is the highest for an iron-containing ternary compound. The results of critical field measurements have been analyzed in terms of WHH theory.
Condensation of metal and semiconductor vapors during nozzle expansion
Authors: Yang S.-N. , Lu T.-M.
Published: July 2002

We consider the homogeneous nucleation and growth of metal and semiconductor clusters resulting from an adiabatic cooling during the expansion of the vapor through a crucible nozzle. It is argued that although the surface tension of most metals and semiconductors is high, the nucleation barrier and critical cluster size are still comparable to that of the gases because of the high vaporization temperature. The nucleation rate is evaluated for a variety of metals and semiconductors based on the classical nucleation theory. Prediction of the lower limit of the Ag pressure for which the vapor attains an appreciable nucleation rate agrees well with a recent cluster beam experiment.
Stability of oxides grown on tantalum silicide surfaces
Authors: Cros A. , Tu K. N.
Published: July 2002

The oxidation of TaSi2 and Ta2Si surfaces prepared under ultrahigh vacuum conditions has been studied by x-ray photoemission spectroscopy. For both silicides, Si and Ta oxides comprise the native oxide formed at room temperature. Annealing at 550 °C induces a solid state reaction within the native oxide. Ta2O5 is partially decomposed, and the liberated oxygen bonds to Si. This reaction and the influence of the silicide stoichiometry are discussed.
Elastic properties of epitaxial ZnSe(001) films on GaAs measured by Brillouin spectroscopy
Authors: Lee S. , Hillebrands B. , Stegeman G. I. , Cheng H. , Potts J. E. , Nizzoli F.
Published: July 2002

Brillouin scattering experiments have been performed on a series of epitaxially grown ZnSe (001) films on GaAs (001) of thicknesses between 0.23 and 2.0 μm. No thickness-dependent modifications of the elastic constants have been found. The obtained room-temperature values of C11=87.0, C12=54.7, and C44=39.1 GPa are in good agreement with literature values of ZnSe bulk material. The calculation of the theoretical cross sections shows a good agreement with the experimental data.
Properties of R2Fe14C compounds and their possible application as permanent magnets
Authors: van Mens R. , de Mooij D. B. , Buschow K. H. J.
Published: July 2002

The formation of the tetragonal compounds R2Fe14C (Nd2Fe14B structure) was studied as a function of the R component. At high temperatures the tetragonal R2Fe14C phase transforms mainly into a rhombohedral phase of the type of R2Fe17Cx in which the C atoms occupy an interstitial position. The transformation temperature is highly dependent on the R component and is fairly low for R=Pr and Nd. The effect of this solid-state transformation on the hard magnetic properties of the R2Fe14C compounds is discussed.
Semiconductor superlattice interference filter design
Authors: Gaylord T. K. , Glytsis E. N. , Brennan K. F.
Published: July 2002

The quantitative analogies that have been previously established [J. Appl. Phys. 65, 814 (1989)] between electron wave propagation in semiconductors and optical wave propagation in dielectrics may be used to translate thin-film optical device designs into semiconductor superlattice device designs. The procedure for this direct mapping is also described in the above reference. The resulting designs, however, have compositions that are not constrained to be within a usable compositional range and they have layer thicknesses that are not constrained to be integer multiples of a monolayer thickness. In the present work, a systematic design procedure is presented that includes these required practical constraints. This procedure is then applied to the design of Ga1−xAlxAs superlattice narrow interference filters. For pass kinetic energies in the range of 0.14–0.20 eV, compositions (values of x) and numbers of monolayer thicknesses needed to produce quarter-wavelength layers are calculated. The detailed design of an example narrow bandpass (15.4 meV) filter with a pass electron energy of 0.20 eV is presented.
Optical and electronic properties of sputtered hydrogenated amorphous silicon-tin alloys
Authors: Girginoudi D. , Georgoulas N. , Thanailakis A.
Published: July 2002

Hydrogenated amorphous silicon-tin alloys (a-Si1−xSnx:H), in the composition range 0≤x≤0.51, have been prepared by rf sputtering. It is shown that the addition of Sn moves the conduction-band edge, thereby closing the optical band gap. The dc dark conductivity increases less than expected, with increasing Sn content, based on a simple decrease in band gap. This and the temperature dependence of the dark conductivity suggest a transition from extended-state free-carrier conduction to localized-state hopping conduction. Photoluminescence measurements show the presence of defect-state radiative recombination.
Absorption enhancement in hydrogenated amorphous silicon-based solar cells
Authors: Morris J. , Arya R. R. , O’Dowd J. G. , Wiedeman S.
Published: July 2002

The enhancement of long wavelength (>600 nm) light absorption by light scattering can result in a significant increase in the short circuit current density of a-Si:H-based solar cells. A model of absorption enhancement is presented and used to predict both light absorption in the active i layer and the total reflectance of single-junction a-Si:H p-i-n solar cells. These results are compared with the corresponding measurements performed on actual solar cells. It is found that the texture imparted to the metallic interface may result in a significant decrease in the reflectance of the rear contact, and in certain of the rear contacts considered, the reflectance is also reduced by interface formation phenomena. These mechanisms reduce the reflectance of the aluminum rear contact to less than 50% in the solar cells studied. In these cells and others where large light absorption occurs in the rear contact, incomplete light randomization is not found to be an important enhancement limiting effect. A thin layer of indium-tin-oxide inserted between the a-Si:H n layer and the contact metal results in critical trapping of scattered light and so reduces light absorption in the rear contact.
Orientation analysis of the anisotropic galvanomagnetism of sintered Bi2Te3
Authors: Ohsugi Isao J. , Kojima Tsutomu , Nishida Isao A.
Published: July 2002

A calculational model is proposed to analyze the anisotropic galvanomagnetism of sintered Bi2Te3 on the basis of the orientation distribution function method. The orientation distribution of crystallites in a specimen is expressed by a distribution function of the Eulerian angle that represents the rotation of the crystallites distributed in the specimen. The distribution can be specified by a single rotation parameter because of the uniaxial anisotropy of Bi2Te3. The rotation parameter can be expressed as the angular width of the distribution by introducing the approximation that the distribution function is a Gaussian function of the tilting angle of the c axis with respect to the pressing direction of the specimen. Resistivity and Hall tensors of sintered specimens are calculated by this method to evaluate the angular width of the distribution. Theoretical curves are calculated from the data of a corresponding single crystal by estimating the distribution parameters. The theoretical curves are in reasonable agreement with the observed data.
Structural and vibrational properties of (InAs)m(GaAs)n strained superlattices grown by molecular beam epitaxy
Authors: Scamarcio G. , Brandt O. , Tapfer L. , Mowbray D. J. , Cardona M. , Ploog K.
Published: July 2002

The structural and vibrational properties of superlattices composed of many periods of highly mismatched InAs and GaAs layers have been studied by means of x-ray diffraction and Raman scattering as a function of the sample geometry. X-ray diffraction measures the average lattice mismatch between the superlattice and the substrate. The long-range order influences the propagative acoustic phonons whereas strain and confinement effects compete in determining the optic vibration frequencies of the InAs layers. The linewidth of the main superlattice peak in the diffraction patterns and the scattering intensities of the acoustic phonons are related to the actual shape of the interfaces. We find that the stability of the structures depends on the total number of periods, in agreement with the predictions of equilibrium elasticity theory. However, the competition between the different relaxation processes is governed by the individual layer thicknesses.
Experimental and numerical study of laser induced spallation into aluminum and copper targets
Authors: Boustie M. , Cottet F.
Published: July 2002

Laser driven shocks can lead to the dynamic failure, called spallation, of materials which undergo a dynamic tension at the crossing of two rarefaction waves. We present here a numerical and experimental study of ductile material scabbing with the particular laser shock conditions (very short durations and very high induced pressure levels). A cumulative damage criterion is implemented into both numerical codes shylac and radioss■. Experiments have been carried out by laser irradiation of thin foils of pure aluminum, copper, and iron by laser pulses with the three durations at medium height 0.6, 2.5, and 27 ns, generating an incident flux ranging from 5×1011 to 5×1012 W/cm2. The validity of the cumulative damage criterion is asserted by the consistency of numerical and experimental results for aluminum whose criterion parameters are known. The unknown constants for copper are determined by fitting the numerical simulations to the experimental data. The material influence on the scab thickness and on the spallation threshold is analyzed. Experimental laser induced spallation data given by irradiation of a rear stepped target have been validated experimentally and numerically.
Al induced crystallization of a-Si
Authors: Radnoczi G. , Robertsson A. , Hentzell H. T. G. , Gong S. F. , Hasan M.-A.
Published: July 2002

The crystallization of amorphous Si induced by Al during heat treatment has been investigated by cross section and plan view transmission electron microscopy. The lowest temperature of Al induced crystallization of amorphous Si was found to be 440 K. The crystallization temperature, however, depends on the thickness of Al layers in layered structures and on the concentration of Al in co-deposited layers below 1-nm-layer thickness and 15 at.% of Al concentration, respectively. Al-induced crystallization in layered structures starts at the Al/amorphous Si interfaces and is located close to them. The amount of crystallized Si depends on the quantity of Al and on the temperature and increases with them. The mechanism of crystallization involves intermixing of Al with Si and the formation of an alloy of high metal concentration in the amorphous/crystalline interface. When the formation of this alloy is not assured due to low Al concentration, then crystallization does not start or the process of crystallization stops. In Al induced crystallization the nucleation of polycrystalline Si grains rather than their crystal growth is affected.
A new kind of light-generation mechanism: Incandescent radiation from clusters
Authors: Weber B. , Scholl R.
Published: July 2002

A novel technique to generate hot refractory metal clusters is explained. A regenerative chemical cycle is used to form small clusters inside a microwave-excited high-pressure discharge which heats the clusters to 4700 K (rhenium) or 3800 K (tungsten). The discharge’s emission spectrum exhibits a strong continuum, which is the incandescent radiation of clusters. The spectral shapes of the continua and the positions of their maxima (Re: 550 nm; W: 700 nm) are characteristic of the refractory metal. A first theoretical description of the clusters’ radiation can be based on the Mie theory. Measurements of emission and absorption coefficients are performed to determine the optical properties and temperatures of the clusters. The existence of clusters with an average size of about 2.5 nm is proved by a laser scattering experiment. The properties of the cluster radiation, especially the continuous spectrum and the high luminous efficiency, are attractive for lighting applications.
Correlation between magnetic interactions and giant magnetoresistance in melt-spun Co10Cu90 granular alloys
Authors: Viegas A. D. C. , Geshev J. , Dorneles L. S. , Schmidt J. E. , Knobel M.
Published: July 2002

The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co10Cu90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters (particle size and density, interparticle distance) and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.
Direct current conductivity and oxygen gas-sensing properties of iron–antimony–tellurite glasses
Authors: Chakraborty S. , Satou H. , Sakata H.
Published: July 2002

Iron tellurite glasses in the system Fe2O3–Sb–TeO2 have been prepared by the melt-quenching technique. The glass forming region for the above system has been determined. A comparative study of the direct current electrical conductivity, σ, of the 15Fe2O3–5Sb–80TeO2 (mol%) glass together with 15Fe2O3–5Sb2O3–80TeO2 and the binary 15Fe2O3–85TeO2 glass has been made in the temperature range 463–598 K. The 15Fe2O3–5Sb–80TeO2 glass was found to be most conductive (σ=1.39×10−5 S cm−1 at 503 K) in this series. The semiconducting behavior of these glasses could be explained by the small polaron hopping model. The oxygen gas-sensing property of the 15Fe2O3–5Sb–80TeO2 glass was investigated. The O2 gas sensitivity at 598 K was found to be 8.1. The variation in conductivity on alternate switching of gas atmospheres between oxygen and argon was found to be reproducible. An attempt has been made to clarify the mechanism of conductivity response considering the phenomenon of surface adsorption followed by bulk diffusion. From the investigation of the gas-sensing behavior, the present glass was found to be a potential candidate for an oxygen gas sensor.
Magnetoexcitons in anisotropic semiconductors
Authors: Zhang Yong , Mascarenhas A. , Jones E. D.
Published: July 2002

Using a numerical technique, we have calculated the exciton ground state in a uniaxially anisotropic semiconductor with and without the presence of a magnetic field. Numerical results are given for the exciton energy level as a function of magnetic field for a wide range of anisotropy parameters 0.001⩽αβ⩽1000 and effective field 0⩽γ⩽10. We have demonstrated that by fitting the experimental data of the field dependence, in a proper field range, with the field aligned in the direction of the uniaxis, the exciton in plane reduced mass μ⊥ and the parameter αβ can be obtained simultaneously. As an example, magnetoluminescence of ordered GaInP2 measured in the field range B⩽14 T is analyzed by applying this method. Analytical formulas have been found for calculating exciton binding energy and the related variational parameters for a well-known method with trial function f(x,y,z)=(πa2b)−1 exp[−(x2+y2)/a2+z2/b2]. Various approximate approaches for calculating the exciton binding energy and the magnetoexciton states are discussed and compared.
High field breakdown of narrow quasi uniform field gaps in vacuum
Authors: Muzykov P. G. , Ma Xianyun , Cherednichenko D. I. , Sudarshan T. S.
Published: July 2002

The challenge in vacuum microelectronic device design is to be able to stress a given micrometric gap to relatively high voltages without threat of a breakdown, which, in effect could destroy the device. In order to obtain basic vacuum insulation data related to the regime of vacuum microelectronics, the prebreakdown and breakdown characteristics of narrow gaps in the range of 3–25 μm were extensively investigated. The observed prebreakdown current was related to field emission from atomic scale microprotrusions or planar emission sites; the emission from these sites eventually produces breakdown. A single spark breakdown caused damage to both the anode and cathode. The dc glow discharge conditioning in air improved the insulation capability of narrow gaps (3–25 μm) significantly. The breakdown strength of a 5 μm gap after conditioning was as high as 5×108 V/m, which is the highest value reported in literature for broad area electrodes. It is shown that the electric field evaporation of metal ions from the electrode surface at an electric field E≅1010 V/m is able to instigate breakdown of the gap even under high vacuum conditions. Electric field evaporation can occur both at the cathode and anode surfaces to create the breakdown conditions. The breakdown data obtained in the micrometric gap regime is very valuable in the design of field emission displays and other vacuum microelectronic devices.