bluuebunny/crossref_metadata_2025 huggingface dataset:
ZnO diode fabricated by excimer-laser doping
Authors: Aoki Toru , Hatanaka Yoshinori , Look David C.
Published: July 2002

A ZnO diode was fabricated by using a laser-doping technique to form a p-type ZnO layer on an n-type ZnO substrate. A zinc-phosphide compound, used as a phosphorous source, was deposited on the ZnO wafer and subjected to excimer-laser pulses. The current–voltage characteristics showed a diode characteristic between the phosphorous-doped p-layer and the n-type substrate. Moreover, light emission, with a band-edge component, was observed by forward current injection at 110 K.
Intersubband absorption in GaN/AlGaN multiple quantum wells in the wavelength range of λ∼1.75–4.2 μm
Authors: Gmachl Claire , Ng Hock M. , Cho Alfred Y.
Published: July 2002

Intersubband optical absorption in narrow, 15–30 Å wide, GaN/AlGaN quantum wells has been measured. The samples were grown by molecular-beam epitaxy on sapphire substrate and the barrier AlN mole fraction was varied from 0.45 to 0.8. Peak absorption wavelengths ranged from 4.2 μm for 30 Å wide wells to 1.77 μm for a 15 Å wide well. Modeling shows that a large contribution to the considerable spectral width of the absorption of ∼150 meV likely results from monolayer fluctuations. In addition to the composition dependent band offset, the intrinsic electric fields in the wells and barriers are the determining factors for the shortest possible wavelength.
Magnetohydrodynamic flows sustaining stationary magnetic nulls
Authors: Titov Vyacheslav S. , Hornig Gunnar
Published: July 2002

Exact solutions of the resistive magnetohydrodynamic equations are derived which describe a stationary incompressible flow near a generic null point of a three-dimensional magnetic field. The properties of the solutions depend on the topological skeleton of the corresponding magnetic field. This skeleton is formed by one-dimensional and two-dimensional invariant manifolds (so-called spine line and fan plane) of the magnetic field. It is shown that configurations of generic null points may always be sustained by stationary field-aligned flows of the stagnation type, where the null points of the magnetic and velocity fields have the same location. However, if the absolute value |j∥| of the current density component parallel to the spine line exceeds a critical value jc, the solution is not unique—there is a second nontrivial solution describing spiral flows with the stagnation point at the magnetic null. The characteristic feature of these new flows is that they cross magnetic field lines but they do not cross the corresponding spine and fan of the magnetic null. Therefore these are nonideal but nonreconnecting flows. The critical value |j∥|=jc coincides exactly with a threshold separating the topological distinct improper radial and spiral nulls. It is shown that this is not an accidental coincidence: the spiral field-crossing flows of the considered type are possible only due to the topological equivalence of the field lines forming the fan plane of the spiral magnetic null. The explicit expression for the pressure distribution of the solution is given and its iso-surfaces are found to be always ellipsoidal for the field-aligned flows, while for the field-crossing flows there are also cases with a hyperboloidal structure.
Generalized master function approach to one-dimensional quasiexactly solvable models
Authors: Jafarizadeh M. A. , Akhtarshenas S. J.
Published: July 2002

By introducing the generalized master function of order up to four together with corresponding weight function, we have obtained all one-dimensional quasiexactly solvable second order differential equations. It is shown that these differential equations have solutions of polynomial type with factorization properties, that is polynomial solutions Pm(E) can be factorized in terms of polynomial Pn+1(E) for m⩾n+1. All known one-dimensional quasiexactly quantum solvable models can be obtained from these differential equations, where roots of polynomial Pn+1(E) are corresponding eigenvalues.
Crystallization kinetics of the bulk-glass-forming Pd43Ni10Cu27P20 melt
Authors: Schroers Jan , Johnson William L. , Busch Ralf
Published: July 2002

The crystallization of undercooled Pd43Ni10Cu27P20 melts is studied under isothermal conditions and at constant heating and cooling rates. Investigations are carried out by fluxing the melt with B2O3 and without any fluxing material. The isothermal experiments allow us to determine the complete time–temperature-transformation diagram with a minimum crystallization time of about 200 s for the fluxed melt and about 130 s for the unfluxed Pd43Ni10Cu27P20 melt. The results of the experiments at constant cooling and heating rates are summarized in a continuous heating and cooling diagram. The critical cooling rate for the fluxed alloy is determined to be 0.09 K/s, whereas the critical heating rate is 6 K/s. For the unfluxed Pd43Ni10 Cu27P20, 0.4 and 9 K/s are found, respectively. This alloy exhibits the most sluggish crystallization kinetics of all metallic systems known so far.
Dissociating the effect of different disturbances on the band gap of a two-dimensional photonic crystal
Authors: Guida G. , Brillat T. , Ammouche A. , Gadot F. , De Lustrac A. , Priou A.
Published: July 2002

In this article, we dissociate the effect of the three generic disturbances on the band gap of a bidimensional metallic photonic crystal made of parallel rods. The disturbances are investigated separately. They are deviations from the perfectly periodic position, the angle between rods constituting the photonic crystal and different sizes in rod diameters. Numerical simulations and experimental measurements have been performed in the microwave region and the results compared. The effects of these disturbances on a localized state are discussed as well. When the disturbance level is weak (i.e., a few percent), the band structure is conserved: the slopes at the edge and the band-gap width are the same as for the perfect structure. By contrast, the bandpass is reduced by about −1 dB. For an important disturbance the crystal becomes practically unusable due to the deep random modifications of its band structure. In all cases when the band gap exists, the transmission peak associated with the localized state is present.
Do thermotropic biaxial nematics exist? A Monte Carlo study of biaxial Gay–Berne particles
Authors: Berardi R. , Zannoni C.
Published: July 2002

We have investigated with extensive Monte Carlo simulations in the isothermal isobaric ensemble a system of N=8192 elongated attractive-repulsive biaxial Gay–Berne (GB) particles. We have found uniaxial and biaxial nematic phases and a biaxial orthogonal smectic phase in this thermotropic model system.
Adjustable resonant cavity for measuring the complex permittivity of dielectric materials
Authors: Gershon D. , Calame J. P. , Carmel Y. , Antonsen T. M.
Published: July 2002

An adjustable resonance cavity was developed to measure the complex permittivity of dielectric materials. The cavity has an inner diameter of 16.400 cm and an inner height of 2.54 cm. The aluminum stationary wall holder was positioned about 10.8 cm above the top of the cavity. It was fixed into place by three 1.27-cm-diam linear shafts. By suspending from the wall holder, the movable wall moved vertically by sliding on 1.27 cm bore-closed ball bushings. By turning a 1 in.-12 nut, the movable wall could be positioned so that the cavity height equaled the height of the sample. Therefore, this enables the measurement of the permittivity of samples with heights between 0.88 and 1.91 cm and radius between 1.27 and 3.18 cm. The complex permittivity of the sample was calculated based upon the sample dimensions, central frequency of TMono modes, and Q factor of the resonance curve using an exact solution. The complex permittivity was measured at the three lowest modes, where the frequency span is 1–4 GHz.
Enhanced diffusion in laser-annealed nonstoichiometric AlAs/GaAs heterostructures
Authors: Balasubramanian S. , Nolte D. D. , Melloch M. R.
Published: July 2002

Transient enhanced intermixing, induced by a focused laser beam, has been observed in arsenic-rich nonstoichiometric AlAs/GaAs quantum wells grown at a low substrate temperature around 300 °C. The intermixing is attributed to a supersaturated concentration of group-III vacancies incorporated into the crystal lattice by the low temperature growth conditions. The intermixing is enhanced by several orders of magnitude relative to diffusion in stoichiometric structures grown at ordinary substrate temperatures. Experimentally observed excitonic blueshifts in laser annealed quantum wells, and the diffusion lengths that are derived from them, are in agreement with those obtained by conventional rapid thermal annealing. Laser annealing provides the capability of spatially patterning integrated optical devices.
Bounded solutions of a second order evolution equation and applications
Authors: Leiva Hugo
Published: July 2002

In this paper we study the following abstract second order differential equation with dissipation in a Hilbert space H: u″+cu′+dA u+kG(u)=P(t), u∈H, t∈R, where c, d and k are positive constants, G:H→H is a Lipschitzian function and P:R→H is a continuous and bounded function. A:D(A)⊂H→H is an unbounded linear operator which is self-adjoint, positive definite and has compact resolvent. Under these conditions we prove that for some values of d, c and k this system has a bounded solution which is exponentially asymptotically stable. Moreover; if P(t) is almost periodic, then this bounded solution is also almost periodic. These results are applied to a very well known second order system partial differential equations; such as the sine-Gordon equation, The suspension bridge equation proposed by Lazer and McKenna, etc.
Aluminum impurities in silicon: Investigation of x-ray Raman scattering in total reflection x-ray fluorescence spectroscopy
Authors: Baur K. , Kerner J. , Brennan S. , Singh A. , Pianetta P.
Published: July 2002

Total reflection x-ray fluorescence using synchrotron radiation from the Stanford Synchrotron Radiation Laboratory has been used to study Al impurities on Si wafer surfaces. For primary excitation energies below the Si K absorption edge an inelastic resonance scattering due to resonant x-ray Raman scattering is observed. This scattering dominates the background behavior of the Al K fluorescence line, and consequently limits the achievable sensitivity for detection of Al surface contaminants. The energy and angle dependence of the resonant x-ray Raman scattering has been investigated to determine the experimental conditions for which the highest sensitivity for Al can be achieved. We find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background has to be taken into account. Our calculations demonstrate a minimum detection limit for Al of 6×109 atoms/cm2 for a 10 000 s count time.
Scaling of mean temperature dissipation rate
Authors: Xu G. , Antonia R. A. , Rajagopalan S.
Published: July 2002

Expressions for Cε and Cεθ, the dimensionless mean energy and temperature dissipation rates, can be reasonably reconciled with measured and numerical data at small to moderate Rλ only if the Kolmogorov constant and more especially the Obukhov–Corrsin constant are assumed to depend on the flow and the Reynolds number.
Thin film semiconductor deposition on free-standing ZnO columns
Authors: Könenkamp R. , Boedecker K. , Lux-Steiner M. C. , Poschenrieder M. , Zenia F. , Levy-Clement C. , Wagner S.
Published: July 2002

We report the deposition of a-Si:H on thin films of free-standing single crystalline ZnO columns. The ZnO columns have a height of several μm and a diameter between 100 and 200 nm. The ZnO films are prepared in electrodeposition and have considerable potential for use in photoelectric thin film devices. Morphology, electronic parameters, and basic optical behavior, such as reflectance and light trapping efficiency, are reported. Amorphous silicon is deposited on the columns as a continuous smooth film with conformal coverage. Some possibilities of using these films in devices are discussed.
Theoretical studies of intersystem crossing effects in the O+H2 reaction
Authors: Hoffmann Mark R. , Schatz George C.
Published: July 2002

We present a general procedure for studying intersystem crossing effects in bimolecular chemical reactions, along with an application of this to the O+H2 reaction. In this procedure, we use previously derived singlet and triplet potential energy surfaces that were based on high quality multireference configuration interaction (MRCI) nonrelativistic electronic structure calculations, and the coupling surface is obtained from lower level complete active space self-consistent field (CASSCF) calculations using the effective nuclear charge one-electron Breit–Pauli expression for the spin-orbit interaction. We find that the resulting spin-orbit splittings match the known values for O(3P), O(1D), and OH(2Π) sufficiently accurately to be useful for dynamics calculations. Also, the electronic basis can be truncated to seven states (1 3A′, 1 3A″, and 1 1A′) without seriously distorting these asymptotic splittings. We show that the seven states may be exactly decoupled into a set of four, which contains the singlet, and a set of three states from the triplets. We find that the spin-orbit matrix elements vary smoothly with geometry, so that a relatively simple function can be used to interpolate matrix elements for all geometries. The cross sections for reaction are calculated using a trajectory surface-hopping (TSH) approach in conjunction with a “diabatic” representation based on the nonrelativistic potentials and the CASSCF spin-orbit coupling matrix. An application of this approach is presented to the O+H2 reaction, using the 1 1A′ state of Dobbyn and Knowles, and 1 3A′ and 1 3A″ states of Walch and Kuppermann [slightly modified so that they are asymptotically degenerate in the product (H+OH) region]. The states show a singlet–triplet (S–T) crossing that is generally on the product side of the barrier on the triplet surfaces. The TSH results indicate that only a few percent of the trajectories undergo intersystem crossing (either from singlet to triplet, or vise versa) at the S–T crossing, so the effect of these transitions on measurable properties of the reaction dynamics is small. However, those trajectories that undergo triplet to singlet transition have much higher product rotational excitation than those that react on the triplet alone. We find that a much larger fraction of trajectories (20%–40%) undergo hopping between the two triplet states, and this leads to an averaging of the dynamical results for the two states.
Laser-induced InAs/GaAs quantum dot intermixing
Authors: Dubowski J. J. , Allen C. Nı̀. , Fafard S.
Published: July 2002

Laser annealing of InAs/GaAs quantum-dot (QD) microstructures has been investigated for selective area tuning of their electronic shell structure. Extensive blueshifts of the QD excited states were observed following 20–40 s laser irradiation. In the most extreme case, we were able to shift the position of the ground state transition by 298 meV, i.e., to the spectral region where the photoluminescence signal originates from the as-grown InAs wetting layer. A reduction from ∼50 to 8 meV of the full width at half maximum of the PL peak corresponding to this transition indicates a drastic change in the structural characteristics of the investigated QD ensemble. The attractive feature of the laser-QD-intermixing technique is that it offers the possibility of obtaining targeted blueshifts and inter-sublevel energy spacing on the lateral scale required in the fabrication of QD-based integrated optoelectronic devices and, possibly, photonic band gap crystals.
Mode-locked operation of the copper-doped germanium terahertz laser
Authors: Hovenier J. N. , de Kleijn R. M. , Klaassen T. O. , Wenckebach W. Th. , Chamberlin D. R. , Bründermann E. , Haller E. E.
Published: July 2002

We have fabricated a copper-doped germanium laser using copper diffusion into commercially available pure germanium. The spectrum and the emission pulses of this laser were investigated. Active mode locking was achieved with pulse lengths of 160 ps at full width half maximum. The current Ge:Cu laser does not yet reach the performance of an earlier studied Ge:Ga laser although mode-locked Ge:Cu lasers promise much shorter pulse lengths and wavelength tunable pulses due to the absence of acceptor related absorption. Possibilities to optimize the laser will be discussed.
Next-order structure-function equations
Authors: Hill Reginald J. , Boratav Olus N.
Published: July 2002

Kolmogorov’s equation [Dokl. Akad. Nauk SSSR 32, 16 (1941)] relates the two-point second- and third-order velocity structure functions and the energy dissipation rate. The analogous next higher-order two-point equation relates the third- and fourth-order velocity structure functions and the structure function of the product of pressure-gradient difference and two factors of velocity difference, denoted Tijk. The equation is simplified on the basis of local isotropy. Laboratory and numerical simulation data are used to evaluate and compare terms in the equation, examine the balance of the equation, and evaluate components of Tijk. Atmospheric surface-layer data are used to evaluate Tijk in the inertial range. Combined with the random sweeping hypothesis, the equation relates components of the fourth-order velocity structure function. Data show the resultant error of this application of random sweeping. The next-order equation constrains the relationships that have been suggested among components of the fourth-order velocity structure function. The pressure structure function, pressure-gradient correlation, and mean-squared pressure gradient are related to Tijk. Inertial range formulas are discussed.
Undulation, dilation, and folding of a layered block copolymer
Authors: Cohen Yachin , Brinkmann Martin , Thomas Edwin L.
Published: July 2002

The response of a mesoscopic layered structure to deformation normal to the layers is prevalently accepted to occur by an undulation instability, a model which was developed for and tested extensively in liquid-crystalline systems. This model has been applied to a layered block copolymer melt by theoretical considerations [Z.G. Wang, J. Chem. Phys. 100, 2298 (1994)] and extended to alternating glassy/rubbery layers by simulation and theory [D.J. Read et al., J. Phys. D. 32, 2087 (1999)]. In both cases, an undulation instability at extremely small strain is predicted. In this study we investigate the deformation mechanism of a lamellar block copolymer structure, stretched in the direction perpendicular to the layers, by simultaneous in situ measurements of sample stress and elongation and two dimensional small-angle x-ray scattering, at a strain resolution of 0.5%. Deformed microstructures were also observed ex-situ with scanning probe microscopy. A styrene-butadiene-styrene triblock copolymer of total molecular weight 82 000 g/mol containing 45% styrene, was used. Specimens were deformed at ambient conditions, where the system is characterized as alternating glassy/rubbery layers, at the glass transition temperature (Tg) of polystyrene (PS) where the system is viscoelastic/rubbery and well above the Tg of polystyrene where the system is viscous/rubbery. At ambient temperature, the constraints on the rubbery layers due to covalent bonding to the glassy layers at the interface, do not allow any significant dilation of the layer spacing. The results indicate that the yield point in the mechanical response does not correspond to a critical undulation instability. Rather, the low-strain behavior is dominated by tilting of layers in the vicinity of defects. Deformation of the block copolymer melt, above the Tg of PS, causes the layers to dilate affinely with the macroscopic elongation. The stability of dilated layers with respect to tilting is explained by a model which considers the dominating influence of interfacial tension. The undulation instability is observed only during deformation at about the Tg of PS, in which the system can be described as alternating viscoelastic/rubbery layers.
Dislocation density in GaN determined by photoelectrochemical and hot-wet etching
Authors: Visconti P. , Jones K. M. , Reshchikov M. A. , Cingolani R. , Morkoç H. , Molnar R. J.
Published: July 2002

Defects in GaN layers grown by hydride vapor-phase epitaxy have been investigated by photoelectrochemical (PEC) etching, and by wet etching in hot H3PO4 acid and molten potassium hydroxide (KOH). Threading vertical wires (i.e., whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic-force microscopy, we find the density of “whisker-like” features to be 2×109 cm−2, the same value found for the etch-pit density on samples etched with both H3PO4 and molten KOH. This value is comparable to the dislocation density obtained in similar samples with tunneling electron microscopy, and is also consistent with the results of Youtsey and co-workers [Appl. Phys. Lett. 73, 797 (1998); 74, 3537 (1999)].
Intersubband absorption at λ∼1.55 μm in well- and modulation-doped GaN/AlGaN multiple quantum wells with superlattice barriers
Authors: Gmachl Claire , Ng Hock M. , George Chu S.-N. , Cho Alfred Y.
Published: July 2002

Intersubband optical absorption around 1.55 μm has been measured in GaN/AlGaN multiple quantum wells (MQWs) grown by molecular-beam epitaxy. First, peak absorption wavelengths as short as 1.41 μm are reported for ultranarrow, ⩾11 Å wide, well-doped MQWs with high, 85%, AlN mole-fraction barriers. Second, in order to enable modulation doping as well as the use of lower AlN mole-fraction barriers, we designed and fabricated QWs embedded in barriers consisting of a short period superlattice of narrow GaN QWs and only 65% AlN mole-fraction barriers. The resulting electron Bragg confinement allows peak absorption wavelengths as short as 1.52 μm. Furthermore, the structures can now be modulation doped through doping of the narrow superlattice wells and subsequent charge transfer into the active well. We observe a reduction of the absorption linewidth, from ∼200 to ∼130 meV, for these structures.
Heterogeneous nucleation on aerosol particles
Authors: Padilla Kira , Talanquer V.
Published: July 2002

We have applied density functional theory in statistical mechanisms to study the heterogeneous nucleation of supersaturated vapors on spherical aerosol-like substrates. Our calculations reveal the inadequacy of the classical nucleation theory in describing the condensation of droplets on very small particles. The latter approach is particularly inaccurate both at high supersaturations and in the vicinity of the wetting transition. Comparisons are also made with recent experimental results for the condensation of supersaturated vapors on submicrometer particles.
Microscopic relaxation in supercritical and liquid neon
Authors: Cunsolo A. , Pratesi G. , Verbeni R. , Colognesi D. , Masciovecchio C. , Monaco G. , Ruocco G. , Sette F.
Published: July 2002

The high frequency behavior of the dynamic structure factor, S(Q,ω), of liquid and supercritical neon is investigated by inelastic x-ray scattering at different temperatures and pressure. The spectral evolution is described in terms of a single-relaxation-time viscoelastic model. The occurrence of a positive dispersion in the sound velocity is clearly visible in both investigated thermodynamic phases. The anomalies in the dispersive behavior deeply reduce at the higher temperatures, probably, as a consequence of important changes in the first shell interactions. More generally, the atomic dynamics is dominated by a relaxation process whose time scale is in the range of fast microscopic degrees of freedom (≈10−13 s), and whose strength and typical time scale stay constant over all the explored liquid and supercritical regions.
Interfacial capacitance effects in magnetic tunneling junctions
Authors: Landry G. , Dong Y. , Du J. , Xiang X. , Xiao John Q.
Published: July 2002

We have fabricated magnetic tunneling junctions by oxidizing a wedge-shaped aluminum layer to produce junctions with ideal oxidized as well as under and over oxidized junctions on a single wafer. By investigating the capacitance spectra, we are able to study the effects due to interface charge accumulation. The electron–electron interaction among accumulated interface charges leads to a voltage drop inside the magnetic electrodes, resulting in the measured capacitance differing from the geometric capacitance. We have extracted an interfacial capacitance of 16 μF/cm2 per interface and a screening length of 0.55 Å for FeNi electrodes.
A fast-scanning, low- and variable-temperature scanning tunneling microscope
Authors: Petersen L. , Schunack M. , Schaefer B. , Linderoth T. R. , Rasmussen P. B. , Sprunger P. T. , Laegsgaard E. , Stensgaard I. , Besenbacher F.
Published: July 2002

The design and performance of a fast-scanning, low- and variable-temperature, scanning tunneling microscope (STM) incorporated in an ultrahigh vacuum system is described. The sample temperature can be varied from 25 to 350 K by cooling the sample using a continuous flow He cryostat and counter heating by a W filament. The sample temperature can be changed tens of degrees on a time scale of minutes, and scanning is possible within minutes after a temperature change. By means of a software implemented active drift compensation the drift rate can be as low as 1 nm/day. The STM is rigid, very compact, and of low weight, and is attached firmly to the sample holder using a bayonet-type socket. Atomic resolution on clean metal surfaces can be achieved in the entire temperature range. The performance of the instrument is further demonstrated by images of adsorbed hexa-tert-butyl-decacyclene molecules on Cu(110), by STM movies, i.e., sequential STM images with a time resolution down to 1 s/image (100×100 Å2 with 256×256 pixels), of the mobility of these molecules, and finally by constant current images of standing waves in the electronic local density of states on Cu(110).
Beat frequencies in a ring laser gyro with its refractive index over unity
Authors: Numai Takahiro
Published: July 2002

A frequency shift Δf during rotation in a ring laser gyro with its refractive index of unity was already well known. However, when the refractive index nr is over unity, several expressions for a beat frequency such as 2Δf∝nr, nr0, nr−1, and nr−2 were proposed. In this article, a beat frequency in a uniformly rotating ring laser gyro with nr>1 is calculated using nonrelativistic, special relativistic, and general relativistic theories. To the first order, all the results obtained from these theories show a common beat frequency, which is proportional to the refractive index nr, as far as the dispersion is neglected. When the dispersion is considered, the beat frequency is in proportion to nr0[1−(rΩ/λ0)2(dnr/df)2]−1, where f is a frequency of a laser beam, λ0 is a wavelength of a laser beam when the ring laser stands still, and nr0 is a refractive index at λ0. If nr0=1−(rΩ/λ0)2(dnr/df)2 is satisfied, the beat frequency seems to be independent of the refractive index.
Spatial characteristics of electron beams from symmetric and nonsymmetric crossed-field secondary emission sources
Authors: Saveliev Y. M. , Sibbett W. , Parkes D. M.
Published: July 2002

The crossed-field secondary emission (CFSE) diode is a cold electron source based on a self-sustained secondary electron emission. The output electron beams are tubular and could be generated in a wide range of currents up to several hundred amperes. In this study, radial and azimuthal current density distributions of electron beams produced by symmetric and nonsymmetric CFSE diodes have been investigated. The electron beams are characterized by extremely high temporal stability. The wall thickness of the tubular beam with a current of >102 A from the diode with a 5 mm anode–cathode gap was measured to be as small as 1.4±0.2 mm. In axisymmetric diodes, the azimuthal current distribution j(φ) is uniform but this is only achieved by careful adjustment of the cathode–anode assembly. In nonaxisymmetric diodes, the j(φ) distributions are strongly nonuniform and depend not only on the magnitude but also on the direction of the magnetic field. Results of the present research show that the CFSE electron sources are potent candidates for incorporation into medium and high power microwave devices.
Ultrafast nonlinear optical method for generation of planar shocks
Authors: Moore D. S. , Gahagan K. T. , Reho J. H. , Funk David J. , Buelow S. J. , Rabie R. L. , Lippert T.
Published: July 2002

Planar shocks generated by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of 120–400 fs duration Gaussian spatial mode incident laser pulses are reproducibly flattened via surface optical breakdown and Kerr focusing in thin dielectric substrates at fluences just above the ablation threshold. These flat laser profiles have been used to produce planar shocks that are flat to 0.7 nm root-mean-square over a 75–100 μm diameter.
Effect of carrier-gas pressure on barrier to nucleation: Monte Carlo simulation of water/nitrogen system
Authors: Oh K. J. , Zeng X. C.
Published: July 2002

Carrier gases are used in most nucleation experiments for releasing the latent heat generated during vapor condensation. In the analysis of experimental data it is often assumed that the carrier gas is inert and would not participate in the nucleation process of the target gas. Several recent nucleation experiments show that the influence of carrier gases to nucleation rate is not negligible under certain conditions. To gain more insight into the carrier-gas effect, we carry out Monte Carlo simulation to compute the free energy of formation of water clusters in the presence of a nitrogen carrier gas. At fixed temperature (240 K) and chemical potential, it is found that the barrier height to nucleation increases with the carrier-gas pressure. This barrier enhancement is attributed to the increase of equilibrium vapor pressure of water in the presence of carrier gas, which results in a decrease of supersaturation. It is also found that the simulation results are consistent with the binary-nucleation theorem.
Ultraviolet photoluminescence from nonbridging oxygen hole centers in porous silica
Authors: Yao Baodian , Shi Huazhong , Zhang Xinyi , Zhang Lide
Published: July 2002

In this letter, we report the observation of an ultraviolet (UV) photoluminescence (PL) emission at around 330 nm in porous silica prepared by the sol–gel process. Upon a posttreatment which leads to OH adsorption, the intensity of the observed UV PL emission increases significantly. It is shown that this behavior is associated with the increase of OH groups adsorbed on the surface of porous silica. We suggest that the observed UV PL emission originates from the nonbridging oxygen hole centers generated from surface hydroxyls. The mechanism of the UV PL emission is expressed tentatively by combining the nonbridging oxygen centers charge modification model and the vibrative absorption of a SiO2 network containing water.
Planar shock wave generated by uniform irradiation from two overlapped partially coherent laser beams
Authors: Ozaki N. , Sasatani Y. , Kishida K. , Nakano M. , Miyanaga M. , Nagai K. , Nishihara K. , Norimatsu T. , Tanaka K. A. , Fujimoto Y. , Wakabayashi K. , Hattori S. , Tange T. , Kondo K. , Yoshida M. , Kozu N. , Ishiguchi M. , Takenaka H.
Published: July 2002

Two partially coherent light laser beams, coupled with a random phase plate were focused at an angle of 31.7° with the centers of the beams offset by 250 μm. This produced a relatively uniform (7% root-mean-square) irradiated spot of 400 μm. When this technique was used to produce a shock wave in a copper wedge, a relatively uniform shock of 2 ns duration was produced. In addition, a multilayered flyer plate was accelerated using this method to an average speed of 21 km/s and produced a planar impact on a glass window. It is anticipated that this technique can be used to produce important multi-Mbar equation of state information in future experiments.
The CO oxidation kinetics on supported Pd model catalysts: A molecular beam/in situ time-resolved infrared reflection absorption spectroscopy study
Authors: Libuda J. , Meusel I. , Hoffmann J. , Hartmann J. , Piccolo L. , Henry C. R. , Freund H.-J.
Published: July 2002

Combining molecular beam techniques and time-resolved infrared reflection absorption spectroscopy (TR-IRAS) we have studied the kinetics of the CO oxidation reaction on an alumina-supported Pd model catalyst. The Pd particles are deposited by metal evaporation under ultrahigh vacuum (UHV) conditions onto a well-ordered alumina film, prepared on a NiAl(110) single crystal. Particle size, density and structure of the Pd deposits have been characterized in previous studies. In the low temperature region, transient and steady-state experiments have been performed over a wide range of CO and oxygen fluxes by crossing two effusive molecular beams on the sample surface. We determine the steady-state CO2 production rate as a function of the CO fraction in the impinging gas flux. Simultaneously, the occupation of CO adsorption sites under steady-state conditions is monitored by in situ IR spectroscopy. The origin of different types of CO2 transients is discussed. In particular we focus on the transient CO2 production after switching off the CO beam. For the model catalyst investigated, detailed reaction rate measurements in combination with time-resolved IRAS show that the origin of the particular transient behavior of the supported model system is not due to the presence of specific adsorption sites on small particles, as has been proposed previously. Instead, we show that the transient behavior can be semiquantitatively simulated on the basis of a simple kinetic model considering a homogeneous surface, and accounting for the inhibition of the dissociative adsorption of O2 at high CO coverage. Moreover, it is discussed how the inherent heterogeneity of the supported particle system can additionally enhance the observed effect.
Strain-induced transformation of amorphous spherical precipitates into platelets: Application to oxide particles in silicon
Authors: Voronkov V. V. , Falster R.
Published: July 2002

The spherical shape of an amorphous precipitate becomes unstable if the combination P2R of precipitate radius R and pressure P exceeds some critical value. This critical value was found to be about 4.44 Gσ, where G is the matrix shear modulus and σ is the specific energy of the precipitate/matrix interface. Once this instability criterion is fulfilled, the initially spherical particle will reduce the total free energy (the sum of strain energy and the surface energy) by becoming a thin oblate spheroid (effectively, a platelet). The actual pressure P in the course of oxygen precipitation in silicon is controlled by a high self-interstitial supersaturation caused by emission of self-interstitials by growing precipitates. The duration of annealing necessary to reach the stage of collapse of spheres into platelets is calculated as a function of temperature and the precipitate density. Calculated results are compatible with the experimentally observed annealing conditions for platelet formation. Another important example of sphere to platelet transformation is microdefect formation in vacancy-type silicon. In this case a large negative value of P is sufficient to induce collapse.
Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS
Authors: Weller A. , Anton M. , Geiger J. , Hirsch M. , Jaenicke R. , Werner A. ,   W7-AS Team, Nührenberg C. , Sallander E. , Spong D. A.
Published: July 2002

Magnetohydrodynamic (MHD) instabilities in the Wendelstein 7-AS stellarator (W7-AS) [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] are characterized experimentally in various plasma parameter regimes and heating scenarios. The observations are compared with theoretical predictions for particular cases. In the high-β range (〈β〉⩽2%) no clear evidence of a stability β-limit could be found yet. In the lower β regime fast particle driven global Alfvén modes are the most important instabilities during neutral beam injection (NBI). Besides of coherent modes with almost no effect on the plasma performance additional Alfvén modes appear at higher frequencies up to 400 kHz, which show nonlinear phenomena-like bursting, frequency chirping, and MHD induced energy and fast particle losses. The activity of edge localized modes (ELMs) is investigated in NBI heated discharges. The issue of current driven instabilities and their potential stabilization by a stellarator field has been investigated with regard to the design of compact hybrid stellarator systems.
Predictability of the inverse energy cascade in 2D turbulence
Authors: Boffetta G. , Musacchio S.
Published: July 2002

The predictability problem in the inverse energy cascade of two-dimensional turbulence is addressed by means of high resolution direct numerical simulations. The growth rate as a function of the error level is determined by means of a finite size extension of the Lyapunov exponent. For errors within the inertial range, the linear growth of the error energy, predicted by dimensional argument, is verified with great accuracy. Our numerical findings quantitatively confirm the results of the classical TFM closure approximation.
Atomic force microscope cantilevers for combined thermomechanical data writing and reading
Authors: King William P. , Kenny Thomas W. , Goodson Kenneth E. , Cross Graham , Despont Michel , Dürig Urs , Rothuizen Hugo , Binnig Gerd K. , Vettiger Peter
Published: July 2002

Heat conduction governs the ultimate writing and reading capabilities of a thermomechanical data storage device. This work investigates transient heat conduction in a resistively heated atomic force microscope cantilever through measurement and simulation of cantilever thermal and electrical behavior. The time required to heat a single cantilever to bit-writing temperature is near 1 μs and the thermal data reading sensitivity ΔR/R is near 1×10−4 per vertical nm. Finite-difference thermal and electrical simulation results compare well with electrical measurements during writing and reading, indicating design tradeoffs in power requirements, data writing speed, and data reading sensitivity. We present a design for a proposed cantilever that is predicted to be faster and more sensitive than the present cantilever.
Effect of graphitic inclusions on the optical gap of tetrahedral amorphous carbon films
Authors: Teo K. B. K. , Rodil S. E. , Tsai J. T. H. , Ferrari A. C. , Robertson J. , Milne W. I.
Published: July 2002

High sp3 fraction tetrahedral amorphous carbon (ta-C) films can be prepared using the filtered cathodic vacuum arc (FCVA). A by-product of the deposition process are small micrometer sized graphitic particles which are also incorporated into the film. The particle coverage of FCVA films is typically <5%, and thus the effect of these graphite inclusions have been largely ignored in earlier optical gap measurements of ta-C. By incorporating a better filter design (e.g., S-bend filter), the particle coverage can be reduced to 0.1%. In this article, we show that the effect of these graphitic inclusions is to scatter or absorb light which significantly affects the optical gap measurement and hence reduces the “apparent” optical gap of the ta-C film. By comparing two ta-C films with different particle coverage but the same sp3 content of 85%, we show that we can correct for the effect of these inclusions. Our results confirm that the E04 gap of a 85% sp3ta-C matrix is 3.6 eV. The importance of considering these micro particles is emphasized as we find that for every 1% of area covered by particles, there is a 3–4 fold percentage difference between the corrected optical gap and measured gap of the film.
Magnetic polaritons in XF2 (X=Mn, Fe) canted antiferromagnets
Authors: Guimarães E. S. , Albuquerque E. L.
Published: July 2002

We discuss the magnetic polariton modes which propagate in a two-sublattice, uniaxial Heisenberg antiferromagnet, for the case where an external magnetic field is applied perpendicular to the easy axis of the material. At high magnetic fields, a large magnetization component normal to the easy axis will be induced, and we may expect to find a magnetic field induced surface polariton mode, which propagates parallel to the easy axis, with nonreciprocal propagation characteristics. We confine our discussion to antiferromagnets, which have a body-centered tetragonal structure (e.g., XF2, with X=Mn and Fe). The spectra show that the nonreciprocal propagation of the surface polariton is much more evident in MnF2 crystals.
Angular dependence of remanence of exchange-coupled CoFe/NiO bilayers
Authors: Zhao Tong , Zhang Kunliang , Fujiwara Hideo
Published: July 2002

The magnetic properties of Co90Fe10(5 nm)/NiO(1–5 nm) bilayers fabricated by rf/dc magnetron sputtering were studied. The bilayers show enhanced uniaxial anisotropy, but no unidirectional anisotropy. The angular dependence of the remanence of these samples shows hysteresis around the hard direction, which increases with increasing NiO layer thickness and was ascribed to switching of the surface spins of the antiferromagnetic layer driven by the exchange coupling between CoFe and NiO layers. This phenomenon can be simulated using a model considering the preferential distribution of the easy axes of the antiferromagnetic grains. A method for extracting information about interlayer coupling and anisotropy of antiferromagnetic layers from the angular dependence of remanence is proposed.
Energy considerations in vector magnetization models
Authors: DellaTorre Edward , Reimers Ann
Published: July 2002

Scalar magnetization model energy calculations, which are useful in calculating energy losses in open magnetization processes, are extended here to vector models. The energy loss is calculated as the difference between the energy input to the magnetic medium and the change in energy stored in the locally reversible component of the total magnetization. It is shown that under rotating magnetizing processes, the energy loss decreases as the material is saturated. For smaller rotating fields in anisotropic media, the magnetization lies between the applied field and the easy axis and the energy loss is not constant with rotation even if the magnitude of the field is constant.
Effect of tensile stress on the 3D reversible and irreversible differential magnetic susceptibilities
Authors: Mao Weihua , Atherton David L.
Published: July 2002

Magnetic hysteresis loops in three orthogonal directions are measured for a line pipe steel sample while the external magnetic field is applied in a direction perpendicular to the tensile stress direction. The total magnetization vector is calculated. This tends to the stress direction when tensile stress is applied. The reversible and irreversible differential magnetic susceptibilities are derived. It is found that the susceptibilities in all three directions are enhanced with increasing tensile stress, although the increase in the stress direction is much larger than in the other directions.
Vortex formation in magnetic narrow rings: The role of magneto-crystalline anisotropy
Authors: Lopez-Diaz L. , Rothman J. , Kläui M. , Bland J. A. C.
Published: July 2002

Hysteresis loops in mesoscopic narrow rings with cubic anisotropy have been computed. A two-step switching process is found. The first transition corresponds to vortex formation and has been studied in detail. Starting from a well defined remanent state with two opposite head-to-head walls, the vortex is created when, due to any asymmetry, one wall depins first, propagates, reaches the other and they annihilate one another. It is also found that, in this geometry, the crystallographic hard axis becomes the global easy axis due to the combined action of the magneto-crystalline anisotropy and the magnetostatic dipolar fields.
Quantum Monte Carlo study of a Heisenberg spin system in two dimensions with long-range interactions
Authors: Vassiliev O. N. , Cottam M. G. , Rojdestvenski I. V.
Published: July 2002

The thermodynamic properties and critical behavior of a Heisenberg ferromagnet in two dimensions are studied using the Handscomb quantum Monte Carlo method. The spins (with quantum number S=12) are arranged on a square lattice and have a long-range interaction with a power-law decrease, J(r)∝r−p, with distance r. The parameter p is taken to be in the range 2<p⩽6. The system has an ordering transition at a nonzero temperature Tc when 2<p<4. The dependence of Tc on p and the critical behavior of the susceptibility are investigated. Properties of spin correlation functions in different regimes (of p and T) are discussed. The results are compared with previous approximate analytic quantum theories.
Electroabsorption measurements and built-in potentials in amorphous silicon–germanium solar cells
Authors: Lyou J. H. , Schiff E. A. , Guha S. , Yang J.
Published: July 2002

We report electromodulated reflectance spectra in n-i-p solar cells with hydrogenated amorphous silicon–germanium alloy absorber layers. At lower photon energies the spectra are determined by bulk electroabsorption, and exhibit peaks near the optical gap of the absorber layers. Voltage scaling of the electroabsorption spectra indicate a built-in potential of Vbi=1.17 V in cells with absorber layer band gaps of 1.50 eV; in conjunction with earlier work, this value argues against a systematic decline in Vbi with an absorber layer band gap. At higher photon energies the spectra are due to direct electroreflectance; the voltage scaling was consistent with model predictions for the electric field at the interface of the p-type and absorber layers.
Magnetoresistance anomaly in DyFeCo thin films
Authors: Wu J. C. , Wu C. S. , Wu Te-ho , Chen Bing-Mau , Shieh Han-Ping D.
Published: July 2002

Microstructured rare-earth–transition-metal DyFeCo films have been investigated using magnetoresistance and extraordinary Hall-effect measurements. The Hall loops reveal variation of coercive fields depending on the linewidth and the composition of the films. The magnetoresistance curves, with changes up to as high as 1.3%, show positive/negative magnetoresistance peaks centered on the coercive fields depending on the linewidth of the films only. The variation of the coercivity can be attributed to the magnetic moment canting between the Dy and FeCo subcomponents and the existence of the diverged magnetization on the edges, and the anomalous magnetoresistance peaks observed are discussed with the existing theories.
Spatial mapping of activation magnetic moment in ferromagnetic films
Authors: Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

We report a technique to simultaneously determine the local activation magnetic moment with a spatial resolution of a submicrometer scale. The local activation magnetic moment is determined from the magnetic-field dependence of the magnetization reversal at the corresponding region observed by a magneto-optical microscope system. The present method has been applied to measure the activation magnetic moment distribution of Co/Pd multilayers. It was found that the activation magnetic moment was nonuniform on a submicrometer scale and that the distribution of the activation magnetic moment enhanced with increasing the film thickness.