bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromagnetic levitation apparatus for investigations of the phase selection in undercooled melts by energy-dispersive x-ray diffraction
Authors: Notthoff C. , Franz H. , Hanfland M. , Herlach D. M. , Holland-Moritz D. , Petry W.
Published: July 2002

The metastable state of an undercooled melt can provide access to the formation of metastable phases whose crystallographic structures differ from their stable counterparts. The electromagnetic levitation technique is used here to undercool bulk melts without containers. This technique, combined with energy dispersive x-ray diffraction of synchrotron radiation, allows in situ study of the rapid crystallization of undercooled Ni–V alloys. The binary Ni–V alloy was chosen as a model system, because it shows phase competition of three different crystallographic phases, bcc, fcc, and an intermetallic phase of tetragonal structure at intermediate concentrations. The diffraction experiments provide for direct identification of the various crystalline phases formed from the nonequilibrium state of the undercooled melt. The phase selection is investigated as a function of undercooling temperature for Ni–V alloys of different concentrations.
Ion-beam-induced porosity of GaN
Authors: Kucheyev S. O. , Williams J. S. , Jagadish C. , Zou J. , Craig V. S. J. , Li G.
Published: July 2002

Wurtzite GaN films bombarded with heavy ions (197Au+) show anomalous swelling of the implanted region with corresponding volume expansion up to ∼50%. Results show that this phenomenon is due to the formation of a porous layer of amorphous GaN. An important implication of this study for the fabrication of GaN-based devices is that amorphization of GaN should be avoided during ion implantation.
Electrochemically etched nickel tips for spin polarized scanning tunneling microscopy
Authors: Cavallini Massimiliano , Biscarini Fabio
Published: July 2002

A fast and simple method for the electrochemical preparation of sharp Ni tips for spin polarized scanning tunneling microscopy (STM) is reported. These Ni tips perform well also in conventional STM experiments, being able to achieve reproducible atomic resolution on graphite. Ni tips combine the advantages of both Pt/Ir tips, since they are oxide free, and of W tips because of the greater reproducibility and control of the tip apex by means of the etching protocol, as compared to freshly cut tips.
Nanostructured FePt:B2O3 thin films with perpendicular magnetic anisotropy
Authors: Luo C. P. , Liou S. H. , Gao L. , Liu Y. , Sellmyer D. J.
Published: July 2002

FePt/B 2 O 3 multilayers were deposited by magnetron sputtering onto 7059 glass substrates. By annealing the as-deposited films at 550 °C, nanostructured FePt:B2O3 films consisting of FePt grains with L10 structure, embedded in a glassy B2O3 matrix, were obtained. The c axes of the FePt grains can be made to align with the film normal direction, which results in a perpendicular anisotropy constant of 3.5×107 erg/cc. The films remain layered structures after annealing when the B2O3 layer thickness exceeds 16 Å. The nanostructure of the films was investigated by transmission electron microscopy. The coercivities and the average grain sizes of the films are dependent on the B2O3 concentrations, with coercivities varying from 4 to 12 kOe, while average grain sizes vary from 4 to 17 nm. Strong perpendicular anisotropy, adjustable coercivity, and fine grain size suggest this nanocomposite system might have significant potential as recording media at extremely high areal density.
Ultrafast scanning tunneling microscopy with 1 nm resolution
Authors: Khusnatdinov N. N. , Nagle T. J. , Nunes G.
Published: July 2002

We present data demonstrating that junction-mixing scanning tunneling microscopy (JM–STM) can provide simultaneous picosecond time resolution and nanometer spatial resolution. Experiments were performed on an Au surface with a patterned Ti overlayer. Our measurements under ultrahigh vacuum conditions achieve a spatial resolution of 1 nm using the tunneling currents generated by 20 ps voltage transients. The observed contrast in a JM–STM signal is demonstrated to arise entirely from the difference in electronic structure between the Au and Ti surfaces. These results confirm that JM–STM signals originate in the tunnel junction and maintain the atomic-scale spatial resolution inherent in STM.
Electromigration of copper in Al(0.25 at. % Cu) conductor lines
Authors: Kao H.-K. , Cargill G. S. , Hu C.-K.
Published: July 2002

Electromigration and diffusion of Cu have been investigated for polycrystalline Al(0.25 at. % Cu) conductor lines. In situ measurements of the evolution of Cu concentration profiles along 200 μm long, 10 μm wide conductor lines with 1.5 μm thick SiO2 passivation during electromigration have been obtained by synchrotron-based white x-ray microbeam fluorescence. The apparent effective charge ZCu* of Cu in Al(Cu) has been found to be −8.6±1.0. The evolution of Cu concentration profiles can be manipulated by controlling the direction and magnitude of the current flow at different temperatures. The effective grain boundary diffusivity DCueff has been determined by fitting the time dependent experimental Cu concentration profiles. The results show Arrhenius behavior of DCueff=D0 exp(−Q/kT) for T=275–325 °C with D0=10−(2.3±1.6) cm2/s and Q=0.76±0.19 eV.
Optical properties and structure of tellurium–germanium– bismuth–antimony compounds with fast phase-change capability
Authors: Lee C. M. , Chin T. S. , Huang E. Y.
Published: July 2002

Quaternary TeGeBiSb amorphous films, 100 nm in thickness, were prepared by rf sputtering onto either a Si (100) or polycarbonate substrate. Crystalline films were obtained by subsequent annealing at 300 °C for 10 min. At least two phases with hexagonal and rhombohedral structures were identified by x-ray diffractometry in the quaternary system. Their compositions are close to formulas Te5Ge4Bi4Sb and Te5Ge4Bi7Sb, with c/a ratios of 5.5 and 9.4, respectively. The Te5Ge4Bi4Sb film shows a reasonable reflectivity contrast ranging from 20% to 25% in the whole visible spectrum, while the reflectivity contrast of Te5Ge4Bi7Sb film shows a sharp lowering tendency with decreasing wavelength and turns negative at wavelengths smaller than 470 nm. Results of thermal analyses show that the films crystallize at around 236–266 °C with corresponding activation energies 2.67–2.88 eV. The change in reflectivity contrast is explained based on the variation in optical band gaps of the TeGeBiSb materials with either Bi content or annealing temperature. These materials may find applicability in reversible type phase-change optical recording.
Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films
Authors: Wu X. L. , Siu G. G. , Fu C. L. , Ong H. C.
Published: July 2002

Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle.
Nonrelativistic dynamics of particles with characteristic spins and anomalous magnetic moments in a homogeneous magnetic field
Authors: Datta Sambhu N. , Misra Anirban
Published: July 2002

A detailed theoretical treatment of the effect of a homogeneous external magnetic field on a general molecule is carried out in this work at the nonrelativistic level. The Hamiltonian considered here is, although nonrelativistic, explicitly spin-adapted through order v2/c2. The effects of the anomalous magnetic moment of each particle, hyperfine interactions and spin–orbit interactions have been systematically included in it. This Hamiltonian is reexpressed in a preferred set of coordinates so as to facilitate the removal of the center of mass motion by exploiting the properties of the total pseudomomentum. The reduced Hamiltonian operator at zero total pseudomomentum represents the dynamics of the electrons and the nuclei belonging to a neutral molecule in relative coordinates. The same operator can be separated as a sum of the electronic Hamiltonian, the nuclear Hamiltonian, and the electronic–nuclear coupling. The effect of a strong magnetic field on molecular vibration and rotation is then described for a general diatomic molecule. The interelectronic and the electron–nuclear kinetic coupling terms are obviously responsible for the known phenomenal increase in the bond dissociation energy at a strong magnetic field under the Born–Oppenheimer approximation. The case of nuclear spin isomerism has been discussed in the specific context of ortho and para hydrogen. This leads to a prediction of the critical field strength beyond which ortho hydrogen may be formed as the major product even at an extremely low temperature. The basic tenets of the adiabatic approximation are examined, and the influence of the electronic–nuclear kinetic coupling on the evaluation of the vibronic interaction parameters is discussed in an Appendix.
First high resolution analysis of the absorption spectrum of propane in the 6.7 μm to 7.5 μm spectral region
Authors: Flaud J.-M. , Lafferty W. J. , Herman M.
Published: July 2002

The rotational structure of the v19 (CH2− wagging, A-type band), v18 (CH3− s-deformation, A-type band), v24 (CH3− d-deformation, C-type band), and v4 (CH3− d-deformation, B-type band) vibrational fundamentals of propane, were recorded using a molecular beam coupled to a Fourier-transform spectrometer working at 0.005 cm−1 resolution. The analysis was performed using Watson Hamiltonians for the 191 and 181 states. However, to properly calculate the rotational levels of the 41 and 241 vibrational levels, it is necessary to include in the Hamiltonian matrix not only the strong A-type Coriolis interaction which couples them but also the various interactions (Coriolis or Fermi-type) which link them to the levels of the dark 51 and 171 vibrational states. It is then possible to calculate the upper-state levels to within an average uncertainty of 2 to 5×10−3 cm−1 depending on the state. These results are satisfactory given the fact that (i) possible perturbations with nearby combination states were not considered and (ii) several torsional splittings were not accounted for. The band centers derived from the fits are ṽ0(v19)=1338.965 cm−1, ṽ0(v18)=1376.850 cm−1, ṽ0(v24)=1471.874 cm−1, and ṽ0(v4)=1476.710 cm−1. The standard uncertainties of these values are estimated to be about 0.002 cm−1, which includes the calibration errors as well as the statistical uncertainty of the fittings.
Effective dynamics for ferromagnetic thin films
Authors: Garcı́a-Cervera Carlos J. , E Weinan
Published: July 2002

In a ferromagnetic material, the dynamics of the relaxation process are affected by the presence of a strong shape or material anisotropy. In this article, we systematically explore this fact to derive the effective dynamical equation for a soft ferromagnetic thin film. We show that, as a consequence of the interplay between shape anisotropy and damping, the gyromagnetic term is effectively also a damping term for the in-plane components of the magnetization distribution. We validate our result through numerical simulation of the original Landau–Lifshitz equation and our effective equation.
Separation of enantiomers by ultraviolet laser pulses in H2POSH: π pulses versus adiabatic transitions
Authors: González Leticia , Kröner Dominik , Solá Ignacio R.
Published: July 2002

Different strategies to separate enantiomers from a racemate using analytical laser pulses in the ultraviolet frequency domain are proposed for the prototype model system H2POSH. Wave-packet propagations on ab initio ground- and electronic-excited state potentials show that it is possible to produce 100% of enantiomeric excess in a sub-picosecond time scale using a sequence of π and half-π pulses. Alternatively, the previous transitions can be substituted by adiabatic counterparts, using chirped laser pulses and a half-STIRAP (stimulated Raman adiabatic passage) method which only transfers half of the population between appropriate levels. Such an overall adiabatic mechanism gains stability concerning the pulse areas and frequencies at the expense of introducing new control variables, like the chirp and time delay.
Reduction of Sasakian manifolds
Authors: Grantcharov Gueo , Ornea Liviu
Published: July 2002

We show that the contact reduction can be specialized to Sasakian manifolds. We prove that the Sasakian reduction is compatible with the Kähler reduction both in the cone construction and in the Boothby–Wang fibration. In particular, applying Futaki’s results, we obtain a sufficient condition for the reduced space of a regular Sasakian–Einstein manifold to be Sasakian–Einstein. We present examples of Sasakian–Einstein manifolds obtained by S1 reduction of standard Sasakian spheres.
Anomalous increase of resistivity against thermal cycling in some of the phase separated manganites
Authors: Mahendiran R. , Maignan A. , Hervieu M. , Martin C. , Raveau B.
Published: July 2002

We investigate the effect of repeated thermal cycles in some of the Pr0.5Ca0.5Mn1−xMxO3 (M=Cr, Co, and Ni and x=0.02–0.05) compounds which undergo a spontaneous (H=0 T) insulator to metal transition below the temperature Tp driven by percolation of ferromagnetic clusters in a charge ordered background. The zero field resistivity (ρ) shows an anomalous behavior upon thermal cycling between a high-(TS)- and a low (TF)- temperature limit: While ρ for T≫Tp is unaffected, it increases significantly for T⩽Tp and Tp shifts down each time upon thermal cycling. In contrast, ρ at a given temperature decreases slowly with time. The anomalous increase of resistivity depends upon the ratio of the charge ordered to ferromagnetic phase fraction and decreases with increasing x and the applied magnetic field (H). We also show that a similar effect can be obtained for Pr0.5Ca0.5Mn0.97Al0.03O3 and Pr0.55Ca0.45MnO3 under H=5 T although these compounds do not exhibit insulator–metal transition in zero field. The low field magnetization study on Pr0.5Ca0.5Mn0.98Cr0.02O3 suggests that the ferromagnetic Curie temperature remains unaffected, whereas the ferromagnetic phase fraction decreases with repeated cycles. We suggest that the observed anomaly is caused by an increase of interfacial strain between charge ordered and ferromagnetic phases.
Improved energy resolution of x-ray single photon imaging spectrometers using superconducting tunnel junctions
Authors: Li L. , Frunzio L. , Wilson C. , Prober D. E. , Szymkowiak A. E. , Moseley S. H.
Published: July 2002

We present measurements of the energy resolution of improved single-photon imaging x-ray spectrometers based on superconducting tunnel junctions. The devices have a Ta film absorber with an Al/AlOx/Al tunnel junction on each end. Recent device designs optimized for better quasiparticle cooling in the Al trap obtain an energy resolution of 13 eV full width at half maximum for a photon energy E=5.9 keV, an improvement of a factor of two over earlier devices. We also determined that the niobium contact used in previous devices degraded the energy resolution in the center section of the absorber. With a different contact configuration, we have eliminated this spatial broadening.
Depth dependence of {311} defect dissolution
Authors: Venezia V. C. , Kalyanaraman R. , Gossmann H.-J. L. , Rafferty C. S. , Werner P.
Published: July 2002

A deep band of {311} defects was created 520 nm below the silicon surface with a 350 keV Si implant followed by a cluster-forming rapid thermal anneal (800 °C, 1000 s). Chemical etching was used to vary the depth to the surface of the {311}-defect band. Afterwards, the defect dissolution was investigated at 750 °C for different times. Varying the depth in this fashion assures that only the depth and no other feature of the cluster distribution is changed. The {311} defects were analyzed by plan-view, transmission electron microscopy. We show that the dissolution time of the {311}-defect band varies linearly with depth, confirming that surface recombination controls the dissolution and is consistent with analogous observations of transient enhanced diffusion.
Noncollisional heating and electron energy distributions in magnetically enhanced inductively coupled and helicon plasma sources
Authors: Kinder Ronald L. , Kushner Mark J.
Published: July 2002

The ability to deposit power in the volume of plasma reactors at locations deeper than the conventional skin depth makes magnetically enhanced inductively coupled plasma (MEICP) and helicon sources appealing for use in materials processing. Mechanisms for power deposition and electron energy transport in MEICPs have been computationally investigated using a two-dimensional (2D) plasma equipment model. Using a tensor conductivity in the solution of Maxwell’s equations, three-dimensional components of the inductively coupled electric field are produced from an m=0 antenna and 2D applied magnetic fields. These fields are then used in a Monte Carlo simulation to generate electron energy distributions (EEDs), transport coefficients, and electron impact source functions. The electrostatic component of the wave is resolved by estimating the charge density using an oscillatory perturbed electron density. For MEICPs operating at pressures less than 10 mTorr in Ar, significant power deposition occurs downstream when the radial and axial components of the electric field are commensurate with the azimuthal component. For magnetic fields above 100 G, the tail of the EED (>20–30 eV) is enhanced in the downstream region. This enhancement results from noncollisional heating by the axial electric field for electrons in the tail of the EED which have long mean free paths, while lower energy electrons are still somewhat collisional.
Size dependent magnetic phase of nanocrystalline Co0.2Zn0.8Fe2O4
Authors: Dey S. , Roy A. , Ghose J. , Bhowmik R. N. , Ranganathan R.
Published: July 2002

Co 0.2 Zn 0.8 Fe 2 O 4 nanoparticles have been synthesized by the coprecipitation method. Nanoparticles with average particle size of 5–∼65 nm were obtained by sintering the samples from 393 to 1173 K. Magnetic measurements show that samples with particle size ⩽13 nm are superparamagnetic above the blocking temperature TB. Samples with average particle size larger than 13 nm have a wide particle size distribution and show the presence of two magnetic phases: superparamagnetic and cluster glass. Cation distribution of the samples show that in the samples with particle size <13 nm Fe ions are distributed between the tetrahedral and octahedral sites of the spinel lattice but in the larger particle size samples the tetrahedral sites have very few Fe ions that form Fe–Fe clusters and hence the high temperature sintered samples show a mixed phase, consisting of superparamagnetic and cluster glass.
Performance of a grating monochromator at BL27SU beamline of SPring-8 in the higher energy region
Authors: Tamenori Y. , Ohashi H. , Ishiguro E. , Ishikawa T.
Published: July 2002

Performance tests were carried out on a varied line-spacing plane grating monochromator at BL27SU of SPring-8 in the higher energy region using photoabsorption spectra of gas and solid samples and photoelectron spectra of Xe 5p3/2. A flux of 1×1011 photons/s/100 mA/0.02% b.w. was achieved at 2 keV. The photon flux steeply decreased above 2 keV to 5×109 photons/s/100 mA/0.02% b.w. at 2.8 keV.
Dopant and thickness dependence of metal-induced lateral crystallization of amorphous silicon films
Authors: Ma Tianfu , Wong Man
Published: July 2002

The thickness and dopant dependence of nickel-based, metal-induced laterally crystallized (MILC) polycrystalline silicon has been systematically studied. With decreasing thickness of the starting amorphous silicon films, the MILC rate decreases and the crystal morphology deteriorates with an increasing number of nickel silicide nodules trapped in the MILC region. Doping with phosphorus or arsenic and damage introduced during ion implantation are found to slow down the MILC rate and to degrade the film morphology, whereas heavy doping with boron is found to speed up the MILC rate. Possible mechanisms responsible for the observed dependence are discussed.
Negative ion production in multicusp sources (abstract)
Authors: Belchenko Y. I. , Oka Y. , Kaneko O. , Takeiri Y. , Tsumori K.
Published: July 2002

Cesium seed improves multicusp negative ion (NI) Source performance. Experiments display the surface origin of NI production in the multicusp source (MS): a strong NI yield dependence on plasma grid (PG) temperature and work function, a low NI yield sensitivity to Cs density in plasma, etc. On other hand, cesium seed changes the MS plasma potential, electron density, and temperature, decreases the coextracted electron current, and produces a NI density increase in the source volume. Cesium ions can contribute to the slow positive charge in the plasma presheath and extraction region, and improve the NI extraction. The study of NI production was done by the directed deposition of a well-defined amount of cesium, by cesium recovery in the MS. The obtained data evidences the surface origin of H− yield enhancement 3–10 mg cesium deposition was enough for producing a NI yield increase due to hydrogen conversion on the cesiated PG surface. The long-term H− yield enhancement with larger cesium deposition and no change of NI yield with the PG temperature change displays the maintenance of the thick Cs+W coverage, which governs the enhanced surface H− production in the MS. The experimental data and the MS surface–plasma mechanism features will be presented and discussed.
Shot noise in low-resistance magnetic tunnel junctions
Authors: George Peter K. , Wu Y. , White R. M. , Murdock Ed , Tondra Mark
Published: July 2002

Shot noise measurements made on low-resistance magnetic tunnel junctions show results inconsistent with the resistance of the samples examined. The results yield lower than expected shot noise which is consistent with parallel path conduction (pinholes). A simple electrical model shows this should be true for noise measurements as well as for the tunneling magnetoresistance (TMR) as the resistance-area product (R×A) is reduced. The model suggests that a correlation between TMR and shot noise should exist assuming the presence of pinholes and that shot noise offers a useful experimental monitoring technique. The difficulties of making these measurements related to high frequency 1/f noise are discussed. Noise results are presented for high R×A and low R×A low-resistance samples which, in this particular case, both show the influence of pinhole shunting.
Reactions of Pt+ with H2, D2, and HD: Effect of lanthanide contraction on reactivity and thermochemistry
Authors: Zhang Xiao-Guang , Armentrout P. B.
Published: July 2002

A guided ion beam tandem mass spectrometer is used to examine the kinetic energy dependence of reactions of the third-row transition metal cation, Pt+, with molecular hydrogen and its isotopologs. A flow tube ion source produces Pt+ ions in its electronic ground state term and primarily in the lowest spin–orbit level. Corresponding state-specific reaction cross sections are obtained. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energy of D0(Pt+–H)=2.81±0.05 eV (271±5 kJ/mol). The experimental thermochemistry is consistent with ab initio calculations, performed here and in the literature. Theory also provides the electronic structures of these species and is used to examine the reactive potential energy surfaces. Results from reactions with HD provide insight into the reaction mechanisms and indicate that the late metal ion, Pt+, reacts largely via a direct mechanism. Results for this third-row transition metal system are compared with the first-row and second-row congeners and found to have higher reactivity towards dihydrogen and stronger M+–H bonds. These differences can be attributed to lanthanide contraction and relativistic effects.
A self-consistent mode-coupling theory for dynamical correlations in quantum liquids: Rigorous formulation
Authors: Rabani Eran , Reichman David R.
Published: July 2002

A quantum molecular hydrodynamic formalism is developed for the study of dynamical correlations in dense quantum liquids. The approach is based on augmenting an exact closed, self-consistent quantum generalized Langevin equation for the Kubo transform of the dynamical correlation of interest, with a suitable approximation for the memory kernel obtained within the framework of a quantum mode-coupling theory. The solution to the quantum generalized Langevin equation requires as input static equilibrium information which is generated from a path-integral Monte Carlo method. Examples are given for the intermediate and self-intermediate scattering functions, and for the velocity autocorrelation function. The attractive advantages of the present approach are discussed.
The effect of current density, stripe length, stripe width, and temperature on resistance saturation during electromigration testing
Authors: Filippi R. G. , Wachnik R. A. , Eng C.-P. , Chidambarrao D. , Wang P.-C. , White J. F. , Korhonen M. A. , Shaw T. M. , Rosenberg R. , Sullivan T. D.
Published: July 2002

Resistance saturation as a function of current density, stripe length, stripe width, and temperature is investigated for a two-level structure with Ti/AlCu/Ti/TiN stripes and interlevel W stud vias. A simple model based on first principles is presented, which relates the maximum fractional resistance change to the current density and stripe length. Experimental results for stripe lengths of 30, 50, 70, and 100 μm are in good agreement with the model predictions. Estimated void sizes based on the resistance saturation data are consistent with the actual void sizes determined from scanning electron microscopy analysis. A weak temperature dependence is found for 0.33 μm-wide samples in the range 170–250°C, while a strong width dependence is observed between 0.33 and 1.50 μm- wide samples. The width dependence is qualitatively explained in terms of a relaxed bulk modulus that depends on the aspect ratio of the interconnect lines.
Effects of thin high-Z layers on the hydrodynamics of laser-accelerated plastic targets
Authors: Obenschain S. P. , Colombant D. G. , Karasik M. , Pawley C. J. , Serlin V. , Schmitt A. J. , Weaver J. L. , Gardner J. H. , Phillips L. , Aglitskiy Y. , Chan Y. , Dahlburg J. P. , Klapisch M.
Published: July 2002

Experimental results and simulations that study the effects of thin metallic layers with high atomic number (high-Z) on the hydrodynamics of laser accelerated plastic targets are presented. These experiments employ a laser pulse with a low-intensity foot that rises into a high-intensity main pulse. This pulse shape simulates the generic shape needed for high-gain fusion implosions. Imprint of laser nonuniformity during start up of the low intensity foot is a well-known seed for hydrodynamic instability. Large reductions are observed in hydrodynamic instability seeded by laser imprint when certain minimum thickness gold or palladium layers are applied to the laser-illuminated surface of the targets. The experiment indicates that the reduction in imprint is at least as large as that obtained by a 6 times improvement in the laser uniformity. Simulations supported by experiments are presented showing that during the low intensity foot the laser light can be nearly completely absorbed by the high-Z layer. X rays originating from the high-Z layer heat the underlying lower-Z plastic target material and cause large buffering plasma to form between the layer and the accelerated target. This long-scale plasma apparently isolates the target from laser nonuniformity and accounts for the observed large reduction in laser imprint. With onset of the higher intensity main pulse, the high-Z layer expands and the laser light is transmitted. This technique will be useful in reducing laser imprint in pellet implosions and thereby allow the design of more robust targets for high-gain laser fusion.
Open problems in active chaotic flows: Competition between chaos and order in granular materials
Authors: Ottino J. M. , Khakhar D. V.
Published: July 2002

There are many systems where interaction among the elementary building blocks—no matter how well understood—does not even give a glimpse of the behavior of the global system itself. Characteristic for these systems is the ability to display structure without any external organizing principle being applied. They self-organize as a consequence of synthesis and collective phenomena and the behavior cannot be understood in terms of the systems’ constitutive elements alone. A simple example is flowing granular materials, i.e., systems composed of particles or grains. How the grains interact with each other is reasonably well understood; as to how particles move, the governing law is Newton’s second law. There are no surprises at this level. However, when the particles are many and the material is vibrated or tumbled, surprising behavior emerges. Systems self-organize in complex patterns that cannot be deduced from the behavior of the particles alone. Self-organization is often the result of competing effects; flowing granular matter displays both mixing and segregation. Small differences in either size or density lead to flow-induced segregation and order; similar to fluids, noncohesive granular materials can display chaotic mixing and disorder. Competition gives rise to a wealth of experimental outcomes. Equilibrium structures, obtained experimentally in quasi-two-dimensional systems, display organization in the presence of disorder, and are captured by a continuum flow model incorporating collisional diffusion and density-driven segregation. Several open issues remain to be addressed. These include analysis of segregating chaotic systems from a dynamical systems viewpoint, and understanding three-dimensional systems and wet granular systems (slurries). General aspects of the competition between chaos-enhanced mixing and properties-induced de-mixing go beyond granular materials and may offer a paradigm for other kinds of physical systems.
Superheterodyne light beating spectroscopy for Rayleigh–Brillouin scattering using frequency-tunable lasers
Authors: Tanaka Hajime , Sonehara Tsuyoshi
Published: July 2002

We developed a new optical superheterodyne method for light scattering using frequency tunable lasers and succeeded in measuring Brillouin spectra of simple liquids in a wide frequency range from 10 MHz to 3 GHz. In this method the speed of a photodetector and electric circuits does not limit the upper frequency bound, since the superheterodyne detection of the signal allows us to shift the optical beat frequency down to a desired fixed frequency suitable for electric signal processing. Thus an almost ultimate signal-to-noise ratio is realized at any phonon frequency. This is a great advantage over conventional optical beating methods. It is realized by the continuous tunability of the frequency of the laser used as a local oscillator light for superheterodyne detection. Our method provides an extremely high frequency resolution (∼300 kHz), which is determined by the stability of an optical frequency difference between the two lasers under its feedback control. We call this method “superheterodyne Brillouin spectroscopy.” The method has a high potential for studying the dynamics in a wide class of transparent condensed matters.
F
+
D
2
reaction at ultracold temperatures
Authors: Bodo E. , Gianturco F. A. , Dalgarno A.
Published: July 2002

We present a quantum mechanical study of the reaction F+D2 at ultracold temperatures based on the potential energy surface of Stark and Werner. The reaction cross section at low energies is controlled by the tunneling through the activation barrier, a mechanism that is favored by the long duration of the collision at those energies. Differences are found in the behavior of the reactive cross section compared to that for F+H2, due to the changed mass and zero point energy. A new feature is detected in the reaction probabilities and is attributed to a Feshbach resonance corresponding to a metastable state in the exit channel.
Transformation kinetics for the shock wave induced phase transition in cadmium sulfide crystals
Authors: Knudson M. D. , Gupta Y. M.
Published: July 2002

Initial stage kinetics of the cadmium sulfide (CdS) phase transition was investigated using picosecond time-resolved electronic spectroscopy in plate-impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in CdS single crystals shocked along a and c axes to stresses ranging between 35 and 90 kbar, which is above the phase-transition threshold stress of approximately 30 kbar. Significant difference in the transformation kinetics was observed for the two crystal orientations. At sufficiently high instantaneous stress, above approximately 60 to 70 kbar for a axis and 50 kbar for c axis, transformation to a metastable state appears to reach a constant state within the 100 ps time resolution. At lower instantaneous stresses, an incubation period on the order of several nanoseconds is observed prior to the onset of electronic changes that mark the onset of the structural change. The subsequent increase in absorbance was quite rapid, with a constant state being reached within the first few nanoseconds after the onset of the structural changes. These results suggest that the nucleation process determines the transformation rate. This insight into transformation kinetics, along with the transformation mechanism obtained from the high-stress experiments, was used to develop a phenomenological model, incorporating ideas of nucleation and growth in martensitic transformations, to simulate the time-dependent extinction of light observed in our experiments. The calculational results incorporating both extinction due to light absorption by the daughter phase volumes and scattering of light by small volumes of the daughter phase were in good agreement with experimental observations. Finally, the orientational differences observed in the transformation kinetics were interpreted in terms of the differences in the elastic-plastic response for the two orientations.
Kinetic processes of NF3 etchant gas discharges
Authors: Greenberg K. E. , Verdeyen J. T.
Published: July 2002

Some of the fundamental properties of the NF3 etchant gas discharge were investigated. The dissociation of the parent gas molecules was inferred using mass spectrometric methods, and it was found that the discharge is much more effective at dissociating NF3 than CF4. Furthermore, the percentage dissociation of the NF3 was extremely large, ∼50%. Using a pulsed discharge it was possible to observe the time evolution of the dissociation products. These studies have led to a mathematical description of the dissociation and product formation kinetics.
A new correction to Schottky barrier lowering in cathodes
Authors: Eng Genghmun
Published: July 2002

A new technique is developed for approximating cathode current-voltage (I-V) and current-temperature (I-T) characteristics in a planar diode configuration. For a uniform work function surface, this method allows both for space-charge effects and for Schottky barrier lowering. Using this method, we derive an explicit first correction to the Schottky effect: ΔΦnew =ΔΦ0[1−(Jnet/JC)]1/2, where ΔΦ0 is the standard Schottky barrier lowering (eV), Jnet is the net emission current density, and JC is the Child’s law current density. The following closed form expression then approximates the net emission current density: Jnet≂JC (JR>JC), and Jnet≂JR exp(ΔΦnew/kT) (JR<JC), where JR is the Richardson’s current density. This equation for Jnet connects the space-charge-limited (SCL) emission regime to the deep temperature-limited (TL) regime. The SCL-to-TL transition point is found to be JR, and the above equations for emission current are continuous with a continuous slope at the transition point.
Simultaneous excitation of magnetostatic and exchange modes in thin circular yttrium iron garnet films
Authors: Barak J. , Bhagat S. M. , Vittoria C.
Published: July 2002

The excitation of magnetostatic modes and exchange modes in thin YIG disks has been studied over a wide range of fields. The spectra of films magnetized normally (in magnetic field perpendicular to sample plane) are well described by the model of magnetostatic waves, modified by the exchange interactions. The exchange modes are weak, presumably due to weak pinning at the surfaces of the films. They follow the Dk2 law. The spectrum obtained in the parallel configuration is not well understood.
Brillouin scattering in thin samples: Observation of backscattering components by 90° scattering
Authors: Takagi Yasunari , Gammon Robert W.
Published: July 2002

Brillouin scattering from thin samples including thin glass, polymer film, and optical fiber, has been studied. Edge illumination of the samples, as opposed to the ordinary method, was used because of the increased scattering intensity. With either method, two inner scattering angles exist between incident and scattered light because of reflections inside the samples, but only one corresponds to the backscattering component. The extra peak in the Brillouin spectrum can be identified as another longitudinal mode of the backscattering component, rather than as a transverse or surface mode. The spectra showed a clear angular dependence of the Brillouin shift and the intensity.
Spatial system of the Wiser model of coal structure according to the second moment of the nuclear magnetic resonance line
Authors: Jurkiewicz Antoni
Published: July 2002

The second moment of the proton nuclear magnetic resonance line was calculated for the Wiser structure of coal according to the Van Vleck theory. The calculations were carried out assuming that the predominant interaction was the dipole interaction between protons. The interaction between protons and paramagnetic centers was estimated as minor, in the range of 0.15–0.27 G2. The result of the calculations both for the rigid structure and for the one with rotating fragments is similar to the experimental spin-spin relaxation time of the Gaussian component of free-induction decay observed for some coals. This result suggests that the Wiser molecule presents the macromolecular part in the molecular-macromolecular model of coal. Its maximal dimensions are X=24.1, Y=29.3, and Z=11.29 Å and the distance between the two remotest protons is 34.5 Å.
Micromagnetics of magnetization reversal in amorphous Tb-Fe films by Lorentz microscopy
Authors: Lin C.-J. , Suit J. C. , Geiss R. H.
Published: July 2002

Lorentz electron microscopy has been used to investigate the micromagnetic mechanism of M-H hysteresis loop behavior of thin amorphous TbFe films with perpendicular easy-axis magnetic anisotropy. Detailed nucleation and growth behavior of reverse domains was observed while an applied field provided by the microscope objective lens along the film normal was slowly changed. Very different results between Tb19Fe81 and Tb32Fe68 films have been observed. The reverse domain grew more or less radially in the former case, while a dendritelike domain pattern was observed for the latter case.
The anomalous phase instability near the Curie temperature of the Fe-Cu system
Authors: Chen L. H. , Chin T. S. , Hung M. P.
Published: July 2002

Taking into account the effect of the magnetic free energy, the phase stability (i.e., the onset of Cu-rich precipitation) at the Fe-rich end of the Fe-Cu binary system near the magnetic ordering (Curie) temperature has been evaluated with a regular solution model and experimental verification. The calculated results show that the stability of alpha iron is significantly lowered near the Curie temperature, and the solvus of copper in alpha iron is bent toward the Curie temperature line, hence a protrusion should be added to the known phase diagram. Microhardness indentation tests, lattice constant determination by the careful x-ray diffraction (XRD), and transmission electron microscopy (TEM) were carried out on the solution heat treated and then aged Fe-Cu alloys containing 0.25-, 0.95-, and 1.5-at. % Cu. All the three experimental results confirm the calculated ones. The Cu-rich precipitate is identified by TEM on an alloy quenched from the protruded region.
Dynamic unloading behavior of soda lime glass
Authors: Lopatin C. M. , Bless S. J. , Brar N. S.
Published: July 2002

An experimental study was conducted to see if the reverberation plate technique, using commercial manganin stress gauges, could be used to determine the dynamic unloading behavior of brittle materials. Using a 50-mm powder gun, plate impact experiments were conducted on soda lime glass, providing unloading data from stress states from below the Hugoniot elastic limit to 17.0 GPa. The slopes of the unloading paths were found to increase as the peak compressive stress was increased. Such unloading behavior is similar to that of quartz, suggesting that the technique is viable.
Structural and magnetic properties of Fe-W alloys
Authors: Lu M. , Chien C. L.
Published: July 2002

FexW100−x alloys have been fabricated by vapor quenching (i.e., sputtering). The compositions with 40≤x≤70 are amorphous, whereas others with x≤30 and x≥80 are crystalline bcc alloys. Extreme sensitivity of magnetic ordering on structure has been observed. Magnetic ordering occurs in the bcc state, for both the Fe- and W-rich alloys. No magnetic ordering can be detected down to 4.2 K for all the amorphous alloys despite their high Fe content.
The use of Einstein’s coefficients to predict the theory of operation of a semiconductor laser
Authors: Taylor G. W.
Published: July 2002

The rate equation for the semiconductor laser is formulated in terms of the appropriate Einstein coefficients. The sum of the quasi-Fermi levels is determined to be the appropriate parametric variable to characterize the frequency dependence of the laser output spectrum. The photon lifetime becomes an important parameter which connects the photon density, the electron density, and the linewidth.
Hysteresis properties of ultrathin ferromagnetic films
Authors: Bruno P. , Bayreuther G. , Beauvillain P. , Chappert C. , Lugert G. , Renard D. , Renard J. P. , Seiden J.
Published: July 2002

We have investigated the hysteresis properties of Au/Co/Au films with ultralow Co thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.
A fluid model for electron dynamics in the vacuum gap of a plasma opening switch
Authors: Ng C.-K. , Sudan R. N.
Published: July 2002

A two-dimensional fluid model is developed to study the equilibrium flow of electrons in the vacuum gap between the cathode and the plasma of a plasma opening switch. The dynamics of electron flow in the vacuum gap region is illustrated by the distributions of the magnetic field, the electrostatic potential, and the electron density. The dependencies of the switch current and its interruption in the vacuum gap on the dimensions of the vacuum gap, the load impedance, and the electrostatic potential across the gap are investigated. The two-dimensional aspect of the electron flow in the vacuum gap and its effects on the performance of a plasma opening switch are emphasized.
Spin-dependent x-ray absorption in Co/Pt multilayers
Authors: Rüegg S. , Schütz G. , Fischer P. , Wienke R. , Zeper W. B. , Ebert H.
Published: July 2002

Spin-dependent absorption spectroscopy or circular magnetic x-ray dichroism studies at the L2,3 edges of 5d atoms in ferromagnetic hosts have been used to investigate the spin-density distribution of the empty d-projected states and thus the local magnetic moment of the absorbing 5d atoms. This method was applied to study the average polarization of the Pt atom in Co/Pt multilayers with different modulation lengths. The spin-dependent absorption profiles for vapor-deposited [25(4 Å Co+x Å Pt), x=9, 16, 23, 30, 40] multilayers indicate a significant average Pt polarization decreasing with the layer thickness. The experimental results are discussed in comparison with spin-polarized relativistic band-structure calculations (linear muffin-tin-orbital calculations).
Magnetism of RFe2 compounds
Authors: Brooks M. S. S. , Nordström L. , Johansson B.
Published: July 2002

The combination of itinerant transition metal (M=Fe) and localized rare earth (R=Gd-Yb) magnetism in RFe2 compounds has been investigated in self-consistent energy band calculations. The computed and measured total moments are in good agreement. We find, however, that there is a significant contribution to the moment from the R-5d partial moments coupled antiparallel to the Fe-3d moment which results from 3d-5d hybridization. The R-4f moments interact with the conduction band system solely by local exchange interactions which are calculated ab initio from density functional theory. A sum rule for the total 3d+5d moment is shown to be obeyed and an expression for the effective ferrimagnetic exchange interaction between 3d and 4f electrons is derived.
Preparation of polycrystalline Bi3Fe5O12 garnet films
Authors: Okuda T. , Katayama T. , Satoh K. , Yamamoto H.
Published: July 2002

Polycrystalline films of nonthermodynamical garnet Bi3Fe5O12 were synthesized by direct epitaxial growth from the vapor phase. Polycrystalline thin layers of various kinds of thermodynamical garnets prepared on fused quartz substrates were employed as the substrate materials. The saturation magnetization of the film grown onto the polycrystalline GGG layer was 140 mT at 5 K. The films showed quite large magneto-optical effects. In the film on the GGG layer, the Faraday rotation angle at a wavelength of 633 nm was −5.5 deg/μm and the magneto-optical Kerr rotation angle at 455 nm was −0.9°.
Preparation and magneto-optic properties of Bi-substituted yttrium iron garnet thin films by metalorganic chemical vapor deposition
Authors: Okada Masaru , Katayama Shigehisa , Tominaga Koji
Published: July 2002

Magneto-optic bismuth-substituted yttrium iron garnet thin films were epitaxially grown during deposition for the first time by metalorganic chemical vapor deposition. Triphenyl bismuth and both yttrium and iron tridipivaloylmethane were used as source materials. Prior to the preparation of garnet films, the deposition behavior of Y2O3, Fe2O3, and Bi2O3 films was examined separately. Epitaxially grown films were as-deposited on a Gd3Ga5O12(111) substrate at 750 °C under a reduced pressure of 6 Torr with a deposition rate of 200–500 Å/min, which is about 10 times faster than that obtained by the conventional sputtering method. A film with a bismuth content of 2.5 atoms per formula unit showed a large Faraday rotation, − 7.5 × 104 deg/cm at λ=633 nm, because of the high bismuth substitution and the crystalline perfection.