bluuebunny/crossref_metadata_2025 huggingface dataset:
Modified superconductor–insulator–normal metal–insulator– superconductor Josephson junctions with high critical parameters
Authors: Nevirkovets I. P. , Ketterson J. B. , Rowell J. M.
Published: July 2002

Superconductor–insulator–normal metal–insulator–superconductor (SINIS) and SINS′NIS junctions (here S, I, and N denote superconductor, insulator, and normal metal, respectively) were fabricated using Nb/Al technology and characterized at low temperatures. It is shown that asymmetric SINIS junctions with different transparency of the two tunnel barriers may have higher critical voltages than analogous symmetric junctions at 4.2 K. Also, SINS′NIS junctions were fabricated and investigated. If the thickness of the S′ layer is very thin, these junctions have quasiparticle current–voltage characteristics similar to those of conventional SINIS junctions at 4.2 K, but the Josephson critical current densities, and therefore, the critical voltages, are much higher than the corresponding values for conventional SINIS junctions.
Linear stability of laminar plane Poiseuille flow of helium II under a nonuniform mutual friction forcing
Authors: Godfrey Simon P. , Samuels David C. , Barenghi Carlo F.
Published: July 2002

The calculation of the stability characteristics of superfluid helium II is complicated by the fact that we have to consider the interaction of two velocity fields: the normal fluid and the superfluid. We consider 2D channel flow and concentrate on the linear stability of the normal fluid under a mutual friction forcing from the superfluid. The linear stability of the superfluid component is also briefly discussed. We consider nonuniform distributions of superfluid vorticity, leading to a nonuniform mutual friction forcing. We derive a modified Orr–Sommerfeld equation for the stability of the normal fluid component and find the neutral stability curves for the normal fluid. We identify a new branch to the neutral stability curve, which can significantly lower the critical Reynolds number of the normal fluid flow.
High-Reynolds-number flow in a narrow gap driven by solidification. II. Planar-flow casting application
Authors: Karcher Ch. , Steen P. H.
Published: July 2002

Flow driven by solidification is studied in the limit when fluid dynamics and heat transfer decouple. This occurs for processes that are characterized by a long flat solidification front and/or thin molten zones. The gap-averaged governing equations, derived in part I [Phys. Fluids 13, 826 (2001)], are applied to planar-flow melt-spinning where liquid metal, held by surface tension in the gap between the injection nozzle and spinning chill-wheel, forms a long thin puddle. A Karman–Pohlhausen approach that includes mean flow and shear flow components leads to velocity and pressure distributions consistent with observation from experiment. A pressure rise along the flow path due to solidification suction competes with the pressure decrease due to convergent Bernoulli flow.
Interference pattern of a coherent electron beam by localized leakage magnetic field
Authors: Park Jeong Young , Kim S. H. , Park W. G. , Kuk Y.
Published: July 2002

We report on the origin of interference patterns at the edge of nanometer-scale Co protrusions observed by low-energy electron point source (LEEPS) microscopy. We find evidence that those interference patterns are due to the phase shift of a coherent electron beam by a localized magnetic field. Typical interference patterns have an apparent size of 10–100 nm and a star-like shape, which are dependent on the sharpness of the Co protrusion. After preparing a ferromagnetic nanoparticle in a saturation remanent state by applying a strong magnetic field, we observed the deflection of the interference pattern. This phenomenon is consistent with the theoretical prediction based on a magnetostatic model. The capability of mapping the local magnetic field suggests that LEEPS microscopy is potentially applicable as an imaging tool of magnetic field with nanometer-scale resolution.
Mobilities of NH4+(NH3)n clusters in helium from 100 K to 298 K
Authors: Midey Anthony J. , Viggiano A. A.
Published: July 2002

A variable temperature-selected ion flow drift tube (VT-SIFDT) has been used to measure the mobilities of NH4+(NH3)n clusters drifting in He. The mobilities have been measured for n=0–2 at 298 K, n=0–3 at 200 K, and n=0–5 at 100 K, marking the first mobilities experiments for these clusters below room temperature. The reduced mobilities measured at 298 K are compared to the previous SIFDT results of Krishnamurthy et al. [J. Chem. Phys. 106, 530 (1997)] for n=0–2. While the current results compare quite favorably for n=0–2, there is a discrepancy for n=3 which is addressed by current 100 K and 200 K data where these species are thermally stable. The trends in the mobilities from 100 K to 298 K as a function of E/N and effective temperature reveal that the repulsive part of the He–NH4+(NH3)n interaction potential is sampled predominantly for n=1–5. However, the attractive part of the potential is accessed at the lowest temperatures for He–NH4+.
Long term instability growth of radiatively driven thin planar shells
Authors: Mason R. J. , Hollowell D. E. , Schappert G. T. , Batha S. H.
Published: July 2002

The Rayleigh–Taylor instability of radiatively driven thin copper foils is studied under pure ablation, as well as with beryllium buffers to provide additional pressure drive, in support of the target design for Inertial Confinement Fusion. Modeling was done with the RAGE adaptive mesh refinement code [R. M. Baltrusaitis, M. L. Gittings, R. P. Weaver, R. F. Benjamin, and J. M. Budzinski, Phys. Fluids 8, 2471 (1996)] of experiments done on the OMEGA [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] laser. The copper foils were typically 11.5 μm thick with 0.45 μm amplitude and 45 μm wavelength cosine surface perturbations. The beryllium layer was 5 μm thick. The drive was a “PS26”-like [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)] laser pulse delivering peak 160–185 eV radiation temperatures. Good agreement between experiment and simulation has been obtained out to 4.5 ns. Mechanisms for late time agreement are discussed.
Physical origin of negative persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction
Authors: Prasad S.
Published: July 2002

When a Si-doped GaAs–AlAs/GaAs heterostructure is illuminated with 1.98 eV light at 4.2 K, a reduction in the two-dimensional (2D) electron density (n2D) is observed. This reduction is followed at times by a small increase in n2D even after the illumination has been switched off. This change is observed on the time scale of minutes and can be explained based on the band bending that results after a reduction in n2D. The negative persistent photoconductivity (NPPC) effect characterized by a persistent reduction in n2D, a postillumination change in n2D, and a long persistence time for T⩽40 K has been investigated. I have used Shubnikov–de Haas oscillations and time-resolved, as well as temperature-dependent, Hall-effect measurements to investigate the origin of this phenomenon. The illumination generates electron–hole (e–h) pairs in the superlattice, where the electrons are trapped into the shallow donor state (SDS) of Si and the holes drift to the two-dimensional channel to recombine with the 2D electrons. All the trapped electrons can be recovered by heating the sample to 60 K. The temperature dependence of the NPPC effect is determined only by the binding energy of the SDS of Si, which is found to be about 5 meV. The e–h recombination in the 2D channel is caused by negatively charged defects, which temporarily bind the holes. This fact is manifested also in the optical quenching of this effect by photons with 1.41 eV or larger energy. The saturation values of n2D(n2Dsat) obtained for 0.8, 1.41, or 1.98 eV illumination at 4.2 K have been investigated and the results confirm the presence of these fixed negative charges (FNCs) near the 2D channel. The change in n2Dsat for 0.8 eV illumination, caused by 1.98 eV illumination, also confirms the presence of FNCs.
Patterning of sp3- and sp2-bonded carbon by atomic-force microscopy
Authors: Zhao J. P. , Chen Z. Y. , Yu Y. H. , Wang X. , Shi T. S. , Wong S. P. , Wilson I. H. , Yano T.
Published: July 2002

We report the pattering of sp3- and sp2-bonded carbon by using conducting atomic-force microscopy (AFM) working in the noncontact mode. A ta-C film with ∼80% sp3 bonds was scanned by the conducting AFM with a biased tip. A current image that clearly shows gray/white and black features was obtained while scanning in the noncontact mode. These features were proposed to be the result of the different electron emission abilities of the sp3- and sp2-bonded carbon in the film. The result not only enables us to image the distribution of the sp3 and sp2 bonds of a-C, but also extends the AFM to provide the electron emission information of a-C on a nanometer scale.
Mass spectrometric investigation of the positive ions formed in low-pressure oxygen/tetraethoxysilane and argon/tetraethoxysilane plasmas
Authors: Aumaille K. , Granier A. , Grolleau B. , Turban G.
Published: July 2002

In this article, we report results on mass spectrometric investigation of the positive ions created in a low-pressure radio frequency helicon plasma reactor using oxygen/tetraethoxysilane (O2/TEOS) and argon/tetraethoxysilane (Ar/TEOS) mixtures. It is shown that the variety of ions is much greater in the Ar/TEOS plasma than in the O2/TEOS plasma. In the case of the Ar/TEOS plasma, ions are observed up to 343 amu whereas in the case of the O2/TEOS plasma, ions are observed up to 211 amu. Ion/molecule reaction rates between TEOS parent positive ions and neutral TEOS molecules are considerably less important in the O2/TEOS plasma as compared with the Ar/TEOS plasma. Using the values of the TEOS dissociation degree measured for O2/TEOS and Ar/TEOS plasmas, the observed ion/molecule reactions might be explained by the higher concentration of TEOS molecules in the Ar/TEOS plasma.
Cage effect in the viscoelasticity of a suspension of hard spheres without hydrodynamic interaction
Authors: Felderhof B. U.
Published: July 2002

The linear viscoelastic response of a suspension of Brownian hard spheres without hydrodynamic interaction is studied in an approximation in which the influence of neighboring particles on the dynamics of a selected pair is taken into account via the potential of mean force. It is shown that the cage effect due to the barrier imposed by neighbors has a profound influence on the frequency-dependence of the viscoelastic response. Analytic calculations are performed for a square well-barrier model of the potential of mean force.
Using the symmetric quasiminimal residuals method to accelerate an inexact spectral transform calculation of energy levels and wave functions
Authors: Huang Shi-Wei , Carrington Tucker
Published: July 2002

We demonstrate that by using the symmetric quasiminimal residuals method one can reduce the number of Hamiltonian matrix–vector products required to compute converged energy levels and wave functions from an inexact spectral transform by about a factor of 2.
Nonlinear solutions of a Maxwellian type for the Vlasov–Poisson equations
Authors: Karimov A. R.
Published: July 2002

The nonlinear dynamics of collisionless unmagnetized plasma is considered. Time-dependent one-particle distribution functions of a Maxwellian type that can be expressed as a series in positive powers of the electrostatic potential are obtained. The influence of conditions on the formation and existence of Maxwellian equilibria is discussed.
Effect of branches on the structure of narrowly confined alkane fluids: n-hexadecane and 2,6,11,15-tetramethylhexadecane
Authors: Cui S. T. , Cummings P. T. , Cochran H. D.
Published: July 2002

Using realistic molecular models and accurately calculating the state condition of the fluids of linear and branched alkanes confined between two strongly adsorbing solid surfaces, we obtained strikingly different structure for the two confined fluids. The structure of the n-hexadecane is solid-like with the molecules clearly forming a layered structure. Within each molecular layer, the molecules are packed predominantly parallel to each other. The structures of adjacent layers are correlated such that the main chain axes of the molecules pack into a local hexagonal array. In contrast, the structure of the branched alkane (2,6,11,15-tetramethylhexadecane) fluid remains liquid-like. Although exhibiting density oscillations normal to the confining surfaces, the structure of the confined fluid of the branched alkane is disordered and the molecules are not confined to a particular layer. The amplitude of the density oscillations is much smaller than for the n-hexadecane. The in-plane positional correlation shows isotropic structure characteristic of liquid, and the orientation of the molecules shows very little correlation.
Compositional dependence of the strain-free optical band gap in InxGa1−xN layers
Authors: Pereira S. , Correia M. R. , Monteiro T. , Pereira E. , Alves E. , Sequeira A. D. , Franco N.
Published: July 2002

The effect of strain on the compositional and optical properties of a set of epitaxial single layers of InxGa1−xN was studied. Indium content was measured free from the effects of strain by Rutherford backscattering spectrometry. Accurate knowledge of the In mole fraction, combined with x-ray diffraction measurements, allows perpendicular strain (εzz) to be evaluated. Optical band gaps were determined by absorption spectroscopy and corrected for strain. Following this approach, the strain free dependence of the optical band gap in InxGa1−xN alloys was determined for x⩽0.25. Our results indicate an “anomalous,” linear, dependence of the energy gap on the In content, at room temperature: Eg(x)=3.39–3.57x eV. Extension of this behavior to higher concentrations is discussed on the basis of reported results.
Photodissociation spectroscopy of benzene cluster ions in ultraviolet and infrared regions: Static and dynamic behavior of positive charge in cluster ions
Authors: Inokuchi Yoshiya , Nishi Nobuyuki
Published: July 2002

Photodissociation spectroscopy is applied to benzene cluster ions in ultraviolet and infrared regions. In the ultraviolet photodissociation spectrum of (C6H6)3+ , a characteristic broad band emerges at 255 nm. This band is assigned to a π*←π transition of a solvent benzene molecule that exists in the trimer. This is in accordance with the previous model of the ion cluster with a dimer ion core and a solvent benzene molecule. The infrared photodissociation spectra of (C6H6)n+ (n=3–5) show a sharp band at 3066 cm−1. The band is attributed to a C–H stretching vibration of the dimer ion core. The infrared spectra of (C6H6)n+ (n=3–5) are fitted to the model spectra reproduced by combining the C–H stretching bands of the dimer ion core and the solvent benzene molecule. The infrared photodissociation spectra of mixed benzene trimer ions with one or two benzene-d6 molecules demonstrate that there is no correlation between the excited dimer ion core site in the trimer and the photofragment dimer ion species. This implies that a dimer ion core switching occurs in photoexcited vibrational states prior to the dissociation.
An investigation on the modeling of transient enhanced diffusion of ultralow energy implanted boron in silicon
Authors: Mannino G. , Whelan S. , Schroer E. , Privitera V. , Leveque P. , Svensson B. G. , Napolitani E.
Published: July 2002

We have investigated and modeled the B diffusion in Si following ultralow energy implantation. Secondary ion mass spectrometry measurements revealed that B diffusion is transient enhanced. For the simulation we have used a kick-out model which requires only two uncorrelated parameters able to describe the microscopical processes involved. By optimizing the parameters, an excellent agreement between the simulated and the experimental profile broadening is achieved. Moreover, an extension of the previous model that accounts for interstitial cluster formation and dissolution was implemented in order to achieve a better description of B diffusion. The extracted parameters are discussed and compared with published values.
Multi-parameter-fitting procedure for photothermal infrared radiometry on multilayered and bulk-absorbing solids
Authors: Dörr Peter , Gruss Christian
Published: July 2002

Photothermal infrared radiometry has been used for the measurement of thermophysical, optical, and geometrical properties of multilayered samples of paint on a metallic substrate. A special data normalization is applied to reduce the number of sensitive parameters which makes the identification task for the remaining parameters easier. The normalization stabilizes the evaluation of the photothermal signal and makes the infrared radiometry more attractive for applications in the industrial environment. It is shown that modeling and multi-parameter-fitting can be applied successfully to the normalized data for the determination of layer thicknesses. As a side product we can calculate some other physical properties of the sample.
Thermal annealing effect on the intersublevel transitions in InAs quantum dots
Authors: Berhane Y. , Manasreh M. O. , Yang H. , Salamo G. J.
Published: July 2002

Isochronal thermal annealing effect on the photoluminescence (PL) spectra of intersublevel transitions in InAs self-assembled quantum dots was investigated. Several peaks due to intersublevel transitions in the quantum dots were observed in the PL spectra of two samples consisting of 10 stacks of InAs quantum dots and InP barriers. Isochronal furnace annealing in the temperature range of 500–800 °C was conducted on the two samples. The results show that the intensity of the PL peaks was dramatically reduced, and a new peak attributed to the wetting layer was observed after the samples were thermally annealed above 550 °C. A small blue shift of the PL peaks due to intermixing was observed.
Roughness evolution in polyimide films during plasma etching
Authors: Agarwal Navnit , Ponoth Shom , Plawsky Joel , Persans P. D.
Published: July 2002

We report an experimental study on the evolution of etch front roughness in fluorinated polyimide films in oxygen based plasmas. For standard low-pressure (40 mT) etching conditions, the root-mean-square roughness, w, of the polymer surface increases with the amount of material etched, d, as w=0.0265(d−116)β with β=1, independent of etch rate, rf power, and gas composition. The etched surfaces can be described by the statistics of self-affine surfaces with scaling exponent, α=0.6±0.1 and lateral correlation length, ξ, of ∼0.3 μm. A dramatic reduction in roughness is observed under higher pressure etching conditions of 1000–2000 mT.
Novel ion-molecular surface reaction to result in CH3 adsorbates on (111) surface of chemical vapor deposition diamond from ethane and surface anionic sites
Authors: Komatsu Shojiro , Okada Katsuyuki , Shimizu Yoshiki , Moriyoshi Yusuke
Published: July 2002

The existence of CH3 adsorbates on (111) surface of chemical vapor deposited diamond, which was observed by scanning tunneling microscopy, was explained by the following SN2 (bimolecular, substitutional, and nucleophilic) type surface reaction; C(s)−+C2H6→C(s)−CH3+CH3−, where C(s) denotes a surface carbon atom. The activation energy was estimated to be 36.78 kcal/mol and the reaction proved to be exothermic with the enthalpy change of −9.250 kcal/mol, according to ab initio molecular orbital calculations at MP2/3-21+G*//RHF/3-21G* level; this result is consistent with typical substrate temperatures, namely about 900 °C, for chemical vapor deposition of diamond. Charge transfer from the highest occupied molecular orbital of the surface anionic site to the lowest unoccupied molecular orbital of ethane, that is antibonding at the CH3–CH3 bond, has been clearly visualized. A characteristic configuration of an ethane molecule which is associated with an anionic vacant site C(s)− on hydrogenated (111) surface of diamond was also found.
Magnetic properties and magneto-optical Kerr effect of Mn/Sb multilayer films on various substrates
Authors: Chen Chenjia , Wang Xuezhong , Cai Ming , Wu Ke , Ding Xiaomin , Zhang Zhaohui , Sun Yunxi
Published: July 2002

Mn/Sb multilayer films were grown on GaAs (100), Si (100), sapphire (0001), and glass by an ultrahigh vacuum evaporation technique and subsequent annealing for a short duration. Magnetization measurements reveal strong room-temperature ferromagnetism even for unannealed films. After annealing the films show increased saturation magnetization Ms and decreased coercivity Hc. X-ray diffraction and magnetic force microscope patterns show a high-quality crystal structure of MnSb is formed and that the magnetic force distribution becomes uniform. Both polar and longitudinal magneto-optical Kerr rotations were observed for all the annealed films on various substrates and the dependence of the Kerr rotation angle on the magnetic field is similar to the magnetization hysteresis loop.
Ablation of aluminum thin films by ultrashort laser pulses
Authors: Le Drogoff B. , Vidal F. , von Kaenel Y. , Chaker M. , Johnston T. W. , Laville S. , Sabsabi M. , Margot J.
Published: July 2002

In this study, various results are presented for laser ablation experiments on aluminum and silicon, made in ambient air by means of subpicosecond laser pulses. These results include threshold fluences for plasma formation and for the appearance of various spectral lines, and the single shot fluence required to remove aluminum layers of various specific thicknesses (ranging from 10 to 500 nm) deposited on a silicon substrate. The threshold fluence for plasma formation is of the order of 0.1 J/cm2. Threshold fluences for the appearance of the spectral lines considered vary from 0.1 to about 5 J/cm2. Finally, our results suggest that for high fluences, even for ultrashort laser pulses, the ablation depth is essentially determined by a long-range process, such as thermal conduction in the solid, rather than by the short range optical depth.
Inert gas beam delivery for ultrafast laser micromachining at ambient pressure
Authors: Sun J. , Longtin J. P.
Published: July 2002

Ultrafast laser micromachining is realized by focusing a femtosecond laser beam to a small spot, where very high optical intensity is achieved at the workpiece. Often, however, the beam must pass through a gas, e.g., air, before reaching the workpiece. At the very high laser intensities associated with ultrafast lasers, the gas can ionize, resulting in a rapid increase in free electron (plasma) density, which decreases the gas refractive index, resulting in plasma defocusing and self-phase modulation. Plasma-induced effects distort the temporal and spatial profile of the laser beam, which degrade feature quality and repeatability for ultrafast laser micromachining. In addition, plasma absorption reduces the energy available for materials processing, resulting in a decreased material removal rate. To avoid these effects, processing has traditionally been performed in a vacuum chamber, however this makes real-time processing on a large scale impractical. This article presents a beam delivery technique that uses inert gas as the beam propagation environment instead of air or a vacuum chamber. Plasma defocusing, self-phase modulation, and shielding effects are minimized due to the higher ionization potential of inert gas and thus less plasma forms along the beam path. Experiments were performed by delivering Ti:Sapphire femtosecond laser pulses in four different environmental gases: air, nitrogen, neon, and helium, to machine holes through a copper plate, with the best feature quality and machining efficiency obtained in helium and the worst in air. This technique shows potential as an innovative method to maintain high beam quality without the need for a vacuum chamber, which significantly improves processing throughput in practical ultrafast laser applications.
Velocity control and staging in laser wakefield accelerators using segmented capillary discharges
Authors: Kaganovich D. , Zigler A. , Hubbard R. F. , Sprangle P. , Ting A.
Published: July 2002

To achieve multi-GeV electron energies in the laser wakefield accelerator, it is necessary to propagate an intense laser pulse long distances in plasma channels while maintaining a proper phase with the accelerated electrons. We have demonstrated a method that allows control of the laser group velocity in long, multistage plasma channels. The control is achieved by modifying the index of refraction through a variation of the plasma density using a segmented capillary discharge.
Cathodoluminescence of epitaxial GaN laterally overgrown on (0001) sapphire substrate. Time evolution with low energy electron beam
Authors: Dassonneville S. , Amokrane A. , Sieber B. , Farvacque J.-L. , Beaumont B. , Gibart P. , Ganiere J.-D. , Leifer K.
Published: July 2002

The evolution of GaN luminescence under electron beam injection has been studied by means of in situ cathodoluminescence experiments on various epitaxial lateral overgrown samples. It is shown that the ultraviolet (UV) peak of undoped materials experiences a decrease of its intensity as well as a noticeable redshift, while the other extrinsic peaks only experience an intensity decrease. However, in Mg doped materials the UV peak intensity decrease is followed by an increase of its intensity which can even reach larger values than the initial one. We suggest that all these features are self-consistently explained by the occurrence of strain relaxation resulting from the beam enhanced diffusion of vacancies from the free surface, and from the coalescence boundaries towards the bulk.
Traversal time for electron tunneling in water
Authors: Galperin Michael , Nitzan Abraham , Peskin Uri
Published: July 2002

The traversal time for tunneling is a measure of the time during which the transmitted particle can be affected by interactions localized in the barrier. The Büttiker–Landauer approach, which estimates this time by imposing an internal clock on the system, has been applied so far for relatively simple one-dimensional models. Here we apply this approach to estimate the traversal time for electron tunneling through a realistic three-dimensional model of a water layer. Observed structure in the energy dependence of times computed reflects the existence of transient tunneling resonances associated with instantaneous water structures.
Effective theories and black hole production in warped compactifications
Authors: Giddings Steven B. , Katz Emanuel
Published: July 2002

We investigate aspects of the four-dimensional (4D) effective description of brane world scenarios based on warped compactification on anti-de Sitter space. The low-energy dynamics is described by visible matter gravitationally coupled to a “dark” conformal field theory. We give the linearized description of the 4D stress tensor corresponding to an arbitrary 5D matter distribution. In particular a 5D falling particle corresponds to a 4D expanding shell, giving a 4D interpretation of a trajectory that misses a black hole only by moving in the fifth dimension. Breakdown of the effective description occurs when either five-dimensional physics or strong gravity becomes important. In scenarios with a TeV brane, the latter can happen through the production of black holes near the TeV scale. This could provide an interesting experimental window on quantum black hole dynamics.
Dichroic Bragg reflectors based on birefringent porous silicon
Authors: Diener J. , Künzner N. , Kovalev D. , Gross E. , Timoshenko V. Yu. , Polisski G. , Koch F.
Published: July 2002

Multilayers of anisotropically nanostructured silicon (Si) have been fabricated and studied by polarization-resolved reflection measurements. Alternating layers having different refractive indices exhibit additionally a strong in-plane anisotropy of their refractive index (birefringence). Therefore, a stack of layers, acting as a distributed Bragg reflector, has two distinct reflection bands, depending on the polarization of the incident linearly polarized light. This effect is governed by a three-dimensional (in-plane and in-depth) variation of the refractive index. These structures can yield optical effects which are difficult to achieve with conventional Bragg reflectors.
Functional differentiation, line integration, and departures from homogeneity of the single-particle kinetic energy functional for one-dimensional systems of N fermions
Authors: March N. H. , Nieto L. M.
Published: July 2002

The differential virial theorem of March and Young for N fermions moving in a common one-dimensional potential energy V(x) is here combined with the Euler equation of density functional theory expressing the constancy of the chemical potential throughout the entire inhomogeneous particle density. The functional derivative of the single-particle kinetic energy is thereby expressed directly in terms of the kinetic energy density; a line integral being involved in establishing the connection. This result is then used to establish a formula measuring departures from simple homogeneity of the kinetic energy functional: a matter of current interest in density functional theory. Finally, the general theory of the functional derivative of the single-particle kinetic energy with respect to the particle density is exemplified for the case of harmonic confinement of fermions in one dimension.
Diagnostics of inductively coupled chlorine plasmas: Measurement of electron and total positive ion densities
Authors: Malyshev M. V. , Donnelly V. M.
Published: July 2002

This work is part of a broader study that creates a set of experimentally measured chlorine plasma parameters (electron and gas temperatures, electron energy distribution function, electron density, densities of ion fractions, and total ion density). This set is obtained over a broad range of operating conditions (1–20 mTorr, 5–1000 W) of an inductively coupled plasma. In this part, we present the electron (ne) and total positive ion (ni+=nCl2++nCl+) densities. ne and ni+ were measured with a Langmuir probe across the diameter or in the middle of the reactor. Line integrated values of ne were independently obtained with microwave interferometry, and converted into the spatially resolved data using the Langmuir probe ne profiles. Finally, a method is presented for measuring relative positive ion densities, based on optical emission at 7504 Å from trace amounts of Ar.
Time constant for relaxation of n=2 excitons into n=1 continuum states in GaAs quantum wells
Authors: Pal Bipul , Vengurlekar A. S.
Published: July 2002

We resonantly excite excitons associated with the second electron and heavy hole subbands (n=2 excitons) in 13 and 17.5 nm GaAs quantum wells (QWs) at 8 K to perform femtosecond degenerate four wave mixing measurements. We deduce the lifetime T1 of the n=2 excitons to be ≈890 fs and ≈2.63 ps, respectively, for the 13 and 17.5 nm QWs.
In situ studies of electrodic materials in Li-ion cells upon cycling performed by very-high-energy x-ray diffraction
Authors: Rossi Albertini V. , Perfetti P. , Ronci F. , Reale P. , Scrosati B.
Published: July 2002

A very high-energy synchrotron radiation source (87 keV) was utilized for in situ sampling of the structural changes occurring in the electrodic materials of a Li-ion cell during charge–discharge cycling. The real-time evolution of their crystal lattice was obtained as a function of the degree of Li intercalation. As a result, new information on two electrodic materials, Li–Ti “zero strain” and Li–Ni–Co oxide, both of extreme interest for generation of rechargeable batteries, was gained. The actual change of the Li–Ti oxide lattice parameter upon cycling was observed in greater detail than before, and provided evidence of unexpected behavior in some intervals of the cycle. In the Li–Ni–Co sample, a new phase formed during the early stages of cycling that remained stable in the subsequent cycles was revealed.
Nanoparticles and nanoballoons of amorphous boron coated with crystalline boron nitride
Authors: Komatsu Shojiro , Shimizu Yoshiki , Moriyoshi Yusuke , Okada Katsuyuki , Mitomo Mamoru
Published: July 2002

Solid- and hollow-cored nanoparticles of amorphous boron coated with crystalline boron nitride (BN) have been synthesized by pulsed-laser vaporization of BN, where the laser plume was controlled with a modulated plasma jet. The hollow particles (nanoballoons) were coated with BN both on the interior and exterior surfaces. The solid particles ranged from a few to 40 nm in their size. The typical diameter of the hollow particles and their wall thickness were about 200 and 30 nm, respectively. The nanoballoons were obtained only when the plasma modulation was synchronized with the ArF excimer-laser pulses.
Etching voltage control technique for electrochemical fabrication of scanning probe microscope tips
Authors: Kim Doo-In , Ahn Hyo-Sok
Published: July 2002

A computer-controlled electrochemical etching system has been developed to fabricate sharp tips for scanning probe microscopy (SPM). In the system developed, an etching voltage control technique has been proposed. The proposed technique comprises two steps in a continuous electrochemical etching process: in the first step, a high etching voltage is applied to produce a tip having a low tip aspect ratio and in the second step, a low etching voltage is applied to prevent buildup of an oxide layer. Scanning electron microscope images of the SPM tips confirmed that the tips with a low aspect ratio and an almost oxide-free surface were successfully fabricated by the system developed.
Comment on “Method of handling the divergences in the radiation theory of sources that move faster than their waves” [J. Math. Phys. 40, 4331 (1999)]
Authors: Hannay J. H.
Published: July 2002

The “method” in question1 has repeatedly led to results which I proved false in 1996 by a short, transparent, and rigorous theorem.2 Even shorter is the remark here that inspection of the standard formula for the electromagnetic field suffices to prove the results of the “method” false.
Effect of discrete wires on the implosion dynamics of wire array Z pinches
Authors: Lebedev S. V. , Beg F. N. , Bland S. N. , Chittenden J. P. , Dangor A. E. , Haines M. G. , Kwek K. H. , Pikuz S. A. , Shelkovenko T. A.
Published: July 2002

A phenomenological model of wire array Z-pinch implosions, based on the analysis of experimental data obtained on the mega-ampere generator for plasma implosion experiments (MAGPIE) generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)], is described. The data show that during the first ∼80% of the implosion the wire cores remain stationary in their initial positions, while the coronal plasma is continuously jetting from the wire cores to the array axis. This phase ends by the formation of gaps in the wire cores, which occurs due to the nonuniformity of the ablation rate along the wires. The final phase of the implosion starting at this time occurs as a rapid snowplow-like implosion of the radially distributed precursor plasma, previously injected in the interior of the array. The density distribution of the precursor plasma, being peaked on the array axis, could be a key factor providing stability of the wire array implosions operating in the regime of discrete wires. The modified “initial” conditions for simulations of wire array Z-pinch implosions with one-dimension (1D) and two-dimensions (2D) in the r–z plane, radiation-magnetohydrodynamic (MHD) codes, and a possible scaling to a larger drive current are discussed.
Rayleigh–Taylor spike evaporation
Authors: Schappert G. T. , Batha S. H. , Klare K. A. , Hollowell D. E. , Mason R. J.
Published: July 2002

Laser-based experiments have shown that Rayleigh–Taylor (RT) growth in thin, perturbed copper foils leads to a phase dominated by narrow spikes between thin bubbles. These experiments were well modeled and diagnosed until this “ spike” phase, but not into this spike phase. Experiments were designed, modeled, and performed on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] to study the late-time spike phase. To simulate the conditions and evolution of late time RT, a copper target was fabricated consisting of a series of thin ridges (spikes in cross section) 150 μm apart on a thin flat copper backing. The target was placed on the side of a scale-1.2 hohlraum with the ridges pointing into the hohlraum, which was heated to 190 eV. Side-on radiography imaged the evolution of the ridges and flat copper backing into the typical RT bubble and spike structure including the “ mushroom-like feet” on the tips of the spikes. RAGE computer models [R. M. Baltrusaitis, M. L. Gittings, R. P. Weaver, R. F. Benjamin, and J. M. Budzinski, Phys. Fluids 8, 2471 (1996)] show the formation of the “ mushrooms,” as well as how the backing material converges to lengthen the spike. The computer predictions of evolving spike and bubble lengths match measurements fairly well for the thicker backing targets but not for the thinner backings.
Effect of nonzero dawn–dusk magnetic field component on particle acceleration in three-dimensional geomagnetic reconnection
Authors: Lee E. , Lee D.-Y. , Min K. W. , Seon J.
Published: July 2002

The acceleration of protons is investigated by tracing their trajectories in the electric and magnetic fields obtained from a three-dimensional magnetohydrodynamic simulation of local magnetic reconnection. The magnetic reconnection is induced by imposing a localized anomalous resistivity in two different initial configurations, the first one without a By component and the second with a positive By superimposed in the two-dimensional Harris field lying in the xz plane. Applying the trajectory calculations to the geotail condition, the previously published results have been mostly recovered for the By=0 case. For the nonzero By case, remarkable asymmetry about the z=0 plane appears in the trajectories, and the pitch angle and spatial dependencies of energy gain. For the particles found above the z=0 plane at the earthward edge from the X-line, results are similar to those in the By=0 case, while they are quite different for the particles found below the z=0 plane. The trajectories of the particles exiting the simulation domain below the z=0 plane are much more complicated than those of the particles exiting above the z=0 plane. Consequently, the duskward skewness of the energy gain, which is well defined above the z=0 plane, is no longer a general and robust feature below the z=0 plane. It is expected that this up–down asymmetry appears not only in the geotail condition but also in other systems as long as the symmetry about the reconnection current sheet plane breaks down by the presence of By, i.e., the magnetic field component aligned with the X-line.
The topology of guiding center orbits in a linear magnetic cusp
Authors: Egedal J. , Fasoli A.
Published: July 2002

Particle orbits for a linear magnetic cusp configuration, characteristic for magnetic reconnection, are analyzed within the guiding center approximation. A class of particle orbits encircling the magnetic X-line is identified and the orbit phase space partition is obtained and characterized as a function of dimensionless parameters controlling the orbit topology. The large majority of particles are found to be trapped in the direction of the X-line implying large neoclassical corrections to the plasma resistivity. This may be the key in explaining the high rates of collisionless magnetic reconnection observed in nature.
A dynamic light scattering study of the hypersonic relaxation in liquid toluene
Authors: Rubio J. E. F. , Baonza V. G. , Taravillo M. , Núñez J. , Cáceres M.
Published: July 2002

Brillouin spectra of liquid toluene have been measured between 288 and 358 K using a dynamic light scattering technique. The polarized spectra have been modeled according to Mountain’s theory. The most relevant quantity of the theory, namely the relaxation time, has been used to check the validity of several molecular models proposed to interpret vibrational–translational energy exchanges observed in nonassociated liquids.
Chain folding in polymer melt crystallization studied by dynamic Monte Carlo simulations
Authors: Hu Wenbing
Published: July 2002

The morphological metastability of spontaneous crystallization from the melt of short-chain semiflexible homopolymers was studied through dynamic Monte Carlo simulations of a lattice multiple-chain system. Frictional hindrance for the sliding diffusion of the chains in the crystallites was employed to enhance the metastability of folded-chain crystallites, and distinguish the polymer crystallite from its mesophase, though their phase transitions have the similar driving forces. The integral folding of short chains in the crystallites and the constant linear crystal growth rate were identified with the actual polymers. In addition, the roughness of the local growth front accompanied with the occasional reversals and jumplike advancing was observed, which cannot be explained by current models. The crowding of the dangling ends on the fold surface seems to be the main reason for suppressing the lateral growth front, and the mechanism of chain folding was proposed. Its implications to the special behaviors of polymer melt crystallization, such as the semicrystalline state, the effect of the chain rigidity and molecular weight to crystal growth, the reversible premelting, and molecular segregation are briefly discussed.
Radiotracer investigation of a deep Be-related band gap state in 4H-SiC
Authors: Albrecht F. , Grillenberger J. , Pasold G. , Witthuhn W. , Achtziger N.
Published: July 2002

One Be-related deep level in the band gap of 4H-SiC was identified by radiotracer deep level transient spectroscopy (DLTS). The radioactive isotope Be7 was recoil implanted into p-type as well as n-type 4H-SiC for these radiotracer experiments. DLTS spectra were taken repeatedly during the elemental transmutation of Be7 to Li7. In the case of p-type 4H-SiC, they exhibit one peak of time-dependent height. Its concentration decreases with the halflife of the nuclear decay of Be7 (T1/2=53.3 d). Thus, this level at 1.06 eV above the valence band edge is identified as Be-related. In n-type 4H-SiC, neither Be- nor Li-correlated deep levels have been found in the investigated part of the band gap within the measurement accuracy.
Superluminality and amplification of signals propagating in an unstable plasma–beam system
Authors: Petrillo V.
Published: July 2002

In an unstable magnetized plasma–beam system, electromagnetical wave packets can travel with superluminal velocities, being amplified at the same time. The possible production of superluminal signals by means of layers of plasma is analyzed in various concrete situations.
Thermal relaxation of electron spin motion in a thermal equilibrium ensemble: Relation to paramagnetic nuclear magnetic resonance relaxation
Authors: Sharp Robert , Lohr Lawrence
Published: July 2002

The electron spin relaxation times measured in ESR spectroscopy are physically distinct from the electron spin relaxation times which appear in the theory of NMR Paramagnetic Relaxation Enhancement (NMR-PRE). ESR involves decay of a perturbed spin density matrix toward thermal equilibrium, while in NMR-PRE measurements, the electron spin density matrix remains at thermal equilibrium throughout the NMR experiment. The pertinent spin relaxation involves the thermal decay of the time correlation functions, Gr(τ)≡〈Sr(0)⋅Sr(τ)〉 (r=x,y,z), of the spin components, quantities which describe the persistence in microscopic correlation of the spin motion in the thermal equilibrium sample. The decay of the Gr(τ) is shown to be level-specific; i.e., Gr(τ) is composed of a sum of contribution associated with individual eigenstates, each of which decays exponentially via a process that is uncoupled to the decay in other eigenstates. This behavior differs markedly from the decay of the nonequilibrium parts of a perturbed density matrix, which involves coupled degree of freedom of the electron spin system. An expression for the level-specific relaxation times has been derived in terms of Redfield matrix elements. This expression is valid for any S⩾1 when the static spin Hamiltonian consists of Zeeman and zfs contributions of arbitrary magnitude. Simple closed-form expressions are given for level-specific relaxation times in the cylindrical and orthorhombic zfs limits for S=1 and S=3/2. The theory is used to interpret electron and nuclear spin relaxation for S=3/2 with specific reference to high-spin Co(II), for which the zfs splittings are typically large. For this spin system, the presence of orthorhombic terms in the zfs tensor causes profound shortening of the electron spin relaxation times relative to the reference cylindrical zfs case and, in consequence, a comparably large rhombicity-induced depression of the NMR relaxation efficiency.
A comprehensive thermodynamic theory of the cross-effect in isothermal diffusion (“Hertz effect”) in a multicomponent gas, liquid, or solid
Authors: Kempers L. J. T. M.
Published: July 2002

The cross-effect between diffusion fluxes in a nonuniform, multicomponent mixture is the cause of diffusion of a component against its own concentration gradient. Gustav Hertz was the first to use this cross-effect for the separation of isotopes. The calculation of the magnitude of the cross-effect, which we call Hertz effect, is possible if all coefficients of the diffusion tensor of the mixture are known. For dilute gases, kinetic gas theory enables the calculation of the diffusion tensor, but calculation methods for dense gases, liquids, and solids require input of measured binary or tracer diffusion coefficients. This paper presents for the first time a calculation method for the magnitude of the Hertz effect without using input of any measured diffusion coefficient. Input is required from the equation-of-state of the mixture and from kinetic gas theory. The theory is based on the principle of a maximum number of possible microstates in the stationary, nonequilibrium state. The theory has been compared with all measured diffusion data of mixtures that can be represented by our phase-behavior package. There is fair to good agreement between measurement and theory, but the Hertz effect in these mixtures is small. A test against diffusion data of highly nonideal mixtures is therefore recommended.
Validation of current density measurements with a B-dot probe
Authors: Piejak R. , Godyak V. , Alexandrovich B.
Published: July 2002

The time varying magnetic flux density in an electrodeless high frequency discharge was measured with a bare wire B-dot probe immersed in plasma. Based on B-dot measurements, the spatial distribution of the current density was determined and integrated over the cross section of the discharge to determine the total discharge current. The total current determined from B-dot measurements was compared with total current measured externally with a current transformer. Agreement between the two methods was very good over a wide range of gas pressures and discharge currents. This work validates that a small, immersed B-dot probe does not significantly perturb the discharge current and can accurately determine current density in a low-pressure gas discharge.