bluuebunny/crossref_metadata_2025 huggingface dataset:
Transmission Mössbauer spectroscopy on pulsed-laser-deposited thick Fe and Ni–Zn ferrite films
Authors: Oliver F. W. , Seifu D. , Hoffman E. , Chrisey D. B. , Horwitz J. S. , Dorsey P. C.
Published: July 2002

Mössbauer spectroscopy with Fe57 is used to characterize pulsed-laser-deposited (PLD) Ni0.6Zny–Fe2O4 films is demonstrated. Mössbauer spectra were recorded as a function of film thickness (3–16 μm) for films deposited in background O2 pressures of 50 and 200 mTorr, and for a zinc concentration of y=0.72. A spectrum was also recorded for y=0.4 deposited in a background pressure of 200 mTorr. The site occupancy was determined for the tetrahedral (A) and octahedral (B) sites. Results obtained here show that the effect of increasing the Zn is to shift Fe(tet) to Fe(oct). When compared with previous Mössbauer measurements on the bulk NiZn ferrites, the ferrite films give spectra of similar quality with no visible evidence of clustering or multiple phases present. The same linewidth was found for a PLD processed Fe film and a Mössbauer standard 70-μm-thick α-Fe enriched foil. The Fe film had a magnetic hyperfine field of 334 kOe and no evidence of the presence of any iron oxides. Good spectra may be obtained by selecting substrates with low mass and low atomic number. We were successful in obtaining measurable Mössbauer spectra for films deposited on 0.5-mm-thick MgO, sapphire, and alumina substrates whereas films deposited on Si, GaAs, and cubic zirconia substrates did not give good spectra.
Selective excitation and thermal quenching of the yellow luminescence of GaN
Authors: Colton J. S. , Yu P. Y. , Teo K. L. , Weber E. R. , Perlin P. , Grzegory I. , Uchida K.
Published: July 2002

We report the observation of narrower structures in the yellow luminescence of bulk and thin-film n-type GaN, using the technique of selective excitation. These fine structures exhibit thermal quenching associated with an activated behavior. We attribute these fine structures to phonons and electronic excitations of a shallow donor-deep acceptor complex, and determine its activation energy for delocalization. Our results suggest that in addition to distant donor-acceptor pairs, the yellow luminescence can also involve emission complexes of shallow donors and deep acceptors.
High-temperature characteristics of 1.3 μm InGaAsN:Sb/GaAs multiple-quantum-well lasers grown by molecular-beam epitaxy
Authors: Yang X. , Heroux J. B. , Jurkovic M. J. , Wang W. I.
Published: July 2002

1.3 μm InGaAsN:Sb/GaAs multiple-quantum-well laser diodes have been grown by solid-source molecular-beam epitaxy using Sb as a surfactant. A low threshold of 1.1 kA/cm2 was achieved for broad-area laser diodes under pulsed operation at room temperature. High-temperature device characterization revealed characteristic temperatures (T0) of 92 and 54 K for operating temperatures below and above 75 °C, respectively, as well as a lasing-wavelength temperature dependence of 0.36 nm/ °C.
Polarity determination and atomic arrangements at a GaN/SiC interface using high-resolution image matching
Authors: Stirman J. N. , Ponce F. A. , Pavlovska A. , Tsong I. S. T. , Smith David J.
Published: July 2002

High-resolution electron microscopy and matching simulations were used to investigate structural features of a GaN/SiC heterointerface. The polarity of the (0001)-oriented SiC substrate was confirmed and it was shown that the polarity of the GaN epilayer corresponded to Ga-terminated (0001) growth. From measurement of average (11̄00) rather than (000l) interplanar spacings it was established that the GaN/SiC interface was abrupt to within one atomic plane. It was concluded that the atomic arrangements at the GaN/SiC interface most likely consisted of N bonded with Si, but with some Ga bonded to C in order to maintain charge balance.
Enhanced quality of epitaxial AlN thin films on 6H–SiC by ultra-high-vacuum ion-assisted reactive dc magnetron sputter deposition
Authors: Tungasmita S. , Birch J. , Persson P. O. Å. , Järrendahl K. , Hultman L.
Published: July 2002

Epitaxial AlN thin films have been grown on 6H–SiC substrates by ultra-high-vacuum (UHV) ion-assisted reactive dc magnetron sputtering. The low-energy ion-assisted growth (Ei=17–27 eV) results in an increasing surface mobility, promoting domain-boundary annihilation and epitaxial growth. Domain widths increased from 42 to 135 nm and strained-layer epitaxy was observed in this energy range. For Ei>52 eV, an amorphous interfacial layer of AlN was formed on the SiC, which inhibited epitaxial growth. Using UHV condition and very pure nitrogen sputtering gas yielded reduced impurity levels in the films (O: 3.5×1018 cm−3). Analysis techniques used in this study are in situ reflection high-energy electron diffraction, secondary-ion-mass spectroscopy, atomic-force microscopy, x-ray diffraction, and cross-section high-resolution electron microscopy.
Temperature dependence of the responsivity of II–VI ultraviolet photodiodes
Authors: Sou I. K. , Ma Z. H. , Zhang Z. Q. , Wong G. K. L.
Published: July 2002

High-temperature dependence, up to 150 °C, of the photoresponsivity of ZnS, ZnSTe, and ZnSSe photodiodes was investigated in this study. It was found that, in general, the responsivity at higher temperatures will shift to longer wavelengths because of band-gap narrowing. A remarkable observation is that the near-band-edge responsivities of these diodes increase at higher temperature. We believe that this observation is attributed to the change of the density-of-state distribution due to lattice expansion at high temperatures, and a simplified model is used to illustrate this hypothesis.
Scanning SQUID microscopy of integrated circuits
Authors: Chatraphorn S. , Fleet E. F. , Wellstood F. C. , Knauss L. A. , Eiles T. M.
Published: July 2002

We have used a scanning YBa2Cu3O7 superconducting quantum interference device (SQUID) at 77 K to image currents in room-temperature integrated circuits. We acquired magnetic field data and used an inversion technique to convert the field data to a two-dimensional current density distribution, allowing us to locate current paths. With an applied current of 1 mA at 3 kHz, and a 150 μm separation between the sample and the SQUID, we found a spatial resolution of 50 μm in the converted current density images. This was about three times smaller than the SQUID–sample separation, i.e., three times better than the standard near-field microscopy limit, and about 10 times sharper than the raw magnetic field images.
Control of the expression of anchored genes using micron scale heater
Authors: Shivashankar G. V. , Liu S. , Libchaber A.
Published: July 2002

We control in vitro gene expression, using DNA (gene) sequences immobilized on miniaturized heaters. For this, electronically addressable indium–tin–oxide heating pads, 100 μm2 in size, are fabricated on a glass substrate to locally define temperatures and thus control transcription and translation of DNA gene sequences attached to the pad. Wheat germ cell extract is the reaction medium. We use luciferase gene and monitor its temperature-controlled expression by luminescence. Various constructs are tested yielding either luciferase free or bound to the coding DNA. Control of protein production at precise sites and rates, using temperature, opens the possibility to engineer various protein networks with different time scales. Working with constructs where the protein stays bound, gene product micro-arrays can be realized.
Simulations of anisotropic front propagation in the H2+O2 reaction on a Rh(110) surface
Authors: Makeev A. , Imbihl R.
Published: July 2002

A mathematical model is presented which reproduces the experimental results of anisotropic front propagation in the bistable H2+O2 reaction on a Rh(110) surface. A model represented by a system of two coupled nonlinear reaction–diffusion equations incorporates the chemical diffusion of adsorbed hydrogen and oxygen. In previous experiments with a photoelectron emission microscope (PEEM) it had been demonstrated that in the system H2+O2/Rh(110) the front anisotropy varied strongly with the experimental parameters. Depending upon temperature and hydrogen partial pressure the reaction fronts were elongated in the [11̄0]-direction or in the [001]-direction of Rh(110). Key features of the mathematical model are diffusion of hydrogen and oxygen and the strong inhibitory site-blocking effect of adsorbed oxygen on the adsorption and diffusion of hydrogen. The model reproduces well the experimental data concerning the bistability range, the dependence of the front propagation velocity on the hydrogen partial pressure and temperature, and the parameter-dependent change in front anisotropy. The simulations demonstrate that oxygen diffusion cannot be neglected despite the fact that under typical conditions the rate of oxygen diffusion is several orders-of-magnitude slower than that of hydrogen.
Orientation of thin liquid crystal films on buffed polyimide alignment layers: A near-edge x-ray absorption fine structure investigation
Authors: Weiss Klaus , Wöll Christof , Johannsmann Diethelm
Published: July 2002

Near-edge x-ray absorption fine structure investigations were performed on thin films of the liquid crystal n-octyl-cyanobiphenyl (8CB) evaporated onto buffed PMDA-ODA polyimide alignment layers and onto highly oriented pyrolytic graphite. The tensor nematic order parameter was derived from the x-ray dichroism as a function of layer thickness and thermal history. The liquid crystal orientation on buffed polyimide is markedly different from that observed on graphite. While on graphite the aromatic rings are oriented parallel to the substrate, there is—on average—a cylindrical symmetry around the molecular main axes for the orientational distribution function of 8CB on polyimide. The main axes are oriented parallel to the substrate in both cases. A brief heat treatment induces desorption of most of the LC material, leaving only the most tightly bound molecules behind. On graphite the remaining 8CB layer is highly oriented, whereas annealing does not significantly change the orientational parameters on polyimide. LC films adsorbed on rubbed polyimide reveal an alignment of the molecular main axes with the rubbing direction.
Densification effects on the Boson peak in vitreous silica: A molecular-dynamics study
Authors: Jund P. , Jullien R.
Published: July 2002

We perform classical molecular-dynamics simulation to study the effect of densification on the vibrational spectrum of a model silica glass. We concentrate this study on the so-called Boson peak and compare our results, obtained from a direct diagonalization of the dynamical matrix, with experimental Raman data. We show that, upon densification, the position of the Boson peak shifts towards higher frequencies while its magnitude decreases which is in agreement with a recent experimental study.
Recent progress in soliton transmission technology
Authors: Nakazawa Masataka , Kubota Hirokazu , Suzuki Kazunori , Yamada Eiichi , Sahara Akio
Published: July 2002

Recent progress on time-division multiplexed (TDM) and wavelength-division multiplexed (WDM) soliton transmission is described, in which dispersion management plays an important role in increasing the power margin and the dispersion tolerance. The characteristics of the dispersion-managed soliton are compared with those of return to zero and nonreturn to zero pulses. With a small dispersion swing, the system can still be described as an average soliton using the nonlinear Schrödinger equation, while with a large dispersion swing, the solitonlike steady-state pulse becomes a chirped Gaussian pulse, in which the governing equation is closer to a linear Schrödinger equation with a parabolic potential well. We describe an in-line modulation scheme for up to 80 Gbit/s per channel and its two channel WDM transmission over 10 000 km. Finally, we describe 640 Gbit/s (40 Gbit/s×16 channels) WDM soliton transmission over 1000 km with a dispersion-managed single-mode fiber.
Photochromic effect in near-stoichiometric LiNbO3 and two-color holographic recording
Authors: Lee Myeongkyu , Takekawa Shunji , Furukawa Yasunori , Uchida Yoshishige , Kitamura Kenji , Hatano Hideki , Tanaka Satoru
Published: July 2002

We have observed that newly developed near-stoichiometric LiNbO3 crystals have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, metastable shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV below the conduction band. Irradiation with UV light induced a stable absorption band extending from λ ≈650 nm to the absorption edge, which is caused by the photoinduced charge transfer from UV-sensitive absorption centers to deep traps via the conduction band. The electron lifetimes at shallow and deep traps could be controlled by doping elements and concentrations. Based on these favorable energy states, nonvolatile two-color holographic recording has been carried out by use of 852 nm recording beams and UV gating light. Nonvolatile readout by a single 852 nm beam was demonstrated for several hours. It was found that two-color recording from the pre-exposed, colored state can also be an attractive way of achieving nonvolatile readout. In this article, we investigate the photochromic effect observed in near-stoichiometric LiNbO3 and its use in two-color holographic recording.
Calculation of the two-dimensional vibrational response function
Authors: Sung Jaeyoung , Cho Minhaeng
Published: July 2002

We present a complete analytical expression of the two-dimensional (2D) vibrational response function without invoking the factorization approximations based on the Wick’s theorem that have been used in most previously reported theories. Since the harmonic approximation to the vibrational degrees of freedom is not required in this new formulation, the vibrational-level dependencies of the transition frequencies and the vibrational relaxation rates are fully incorporated in the obtained formula for the 2D vibrational response function. Furthermore, the non-Markovian nature of the vibrational dephasing process in condensed phases is also fully taken into account by carrying out the resummation of the associated diagrams with the linked diagram theory. It is found that there exists an additional contribution to the 2D vibrational response function, which was completely ignored due to the approximations based on the Wick’s theorem.
Laser frequency upshift, self-defocusing, and ring formation in tunnel ionizing gases and plasmas
Authors: Liu C. S. , Tripathi V. K.
Published: July 2002

The combined effects of tunnel ionization of gases on laser frequency upshift, defocusing, and ring formation are considered self-consistently. A high-intensity short pulse laser causes rapid tunnel ionization of a gas. The increasing plasma density leads to a decreasing refractive index, modulating the phase of the laser as it propagates and causing frequency upshift and supercontinuum generation. For laser intensity profile peaking on axis, the tunnel ionization produces a minimum of refractive index on axis, thus defocusing the laser. The defocusing reduces the ionization rate and frequency upshift. As the laser propagates over a Rayleigh length its trailing portion develops a ring shape distribution due to stronger defocusing of rays on axis than the off-axis rays, as seen in a recent experiment.
Numerical study of singularity formation in a class of Euler and Navier–Stokes flows
Authors: Ohkitani Koji , Gibbon John D.
Published: July 2002

We study numerically a class of stretched solutions of the three-dimensional Euler and Navier–Stokes equations identified by Gibbon, Fokas, and Doering (1999). Pseudo-spectral computations of a Euler flow starting from a simple smooth initial condition suggests a breakdown in finite time. Moreover, this singularity apparently persists in the Navier–Stokes case. Independent evidence for the existence of a singularity is given by a Taylor series expansion in time. The mechanism underlying the formation of this singularity is the two-dimensionalization of the vorticity vector under strong compression; that is, the intensification of the azimuthal components associated with the diminishing of the axial component. It is suggested that the hollowing of the vortex accompanying this phenomenon may have some relevance to studies in vortex breakdown.
Is noise a crucial factor in rate equation modeling of nonlinear dynamics in laser diodes?
Authors: Lim C. G. , Iezekiel S. , Snowden C. M.
Published: July 2002

The effect of noise on the accuracy of single-mode rate equations in predicting laser diode (LD) behavior is evaluated. Inclusion of noise increases the relaxation oscillation damping. Nonlinear dynamics analysis reveals that noise has disguised an important factor that governs the agreement between calculated and measured results. Instead, simulations show that the LD active layer temperature has a major impact on the accuracy of modeling LD behavior.
Kinetic of the NO removal by nonthermal plasma in N2/NO/C2H4 mixtures
Authors: Fresnet F. , Baravian G. , Magne L. , Pasquiers S. , Postel C. , Puech V. , Rousseau A.
Published: July 2002

NO removal is studied in N2/NO and in N2/NO/C2H4 mixtures through time-resolved laser-induced fluorescence in the afterglow of a pulsed homogeneous discharge. NO density measurements are compared with predictions of a 0D model on a large range of parameter values, such as the specific deposited energy and the ethene initial concentration. It is shown that dissociation of NO through collision with the N2(a′1Σu−) state play the main part in the NO removal kinetic. Moreover, quenching of N2(a′1 Σu−) by C2H4 leads to a drastic decrease of the NO removal efficiency when ethene is added to N2/NO. The determined rate coefficient value for the quenching mechanism is (4±2)×10−10 cm3 s−1.
Investigations of electrostrictive Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics under high-power drive conditions: Importance of compositional fluctuations on residual hysteresis
Authors: Viehland D. , Li J. F.
Published: July 2002

The high-field polarization behavior of electrostrictive Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics has been investigated as a function of measurement frequency, drive field, and temperature. It was observed that hysteretic losses increase with increasing frequency. Significant variations in loss were found between specimens of similar composition obtained from different sources, which became more pronounced with increasing temperature. The results indicate that careful synthesis and processing are crucial to the thermal stability of electrostrictive ceramics in high-power source applications. Further investigations revealed the potential use of accelerated aging by substituent introduction for mitigation of thermal stability concerns.
LIGA for lobster: First observation of lobster-eye focusing from lithographically produced optics
Authors: Peele Andrew G. , Irving Thomas H. K. , Nugent Keith A. , Mancini Derrick C. , Christenson Todd R. , Petre Rob , Brumby Steven P. , Priedhorsky William C.
Published: July 2002

The prospect of making a lobster-eye telescope is drawing closer with recent developments in the manufacture of microchannel-plate optics. This would lead to an x-ray all-sky monitor with vastly improved sensitivity and resolution over existing and other planned instruments. We consider a new approach, using deep etch x-ray lithography, to making a lobster-eye lens that offers certain advantages even over microchannel-plate technology.
Analysis of interfacial thermal stresses in a trimaterial assembly
Authors: Suhir E.
Published: July 2002

The interfacial stresses in, and the curvature of, an elongated trimaterial assembly, subjected to the change in temperature, are predicted based on an approximate structural analysis (strength-of-materials) model. The obtained results can be helpful in stress–strain evaluations and physical design of trimaterial assemblies in microelectronic and photonic packaging.
Calculation of excitonic properties of conjugated polymers using the Bethe–Salpeter equation
Authors: van der Horst J.-W. , Bobbert P. A. , Michels M. A. J. , Bässler H.
Published: July 2002

The excitonic spectra of a number of conjugated polymers, polythiophene (PT), polyphenylenevinylene (PPV), ladder-type poly-para-phenylene (LPPP), trans-polyacetylene (PA), and polydiacetylene (PDA), are calculated by solving the Bethe–Salpeter equation for the two-body Green function of the electron–hole pair. The only input to the calculation is the bulk dielectric constant. We find intermediate binding energies of 0.4–0.6 eV for the lowest optical 1 1Bu exciton, obtaining good agreement with recent experiments for each polymer individually. The agreement for the singlet–triplet splittings and the 1 1Bu polarizabilities is satisfactory. This also holds for the 1 1Bu–2 1Ag splittings in PT, LPPP, and PPV. For PA and PDA we find the 2 1Ag exciton above the 1 1Bu exciton, in contrast with experiment; a possible explanation for this discrepancy is the absence of lattice relaxation effects and double excitations in our calculations.
Vibrational mode-specific photochemical reaction dynamics of chlorine dioxide in solution
Authors: Fidder Henk , Tschirschwitz Frank , Dühr Oliver , Nibbering Erik T. J.
Published: July 2002

We study the reaction dynamics of OClO in cyclohexane, acetonitrile, and water by femtosecond pump–probe spectroscopy. In all solvents we observe a quantum beat in a 403 nm one-color pump–probe experiment with 55 fs temporal resolution, that decays with a 1.3–1.5 ps time constant. From this we conclude that, in contrast to previous reports, not all OClO molecules dissociate after excitation with 403 nm light. In both cyclohexane and water we observe in the 403 nm experiment an increase in stimulated emission between 0.5 and 2 ps that appears to be connected to the quantum beat decay. We explain these results as the consequence of vibrational relaxation of the bending mode of OClO. Relaxation from (ν1,1,0) to (ν1,0,0) leads to population of a state with a two times higher transition dipole moment, which accounts for the increased stimulated emission. Further proof that not all OClO molecules dissociate immediately after excitation is found in the identification of a stimulated emission contribution in femtosecond 400 nm pump/800 nm probe experiments, which also decays with about a 1.5 ps time constant. Femtosecond 400 nm pump/267 nm probe measurements indicate that a fraction of the OClO molecules dissociate very rapidly, with dissociation times of ⩽60, 80, and 140 fs, in acetonitrile, water, and cyclohexane, respectively. An anisotropy decay is resolved at 267 nm of the formed ClO in water and cyclohexane, with anisotropy decay times of 0.17 and 0.27 ps, respectively. In all solvents a fraction of the ClO+O fragments recombine, with time constants of 1.2 and 4.1 ps in water, 6.0 ps in acetonitrile, and 8.9 ps in cyclohexane. In acetonitrile a secondary dissociation pathway is identified with a 2.1 ps time constant. This pathway might also be responsible for the biexponentiality of the recombination process in water. In particular, in acetonitrile and cyclohexane the data indicate cage escape of a significant amount of fragments.
Giant Faraday rotation of blue light in epitaxial CexY3−xFe5O12 films grown by pulsed laser deposition
Authors: Kim Hyonju , Grishin Alex , Rao K. V.
Published: July 2002

We report crystalline, magnetic, and magneto-optic (MO) properties of Ce-substituted yttrium iron garnet (CexY3−xFe5O12) thin films epitaxially grown onto single crystal Gd3Ga5O12 (111) substrates using Nd:YAG pulsed laser deposition technique. Oxygen ambient pressure used for the growth is found to be the critical parameter to prepare CexY3−xFe5O12 films with good crystalline and magnetic properties as well as large MO effect. The film fabricated at 50 m Torr oxygen pressure exhibits a maximum Faraday rotation (FR) θF=1.78 and 4°/μm at λ=633 and 430 nm, respectively, a minimum in-plane coercivity Hc=35 Oe, and the narrowest full width at half maximum = 0.06° for the (444) x-ray Bragg reflection rocking curve. The analog of the Verdet constant V=θF/4πMs also found to be dependent on the oxygen ambient pressure reaches a value as high as 1.41°/μm kG at 633 nm, suggesting that this material is useful for MO applications. The energy dispersion FR spectra, measured over visible region 400 to 840 nm, clearly demonstrate that Ce substitution prominently enhances Faraday effect at 690 and at 430 nm blue-wavelength region.
Nonlinear stability analysis of the thin pseudoplastic liquid film flowing down along a vertical wall
Authors: Cheng Po-Jen , Chen Cha’o-Kuang , Lai Hsin-Yi
Published: July 2002

This article investigates the weakly nonlinear stability theory of a thin pseudoplastic liquid film flowing down on a vertical wall. The long-wave perturbation method is employed to solve for generalized nonlinear kinematic equation with free film interface. The normal mode approach is used to compute the linear stability solution for the film flow. The method of multiple scales is then used to obtain the weak nonlinear dynamics of the film flow for stability analysis. It is shown that the necessary condition for the existence of such a solution is governed by the Ginzburg–Landau equation. The modeling results indicate that both subcritical instability and supercritical stability conditions are possible to occur in a pseudoplastic film flow system. The results also reveal that the pseudoplastic liquid film flows are less stable than Newtonian’s as traveling down along the vertical wall. The degree of instability in the film flow is further intensified by decreasing the flow index n.
The Hausdorff entropic moment problem
Authors: Romera E. , Angulo J. C. , Dehesa J. S.
Published: July 2002

Our aim in this paper is twofold. First, to find the necessary and sufficient conditions to be satisfied by a given sequence of real numbers {ωn}n=0∞ to represent the “entropic moments” ∫[0,a][ρ(x)]ndx of an unknown non-negative, decreasing and differentiable (a.e.) density function ρ(x) with a finite interval support. These moments are called entropic moments because they are closely connected with various information entropies (Renyi, Tsallis, …). Second, we outline an efficient method for the reconstruction of the density function from the knowledge of its first N entropic moments.
Multilayer adsorption model for the protein–ligand interaction
Authors: Varela L. M. , Garcia M. , Perez-Rodriguez M. , Taboada P. , Ruso J. M. , Mosquera V.
Published: July 2002

In the present work we present a theoretical formalism based on the combination of the Brunauer–Emmet–Teller multilayer adsorption model with an electrolytic adsorbate, and the results are used to predict binding isotherms of several synthetic penicillin drugs onto human serum albumin. The occurrence of adsorption maxima in these binding processes is correctly predicted by this noncooperative binding model and it is demonstrated to be due to the ionic character of the adsorbate. The effect of the hydrophobic interactions between adsorbate monomers on the value of the maximum number of adsorbed particles is also a matter of study, and it is proven that this number increases with increasing hydrophobic character of the adsorbate.
Diffusivity measurements of silicon in silicon dioxide layers using isotopically pure material
Authors: Tsoukalas D. , Tsamis C. , Normand P.
Published: July 2002

We report measurement of the silicon diffusion coefficient in silicon dioxide films using isotopically enriched Si28 silicon dioxide layers that enable relatively low Si30 concentration measurements to be performed using secondary ion mass spectrometry. Two types of experiments are made. Si30 atoms are introduced in excess in a stoichiometric isotopically pure silicon dioxide layer either by ion implantation or by a predeposition technique. These experiments are representative of any physical situation in which excess silicon atoms are introduced into silicon dioxide layers during silicon processing. The estimated diffusivity values are significantly higher than previously reported values for Si diffusion within a stoichiometric oxide and closer to reported values for excess Si diffusion within an oxide. The activation energy of the diffusivity is found to be 4.74 eV.
(Fe–Co)–(Mg-fluoride) insulating nanogranular system with enhanced tunnel-type giant magnetoresistance
Authors: Kobayashi Nobukiyo , Ohnuma Shigehiro , Masumoto Tsuyoshi , Fujimori Hiroyasu
Published: July 2002

Insulating nanogranular-type tunnel magnetoresistive thin films made of (Fe or Fe–Co)–(Mg-fluoride) have been investigated. The films were prepared by a tandem deposition method, using Fe, Co, or Fe+Co metal and MgF2 insulator targets. The granular structure was found to consist of Fe or Fe–Co based nanogranules surrounded by thin intergranules of Mg based fluoride with the MgF2 crystal structure. A magnetoresistance value of 13.3% at room temperature and 10 kOe, the largest values ever reported, were obtained at the compositions of 32 vol %(Fe0.51Co0.49)–(Mg–F). To increase the magnetic field sensitivity of the magnetoresistance, a granular-in-gap film consisting of an (Fe–Co)–(Mg–F) granular thin film filling a narrow gap in a soft magnetic Permalloy thin film was prepared. A remarkably high magnetoresistance of 4% or more at 1–2 Oe was obtained.
72 W Nd:Y3Al5O12 ceramic laser
Authors: Lu Jianren , Murai T. , Takaichi K. , Uematsu T. , Misawa K. , Prabhu M. , Xu J. , Ueda K. , Yagi H. , Yanagitani T. , Kaminskii A. A. , Kudryashov A.
Published: July 2002

A high-power continuous-wave polycrystalline 1% Nd:Y3Al5O12 (Nd:YAG) ceramic rod laser was demonstrated. With 290 W/808 nm laser diode pumping, cw laser output of 72 W was obtained at 1064 nm. The optical-to-optical conversion efficiency is 24.8%. Thermally induced birefringence properties of Nd:YAG ceramic was also investigated.
Maskless etching of silicon using patterned microdischarges
Authors: Sankaran R. M. , Giapis K. P.
Published: July 2002

Microdischarges in flexible copper-polyimide structures with hole diameters of 200 μm have been used as stencil masks to pattern bare silicon in CF4/Ar chemistry. The discharges were operated at 20 Torr using the substrate as the cathode, achieving etch rates greater than 7 μm/min. Optical emission spectroscopy provides evidence of excited fluorine atoms. The etch profiles show a peculiar shape attributed to plasma expansion into the etched void. Forming discharges in multiple hole and line shapes permits direct pattern transfer in silicon and could be an alternative to ultrasonic milling and laser drilling.
Holographic surface gratings in iron-doped lithium niobate
Authors: Sarkisov S. S. , Curley M. J. , Kukhtarev N. V. , Fields A. , Adamovsky G. , Smith C. C. , Moore L. E.
Published: July 2002

Surface gratings associated with holographic volume gratings in photorefractive crystals of iron-doped lithium niobate have been studied using diffraction of a reflected probe beam and high-resolution phase-shifted interferometric profilometry. Both techniques show that the surface gratings exist in the form of periodical corrugations of the same period as that of the volume grating. The maximum amplitude of the periodical surface relief measured by both techniques is close to 6.5 nm. We also demonstrated that the periodical electric forces on the surface were capable of assembling polystyrene microspheres along the fringes of the grating. Large amplitude of the periodic electric field (1.6×104 V/cm) is associated with the photogalvanic effect.
Electronic and vibrational dynamic solvent effects on Raman spectra
Authors: Cappelli Chiara , Corni Stefano , Tomasi Jacopo
Published: July 2002

We report a methodology for the theoretical evaluation of nonequilibrium solvent effects on Raman intensities within the nonequilibrium Polarizable Continuum Model. Effects due to an incomplete solvent response both to the external field-induced oscillation in the solute electronic density and to molecular vibrations are considered. The extent of such effects on absolute scattering factors and relative intensities of few simple molecules (HCHO, FCHO, FCFO, ClCHO, ClCClO) in various solvents are discussed.
Measurement of the in-depth stress profile in hydrogenated microcrystalline silicon thin films using Raman spectrometry
Authors: Paillard V. , Puech P. , Sirvin R. , Hamma S. , Roca i Cabarrocas P.
Published: July 2002

Raman spectrometry is used to measure stress in hydrogenated microcrystalline silicon thin films. Moreover, by the use of different excitation wavelengths, from red to near ultraviolet, we can probe different film depths and get information on the stress distribution along the growth direction. For films deposited by standard rf glow discharge at different substrate temperatures, on glass substrates, we found large stress gradients. Indeed, the high compressive stress (up to 1 GPa) in the bulk of the film, close to the glass substrate, reduces and becomes tensile as the film free surface is approached. Moreover, the higher the substrate temperature, the higher the stress gradient.
Creation operators and Bethe vectors of the osp(1|2) Gaudin model
Authors: Kulish P. P. , Manojlović N.
Published: July 2002

A Gaudin model based on the orthosymplectic Lie superalgebra osp(1|2) is studied. The eigenvectors of the osp(1|2) invariant Gaudin Hamiltonians are constructed by algebraic Bethe ansatz. Corresponding creation operators are defined by a recurrence relation. Furthermore, explicit solution to this recurrence relation is found. The action of the creation operators on the lowest spin vector yields Bethe vectors of the model. The relation between the Bethe vectors and solutions to the Knizhnik–Zamolodchikov equation of the corresponding super-conformal field theory is established.
Ion-channeling analysis of boron clusters in silicon
Authors: Selen L. J. M. , Janssen F. J. J. , van IJzendoorn L. J. , de Voigt M. J. A. , Theunissen M. J. J. , Smulders P. J. M. , Eijkemans T. J.
Published: July 2002

We have measured axially channeled Rutherford backscattering spectra of Si1−xGex nanofilms in silicon(001). A step in the yield of the host crystal was found for off-normal axes at the depth of the nanofilm. The step was measured as a function of the angle between the incoming beam and the [011] axis and shows two maxima. It is found that Monte Carlo simulations assuming tetragonal distortion reproduce the experimental results. A universal curve was derived which enables determination of the tetragonal distortion from ion-channeling experiments, for a given film thickness. The results are compared with XRD measurements.
Operator formulation of centroid dynamics for Bose–Einstein and Fermi–Dirac statistics
Authors: Blinov Nicholas , Roy Pierre-Nicholas
Published: July 2002

This paper is devoted to the development of an operator formulation of the recent extension of the centroid molecular dynamics method [J. Chem. Phys. 110, 3647 (1999); 111, 5303 (1999)] to boson and fermion systems. An operator calculus is used to rederive the basic equations of centroid dynamics. The following generalization to the case of systems of many indistinguishable particles is based on the use of a projection operator. Two different definitions of the quasi-density operator for bosonic and fermionic systems are suggested. The first definition allows an exact evaluation of equilibrium properties for systems with exchange effects using classical-like molecular dynamics calculations. The second one provides a formal justification of Bose–Einstein/Fermi–Dirac centroid dynamics with the same set of approximations as for Boltzmann statistics, and can be used to extract quantum dynamical information. In this case, the corresponding centroid correlation function can be related to a double Kubo transformed quantum mechanical one.
High transport critical current density obtained for powder-in-tube-processed MgB2 tapes and wires using stainless steel and Cu–Ni tubes
Authors: Kumakura H. , Matsumoto A. , Fujii H. , Togano K.
Published: July 2002

MgB 2 tapes and wires were fabricated by the powder-in-tube method. Stainless steel and Cu–Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10 000 A/cm2 at 4.2 K and 5 T. A high Jc of about 300 000 A/cm2 was obtained by extrapolating the Jc–B curves to zero field. Multifilamentary (seven-core) MgB2 wire was successfully fabricated using Cu–Ni tubes. For both tapes and wires, the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu–Ni sheathed wire was lower than the stainless steel sheathed tape due to the lower packing density of MgB2.
Breakdown of adiabatic invariance in spherical tokamaks
Authors: Carlsson Johan
Published: July 2002

Due to the weak but strongly varying magnetic field, fast ions in spherical tokamaks are susceptible to the breakdown of adiabatic invariance. Beam ions in present-day spherical tokamaks are typically marginally adiabatic or super-adiabatic. Super-adiabatic ions have energies in the intermediate range where the longitudinal invariant is still adiabatically invariant, but the magnetic moment is not, and their motion is integrable. Fusion alphas in proposed burning-plasma experiments would, however, to a large extent be nonadiabatic, and follow chaotic orbits.
An improved method to spot-weld difficult junctions
Authors: Ferrenz Elizabeth E. , Amare Andinet , Arumainayagam Christopher R.
Published: July 2002

Recent advances in spot-welding technology such as high frequency direct current inverter welders provide an improved and reproducible method to spot-weld difficult junctions. The importance of removing the oxide layers on metal surfaces, accurately delivering the weld pulse profile, and controlling the force applied to the materials during the welding process are discussed in the context of resistance spot-welding a molybdenum crystal to a tantalum loop and attaching a tungsten–rhenium thermocouple to the crystal.
Fabrication of two-dimensional photonic crystals using interference lithography and electrodeposition of CdSe
Authors: Divliansky Ivan B. , Shishido Atsushi , Khoo Iam-Choon , Mayer Theresa S. , Pena David , Nishimura Suzushi , Keating Christine D. , Mallouk Thomas E.
Published: July 2002

This letter describes a simple synthetic approach to fabricate two-dimensional midinfrared CdSe photonic crystals (PC) by electrodeposition of CdSe in a polymer template defined using interference lithography. Characterization of the transmission spectra of CdSe PCs with a hexagonal array of 1.3 μm diameter and 2.7 μm pitch air voids showed a well-defined drop in transmission at 4.23 μm. The drop in transmission increased with incident angle, reaching a maximum of approximately 2.6 dB at 40° relative to the surface normal. This two-step synthetic approach can be used to incorporate photonic crystals onto arbitrary substrates for integration into future advanced optical circuits.
Scaling behavior in the Einstein–Yang–Mills monopoles and dyons
Authors: Hosotani Yutaka
Published: July 2002

Scaling behavior in the moduli space of monopole and dyon solutions in the Einstein–Yang–Mills theory in the asymptotically anti-de Sitter space is derived. The mass of monopoles and dyons scales with respect to their magnetic and electric charges, independent of the values of the cosmological constant and gauge coupling constant. The stable monopole and dyon solutions are approximated by solutions in the fixed anti-de Sitter spacetime. Unstable solutions can be viewed as the Bartnik–McKinnon solutions dressed with monopole and dyon solutions in the fixed anti-de Sitter space.
Laser ion source development for the Columbia University microbeam
Authors: Bigelow A. W. , Randers-Pehrson G. , Brenner D. J.
Published: July 2002

A design is given of a laser ion source for the 4.2 MV Van de Graaff accelerator at the Columbia University Radiological Research Accelerator Facility (RARAF). The source has been designed with application in mind for the RARAF single-particle single-cell microbeam, though it will also be used for broad-beam irradiations. The operating principle, laser ablation, can produce heavy ions with high charge states so that their energies will be high enough to provide sufficient range—at least 20 μm—for irradiating cells on a thin surface at atmospheric pressure. The laser ion source being implemented at RARAF is based on the laser operated ion source used by Hughes at the University of Arkansas and consists of three main components: laser generator, source vacuum chamber, and spherical electrostatic analyzer.
Molecular hydrodynamic theory of nonresonant Raman spectra in liquids: Fifth-order spectra
Authors: Denny R. Aldrin , Reichman David R.
Published: July 2002

Building upon the framework of the preceding paper, a molecular hydrodynamic theory of the fifth-order (two-dimensional) nonresonant Raman spectrum in a simple liquid is presented. A multi-time mode-coupling-like theory is developed and compared with recent computer simulations for liquid Xe. The theory is able to provide a microscopic rationale for the absence of an echo in this system. Experimental predictions for the temperature and density dependence of the signal are presented. Comparison is made with the instantaneous-normal-mode theory. The limitations of the present approach are discussed.
Stacking faults and their effects on ferroelectric properties in strontium bismuth tantalate
Authors: Ding Y. , Liu J. S. , Zhu J. S. , Wang Y. N.
Published: July 2002

The stacking faults and their effects on ferroelectric properties in strontium–bismuth–tantalate SrBi2Ta2O9 have been studied by transmission electron microscopy (TEM) and ferroelectric hysteresis loop measurement. The structure of SrBi2Ta2O9 consists of Bi2O2 layers and double perovskite type TaO6 octahedral units. There are four possible types of nonstiochiometric stacking faults: two intrinsic faults by removing either one or two perovskite layers and two extrinsic faults by inserting either one or two perovskite layers. TEM investigation reveals that the stacking faults are the extrinsic type. The extrinsic stacking faults in high density might destroy the ferroelectricity, and the reasons are discussed. With excess Bi, the density can be held low efficiently, and with deficient Sr the stacking faults change its form.
The steady propagation of a bubble in a flexible-walled channel: Asymptotic and computational models
Authors: Jensen O. E. , Horsburgh M. K. , Halpern D. , Gaver D. P.
Published: July 2002

The steady motion at zero Reynolds number of a semi-infinite bubble through a fluid-filled, flexible-walled channel, a model for the reopening of a collapsed lung airway, is described using asymptotic and numerical methods. The channel walls are membranes that are supported by external springs and are held under large longitudinal tension. An asymptotic analysis is presented under the assumption that membrane slopes are uniformly small. Near the bubble tip, the flow is equivalent to that of a semi-infinite bubble in a weakly tapered channel. Key features of this two-dimensional flow are matched to long-wavelength approximations describing the remainder of the solution domain. The analysis is valid for a wide range of bubble speeds, and it takes a particularly simple form when the bubble peels apart the channel walls as it advances. Predictions of bubble pressure as a function of bubble speed are validated by comparison with existing computations, new boundary-element simulations describing bubble motion in a channel with one rigid and one deformable wall, and experiments.
Two-photon-excited green emission and its dichroic shift of oriented thin-film CdS on glass formed by laser deposition
Authors: Ullrich B. , Schroeder R. , Sakai H. , Zhang A. , Cheng S. Z. D.
Published: July 2002

The photoluminescence of oriented thin-film CdS on glass formed by laser deposition was investigated employing 200 fs, 1.54 eV laser pulses at room temperature. The ultrafast excitation caused a two-photon absorption process, which results in purely green emission at the band gap. The spectra are fitted very well by the application of the van Roosbroeck–Shockley relation, density of states, and Urbach’s rule demonstrating the intrinsic character of the radiative recombination. It is further shown that the energy position of the emission peak depends on the polarization of the impinging laser beam due to the dichroism of the highly oriented films.
Rotationally resolved infrared spectroscopy of h2- and d1-formic acid monomer in liquid He droplets
Authors: Madeja Frank , Markwick Phineus , Havenith Martina , Nauta K. , Miller R. E.
Published: July 2002

Rotationally resolved spectra of h2- and d1-formic acid embedded in liquid helium droplets have been recorded in the spectral region of the O–H (ν1) and C–H (ν2) stretch vibrations. Accidental resonant mixing between the ν1-band of h2-formic acid and the combination bands (ν2+ν7) and (ν2+ν9) has been observed. The fundamental ν1 band can interact via Fermi resonance with the (ν2+ν7), and the (ν2+ν7) via Coriolis coupling with the (ν2+ν9) band. Examination of the resonance induced line-broadening effects suggests that the helium environment modifies vibrational relaxation dynamics. The 2ν3 C=O stretch overtone is also observed and was assigned by a density functional theory (DFT) ab initio calculation. The spectroscopic constants are determined by fitting all spectra with a standard gas phase Hamiltonian.
Low voltage I–V characteristics in magnetic tunneling junctions
Authors: Cabrera G. G. , Garcı́a N.
Published: July 2002

We show that elastic currents, which take into account variations of the tunneling transmission with voltage and a large ratio of majority to minority spin densities of states of the conduction band at the Fermi level, can account for the low voltage current anomalies observed in magnet–oxide–magnet junctions.
Evaluation of the small-angle x-ray scattering of carbons using parametrization methods
Authors: Smarsly Bernd , Antonietti Markus , Wolff Thomas
Published: July 2002

A parametrization algorithm to determine chord-length distributions (CLD) from small-angle scattering data is used to characterize details of the pore structure, pore size, and morphology of porous carbon materials. This approach is applied to various carbon processing procedures, such as steam activation, tempering, and contacting with polymer binders. It is found that this CLD method reveals, compared to previous small angle x-ray scattering evaluation approaches, fine changes in the pore structure and morphology of porous carbons with higher accuracy and physical significance.
Structure of aqueous ZnBr2 solution probed by x-ray absorption spectroscopy in normal and hydrothermal conditions
Authors: Simonet V. , Calzavara Y. , Hazemann J. L. , Argoud R. , Geaymond O. , Raoux D.
Published: July 2002

Local-order evolution around ions in aqueous solutions has been investigated between normal and hydrothermal conditions. The behavior of cations and anions in aqueous ZnBr2 solution were studied by performing x-ray absorption spectroscopy experiments at both Br and Zn edges. Extended x-ray absorption fine structure analyses are made on account of anharmonic treatment and multiple scattering contributions involving H atoms at the Br edge. The extended x-ray absorption fine structure results are coupled to x-ray absorption near-edge structure simulations in order to identify the complexes formed under normal and hydrothermal conditions. It appears that both Zn and Br ions are largely hydrated under normal conditions and that Zn–Br pairs are formed in hydrothermal conditions. This is related to an octahedral-to-tetrahedral evolution of the Zn local environment, the majority of Zn atoms being surrounded by water octahedra in normal conditions and by distorted tetrahedra involving Br and O atoms in hydrothermal conditions.