bluuebunny/crossref_metadata_2025 huggingface dataset:
New broad spectrum autocorrelator for ultrashort light pulses based on multiphoton photoemission
Authors: Dourandin Alexander , Brenner Henry C. , Pope Martin
Published: July 2002

A prototype autocorrelator device for measuring the pulse width of ultrashort laser pulses in the picosecond and femtosecond regimes has been constructed. The device is based on multiphoton induced ionization of electrons from a charged metal surface mounted on a vibrating silica fiber, which is an entirely different physical principle from those employed by existing methods for ultrashort pulse characterization, e.g., streak cameras or second harmonic generation. The new method distinguishes itself from existing techniques in that it is, in principle, applicable to a very wide range of wavelengths, from the far ultraviolet to the infrared, requires no special optical orientation of a nonlinear crystal, and can be used for both femtosecond and picosecond pulses. The prototype device has been successfully applied to 532 nm laser pulses with a ∼20 ps pulse width, and preliminary work shows it is applicable to femtosecond pulses as well
Electrical probe with guard and automatic positioner
Authors: Garcı́a-Hernández J. M. , López-Callejas R. , Camps-Carvajal E. , Benı́tez-Read J. S.
Published: July 2002

We present the design and construction of a diagnostic system of the plasma parameters by means of electrical probes. The novelties of the system are the protection provided to the electrical probe with a guard, thus minimizing the probe deterioration, and the automatic positioning device associated with the probe. This system consists of: the electronic instrumentation for the electrical probe bias; two control systems, one for the guard’s motion, and another for the positioning of the electrical probe; and finally, a data acquisition system. During the system’s operation, and based on the data measured by the electrical probe at different positions where the data acquisition is carried out, the following parameters are available in real time: floating voltage, ion saturation voltage and current, electron temperature, and plasma’s voltage and density. The motion of the guard and electrical probe array is programmable up to a maximum speed of 1 cm/s, and the time required for the bias of the probe, the measuring and storing of the signal, and the generation of an ASCII data file is approximately 10 ms. With this system, the duty cycle of the probe is extended due to the shielding provided by the guard. Likewise, the reduction of deposits of spurious materials on the probe results in a more reliable measuring process of the plasma parameters.
Multilayer mirror soft x-ray spectrometer for fast electron temperature measurement on the compact helical system
Authors: Lee S. , Duorah S. , Ejiri A. , Iguchi H. , Fujisawa A. , Ishiyama E. , Takase Y. , Toyama H. , Aramaki M. , Kojima M. , Okamura S. , Matsuoka K.
Published: July 2002

A multilayer mirror (MLM) soft x-ray spectrometer has been installed on the Compact Helical System. The x-ray energy spectrum from ECH and neutral beam injected heated low beta plasmas was measured as a function of radiation energy. Modulations of the x-ray intensity associated with magnetohydrodynamic instability were observed. The electron temperature measurement was also investigated using the MLM x-ray spectrometer. The data suggest the possibility of a fluctuation measurement of the electron temperature Te with fast time resolution using the MLM spectrometer.
Measurements of the double piezoelectric effect
Authors: Smith W. F. , Axelrod B. W.
Published: July 2002

For a piezoelectric device with multiple electrodes, it should be possible to quantitatively and conveniently measure the piezoelectric coefficient d31 by use of the double piezoelectric effect, without the need for any unusual apparatus or a calibration sample. One pair of electrodes is used to excite the piezo, and another is used to measure the response. For example, for a piezoelectric tube this should allow measurement of d31 as a function of temperature or of lateral offset voltage. An important correction to the current theory is described. Measurements on two piezo tubes are presented for excitation voltages in the range of 0.5–110 Vpp. An inductive proximity sensor was used to measure the actual piezo motion. It is shown that the current theory disagrees with this and previous experiments by a simple factor of 2 for symmetric excitations. However, for asymmetric excitations the disagreement with theory is more complex, and the current theory does not accurately predict the effects of varying tube geometry.
Instrumentation for wide frequency range measurements of epoxy resin during microwave curing
Authors: Meyer O. , Chevalier S. , Belhadj-Tahar N. , Fourrier M. , Fourrier–Lamer A.
Published: July 2002

A new instrumentation system for measuring wide frequency band complex permittivity (1 MHz–2 GHz) during the microwave curing process has been developed using an original system coupled with coaxial discontinuity. The instrumentation operates through the automatic commutation of the wide frequency band cell on a power supply or a probe supply. This system allows the use of different heating frequencies (215 MHz, 434 MHz, or 2.45 GHz) with the same cell. This instrumentation has been used to study microwave curing compared with conventional heating, but it can also be applied to the use of microwaves in chemical processes or to study the biological effects of microwaves.
A physical model of the beam impedance method within ideal nested magnetic surfaces
Authors: Hamada Takateru , Obiki Tokuhiro , Mizuuchi Tohru , Sano Fumimichi , Nakasuga Masahiko
Published: July 2002

The beam impedance method is used to measure the vacuum magnetic structure of heliotron/torsatron systems. It measures the current that flows from an electron gun inserted into the vacuum chamber to the chamber wall. To interpret experimental results, we have been developing a physical model of the method and a numerical code based on the model. For simplicity, we have focused on a limited model in which only ideal nested magnetic surfaces exist. As a new approach to reproduce the real physical condition, we treat the current that flows across magnetic surfaces and that which is emitted from the electron gun separately. Radial electron distribution forms for the two currents to match each other. The characteristic of electron emission from the gun is obtained experimentally and included in the code. The diffusion process is assumed to obey the neoclassical diffusion theory. The numerically obtained current profile agrees well with the measurement if the suppression of diffusion due to a radial electric field is taken into account.
Very high resolution measurement of the penetration depth of superconductors by a novel single-coil inductance technique
Authors: Gauzzi A. , Le Cochec J. , Lamura G. , Jönsson B. J. , Gasparov V. A. , Ladan F. R. , Plaçais B. , Probst P. A. , Pavuna D. , Bok J.
Published: July 2002

We describe a novel single-coil mutual inductance technique for measuring the magnetic penetration depth λ of superconductors at 2–4 MHz as a function of temperature in the 4–100 K range. We combine a single-coil configuration with a high-stability marginal oscillator; this enables us to measure the absolute value of λ on both bulk samples and thin films with very high resolution (δλ=10 pm) and a precision of 30 nm. As example of application, we report measurements on NbTi bulk samples and Nb films. This contactless technique is suited for probing the superconducting properties of samples over large surfaces.
Further considerations on the preparation of small electrical contacts by spot welding
Authors: Walker I. R. , Moss C. J.
Published: July 2002

A recent publication by the authors describes a spot welder which can be used to join small wires to samples of metallic materials. The article also discusses the selection of contact wires, the preparation of samples prior to welding, and other details of the welding method. In the present work, we give a much improved version of the power supply circuit, and offer additional information on the technique that was not discussed previously. A “universal” method of electropolishing is also described which may be used to remove oxides and other surface contaminants from samples prior to spot welding. This technique can be applied to a variety of materials, including rare earth elements and compounds, which are generally difficult to polish or etch using other chemical or electrochemical means.
Automated system for measuring optical parameters for coherence-modulated optical links
Authors: Gutiérrez-Martı́nez C. , Rodrı́guez-Asomoza J.
Published: July 2002

A computer-controlled system allowing automated measurements of coherence lengths and optical delays as starting data for designing coherence modulated optical links is described here. Modulation using optical delays as information carriers is currently known as coherence modulation. Main characteristics of coherence modulated transmissions include the need of using low-coherence optical sources and integrated optics retarders. The reported results include characterization of low-coherence semiconductor optical sources and lithium niobate (LiNbO3) integrated optics coherence modulators.
Flux dam, a method to reduce extra low frequency noise when a superconducting magnetometer is exposed to a magnetic field
Authors: Koch R. H. , Sun J. Z. , Foglietti V. , Gallagher W. J.
Published: July 2002

This letter discusses the effects of a moderate magnetic field applied to a SQUID magnetometer and introduces the invention of the ‘‘flux dam.’’ A flux dam is a weak link or Josephson junction in series with the pickup loop which allows magnetic flux to enter and leave the inside of the superconducting loop easily for moderate changes in magnetic field. This largely prevents the flux lines from entering and being pinned in the superconducting parts of the magnetometer which would create extra low frequency noise. SQUID magnetometers with flux dams in the pickup loop can be operated in the Earth’s magnetic field without a significant increase in low frequency flux noise.
Ferroelectric thin films with polarization gradients normal to the growth surface
Authors: Mantese Joseph V. , Schubring Norman W. , Micheli Adolph L. , Catalan Antonio B.
Published: July 2002

Thin film ferroelectrics with polarization gradients normal to the growth surface readily form when gradients in temperature, strain, or composition are coupled to the polarization vector in ferroelectric materials. This letter describes the formation of thin films of potassium tantalum niobate with graded polarizations obtained by grading the tantalum to niobium ratio of the ferroelectric phase. Unlike a simple structure consisting of laminated layers of ferroelectric material, the polarization gradient which forms breaks the natural symmetry of the ferroelectric material at any given plane, resulting in a self-poling of the structure subsequent to excitation by an oscillatory electric field. Once poled, the devices reveal a measurable potential across the structure which varies with temperature.
Determination of the dielectric tensor in anisotropic materials
Authors: Alonso M. I. , Garriga M. , Alsina F. , Piñol S.
Published: July 2002

We demonstrate a new approach to determine the dielectric tensor of anisotropic materials using a rotating polarizer (or analyzer) ellipsometer. The dependence of the ellipsometric parameters as a function of azimuthal angle shows characteristic patterns very sensitive to the magnitude of the dielectric tensor and its orientation with respect to the sample surface. These patterns are in general nonsinusoidal and depend strongly on the angle setting of the fixed analyzer (or polarizer). We illustrate the method with results obtained on a generic crystallographic face of a uniaxial material, namely, a single crystal of the electron-doped high-temperature superconductor Nd1.85Ce0.15CuO4−δ .
High-resolution x-ray diffraction measurements of residual stresses in polycrystalline thin films deposited on single crystalline substrates
Authors: Levit M. , Zolotoyabko E. , Weiss B.-Z.
Published: July 2002

A nondestructive method for stress determination in polycrystalline/amorphous thin films deposited on the nearly perfect crystals was developed. High-resolution x-ray diffraction rocking curves are taken from the substrate using a monochromatized and quasiparallel incident x-ray beam. Changes in the rocking curve shape, caused by stress-induced bending of the substrate, are compared with the newly derived analytical expression of the diffraction spectrum, and as a result the radius of curvature and hence the biaxial stress is obtained. For the (004) Si reflection and the Cu Kα1 radiation the radii of curvature within the interval, 3 m<R<500 m, can be resolved. The method was successfully applied to measure stresses in heat-treated Ni90Ti10 alloy films deposited on a Si (001) substrate. The stress behavior (as a function of the annealing temperature) was found to reflect the sequence of the formation of silicides and enabled the evaluation of their thermal expansion coefficients.
Incoherent interface of InAs grown directly on GaP(001)
Authors: Chang J. C. P. , Chin T. P. , Woodall J. M.
Published: July 2002

We report molecular beam epitaxial growth of InAs on GaP(001), which has the largest lattice mismatch (11%) among all the arsenides and phosphides. Reflection high-energy electron diffraction and high-resolution transmission electron microscopy were used to optimize the growth and characterize the epilayer. It is found that the growth mode can be controlled by the surface V/III ratio: three-dimensional and two-dimensional layer-by-layer growths under As-stable and In-stable conditions, respectively. In both cases, a regular network of pure edge-type (90°) misfit dislocations with a spacing of 4 nm was formed directly at the heterointerface, which corresponds to 85% of degree of strain relaxation. The epilayers grown under In-stable conditions have relatively smooth surfaces with low threading dislocation densities. This is owing to the fact that the interface misfit dislocations were exclusively of the edge-type which have no threading component and which relieve strain most effectively. The results demonstrate the ability to control the growth mode as well as the misfit dislocation nucleation type.
An investigation on the leakage current and time dependent dielectric breakdown of ferroelectric lead–zirconate–titanate thin film capacitors for memory device applications
Authors: Chen Jia-lin , Chen Hong-ming , Lee Joseph Ya-min
Published: July 2002

Lead–zirconate–titanate (PZT) is an interesting ferroelectric material for the application of DRAM and nonvolatile memory devices. In this work, metal-PZT-metal capacitors with Au and Pt as the top and the bottom electrodes are fabricated. The leakage current, time dependent dielectric breakdown (TDDB) lifetime and the correlation between them are studied. The leakage current is found to depend on the applied electric field in a power law relationship. The exponent in the power law relation is found to be 0.88 in the low field region (lower than 100 kV/cm) and 9.6 in the high field region (larger than 100 kV/cm). The TDDB of the PZT capacitors is measured. An extrapolation method is proposed to obtain the projected TDDB lifetime from the time to breakdown (tBD)data. The power law exponential extracted from the TDDB measurement in this method is found to be in good agreement with that obtained from the leakage current measurement.
Relaxation process of Fe(CuNb)SiB amorphous alloys investigated by dynamical calorimetry
Authors: Zhu J. , Clavaguera-Mora M. T. , Clavaguera N.
Published: July 2002

Differential scanning calorimetry and dynamic differential scanning calorimetry were used to analyze the relaxation process of Fe(CuNb)SiB amorphous alloys. The Curie temperature (TC) evolution of the amorphous phase during relaxation as a function of heating rate, time and pre-annealing temperature were measured. Two distinct relaxation processes are observed, consequent with topological and chemical short range order changes.
Spin-on doping of porous silicon and its effect on photoluminescence and transport characteristics
Authors: Sen S. , Siejka J. , Savtchouk A. , Lagowski J.
Published: July 2002

We have investigated a novel way to dope porous Si layers with (B) and/or (P) using the spin-on doping technique. Under certain conditions, pore filling by the dopant solution was measured to be near 90%–95% leading to a homogeneous coverage of the porous skeleton. Near two orders of magnitude decrease in diode resistance was achievable following rapid thermal activation in a N2 atmosphere of B only or B+P double doped porous Si. Photoluminescence (PL) intensities observed in B+P double doped porous layers were significant. Relative to as-prepared samples, the PL intensities of double doped samples were weaker for porous Si on n-type and stronger for porous Si formed on p-type Si. In both cases, the PL magnitudes after double doping were comparable.
Improved quality of CuGaSe2 and CuAlSe2 epilayers grown on CuGa0.96In0.04Se2 substrates
Authors: Chichibu Shigefusa , Nakanishi Hisayuki , Shirakata Sho , Isomura Shigehiro , Miyake Hideto , Sugiyama Koichi
Published: July 2002

Nearly strain free CuGaSe2 and CuAlSe2 thin films were successfully grown by metalorganic vapor phase epitaxy on CuGa0.96In0.04Se2 substrates prepared by the traveling heater method (THM). CuAlSe2 (112) epilayers exhibited fine surface structures which reflected the crystal geometry. The CuGaSe2 epilayers exhibited intense near-band-edge emission even at room temperature. THM-grown single crystals were demonstrated to be a suitable substrates for epitaxy of chalcopyrite compounds.
Efficient wavelength shifting over the erbium amplifier bandwidth via cascaded second order processes in lithium niobate waveguides
Authors: Gallo Katia , Assanto Gaetano , Stegeman George I.
Published: July 2002

We investigate two schemes for wavelength conversion based on the cascading of two successive second order processes in a quasi-phase-matched lithium niobate channel waveguide. Efficient conversion over the full erbium amplifier bandwidth is possible with a single multi-hundred milliwatt pump laser operating around 1.55 μm.
Electrode dependence of hydrogen-induced degradation in ferroelectric Pb(Zr,Ti)O3 and SrBi2Ta2O9 thin films
Authors: Han Jin-Ping , Ma T. P.
Published: July 2002

Forming gas annealing causes changes in the remanent polarization (Pr), coercive field (Ec), and leakage current (I) in both PZT [Pb(Zr,Ti)O3] and SBT (SrBi2Ta2O9) samples with a variety of top electrode materials (Pt, Au, Ag, Cu, Ni, and In2O3), and the degree of degradation depends strongly on the top electrode material. These results may be explained by a model that is based on the catalytic activities of the top electrode to dissociate hydrogen molecules into hydrogen atoms, with the latter subsequently migrating into PZT or SBT films to cause oxygen deficiency and its associated property degradation. This model can be expanded to explain the recovery phenomenon resulting from oxygen annealing, which also depends on the catalytic activity of the top electrode to produce atomic oxygen from molecular oxygen.
Perfect spatial ordering of self-organized InGaAs/AlGaAs box-like structure array on GaAs (311)B substrate with silicon nitride dot array
Authors: Kuramochi Eiichi , Temmyo Jiro , Tamamura Toshiaki , Kamada Hidehiko
Published: July 2002

The metalorganic vapor phase epitaxial growth of strained InGaAs/AlGaAs box-like structure self-organized on GaAs (311)B substrate was investigated using fine silicon nitride (SiN) dot arrays for improving the controllability of self-organization phenomena. AlGaAs barrier layer grown at 750 °C buries SiN dots, forming novel pentagonally shaped hollows on (311)B substrate due to the (−100) facet growth and lateral growth. The In0.3Ga0.7As layer is preferentially grown in these hollows, then box-like structure is formed in these hollows during the growth interruption. Successive growth of AlGaAs/In0.3Ga0.7As epilayers induces the stacking of box-like structures just on top of the bottom boxes. The pairing probability of bottom and upper boxes is strongly dependent on the SiN dot array pitch, and the perfect spatial ordering of upper box arrays is achieved when the SiN dot pitch is in a range of 250–300 nm. This approach allows the exact positioning of self-formed box structure.
Nanoscale metal/self-assembled monolayer/metal heterostructures
Authors: Zhou C. , Deshpande M. R. , Reed M. A. , Jones L. , Tour J. M.
Published: July 2002

We present the investigation of novel metal/organic monolayer/metal heterostructure diodes. Our technique provides well-defined, stable, and reproducible metallic contacts to a self-assembled monolayer of 4-thioacetylbiphenyl with nanoscale area. Electronic transport measurements show a prominent rectifying behavior arising from the asymmetry of the molecular heterostructure. Variable-temperature measurements reveal that thermal emission of electrons over a barrier of 0.22 eV dominates for electron injection from Ti into the organic layer while the transport for electron injection from Au into the organic layer satisfies the formula for hopping conduction.
Vibrational dynamics of force microscopy: Effect of tip dimensions
Authors: Wright Oliver B. , Nishiguchi Norihiko
Published: July 2002

The dynamics of a vibrating cantilever with an attached tip in contact with a solid is treated analytically. The tip length is shown to be crucial in determining the resonant response. The finite tip size changes the boundary conditions for the flexural motion, rendering the cantilever-tip-sample combination more rigid and implicating both the normal and lateral stiffnesses of the sample in the analysis. This is confirmed in an experiment with a silica sample, a sapphire tip, and a silicon cantilever. The theory has implications in the field of quantitative analysis with atomic ac force microscopy.
Characterization of GaAs surfaces treated with phosphine gas photodecomposed by an ArF excimer laser
Authors: Sugino Takashi , Ninomiya Hideaki , Shirafuji Junji , Matsuda Koichiro
Published: July 2002

Phosphidization of GaAs surfaces is attempted with phosphine gas photodecomposed by an ArF excimer laser. Electron traps at and near the phosphidized GaAs surfaces are characterized by isothermal capacitance transient spectroscopy measurements. Phosphidization leads to a reduction in the trap (Ec−0.81 eV) known as an EL2 center and generation of two traps (Ec−0.24 eV and Ec−0.49 eV), which are designated as NL1 and NL2, respectively. A significant metal work-function dependence of the barrier height is demonstrated for Schottky junctions formed on the GaAs surfaces phosphidized under optimum condition, suggesting that phosphidization is effective in reducing surface states of GaAs.
Structural and magneto-transport properties of electrodeposited bismuth nanowires
Authors: Liu Kai , Chien C. L. , Searson P. C. , Yu-Zhang Kui
Published: July 2002

Arrays of semimetallic Bi nanowires have been successfully fabricated by electrodeposition. Each nanowire consists of elongated Bi grains along the wire direction. Very large positive magnetoresistance of 300% at low temperatures and 70% at room temperature with quasilinear field dependence has been observed. These features are desirable for wide-range field sensing applications.
The conductivity dependence of the shear stress in electrorheological fluids
Authors: Lan Yucheng , Xu Xiaoyu , Men Shouqiang , Lu Kunquan
Published: July 2002

A ferroelectric KNO3/silicone oil electrorheological (ER) fluid is introduced to investigate the conductivity dependence of the ER effect under dc electric fields where the ER effect is conductivity dominated. By measuring the temperature dependence of the shear stress across the Curie temperature of particles, the dependence of the ER effect on conductivity has been quantitatively obtained in experiments. There is a critical conductivity ratio Γc (or mismatch factor βc2): when Γ<Γc, the shear stress increases with Γ; when Γ>Γc, the shear stress decreases with Γ. An agreement is obtained between theory and experiment when Γ (or β2) is lower. In the higher Γ(or β2) range, the experimental data are not in agreement with the theoretical prediction and the interfacial effect should be taken into account. The experimental data are more reliable due to the same conditions, such as the chemical nature, the surfacial property of particles, and the interfacial property between particles and suspending liquid as well as the size and shape of the particles.
Measurement of far-infrared intersubband spontaneous emission from optically pumped quantum wells
Authors: Lyubomirsky I. , Hu Q. , Melloch M. R.
Published: July 2002

We present the results of an experiment to measure far-infrared intersubband spontaneous emission from optically pumped quantum wells. A four-level laser design scheme was implemented using a coupled triple quantum well structure for optical pumping from E1 to E4 with a CO2 laser. The phonon scattering rates were designed to optimize the far-infrared emission efficiency and population inversion between E3−E2∼30 meV. The spontaneous emission spectrum was successfully resolved to reveal an emission peak at 30.8 meV with a linewidth of 7 meV.
J-aggregate electroluminescence in dye doped polymer layers
Authors: Mal’tsev E. I. , Lypenko D. A. , Shapiro B. I. , Brusentseva M. A. , Berendyaev V. I. , Kotov B. V. , Vannikov A. V.
Published: July 2002

The electroluminescence (EL) of a dye nanocrystalline phase was revealed in single layer light-emitting diodes based on polymers doped with cyanine dye molecules. Observation of light emission in the visible range depended on the redox potentials of the dyes used. The 9,10-dithioanthracene-containing polyimides exhibiting efficient electron-hole transport appeared to be appropriate media for the generation of J-aggregate EL.
Large enhancement in quasiphase matched second-harmonic generation efficiency by an external temperature gradient
Authors: Pradhan Ranjit D. , Melikechi Noureddine
Published: July 2002

We show that for continuous wave second-harmonic generation using quasiphase matching in periodically segmented waveguides, thermal loading caused by the fundamental and the generated beams degrades the quasiphase matching condition which corrupts the spectral quality of the second-harmonic field. We also show that a carefully designed external temperature gradient leads to an enhancement of the conversion efficiency and control of the spectral line shape of the output field. Compared to earlier work, our simple scheme yields an enhancement of the second-harmonic conversion efficiency by a factor of 2–4 depending on the input fundamental power.
Blockage of the annealing-induced Si/SiO2 degradation by helium
Authors: Afanas’ev V. V. , Stesmans A.
Published: July 2002

Electrical degradation of Si/SiO2 structures caused by postoxidation annealing was comparatively studied in noble gas (He, Ne, Ar), vacuum, and N2 ambient. Helium is found to significantly retard the generation of defects responsible for the low-field conductivity of ultrathin oxides and the hole trapping in SiO2. The physical mechanism of the blockage effect is attributed to the occupation of interstitial cavities in SiO2 by the noble gas atoms that prevent interfacial reaction between Si and SiO2.
Structure control and characterization of SrBi2Ta2O9 thin films by a modified annealing method
Authors: Hu G. D. , Wilson I. H. , Xu J. B. , Cheung W. Y. , Wong S. P. , Wong H. K.
Published: July 2002

SrBi 2 Ta 2 O 9 (SBT) ferroelectric thin films were prepared by metalorganic decomposition on Pt/Ti/SiO2/Si substrates at annealing temperatures ranging from 600 to 750 °C. The SBT thin films were annealed layer by layer during the spin-coating process using a rapid thermal annealing (RTA) furnace. The relative intensity of (200) peak in x-ray diffraction increased with the increase of the annealing temperature. A (200)-predominant film can be formed at 700 and 750 °C. For the film annealed by RTA furnace at 650 °C, the remanent polarization (2Pr) and coercive field (2Ec) were 19.8 μC/cm2 and 116 kV/cm, respectively.
Inversion of surface composition and evolution of nanostructure in Cu/Co nanocrystals
Authors: Edelstein A. S. , Harris V. G. , Rolison D. R. , Kurihara L. , Smith David J. , Perepezko J. , da Silva Bassani M. H.
Published: July 2002

Nanocrystals of Cu/Co with a nearly pure Cu core and a Co-rich shell have been chemically synthesized. This structure results from reaction kinetics and represents an inversion of surface composition since the surface energy of Co is larger than that of Cu. Both the Cu core and the Co-rich shell initially have an fcc structure and a common lattice constant. Annealing at temperatures in the range of 300 to 600 °C causes the material to phase separate, forming small, increasingly pure Co nanocrystals which aggregate on the surface of the pure Cu nanocrystals.
Evidence for localized Si-donor state and its metastable properties in AlGaN
Authors: Skierbiszewski C. , Suski T. , Leszczynski M. , Shin M. , Skowronski M. , Bremser M. D. , Davis R. F.
Published: July 2002

Transport studies of AlxGa1−xN (0.5<x<0.6) doped with Si have been performed in the pressure range up to 1.4 GPa. For these alloys, the Si dopant forms two donor states. One of them has an effective mass character and the other one represents the localized state strongly coupled to the crystal lattice (metastable state). The localized state of Si forms the corresponding level in the gap for x exceeding 0.5. For the higher x, an increase of the activation energy of this state occurs. Metastable properties of the localized state of Si lead to a persistent photoconductivity effect and to a pressure induced freeze-out of electrons.
Stimulated emission and lasing in whispering-gallery modes of GaN microdisk cavities
Authors: Chang Seongsik , Rex Nathan B. , Chang Richard K. , Chong Gabel , Guido Louis J.
Published: July 2002

We report optically pumped, pulsed lasing action in whispering-gallery modes of GaN microdisk cavities at room temperature. The microdisk structure was fabricated by reactive-ion etching a 2-μm-thick GaN epitaxial layer grown via metalorganic chemical vapor deposition. Below the lasing threshold, stimulated emission with superlinear pump-intensity dependence is observed. Spontaneous-to-stimulated emission transition occurs at a pump intensity that is 10× lower than that for a GaN sample without a cavity structure. Above the lasing threshold, the pump-intensity dependence is almost linear and gain pinning is observed. In addition, whispering-gallery modes are observed with the linewidth of individual peaks being as narrow as 0.1 nm.
Blue photoluminescence from thermally grown SiO2 on micromachined arrays of silicon planes
Authors: Diligenti A. , Pieri F. , Piotto M. , Fuso F. , Allegrini M.
Published: July 2002

SiO 2 was thermally grown on arrays of silicon planes obtained from 〈110〉 substrates by means of micromachining. Blue photoluminescence was observed under pulsed-laser excitation from SiO2 grown on these planes. Experiments revealed that this emission was not affected by the Si/SiO2 interface properties or the silicon thickness, whereas its intensity and spectral features depended on the oxide thickness. Moreover, no detectable luminescence was observed from the oxide grown on unpatterned regions, where a smaller amount of oxide was excited by the laser beam. The photoluminescence disappeared when the oxide was removed.
LaNiO
3
buffer layers for high critical current density YBa2Cu3O7−δ and Tl2Ba2CaCu2O8−δ films
Authors: Carlson C. M. , Parilla P. A. , Siegal M. P. , Ginley D. S. , Wang Y.-T. , Blaugher R. D. , Price J. C. , Overmyer D. L. , Venturini E. L.
Published: July 2002

We demonstrate high critical current density superconducting films of YBa2Cu3O7−δ (YBCO) and Tl2Ba2CaCu2O8−δ (Tl-2212) using LaNiO3 (LNO) buffer layers. YBCO films grown on an LNO buffer layer have only a slightly lower Jc (5 K, H=0) than films grown directly on a bare LaAlO3 substrate. YBCO films grown on LNO buffer layers exhibit minor microstructural disorder and enhanced flux pinning. LNO-buffered Tl-2212 samples show large reductions in Jc at all temperatures and fields compared to those grown on bare LaAlO3, correlating to both a-axis grain and nonsuperconducting phase formation. LNO could be a promising buffer layer for both YBCO and Tl-based superconducting films in coated conductor applications.
Leakage current of sol-gel derived Pb(Zr, Ti)O3 thin films having Pt electrodes
Authors: Shin Ju Cheol , Hwang Cheol Seong , Kim Hyeong Joon , Park Soon Oh
Published: July 2002

The electrical conduction behaviors of sol-gel derived Pb(Zr, Ti)O3 (PZT) thin films on Pt electrodes were analyzed based on a fully depleted film, thermionic field emission, and space charge limited conduction model in the low and high electric field regions, respectively. For films having thicknesses ranging from 150 to 250 nm, no thickness-dependent variation in the dielectric constant was observed due to the relatively large thicknesses. The rather small film-thickness-dependent leakage current characteristics in the low-field region elucidates that the positive space charge density in the film is about 1018 cm−3, which is a smaller value than that of the sputter deposited (Ba, Sr)TiO3 thin films by an order of magnitude. The calculated interfacial potential barrier height and effective mass of electrons were 0.93 eV and 0.09m0, respectively. The slope larger than 2 from the log J vs log V plot in the high-field region implies that the energy level of electron traps are continuously distributed in the energy band gap.
Subpicosecond spin relaxation in GaAsSb multiple quantum wells
Authors: Hall K. C. , Leonard S. W. , van Driel H. M. , Kost A. R. , Selvig E. , Chow D. H.
Published: July 2002

Spin relaxation times in GaAsxSb1−x quantum wells are measured at 295 K using time-resolved circular dichroism induced by 1.5 μm, 100 fs pulses. Values of 1.03 and 0.84 ps are obtained for samples with x=0 and 0.188, respectively. These times are >5 times shorter than those in InGaAs and InGaAsP wells with similar band gaps. The shorter relaxation times are attributed to the larger spin-orbit conduction-band splitting in the Ga(As)Sb system, consistent with the D’yakonov–Perel theory of spin relaxation [M. I. D’yakonov and V. I. Perel, Sov. Phys. JETP 38, 177 (1974)]. Our results indicate the feasibility of engineering an all-optical, polarization switch at 1.5 μm with response time <250 fs.
SrRuO
3
/(Ba, Sr)TiO
3
/SrRuO
3
capacitor annealed in the forming gas with and without oxygen addition
Authors: Lin Jun , Natori Katsuaki , Fukuzumi Yoshiaki , Izuha Mitsuaki , Tsunoda Kohji , Eguchi Kazuhiro , Hieda Katsuhiko , Matsunaga Daisuke
Published: July 2002

Forming gas (3%H2+97%N2) anneals result in decomposition of SrRuO3 and increase the leakage current of the SrRuO3/(Ba, Sr)TiO3/SrRuO3 capacitor. However, we show that 0.5% O2 addition to the forming gas (3%H2+0.5%O2+96.5%N2) does not cause degradation of the SrRuO3/(Ba, Sr)TiO3/SrRuO3 capacitor, and can also enhance the performance of the transistor effectively. To correctly study the effect of a forming gas anneal on the SrRuO3/(Ba, Sr)TiO3/SrRuO3 capacitor, efforts should be made to avoid the possible O2 diffusion from air into furnace.
Kadanoff–Baym equations and non-Markovian Boltzmann equation in generalized T-matrix approximation
Authors: Semkat D. , Kremp D. , Bonitz M.
Published: July 2002

A recently developed method [Semkat et al., Phys. Rev. E 59, 1557 (1999); Kremp et al., in Progress in Nonequilibrium Green’s Functions (World Scientific, Singapore, 2000), p. 34] for incorporating initial binary correlations into the Kadanoff–Baym equations (KBE) is used to derive a generalized T-matrix approximation for the self-energies. It is shown that the T-matrix obtains additional contributions arising from initial correlations. Using these results and taking the time-diagonal limit of the KBE, a generalized quantum kinetic equation in binary collision approximation is derived. This equation is a far-reaching generalization of Boltzmann-type kinetic equations: It self-consistently includes memory effects (retardation, off-shell T-matrices) as well as many-particle effects (damping, in-medium T-matrices) and spin-statistics effects (Pauli-blocking).
Evolution of the vorticity-area density during the formation of coherent structures in two-dimensional flows
Authors: Capel H. W. , Pasmanter R. A.
Published: July 2002

It is shown: (1) that in two-dimensional, incompressible, viscous flows the vorticity-area distribution evolves according to an advection-diffusion equation with a negative, time dependent diffusion coefficient and (2) how to use the vorticity-stream function relations, i.e., the so-called scatter-plots, of the quasi-stationary coherent structures in order to quantify the experimentally observed changes of the vorticity distribution moments leading to the formation of these structures.
A corotating two-roll mill for studies of two-dimensional elongational flows with vorticity
Authors: Reyes Marco A. H. , Geffroy Enrique
Published: July 2002

A closed-form analytical solution for the Stokes flow generated by a corotating two-roll mill is used to generate two-dimensional elongational flows with well-defined amounts of vorticity. These flow devices can generate conditions found between simple shear and purely elongational flows, and are among the few systems that can take into account the presence of nearby boundaries. In the laboratory, these devices have been used for studies of the microstructural dynamics of complex fluids such as polymeric solutions, colloids, bubbles and drops, etc., and allow pointwise optical studies of the flow parameters. Hence, a comparison is presented of available experimental and numerical results for the stagnation point versus the analytical solution presented here. Based upon this solution, a new design for the two-roll mill is presented for the purpose of minimizing the disadvantages of current mills.
Photovoltaic heterostructure devices made of sequentially adsorbed poly(phenylene vinylene) and functionalized C60
Authors: Mattoussi H. , Rubner M. F. , Zhou F. , Kumar J. , Tripathy S. K. , Chiang L. Y.
Published: July 2002

We report on the preparation and characterization of rectifying photovoltaic heterostructure devices made of poly(phenylene vinylene), PPV, and C60. The heterojunctions were built from solution using the technique of layer-by-layer sequential adsorption. This technique permits one to control the heterostructure at the molecular scale. Upon illumination with a laser beam, the devices showed large photoresponses (current and voltage) that resulted from a photoinduced electron transfer between the PPV (donor layer) and the C60 (acceptor layer). The photocurrent was found to increase with the laser power and with the photon energy of the incident radiation. Also, a constant high photovoltage response of ∼700–800 mV was measured. Analysis of the time dependence of the photocurrent rise and decay, when the device was illuminated with a modulated square wave signal (chopped laser beam), permitted us to draw an analogy between the present heterojunction and a circuit made of a capacitor and a resistance in series.
Response to “Comment on ‘Statistical theory of time-dependent diffusion-controlled reactions in fluids and solids’ ” [J. Chem. Phys. 113, 2932 (2000)]
Authors: Felderhof B. U. , Jones R. B.
Published: July 2002

The authors of the Comment consider a limiting case for which the theory is not intended. The theory does not take account of screening effects. It is expected to apply at high volume fractions, between approximately 10% and 40%.
Obtaining frequency markers of variable separation with a spherical mirror Fabry-Perot interferometer
Authors: Budker Dmitry , Rochester Simon M. , Yashchuk Valeriy V.
Published: July 2002

A compact (L≈5 cm) spherical Fabry-Pérot interferometer with adjustable mirror spacing is used to produce interference fringes with frequency separation (c/2L)/N, N≲20. These fringes are used as frequency markers in a Doppler-free saturation spectroscopy experiment. The conditions for observation of the fringes are discussed in terms of the eigenmodes of the cavity with high transverse indices.
A highly sensitive electron spectrometer for crossed-beam collisional ionization: A retarding-type magnetic bottle analyzer and its application to collision-energy resolved Penning ionization electron spectroscopy
Authors: Yamakita Yoshihiro , Tanaka Hideyasu , Maruyama Ryo , Yamakado Hideo , Misaizu Fuminori , Ohno Koichi
Published: July 2002

A highly sensitive electron energy analyzer which utilizes a “magnetic bottle” combined with a retarding electrostatic field has been developed for Penning ionization electron spectroscopy. A beam of metastable rare-gas atoms is crossed with a continuous supersonic sample beam in the source region of the analyzer. The emitted electrons are collected by an inhomogeneous magnetic field (the magnetic bottle effect) with a high efficiency of nearly 4π solid angle, which is more than 103 times higher than that of a conventional hemispherical analyzer. The kinetic energy of electrons is analyzed by scanning the retarding field in a flight tube of the analyzer in the presence of a weak magnetic field. The velocity of the metastable atoms can also be resolved by a time-of-flight method in the present instrument. Examples of Penning ionization electron energy spectra as a function of collision energy are presented for Ar and N2 with metastable He*(2 3S) atoms. This instrument has opened the possibility for extensive studies of Penning ionization electron spectroscopy for low-density species, such as clusters, ions, electronically excited species, unstable or transient species, and large molecules with low volatility.
Structure and magnetic properties of Gd3Fe28.2W0.8
Authors: Liu Q. L. , Rao G. H. , Liang J. K. , Shen B. G.
Published: July 2002

The ternary intermetallic compound Gd3Fe28.2W0.8 with a Nd3Fe29−xTix-type structure was synthesized with W as a stabilizing element, and its crystal structure and magnetic properties were investigated by means of x-ray powder diffraction and magnetic measurements. The space group of Gd3Fe28.2W0.8 is A2/m with lattice parameters a=10.5812 Å, b=8.5338 Å, c=9.6986 Å, and β=96.86°. Rietveld refinement of the crystal structure reveals that W atoms occupy exclusively three crystallographic positions (4g, 4i3, 4i4) with only one neighboring Gd atom. The preferential occupation of W atoms can be understood in terms of the site volume and enthalpy of mixing. The compound exhibits a planar magnetocrystalline anisotropy perpendicular to the [102] direction. The Curie temperature Tc, saturation moment Ms, and anisotropy field HA are 502 K, 38.6 μB/f.u., and 7.5 T, respectively.
X-ray standing wave study of MnTe fractional monolayers inserted in CdTe (001)
Authors: Hartmann J. M. , Martrou D. , Boulliard J. C. , Capelle B. , Cibert J.
Published: July 2002

The x-ray standing wave method is used in correlation with reflection high-energy electron diffraction and scanning tunneling microscopy to investigate the crystallographic features of MnTe monolayers inserted in CdTe (001). Either conventional molecular beam epitaxy or atomic layer epitaxy were employed for the formation of the CdTe starting surface, the deposition of the MnTe fractional monolayer, and its encapsulation by CdTe. Significant differences concerning the ratio of Mn atoms involved in MnTe clusters to those incorporated as part of a CdMnTe alloy are observed between the samples. Those differences are due to differing CdTe starting surface roughness.
Hydration structure and stability of Met-enkephalin studied by a three-dimensional reference interaction site model with a repulsive bridge correction and a thermodynamic perturbation method
Authors: Kovalenko Andriy , Hirata Fumio , Kinoshita Masahiro
Published: July 2002

We study the hydration structure and free energy of several conformations of Met-enkephalin in ambient water by employing the one-dimensional (1D) as well as three-dimensional (3D) reference interaction site model (RISM) integral equation theories, complemented by the hypernetted chain (HNC) closure with the repulsive bridge correction (RBC). The RBC contribution to the excess chemical potential of solvation is calculated by means of the thermodynamic perturbation theory (TPT), which crucially reduces computational burden and thus is especially important for a hybrid algorithm of the RISM with molecular simulation. The 3D-RISM/HNC+RBC-TPT approach provides improved prediction of the solvation thermodynamics and gives a detailed description of the solvation structure of a biomolecule. The results obtained are discussed and compared to those following from the 1D-RISM/HNC theory. The latter yields physically reasonable results for the conformational stability of biomolecules in solution, which is further improved by adding the 1D-RBC. The modified, 1D-RISM/HNC+RBC-TPT integral equation theory combined with the simulated annealing or generalized-ensemble Monte Carlo simulation methods is capable of reliable prediction of conformations of biomolecules in solution with due account for the solvent effect at the microscopic level.
Digital data readback for a probe storage device
Authors: Lapshin Rostislav V.
Published: July 2002

An experimentally proved method is described for data readback from an information track using separate atoms on a crystal surface as memory elements. The key idea consists of local scanning and recognition of memory elements on the carrier surface followed by attaching the device probe to them so as to keep the probe position over the track.
Anisotropic tuning behavior in epitaxial Ba0.5Sr0.5TiO3 thin films
Authors: Hyun S. , Lee J. H. , Kim S. S. , Char K. , Park S. J. , Sok J. , Lee E. H.
Published: July 2002

The tuning properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films were investigated by a scanning microwave microscope (SMM) and an LCR meter. Although the BST films on LaAlO3 and MgO substrates showed similar tuning behavior when measured by the LCR meter at 1 MHz, remarkably different tuning properties were observed in the planar capacitors measured by SMM. The BST films on LaAlO3 substrates were hardly tuned when measured by SMM, while the BST films on MgO showed significant tuning behavior between the electrodes. We attribute these different tuning properties to the anisotropic tuning caused by the strain in BST films. This will enable the design of much improved tunable devices while minimizing the loss associated with the dielectric.
Multi-aperture gasket for experiments at high pressure in a diamond-anvil cell
Authors: Zhao J. , Hearne G. , Maaza M. , Nieuwoudt M. K. , Comins J. D.
Published: July 2002

It is possible to study multiple samples which are concurrently at high pressure in a diamond-anvil cell, being convenient for investigations in a single compression or decompression sequence of pressures, if more than one sample cavity is drilled in the gasket. This has been demonstrated in “routine” sub-megabar luminescence experiments to 30–40 GPa and may potentially be extended to the megabar regime under a variety of P-T conditions if smaller sample cavities are utilized. The idea of multiple samples concurrently under high pressure in cavities of the same gasket may find fruitful application in experiments involving, micro-optical (Raman or infrared) spectroscopy, synchrotron x-ray diffraction, and laser-heating investigations, where the probe radiation may be confined to sample regions of less than 50 μm diameter.