bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer-displacement detection of optically trapped metallic particles based on critical angle method for small force detection
Authors: Higurashi Eiji , Sawada Renshi , Ito Takahiro
Published: July 2002

We have developed a method of measuring the displacement of optically trapped metallic particles (10 μm in diameter) in water with nanometer resolution to detect small forces. Metallic particles were optically trapped in two dimensions by focusing a laser beam below the particle using an objective lens with a numerical aperture of 0.9. Displacement of a trapped metallic particle was detected using the light reflected from the particle based on the critical angle method. The lateral spring constant was estimated from the equipartition theorem to be on the order of 10−6 N/m and found to increase as the incident laser power increased. Consequently, a trapped metallic particle can be used to detect small forces (10−13 N).
The operation and modeling of transmission line transformers using a referral method
Authors: Graneau P. N. , Rossi J. O. , Smith P. W.
Published: July 2002

This article describes the modeling of transmission line transformers using methods which are analogous to the methods used to analyze both the ac frequency and transient response of conventional transformers. Transformers in which the lines used to construct them are wound inductively, in order to suppress parasitic short circuit paths within the transformers, are analyzed. It is shown that by using this technique the resulting inductive isolation of the secondary circuit from the primary substantially reduces pulse droop and pulse distortion. Despite the apparent complexity of these transformers, a method by which circuit models of these transformers can be deduced is given. From these models very simple equivalent circuits can be derived which can then be used to calculate accurately the performance characteristics of the transformers and, in particular, predict the pulse distortion characteristics of these devices. Different winding configurations are also considered and it is shown that, by the use of mutually coupled winding of the transmission lines in the transformer, it is possible to minimize pulse droop. Finally it is shown that the modeling technique can be used, in modified form, to analyze the ac frequency response of this type of transformer.
A HTc superconducting quantum interference device preamplifier stage to detect 3He nuclear precession
Authors: Sassier E. , Monfort Y. , Gunther C. , Robbes D. , Moreau O. , Gilles H.
Published: July 2002

We built a HTc superconducting quantum interference device (SQUID) preamplifier stage to pick up the free precession signal of the 3He nucleus in Earth’s magnetic field at room temperature. The SQUID is coupled to the precession field by a copper-made flux transformer. It is operated without any magnetic shield.
Development of zone plates with a blazed profile for hard x-ray applications
Authors: Yun W. , Lai B. , Krasnoperova A. A. , Di Fabrizio E. , Cai Z. , Cerrina F. , Chen Z. , Gentili M. , Gluskin E.
Published: July 2002

A linear and a circular zone plate with a blazed zone profile (ZPBP) have been fabricated and characterized using synchrotron x rays. The ZPBPs have significantly improved performances in terms of focusing efficiency and the background near the focus compared to those of a zone plate with a square profile, of which the transmission function can be characterized by a binary square wave. In many respects and practical cases, an x-ray ZPBP may be used in a way analogous to an optical lens in the visible light region. In this article, the experimental characterization of the ZPBPs is presented and some special applications are discussed.
A cryogenic microwave scanning near-field probe: Application to study of high-Tc superconductors
Authors: Lann A. F. , Abu-Teir M. , Golosovsky M. , Davidov D. , Djordjevic S. , Bontemps N. , Cohen L. F.
Published: July 2002

We report a vacuum cryogenic (80 K<T<350 K), near-field microwave scanning system based on a 90 GHz transmitting/receiving resonant slit antenna with a capacitive measurement of the probe-sample separation. The probe allows local measurement of resistance as a function of temperature with the spatial resolution of 20-50 μm. The mm-wave probe is integrated with the eddy-current probe which allows global measurement of resistance of conducting and superconducting samples. This integrated probe is used for local study of the superconducting transition in high-Tc superconducting thin films. The sensitivity of our present mm-wave probe is sufficient for probing conductivity in the normal state and in the superconducting state close to Tc however not yet sufficient for probing conductivity variations far below superconducting transition temperature.
Novel method to measure bulk absorption in optically transparent materials
Authors: Guntau M. , Triebel W.
Published: July 2002

A novel method to measure low absorption of pulsed laser radiation by materials with high optical transparency is demonstrated. The absorbed energy generates a temperature gradient and thus a refractive index profile. A probe laser beam is deflected by the refractive index profile and its deflection is a measure of pure volume absorption without interference from surface effects. Numerical calculations were carried out to estimate the sensitivity of the discussed laser induced deflection (LID) arrangement and to optimize the path of the probe beam in relation to the irradiated field within the sample. The experimental results agree sufficiently with numerical calculations by finite element method combined with raytracing procedures. Because the transparency is not measured directly a calibration is necessary. To provide this, a sample with known bulk absorption coefficient and thermal properties was used. In order to compensate probe beam fluctuations in air outside the sample, a double beam arrangement was designed. The LID method is applied to investigate fused silica with high ultraviolet (UV) transparency under KrF excimer laser irradiation (λ=248 nm). A He/Ne laser serves as probe beam source. Using sample dimensions of 20×20×10 mm3 and UV beam cross sections of 5×5 mm2 absorbed power down to 1 mW is detected. At typical irradiation conditions (50 s−1, 300 mJ/cm2), this value corresponds with an absorption coefficient of around 2.5×10−4 cm−1. The limitation of the absorption measurement depends on the input power of the pump laser.
Real-time optical measurements with picosecond resolution during laser induced transformations
Authors: Solis J. , Siegel J. , Afonso C. N.
Published: July 2002

This work describes a novel technique allowing to measure with picosecond resolution the time evolution of the optical properties (reflectivity/transmission) of a material surface exposed to a single laser pulse. The experimental setup is based on the use of a streak camera in combination with a single-mode probe cw laser. The maximum achievable time resolution is about 1 ps for the acquisition of a single event. Results concerning melting of Ge films under ps pulses will be used to illustrate the potential of this measurement technique for resolving single optical transient events in the ps time scale. The advantages of the system with respect to the use of pump-and-probe optical measurements are especially remarkable for the study of events occurring in time windows with full widths from 1 to 10 ns, as it is the case for rapid solidification phenomena induced by ultrashort laser pulses. This has allowed us to investigate in detail, for instance, the occurrence of bulk and surface initiated solidification phenomena in thin amorphous Ge films.
Instrumental noise and detectivity analysis of photopyroelectric destructive thermal-wave interferometry
Authors: Wang Chinhua , Mandelis Andreas
Published: July 2002

A complete noise analysis of a two-beam photopyroelectric (thermal-wave) destructive interferometric sensor instrument is presented and compared to its single-beam, noninterferometric counterpart. The noise analysis is performed using a Green-function formalism applied to experimental observations. The instrumental background noise contribution from the detector and the amplifier is separated from the laser noise and the instrumental noise due to amplification associated with different sensitivity scales. The latter serves as the source of comparison between the two sensor configurations. It is found that the dc laser drift noise and low-frequency fluctuation noise, which are dominant in the single-beam mode, are greatly reduced to the same order of magnitude as the instrumental background noise in the two-beam mode. The system white noise resulting from the incident laser beam and from the sensitivity scale (amplification) of the demodulating lock-in amplifier are also examined in light of the experimental data. It is found that the detectivity D* (the inverse of the noise equivalent power), of the instrument is enhanced by at least 1 order of magnitude in the interferometric mode.
Novel results on thermal diffusivity measurements on anisotropic materials using photothermal methods
Authors: Salazar A. , Sánchez-Lavega A. , Ocáriz A. , Guitonny J. , Pandey J. C. , Fournier D. , Boccara A. C.
Published: July 2002

We have applied two photothermal techniques (infrared radiometry and mirage) and conventional procedures to measure the thermal diffusivity of a highly anisotropic sample of pyrolytic graphite. Unexpected and strongly differing results are obtained with each technique. A theoretical analysis indicates that whereas the mirage technique gives a measure of the elements of the thermal diffusivity tensor, infrared radiometry yields the elements of another thermal magnitude represented by the inverse of this tensor.
Characterization of nanocrystalline diamond films by core-level photoabsorption
Authors: Gruen D. M. , Krauss A. R. , Zuiker C. D. , Csencsits R. , Terminello L. J. , Carlisle J. A. , Jimenez I. , Sutherland D. G. J. , Shuh D. K. , Tong W. , Himpsel F. J.
Published: July 2002

Core-level photoabsorption has been used to determine the sp2 and sp3 bonding content of nanocrystalline diamond thin films grown using C60 or CH4 precursors. The C(1s) absorption spectra show clear bulk diamond excitonic and sp3 features with little evidence of sp2 bonding, while the Raman spectra measured from these same films are ambiguous and indeterminate. This result can be attributed to the local structure (near-neighbor bonding) sensitivity of core-level photoabsorption that is insensitive to domain size, unlike Raman spectroscopy.
CeO2 as insulation layer in high T
c superconducting multilayer and crossover structures
Authors: van Wijck M. A. A. M. , Verhoeven M. A. J. , Reuvekamp E. M. C. M. , Gerritsma G. J. , Blank D. H. A. , Rogalla H.
Published: July 2002

We present a study of the electrical properties of insulating CeO2 layers in combination with superconducting (Y/Dy) Ba2Cu3O7−δ (RBCO) films over ramps and in crossover structures. CeO2 is frequently used as a buffer layer, or template layer for biepitaxial grain boundary junctions, but can also be used as an insulating layer in ramp-type junctions and other multilayer structures. Epitaxial thin films of CeO2 were deposited by pulsed laser ablation using SrTiO3 substrates. We characterized the insulating performance of CeO2 thin films in terms of breakdown field Ebd and the relative dielectric constant εr. For 80 nm thick CeO2 at 77 K we found Ebd=1×106 V/cm, using a 1 nA/100 μm2 breakdown criterion, which gives us a specific resistivity of ρ≥109 Ω cm up to breakdown. From capacitance measurements on planar RBCO/CeO2/RBCO structures we obtained for the dielectric constant: εr≊15. The texture of CeO2 in combination with RBCO on ramped surfaces, simulated by SrTiO3 (STO) (103) substrates having their normal tilted by 18° away from the STO[001] direction, has been studied by x-ray diffraction.
Improved electron-beam patterning of Si with self-assembled monolayers
Authors: Whelan C. S. , Lercel M. J. , Craighead H. G. , Seshadri K. , Allara D. L.
Published: July 2002

A set of processes has been explored that enhances the utility of self-assembled-monolayer electron-beam resists for patterning silicon. A self-assembled monolayer resist of octadecylsiloxane was exposed using a scanning electron microscope with a 20 keV beam energy and dose of 320 μC/cm2. After the patterned monolayer was developed using ultraviolet light and ozone, it served as a wet etch mask for the underlying native oxide. Linewidths of ∼30 nm were etched in silicon using the patterned oxide as a reactive ion etch mask. The maximum etch depth achieved in silicon was 90 nm using this process.
Direct visualization of the oscillation of Au (111) surface atoms
Authors: Hesjedal T. , Chilla E. , Fröhlich H.-J.
Published: July 2002

A high frequency oscillating Au (111) surface was measured with atomic resolution using a modified scanning tunneling microscope. On the atomic scale propagating surface acoustic waves lead to oscillations of atoms on elliptical trajectories, with the axes being determined by the material parameters of the surface. Since those oscillation frequencies are much higher than the scan frequencies the topography contrast is reduced. This basic problem is solved by measuring a stroboscopic snapshot seeing a defined state of oscillation. The atomic resolution of the phase and the amplitude contrast is explained by the superposition of the surface topography and the oscillation trajectory.
Magnetic alignment of mesophase pitch-based carbon fibers
Authors: Matthews M. J. , Dresselhaus M. S. , Dresselhaus G. , Endo M. , Nishimura Y. , Hiraoka T. , Tamaki N.
Published: July 2002

Mesophase pitch-based carbon fibers (MPCFs) have recently been developed for use as high performance anode materials in Li ion secondary batteries, having a microscopic as well as macroscopic structure especially suitable for Li storage. Because of the highly anisotropic diamagnetic moment observed between 50 and 310 K in pristine milled MPCF segments, they can easily be oriented parallel to an applied magnetic field, as observed by scanning electron microscopy. A simple model is proposed to explain both the observed alignment of undoped fibers and the suppression of alignment in B-doped MPCFs for relatively small applied magnetic fields, because of their smaller diamagnetic moment.
Magnetostrictive SmFe2/metal composites
Authors: Pinkerton F. E. , Capehart T. W. , Herbst J. F. , Brewer E. G. , Murphy C. B.
Published: July 2002

We have fabricated novel magnetostrictive composites consisting of SmFe2 embedded in an Fe or Al matrix which feature high magnetostriction and good mechanical strength. Hot pressed composites of 50% SmFe2/50% Fe or Al by volume have magnetostrictive strains parallel to the applied magnetic field, λ∥, between −280 and −440 ppm when consolidated at 610 (Fe) or 540 °C (Al). One unique feature of this work is the production of the magnetostrictive SmFe2 component by melt spinning; ribbon-based composites with Al combine significant magnetic coercivity, Hci=3 kOe, with high magnetostriction (λ∥=−280 ppm). Unlike their brittle SmFe2 parent materials, the composites are easily machinable.
Properties of strained (In, Ga, Al)As lasers with laterally modulated active region
Authors: Ledentsov N. N. , Bimberg D. , Shernyakov Yu. M. , Kochnev V. , Maximov M. V. , Sakharov A. V. , Krestnikov I. L. , Egorov A. Yu. , Zhukov A. E. , Tsatsul’nikov A. F. , Volovik B. V. , Ustinov V. M. , Kop’ev P. S. , Alferov Zh. I. , Kosogov A. O. , Werner P.
Published: July 2002

Transmission electron microscopy studies of low indium composition InxGa1−xAs insertions (x<0.4) in a GaAs matrix deposited by molecular beam epitaxy or by metal-organic chemical vapor deposition reveal nanoscale quasiperiodic compositional and morphological modulations. The luminescence peak wavelength is found to be a strong function of deposition parameters and can be tuned from 1.1 to 1.3 μm for the same x and average thickness of the deposit. Strained (In, Ga, Al)As lasers with such a laterally modulated InxGa1−xAs active region demonstrate significant depolarization of the electroluminescence. For short cavity length, lasing occurs at energies corresponding to transitions between InxGa1−xAs conduction band and light holelike valence band states. Devices lase at low threshold current densities and demonstrate continuous wave operation up to 3 W at room temperature.
Numerical analysis of excimer-laser-induced melting and solidification of thin Si films
Authors: Gupta Vikas V. , Jin Song H. , Im James S.
Published: July 2002

We have developed an efficient two-dimensional numerical model, based on the finite difference method and utilizing the alternate-direction explicit scheme, that can simulate excimer laser melting and solidification of thin Si films on SiO2. The model takes into account important aspects of the process such as undercooling and the temperature-dependent velocity of the solidifying interface, supercooling of liquid Si, and the inert nature of the underlying oxide interface. We demonstrate the unique capability of the model by simulating spatially confined beam-induced localized complete melting of the irradiated portion of the film, and the ensuing lateral solidification, which initiates from the unmelted regions of the film into the completely molten area.
Reversible transition between InGaAs dot structure and InGaAsP flat surface
Authors: Ozasa Kazunari , Aoyagi Yoshinobu , Park Young Ju , Samuelson Lars
Published: July 2002

We have studied the in situ modification of coherently grown InGaAs dots by interaction with phosphorus. By monitoring the intensity of reflection high-energy electron diffraction transmission spot, the in situ phosphorus (precracked PH3) supply on the InGaAs dots was examined at 480 °C. It was found that the phosphorus exposure induces a surface structure change from a dot structure to a flat surface. The change is caused by the replacement of arsenic in the dots by phosphorus, which reduces the strain between the InGaAs(P) dots and the GaAs substrate. By switching AsH3/PH3 beams in situ, a reversible transition of the surface structure between the InGaAs dot structure and the InGaAsP flat surface was observed. A transitional state between the dot structure and the flat surface was metastabilized by tuning the AsH3/PH3 beam ratio. The metastabilized surface was observed ex situ using a high-resolution scanning electron microscope.
Bright blue–green electroluminescence from aromatic polyimides
Authors: Mal’tsev E. I. , Brusentseva M. A. , Kolesnikov V. A. , Berendyaev V. I. , Kotov B. V. , Vannikov A. V.
Published: July 2002

New polyimide type of polymers, aromatic polyimides (APIs) containing sulphur atoms in the backbone, were investigated as electron-hole transporting and light-emitting materials for use in unilayer electroluminescent diodes. The simplicity of synthesis, high thermal stability, organic solvent solubility together with excellent film-forming properties make the APIs potentially of interest for technological applications.
The formation of GaN dots on AlxGa1−xN surfaces using Si in gas-source molecular beam epitaxy
Authors: Shen Xu-Qiang , Tanaka Satoru , Iwai Sohachi , Aoyagi Yoshinobu
Published: July 2002

Nanoscale GaN dots were successfully formed on AlxGa1−xN/6H-SiC(0001) surfaces by gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the AlxGa1−xN surface free energy. Without introducing Si, the GaN growth mode was two dimensional and (1×3) reconstruction was observed. The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the AlxGa1−xN surface. In situ reflection high-energy electron diffraction and atomic force microscopy observations were used to monitor and characterize the growth processes and surface morphology.
Improved low frequency and microwave dielectric response in strontium titanate thin films grown by pulsed laser ablation
Authors: Dalberth M. J. , Stauber R. E. , Price J. C. , Rogers C. T. , Galt David
Published: July 2002

We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured as a function of temperature and electric field bias using a microwave ring resonator and a flip-chip technique. The films having the highest dielectric constant were grown with an oxygen pressure of 600 mTorr and showed large grains in the plane of the film. The small-signal dielectric constant of these films could be changed by a factor of 4 by applying an electric field. The films with the highest dielectric constant showed increased losses, but an improved figure of merit for application to tunable circuits.
Effects of persistent photoconductivity on the characteristic performance of an AlGaN/GaN heterostructure ultraviolet detector
Authors: Li J. Z. , Lin J. Y. , Jiang H. X. , Khan M. A.
Published: July 2002

Photocurrent (PC) transient characteristics of an AlGaN/GaN heterostructure UV detector have been studied. We observed that the PC transients of the AlGaN/GaN heterostructure depended strongly on its initial conditions. Under a pulsed laser excitation, the PC responsivity, dark current level, and decay time constant all increased progressively with the number of successive excitation pulses and eventually saturated at constant values after about 30 pulsed laser exposures. Our results indicate that the observed PC transient characteristics are directly correlated with the effect of persistent photoconductivity in the two-dimensional electron gas region caused by deep level impurities and can have a significant influence on the performance of the UV photodetectors based on AlGaN/GaN heterostructures.
Ion-induced electron emission from diamond
Authors: Kalish R. , Richter V. , Cheifetz E. , Zalman A. , Yona P.
Published: July 2002

The electron emission yields (γ) from conductive (B doped) and undoped chemical vapor deposited diamond caused by light (protons) and heavy (argon) ion impact were measured as functions of ion dose and energy (40–300 keV). Very large values of γ are obtained for the case of B doped diamond for both ions. Whereas the emission due to Ar is found to decay very rapidly towards the low γ value measured for graphite, it remains persistently very high (γ∼25) for the case of protons, indicating possible application of diamond as a sensitive detector for light ions.
Anomalous high carrier mobility in smectic E phase of a 2-phenylnaphthalene derivative
Authors: Funahashi Masahiro , Hanna Jun-ichi
Published: July 2002

Ambipolar carrier transport properties in different phases of a liquid-crystalline photoconductor, 2-(4-octylphenyl)–6-n-butoxynaphthalene, were investigated by a time-of-flight technique. Carrier mobilities were increased stepwise when phase transition took place as the temperature decreased. The smectic E phase in the range of 55–125 °C exhibited nondispersive ambipolar carrier transport with an anomalous high carrier mobility of 1.0×10−2 cm2/V s, while the smectic A phase between 125 and 129 °C had similar carrier transport with a smaller mobility of 4×10−4 cm2/V s. In contrast to the crystalline phase, structural defects in the smectic E phase, which were obvious under microscopic observation with polarized illumination, did not deteriorate the carrier transport properties.
Conducting barriers for vertical integration of ferroelectric capacitors on Si
Authors: Aggarwal S. , Dhote A. M. , Li H. , Ankem S. , Ramesh R.
Published: July 2002

One of the issues impeding the commercialization of nonvolatile ferroelectric memories is the identification and development of a conducting diffusion barrier for the vertical integration of ferroelectric capacitors on Si-based transistors. We report results on the use of Ti–Al alloys as such a diffusion barrier. Our results indicate that it maintains its structural integrity and electrical conductivity after the deposition of a lead-based ferroelectric stack at 650 °C. The electrical properties of the capacitors were measured through the barrier layer by making direct contact to the diffusion barrier. The properties of the capacitors measured by direct contact to the Ti–Al alloy show clearly the absence of an insulating interfacial layer. Transmission electron microscopy and Rutherford backscattering studies confirm that there is no oxidation of the conducting barrier and no reaction between it and the electrode.
An ion-beam technique for measuring surface diffusion coefficients
Authors: DeLuca P. M. , Labanda J. G. C. , Barnett S. A.
Published: July 2002

The effective surface diffusion coefficient of Ga along the [110] direction on vicinal GaAs(001)2×4 surfaces during molecular-beam epitaxy was measured using specular ion current measurements. In this technique, 3 keV Ar ions were impinged upon the surface at a glancing angle (typically 3°), and the specularly scattered ion current was measured. Since specular reflections require a locally flat surface, adatoms cause a decrease in the measured current, allowing an average adatom density measurement. The time dependence of the Ga adatom population was measured during and after Ga deposition. Diffusion coefficients, obtained from the adatom lifetimes using a simple model of diffusion to the step edges, were fit well by the expression D=2×10−9 exp(−0.73 eV/kT)cm2/s from 400 to 600 °C.
Dependence of magnetic properties on grain size of α-Fe in nanocomposite (Nd, Dy)(Fe, Co, Nb, B)5.5/α-Fe magnets
Authors: Sun X. K. , Zhang Jian , Chu Yelong , Liu Wei , Cui Baozhi , Zhang Zhidong
Published: July 2002

For the nanocomposite magnets (Nd0.9Dy0.1)(Fe0.77Co0.12Nb0.03B0.08)5.5/α-Fe, the best magnetic properties were obtained with the average grain size of α-Fe d=15 nm and with the content of α-Fe cm=45 wt % that is well below the structural percolation threshold of the soft magnetic phase. The measured dependence of the coercivity on the grain size of α-Fe tends to indicate that the optimum grain size dop increases with increasing α-Fe content, for cm>30 wt %. If d>dop, contracted hysteresis is formed; if d<dop/2, the coercivity rapidly decrease due to possible coupling between hard magnetic grains situated at two sides of a soft grain.
Recalescence after solidification in Ge films melted by picosecond laser pulses
Authors: Siegel J. , Solis J. , Afonso C. N.
Published: July 2002

Thin amorphous Ge films on glass substrates are irradiated by single picosecond (ps) laser pulses and the induced melting and solidification process is followed by means of real-time reflectivity measurements with ps resolution using a setup based on a streak camera. Due to the excellent time resolution achieved in single exposure, the recalescence process occurring upon solidification can be completely resolved by means of an all-optical technique. The results are consistent with the bulk nucleation of the amorphous phase in the supercooled liquid at an extremely large nucleation rate. The massive release of solidification heat causes the reheating and partial remelting of the film after its complete solidification. The occurrence of recalescence after solidification is responsible for the formation of the crystalline phase finally obtained.
Magnetic imaging of perovskite thin films by ferromagnetic resonance microscopy—La0.7Sr0.3MnO3
Authors: Lofland S. E. , Bhagat S. M. , Shu Q. Q. , Robson M. C. , Ramesh R.
Published: July 2002

We present ferromagnetic resonance scans of epitaxial thin films of La0.7Sr0.3MnO3. In most films, the resonance field indicates the presence of large stresses, the line is wide and both the field and width vary significantly with position. However, in the perpendicular geometry (field normal to the film plane) the best films show narrow lines (<20 Oe) which are relatively independent of position, indicative of magnetic homogeneity. The experiments show that this technique can be used as a contactless, nondestructive diagnostic tool for assessing the magnetic quality of colossal magnetoresistance thin films.
Blue cooperative upconversion in Yb3+-doped multicomponent sol-gel-processed silica glass for three-dimensional display
Authors: Maciel G. S. , Biswas A. , Kapoor R. , Prasad P. N.
Published: July 2002

We propose using blue cooperative upconversion of ytterbium (Yb3+) ions produced by a one-color one-beam pumping scheme for a three-dimensional fluorescence display. The results are presented using a diode laser as a pump source emitting at 973 nm, and the sample used was a multicomponent sol-gel-processed silica glass (1 mol. % of Yb3+). The maximum absolute blue power emitted was ∼1 μW for an excitation power of ∼900 mW.
Infrared photoluminescence and optical absorption characteristics in Ge-doped ZnSe crystals
Authors: Mita Yoh , Akami Masayuki , Maruyama Susumu
Published: July 2002

Intense infrared emission peaking at a wavelength of 1.35 μm has been observed in Ge-doped ZnSe crystals at low temperatures. The emission was particularly strong in the crystals doped with Ge in Se atmosphere and was then rapidly quenched. The emission was accompanied by a characteristic additive absorption band centered around a wavelength of 500 nm, which showed a partial but distinct metastable change upon light irradiation. The excitation spectrum and the preparation conditions of the infrared emission corresponded to those of electron spin resonance signals, owing to isolated, trivalent Ge ions in ZnSe. The nature and the possible applications of the emission are discussed.
Interfacial properties of (111)A GaAs/AlGaAs multiquantum-well structures grown by metalorganic vapor phase epitaxy
Authors: Sanz-Hervás A. , Cho Soohaeng , Majerfeld A. , Kim B. W.
Published: July 2002

We present the heterointerfacial properties of GaAs/AlGaAs multiquantum-well structures grown by atmospheric-pressure metalorganic vapor phase epitaxy on (111)A GaAs substrates at a relatively low growth temperature of 600 °C. For a 25-period GaAs/Al0.27Ga0.73As multiquantum-well structure with a well width of 44 Å, a photoluminescence linewidth of 10.5 meV was observed, which is smaller than previously reported for a similar GaAs/AlGaAs multiquantum-well structure grown by molecular beam epitaxy on (111)A GaAs. As this linewidth corresponds to a combined well-width fluctuation and interfacial roughness throughout the 25 periods of at most ±1 monolayer, it is concluded that epitaxial growth on the (111)A surface can result in high-quality heterointerfaces, particularly at low growth temperatures.
Hall cross size scaling and its application to measurements on nanometer-size iron particle arrays
Authors: Wirth S. , Molnár S. von
Published: July 2002

Hall crosses were used to measure the magnetic properties of arrays of ferromagnetic, nanometer-scale iron particles. The arrays typically consist of several hundred particles of 9–20 nm in diameter. It is shown that the sensitivity of the measurements can be improved by matching the areas of the Hall cross and the array grown onto it by at least an order of magnitude. We predict that single particles of diameter as small as 10 nm can be measured if grown onto Hall crosses of appropriate size.
Colloid-facilitated tracer transport by steady random ground-water flow
Authors: Cvetkovic V.
Published: July 2002

We study the transport of reactive solute in a three-phase system (water–solid matrix-colloids) in natural porous media. Semianalytical (integral) solutions are derived for the first time, which can be used for computing expected concentration, mass flux, or discharge for the dissolved as well as for colloid-bounded tracer. The results are based on a few simplifying assumptions: advection-dominated transport, linear mass transfer reactions, and steady-state colloidal concentration. Derived semianalytical expressions capture the main features of colloid-facilitated transport (the reversible-equilibrium and irreversible-kinetic sorption of tracers on colloids), and are applicable for the general class of linear sorption processes on the porous matrix. Derived solutions account for spatial variability of flow and sorption parameters, which is relevant for field-scale applications. We apply the theoretical results to the transport of neptunium and plutonium, using flow and transport data from the alluvial aquifer near Yucca Mountain, Nevada. Based on the zeroth and first temporal moment, dimensionless indicators are proposed for assessing the potential impact of colloid-facilitated tracer transport in aquifers. Generic sensitivity curves show the importance of tracer-colloid kinetic rates. Even very low irreversible rates (which will generally be difficult to determine in the laboratory) may yield observable effects for sufficiently long transport times. The obtained results can be used for assessing the significance of colloid-facilitated tracer transport under field conditions, as well as for setting further constraints on relevant parameters which need to be estimated in the field.
Effective interactions, structure, and isothermal compressibility of colloidal suspensions
Authors: Dijkstra Marjolein , van Roij René , Evans Robert
Published: July 2002

We study the effective interactions, structure, and the isothermal compressibility of a binary mixture interacting with pairwise additive pair potentials. By integrating out the degrees of freedom of species 2 in the partition sum we first show that a binary mixture can be mapped formally onto an effective one-component system with an effective Hamiltonian consisting of a structure-independent term, which contributes to the total pressure and chemical potential of the system, but does not affect the phase behavior, and a structure-dependent potential of mean force, which contains pair–, triplet–, and higher–body interactions. We then show that the 1-1 structure factor and pair correlation function, and the total isothermal compressibility of the mixture are equal to those of the effective one-component system, provided the mapping is exact. We illustrate and confirm these results by calculating the structure factors and pair correlation functions of the binary Asakura–Oosawa model, which is a simple model for colloid–polymer mixtures, and those of the corresponding one-component system for a size ratio such that the mapping onto an effective one-component Hamiltonian with a strictly pairwise potential of mean force is exact. The distinction between the osmotic and total compressibility of the mixture is emphasized.
Optical properties of multiple layers of self-organized InAs quantum dots emitting at 1.3 μm
Authors: Bloch J. , Shah J. , Pfeiffer L. N. , West K. W. , Chu S. N. G.
Published: July 2002

We report the growth and characterization of multiple layers of self-organized InAs quantum dots emitting near 1.3 μm. We analyze their optical properties as a function of the number of dot layers and investigate how the vertical stack modifies the dot size distribution.
Er
3+
in strong light-confining microcavity
Authors: Lipson M. , Kimerling L. C.
Published: July 2002

Er 3+ ions produce a sharp line emission that has small absorption cross section, long radiative lifetime, and is independent of external perturbations. These optical properties of the Er3+ ions limit the pumping efficiency and tunability of the emission line. In this work we present a study in which Er3+ emission was modified by coupling an ensemble of Er3+ ions to a microcavity resonance. The optical response of an optical cavity that consists of an Er2O3 layer confined between Si/SiO2 distributed Bragg reflectors is studied. When the cavity is in resonance with the optical transition of the Er3+ ions, the resonances in the reflectivity spectra split. The results are explained by a standard semiclassical model of atom–photon coupling.
Self-adaptive quadrature and numerical path integration
Authors: Sabo Dubravko , Doll J. D. , Freeman David L.
Published: July 2002

In the present paper we explore the use of generalized Gaussian quadrature methods in the context of equilibrium path integral applications. Using moment techniques, we devise a compact, self-adaptive approach for use in conjunction with selected classes of interaction potentials. We demonstrate that, when applicable, the resulting approach reduces appreciably the number of potential energy evaluations required in equilibrium path integral simulations.
Photoresponsivity of ultraviolet detectors based on InxAlyGa1−x−yN quaternary alloys
Authors: Oder T. N. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

We describe the growth, fabrication, and characterization of an ultraviolet (UV) photoconductive detector based on InxAlyGa1−x−yN quaternary alloy that is lattice matched to GaN. The detector consisted of 0.1 μm InxAlyGa1−x−yN alloy grown on 0.5–1.0 μm GaN epilayer by metalorganic chemical vapor deposition. With varying indium concentration, the cut-off wavelength of the InxAlyGa1−x−yN detectors could be varied to the deep UV range. The most important and intriguing result is that the responsivity of the InxAlyGa1−x−yN quaternary alloy exceeded that of AlGaN alloy of a comparable cutoff wavelength by a factor of five. This makes the nitride quaternary alloy very important material for solar blind UV detectors applications particularly in the deep UV range where Al rich AlGaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN. The advantages of InxAlyGa1−x−yN quaternary over AlGaN ternary alloys for UV detector applications are also discussed.
Low-resistance Ti/Au ohmic contacts to Al-doped ZnO layers
Authors: Kim Han-Ki , Han Sang-Heon , Seong Tae-Yeon , Choi Won-Kook
Published: July 2002

We report on low-resistance ohmic contacts to the moderately doped n-type ZnO:Al(nd=2×1017 cm−3) obtained using Ti (30 nm)/Au (50 nm) metallization schemes. Annealed Ti/Au contacts exhibit linear current–voltage characteristics, showing that high-quality ohmic contacts are formed. The Ti/Au scheme produces a specific contact resistance of 2×10−4 Ω cm2 when annealed at 300 °C for 1 min in a N2 atmosphere.
Chaining in magnetic colloids in the presence of flow
Authors: Pérez-Castillo I. , Pérez-Madrid A. , Rubı́ J. M. , Bossis G.
Published: July 2002

We discuss the effect of an homogeneous flow in the aggregation process of colloidal magnetic particles at moderate concentration. Situations in which the presence of flow acts in favor of the chaining process: particles assemble into chains larger than the ones emerging in the absence of flow, under the only influence of an externally imposed field, have been analyzed. The results we obtain follow from the analysis of the pair correlation function which, owing to the potential character of the flow we consider, can be interpreted in terms of a Boltzmann-type stationary distribution function. To render the influence of the flow on the resulting structures explicit, we study the particular cases of axisymmetric and planar elongational flows.
Emission behavior of molecularly doped electroluminescent device using liquid-crystalline matrix
Authors: Mochizuki Hiroyuki , Hasui Takahiro , Shiono Takeshi , Ikeda Tomiki , Adachi Chihaya , Taniguchi Yoshio , Shirota Yasuhiko
Published: July 2002

Electroluminescent (EL) behavior has been evaluated in a cell which contained molecular dispersion of 1,4-bis(N,N-diphenylaminophenyl-1,3,4-oxadiazolyl)benzene (OXD) in a low-molecularweight liquid crystal, 4-octyloxy-4′-cyanobiphenyl (8OCB). EL emission was observed above the temperature where the OXD/8OCB mixture showed a liquid-crystalline phase. Furthermore, we fabricated an EL device which consisted of interdigital electrodes coated onto a glass substrate. It was found that the EL emission from the device was highly polarized.
Determination of thermal conductivity of natural silicate melts
Authors: Büttner R. , Zimanowski B. , Lenk C. , Koopmann A. , Lorenz V.
Published: July 2002

Cooling of natural silicate melts and energy transfer to the environment are controlled by the temperature-dependent thermal conductivity. We describe a method that allows a correlation between temperature-dependent strain rate and a bulk temperature during a cooling or heating process under quasisteady state conditions in a Newtonian flow regime. A rotational viscometer measured data for experimental cooling curves of remelted volcanic rock materials. From these data we can calculate the thermal conductivity of an unknown melt after we calibrate the setup with a melt of known thermal conductivity as a by-product of viscosimetry.
Modeling of a GaN-based light-emitting diode for uniform current spreading
Authors: Kim Hyunsoo , Lee Ji-Myon , Huh Chul , Kim Sang-Woo , Kim Dong-Joon , Park Seong-Ju , Hwang Hyunsang
Published: July 2002

The characteristics of the GaN/InGaN multiquantum-well light-emitting diode (LED) have been examined from the view point of uniform current spreading. By means of simple modeling, it was found that the current density and the length of the lateral current path through the transparent layer represent dominant parameters in determining uniform current spreading. In this regard, we studied the effect of current density on the reliability characteristics of the LED. We were able to significantly improve the electrical, optical, and reliability characteristics of the LED in terms of reducing the length of the lateral current path through the transparent layer.
Quantum box energies as a route to the ground state levels of self-assembled InAs pyramidal dots
Authors: Califano M. , Harrison P.
Published: July 2002

A theoretical investigation of the ground state electronic structure of InAs/GaAs quantum confined structures is presented. Energy levels of cuboids and pyramidal shaped dots are calculated using a single-band, constant-confining-potential model that in former applications has proved to reproduce well both the predictions of very sophisticated treatments and several features of many experimental photoluminescence spectra. A connection rule between their ground state energies is found which allows the calculation of the energy levels of pyramidal dots using those of cuboids of suitably chosen dimensions, whose solution requires considerably less computational effort. The purpose of this work is to provide experimentalists with a versatile and simple method to analyze their spectra. As an example, this rule is then applied to successfully reproduce the position of the ground state transition peaks of some experimental photoluminescence spectra of self-assembled pyramidal dots. Furthermore the rule is used to predict the dimensions of a pyramidal dot, starting from the knowledge of the ground state transition energy and an estimate for the aspect ratio Q.
Nuclear magnetic resonance proton-spin relaxation study of the local director fluctuations in the lyotropic liquid crystal: Potassium laurate/1-decanol/water
Authors: Rodrı́guez C. R. , Vaca Chávez F. , Pusiol D. J. , Figueiredo Neto A. M. , Seitter R.-O.
Published: July 2002

Field cycling nuclear magnetic resonance relaxometry was used to study the slow molecular dynamics in the nematics and isotropic phases and polyphasic region of the phase diagram in the thernary mixture: potassium laurate/1-decanol/water. The experiment has been performed over a broad range of Larmor frequencies (2×103−6.6×106 Hz). The first experimental evidence of director fluctuations in a micellar lyotropic nematic liquid crystal, studied by H1 spin-lattice relaxation rate, is reported. The results evidence that in the nematic mesophases, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range (≲105 Hz). By increasing the intermicellar water content, a crossover was found between a quasi-isotropic three-dimensional (3D) director fluctuation behavior and a two-dimensional (2D) pseudo-lamellar undulation fluctuation. In spite of the fact that no menatic phases of this micellar complex fluid are isotropic with respect to light scattering, they present spin-lattice relaxation profiles as driven by local director fluctuations. The polyphasic region, at lower temperatures, shows a quasinematic 3D director fluctuation behavior; meanwhile, the isotropic phase, at higher temperatures, presents 2D pseudo-lamellar undulation fluctuation modes. We conclude that the micelles, in the isotropic phase, preserve the pseudo-lamellar structure, already found in the nematic phases but forming nematic domains with the directors randomly oriented. In order to explain the higher frequency range (≳105 Hz), two relaxation mechanisms are assigned: (i) molecular reorientation by translational diffusion on the micellar surface and (ii) molecular exchange between the micelle and the bulk.
The role of defects at low concentrations in the NH3/TiO2(110) adsorption system: An Auger–photoelectron coincidence spectroscopy study
Authors: Siu W. K. , Bartynski R. A. , Hulbert S. L.
Published: July 2002

We present Auger–photoelectron coincidence spectra from the stoichiometric and reduced (by vacuum annealing) TiO2(110) surfaces after saturation exposure to NH3 at room temperature. The Ti 3p coincidence photoemission spectrum shows substantial emission from residual (or intrinsic) defects on the stoichiometric surface and additional emission from thermally-induced bridging oxygen vacancies on the reduced surface. Changes in the line shape of the coincidence Ti 3p spectrum upon adsorption show that NH3 binds to the five-fold coordinated Ti ions and the rosette structures of both surfaces, and at bridging oxygen vacancies of the reduced surface, but not at intrinsic defects. Quantitative analysis of the spectra supports a structural model where on the stoichiometric surface NH3 adsorbs at every other five-fold coordinated Ti site and at every other Ti site of the rosettes. On the reduced surface, NH3 adsorbs at bridging oxygen vacancies preventing adsorption on neighboring five-fold coordinated Ti sites, suggesting that the saturation coverage of NH3 is smaller for the reduced surface. We observe a correlation between the density of bridging oxygen vacancies and the coincidence intensity from five-fold coordinated Ti ions that gives an internally consistent account of these data.
Squeeze elastic deformation and contact area of a rubber adhesive
Authors: Tordjeman P. , Papon E. , Villenave J-J.
Published: July 2002

New experimental results show that the tack energy of a nonstringing rubber adhesive is proportional to the square function of the contact area. However, this area seems only to be controlled by the contact force and the thickness of the adhesive. A study of how the contact area depends on physical parameters is of great interest for the modeling of the tack properties of pressure-sensitive adhesives (PSAs). With this objective, we give a mechanical analysis of the tack test in the framework of elasticity. This analysis leads to an analytical expression of force versus thickness of material that is in agreement with the experimental data. Based on this mechanical analysis, a model is proposed to take into account the dependence of the contact area with the contact force and the adhesive thickness. This model is based on the idea that, in confined geometry, the adhesive behaves like an elastic solid and the contact area is a function of the elastic squeeze deformation close to the probe surface. The confrontation with experimental results is good and shows the relevance of this approach. Finally, the model underlines the importance of the roughness, the thickness and the Young’s modulus of the adhesive according to the experimental results.