bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and optical properties of vertically aligned InP quantum dots
Authors: Zundel M. K. , Specht P. , Eberl K. , Jin-Phillipp N. Y. , Phillipp F.
Published: July 2002

Stacked layers of self-assembled InP quantum dots embedded in Ga0.52In0.48P have been prepared by solid source molecular beam epitaxy. Thereby the distance between the dot layers has been varied from 2 to 16 nm. Cross sectional transmission electron microscopy shows that the InP dots are aligned in the growth direction [100]. As the distance between the dot layers is reduced, each dot of the first dot layer is reproduced in the upper layers, and this leads to an improvement of the dot size homogeneity of the stacked InP dot system. This is confirmed by photoluminescence (PL) measurements, which demonstrate a very narrow linewidth of 26 meV for a triple layer with 2 nm separation between the dot layers in comparison with a linewidth of 41 meV for a single layer sample. At the same time, the PL peak of the dots is shifted by 72 meV to lower energies which is ascribed to a reduced strain and strong electrical coupling between the densely stacked InP dots.
Organic light-emitting diodes using 3- or 5-hydroxyflavone–metal complexes
Authors: Hamada Yuji , Sano Takeshi , Fujii Hiroyuki , Nishio Yoshitaka , Takahashi Hisakazu , Shibata Kenichi
Published: July 2002

3- or 5-hydroxyflavone-metal complexes were synthesized and applied to organic light-emitting diodes (OLEDs) as an emitting material and an electron transport material. When bis(5-hydroxyflavonato)beryllium [Be(5Fla)2] was used as an emitting material, the OLED showed a luminance of 1900 cd/m2 with yellow. On the other hand, when used as an electron transport material in an organic three-layer cell, a high luminance of more than 10 000 cd/m2 was obtained. Furthermore, the turn-on voltage of the OLED could be lowered to 2.4 V and its lifetime was more than 1300 h. Be(5Fla)2 was found to have a good electron transporting capability and good stability.
Ion implantation as a tool for controlling the morphology of porous gallium phosphide
Authors: Tiginyanu I. M. , Schwab C. , Grob J.-J. , Prévot B. , Hartnagel H. L. , Vogt A. , Irmer G. , Monecke J.
Published: July 2002

We investigate the morphology of porous layers obtained by electrochemical anodization of (100)-oriented n-type GaP substrates before and after a preliminary 5-MeV Kr+ implantation. Apart from favoring the observation of a surface-related phonon in the frequency gap between the bulk optical phonons, ion implantation appears to be an effective means of controlling the morphology of porous GaP, irrespective of initial substrate material features.
Sensitivity-enhanced reflection Z-scan by oblique incidence of a polarized beam
Authors: Martinelli M. , Bian S. , Leite J. R. , Horowicz R. J.
Published: July 2002

We demonstrate a new experimental method which allows the measurement of the nonlinear optic index in absorptive media with great sensitivity. In this technique the reflection from a polarized Gaussian laser beam close to the Brewster angle is measured. A sensitivity enhancement factor of 30 with respect to other techniques is observed for an optical crystal, and higher values are possible to be obtained.
Excitation density dependence of photoluminescence in GaN:Mg
Authors: Oh Eunsoon , Park Hyeongsoo , Park Yongjo
Published: July 2002

Continuous redshift of an emission from 3.23 to 2.85 eV with decreasing excitation density is observed in the photoluminescence spectra of highly doped GaN:Mg. It has been explained by the formation of minibands originating from the overlap of deep levels and shallow acceptor levels. For nominally undoped samples the intensity ratio of a donor-bound exciton line and a donor–acceptor pair line changes dramatically with excitation density due to the limited number of acceptors.
Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells
Authors: Wu X. H. , Elsass C. R. , Abare A. , Mack M. , Keller S. , Petroff P. M. , DenBaars S. P. , Speck J. S. , Rosner S. J.
Published: July 2002

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {101̄1} planes) and an open hexagonal inverted pyramid which is defined by the six {101̄1} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101̄1} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the following characteristics: (i) its intensity is correlated with the side-wall quantum wells; (ii) the temperature independence of the full width at half maximum strongly supports a localized exciton recombination process.
Thermal characteristics of silicon nitride membranes at sub-Kelvin temperatures
Authors: Leivo M. M. , Pekola J. P.
Published: July 2002

We have performed calorimetric measurements on 200 nm thin silicon nitride membranes at temperatures from 0.07 to 1 K. Besides full windows, membranes cut into a thermally isolating suspended bridge geometry were investigated. Based on dc and ac measurements employing normal-metal/insulator/superconductor (NIS) tunnel junctions both as a thermometer and a heater, we report on heat transport and thermal relaxation in silicon nitride films. The bridge structure improves thermal isolation and, consequently, energy sensitivity by two orders of magnitude over those of the full membrane with the same size, and makes such a structure very attractive for bolometric and microrefrigeration applications.
Phase separation in InGaN/GaN multiple quantum wells
Authors: McCluskey M. D. , Romano L. T. , Krusor B. S. , Bour D. P. , Johnson N. M. , Brennan S.
Published: July 2002

Evidence is presented for phase separation in In0.27Ga0.73N/GaN multiple quantum wells. After annealing for 40 h at a temperature of 950 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of In-rich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase. A diffraction peak corresponding to an In-rich phase is also present in the annealed material. Nanoscale In-rich InGaN precipitates are observed by transmission electron microscopy and energy dispersive x-ray chemical analysis.
The role of nonstoichiometry in 180° domain switching of LiNbO3 crystals
Authors: Gopalan Venkatraman , Mitchell Terence E. , Furukawa Y. , Kitamura K.
Published: July 2002

We show here conclusively that the internal field originates from nonstoichiometric point defects in LiNbO3 crystals. The switching fields required for 180° domain reversal in congruent crystals [C=Li2O/(Li2O+Nb2O5)=0.484] are ∼4–5 times larger than the switching fields for nearstoichiometric crystals (C=0.498). An internal field of ∼2.5 kV/mm observed in congruent crystals disappears in stoichiometric crystals. The concentration of hydrogen incorporated during crystal growth has no effect on the switching or internal fields. The measured spontaneous polarization, Ps=80±5 μC/cm2 is relatively insensitive to the crystal nonstoichiometry and the hydrogen content.
Strain compensated In1−xGaxAs(x<0.47) quantum well photodiodes for extended wavelength operation
Authors: Dries J. Christopher , Gokhale Milind R. , Thomson K. John , Forrest Stephen R. , Hull Robert
Published: July 2002

The use of highly strained (−2.0%) In0.83Ga0.17As quantum wells for the detection of light to a wavelength of λ∼2.0 μm is reported. Crystal quality for a 50 period multiple quantum well (MQW) detector grown on InP substrates is maintained through strain compensation using tensile strained In0.83Ga0.17P barriers. Transmission electron microscopy and double crystal x-ray diffraction reveal smooth interfaces and no observable defects for In0.83Ga0.17As layers with widths less than 80 Å. Single-pass quantum efficiencies of 30% have been achieved at λ=1.95 μm, using a 75 μm diam MQW, strain-compensated, top illuminated, low dark current (∼250 pA at 20 V) p-i-n detector. The theoretical cutoff wavelength limit for diodes fabricated using this technique is calculated to be λ=2.15 μm.
Visualization of 10 μm surface acoustic waves by stroboscopic x-ray topography
Authors: Zolotoyabko E. , Shilo D. , Sauer W. , Pernot E. , Baruchel J.
Published: July 2002

X-ray topographs under acoustic wave excitation were taken from LiNbO3-based surface acoustic wave devices with He-implanted waveguide layers, using monochromatized synchrotron radiation from the European Synchrotron Radiation Facility (ESRF), Grenoble. Measurements were performed in the stroboscopic mode, i.e., by synchronizing the electron bunch frequency with the resonant frequency of the acoustic wave excitation in the 300 MHz range. These x-ray diffraction images showed plane acoustic wave propagation through the LiNbO3 crystals as well as a weak wavefront distortion due to the scattering on dislocation deformation fields. In some images secondary spherical waves were observed as a result of the strong acoustic wave interaction with the submicron size density perturbations.
Surface photovoltage spectroscopy of a GaAs/AlGaAs heterojunction bipolar transistor
Authors: Mishori B. , Leibovitch M. , Shapira Yoram , Pollak Fred.-H. , Streit Dwight C. , Wojtowicz Michael
Published: July 2002

The electronic properties of a GaAs/AlGaAs heterojunction bipolar transistor (HBT) structure have been studied by surface photovoltage spectroscopy. The p-base band-gap narrowing has been determined and confirmed by numerical simulation. Based on the shape of the surface photovoltage spectrum, it is possible to monitor the doping level and evaluate the minority-carrier mobility. This work demonstrates the power of the technique as a precision tool for HBT quality control.
Nanoscale surface clustering on CdTe epilayers
Authors: Marsal L. , Mariette H. , Samson Y. , Rouvière J. L. , Picard E.
Published: July 2002

We have studied the surface morphology of two dimensional CdTe films grown by atomic layer epitaxy on CdZnTe buffers. Atomic force microscopy observations in air show a strong evolution of the surface after the growth by forming clusters of around 20 nm in diameter and 3 nm in height. The size increase of these clusters as time proceeds is directly related to the formation of tellurium and cadmium oxides on the surface as revealed by x-ray photoelectron spectroscopy data. This nanoscale clustering then occurs ex situ, due to exposure to atmosphere which induces an oxidation of the CdTe epilayers.
Characteristics of InGaAs quantum dot infrared photodetectors
Authors: Xu S. J. , Chua S. J. , Mei T. , Wang X. C. , Zhang X. H. , Karunasiri G. , Fan W. J. , Wang C. H. , Jiang J. , Wang S. , Xie X. G.
Published: July 2002

A quantum dot infrared photodetector (QDIP) consisting of self-assembled InGaAs quantum dots has been demonstrated. Responsivity of 3.25 mA/W at 9.2 μm was obtained for nonpolarized incident light on the detector with a 45° angle facet at 60 K. The QDIPs exhibit some unique electro-optic characteristics such as a strong negative differential photoconductance effect and blueshift of the response peak wavelength.
Selective H atom sensors using ultrathin Ag/Si Schottky diodes
Authors: Nienhaus Hermann , Bergh Howard S. , Gergen Brian , Majumdar Arun , Weinberg W. Henry , McFarland Eric W.
Published: July 2002

Schottky diodes with ultrathin silver films on n-Si(111) are used for selective detection of atomic hydrogen. The exothermic adsorption of H atoms on the Ag surface creates hot electrons which may travel ballistically through the metal film and traverse the Schottky barrier. The chemically induced current is measurable (≈0.001 electrons/H atom) as a chemicurrent. After saturation of the adsorption sites, this chemicurrent achieves a steady-state value due to a balance of removal of adsorbed hydrogen and readsorption. The detection limit of the sensors is approximately 1010 H atoms cm−2 s−1. The detectors are completely insensitive to H2 molecules.
Low-resistance Ta/Ti Ohmic contacts for p-type GaN
Authors: Suzuki Masaaki , Kawakami T. , Arai T. , Kobayashi S. , Koide Yasuo , Uemura T. , Shibata N. , Murakami Masanori
Published: July 2002

Although extensive efforts have continued to develop Ohmic contacts for p-type GaN, which have specific contact resistance (ρc) lower than that (ρc∼10−2 Ω cm2) of conventional Ni/Au contacts, to the best of our knowledge no breakthrough has been reported in open literature. We demonstrated that bilayered Ta/Ti contacts have a ρc value of around 3×10−5 Ω cm2 for p-type GaN with a hole concentration of 7×1017 cm−3. This contact has resistance low enough to manufacture blue laser diodes, but deterioration of the ρc value during room-temperature storage is the key issue.
Coupling superconducting-ferromagnetic point contacts by Andreev reflections
Authors: Deutscher Guy , Feinberg Denis
Published: July 2002

The properties of a device made of two point contacts between normal (N) or ferromagnetic (F) tips, and a superconductor (S), are discussed as a function of the spin polarization and the distance L between the contacts. When L is smaller than the superconductor coherence length ξ, nonlocal Andreev reflections occur: for opposite spin polarizations of the contacts, “mixed” Cooper pairs made of electrons coming one from each tip can be injected into the superconductor. This leads to unusual properties, for instance, the parallel resistance of two S/F contacts goes from infinity for full and equal polarizations, to a finite Andreev value for opposite ones.
Self-assembled GaInNAs quantum dots for 1.3 and 1.55 μm emission on GaAs
Authors: Sopanen M. , Xin H. P. , Tu C. W.
Published: July 2002

Ga 0.3 In 0.7 N x As 1−x  (x⩽0.04) and InNxAs1−x (x⩽0.02) self-assembled quantum dots are fabricated on GaAs by gas-source molecular beam epitaxy. The effect of growth temperature on island density, island size, and optical properties is studied in detail. From a single layer of Ga0.3In0.7N0.04As0.96 dots embedded in GaAs, peak photoluminescence wavelength of up to 1.52 μm is detected at room temperature. Thus, the fabrication of 1.3 and 1.55 μm GaInNAs quantum dot lasers on GaAs becomes feasible.
A model for fatigue in ferroelectric perovskite thin films
Authors: Dawber Matthew , Scott J. F.
Published: July 2002

An analytic expression in closed form is given for “fatigue,” the dependence of P(N), the switched charge per unit area P versus switching event number N, in ABO3 perovskite structure ferroelectric thin films is given. The analysis is based upon Arlt’s model for fatigue in bulk perovskites, which involves preferential electromigration of oxygen vacancies to sites parallel to the electrode–ferroelectric interface plane, together with some arguments by Brennan on the effectiveness of such defect planes in pinning domain walls. The model is applied to PZT/Pt with no adjustable parameters and yields in complete agreement with experimental data the dependence of P(N) at different frequencies, different voltages, and different temperatures. Notably, unlike other proposals in the literature, the model does not involve any charge injection from the electrodes.
InAs(PSb)-based “W” quantum well laser diodes emitting near 3.3 μm
Authors: Joullié A. , Skouri E. M. , Garcia M. , Grech P. , Wilk A. , Christol P. , Baranov A. N. , Behres A. , Kluth J. , Stein A. , Heime K. , Heuken M. , Rushworth S. , Hulicius E. , Simecek T.
Published: July 2002

Mid-infrared laser diodes with an active region consisting of five “W” InAsSb/InAsP/InAsSb/InAsPSb quantum wells and broad InAsPSb waveguide were fabricated by metalorganic vapor phase epitaxy on InAs substrates. Laser emission was demonstrated at 3.3 μm up to 135 K from asymmetrical structures having n-type InAsPSb and p-type InPSb cladding layers. The devices operated in pulsed regime at 3.3 μm, with a lowest threshold current density of 120 A/cm2 at 90 K, and an output power efficiency of 31 mW/facet/A. The characteristic temperature was 35 K.
Impact of face-to-face annealing in preparation of sol-gel-derived SrBi2Ta2O9 thin films
Authors: Aizawa Koji , Tokumitsu Eisuke , Okamoto Kojiro , Ishiwara Hiroshi
Published: July 2002

An annealing method for preparing SrBi2Ta2O9 (SBT) thin films, a face-to-face annealing method, is proposed, and its effectiveness is demonstrated. In this method, an SBT film deposited on a substrate using a sol-gel technique was directly put with the film side down on the other SBT film during crystallization process. The SBT films were crystallized at 750 °C in an oxygen flow and successively annealed at 400 °C in a 6.2 wt % ozone flow. It was found that remanent polarization value (2Pr) in a 175-nm-thick SBT film was as large as 23 μC/cm2 when the face-to-face annealing method was conducted. It was also found from time-dependent leakage current measurement that the leakage current density of the films was lower than 1×10−9 A/cm2 at an electric field of 65 kV/cm.
Investigation of the cleaved surface of a p–i–n laser using Kelvin probe force microscopy and two-dimensional physical simulations
Authors: Robin F. , Jacobs H. , Homan O. , Stemmer A. , Bächtold W.
Published: July 2002

We have investigated the cross-sectional electric field and potential distribution of a cleaved n+-InP/InGaAsP/p+-InP p–i–n laser diode using Kelvin probe force microscopy (KFM) with a lateral resolution reaching 50 nm. The powerful characterization capabilities of KFM were compared with two-dimensional (2D) physics-based simulations. The agreement between simulations and KFM measurements regarding the main features of the electric field and potential is very good. However, the KFM yields a voltage drop between n- and p-doped InP regions which is 0.4 times the one simulated. This discrepancy is explained in terms of surface traps due to the exposure of the sample to the air and in terms of incomplete ionization. This hypothesis is confirmed by the 2D simulations.
Softening and mode crossing of the lowest-frequency A1(transverse-optical) phonon in single-crystal PbTiO3
Authors: Cho Seong M. , Jang Hyun M.
Published: July 2002

The polarized Raman spectra of high-quality single-crystal PbTiO3 were measured in the temperature range between 300 and 700 K. The soft-mode behavior of the A1(1TO) (transverse- optical) phonon was observed directly. Above 400 K, however, an intensity transfer between the subpeaks of the A1(1TO) mode was observed, and the line shape of the A1(1TO) mode strongly deviated from the theoretical prediction based on the harmonic approximation in the interatomic potential. In addition, the mode crossing between the A1(1TO) phonon and the E(1LO) (longitudinal optical) phonon was observed. The observed mode crossing behavior clearly supports the assignment of the E(1LO) and A1(1TO) modes, which was originally suggested by Foster and co-workers [Phys. Rev. Lett. 71, 1258 (1993)].
Wave front control systems based on modal liquid crystal lenses
Authors: Loktev M. Yu. , Belopukhov V. N. , Vladimirov F. L. , Vdovin G. V. , Love G. D. , Naumov A. F.
Published: July 2002

We describe two electro-optical systems for adaptive focusing of linearly polarized light. The aperture size is 5 mm and the focal length can be varied from 1 to 4 m for wavelengths from 0.663 to 0.85 μm. The first is a commercially available system including a PC compatible control unit and software, and an adaptive liquid crystal lens. The other is an experimental system consisting of a self-contained unit with an autonomous power supply and an adaptive lens. The safe operating limit in the visible region is 10 W/cm2 with a transmission of 70% without antireflection coating. The switching speed of focus variation from 2 to 1 m and from 1 to 2 m is 780 and 860 ms, respectively. The operating principles of the spherical adaptive lenses and their control units are described. Phase aberrations of the lenses were measured by a Zygo phase shifting interferometer, and the results are presented.
Convergence of the multipole expansion for electrostatic potentials of finite topological atoms
Authors: Kosov D. S. , Popelier P. L. A.
Published: July 2002

The exact atomic electrostatic potential (AEP) and atomic multipole moments are calculated using the topological partitioning of the electron density. High rank (l⩽20) spherical tensor multipole moments are used to examine the convergence properties of the multipole expansion. We vary independently the maximum multipole rank, lmax, and the radius of the spherical grid around an atom in a molecule where we measure the discrepancy between the exact AEP and the one obtained via multipole expansion. The root mean square values are between 0.1 and 1.6 kJ/mol for four atoms (C, N, O, S) on a spherical grid with the ρ=0.001 a.u. convergence radius and for lmax=4. Our calculations demonstrate that this fast convergence is due to the decay of the electron density. We show that multipole moments generated by finite atoms are adequate for use in the multipole expansion of the electrostatic potential, contrary to some claims made in the literature. Moreover they can be used to model intermolecular and in principle intramolecular interactions as well.
Anisotropic scaling of hard disk surface structures
Authors: Karabacak T. , Zhao Y.-P. , Liew T. , Wang G.-C. , Lu T.-M.
Published: July 2002

We present a detailed study of the surface roughness of a computer hard disk using atomic force microscopy and light scattering. The power spectrum density analysis showed that the surface possesses an anisotropic scaling behavior and has both random roughness and periodic roughness components. Quantitative anisotropic correlation length (ξ) and roughness exponent (α) for the directions parallel and perpendicular to the grooves are obtained. A novel in-plane (measurement parallel to the sample surface) light scattering technique is shown to be particularly useful for the analysis of the rough surface parallel to the grooves.
Detection of terahertz radiation with low-temperature-grown GaAs-based photoconductive antenna using 1.55 μm probe
Authors: Tani Masahiko , Lee Kwang-Su , Zhang X.-C.
Published: July 2002

THz radiation is detected by a low-temperature-grown GaAs (LT-GaAs) photoconductive antenna probed with a 1.55 μm probe laser. The detection efficiency is found to be approximately 10% of that obtained with a 780 nm probe. From the nonquadratic dependence of photoconductivity on laser intensity, two-step photoabsorption mediated by midgap states in LT-GaAs is suggested, instead of the two-photon absorption, as the primary process for the photoconductivity.
The reaction of BF3 with H2O: Infrared spectrum of BF2OH trapped in solid neon
Authors: Jacox Marilyn E. , Irikura Karl K. , Thompson Warren E.
Published: July 2002

When a Ne:BF3 sample is passed through a long stainless steel deposition line before being frozen onto a cryogenic observation surface maintained at approximately 5 K, the infrared spectrum of the resulting deposit includes prominent absorptions assigned to BF2OH and weaker absorptions contributed by F311B–OH2. Pretreatment of the deposition line with isotopically substituted water leads to the appearance of absorptions of the O18- and D-substituted products. The assignments are supported by the results of ab initio calculations at several different levels and by a least-squares force constant fit to the infrared absorptions of the isotopomers of BF2OH. The thermochemistry of the reactions of BF3 and H2O to form F3B–OH2 and to form BF2OH+HF has been calculated at the G2 level. Although the heats of reaction calculated for 298 K somewhat favor formation of the complex, when the entropy is also considered the two reactions become more competitive.
Characterization of Si nanocrystals grown by annealing SiO2 films with uniform concentrations of implanted Si
Authors: Guha S. , Qadri S. B. , Musket R. G. , Wall M. A. , Shimizu-Iwayama Tsutomu
Published: July 2002

We have performed physical and optical characterization of Si nanocrystals grown by ion implantation of Si+ ions at multiple energies with varying doses into thermally grown SiO2 films. The purpose of multiple implants was to achieve uniform composition of the added Si profile throughout the SiO2 film to produce Si particles with a narrow size distribution upon annealing at 1000 °C in a nitrogen atmosphere. The depth distribution of the composition and sizes of the Si particles in SiO2 films before and after the anneal were determined using Rutherford backscattering (RBS), forward recoil spectroscopy, small-angle x-ray diffraction (SXRD), and high-resolution transmission electron microscopy (HRTEM). From RBS we concluded that the amount of free silicon was reduced by annealing, presumably due to oxidation in the annealing process. The mean cluster sizes of the annealed samples were determined by SXRD. HRTEM was also employed to determine the average size of Si particles. Photoluminescence spectra (PL) from these samples were broad and the peak positions of the PL spectra were blue-shifted with decreasing cluster size. The line shapes of the PL spectra were calculated with a quantum confinement model assuming a log-normal size distribution of Si nanoparticles and (1/D)1.25 dependence of the band gap energy as a function of particle size D. The band gap energy and the average particle size obtained from the calculated line shape spectra agree well with the quantum confinement model.
Parametric amplification in a torsional microresonator
Authors: Carr Dustin W. , Evoy Stephane , Sekaric Lidija , Craighead H. G. , Parpia J. M.
Published: July 2002

We observe parametric amplification in a torsional micron-scale mechanical resonator. An applied voltage is used to make a dynamic change to the torsional spring constant. Oscillating the spring constant at twice the resonant frequency results in a phase dependent amplification of the resonant motion. Our results agree well with the theory of parametric amplification. By taking swept frequency measurements, we observe interesting structure in the resonant response curves.
Holographic Bragg grating generation in photorefractive polymer-dissolved liquid-crystal composites
Authors: Ono Hiroshi , Kawamura Tomomi , Mocam Frias Nazarene , Kitamura Keiko , Kawatsuki Nobuhiro , Norisada Hideki , Yamamoto Tohei
Published: July 2002

A large optical nonlinearity of n2=0.57 cm2/W in photorefractive mesogenic composites was achieved with low applied dc fields (0.30 V/μm) and a fringe spacing of 2.8 μm. The resultant diffraction efficiency from the Bragg gratings, which were written by a less intense laser (frequency-doubled Nd-YAG laser with a 532 nm wavelength), was around 39%. The limiting factors underlying these nonlinearities and their dynamics are also discussed.
Plastic ribbon THz waveguides
Authors: Mendis R. , Grischkowsky D.
Published: July 2002

We report an experimental and theoretical study of single mode, guided wave propagation in plastic ribbons. Using cylindrical THz optics, we obtain excellent quasioptic coupling between freely propagating THz waves and a single TM0 waveguide mode. Dispersive, low-loss propagation is observed within the experimental bandwidth of 0.1–3.5 THz for 2-cm-wide, high-density polyethylene ribbon waveguides with dimensions of 150 μm thick by 10 mm long, and 120 μm thick by 20 mm long. Such ribbons show promise as a flexible THz interconnect with a controllable group velocity dispersion.
Green’s function for steady flow over a small two-dimensional topography
Authors: Hayes M. , O’Brien S. B. G. , Lammers J. H.
Published: July 2002

We consider steady flow of a thin viscous liquid film over a small two-dimensional topography and develop a Green’s function for the linearized problem. The solutions so obtained can be used to analyze the effect of arbitrary small substrate defects on the coating applied to a substrate.
Thermal properties of Nb2SnC
Authors: Barsoum M. W. , El-Raghy T. , Porter W. D. , Wang H. , Ho J. C. , Chakraborty S.
Published: July 2002

In this article we report on the thermal properties of the ternary carbide Nb2SnC. Its heat capacity, cp, is 75 J/mol K at ambient temperatures and increases to ≈115 J/mol K at 1300 K. Analysis of cp measurements in the 4–15 K temperature range yields a Debye temperature of 324 K and a density of states at the Fermi level of 2.66 (eV unit cell)−1. At room temperature, the thermal conductivity is 17 W/m K and it increases to ≈31 W/m K at 1500 K. At temperatures greater than 600 K, roughly 85% of the heat is transported by electronic defects (i.e., electrons and/or holes). The thermal properties of Nb2SnC are quite comparable to those of near-stoichiometric NbCx.
Chirped pulse excitation in condensed phases involving intramolecular modes studied by double-sided Feynman diagrams for fast optical dephasing
Authors: Fainberg B. D. , Narbaev V.
Published: July 2002

The effect of the quantum intramolecular modes on the chirped pulse excitation in condensed phase has been studied. Nonperturbative equations for the populations of molecular electronic states under the action of intense chirped pulses have been obtained using the double-sided Feynman diagrams. We have shown that the application of this technique to systems with fast electronic dephasing enables us to include strong system–bath interactions (non-Markovian relaxation) and to perform the summation of diagrams. We have studied the influence of the chirp rate on the integral population of the excited state n2 after the completion of pulse action. We have shown that the effect of the quantum intramolecular modes strongly depends on the carrier pulse frequency. Incorporating these modes increases n2 when a molecule is excited near the 0→1 transition with respect to the quantum intramolecular vibration. If the molecule is excited near the 0→0 transition with respect to the intramolecular mode, the effect is opposite.
High coercivity in mechanically milled ThMn12-type Nd–Fe–Mo nitrides
Authors: Zhang Xiao-dong , Cheng Ben-pei , Yang Ying-chang
Published: July 2002

Starting from carefully homogenized Nd10Fe90-yMoy (y=12, 10, 7) alloys and by appropriate mechanical milling, the as-milled microstructure consisting of a nanoscale mixture of severely distorted 1:12 phase and substitutional α-Fe-based solid solution was obtained. This kind of as-milled microstructure was thought to have a critical effect on the formation of iron-free nanocrystalline 1:12 phase during subsequent annealing. Upon nitrogenation, the sample of Nd10Fe78Mo12Nx exhibited a record-high coercivity of 13.1 kOe at 293 K. Measurements of initial magnetization curve and a family of demagnetization curves engendered under different maximum applied fields Hm were carried out, and the results revealed the domain-wall pinning at grain boundaries as the coercivity mechanism. A low Mo-content sample of Nd10Fe83Mo7Nx with Hci∼5.8 kOe, Br∼6.8 kG, and (BH)max∼7.0 MG Oe was made by optimizing the preparation conditions.
Enhancement of dielectric and ferroelectric properties by addition of Pt particles to a lead zirconate titanate matrix
Authors: Duan N. , ten Elshof J. E. , Verweij H. , Greuel G. , Dannapple O.
Published: July 2002

Ferroelectric/metal composites have been characterized for their electric field dependence of the dielectric and ferroelectric properties. The composites show an interesting change of dielectric properties with increasing metal concentration. A relatively high dielectric constant, up to four times as high as that of monolithic lead zirconate titanate (PZT), is observed in the investigated composition range. The dielectric properties are clearly electric field dependent. The tunability of the composite, defined as the difference between the dielectric constant with no applied voltage and the dielectric constant with an applied voltage, divided by the dielectric constant with no applied voltage, was found to be ∼17, whilst for the monolithic PZT ceramics it is about 3 only. This offers perspectives for application of these composites in lighting electronics and voltage controllers. Ferroelectric hysteresis measurements on these composites revealed a square-shaped hysteresis loop with remnant polarization comparable with single PZT ceramics, while the coercive field was much lower than that of pure PZT ceramics. This might be interesting for application of these composites as a ferroelectric memory material with decreased switching voltage.
An investigation of the F+H2 reaction based on a full ab initio description of the open-shell character of the F(2P) atom
Authors: Alexander Millard H. , Manolopoulos David E. , Werner Hans-Joachim
Published: July 2002

Expanding on an earlier Communication [M. H. Alexander, H.-J. Werner, and D. E. Manolopoulos, J. Chem. Phys. 109, 5710 (1998)], we present here the full framework for the quantum treatment of reactions of the fluorine atom with molecular hydrogen. This involves four potential energy surfaces (PESs) and two, coordinate-dependent spin–orbit interaction terms, all of which were fitted to the results of ab initio calculations. Quantum scattering calculations, based on a time-independent method formulated in hyperspherical coordinates, were carried out to determine initial and final state-resolved reactive cross sections, for reaction of F in its ground (2P3/2) and excited (2P1/2) spin–orbit state with H2 in j=0 and j=2(pH2) and j=1(oH2). The overall reactivity of the excited state of F, which can occur only through nonadiabatic transitions, is found to be small, at most 25% of the reactivity of the ground spin–orbit state, which is adiabatically allowed. In addition, when compared with results of earlier calculations, based on a single, electronically adiabatic, PES, our calculations show that even fine details of the dynamics of the F+H2 reaction will be well described by calculations on a single PES. The contribution of the excited spin–orbit state can be seen most clearly in the formation of HF products in the v=3 vibrational manifold, which are nearly thermoneutral (or even slightly endoergic) in the reaction of ground-state F atoms. The cross section for the near resonant electronic-rotational process [F*+H2(j=0)→F+H2(j=2)] is found to be large, in confirmation of earlier work.
Evidence of light-emitting amorphous silicon clusters confined in a silicon oxide matrix
Authors: Rinnert H. , Vergnat M. , Burneau A.
Published: July 2002

Amorphous silicon oxide thin films were prepared by the coevaporation technique in ultrahigh vacuum. Different compositions were obtained by changing the evaporation rate of silicon. The samples were then annealed to different temperatures up to 950 °C. The composition and the structure were investigated using energy dispersive x-ray spectroscopy, infrared absorption measurements, and Raman spectroscopy. This study attests the presence of amorphous silicon clusters in a silicon oxide matrix. Optical transmission measurements were performed and interpreted in the field of the composite medium theory. The obtained results are in good agreement with the presented structural model. The photoluminescence in the red-orange domain was studied in relation with the structure. The correlation between the photoluminescence energy and intensity and the structure shows that the light emission originates from the silicon clusters embedded in the silicon oxide matrix. Moreover the dependence of the photoluminescence energy with the silicon volume fraction suggests the origin of the light emission could be due to a quantum confinement effect of carriers in the amorphous silicon clusters.
On the use of graph invariants for efficiently generating hydrogen bond topologies and predicting physical properties of water clusters and ice
Authors: Kuo Jer-Lai , Coe James V. , Singer Sherwin J. , Band Yehuda B. , Ojamäe Lars
Published: July 2002

Water clusters and some phases of ice are characterized by many isomers with similar oxygen positions, but which differ in direction of hydrogen bonds. A relationship between physical properties, like energy or magnitude of the dipole moment, and hydrogen bond arrangements has long been conjectured. The topology of the hydrogen bond network can be summarized by oriented graphs. Since scalar physical properties like the energy are invariant to symmetry operations, graphical invariants are the proper features of the hydrogen bond network which can be used to discover the correlation with physical properties. We demonstrate how graph invariants are generated and illustrate some of their formal properties. It is shown that invariants can be used to change the enumeration of symmetry-distinct hydrogen bond topologies, nominally a task whose computational cost scales like N2, where N is the number of configurations, into an N ln N process. The utility of graph invariants is confirmed by considering two water clusters, the (H2O)6 cage and (H2O)20 dodecahedron, which, respectively, possess 27 and 30 026 symmetry-distinct hydrogen bond topologies associated with roughly the same oxygen atom arrangements. Physical properties of these clusters are successfully fit to a handful of graph invariants. Using a small number of isomers as a training set, the energy of other isomers of the (H2O)20 dodecahedron can even be estimated well enough to locate phase transitions. Some preliminary results for unit cells of ice-Ih are given to illustrate the application of our results to periodic systems.
Radiative recombination and filling effect of surface states in porous InP
Authors: Liu Aimin , Duan Changkui
Published: July 2002

Porous InP samples prepared by wet electrochemical anodization techniques are studied by photoluminescence (PL) spectroscopy at 25 K. The PL of as-grown porous InP shows redshift with respect to that of bulk InP. The PL emission is very sensitive to chemical and heat treatment. For instance, the PL obtained from the AgNO3 treated sample shows a blueshift with respect to that of the bulk InP. Moreover the PL spectra from the as-grown porous sample and the AgNO3 treated sample exhibit opposite shift trend on excitation levels. The PL of the AgNO3 treated sample shifts slightly to lower energies with increasing excitation levels, which is similar to that of the bulk InP, while the PL of the as-grown porous InP shifts obviously to higher energies, which is explained by the filling effect of surface states.
Correlation between morphology and cathodoluminescence in porous GaP
Authors: Stevens-Kalceff M. A. , Tiginyanu I. M. , Langa S. , Föll H. , Hartnagel H. L.
Published: July 2002

Porous layers fabricated by anodic etching of n-GaP substrates in a sulfuric acid solution were studied by electron microscopy and cathodoluminescence (CL) microanalysis. The morphology of porous layers was found to depend strongly upon the anodization conditions. When the etching process starts at the initial surface, “catacomb-like” pores and current-line oriented pores are introduced at low and high anodic current densities, respectively. After the initial development of either kind of pore, further anodization at the current density of about 1 mA/cm2 favors the propagation of pores along 〈111〉 crystallographic directions. The spatial and spectral distribution of CL in bulk and porous samples is presented. A comparative analysis of the secondary electron and panchromatic CL images evidenced a porosity induced increase in the emission efficiency.
Neutron diffraction study on the defect structure of indium–tin–oxide
Authors: González Gabriela B. , Cohen Jerome B. , Hwang Jin-Ha , Mason Thomas O. , Hodges Jason P. , Jorgensen James D.
Published: July 2002

The defect structure of undoped and Sn-doped In2O3 (ITO) materials was studied by preparing powders under different processing environments and performing neutron powder diffraction. The effect of tin doping and oxygen partial pressure was determined. Structural information was obtained by analyzing neutron powder diffraction data using the Rietveld method. The results include positions of the atoms, their thermal displacements, the fractional occupancy of the interstitial oxygen site, and the fractional occupancies of Sn on each of the two nonequivalent cation sites. The tin cations show a strong preference for the b site versus the d site. The measured electrical properties are correlated with the interstitial oxygen populations, which agree with the proposed models for reducible (2SnIn•Oi″)x and nonreducible (2SnIn•3OOOi″)x defect clusters.
Infrared spectroscopy and photochemistry at 266 nm of the ozone dimer trapped in an argon matrix
Authors: Bahou M. , Schriver-Mazzuoli L. , Schriver A.
Published: July 2002

Infrared spectroscopy has been coupled with the matrix isolation technique. First, to identify the ozone dimer isolated in solid argon, and second, to investigate the mechanism of its photodissociation at 11 K using the 266 nm laser line. Identification of dimers, which are trapped in two different sites, was performed through the observation of the ν3 asymmetric stretching modes. By absorption of one photon, dimers are photodissociated with a high photolysis rate when compared with that of ozone monomers, suggesting the absence of cage recombination of the photofragments. Experimental observations suggest that the major pathway after photolysis is the formation of an excited transient species O4*, which fragments into O+O3. A two-step mechanism, involving the subsequent reaction of O(1D) with O3 formed in a first step appears unlikely.
The extended surface forces apparatus. Part I. Fast spectral correlation interferometry
Authors: Heuberger M.
Published: July 2002

This article describes details of the operation and performance of fast spectral correlation interferometry, which allows for considerably improved operation of the surface forces apparatus. Up to 150 interference fringes can be simultaneously tracked over a wide spectral range, leading to considerably more precise and simultaneous extraction of multiple optical quantities, over a greatly extended distance range. When used for surface-separation measurement, a precision of 25 pm is readily achievable over a distance ranging from 0 to >10 μm. Data acquisition rates are also considerably improved, allowing for dynamic measurements. Automated actuation of optical deflectors introduces a multidimensional scanning capability to the optical probe with a lateral resolution of 1 μm. The entire process is computer controlled and features unattended batch processing of complex measurements. This publication illustrates experimental setup, methodology, measurements, and detailed error calculation for a selection of practically relevant situations in the extended surface forces apparatus.
Critical cooling rate and thermal stability of Zr–Ti–Cu–Ni–Be alloys
Authors: Waniuk Theodore A. , Schroers Jan , Johnson William L.
Published: July 2002

The critical cooling rate as well as the thermal stability are measured for a series of alloys in the Zr–Ti–Cu–Ni–Be system. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr70Ti30)55(Ni39Cu61)25Be20 to (Zr85Ti15)55(Ni57Cu43)22.5Be27.5 show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for some compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the composition-dependent glass forming ability and thermal stability is discussed with reference to a chemical decomposition process.
Coupled molecular dynamics/semiempirical simulation of organic solutes in polar liquids. II. Coumarin 153 in methanol and acetonitrile
Authors: Cichos F. , Brown R. , Bopp Ph. A.
Published: July 2002

In this paper we present a coupled molecular dynamics/semiempirical simulation of the solvation of the dye Coumarin 153 (C153) in two solvents, methanol and acetonitrile. In order to account for the solute electronic polarizability we use a semiempirical description to determine the charge distribution of the dye during the simulation. Solute–solvent and solvent–solvent interactions are described by empirical potentials. We examine the structure of the solvation shell, the purely electrostatic part of the solute–solvent interactions, shifts of the absorption and emissions spectra and the solvation dynamics of C153 in both solvents. In contrast with our first study of naphthalene in acetonitrile, the equilibrium simulations show structural changes in the solvation shell when electronic polarizability is included. The inclusion of electronic polarizability also enhances solute–solvent electrostatic interactions. Therefore, an increase of absorption and emission redshifts is observed compared to simulations with a nonpolarizable solute. While the computed absorption shift is in excellent agreement with experimental data, the emission shift calculations fail. Possible reasons for this failure are discussed. The solvation dynamics shows a considerable slowing down compared to the nonpolarizable solute. Nonetheless, linear response is still valid in these systems. Some differences with the continuum model of solvation are pointed out.
Symmetry and site selectivity in molecular chemisorption: Benzene on Ni{111}
Authors: Yamagishi S. , Jenkins S. J. , King D. A.
Published: July 2002

We discuss the results of large-scale density functional calculations for (7×7) benzene adsorption on the ferromagnetic substrate Ni{111}. Adsorption at the bridge site, with C–C bonds parallel to the [2̄11] direction, is found to be energetically preferred over the [1̄10]-oriented hcp site suggested by electron diffraction experiments. We discuss the roles of intermolecular forces and anisotropic molecular vibrations in accounting for this apparent discrepancy. Furthermore, our analysis of charge and spin densities clarifies the relationship between preferred adsorption sites and the symmetry of frontier molecular orbitals.
Coexistence of superconductivity and antiferromagnetism in the La1−xPrxBa2Cu3O7 system
Authors: Luo H. M. , Ding S. Y. , Wu X. S.
Published: July 2002

Structural, magnetic, and calorimetric properties are reported for the La1−xPrxBa2Cu3O7 solid solution. The structure remains orthorhombic with lattice parameter increases from a=0.3872, b=0.3927, and c=1.1709 nm for PrBa2Cu3O7 to a=0.3903, b=0.3944, and c=1.1801 nm for LaBa2Cu3O7. Substitution of Pr for La depresses Tc from 90 to 52 K for La0.5Pr0.5Ba2Cu3O7, which was reported to be a nonsuperconductor. The Pr antiferromagnetic ordering temperature TN decreases monotonically from 18 K for PrBa2Cu3O7 to 6 K for La0.8Pr0.2Ba2Cu3O7. Thus there is a large range of coexistence of superconductivity and antiferromagnetism for 0.2⩽x⩽0.5. The superconductivity and anomalous Pr ordering will be discussed with the R1−xPrxBa2Cu3O7 systems (R=Y, Nd–Lu).