bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultraviolet (340–390 nm), room temperature, photoluminescence from InAs nanocrystals embedded in SiO2 matrix
Authors: Shi Jianzhong , Zhu Kaigui , Zheng Qingqi , Zhang Lide , Ye Ling , Wu Jianxin , Zuo Jian
Published: July 2002

InAs nanocrystals embedded in SiO2 matrix have been fabricated by a radio-frequency magnetron co-sputtering technique without postannealing. X-ray photoelectron spectra and Raman spectroscopy strongly suggest the existence of InAs nanocrystals in the SiO2 matrix. From the optical absorption spectrum, the absorption edge exhibits a very large blueshift of 3.3 eV with respect to that of bulk InAs. The double-peak ultraviolet photoluminescence is observed. Our experimental results show that this double-peak phenomenon originates from the radiative recombination of the quantum-confined electron-heavy hole excitons and electron-split-off hole excitons.
Carrier trapping due to Fe3+/ Fe2+ in epitaxial InP
Authors: Söderström D. , Marcinkevićius S. , Karlsson S. , Lourdudoss S.
Published: July 2002

Time-resolved photoluminescence studies were performed on epitaxially grown InP either doped with iron or with iron and sulphur to gain information on the carrier trapping characteristics of Fe3+/Fe2+. The carrier trapping time was found to be dependent on both the iron and sulphur concentrations, from which the electron and hole capture cross sections of the Fe deep centers are determined as σe=1×10−15 cm2 and σh=6×10−15 cm2, respectively.
Improved performance of oxide-confined vertical-cavity surface-emitting lasers using a tunnel injection active region
Authors: Huffaker D. L. , Deppe D. G.
Published: July 2002

Data are presented on vertical-cavity surface-emitting lasers that use tunneling for injection of electrons into a quantum well active region. The tunneling injection scheme increases the electronic confinement and improves the lasing performance for small active areas. A lasing threshold of 36 μA with a slope efficiency of 23% is achieved for a 2.5 μm device, and a 52% slope efficiency is achieved for a 4 μm device with a 92 μA threshold.
Atomic force and scanning electron microscopic observations of surface and domain structures of BaTiO3 films and bulk crystals
Authors: Tsunekawa S. , Fukuda T. , Ozaki T. , Yoneda Y. , Terauchi H.
Published: July 2002

Tapping-mode atomic force and in situ scanning electron microscopic observations of surface and ferroelectric domain structures are performed, respectively, for BaTiO3 crystalline films grown on {100}SrTiO3 substrates by molecular beam epitaxy and for bulk crystals prepared by the top-seeded solution growth technique. Reconstructionlike structures of very long periods are found for the first time in both thin (20 nm) and thick (1000 nm) films. A dependence of the end size of pole-type 180° domains observed in films and bulk crystals on their thickness agrees with the relation expected theoretically.
Ion-cut silicon-on-insulator fabrication with plasma immersion ion implantation
Authors: Lu Xiang , Iyer S. Sundar Kumar , Hu Chenming , Cheung Nathan W. , Min Jing , Fan Zhineng , Chu Paul K.
Published: July 2002

We report the implementation of ion-cut silicon-on-insulator (SOI) wafer fabrication technique with plasma immersion ion implantation (PIII). The hydrogen implantation rate, which is independent of the wafer size, is considerably higher than that of conventional implantation. The simple PIII reactor setup and its compatibility with cluster-tools offer other ion-cut process optimization opportunities. The feasibility of the PIII ion-cut process is demonstrated by successful fabrication of SOI structures. The hydrogen plasma can be optimized so that only one ion species is dominant. The feasibility of performing ion-cut using helium PIII is also demonstrated.
Resonant cavity enhanced detectors embedded in photonic crystals
Authors: Temelkuran B. , Ozbay E. , Kavanaugh J. P. , Tuttle G. , Ho K. M.
Published: July 2002

We report a resonant cavity enhanced (RCE) detector built around a three-dimensional photonic band gap crystal. The RCE detector was built by placing a monopole antenna within the localized modes of planar and boxlike defect structures. The enhanced electric field around these defect structures were then measured by a microwave detector and a network analyzer. We measured a power enhancement factor of 3450 for planar cavity structures. A Fabry–Perot cavity model was used to understand and predict resonant cavity enhancement in this structure. The tuning bandwidth of the RCE detector extends from 10.5 to 12.8 GHz, which corresponds to the full photonic band gap by the crystal. These RCE detectors have increased sensitivity and efficiency when compared to conventional detectors, and can be used for various applications.
Electromechanical coupling coefficient for surface acoustic waves in proton-exchanged 128°-rotated Y-cut lithium niobate
Authors: Čiplys D. , Rimeika R.
Published: July 2002

The attenuation of surface acoustic waves propagating along crystallographic X axis in proton-exchanged 128° rotated Y-cut lithium niobate during the evaporation of copper film on the crystal surface has been measured as a function of sheet resistivity of the film. From the magnitude of attenuation maxima, the effective electromechanical coupling coefficients for samples with different thicknesses of HxLi1−xNbO3 layers have been evaluated, and from the position of maxima in the resistivity scale, the effective dielectric permittivities of the structure have been estimated. The electromechanical coupling coefficient is found to be reduced practically to zero with increasing thickness of protonated layer and acoustic frequency, and the dielectric permittivity is not significantly affected by the proton exchange.
Low-pressure transformation of graphite to diamond under irradiation
Authors: Lyutovich Y. , Banhart F.
Published: July 2002

It is demonstrated experimentally that an irradiation-induced transformation of planar as well as curved graphite to diamond can be carried out without applying pressure. A specimen containing both graphite and diamond is irradiated at high temperature with an electron beam in an electron microscope. It is observed that the diamond crystals grow on the expense of graphite, although graphite is, at the low pressure prevailing here, the stable phase of carbon.
Observation of net gain in the yellow band of K2 excimer by electron-beam pumping
Authors: Xing Da , Wang Qi , Tan Shi-ci , Ueda Ken-ichi
Published: July 2002

The design of a high-temperature cell appropriate for an electron-beam transverse pump of alkali dimer excimer lasers is described. By using the cell, an amplified spontaneous emission and a maximum net gain coefficient of 3% cm−1 of the K2 yellow band (574 nm) were observed from the e-beam-excited mixture of K/K2 vapor with argon buffer gas. The dissociative recombination of K3+ is discussed as an efficient formation process of the upper state by the electron-beam pumping.
Growth of high mobility GaN by ammonia-molecular beam epitaxy
Authors: Tang H. , Webb James B.
Published: July 2002

The growth of high electron mobility GaN on (0001) sapphire by ammonia molecular beam epitaxy is reported. A buffer layer of AlN <300 Å is initially deposited by magnetron sputter epitaxy, a technique where the aluminum source is a planar dc magnetron sputter cathode and ammonia is used for the nitrogen source. The GaN epilayer is deposited using a conventional K cell for the gallium source and ammonia for the nitrogen source. The layers were doped using silane. Measured room temperature electron mobilities of 560 cm2/V s were observed for layers with carrier densities of ∼1.5×1017 cm−3. The 4 K photoluminescence spectrum showed a very strong donor bound exciton at 3.48 eV with a full width at half maximum (FWHM) of 4.9 meV. X-ray diffraction studies showed the layers to have good crystallinity with FWHM of the ω–2θ and ω scans as low as 13.7 and 210 arcsec, respectively.
GaN avalanche photodiodes grown by hydride vapor-phase epitaxy
Authors: McIntosh K. A. , Molnar R. J. , Mahoney L. J. , Lightfoot A. , Geis M. W. , Molvar K. M. , Melngailis I. , Aggarwal R. L. , Goodhue W. D. , Choi S. S. , Spears D. L. , Verghese S.
Published: July 2002

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.
Room-temperature photoluminescence spectroscopy of self-assembled In0.5Ga0.5As single quantum dots by using highly sensitive near-field scanning optical microscope
Authors: Matsuda K. , Saiki T. , Saito H. , Nishi K.
Published: July 2002

We have studied the optical properties of self-assembled In0.5Ga0.5As single quantum dots (QDs) at room temperature with a near-field scanning optical microscope. Successful detection of a weak photoluminescence (PL) signal from a single QD at room temperature could be achieved by using a double-tapered fiber probe having the advantages of both high collection efficiency and high spatial resolution. Through the precise examination of PL spectra of many QDs, including broadening and saturation behaviors, the homogeneous linewidth of the ground state emission is evaluated as from 9.8 to 14.5 meV.
A quasicontinuous wave, optically pumped violet vertical cavity surface emitting laser
Authors: Song Y.-K. , Zhou H. , Diagne M. , Nurmikko A. V. , Schneider R. P. , Kuo C. P. , Krames M. R. , Kern R. S. , Carter-Coman C. , Kish F. A.
Published: July 2002

We have fabricated and studied a violet (λ=403 nm) vertical cavity surface emitting laser structure, composed of an InGaN multiple quantum well active medium and a pair of high reflectivity dielectric mirrors. Lasing under high repetition rate (76 MHz) pulsed optical pumping has been achieved at temperatures up to T=258 K at average pump power of approximately 30 mW.
Epitaxial relationship in the AlN/Si(001) heterosystem
Authors: Lebedev V. , Jinschek J. , Kaiser U. , Schröter B. , Richter W. , Kräußlich J.
Published: July 2002

The epitaxial growth of crystalline wurtzite AlN thin films on (001) Si substrates by plasma-assisted molecular-beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. Cross-sectional transmission electron microscopy and x-ray diffraction investigations revealed a two-domain film structure (AlN1 and AlN2) with a 30° rotation between neighboring domain orientations and an epitaxial orientation relationship of [0001]AlN∥[001]Si and 〈011̄0〉AlN1∥〈2̄110〉AlN2∥[110]Si. A model for the nucleation and growth mechanism of 2H–AlN layers on Si(001) is proposed.
Nucleation-enhancing treatment for diamond growth over a large-area using magnetoactive microwave plasma chemical vapor deposition
Authors: Jeon Hyeongmin , Wang Chunlei , Hatta Akimitsu , Ito Toshimichi
Published: July 2002

A novel pretreatment enhancing diamond nucleation has been developed for diamond growth over a large area using a magnetoactive microwave plasma chemical vapor deposition method. After the predeposition of carbon films on Si(100) substrates using CH4/CO2/He gas mixtures, diamond films with high nucleation densities were obtained after a subsequent 2 h growth process commonly employed using a CH4/CO2/H2 gas mixture. In the present study, especially, the effect of CO2 concentration in the CH4/CO2/He gas mixture in the pretreatment process has been examined on the carbon film growth. The results show that the diamond nucleation with densities as high as ∼109/cm2 was attained for small CO2 concentrations of 1%–2% during the pretreatment process, while no successful enhancement was enabled for Si substrates pretreated at high CO2 concentrations beyond 3.7%. The structural property of the predeposited carbon films significantly influenced the diamond nucleation. This was evidenced by in situ data of optical emission spectroscopy and quadrupole mass spectroscopy during the pretreatment process, as well as by ex situ data of morphology and composition of the specimens. The volume density of the carbon films obtained after the pretreatment was maximized at a CO2 concentration of 1.9%. The bonding nature of the carbon atoms deduced from the related Raman scattering spectra apparently changed with CO2 concentration. The role of the predeposited carbon films is discussed in relation to etching and agglomeration phenomena during the subsequent diamond growth process.
Selective dip-coating of chemically micropatterned surfaces
Authors: Darhuber Anton A. , Troian Sandra M. , Davis Jeffrey M. , Miller Scott M. , Wagner Sigurd
Published: July 2002

We characterize the selective deposition of liquid microstructures on chemically heterogeneous surfaces by means of dip coating processes. The maximum deposited film thickness depends critically on the speed of withdrawal as well as the pattern size, geometry, and angular orientation. For vertically oriented hydrophilic strips, we derive a hydrodynamic scaling relation for the deposited film thickness which agrees very well with interferometric measurements of dip-coated liquid lines. Due to the lateral confinement of the liquid, our scaling relation differs considerably from the classic Landau–Levich formula for chemically homogeneous surfaces. Dip coating is a simple method for creating large area arrays of liquid microstructures for applications involving chemical analysis and synthesis, biochemical assays, or wet printing of liquid polymer or ink patterns.
Crystallization in Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass under pressure
Authors: Jiang J. Z. , Zhou T. J. , Rasmussen H. , Kuhn U. , Eckert J. , Lathe C.
Published: July 2002

The effect of pressure on the crystallization behavior of the bulk metallic glass-forming Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy with a wide supercooled liquid region has been investigated by in situ high-pressure and high-temperature x-ray powder diffraction measurements using synchrotron radiation. In the pressure range from 0 to 3 GPa, the crystallization temperature increases with pressure having a slope of 19 K/GPa, which can be explained by the suppression of atomic mobility. This observation is opposite to the results of W.H. Wang, D.W. He, D.Q. Zhao, and Y.S. Yao [Appl. Phys. Lett. 75, 2770 (1999)], reporting a decrease of the crystallization temperature under pressure in a pressure range of 0–6 GPa for the bulk glass Zr41Ti14Cu12.5Ni9Be22.5C1 alloy. Compressibility with a volume reduction of approximately 22% at room temperature does not induce crystallization in the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk glass alloy. This indicates that the densification effect induced by pressure in the pressure range investigated plays a minor role in the crystallization behavior of this bulk glass alloy. The different crystallization behavior of the carbon-free and the carbon-containing glassy alloys has been critically assessed.
High-speed GaAs-based resonant-cavity-enhanced 1.3 μm photodetector
Authors: Kimukin Ibrahim , Ozbay Ekmel , Biyikli Necmi , Kartaloǧlu Tolga , Aytür Orhan , Unlu Selim , Tuttle Gary
Published: July 2002

We report GaAs-based high-speed, resonant-cavity-enhanced, Schottky barrier internal photoemission photodiodes operating at 1.3 μm. The devices were fabricated by using a microwave-compatible fabrication process. Resonance of the cavity was tuned to 1.3 μm and a nine-fold enhancement was achieved in quantum efficiency. The photodiode had an experimental setup limited temporal response of 16 ps, corresponding to a 3 dB bandwidth of 20 GHz.
Energy spectrum in the enstrophy transfer range of two-dimensional forced turbulence
Authors: Ishihara Takashi , Kaneda Yukio
Published: July 2002

Numerical simulations of two-dimensional forced turbulence suggest that the enstrophy transfer range energy spectrum E(k) a little steeper than k−3 is robust in the sense that it may be realized in a wave number range under different run conditions. It is shown that such energy spectra fit well E(k)=CKη2/3k−3[ln(k/k1)]−1/3, where CK is a dimensionless constant, η the enstrophy transfer rate per unit mass and k1 a wave number at the bottom of the range. The simulations give CK≈1.9 in fairly good agreement with the existing theoretical estimates.
Quantum dot and hole formation in sputter erosion
Authors: Kahng B. , Jeong H. , Barabási A.-L.
Published: July 2002

Recently, it was experimentally demonstrated that sputtering under normal incidence leads to the formation of spatially ordered uniform nanoscale islands or holes. Here, we show that these nanostructures have inherently nonlinear origin, first appearing when the nonlinear terms start to dominate the surface dynamics. Depending on the sign of the nonlinear terms, determined by the shape of the collision cascade, the surface can develop regular islands or holes with identical dynamical features, and while the size of these nanostructures is independent of flux and temperature, it can be modified by tuning the ion energy.
Mechanism of electroluminescence in dye doped thiophene based conjugated polymer
Authors: Kaur Amarjeet , Cazeca Mario J. , Chittibabu K. G. , Kumar Jayant , Tripathy Sukant K.
Published: July 2002

The optical and electrical properties of polymer light emitting diodes based on poly [2-(3-thienyl) ethanol butoxy carbonyl-methyl urethane] as emitting layer have been studied. The diode structures have been made by utilizing indium tin oxide coated glass as an anode and aluminum as a cathode. Polyaniline and tris(8-hydroxy-quinolino) aluminum have been employed as hole and electron transport layers, respectively. Enhanced electroluminescence (EL) near the turn-on voltage has been observed on addition of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye. On the basis of current–voltage, luminance–voltage, capacitance–voltage, and impedance measurements, an attempt has been made to understand the mechanism of EL emission in these polymeric light emitting diodes.
Structures and properties of hard sphere mixtures based on a self-consistent integral equation
Authors: Lee Lloyd L. , Malijevsky Anatol
Published: July 2002

A new self-consistent closure is formulated for the additive hard sphere mixtures at high densities (η=0.49) within the Ornstein–Zernike integral equation approach. Diameter ratios (σSS/σBB) from 0.3 to 0.9 and several compositions are examined. The consistencies include the thermodynamic ones (e.g., pressure consistency, and Gibbs–Duhem relation), and the structural ones (e.g., the zero-separation theorems). The bridge functions have built-in “flexibility” that can be adapted to the consistency requirements. Comparison with Monte Carlo simulation shows that the present closure yields highly accurate results. The contact values and zero-separation values are more accurate than those obtained from the conventional closures, such as the Percus–Yevick and Martynov–Sarkisov closures. A structural theory for hard sphere mixtures has been formulated that is accurate and consistent at the same time.
Metallization of magnetite (Fe3O4) under high pressure
Authors: Todo S. , Takeshita N. , Kanehara T. , Mori T. , Môri N.
Published: July 2002

Electrical resistivity measurements have been made on a good qualified single crystalline magnetite (Fe3O4) at temperatures from 300 down to 3.0 K under pressure up to 10 GPa. A steep change in resistivity at the Verwey transition temperature has been observed at pressure below 6.5 GPa, which shows a quite distinct result reported in prior work. Moreover, the Verwey transition temperature has been found to decrease nonlinearly with increasing pressure and it disappears at around 8 GPa. Above 8 GPa magnetite exhibits metallic behavior. The residual-resistivity ratio of the metallic state is observed to be more than 350. This finding of a metallic ground state in magnetite provides insight into the understanding of the Verwey transition in magnetite.
Charge transport and colossal magnetoresistance phenomenon in La1−xZrxMnO3
Authors: Roy Sujoy , Ali Naushad
Published: July 2002

In this study we have investigated the magnetic and electrical transport properties of Zr doped lanthanum manganite perovskite. The structural, magnetic, and transport properties of the Zr doped compounds were determined using x-ray diffraction, dc magnetic susceptibility, and a four probe method for electrical resistivity and magnetoresistance measurements in the temperature range of 5–400 K. The structure of the compounds was found to be rhombohedral. The magnetization versus temperature curves show ferromagnetic regions with the magnetic transition temperatures getting saturated for x⩾0.07 compounds. The resistivity curves show decreasing resistivity with increasing Zr content in the compound. The resistivity of the compounds is very high and is explained as due to the localization tendency of the electrons. The metal–insulator transition temperature shows a compositional dependence and has additional contributions apart from magnetism. The results are explained by the double exchange interaction and Mn2+/Mn3+ ratio, and also by taking into account the competition between the core-spin interaction and double exchange interaction.
Extraction of oxide trap properties using temperature dependence of random telegraph signals in submicron metal–oxide–semiconductor field-effect transistors
Authors: Amarasinghe Nuditha Vibhavie , Çelik-Butler Zeynep , Keshavarz Abdol
Published: July 2002

Random telegraph signals (RTS) in the drain voltage of light-doped drain n-metal–oxide–semiconductor field effect transistors with W×L=0.5×0.35 μm2 were investigated in the 300–230 K temperature range. The mean capture and emission times were studied as a function of gate voltage as well as temperature in the RTS. A method was developed where several trap characteristics can be extracted, including the position, barrier energy for capture, enthalpy, and entropy associated with emission of an electron, as well as screened scattering coefficient for carrier mobility as a function of temperature. This article reports on a single trap as an example. The position of the trap in the oxide (Tox=70 Å) was found to be 12 Å and, as expected, independent of temperature. The mean capture and emission times exhibited an increase as the temperature is decreased, following a thermally activated process. Utilizing these observations and the temperature dependence of the drain current, the gate voltage dependence of the trap binding energy, and capture activation energy was evaluated. The capture cross-section prefactor (σ0) was calculated using temperature dependence of capture and emission times. These two independent calculations showed almost two orders of magnitude difference in σ0, which is unrealistic. In order to remedy this, the trap binding energy was resolved into entropy and enthalpy components, which were evaluated as a function of gate voltage using the RTS data. The relative contributions of charge carrier mobility and number fluctuations on RTS amplitude were studied as a function of temperature. The mobility fluctuations were found to dominate over number fluctuations over the whole temperature range, and this dominance was found to be more pronounced at higher temperatures. The scattering coefficient due to screened Coulomb scattering effect was computed from the measured RTS data as a function of channel carrier density.
Spontaneous polarization-vector-reorientation photorefractive effect in ferroelectric liquid crystals
Authors: Sasaki Takeo , Kino Yuji , Shibata Minoru , Mizusaki Naoko , Katsuragi Atsushi , Ishikawa Yuichi , Yoshimi Takeshi
Published: July 2002

The photorefractive effect of a ferroelectric liquid crystal doped with a photoconductive compound was investigated. The photorefractive effect appeared only at the temperature at which the sample exhibits ferroelectricity. The refractive index grating formation time was measured to be ∼30 ms. In the ferroelectric phase, reorientation of the spontaneous polarization vector was found to be induced by the internal space-charge field.
Cavity-enhanced and quasiphase-matched multi-order reflection-second-harmonic generation from GaAs/AlAs and GaAs/AlGaAs multilayers
Authors: Mu Xiaodong , Ding Yujie J. , Yang Haeyeon , Salamo Gregory J.
Published: July 2002

We have observed quasiphase-matched second-harmonic generation in the reflection geometry from GaAs/AlAs multilayers. By using GaAs/AlGaAs multilayers and Fresnel reflection as a cavity, we have also achieved cavity-enhanced nonphase-matched second-harmonic generation from GaAs/AlAs multilayers. The linewidth for the first order reflection-second-harmonic generation is limited only by wave-vector mismatch. In addition, we have demonstrated two-order-of-magnitude enhancement on the conversion efficiency by using the cavity.
Exchange energy density of an atom as a functional of the electron density
Authors: Tao Jianmin
Published: July 2002

An electron-density functional for the conventionally defined exchange energy density of an atom is constructed using Becke’s inhomogeneity parameter QB based on the density matrix expansion of the exchange hole. The proposed functional (the energy density meta-generalized gradient approximation or EDMGGA) has the following properties: (i) The exchange energy density εxEDMGGA(r) has correct nuclear cusp and density-tail behaviors. (ii) The corresponding exchange potential δEx[n]/δn(r) is finite near the nucleus and decays asymptotically as −k/r in the tail. Numerical results show that our functional yields total exchange energies for atoms with about the same accuracy as Becke’s widely used functional B88, but significantly improves the local description of the exchange energy density. In one Appendix, by introducing a general coordinate transformation, we show that the asymptotic behavior of the conventionally defined exchange energy density depends upon the choice of the coordinate transformation and the established tail behavior, −1/2r, for a finite system is only a special case in the general coordinate transformation. In another Appendix, we discuss alternative definitions of the exchange energy density.
Shock wave propagation and dispersion in glow discharge plasmas
Authors: Macheret Sergey O. , Ionikh Yuri Z. , Chernysheva Naira V. , Yalin Azer P. , Martinelli Luigi , Miles Richard B.
Published: July 2002

Spark-generated shock waves were studied in glow discharges in argon and argon–nitrogen mixtures. Ultraviolet filtered Rayleigh scattering was used to measure radial profiles of gas temperature, and the laser schlieren method was used to measure shock arrival times and axial density gradients. Time accurate, inviscid, axisymmetric fluid dynamics computations were run and results compared with the experiments. Our simulation show that changes in shock structure and velocity in weakly ionized gases are explained by classical gas dynamics, with the critical role of thermal and multi-dimensional effects (transverse gradients, shock curvature, etc.). A direct proof of the thermal mechanism was obtained by pulsing the discharge. With a sub-millisecond delay between starting the discharge and shock launch, plasma parameters reach their steady-state values, but the temperature is still low, laser schlieren signals are virtually identical to those without the discharge, differing dramatically from the signals in discharges with fully established temperature profiles.
Tunneling electron loss from isolated platinum tetrahalide dianions
Authors: Blom Martine N. , Hampe Oliver , Gilb Stefan , Weis Patrick , Kappes Manfred M.
Published: July 2002

Electron detachment from doubly charged anions PtClxBr4−x2− to form PtClxBr4−x− (x=0–4) was probed for in a Fourier-transform ion-cyclotron-resonance mass spectrometer at 288 K. The corresponding decay rates were measured as a function of collision gas pressure. In the zero-pressure limit, nonzero unimolecular decay rate constants were observed for x=4 and 3 (0.273±0.035 and 0.056±0.019 s−1 for 288 K, respectively). These show a strong temperature dependence suggesting the new phenomenon of blackbody radiation induced tunneling electron loss. Comparison to electron tunneling rates calculated by means of the Wentzel–Kramers–Brillouin approximation using Coulomb barrier properties and electron detachment energies derived from ab initio quantum chemical calculations provides a qualitative rationalization of the observed trends.
Exponentiation of the spinor representation of the Lorentz group
Authors: Fredsted John
Published: July 2002

An exact finite expression for the exponentiation of the (spin 12) spinor representation of the Lorentz group is obtained. From this expression an exact finite expression for the exponentiation of the vector representation of the Lorentz group is derived. The two expressions are compared with the literature in the special cases of either spatial rotations or boosts, only.
Nonlocal sausage-like instability of current channels in electron magnetohydrodynamics
Authors: Das Amita , Kaw Predhiman
Published: July 2002

The question of stability of electron current channels in a plasma to rapidly growing modes on electron time scales is of considerable interest in a wide range of physical problems, e.g., fast ignitor concept of laser fusion, plasma opening switches, Z pinches, etc. Earlier studies have demonstrated the instability of such current channels to kink modes. However, the stability to sausage-like modes has not been investigated so far. In this paper such an investigation has been carried out and it is shown that the current channels are unstable to sausage-like modes with parallel scale lengths longer than the current channel width. These instabilities are closely related to the Kelvin–Helmholtz (K-H) instability of a sheared electron layer.
Large eddy simulation of turbulent flow in a square annular duct
Authors: Xu Hongyi , Pollard Andrew
Published: July 2002

The fully developed turbulent flow in a straight square annular duct is simulated using large eddy simulation (LES). The flow Reynolds number is 200 based on the mean friction velocity and half the hydraulic diameter of the annular duct. A grid refinement study is carried out using 130×66×66 (0.57 million) and 130×130×130 (2.2 million) control volumes, respectively. The mean flow field and the turbulent statistics are compared with existing experimental and numerical data for square duct flow. Acceptable agreement is obtained. A universal relation between the mean streamwise velocity and the distance away from a concave 90° corner along the corner bisector is derived by curve-fitting the results from both LES in a square annular duct and direct numerical simulation in a square duct. The results from the present investigation exhibit the distinctive secondary flow of Prandtl second kind, which consist of a chain of counter-rotating vortex pairs around both convex and concave 90° corners in the annular duct. Mechanisms responsible for the generation of the secondary flow are interrogated by analyzing the anisotropy of the Reynolds stress distributions.
High-speed visible-blind GaN-based indium–tin–oxide Schottky photodiodes
Authors: Biyikli Necmi , Kartaloglu Tolga , Aytur Orhan , Kimukin Ibrahim , Ozbay Ekmel
Published: July 2002

We have fabricated GaN-based high-speed ultraviolet Schottky photodiodes using indium–tin–oxide (ITO) Schottky contacts. Before device fabrication, the optical transparency of thin ITO films in the visible-blind spectrum was characterized via transmission and reflection measurements. The devices were fabricated on n−/n+ GaN epitaxial layers using a microwave compatible fabrication process. Our ITO Schottky photodiode samples exhibited a maximum quantum efficiency of 47% around 325 nm. Time-based pulse-response measurements were done at 359 nm. The fabricated devices exhibited a rise time of 13 ps and a pulse width of 60 ps.
Ultrasonic characterization of poling in lead zirconate titanate ceramics
Authors: Friend James , Jamieson Eric , Pennell Marvin , Huebner Wayne
Published: July 2002

A method for measuring the spatial variation of the macroscopic poling state of a piezoelectric material using an ultrasonic transducer is described. The relatively simple method clearly indicates the distribution of poling within the material, demonstrated by the testing of a collection of partially poled lead zirconate titanate samples.
Optical properties of BeCdSe/ZnCdMgSe strained quantum well structures
Authors: Maksimov O. , Guo S. P. , Muñoz Martin , Tamargo M. C.
Published: July 2002

We report the optical properties of BeCdSe/ZnCdMgSe single quantum well (QW) structures that consist of closely lattice matched ZnCdMgSe barrier layers and a strained BeCdSe QW layer (Δa/a=1.95%) grown on InP substrates. Emission from the red to the green regions of the visible spectrum was obtained from the structures with the QW thickness varying from 95 to 12 Å. Efficient QW emission, dominated by an exciton recombination behavior, was observed. From the Arrhenius plot of the integrated emission intensity as a function of temperature, an activation energy of 61 meV was obtained for a BeCdSe QW structure with a 48 Å thick QW layer. Parameters that describe the temperature dependence of the near band edge emission energy and the broadening of the excitonic emission were evaluated. Our results indicate that the BeCdSe-based QW structures are attractive for application as red light emitters.
Observation of narrow-band Si L-edge Čerenkov radiation generated by 5 MeV electrons
Authors: Knulst W. , Luiten O. J. , van der Wiel M. J. , Verhoeven J.
Published: July 2002

Narrow-band Čerenkov radiation at 99.7 eV has been generated by 5 MeV electrons in a silicon foil, with a yield ∼1×10−3 photon/electron. These measurements demonstrate the feasibility of a compact, narrow-band, and intense soft x-ray source based on small electron accelerators. The observed yield and dependence of the photon spectrum on emission angle are in agreement with theoretical predictions for Čerenkov radiation based on refractive index data of silicon.
Absorption of microwaves in La1−xSrxMnO3 manganese powders over a wide bandwidth
Authors: Li G. , Hu G.-G. , Zhou H.-D. , Fan X.-J. , Li X.-G.
Published: July 2002

We present the frequency dependence of microwave-absorbing properties of La1−xSrxMnO3 (x=0.4, 0.5, 0.6, and 0.7) powders at room temperature. The absorbing properties change gradually with x in the frequency range of 8–12 GHz. The optimal absorption can be achieved for a x=0.4 sample and its microwave loss peak value is about 25 dB. Further experimental results show that the absorption can be attributed to magnetic and dielectric losses and the microwave loss peak corresponds to the maximum dielectric loss tangent tan δe near 10.5 GHz. Furthermore, the absorbing properties of the oxides mixed with carbonic fiber and Y-type planar hexagonal ferrite have been rudimentarily studied. Results show that these additives greatly enhance the microwave-absorbing properties of the oxides.
Rayleigh–Taylor growth at decelerating interfaces
Authors: Drake R. P. , Keiter P. A.
Published: July 2002

The number of linear e-foldings of Rayleigh–Taylor instability growth is calculated for several cases of interest to experiment design. The planar, Sedov–Taylor case produces maximum Rayleigh–Taylor growth.
Femtosecond all-optical polarization switching based on the virtual excitation of spin-polarized excitons in quantum wells
Authors: Gansen Eric J. , Jarasiunas K. , Smirl Arthur L.
Published: July 2002

An optically addressed nonlinear polarization switch based on the near-resonant excitation of a spin-polarized population of virtual excitons is demonstrated in a multiple quantum well. Pulse-width-limited switching (∼400 fs full width at half maximum) and high-contrast performance (194:1) are achieved in a thin (40 well) sample at 100 K. Differential absorption measurements identify the dominant switching mechanisms.
Three-dimensional arrays of BiSrCaCuO-2212 intrinsic Josephson junctions and zero-crossing Shapiro steps at 760 GHz
Authors: Wang H. B. , Wu P. H. , Chen J. , Maeda K. , Yamashita T.
Published: July 2002

On a slice of 200–300-nm-thick Bi2Sr2CaCu2O8+x, stacks of intrinsic Josephson junctions are patterned, connected to each other, and integrated with a bow-tie antenna and some chokes. Over an area of 150 μm by 170 μm, we have been able to fabricate up to 256 stacks with over 11 000 junctions. Such three-dimensional arrays of intrinsic Josephson junctions respond sensitively to irradiation at 760 GHz from a far-infrared laser, showing clear zero-crossing Shapiro steps up to 0.75 V. In addition, possible applications in quantum voltage standards are discussed here. The high density of integration of the Josephson junctions is also promising for many other applications.
Spectral dependence and Hall effect of persistent photoconductivity in polycrystalline Cu(In,Ga)Se2 thin films
Authors: Meyer Th. , Engelhardt F. , Parisi J. , Rau U.
Published: July 2002

We investigate persistent photoconductivity in polycrystalline Cu(In,Ga)Se2 thin films by measuring the transient behavior of their electrical conductivity under and after illumination. Characteristic nonexponential transients of the conductivity under and after illumination extend over more than four orders of magnitude in time. From this result, we conclude that the persistent photoconductivity in Cu(In,Ga)Se2 cannot be explained by the mesoscopic barrier model. Rather, it is a microscopic effect which can be understood in the frame of a large lattice relaxation model. Investigation of the conductivity under monochromatic subband gap illumination determines the minimum photon energy for the direct excitation of the persistent photoconductivity to be approximately 0.6 eV. Hall effect measurements identify the persistent photoconductivity as a hole density effect in the bulk of the Cu(In,Ga)Se2 grains, consistent with the lattice relaxation model.
Intermolecular interaction energies from the total energy bifunctional: A case study of carbazole complexes
Authors: Wesolowski T. A. , Morgantini P.-Y. , Weber J.
Published: July 2002

An approach in which the total energy of interacting subsystems is expressed as a bifunctional depending explicitly on two functions: electron densities of the two molecules forming a complex (ρ1 and ρ2) was used to determine the equilibrium geometry and the binding energy of several weak intermolecular complexes involving carbazole and such atoms or molecules as Ne, Ar, CH4, CO, and N2. For these complexes, the experimental dissociation energies fall within the range from 0.48 to 2.06 kcal/mol. Since the effect of the intermolecular vibrations on the dissociation energy is rather small, the experimental measurements provide an excellent reference set. The obtained interaction energies are in a good agreement with experiment and are superior to the ones derived from conventional Kohn–Sham calculations. A detailed analysis of relative contribution of the terms which are expressed using approximate functionals (i.e., exchange-correlation Exc[ρ1+ρ2] and nonadditive kinetic energy Tsnad[ρ1,ρ2]=Ts[ρ1+ρ2]−Ts[ρ1]−Ts[ρ2]) is made. The nonvariational version of the applied formalism is also discussed.
Silicon nanostructures fabricated by scanning probe oxidation and tetra-methyl ammonium hydroxide etching
Authors: Chien F. S.-S. , Hsieh W.-F. , Gwo S. , Vladar A. E. , Dagata J. A.
Published: July 2002

Fabrication of silicon nanostructures is a key technique for the development of monolithically integrated optoelectronic circuits. We demonstrated that the process of scanning probe microscope (SPM) oxidation and anisotropic tetra-methyl ammonium hydroxide (TMAH) etching is a low-cost and reliable method to produce smooth and uniform silicon nanostructures on a variety of silicon substrates. Etched structures with a pitch of 100 nm, positive- and negative-contrast structures, and features height greater than 100 nm have been produced on bare silicon, and Si3N4-coated and silicon-on-insulator wafers. Evolution of hexagonal pits on two-dimensional grid structures were shown to depend on the pattern spacing and orientation with respect to Si(110) crystal directions. We successfully combined SPM oxidation with traditional optical lithography in a mixed, multilevel patterning method for realizing micrometer-and nanometer-scale feature sizes, as required for photonic device designs. The combination of SPM oxidation and TMAH etching is a promising approach to rapid prototyping of functional nano-photonic devices.
A new method for deflecting liquid microjets
Authors: Chwalek James M. , Trauernicht David P. , Delametter Christopher N. , Sharma Ravi , Jeanmaire David L. , Anagnostopoulos Constantine N. , Hawkins Gilbert A. , Ambravaneswaran Balasubramanian , Panditaratne Jayanta C. , Basaran Osman A.
Published: July 2002

A new method is reported for deflecting a microscopic jet emanating from a nozzle away from the nozzle’s axis of symmetry. It relies on putting energy into the jet through an asymmetric heater embedded in the nozzle. This novel phenomenon is probed theoretically. It is shown that jet deflection is set by the competition among three effects. Two of these can be attributed to the variation with temperature of surface tension and the third to that of viscosity. Whether the contact line is fixed or free is shown to profoundly impact the extent of jet deflection at a given flow rate.
Measurement of particle motions within tumbling granular flows
Authors: Metcalfe Guy , Graham Lachlan , Zhou James , Liffman Kurt
Published: July 2002

Flowing granular materials are complex, industrially important, and scientifically provocative. In this paper we report measurements of granular transport in 3-dimensional tumbling containers. We use magnetic resonance imaging techniques for direct tracking of particles and measure the interior flows of granular materials. One goal is to measure industrial mixer performance over a wide range of conditions. As the mixer geometries are relatively simple, such measurements could serve as incisive tests during development of better granular equations of motion.
Generalized nonstandard finite differences and physical applications
Authors: Cole James B.
Published: July 2002

Nonstandard finite differences can be used to construct exact algorithms to solve some differential equations of physical interest such as the wave equation and Schrödinger’s equation. Even where exact algorithms do not exist, nonstandard finite differences can greatly improve the accuracy of low-order finite-difference algorithms with a computational cost low compared to higher-order schemes or finer gridding. While nonstandard finite differences have been applied successfully to a variety of one-dimensional problems, they cannot be directly extended to higher dimensions without modification. In this article we generalize the nonstandard finite-difference methodology to two and three dimensions, give example algorithms, and discuss practical applications. © 1998 American Institute of Physics.
Formation mechanism of drawing-induced E′ centers in silica optical fibers
Authors: Hanafusa H. , Hibino Y. , Yamamoto F.
Published: July 2002

The formation mechanism of drawing-induced E′ centers in silica optical fibers is clarified by analyzing the thermodynamic behavior of point defects during the drawing process. The E′ center concentration in the fibers is given by nd=np  exp(−Ef/kTd−A/V1), where Td is the drawing temperature, V1 the drawing speed, and A a constant. The formation energy Ef is 3.8 eV while the precursor concentration np is 7×1022 g−1. The E′ center concentration at the fiber surface is greater than that at the center because of the transverse difference in the quenching rate during the drawing process. The annealing behavior of the E′ center concentration is expressed as nd(t)=nd(0)exp[−νt  exp(−Ed/kTa)], where Ta is the annealing temperature. The activation energy for the motion of the E′ centers Ed is 0.2 eV while the frequency factor ν is 8×10−3 sec.−1.
Spin-polarized secondary electrons for nondestructive magnetic depth profiling
Authors: Mauri D. , Allenspach R. , Landolt M.
Published: July 2002

The spin polarization of true secondary electrons is shown to provide an efficient surface magnetometer with the unique feature of a tunable probing depth up to 10–20 Å. The relation between polarization and magnetization profiles as well as the polarization spectra is discussed.
Optical response of switching SmS in thin films prepared by reactive evaporation
Authors: Hickey C. F. , Gibson U. J.
Published: July 2002

We describe a simple technique for fabrication of samarium monosulfide films using reactive evaporation and unheated substrates, resulting in films which consist of highly ordered SmS grains in an amorphous dielectric matrix. Mechanical polishing of the films results in a semiconductor-metal transition in a fraction of the grains, with an associated large change in the optical response. The films are stable in the switched state, and may be locally annealed back to the semiconducting state using a laser, thus suggesting application in optical data storage. The optical properties in both states are successfully modeled using a Bruggeman effective medium theory and experimentally determined values of the input parameters.
Comparison between two models of nonlocal heat flux in laser-generated plasma
Authors: Holstein P. A. , Decoster A.
Published: July 2002

Two models of nonlocal heat flux in laser-driven plasmas have been presented by Luciani and Mora, and by Albritton et al. The disagreement between them is shown to be only apparent: when used in exactly the same conditions, they actually give the same answer, provided that the correct tabulated propagator is used in the integral giving the flux. A steepening of the temperature profile at critical density still exists for both models.
Lattice imaging of metastable TiSi2
Authors: Chou T. C. , Wong C. Y. , Tu K. N.
Published: July 2002

The microstucture of metastable C49-TiSi2 was studied by high-resolution transmission electron microscopy in a bilayer thin film of Ti and Si annealed at 700 °C. Large grains (300–400 nm) of C49-TiSi2 phase with high density of stacking faults on (010) planes were observed. A preferred orientation among the C49-TiSi2 grains was identified, and the grains were aligned along the [010] with slight misorientation. The diffraction pattern from the metastable TiSi2 showed continuous streaks superimposed by discrete spots along hk0 reciprocal lattice rows, indicating the presence of polytypism. Lattice imaging of polytypic TiSi2 has been obtained and it showed that the structure is nonperiodic and one dimensionally disordered due to the stacking faults. The finding of metastable TiSi2 associated with polytypism suggests that its structure could be conveniently represented by various stackings of atomic planes along the [010] direction. An atomic model is proposed to explain the origin of polytypism. It is shown that the arrangement of atoms in C49-TiSi2 structure has four equivalent positions.
The prediction and confirmation of critical epitaxial parameters
Authors: van der Merwe Jan H. , Jesser W. A.
Published: July 2002

The coherency-incoherency transition in epitaxial crystals is said to take place at a critical misfit fc or, for a system in which a monolayer is subcritical, at a critical thickness hc. In this paper, we analyze the physical principles and models used to predict critical parameters and put them into perspective. We stress the dependence of the relevant principles on the equilibrium-nonequilibrium conditions under which the quantities are measured in practice. The advantages and disadvantages of the models used—essentially the Frenkel–Kontorowa and Volterra models—are highlighted.