bluuebunny/crossref_metadata_2025 huggingface dataset:
Periodic order induced by horizontal vibrations in a two-dimensional assembly of heavy beads in water
Authors: Wunenburger R. , Carrier V. , Garrabos Y.
Published: July 2002

We report on the observation of the periodic pattern occuring in a two-dimensional (2D) assembly of heavy beads (of diameter D=280–630 μm) immersed in water when the vessel containing the fluid and the beads is submitted to horizontal vibrations (of frequency f and oscillation amplitude A). Under vibrations, regularly spaced, one bead thick lines perpendicular to the direction of vibrations are formed at small surface fraction. In the ranges f=10–20 Hz and A=0.7–3.5 mm, the amplitude of the oscillatory motion of the beads relative to the fluid B is found to vary linearly with A. A simple phenomenological model based on dynamic solid friction and viscous drag gives qualitatively account for this linear dependence. The period of the pattern λ is found to vary with viscosity v, ω=2πf, D, and B according to the law λ/D∼(B/D)0.5(BωD/v)−0.21. The dynamics of pattern formation exhibits several scenarii, depending on the initial bead distribution, as well as several characteristic time scales. We review and discuss possible mechanisms of pattern formation reported in literature and proposed for similar phenomena observed in the frame of rheology of suspensions and acoustic streaming flows.
Modulated plasma potentials and cross field diffusion in a Helicon plasma
Authors: Perry Andrew , Conway Garrard , Boswell Rod , Persing Harold
Published: July 2002

As the input rf power to a helicon plasma is increased there is a discontinuous increase in the density which is identified with a change from a capacitive to an inductive/wave coupling to the plasma. The radial distribution of the density is much narrower in the inductive (high) mode than in the capacitive (low) mode. Although the time average plasma potential (Vp) decreases markedly at this mode change, the radial profile of Vp is flat in both modes. Measurements with an emissive probe show that in the capacitive mode Vp is strongly modulated (ΔV is order of Vp) at the rf drive frequency indicating the presence of large rf fields in the source. There is also a large radial gradient in the amplitude of this modulation. In the inductive mode the modulation decreases significantly but can still be observed in measurements of the ion energy distribution. It is suggested that in the capacitive mode, the radial diffusion of electrons is driven by the large gradients in the rf fields. As the density increases, the fields are screened out, diminishing the radial electron loss and consequently, the radial ion loss. The reduced radial diffusion of both charged species produces a still higher density as the major loss surface is now reduced to the ends only.
Dynamics of anions and cations in hydrogensulfides of alkali metals (NaHS, KHS, RbHS): A proton nuclear magnetic resonance study
Authors: Haarmann F. , Jacobs H. , Senker J. , Rössler E.
Published: July 2002

Second moments (M2) and spin-lattice relaxation times (T1) of proton nuclear magnetic resonance (NMR) measurements were analyzed for the hydrogensulfides of sodium, potassium, and rubidium. Three modifications are known at ambient pressure for each of MHS with M=Na, K,Rb. They are isotypic for the different cations, with an ordered monoclinic low temperature modification, a dynamically disordered rhombohedral middle temperature modification (MTM), and a cubic high temperature modification (HTM). The number of proton sites for one proton increases from two in the MTM to at least eight in the HTM. The title compounds were investigated by proton NMR in the temperature range 180 K⩽T⩽560 K with a resonance frequency of ν0=400 MHz. In addition, KHS was measured from T=90 K up to T=297 K, using proton resonance frequencies ν0=45 MHz and ν0=96 MHz. The M2 calculated from the absorption signal of the individual modification for the various cations do not differ from those calculated for the known crystal structures. The minima of T1 are in good agreement with results of calculations based on the crystal structure of the MTM of these compounds and two-site 180° reorientations of the anions. Activation enthalpies and attempt frequencies (NaHS:26.2(4) kJ mol−1, 6.1(2)×1014 s−1, KHS:19.0(4) kJ mol−1, 4.0(7)×1013 s−1, RbHS:16.3(4) kJ mol−1, 2.0(2)×1013 s−1) agree with those determined by quasielastic neutron scattering [Haarmann et al., J. Chem. Phys. 113, 8161 (2000)]. For the cubic HTM, translational motion of the cations is probable. Only in the case of KHS and RbHS, was the onset of this translational motion already observed in the MTM at temperatures close to the phase transition into the HTM. This seems to be a difference in the mechanism of the MTM⇄HTM phase transition for NaHS, on the one hand, and KHS, RbHS, on the other hand.
Effective macroscopic symmetries and materials properties of multidomain 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 single crystals
Authors: Yin Jianhua , Cao Wenwu
Published: July 2002

The crystal symmetry of a 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 (PZN-4.5%PT) single crystal is rhombohedral 3m. It is in multidomain state after being poled along [001] of its original cubic coordinates. Although most studies on this system assumed the tetragonal 4mm effective symmetry, experimental observations show that many of the crystals actually have effective orthorhombic mm2 symmetry. Some crystal domain patterns may even become monoclinic m or even lower in symmetry. In other words, domains form a hierarchical symmetry, which controls the effective materials properties of the multidomain system. We report an analysis of the domain symmetry involved and a full set of materials properties measured in crystals having only two of the four possible domains based on mm2 symmetry.
Change of conduction mechanism by microstructural variation in Pt/(Ba,Sr)TiO3/Pt film capacitors
Authors: Ahn Kun Ho , Kim Sang Sub , Baik Sunggi
Published: July 2002

The effects of microstructure on the leakage current behaviors in Pt/(Ba,Sr)TiO3/Pt thin film capacitors prepared by a radio-frequency magnetron sputtering technique were studied. Both applying an epitaxial seed layer and controlling post-annealing processes enabled us to obtain (Ba,Sr)TiO3 films of three different microstructures (granular, columnar, and epitaxial) but possessing an identical interface state with the Pt electrode. The Schottky emission dominates in the film composed of granular grains. In contrast, the Fowler–Nordheim tunneling governs in the epitaxial film with much elevated leakage current levels. In the case of the columnar film, at low temperatures the tunneling dominates, while at high temperatures the Schottky emission governs with intermediate leakage levels. An energy band model accounting for the change of leakage current mechanisms by microstructure variation is presented.
Precise positioning control with double feedback loop for ultralarge scale integrated manufacturing machine
Authors: Takahashi Yoshihiko , Takahashi Hideyuki
Published: July 2002

This article presents a new positioning control system for ultralarge scale integrated (ULSI) circuit manufacturing machines. Both accuracy and high speed are required in the positioning control of ULSI manufacturing. Two major problems exist in the precise positioning control: mechanical resonance and steady state error induced by solid friction. The proposed control system utilizes H2 control to compensate for the mechanical resonance, and an integral action was combined with the H2 control in order to compensate for the steady state error. The proposed control system is composed of a double feedback control loop in which the inner control loop is the H2 controller, and the outer control loop is the integral action controller. A desired position is input to the integral action controller, and then the output signal of the integral action controller becomes the desired value of the H2 controller. In the controller design stage of the proposed control system, the H2 control gain and the integral action gain can be chosen independently. The validity of the proposed control system has been confirmed by the simulation and experimental results. The response was converged quickly to the sensor resolution of 50 nm with less vibration.
Circuit for continuous motional series resonant frequency and motional resistance monitoring of quartz crystal resonators by parallel capacitance compensation
Authors: Arnau A. , Sogorb T. , Jiménez Y.
Published: July 2002

A deep analysis of the problem associated with oscillators as interface circuits for quartz-crystal-microbalance sensors, reveals that the so-called static capacitance of the sensor is one of the elements that makes the use of oscillators more critical for sensors applications. A phase-locked-loop based circuit specifically designed for compensating the parallel capacitance effects in quartz crystal resonator sensors is presented. This circuit permits the calibration of the external circuitry to the sensor and an accurate determination of the effective capacitive compensation. The system provides a continuous measurement of the motional series resonant frequency and motional resistance. An extension and automation of the proposed system for multiple sensor characterization is introduced. The theoretical analysis of the circuit along with the experimental results presented prove that the proposed system is a good alternative for quartz sensors characterization.
Molecular-beam epitaxial growth of GaAs and InGaAs/GaAs nanodot arrays using anodic Al2O3 nanohole array template masks
Authors: Mei X. , Kim D. , Ruda H. E. , Guo Q. X.
Published: July 2002

Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth. Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates. Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated. In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated. Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm. Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array.
Quantum-mechanical calculations on termolecular association reactions XY+Z+M→XYZ+M: Application to ozone formation
Authors: Charlo David , Clary David C.
Published: July 2002

We present a quantum-mechanical model for termolecular association reactions XY+Z+M→XYZ+M involving the formation of a long-lived complex XYZ*. The rotation of the molecule XYZ is treated in the infinite order sudden approximation (IOS) and its vibrations are treated by the coupled-channel method (VCC). Resonances featuring the XYZ* long-lived complex formation are first computed by means of the stabilization method and are then included in the vibrational basis functions used for the inelastic VCC-IOS scattering calculation. The method yields rate constants for the association process selected in resonance and bound states of XYZ. We apply the method to the formation of ozone and investigate isotope effects. Calculations of energy transfer and collision-induced recombination of O+O2 in collision with Ar are reported for a range of ozone isotopomers. The bending mode of O3 is not treated explicitly in these computations. The results establish a strong selectivity in vibrational state-to-state cross sections for the deactivation of O3 during the collisional energy transfer process with Ar. The present calculations also account for the high sensitivity of rate constants with respect to the isotopic composition of ozone molecules but not in the same proportion as experiments. The energy transfer from selected initial vibrational states is also calculated as a function of the initial relative kinetic energy.
Influence of Cr additions in magnetic properties and crystallization process of amorphous iron based alloys
Authors: Marin P. , Lopez M. , Hernando A. , Iqbal Y. , Davies H. A. , Gibbs M. R. J.
Published: July 2002

Amorphous Fe63.5Cr10Si13.5B9Cu1Nb3 ribbon samples have been prepared by melt spinning. The addition of Cr to a FINEMET-type alloy decreases the Curie temperature of the amorphous matrix and also modifies the alloy crystallization temperature. The evolution of the microstructure during crystallization has been analyzed by means of x-ray diffractometry and transmission electron microscopy. An interesting dependence of the intensity of the amorphous diffraction halo was observed for temperatures below 818 K, where a large fraction of glassy phase was still present in the sample. The evolution of the coercivity with annealing temperature for various measurement temperatures has also been studied and, in conjunction with the microstructural observations, is interpreted within the framework of the two-phase theory for nanocrystalline materials.
New experimental evidence for the stabilizing effect of a superthermal electron avalanche during electron cyclotron resonant heating
Authors: Lazaros Avrilios , Shimizu Akihiro , Yoshimura Yasuo , Matsuoka Keisuke , Okamura Shoichi , Suzuki Chihiro ,   CHS Group
Published: July 2002

New experimental evidence for the stabilizing effect of the superthermal electrons [A. Lazaros, Phys. Plasmas 6, 148 (1999)], which are produced through the avalanche effect [A. Lazaros, Phys. Plasmas 8, 1263 (2001)] during electron cyclotron resonant heating (ECRH), is provided by the suppression of the “fishbone” mode with ECRH in the “compact helical system” [S. Okamura et al., Nucl. Fusion 39, 1337 (1999)], in an experimental arrangement which favors the development of a superthermal electron avalanche and excludes any other stabilizing effect. The significance of the radial location of the EC resonance and the electron density, for the development of the superthermal electron avalanche, is confirmed by radial scans of the EC resonance and by density scans.
Microcrystalline silicon thin films studied by atomic force microscopy with electrical current detection
Authors: Rezek B. , Stuchlı́k J. , Fejfar A. , Kočka J.
Published: July 2002

Hydrogenated microcrystalline silicon (μc-Si:H) layers with thickness from 100 to 540 nm were prepared in situ by plasma enhanced chemical vapor deposition. The growth of μc-Si:H on various substrates [NiCr, device quality, and laser annealed amorphous silicon (a-Si:H)] was studied in ultrahigh vacuum by atomic force microscope using a conductive cantilever which enabled simultaneous measurement of morphology and local current with lateral resolution below 5 nm. The effect of barriers, voltage, and time on contrast in local current map is discussed in detail. Coexistent amorphous and microcrystalline regions are clearly identified due to their different conductivity. Laser annealing of the a-Si:H substrate significantly increases the crystalline fraction at the same layer thickness. Grains as small as 10–30 nm separated by less conductive grain boundaries were revealed in microcrystalline regions.
Comparison of surface interactions for NH and NH2 on polymer and metal substrates during NH3 plasma processing
Authors: Steen Michelle L. , Kull Kristen R. , Fisher Ellen R.
Published: July 2002

How different plasma species influence the reactions and reactivity of each other during plasma processing is not fully understood, especially with respect to surface interactions. The goal of this study is to provide insight into the relationships between gas-phase NHx species in NH3 plasmas. Specifically, formation of NHx, their relationship with the surface during plasma processing, and the effects of charged species on these interactions are discussed. The surface reactivities of NH and NH2 radicals with different substrate materials during NH3 plasma processing were investigated using the imaging of radicals interacting with surfaces technique. Scatter coefficients, S, for NHx species were obtained as a function of applied rf power for polyimide, polytetrafluoroethylene, and platinum substrates. To investigate the role of ions on the formation of NHx radicals in the gas phase and at the surface, ions were removed (>98%) from the plasma molecular beam. Results from the “ion-free” conditions suggest that ions enhance NH2 surface generation, but suppress NH-forming mechanisms at high rf powers. Surface interaction results for NH and NH2 with and without ions provide the basis for a discussion of possible surface interaction mechanisms for the different substrates examined.
Gaussian effective interaction between flexible dendrimers of fourth generation: A theoretical and experimental study
Authors: Likos C. N. , Rosenfeldt S. , Dingenouts N. , Ballauff M. , Lindner P. , Werner N. , Vögtle F.
Published: July 2002

We propose a theory for the effective interaction between soft dendritic molecules that is based on the shape of the monomer density profile of the macromolecules at infinite dilutions. By applying Flory-type arguments and making use of the experimentally measured density profiles, we derive a Gaussian effective interaction whose parameters are determined by the size and monomer number of the dendrimers that are derived from small-angle neutron scattering (SANS) measurements. By applying this theory to concentrated dendrimer solutions we calculate theoretical structure factors and compare them with experimental ones, derived from a detailed analysis of SANS-data. We find very good agreement between theory and experiment below the overlap concentration, where drastic shape deformations of the dendrimers are absent.
Two crossover regions in the dynamics of glass forming epoxy resins
Authors: Corezzi S. , Beiner M. , Huth H. , Schröter K. , Capaccioli S. , Casalini R. , Fioretto D. , Donth E.
Published: July 2002

Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)≈10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)≈10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossover region is characterized by onset of the (a′,α) process, with a relaxation time about one decade greater than that of the quasicontinuous (a,γ) trace. The β-crossover region is characterized, besides splitting of main andβ relaxation times, by a change in the temperature dependence of the main-relaxation time as reflected by a bend in the Stickel plot of the continuous (a′,α) trace, the separation of individual temperature dependences of different transport properties such as impurity-ions diffusion coefficient and viscosity, and a temperature-dependent main relaxation time that starts to be in accordance (at lower temperatures) with the Adam–Gibbs model. The cooperativity of the main process between the γ and β crossover seems to be small. Below the β crossover, cooperativity increases up to values of order Nα∼100 near Tg, and configurational entropy seems to correlate with the main relaxation time.
Method for evaluation of the emissivity spectrum of a localized vibrational absorption band
Authors: Oheda Hidetoshi
Published: July 2002

Even when a material is in thermal equilibrium with its surroundings, the thermal radiation that it emits can be made evident by detecting a difference between its radiation power and that of some reference material, such as a black chopper wheel. Such a way of revealing thermal radiation from a material provides us with a possibility of characterizing radiant properties of that material without requiring any special radiant standard. In this work I develop a semiquantitative analysis of such a difference signal, which provides us with a simple relationship between the emissivity of a material and measurable quantities. Using this result, I propose a method for determining the spectral dependence of emissivity of a material. The validity and usefulness of the method are confirmed by applying it to the Si–H stretching band in two amorphous films of undoped hydrogenated amorphous silicon and of its alloy with nitrogen.
Interface properties of SiO2/n-GaN metal–insulator–semiconductor structures
Authors: Nakano Yoshitaka , Jimbo Takashi
Published: July 2002

Electrical characterization of SiO2/n-GaN metal–insulator–semiconductor structures fabricated on sapphire substrates was performed by using high-frequency pulsed capacitance–voltage and capacitance-transient techniques. Fast and slow capacitance transients are clearly seen after applying reverse voltages, reflecting thermal emissions of carriers from the SiO2/GaN interface. The temperature dependence of the capacitance–voltage characteristics shows capacitance saturation in deep depletion (>15 V), which is probably associated with the slow capacitance transient. Deep-level transient spectroscopic measurements reveal two interface traps with activation energies of 0.71 and ∼0.76 eV from the conduction band, corresponding to the fast and slow capacitance transients, respectively. Therefore, the observed capacitance saturation may be due to Fermi-level pinning induced by the latter interface trap.
Laser induced reactions in a 22-pole ion trap: C2H2++hν3+H2→C2H3++H
Authors: Schlemmer Stephan , Lescop Emmanuelle , von Richthofen Jan , Gerlich Dieter , Smith Mark A.
Published: July 2002

A sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described, briefly called laser induced reactions (LIR). The technique is based on (i) trapping ions over a long time in a cold 22-pole rf ion trap followed by mass spectrometric detection, (ii) providing a suitable low density gas environment for collisions, (iii) modifying the low temperature chemical kinetics using selective excitation via a tunable radiation source. In this paper, the H-atom transfer reaction C2H2+ (v3=1,J)+H2→C2H3++H, is used to monitor the infrared excitation of acetylene ions. Rotationally resolved spectra are presented for the antisymmetric C–H stretching vibration. For recording a spectrum, it is sufficient to fill the trap with a few thousand parent ions. Differences with respect to conventional IR spectroscopy are discussed, especially the processes which influence the LIR signal. From the measured intensities and their dependence on parameters such as storage time, laser fluence and target gas density, information on state specific rate coefficients has been obtained at an ambient temperature of 90 K. Based on a model simulating the kinetics, rate coefficients for various inelastic and reactive collisions are derived. Vibrational excitation of C2H2+ (v3=1,J) increases the rate of the title reaction by more than three orders of magnitude, while rotation hinders the reaction. The fine-structure state of the parent ion does not affect its reactivity. Ways are pointed out to apply the method to various classes of molecular ions.
Low-emittance electron-beam generation with laser pulse shaping in photocathode radio-frequency gun
Authors: Yang J. , Sakai F. , Yanagida T. , Yorozu M. , Okada Y. , Takasago K. , Endo A. , Yada A. , Washio M.
Published: July 2002

A technique of laser-pulse shaping was developed for low-emittance electron-beam generation in a photocathode radio-frequency (rf) gun. The emittance growth due to space charge and rf effects in the rf gun was experimentally investigated with square and gaussian temporal pulse shapes. It was found that the square pulse shaping was a useful tool for both the reduction of nonlinear space-charge force and the correction of linear space charge. The normalized transverse rms emittance at 1 nC was obtained to be 1.20 πmm-mrad for the square pulse shape with pulse length of 9 ps full width at half maximum. The emittance was measured as a function of the electron bunch charge and the laser-pulse length.
Structure of the exact wave function. V. Iterative configuration interaction method for molecular systems within finite basis
Authors: Nakatsuji Hiroshi , Ehara Masahiro
Published: July 2002

The iterative configuration interaction (ICI) method is applied to molecular systems within finite basis using only few (1–3) variables and shown to give the exact results that are identical to the full CI (FCI) ones. Since each iteration step of ICI is variational, the ICI converges monotonically to the exact solution from above. The diagonalization in ICI is so slight as the number of variables is so small, in contrast to the huge number of variables of FCI. We calculated the molecular ground states of various spin-space symmetries using minimal basis and double zeta basis. The number of iterations for convergence was small for minimal basis but moderate for double zeta basis, considering that only 1–3 variables are optimized in each iteration step.
Controlled alignment of carbon nanofibers in a large-scale synthesis process
Authors: Merkulov Vladimir I. , Melechko A. V. , Guillorn M. A. , Simpson M. L. , Lowndes D. H. , Whealton J. H. , Raridon R. J.
Published: July 2002

Controlled alignment of catalytically grown carbon nanofibers (CNFs) at a variable angle to the substrate during a plasma-enhanced chemical vapor deposition process is achieved. The CNF alignment is controlled by the direction of the electric field lines during the synthesis process. Off normal CNF orientations are achieved by positioning the sample in the vicinity of geometrical features of the sample holder, where bending of the electric field lines occurs. The controlled growth of kinked CNFs that consist of two parts aligned at different angles to the substrate normal also is demonstrated.
Detailed investigations on the photophysical properties of poly(2,5-pyridine diyl)
Authors: Sinha S. , Rothe C. , Beeby A. , Horsburgh L. E. , Monkman A. P.
Published: July 2002

The photophysical properties of poly(2,5-pyridine diyl) (PPY) are characterized in detail using steady state measurements, time correlated single photon counting techniques, and excitation pulse energy dependence of the photoluminescence intensity. Our results show that the blue emission from dilute solution is prompt fluorescence (lifetime ∼10 ps). On the other hand, the green emission from thin solid film consists of phosphorescence as well as delayed fluorescence with lifetimes of ∼4.8 ns and 1.1 ns, respectively. Oxygen complexation plays an important role and increases triplet population in thin film. At low excitation pulse energies, emission in thin film originates mostly from the triplet state, while at higher excitation pulse energies, delayed fluorescence becomes the major part in the emission. Also, at low temperature (20 K), an excimerlike emission is observed to the red side of the PPY emission spectrum in thin film, which is only induced by pressure.
The effect of pressure on the structural and secondary relaxations in 1,1′-bis (p-methoxyphenyl) cyclohexane
Authors: Hensel-Bielowka S. , Ziolo J. , Paluch M. , Roland C. M.
Published: July 2002

The dielectric spectrum of the organic glass former 1,1′-bis (p-methoxyphenyl) cyclohexane was measured over a range of temperatures and pressures, corresponding to a variation of the structural relaxation time, τα, by 8 decades. The temperature dependence of τα corresponded to a fragility equal to 72, which is consistent with the correlation of same with the shape (breadth) of the relaxation function. The dependence on pressure of τα could be described as a simple activated process, with a pressure-independent activation volume equal to ∼230 cm3/mol, varying inversely with temperature. The pressure coefficient of the glass temperature, Tg, was 240 K/GPa. At frequencies beyond the structural relaxation peak, there is a second, thermally activated process, having an activation energy=74.2 kJ/mol. The relaxation times for this process were invariant to pressure. Extrapolation of these relaxation times (measured below the Tg) intersects the ambient pressure structural relaxation data at a temperature, Tβ=268 K. This is about equal to the temperature, TB, at which the structural relaxation times deviated from a single Vogel–Fulcher–Tamman curve. In this respect, the secondary process exhibits the properties of a Johari–Goldstein relaxation. However, since Tβ varies with pressure, it remains to be seen whether its equivalence to TB is maintained at elevated pressure.
Three-dimensional dielectric photonic crystals of body-centered-tetragonal lattice structure
Authors: Tao R. , Xiao D.
Published: July 2002

Numerical calculation has been carried out for three-dimensional (3D) dielectric photonic crystals of body-centered-tetragonal (bct) lattice structure, which can be produced by utilizing the electrorheological and magnetorheological effect. The calculation shows that there is a complete band gap between the fifth band and the sixth band if the ratio of the dielectric constant of the host medium εh to the dielectric constant of the spheres εa is high enough, εh/εa>15.7. Therefore, 3D bct dielectric photonic crystals will be suitable for various optical applications.
Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys
Authors: Bhattacharya S. , Tritt Terry M. , Xia Y. , Ponnambalam V. , Poon S. J. , Thadhani N.
Published: July 2002

Half-Heusler alloys with the general formula TiNiSn1−xSbx are currently being investigated for their potential as thermoelectric (TE) materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity κL appears to increase somewhat randomly with small amounts (x<5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of κL. Furthermore, effects of submicron grain sizes on κL in ball milled and shock compressed samples are also presented.
Self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La0.8Sr0.2MnO3 thin films
Authors: Jiang J. C. , Meletis E. I. , Gnanasekar K. I.
Published: July 2002

We report direct transmission electron microscopy evidence of self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La0.8Sr0.2MnO3 (LSMO) thin films grown on (001) LaAlO3 (LAO) using pulsed-laser ablation. The orthogonal column nanostructures have an orthorhombic structure and are epitaxially grown on a continuous cubic perovskite LSMO thin-film layer that is epitaxially grown on (001) LAO substrate. The orthogonal column nanostructures exhibit a narrow size distribution, with the short edges having a length of about 25 nm and the long edge (growth direction) of ∼70 nm. The short edges are parallel to the [100] and [010] directions of LAO. All columns are encapsulated by uniformly thick amorphous-like grain boundaries and are “quasi” periodically arranged along the [100] and [010] directions of LAO. The continuous epitaxial layer of cubic perovskite structure grows along the [001] direction with an in-plane orientation relationship with respect to the substrate of 〈100〉LSMOC∥〈100〉LAO, while the orthorhombic column structures grow along its [11̄0] direction with an in-plane orientation relationship of [110]LSMOO∥[100]LAO and [001]LSMOO∥[010]LAO. The spontaneous formation of such self-organized, coherent column nanostructures can be considered to follow a type of Stranski–Krastanov growth mode without Oswald ripening.
Spectral-dependent cross-section determination for singlet excited states using z scan
Authors: Huang Yuhua , Yuan Yizhong , Ding Jingxin , Sun Zhenrong , Zeng Heping
Published: July 2002

We report a method to determine the spectral-dependent cross section of the first excited singlet state (S1) of dye molecules in solution. The method is based on the z-scan technique and requires the duration of the excitation laser pulses much shorter than the interband relaxation of S1 of the dye molecule. The method was used to determine the cross-section spectra of the first singlet excited state of an indoletricarboncyanine dye using 35 ps laser pulses.
Effect of Mn substitution on the volume and magnetic properties of Er2Fe17
Authors: Wang J. L. , Ibarra M. R. , Marquina C. , Garcı́a-Landa B. , Li W. X. , Tang N. , Wang W. Q. , Yang F. M. , Wu G. H.
Published: July 2002

The effects of Mn substitution on the magnetic properties of Er2Fe17−xMnx compounds have been investigated by different experimental techniques. An unusual composition dependence of the unit cell volume at room temperature occurs due to a strong magnetovolume effect, which has been confirmed by high temperature linear thermal expansion measurements. From magnetization measurements the composition dependence of the spontaneous magnetization of the 3d-sublattice magnetic moment and of the Curie temperature has been determined. Spin reorientation has been detected for compounds with 3⩽x⩽5.5, as well as remarkable magnetic history effects.
Dynamic light scattering studies of supercooled phenylphthalein–dimethylether dynamics under high pressure
Authors: Patkowski Adam , Paluch Marian , Kriegs Hartmut
Published: July 2002

The behavior of the α-relaxation process in phenylphthalein–dimethylether (PDE) under high pressure was studied by means of dynamic light scattering–photon correlation spectroscopy (DLS-PCS). The temperature dependence of the relaxation time of the α-process at ambient pressure was found to resemble that obtained from dielectric relaxation (DR) measurements. On the basis of the DLS measurements one can conclude that PDE satisfies the correlation between nonexponentiality and fragility established for low-molecular weight glass forming liquids and polymers. On the other hand, dielectric relaxation studies indicate that PDE is an exception from this correlation. These differences between the DLS and DR studies do not result from the overlap of the α- and β-processes in the DLS experiments but might be due to the different coupling of the probes seen in these experiments (optical anisotropy and dipole moment) to the dynamics of the entire PDE molecule. We also studied the effect of pressure on fragility and the glass transition temperature in PDE and we found that fragility is practically independent of pressure.
Photoluminescence spectrum of highly excited single CdS nanocrystals studied by a scanning near-field optical microscope
Authors: Kanemitsu Yoshihiko , Inagaki Takeshi J. , Ando Masanobu , Matsuda Kazunari , Saiki Toshiharu , White C. W.
Published: July 2002

Many-body effects in strongly photoexcited single CdS nanocrystals embedded in Al2O3 matrices have been studied by a scanning near-field optical microscope at low temperatures. Under weak excitations, we find a sharp photoluminescence (PL) band originating from the exciton recombination. With an increase of the excitation laser energy, the PL intensity increases and the broad PL band is superimposed on the exciton-related sharp PL band. From a theoretical analysis, it was concluded that the broadening of the PL band is due to electron-hole pairs in CdS nanocrystals. The many-body effects of correlated exciton systems in CdS nanocrystals will be discussed.
Self-diffusion of fluids in narrow cylindrical pores
Authors: Mon K. K. , Percus J. K.
Published: July 2002

Fluids under stochastic dynamics in narrow cylindrical pores exhibit a dynamical transition from single-file diffusion (SFD) to Fickian bulk diffusion. For long time, the mean square displacement will change as the pore size increases, with a transition from SFD, ∼t1/2, to Fickian, ∼t, while the diffusion coefficient (Dxx) increases from zero. We present a theory of this important process in terms of a hopping time, τhop, leading to Dxx∝(τhop)−1/2 which is verified with simulation. While the crossover is to be expected, the simple form is a priori unanticipated and is likely to be universal.
Single-crystalline gallium nitride nanobelts
Authors: Bae Seung Yong , Seo Hee Won , Park Jeunghee , Yang Hyunik , Park Ju Chul , Lee Soun Young
Published: July 2002

Single-crystalline wurtzite gallium nitride nanobelts were synthesized by thermal reaction of gallium, gallium nitride, and ammonia using iron and boron oxide as catalysts. The structure of nanobelts was investigated by high-resolution transmission electron microscopy with electron energy-loss spectroscopy. They have a distinctive triangle tip and thick side edges. The widths are 200–300 nm, the thickness of belt plane is about 1/10 of the width, and the lengths are up to a few tens μm. The growth direction is uniformly perpendicular to the [010] direction.
Periodically poled structures in doped lithium niobate crystals
Authors: Kokanyan Edvard P. , Babajanyan Vahan G. , Demirkhanyan Gagik G. , Gruber John B. , Erdei Sandor
Published: July 2002

Periodically poled lithium niobate crystals doped with transition-metal ions Cr3+ and Hf4+, and rare earth ions Er3+ and Yb3+ were grown by the Czochralski method. Periodic domain structures were obtained either by rotation-induced growth striations (during off-centered growth) or by the application of a periodic electric current passed through the crystal-melt system during growth. We achieved controlled formation of domains having a width from 4 to 50 μm. The influence of dopant ion concentrations and the type of these dopant impurities were investigated relative to the formation of the periodic domain structure. Methods that involve the use of scanning electron microscopy and single-pass second harmonic generation show a high degree of domain structure uniformity over a large section (∼3 mm) of the crystals grown. Samples doped with HfO2 show reasonable photorefractive resistance, an important feature in the application of these materials in photonic devices.
Pyroelectric properties of electrically poled photoaddressable polymers
Authors: Korneev N. , Ramirez O. Flores , Bertram R. P. , Benter N. , Soergel E. , Buse K. , Hagen R. , Kostromine S. G.
Published: July 2002

In this article, we present pyroelectric investigations of electrically poled side-chain polymers containing mesogenic disazobenzene chromophores using a very simple setup. Heating above the glass temperature (about 104°C) usually eliminates the pyroelectric effect of the sample. However, in samples with an indium tin oxide/aluminum electrode design, a self-poling effect is observed. The pyroelectric coefficients of electrically poled samples reach values of up to 0.2 μC/(m2K). From this, we conclude that the strongly anisometric side chains can be reoriented, and that their degree of alignment is about 3%.
Oxadiazole-containing material with intense blue phosphorescence emission for organic light-emitting diodes
Authors: Liang Fushun , Wang Lixiang , Ma Dongge , Jing Xiabin , Wang Fosong
Published: July 2002

2-(2-hydroxyphenyl)-5-phenyl-1, 3, 4-oxadiazole (HOXD), characteristic of excited state intramolecular proton-transfer (ESIPT), was synthesized and found to emit strong blue phosphorescence in the solid state at room temperature and at low temperature (77 K). The photoluminescent spectrum measurement in solution showed that there are two kinds of emission: fluorescence originated from the singlet state and phosphorescence derived from the triplet state in HOXD formed by ESIPT. For the photoluminescent spectrum in the solid state, only phosphorescence emission with the lifetime of 66 μs was observed. Multiple-layer light-emitting diodes with the configuration of ITO/NPB/HOXD/BCP/Alq3/Mg:Ag were fabricated using HOXD as emitter and the maximum brightness of 656 cd/m2 and the luminous efficiency of 0.14 lm/W was obtained.
Thermally-stable low-resistance Ti/Al/Mo/Au multilayer ohmic contacts on n–GaN
Authors: Kumar V. , Zhou L. , Selvanathan D. , Adesida I.
Published: July 2002

A metallization scheme consisting of Ti/Al/Mo/Au with excellent edge acuity has been developed for obtaining low-resistance ohmic contacts to n–GaN. Excellent ohmic characteristics with a specific contact resistivity as low as 4.7×10−7 Ω-cm2 were obtained by rapid thermal annealing of evaporated Ti/Al/Mo/Au at 850 °C for 30 sec in a N2 ambient. Additionally, no degradation in specific contact resistivity was observed for these contacts subjected to long-term annealing at 500 °C for 360 h.
High-density plasma-induced etch damage of InGaN/GaN multiple quantum well light-emitting diodes
Authors: Hahn Y. B. , Choi R. J. , Hong J. H. , Park H. J. , Choi C. S. , Lee H. J.
Published: July 2002

Plasma-induced damage of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) has been studied in terms of forward turn-on and reverse breakdown voltages, together with etch rate and surface morphology. The physical degradation of sidewall along with rough surface morphology of n–GaN caused by increased ion scattering induced the deterioration of the forward and reverse voltages. The forward turn-on voltage was relatively independent of the pressure up to 20 mTorr. The reverse breakdown voltage showed the worst degradation at 75% Cl2 mainly because of a sidewall contamination. It was found that the turn-on voltage is sensitive to the surface roughness of the etched n–GaN and the breakdown voltage is strongly affected by the sidewall contamination. Annealing under nitrogen after the mesa etching improved the electrical properties of the InGaN/GaN MQW LEDs.
Glucose biosensing using an enzyme-coated microcantilever
Authors: Subramanian A. , Oden P. I. , Kennel S. J. , Jacobson K. B. , Warmack R. J. , Thundat T. , Doktycz M. J.
Published: July 2002

A microcantilever-based biosensor is described. The enzyme glucose oxidase was immobilized on a micromachined silicon cantilever containing a gold coating, such as those used for atomic force microscopy. Specific, quantifiable deflection of the derivatized cantilevers was observed in the presence of the appropriate analyte. An analysis of the reaction energetics and the expected thermal response of the cantilever indicates that cantilever deflection is not simply a result of reaction-generated heat. This deflection appears to result from surface induced stresses. The combination of a highly specific enzyme and the microcantilever platform provides a unique approach for quantifying enzyme substrates without the complication of sample labeling.
Magnetic-domain-controlled vortex pinning in a superconductor/ferromagnet bilayer
Authors: Lange M. , Van Bael M. J. , Moshchalkov V. V. , Bruynseraede Y.
Published: July 2002

Vortex pinning in a type-II superconducting Pb film covering a Co/Pt multilayer with perpendicular magnetic anisotropy is investigated. Different stable magnetic-domain patterns like band and bubble domains can be created in the Co/Pt multilayer, clearly influencing the vortex pinning in the superconducting Pb layer. The most effective pinning is observed for the bubble domain state. We demonstrate that the pinning properties of the superconductor/ferromagnet bilayer can be controlled by tuning the size, density, and magnetization direction of the bubbles.
Rapid eutectic growth under containerless condition
Authors: Han X. J. , Wang N. , Wei B.
Published: July 2002

Rapid eutectic growth in a highly undercooled liquid was accomplished by containerlessly processing Co–Mo eutectic alloy in a drop tube. The containerless state during free fall produces substantial undercoolings up to 391 K (0.24TE) in falling droplets before crystallization. The eutectic growth mechanism is found to transform from lamellar eutectic to anomalous eutectic if the droplet undercooling exceeds a critical value of about 56 K, which coincides well with the lower boundary of the calculated eutectic coupled zone. Although the reduced gravity level of 10−2–10−3 g has little influence on this eutectic growth mechanism transition, it frequently results in the formation of spherical anomalous eutectic grains owing to the symmetrical temperature field and concentration field surrounding solid/liquid interface. Both theoretical analyses and experimental observations indicate that the independent nucleation and cooperative branched growth of two eutectic phases are responsible for the eutectic growth mechanism transition. The three-dimensional structural model of anomalous eutectic grain is a two-phase composite dendrite.
Transient reflecting grating spectroscopy for defect analysis of surface region of semiconductors
Authors: Donen Hiroshi , Katayama Kenji , Sawada Tsuguo
Published: July 2002

Ultrafast transient reflecting grating (TRG) spectroscopy was applied to investigate the influence of various defect states on ultrafast carrier dynamics of up to 3 ps duration in an ion-implanted silicon surface region. The TRG spectra revealed the energy-state distribution of two kinds of defect states, and it was observed that photoexcited carriers were trapped in each state depending on annealing time. It was proposed that TRG spectroscopy can be used as an analytical method for characterizing defects in the surface region of semicondcutors.
Investigation on indium diffusion in silicon
Authors: Solmi S. , Parisini A. , Bersani M. , Giubertoni D. , Soncini V. , Carnevale G. , Benvenuti A. , Marmiroli A.
Published: July 2002

The diffusion of indium in silicon has been investigated in the temperature range of 800 to 1000 °C by using secondary ion mass spectroscopy and transmission electron microscopy. Our data indicate that, for implants at 150 keV through a thin oxide layer (19 nm), the amount of dopant that leaves the silicon is only controlled by the flow of indium that reaches the surface, being both the segregation coefficient at the interface SiO2/Si and the indium diffusion coefficient in the oxide favorable to the out-diffusion. Comparison between experimental and simulated profiles has evidenced that, besides the expected transient enhanced diffusion occurring in the early phases of the annealing, a heavy loss of dopant by out-diffusion was associated with a high In diffusivity near the surface. Measurements of the hole concentration in uniformly doped silicon on insulator samples performed in the temperature range of 700 to 1100 °C indicate that indium solubility is equal or greater than 1.8×1018 cm−3; this value is higher than those previously proposed in literature.
High power and high brightness from an optically pumped InAs/InGaSb type-II midinfrared laser with low confinement
Authors: Kaspi R. , Ongstad A. , Dente G. C. , Chavez J. , Tilton M. L. , Gianardi D.
Published: July 2002

We report on optically pumped semiconductor lasers emitting near 3.8 μm that exhibit high power and low output divergence. The lasers incorporate multiple InAs/InGaSb/InAs type-II wells imbedded in an InGaAsSb waveguide that is designed to absorb the pump emission. When operated at 85 K, 0.25 mm×2.5 mm broad area devices produce >5 W of peak power under long pulse conditions. Moreover, these extremely bright devices exhibit a fast axis divergence of only ∼15° full width at half maximum (FWHM), coupled with a slow axis divergence of ∼6° FWHM. The first is due to the reduced optical confinement in the transverse direction, while the latter is attributed to the suppression of filament formation, which is another beneficial consequence of the low optical confinement.