bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and properties of polymethylene melt surfaces from molecular dynamics simulations
Authors: Chang Jaeeon , Han Jie , Yang Liu , Jaffe Richard L. , Yoon Do Y.
Published: July 2002

Thermodynamic, structural, and dynamic properties of polymethylene melt surfaces are studied by molecular dynamics simulations using both an explicit atom and a united atom model. N-tridecane (C13H28) melt films with a thickness of about 30 Å are studied by NVT-MD simulation method at the temperatures from 300 K to 450 K. We obtain stable surface properties such as surface tension, density profile, order parameter, and diffusivity upon performing the simulation on these films for 1 or 2 ns. When compared with experiment, simulations give a reasonable agreement for the surface tension with error of ca. 20%. It is observed that the density of chain-end group (methyl) is enhanced near the free surface, while it is depleted in the region below the surface. The interfacial thickness of the density transition region defined as liquid density divided by maximum density gradient is estimated to be about 5 Å at room temperature. In this interfacial region, a slight preference for chain segments to orient along the direction parallel to the surface is observed with practically no difference in the chain conformation from the bulk value. The molecular diffusivity along the film surface is enhanced by a factor of ca. 3 compared with the diffusivity along the surface normal in the interfacial region. Both the explicit atom and the united atom model show almost the same thermodynamic and structural properties near the surface.
Rhodium doped manganites: Ferromagnetism and metallicity
Authors: Raveau B. , Hébert S. , Maignan A. , Frésard R. , Hervieu M. , Khomskii D.
Published: July 2002

It is shown that ferromagnetism and insulator to metal transitions in small A site cation manganites Pr1−xCaxMnO3 are induced by rhodium doping. Colossal magnetoresistance properties are evidenced for a large compositional range (0.35⩽x<0.60). The ability of rhodium to induce such properties is compared to the results obtained by chromium and ruthenium doping. Models are proposed to explain this behavior.
High thermal stability of exchange-biased bilayers and bottom giant magnetoresistive spin valves using an α-Fe2O3 antiferromagnetic layer
Authors: Bae Seongtae , Judy Jack H. , Chen P. J. , Egelhoff William F. , Zurn Shayne
Published: July 2002

The thermal stability of antiferromagnetic α-Fe2O3 exchange-biased bilayers and bottom giant magnetoresistive (GMR) spin valves has been investigated experimentally at various ambient temperatures. An α-Fe2O3 exchange-biased bilayer exhibited a high blocking temperature, Tb, of 390 °C, and a bottom spin valve showed stable GMR performance above 350 °C. The Tb of α-Fe2O3 exchange-biased bilayers depended strongly on the adjacent ferromagnetic material and the number of measurements. In addition, the increasing mean grain size and enhanced (104) and (110) crystalline texture of α-Fe2O3 increased Tb by up to 23%.
Computer simulation of void growth dynamics under the action of electromigration and capillary forces in narrow thin interconnects
Authors: Ogurtani Tarik Omer , Oren Ersin Emre
Published: July 2002

A comprehensive picture of void dynamics in connection with the critical morphological evolution has been developed in order to understand the conditions under which premature failure of metallic thin interconnects occurs. Our mathematical model on the mass flow and accumulation on void surfaces, under the action of applied electrostatic and elastostatic force fields, and capillary effects, follows an irreversible but discrete thermodynamic formulation of interphases and surfaces. This formalism also takes into account in a natural way the mass transfer process (the void growth), between bulk phase and the void region in multi-component systems, in terms of the normalized local values of Gibbs free energy of transformation with respect to the specific surface Gibbs free energy, in addition to the contribution due to local curvature of the advancing reaction front, rather rigorously.
In-plane optical anisotropy of symmetric and asymmetric (001) GaAs/Al(Ga)As superlattices and quantum wells
Authors: Ye Xiaoling , Chen Y. H. , Wang J. Z. , Xu B. , Wang Z. G. , Yang Z.
Published: July 2002

Two sensitive polarized spectroscopies, reflectance difference spectroscopy and photocurrent difference spectroscopy, are used to study the characteristic of the in-plane optical anisotropy in the symmetric and the asymmetric (001) GaAs/Al(Ga)As superlattices (SLs). The anisotropy spectra of the symmetric and the asymmetric SLs show significant difference: for symmetric ones, the anisotropies of the 1HH→1E transition (1H1E) and 1L1E are dominant, and they are always approximately equal and opposite; while for asymmetric ones, the anisotropy of 1H1E is much less than that of 1L1E and 2H1E, and the anisotropy of 3H2E is very strong. The calculated anisotropy spectra within the envelope function model agree with the experimental results, and a perturbation approach is used to understand the role of the electric field and the interface potential in the anisotropy.
Inverse bremsstrahlung of hot, weakly coupled plasmas
Authors: Wierling A. , Millat Th. , Röpke G. , Redmer R. , Reinholz H.
Published: July 2002

The free–free absorption coefficient for radiation in hot, weakly coupled plasmas is determined from a systematic approach to the dynamical conductivity. Based on a generalized linear-response theory, it is expressed in terms of determinants of equilibrium correlation functions permitting a perturbative treatment. Within a Gould–DeWitt-type approach, dynamical screening is accounted for by a partial summation of loop diagrams, while strong collisions are treated by a ladder approximation. Known expressions for the absorption coefficient are reproduced when considering certain limits. A comparison is made with simulation results.
Bremsstrahlung emission from interactions between two dressed dust grains in dusty plasmas
Authors: Jung Young-Dae
Published: July 2002

Bremsstrahlung emission from interactions between two dressed grains charged with the same sign in dusty plasmas are investigated using the classical trajectory method. An interaction potential model taking into account the cross terms of shield effects is applied to describe the interaction potential between dressed dust grains in dusty plasmas. The classical straight-line trajectory method is applied to represent the bremsstrahlung cross section as a function of the scaled impact parameter, collision energy, and photon energy. The result shows that the potential well in the interaction potential plays an important role in the bremsstrahlung spectrum. It is also found that there exists a nonradiation position in the bremsstrahlung radiation due to the balance between the repulsive and attractive interactions.
Coherent states for SU(3)
Authors: Mathur Manu , Sen Diptiman
Published: July 2002

We define coherent states for SU(3) using six bosonic creation and annihilation operators. These coherent states are explicitly characterized by six complex numbers with constraints. For the completely symmetric representations (n,0) and (0,m), only three of the bosonic operators are required. For mixed representations (n,m), all six operators are required. The coherent states provide a resolution of identity, satisfy the continuity property, and possess a variety of group theoretic properties. We introduce an explicit parametrization of the group SU(3) and the corresponding integration measure. Finally, we discuss the path integral formalism for a problem in which the Hamiltonian is a function of SU(3) operators at each site.
On the X 1∑+ state of KLi
Authors: Martin F. , Crozet P. , Ross A. J. , Aubert-Frécon M. , Kowalczyk P. , Jastrzȩbski W. , Pashov A.
Published: July 2002

Laser-induced fluorescence spectra of the B–X system of K39Li7, recorded on a Fourier transform interferometer, have allowed 47 vibrational levels of the electronic ground state to be observed. The ground state energies have been fitted to a Dunham polynomial expansion, and also directly to a numerical potential curve. Both fits reproduce the data to within the experimental accuracy of measurement (0.005 cm−1).
Asymmetric vortex merger: Experiments and simulations
Authors: Amoretti M. , Durkin D. , Fajans J. , Pozzoli R. , Romé M.
Published: July 2002

The two-dimensional (2-D) merging of an intense, pointlike vortex with a diffuse, extended vortex is investigated with experiments using strongly magnetized electron columns in a Malmberg–Penning trap, and with numerical simulations using a 2-D particle-in-cell code. The study is restricted to highly nonlinear conditions, where the perturbative approach does not apply. A very good agreement between experiment and simulation is obtained. The pointlike vortex wraps the extended vortex about itself, moving toward the center of the system during the process. The interaction generates filaments of zero vorticity within the extended vortex that subsequently evolve into vorticity holes. During the evolution, energy is fed to the extended vortex from the background curl-free flow via the stirring action of the pointlike vortex, whose energy remains approximately constant.
The bridge functions of molten salts
Authors: Taşseven Çetin , Enrique González Luis , Silbert Moises , Alcaraz Olga , Trullàs Joaquim
Published: July 2002

The bridge functions of molten NaCl and AgI near melting were obtained by using the model potentials of Born–Huggins–Mayer for NaCl and Vashishta–Rahman for AgI. The calculations of the bridge functions involved molecular dynamics simulations, the extension of the procedure originally proposed by Poll et al. [Phys. Rev. A 37, 1672 (1988)] and the numerical solution of the Ornstein–Zernike equations. The calculated bridge functions do not conform with the universality ansatz. They also differ from the bridge functions obtained for model electrolytes. Following the results obtained for electrolytes and those of this work for molten salts, it is conjectured that the universality ansatz for the bridge functions does not apply for systems whose attractive interactions play a decisive role in their structural ordering.
The structure of a fluid mixture of deuterated ethane and deuterated methane by high-pressure neutron diffraction experiments
Authors: Pfleiderer Till , Bertagnolli Helmut , Tödheide Klaus , Fischer Henry E.
Published: July 2002

High-pressure neutron diffraction data on a mixture of 33.3 mole % deuterated ethane and 66.7 mole % deuterated methane at six supercritical states are presented. The density could be varied by a factor larger than 2. The density dependence of the intra- and intermolecular structure is determined. The resulting total atomic pair correlation functions are compared with the results of Monte Carlo computer simulations with effective pair potentials.
Design and construction of a space-borne optical tweezer apparatus
Authors: Resnick Andrew
Published: July 2002

A compact optical tweezer package has been developed for use on a microscope to be flown on the International Space Station as part of a series of experiments in colloid crystallization. A brief introduction to the principles of single-beam optical tweezer operation will be presented, after which a detailed system layout will be shown. Special design requirements due to the spaceflight nature of the hardware will also be discussed. The tweezer apparatus is capable of trapping many particles through use of a two-axis acousto-optical deflector. The trap strength is sufficient to perform the required science (50 pN at Δn=0.2). The trap beam behaves approximately as a diffraction limited single mode Gaussian beam of numerical aperture, NA=1.4, as shown through spot size measurements and confocal-type images of the focal region. This is the first time optical tweezers will be deployed in a microgravity environment.
How does the closing of the ring affect the electric properties of sulphur dioxide? A comparison with the open and closed form of ozone
Authors: Xenides Demetrios , Maroulis George
Published: July 2002

We have calculated the static polarizability and hyperpolarizability of the ring structure of sulphur dioxide. Our best values for the mean dipole polarizability (ααβ), the first (βαβγ) and second (γαβγδ) hyperpolarizability, obtained at the CCSD(T) level of theory with α carefully optimized basis set, are ᾱ=28.94 e2a02Eh−1, β̄=110.6 e3a03Eh−2 and γ̄=6117 e4a04Eh−3. At the same level of theory with a basis set of exactly the same size, the relevant quantities for the open structure are ᾱ=25.84 e2a02Eh−1, β̄=30.1 e3a03Eh−2 and γ̄=3390 e4a04Eh−3 [D. Xenides and G. Maroulis, Chem. Phys. Lett. 319, 618 (2000)]. Thus, the ring structure of sulphur dioxide is significantly more polarizable than the isoelectronic open one. This is in marked contradistinction to ozone, in which case the open system represents the more polarizable form. Electric polarizability and hyperpolarizability increases as O3(ring)<O3(open)<SO2(open)<SO2(ring).
Trions in a periodic potential
Authors: Florek Wojciech
Published: July 2002

The group-theoretical classification of trion states (charged excitons X±) is presented. It is based on considerations of products of irreducible projective representations of the two-dimensional translation group. For a given BvK period N degeneracy of obtained states is N2. Trions X± consist of two identical particles (holes or electrons), so the symmetrization of states with respect to particles transposition is considered. There are N(N+1)/2 symmetric and N(N−1)/2 antisymmetric states. Completely antisymmetric states can be constructed by introducing antisymmetric and symmetric spin functions, respectively. Two symmetry-adapted bases are considered: The first is obtained from a direct conjugation of three representations, whereas in the second approach the states of an electrically neutral pair hole–electron are determined in the first step. The third possibility, a conjugation of representations corresponding to identical particles in the first step, is presented elsewhere.
Role of the attractive portion of the Lennard-Jones potential in the homogeneity of melts of isotactic and syndiotactic polypropylene
Authors: Clancy Thomas C. , Mattice Wayne L.
Published: July 2002

Recently Clancy et al. reported a Monte Carlo simulation that reproduces the tendency, observed by Mülhaupt and co-workers, for demixing of melts of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP). The short-range intermolecular interactions in the coarse-grained chains in the simulation are controlled by a rotational isomeric state model for PP that was described by Suter et al. The intermolecular interactions of the beads are controlled by a discretized representation of a continuous Lennard-Jones (LJ) potential that describes the interaction of two molecules of propane. The work reported here reveals that the outcome is sensitive to the truncation of this LJ potential. A weak tendency for demixing of the 50:50 iPP:sPP melt is seen if the LJ potential is truncated so that only its repulsive part is used in the simulation. Inhomogeneity is enhanced if the truncation of the LJ potential is shifted to larger distances, so that the attractive branch of the LJ potential is incorporated in the simulation. This observation in the simulation leads to conclusive identification of the mechanism responsible for demixing of the 50:50 melt. It also implies that the same mechanism may affect the miscibility of melts of other vinyl polymers in which the chains differ in stereochemical composition.
Localized oscillations and Fraunhofer diffraction in crystalline phases of a monolayer
Authors: Galvan-Miyoshi Julián , Ramos Salvador , Ruiz-Garcia Jaime , Castillo Rolando
Published: July 2002

Localized oscillations present in the crystalline phases of the heneicosanoic acid Langmuir monolayer were studied in detail. They appear like blinking interference rings, when observed with Brewster angle microscopy. Monolayers with localized oscillations were transferred on mica to be characterized by atomic force microscopy. We found granules produced by the expulsion of matter from the monolayer. However, these granules are too short to produce Newton’s rings; the common belief of the origin of the interference rings in the field of Langmuir monolayers. The analysis of the light intensity distribution and the sizes of the rings are consistent with Airy patterns produced by Fraunhofer diffraction due to the reflected light from the multilayer granules. The origin of the blinking of these patterns is still unclear.
Electron transfer through fluctuating bridges: On the validity of the superexchange mechanism and time-dependent tunneling matrix elements
Authors: Skourtis Spiros S. , Archontis Georgios , Xie Qian
Published: July 2002

The superexchange mechanism of electron-transfer reactions is studied for time-dependent donor–bridge–acceptor systems. It is shown that superexchange may not be a relevant mechanism in a situation where donor and acceptor states are off-resonant to the bridge with an energy gap much greater than KBT. The competing mechanism in this case involves coherent through-bridge transfer. Methods for estimating its contribution to the electron-transfer probability are presented. It is also shown that the superexchange component of the electron-transfer probability can generally be described by a time-dependent two-state effective Hamiltonian. The off-diagonal element of this Hamiltonian is a generalized superexchange matrix element applicable to time-dependent donor–bridge–acceptor systems. It is nonperturbative and should be used to compute time-dependent superexchange pathways. The derivation of the effective Hamiltonian also applies to time-dependent superexchange systems with multiple donor (acceptor) states. All calculations are carried out on azurin and they involve molecular dynamics simulations coupled to electronic-structure/dynamics calculations using the complete neglect of differential overlap method.
Interaction of F2 excimer laser pulses with hydroxy groups in SiO2 glass: Hydrogen bond formation and bleaching of vacuum ultraviolet absorption edge
Authors: Kajihara Koichi , Ikuta Yoshiaki , Hirano Masahiro , Ichimura Tomoaki , Hosono Hideo
Published: July 2002

Excitation of vacuum ultraviolet absorption band of SiOH groups by F2 excimer laser photons (7.9 eV) was performed for synthetic SiO2 glasses. Irradiation induced two concurrent optical changes; red-shift of SiOH infrared absorption band and bleaching of vacuum ultraviolet absorption edge which is primarily controlled by SiOH absorption. These optical changes have the same origin, that is, the structural alternation of SiOH from isolated to metastable hydrogen-bonded states. The F2-induced hydrogen-bonded state was thermally stable to ∼300 °C and the kinetic barrier for the thermal relaxation was ∼1.7 eV. The formation of hydrogen-bonded state proceeded via one-photon absorption processes for F2 laser irradiation, but did not occur for ArF laser (6.4 eV) irradiation. This F2-induced hydrogen-bonded state is distinguished from laser-induced compacted state because the latter occurs for ArF or F2 laser irradiations via two-photon absorption processes.
First step towards the growth of single-crystal oxides on Si: Formation of a two-dimensional crystalline silicate on Si(001)
Authors: Liang Y. , Gan S. , Engelhard M.
Published: July 2002

We have investigated the structural and chemical properties of reconstructed Sr/Si(001) surfaces at different Sr coverages using low energy electron diffraction, x-ray photoelectron spectroscopy, and scanning tunneling microscopy. The results show that upon low temperature oxidation and subsequent UHV annealing of the Sr/Si(001)-(2×1) surface, a crystalline Sr2SiO4 silicate-like layer formed. Using this layer as a template, single-crystal SrO thin films were grown on Si(001) substrates. Our results provide microscopic and spectroscopic evidence of the formation of a uniform, stable, two-dimensional crystalline silicate that can be used for growth of single-crystal oxides on Si(001) substrates.
Metallic ferromagnetism in Ni-doped La0.7Sr0.3CoO3
Authors: Wang Z. H. , Wang F. W. , Chen X. , Li R. W. , Shen B. G.
Published: July 2002

The structure, magnetization, and electrical transport of perovskite cobaltites La0.7Sr0.3Co1−yNiyO3 have been studied. It is found that the substitution of Ni for Co induces no apparent changes in bond length and angle in the Co(Ni)–O network, and only moderately suppresses the ferromagnetism. Moreover, a retention of the metallic conduction has been observed from y=0 to 0.2, which is in strong contrast to the reported insulating state of the other Co site doped La0.7Sr0.3CoO3. A double-exchange-like interaction between Ni ion and its neighboring Co ion is suggested to explain the experimental results.
Measurement of ion temperatures in a large-diameter electron cyclotron resonance plasma
Authors: Koga Mayuko , Yoshizawa Takenori , Ueda Yoko , Kawai Yoshinobu , Yonesu Akira
Published: July 2002

The ion temperature in a large-diameter electron cyclotron resonance plasma is measured using high-resolution optical emission spectroscopy, and the correlation between the ion temperature and fluctuations observed near the chamber wall is investigated. Furthermore, the effect of multicusped fields on the ion temperature is examined. The ion temperature and the amplitude of ion saturation current fluctuations are found to decrease when multicusped fields are applied. The ion temperature and fluctuations increase with increasing incident microwave powers from 2.0 to 2.5 kW, indicating that the ion temperature is correlated with the fluctuation amplitude. The measurement of the ion saturation current fluctuation and floating potential fluctuation suggests that the fluctuations are excited by flute instability.
Ionizing radiation effects in copper indium gallium diselenide thin-film solar cells
Authors: Tringe J. , Nocerino J. , Tallon R. , Kemp W. , Shafarman W. , Marvin D.
Published: July 2002

Cu(In, Ga)Se 2 /CdS/ZnO thin-film solar cells were exposed to doses up to 5×108 rad(Si) of 50 keV x rays. Device performance consistently showed very little degradation, implying that previously observed radiation-induced performance reductions were likely the result of displacement damage. Subsequent experiments showed that cells recovered to near beginning-of-life performance with only short anneals under 50 °C.
Nanomechanical properties and morphology of thick polyurethane films under contact pressure and stretching
Authors: Amitay-Sadovsky E. , Ward B. , Somorjai G. A. , Komvopoulos K.
Published: July 2002

The surface nanomechanical properties and morphology of ∼500-μm-thick polyurethane films subjected to normal loads in the range of 3–35 nN and longitudinal strains up to 150% were investigated with an atomic force microscope. Nanoindentation experiments performed at very low loads and small penetration depths demonstrated that the elastic behavior of the polyurethane surface is remarkably different from that of the bulk material. The distinctly different mechanical response to applied pressure between surface and near-surface (bulk) regions of the polymer is attributed to packing of the polymer chains at the contact region. The polyurethane copolymer consists of two immiscible hard and soft segments. It is suggested that upon stretching, randomly oriented hard segments migrate to the surface, leading to an increase of the hard-to-soft segment concentration and a decrease of the surface roughness. A general description of the polyurethane surface and bulk behavior under stretching and compression is presented in the context of the obtained results.
Photoinduced nonadiabatic dynamics in quartet Na3 and K3 formed using helium nanodroplet isolation
Authors: Reho J. H. , Higgins J. , Nooijen M. , Lehmann K. K. , Scoles G. , Gutowski M.
Published: July 2002

Helium nanodroplet isolation is used to produce van der Waals-bound quartet state alkali trimers (Na3 and K3) selectively over the corresponding chemically bound doublet trimers. Frequency-resolved excitation and emission spectroscopy reveals the presence of nonadiabatic spin–flip processes in the electronically excited states. A total of four quartet to quartet electronic transitions are observed: the 2 4E′,1 4E←1 4A2′ transitions of Na3 and the 1 4A1″,2 4E′←1 4A2′ transitions of K3. Time-resolved spectroscopy reveals that intersystem crossing times in Na3 decrease from 1.4 ns after excitation of the 0–0 band to approximately 400 ps for the higher vibronic levels (3,5/2). Analysis of the resonant quartet fluorescence reveals that the excited electronic state cools vibrationally on a time scale that is comparable to, but slower than, the intersystem crossing time.
Spatial variation of photoluminescence and related defects in InGaN/GaN quantum wells
Authors: Jeong M. S. , Kim Y.-W. , White J. O. , Suh E.-K. , Cheong M. G. , Kim C. S. , Hong C.-H. , Lee H. J.
Published: July 2002

Spatially and spectrally resolved photoluminescence of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition is studied with near-field scanning optical microscopy (NSOM) and transmission electron microscopy (TEM). High-spatial-resolution NSOM images show bright blue quantum well emission around V defects and yellow emission inside the defects. TEM data suggest that the spatial distribution of blue luminescence is partly due to dislocation gettering by V defects. The yellow emission is attributed to the Ga vacancy-impurity complexes trapped inside V defects.
Effects of structural fluctuations on three-dimensional photonic crystals operating at near-infrared wavelengths
Authors: Ogawa Shinpei , Tomoda Katsuhiro , Noda Susumu
Published: July 2002

We investigate the effects of structural fluctuations on three-dimensional photonic crystals with stacked-stripe (woodpile) structures at near-infrared wavelengths. Two photonic crystals are prepared: one with a perfect structure and the other with significant structural irregularities artificially introduced. Through an experimental comparison of the optical properties and photonic band gap structures of these two photonic crystals, it is shown that the structure treated here is very robust to structural fluctuations.
Monte Carlo simulation of ions inside a cylindrical bore for plasma source ion implantation
Authors: Liu Chengsen , Wang Dezhen
Published: July 2002

The ion impact energy and angle distributions at the interior sidewall of a large cylindrical bore in the presence of an auxiliary electrode for plasma source ion implantation are determined. A collisional model is developed for cylindrical bore geometry by using Monte Carlo techniques. The ion-neutral charge exchange and momentum-transfer cross sections that depend on the ion energy are taken into account precisely. The simulation results for ions (Ar+) implanting into the target for different pressures are given and the relationship between the distributions and the pressure of the neutral gas is investigated.
Higher-order nonlinear dielectric microscopy
Authors: Cho Yasuo , Ohara Koya
Published: July 2002

A higher-order nonlinear dielectric microscopy technique with higher lateral resolution than conventional nonlinear dielectric imaging is investigated. The proposed scheme involves the measurement of higher-order nonlinear dielectric constants, with sensitivity to the first 0.75 nm in depth for LiNbO3 substrate surface. This technique is demonstrated to be very useful for observing surface layers of the order of unit cell thickness on ferroelectric materials.
Effect of surfactant Sb on carrier lifetime in GaInP epilayers
Authors: Fetzer C. M. , Lee R. T. , Stringfellow G. B. , Liu X. Q. , Sasaki A. , Ohno N.
Published: July 2002

Samples of Ga0.52In0.48P grown on (001) GaAs with small amounts of surfactant Sb were investigated using time-resolved photoluminescence. All samples show a luminescence that may be fit to a two-stage exponential decay with a fast and a slow lifetime. For growth without Sb (Sb/III(v)=0), the sample shows a strong CuPtB ordering and a fast component lifetime of 7 ns. As the Sb concentration is increased, the degree of order is reduced, with a consequent increase in band gap energy. In the highest band gap material, produced at Sb/III(v)=0.016, the fast lifetime is 2.9 ns, an order of magnitude larger than published values for GaInP disordered by misorienting the substrate. Increasing the Sb further causes the band gap energy to decrease due to the onset of composition modulation. At Sb/III(v)=0.064, the fast component lifetime decreases to 0.79 ns. Samples grown with Sb/III(v)>0.016 show a lifetime that depends on energy and is fit well by a model of localized excitons.
Preliminary experiment for residual stress analysis at the advanced photon source
Authors: Xiao Yanan , Lee Myungae , Wittmer Dale E. , Graber Tim , Gebhardt Jeffrey , Viccaro P. James , Mini Susan M. , Linton Jennifer , Beno Mark
Published: July 2002

In order to evaluate and find the proper conditions for residual stress (RS) measurements, two kinds of preliminary x-ray diffraction experiments were conducted at the Advanced Photon Source: angle-dispersive x-ray diffraction and energy-dispersive x-ray diffraction. These two experimental methods and their precision were investigated for RS measurement. The results obtained and various factors that may influence the accuracy of these experiments are discussed.
Monte Carlo ray-tracing error analysis of a sagittal-focusing optical system as applied to synchrotron radiation
Authors: Bai Jianming , Ice Gene , Sparks Cullie
Published: July 2002

A Monte Carlo ray-tracing program was developed to analyze the geometrical defects and alignment errors on the shape of the x-ray beam image and focusing efficiency of a conical sagittal-focusing monochromator. The geometrical defects and alignment errors were represented with measurable and controllable parameters so that their effects can be analyzed quantitatively. The programmed analysis can be used as a guide to find the precision needed for an optimal sagittal-focusing optical system in terms of flux and focus for large horizontal divergences.
Versatile focusing using a combination of toroidal and Kirkpatrick–Baez mirrors
Authors: Heald S. M.
Published: July 2002

Toroidal mirrors can be used to efficiently focus synchrotron radiation for magnifications near 1. At the Pacific Northwest Consortium-Collaborative Access Team (PNC-CAT) insertion device beamline, a toroidal mirror is used to focus the entire undulator beam to a spot size of 400(H)×100(V) μm2. For large demagnifications, Kirkpatrick–Baez (KB) mirrors are a good choice. Spot sizes down to 1 μm have been achieved at PNC-CAT. However, these mirrors only collect a fraction of the undulator beam, and result in a highly divergent beam with a short working distance. To achieve an intermediate beam size with less divergence, the toroidal and KB mirrors can be combined. The toroidal mirror reduces the beam to match the entrance aperture of the KB mirrors. This combination can provide beam sizes below 100 μm while collecting the entire undulator beam and providing a convenient working distance. Thus, the beam size, working distance, and divergence can be tailored to the needs of the experiment.
Doping mechanism of antinomy in PbWO4
Authors: Li Wensheng , Tang Tong B. , Feng Xiqi
Published: July 2002

Sb doped PbWO4 (Sb:PWO) shows unique features in its dielectric and visible spectra. We propose that, in low concentration, the dopant enters the lattice as interstitial ions, and at high level it also substitute for W6+ sties. The existence of interstitial ions with relatively high mobility leads to non-negligible dc conductivity, whereas the substitutional impurity produces O23− color centers, which results in absorption at 420 nm, as well as holes hopping among oxygen ions in the Sb–O tetrahedra, that is the origin for the observed dielectric relaxation with an unusually low activation energy of 30±2 meV.
Luminescence from stacking faults in 4H SiC
Authors: Sridhara S. G. , Carlsson F. H. C. , Bergman J. P. , Janzén E.
Published: July 2002

A previously unreported photoluminescence spectrum observed in certain 4H SiC bipolar diodes after extended forward voltage operation is reported. We assign this emission to exciton recombination at local potential fluctuations caused by stacking faults, which are created during operation of the diodes. Possible recombination mechanisms responsible for the spectrum are discussed.
Performance of a rapid-scan vacuum Michelson interferometer at the NSLS
Authors: Brierley P. , Badeau B. , Dumas P. , Smith M. , Williams G. P.
Published: July 2002

A commercial Nicolet Magna-IR series rapid-scan Michelson Fourier transform infrared was installed in a vacuum housing and integrated into the U4IR beamline at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The frequency reference laser was mounted outside the vacuum, but the moving mirror mechanism and the dynamic alignment system for the fixed mirror were in a vacuum. The performance of the instrument was measured in the usual way by measuring the repeatability of data collected under specific conditions of aperture, resolution, and mirror scanning velocity. We briefly discuss the beamline design, to put the interferometer in context, then present signal to noise data which we discuss in terms of both instrument performance and also storage ring beam stability. Under optimal conditions, the instrument has a reproducibility of 0.01% in 1 min of measuring time at a resolution of 2 cm−1, over a range from 100–3000 cm−1.
Modeling and analysis of the electron cyclotron resonance diamond-like carbon deposition process
Authors: Yoon S. F. , Tan K. H. , Rusli  , Ahn J.
Published: July 2002

Diamond-like carbon (DLC) films were deposited using the electron cyclotron resonance (ECR) chemical vapor deposition process. The behavior of the ECR plasma was formulated using deposition conditions such as microwave power, pressure, and hydrogen/methane (H2/CH4) ratio as input parameters. Thereafter, the outputs were used to formulate a DLC film deposition model, which takes into account the ion bombardment at the film surface, attachment of carbon-carrying ions, and chemisorption of hydrocarbon radicals on the film and hydrogen–surface reactions. The DLC film deposition model suggests that under conditions of high hydrogen atom flux, the main precursors are carbon-carrying ions. Hydrocarbon radicals, such as CH3, only contribute to ∼20% of the total film deposition rate.
A new cylindrical photoacoustic cell with improved performance
Authors: Hao Lu-Yuan , Ren Zhi , Shi Qiang , Wu Jian-Lan , Zheng Yan , Zheng Jing-Jing , Zhu Qing-Shi
Published: July 2002

We report a new design for an acoustically resonant photoacoustic cell with high performance. A cylindrical cell with suitable size was selected so that the resonant frequency of the first radial mode was equal to that of a longitudinal higher mode. The ends of the cell have a pair of thin coaxial tubes which lengths are 1/2 and 1/4 of acoustic wavelength λ, respectively. The λ/4 acoustic resonance makes a coupling between the first radial resonance and the higher longitudinal resonance, then leads to an acoustic energy concentrating in the middle of the cell. Thus, the surface loss was decreased, the acoustic quality factor and pressure amplitude increased obviously as compared with conventional radial resonant cylindrical cell. The ability to measure absorption coefficients as low as 2.43×10−9 cm−1 (1:1 signal to noise level) has been achieved.
Observation of in plane magnetization reversal using polarization dependent magneto-optic Kerr effect
Authors: Ohldag H. , Weber N. B. , Hillebrecht F. U. , Kisker E.
Published: July 2002

We present an experimental setup for in plane two axis magnetometry using the polarization dependent magneto–optic Kerr effect (MOKE). A conventional setup to measure longitudinal MOKE with crossed polarizers is extended by a Faraday cell to compensate for the rotation of the polarization vector caused by a magnetized sample. The shape of the hysteresis loops measured on thin FeNi alloy films depends strongly on the angle between the optical axis of the analyzer and the plane of incidence. We derive expressions for the compensation angle which allow for extraction of vectorial magnetic information from loops detected with oblique polarization. For a small deviation from pure s or p polarization the transverse magnetization is found to be proportional to the difference between the loop obtained with oblique polarization and the one obtained with pure s or p polarization. Thus the complete in plane reversal process split up into longitudinal and transverse components can be observed.
Photocarrier generation and bipolar transport in diphenoquinone doped polymethylphenylsilane thin films
Authors: Ohsawa Yoshikazu , Naito Hiroyoshi
Published: July 2002

Bipolar transport and photocarrier generation processes in thin films of 3,5-dimethyl-3′,5′-di- tert-butyl-4,4′-diphenoquinone (MBDQ) doped polymethylphenylsilane (PMPS) are studied in terms of optical absorption, photoluminescence, the standard time-of-flight, and electroabsorption experiments. With increasing MBDQ concentration, the electron drift mobility is observable above 30 wt. % MBDQ addition (5.0×10−7 cm2/Vs at 40 wt. % MBDQ addition) and the hole drift mobility is unchanged (∼1.0×10−4 cm2/Vs) for 0–40 wt. % MBDQ addition. The photocarrier generation efficiency is increased by an order of magnitude over pristine PMPS for 30–40 wt. % MBDQ addition in a visible spectral range. The electroabsorption experiments of MBDQ/PMPS composites show that the existence of charge transfer states becomes evident with increasing MBDQ concentration, which facilitate the photocarrier generation in the spectral range.
Si
3
N
4
/
SiC
interface structure in SiC-nanocrystal-embedded α-Si3N4 nanorods
Authors: Gao Y. H. , Bando Y. , Kurashima K. , Sato T.
Published: July 2002

Si 3 N 4 / SiC interface structure in SiC-nanocrystal-embedded α-Si3N4 nanorods was studied by high-resolution transmission electron microscopy. The SiC-nanocrystal-embedded α-Si3N4 nanorods were synthesized by the method of carbothermal reduction of SiO in pure N2 atmosphere, while the SiC nanocrystals were produced from a substitution of SiC for Si3N4. Between SiC and Si3N4, there are three kinds of plane configurations and a set of orientation relationships, i.e., [11̄0]SiC//[0001]Si3N4 and nearly (111)SiC//(101̄0)Si3N4 with low-angle discrepancy of either 3° or 5°. The origin of the low-angle discrepancies was explained in terms of a reciprocal lattice theory for heteroepitaxial interfaces.
Ionization potentials and radii of neutral and ionized species of elements 107 (bohrium) and 108 (hassium) from extended multiconfiguration Dirac–Fock calculations
Authors: Johnson E. , Fricke B. , Jacob T. , Dong C. Z. , Fritzsche S. , Pershina V.
Published: July 2002

Large multiconfiguration Dirac–Fock calculations were performed for all neutral and up to 7 and 8 times ionized species of elements 107 and 108, respectively, as well as all the homologous elements Mn, Tc, Re, Fe, Ru, and Os for a number of different total angular momenta. Comparison of the resulting ionization potentials to the known values of the homologues allow an extrapolation and thus a prediction of the chemically important seven or eight ionization potentials of elements 107 or 108 as well as a prediction of some ionic radii.
Effect of cosubstitution of La and V in Bi4Ti3O12 thin films on the low-temperature deposition
Authors: Watanabe Takayuki , Funakubo Hiroshi , Osada Minoru , Noguchi Yuji , Miyayama Masaru
Published: July 2002

The ferroelectricity of Bi4Ti3O12 (BIT), (Bi3.2La0.8)Ti3O12 (BLT), Bi4(Ti2.97V0.03)O12 (BTV), and (Bi3.2La0.8)(Ti2.97V0.03)O12 (BLTV) films prepared at 600 °C by metalorganic chemical vapor deposition was compared. For the BIT, BLT, BTV films deposited on (111)Pt/TiO2/SiO2/Si substrates, the ferroelectricity was not obtained even though the films consisted of a single phase. On the other hand, the BLTV films exhibited clear ferroelectricity. Furthermore, the degree of squareness of the hysteresis loops of the BLTV films was improved by changing the bottom electrode from Pt to Ir, (111)Ir/TiO2/SiO2/Si substrate; the remanent polarization and the coercive field became 8.5 μC/cm2 and 48 kV/cm, respectively. Moreover, good fatigue endurance up to 109 switching cycles was confirmed for the (Bi3.2La0.8)(Ti2.97V0.03)O12 thin film. Therefore, cosubstitution of La and V in BIT thin films is effective for lowering the deposition temperature of the film.
ECR ion sources and rare isotope beams at Louvain-la-Neuve
Authors: Gaelens Michel , Loiselet Marc , Ryckewaert Guido
Published: July 2002

Electron cyclotron resonance (ECR) ion sources are used in a variety of applications. A lot of them are put to work to ionize abundant isotopes at high intensity and/or high charge states. But another important field for these kinds of sources is also to ionize, with high efficiency, isotopes which are available only in very small quantities, e.g., radioactive isotopes or stable isotopes with very small abundances. At Louvain-la-Neuve a range of different “exotic” beams are accelerated using two types of ECR ion sources. Each element requires the use of some unique method in preparing, operating, or tuning the source. Examples of the use of ECR sources for gaseous exotic isotope ionization include: the production of low charge state Ar37 (t1/2=35 days) and O17 (natural abundance 0.038%) for implantation purposes and short-lived He6 (t1/2=0.8 s) for postacceleration. The sputtering method has been used for the production of Ni64 beams (0.91% natural abundance) and He7 beams (t1/2=53 days). Some aspects of the tuning, preparation, and performance of the ECR sources and the configuration and preparation of the sputtering apparatus will be discussed.
An arbitrary function generator for the electromagnetic insertion devices at the APS
Authors: Deriy B. N. , Makarov O. A. , Vasserman I. B.
Published: July 2002

An arbitrary function generator has been designed for electromagnetic insertion devices at the Advanced Photon Source (APS). It allows control of up to eight correction coils and provides switching signals to the main power supply of the insertion device. In this article, we discuss two applications of the arbitrary function generator with the electromagnetic insertion devices at the APS. The effectiveness of the arbitrary function generator for both magnetic field correction and minimizing orbit distortion in the storage ring is shown.
Confined polymer networks: The harmonic approach
Authors: Allegra Giuseppe , Raos Guido
Published: July 2002

Confinement of phantom networks of Gaussian chains between parallel surfaces is simulated through the application of a harmonic potential 12Hx2 to each chain bead at a distance x from the mean plane. We consider infinite, square (cubic) networks with a topological dimensionality of two (three), in addition to three-dimensional networks with a finite number of layers along one dimension. The partition function stays Gaussian and its integration reduces to the search of the eigenvalues of cyclic, or quasicyclic matrices. As applied to an isolated chain the confinement energy is 75% the exact value obtained by Casassa’s exact approach, and the accuracy is expected to improve in the network case. At strong compressions, the confinement energy per chain EH is about the same for all the networks, but smaller than for a single chain with the same contour length. Taking EH in kBT units, M=EH−1 gives the number of chains in the correlation domain. Therefore, the external potential breaks up the network into smaller and smaller correlated domains, whose statistics remain unperturbed (“blobs”). The unperturbed mean-square radius of gyration reproduces previous results on the two- and the three-dimensional networks. The enormous collapse in the three-dimensional case is evidenced once again.
A 20-channel electron cyclotron emission detection system for a grating polychromator in JT-60U
Authors: Isayama A. , Isei N. , Ishida S. , Sato M.
Published: July 2002

A 20-channel electron cyclotron emission (ECE) detection system for the grating polychromator in JT-60U has been developed to measure electron temperature and its perturbations. Indium antimonide (InSb) detectors aligned in a row are cooled by liquid helium held by a 50 cm-diam cryostat, in which a hold time of the liquid helium reaches 7 weeks. Twenty-channel preamplifiers are dc-coupled ones, which enable to measure low-frequency ECE. From the comparison between the new system and the existing 20-channel detector system, where a refrigerator for cooling InSb detectors and ac-coupled preamplifiers are used, it is found that the signal-to-noise ratio is improved, and that small perturbations can be detected. By using this system, the electron temperature and its perturbations near the internal transport barrier in a reversed shear discharge are measured in detail.
An insertion device beamline for 5–250 eV at the Canadian Light Source
Authors: Reininger R. , Tan K. , Coulthard I.
Published: July 2002

An undulator-based beamline designed to cover the energy range between 5 and 250 eV at the Canadian Light Source is described. The 8 m long monochromator is based on a spherical mirror that provides a converging beam to a variable line spacing plane grating. Three spherical mirrors and three gratings are used to cover the energy range of the beamline. A plane mirror that can be inserted into the beam before the exit slit allows diverting the beam to a second branch. The beamline is expected to deliver more than 1.5×1012 photons/s at a resolving power of 104 between 11 and 230 eV onto a spot size of less than 200 μm in diameter with low contribution of higher orders.
A microtomography beamline at the Louisiana State University Center for Advanced Microstructures and Devices synchrotron
Authors: Ham Kyungmin , Jin Hua , Butler Leslie G. , Kurtz Richard L.
Published: July 2002

A microtomography beamline has been recently assembled and is currently operating at the Louisiana State University’s Center for Advanced Microstructures and Devices synchrotron (CAMD). It has been installed on a bending magnet white-light beamline at port 7A. With the storage ring operating at 1.5 GeV, this beamline has a maximum usable x-ray energy of ∼15 keV. The instrumentation consists of computer-controlled positioning stages for alignment and rotation, a CsI(Tl) phosphor screen, a reflecting mirror, a microscope objective (1:1, 1:4), and Linux/LabVIEW-controlled charge coupled device. With the 1:4 objective, the maximum spatial resolution is 2.25 μm. The positioning and image acquisition computers communicate via transfer control protocol/internet protocol (TCP/IP). A small G4/Linux cluster has been installed for the purpose of on-site reconstruction. Instrument, alignment and reconstruction programs are written in MATLAB, IDL, and C. The applications to date are many and we present several examples. Several biological samples have been studied as part of an effort on biological visualization and computation. Future improvements to this microtomography station include the addition of a double-multilayer monochromator, allowing one to evaluate the three-dimensional elemental composition of materials. Plans also include eventual installation at the CAMD 7 T wiggler beamline, providing x rays in excess of 50 keV to provide better penetration of higher mass-density materials.