bluuebunny/crossref_metadata_2025 huggingface dataset:
First results from the SpectroMicroscopy Beamline at the Advanced Light Source
Authors: Denlinger J. D. , Rotenberg E. , Warwick T. , Visser G. , Nordgren J. , Guo J.-H. , Skytt P. , Kevan S. D. , McCutcheon K. S. , Shuh D. , Bucher J. , Edelstein N. , Tobin J. G. , Tonner B. P.
Published: July 2002

The SpectroMicroscopy Facility at the Advanced Light Source is based on a high brightness, high-resolution beamline, and includes a collection of projects designed to exploit the unique characteristics of the soft x-ray beam. The beamline itself is comprised of a 5-m long, 5-cm-period undulator, a spherical-grating monochromator with water-cooled gratings. Adaptive optics refocus the monochromatic beam to two ‘‘microfocus’’ experimental stations with spot sizes less than 50 μm diameter and a third ‘‘nanofocus’’ station uses a zone-plate lens to further demagnify the microfocus spot. Experimental stations include an ‘‘ultraESCA’’ spectrometer for small-area spectroscopy and photoelectron diffraction, a scanning transmission x-ray microscope, and photoelectron microscopes. Commissioning experiments of microscopic actinide photoemission, surface-core-level photoelectron diffraction, and high-resolution soft x-ray fluorescence demonstrate dramatic improvements in sensitivity due to the high brightness and small focus of the beamline.
Local sensitivity of an electrochemical quartz crystal microbalance: Spatial localization of the low frequency mode
Authors: Oltra R. , Efimov I. O.
Published: July 2002

The frequency response of 5 and 6 MHz AT-cut electrochemical quartz-crystal microbalances to the highly nonuniform distribution of mass loading appearing in localized corrosion or local deposition was investigated. It was shown that the evolution of the frequency deviates from the theoretical one calculated using the conventional sensitivity. This behavior is described in terms of localization of resonance modes under the heavy loaded part of crystal. Mass-dependent corrections to the conventional sensitivity were calculated.
Heavy ion beam probe diagnostics on TJ-1 tokamak and the measurements of the plasma potential and density profiles
Authors: Bondarenko I. S. , Khrebtov S. M. , Krupnik L. I. , Nedzelskij I. S. , Gordeev O. A. , Kharchev N. K. , Melnikov A. V. , Tarasyan K. N. , Zimeleva L. G. , Hidalgo C. , Garcia-Cortes I.
Published: July 2002

For the investigation of the plasma density and potential in the TJ-1 tokamak, we applied the heavy ion beam probe (HIBP) diagnostics with the beam energy up to 100 keV. The geometry of the experiment allows us to observe the whole plasma cross section and to catch the primary beam for direct calibration. HIBP on TJ-1 operates in two modes: shot-by-shot measurements which allow us to obtain the time evolution of the plasma parameters at every desired point of the detector grid, and scanning along the detector line (generally passing through the center) in the single shot which allows us to get a few profiles of the plasma parameters. When scanning we varied the injection angle with a period less than 2 ms. The spatial resolution of measurements was 2–5 mm. The bandwidth used (≈20 kHz) allows us to see the slow oscillations of the potential and density. The measurements were performed in the regimes with the magnetic field B=1.1–1.4 T, current I=26–32 kA, and the shot duration up to 25 ms. HIBP on TJ-1 enables us to measure simultaneously the time-spatial evolution of the potential and density; that makes HIBP a powerful tool for direct investigation of the transport processes in the plasma driven by the radial electric field.
Effect of the sample volume splitting in heavy ion-beam probing (HIBP) for spherical tokamaks (by the example of HIBP for mega ampere spherical tokamak)
Authors: Melnikov A. V. , Perfilov S. V.
Published: July 2002

Here, we propose the heavy ion-beam probing (HIBP) project for the mega ampere spherical tokamak (MAST) to establish systematic research of the radial electric field as an idea to investigate both the reactor-related physics (H mode and advanced tokamak mode with reversed shear) and basic plasma physics (the nature of anomalous transport). The calculations of the probing beam trajectories made for typical MAST parameters: B0=0.6 T, Ipl=1 MA, and Cs+ probing ions show the applicability of HIBP for the spherical torus MAST with energy of about a few hundred keV. The detector line connecting the center and the edge of the plasma allows us to obtain a radial profile by variation of the energy shot by shot. A detector line of equal energy E=110 keV allows us to get a series of profiles in the internal part of the plasma 0<ρ<0.4 during a single shot by a fast scan of the injection angle. The effect of splitting of the sample volume due to the intersection of the secondary fan in a rapidly decreasing toroidal magnetic field of the spherical tokamak was observed: two sample volumes can appear instead of the conventional one. It changes the principles of HIBP: the notion of a detector grid became more general and a new problem appears: how to separate the signals coming from different sample volumes. Using this effect we can spread the detector grid and observe the outer part of the plasma, 0.4<ρ<1. It was shown that these two parts can be united and the total plasma profile can be observed in one shot.
Generation of terahertz radiation from a poled polymer
Authors: Nahata Ajay , Auston David H. , Wu Chengjiu , Yardley James T.
Published: July 2002

We report the generation of terahertz radiation via optical rectification from a poled polymer using femtosecond optical pulses. We have measured the refractive index and power absorption in the terahertz frequency range. The corresponding difference frequency mixing coherence length in the poled polymer (1 mm at 1 THz) is ∼20 times greater than that of LiNbO3 (50 μm at 1 THz). The observed far-infrared electric field radiated from a 16 μm thick polymer sample is only 4 times smaller than that from a 1 mm thick y-cut LiNbO3 crystal. We discuss conversion efficiencies for thicker polymer samples.
High average power, high-repetition rate femtosecond pulse generation in the 1–5 μm region using an optical parametric oscillator
Authors: Spence D. E. , Wielandy S. , Tang C. L. , Bosshard C. , Günter P.
Published: July 2002

We describe the operation of a high average power, high-repetition rate, femtosecond optical parametric oscillator in the important 3–5 μm spectral region. The Ti:sapphire pumped optical parametric oscillator was based on the nonlinear material potassium niobate. The idler wavelength was continuously tunable over the 2.3–5.2 μm region and average powers as high as 170 mW were measured. At 5.2 μm, average powers of more than 20 mW were recorded. Idler pulse durations were determined using cross-correlation techniques and pulses as short as 60 fs were measured in the midinfrared.
Insulator–metal transitions in Pr0.7Ca0.3MnO3 induced by a magnetic field
Authors: Barratt J. , Lees M. R. , Balakrishnan G. , McK Paul D.
Published: July 2002

A magnetic field induced insulator to metal transition has been observed in both polycrystalline and single crystals samples of Pr0.7Ca0.3MnO3. Application of a magnetic field leads to a first-order phase transition from an insulating to a conducting state at low temperatures. The hysteresis associated with this transition allows the resistivity at 4 K to be varied by more than eight orders of magnitude depending on the field history of the sample.
Second harmonic generation in a domain-inverted MgO-doped LiNbO3 waveguide by using a polarization axis inclined substrate
Authors: Sonoda Shinichiro , Tsuruma Isao , Hatori Masami
Published: July 2002

We demonstrate quasiphase matched second harmonic generation by use of annealed proton-exchanged 87° Z-cut MgO 5 mol % doped LiNbO3 (MgO:LN) waveguides supporting a TE-mode guided wave. Formation of a domain inversion was achieved by applying electric fields in 3° Y-cut and 87° Z-cut MgO:LN. In 3° Y-cut and 87° Z-cut MgO:LN substrates, a polarization axis inclines at an angle θ equal to 3° to the surface. The domain inversion was formed in a direction parallel to the polarization axis like needles stuck into the substrate. The depth of the domain inversion was 2.5 μm with a period of 4.75 μm. The depth was about 2.5 times deeper than that in X- and Y-cut substrate at an angle θ equal to 0°. By forming a waveguide on the domain-inverted substrate, a blue light of wavelength 475 nm, and a power of 37 mW has been obtained and a normalized second harmonic conversion efficiency was 300%/W cm2 with interaction length of 10 mm, using a tunable external cavity laser diode as a fundamental wave source.
Deposition and doping of silicon carbide by gas-source molecular beam epitaxy
Authors: Kern R. S. , Davis R. F.
Published: July 2002

Thin films of silicon carbide (SiC) have been deposited at 1400–1450 °C on vicinal and on-axis 6H-SiC(0001) substrates by gas-source molecular beam epitaxy using the SiH4-C2H4-H2 gas system. Polytype control (6H- or 3C-SiC) was established by utilizing substrates of particular orientations. Residual, unintentionally incorporated nitrogen impurity levels were affected by changing the SiH4/C2H4 gas flow ratio, in agreement with the “site-competition epitaxy” model. In situ doping was achieved by intentional introduction of nitrogen and aluminum into the growing crystal.
Fabrication of photonic crystals by deep x-ray lithography
Authors: Feiertag G. , Ehrfeld W. , Freimuth H. , Kolle H. , Lehr H. , Schmidt M. , Sigalas M. M. , Soukoulis C. M. , Kiriakidis G. , Pedersen T. , Kuhl J. , Koenig W.
Published: July 2002

We have developed a new microfabrication technique for the construction of three-dimensional photonic crystals. In particular, we used multiple tilted x-ray lithography exposures in order to construct structures with photonic band gaps in the infrared region. First polymethylmethacrylate (PMMA) resist layers with a thickness of 500 μm were irradiated, then the holes in the resist structure were filled with preceramic polymer and subsequent pyrolysis converts the preceramic polymer into a SiCN ceramic. Theoretical results with fitted values of the dielectric constant are in good agreement with the transmission measurements.
Hard hydrogenated carbon films with low stress
Authors: Lacerda R. G. , Marques F. C.
Published: July 2002

Analysis of hard a-C:H films with low stress prepared by methane plasma decomposition is reported. Films with hardness as high as 14 GPa and stress as low as 0.5 GPa were obtained. These films have a high Raman Id/Ig ratio (∼1.0), and small Tauc’s band gap (∼0.4 eV). This letter also supplies strong evidence that the subimplantation deposition model, used to explain the formation of ta-C and ta-C:H films, is also valid for a-C:H films deposited by methane plasma decomposition. It is proposed that the rigidity of the films is basically provided by a matrix of dispersed cross-linked sp2 sites, in addition to the contribution of the sp3 sites.
Three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 thin films
Authors: Rao R. A. , Lavric D. , Nath T. K. , Eom C. B. , Wu L. , Tsui F.
Published: July 2002

The evolution of three-dimensional strain states and crystallographic domain structures of epitaxial colossal magnetoresistive La0.8Ca0.2MnO3 films have been studied as a function of film thickness and lattice mismatch with two types of (001) substrates, SrTiO3 and LaAlO3. In-plane and out-of-plane lattice parameters and strain states of the films were measured directly using normal and grazing incidence x-ray diffraction techniques. The unit cell volume of the films is not conserved, and it exhibits a substrate-dependent variation with film thickness. Films grown on SrTiO3 substrates with thickness up to ∼250 Å are strained coherently with a pure (001)T orientation normal to the surface. In contrast, films as thin as 100 Å grown on LaAlO3 show partial relaxation with a (110)T texture. While thinner films have smoother surfaces and higher crystalline quality, strain relaxation in thicker films leads to mixed (001)T and (110)T textures, mosaic spread, and surface roughening. The magnetic and electrical transport properties, particularly Curie and peak resistivity temperatures, also show systematic variations with respect to film thickness.
Improved performance of photorefractive polymers based on merocyanine dyes in a polar matrix
Authors: Meerholz Klaus , De Nardin Yessica , Bittner Reinhard , Wortmann Rüdiger , Würthner Frank
Published: July 2002

Recently, chromophores with a large figure-of-merit for incorporation into photorefractive polymers with low glass-transition temperature have become available. However, their rather polar nature so far limited their use in typical nonpolar photoconducting matrices due to dye aggregation. By incorporation of an additional polar compound we were able to influence this situation favorably. The material we report here has a factor 4 to 5 (2) improved index modulation amplitude (gain coefficient) compared to the best previously known materials. The material is sufficiently resistant against phase separation and operates in the near infrared at typical wavelengths of commercially available high-power laser diodes.
A fully lithographed voltage-biased superconducting spiderweb bolometer
Authors: Gildemeister J. M. , Lee Adrian T. , Richards P. L.
Published: July 2002

We describe the fabrication and characterization of superconducting transition-edge bolometers for astrophysical applications at far-infrared and mm wavelengths. The sensor is voltage biased and the current is measured with a superconducting quantum interference ammeter. Strong negative electrothermal feedback keeps the sensor temperature nearly constant, reduces the response time significantly, and improves linearity. It also makes the responsivity relatively insensitive to changes in optical background loading and refrigerator temperature. The bolometers are made using standard microlithographic techniques suitable for fabrication of large scale arrays. Detailed measurements of optical response are presented for a range of bias conditions and are compared with theory. Measured noise spectra are shown and a model for the noise is presented.
Magnetic phase diagram of ultrathin Co/Si(111) film studied by surface magneto-optic Kerr effect
Authors: Tsay Jyh-Shen , Yao Yeong-Der
Published: July 2002

A magnetic phase diagram of the ultrathin Co/Si(111) film deposited at 300 K has been established by the surface magneto-optic Kerr effect technique. The temperature, where ferromagnetism vanishes, increases from 375 to 625 K as the coverage of the Co film increases from 3.5 to 16 monolayers. A quantitative calculation of the normalized Auger signal of CoSi2 shows that the calculated values lie between the experimental measured Auger signals before and after ferromagnetism vanishes for films with coverage between 3.5 and 9.1 monolayers. For samples with higher coverage, the experimental data are smaller than that by calculation. This may be qualitatively explained by Co atoms escaping from the CoSi2 phase to diffuse into the Si substrate. The disappearance of ferromagnetism is mainly attributed to silicide formation.
Light-emitting diodes based on an alternating copolymer containing triphenylamine and phenylene units
Authors: Yu Gui , Liu Yunqi , Wu Xia , Zheng Min , Bai Fenglian , Zhu Daoben , Jin Linpei , Wang Mingzhao , Wu Xiuni
Published: July 2002

The optical and electroluminescence properties of a light-emitting poly(arylene vinylene) derivative comprising triphenylamine as the arylene unit (TPA–PPV) are demonstrated. TPA–PPV is soluble in common organic solvents, and has a high photoluminescent (PL) quantum yield (ΦPL=0.94 in benzene). A single-layer light-emitting diode Indium–tin–oxide/TPA–PPV/Al utilizing blue–green light-emitting TPA–PPV as the emissive layer is fabricated, and luminance in excess of 612 cd/m2, turn-on voltages of 1.5 V, and high luminous efficiency (3.0 lm/w) are reported.
Photoinduced liquid crystal alignment based on a surface relief grating in an assembled cell
Authors: Li Xiang Tong , Natansohn Almeria , Rochon Paul
Published: July 2002

Surface relief gratings have been optically inscribed on azopolymer films on the interior surface of an empty liquid crystal (LC) cell. Uniform LC alignment is observed in the inscribed regions when the cell is filled. Since the microgrooves are on both sides, this may provide a higher degree of order than other techniques. The stability of the relief gratings and the transmittance of the azopolymer films can be further improved by photobleaching. This alignment technique could have useful applications in the in-plane switching LC displays.
Epitaxial ferroelectric Ba0.5Sr0.5TiO3 thin films for room-temperature tunable element applications
Authors: Chen C. L. , Feng H. H. , Zhang Z. , Brazdeikis A. , Huang Z. J. , Chu W. K. , Chu C. W. , Miranda F. A. , Van Keuls F. W. , Romanofsky R. R. , Liou Y.
Published: July 2002

Perovskite Ba0.5Sr0.5TiO3 thin films have been synthesized on (001) LaAlO3 substrates by pulsed laser ablation. Extensive x-ray diffraction, rocking curve, and pole-figure studies suggest that the films are c-axis oriented and exhibit good in-plane relationship of 〈100〉BSTO//〈100〉LAO. Rutherford backscattering spectrometry studies indicate that the epitaxial films have excellent crystalline quality with an ion beam minimum yield χmin of only 2.6%. The dielectric property measurements by the interdigital technique at 1 MHz show room-temperature values of the relative dielectric constant, εr, and loss tangent, tan δ, of 1430 and 0.007 with no bias, and 960 and 0.001 with 35 V bias, respectively. The obtained data suggest that the as-grown Ba0.5Sr0.5TiO3 films can be used for development of room-temperature tunable microwave elements.
Dynamical diffraction in metallic optical gratings
Authors: Treacy M. M. J.
Published: July 2002

I show that dynamical diffraction of light in thin metallic gratings causes strong fluctuations in the transmitted intensity when the optical wavelength is comparable to the grating periodicity. The relative excitation of propagating Bloch waves in the grating depends on the incident wave vector. Bloch waves that peak in amplitude in the metal are strongly absorbed, whereas Bloch waves that peak in the holes are weakly absorbed. Anomalously high transmission occurs when the latter Bloch waves are strongly excited. Since surface plasmons are associated with the evanescent modes of the diffracted wavefield, I argue that the propagating diffracted beams and the surface plasmons are both part of the same dynamical scattering process.
Application of an AlGaAs/GaAs/InGaAs heterostructure emitter for a resonant-tunneling transistor
Authors: Tsai Jung-Hui
Published: July 2002

An AlGaAs/GaAs/InGaAs resonant-tunneling heterostructure-emitter bipolar transistor with negative-differential-resistance (NDR) behavior has been fabricated and demonstrated. Typical device performances with current gain of 140 incorporating an N-shaped NDR with a peak-to-valley current ratio of 5.3 are obtained at room temperature. The NDR behavior is believed to mainly result from a double-barrier-like resonant-tunneling effect. In other words, the on and off electron resonant tunneling from a depleted GaAs emitter through an InGaAs quantum well and ultrathin base toward a collector layer yield the interesting NDR behavior. Consequently, the proposed device provides a good potential for applications in amplifiers, low-power consumption, and logic currents.
Fiber-optic oxygen sensor using molybdenum chloride cluster luminescence
Authors: Ghosh Ruby N. , Baker Gregory L. , Ruud Cory , Nocera Daniel G.
Published: July 2002

We report on a reflection-mode fiber-optic oxygen sensor based on the O42 quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0%–21% gaseous oxygen environment have been obtained, a range suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0%–1.0% range. The response time of the fiber probe is theoretically predicted to be 1 s.
Time-resolved study of yellow and blue luminescence in Si- and Mg-doped GaN
Authors: Kwon Yong-Hwan , Shee S. K. , Gainer G. H. , Park G. H. , Hwang S. J. , Song J. J.
Published: July 2002

Time-resolved photoluminescence has been employed to study the donor-acceptor pair recombination kinetics of the yellow (∼2.3 eV) and blue (∼2.8 eV) luminescence bands in Si- and Mg-doped GaN layers, respectively. As the Si doping concentration in Si-doped GaN increases, the lifetime τ1/e of the yellow luminescence decreases, indicating that a shallow Si donor is the origin of the yellow luminescence. The blue luminescence is most likely due to a shallow Mg acceptor and a deep donor composed of a Mg acceptor-nitrogen vacancy complex, as seen by the independence of τ1/e on the Mg concentration measured by secondary ion mass spectroscopy in the range (2.5–6.0)×1019 cm−3. As the temperature is increased from 10 to 300 K, the lifetimes for the yellow and blue luminescence remain nearly constant, indicating that the distribution of electrons and holes bound to donors and acceptors does not change much with increasing temperature.
Strain-dependent vacuum annealing effects in La0.67Ca0.33MnO3−δ films
Authors: Sun J. R. , Yeung C. F. , Zhao K. , Zhou L. Z. , Leung C. H. , Wong H. K. , Shen B. G.
Published: July 2002

The effects of vacuum annealing on the structural and transport properties of the La0.67Ca0.33MnO3−δ films grown on SrTiO3 (LCMO/STO) and NdGaO3 (LCMO/NGO) substrates have been studied. A lattice expansion due to oxygen release during the annealing is observed. Under the same condition, the change of the out-of-plane lattice parameter in LCMO/STO is two to three times larger than that in LCMO/NGO, indicating a strong tendency for the oxygen in the former to escape. Correspondingly, the metal-to-semiconductor transition shifts to lower temperatures, linearly with lattice constant until a critical value, Δd=0.03 Å for LCMO/STO and Δd=0.05 Å for LCMO/NGO, after which a sudden drop of the transition temperature to zero occurs. The different lattice strains in both films are presumably responsible for the different critical oxygen contents for the occurrence of the resistive transition.
Microsolvation of similar-sized aromatic molecules: Photoelectron spectroscopy of bithiophene–, azulene–, and naphthalene–water anion clusters
Authors: Schiedt J. , Knott W. J. , Le Barbu K. , Schlag E. W. , Weinkauf R.
Published: July 2002

We perform a comparison of electron affinities (EA) of the conjugated molecules bithiophene, azulene, naphthalene, and their water clusters. Bithiophene and azulene monomers have positive EAs of +49±5 meV and +790±8 meV, but naphthalene has a negative EA. Despite their different EAs and their different molecular orbital energies the three molecules show very similar microsolvation shifts per water unit. This is explained by similar sizes of the π orbitals in which the surplus electron is delocalized leading to a similar electrostatic water to charge interaction. This qualitative dependence of solvation energy on anion size agrees well with classical solvation concepts. A comparison of our binding energies with previous calculations for other systems shows that formation of a water subcluster can be assumed. For all three molecules the cluster EAs increase nearly linearly with the number of waters. Using a linear approach and a calibration for the error in the first solvation step we extrapolated the naphthalene (H2O)n cluster series to a monomer EA−200 meV±50 meV, in good agreement to previous measurements. To become new insights B3LYP/6-31++G** structures and energies have been calculated for azulene, naphthalene, and their clusters with one water and compared with experimental EAs.
Thin film polymer blends undergoing phase separation and wetting: Identification of early, intermediate, and late stages
Authors: Wang Howard , Composto Russell J.
Published: July 2002

Using forward recoil spectrometry and atomic force microscopy, the phase evolution of a critical blend thin film of deuterated poly(methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) is found to develop by three distinct stages. During the early stage, dPMMA-rich wetting layers rapidly grow at the air/polymer and polymer/substrate interfaces. A hydrodynamic flow mechanism is proposed based on the scaling of the layer thickness with time, t−1, and the direct observation of an interconnected, bicontinuous morphology across the depletion zone. The lateral wave number of this morphology grows rapidly as t−1 but slows down to t−1/3 when the phase size approaches the film thickness. During the intermediate stage, the wetting layer thins and, concurrently, dPMMA-rich domains spanning the SAN-rich middle grow as t−0.41 in good agreement with an interfacially driven growth model. During the late stage, these capillary fluctuations eventually cause spontaneous rupturing of the middle layer resulting in an interconnected 2D network which eventually coarsens into isolated SAN-rich droplets encapsulated by a thick dPMMA-rich wetting layer. The surface roughness increases rapidly initially, reaches a constant value, and then increases at a much faster rate than that predicted by a trilayer model based on capillary fluctuations.
Ion-beam-induced dissociation and bubble formation in GaN
Authors: Kucheyev S. O. , Williams J. S. , Zou J. , Jagadish C. , Li G.
Published: July 2002

Structural studies reveal that heavy ion bombardment of GaN causes amorphization and anomalous swelling of the implanted region as a result of the formation of a porous structure. Results strongly suggest that such a porous structure consists of N2 gas bubbles embedded into a highly N-deficient amorphous GaN matrix. The evolution of the porous structure in amorphous GaN appears to be a result of stoichiometric imbalance where N- and Ga-rich regions are produced by ion bombardment. Prior to amorphization, ion bombardment does not produce a porous structure due to very efficient dynamic annealing processes in the crystalline phase.
Dimers of alkaline earth metal halide radicals, (MX)2 (M=Be, Mg, Ca; X=F, Cl): A theoretical study
Authors: Li G. P. , Hamilton I. P.
Published: July 2002

Alkaline earth metal halide radical dimers, (MX)2 (M=Be, Mg, Ca and X=F, Cl) have not yet been observed experimentally and their existence could be doubted since MX is known to disproportionate in the bulk. We study the (MX)2 species using DFT and MP2 methods and show that they are either thermodynamically or kinetically stable with respect to various reactions including disproportionation. Energetics, geometries, and vibrational frequencies of singlet and triplet equilibrium structures and some transition states are calculated. The ground states of (MX)2 are all singlet except (CaF)2 which is triplet and, with the possible exception of (CaF)2, the ground states have a rhombic structure in which the X atoms are shared equally with each M atom. The ionization potentials of the (MX)2 species are significantly lower than those of the corresponding MX species suggesting that their chemistry may be significantly different.
Nonperiodic flux to voltage conversion of series arrays of dc superconducting quantum interference devices
Authors: Häussler Ch. , Oppenländer J. , Schopohl N.
Published: July 2002

We present a theoretical study on the voltage response function 〈V〉 of series arrays of dc superconducting quantum interference devices (SQUIDs) for which the elementary dc SQUID loops vary in size and, possibly, in orientation. If the distribution of the array loop sizes is chosen according to an arithmetic relation, 〈V〉 is not a Φ0-periodic function of the strength of external magnetic field B. For arithmetic arrays the periodicity of 〈V〉 is controlled by the geometry of the array alone and does not depend on spreads in the array junction parameters. If small fluctuations are added to the loop size distribution, 〈V〉 becomes a unique function around a global minimum at B=0 without possessing significant additional minima for finite B. This filter property does not apply for conventional SQUIDs and series arrays of identical or nearly identical SQUID loops. Applications of arithmetic series arrays include the design of current amplifiers and novel quantum interference filters, which possess large voltage swing and low noise level. In particular, quantum interference filters should provide a rather simple and extremely sensitive measurement of the absolute strength of magnetic fields.
SiGe-on-insulator prepared by wafer bonding and layer transfer for high-performance field-effect transistors
Authors: Huang L. J. , Chu J. O. , Canaperi D. F. , D’Emic C. P. , Anderson R. M. , Koester S. J. , Wong H.-S. Philip
Published: July 2002

SiGe-on-insulator material was fabricated by wafer bonding and hydrogen-induced layer transfer techniques. The transferred SiGe layer is strain relaxed and has a Ge content ranging from 15% to 25%. High-quality strained Si layers were grown on the SiGe-on-insulator substrates by the UHV/chemical vapor deposition process at 550 °C. An electron mobility of 40 000 cm2/V s in a modulation-doped Si/SiGe heterostructure was achieved at 30 K on a SiGe-on-insulator substrate.
A switchable polymer layer: Chain folding in end-charged polymer brushes
Authors: Heine David , Wu David T.
Published: July 2002

We use a self-consistent field approximation to model the configurations of end-charged homopolymer and block copolymer brushes in response to an external electric field due to charges on the grafting surface. By varying the charge density on the grafting surface, we can cause the chains either to extend outward, greatly increasing the brush height, or to loop back to the grafting surface. We show that such a copolymer brush can present one block at the exposed surface in the extended state and present the other block in the retracted state. This occurs for both a solvated brush and a dry brush. We also compare these results to those of a modified Alexander–de Gennes model for the end-charged homopolymer brush.
Actuation and characterization of atomic force microscope cantilevers in fluids by acoustic radiation pressure
Authors: Degertekin F. L. , Hadimioglu B. , Sulchek T. , Quate C. F.
Published: July 2002

An actuation method for atomic force microscope (AFM) cantilevers in fluids is reported. The radiation pressure generated by a focused acoustic transducer at radio frequency (rf) (100–300 MHz) exerts a localized force of controlled amplitude at a desired location on the AFM cantilever. This force can be used to measure the spring constant and other dynamic properties of the cantilever. Furthermore, by amplitude modulating the rf signal input to the acoustic transducer, the cantilever is actuated in the dc–MHz frequency range. This provides a broadband actuation and characterization method for AFM cantilevers with arbitrary geometry. The technique is demonstrated on AFM cantilevers with spring constants in the 0.01–10 N/m range using a micromachined acoustic transducer/Fresnel lens structure operating at 179 MHz in water.
Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique using monoenergetic positron beams
Authors: Uedono A. , Chichibu S. F. , Chen Z. Q. , Sumiya M. , Suzuki R. , Ohdaira T. , Mikado T. , Mukai T. , Nakamura S.
Published: July 2002

Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and acceptor-type defects in −c GaN.
Electronic properties of mechanically induced kinks in single-walled carbon nanotubes
Authors: Bozovic Dolores , Bockrath M. , Hafner Jason H. , Lieber Charles M. , Park Hongkun , Tinkham M.
Published: July 2002

We have used an atomic-force microscope tip to mechanically buckle single-walled carbon nanotubes. The resistance of the induced defects ranged from 10 to 100 kΩ and varied with the local Fermi level, as determined by scanned-gate microscopy. By forming two closely spaced defects on metallic nanotubes, we defined quantum dots less than 100 nm in length. These devices exhibited single-electron charging behavior at temperatures up to ∼165 K.
Ti
3
SiC
2
and ice
Authors: Barsoum M. W. , Radovic M. , Finkel P. , El-Raghy T.
Published: July 2002

Despite vast differences in their other properties, the responses of Ti3SiC2 and ice to stress are quite similar in that they are quite strain-rate sensitive; if loaded rapidly, both are brittle, but if loaded slowly they are quite plastic. This stems from the fact that both are plastically very anisotropic; deformation occurs overwhelmingly, if not exclusively, by slip along basal planes. In both cases, stress concentrations and the rate at which they are relaxed dictate the nature of the mechanical response. In the brittle regime, microcracks and their linkage play a dominant role in both cases.
Spinodal decomposition of a binary fluid with fixed impurities
Authors: Qiu Feng , Peng Gongwen , Ginzburg Valeriy V. , Balazs Anna C. , Chen Hsuan-Yi , Jasnow David
Published: July 2002

The phase separation dynamics of a binary fluid containing randomly distributed fixed impurities is studied in two dimensions (d=2). The impurities act as osmotic force centers and favor one component of the fluid. We found, as expected, that hydrodynamic flow promotes the coalescence of the domains in the early stage of phase separation; at later stages for sufficiently high particle density and strong preferential interaction strength, the domain growth slows down and finally is pinned at a finite domain size independent of the hydrodynamics. The density of impurities in the unfavorable phase is shown to satisfy a scaling form involving the total impurity density n0 and the ratio R/R0 with R the domain size and R0=n0−1/d the average distance between the impurities.
High quality factor measured in fused silica
Authors: Penn Steven D. , Harry Gregory M. , Gretarsson Andri M. , Kittelberger Scott E. , Saulson Peter R. , Schiller John J. , Smith Joshua R. , Swords Sol O.
Published: July 2002

We have measured the mechanical dissipation in a sample of fused silica drawn into a rod. The sample was hung from a multiple-bob suspension, which isolated it from rubbing against its support, from recoil in the support structure, and from seismic noise. The quality factor, Q, was measured for several modes with a high value of 57.1±0.1 million found for the mode number 2 at 726 Hz. This result is about a factor of 2 higher than previous room temperature measurements. The measured Q was strongly dependent on handling, with a pristine flame-polished surface yielding a Q three to four times higher than a surface which had been knocked several times against a copper tube.
Salt effects over the swelling of ionized mesoscopic gels
Authors: Fernández-Nieves A. , Fernández-Barbero A. , de las Nieves F. J.
Published: July 2002

In this work, the effects of salt concentration over the swelling of ionic mesoscopic gels will be studied theoretically and verified with experiments. We will restrict ourselves to the weak screening limit where the Debye screening length is larger than the mesh size of the gel. Under this condition, direct electrostatic interactions are negligible and the swelling is driven by the osmotic pressure of the ions. The swelling response of a mesoscopic gel is strongly dependent on the ionization degree of the gel. In particular, a maximum in the size–salt concentration curve appears for a partially ionized gel, when the salt concentration equals the network charge concentration. This maximum is removed for a totally ionized polymer network. Despite these facts, a charge independent asymptotic behavior between size and salt concentration becomes apparent, at sufficiently high values of the latter. The Flory–Huggins mean-field approach together with the Donnan relations describe the observed swelling adequately, thus accounting for the essential physics of the problem. As a final remark, an experimental method for determining gel charge will be inferred from the results.
Theory of enantiomeric control in dimethylallene using achiral light
Authors: Gerbasi David , Shapiro Moshe , Brumer Paul
Published: July 2002

Extensive control over enantiomer populations using achiral light is computationally demonstrated for J, MJ-selected 1,3 dimethylallene. In particular, by altering the detuning of one of three lasers incident on an J, MJ-polarized racemic mixture, one can alter the enantiomeric excess from ≈93% of the L enantiomer to ≈93% of the D enantiomer.
Time-resolved sensing of organic vapors in low modulating porous silicon dielectric mirrors
Authors: Allcock P. , Snow P. A.
Published: July 2002

Time-resolved vapor diffusion into low modulating porous silicon dielectric mirrors has uncovered salient information about the spatial concentration distribution of the vapor within the porous matrix. Results indicate that relatively high concentrations of vapor result in instantaneous capillary condensation in the pores and yield a uniform concentration distribution. Results are modeled for both a concentration gradient and uniform filling of the porous matrix and compared to the experimental observations. Experiments with acetone, butanone, and pentanone have shown a linear dependence of the capillary condensation saturation time on the molecular weight of the organic solvents. An indicator of a concentration gradient within the matrix has been identified in the form of the full width at half maximum of the first-order reflectivity peak.
Periodic density functional embedding theory for complete active space self-consistent field and configuration interaction calculations: Ground and excited states
Authors: Klüner Thorsten , Govind Niranjan , Wang Yan Alexander , Carter Emily A.
Published: July 2002

We extend our recently reported embedding theory [J. Chem. Phys. 110, 7677 (1999)] to calculate not only improved descriptions of ground states, but now also localized excited states in a periodically infinite condensed phase. A local region of the solid is represented by a small cluster for which high quality quantum chemical calculations are performed. The interaction of the cluster with the extended condensed phase is taken into account by an effective embedding potential. This potential is calculated by periodic density functional theory (DFT) and is used as a one-electron operator in subsequent cluster calculations. Among a variety of benchmark calculations, we investigate a CO molecule adsorbed on a Pd(111) surface. By performing complete active space self-consistent field, configuration interaction (CI), and Møller–Plesset perturbation theory of order n (MP-n), we not only were able to obtain accurate adsorption energies via local corrections to DFT, but also vertical excitation energies for an internal (5σ→2π*) excitation within the adsorbed CO molecule. We demonstrate that our new scheme is an efficient and accurate approach for the calculation of local excited states in bulk metals and on metal surfaces. Additionally, a systematic means of improving locally on ground state properties is provided.
Simulation of proton transfer along ammonia wires: An “ab initio” and semiempirical density functional comparison of potentials and classical molecular dynamics
Authors: Meuwly Markus , Karplus Martin
Published: July 2002

Protonated ammonia clusters of the composition (NxH3x+1)+ with x=2,3,4 are investigated by using the gradient corrected, three-parameter functional by Becke based on the functional by Lee, Yang, and Parr (B3LYP/6-31G**) and self-consistent charges density functional tight-binding (SCC–DFTB) methods for calculating the potential energy surface and forces in the Born–Oppenheimer approximation. They are used for classical molecular dynamics simulations at temperatures ranging from 5 K to 600 K. Results from the two methods are compared for proton transfer in N2H7+. The number of proton transfer events as a function of temperature is similar, although at low temperatures, SCC–DFTB cuts off more rapidly than B3LYP/6-31G**. Calculated vibrational spectra agree well for the intermolecular N–N and intramolecular N–H stretch excitations. Both approaches lead to broad, relatively unstructured bands extending over about 1500 cm−1 for the proton transfer coordinate. Simulations at the SCC–DFTB/MD level for larger (NxH3x+1)+ (x⩽4) clusters are presented and discussed. They show significant structural reorganization within the cluster. Consecutive proton hops within a few tenths of a fs are observed. A N2H7+ cluster immersed in a water shell containing 25 water molecules was studied by the mixed quantum mechanical/molecular mechanics (QM/MM) method with SCC–DFTB for the QM part. The presence of water appears to impede proton transfer. Including corrections for basis set superposition error in the MP2/aug-cc-pVTZ and B3LYP/6-31G** calculations has a small effect. It increases the barrier heights from 0.78 kcal/mol to 1.28 kcal/mol (MP2) and from 0.10 kcal/mol to 0.27 kcal/mol (B3LYP), respectively.
Shock wave demagnetization of BaFe12O19 hard ferrimagnetics
Authors: Shkuratov S. I. , Talantsev E. F. , Dickens J. C. , Kristiansen M.
Published: July 2002

A study of the effect of shock waves on the phase state of a hard ferrimagnetic material has been performed. A plane shock wave was passed along the axis of a cylindrical BaFe12O19 hard ferrite magnet. The shock wave demagnetized the cylinder, reducing the magnetic flux. This change in magnetic flux generated an electromotive force (EMF) in a coil wound around the ferrite. The value of the EMF calculated on the assumption that the ferrite was completely demagnetized by the shock wave is in good agreement with the peak EMF value obtained experimentally.
HKrF in solid krypton
Authors: Pettersson Mika , Khriachtchev Leonid , Lignell Antti , Räsänen Markku , Bihary Z. , Gerber R. B.
Published: July 2002

A new krypton-containing compound, HKrF, has been prepared in a low-temperature Kr matrix via VUV photolysis of the HF precursor and posterior thermal mobilization of H and F atoms. All three fundamental vibrations have been observed in the FTIR spectra at ∼1950 cm−1 (H–Kr stretch), ∼650 cm−1 (bending), and ∼415 cm−1 (Kr–F stretch). Two distinct sites of HKrF have been identified. The energy difference between the H–Kr stretching vibrations for the two sites is remarkably large (26 cm−1), indicating a strong influence of the environment. In annealing after the photolysis of the precursor, HKrF is formed in two different stages: at 13–16 K from closely trapped H+F pairs and at T>24 K due to more extensive mobility of H and F atoms in the matrix. HKrF in a less stable site decreases at temperatures above 32 K, the other site being stable up to the sublimation temperature of the matrix. The photodecomposition cross section for HKrF has been measured between 193 and 350 nm and compared with the cross sections of the previously reported HArF and HKrCl molecules. The condensed-phase VSCF (vibrational self-consistent field) calculations suggest that the more stable form is a single-substitutional site and the less stable form is a double-substitutional site of HKrF in solid Kr. The gas to matrix shifts for these sites are predicted to be +(9–26) cm−1 for the H–Kr stretching and the bending vibrations and −(7–10) cm−1 for the Kr–F stretching vibrations.
Lattice-matched, epitaxial, silicon-insulating lanthanum yttrium oxide heterostructures
Authors: Guha Supratik , Bojarczuk Nestor A. , Narayanan Vijay
Published: July 2002

We demonstrate a ternary (LaxY1−x)2O3 thin-film oxide that can be grown epitaxially on Si(111) substrates with a lattice constant that can be matched to twice the lattice constant of silicon. We further show that silicon can then be deposited epitaxially (though with a high defect density) on this oxide such that epitaxial silicon/oxide/silicon structures may be grown. We discuss the microstructural relationships and the growth modes for the oxide on silicon and silicon on oxide growths.
The effect of media background on reading and writing in perpendicular recording
Authors: van der Heijden P. A. A. , Karns D. W. , Clinton T. W. , Heinrich S. J. , Batra S. , Karns D. C. , Roscamp T. A. , Boerner E. D. , Eppler W. R.
Published: July 2002

The effect of stray fields from the recording layer on the reading and writing process has been studied for a perpendicular recording system. The system consists of a single pole writer with a wide return pole, a conventional spin-valve reader and a double layer recording medium. A writer- and reader-induced asymmetry is observed in the recording process for a dc erased background. The asymmetry is interpreted as stray fields of the dc background giving rise to reader saturation and an offset in the current driven inductive write field.
Individually addressable vertically aligned carbon nanofiber-based electrochemical probes
Authors: Guillorn M. A. , McKnight T. E. , Melechko A. , Merkulov V. I. , Britt P. F. , Austin D. W. , Lowndes D. H. , Simpson M. L.
Published: July 2002

In this paper we present the fabrication and initial testing results of high aspect ratio vertically aligned carbon nanofiber (VACNF)-based electrochemical probes. Electron beam lithography was used to define the catalytic growth sites of the VACNFs. Following catalyst deposition, VACNF were grown using a plasma enhanced chemical vapor deposition process. Photolithography was performed to realize interconnect structures. These probes were passivated with a thin layer of SiO2, which was then removed from the tips of the VACNF, rendering them electrochemically active. We have investigated the functionality of completed devices using cyclic voltammetry (CV) of ruthenium hexammine trichloride, a highly reversible, outer sphere redox system. The faradaic current obtained during CV potential sweeps shows clear oxidation and reduction peaks at magnitudes that correspond well with the geometry of these nanoscale electrochemical probes. Due to the size and the site-specific directed synthesis of the VACNFs, these probes are ideally suited for characterizing electrochemical phenomena with an unprecedented degree of spatial resolution.
Direct measurement of the binding energy of the NO dimer
Authors: Wade Elisabeth A. , Cline Joseph I. , Lorenz K. Thomas , Hayden Carl , Chandler David W.
Published: July 2002

The binding energy of the NO dimer has been measured directly using velocity-mapped ion imaging. NO dimer is photodissociated to produce NO(X) and NO(A), and the NO(A) is then nonresonantly ionized to NO+. The threshold for production of NO+ ions is measured at 44 893±2 cm−1, which corresponds to a binding energy of 696±4 cm−1.