bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of thickness and upper electrode diameter on polarization switching and electron emission of Pb(Zr0.8Ti0.2)O3 ferroelectric cathode
Authors: Park Jihoon , Kim Yong Tae , Yoon Ki Hyun
Published: July 2002

To investigate the structural effect on the ferroelectric electron emission, the electric field distribution in a two-dimensional structure was calculated as a function of thickness and upper electrode diameter, and the switching charge density and emission charge were measured simultaneously. The simulation results of the electric field distribution showed that an asymmetric electrode structure caused a stray field on the bare surface of the ferroelectric cathode near the edge of upper electrode. The distance of the stray field from the electrode edge increased with increasing ferroelectric thickness, but it did not depend on the upper electrode diameter. The switching charge density increased more on the cathode with thicker ferroelectric and smaller upper electrode diameter. This was attributed to the difference of field distribution on the bare ferroelectric surface near the electrode edge. The expanded stray field for the thicker ferroelectric cathode enhanced polarization switching near the electrode edge. As a result, with increasing ferroelectric thickness, the emission charge increased and the threshold field decreased. However, there was no apparent difference between the emission charges from the different upper electrode diameters.
Anisotropic diffusion of spheroids in liquids: Slow orientational relaxation of the oblates
Authors: Vasanthi R. , Bhattacharyya Sarika , Bagchi Biman
Published: July 2002

We carry out extensive molecular dynamics (MD) simulations of translation and rotation of spheroids in the suspension of spheres, for a wide range of values of the aspect ratio, κ ranging from the extreme limits of long rodlike molecules to the disc-shaped ones. While diffusion is anisotropic in short times (compared to the orientational correlation time), it is isotropic in the very long time and the ratio of the two components of short time translational diffusion (D∥ and D⊥, where ∥ and ⊥ refer to the motion parallel and perpendicular to the major axis) scales with the aspect ratio κ over a large range of κ. For thin discs, the preferred mode of motion is in the plane of the disc. The oblates show a very long-time orientational correlation which is reflected by the slow decay of its time correlation function. The correlation time becomes progressively longer and finally diverges as the spherical limit is approached. This is expected because the angular momentum is conserved in the limit of fully spherical molecules (κ=1). For both prolates and oblates, the crossover from the anisotropic to the isotropic motion is found to correlate with the orientational correlation time of the spheroid.
Hysteresis loop of a ferroelectric bilayer with an antiferroelectric interfacial coupling
Authors: Wu Yin-Zhong , Yao Dong-Lai , Li Zhen-Ya
Published: July 2002

The intrinsic hysteresis of a ferroelectric bilayer with an antiferroelectric interfacial coupling is studied within the framework of the Transverse Ising Model. We find it is possible to exit triple hysteresis loops once the parameters of the two slabs are selected appropriately. The effects of the thickness, the interfacial coupling constant and the temperature on the hysteresis of the bilayer are investigated. The quantum effect on the hysteresis is also examined; it is found that the quantum effect can result in the disappearance of the hysteresis.
Ion-beam processing effects on the thermal conductivity of n-GaN/sapphire (0001)
Authors: Florescu D. I. , Pollak Fred H. , Lanford William B. , Khan Farid , Adesida I. , Molnar R. J.
Published: July 2002

We have measured high spatial/depth resolution (2–3 μm) thermal conductivity (κ) at 300 K before and after plasma-induced effects on two series of n-GaN sapphire (0001) samples fabricated by hydride vapor phase epitaxy using scanning thermal microscopy. The sample thicknesses were 50±5 μm for one set and 25±5 μm for the second. The carrier concentrations were ∼8×1016 cm−3 and ∼1.5×1017 cm−3, respectively, as determined by Hall effect measurements. The thermal conductivity before treatment was similar to that previously reported for hydride vapor phase epitaxy material with comparable carrier concentration and thickness [D. I. Florescu et al., J. Appl. Phys. 88, 3295 (2000)]. Damage was induced by ion-beam processing the samples under constant Ar+ gas flow and pressure for a fixed period of time (5 min), with the dc bias voltage (Vdc) being the only variable processing parameter (125–500 V). The thermal conductivity near the surface, κ, was found to exhibit a linear decrease with Vdc in the investigated range after this procedure. A second process was then applied in order to remove some damage. In this case the samples were processed under a constant mixture of Cl2 and Ar+ gas flow and Vdc′ of 50 V. For the samples with Vdc in the range 125 V<Vdc⩽250 V, κ was found to be actually lower after the damage removal process. The minimum κ was found at 250 V. This is probably due to Ar+ beam channeling [O. Breitschadel et al., Appl. Phys. Lett. 76, 1899 (2000)], which has been reported on similar structures at this voltage. When the initial processing voltage was 250 V<Vdc<500 V, κ showed a tendency to recover somewhat.
Improved model for the stress-induced leakage current in thin silicon dioxide based on conduction-band electron and valence-band electron tunneling
Authors: Chim W. K. , Lim P. S.
Published: July 2002

This article presents a detailed investigation on the stress-induced leakage current (SILC) conduction mechanism via conduction-band electron (CBE) and valence-band electron (VBE) tunneling in thin oxides. An improved SILC model that is able to reproduce the experimental SILC over a wide range of oxide fields, and yet give a realistic level of extracted neutral trap concentration, is proposed. Calculations performed with the improved SILC model suggest that SILC conduction via neutral traps is accompanied by energy relaxation (i.e., an inelastic mechanism), irrespective of the origin (i.e., whether CBE or VBE) of the tunneling species. For both CBE and VBE tunneling, inelastic tunneling with energy relaxation (Erelax) of 1.5 and 0.8 eV, was found to fit the experimental measurements well. These values of Erelax agree with those reported in the literature.
Performance and application of a superlattice infrared photodetector with a blocking barrier
Authors: Chen C. C. , Chen H. C. , Hsu M. C. , Hsieh W. H. , Kuan C. H. , Wang S. Y. , Lee C. P.
Published: July 2002

An infrared photodetector that contains a superlattice structure and a blocking barrier was investigated. The photodetector shows advantages including a low operating voltage (<0.7 V), wide detection bandwidth, flexible miniband engineering, and in particular, voltage-tunable spectral responsivity. The blocking barrier not only prohibits the dark current but also acts as a high-pass energy filter to tune the spectral responsivity. The zero background peak detectivity of our detector is 3.7×109 cm Hz0.5/W (9.7 μm) at −0.5 V and 2.2×1010 cm Hz0.5/W (6.7 μm) at −0.1 V. Since the spectral responsivity at different biases is independent, temperature sensing is feasible by taking the ratio of the measured photocurrent at different biases. A demonstration of temperature sensing by using our superlattice infrared photodetector with a blocking barrier is also shown in this article. Those results show the superlattice is a simple and efficient structure for the design of a multicolor infrared photodetector.
Enthalpy and entropy contributions to the pressure dependence of hydrophobic interactions
Authors: Ghosh Tuhin , García Angel E. , Garde Shekhar
Published: July 2002

We use long molecular dynamics simulations of methane molecules in explicit water at three different temperatures at pressures of 1 and 4000 atm to calculate entropic and enthalpic contributions to the free energy of methane–methane association. In agreement with previous simulation studies, we find that the contact minimum is dominated by entropy whereas the solvent-separated minimum is stabilized by favorable enthalpy of association. Both the entropy and enthalpy at the contact minimum change negligibly with increasing pressure leading to the relative pressure insensitivity of the contact minimum configurations. In contrast, we find that the solvent-separated configurations are increasingly stabilized at higher pressures by enthalpic contributions that prevail over the slightly unfavorable entropic contributions to the free energy. The desolvation barrier is dominated by unfavorable enthalpy of maintaining a dry volume between methanes. However, the increasing height of the desolvation barrier with increasing pressures results from entropy changes at the barrier configurations. Further resolution of the enthalpy of association shows that major contributions to the enthalpy arise from changes in water–water interactions and the mechanical work (PΔV) expended in bringing the methanes to a separation of r. A connection of these thermodynamic features with the underlying changes in water structure is made by calculating methane–methane–water oxygen triplet correlation functions.
The structure and magnetic properties of FeNiN films
Authors: Wang H. Y. , Jun-Liu  , Huang H. S. , Mao W. H. , Chen H. , Zhang H. Y. , He Y. J. , Jiang E. Y.
Published: July 2002

FeNiN films with 0–56 at % Ni concentrations were prepared by dual ion beam sputtering in a nitrogen–argon gas mixture. Effects of nitrogen and nickel concentrations on the structure and magnetic properties of FeNiN films have been studied. FeNiN films with lower nickel and nitrogen concentrations contained the bcc α-(FeNi) phase, films with 8–12 at % nitrogen and 5–25 at % Ni concentrations contained α″-(Fe, Ni)16N2 and γ′-(Fe, Ni)4N phases, and films with 30–56 at % Ni concentrations consisted of a mixture of fcc γ-FeNi alloy and γ′-(Fe, Ni)4N phases. The FeNiN films with 8–12 at % nitrogen and 5–15 at % nickel concentrations showed high saturation magnetization of 2.2–2.3 T and low coercivity of 96–110 A/m. The variation of the magnetic properties was attributed to the change in the structure of the films.
Reactivity of MgB2 with common substrate and electronic materials
Authors: He T. , Cava R. J. , Rowell John M.
Published: July 2002

The reactivity of MgB2 with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 and 800 °C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for reactivity were ZrO2, yttria stabilized zirconia, MgO, Al2O3, SiO2, SrTiO3, TiN, TaN, AlN, Si, and SiC. At 600 °C, MgB2 reacted only with SiO2 and Si. At 800 °C, however, reactions were observed for MgB2 with Al2O3, SiO2, Si, SiC, and SrTiO3. The Tc of MgB2 decreased in the reactions with SiC and Al2O3.
Dynamical ordering of symmetric non-Birkhoff periodic points in reversible monotone twist mappings
Authors: Tanikawa Kiyotaka , Yamaguchi Yoshihiro
Published: July 2002

We study the coexistence of symmetric non-Birkhoff periodic orbits of C1 reversible monotone twist mappings on the cylinder. We prove the equivalence of the existence of non-Birkhoff periodic orbits and that of transverse homoclinic intersections of stable and unstable manifolds of the fixed point. We derive the positional relation of symmetric Birkhoff and non-Birkhoff periodic orbits and obtain the dynamical ordering of symmetric non-Birkhoff periodic orbits. An extension of the Sharkovskii ordering to two-dimensional mappings has been carried out. In the proof of various properties of the mappings, reversibility plays an essential role.
Electrostatic interactions in a neutral model phospholipid bilayer by molecular dynamics simulations
Authors: Saiz Leonor , Klein Michael L.
Published: July 2002

The organization of the lipid headgroups in a neutral model membrane is studied by atomistic simulations in the fluid lamellar phase, Lα. In particular, we report the results obtained for a fully hydrated 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine lipid bilayer at room temperature. The orientational distribution of the lipid dipole moments with respect to the membrane normal presents a maximum at 70° (20° above the plane of the interface, pointing toward the water region). We also found another smaller peak at 110° (−20° with respect to the membrane plane). This preferential orientation of the lipid headgroup dipoles with respect to the bilayer normal obtained at 303 K is qualitatively different from previous calculations at higher temperatures in the fluid lamellar phase, where headgroup dipoles were uniformly distributed with orientations spanning 0°–135°. Despite their differences, both situations give rise to a similar mean orientation of ∼70°, which is in excellent agreement with experiment. The statistics of the main lipid–lipid interactions, the charge density profiles, the electrostatic potential along the bilayer normal, and the polarization of water molecules at the interfacial plane are also analyzed.
A new dynamics facility combining a storage ring with a synchronized free electron laser
Authors: Dylla H. F. , Hutton A. , Neil G. , Williams G. P.
Published: July 2002

Jefferson Laboratory currently operates a kilowatt average power, subpicosecond mid-infrared free electron laser as a user facility. We present plans to add an electron storage ring to this facility. Pulses of light from both sources will be synchronized at repetition rates up to 125 MHz to allow pump–probe dynamics experiments to be performed. We will present an outline of the new facility together with the operational parameters of the two sources. The new facility could be on line as a user facility in 2004.
Design, fabrication, and evaluation of an internally cooled silicon carbide mirror
Authors: Khounsary A. , Fernandez P. , Assoufid L. , Mills D. , Walters D. , Schwartz J. , Robichaud J.
Published: July 2002

In this article, the design, fabrication, prepolish coating, and polishing of a reaction-bonded (RB) internally cooled silicon carbide (SiC) mirror is described. The mirror was developed from a mold of SiC powder in a near-net shape and then infused with silicon vapor to make a dense mirror substrate. The mirror surface was then rough polished, coated with a thin layer of SiC, and polished to a final fine finish. The design and manufacturing of this mirror—intended to be used as a multilayer substrate on a high-heat-load undulator beamline—are described, and data on the surface figure and finish are provided. This type of mirror can provide an attractive alternative to internally cooled silicon mirrors. Because the substrate is made in one piece, it avoids the frit or metal bonding that is usually necessary with silicon substrates. Advantages of RB SiC mirrors include lower cost and higher reliability.
Performance of an advanced elliptically polarized undulator with shimming
Authors: Hwang C. S. , Chang C. H. , Lin F. Y. , Chen H. H. , Fan T. C. , Wang Ch. , Chang H. P. , Chen Jenny , Hsu K. T.
Published: July 2002

A 4-m-long elliptically polarizing undulator (EPU5.6) device with an Apple-II structure is installed in the Synchrotron Radiation Research Center storage ring. This is the longest EPU device with an Apple-II structure worldwide. The errors of the magnet construction and the assembly process are among the several factors that can degrade the spectral performance. This study describes the mechanical design concept and construction that reduce magnetic field errors to keep high performance of the magnet. The electron orbit distortion in various polarization radiation modes is discussed with reference to a global feedback system and a feed forward table with following gap and phase. The beam dynamic effects are also investigated when the phase and magnetic gap are changed.
A far-infrared beamline for ultrahigh vacuum surface vibrational spectroscopy at Aladdin
Authors: Hansen Roger W. C. , Julian Robert , Kubala Tim , Bosch Robert A. , May Tim , Hirschmugl Carol
Published: July 2002

A far-infrared beamline has been implemented on port 031 of the Aladdin Storage Ring. Port 031 has been designed to capture the edge radiation produced by the fringe field of a bending magnet rather than normal bending magnet radiation. Calculations indicate that in cases where the aperture is limited edge radiation provides higher flux than bending magnets, particularly at the longer wavelengths. The far-infrared (IR) beamline shares this port with a mid-IR microscope beamline and uses the same optics to collect and collimate the light. To install the far-IR beamline, a special mirror was designed to deflect light to the purged bench and microscopes or to move out of the light path and allow the light to continue to the far-IR beamline. The vacuum bench employs a Thermo Nicolet interferometer with a solid substrate beamsplitter modified for use under rough vacuum. The light from the interferometer is focused through CsI windows on a sample in an ultrahigh vacuum chamber. The specular reflected light from the sample is collected by an elliptical mirror and focused onto a bolometer detector. The details of the beamline will be discussed as well as the first experimental results.
Etching of diamonds for x-ray monochromators
Authors: Maj J. A. , Macrander A. T. , Krasnicki S. F. , Fernandez P. B. , Erck R. A.
Published: July 2002

Results of a study of etching of synthetic diamond single crystals are reported. The studies were done to examine the efficacy for improvements in the performance of synchrotron based x-ray monochromators. The three diamonds that we studied were all type Ib and were all (111) oriented. Synthetic diamonds are commonly type Ib and are yellow in color due to nitrogen impurities. Such diamonds are good candidate crystals for use as synchrotron-based x-ray monochromators because reasonably low dislocation densities can result. X-ray topography and x-ray double-crystal diffractometry with the diamond (111) reflection in a slightly dispersive geometry and Cu Kα (8 keV) radiation were used to assess the diffraction properties. Two separate etching procedures were studied. In the first, 1 keV oxygen+argon ion bombardment was used to sputter clean the surface and to introduce oxygen, followed by chemical oxidation at 720–730 °C. The oxidation was performed with KOH and Na2O2. Rocking curve full width at half maximum (FWHM) was improved from 7.3 to 6.4 arc sec and from 8.1 to 6.7 arc sec for two sides of the same slab that were treated separately. The theoretical ideal value is 5.8 arc sec. For the second technique, a patented and commercially available procedure that involves plasma deposition of SiOk compounds on a platter followed by abrasion of the diamond against this platter was studied. Two separate diamonds were treated with this second process, and, for one, the FWHM was slightly improved from 7.2 to 6.6 arc sec, but, for the other, the FWHM values were found to be increased from 8.0 to 8.5 arc sec.
The new beamline 3 at SURF III for source-based radiometry
Authors: Shaw Ping-Shine , Shear David , Stamilio Rebecca J. , Arp Uwe , Yoon Howard W. , Saunders Robert D. , Parr Albert C. , Lykke Keith R.
Published: July 2002

The Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology provides a unique opportunity for high-accuracy ultraviolet (UV) to infrared radiometry due to the 70-fold improvement in the uniformity of the magnetic field from the previous generation of SURF. This improvement enables the properties of the output radiation, such as spectral power, angular spread, and polarization, to be more accurately predicted based on the use of the Schwinger’s equation. The radiation from SURF III is completely characterized by only three parameters, the magnetic field, the radius of the electron beam trajectory, and the electron beam current. For radiometry, the calculability of SURF III provides an important standard light source for source intercomparison. In contrast to the widely used blackbody source where the thermal radiation is completely characterized by the temperature and the emissivity of the blackbody walls, synchrotron radiation extends the wavelength range to UV and x ray which is impractical for blackbody sources. At SURF III, a new beamline, beamline 3, is constructed as a white light beamline for source-based radiometry. We describe the design of the new beamline 3 and its front-end high accuracy electron beam current monitor. This monitor not only measures one of the three fundamental parameters, the electron beam current, it also serves as an electron beam diagnostic tool. We also discuss ways to verify the calculability of SURF III using filter radiometers.
Solving the crystallographic phase problem with reference-beam diffraction
Authors: Shen Qun , Pringle Daniel , Szebenyi Marian , Wang Jun
Published: July 2002

By using a reference-beam diffraction data-collection technique, it is possible to directly measure a large number of relative phases of Bragg reflections on an area detector in a typical protein crystallography experiment. The technique, being developed at Cornell, incorporates the principle of three-beam diffraction into the most common method of data collection, i.e., the oscillating-crystal method, and allows recordings of many phase-sensitive three-beam interference profiles simultaneously. Recent advances include a dedicated five-circle κ diffractometer and new data acquisition and analysis algorithms. Experimental results on a protein crystal are presented and the strategies of using the measured phases for solving crystal structures are discussed.
Synchrotron ultraviolet radiation facility SURF III
Authors: Arp U. , Clark C. W. , Farrell A. P. , Fein E. , Furst M. L. , Hagley E. W.
Published: July 2002

The National Institute of Standards and Technology (NIST) has operated the Synchrotron Ultraviolet Radiation Facility (SURF) continuously since the early 1960s. The original accelerator was converted into a storage ring, called SURF II, in 1974. Then in 1998, motivated mainly by limitations in the accuracy of radiometric calibrations and the wish to extend the spectrum of the emitted synchrotron radiation to shorter wavelengths, a second major upgrade was performed. This time the whole magnet system was replaced to improve the calculability and allow for higher magnetic fields. Since the recommissioning of SURF III we have been working to improve the stability of the stored electron beam through modifications of the radio-frequency system, leading to operations with unprecedented stability and new record injection currents topping 700 mA.
An open-flow cryostat using helium gas for cryogenic x-ray diffraction experiments
Authors: Nakasako Masayoshi , Sawano Masahito , Kawamoto Masahide
Published: July 2002

An open-flow cryostat using helium gas has been developed for cryogenic x-ray crystallography. The device is composed of a cryostat and a heat-exchange apparatus, and dry nitrogen gas is supplied for shielding the cooled helium gas from atmosphere. The lowest temperature reached by the device is 35 K within a fluctuation of ±0.5 K at a flow rate of 6 l/min. This device is easy to operate and enables to study crystal structures and the temperature-dependent phenomena occurring in biological macromolecules and materials above 35 K.
Spatio-temporal pattern formation during CO oxidation on Pt(100) at low and intermediate pressures: A comparative study
Authors: Lele T. , Lauterbach J.
Published: July 2002

Experimental studies of CO oxidation on Pt(100) over two different ranges of reactant pressures will be reviewed. Using photoemission electron microscopy (PEEM), spatio-temporal pattern formation was observed at temperatures between 420 and 540 K in the 10−5 mbar pressure range. In an attempt to bridge the “pressure-gap,” ellipsomicroscopy for surface imaging was used to follow pattern formation at temperatures around 600 K in the 10−2 mbar pressure range. The features of the nonlinear phenomena, observed in these two different pressure regimes, are markedly different. This is shown by comparison of various qualitative and quantitative features of spatio-temporal pattern formation as well as the dynamics of the macroscopic reaction rate. Subsurface oxygen is proposed as a tentative alternative to the surface phase transition for oscillations in the reaction rate at higher temperatures and intermediate pressures.
Blueshift of magneto-optical spectra and ferromagnetic ordering in (GaMn)As/AlAs ultrathin (<5 nm) heterostructures
Authors: Shimizu H. , Tanaka M.
Published: July 2002

We have studied transmission magneto-optical spectra of (GaMn)As/AlAs ultrathin heterostructures. In the heterostructures whose (GaMn)As thickness d is thinner than 5 nm, the magneto-optical spectra are significantly blueshifted with the decrease of d, whereas the heterostructures with thicker (GaMn)As (>12 nm) show magneto-optical spectra similar to that of bulk (GaMn)As, and the mechanisms of the blueshift are discussed. Furthermore, we have observed ferromagnetic ordering even in the ultrathin (GaMn)As (d=2 and 5 nm) samples with their transition temperature of 70 K.
Asymmetry in the magnetoresistance ratio in magnetic tunnel junctions
Authors: Davis A. H. , MacLaren J. M.
Published: July 2002

Theoretical treatment of spin dependent tunneling is used to reveal how asymmetry in the bias dependence of the tunneling magnetoresistance ratio (TMR) arises. The tunneling states in the two ferromagnetic electrodes are taken from the results of ab initio electronic structure calculations. We show that differences between the electronic structure of the electrodes at the two interfaces and/or an asymmetric barrier can cause an asymmetric TMR. Most dramatically, we treat the case of nominally identical electrodes but with different crystalline structures, specifically a Co junction where one electrode is hexagonal-close-packed Co and the other is face-centered-cubic (fcc) Co. In this case we correlate a feature common to both the experiment and the calculations with a specific feature found in the calculated band structure of fcc Co, suggesting the band structure is at the root of the asymmetry.
Development of an integrated magnetic sensor with linear output based on spin valves
Authors: Prieto J. L. , Evetts J. E. , Blamire M. G. , Rouse N.
Published: July 2002

The particular objective in this article is to develop a sensor with high sensitivity and linear output that crosses zero at zero field. The sensor is an integrated magnetic device based on a Wheatstone bridge of micropatterned spin valve resistors with the addition of a soft magnetic structure that determines the principal characteristics of the sensor. This magnetic structure is a soft layer shaped to guide the magnetic flux lines of the external magnetic field to the proximity of the bridge elements with the desired angle. With this configuration we change the local field direction rather than the local anisotropy of the spin valve material in order to set up the bridge. The sensitivity of the sensor strongly depends on the permeability of the soft adjacent layer, hence it is significantly increased even in a nonoptimized device, considering other magnetoresistance devices currently available. The sensitivity reported for this device is ∼0.2 %/μT.
An approach to three-dimensional microstructure fabrication utilizing hard x-ray lithography of synchrotron radiation
Authors: Hattori Tadashi , Mekaru Harutaka , Utsumi Yuichi
Published: July 2002

A three-dimensional (3D) microstructure fabrication technique utilizing hard x-ray lithography has been developed. In this technique, as the intensity distribution of the x rays is controlled by a newly developed bending mirror, the exposure residual depth of polymethylmethacrylate (PMMA) resist is controlled over the exposed area. The maximum difference of depths was approximately 50 μm over the large area more than 60 mm (horizontal)×5 mm (vertical). We also investigated the effects of controlling the beam intensity distribution on the obtained 3D resist pattern shapes by changing x-ray mask absorber shapes and angle. 3D PMMA patterns with inclined shape sidewall and graded depths were successfully fabricated. We believe this technique greatly expands applications of the LIGA process.
Ultrafast optical Kerr effect measurements of third-order nonlinearities in cross-conjugated iso-polydiacetylene oligomers
Authors: Slepkov Aaron D. , Hegmann Frank A. , Zhao Yuming , Tykwinski Rik R. , Kamada Kenji
Published: July 2002

We present a study on the ultrafast third-order nonlinearities of a novel series of iso-polydiacetylene oligomers (iso-PDAs). Unlike polydiacetylenes that contain a linearly-conjugated backbone, iso-PDAs have a backbone that is cross-conjugated. A new Kerr-gate technique, differential optical Kerr effect (DOKE) detection, is used to measure third-order nonlinear susceptibilities, χ(3), and second hyperpolarizabilities, γ, of monomer, dimer, trimer, pentamer, and heptamer samples in tetrahydrofuran (THF) solutions. A linear increase in γ as a function of the number of repeat units is observed for all samples except the monomer, suggesting that the fixed-length linearly conjugated segments dominate the electronic polarizability. An added increase to the oligomer nonlinearities due to communication along the cross-conjugated path is not observed. The largest nonlinearity was observed in the heptamer sample, displaying a second hyperpolarizability relative to the THF solvent of γheptamer/γTHF=181±9. In addition, an interesting feature arising out of the signal decay tail is present in the samples but absent in our THF solvent reference.
High-energy x-ray optics developments at Advanced Photon Source beamline 1-ID (abstract)
Authors: Shastri S. D. , Fezzaa K. , Haeffner D. R. , Lai B. , Lee W.-K. , Maser J. M.
Published: July 2002

Three high-energy optics accomplishments are presented. The first is a cryogenically cooled, bent, double-Laue monochromator for 50–200 keV x rays, which delivers over ten times more flux than that from a flat crystal monochromator, but without an increase in energy width. Second, the microfocusing of 50 keV undulator radiation using two stacked Fresnel zone plates is described. Third, a high-resolution inverse-Cauchois scanning analyzer, consisting of a bent Laue crystal, is presented for Compton and fluorescence spectroscopy applications.
Nonlinear magneto-optical properties of Pr1−xCaxMnO3 and Nd1−xSrxMnO3
Authors: Fiebig M. , Miyano K. , Tomioka Y. , Kuwahara H. , Tokura Y. , Reimann K.
Published: July 2002

Optical three-photon difference frequency generation (DFG) was observed in Pr1−xCaxMnO3 (x=0.30, 0.40) and Nd1−xSrxMnO3 (x=0.48, 0.50). In the paramagnetic phase, the DFG signal reflects the fourfold anisotropy of the quasicubic crystal, and all the independent tensor elements for DFG near 2.0 eV are determined. Below TN≈150 K, a two-order-parameter coupling to both antiferromagnetism and charge ordering leads to additional DFG contributions which allow to observe the formation of ∼100 μm large magnetic domains.
Sulphur passivation of the InGaAsSb/GaSb photodiodes
Authors: Wu BinHe , Xia GuanQun , Li ZhiHuai , Zhou Jian
Published: July 2002

Sulphur passivation effects on InGaAsSb/GaSb mesa-type photodiodes were investigated. Measurement results showed zero-bias resistance increased up to 40 times and reverse currents decreased greatly, as a result of sulphur passivation. Current–voltage characteristics showed no sign of degradation of InGaAsSb surface. Auger electron spectroscopy and x-ray photoelectron spectroscopy results demonstrated that the passivation layer is not in the form of S compounds but S element, S8 or Sn.
Crossover among structural motifs in transition and noble-metal clusters
Authors: Baletto F. , Ferrando R. , Fortunelli A. , Montalenti F. , Mottet C.
Published: July 2002

The energetics of nanoclusters is investigated for five different metals (Ag, Cu, Au, Pd, and Pt) by means of quenched molecular dynamics simulations. Results are obtained for two different semiempirical potentials. Three different structural motifs are considered: icosahedra (Ih), decahedra (Dh), and truncated octahedra (TO). The crossover sizes among structural motifs are directly calculated, considering cluster up to sizes N≃40 000. For all the systems considered, it is found that icosahedra are favored at small sizes, decahedra at intermediate sizes, and truncated octahedra at large sizes. However, the crossover sizes depend strongly on the metal: in Cu, the icosahedral interval is rather large, and it is followed by a very wide decahedral window; on the contrary, in Au, the icosahedral interval is practically absent, and the decahedral window is narrow. The other metals display intermediate behaviors, Ag being close to Cu, and Pd and Pt being close to Au. A simple criterion, which is based on the ratio between the bulk modulus and the cohesive energy per atom, is developed to account for the differences among the metals.
Magnetic field dependence of micromachined Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions with a submicron loop
Authors: Kim S.-J. , Chen J. , Nakajima K. , Yamashita T. , Takahashi S. , Hatano T.
Published: July 2002

Two-stacked intrinsic Josephson junctions (IJJs) on a Bi2Sr2CaCu2O8+δ(Bi-2212) whisker with a submicron hole are micromachined by a 3D focused ion beam (FIB) etching method. The junction area, the number of elementary junctions, and the loop area of the SQUIDs are 3, 20, and 0.42 μm2, respectively. The SQUIDs show typical I–V characteristics of IJJs with a critical current IC of 12 μA at 4.2 K. The modulation parameter βL(=2ICL/Φ0) is 6.4, and the inductance L of the SQUID is 1120 pH. That includes the inductances of the elementary junction around a SQUID loop. In a strong magnetic field, the Fraunhofer pattern is obtained and well fitted to the calculated value from the junction size, ΔB=Φ0/tW; t is the interlayer distance, and W is the junction width. In a low magnetic field, approximately 10% of the IC modulation corresponding to the value of βL is observed. Eighty G of the modulation period approximately agrees with the value calculated from the SQUID.
Pattern formation during the CO-oxidation involving subsurface oxygen
Authors: Rotermund Harm Hinrich , Pollmann Michael , Kevrekidis Ioannis G.
Published: July 2002

This paper focuses on subsurface oxygen and its influence on pattern formation during CO-oxidation on platinum surfaces. For the observation of spatiotemporal pattern formation during catalytic reactions the photoelectron emission microscope (PEEM) has proven to be an excellent real-time imaging instrument, capable of tracking local work function changes. The existence of subsurface oxygen on platinumlike surfaces has been extensively discussed and for palladium its presence has been clearly established during rate oscillations. Subsurface oxygen is defined at this point as an atomic O species located directly underneath the uppermost metal crystal layer; its dipole moment therefore considerably lowers the work function of the surface. Here we review some of the investigations involving subsurface oxygen, focusing on the role subsurface oxygen might play in pattern formation during CO-oxidation on platinum. We will also present some new results, where this species clearly interacts with chemisorbed oxygen under restrictions by boundary conditions on the Pt(110) single crystal. These previously (through microlithography) constructed domain boundaries on the surface are made out of Rh or Pd, and they are acting as an additional source of CO molecules for the Pt surface.
Use of a four detector photopolarimeter for Kerr effect measurements
Authors: Dumelow T. , da Silva M. P. , Chianca C. V. , Spinelli R. M. P.
Published: July 2002

A new type of Kerr effect polarimeter, making use of four detectors, is described. The light is reflected at oblique incidence off each detector in turn, in such a way that the plane of incidence changes on each reflection. Since the reflections are polarization dependent, each detector signal depends on the Kerr rotation and, since the detector outputs can be normalized, this rotation may be obtained without the need of either a particularly stable laser source nor any type of modulation. The method thus provides a cheap alternative to more traditional Kerr techniques. We demonstrate its use in obtaining hysteresis loops for a thin film sample of CoO deposited on Co.
An optical mounting system for cavity ring-down polarimetry
Authors: Müller Thomas , Wiberg Kenneth B. , Vaccaro Patrick H.
Published: July 2002

A mounting system is presented for the manipulation and alignment of optical components comprising the heart of a polarization-sensitive ring-down spectrometer. Development of this unique apparatus has been motivated by our recent introduction of cavity ring-down polarimetry which affords a sensitive probe of circular birefringence (nonresonant polarization rotation) and circular dichroism (resonant differential absorption) in rarefied (gas-phase) media. In particular, the mounting hardware described in this note permits facile adjustment of cavity mirrors and intracavity polarization elements while still maintaining overall vacuum integrity of the polarimeter vessel.
Photoinduced magnetic effects in bulk single-crystalline, polycrystalline, and LPE films of Yttrium Iron Garnet
Authors: Hisatake K. , Matsubara I. , Maeda K. , Fujiwara T. , Peters M. , Abe S. , Kudo K. , Kainuma S. , Tamaki T. , Kino Y. , de Francisco C. , Munoz J. M. , Alejos O. , Hernandez P. , Torres C. , Shinagawa K.
Published: July 2002

The effect of light on magnetic initial permeability (the so-called photomagnetic effect) and photoinduced disaccommodation were studied in samples of yttrium iron garnet over the temperature range 77–300 K. The samples studied were bulk crystalline, produced by the floating zone method and the flux method, polycrystalline, produced by the usual sintering method, and single-crystalline films, produced by in situ liquid-phase epitaxis. One or both of the studied phenomena were observed in our experimental samples. To explain the experimental results, we suggest there is photoinduced structural distortion around oxygen vacancies due to a photoinduced change of charge imbalance.
Correlations between spatially resolved Raman shifts and dislocation density in GaN films
Authors: Nootz G. , Schulte A. , Chernyak L. , Osinsky A. , Jasinski J. , Benamara M. , Liliental-Weber Z.
Published: July 2002

Spatially resolved Raman spectra were measured on thick GaN samples with known dislocation density grown by hydride vapor phase epitaxy. The frequencies of the E2 (high) and E1 (transverse optical) phonons shift to lower wave number over a distance of 30 μm from the sapphire substrate/GaN interface. The shifts are linearly correlated with the dislocation density suggesting that the strain due to the lattice mismatch at the interface determines both quantities.
Electronic structure and magnetic character of UGe2 (invited)
Authors: Shick A. B. , Pickett W. E.
Published: July 2002

We review the experimental data on UGe2, provide an overview of some theoretical questions regarding coexistence of superconductivity with ferromagnetism, and then describe our application of a generalized local density approximation (LDA)+U method to this unusual compound. This picture confirms that both ferromagnetism and superconductivity arises from the same U 5f states. Both the exchange splitting and spin–orbit coupling (at ambient pressure) are larger than individual 5f bandwidths, but some simplification occurs: the states at the Fermi level are primarily majority spin, and (what appears to be) the major piece of Fermi surface has strong orbital ml=0 character with respect to the spin direction, and this surface is quasi-two-dimensional. These results may assist in building models of superconducting pairing in UGe2.
Numerical simulation of the aftereffect of a Co/Pt bimodal magneto-optical medium
Authors: Chen Andrew , Fry Richard A. , Torre Edward Della
Published: July 2002

Moving version of the Preisach–Arrhenius (PA) model function is used to simulate aftereffect in bimodal Co/Pt multilayers. The Co/Pt multilayers are assumed to have two kinds of magnetic phases, each characterized by its own magnetic properties. The Co/Pt bimodal major loop is the sum of the conventional (unimodal) loops of each of the two constituent phases. The aftereffect of each of these loops is calculated separately using its own parameters obtained from the major loop. The individual models are then combined into a single bimodal aftereffect model.
Exchange-controlled magnetic anisotropy
Authors: Skomski Ralph
Published: July 2002

The magnetocrystalline anisotropy of 5f transition-metal atoms (actinides) is investigated. A simple model Hamiltonian reproduces the observed huge low-temperature anisotropy of cubic actinide compounds such as US and predicts the temperature dependence of the anisotropy. The dominance of the spin–orbit and crystal–field interactions means that the magnitude of the anisotropy is limited only by interatomic exchange. One consequence is that cubic and uniaxial 4f magnets have similar magnitudes of the anisotropy and similar temperatures dependencies.
Mössbauer and magnetic properties of Co–Ti substituted barium hexaferrite nanoparticles
Authors: An Sung Yong , Shim In-Bo , Kim Chul Sung
Published: July 2002

Co–Ti substituted M-type hexagonal barium ferrite nanoparticles BaFe12−2xCoxTixO19 (0⩽x⩽1.0) have been prepared by a sol-gel method. Magnetic and structural properties of the powders were characterized with a Mössbauer spectroscopy, vibrating sample magnetometer, x-ray diffraction (XRD), thermogravimetry (TG), and differential thermal analysis (DTA). The decomposition of amorphous hydroxides in the dried precipitate continued until 570 °C, according to a TG-DTA analysis. The result of XRD measurements shows that the a and c lattice parameters increase with increasing x from a=5.882 and c=23.215 Å for x=0.0, to a=5.895 and c=23.295 Å for x=1.0. The Fe57 Mössbauer spectra were fitted by a least-squares technique with four subpatterns of Fe sites in the structure and corresponding to the 4f2, 4f1+2a, 12k, and 2b sites. The relative spectra areas of BaFe10CoTiO19 at 295 K were 15%, 30%, 50%, and 5% for 4f2, 4f1+2a, 12k, and 2b subspectra, respectively. The 2b site had a very large quadrupole splitting. The isomer shifts indicated that the valence states of the Fe ions were ferric. The magnetization slightly decreases and the coercivity, HC, drops dramatically from about 5014 to 228 Oe as x increases from 0.0 to 1.0. Co–Ti substituted barium hexaferrite to be controlled to reduce their coercivities with a small decrease of their magnetization.
Magnetic and structural properties of ultrafine CoFe1.9RE0.1O4 (RE=Gd, Nd) powders grown by using a sol-gel method
Authors: Kim Woo Chul , Kim Sam Jin , Kim Chul Sung
Published: July 2002

Ultrafine CoFe1.9RE0.1O4 (RE=Gd, Nd) powders have been fabricated by a sol-gel method. Magnetic and structural properties of the powders were investigated by x-ray diffractometer, Mössbauer spectroscopy, and vibrating sample magnetometer. The CoFe1.9Gd0.1O4 powders that were fired at and above 923 K contained only a single spinel phase and behaved ferrimagnetically. The grain diameters were estimated to be 11–30 nm for the Co-Gd ferrite powders fired in 773–1123 K. Mössbauer spectra measurements showed that the CoFe1.9Gd0.1O4 powders fired at 723–823 K and the CoFe1.9Nd0.1O4 powders fired at 523–1023 K had a spinel structure and were mixed paramagnetic and ferrimagnetic in nature. Mössbauer spectra of the Co–Gd ferrite powder fired at 923 K were taken at various temperatures ranging from 14 to 875 K. The iron ions at both A (tetrahedral) and B (octahedral) sites were found to be in ferric high-spin states. The Néel temperature TN was found to be 875±2 K. Debye temperatures for A and B sites were found to be ΘA=640±5 K and ΘB=217±5 K, respectively. The magnetic behaviors of the CoFe1.9Gd0.1O4 powders fired at and above 723 K, and CoFe1.9Nd0.1O4 powders fired at and above 923 K, respectively, showed that an increase of the firing temperature yielded a decrease in the coercivity and an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization were Hc=1,149 Oe and Ms=72 emu/g in the CoFe1.9Gd0.1O4 samples and Hc=959 Oe and Ms=63 emu/g in the CoFe1.9Nd0.1O4 samples.
Nanotribological studies of thin CrNx, CrOx and CNx overcoats for magnetic recording applications
Authors: Wei G. , Barnard J. A.
Published: July 2002

This work presents systematic nanotribological studies of thin CrNx, CrOx and CNx overcoats for magnetic recording applications over a wide range of N and O concentrations. Composition and crystal structure were determined by x-ray diffraction and x-ray photoelectron spectroscopy. The nanohardness and modulus of the thin films were measured with a Hysitron nanomechanical testing system. Nanoscratch tests were made using a Nano Indenter II system and used to assess the wear resistance and adhesion of the thin films. AFM was used to evaluate the surface morphology and the details of the wear track profiles. The hardness, modulus, tribological properties (wear resistance, elastic recovery, adhesion, friction coefficient) and surface roughness of CrNx, CrOx and CNx have been compared. The hardness and adhesion of CrNx is comparable to CNx, and the roughness of CrNx is lower than CNx. With a very thin Cr underlayer, CrNx appears to have the potential for magnetic recording applications.
A comparison of the thermal stability factors in single layer and exchange-coupled longitudinal media
Authors: Voznyuk V. G. , Doyle W. D. , Abarra E. N.
Published: July 2002

We have measured the time-dependent remanent coercivity in a series of films to determine the thermal stability of antiferromagnetically exchange coupled multilayers [synthetic ferrimagnetic media (SFM)]. Samples were deposited by dc magnetron sputtering on glass substrates. Antiferromagnetic coupling between the magnetic layers was induced by a 0.7-nm-thick Ru spacer. The measurements were done using an alternating gradient magnetometer and in the series of samples the upper layer thickness was changed. The thermal stability factor (KV/kT) and the intrinsic switching field (H0) were extracted from the fit of the experimental data points to Sharrock’s formula. Single layer samples showed a linear growth of H0 with the magnetic layer thickness with some increase in KV/kT. In contrast, no substantial change in H0 with thickness was observed in the SFM. However, a significant increase in the stability factor of 30% and 44% was observed in SFM samples with effective thicknesses of 7 and 12 nm, respectively, compared to single layer media samples with comparable thicknesses. Data confirm the strong dependence of the stability factor on the properties of the top layer.
Magnetization reversal analysis for antiferromagnetic-coupled films with magnetic field angle dependence
Authors: Ren H. B. , Wang J. P. , Ng Vivian
Published: July 2002

In this article, theoretical analysis for the antiferromagnetically coupled films (two layers system) with magnetic field angle dependence is presented. The nonlinear simulative equations for such medium have been deduced and solved by the graphic solution method. Through the analysis of the solutions of different magnetic parameters (K1, K2, M1, M2, t1, t2, and J) and applied magnetic field angle (θ), the relationship between the magnetic switching fields (h1n,h2n) for each layer and the above magnetic parameters have been determined. The hysteresis loop with an applied field angle can be calculated and one example is plotted. By comparing the magnetic switching properties (h1n,h2n) with that of single layer (hn), it has been found that for both the top-magnetic layer and bottom-magnetic layer, the difference of the switching field (h1n−hn) and (h2n−hn), which are brought by the antiferromagnetical coupling effect (J), are closely correlated with the applied field angle.