bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of photoinduced poling and thermal poling of azo-dye-doped polymer films for second order nonlinear optical applications
Authors: Blanchard P. M. , Mitchell G. R.
Published: July 2002

Photoinduced poling (PIP) is a new technique which allows the room-temperature preparation of guest/host polymer films exhibiting significant polar order for nonlinear optical applications. We report a comparison of this novel technique with the conventional electrode poling procedure performed at the glass transition temperature of the polymer using disperse red 1/poly(methylmethacrylate) films. In particular, in situ second harmonic generation measurements show that levels of polar order achieved using these two techniques are similar. In contrast, the stability of the polar order is reduced by up to 20 times in terms of the decay time constant in films prepared using PIP although the stability is very dependent upon the temperature at which the poling was performed.
Monolayer analysis in Rutherford backscattering spectroscopy
Authors: Kimura Kenji , Ohshima Kazuomi , Mannami Michi-hiko
Published: July 2002

Energy spectra of 300-keV He ions backscattered from single-crystal surfaces are measured with a 90° magnetic spectrometer at grazing exit angles. The ions scattered from successive atomic layers can be resolved as separated peaks in the energy spectra. It is shown that both high energy resolution and the grazing angle technique are essential to the monolayer analysis.
Submicron studies of recording media using thin-film magnetic scanning probes
Authors: Manalis Scott , Babcock Kenneth , Massie James , Elings Virgil , Dugas Matthew
Published: July 2002

We present a technique for measuring the coercivity of magneto-optical and perpendicular recording media on a submicron scale using a magnetic force microscope and an electromagnet. Co–Cr coated silicon tips were used to write and image magnetic bits down to 150 nm in diameter. Bits were written when the sum of the tip stray field and an external field Hext exceeded the local, or ‘‘point’’ coercivity. Media can be characterized by a continuous write probability Pw(Hext) which differs significantly from bulk hysteresis loops. For an intermediate field range, writing was intermittent (0<Pw<1) due to spatial variations in media properties.
Broadband digital lock-in amplifier techniques
Authors: Dixon Paul K. , Wu Lei
Published: July 2002

Using only commercially available equipment, we have developed simple digital lock-in techniques that surpass the capabilities of analog lock-in amplifiers. We describe both dual-channel and single-channel digital lock-ins that are capable of measurement at any harmonic or subharmonic of the reference frequency. The best of these digital lock-ins operates over a frequency range from dc to 400 kHz with lower phase noise and better flexibility, stability, and orthogonality than any analog lock-in amplifier.
ac loss measurements on superconductors in the microwatt range
Authors: Schmidt C. , Specht E.
Published: July 2002

Technical superconductors for 50-Hz application, operated at liquid-helium temperature, are designed for very low ac losses. These low losses cannot be measured calorimetrically on short samples of conductors with the conventional measuring techniques, but only on coils wound of several hundreds of meters of wire. We describe here the development of a technique which allows the measurement of ac losses with a resolution of 10−8 W. The method is similar to a thermal conductivity measurement. The sample is placed into a vacuum vessel and connected via a thermal resistance to the liquid-helium bath. Losses in an alternating magnetic field lead to a temperature increase which is a measure of the dissipated power. Calibration is done with an ohmic heater connected to the sample.
Simple adapter for a closed-loop helium cryostat in ultrahigh-vacuum application
Authors: Milun M. , Pervan P. , Valla T. , Dukić P.
Published: July 2002

An adapter is described that enables one to use a nonbakable closed-loop helium cryostat in ultrahigh-vacuum experiments. The adapter is fully bakable, simple and cheap, and has been designed specifically for systems where a sample holder is mounted to an x,y,z-rotational manipulator. During bakeout of the ultrahigh-vacuum apparatus the cold head of the cryostat is removed from the system.
The angular resolution of Lamb mode detection
Authors: Laperre J. , Thys W. , Notebaert H.
Published: July 2002

It is shown experimentally that the angular resolution of Lamb mode detection can be strongly enhanced by properly positioning the detector in the ‘‘leaky’’ field. For this purpose we studied a 1-mm-thick aluminum plate (140 mm×200 mm) which has two pairs of closely spaced Lamb modes (s1,a1 and s0,a0) in the frequency region between 3 and 4 MHz. In order to obtain the necessary spatial resolution, the Lamb spectrum is measured by means of an acousto-optical technique with a small aperture photodiode as detector. It turns out that for each pair of closely spaced modes, there is a detector position in the leaky field where the angular resolution is optimal. These results are explained by analyzing the acoustic profiles transmitted by the plate at different angles of incidence.
High-performance liquid chromatography fraction marker-timer controller
Authors: Kath Gary S. , DiSalvo Jerry , Feygin Ilya
Published: July 2002

A fraction marker-timer controller has been designed that partially automates peak collection and provides event marking during high-performance liquid chromatography of partially purified samples having complex chromatograms. The device is activated via a pushbutton switch when a peak of interest elutes. The apparatus places an event mark on the detector’s chart recording, provides a time delay to account for detector-to-fraction-collector dead volume, and then advances a fraction collector to capture the peak in a test tube. If more than one peak elutes in the dead volume window, up to three independent time delays may be triggered.
Inductively driven gas-breakdown plasma source for intense ion beam production
Authors: Ueda M. , Greenly J. B. , Rondeau G. D. , Hammer D. A.
Published: July 2002

A plasma source based on an inductive breakdown of a supersonic gas puff is described. The source was developed to provide an anode plasma for an annular, extraction geometry, magnetically insulated ion diode. In this source, plasmas with densities of 1013 cm−3 were generated and accelerated to velocities of 20–30 cm/μs; plasma fluxes of 10–40 A/cm2 were obtained. Operating the source under the diode insulating field effect, plasma fluxes above 100 A/cm2 were observed. When the plasma source was used in conjunction with a magnetically insulated diode gap, intense ion beams with proton fluxes of more than 100 A/cm2, energies of 100 keV, and beam pulses longer than 1 μs were extracted.
Phase-frequency relationships in oscillating quartz microbalance electrodes: Determination of an optimal operating frequency for solution-phase microgravimetry
Authors: Komplin Glenn C. , Pietro William J.
Published: July 2002

A systematic study of the effects of overtone harmonic, operating frequency, and electrode surface area on mass sensitivity of liquid-phase quartz-crystal microbalance (QCM) operation is presented. Although the Sauerbrey equation implies that sensitivity should increase as the square of the operating frequency, the effects of mechanical damping induced by operating the crystal under liquid media causes a loss of phase/mass sensitivity at higher nominal frequencies. The competing effects provide a optimum frequency at which to operate the crystal for maximum mass sensitivity under liquids. In the case of water, this optimum frequency is 12.4 MHz. As the phase of the complex impedance is used by most series resonance crystal maintaining circuits as an error signal in stabilizing frequency, progressively higher operating frequencies are not desirable for QCM operation under liquids.
Digital interface for quadrature demodulation of interferometer signalsa)
Authors: Waller J. , Shi X. H. , Altoveros N. C. , Howard J. , Blackwell B. D. , Warr G. B.
Published: July 2002

We describe a digital interface for processing signals produced by a scanning multichannel far-infrared interferometer/polarimeter for plasma density measurements. The interface samples the interferometer signals in quadrature before digital filtering, demodulation and downloading to a transputer array for real-time tomographic inversion and display.
A multichord spectrometer using an 8×8 anode photomultipliera)
Authors: Carolan P. G. , O’Connell R.
Published: July 2002

A multianode photomultiplier (8×8 anodes of 2.5×2.5 mm2) is used to detect a collection of spectra in a high dispersion echelle spectrometer. A cylindrical lens is placed at the output slit to increase the dispersion at the photomultiplier. The cross talk between adjacent spectra is non-negligible (although <5%), resulting in some interspectral distortion. This is removed by solving a system of simultaneous equations, one for each channel, obtained from the measured cross-talk coefficients. The spectrometer has been used on the COMPASS-D tokamak to measure ion temperatures and fluid velocities.
Effect of high-energy neutron flux on fiber optics in an active diagnostic on TFTRa)
Authors: Paul S. F. , Goldstein J. L. , Durst R. D. , Fonck R. J.
Published: July 2002

A bundle of 1-mm-diam fused silica optical fibers on an existing TFTR diagnostic has been exposed to 11 high-power DT discharges. Each shot subjected the fibers to a peak fast (14.7 MeV) neutron flux of ≊2×1012 n/cm2/s and a γ-dose rate of 500 rad(Si)/s for 0.75–1.0 s. The total fast-neutron fluence for these shots was ≊5×1012 n/cm2. A 15-m-long section of the bundle ran along the tokamak’s toroidal field coils and the remaining 15 m ran radially away from the reactor. Fiber luminescence at 660 nm was ≊1010 photons/s/sr/cm2/Å for the above flux (≊5%–10% of the bremsstrahlung emission), and varied linearly with DT neutron rate. Luminescence at 530 nm was 50% stronger, consistent with a Cerenkov radiation spectrum. Sensitivity to 3.5 MeV DD neutrons was ≊1/3 to 1/2 of that for DT neutrons. Fiber transmission decreased with the time integral of the neutron source rate and was reduced by 4% for the above flux. The fiber recovered rapidly: within 10 s, the transmission loss was only 2.5%. Shortly thereafter, the rate of recovery slowed to ≊0.05% per minute, but was sufficient to restore 75% of the transmission loss within two to four discharges. Recovery continued at ≊0.1% per hour and slowed overnight to ≊0.1% per day. Within the relative error of <0.2%, full transmission was recovered after five days.
Piezoelectric push–pull micropositioner for ballistic electron emission microscope
Authors: Shang Guangyi , Qiu Xiaohui , Wang Chen , Bai Chunli
Published: July 2002

Based on the push–pull mechanism, we have developed a piezoelectric micropositioner, with a new mechanical construction. The positioner is basically composed of two piezoelectric actuators as the source of the motion and three permanent magnet feet to produce enough friction force for clamping. The movement of more than 5 mm with the steps of 150–700 nm at the speed of 50–200 steps per second can be accomplished by applying very low driving voltage to the positioner. The absence of clamping voltage in this positioner makes it well suited for use in ballistic electron emission microscope (BEEM). A BEEM operated in a gas-controlled environment based on it has been built and its performances have been tested.
A low-cost GPIB programmable precision voltage source
Authors: Borghesani A. F. , Delfitto G. , Sambo A.
Published: July 2002

We have developed a simple, low-cost, GPIB programmable precision voltage source for laboratory use with two outputs in the range [0,−5]V and [0,+5]V. The original goal was to allow the interfacing to personal computer of older high-voltage sources that can be remotely driven by an analog signal. This voltage source can be more generally used whenever a GPIB programmable low-power precision voltage source is needed.
A high-precision heat-flow calorimeter for measurements up to 1300 K
Authors: Zubkov Alexey , Fujino Takeo , Sato Nobuaki , Yamada Kohta
Published: July 2002

A new high-precision modification of the Setaram heat-flow calorimeter operating at temperatures up to 1300 K has been designed and built. Particular attention was paid in constructing a calorimetric detector providing integration of the major part of heat flux from the surface of the crucible in which a reaction takes place. The calibration parameter of the calorimeter is reproducible within ±1% in the whole volume of the working crucible. The calorimeter can be used for accurate measurements of enthalpy changes as small as 3 J.
A simple method to determine the purity of an inert gas
Authors: Mao O. , Altounian Z. , Ström-Olsen J. O.
Published: July 2002

A simple and economical method to determine the overall purity of inert gases such as Ar and He is described which uses the filament of commercial light bulbs. The lifetime of the incandescent filament is sensitive to gas impurities such as O2, H2O, and some hydrocarbons, but insensitive to N2 and, to first order, to the applied voltage. The relationship between the filament lifetime, τ, and the O2 and H2O concentrations was carefully calibrated. The relationship follows a power law: τ=AC−γ, where A and γ are positive constants depending on the impurity type. For mixtures of O2 and H2O, a simple superposition rule for the overall lifetime, τoverall, as 1/τoverall = 1/τO2 + 1/τH2O, applies. A mechanism for the concentration dependence of the filament lifetime is proposed in which the creep of the filament is enhanced by the impurity solutes in the filament. With a lifetime spanning from a few seconds to a few days, the filament acts as a simple gauge for the determination of impurity concentrations in an inert atmosphere. As an example, for O2, the measurable concentrations vary from 105 ppm to 10−2 ppm.
Time-resolved fluorescence polarization measurements for entire emission spectra with a resistive-anode, single-photon-counting detector: The Fluorescence Omnilyzer
Authors: Kelly Lisa A. , Trunk John G. , Sutherland John C.
Published: July 2002

We report a fluorescence analyzer that records simultaneously the temporal profiles for both orthogonal linear polarizations for all wavelengths in a fluorescence emission spectrum. The Analyzer combines a resistive-anode single-photon-counting photomultiplier, imaging spectrograph, Wollaston polarizer, multiparameter analyzer with histograming memory, and standard timing electronics. The spectrograph disperses the fluorescence spectrum across the photocathode of the photomultiplier, and the Wollaston polarizer separates the spectra of the two polarizations in opposite directions from the center of the photocathode perpendicular to the direction of spectral dispersion. The locations at which each photon reaches the photocathode is determined by the ratios of the charges read from the four corners of the resistive anode. One of the two address coordinates that determine where in histogramming memory each photon is recorded is obtained by measuring the time of arrival of the photon at the detector relative to the pulse of light that excites the fluorescence. The second address coordinate is obtained by combining the most-significant bit of the location of the event along the direction on the resistive anode corresponding to the polarization of the photon with the multibit digital value indicating photon wavelength. Storing the data directly into histogramming memory permits display of the data set as it is recorded. Both the spectral and temporal calibrations of the fluorescence analyzer are independent of the polarization of the fluorescence. The ≈100 ps temporal resolution of the resistive-anode detector is well matched to the ≈1 ns full width at half-maximum pulses of light produced by the synchrotron storage ring that we use as the excitation source, but laser excitation could also be used with this detector. Recording simultaneously all of the data required for the global analysis of the time evolution of both linear polarization components of fluorescence, and thus, time-resolved anisotropy, reduces the duration of exposure of the sample to the excitation beam, hence, facilitating studies of fragile or photosensitive biological specimens.
Spurious electrons in electron spectrometers and their effect on differential electron impact ionization cross-section measurements
Authors: Rudd M. E.
Published: July 2002

The effect of the nonideal response characteristics of electrostatic analyzers is described and equations are derived, which allow a calculation of the distortion that this causes in measurements of angular and energy distributions of electrons from atomic collisions, especially in electron impact. Examples are given that show how a contamination as small as 10−5 from spurious electrons generated inside an analyzer can explain the peak seen at zero angle in some angular distribution measurements and the filling in of the minimum in the energy distributions in others.
Plasma wakefield klystron
Authors: Pasour J. A. , Seeley R. , Smithe D. , Nguyen K.
Published: July 2002

We describe a microwave source that utilizes a plasma to bunch the electron beam. The electron beam interacts with the plasma via the wakefield produced by the head of the beam. The resulting plasma oscillations bunch the beam at a frequency proportional to the plasma frequency. Thus the device can be tuned by varying the plasma density. The theory of the device is described and numerical simulations of the interaction are presented. The theory and simulations were validated by a proof-of-principle experiment using a simple klystron configuration.
Sensing position and speed by recording magnetization transitions on mechanically functional machine members
Authors: Garshelis I. J.
Published: July 2002

A means for sensing relative position and speed of mechanically functional machine members having accessible ferromagnetic portions is described. The method is based on creating, sensing, and annihilating local transitions in the polarity of remanent magnetization in surface regions of such parts while in motion, in analogous fashion to conventional magnetic recording. The presence and location of a transition, sensible by its associated external field, constitutes stored information carried by the member. Bias and “erasure” are provided by a permanent magnet; transitions are “written” by pulsed currents through a coil on a gapped core and “read” by Hall-effect field sensors located at purposeful distances downstream. Transition position is determinable within ±0.1 mm. Velocity is determined by the elapsed time between a recording pulse and its detection. Experimental data from a polyurethane timing belt reinforced with cabled steel wires demonstrates system operation.
Optical activity in the vacancy ordered III2VI3 compound semiconductor (Ga0.3In0.7)2Se3
Authors: Ye J. , Yoshida T. , Nakamura Y. , Nittono O.
Published: July 2002

Great optical activity is realized by a vacancy ordered III2VI3 compound (Ga0.3In0.7)2Se3 with point group 6 which is based on wurtzite structure and characterized by the screw arrangement of cation atoms along the c axis. The transition of the fundamental absorption edge is direct and the band gap is estimated to be 2.05 eV. An anomalous optical rotatory dispersion around the absorption edge is observed and the maximum rotatory power of 125°/mm is obtained at λ=620 nm. The optical activity for red light is always above 60°/mm, that is 4–6 times as large as that of α quartz. (Ga0.3In0.7)2Se3 single crystal is very useful, especially for the He–Ne laser as an optically active substance; the rotatory power reaches 103°/mm, being more than 5 times of α quartz.
A high pressure permeameter for the measurement of liquid conductivity of porous construction materials
Authors: Green K. M. , Hoff W. D. , Carter M. A. , Wilson M. A. , Hyatt J. P.
Published: July 2002

Equipment is described which provides measurements of the pressure and volume of unidirectional liquid flow through sample cores of porous solids. The real-time measurement of flow pressures enables any changes in conductivity resulting either from interactions with the flow liquid or from changes in effective stress levels to be monitored directly. The equipment can use a range of flow liquids and be adapted for miscible and immiscible displacement studies. It is particularly suitable for cement-based and stone construction materials.
Laser frequency stabilization using linear magneto-optics
Authors: Yashchuk Valeriy V. , Budker Dmitry , Davis John R.
Published: July 2002

The design of a diode laser frequency stabilization system using the Zeeman effect is described. Various regimes of operation are analyzed using the Jones matrix approach. The system is different from the original Joint Institute for Laboratory Astrophysics design in that the magnetic fields are fully contained and thus it can be used in proximity of magnetically sensitive instruments.
Influence of substrate-induced biaxial compressive stress on the optical properties of thin GaN films
Authors: Rieger W. , Metzger T. , Angerer H. , Dimitrov R. , Ambacher O. , Stutzmann M.
Published: July 2002

The influence of biaxial stress on the optical properties of thin GaN films is studied by x-ray diffraction and Raman and photoluminescence spectroscopy. The stress is caused by differences in the thermal expansion coefficient and lattice mismatch between the film and c-plane sapphire substrates. In particular, the influence of various thicknesses of AlN buffer layers on the strain in GaN films is studied. GaN/AlN films were deposited by low pressure metal organic chemical vapor deposition using triethylgallium and tritertbutylaluminum and ammonia. We observe a pronounced reduction of strain in the GaN films with increasing buffer thickness: An AlN buffer layer thicker than 200 nm eliminates the stress completely. Estimates of the linear coefficient for the near band gap luminescence shift due to biaxial compressive strain yield a value of 24 meV/GPa.
Domain inversion in LiTaO3 and LiNbO3 by electric field application on chemically patterned crystals
Authors: Baron Cynthia , Cheng Hsing , Gupta Mool C.
Published: July 2002

We report a process for the formation of inverted ferroelectric domain regions in LiTaO3 and LiNbO3. This process involves the chemical patterning of the material surface by diffusion of Ti or ion exchange followed by domain inversion by a uniform electric field. The resultant periodic domain structure is applicable to first order quasi-phase-matched second-harmonic generation (QPMSHG). In first order periodically poled crystals of Z-cut LiTaO3, initial results have shown bulk conversion efficiencies of up to 2%/(W cm). A second-harmonic conversion of 3 mW of blue wavelength power was observed from waveguide devices made on these crystals.
Room temperature photopumped laser operation of native-oxide-defined coupled GaAs–AlAs superlattice microrings
Authors: Evans P. W. , Holonyak N.
Published: July 2002

Data are presented on the room temperature photopumped laser operation of GaAs–AlAs superlattice microrings in single, coupled pair, and coupled hexagonal-ring configurations. The lasers are fabricated from a GaAs–AlAs superlattice (Lz∼30 Å, LB∼70 Å) that is selectively disordered and deep oxidized (H2O vapor + N2) through its entire thickness to form high-index- contrast planar microrings (11 μm o.d., 3 μm i.d.). Thresholds are ∼1.7×105 W/cm2 (∼150 mW, pump power) for single microrings and 4.3×105 W/cm2 for the pair and hexagonal-ring geometries. Single, coupled pair, and even-numbered coupled microrings arranged in a ‘‘ring’’ (circular) configuration exhibit multiple single modes or more typically twin modes at microring whispering-gallery mode spacings. Microrings are of interest as basic elements in more extended photonic structures, and should be compatible with diode operation of micro lasers.
Generation and detection of coherent terahertz waves using two photomixers
Authors: Verghese S. , McIntosh K. A. , Calawa S. , Dinatale W. F. , Duerr E. K. , Molvar K. A.
Published: July 2002

A general technique has been demonstrated at microwave and submillimeter-wave frequencies for photoconductive sampling in the frequency domain using photomixers and continuous-wave laser diodes. A microwave version in which two photomixers were coupled by a transmission line was developed to quantitatively test the concept from 0.05 to 26.5 GHz. A quasioptical version using antenna-coupled photomixers was demonstrated from 25 GHz to 2 THz. Such a system can outperform systems based on time-domain photoconductive sampling in frequency resolution, spectral brightness, system size, and cost.
Elastic strain of freely suspended single-wall carbon nanotube ropes
Authors: Walters D. A. , Ericson L. M. , Casavant M. J. , Liu J. , Colbert D. T. , Smith K. A. , Smalley R. E.
Published: July 2002

We have induced large elastic strains in ropes of single-wall carbon nanotubes, using an atomic force microscope in lateral force mode. Freely suspended ropes were observed to deform as elastic strings with tension proportional to elongation. Ropes were elastically deformed over >10 cycles without showing signs of plastic deformation. The maximum strain observed, 5.8±0.9%, gives a lower bound of 45±7 GPa for the tensile strength (specifically, yield stress) of single-wall nanotube ropes.
Writing nanometer-scale pits in sputtered carbon films using the scanning tunneling microscope
Authors: Eagle S. C. , Fedder G. K.
Published: July 2002

A reproducible method of the formation of pits in a sputtered carbon surface with a platinum-iridium tunneling tip is presented for possible use in lithography or data storage applications. Thin carbon films are sputtered on top of chrome and gold metallic underlayers on a silicon substrate. Overall surface roughness of the carbon films is under 3 Å. Holes are produced in the carbon film by applying short voltage pulses (4–8 V in height, 250 ns–100 μs in length) across the tunneling gap. An array of holes written in the carbon demonstrate reproducibility and the feasibility of using this multilayered structure in a data storage system.
In situ video observation of 180° domain kinetics in congruent LiNbO3 crystals
Authors: Gopalan Venkatraman , Jia Q. X. , Mitchell T. E.
Published: July 2002

We report in situ observation of the nucleation and growth of ferroelectric 180° domains in congruent LiNbO3 crystals. The domains nucleate and grow as six-sided polygons. The wall velocity measured at a constant field shows a spike-like behavior with time, suggesting a strong pinning–depinning type of growth mechanism. This behavior is similar to the corresponding transient current measurement which also shows current spikes during domain reversal.
Probing the near fields of the super-resolution near-field optical structure
Authors: Tsai Din Ping , Lin Wei Chih
Published: July 2002

Super-resolution near-field structure, glass/SiN (170 nm)/Sb (15 nm)/SiN (20 nm), a promising structure for near-field ultrahigh-density optical data storage, has been studied by a tapping-mode tuning-fork near-field scanning optical microscope in transmission mode. Both propagating and evanescent field intensities were found at the focused spots of the surface of the super-resolution near-field structure. Images of the near-field intensity gradients at different excited laser powers (0.42–2.43 μW) showed that the area of the static evanescent intensity could be stably controlled. The enhancement of the near-field intensity, and the reduction of the focused spot through the super-resolution near-field structure, glass/SiN (170 nm)/Sb (15 nm)/SiN (20 nm) have been observed.
Analysis of the temperature–field dependence of the magnetostriction in the antiferromagnetic phase of the easy-plane antiferromagnet CoCl2
Authors: Kalita V. M. , Lozenko A. F. , Ryabchenko S. M.
Published: July 2002

An analysis is made of the experimental magnetic-field curves of the anisotropic (in the easy-plane) magnetostriction of the antiferromagnet CoCl2 at different temperatures in the ordered phase. It is shown that the field dependence of the magnetostriction in a single-domain antiferromagnetic state remains proportional to the square of the applied field over the entire temperature range; this behavior corresponds well to the classic (Néel) scheme of collapse of the sublattice spins (spin flip). The temperature dependences of the intrasublattice and intersublattice contributions to the magnetoelasticity are proportional to each other. The temperature dependence of the spin-flip field obtained from the magnetostriction data agrees with that obtained previously from observations of the antiferromagnetic resonance (AFMR). The temperature dependence of the spontaneous anisotropy of the magnetostriction, obtained by extrapolating the magnetostriction in the single-domain state to zero field, is close to M∼T4.
Vortex morphologies on reaccelerated interfaces: Visualization, quantification and modeling of one- and two-mode compressible and incompressible environments
Authors: Kotelnikov Alexei D. , Ray Jaideep , Zabusky Norman J.
Published: July 2002

We examine the vortex dynamics and interfacial evolution of “reacceleration” and “reshock” for single interface density-stratified fluid (Richtmyer–Meshkov) environments. In the former case, we simulate, visualize, and quantify the parameter range of the free falling tank laboratory experiment of J. Jacobs and C. F. Niederhaus, where the impulsive subsequent acceleration of the interface occurs after several rolls from the initial impulsive acceleration. We interpret the rapid onset of chaotic motion in the roll-up region as due to the formation of intertwined close-laying layers of opposite-signed vorticity. In the latter case, we make compressible simulations at low Mach number (M=1.2) and compare with incompressible simulations. We juxtapose the results and find an excellent agreement in large and intermediate size features and their magnitudes before the “reshock.” At the “reshock,” discrepancies arise due to highly compressible regime. The simulations were made with incompressible vorticity-based methods, vortex-in-cell (VIC) and vortex blob (VB), and compressible second-order Godunov codes.
Highly parallel data storage system based on scanning probe arrays
Authors: Lutwyche M. I. , Despont M. , Drechsler U. , Dürig U. , Häberle W. , Rothuizen H. , Stutz R. , Widmer R. , Binnig G. K. , Vettiger P.
Published: July 2002

This letter discusses an alternative storage approach to conventional magnetic data storage. The approach uses a 32×32 array of scanning probe microscopes working in parallel to read and write data as small indentations in a polymer storage medium. The results have densities of 100–200 Gbit/in.2. At such densities, it is shown that well over half the array works, and at lower densities more than 80% of levers are working.
Viscoelectroelastic behavior of heterogeneous piezoelectric solids
Authors: Li JiangYu , Dunn Martin L.
Published: July 2002

We study the viscoelectroelastic behavior of heterogeneous piezoelectric solids, focusing on the connection between heterogeneity and coupled mechanical and electrical relaxations. Our approach is based on the existence of a correspondence between quasistatic viscoelectroelasticity and static piezoelectricity when linear constitutive response exists. We couple this correspondence principle with micromechanics models to predict the overall behavior of heterogeneous piezoelectric solids in terms of microstructural details. We devote specific attention to a class of two-phase materials consisting of a lossless piezoelectric phase embedded in a lossy (mechanically and electrically) matrix and obtain closed form expressions for the effective complex electroelastic moduli. Numerical results are presented and discussed, and qualitative agreement with experiment is observed.
Disclination renormalization of the disjoining pressure between walls bounding a nematic liquid crystal
Authors: Braun F. N.
Published: July 2002

Nematics confined between antagonistically anchoring walls exert a positive disjoining pressure derived from the elastic energy of rotation of the director. We calculate negative corrections due to the presence of thermally excited disclinations in the director field.
Solvent effects on the collapse dynamics of polymers
Authors: Chang Rakwoo , Yethiraj Arun
Published: July 2002

The effect of solvent on the collapse dynamics of polymers is studied using computer simulation. Two cases are investigated, one where the solvent is incorporated through a pairwise additive attraction between the polymer beads and a random force on each polymer bead, and another where the solvent is incorporated in an explicit fashion as a second component. Brownian dynamics and molecular dynamics simulations are used in the former and latter model, respectively, with intermolecular interactions chosen so that the equilibrium size of the polymer is similar in both models at similar conditions. In the Brownian dynamics simulations, at short times local blobs of monomers are found separated by linear segments. With time the blobs grow in size and coalesce to form sausage like shapes. These sausages gradually become thicker and shorter until the final shape of a spherical globule is reached. The first stage is rapid whereas the second sausage-sphere stage is slow. In this stage the polymer often gets trapped in local minima and the change in size with time occurs through discrete jumps, and the equilibrium conformation is often not reached. In contrast, in the molecular dynamics simulations with explicit solvent, the size of the polymer changes smoothly with time, and the polymer does not get trapped in local minima for the cases investigated, although the sequence of polymer shapes is similar. This suggests that incorporating solvent molecules explicitly is important in the computer simulations of collapse and folding of polymers.
Linear chain polymerization initiated by a scanning tunneling microscope tip at designated positions
Authors: Okawa Y. , Aono M.
Published: July 2002

A conjugated polymer nanowire can be created at any designated position in a monomolecular layer by initiating chain polymerization using a scanning tunneling microscope tip with a spatial precision of the order of 1 nm. The demonstration has been presented for a self-ordered monomolecular layer of 10,12-pentacosadiynoic acid, which is a diacetylene compound, adsorbed on a graphite surface. The polymer nanowires created have a length ranging from 5 to 300 nm, the length being controlled by domain boundaries or artificial defects in the molecular layer. The frequency of occurrence of chain polymerization is measured against the pulsed bias voltage, which suggests that the excitation of the molecule is caused by the inelastically tunneling electrons.
Selenium-related luminescent centers in metalorganic chemical-vapor-phase deposition grown ZnSe epilayers on GaAs
Authors: Zhang X. B. , Ha K. L. , Hark S. K.
Published: July 2002

ZnSe epilayers were grown on a (001) GaAs substrate by metalorganic chemical-vapor-phase deposition. An interruption of the Zn source (i.e., Se passivation) was purposely introduced during the growth. The optical properties of the epilayers grown were studied by photoluminescence (PL) spectroscopy. We show that Se passivation during the growth interruption introduces luminescent centers in the epilayers. Evidence of this assignment comes from the characteristic temperature and excitation wavelength dependence of the PL spectra, which are distinctly different from those of commonly observed deep-level emissions associated with the so-called self-activated centers. Moreover, the PL peak energy of the centers depends strongly on the coverage of Se: the longer the time or the higher the flow rate of the Se precursor used for the passivation, the lower the energy of its PL peak. The possible origin of this luminescence is discussed.
Orientational effect in the magnetoresistance of organic conductors
Authors: Peschanskiı̆ V. G. , Atalla R.
Published: July 2002

The asymptotic behavior of the magnetoresistance of layered conductors of organic origin is discussed for different orientations of a high magnetic field H relative to the layers. It is shown that when current flows along the normal n to the layers, the amplitude of the Shubnikov–de Haas quantum oscillations and the smoothly varying part of the magnetoresistance increase sharply at certain angles θ between the vectors H and n. An experimental study of the orientational effect in a high magnetic field yields detailed information about the electron energy spectrum of quasi-two-dimensional conductors.
Single layer light-emitting devices with high efficiency and long lifetime based on tris(2,2′ bipyridyl) ruthenium(II) hexafluorophosphate
Authors: Rudmann H. , Rubner M. F.
Published: July 2002

In this work, single layer, light-emitting devices have been prepared from pristine tris(2,2′ bipyridyl) ruthenium(II) hexafluorophosphate ([Ru(bpy)3](PF6)2) and blends of [Ru(bpy)3](PF6)2 with glassy polymers such as poly(methylmethacrylate) (PMMA), polycarbonate (PC), and polystyrene. Due to the electrochemical nature of the device operation, a high external quantum efficiency at a low operating voltage is achieved. For pristine devices fabricated with an Al cathode, external quantum efficiencies in the range of 1.2%–1.5% at 100–1000 cd/m2 have been achieved. Such devices, however, show signs of degradation in time when stored in the off state in inert atmosphere. Blending with glassy polymers such as PMMA results in an improved film quality and a slowing of the device degradation which, in return, decreases the leakage current during device operation. Therefore, external quantum efficiencies of 2%–2.5% at a light output of 200 cd/m2 are observed when the electroluminescent tris(2,2′ bipyridyl) ruthenium(II) complex is blended with PMMA or PC. In addition, increased efficiency and lifetime are found when the devices are operated under a 50% duty cycle at 5 V and 1 kHz compared to the operation under constant voltage. With a 50% duty cycle, half lives of around 500–1100 h continuous operation have been achieved at luminance levels in the range of 200–350 cd/m2. When Ag is used as the cathode material, PMMA blend devices exhibit external quantum efficiencies in the range of 2.5%–3.0% at luminance levels of around 50 cd/m2. In addition, devices with a Ag cathode show no signs of degradation when stored in the off state.
On the nature of the half-integer quantum features on the transport and Hall resistances of 2D electron systems in a quantizing magnetic field
Authors: Pashitskiı̆ E. A.
Published: July 2002

It is shown that in two-dimensional (2D) electron systems in a quantizing magnetic field, in addition to electron–hole (excitonic) pairing through the Coulomb interaction it is also possible to have Cooper pairing of the 2D electrons due to their interaction with 2D phonons and 2D plasmons localized at the interface of the crystals in semiconductor heterostructures. By summing the divergent (as T→0) ladder diagrams in the zero-sound and Cooper channels, the critical temperatures of the transition to the excitonic and Cooper phases are found. It is shown that the excitonic phase can exist only in comparatively narrow regions near half-integer values of the filling factor ν=(2n−1)/2 (n=1,2,3,…), a finding which is in qualitative agreement with the experimentally observed anisotropic features in the transport resistance of 2D systems in the integer quantum Hall effect regime. By taking into account a superposition of states of bound electron pairs and unpaired 2D electrons in the Cooper phase, one can describe practically all of the experimentally observed quantum features in the fractional quantum Hall effect regime, including at values of ν that are not described by the composite fermion model. At the same time, the interelectron attraction can promote triplet Cooper pairing of composite fermions, which is accompanied by the appearance of an “exotic” quantum feature at ν=5/2. Arguments supporting the possibility of experimental observation of the Cooper phase are presented.
Bound spin-excitons in two-dimensional electron gas
Authors: Fleurov V. , Kikoin K.
Published: July 2002

A theory of the spin exciton capture by a magnetic impurity in a 2D electron gas is developed. We consider a resonance model for the electron scattering by a transition metal impurity and calculate the binding potential for spin excitons. This potential is spin selective and is capable of binding a spin exciton with zero angular momentum. In order to trap an exciton with a nonzero angular momentum m, the potential must exceed a certain threshold value that depends on m.
Shell effects in alkali metal nanowires
Authors: Yanson A. I. , van Ruitenbeek J. M. , Yanson I. K.
Published: July 2002

After making a cold weld by pressing two clean metal surfaces together and then gradually separating the two pieces, a metallic nanowire is formed, which progressively thins down to a single atom before the contact is lost. We show evidence that the radii of such nanowires are influenced by electronic shell filling effects, in analogy to electronic shell structure in metal clusters. A modulation of the shell effect, known as supershell structure, is clearly observed. For sodium and potassium at larger diameters there is a crossover to crystalline facets with shell-closings corresponding to the completion of an additional atomic layer. We conclude that a complete analogy exists between shell structure in metallic clusters and nanowires.
Magnetic susceptibility of La0.875Sr0.125MnO3 under hydrostatic pressure to 1 GPa
Authors: Petrova A. E. , Itskevich E. S. , Ventcel’ V. A. , Kraidenov V. F. , Rudnev A. V.
Published: July 2002

The magnetic susceptibility of single-crystal La0.875Sr0.125MnO3 is measured under hydrostatic pressure up to 1 GPa. The Curie temperature TC and the temperatures TM and TOO of quantum phase transitions are determined. The pressure dependence of these transition temperatures is determined: dTC/dP=18 K/GPa, dTM/dP=16 K/GPa, and dTOO/dP=22 K/GPa. The phase diagram obtained demonstrates that an applied pressure can have the same effect on the phase transitions as a change in the degree of doping x.
Relaxation processes in amorphous films of monoatomic metals
Authors: Kuz’menko V. M. , Lazarev B. G.
Published: July 2002

The relaxation of the electrical resistance in amorphous films of Bi, Yb, Be, and Mn is investigated at low temperatures. It is shown that this process during an isothermal hold is ordinarily a simple exponential process with a single characteristic relaxation time. In amorphous metal–hydrogen films a subbarrier tunneling of hydrogen atoms apparently occurs. It is conjectured that this is the cause of the delay in the onset of superconductivity at T=4.2 K after completion of the condensation of amorphous Be–H films.
Lattice modes of solid nitrogen to 104 GPa
Authors: Olijnyk H. , Jephcoat A. P.
Published: July 2002

Lattice modes of solid nitrogen are studied by Raman spectroscopy at room temperature to 104 GPa using the diamond-anvil technique. Changes in the lattice mode spectral features correlate with those observed in the vibronic spectra suggesting symmetry changes of the crystal lattice. The changes in the spectral features mainly appear as branchings of existing modes, supporting the view of a close structural relationship among these high-pressure phases.
Fast tree search for enumeration of a lattice model of protein folding
Authors: Cejtin Henry , Edler Jan , Gottlieb Allan , Helling Robert , Li Hao , Philbin James , Wingreen Ned , Tang Chao
Published: July 2002

Using a fast tree-searching algorithm and a Pentium cluster, we enumerated all the sequences and compact conformations (structures) for a protein folding model on a cubic lattice of size 4×3×3. We used two types of amino acids—hydrophobic (H) and polar (P)—to make up the sequences, so there were 236≈6.87×1010 different sequences. The total number of distinct structures was 84 731 192. We made use of a simple solvation model in which the energy of a sequence folded into a structure is minus the number of hydrophobic amino acids in the “core” of the structure. For every sequence, we found its ground state or ground states, i.e., the structure or structures for which its energy is lowest. About 0.3% of the sequences have a unique ground state. The number of structures that are unique ground states of at least one sequence is 2 662 050, about 3% of the total number of structures. However, these “designable” structures differ drastically in their designability, defined as the number of sequences whose unique ground state is that structure. To understand this variation in designability, we studied the distribution of structures in a high dimensional space in which each structure is represented by a string of 1’s and 0’s, denoting core and surface sites, respectively.
Xe nuclear magnetic resonance line shapes in nanochannels
Authors: Jameson Cynthia J. , de Dios Angel C.
Published: July 2002

The intermolecular nuclear magnetic resonance (NMR) chemical shifts of Xe in nanochannels of various dimensions and shapes are considered. Predictions of the line shapes that result from anisotropic averaging of the shielding tensor for Xe atoms in various limiting cases of nanochannels are made, based on ab initio calculations of Xe129 shielding surfaces. Variation of the line shapes with channel cross sections, Xe loading, and with temperature are predicted for channels having cross-sectional areas that do not permit two or more Xe centers to be located on the same cross-sectional plane. It is shown that Xe in effectively one-dimensional channels should be expected to exhibit signature line shapes in Xe129 NMR spectra, provided that the cross-sectional dimensions of the channels are sufficiently small that Xe atoms do not pass each other during diffusion. These predictions are tested against experimental examples of anisotropic Xe line shapes in various cavities and nanotubes.
Mean excitation energy, static polarizability, and hyperpolarizability of the spherically confined hydrogen atom
Authors: Banerjee Arup , Sen K. D. , Garza Jorge , Vargas Rubicelia
Published: July 2002

Calculations of mean excitation energy, Im, static polarizability, α, and hyperpolarizability, γ, using the variation perturbation procedure are reported for the spherically confined hydrogen atom. The electric response properties α and γ have been found to strongly depend upon the radius of confinement. The hyperpolarizabilty changes sign and becomes negative under strong confinement.
Behavior of some heavy and light rare earth-cobalt magnets at high temperature
Authors: Chen Christina H. , Gong Wei , Walmer Michael H. , Liu Sam , Kuhl G. Edward
Published: July 2002

This article reports our investigation on the behavior of some heavy and light rare earth-cobalt magnets at high temperatures. Magnetic properties and the temperature coefficients of magnetization, α, over a wide temperature range have been studied for various magnets, which include RE(Co0.74Fe0.14Cu0.09Zr0.03)7.4 for RE=Er, Dy, Tm, Ho, and Tb, as well as RE(Co0.68Fe0.22Cu0.07Zr0.03)7.4 for RE=Sm, Ho, and Gd. The results show that the temperature for magnetization peak is 480 °C for the Er–Co magnet, and 370 °C for the Ho–Co magnet. Our research indicates that there is good potential for making temperature compensated, high temperature magnets with Ho, Er, Gd, or combinations of these rare earths to satisfy certain applications. The Tm–Co magnet does not show the typical characteristic shown by other heavy rare earth cobalt magnets, which is a positive α at points above room temperature. However, Tm–Co shows a small and near constant negative α value, which may be useful for some applications. Magnetic properties and the temperature dependence for temperature compensated magnets with high coercivity are also reported in this article, which have the composition of Sm0.6RE0.4(Co,Fe,Cu,Zr)7.4 with RE=Er, Ho, Tm, Dy, and Tb.