bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Tc superconducting gradiometer with a long baseline asymmetric flux transformer
Authors: Dantsker Eugene , Froehlich O. M. , Tanaka S. , Kouznetsov K. , Clarke John , Lu Z. , Matijasevic V. , Char K.
Published: July 2002

The planar gradiometer consists of a directly coupled magnetometer inductively coupled to the smaller loop of an asymmetric, two-loop flux transformer patterned in a single film of YBa2Cu3O7−x. The coupling is adjusted mechanically to balance the gradiometer to about 1 part in 3000 with respect to fields perpendicular to its plane. The baseline is 48 mm and the intrinsic balance with respect to in-plane fields about 1 part in 1400. When a magnetic field is applied solely to the magnetometer, its sensitivity is reduced by only 5% by the presence of the flux transformer.
Interstitial copper-related center in n-type silicon
Authors: Istratov A. A. , Hieslmair H. , Flink C. , Heiser T. , Weber E. R.
Published: July 2002

n-type silicon samples were measured by deep level transient spectroscopy (DLTS) immediately (within one hour of storage at room temperature, required for the preparation of Schottky-diodes) after copper diffusion and quench. A donor level at Ec-(0.15±0.01) eV with a concentration of up to 1013 cm−3 was detected. The amplitude of the DLTS peak decreased with the time of storage at room temperature, and stabilized at a concentration (4 to 7)×1011 cm−3 after 15–20 h. The activation energies and prefactors of the decay of the DLTS peak in n-type Si and the reactivation of copper-compensated boron in p-type Si concur. This correlation suggests that the deep level is interstitial copper itself or a complex of interstitial copper.
Mapping of electrical double-layer force between tip and sample surfaces in water with pulsed-force-mode atomic force microscopy
Authors: Miyatani Tatsuya , Horii Miki , Rosa Armin , Fujihira Masamichi , Marti Othmar
Published: July 2002

Pulsed-force-mode atomic force microscopy (PFM-AFM) using a cantilever with a Si3N4 tip was applied to map charge distribution on a sample surface in water. In order to confirm the applicability of the present PFM, we prepared a patterned sample by vapor deposition of Al on a quartz plate covered with silica beads, followed by oxidation of Al with O2 and removal of the beads with ultrasonication. The two different areas of Al2O3 and SiO2 had different isoelectric points and bore positive and negative charge, respectively, at pH 8.6. The lateral resolution of the present method was found to be ca. 30 nm.
Whispering-gallery-mode microring laser using a conjugated polymer
Authors: Kawabe Y. , Spiegelberg Ch. , Schülzgen A. , Nabor M. F. , Kippelen B. , Mash E. A. , Allemand P. M. , Kuwata-Gonokami M. , Takeda K. , Peyghambarian N.
Published: July 2002

We observed laser emission in whispering gallery modes using a microring composed of a semiconducting polymer poly[2,5-bis-(2′-ethylhexyloxy)-p-phenylenevinylene coated on an etched fiber under transient and quasisteady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 μJ/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasisteady-state pumping shows the possibility to develop electrically pumped polymer lasers.
Gas source molecular beam epitaxy of GaN with hydrazine on spinel substrates
Authors: Nikishin S. A. , Temkin H. , Antipov V. G. , Guriev A. I. , Zubrilov A. S. , Elyukhin V. A. , Faleev N. N. , Kyutt R. N. , Chin A. K.
Published: July 2002

Growth of high quality wurtzite-structure GaN layers on (111) MgAl2O4 by gas source molecular beam epitaxy is described. Hydrazine was used as a source of active nitrogen. In situ reflection high energy electron diffraction was used to monitor the growth mode. Two-dimensional growth was obtained at temperatures above 750 °C on multi-step GaN buffer layers. The resulting GaN films show excellent luminescence properties.
Broadening of near-band-gap photoluminescence in n-GaN films
Authors: Iliopoulos E. , Doppalapudi D. , Ng H. M. , Moustakas T. D.
Published: July 2002

This letter addresses the broadening mechanism of the near-band-gap photoluminescence in GaN films doped n type with silicon. The films were produced by plasma assisted molecular beam epitaxy and their carrier concentration was varied systematically from 1015 to 1020 cm−3. The photoluminescence was excited with a 10 mW He–Cd laser at 77 K. At low carrier concentration (<1017 cm−3) the photoluminescence peak has a full width at half maximum of about 18 meV, while at high carrier concentration (>1018 cm−3) the full width at half maximum increases monotonically with carrier concentration up to about 120 meV. The broadening of the line at high carrier concentration is attributed to tailing of the density of states caused by potential fluctuations due to randomly distributed impurities. The data were quantitatively analyzed, as a function of carrier concentration and compensation ratio, using the impurity band broadening model of Morgan [Phys. Rev. 139, A343 (1965)], and the agreement between model and experimental data supports the model’s validity and suggests a potential method of determining the compensation in degenerate nitride semiconductors.
Photorefractive Bragg gratings in nematic liquid crystals aligned by a magnetic field
Authors: Wiederrecht Gary P. , Wasielewski Michael R.
Published: July 2002

Photorefractive Bragg gratings are observed in low-molar-mass nematic liquid crystals doped with electron donor and acceptor molecules. This is accomplished by alignment of the nematic liquid crystals in a 0.3 T magnetic field, which produces thicker homeotropic aligned samples than traditional surfactant techniques. Grating fringe spacings as low as 3.7 μm are achieved with 176-μm-thick samples, producing grating Q values of 33. Up to this point, low molar mass nematic liquid crystals have exhibited photorefractive gratings with Q⩽1. Asymmetric two-beam coupling and photoconductivity experiments are performed to verify the photorefractive origin of the gratings.
Epitaxial growth of metastable Ba2RuO4 films with the K2NiF4 structure
Authors: Jia Y. , Zurbuchen M. A. , Wozniak S. , Carim A. H. , Schlom D. G. , Zou L-N. , Briczinski S. , Liu Y.
Published: July 2002

Epitaxial Ba2RuO4 films with the K2NiF4 structure have been grown by pulsed laser deposition on (100) SrTiO3 substrates. X-ray diffraction and cross-sectional transmission electron microscopy results indicate that the films are c-axis oriented, single-domain, and contain relatively few stacking faults and intergrowths. Electrical measurements indicate metallic conductivity to low temperatures, but no evidence of superconductivity.
The self-assembly of uniform heteroepitaxial islands
Authors: Chiu Cheng-hsin
Published: July 2002

We present simulation for the morphological evolution of a Stranski–Krastanow system during the annealing process. By considering the strain energy, the surface-energy anisotropy, and the film–substrate interaction, the simulation demonstrates that the morphological evolution of the system can lead to an array of nearly uniform islands that is stable against coarsening. The stability results from the suppression of strain energy relaxation by strong surface-energy anisotropy and strong film–substrate interaction.
1.5-Mbit/s direct readout of line-and-space patterns using a scanning near-field optical microscopy probe slider with air-bearing control
Authors: Issiki F. , Ito K. , Etoh K. , Hosaka S.
Published: July 2002

We have demonstrated 1.5-Mbit/s signal readout of a 0.25 μm line-and-space (L&S) pattern on a rotating disk by using a scanning near-field optical microscopy (SNOM) probe slider with air-bearing control. The light transmittance of the probe was greatly increased through direct irradiation of a focused light to a pyramidal probe tip. The bit rate of the signal was enhanced by two orders of magnitude compared to that of existing SNOM systems that use a tapered-fiber probe. The signal contrast, signal-to-noise ratio, and carrier-to-noise ratio for the 0.25 μm L&S pattern were 19%, 17 dB, and 37 dB, respectively. The estimated resolution limit of the probe corresponded to a L&S width of ∼130 nm.
Raman study of ZnxBe1−xSe alloy (100) epitaxial layers
Authors: Pagès O. , Ajjoun M. , Laurenti J. P. , Bormann D. , Chauvet C. , Tournié E. , Faurie J. P.
Published: July 2002

Long wavelength longitudinal optical (LO) and transverse optical (TO) phonons of BeSe and ZnxBe1−xSe layers are identified in a wide composition range by using Raman spectroscopy. A two-mode behavior is clearly evidenced. As predicted by the dielectric model of Hon and Faust, the eigenfrequencies of the BeSe- and ZnSe-like LO modes correspond to the maxima of Im〈−ε(ω, x)−1〉. Excellent agreement is obtained with a model where the calculations are performed by using the equations of motion and polarization derived from the modified random element isodisplacement model. Besides, the TO and LO frequencies for BeSe are determined to be 501 and 579 cm−1, respectively.
On the dispersion managed soliton
Authors: Cautaerts Vincent , Maruta Akihiro , Kodama Yuji
Published: July 2002

We review various methods used to study the dispersion managed soliton for nonlinear return-to-zero pulse propagation in optical fibers. A numerical averaging method, the guiding center soliton, the variational method both with a simple and with an extended ansatz, as well as the multiscale theory are discussed and numerically compared, allowing us to show their domains of applicability. Their relative merits and demerits are then exposed.
Dimerization and solvation equilibria: The direct correlation function
Authors: Mileva Elena , Evans Glenn T.
Published: July 2002

The Wertheim method is applied to the calculation of thermodynamic and structural properties of a fluid that undergoes a reversible dimerization reaction a+b→ab. The a and b particles are hard spheres with a single sticky spot that allows a,b dimerization, but no additional association. For this mixture of a’s, b’s and ab particles, one can add the direct correlation functions to the list of previously calculated quantities, such as the pressure, and the contact values of the pair correlation functions.
Analysis of “equation of state” for supercooled liquid
Authors: Paluch Marian , Hensel-Bielowka Stella , Psurek Tatiana
Published: July 2002

The pressure and temperature dependent dielectric relaxation times are compared with the predictions of the model proposed by Avramov. Remarkable agreement is found in whole P and T plane. It was also pointed out that the tested model predicts no pressure effect on fragility and nonlinear character of pressure dependence of glass transition temperature. Our data provide also an assessment of the generalized Vogel–Fulcher–Tammann (VFT) law. However, it is inferred that the generalized VFT law is not able to accurately reproduce the surface described by experimental relaxation times. Additionally, isothermal data are analyzed by the pressure counterpart of the temperature VFT law. The isothermal data expressed in terms of the reduced relaxation time and reduced pressure form a master curve.
Optical observation of contrast agent destruction
Authors: Chomas James E. , Dayton Paul A. , May Donovan , Allen John , Klibanov Alexander , Ferrara Katherine
Published: July 2002

Fragmentation of an ultrasound contrast agent on the time scale of microseconds provides opportunities for the advancement of microvascular detection, blood flow velocity estimation, and targeted drug delivery. Images captured by high-speed imaging systems show destruction of a microbubble during compression. Peak wall velocity of −700 m/s and peak acceleration of 1.2×1012 m/s2 is observed for insonation with a peak pressure of −1.1 MPa and a center frequency of 2.4 MHz. Theoretical calculations of wall velocity and acceleration using a modified Rayleigh–Plesset model predict a peak negative wall velocity of −680 m/s and peak acceleration of 2×1012 m/s2.
Characterization of nuclear wave packets describing molecular photodissociation
Authors: Lein M. , Erdmann M. , Engel V.
Published: July 2002

A bound-to-free transition initiated by femtosecond excitation of diatomic molecules results in photofragments with a distribution of kinetic energies. A measurement of the kinetic-energy distribution yields the modulus squared of the asymptotic momentum-space wave packet prepared in the laser excitation process. On the other hand, the coordinate-space density of the wave packet entering the interaction-free region can be determined from pump–probe integrated fluorescence spectroscopy. We provide several numerical examples to show that this information can be used to determine the phase of the asymptotic wave packet so that this particular quantum-mechanical wave function can be characterized completely. To achieve this aim we use an iteration scheme (Gerchberg–Saxton algorithm) which does not require any further information about the system or the laser pulses.
Growth of ferromagnetic Nd0.7Sr0.3MnO3 films with an off-axis sputtering configuration
Authors: Wang L. M. , Sung H. H. , Su B. T. , Yang H. C. , Horng H. E.
Published: July 2002

Ferromagnetic Nd0.7Sr0.3MnO3 (NSMO) films have been prepared onto (001) SrTiO3 by the off-axis magnetron sputtering technique. The NSMO films are characterized by resistivity measurements, x-ray diffraction, and the atomic force microscope. Under our deposition conditions the films show a maximum resistivity (ρmax) at the highest peak temperature of Tp=180–240 K. By increasing the deposition temperature, Tp shifts to lower temperature and ρmax increases. In an applied magnetic field, large magnetoresistance (MR) ratios >2200% for several in situ deposited films at 180 K and H=6 T are obtained. The influence of growth conditions on the magnetotransport properties of NSMO films and their MR characteristics are discussed.
Interface contribution to giant magnetoresistance in granular AgFe studied with Mössbauer spectroscopy
Authors: Alof C. , Stahl B. , Ghafari M. , Hahn H.
Published: July 2002

The magnetotransport properties of granular thin AgFe films, prepared by codeposition of the constituent metals in an ultrahigh vacuum have been investigated. Mössbauer spectroscopy was employed to investigate the role of scattering of conduction electrons at the interface between the magnetic Fe particles and the Ag matrix. It is possible to determine the ratio of Fe atoms located at the Ag/Fe interface and in the particles (bulk atoms). The giant magnetoresistance effect correlates with the ratio of interface/bulk atoms.
Confinement effects on the electronic and vibronic properties of CdS0.65Se0.35 nanoparticles grown by thermal annealing
Authors: Verma Prabhat , Gupta Lalita , Abbi S. C. , Jain K. P.
Published: July 2002

CdS 0.65 Se 0.35 nanoparticles are grown in a glass matrix by thermally annealing a base glass material, in which Cd, S, and Se were introduced by diffusion. The starting base material contains no crystalline structure. A comparative study of the confinement effects on the annealed samples using photoluminescence, low-frequency Raman, and optical Raman scattering experiments is presented. Growth of nanoparticles is observed with the three independent experimental techniques as the annealing temperature is varied from 550 to 800 °C. Radii of the thermally grown nanoparticles calculated from the three independent techniques are found to be in good agreement. In addition, surface phonon modes are observed in the optical spectral range, the frequencies of which agree well with those calculated theoretically. As expected from the theory, the positions of the surface phonons are found to be independent of the particle size.
Gain characteristic of continuous-wave InGaN multiple quantum well laser diodes during life testing
Authors: Goddard Lynford L. , Kneissl Michael , Bour David P. , Johnson N. M.
Published: July 2002

A method to measure small changes in gain and distributed loss in laser diodes during life testing is described. The evolution of the gain spectrum was computed by measuring the true spontaneous emission spectrum and the lasing wavelength of the diode with time. This method was applied to investigate the gain spectrum of InGaN multiple quantum well ridge waveguide laser diodes operating under continuous-wave (cw) conditions during diode life testing. During lasing there were large changes in the light output power versus current characteristic, but no significant change in the forward current-voltage characteristic. No catastrophic failure occurred; the light output power decreased continuously during life testing until the device stopped lasing under cw conditions. We observed an increase in pulsed threshold current of 15(±1)%, a decrease in gain at fixed current of 11(±2)%, and a slight increase in the total optical losses of 4(±2)%. These changes occurred primarily while the device was lasing under cw conditions.
Charging in gate oxide under irradiation: A numerical approach
Authors: Fourches Nicolas T.
Published: July 2002

To complement the numerous experimental studies devoted to the gate oxide of irradiated metal–oxide–semiconductor structures a code was developed to compute the net oxide charge growth caused by irradiation. Intrinsic parameters concerning deep levels and transport were introduced. The validity of the code was assessed by a comparison with experimental data. With this tool a predictive approach can be taken. The role of compensating deep electron levels is revealed here, giving an interpretation of the experimental results at relatively high doses. The consequences of the temperature of irradiation can also be directly observed.
Raman scattering study of GaAs crystalline layers grown by molecular beam epitaxy at low temperature
Authors: Sano H. , Suda A. , Hatanaka T. , Mizutani G. , Otsuka N.
Published: July 2002

Raman scattering, x-ray diffraction, and transmission electron microscopy (TEM) were used to study GaAs layers grown by molecular beam epitaxy at low substrate temperatures (LT-GaAs). The intensity of forbidden Raman scattering of longitudinal optical and transverse optical phonons linearly increases as a function of the concentration of excess As in the range of [AsGa]=0.04×1020–1.175×1020 cm−3. Concentrations of excess As in LT-GaAs layers were estimated from the lattice spacings measured with an x-ray diffractometer. No obvious defect was seen in cross-sectional TEM images of these nonstoichiometric As-rich GaAs layers. The origin of the forbidden Raman scattering of the nonstoichiometric LT-GaAs layers is explained as the strain induced by AsGa (As antisite)-related defects with low structural symmetry.
Dynamic characteristics of weakly coupled GaAs/AlGaAs superlattices
Authors: Rasulova G. K.
Published: July 2002

Dynamic characteristics of weakly coupled superlattices have been experimentally studied by investigation of the transient current responses and an oscilloscopic study of the hysteresis loop. The observed transient current responses are classified into two categories: the switching current response with different levels of the initial and final current, and the transient current response retaining the same value of the initial and final current. The study of the oscilloscope images of the current–voltage characteristics produced by different types of an external sweep allowed us to reproduce the current traces for closed cycle current oscillations. Damped current oscillations with a frequency of 83 kHz have been observed.
Magnetic domain structures and giant magnetoresistance of granular (Ni74Fe16Co10)35Ag65 films
Authors: Wang H. , Lu X. , Yan X. , Wong S. P. , Cheung W. Y. , Ke N. , Xu J. B. , Hu S. J. , Zeng D. C. , Liu Z. Y.
Published: July 2002

In this article, granular (NiFeCo)35Ag65 films having a nominal atomic ratio of Ni:Fe:Co=74:16:10, near the zero magnetostriction ternary alloy, were prepared by dc magnetron co-sputtering. Subsequent thermal annealing was performed in a vacuum (<2×10−3 Pa) furnace for 1 h at various temperatures ranging from 250 to 500 °C. The crystalline structure and magnetic structure of the as-deposited and annealed films were characterized by Rutherford backscattering spectroscopy, x-ray diffraction, atomic force microscopy, and magnetic force microscopy. The giant magnetoresistance (GMR) for the films was measured as a function of temperature between 20 and 300 K using a conventional four-point probe dc technique in the presence of a magnetic field up to 7.6 kOe. It was found that, the crystalline structure, the magnetic structure, and consequently the GMR were closely related to the thermal treatment. The film annealed at the temperature of 300 °C has the isolated single domain structure and the highest GMR value of 19.1% at 20 K, while the film annealed at 500 °C has the long-range domain structure and the lowest GMR value of 9.9% at 20 K.
Modeling of the I–V characteristics in amorphous silicon n+-i-n+ devices
Authors: Cech V.
Published: July 2002

Both model and experimental results of electron injection in amorphous silicon n+-i-n+ devices with heavily doped n+ layers are presented using a realistic model of such a structure, developed by the author. Spatial profiles of transport parameters were calculated changing the undoped layer (i layer) thickness and the energy and spatial distributions of the density of localized states in undoped film. Simulated current–voltage (I–V) characteristics were compared with space-charge-limited current (SCLC) dependences given by the drift currents. The effect of diffusion currents on the I–V characteristics was studied to determine the criteria for a correct application of the SCLC technique. As follows from numerical simulations, only the characteristics measured on a device with a sufficiently “thick” i layer can be used to apply the SCLC technique. The effect of contacts on the I–V dependence can be checked by the scaling law.
Suppression of silicide formation in Fe films grown on Si(001)
Authors: Zavaliche Florin , Wulfhekel Wulf , Xu Hai , Kirschner Jürgen
Published: July 2002

Thin ferromagnetic Fe films were grown at both room and low temperatures (150 K) on (2×1) and on Au-passivated Si(001). In the case of Fe grown on (2×1) Si(001), the magnetization sets in at an Fe coverage of 3.6 monolayers (ML) at room temperature and of 2.3 ML at 150 K, indicating a reduced magnetization due to silicide formation. To suppress Fe–Si reactions we used a 2 ML thick Au buffer layer deposited in two steps. We performed a reconstruction by the deposition of about 0.5 ML of Au at ∼1000 K followed by the growth of another 1.5 ML at room temperature or at 150 K. Room temperature growth results in rough Au films and no earlier onset of magnetization in comparison to the case of unbuffered Fe films. However, a strong out-of-plane magnetization is measured at the onset coverage. By contrast, at 150 K, smooth Au buffer films are observed, acting as protective layers against Fe and Si intermixing. Growth on these buffer layers results in an early onset of the out-of-plane magnetization at 1.4 ML of Fe and a larger Kerr signal. Both these findings point at a significant reduction of silicide formation. A 2.3 ML thick Fe film grown on Au passivated Si(001) shows a reversible magnetic behavior upon annealing to 280 K and cooling back to 150 K, indicating a thermally stable system.
Effect of deposition temperature on the optical transmission and paramagnetic centers in pulsed laser deposited amorphous silicon carbide thin films
Authors: Tabbal M. , Isber S. , Christidis T. C. , El Khakani M. A. , Chaker M.
Published: July 2002

The optical transmission and paramagnetic centers in pulsed laser deposited amorphous silicon carbide films were investigated as a function of the deposition temperature (Td). As Td is raised from 200 to 650 °C, the optical gap of the films increases from about 2.0 eV up to a value of 2.2 eV, while the density of paramagnetic defects, measured by electron paramagnetic resonance (EPR), diminishes from (5.6 to 3.6)×1020 spins/cm3. EPR line shape analyses and g-value measurements indicate that the EPR active centers are clustered sp2 carbon related defects. At a measurement temperature of −190 °C, the exchange narrowed EPR linewidth, evaluated at 5.0±0.3 G, is nearly independent of Td. However, at room temperature, the EPR linewidth was found to increase with deposition temperature, thus suggesting a decrease in the size of the sp2 bonded clusters. All the structural changes revealed by EPR, as Td was raised from 200 to 650 °C, correlate well with the optical transmission data and with the increase in heteronuclear SiC bonding revealed by Fourier transform infrared spectroscopy. Finally, analysis of the thermal dependence of the EPR linewidth together with the conductivity of the a-SiC films have revealed the existence of different conduction mechanisms in the layers, depending on the deposition temperature.
A comparison of impurity-free and ion-implantation-induced intermixing of InGaAs/InP quantum wells
Authors: Dao L. V. , Gal M. , Carmody C. , Tan H. H. , Jagadish C.
Published: July 2002

We have compared the time integrated photoluminescence (PL) and the time resolved PL of several lattice matched InGaAs/InP quantum wells intermixed either by ion implantation or an impurity-free method. We have found that the carrier capture rates into quantum wells and carrier relaxation from the wells depend on the type of intermixing used. Our results indicate that the carrier lifetimes are significantly longer in samples intermixed by the impurity-free methods, while the carrier collection efficiency of the quantum wells is more efficient in samples intermixed by ion implantation.
Spin and orbital excitations in undoped manganites
Authors: Gu R. Y. , Shen Shun-Qing , Wang Z. D. , Xing D. Y.
Published: July 2002

We develop a theory for spin and orbital excitations in undoped manganites to account for the spin and orbital orderings observed experimentally. It is found that the anisotropy of the magnetic structure is closely related to the orbital ordering, and the Jahn-Teller effect stabilizes the orbital ordering. The phase diagram and the low-energy excitation spectra for both spin and orbital orderings are obtained. The calculated critical temperatures can be quantitatively comparable to the experimental data.
Principles for the design and operation of a molecular wire transistor
Authors: Emberly Eldon , Kirczenow George
Published: July 2002

We explore theoretically the fundamental principles of design and operation of a three-terminal molecular wire transistor that consists of a single π-conjugated carbon chain molecule with thiol end groups (CnHn−2S4), self-assembled on the cleaved edge of a multilayer of alternating thin gold and insulating films. The ends of the chain bond to two outer gold layers that act as source and drain, and the chain bridges a third (inner) gold layer that acts as a gate. We show that transistor action should occur in this device if sulfur atoms are adsorbed on the surface of the gold gate. The sulfur atoms acquire charge as the gate voltage is increased, thereby enhancing the interaction between the gate and molecule and creating a strong potential barrier that hinders electron flow along the molecular wire.
The effects of temperature and morphology on electron transmission and stimulated desorption of H− from thin hydrocarbon films
Authors: Bass A. D. , Parenteau L. , Weik F. , Sanche L.
Published: July 2002

We have measured for the 0–20 eV incident energy range, the low-energy electron transmission (LEET) spectra and electron stimulated desorption (ESD) yields of H− from pure films of n-hexane and ethyl-benzene from 20 K to sublimation. The ESD yields from both dissociative electron attachment and dipolar dissociation initially increase as the films are heated. This effect is attributed to the collapse of pores within the films. For n-hexane, a marked decrease in ESD yields is observed at film temperatures ⩾80 K, which correlates to the onset of crystallization as evidenced in LEET spectra. This decrease in anion yield is associated with the formation of a well-defined band structure that reduces the time period an excitation and/or electron resides on a particular molecule. This behavior in the ESD yields from ethyl-benzene films is not observed prior to sublimation in agreement with LEET spectra which show negligible change with film temperature indicating that there is no change of phase prior to sublimation (i.e., the film remains amorphous at all temperatures).
Resistance and tunneling spectra of aligned multiwalled carbon nanotube arrays
Authors: Davydov D. N. , Li J. , Shelimov K. B. , Haslett T. L. , Moskovits M. , Statt B. W.
Published: July 2002

The resistance and tunneling spectra of samples formed by depositing silver electrodes at the two ends of aligned, template-grown, carbon nanotube arrays were measured in the temperature range 0.67–440 K. Two types of samples were fabricated, one with small oxide tunnel junctions separating the carbon nanotubes from the metal electrodes, the other with a significant Al2O3 tunnel barrier. The measurements indicate the presence of three regimes for dI/dV(V). For T>220 K, dI/dV(V) and the zero-bias conductivity show a broad minimum and an activation temperature dependence suggesting semiconductor behavior. In the temperature range 10<T<140 K, the zero-bias conductivity shows a square-root temperature dependence. For T<2 K, a very steep rise in the zero-bias tunneling resistance is observed with a strong simultaneous suppression of the tunneling conductivity near the Fermi energy. Coulomb blockade is suggested as a plausible explanation of the observed behavior.
Fourier transform infrared and dielectric study of water–C12E4 liquid crystals
Authors: Caponetti E. , D’Angelo P. , Pedone L. , Liveri V. Turco
Published: July 2002

Fourier transform infrared (FT-IR) spectroscopy and time domain reflectometry (TDR) have been employed to study nano-confinement effects in water–tetraethylene glycol monododecyl ether (C12E4) system as a function of the water to surfactant molar ratio (R) at 25 °C. The study emphasises that, at low R values, the vibrational dynamics of water entrapped between equidistant parallel bilayers of oriented surfactant molecules is significantly different from that of pure water whereas at R>7 it becomes practically indistinguishable. On the other hand, even at the higher R values investigated, the dielectric properties of the aqueous domain confined in water–C12E4 liquid crystals are found to be strongly affected by confinement effects. The observed single relaxation processes with a symmetrical distribution of relaxation time are discussed in terms of effects due to interfacial interactions and topological restrictions on water-surfactant head group dynamics in liquid crystals. Correlation between the structural and dynamical properties of water–C12E4 liquid crystals and thermal effects due to microwave irradiation on these highly viscous microheterogeneous systems are also discussed.
Temperature dependence of the threshold for laser emission in polymer microlasers
Authors: Ramos-Ortiz G. , Spiegelberg Ch. , Peyghambarian N. , Kippelen B.
Published: July 2002

We studied the temperature dependence of the laser emission threshold in microring polymer lasers. For microring lasers with diameters between 16 and 120 μm, a weak temperature dependence was observed when the temperature was varied between 300 and 10 K. These experimental results are explained within a four-level model. Our results suggest that a significant reduction of threshold can be achieved at low temperatures if the quality factor Q of the microcavities is improved.
Optimization of a one-dimensional time-of-flight mass spectrometer
Authors: Even Uzi , Dick Bernhard
Published: July 2002

Space focusing of a linear time-of-flight mass spectrometer (TOFMS) with an arbitrary number N of ion acceleration regions, a field free drift tube, and a reflectron is analyzed. A quality function Q(d,u0) is defined as the fractional error in the flight time of an ion with initial position d and initial velocity u0 with respect to an ion with d=0 and u0=0. This quality function has the form Q(d,u0)=−u0/f(0)+F(u02−d), where the function f(y) depends on the dimensions and the field strengths chosen for the instrument, and F(y)=f(y)/f(0)−1. The quality function is optimized up to the order k by setting all Taylor coefficients of F(y) up to and including yk equal to zero and solving this system of equations for the design parameters of the instrument. A linear TOFMS with N acceleration regions can be optimized in this way with respect to N design parameters. An additional reflectron will not add another optimizable parameter, i.e., no further Taylor coefficient can be made to vanish. After optimization of the TOFMS with respect to all field strengths including that in the reflectron, the quality function becomes independent of the length of the field free drift tube. Hence, the effect of the reflectron is to make space focusing independent from the drift tube length. The quality function for a fully optimized TOFMS depends only on the number N of acceleration stages and is, for a given N, identical for the designs with and without a reflectron. However, the design containing a reflectron has a smaller value of the factor 1/f(0) which determines the error introduced by the initial velocity distribution of the ions. A space focused TOFMS cannot be further focused with respect to the initial velocity u0, since the first term in the Taylor expansion of the quality function in the variable u0 is proportional to the inverse of the total flight time. For a TOFMS with two or more acceleration regions this is the dominant source of error that remains after space focusing. This situation cannot be improved by delayed or pulsed extraction of the ions from the ionization region nor by deceleration of the ions before they enter the field free region.
Magnetic properties of Co–P powders produced by chemical reduction
Authors: Saito T. , Igarashi M. , Kobayashi M.
Published: July 2002

Co–P powders were produced by chemical reduction. The powders had a spherical shape with an average diameter of about 1 μm. X-ray diffraction and differential scanning calorimetry studies confirmed that the powders were amorphous. The amorphous powders showed higher saturation magnetization than the crystalline counterparts. Heat treatment of the powders above the crystallization temperature resulted in the formation of fcc Co, hcp Co, and Co2P phases. The saturation magnetization of the annealed powders monotonically decreased as the annealing temperature increased. On the other hand, the coercivity of the annealed powders rapidly increased with increasing annealing temperature. The powders annealed at 600 °C had a saturation magnetization of 100 emu/g with a coercivity of 500 Oe.
Crystallization and optical nonlinearity in GeO2–SiO2 glass poled with ArF excimer-laser irradiation
Authors: Matsumoto Syuji , Fujiwara Takumi , Seno Yoshiki , Hirose Yosiharu , Ohama Motoshi , Ikushima Akira J.
Published: July 2002

We report on crystallization and second- and third-order optical nonlinearities of GeO2–SiO2 glass poled with ArF laser irradiation. With laser power and 100 mJ/cm2/pulse, the treatment generates crystallites in the glass, provided that the poling field strength is greater than 0.5×105 V/cm. Its crystal structure is possibly the cristobalite phase of the Ge–Si–O system. A large coefficient of second-harmonic generation (SHG), d33, is induced in the glass with the crystallite generation, and its value is comparable to d22 of LiNbO3. Moreover, the third-order optical susceptibility χ(3) increases by a factor of approximately 15 compared to that of glass without the crystallites. Even after the d33 coefficient decays out, the crystallites and χ(3) are retained. The result suggests that the large SHG in the glass is not induced from inert second-order nonlinearity of the crystallites, but the main origin of the induction is the associated effective second-order nonlinearity with the formation of the space-charge field.
Electrical and optical properties of CNx(0⩽x⩽0.25) films deposited by reactive magnetron sputtering
Authors: Broitman E. , Hellgren N. , Järrendahl K. , Johansson M. P. , Olafsson S. , Radnóczi G. , Sundgren J.-E. , Hultman L.
Published: July 2002

The electrical and optical properties of carbon-nitride CNx films (0⩽x⩽0.25) deposited by unbalanced reactive magnetron sputtering from a graphite target in mixed Ar/N2 discharges at a substrate temperature of 350 °C have been investigated. Pure C films exhibit a dark conductivity at room temperature of 25 Ω−1 cm−1, which grows up to 250 Ω−1 cm−1 for CNx films with N content of 20%. For CNx films, temperature-dependent conductivity measurements suggest that two electron conduction processes exist in the investigated temperature range 130<T<300 K. Under white-light illumination, photoconductivity is observed. The film optical properties obtained from spectroscopic ellipsometry measurements between 1.24 and 5 eV indicate a semimetallic behavior with a large amount of sp2 bonds in the material. The measured electrical and optical properties of the films are related to the apparent film microstructure and bonding nature. Electron microscopy show that the addition of N2 in an Ar discharge leads to a transformation from amorphous to a fullerene-like microstructure consisting of curved, frequently intersecting, and highly in-plane oriented basal lattice planes.
Simple method for estimating linear attenuation coefficients from x-ray diffraction tomography data
Authors: Pavlov K. M. , Kewish C. M. , Davis J. R. , Morgan M. J.
Published: July 2002

In this article, we demonstrate how the spatial distribution of x-ray linear attenuation coefficients within an object can be estimated from x-ray diffraction tomography data. The experimental arrangement to achieve this exploits a position sensitive detector and an analyzer crystal. The quality of reconstructed maps of linear attenuation coefficients is comparable to results obtained from conventional transmission computed tomography.
The X̃ 2Π and Ã 2Σ+ electronic states of the HCSi radical: Characterization of the Renner–Teller effect in the ground state
Authors: Sari Levent , Gonzales Jason M. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner–Teller splitting are reported. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X̃ 2Π) has a linear geometry with re(CH)=1.0781 Å and re(CSi)=1.6956 Å. This is in good agreement with the experimental values of r0(CH)=1.0677 Å and r0(CSi)=1.6925 Å, respectively. In the Ã 2Σ+ state, HCSi is also found to have a linear geometry with re(CH)=1.0737 Å and re(CSi)=1.6130 Å at the [CCSD(T)/cc-pVQZ] level, confirming experimental values of r0(CH)=1.0625 Å, r0(CSi)=1.6118 Å, and the observation of C–Si triple bond character. With the same method, the X̃ 2Π and Ã 2Σ+ state C–H stretching vibrational frequencies are predicted to be 3271 and 3319 cm−1, respectively, for which experimental values are not available. The classical X̃–Ã splitting (Te value) was determined to be 32.6 kcal/mol (1.41 eV, 11 730 cm−1) and quantum mechanical splitting (T0 value) to be 33.5 kcal/mol (1.45 eV, 11726 cm−1), which are in excellent agreement with the experimental T0 value of 11 766.721 cm−1 (33.64 kcal/mol, 1.459 eV). The linear excited Ã 2Σ+ state of the molecule has a real degenerate bending vibrational frequency, whereas the ground state (X̃ 2Π) is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies. The Renner parameter (ε) and average harmonic bending frequency (ω2) of the X̃ 2Π state are predicted to be ε=−0.114 and ω2=518 cm−1 at the CCSD(T)/cc-pVQZ level of theory. The electronic structure analysis of the ground state showed that the HCSi radical is an A-type Renner–Teller molecule.
Fabrication and temperature dependence of the resistance of single-crystalline Bi nanowires
Authors: Wang X. F. , Zhang J. , Shi H. Z. , Wang Y. W. , Meng G. W. , Peng X. S. , Zhang L. D. , Fang J.
Published: July 2002

Single-crystalline Bi nanowires with diameters ranging from 20 to 70 nm were prepared by electrodeposition using nanoporous aluminum oxide membranes rather than the more usual track-etched polycarbonate membranes. X-ray diffraction and selected area electron diffraction investigations revealed that the nanowires are essentially single crystalline and highly oriented. The temperature dependence of zero-field resistance of different diameter nanowires indicated that these Bi nanowires undergo a semimetal–semiconductor transition due to two-dimensional quantum confinement effects. The resistance maximum was observed at 50 K in zero magnetic field for 20 nm Bi nanowires, while the resistance minimum at 258 K for 50 nm Bi nanowires, due to the quantum size effect.
Co cluster coalescence behavior observed by electrical conduction and transmission electron microscopy
Authors: Peng D. L. , Konno T. J. , Wakoh K. , Hihara T. , Sumiyama K.
Published: July 2002

We deposited monodispersed Co clusters with mean diameters d=6, 8.5, and 13 nm on quartz and microgrid substrates using a plasma-gas-condensation-type cluster beam deposition system. The cluster–cluster coalescence behavior of the Co cluster assemblies was investigated by in situ electrical conductivity measurements and ex situ transmission electron microscopy (TEM). The electrical conductivity measurement indicates that, below temperature T≈100 °C, the Co clusters with d=8.5 nm maintain their original size as deposited at room temperature, while the cluster–cluster coalescence takes place at their interface at T>100 °C. The TEM observation indicates that the morphology of the cluster distribution shows no marked change at substrate temperatures Ts<250 °C. Above Ts=300 °C, the interfacial area of coalesced clusters is crystalline, and has its own orientation, different from that of two connected cluster cores.
Mechanism controller system for the optical spectroscopic and infrared remote imaging system instrument on board the Rosetta space mission
Authors: Castro Marı́n J. M. , Brown V. J. G. , López Jiménez A. C. , Rodrı́guez Gómez J. , Rodrigo R.
Published: July 2002

The optical, spectroscopic infrared remote imaging system (OSIRIS) is an instrument carried on board the European Space Agency spacecraft Rosetta that will be launched in January 2003 to study in situ the comet Wirtanen. The electronic design of the mechanism controller board (MCB) system of the two OSIRIS optical cameras, the narrow angle camera, and the wide angle camera, is described here. The system is comprised of two boards mounted on an aluminum frame as part of an electronics box that contains the power supply and the digital processor unit of the instrument. The mechanisms controlled by the MCB for each camera are the front door assembly and a filter wheel assembly. The front door assembly for each camera is driven by a four phase, permanent magnet stepper motor. Each filter wheel assembly consists of two, eight filter wheels. Each wheel is driven by a four phase, variable reluctance stepper motor. Each motor, for all the assemblies, also contains a redundant set of four stator phase windings that can be energized separately or in parallel with the main windings. All stepper motors are driven in both directions using the full step unipolar mode of operation. The MCB also performs general housekeeping data acquisition of the OSIRIS instrument, i.e., mechanism position encoders and temperature measurements. The electronic design application used is quite new due to use of a field programmable gate array electronic devices that avoid the use of the now traditional system controlled by microcontrollers and software. Electrical tests of the engineering model have been performed successfully and the system is ready for space qualification after environmental testing. This system may be of interest to institutions involved in future space experiments with similar needs for mechanisms control.
Why the anisotropic planar rotor model is nearly second order
Authors: Milner Scott T. , Frischknecht Amalie L.
Published: July 2002

Despite extensive Monte Carlo (MC) simulations, the nature of the phase transition in the anisotropic planar rotor (APR) model remains elusive. The ground state is sixfold degenerate, which would naively suggest strongly first-order q=6 Potts behavior. Extensive MC simulations indicate either a second-order transition with q=3 Potts exponents, or a very weakly first-order transition. We show that the APR model maps to a generalized six-state Potts model, with a bond energy between pairs of Potts states q and (q+3) mod 6 larger by a factor α=αAPR⩾2. For α=αT≈2.5, there exists a tricritical point separating first-order behavior (including q=6 Potts at α=1) from second-order behavior (including q=3 Potts at large α). Thus the APR model is weakly first order because of the proximity to this tricritical point.
Linear stability and receptivity analyses of the Stokes layer produced by an impulsively started plate
Authors: Luchini Paolo , Bottaro Alessandro
Published: July 2002

The stability and the receptivity of the boundary layer produced by the impulsive motion of a flat plate in its plane is studied. The evolution of two-dimensional traveling disturbance waves for this physical situation, known as Stokes’ first problem, is treated by integrating directly the (parabolic in time) linearized Navier–Stokes equation and by a multiple-scale approach. In the asymptotic analysis, the Orr–Sommerfeld equation is found at leading order. Through the compatibility condition for the equation at next order, an O(1) correction to growth rates and frequencies is achieved. Such corrections are found to be very mild. After having established that the leading-order results are adequate when looking at the stability characteristics of the flow for a given (large) time, a receptivity analysis is performed. The adjoint of the parabolic system is obtained, and through its backward-in-time integration, the initial and wall Green’s functions are obtained. These are then compared to the results of the multiple-scale receptivity analysis.
Relations between parameters of density functional theories through exactly solvable many-fermion models
Authors: Rau A. R. P. , Hinze Jürgen
Published: July 2002

By considering exactly solvable many-fermion models in different spatial dimensions and with different interaction potentials, the chemical potential and Lagrange multiplier for the normalization requirement are identified with the energy of the last particle, and their relation to the average energy per particle is established.
Infrared photodetector based on intersubband transitions to minigap-confined states in doped quantum wells
Authors: Leavitt R. P. , Little J. W.
Published: July 2002

We have demonstrated an infrared photodetector based on intersubband transitions in doped (In, Ga)As quantum wells in which the excited state is confined by the minigap between two minibands in a surrounding superlattice. Sharp peaks were observed at 78 K in the infrared photocurrent spectra in the 2.7–3.6 μm wavelength region corresponding to transitions in which the excited state energy is substantially above the (In, Al)As conduction-band edge. Broader, longer-wavelength photocurrent features corresponding to transitions into the lower-lying miniband states were also observed. The spectral features in the photocurrent spectra are well described by calculations of the optical absorption cross section.