bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of Si∕β-FeSi2∕Si double-heterostructures on Si(111) substrates by molecular-beam epitaxy and photoluminescence using time-resolved measurements
Authors: Takauji M. , Seki N. , Suemasu T. , Hasegawa F. , Ichida M.
Published: September 2004

Highly [110]∕[101]-oriented semiconducting iron disilicide β-FeSi2 continuous films were grown on Si(111) by molecular-beam epitaxy (MBE) using a β-FeSi2 epitaxial template formed by reactive deposition epitaxy. The optimum MBE growth temperature was determined to be about 750°C. At this temperature, the full width at half maximum β-FeSi2(220)∕(202) x-ray diffraction peak was at a minimum. Subsequent MBE overgrowth of an undoped Si layer was performed on the β-FeSi2 at 500°C, resulting in the Si∕β-FeSi2∕Si double heterostructure. After annealing the wafers at 800°C in Ar for 14h, 1.55μm photoluminescence (PL) was obtained at low temperatures. Time-resolved PL measurements elucidated that the luminescence originated from two sources, one with a short decay time (τ∼10ns) and the other with a long decay time (τ∼100ns). The short decay time was thought to be due to carrier recombination in β-FeSi2, whereas the long decay time was due probably to a dislocation-related D1 line in Si.
Polarization tuning the Stokes photoluminescence spectra of erbium doped KNbO3 ceramics
Authors: Chu Sheng-Yuan , Wen Cheng-Hung , Tyan Sing-Long , Lin Yuu-Ging , Juang Yung-Der , Wen Cheng-Kuo
Published: September 2004

The poling effect in Stokes photoluminescence (PL) spectroscopy of erbium doped potassium niobate (KNbO3) ceramics has been investigated by studying the relations between PL spectra with the poling electric field, poling time, and annealing conditions, respectively. All the Stark splitting components of S3∕24→I13∕24 transition were evident after the poling process. The experiments also showed that the polarization enhances the fine structure and modifies the intensity ratio of the S3∕24→I15∕24, F9∕24→I15∕24, and S3∕24→I15∕24 transitions at different poling electric fields and poling times for the samples doped with 5% of erbium. Comparing the poling spectra with annealing spectra, we can see that both polarization and annealing can modify the intensity of PL. However, the mechanisms of the two effects are different. The frequency shift Δν of the peaks depended on the wavelength after poling demonstrates that the Stark splitting was produced by the dipole moment of erbium ions interacting with the internal local electric field due to polarization.
Charge ordering induced by phase separation in polycrystalline La0.7−xLuxSr0.3MnO3 perovskites
Authors: Song Wei , Luo Feng , Huang Yun-Hui , Yan Chun-Hua , Sun Ben-Zhe , He Lian-Long
Published: September 2004

Multiphase separation with hexagonal (H), rhombohedral (R), and cubic (C) phase was observed in polycrystalline La0.7−xLuxSr0.3MnO3 series by high-resolution transmission electron microscopy. Particularly, in La0.25Lu0.45Sr0.3MnO3, an abnormal lattice distortion of R phase was confirmed by lattice parameter calculation from x-ray diffraction data, as a character of a charged ordered (CO) state. This CO state, a transitional state induced by the phase-separation process of cation-out diffusion, was identified by magnetization and electron spin resonance measurements.
Influence of dielectric capping layers on the crystallization kinetics of Ag5In6Sb59Te30 films
Authors: Njoroge Walter K. , Dieker Henning , Wuttig Matthias
Published: September 2004

AgInSbTe alloys are attractive storage materials for phase change recording utilizing both optical and electronic contrast. The demand to decrease the thickness of such storage layers increases the significance of the surrounding dielectric layers, which can have a profound impact on the crystallization process. Hence we have investigated the influence of different capping layers on the crystallization kinetics of Ag5In6Sb59Te30 films by measuring the electrical resistivity of the films as a function of temperature employing the van der Pauw method. While the phase transition temperature was found to only increase slightly for capped Ag5In6Sb59Te30 films, a profound impact on the activation barrier for crystallization was observed. Activation energies of 2.39±0.10eV and 3.24±0.12eV were determined for films capped with 5nm of ZnS-SiO2 and Si3N4, respective1y, in comparison with a value of 3.03±0.17eV for an uncapped film. Additional stress measurements reveal that this change in activation barrier is not caused by stresses induced by the dielectric films but must rather be due to the different activation barriers for heterogeneous nucleation at the different interfaces. In the amorphous state the temperature dependence of the sheet resistance follows an activation-type relation, where the activation energy is not affected by the capping layer.
Dependence of acceptor levels and hole mobility on acceptor density and temperature in Al-doped p-type 4H-SiC epilayers
Authors: Matsuura Hideharu , Komeda Masahiko , Kagamihara Sou , Iwata Hirofumi , Ishihara Ryohei , Hatakeyama Tetsuo , Watanabe Takatoshi , Kojima Kazutoshi , Shinohe Takashi , Arai Kazuo
Published: September 2004

The temperature-dependent hole concentration p(T) and hole mobility μp(T) are obtained in p-type 4H-SiC epilayers with several Al-doping densities. From p(T), the densities and energy levels of acceptors are determined by the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the acceptor species. In the heavily Al-doped case, the excited states of acceptors affect p(T) because the Fermi level is located between the valence band maximum and the acceptor level (i.e., the ground state level of the acceptor), indicating that a distribution function for acceptors, which includes the influence of excited states of acceptors, should be required. Here, FCCS can determine acceptor densities and acceptor levels using any distribution function (e.g., the Fermi-Dirac distributing function or the distribution function including the influence of excited states). Two types of acceptor species are detected in the lightly Al-doped epilayers, while only one type of acceptor species is found in the heavily Al-doped epilayer. Some of the parameters required to simulate electric characteristics of 4H-SiC power electronic devices are obtained; (1) the dependence of each acceptor level on a total acceptor density and (2) the dependence of the hole mobility on temperature and total impurity density.
An empirical formula for negative corona discharge current in point-grid electrode geometry
Authors: Yamada Kimio
Published: September 2004

A series of measurements carried out with a point-grid electrode system has clarified that the current-voltage characteristics of the negative corona discharge obey the Townsend relation, further, in which the coefficient is proportional to temperature and the power of the electrode gap independently, and the corona onset voltage is proportional to the product of the reciprocal of temperature and the power of the electrode gap. The empirical formula derived from their relations has estimated negative corona currents with an accuracy of 1μA for a given parameter set, which will allow application to ion generator design.
Intersection of a domains in the c-domain matrix driven by electric field in tetragonal ferroelectric crystal
Authors: Tan X. , Shang J. K.
Published: September 2004

Domain structures in a tetragonal ferroelectric crystal were examined by transmission electron microscopy (TEM) before and after application of bipolar cyclic electric fields. Prior to the application of the bipolar field, the crystal was poled to an initial domain structure which consisted of a high volume fraction of c domains. Dispersed in the matrix of the c domains were two orthogonal sets of a-domain strips. These two sets of a-domain strips stayed apart to avoid direct contact. Upon application of bipolar cyclic electric fields, intersections of the a domains were observed in the ⟨001⟩-oriented tetragonal ferroelectric crystal. These intersections were formed as one set of the a domains grew under the influence of the in-plane electric field. As a result of the domain wall intersection, segments of the domain wall between two intersecting a domains carried excess electric charges. In the successive TEM examination, domain wall distortion and microcracks were found at these intersections.
Ultrafast phase transitions after femtosecond laser irradiation of indium phosphide
Authors: Bonse J. , Wiggins S. M. , Solis J.
Published: September 2004

The structural transformation dynamics of single-crystalline indium phosphide irradiated with 150fs laser pulses at 800nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150fs up to 500ns. The results obtained show that for fluences above the threshold of 0.16J∕cm2 thermal melting of the material occurs on the time scale of 1–2ps, while ablation at higher fluences (>0.23J∕cm2) is preceeded by an ultrafast phase transition (“nonthermal melting”) occurring within 400fs after the arrival of the pulse to the surface. The evolution of the reflectivity on a longer time scale shows a resolidification time typically around 25ns after which an amorphous layer several tens of nanometer thick is formed on the surface.
Impact of ultraviolet light on coercive field, poling dynamics and poling quality of various lithium niobate crystals from different sources
Authors: Wengler M. C. , Fassbender B. , Soergel E. , Buse K.
Published: September 2004

Ferroelectric domain reversal by electric field poling of lithium niobate crystals (LiNbO3) with varying stoichiometry and magnesium (MgO) doping level obtained from various commercial suppliers is investigated. Magnesium doping lowers the domain-wall velocity, increases the uniformity of the growth of the domains, and reduces the impact of crystal symmetry on the shape of the domains. Illumination with ultraviolet (UV) laser light (305nm) reduces the coercive field by up to 34% in MgO-doped crystals, but is accompanied by a degradation of poling quality. UV light of longer wavelengths (334nm) has no influence on the coercive field except for the MgO:LiNbO3 material of one supplier, where the field is reduced by 27%. In this case the poling quality is excellent. UV-induced reduction of stress-induced birefringence is observed in some samples. The results are of crucial relevance for light-induced domain engineering of LiNbO3 crystals.
Differences in the metallic plasma-neutral gas structure in a vacuum arc operated with nitrogen and argon
Authors: Grondona Diana , Márquez Adriana , Minotti Fernando , Kelly Héctor
Published: September 2004

Ion current and electron temperature are measured using electrostatic probes in a dc, nonfiltered vacuum arc operated with argon and nitrogen as filling gases in the pressure range 0.001–1mbar. It is found that the measured ion current for argon is between two and six times larger than for nitrogen, for similar operating conditions. Also, the electron temperature is smaller for Ar. These differences can be satisfactorily explained with a simple one-dimensional model, which includes the most relevant elastic and inelastic processes that take place in the interelectrodic plasma: elastic scattering of metallic ions by neutral gas, charge exchange, electron impact ionization of gas, dissociative recombination of gas ions, and conversion of atomic ions into molecular ions. The observed differences between nitrogen and argon are attributed to the low rate of conversion of atomic argon into molecular argon, that makes inoperative the channel of dissociative recombination for this gas, together with the higher ionization rate of argon.
A quantitative model for the blueshift induced by rapid thermal annealing in GaNAs∕GaAs triple quantum wells
Authors: Sun Yijun , Egawa Takashi , Ishikawa Hiroyasu
Published: September 2004

The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs∕GaAs triple quantum wells grown by chemical beam epitaxy are studied in detail by photoluminescence (PL) spectroscopy at 77K. Special emphasis is put on the RTA-induced PL peak blueshift. It is found that the blueshift is neither due to nitrogen diffusion from well layer to barrier layer nor due to homogenization of nitrogen composition fluctuations. The blueshift is due to the coupling between the radiative recombination of PL emission and the nonradiative recombination of nonradiative centers. A quantitative model is proposed in which the blueshift is proportional to the relative change of the concentration of nonradiative centers. This model quantitatively explains not only our present results but also previous observations.
Characterization of sulfur passivated n-GaSb using transmission electron microscopy and the influence of passivationon ohmic contact resistance
Authors: Robinson J. A. , Mohney S. E.
Published: September 2004

Transmission electron microscopy has been used to study sulfur passivation of (100) n-GaSb. An amorphous Ga-S layer forms during passivation, with a thickness dependent on the duration of passivation. Minimal oxygen is present in the sulfide layer. The sulfide layer remains amorphous when annealed at 350°C for 10min. After 20min at 500°C, the layer becomes nanocrystalline and is enriched with Ga from the GaSb, with voids and Sb precipitates forming at the sulfide/GaSb interface. If used as a premetallization surface preparation, sulfur passivation can inhibit reaction between a contact metallization and GaSb; however, with an appropriate choice of passivation conditions to minimize the thickness of the sulfide layer, it can instead promote a more uniform reaction between the metal and semiconductor than is achieved with a conventional surface preparation. It can also lead to a reduced ohmic contact resistance.
Power consideration in the pulsed dielectric barrier discharge at atmospheric pressure
Authors: Laroussi M. , Lu X. , Kolobov V. , Arslanbekov R.
Published: September 2004

Nonequilibrium, atmospheric pressure discharges are rapidly becoming an important technological component in material processing applications. Amongst their attractive features is the ability to achieve enhanced gas phase chemistry without the need for elevated gas temperatures. To further enhance the plasma chemistry, pulsed operation with pulse widths in the nanoseconds range has been suggested. We report on a specially designed, dielectric barrier discharge based diffuse pulsed discharge and its electrical characteristics. Two current pulses corresponding to two consecutive discharges are generated per voltage pulse. The second discharge, which occurs at the falling edge of the voltage pulse, is induced by the charges stored on the electrode dielectric during the initial discharge. Therefore, the power supplied to ignite the first discharge is partly stored to later ignite a second discharge when the applied voltage decays. This process ultimately leads to a much improved power transfer to the plasma.
Pulsed Orotron—A new microwave source for submillimeter pulse high-field electron paramagnetic resonance spectroscopy
Authors: Grishin Yu. A. , Fuchs M. R. , Schnegg A. , Dubinskii A. A. , Dumesh B. S. , Rusin F. S. , Bratman V. L. , Möbius K.
Published: September 2004

A vacuum-tube device for the generation of pulsed microwave radiation in the submillimeter range (up to 380 GHz) is presented, designed for use as a source in a 360 GHz high-field/high-frequency electron paramagnetic resonance (EPR) spectrometer—the pulsed Orotron. Analogous to the known continuous wave (cw) version, in the pulsed Orotron microwave radiation is generated by the interaction of a nonrelativistic electron beam with a diffraction grating (stimulated Smith–Purcell radiation) in feedback with an open Fabry–Pérot resonator construction. The presented design extends the cw Orotron by a gate electrode and a high-voltage pulsing unit to control the electron beam current. The generated pulses at 360 GHz have pulse lengths from 100 ns–10 μs and a pulse power of (22±5) mW. The output in a broader frequency band between 320 and 380 GHz ranges from 20 up to 60 mW. Within a 10 μs time slot, incoherent pulse trains of arbitrary duration can be generated. The pulsed Orotron has been incorporated in the quasioptical microwave bridge of a heterodyne induction mode EPR spectrometer. The first free induction decay measurements at a microwave frequency of 360 GHz and a magnetic field of 12.8 T on a polycrystalline perylenyl–ion sample are presented and future applications and extensions of Orotron-EPR spectroscopy are discussed.
Water bath calorimetric study of excess heat generation in “resonant transfer” plasmas
Authors: Phillips Jonathan , Mills Randell L. , Chen Xuemin
Published: September 2004

Water bath calorimetry was used to demonstrate one more peculiar phenomenon associated with a certain class of mixed gas plasmas, termed resonant transfer (RT) plasmas. Specifically, He∕H2(10%) (500mTorr), Ar∕H2(10%) (500mTorr), and H2O(g) (500 and 200mTorr) plasmas generated with an Evenson microwave cavity consistently yielded on the order of 50% more heat than non-RT plasma (controls) such as He, Kr, Kr∕H2(10%) under identical conditions of gas flow, pressure, and microwave operating conditions. The excess power density of RT plasmas was of the order 10Wcm−3. In earlier studies with these same RT plasmas it was demonstrated that other unusual features were present including dramatic broadening of the hydrogen Balmer series lines, unique vacuum ultraviolet lines, and, in the case of water plasmas, population inversion of the hydrogen excited states. Both the current results and the earlier results are completely consistent with the existence of a hitherto unknown exothermic chemical reaction, such as that predicted by Mills, occurring in RT Plasmas.
Glucose biosensors based on organic light-emitting devices structurally integrated with a luminescent sensing element
Authors: Choudhury Bhaskar , Shinar Ruth , Shinar Joseph
Published: September 2004

A platform for photoluminescence (PL) based biosensing is demonstrated for glucose. The sensor is structurally integrated, i.e., individually addressable organic light-emitting device (OLED) pixels (serving as the light source) and the sensing element are fabricated on glass or plastic substrates attached back-to-back. This results in a very compact, potentially miniaturizable sensor, which should strongly impact PL-based biosensor technology. The sensing element is an oxygen-sensitive dye coembedded with glucose oxidase in a thin film or dissolved in solution. The glucose biosensor is demonstrated for two OLED∕dye pairs: [blue OLED]∕[Ru dye] and [green OLED]∕[Pt dye]. Both PL-intensity and PL-lifetime modes are demonstrated for each pair; the lifetime mode eliminates the need for frequent sensor calibration. The sensor performance is evaluated in terms of design, dynamic range, limit of detection, and stability. The use of the glucose biosensor in conjunction with an oxygen sensor is also discussed.
Observation of dendritic flux instabilities in YNi2B2C thin films
Authors: Wimbush S. C. , Holzapfel B. , Jooss Ch.
Published: September 2004

Magneto-optical imaging and magnetization measurements performed on thin films of the borocarbide superconductor YNi2B2C reveal the occurrence of magnetic flux instabilities upon reducing the applied magnetic field towards the remanent state. In contrast to other low-Tc materials such as Nb and MgB2, where similar instabilities occur in both increasing and decreasing magnetic fields, dendritic flux patterns are observed in YNi2B2C for decreasing fields only. Also in the magnetization measurements, a distinct asymmetry is evident between increasing and decreasing fields. The effect does not depend on the sweep rate of the field, but is strongly dependent on the maximum field applied before reduction. The observation of spontaneous flux instabilities in this additional family of low-temperature superconductors suggests that the responsible mechanism is universal to this class of materials.
Signal processing in photoacoustic detection of phase transitions by means of the autospectra correlation-based method: Evaluation with ceramic BaTiO3
Authors: Mejía-Uriarte E. V. , Navarrete M. , Villagrán-Muniz M.
Published: September 2004

This work describes a simple numerical procedure which, when applied to digitally recorded photoacoustic (PA) signals, allows the construction of thermal profiles (rS,drS/dT) to determine: the transition order, the phase transition temperature (Tc), and the phase transformation temperature range (ΔT), on samples, which undergo low–high transitions. During continuous heating of the sample, the ultrasonic signal was generated using a pulsed laser beam incident on a surface and detected on the opposite surface of the sample using a long quartz bar attached to a piezoelectric sensor. The thermal profile, rS, is built from a sequence of the ordinary correlation coefficients ri associated with an interval of temperature. The ri coefficients are calculated from amplitude spectra pairs. The amplitude spectra are obtained via fast Fourier transforms from original PA records. This procedure is applied on samples of bulk ceramic BaTiO3 to obtain their thermal behavior. The PA signal and temperature sample were registered every 0.2°. The samples were heated from room temperature to 137 °C at a rate of 0.1 °C min−1. The thermal profile rS shows the entire thermal history including the structural phase transition from tetragonal to cubic (T-C), which appears as a jump on the graph within an uncertainty of 2%. The drS/dT profile shows that the T-C phase transformation occurs over a range of temperatures. The results are in agreement with those reported in the literature.
A power-efficient thermocycler based on induction heating for DNA amplification by polymerase chain reaction
Authors: Pal Debjani , Venkataraman V. , Mohan K. Naga , Chandra H. Sharat , Natarajan Vasant
Published: September 2004

We have built a thermocycler based on the principles of induction heating for polymerase chain reaction (PCR) of target sequences in DNA samples of interest. The cycler has an average heating rate of ∼0.8 °C/s and a cooling rate of ∼0.5 °C/s, and typically takes ∼4 h to complete a 40-cycle PCR protocol. It is power-efficient (∼6 W per reaction tube), micro-processor controlled, and can be adapted for battery operation. Using this instrument, we have successfully amplified a 350 bp segment from a plasmid and SRY, the human sex determining gene, which occurs as a single-copy sequence in genomic DNA of human males. The PCR products from this thermocycler are comparable to those obtained by the use of commercially available machines. Its easy front-end operation, low-power design, portability and low cost makes it suitable for diagnostic field applications of PCR.
Phase mask based interferometer: Operation principle, performance, and application to thermoelastic phenomena
Authors: Glorieux C. , Beers J. D. , Bentefour E. H. , Van de Rostyne K. , Nelson Keith A.
Published: September 2004

A simple, versatile, sensitive optical interferometer based on diffractive optics is presented. The absence of a need for active stabilization, and a compact common-path design requiring two optical elements, make the interferometer ideal for time-resolved measurements in the picosecond through millisecond regimes. Its performance is characterized quantitatively, and its utility for local detection and scanning as well as spatially resolved imaging of thermoelastically induced strain is demonstrated.
One-dimensional numerical simulation of laser-driven flyer plates
Authors: Gu Zhuowei , Sun Chengwei , Zhao Jianheng , Zhang Ning
Published: September 2004

A one-dimensional (1D) bulk absorption model to simulate the accelerating process of laser-driven flyer plate is presented, in which the laser energy deposition and acceleration of flyer plates are included. In the laser intensity range of 108–1010W∕cm2, the gasdynamical behavior of the ablated foil plasma is described with a modified 1D Lagrangian hydrodynamic code. The histories and profiles of physical parameters including pressure, temperature, velocity, opacity, and average ionization charge number are given by this model. According to the calculation, an aluminum foil of original thickness of 10μm may be accelerated to a velocity of about 2km∕s, the calculated average ionized charge number of plasma Z is >3 peak temperature and plasma pressure are in the order of 2.1×105K and 1.5×103GPa, respectively, and the maximum Rosseland opacity is about 1.9×105cm−1. The calculated particle velocity distribution in plasma is linear, which confirms the assumption employed in the Gurney model.
Structural phase transitions of BaNbxTi1−xO3(0.0⩽x⩽0.5) thin films
Authors: Guo Haizhong , Liu Lifeng , Ding Shuo , Lu Huibin , Zhou Yueliang , Cheng Bolin , Chen Zhenghao
Published: September 2004

The phase transition behavior of BaNbxTi1−xO3 (BNTO) (0.0⩽x⩽0.50) thin films grown on MgO substrates by laser molecular beam epitaxy was systematically investigated by using x-ray diffraction (XRD) and micro-Raman spectroscopy. The asymmetric rocking XRD scan measurements show that with an increase of Nb-doped content, the lattice parameters c and a increase while c∕a ratio decreases, indicating a decrease of tetragonality of the BNTO films. The intensity of Raman signal decreases and the width of the bands broaden with increase of Nb-doped content. The results of XRD and Raman spectra indicate that at room temperature BNTO thin films with Nb⩽10 at. % have tetragonal structure, however, for Nb⩾20 at. %, BNTO thin films exhibit typical disordering cubic structure.
THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates
Authors: Stepantsov E. , Tarasov M. , Kalabukhov A. , Kuzmin L. , Claeson T.
Published: September 2004

Superconducting Josephson junctions with high characteristic voltages (IcRn larger than 4 mV at 4.2 K) are fabricated by depositing YBa2Cu3O7−x on tilted sapphire bicrystal substrates, where the tilting axis is along the grain boundary. The good junction quality and low microwave losses in sapphire gave high frequency response well into the THz region. High quality YBa2Cu3O7−x epitaxial films were deposited on tilted sapphire substrates with CeO2 buffer layers by pulsed laser deposition. YBaCuO films have smaller tilt angles, from 1.0° up to 10.3°, compared to inclination angles of the substrates from 1.5° to 13.6°. X-ray diffraction shows only a single orientation of the films in the a-b plane, as well as an absence of a-axis particles and outgrowths. Critical temperatures as high as Tc=88.5−89.0K and ΔTc⩽1.5K were obtained in all films. The grain boundary in a common high-Tc superconducting junction is usually much less straight than in the in-plane rotated bicrystal substrate, depressing Josephson currents. The waviness of the artificial grain boundary in a tilted bicrystal film is three times less than in an in-plane (untilted) bicrystal. Tilted Josephson junctions of widths from 1.5 to 6 μm were tested at temperatures from 0.26 to 77 K. IcRn products as high as 4.5 mV were observed at T=4.2K. Shapiro steps were observed at voltages over 3 mV under 300 GHz irradiation. Josephson radiation from the tilted junction was measured at frequencies up to 1.7 THz by a cryogenic bolometer. Suppressing the critical current with a magnetic field can separate Josephson radiation and thermal radiation. A parabolic dependence of the response on bias voltage for thermal radiation corresponds to an increase of junction temperature from 260 mK at zero bias to 3 K at 1 mV bias.
Optical properties of SiOx nanostructured films by pulsed-laser deposition at different substrate temperatures
Authors: Chen X. Y. , Lu Y. F. , Wu Y. H. , Cho B. J. , Song W. D. , Dai D. Y.
Published: September 2004

Silicon oxide (SiOx) nanostructured films have been formed by pulsed-laser deposition of Si in oxygen at different substrate temperatures, in order to study the structures and optical properties related to quantum confinement effects. After laser ablation, the single-crystal Si(100) target is converted to a polycrystal structure and shows weak photoluminescence (PL). The as-deposited SiOx nanostructured films show large particles (i.e., droplets) on a uniform background film. The droplets with weak PL emission have the same high crystallinity as the Si(100) target. Strong PL is observed from the amorphouslike background films rather than from the crystalline droplets. As substrate temperatures increase from room temperature (23°C)to800°C, the PL band continually redshifts from 1.9to1.6eV and the optical band gap decreases from 2.9to2.1eV due to the increased Si concentration in the films. After high-vacuum annealing at 800°C, both the PL and optical absorption are enhanced. The optical band gap also decreases after annealing. Combined with the progressive PL redshifts of the SiOx films with increasing Si concentration by plasma-enhanced chemical vapor deposition, the results support the quantum confinement theory.
Two-process model of electron field emission from nanocarbons: Temperature effect
Authors: Altman Igor S. , Pikhitsa Peter V. , Choi Mansoo
Published: September 2004

The two-process model on electron field emission from nanocarbons that we recently suggested [I. S. Altman, P. V. Pikhitsa, and M. Choi, Appl. Phys. Lett. 84, 1126 (2004)] has solved the existing experimental puzzles such as an occurrence of the sharp knee in the Fowler-Nordheim (FN) plot. Our model implies two successive processes: (1) Tunneling from the metallic region into the semiconducting region under the external macroscopic electric field and (2) tunneling from the semiconducting region into vacuum under the Coulomb field of an additional electron appearing in the first process. However, this model in its original form was inapplicable at finite temperatures. We develop the model (remaining within the framework of the two-process approach, which allows the knee occurrence in the FN plot) in order to describe temperature effects in field emission from nanocarbons. Fitting of the recent experimental data on the temperature behavior of field emission from carbon nanotubes allowed us to extract parameters corresponding to the first process in our model.
Incorporating the gas flow in a numerical model of rf discharges in methane
Authors: Okhrimovskyy A. , Bogaerts A. , Gijbels R.
Published: September 2004

A two-dimensional modified fluid model for a capacitively coupled rf discharge in methane, used for the deposition of diamond-like carbon layers, is presented. The gas velocity calculated with a computational fluid dynamics code is used as an input for the modified fluid model. Convection is taken into account as an additional transport mechanism as well as diffusion and migration. The calculations show that the gas flow results in a shift of the maximum of the densities of the plasma species toward the grounded electrode. It is shown that this shift has a large effect on the fluxes of the ions and radicals toward the substrate where the deposition occurs. As a result, the deposition rate will increase if the gas velocity has a component directed to the surface and it will decrease in the opposite case. However, the uniformity of the layer can become worse when the convection velocity is too high for the reactor geometry studied here.
Comparative analysis of gyrotron backward-wave oscillators operating at different cyclotron harmonics
Authors: Yeh Y. S. , Chang T. H. , Wu T. S.
Published: September 2004

A comparative analysis between the fundamental and second cyclotron harmonics of gyrotron backward-wave oscillators (gyro-BWOs) is presented. The simulation results reveal that nonlinear field contraction is a common feature for both harmonic interactions. Besides, the electron transit angle, used to characterize the axial modes of the fundamental harmonic TE11 mode at the start-oscillation conditions, is found to be applicable even for the second harmonic TE21 mode. Each axial mode of either the fundamental harmonic TE11 or the second harmonic TE21 modes is maintained at a constant value of the electron transit angle while changing the operating parameters, such as magnetic field and beam voltage. Extensive numerical calculations are conducted for the start-oscillation currents and tuning properties. Moreover, single-mode operating regimes are suggested where the second harmonic TE21 gyro-BWO could generate a considerable output power, comparing with the fundamental harmonic TE11 gyro-BWO.
Multiple cracking of brittle coatings on strained substrates
Authors: Hsueh C. H. , Wereszczak A. A.
Published: September 2004

Multiple cracking of well-adhered brittle coatings on strained substrates has been commonly observed in many coating applications. Here we found two significant uses for this cracking phenomenon. First, if crack density as a function of applied strain is measured at one coating thickness, it can be used to predict those for other coating thicknesses. Second, if the residual stress in the coating is unknown, the crack density versus applied strain relation can be used to deduce this residual stress. The key to our method is to obtain a master curve by normalizing the crack density and the applied strain, respectively, by the coating thickness and the critical applied strain in initiating cracking.
Enhanced microchannel plate performance at high particle fluxes by pulsed exposure mode of operation
Authors: Tournianski M. R. , Carolan P. G. , Akers R. J.
Published: September 2004

Operating a microchannel plate (MCP) in saturated mode provides a simple photon and particle counting detector. However, there is a finite recovery time after individual events during which individual microchannels no longer respond, reducing the overall sensitivity. At continuous high flux levels, the corrections from measured to true flux become increasingly large as the fraction of live microchannels rapidly decays to low values. Gating the flux arriving at the MCP greatly increases the proportion of live microchannels allowing periodic measurements to be made that accommodate high fluxes and associated low errors. Such improvements have been observed in a neutral particle analyzer on the Mega Ampere Spherical Tokamak. A simple analytical treatment accounts for the measured improvements.
Characteristics of a miniature parallel-plate free-air ionization chamber for measuring the intensity of synchrotron radiation from an undulator
Authors: Nariyama Nobuteru
Published: September 2004

In order to develop an absolute intensity monitor for synchrotron radiation from an undulator, the characteristics of a parallel-plate free-air ionization chamber with a plate separation of 4.2 mm, which was considered to be as narrow as possible, were investigated using 8–30 keV monoenergetic photons at SPring-8. Using a Si-PIN photodiode as the reference monitor, saturation was confirmed at 8–30 keV at a photon intensity of an order of 1013 photons/s. The collection efficiency became almost unity at 8 and 10 keV within 3.8% and 1.1%, respectively, which gradually decreased with increasing energy and attained 0.52 at 30 keV because some of the high-energy electrons escaped from the sensitive volume. When the pair of electrodes was transferred from the upper and lower sides to the left and right sides of the beam axis in order to investigate the influence of linear polarization of synchrotron radiation, a decrease in the collection efficiency was observed. Monte Carlo simulations considering linear polarization showed that the plate separation required for no electron loss was 26, 14, and 8 mm for 30, 20, and 15 keV photons, respectively. For 20 keV photons, saturation characteristics were investigated using an ionization chamber with 14 mm plate separation.
Atomistic structure of SiO2∕Si∕SiO2 quantum wells with an apparently crystalline silicon oxide
Authors: Cho Eun-Chel , Green Martin A. , Xia James , Corkish Richard , Nikulin Andrei
Published: September 2004

Thermal oxidation of a silicon-on-insulator substrate produces evidence that an ordered SiO2 structure can exist on thermally oxidized SiO2–Si interfaces. An apparently ordered SiO2 layer was observed by a high-resolution transmission electron microscope (HRTEM) when a thin silicon layer enclosed by SiO2 was less than 3.0nm thick. X-ray diffraction of the ultrathin Si (<3nm) samples showed diffractions from an ordered SiO2 phase, first-order Bragg reflection peaks with a lattice spacing of 4.1±0.15Å, and second-order Bragg reflection peaks with 2.03±0.15Å, in addition to the peaks from the Si substrate and the thin Si layer. Even in samples with thick Si layers enclosed by SiO2, which did not show the apparently ordered silicon oxide layer by the HRTEM, x-ray results showed a weak diffraction as if from a crystalline silicon oxide. The disappearance of the second-order Bragg reflection at higher energies indicates that the lattice structure of any crystalline SiO2 phase is far from perfect.
Human stick balancing: Tuning Lèvy flights to improve balance control
Authors: Cabrera Juan Luis , Milton John G.
Published: September 2004

State-dependent, or parametric, noise is an essential component of the neural control mechanism for stick balancing at the fingertip. High-speed motion analysis in three dimensions demonstrates that the controlling movements made by the fingertip during stick balancing can be described by a Lévy flight. The Lévy index, α, is approximately 0.9; a value close to optimal for a random search. With increased skill, the index α does not change. However, the tails of the Lévy distribution become broader. These observations suggest a Lévy flight that is truncated by the properties of the nervous and musculoskeletal system; the truncation decreasing as skill level increases. Measurements of the cross-correlation between the position of the tip of the stick and the fingertip demonstrate that the role of closed-loop feedback changes with increased skill. Moreover, estimation of the neural latencies for stick balancing show that for a given stick length, the latency increases with skill level. It is suggested that the neural control for stick balancing involves a mechanism in which brief intervals of consciously generated, corrective movements alternate with longer intervals of prediction-free control. With learning the truncation of the Lévy flight becomes better optimized for balance control and hence the time between successive conscious corrections increases. These observations provide the first evidence that changes in a Lévy flight may have functional significance for the nervous system. This work has implications for the control of balancing problems ranging from falling in the elderly to the design of two-legged robots and earthquake proof buildings.
Optical microphone: New results
Authors: de Paula Mauro Henrique , Omido Agleison Ramos , Schumaher Daniel Freitas de Queiroz , da Gama Hevelyne Henn
Published: September 2004

Despite the fact that the optical microphone increased the sensitivity of the photoacoustic technique by 4 orders of magnitude, it was not used for a period of more than 10 yrs after it was published. This is because the manufacture of this microphone demands a great ability and some of its details are not fully described. Due to the importance of the optical microphone for photoacoustics we report some improved results and try to describe the details more adequately.
Chromosome painting using repetitive DNA sequences as probes for somatic chromosome identification in maize
Authors: Kato Akio , Lamb Jonathan C. , Birchler James A.
Published: September 2004

Study of the maize (
Zea mays
L.) somatic chromosomes (2
n
= 20) has been difficult because of a lack of distinguishing characteristics. To identify all maize chromosomes, a multicolor fluorescence
in situ
hybridization procedure was developed. The procedure uses tandemly repeated DNA sequences to generate a distinctive banding pattern for each of the 10 chromosomes. Fluorescence
in situ
hybridization screening trials of nonsubtracted or subtracted PCR libraries resulted in the isolation of microsatellite 1-26-2, subtelomeric 4-12-1, and 5S rRNA 2-3-3 clones. These three probes, plus centromeric satellite 4 (Cent4), centromeric satellite C (CentC), knob, nucleolus-organizing region (NOR), pMTY9ER telomere-associated sequence, and tandemly repeated DNA sequence 1 (TR-1) were used as a mixture for hybridization to root-tip chromosomes. All 10 chromosomes were identified by the banding and color patterns in the 14 examined lines. There was significant quantitative variation among lines for the knob, microsatellite, TR-1, and CentC signals. The same probe mixture identifies meiotic pachytene, late prophase I, and metaphase I chromosomes. The procedure could facilitate the study of chromosomal structure and behavior and be adapted for other plant species.
Proteasomal degradation of the FoxO1 transcriptional regulator in cells transformed by the P3k and Akt oncoproteins
Authors: Aoki Masahiro , Jiang Hao , Vogt Peter K.
Published: September 2004

The P3k oncoprotein [homolog of the catalytic subunit p110α of class 1A phosphoinositide 3-kinase (PI3K)] and its downstream effector Akt induce oncogenic transformation in cultures of chicken embryo fibroblasts (CEF). The winged helix transcription factor FoxO1 is a growth-attenuating and proapoptotic protein and serves as a substrate of Akt. Here we show that FoxO1 expression is constitutively suppressed in CEF transformed by P3k or Akt. The FoxO1 protein level is high in serum-starved normal CEF, but platelet-derived growth factor treatment induces rapid phosphorylation and disappearance of FoxO1. PI3K inhibitors or the proteasome inhibitor lactacystin interfere with this process. These data suggest that phosphorylation-dependent degradation of FoxO1 by means of proteasomes plays a role in oncogenic transformation by P3k and Akt. A dominant negative mutant of FoxO1 containing the repressor domain of the
Drosophila
Engrailed protein induces partial oncogenic transformation of CEF and interferes with FoxO1-dependent transcriptional activation. The FoxG1 oncoprotein also inhibits transcriptional activation by FoxO1. Inhibition of FoxO1, albeit by different mechanisms, appears to be a common denominator of the PI3K and FoxG1 oncogenic pathways.
Tumor-promoting phorbol esters and activated Ras inactivate the tuberous sclerosis tumor suppressor complex via p90 ribosomal S6 kinase
Authors: Roux Philippe P. , Ballif Bryan A. , Anjum Rana , Gygi Steven P. , Blenis John
Published: September 2004

Tuberous sclerosis complex (TSC) is a genetic disorder caused by mutations in either of the two tumor suppressor genes
TSC1
or
TSC2
, which encode hamartin and tuberin, respectively. Tuberin and hamartin form a complex that inhibits signaling by the mammalian target of rapamycin (mTOR), a critical nutrient sensor and regulator of cell growth and proliferation. Phosphatidylinositol 3-kinase (PI3K) inactivates the tumor suppressor complex and enhances mTOR signaling by means of phosphorylation of tuberin by Akt. Importantly, cellular transformation mediated by phorbol esters and Ras isoforms that poorly activate PI3K promote tumorigenesis in the absence of Akt activation. In this study, we show that phorbol esters and activated Ras also induce the phosphorylation of tuberin and collaborates with the nutrient-sensing pathway to regulate mTOR effectors, such as p70 ribosomal S6 kinase 1 (S6K1). The mitogen-activated protein kinase (MAPK)-activated kinase, p90 ribosomal S6 kinase (RSK) 1, was found to interact with and phosphorylate tuberin at a regulatory site, Ser-1798, located at the evolutionarily conserved C terminus of tuberin. RSK1 phosphorylation of Ser-1798 inhibits the tumor suppressor function of the tuberin/hamartin complex, resulting in increased mTOR signaling to S6K1. Together, our data unveil a regulatory mechanism by which the Ras/MAPK and PI3K pathways converge on the tumor suppressor tuberin to inhibit its function.
The moduli space of isometry classes of globally hyperbolic spacetimes
Authors: Luca Bombelli  , Johan Noldus
Published: September 2004

This paper is part of a research programme on the structure of the moduli space of Lorentzian geometries, a Lorentzian analogue of Gromov–Hausdorff theory based on the use of the Lorentz distance as basic kinematical variable. We first prove results aimed at a better understanding of the tools available in this framework, such as the relationship between notions of closeness used to define limit spaces, and the properties of the auxiliary ‘strong’ Riemannian metric defined on each Lorentz space. Then we examine concepts motivated by applications to quantum gravity, namely causality of the limit spaces and compactness of classes of Lorentz spaces.
SMP-1, a Member of a New Family of Small Myristoylated Proteins in Kinetoplastid Parasites, Is Targeted to the Flagellum Membrane inLeishmania
Authors: Tull Dedreia , Vince James E. , Callaghan Judy M. , Naderer Thomas , Spurck Tim , McFadden Geoffrey I. , Currie Graeme , Ferguson Kris , Bacic Antony , McConville Malcolm J.
Published: September 2004

The mechanisms by which proteins are targeted to the membrane of eukaryotic flagella and cilia are largely uncharacterized. We have identified a new family of small myristoylated proteins (SMPs) that are present in Leishmania spp and related trypanosomatid parasites. One of these proteins, termed SMP-1, is targeted to the Leishmania flagellum. SMP-1 is myristoylated and palmitoylated in vivo, and mutation of Gly-2 and Cys-3 residues showed that both fatty acids are required for flagellar localization. SMP-1 is associated with detergent-resistant membranes based on its recovery in the buoyant fraction after Triton X-100 extraction and sucrose density centrifugation and coextraction with the major surface glycolipids in Triton X-114. However, the flagellar localization of SMP-1 was not affected when sterol biosynthesis and the properties of detergent-resistant membranes were perturbed with ketoconazole. Remarkably, treatment of Leishmania with ketoconazole and myriocin (an inhibitor of sphingolipid biosynthesis) also had no affect on SMP-1 localization, despite causing the massive distension of the flagellum membrane and the partial or complete loss of internal axoneme and paraflagellar rod structures, respectively. These data suggest that flagellar membrane targeting of SMP-1 is not dependent on axonemal structures and that alterations in flagellar membrane lipid composition disrupt axoneme extension.
Comparative analysis of dioxin response elements in human, mouse and rat genomic sequences
Authors: Sun Y. V. , Boverhof D. R. , Burgoon L. D. , Fielden M. R. , Zacharewski T. R.
Published: September 2004

Comparative approaches were used to identify human, mouse and rat dioxin response elements (DREs) in genomic sequences unambiguously assigned to a nucleotide RefSeq accession number. A total of 13 bona fide DREs, all including the substitution intolerant core sequence (GCGTG) and adjacent variable sequences, were used to establish a position weight matrix and a matrix similarity (MS) score threshold to rank identified DREs. DREs with MS scores above the threshold were disproportionately distributed in close proximity to the transcription start site in all three species. Gene expression assays in hepatic mouse tissue confirmed the responsiveness of 192 genes possessing a putative DRE. Previously identified functional DREs in well-characterized AhR-regulated genes including Cyp1a1 and Cyp1b1 were corroborated. Putative DREs were identified in 48 out of 2437 human–mouse–rat orthologous genes between −1500 and the transcriptional start site, of which 19 of these genes possessed positionally conserved DREs as determined by multiple sequence alignment. Seven of these nineteen genes exhibited 2,3,7,8-tetrachlorodibenzo- p -dioxin-mediated regulation, although there were significant discrepancies between in vivo and in vitro results. Interestingly, of the mouse–rat orthologous genes with a DRE between −1500 and +1500, only 37% had an equivalent human ortholog. These results suggest that AhR-mediated gene expression may not be well conserved across species, which could have significant implications in human risk assessment.