bluuebunny/crossref_metadata_2025 huggingface dataset:
Design, fabrication, and test of a fast scintillation detector preamplifier
Authors: Bezboruah T. , Boruah K. , Boruah P. K.
Published: July 2002

A fast scintillation detector preamplifier is designed, which uses an operational video amplifier (type: LM733). Despite its simplicity, the preamplifier exhibits good noise and speed parameters. The amplifier is specially designed for an extensive air shower experiment [T. Bezboruh, K. Boruah, and P. K. Boruah, Nucl. Instrum. Methods Phys. Res. A 410, 206 (1998); Astropart. Phys. 11, 395 (1999)], Gauhati University, Assam, India. Plastic scintillators and fast photomultiplier tubes are used in the experiment to detect ultra high energy cosmic ray events. Here we report the characteristics of the amplifier including the hardware and its performances in the experiment.
Two-photon excitation microfluorometer for multiplexed single-step bioaffinity assays
Authors: Soini Juhani T. , Soukka Jori M. , Soini Erkki , Hänninen Pekka E.
Published: July 2002

A new type of instrumentation for single-step bioaffinity assays and microvolume fluorometry is presented. The concept is based on the use of two-photon excitation by a low-cost near-infrared laser and individual observation of bioactive fluorescent microparticles. The applicability of the instrument is demonstrated by a microparticle based multiplexed bioaffinity assay where several fluorescent markers are simultaneously excited. This instrument can be applied in the growing fields of drug discovery, in life science research, and in routine laboratory diagnostics.
Electrochemical tuning of band structure of single-walled carbon nanotubes probed by in situ resonance Raman scattering
Authors: Ghosh Shankar , Sood A. K. , Rao C. N. R.
Published: July 2002

In situ resonance Raman scattering of single-walled carbon nanotubes investigated under electrochemical biasing demonstrates that the intensity of the radial breathing mode varies significantly in a nonmonotonic manner as a function of the cathodic bias voltage, but does not change appreciably under anodic bias. The tangential mode is, however, not affected. These results can be quantitatively understood in terms of the changes in the energy gaps between the one-dimensional van Hove singularities in the electron density of states arising possibly due to the alterations in the overlap integral of π bonds between the p orbitals of the adjacent carbon atoms.
Temperature dependence of electrostriction in rhombohedral Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Liu Shi-Fang , Park Seung-Eek , Cross L. Eric , Shrout Thomas R.
Published: July 2002

The temperature dependent electrostrictive behavior of rhombohedral Pb(Zn1/3Nb2/3)O3-xPbTiO3 crystals (PZN-PT, x<0.09) was investigated. Although the application of an E field induces a macroscopic ferroelectric phase at room temperature, rhombohedral crystals still exhibit relaxor characteristics, such as polarization modulation. The polarization is believed to become homogeneous in association with the E-field induced phase transition into the tetragonal phase. Electrostrictive coefficients, Q11∼9.5×10−2 m4/C2 and Q12∼−3.6 m4/C2 were calculated at the high fields. Strain values calculated based upon these coefficients were found to be significantly greater than that measured at low fields, being attributed to polarization modulation. With decreasing temperature, the theoretical strain values came closer to measured strain values. Theoretical and calculated values are expected to be equal at temperatures lower than 150 K, where the polarization in the E-field exposed crystal is homogeneous and collinear.
Short-range compositional randomness of hydrogenated amorphous silicon–germanium films
Authors: Chapman B. D. , Han S.-W. , Seidler G. T. , Stern E. A. , Cohen J. David , Guha S. , Yang J.
Published: July 2002

The optical absorption spectrum of hydrogenated amorphous silicon–germanium (a-Si1−xGex:H) films can be tuned by the Ge content. As a result, there is considerable interest in applications of this photovoltaic alloy in solar cells. However, some aspects of the relationship between microstructure and optoelectronic properties are not yet fully understood. We report here a study of the local Ge environment in a-Si1−xGex:H films having demonstrated high photovoltaic efficiency. We present Ge K-edge extended x-ray absorption fine structure (EXAFS) measurements for samples with Ge content ranging from x=0.10 to 0.45. A structural analysis of the EXAFS reveals compositional randomness in the local Ge environment for x<0.4, indicating short-range random miscibility of Ge and Si in the alloying process. This is consistent with measurements of earlier generations of Si-rich a-Si1−xGex:H films, and suggests that one must look beyond the first coordination shell to understand the relationship between microstructure and optoelectronic properties. We also find no compositional dependence in the Ge environment nearest-neighbor bond lengths, indicating that there is little topological rigidity in the amorphous phase. Together, these results exclude any substantial Ge nanocrystallization or segregation into regions of concentrated Ge hydrides, but do not prohibit more complicated compositional inhomogeneity caused by partial phase separation on longer length scales.
Low-temperature dielectric relaxation in the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound
Authors: Ang Chen , Yu Zhi , Youn H. J. , Randall C. A. , Bhalla A. S. , Cross L. E. , Nino J. , Lanagan M.
Published: July 2002

The dielectric behavior of the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound has been studied. A low-temperature dielectric relaxation was observed in a low-permittivity matrix with ε=∼60. The dielectric relaxation process follows a modified Debye model in the vicinity of the relaxation peak, and the relaxation rate follows the Arrhenius law in the wide frequency range 102 to ∼1010 Hz. The temperature intensity of dielectric peaks are independent of dc bias (⩽60 kV/cm). The dielectric relaxation is tentatively attributed to the hopping of Zn/Bi ions at A sites with more than one equivalent potential minima, and the reorientation of the dipoles probably formed through interactions with the “seventh oxygen” and the Bi/Zn A-site ions in the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound.
Imaging charged defects on clean Si(100)-(2×1) with scanning tunneling microscopy
Authors: Brown G. W. , Grube H. , Hawley M. E. , Schofield S. R. , Curson N. J. , Simmons M. Y. , Clark R. G.
Published: July 2002

Scanning tunneling microscopy (STM) has been used to image charged defects on the clean Si(100)-(2×1) surface. Previous studies have shown that, in the absence of “C”-type defects, the surface does not pin the Fermi level, allowing near surface charge to influence the state density contributing to the tunneling current. As in the case of cleavage faces of III–V semiconductor crystals, the charge-induced band bending produces long-range enhancements superimposed on the periodic surface lattice. This is observed in empty-state STM images taken on n-type material. No band bending signature is seen in the filled-state images. This can be understood by considering the band structure at the surface, which has surface states within the band gap. The charged defects observed in this work are of the types commonly observed in clean Si(100)-(2×1) STM studies, however, not all defects of a given type appear charged. This would indicate subtle differences in structure or the influence of impurities. Predictions for p-type material will also be made.
Observations and empirical scalings of the high-confinement mode pedestal on Alcator C-Mod
Authors: Hughes J. W. , Mossessian D. A. , Hubbard A. E. , LaBombard B. , Marmar E. S.
Published: July 2002

On the Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)], radial profiles of electron temperature (Te) and density (ne) are measured at the plasma edge with millimeter resolution Thomson scattering [Rev. Sci. Instrum. 72, 1107 (2001)]. Edge transport barriers in the high confinement regime (H mode) exhibit Te, ne pedestals with typical widths of 2–6 mm, with the Te pedestal on average slightly wider than and inside the ne pedestal. Measurements at both the top and the base of the pedestal are consistent with profiles obtained using other diagnostics. The two primary H-mode regimes on C-Mod, enhanced Dα (EDA) and edge-localized mode free, have been examined for differences in pedestals. EDA operation is favored by high edge collisionality ν*, in addition to high edge safety factor q95. Scaling studies at fixed shape yield little systematic variation of pedestal widths with plasma parameters, though higher triangularity is seen to increase the ne pedestal width dramatically. Pedestal heights and gradients show the clearest dependencies on plasma control parameters. Pedestal ne and Te both scale linearly with plasma current IP, while pedestal Te depends strongly on power flowing from the core plasma into the scrape-off layer PSOL. The electron pressure (pe) pedestal and pe gradient both scale with IP2PSOL1/2. Plasma stored energy WP scales with pedestal pe, implying that pedestal scalings may in large part determine global confinement scalings.
Generation-recombination noise in highly asymmetrical p–n junctions
Authors: Tejada J. A. Jiménez , Godoy A. , Palma A. , Villanueva J. A. López
Published: July 2002

A previous theory for the current noise associated with carrier generation and recombination in the space-charge region of p–n junctions is recalculated for the case of a highly asymmetrical distribution of dopant impurities. We propose a model based on the true concentrations of electrons and holes and electric field in the space-charge layer. Carriers coming from the highly doped region create a layer of mobile charge in the depletion region of the less-doped zone. This charge, usually studied under forward-bias conditions, is neglected under reverse voltages. We have demonstrated that this mobile-charge layer, although located in a very tiny region compared to the width of the space-charge layer, is crucial in the study of charge fluctuations stimulated by generation-recombination centers. Experimental results have been successfully reproduced with our theory for reverse-biased p+–n diodes with deep centers located close to the metallurgical junction.
Positron and positronium chemistry by quantum Monte Carlo. VI. The ground state of LiPs, NaPs, e+Be, and e+Mg
Authors: Mella Massimo , Casalegno Mosé , Morosi Gabriele
Published: July 2002

The ground states of the positronic complexes LiPs, NaPs, e+Be, e+Mg, and of the parent ordinary-matter systems have been simulated by means of the all-electron fixed-node diffusion Monte Carlo (DMC) method. Positron affinities and positronium binding energies are computed by direct difference between the DMC energy results. LiPs was recomputed in order to test the possibility of approximating the electron–positron Coulomb potential with a model one that does not diverge for r=0, finding accurate agreement with previous DMC results. As to e+Be, the effect due to the near degeneracy of the 1s22s2 and 1s22p2 configurations in Be is found to be relevant also for the positron affinity, and is discussed on the basis of the change in the ionization potential and the dipole polarizability. The DMC estimate of the positron affinity of Mg, a quantity still under debate, is 0.0168(14) hartree, in close agreement with the value 0.015 612 hartree computed by Mitroy and Ryzhihk [J. Phys. B. 34, 2001 (2001)] using explicitly correlated Gaussians.
Effect of slowing down on the ripple-induced transport of α particles
Authors: Marchenko V. S.
Published: July 2002

The adiabatic convection of slowing-down α particles that are trapped in the resonance with toroidal field ripples is considered. It is shown that under certain conditions this transport mechanism dominates over the standard “superbanana” diffusion. In particular, it can be responsible for the “delayed” losses of partially thermalized alphas in the Tokamak Fusion Test Reactor [Proceedings of the 14th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Wurzburg, 1992 (IAEA, Vienna, 1992)] deuterium–tritium supershots [Herrmann et al., Nucl. Fusion 37, 293 (1997)].
Detailed study of nuclear fusion from femtosecond laser-driven explosions of deuterium clusters
Authors: Zweiback J. , Cowan T. E. , Hartley J. H. , Howell R. , Wharton K. B. , Crane J. K. , Yanovsky V. P. , Hays G. , Smith R. A. , Ditmire T.
Published: July 2002

Recent experiments on the interaction of intense, ultrafast pulses with large van der Waals bonded clusters have shown that these clusters can explode with sufficient kinetic energy to drive nuclear fusion. Irradiating deuterium clusters with a 35 fs laser pulse, it is found that the fusion neutron yield is strongly dependent on such factors as cluster size, laser focal geometry, and deuterium gas jet parameters. Neutron yield is shown to be limited by laser propagation effects as the pulse traverses the gas plume. From the experiments it is possible to get a detailed understanding of how the laser deposits its energy and heats the deuterium cluster plasma. The experiments are compared with simulations.
Ultrafast electron optics: Propagation dynamics of femtosecond electron packets
Authors: Siwick Bradley J. , Dwyer Jason R. , Jordan Robert E. , Miller R. J. Dwayne
Published: July 2002

Time-resolved electron diffraction harbors great promise for resolving the fastest chemical processes with atomic level detail. The main obstacles to achieving this real-time view of a chemical reaction are associated with delivering short electron pulses with sufficient electron density to the sample. In this article, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model. It is found that space-charge effects can broaden the electron pulse to many times its original length and generate many eV of kinetic energy bandwidth in only a few nanoseconds. There is excellent agreement between the N-body simulation and the mean-field model for both space-charge induced temporal and kinetic energy distribution broadening. The numerical simulation also shows that the redistribution of electrons inside the packet results in changes to the pulse envelope and the development of a spatially linear axial velocity distribution. These results are important for (or have the potential to impact on) the interpretation of time-resolved electron diffraction experiments and can be used in the design of photoelectron guns and streak tubes with temporal resolution of several hundred femtoseconds.
Infrared vibronic absorption spectrum and spin–orbit calculations of the upper spin–orbit component of the Au3 ground state
Authors: Guo Rui , Balasubramanian K. , Wang Xuefeng , Andrews Lester
Published: July 2002

Laser-ablated gold co-deposited with excess neon at 3.5 K produces a new sharp 2025.5-cm−1 absorption. Annealing to 8 K increases this absorption ten-fold and produces six weaker associated vibronic bands with 172- and 118-cm−1 intervals. Relativistic configurations (RCI) computations are carried out for several electronic states lying below 4.4 eV. These calculations show that the spin–orbit effect stabilizes the D3h structure and quenches Jahn–Teller distortion. They also predict a 0.2-eV spin–orbit splitting of the Au3 ground state, which is in excellent agreement with the 2025.5-cm−1 neon matrix band origin. We have also confirmed the assignment of the A-X system at 1.8 eV observed by Bishea and Morse. The observed vibronic intervals are in accord with calculated a1′ and e′ stretching fundamentals and they confirm the D3h geometry for Au3. This work reports the first observation of the ground-state spin–orbit splitting of a heavy metal trimer.
Stripe patterns in frustrated spin systems
Authors: Mu Yan , Ma Yu-qiang
Published: July 2002

We investigate the effects of the long-range dipolar interaction on the formation of the stripe patterns in two-dimensional (2D) spin systems with competing short- and long-range interactions by using the continuous time Monte Carlo technique. We find that there exists an optimal temperature region at which the best stripe patterns are prone to be formed, and the stability and the orientational ordering of striped structures mainly depend on the temperature T, while the width of the striped domains is determined by the strength of the long-range dipolar interaction g. Furthermore, a complete T-g phase diagram is obtained. The results provide a simple and universal picture to account for striking and substantial physics revealed in the prevalent striped morphologies of 2D spin systems.
Thermodynamics of the photoferroelectric effect in smectic-C* liquid crystals
Authors: Langhoff Arne , Giesselmann Frank
Published: July 2002

A general thermodynamic description of the photoferroelectric effect, where the spontaneous polarization of a ferroelectric material is changed by interaction with light, is proposed. This description leads to a distinction between primary and secondary photoferroelectricity. At a certain reduced temperature, the primary photoferroelectric effect originates from the light-induced change in the polar ferroelectric order, whereas the secondary photoferroelectric effect is a purely thermodynamic consequence of the light-induced change in the ferroelectric transition temperature Tc. Using simple approximations, the general theory was applied to photoferroelectric Sm-C* liquid crystals where the photoresponse is induced by the Z,E photoisomerization of azobenzene dopant molecules. The results agree with experimental data obtained for a photoferroelectric Sm-C* mixture containing 5 mol % of the photoisomerizable dopant 4,4′-bis-[(2-methyl)-butyloxy]azobenzene dissolved in the commercial ferroelectric Sm-C* host phase abbreviated FLC 6430.
Corrections to the strong-stretching theory of polymer brushes due to the entropy of the free ends
Authors: Matsen M. W.
Published: July 2002

The entropy associated with the free ends of a polymer brush is incorporated into the standard analytical strong-stretching theory (SST). The resulting corrections are found to have a significant effect on the end-segment distribution function, the self-consistent field, the brush profile, and the polymer trajectories. This alone is enough to bring many of the SST predictions into good agreement with the more complete numerical self-consistent field theory (SCFT). Furthermore, the corrections, in most cases, obey simple scaling expressions that remain accurate down to experimentally relevant grafting densities. This encouraging result suggests that, with further corrections, an analytical SST could be developed capable of quantitatively competing with SCFT.
Polyatomic molecules in strong laser fields: Nonadiabatic multielectron dynamics
Authors: Lezius M. , Blanchet V. , Ivanov Misha Yu. , Stolow Albert
Published: July 2002

We report the observation and characterization of a new nonresonant strong field ionization mechanism in polyatomic molecules: Nonadiabatic multi-electron (NME) dynamics. The strong field response of a given molecule depends on important properties such as molecular geometry and bonding, the path length of delocalized electrons and/or ionization potential as well as on basic laser pulse parameters such as wavelength and intensity. Popular quasi-static tunnelling models of strong field molecular ionization, based upon the adiabatic response of a single active electron, are demonstrated to be inadequate when electron delocalization is important. The NME ionization mechanism greatly affects molecular ionization, its fragmentation and its energetics. In addition, multi-electron effects are shown to be present even in the adiabatic long wavelength limit.
Structural defects in electrically degraded 4H-SiC p+/n−/n+ diodes
Authors: Persson P. O. Å. , Hultman L. , Jacobson H. , Bergman J. P. , Janzén E. , Molina-Aldareguia J. M. , Clegg W. J. , Tuomi T.
Published: July 2002

Triangular structural defects are occasionally generated during the long-term operation of 4H-SiC pin diodes and degrade the forward characteristics of the diode. We have used synchrotron white beam x-ray topography, scanning electron microscopy, in situ cathodo luminescence, and transmission electron microscopy to characterize the structure and formation of these defects. It is shown that the defects are stacking faults on the (0001) basal planes, bound by partial dislocations with Burgers vectors 1/3〈101̄0〉 and 1/3〈011̄0〉. These partials are suggested to form by the dissociation of existing dislocations.
Photoreflectance and photoluminescence spectroscopy of the lattice-matched InGaAs/InAlAs single quantum well
Authors: Wang Y. C. , Tyan S. L. , Juang Y. D.
Published: July 2002

A lattice-matched In0.53Ga0.47As/In0.52Al0.48As single quantum well (SQW) structure grown by gas source molecular beam epitaxy has been investigated by photoreflectance (PR) and photoluminescence (PL). The PR measurements allowed the observation of interband transitions from the heavy- and light-hole valence subbands to the conduction subbands. The transition energies measured from the PR spectra agree with those calculated theoretically. Two features corresponding to the ground state transition coming from the SQW and the band gap transition generated from the buffer layer are observed in the PL spectra and are in good agreement with the PR data. The effect of the temperature on the transition energies is essentially same as that in the gap transition of the bulk structure. The values of the Varshni coefficients of InGaAs/InAlAs were obtained from the relation between the exciton transition energy and the temperature. The built-in electric field could be determined and located from a series of PR spectra by sequential etching processes. The phase spectra obtained from the PR spectra by the Kramers–Kronig transformation were analyzed in terms of the two-ray model, and calculated the etching depth in each etching, and thus leading to the etching rate. The etching rate obtained from phase shift analysis agrees with that measured by atomic force microscopy. The etching results suggest that a built-in electric field exists at the buffer/substrate interface and it also enables us to determine the etching rate.
Simulation of the micromagnetic behavior of arrays of interacting nanoelements
Authors: Ridley P. H. W. , Roberts G. W. , Chantrell R. W.
Published: July 2002

In this work we investigate the behavior of small arrays of interacting permalloy particles at the submicron level. Each individual particle is termed a nanoelement and is rectangular in form with varying elongation. The interest in such structures of magnetic material is increasing, due mainly to the possible potential use in future high-density magnetic storage media applications. To carry out our investigations we have developed a dynamical micromagnetic model based on the use of the finite element method. For our results we investigate the effects of misaligned and aligned anisotropy distributions on arrays with varying size and space of nanoelement. We observe that the reversal mechanism of the arrays is very sensitive to the disorder of the intrinsic material properties. In the case of aligned uniaxial anisotropy a highly symmetric cooperative switching mechanism is observed. The larger anisotropy has the effect of stabilizing states during the reversal process, leading to distinctive switching along the hysteresis curve. A random anisotropy breaks this high symmetry sufficiently to reduce the cooperative switching, leading to a relatively random reversal of individual nanoelements.
Exposure characteristics of cobalt fluoride (CoF2) self-developing electron-beam resist on sub-100 nm scale
Authors: Malac Marek , Schoefield Marvin , Zhu Yimei , Egerton Ray
Published: July 2002

We have studied electron-beam exposure of cobalt fluoride (CoF2) thin films by real-time high-resolution transmission electron microscopy and by electron energy-loss spectroscopy. We were able to remove fluorine completely from an irradiated area and retain metallic cobalt by exposing the area at low dose rate and elevated temperature. The structures were composed of separated single-crystal cobalt nanoparticles with dimensions on the order of 5–10 nm.
Spontaneous edge transport barrier formation due to suppression of edge thermal instabilities as a low–high trigger mechanism in tokamaks
Authors: Stacey Weston M.
Published: July 2002

Models for the calculation of edge temperature gradients, of growth rates of edge thermal instabilities, and of the enhancement of edge transport coefficients due to thermal instability growth rates have been coupled. The composite model predicts the existence of threshold values of the edge power flux at which the edge transport coefficients sharply decrease and the edge temperature gradients sharply increase; i.e., an edge transport barrier forms. Predicted consequences of this transport barrier formation are found to be qualitatively consistent with phenomena observed experimentally in conjunction with the L–H (low-to-high) transition. It is suggested that this spontaneous formation of an edge transport barrier due to suppression of an edge thermal instability could be a trigger mechanism for the L–H transition in tokamaks.
Assessment of calibration procedures for accurate determination of thermal parameters of liquids and their temperature dependence using the photopyroelectric method
Authors: Delenclos Sylvain , Chirtoc Mihai , Sahraoui Abdelhak Hadj , Kolinsky Corinne , Buisine Jean Marc
Published: July 2002

We present a systematic theoretical and experimental investigation on the accuracy of thermal diffusivity α and thermal effusivity e of liquids measured by the photopyroelectric (PPE) method in back-detection configuration (BPPE). Special cases corresponding to different cell structures are analyzed in terms of error determination of α and e for water and ethylene glycol. We propose a new normalization procedure allowing for estimation of these parameters with accuracy of 2% on α and 5% on e over extended frequency range. The normalization eliminates the frequency-dependent influence of the transducer impedance and associated electronics, reduces the errors due to coupling fluid between cell components, and reduces the number of temperature-dependent parameters that must be known in order to characterize the sample. Technical solutions for improving the performances are suggested. Another goal of the study was to demonstrate the possibility of the BPPE method to yield small variations of thermal parameters as a function of temperature. We found good agreement with the literature data for the temperature coefficients (Δα/α)/ΔT=0.267±0.015%/K for water and (Δe/e)/ΔT=0.10±0.05%/K for ethylene glycol, between 20 and 60 °C. The special case implying the normalization to a reference material and with a glass substrate for the pyroelectric sensor is shown to be the best adapted to the determination of both thermal parameters and of their temperature dependence.
Nonlinear behaviors of valence-band splitting and deformation potential in dilute GaNxAs1−x alloys
Authors: Ya M. H. , Chen Y. F. , Huang Y. S.
Published: July 2002

Photoreflectance and piezoreflectance investigations have been performed on a series of GaNAs layers grown by low-pressure metal-organic chemical vapor deposition on Si-doped GaAs substrate. In addition to the observation of the fundamental band-gap and spin-orbit splitting, the valence-band splitting in thin GaNxAs1−x epilayers strained coherently by the GaAs substrate is observed in these modulation spectra. Comparing photoreflectance with piezoreflectance spectra, we clearly establish the transitions involving the heavy-hole and light-hole bands. We find that the valence-band splitting increases with increasing nitrogen composition, and it decreases with increasing temperature. We point out that the underlying origin of our observation can be attributed to the effect of lattice mismatch between GaNAs film and GaAs substrate. We also find that the deformation potential of GaNxAs1−x does not follow the linear interpolation of those for GaAs and GaN. It shows a strong composition dependence with a bowing in the small composition.
Dissociation of nitrogen-oxygen complexes by rapid thermal anneal heat treatments
Authors: Libbert J. L. , Mule’Stagno L. , Banan M.
Published: July 2002

Float zone silicon melt doped or implanted with nitrogen exhibits absorption bands in the midinfrared range that are caused by localized vibration modes of nitrogen pairs. Czochralski-grown silicon crystals melt doped with nitrogen exhibit both these lines and additional absorption lines related to the interaction of nitrogen with oxygen to form nitrogen-oxygen complexes. The stability of these complexes is studied by subjecting silicon samples containing these complexes to rapid thermal heat treatments at several temperatures and for varying times and monitoring the relative absorption by the complexes and the nitrogen pairs. We have found that the complexes begin to dissociate rapidly for temperatures above 700 °C but they are still present to some extent even after anneals at temperatures as high as 1200 °C.
Dynamics of Barkhausen jumps in disordered ferromagnets
Authors: Zheng Guang-Ping , Li Mo , Zhang Jinxiu
Published: July 2002

Evolution of Barkhausen jumps during the magnetization reversal process in disordered magnetic material is investigated. Based on the magnetoelastic effect (ΔE effect), we investigated the dynamics of Barkhausen jumps through an internal friction measurement of amorphous Fe–B–Mo ribbons. The ΔE caused by the Barkhausen jump is found to have a power-law scaling relation with the driving rate of magnetic field. Using numerical simulation, dynamics of Barkhausen avalanches in a realistic spin-lattice model for a disordered ferromagnet is analyzed. The dynamic scaling and inhomogeneous behavior observed in both experiments and theoretical models are presented and discussed.
A theoretical study of ozone isotopic effects using a modified ab initio potential energy surface
Authors: Gao Yi Qin , Chen Wei-Chen , Marcus R. A.
Published: July 2002

A modified ab initio potential energy surface (PES) is used for calculations of ozone recombination and isotopic exchange rate constants. The calculated low-pressure isotopic effects on the ozone formation reaction are consistent with the experimental results and with the theoretical results obtained earlier [J. Chem. Phys. 116, 137 (2002)]. They are thereby relatively insensitive to the properties of these PES. The topics discussed include the dependence of the calculated low-pressure recombination rate constant on the hindered-rotor PES, the role of the asymmetry of the potential for a general X+YZ reaction (Y≠Z), and the partitioning to form each of the two recombination products: XYZ and XZY.
Effect of chemical reactions on the transport coefficients of binary mixtures
Authors: Alves Giselle M. , Kremer Gilberto M.
Published: July 2002

The effects caused by the chemical reactions on the transport coefficients of a mixture of ideal gases which undergo a simple reversible symmetric reaction of the type A+A⇌B+B are analyzed within the framework of a kinetic theory based on the Boltzmann equation. The analyzed system is closed to the final stage of a chemical reaction where the affinity is considered to be a small quantity and the system tends to the chemical equilibrium. This kind of reaction is known as “fast” reactions, because the reactive processes are of the same order as the elastic ones. The internal degrees of freedom of the molecules of the gas are not taken into account. This enables to represent the gas molecules as rigid spheres and therefore to determine the coefficients of thermal conductivity, diffusion, thermal-diffusion ratio, shear viscosity, and the forward reaction rate analytically. It was verified that some factors—among others, the concentration of the reagents (or products of the reaction), the activation energy, the heat of reaction, and the steric factor—can affect in a significant way the transport coefficients in reactive gas mixtures.
Origin of the split Si–H stretch mode on hydrogen terminated 6H-SiC(0001): Titration of crystal truncation
Authors: Sieber N. , Stark T. , Seyller Th. , Ley L. , Zorman C. A. , Mehregany M.
Published: July 2002

Using Fourier-transform infrared absorption spectroscopy, we have studied the Si–H stretch mode on hydrogenated 6H-SiC(0001) and 3C-SiC(111). On 6H-SiC(0001) two signals at 2133.5 and 2128.0 cm−1 are observed. Their intensity ratio varies with sample preparation temperature. On 3C-SiC(111) essentially a single mode is observed at 2128.4 cm−1 at all temperatures. The origin of the two modes is explained in terms of different stacking arrangement below the surface. The possibility for a stacking rearrangement during the hydrogen treatment is discussed.
Kinetics of fluorescence quenching of S2-thioketones studied by the Smoluchowski–Collins–Kimball model: Standard systems
Authors: Krystkowiak Ewa , Maciejewski Andrzej
Published: July 2002

The fluorescence quenching of S2-xanthione and S2-deuterated xanthione by 3,3-diethylpentane, cyclohexane, and deuterated cyclohexane in perfluoro-1,3-dimethylcyclohexane has been studied by means of a picosecond time–resolved, single–photon counting technique. The donor fluorescence decay in the presence of the quencher for six analyzed systems could satisfactorily be fit using the Smoluchowski–Collins–Kimball function. The molecular parameters: R (the sum of the molecular radii), D (the sum of diffusion coefficients), and the specific rate constant of the process (κ) for all electronically excited donor–quencher pairs have been determined. The values of molecular parameter D, obtained from fluorescence quenching studies, are significantly lower than the sum of the macroscopic measured diffusion coefficients values. These results points to substantial differences in the rate of translational diffusion established in the molecular and macroscopic scale. The systems studied are proposed as model systems for further study of the time-dependent fluorescence quenching processes.
Photoelectron spectroscopy of pyrazine anion clusters
Authors: Song Jae Kyu , Lee Nam Ki , Kim Seong Keun
Published: July 2002

We studied the energetics and character of electron binding in the pyrazine anion clusters by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The mass distribution showed that the minimum number of molecules in a neat cluster of pyrazine to form an anion was two, with a single pyrazine molecule incapable of accommodating an excess electron. On the other hand, even the addition of a very weak solvent such as Ar sufficed to bring the affinity level of pyrazine below the vacuum level. Photoelectron spectra of some pyrazine-containing anion clusters, (Pz)1−–Arn (n=1–10) and (Pz)1−–S1 (S=pyrazine, benzene, and water), were obtained. A vibrational progression was observed in the photoelectron spectra of (Pz)1−–Arn. The electron affinity of pyrazine was determined to be −0.01±0.01 eV from extrapolation. A small drop in incremental electron affinity was observed from (Pz)1−–Ar4 to (Pz)1−–Ar5, indicating closure of the first solvation shell by four Ar atoms. The pyrazine dimer anion was found to exist in two isomeric forms of comparable energy, one with a symmetric structure and the other with an asymmetric one. The hydrogen bonding with the ring nitrogen strongly affects the binding of the pyrazine dimer anions. The electron affinity of the pyrazine dimer was estimated to be 0.13–0.15 eV.
Photoluminescence of a third order mode optically pumped semiconductor laser structure
Authors: Semaltianos N. G. , Vinter B. , De Rossi A. , Berger V. , Ortiz V.
Published: July 2002

Semiconductor lasers with third order waveguide mode emission are a promising way in which to achieve microsources of twin photons. The transport of electrons and holes in these particular structures is a critical issue. The temperature and pumping energy dependence of the photoluminescence spectra of a semiconductor structure specially designed for third order mode emission at 775 nm is studied. The analyses show that transfer of carriers from satellite double heterojunctions to the quantum well through a 140 nm Al0.50Ga0.50As barrier is very efficient, and the population inversion required for lasing is always obtained first in the quantum well rather than in the double heterojunctions at both low and room temperature.
Helix formation and folding in an artificial peptide
Authors: Alves Nelson A. , Hansmann Ulrich H. E.
Published: July 2002

We study the relation between α-helix formation and folding for a simple artificial peptide, Ala10–Gly5–Ala10. Our data rely on multicanonical Monte Carlo simulations where the interactions among all atoms are taken into account. The free-energy landscape of the peptide is evaluated for various temperatures. Our data indicate that folding of this peptide is a two-step process. In the first step two α-helices are formed which afterwards re-arrange themselves into a U-like structure.
Charge-enhanced C–H–O interactions of a self-assembled triple helical spine probed by high-pressure
Authors: Chang Hai-Chou , Ming Lee Kwang , Jiang Jyh-Chiang , Lin Ming-Shan , Chen Jen-Shin , Lin Ivan J. B. , Hsien Lin Sheng
Published: July 2002

C–H–O interactions of a self-assembled triple helix based on the 1-acetamido-3(2-pyrazinyl)-imidazolium cation has been probed by high pressure. The infrared spectroscopic profiles and ab initio calculations allow us to make a vibrational assignment of this compound. The C–H bonds forming C–H–O interactions shorten as the pressure was elevated, while free C–H vibration modes show low sensitivity to high pressure. The pressure-dependent results can be attributed to the strengthening of C–H–O electrostatic–dispersion interactions upon increasing pressure. The appearance of the free-NH infrared absorption indicates that the conventional N–H–O hydrogen bond does not dominate the inter-strand packing in the compound. It is proposed that the charge-enhanced C–H–O interactions, forming a helical hydrogen-bonding network, disturb the formation of inter-strand N–H–O hydrogen-bonding in order to form a maximum number of hydrogen bonds. Applying high-pressure seems not to change the C=O bond length in contrast to the trend of blue-shift in frequency of C–H vibrations. London dispersion energy is suggested to be required for understanding the pressure-dependent results, although more additional terms, such as the effect in the presence of charge, are needed for the correct explanation. This work demonstrates that high-pressure studies may have the potential to provide insight into the C–H–O structural properties of biological related systems.
High-resolution operation of frequency-multiplexed transition-edge photon sensors
Authors: Cunningham M. F. , Ullom J. N. , Miyazaki T. , Labov S. E. , Clarke John , Lanting T. M. , Lee Adrian T. , Richards P. L. , Yoon Jongsoo , Spieler H.
Published: July 2002

We present the multiplexed readout of two gamma-ray microcalorimeters made from transition-edge sensors. We use a frequency-domain multiplexing scheme in which each sensor is biased at an identifying frequency. We show that the energy resolution of the sensors is unaffected by multiplexing, and that crosstalk between the sensors is negligible. Our results indicate the feasibility of multiplexing 30 sensors or more to one readout line.
Domain processes in the magnetization reversal of exchange-biased IrMn/CoFe bilayers
Authors: Gogol P. , Chapman J. N. , Gillies M. F. , Vanhelmont F. W. M.
Published: July 2002

We have used transmission electron microscopy to study directly the way magnetization reversal proceeds in the ferromagnetic layer for sets of bilayers in which the ferromagnetic layer (CoFe) was of constant thickness while the thickness of the antiferromagnetic layer (IrMn) was varied. The first set studied was in the as-deposited state while the second was subjected to rapid thermal processing. For IrMn thicknesses of 20 Å, no shift fields were observed, although significant coercivities (∼70 Oe) were recorded and reversal involved rather simple domain processes. By contrast, complex small-scale domains dominated the reversal processes for samples where the IrMn thickness exceeded 60 Å; here, strong exchange biasing and higher coercivities were the norm. For all thicknesses of IrMn, an unexpected variation in the dominant orientation of domain walls on the outward and return parts of the magnetization cycle tended to be observed. A possible origin of this, together with the differences between the as-deposited samples and those subjected to rapid thermal processing, are discussed.
Ballistic transport in silicon vertical transistors
Authors: Nishiguchi K. , Oda S.
Published: July 2002

Clear evidence for ballistic transport has been observed at 5 K from silicon vertical transistors with wrap around gates. The effect of channel shape was investigated experimentally and accounted for theoretically by the anisotropy of the Si conduction band. A reduction in conductance and the appearance of multiple steps were observed when a magnetic field was applied perpendicular to the channel. These results were successfully modeled within the effective mass approximation by including the magnetic vector potential and effects due to series resistance and the spin and valley degeneracy.
High operational stability of electrophosphorescent devices
Authors: Kwong Raymond C. , Nugent Matthew R. , Michalski Lech , Ngo Tan , Rajan Kamala , Tung Yeh-Jiun , Weaver Michael S. , Zhou Theodore X. , Hack Michael , Thompson Mark E. , Forrest Stephen R. , Brown Julie J.
Published: July 2002

Electrophosphorescent devices with fac-tris(2-phenylpyridine)iridium as the green emitting dopant have been fabricated with a variety of hole and exciton blocking materials. A device with aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) demonstrates an efficiency of 19 cd/A with a projected operational lifetime of 10 000 h, operated at an initial brightness of 500 cd/m2; or 50 000 h normalized to 100 cd/m2. An orange-red electrophosphorescent device with iridium(III) bis(2-phenylquinolyl-N,C2′)acetylacetonate as the dopant emitter and BAlq as the hole blocker demonstrates a maximum efficiency of 17.6 cd/A with a projected operational lifetime of 5000 h at an initial brightness of 300 cd/m2; or 15 000 h normalized to 100 cd/m2. The average voltage increase for both devices is <0.3 mV/h. The device operational lifetime is found to be inversely proportional to the initial brightness, typical of fluorescent organic light emitting devices.
Comparative study of InGaAs quantum dot lasers with different degrees of dot layer confinement
Authors: Groom K. M. , Tartakovskii A. I. , Mowbray D. J. , Skolnick M. S. , Smowton P. M. , Hopkinson M. , Hill G.
Published: July 2002

We report a comparative study of the gain and lasing characteristics of two different InGaAs quantum dot (QD) laser designs, with multiple QD layers separated by barriers of (A) GaAs or (B) GaAs/AlGaAs. A higher degree of carrier confinement in structure B results in superior lasing characteristics at elevated temperatures. However, at temperatures below 130 K these devices demonstrate inhomogeneously broadened gain spectra, resulting in lasing over a much wider energy range than for structure A. The results are consistent with inefficient, low temperature interdot carrier transport in devices based on structure B.
High frequency effective viscosities of nematic liquid crystals with tilted orientation
Authors: Domack Arno , Johannsmann Diethelm
Published: July 2002

Using quartz crystal resonators, we have measured the effective viscosities of nematic liquid crystals (LCs) in the MHz domain. The LC orientation was switched via the command-surface effect. The dependence of viscosity on orientation cannot be explained by orientational modes. They are attributed to the emission of longitudinal sound, the latter effect requiring a tilt orientation of the nematic director with respect to the surface. Optical investigations confirm the existence of a finite pretilt angle. Sensor applications are discussed.
Influence of the carrier gas composition on morphology, dislocations, and microscopic luminescence properties of selectively grown GaN by hydride vapor phase epitaxy
Authors: Wagner V. , Parillaud O. , Bühlmann H. J. , Ilegems M. , Gradecak S. , Stadelmann P. , Riemann T. , Christen J.
Published: July 2002

We have studied the influence of the carrier gas (hydrogen versus nitrogen) on the morphology and defect characteristics of GaN grown by epitaxial lateral overgrowth (ELO) using hydride vapor phase epitaxy (HVPE). Growth was carried out on metalorganic vapor phase epitaxy GaN/sapphire patterned with SiO2 stripes, aligned along the 〈11̄00〉 GaN direction. The cross sections of the ELO-grown stripes change from trapezoidal to triangular with an increase in hydrogen content in the carrier gas due to a change of the ratio of the growth velocities on the {112̄2} and (0001) facets. Transmission electron microscopy observations show that while in stripes with trapezoidal morphology, dislocations from the window region still reach the sample surface, for triangular stripes they are bent in the horizontal direction away from the top surface. Cross-sectional cathodoluminescence (CL) microscopy shows basically two distinct regions of luminescence intensity and nature, one showing near-band gap excitonic emission, the other a high intensity blueshifted emission band which we attribute to e–h plasma recombination, indicating a high local free carrier concentration due to intrinsic defects or impurities. These two regions are correlated with different growth facets and not with the dislocation distribution. An original two-step growth method was developed which exploits this dependence of the morphology on the gas phase composition. In the first step the formation of triangular facets is preferential. This is done to bend the dislocations which have propagated vertically from the seed layer into the horizontal direction. In the second step the layers are planarized using conditions which favor lateral growth. Very good control of the faceting and high reproducibility of the selectively grown structures and layers could be obtained. By using spatially resolved CL measurements carried out on cleaved cross sections of the layers, different domains could be identified and correlated with the growth mode. The ELO layers obtained by two-step growth have significantly reduced dislocation densities of around 2–3×107 cm−2 at the surface, which is comparable to the best values of HVPE ELO GaN reported in the literature. The full width half maximum of the x-ray rocking curve gives values below 200 arcsec.
Different pressure coefficients of the light emission in cubic and hexagonal InGaN/GaN quantum wells
Authors: Suski T. , Teisseyre H. , Łepkowski S. P. , Perlin P. , Kitamura T. , Ishida Y. , Okumura H. , Chichibu S. F.
Published: July 2002

We have studied the influence of hydrostatic pressure on the light emission from cubic In0.1Ga0.9N. A qualitative difference between pressure dependence of photoluminescence peak energies for cubic and wurtzite symmetry InGaN/GaN quantum wells (QWs) was found. Cubic samples revealed magnitude of dEE/dP of 26–30 meV/GPa, practically independent of the QW width. Previous studies of the hexagonal InGaN/GaN structures showed that with increasing QW width dEE/dP changed between about 30 meV/GPa and 0 meV/GPa. This different behavior of two types of QWs can be explained by the lack of built-in electric field (along growth direction) in case of cubic structures. To describe pressure evolution of the optical transitions in cubic InGaN/GaN QWs and thick epitaxial layer, we use a simple k×p model based on the linear theory of elasticity. To reproduce the experimental data, it is necessary to invoke presence of In-rich fluctuations in the cubic In0.1Ga0.9N samples.