bluuebunny/crossref_metadata_2025 huggingface dataset:
In-layer conductivity and photoconductivity in MnPSe3, CdPSe3, and CdPS3
Authors: Carnabuci A. , Grasso V. , Silipigni L. , Salvato G.
Published: July 2002

Electrical properties of single crystals of the MnPSe3, CdPSe3, and CdPS3 layered compounds have been investigated by in-layer dc conductivity and photoconductivity measurements as a function of temperature. The observed dc conductivity dependence exhibits a semiconductor behavior for all three compounds in spite of their high resistivity values. At high temperatures the dc conductivity of all three compounds shows an Arrhenius temperature dependence, while photoconductivity is thermally activated only for MnPSe3 and almost temperature independent for CdPSe3 and CdPS3. These results, interpreted in terms of the simplified energy-band schemes existing for CdPX3 and MnPSe3, have allowed us to identify the conduction mechanisms and to deduce more information about the distribution of the electronic states in the gap region.
Piezoelectricity of ordered (Ga0.5In0.5)N alloys
Authors: Al-Yacoub A. , Bellaiche L.
Published: July 2002

First-principles calculations are performed to compare the e33 and e31 piezoelectric coefficients of GaN and InN with those of the (Ga0.5In0.5)N alloy exhibiting an alternation of Ga and In planes along the c axis. The magnitude of e33 and e31 in the ordered alloy is found to be (15%–28%) smaller than the magnitude of the corresponding coefficients resulting from the compositional average over the GaN and InN parent compounds. The microscopic origins for this downward deviation of piezoelectricity from a linear behavior with composition are revealed and discussed.
X-ray phase retrieval in high-resolution refraction data from amorphous materials
Authors: Siu K. , Nikulin A. Y. , Tamasaku K. , Ishikawa T.
Published: July 2002

A recently developed experimental–analytical x-ray diffraction method for the direct, nondestructive characterization of crystalline materials is applied to analyze high-resolution x-ray refraction data from amorphous materials. The method uses a logarithmic dispersion relation to determine the x-ray phase. A priori knowledge of the sample structure is utilized to reconstruct the physical characteristics of the sample with a resolution of 0.25 μm.
Superconducting quantum interference device based on MgB2 nanobridges
Authors: Brinkman Alexander , Veldhuis Dick , Mijatovic Dragana , Rijnders Guus , Blank Dave H. A. , Hilgenkamp Hans , Rogalla Horst
Published: July 2002

The superconductor MgB2, with a transition temperature of 39 K, has significant potential for future electronics. An essential step is the achievement of Josephson circuits, of which the superconducting quantum interference device (SQUID) is the most important. Here, we report Josephson quantum interference in superconducting MgB2 thin films. Modulation voltages of up to 30 μV are observed in an all-MgB2 SQUID, based on focused-ion-beam patterned nanobridges. These bridges, with a length scale <100 nm, have outstanding critical current densities of 7×106 A/cm2 at 4.2 K.
Effects of B and C on the ordering of L1-CoPt thin films
Authors: Yamaguchi H. , Kitakami O. , Okamoto S. , Shimada Y. , Oikawa K. , Fukamichi K.
Published: July 2002

We have investigated the effects of B and C on the ordering of L10-CoPt films and confirmed the following quite different role between B and C in CoPt. A small amount of B considerably decreases the temperature for ordering and is interstitially incorporated into L10-CoPt along its c axis due to the large negative heat of solutions with Co and Pt. X-ray photoelectron spectroscopy analyses have revealed that B atoms combine with Co and Pt. In contrast, C is not incorporated into the CoPt lattice due to its positive heat of solution, resulting in no decrease in the ordering temperature. The present results reveal that CoPt films containing a small amount of B are promising for high density recording media.
Monte Carlo simulations for a fluctuating sphere labeled on a flexible polymer chain in good solvents
Authors: Chen Yong , Shew Chwen-Yang
Published: July 2002

Monte Carlo simulations are conducted to investigate a model composed of a fluctuating sphere labeled on one chain end of an isolated flexible chain polymer in good solvents. The labeled sphere is to model the instantaneous size of a bound flexible chain segment or a vibrating chromophore on a polymer chain. We assume the vibration of the sphere is governed by a harmoniclike potential, and the sphere size stays positive. We first address the issue regarding the confinement effect induced by a flexible chain. To rationalize the simulation results, we carry out a detailed analysis for a simple case containing a dimer grafted onto a fluctuating sphere. Using the sphere with a large size fluctuation, we find that the fluctuating sphere can be confined within the coiled polymer chain, and even trapped inside the grooves between neighboring monomers. The results imply the confinement effects may influence the properties of chromophores labeled on polymers or drugs bound to biopolymers. Moreover, in a separate study, we show the fluctuating sphere model can be used to fit a bound flexible chain segment, and provides a means to parameterize a polymer chain to a dumbbell, with possible applications in the dynamics of dilute polymer solutions.
Intrachain fluorescence quenching in a flexible polymer: A theory for frequency-domain fluorometric experiments
Authors: Sung Jaeyoung , Lee Sangyoub
Published: July 2002

We present a theory for analyzing the frequency-domain fluorometric experiments on intrachain fluorescence-quenching reactions occurring in flexible chain polymers. The results are applied to investigate the qualitative dependence of the modulation and the phase angle on the chain length of the polymer, based on the Rouse chain model.
Near-critical fluids as experimental models for geophysical flows: The case of internal gravity waves
Authors: Carlès Pierre , El Khouri Laurence
Published: July 2002

We perform a theoretical analysis of internal gravity waves in near-critical fluids to determine whether they have the same behavior as in classical fluids. Two points are studied: the relevance of classical incompressible equations and the possibility of a coupling between internal gravity waves and the piston effect. Using asymptotic analysis of Navier–Stokes equations, it is shown that incompressible equations are applicable even very close to the critical point, and that the piston effect and internal gravity waves never couple, unlike what happens in the case of forced oscillations. These two conclusions confirm the relevance of using near-critical fluids to simulate internal gravity waves in large geophysical systems.
Coupling and orientation between anharmonic vibrations characterized with two-dimensional infrared vibrational echo spectroscopy
Authors: Golonzka O. , Khalil M. , Demirdöven N. , Tokmakoff A.
Published: July 2002

Two-dimensional vibrational spectroscopy has been used to characterize transient molecular structure by measuring the couplings and projection angles between two strongly coupled anharmonic vibrations. Two-dimensional Fourier-transform infrared spectra of the coupled carbonyl stretches of Rh(CO)2(C5H7O2) in hexane have been obtained from femtosecond vibrational echo signals detected with spectral interferometry. The eight resonances in the two-dimensional spectrum can be interpreted as two diagonal peaks and two cross peaks, each split into a pair. The splitting between the peak pairs is directly related to the diagonal and off-diagonal anharmonicity of the symmetric and asymmetric carbonyl stretches. The ratio of the amplitude of the cross peaks for two different polarization geometries determines the projection angle between the coupled transition dipoles. The experimental characterization of the vibrational eigenstates allows the local carbonyl structure to be modeled as bilinearly coupled cubic anharmonic oscillators. The interaction between the carbonyl stretches arises from the mutual bonding with the rhodium metal center. This two-dimensional infrared experiment characterizes the structure with a time window of roughly 20 ps, suggesting a general method for capturing transient molecular structure in solution.
Theory and first principles calculations of dissociative resonant photoionization: The evolution of atomic peaks and holes
Authors: Sałek P. , Carravetta V. , Gel’mukhanov F. , Ågren H.
Published: July 2002

We present theory and computational method for analyzing dissociative resonant photoemission from first principles. Particular emphasis is devoted to the conditions for observing so-called atomic peaks and atomic holes. The atomic peaks are connected with photoemission following resonant excitation to dissociative core excited states which show signals from scattering channels involving the dissociation (atomic) fragments in addition to those involving the compound molecule. The holes are the results of continuum–continuum interference effects between these two, atomic and molecular, channels which may act destructively under certain conditions. We apply a novel electronic structure method to compute the transition moments for the resonant and direct photoemission channels including their dependence on internuclear distances and their interference. The relevant matrix elements involving the photoelectron are obtained using similar techniques for the two types of channels, with the scattered electron wave in each case being determined in the full molecular anisotropic potential. A study of resonant photoemission through the core excited σ* states of HF and HCl indicates that the appearance of the atomic peaks and holes is subtly dependent on the nuclear dynamics, the potential energy curves, and the excitation photon frequency. We demonstrate that the resonant contribution and the evolution of the atomic peaks can be subject to strong dynamical suppression, so strong in fact that main state photoionization may constitute the dominating channel even at resonant conditions. It is shown that such dynamical suppression explains that resonant excitation to the F 1s-σ* dissociative state in hydrogen fluoride gives a photoelectron spectrum in which the spectator part contains strong atomic lines but a participator part where such lines are lacking, although they both refer to the same, dissociative, core excited state. The findings in the present work give evidence that both direct and resonant channels should be simultaneously considered in analyses of the dissociative photoemission process even at resonant conditions.
Anomalous photoluminescence behavior in GaAs/AlAs superlattice-like double-barrier diodes under bias
Authors: Vercik Andrés , Gobato Yara Galvão
Published: July 2002

Transport and optical properties of a GaAs/AlAs superlattice-like double-barrier diode were studied by continuous-wave photoluminescence spectroscopy. An enhancement of the quantum-well photoluminescence intensity with increasing temperatures is reported in biased diodes, which is related to transport properties, disappearing at zero-applied voltage. The anomalous observed behavior can be explained in terms of the temperature-dependent transport properties of the minority carriers (holes) in the collector spacer layer, which limits the supply of holes available for tunneling.
A combined quantum chemistry and classical molecular interaction energy method for the determination of crystal geometries and energies
Authors: Ko Gwon Hee , Fink William H.
Published: July 2002

Theoretical molecular solid crystal structure optimization presents many computational challenges. Calculation of both intramolecular interactions and intermolecular interactions are major obstacles. In this study, the intramolecular interactions are treated quantum mechanically and the intermolecular interactions are approximated by a force field, which is in part determined by a partial charge analysis of the quantum treatment. This combined approach, called the quantum coupled unit cell description (QCUCD) method, treats short-range and long-range intermolecular interactions with convergence-accelerated lattice sum techniques. QCUCD finds the internal molecular geometry; then crystal parameters are optimized until a fully consistent solution between the unit cell description and the quantum chemistry of the molecular electronic structure is achieved. The single molecule experiences the periodically repeating potential at one molecular site of the crystal. The solution of the electronic structure of the molecule under the QCUCD Hamiltonian determines the atomic partial charges that are positioned in a periodic array to establish the electrostatic contribution to the potential. The parameters for the Lennard-Jones interactions are taken from the force field of the widely used molecular simulation program AMBER as a convenient source for a large number of atom-based interactions. All geometric parameters for definition of the molecular crystal are optimized: the lattice constants, the orientational and positional degrees of freedom of the molecules at their crystal lattice sites, and the internal molecular geometry. QCUCD correctly determines the crystalline structures of the simple molecular solids N2, CO, and CO2 and also obtains the correct crystal packing orders of two different phases of N2 crystal in terms of crystal packing energies.
Rotational branching in population transfer in H2 by chirped adiabatic Raman passage
Authors: Sen Sanjay , Ghosh Swaralipi , Bhattacharyya S. S. , Saha Samir
Published: July 2002

We have theoretically investigated the branching in population transfer in H2 by chirped adiabatic Raman passage from the ground vg=0, Jg=0 level to the final rotational levels Jf=0 (Q-branch) and Jf=2 (S-branch) of the fundamental transition (vf=1) in the ground X 1Σg+ state via the nonresonant intermediate B 1Σu+ and C 1Πu+ states. The density matrix technique has been employed to describe the time evolution of the process. We have evaluated the rotational branching of transferred population in the final levels using linearly chirped pump and Stokes laser pulses with different chirp rates and chirp widths. Both the pulses are considered to have the same temporal shape, duration, peak intensity, and linear parallel polarizations. It has been observed that for negative chirping of the pump laser and positive chirping of the Stokes laser, branching in population transfer to the Q- and S-branches occur for fast chirping and short pulses while for slow chirping and long pulses, more and more population builds up in the S-branch. When the chirpings of the pulses are reversed, total population is transferred only to the Q-branch. We have endeavored to interpret the results within the framework of the adiabatic energy eigenvalue picture and the relative values of the two-photon Raman matrix elements for the two transitions.
High-repetition-rate ferroelectric-cathode gyrotron
Authors: Einat M. , Jerby E. , Rosenman G.
Published: July 2002

The intensive research on ferroelectric electron-emission mechanisms in the last decade has resulted in a wide understanding of the physics and characteristics of this plasma-assisted electron source. Nevertheless, practical devices employing this cathode were hardly introduced. In this experimental study, a high-repetition-rate microwave oscillator based on a ferroelectric electron gun has been developed. The device operates as a cyclotron-resonance maser in the gyrotron mode. Microwave pulses exceeding 1.5 kW at ∼7 GHz are measured in repetition rates above 3 MHz and duty cycles of ∼50%. These experimental results encourage the implementation of ferroelectric cathodes in practical high-power microwave tubes.
On the Kolmogorov-like generalization of Tsallis entropy, correlation entropies and multifractal analysis
Authors: Mesón Alejandro M. , Vericat Fernando
Published: July 2002

The generalization à la Kolmogorov of Tsallis entropy, introduced by the authors in a previous work [J. Math. Phys. 37, 4480 (1996)], is revisited. Invariance properties are pointed out under weaker conditions than before. This result leads us to wonder if Tsallis entropy at the Kolmogorov abstraction level brings new information with respect to the generalization that Kolmogorov did of Shannon entropy. The negative answer motivates us to look for other generalizations of Tsallis entropy in order to avoid the lack of new information. Correlation entropies seem to be good candidates for this purpose. The relationship of this kind of entropy with the multifractal analysis is studied with the help of the thermodynamic formalism. We also outline its usefulness to generalize properties of Tsallis entropy.
Poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]/tris (8-hydroxyquinoline) aluminum heterojunction electroluminescent devices produced by cluster beam deposition methods
Authors: Kim Jae-Yoo , Kim Eun-Sook , Choi Jong-Ho
Published: July 2002

We have fabricated and characterized double-layer-type electroluminescent devices with the structure of indium-tin-oxide-coated glass/poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (CzEH-PPV)/tris(8-hydroxyquinoline) aluminum (Alq3)/Li:Al, in which CzEH-PPV was used as a hole transport medium, and neutral and ionized cluster beam deposition (NCBD and ICBD) methods were applied to deposit Alq3. The surface morphology observed by atomic force microscopy shows that NCBD and especially ICBD methods are more efficient in producing flat and smooth thin film surfaces in comparison to the conventional physical vapor deposition method. Studies of photoluminescence, electroluminescence (EL), and device characteristics demonstrate that the polymeric thin film is susceptible to ion radiation damage and the NCBD-based devices show better device performance, including lower threshold and turn-on voltages, improved EL intensity-voltage, current density-voltage, and external quantum efficiency (EQE)-current characteristics. In addition, the doping effect of the highly fluorescent dye molecule 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran into the Alq3 layer reveals a complete energy transfer, color-tuning capability and enhanced EQEs.
In situ Raman study of C60 polymerization at high pressure high temperature conditions
Authors: Talyzin A. V. , Dubrovinsky L. S. , Le Bihan T. , Jansson U.
Published: July 2002

C 60 polymerization was studied in situ by Raman spectroscopy in the pressure range 10–27 GPa and temperatures up to 830 K. Quenched samples were studied by XRD using synchrotron radiation. Below 18 GPa only one- and two-dimensional polymers were found to form during heating. During slow cooling and pressure release, only the relative amounts of the different phases were changing. The structure of the 10–18 GPa samples after quenching is rhombohedral. The observed polymerization pathway suggests a gradual increase of the polymerization which occurs in the following sequence monomers→dimers→chains→tetra-coordinated polymer→hexa-coordinated polymer. Above 18 GPa the in situ Raman spectra during heating remained almost unchanged. XRD of the quenched samples revealed that they have a cubic structure with cell parameter above 13 Å. The sample heated at 27 GPa showed elliptical Debye–Scherrer diffraction ring due to the strong anisotropic deformation. No very dense phases reported in previous studies (so called “superhard fullerite”) were observed.
Polarized atomic orbitals for linear scaling methods
Authors: Berghold Gerd , Parrinello Michele , Hutter Jürg
Published: July 2002

We present a modified version of the polarized atomic orbital (PAO) method [M. S. Lee and M. Head-Gordon, J. Chem. Phys. 107, 9085 (1997)] to construct minimal basis sets optimized in the molecular environment. The minimal basis set derives its flexibility from the fact that it is formed as a linear combination of a larger set of atomic orbitals. This approach significantly reduces the number of independent variables to be determined during a calculation, while retaining most of the essential chemistry resulting from the admixture of higher angular momentum functions. Furthermore, we combine the PAO method with linear scaling algorithms. We use the Chebyshev polynomial expansion method, the conjugate gradient density matrix search, and the canonical purification of the density matrix. The combined scheme overcomes one of the major drawbacks of standard approaches for large nonorthogonal basis sets, namely numerical instabilities resulting from ill-conditioned overlap matrices. We find that the condition number of the PAO overlap matrix is independent from the condition number of the underlying extended basis set, and consequently no numerical instabilities are encountered. Various applications are shown to confirm this conclusion and to compare the performance of the PAO method with extended basis-set calculations.
Low-frequency noise in GaN thin films deposited by rf-plasma assisted molecular-beam epitaxy
Authors: Leung B. H. , Fong W. K. , Zhu C. F. , Surya Charles
Published: July 2002

We report detailed investigations of low-frequency excess noise in GaN thin-film cross-bridge structures deposited by rf-plasma assisted molecular-beam epitaxy on top of an intermediate-temperature buffer layer (ITBL) grown at 690 °C. The experimental data indicates strong dependence of the voltage noise power spectra on the thickness of the ITBL with an optimal thickness of 800 nm. A model has been presented to account for the observed noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers. The process results in the correlated fluctuations in both the carrier number and the Coulombic scattering rate. Detailed computation shows that number fluctuation dominates in our samples. Our numerical evaluation indicates a reduction in the trap density by over an order of magnitude with the use of an ITBL in the growth of GaN thin films.
Comparison of radiative properties of InAs quantum dots and GaInNAs quantum wells emitting around 1.3 μm
Authors: Markus A. , Fiore A. , Ganière J. D. , Oesterle U. , Chen J. X. , Deveaud B. , Ilegems M. , Riechert H.
Published: July 2002

The emission properties of self-assembled InAs quantum dots (QDs) and lattice-matched GaInNAs quantum wells (QWs) emitting around 1.3 μm were investigated by temperature-dependent and time-resolved photoluminescence (PL). The QDs have much higher PL efficiency at low excitation, but saturate faster as the excitation is increased, due to the lower density of states. Lifetime measurements show that nonradiative recombination plays a more important role in the GaInNAs QW than in QDs.
Approach to saturation in nanomagnetic systems: Easy axis distribution and interactions
Authors: Iglesias R. , Rubio H.
Published: July 2002

Using the applied field angle dependence of the approach to saturation of the magnetization, a theory for the determination of the distribution of easy axes of anisotropy and the ratio between interaction and anisotropy in bidimensional nanomagnetic materials is proposed. The usual saturation process laws in two dimensions at high and intermediate field ranges are recovered and other dependencies between the former are found and justified in terms of scaling arguments. Finally, an approximate analytical model that provides a simpler method by which to determine the easy axis distribution and interaction strength is discussed.
Self-consistent coupling of cavitation bubbles in aqueous systems
Authors: Strauss Moshe , Kaufman Yitzhak , Sapir Micha , Amendt Peter A. , London Richard A. , Glinsky Michael E.
Published: July 2002

The dynamics of an ensemble of cavitation voids initiated by laser-produced stress waves in aqueous systems is considered. Aqueous systems have large similarity to soft tissues. Laser-initiated stress waves are reflected from tissue boundaries, thereby inducing a tensile stress that is responsible for tissue damage. The early stage of damage is represented by an ensemble of voids or bubbles that nucleate and grow around impurities under stress wave tension. For impurity densities larger than 105 cm−3 the bubbles growth reduces the tensile wave component and causes the pressure to oscillate between tension and compression. For impurity densities below 108 cm−3 the bubbles grow on a long time scale (∼10 μs) relative to the wave interaction time (∼100 ns). For bubble densities above 108 cm−3 the bubble lifetime is greatly shortened because of the reduced tensile component. On a long time scale the growing bubbles cause a significant reduction in the liquid average compression pressure below the ambient atmospheric pressure. This effect increases the bubble lifetime by almost a factor of 2 relative to the low impurity density case when the bubbles are growing independently, in agreement with experiment [Paltauf and Schmidt-Kloiber, Appl. Phys. A: Mater. Sci. Process. 62, 303 (1996)]. As the collapse stage starts, small bubbles collapse first and the compression pressure screening becomes less effective, thereby accelerating the collapse of the larger bubbles and reducing the spread of the bubble lifetimes.
High-resolution infrared spectra of jet-cooled allyl radical (CH2–CH–CH2): ν2, ν3, and ν14 C–H stretch vibrations
Authors: Han Jia-xiang , Utkin Yu. G. , Chen Hong-bing , Hunt Neil T. , Curl R. F.
Published: July 2002

The high-resolution infrared spectrum of jet-cooled allyl radical (CH2–CH–CH2) in the region 3010–3045 cm−1 has been recorded in pulsed slit jet expansions using both modulated discharge and excimer laser flash photolysis methods to produce the radicals. Over 400 transitions are observed and assigned to the ν2, ν3, and ν14 C–H stretch vibrations. The band origins have been determined to be 3033.8745(6), 3023.4605(6), and 3020.32(1) cm−1, respectively. Spectral analysis indicates that all three upper states are perturbed, with the perturbations in the ν14 upper state so pervasive as to make least-squares analysis difficult. The gas phase frequencies reveal a somewhat unusual, in comparison with the other bands, matrix shift to the blue for ν2 of 18 cm−1.
Selective growth of (100)-, (110)-, and (111)-oriented MgO films on Si(100) by pulsed laser deposition
Authors: Chen X. Y. , Wong K. H. , Mak C. L. , Yin X. B. , Wang M. , Liu J. M. , Liu Z. G.
Published: July 2002

Selective growth of singly oriented (110)-, (100)-, and (111)-MgO films on Si(100) substrates were obtained by pulsed laser deposition. The effects of deposition temperature, ambient oxygen pressure, and etching of the substrate on the structural properties of the films were studied. It is found that the crystalline orientations of the MgO films are determined at the initial deposition stage by the substrate temperature only. The ambient pressure during deposition and etching of the Si substrates only effect the crystalline quality. Both (110)- and (111)-oriented films show granular grain structures. The (100)-oriented films grown on etched Si substrates display similar granular structures. Those deposited on nonetched Si substrates, however, reveal distinctive columnar grains. The observed phenomena are discussed based on the theory of crystal growth. The mechanism of the orientation selection is attributed to the energy balance between the surface and the interface energies. The varied grain structures are explained by considering the mobility of adatoms in different situations.
Preferential formation of Al–N bonds in low N-content AlGaAsN
Authors: Geppert T. , Wagner J. , Köhler K. , Ganser P. , Maier M.
Published: July 2002

The bonding of nitrogen in low N-content AlxGa1−xAs1−yNy with x⩽0.05 and y⩽0.04 has been studied by Raman spectroscopy. Upon the addition of Al to GaAsN, additional vibrational modes are observed at around 450 cm−1, which is below the GaN-like longitudinal optical (LO) phonon mode centered at 470 cm−1. These modes are attributed to the formation of Al and N containing complexes with Al-to-N bonding. With increasing Al content the Al–N related modes gain intensity at the expense of the GaN-like mode, and they become the dominant N-related feature for an Al-content of 5% at a fixed N content of 1%. On the other hand, increasing the N content from 0% up to 4% at a constant Al concentration of 5% results first in the appearance and eventual saturation in intensity of the AlN-like modes, accompanied by a steep increase in intensity and eventual dominance of the GaN-like vibrational mode. Simultaneously the AlAs-like LO2 phonon mode shows a drastic decrease in intensity for N contents exceeding 2%. All these observations strongly indicate that there is a preferential formation of Al–N bonds in low N- and Al-content AlGaAsN, which is in direct contrast to GaInAsN, where even after thermal annealing the GaN-like mode remains dominant in the Raman spectrum compared to the InN-like modes.
Critical temperature of superconducting bilayers: Theory and experiment
Authors: Brammertz G. , Golubov A. A. , Verhoeve P. , den Hartog R. , Peacock A. , Rogalla H.
Published: July 2002

A generalized model for the critical temperature TC of superconducting bilayers is presented, which is valid with no restrictions to film thicknesses, TC of the layers, and interface resistivity. The model is verified experimentally on a series of Nb–Al and Ta–Al bilayers with Nb, Ta layer thicknesses of 100 nm and Al layer thicknesses ranging from 5 to 200 nm. Excellent agreement between theory and experiment was found for the energy gap and the TC of bilayers. The results are important for designing practical superconducting devices.
Modeling of the induced charge in semiconductor gamma-ray imaging array
Authors: Ifraimov M. , Ludwig A. , Nemirovsky Y.
Published: July 2002

Pixelated two-dimensional (2D) arrays of gamma-ray spectrometers based on CdZnTe are of great interest due to the high spatial and energy resolution they provide. In contrast to a single spectrometer, in a 2D array, the field is not uniform and the induced charge is not proportional to the carrier distance from the anodes. For complete performance modelling of such an array, there is a need to calculate the charge induced on the anodes by a single photogenerated charge carrier. In this study, we present a solution based on the Moments Method, which considers the gaps between the anodes, i.e. mixed boundary conditions. This solution is compared with an approximation presented in previous studies. Good agreement is found in the region under an anode, however, in between anodes, more accurate results are achieved using the Moments Method.
Threshold reduction in quantum cascade lasers with partially undoped, dual-wavelength interdigitated cascades
Authors: Straub Axel , Mosely Trinesha S. , Gmachl Claire , Colombelli Raffaele , Troccoli Mariano , Capasso Federico , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

A dual-wavelength quantum cascade (QC) laser with an interdigitated cascade is presented. Aside from providing two-wavelength operation at 8.0 and 9.5μm wavelength, this laser design was used to test the role of extrinsic carriers in the injectors. An interdigitated cascade was grown with undoped injectors bridging 9.5 and 8.0 μm active regions, but doped injectors bridging 8.0 and 9.5 μm active regions. Clear laser action on both wavelengths demonstrates that doping of all injector regions is not a firm requirement for QC lasers. Comparison with a conventionally doped interdigitated cascade QC laser shows a threshold reduction by a factor of approximately 2 for the laser based on the active regions preceded by the undoped injector. This can be understood from the absence or strong reduction of impurity scattering related to the dopant ions.
Nonlocal response in CdTe photovoltaics
Authors: Shvydka Diana , Compaan A. D. , Karpov V. G.
Published: July 2002

We have studied the nonlocal photovoltaic response to a laser beam in CdTe/CdS solar cells. The laser-generated plasma is shown to spatially decay over a considerable distance that depends on the device lateral resistance and laser beam power. This affects open circuit voltage far from the laser spot. For the case when the lateral resistance is dominated by the transparent conductive oxide (in completed devices), it is shown that the characteristic decay length may be as long as 1 m. For the alternative case of unfinished devices that do not have a metal layer, the semiconductor layer sheet resistance dominates the nonequilibrium plasma spreading, and the characteristic decay length falls into the range of tenths of a millimeter. Also associated with such nonlocal response are features in photoluminescence mapping, where different excitation powers lead to different map topologies. We have developed a theory that expresses the effects of laser-generated plasma spreading in terms of the semiconductor film photovoltaic parameters.
Beam steering in waveguide arrays
Authors: Pertsch T. , Zentgraf T. , Peschel U. , Bräuer A. , Lederer F.
Published: July 2002

We demonstrate that highly efficient beam steering and switching can be achieved in thermo-optic waveguide arrays. For a given initial excitation, we find the position of the output field to depend on the transverse temperature gradient. By changing this gradient, we manage to steer the beam across the entire array. Diffractive spreading is almost suppressed and the beam retains its initial shape.
Apertureless optical near-field fabrication using an atomic force microscope on photoresists
Authors: Tarun Alvarado , Daza Marlon Rosendo H. , Hayazawa Norihiko , Inouye Yasushi , Kawata Satoshi
Published: July 2002

We present an apertureless optical near-field fabrication using an atomic force microscope. With this technique, we can directly pattern 100 nm lines that are smaller than the diameter of the incident far-field diffraction spot on positive photoresist spin coated over glass substrate. The nanostructure fabrication was carried out by the field enhancement generated at the tip apex of an apertureless probe illuminated with a 403 nm laser light. Results also show that the amount of energy required to induce photolysis seems to decrease in the presence of the tip.
Correlation between internal stress and ferroelectric fatigue in Bi4−xLaxTi3O12 thin films
Authors: Lee Jung-Kun , Kim Chang-Hoon , Suh Hee-Sang , Hong Kug-Sun
Published: July 2002

La-substituted bismuth titanate [Bi4−xLaxTi3O12(BLT)] films were synthesized using pulsed-laser deposition and ferroelectric fatigue phenomenon was investigated. The internal strain present in the films, which was analyzed through the evaluation of the x-ray diffraction peak, was partially responsible for the fatigue in BLT films. When x⩾0.75, the change in the internal strain was saturated and there was no significant degradation of switching charge, at least up to 1010 cycles. It revealed that the internal strain, as well as chemical stability of oxygen ions, contributed to the ferroelectric fatigue of the BLT films. The origin of the internal strain is discussed in terms of the lattice mismatch between bulk materials and thin films.
Iodine-stabilized Nd:YAG laser applied to a long-baseline interferometer for wideband earth strain observations
Authors: Araya Akito , Kunugi Takashi , Fukao Yoshio , Yamada Isao , Suda Naoki , Maruyama Sumitaka , Mio Norikatsu , Moriwaki Shigenori
Published: July 2002

We stabilized a frequency-doubled Nd:YAG laser (λ=532 nm) with reference to an iodine absorption line, and applied it to a long-baseline interferometer for earth strain observations. To obtain unmodulated light, saturated absorption signals of an external iodine cell were detected by the modulation transfer technique using an acousto-optic modulator working as both amplitude and frequency modulators. Two feedback loops, which could control the laser frequency by a piezo-electric actuator and a thermal actuator, realized fast and wide-range frequency stabilization, and ensured long-term stable operation. From a beat-note measurement between two identical systems, we obtained a frequency stability (in Allan variance) of ≲2×10−13 for time intervals of 10 to 1000 s. By applying the stabilized Nd:YAG laser to a light source of a 10 m interferometer, we successfully observed earth tides and earthquakes in strain variations. Other geophysical signals, detectable by this strainmeter, are also discussed.
Molecular theory of hydrophobic mismatch between lipids and peptides
Authors: Duque Daniel , Li Xiao-jun , Katsov Kirill , Schick M.
Published: July 2002

Effects of the mismatch between the hydrophobic length d, of transmembrane alpha helices of integral proteins and the hydrophobic thickness, Dh, of the membranes they span are studied theoretically utilizing a microscopic model of lipids. In particular, we examine the dependence of the period of a lamellar phase on the hydrophobic length and volume fraction of a rigid, integral, peptide. We find that the period decreases when a short peptide, such that dDh, is inserted. The effect is due to the replacement of extensible lipid tails by rigid peptide. As the peptide length is increased, the lamellar period continues to increase, but at a slower rate, and can eventually decrease. The amount of peptide which fails to incorporate and span the membrane increases with the magnitude of the hydrophobic mismatch |d−Dh|. We explicate these behaviors which are all in accord with experiment. Predictions are made for the dependence of the tilt of a single trans-membrane alpha helix on hydrophobic mismatch and helix density.
Dynamical response of the sinusoidally perturbed electrodissolution/passivation of iron in sulfuric acid solutions: Entrainment, spike generation, and quasiperiodicity
Authors: Karantonis Antonis , Pagitsas Michael , Sazou Dimitra
Published: July 2002

The iron/sulfuric acid (Fe/2 M H2SO4) system exhibits periodic current oscillations of relaxation type within the potential transition region formed between the active and passive states of the iron electrode when it is polarized in the 2 M sulfuric acid solution. In the present work the dynamical response of the Fe/2 M H2SO4 electrochemical oscillator is investigated when the applied potential at the iron electrode is sinusoidally perturbed. The behavior of the periodically perturbed Fe/2 M H2SO4 oscillator differs significantly from the response of other forced oscillators, as the potential amplitude Ep and the frequency ratio ωp/ω0 vary. The ωp and ω0 are the angular frequencies of the perturbed applied potential and the unperturbed oscillator, respectively. A special feature of its response is the appearance of a number of spikes, generated within the passive section of a periodic oscillatory cycle for ωp/ω0<2.9, for periods of the autonomous oscillator T0≳3 s. The number of the generated spikes depends on the amplitude and frequency of the perturbed applied potential as well as on the period of the autonomous oscillator. Spikes are not generated for ωp/ω0=1 and the system is harmonically entrained by the forcing frequency. However, when the system is subharmonically entrained for ωp/ω0 close to 2, spike generation does occur. By increasing the perturbation frequency for ωp/ω0≳2.9 and T0≳3 s, or by decreasing the autonomous period for T0<3 s and all the ωp/ω0<2.9 ratios, the spike generation pattern, is replaced by a quasiperiodic pattern. The dynamical response of the perturbed Fe/2 M H2SO4 electrochemical oscillator is characterized by using time-delay reconstructions of the attractors, Poincaré maps, and Fourier power spectra.
The induction of reentry in cardiac tissue. The missing link: How electric fields alter transmembrane potential
Authors: Roth Bradley J. , Krassowska Wanda
Published: July 2002

This review examines the initiation of reentry in cardiac muscle by strong electric shocks. Specifically, it concentrates on the mechanisms by which electric shocks change the transmembrane potential of the cardiac membrane and create the physiological substrate required by the critical point theory for the initiation of rotors. The mechanisms examined include (1) direct polarization of the tissue by the stimulating current, as described by the one-dimensional cable model and its two- and three-dimensional extensions, (2) the presence of virtual anodes and cathodes, as described by the bidomain model with unequal anisotropy ratios of the intra- and extracellular spaces, (3) polarization of the tissue due to changing orientation of cardiac fibers, and (4) polarization of individual cells or groups of cells by the electric field (“sawtooth potential”). The importance of these mechanisms in the initiation of reentry is examined in two case studies: the induction of rotors using successive stimulation with a unipolar electrode, and the induction of rotors using cross-field stimulation. These cases reveal that the mechanism by which a unipolar stimulation induces arrhythmias can be explained in the framework of the bidomain model with unequal anisotropy ratios. In contrast, none of the examined mechanisms provide an adequate explanation for the induction of rotors by cross-field stimulation. Hence, this study emphasizes the need for further experimental and theoretical work directed toward explaining the mechanism of field stimulation.
Energy band-gap discontinuities in GaAs:(Al,Ga)As heterojunctions
Authors: Batey J. , Wright S. L. , DiMaria D. J.
Published: July 2002

The measurement of the energy band discontinuities in GaAs:(Al,Ga)As heterostructures is described. These values are deduced from the activation energy governing current transport in the direction perpendicular to the heterojunction interface. Using complementary structures (fabricated with an Al mole fraction of ≂0.38), we study both electron and hole transport to independently measure the conduction and valence-band discontinuities respectively. The results obtained are self-consistent and indicate that the total band-gap difference distributes approximately in the ratio 60:40 between the conduction band and valence band. Measurement of the conduction-band discontinuity for an Al mole fraction of ≂0.24 yields a similar ratio. Preliminary measurements of the valence-band discontinuity in structures fabricated using (Al,Ga)As with an indirect band gap (Al mole fraction of ≂0.60) are also described.
Diffusion and electrical properties of silicon-doped gallium arsenide
Authors: Greiner Mark E. , Gibbons James F.
Published: July 2002

The amphoteric nature of silicon in gallium arsenide is used to develop diffusion and electrical compensation mechanisms. The diffusion mechanism is based on the formation and diffusion of nearest-neighbor donor-acceptor pairs. General solutions are presented that predict abrupt diffusion fronts for a wide range of pairing conditions. Experiments support the application of this mechanism to Si diffusion in GaAs at high concentrations. A compensation mechanism for amphoteric dopants is developed as well. The compensation process is driven primarily by the free-electron concentration. Nearly complete compensation is predicted for large dopant concentrations.
Fractoemission from single-crystal pentaerythritol tetranitrate
Authors: Miles M. H. , Dickinson J. T. , Jensen L. C.
Published: July 2002

The emission of electrons, photons, and positive ions was monitored during three-point bending, compression, and fast compression (impact loading) of single-crystal pentaerythritol tetranitrate (PETN). The emission appears to be fracture mode dependent with negligible emission from cleavage-type fracture and very intense electron emission accompanying impact. Possible causes of this enhanced emission are discussed in terms of shear effects and frictional grinding of the crystal fragments. Evidence of a microdischarge (electrical breakdown between fracture surfaces) during impact loading of the crystals is presented.
A technique for i
n s
i
t
u calibration of an x-ray streak camera in the nanosecond regime using a high density Z-pinch
Authors: Jones L. A. , Kallne E. , Kania D. R. , Maestas M. , McGurn J. S. , Shepherd R.
Published: July 2002

A technique for calibrating an x-ray streak camera in situ has been developed. It has been applied to an x-ray streak camera installed on a collapsing gas shell Z-pinch. The calibration was done by simultaneously measuring the emitted x-rays using a pinhole camera, an InP:Fe photoconductive detector, and the streak camera. The spatial dimension of the streak image was calibrated by integrating the film density over time and fitting the resulting data to the corresponding information taken from an x-ray pinhole image. The temporal calibration was obtained by similarly fitting the film density from the x-ray streak image as a function of time integrated over the appropriate part of the spatial dimension with the signal from an InP:Fe photoconductive detector. By using this technique we have found a spatial magnification of the streak camera system consistent with the results derived from the geometrical optics to within 10%. A temporal dispersion of 2.5±0.5 ns/mm was obtained, which is in agreement with the nominal speed of 2.5 ns/mm. This technique also yielded an absolute intensity calibration of the streak camera.
Simultaneous electron/hole transport in photorefractive materials
Authors: Valley George C.
Published: July 2002

The steady-state space-charge field and the response time of a photorefractive material illuminated with a sinusoidal interference pattern are derived for two models in which both electron and hole transport are important. In the first model in which electrons and holes are produced from a single set of recombination centers, the sign of the steady-state space-charge field depends on the relative value of the conductivities and/or absorption coefficients of the electrons and holes, and a single response time proportional to irradiance is obtained. In the second model in which electrons are photoionized from one set of recombination centers and holes from another, the sign of the space-charge field depends on the relative concentrations of the empty hole and electron traps, and two time constants, each inversely proportional to irradiance, are obtained.
Electromagnetic particle-in-cell simulations of Applied-B proton diodes
Authors: Slutz S. A. , Seidel D. B. , Coats R. S.
Published: July 2002

Fully electromagnetic particle-in-cell simulations of Applied-B ion diodes have been performed using the magic code. These calculations indicate that Applied-B diodes can be nearly 100% efficient. Furthermore, the simulations exhibit an impedance relaxation phenomenon due to the buildup of electron space charge near the anode which causes a time-dependent enhancement of the ion emission above the Child–Langmuir value. This phenomenon may at least partially explain the rapidly decreasing impedance that has been observed in Applied-B ion diode experiments. The results of our numerical simulations will be compared to experimental data on Applied-B ion diodes and to analytic theories of their operation.