bluuebunny/crossref_metadata_2025 huggingface dataset:
Multidimensional classical Liouville dynamics with quantum initial conditions
Authors: Horenko Illia , Schmidt Burkhard , Schütte Christof
Published: September 2002

A simple and numerically efficient approach to Wigner transforms and classical Liouville dynamics in phase space is presented. (1) The Wigner transform can be obtained with a given accuracy by optimal decomposition of an initial quantum-mechanical wave function in terms of a minimal set of Gaussian wave packets. (2) The solution of the classical Liouville equation within the locally quadratic approximation of the potential energy function requires a representation of the density in terms of an ensemble of narrow Gaussian phase-space packets. The corresponding equations of motion can be efficiently solved by a modified leap-frog integrator. For both problems the use of Monte Carlo based techniques allows numerical calculation in multidimensional cases where grid-based methods such as fast Fourier transforms are prohibitive. In total, the proposed strategy provides a practical and efficient tool for classical Liouville dynamics with quantum-mechanical initial states.
Photodissociative spectroscopy of the hydroxymethyl radical (CH2OH) in the 3s and 3px states
Authors: Feng Lin , Huang Xin , Reisler Hanna
Published: September 2002

The absorption spectrum and photodissociation dynamics of the hydroxymethyl radical via its two lowest excited electronic states, 3s and 3px, are investigated in a supersonic molecular beam by the depletion, resonance enhanced multiphoton ionization, and photofragment yield spectroscopy methods. The measured origins of the electronic transitions to the 3s and 3px states agree with the most recent ab initio calculations. The vibronic bands of the 2 2A′(3px)←1 2A″ transition are much broader than those of the transition terminating in the 2 2A″(3pz) state, while the transition to the 1 2A′(3s) state appears structureless. The investigation of the deuterated analog CH2OD shows that near the onset of the transition to the 3s state, only the O–D bond fission pathway is important, while both H and D products are detected following excitation to the 3px state. The progressive broadening of the absorption features from the uppermost 3pz to the lowest 3s excited state is explained based on recent calculations of surface couplings to lower electronic states. These couplings also control the photodissociation dynamics and the reaction outcomes.
Effects of crystallographic texture on stress-migration resistance in copper thin films
Authors: Koike J. , Wada M. , Sanada M. , Maruyama K.
Published: September 2002

The crystallographic texture of heat-treated Cu thin films and its effects on stress-migration resistance were studied as a function of film thickness within a range of 50–900 nm. All as-deposited films had (111) texture. After heat treatment at 723 K, texture transition from (111) to (100) was observed in films of thickness greater than 300 nm. The (111) texture films after heat treatment showed severe stress migration; in contrast, the (100) texture films showed no noticeable stress migration. The observed stress-migration resistance in the (100) texture films can be attributed to the absence of twins and to lower thermal stress as compared with the (111) texture films.
Bright white small molecular organic light-emitting devices based on a red-emitting guest–host layer and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl
Authors: Cheon K. O. , Shinar J.
Published: September 2002

Bright multilayer white organic light-emitting devices (OLEDs) based on red-emitting [2-methyl-6-[2-∼(2,3,6,7-tetrahydro-1H, 5H-benzo [ij] quinolizin-9-yl)-ethenyl]-4H-pyran4-ylidene] propane-dinitrile (DCM2)-doped N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) are described. The blue emission increased relative to the red emission with increasing bias, but the color coordinates of the total emission were well within the white region at V>10 V. The most lightly doped device, which contained a 5 nm thick layer of 0.5 wt. % of the dye dopant, exhibited the highest power efficiency, 4.1 lm/W, external quantum efficiency of 3.0%, and white brightness Lmax>50 000 Cd/m2 (at ∼1100 mA/cm2).
Unidirectional coercivity enhancement in exchange-biased Co/CoO
Authors: Gredig T. , Krivorotov I. N. , Eames P. , Dahlberg E. D.
Published: September 2002

A unidirectional coercivity enhancement, exhibiting the same behavior as a positive exchange bias, has been discovered in a temperature range below the blocking temperature in Co/CoO bilayers. Below this temperature range, the usual shift of the center of the M–H loops to the negative or antiparallel to the cooling field direction is found. This behavior is observed in both magnetic hysteresis loops and transport properties. The positive exchange bias can be explained by reversible changes in the interfacial pinning by the antiferromagnet causing an asymmetric magnetization reversal and a unidirectional coercivity enhancement along the cooling field direction.
Stress-induced polarization-graded ferroelectrics
Authors: Mantese Joseph V. , Schubring Norman W. , Micheli Adolph L. , Thompson Margarita P. , Naik Ratna , Auner Gregory W. , Misirlioglu I. Burc , Alpay S. Pamir
Published: September 2002

Polarization-graded ferroelectrics and their electrically active embodiments, graded ferroelectric devices and transpacitors, have been formed from a variety of material systems, both by grading the composition of the ferroelectric and by imposing temperature gradients normal to the electrode surfaces. In this letter, we show how these same devices can be formed from homogeneous ferroelectric films of lead strontium titanate by imposing stress gradients on the material normal to their electrode surfaces.
Modeling of adiabatic and diabatic potential energy surfaces of Cl(2P)⋯H2(1∑g+) prereactive complex from ab initio calculations
Authors: Kłos Jacek , Chałasiński Grzegorz , Szczȩśniak M. M.
Published: September 2002

The three lowest diabatic potential energy surfaces for the Cl(2P)+H2(1∑g+) van der Waals complex are derived from accurate ab initio calculations of the T-shaped (C2v) and collinear geometries (C∞v), at the coupled-cluster with the single, double, and noniterative triple excitations level of theory with a large basis set. The nonadiabatic coupling (off-diagonal derivative) matrix element and the fourth, off-diagonal, diabatic surfaces are determined by separate multireference configuration interaction calculations. For the intermediate geometries the angular dependence is modeled by a simple Legendre-polynomial interpolation. Three adiabatic potential energy surfaces (PESs) are also obtained by diagonalizing the total Hamiltonian in the diabatic basis. Both the nonrelativistic and relativistic (including spin–orbit coupling) PESs are evaluated. The nonrelativistic ground state PES agrees well with the entrance channel of the Bian-Werner (BW2) PES [W. Bian and H.-J. Werner, J. Chem. Phys. 112, 220 (2000)]. The dependence on the H2 stretching coordinate is also incorporated.
Orientation dependence of the optical properties in InAs quantum-dash lasers on InP
Authors: Ukhanov A. A. , Wang R. H. , Rotter T. J. , Stintz A. , Lester L. F. , Eliseev P. G. , Malloy K. J.
Published: September 2002

The anisotropy of the modal gain and the linewidth enhancement factor was experimentally measured in InAs/AlGaInAs/InP semiconductor lasers with an active region composed of quantum confined structures in the form of short wires called quantum dashes. This anisotropy is due to the polarization dependence of the transition matrix element in these quantum nanostructures. The spectral dependence of the gain and linewidth enhancement factor was investigated in a wavelength range from 1540 to 1640 nm at subthreshold current densities. The largest gain and the smallest linewidth enhancement factor were obtained when the quantum dashes were oriented perpendicular to the axis of the laser cavity.
Magnetic properties of Sm2(Fe0.95M0.05)17Nx (M=Cr and Mn) anisotropic coarse powders with high coercivity
Authors: Ito Mikio , Majima Kazuhiko , Shimuta Toru , Katsuyama Shigeru , Nagai Hiroshi
Published: September 2002

Sm 2 ( Fe 0.95 Cr 0.05 ) 17 N x and Sm2(Fe0.95Mn0.05)17Nx coarse powders 10–70 μm in size were synthesized by crushing mother alloy ingots into 32–74 μm in particle size and subsequent nitrogenation at 748 K in a flowing mixed gas of 60 vol % H2+40 vol % NH3. The effects of Cr or Mn substitution for Fe on the nitrogenation rate, magnetic properties, and microstructure of the Sm2Fe17Nx hard magnetic material were investigated. Cr and Mn substitution was quite effective for accelerating nitrogenation. When the powders were nitrogenated beyond x=3, amorphous phase formation was observed as the x value increased. The magnetic properties of the nitrogenated powders were significantly improved by Cr and Mn substitution, and these powders also possessed a satisfactory magnetic anisotropy. The maximum coercivity in this study, 0.59 MA/m, was obtained for the Sm2(Fe0.95Mn0.05)17N5.0 powder in spite of its large particle size. The high coercivity of the coarse powders was caused by a cell-like microstructure composed of fine 2–17 crystalline grains 20–30 nm in size surrounded by an amorphous phase.
Nonlinear optical properties of laser ablated silicon nanostructures
Authors: Vijayalakshmi S. , Lan A. , Iqbal Z. , Grebel H.
Published: September 2002

The role of crystallite symmetry on the nonlinear optical properties of laser ablated silicon films is discussed in this article. Laser ablated Si films exhibit a nonlinear refractive index change which may be as high as Δn=−0.5 at wavelength of λ=532 nm for films with an average thickness of 200 nm. However, the origin of this nonlinearity is not known. These films consist of large droplets comprised of hexagonal wurtzite symmetry crystallites and nanoclusters interspersed between them. Using Raman spectroscopy and linear and nonlinear optical measurements, we monitored the crystallographic symmetry of these droplets when annealed under various conditions. Based on the results we attribute the large nonlinear refraction coefficient to the hexagonal wurtzite symmetry of the crystallites, hence raising the possibility for developing very efficient nonlinear optical devices.
Density waves in gravity-driven granular flow through a channel
Authors: Liss Elizabeth D. , Conway Stephen L. , Glasser Benjamin J.
Published: September 2002

A molecular-dynamic computer simulation is used to examine rapid granular flow in a vertical channel. A two-dimensional event driven algorithm is used with periodic boundary conditions in the flow direction and solid walls in the lateral direction. Flow in the channel leads to an inhomogeneous distribution of the particles and two distinct types of density waves are identified: An S-shaped wave and a clump. The density waves are further characterized by quantifying their temporal evolution using Fourier methods and examining local and global flow properties of the system, including velocities, mass fluxes, granular temperatures, and stresses. A parametric study is used to characterize the effect of the system parameters on the density waves. In particular we are able to show that the dynamics of large systems are often qualitatively and quantitatively different from those of small systems. Finally, the types of density waves and dominant Fourier modes observed in our work are compared to those that are predicted using a linear stability analysis of equations of motion for rapid granular flow.
Kerr effect in gas and its application to noncontact measurement of electric field
Authors: Kumada Akiko , Iwata Akirahiko , Ozaki Kouichi , Chiba Masakuni , Hidaka Kunihiko
Published: September 2002

A noncontact measurement of electric field is proposed based on the Kerr effect in gases. With the Kerr sensing technique, the electric field can be measured without causing any field disturbance. Kerr constants of gases are, however, very small compared with those of liquids, and this ideal technique has not been applied to electric field measurement in gas. In this study, a system is developed for a highly sensitive measurement of the small phase retardation caused by the Kerr electro-optic effect in gases. An optical phase modulation technique is effectively adopted to improve the sensitivity of the system. With this optical system, (i) dc stressed uniform electric fields in sulfur hexafluoride, nitrogen, and carbon dioxide gases at the pressure of 0.2–0.5 MPa and (ii) ac stressed uniform electric fields in atmospheric air are measured.
Fluorescence spectroscopy and transmission electron microscopy of the same isolated semiconductor nanocrystals
Authors: Koberling Felix , Mews Alf , Philipp Günther , Kolb Ute , Potapova Inga , Burghard Marko , Basché Thomas
Published: September 2002

We present a method to establish a correlation between the crystalline structure and the fluorescence properties of isolated semiconductor nanocrystals. By using ultrathin silicon nitride substrates with markers, we have localized and investigated the same particles in both a high-resolution transmission electron microscope (TEM) and a confocal optical microscope. We have found that the observation of strong fluorescence emission does not require single domain particles. Additionally, we have correlated the size and shape of a particle as determined by TEM and its spectral properties like emission wavelength and spectral diffusion.
Dielectric characteristics of Al2O3–HfO2 nanolaminates on Si(100)
Authors: Cho M.-H. , Roh Y. S. , Whang C. N. , Jeong K. , Choi H. J. , Nam S. W. , Ko D.-H. , Lee J. H. , Lee N. I. , Fujihara K.
Published: September 2002

The structural characteristics and the chemical state of a HfO2–Al2O3 nanolaminate structure, depending on the postannealing temperature, were examined by x-ray diffraction and x-ray photoelectron spectroscopy. The structural stability is significantly enhanced up to 870 °C and so is able to sustain its amorphous and laminate structure. However, the laminate structure is drastically broken at the annealing temperature of 920 °C and the crystallization is locally generated. In particular, the formation of the interfacial layer during the postannealing treatment is effectively suppressed in the nanolaminated structure. The dielectric constant of the nanolaminate structure calculated from the accumulation capacitance increases from ∼10 to ∼17 as the annealing temperature increases. This change is closely related to the degree of the mixture composed by Al2O3 and HfO2.
Viscoelasticity of silica aerogels at ultrasonic frequencies
Authors: Gómez Álvarez-Arenas T. E. , Montero de Espinosa F. R. , Moner-Girona M. , Rodrı́guez E. , Roig A. , Molins E.
Published: September 2002

In this letter, the possibility to use a technique based on the analysis of thickness resonances of air-surrounded aerogel plates at ultrasonic frequencies to obtain viscoelastic properties is investigated. These resonances were excited and sensed by airborne ultrasonic waves. Toward this purpose, specially designed air-coupled, high-sensitivity, and broadband piezoelectric transducers were used. Precise and simultaneous measurements of the velocity and attenuation of longitudinal and shear waves at different frequencies, as well as aerogel density, were obtained. It allowed us to afford a full characterization of the viscoelastic properties of these materials at ultrasonic frequencies.
Proposed model for calculating the standard formation enthalpy of binary transition-metal systems
Authors: Zhang R. F. , Liu B. X.
Published: September 2002

In this letter, we present a proposed model for calculating the standard formation enthalpy of binary transition-metal systems by adding a prefactor S(c) to the well-documented formula developed by Miedema. The main idea is to take into account the significant effect of the atomic size difference on the contact surface while two dissimilar metals approaching together. Employing this model, the standard formation enthalpies of some 260 intermetallic compounds were calculated and compared with the experimentally measured data. It was found that the precision of the calculated values by the proposed model could be improved 13%–65% and that statistically, over 95% of the calculated values were in agreement with the experimental ones within an error of ±23 kJ/mol, while employing the Miedema’s formula the agreement was about 80%.
Compatibility of polycrystalline silicon gate deposition with HfO2 and Al2O3/HfO2 gate dielectrics
Authors: Gilmer D. C. , Hegde R. , Cotton R. , Garcia R. , Dhandapani V. , Triyoso D. , Roan D. , Franke A. , Rai R. , Prabhu L. , Hobbs C. , Grant J. M. , La L. , Samavedam S. , Taylor B. , Tseng H. , Tobin P.
Published: September 2002

Polycrystalline-silicon (poly-Si) gate compatibility issues with HfO2 and Al2O3 capped HfO2 gate dielectrics are reported. It can be generally stated that chemical vapor deposition (CVD) silicon gates using silane deposited directly onto HfO2 results in electrical properties much worse compared to similar HfO2 films using platinum metal gates. However, depositing CVD silicon gates directly onto Al2O3 capped HfO2 showed greater than a 104 times reduction in gate leakage compared to the poly-Si/HfO2 and poly-Si/SiO2 controls of similar electrical thickness.
Thermal contact conductance of actuated interfaces
Authors: Song Woo-Bin , Sutton Michael S. , Talghader Joseph J.
Published: September 2002

The thermal contact conductance (TCC) of microactuated mechanical interfaces has been characterized using an electronic technique, where micromachined test structures were heated with a current and the TCC was inferred from the change in resistance. For every device tested, the TCC was higher in vacuum than in air. This is in stark contrast to the behavior of bulk interfaces, and several experiments suggest that it may be the result of a decreased solid–solid contact area in air caused by the pressure of the interstitial gas. The average effective TCC of a polysilicon/nitride interface brought together by electrostatic actuation varies about values of 6.0×104 W/(K m2) in air and 9.5×104 W/(K m2) under vacuum for applied pressures of 1 MPa. These values are significantly higher than commonly reported for nonmetallic materials and probably reflect the very smooth surfaces of deposited thin films.
Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN (0.42⩽x<1)
Authors: Taniyasu Yoshitaka , Kasu Makoto , Kobayashi Naoki
Published: September 2002

We have obtained n-type conductive Si-doped AlN and AlXGa1−XN with high Al content (0.42⩽x<1) in metalorganic vapor phase epitaxy by intentionally controlling the Si dopant density, [Si]. Si-doped AlN showed the n-type conduction when [Si] was less than 3×1019 cm−3. When [Si] was more than 3×1019 cm−3, it became highly resistive due to the self-compensation of Si donors. This indicates that the self-compensation plays an important role at higher [Si] and determines the upper doping limit of Si for the AlN and AlXGa1−XN. For x⩾0.49, the ionization energy of Si donors increased sharply with increasing Al content. These resulted in a sharp decrease in the highest obtainable electron concentration with increasing Al content for the Si-doped AlXGa1−XN.
Output enhancement of spin-valve giant magnetoresistance in current-perpendicular-to-plane geometry
Authors: Yuasa H. , Yoshikawa M. , Kamiguchi Y. , Koi K. , Iwasaki H. , Takagishi M. , Sahashi M.
Published: September 2002

In this work, we present a suitable material for metal-based spin-valve in current-perpendicular-to-plane (CPP) geometry. The AΔR (A is the element size and ΔR is the change in resistance) was investigated for three kinds of free and pinned layers material, that is, Co90Fe10, Fe50Co50, and Fe50Co50 with half-atomic Cu layers. When the free and pinned layers are 5 nm, AΔR is 1.0 mΩ μm2, 1.6 mΩ μm2, and 2.9 mΩ μm2, respectively. Moreover, the dual-type spin valve having Fe50Co50 with half-atomic Cu layers achieved 5.2 mΩ μm2. According to the free and pinned layers thickness dependence of AΔR, the spin-dependent resistance at the interfaces between ferromagnetic layers and spacer Cu is enhanced mainly by changing from Co90Fe10 to Fe50Co50, and the spin-dependent bulk resistance in free and pinned layers is enlarged by inserting half-atomic Cu layers.
Optical bandgap energy of wurtzite InN
Authors: Matsuoka Takashi , Okamoto Hiroshi , Nakao Masashi , Harima Hiroshi , Kurimoto Eiji
Published: September 2002

Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy. Growth of a (0001)-oriented single crystalline layer was confirmed by Raman scattering, x-ray diffraction, and reflection high energy electron diffraction. We observed at room temperature strong photoluminescence (PL) at 0.76 eV as well as a clear absorption edge at 0.7–1.0 eV. In contrast, no PL was observed, even by high power excitation, at ∼1.9 eV, which had been reported as the band gap in absorption experiments on polycrystalline films. Careful inspection strongly suggests that a wurtzite InN single crystal has a true bandgap of 0.7–1.0 eV, and the discrepancy could be attributed to the difference in crystallinity.
Strong enhancement of THz radiation intensity from semi-insulating GaAs surfaces at high temperatures
Authors: Nakajima M. , Takahashi M. , Hangyo M.
Published: September 2002

The temperature dependence of THz radiation from semi-insulating GaAs surfaces excited by ultrashort laser pulses has been studied above room temperature up to 900 K. It is found that the radiation power increases with an increase in temperature and has a peak at ∼500 K, and then decreases with a further increase in temperature. The power radiated at 500 K is more than 30 times stronger than that at room temperature. The enhancement mechanism is discussed in relation to the temperature dependence of the built-in surface electric field.
Quantum operations that cannot be implemented using a small mixed environment
Authors: Zalka Christof , Rieffel Eleanor
Published: September 2002

To implement any quantum operation (a.k.a. “superoperator” or “CP map”) on a d-dimensional quantum system, it is enough to apply a suitable overall unitary transformation to the system and a d2-dimensional environment which is initialized in a fixed pure state. It has been suggested that a d-dimensional environment might be enough if we could initialize the environment in a mixed state of our choosing. In this note we show with elementary means that certain explicit quantum operations cannot be realized in this way. Our counterexamples map some pure states to pure states, giving strong and easily manageable conditions on the overall unitary transformation. Everything works in the more general setting of quantum operations from d-dimensional to d′-dimensional spaces, so we place our counterexamples within this more general framework.
Light scattering in transparent glass ceramics
Authors: Hendy S.
Published: September 2002

Transparent glass ceramic materials, with microstructures comprised of dispersed nanocrystallites in a residual glass matrix, offer the prospect of nonlinear optical properties. However, good transparency requires low optical scattering and low atomic absorption. The attenuation of light due to scattering (turbidity) will depend upon the difference in refractive index of the two phases and the size and distribution of crystals in the glass. Here, we model the glass ceramic as a late-stage phase-separated structure, and compute scattering in this model. We find that the turbidity follows a k8R7 relationship, where k is the wave vector of light in the glass ceramic and R is the average radius of the crystals in the glass.
Comparative study of ultrafast intersubband electron scattering times at ∼1.55 μm wavelength in GaN/AlGaN heterostructures
Authors: Heber J. D. , Gmachl C. , Ng H. M. , Cho A. Y.
Published: September 2002

We report on a comparative study of room temperature intersubband electron scattering lifetimes in GaN/AlGaN single and coupled double multiple quantum well (QW) samples with peak absorption wavelengths ranging from 1.4 to 1.7 μm. Using time-resolved pump-probe spectroscopy electron scattering times as short as ∼160 fs have been measured for a coupled QW sample and ≲300 fs for single QW samples. While no significant dependence on the excitation power has been observed, a decrease of the scattering times with increasing probe wavelength has been measured and may be attributed to monolayer fluctuations in the samples.
Unusual lattice distortion in a Ba0.5Sr0.5TiO3 thin film on a LaAlO3 substrate
Authors: Chen J. H. , Lia C. L. , Urban K. , Chen C. L.
Published: September 2002

Transmission electron microscopy (TEM) of a perovskite Ba0.5Sr0.5TiO3 thin film, grown on a (001) LaAlO3 substrate by pulsed-laser ablation, reveals that the film of single-crystal quality has an unusually distorted lattice with lattice parameters a and b (parallel to the interface) larger than c (perpendicular to the interface) by 1.4%. There is evidence that the as-examined Ba0.5Sr0.5TiO3 film is a variant of its high-temperature cubic phase due to its anisotropic thermal contraction during cooling. A large lattice mismatch value of 5.7% (to be compared to the normal value of 4.13%) is observed from selected area electron diffraction patterns and high-resolution TEM images of cross-sectional specimens, which suggests that the growing high-temperature film under the film growth condition may have a larger lattice constant and a different thermal expansion behavior with respect to the bulk material.
Spatial population distribution of laser ablation species determined by self-reversed emission line profile
Authors: Sakka Tetsuo , Nakajima Takashi , Ogata Yukio H.
Published: September 2002

We propose a method for determining the spatial distribution of population densities for the species in laser-produced plasma. Our method relies on the parameter fittings of the experimentally observed self-reversed emission profiles to the model which is based on the calculation of one-dimensional radiative transfer. Employed parameters in the model represent spatial distribution of emitters, absorbers, and plasma free electrons. Since the density of plasma electrons has a spatial dependence, Stark shifts and broadenings are incorporated in a position-sensitive manner. After a general description of the method, we have specifically applied it to the laser-ablated Al plasma, where Al(I) P∘2–2S emission line is employed for the analysis. In this specific example, we find that the accuracy of the fittings is significantly improved due to the presence of two emission lines originating from the fine structure, i. e., P1/2∘2–2S1/2 and P3/2∘2–2S1/2. In particular, the depth of the self-reversed structure turns out to be very sensitive to the position-dependent upper and lower level populations, which enables us to accurately determine the spatial variation of the laser-ablated species in these states. Furthermore, the calculated profile is almost unchanged with temperatures employed for fittings. This means that the present method gives reliable values of the parameters for the spatial distributions, even if the temperature is not precisely known.
Ballistic cooling in a wideband two-dimensional electron gas bolometric mixer
Authors: Lee Mark , Pfeiffer L. N. , West K. W.
Published: September 2002

We show that bolometric mixers fabricated from a high mobility two-dimensional electron gas can reach a regime where electron energy relaxation is dominated by ballistic, rather than diffusive, outflow of excited electrons. This ballistic cooling mechanism establishes the maximum physical limit on mixer speed for a transit-time limited device. Intermediate frequency bandwidths of nearly 40 GHz have been obtained in devices with channel lengths >1 μm in ballistically cooled devices.
Intersubband transition in (GaN)m/(AlN)n superlattices in the wavelength range from 1.08 to 1.61 μm
Authors: Kishino Katsumi , Kikuchi Akihiko , Kanazawa Hidekazu , Tachibana Tetsuo
Published: September 2002

Intersubband transition (ISBT) in the wavelength range from 1.08 to 1.61 μm was systematically investigated in (GaN)m/(AlN)n superlattices (SLs), grown directly on (0001) sapphire substrates by rf-plasma molecular beam epitaxy. The SLs consisted of 90 periods of GaN wells (m=2∼10 monolayers (ML) in thickness) and AlN barriers (n∼11 ML). One ML corresponds to a thickness of around 2.6 Å. For a 3.3 ML GaN well, the ISBT absorption wavelength reached a minimum of 1.08 μm, close to a theoretically predicted limitation, and it increased monotonically up to 1.61 μm with increasing well thickness to 9.5 ML. We observed absorption spectra as narrow as 61 and 66 meV in linewidth for 1.55 and 1.37 μm samples, respectively.
Splitting of the focal spot of a high numerical-aperture objective in free space
Authors: Chon James W. M. , Gan Xiaosong , Gu Min
Published: September 2002

Reported in this letter is a phenomenon that the focal spot of a high numerical-aperture objective in free space can split into two spots if a ring beam is used for illumination. Diffraction by a high numerical-aperture objective results in a depolarization such that the diffracted field in the focal region includes not only a component with the same polarization as the incident beam, but also orthogonal and longitudinal components. The use of a ring beam enhances the relative contribution from the longitudinal component. As a result, a single focal spot splits into two spots along the incident polarization direction. It is revealed theoretically that for an objective of given numerical aperture there is a threshold of the central obstruction size of a ring beam for the appearance of a two-peak focus.
Multi-prism x-ray lens
Authors: Cederström Björn , Lundqvist Mats , Ribbing Carolina
Published: September 2002

Refractive x-ray lenses with a triangular surface profile have been used to focus a synchrotron beam to sub-μm line width. These lenses are free from spherical aberration and work in analogy with one-dimensional focusing parabolic compound refractive lenses. However, the focal length can be easily varied by changing the gap between the two jaws. Silicon lenses were fabricated by wet anisotropic etching, and epoxy replicas were molded from the silicon masters. The lenses provided intensity gains up to a factor of 32 and the smallest focal line width was 0.87 μm. The simplified geometry and associated fabrication technique open possibilities for low-Z materials such as beryllium, which should greatly enhance the performance of refractive x-ray optics.
An alternative interpretation for the Holm “alpha model”
Authors: Montgomery David C. , Pouquet Annick
Published: September 2002

By reinterpreting a recent successful closure procedure in terms of local spatial averaging and the neglect of fluctuations about that average, it is shown how the results of that closure scheme (the “alpha model”) may be more simply derived.
Intersubband electroluminescence from Si/SiGe cascade emitters at terahertz frequencies
Authors: Lynch S. A. , Bates R. , Paul D. J. , Norris D. J. , Cullis A. G. , Ikonic Z. , Kelsall R. W. , Harrison P. , Arnone D. D. , Pidgeon C. R.
Published: September 2002

The quantum cascade laser provides one possible method of realizing high efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions. In surface-normal emission, only light-hole to heavy-hole electroluminescence is observed as predicted by theory. Intersubband emission is demonstrated at 2.9 THz (103 μm wavelength), 8.9 THz (33.7 μm), and 16.2 THz (18.5 μm) from the Si/SiGe quantum cascade heterostructures.
Theoretical and experimental investigation of boron diffusion in polycrystalline HfO2 films
Authors: Liu Chun-Li , Jiang Z. X. , Hegde R. I. , Sieloff D. D. , Rai R. S. , Gilmer D. C. , Hobbs C. C. , Tobin P. J. , Lu Shifeng
Published: September 2002

We present ab initio modeling results including formation, migration, and activation energies for B diffusion through bulk and grain boundaries in polycrystalline HfO2 films. Modeling results clearly indicate that B can penetrate through a 40 Å HfO2 film via grain boundary diffusion, but not by bulk diffusion. Secondary ion mass spectroscopy analysis of B concentration profiles for polysilicon/HfO2/Si gate stacks after different anneals showed double B peaks at the interfaces and thus confirmed the modeling prediction.
Oxygen reaction-diffusion in metalorganic chemical vapor deposition HfO2 films annealed in O2
Authors: Bastos K. P. , Morais J. , Miotti L. , Pezzi R. P. , Soares G. V. , Baumvol I. J. R. , Hegde R. I. , Tseng H. H. , Tobin P. J.
Published: September 2002

Composition, atomic transport, and chemical reaction were investigated following annealing in O2 of ultrathin HfO2 films deposited on Si substrates thermally nitrided in NO. The as-prepared thin film composition was established by Rutherford backscattering spectrometry, nuclear reaction analysis, and x-ray photoelectron spectroscopy as a HfO2 film on an intermediate layer containing silicon oxynitride, hafnium silicate, and possibly hafnium–silicon oxynitride. O penetration, incorporation in the bulk of the HfO2/SiOxNy structure, and oxidation of the substrate forming SiO2 were observed.
Waveguiding effects in the measurement of optical gain in a layer of Si nanocrystals
Authors: Valenta J. , Pelant I. , Linnros J.
Published: September 2002

We discuss applicability of the variable stripe length method to experimental investigation of optical gain in a luminescent layer that behaves like a planar waveguide. We show that an interplay between the output direction of guided light modes and the numerical aperture of the collection optics may lead to an artifact manifesting itself as an apparent but false gain. We propose a way to circumvent this inconvenience by using a “shifting excitation spot” complementary measurement. The method is demonstrated on a layer of Si nanocrystals embedded into a synthetic silica plate.
Widely tunable light-emitting diodes by Stark effect in forward bias
Authors: Le Thomas N. , Pelekanos N. T. , Hatzopoulos Z. , Aperathitis E. , Hamelin R.
Published: September 2002

By band gap engineering of the active region of a light-emitting diode, we were able to induce, in forward bias, large space-charge electric fields that generated Stark shifts over 100 nm in the 900 nm spectral region. These shifts are analyzed in terms of inhomogeneous carrier distribution throughout the active region.
Electro-optic detection of subwavelength terahertz spot sizes in the near field of a metal tip
Authors: van der Valk N. C. J. , Planken P. C. M.
Published: September 2002

We report on a method to obtain a subwavelength resolution in terahertz time-domain imaging. In our method, a sharp copper tip is used to locally distort and concentrate the THz electric field. The distorted electric field, present mainly in the near field of the tip, is electro-optically measured in an (100) oriented GaP crystal. By raster scanning the tip along the surface of the crystal, we find the smallest THz spot size of 18 μm for frequencies from 0.1 to 2.5 THz. For our peak frequency of 0.15 THz, this corresponds to a resolution of λ/110. Our setup has the potential to reach a resolution down to a few μm.
Determination of the shape and indium distribution of low-growth-rate InAs quantum dots by cross-sectional scanning tunneling microscopy
Authors: Bruls D. M. , Vugs J. W. A. M. , Koenraad P. M. , Salemink H. W. M. , Wolter J. H. , Hopkinson M. , Skolnick M. S. , Long Fei , Gill S. P. A.
Published: September 2002

We present a cross-sectional scanning-tunneling microscopy investigation of the shape, size, and composition of InAs quantum dots in a GaAs matrix, grown by molecular beam epitaxy at low growth rate. From the dimensional analysis we conclude that the investigated quantum dots have an average height of 5 nm, a square base of 18 nm oriented along [010] and [100] and the shape of a truncated pyramid. From outward relaxation and lattice constant profiles we conclude that the dots consist of an InGaAs alloy and that the indium concentration increases linearly in the growth direction. Our results justify the predictions obtained from previous photocurrent measurements on similar structures and the used theoretical model.
Partnerships, Power, and Scale in Rural Governance
Authors: Edwards Bill , Goodwin Mark , Pemberton Simon , Woods Michael
Published: September 2002

Partnerships have become established as a significant vehicle for the implementation of rural development policy in Britain. In promoting new working relationships between different state agencies and between the public, private, and voluntary sectors, partnerships have arguably contributed to a reconfiguration of the scalar hierarchy of the state. In this paper we draw on recent debates about the ‘politics of scale’ and on empirical examples from Mid Wales and Shropshire to explore the scalar implications of partnerships. We investigate how discursive constructs of partnership are translated into practice, how official discourses are mediated by local actors, the relationship between partnerships and existing scales of governance, and the particular ‘geometry of power’ being constructed through partnerships. We argue that the existing scalar hierarchy of the state has been influential in structuring the scales and territories of partnerships, and that, despite an apparent devolution of the public face of governance, the state remains crucial in governing the process of governance through partnerships.
Policy options to encourage community support for the sustainable development of Philippine lakes: Insights from Laguna de Bay
Authors: Malayang Ben S. , Briones Nicomedes D. , Catalan Zenaida B.
Published: September 2002

Philippine lakes, though an important feature of the country's landscape and a vital source of food for the population, suffer from degradation by human activities. Based on the experience of Laguna de Bay, the largest lake in Asia, lake degradation can be attributed to insufficient investment, excessive demand, inappropriate technologies, and ineffective institutional arrangements. Policy options should be tailored to address each of these specific views. Picking one policy over another or a standard cocktail of policies may not work best on Philippine lakes, but a specific mix of policies developed for each of the lakes may be effective. This requires putting together a combination of policy instruments to influence users' behavior in different lakes.
Heavy metal distribution in bottom sediments of a Mexican reservoir
Authors: Avila-Pérez P. , Garcia-Aragón J. A. , Díaz-Delgado C. , Tejeda-Vega S. , Reyes-Gutiérrez R.
Published: September 2002

Sediments from the Jose Antonio Alzate Reservoir (México) that receives large amount of industrial and municipal untreated waste water were analyzed in order to explore pollution by heavy metals. Samples from ten locations were analyzed to obtain particle size and concentrations of selected heavy metals in different sieve size fractions. Areas of deposition of fines are illustrated. The analysis of metal concentration by size fraction, the relative presence of metals, the ratio of metal concentrations to sediment particle size and additional information such as anthropogenic inputs, chemical association were used to explain the distribution pattern of heavy metals in the reservoir.
Survival of, and competition between, oligodendrocytes expressing different alleles of thePlpgene
Authors: Edgar J.M. , Anderson T.J. , Dickinson P.J. , Barrie J.A. , McCulloch M.C. , Nave K.-A. , Griffiths I.R.
Published: September 2002

Mutations in the X-linked Plp gene lead to dysmyelinating phenotypes and oligodendrocyte cell death. Here, we exploit the X inactivation phenomenon to show that a hierarchy exists in the influence of different mutant Plp alleles on oligodendrocyte survival. We used compound heterozygote mice to study the long-term fate of oligodendrocytes expressing either the jimpy or rumpshaker allele against a background of cells expressing a Plp-null allele. Although mutant and null oligodendrocytes were generated in equal numbers, the proportion expressing the mutant allele subsequently declined, but whereas those expressing the rumpshaker allele formed a reduced but stable population, the number of jimpy cells fell progressively. The age of decline in the jimpy cells in different regions of the CNS correlated with the temporal sequence of myelination. In compound heterozygotes expressing rumpshaker and jimpy alleles, oligodendrocytes expressing the former predominated and were more abundant than when the rumpshaker and null alleles were in competition. Thus, oligodendrocyte survival is not determined solely by cell intrinsic factors, such as the conformation of the misfolded PLP, but is influenced by neighboring cells, possibly competing for cell survival factors.
Apoptosis-inducing factor is involved in the regulation of caspase-independent neuronal cell death
Authors: Cregan Sean P. , Fortin Andre , MacLaurin Jason G. , Callaghan Steven M. , Cecconi Francesco , Yu Seong-Woon , Dawson Ted M. , Dawson Valina L. , Park David S. , Kroemer Guido , Slack Ruth S.
Published: September 2002

Caspase-independent death mechanisms have been shown to execute apoptosis in many types of neuronal injury. P53 has been identified as a key regulator of neuronal cell death after acute injury such as DNA damage, ischemia, and excitotoxicity. Here, we demonstrate that p53 can induce neuronal cell death via a caspase-mediated process activated by apoptotic activating factor-1 (Apaf1) and via a delayed onset caspase-independent mechanism. In contrast to wild-type cells, Apaf1-deficient neurons exhibit delayed DNA fragmentation and only peripheral chromatin condensation. More importantly, we demonstrate that apoptosis-inducing factor (AIF) is an important factor involved in the regulation of this caspase-independent neuronal cell death. Immunofluorescence studies demonstrate that AIF is released from the mitochondria by a mechanism distinct from that of cytochrome-c in neurons undergoing p53-mediated cell death. The Bcl-2 family regulates this release of AIF and subsequent caspase-independent cell death. In addition, we show that enforced expression of AIF can induce neuronal cell death in a Bax- and caspase-independent manner. Microinjection of neutralizing antibodies against AIF significantly decreased injury-induced neuronal cell death in Apaf1-deficient neurons, indicating its importance in caspase-independent apoptosis. Taken together, our results suggest that AIF may be an important therapeutic target for the treatment of neuronal injury.
A survival pathway forCaenorhabditis eleganswith a blocked unfolded protein response
Authors: Urano Fumihiko , Calfon Marcella , Yoneda Takunari , Yun Chi , Kiraly Moni , Clark Scott G. , Ron David
Published: September 2002

The unfolded protein response (UPR) counteracts stress caused by unprocessed ER client proteins. A genome-wide survey showed impaired induction of many UPR target genes in xbp-1 mutant Caenorhabditis elegans that are unable to signal in the highly conserved IRE1-dependent UPR pathway. However a family of genes, abu (activated in blocked UPR), was induced to higher levels in ER-stressed xbp-1 mutant animals than in ER-stressed wild-type animals. RNA-mediated interference (RNAi) inactivation of a representative abu family member, abu-1 (AC3.3), activated the ER stress marker hsp-4::gfp in otherwise normal animals and killed 50% of ER-stressed ire-1 and xbp-1 mutant animals. Abu-1(RNAi) also enhanced the effect of inactivation of sel-1, an ER-associated protein degradation gene. The nine abu genes encode highly related type I transmembrane proteins whose lumenal domains have sequence similarity to a mammalian cell surface scavenger receptor of endothelial cells that binds chemically modified extracellular proteins and directs their lysosomal degradation. Our findings that ABU-1 is an intracellular protein located within the endomembrane system that is induced by ER stress in xbp-1 mutant animals suggest that ABU proteins may interact with abnormal ER client proteins and this function may be particularly important in animals with an impaired UPR.
Remodeling of organelle-bound actin is required for yeast vacuole fusion
Authors: Eitzen Gary , Wang Li , Thorngren Naomi , Wickner William
Published: September 2002

Actin participates in several intracellular trafficking pathways. We now find that actin, bound to the surface of purified yeast vacuoles in the absence of cytosol or cytoskeleton, regulates the last compartment mixing stage of homotypic vacuole fusion. The Cdc42p GTPase is known to be required for vacuole fusion. We now show that proteins of the Cdc42p-regulated actin remodeling cascade (Cdc42p → Cla4p → Las17p/Vrp1p → Arp2/3 complex → actin) are enriched on isolated vacuoles. Vacuole fusion is dramatically altered by perturbation of the vacuole-bound actin, either by mutation of the ACT1 gene, addition of specific actin ligands such as latrunculin B or jasplakinolide, antibody to the actin regulatory proteins Las17p (yeast Wiskott-Aldrich syndrome protein) or Arp2/3, or deletion of actin regulatory genes. On docked vacuoles, actin is enriched at the “vertex ring” membrane microdomain where fusion occurs and is required for the terminal steps leading to membrane fusion. This role for actin may extend to other trafficking systems.
The human silent information regulator (Sir)2 homologue hSIRT3 is a mitochondrial nicotinamide adenine dinucleotide–dependent deacetylase
Authors: Schwer Björn , North Brian J. , Frye Roy A. , Ott Melanie , Verdin Eric
Published: September 2002

The yeast silent information regulator (Sir)2 protein links cellular metabolism and transcriptional silencing through its nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase activity. We report that mitochondria from mammalian cells contain intrinsic NAD-dependent deacetylase activity. This activity is inhibited by the NAD hydrolysis product nicotinamide, but not by trichostatin A, consistent with a class III deacetylase. We identify this deacetylase as the nuclear-encoded human Sir2 homologue hSIRT3, and show that hSIRT3 is located within the mitochondrial matrix. Mitochondrial import of hSIRT3 is dependent on an NH2-terminal amphipathic α-helix rich in basic residues. hSIRT3 is proteolytically processed in the mitochondrial matrix to a 28-kD product. This processing can be reconstituted in vitro with recombinant mitochondrial matrix processing peptidase (MPP) and is inhibited by mutation of arginines 99 and 100. The unprocessed form of hSIRT3 is enzymatically inactive and becomes fully activated in vitro after cleavage by MPP. These observations demonstrate the existence of a latent class III deacetylase that becomes catalytically activated upon import into the human mitochondria.
Increasing public understanding of transgenic crops through the World Wide Web
Authors: Byrne Patrick F. , Namuth Deana M. , Harrington Judy , Ward Sarah M. , Lee Donald J. , Hain Patricia
Published: September 2002

Transgenic crops are among the most controversial “science and society” issues or recent years. Because of the complex techniques involved in creating these crops and the polarized debate over their risks and benefits, a critical need has arisen for accessible and balanced information on this technology. World Wide Web sites offer several advantages for disseminating information on a fast-changing technical topic, including their global accessibility; and their ability to update information frequently, incorporate multimedia formats, and link to networks of other sites. An alliance between two complementary web sites at Colorado State University and the University of Nebraska-Lincoln takes advantage of the web environment to help fill the need for public information on crop genetic engineering. This article describes the objectives and features of each site. Viewership data and other feedback have shown these web sites to be effective means of reaching public audiences on a complex scientific topic.