bluuebunny/crossref_metadata_2025 huggingface dataset:
Stabilizing impact of high gradient of β on microturbulence
Authors: Bourdelle C. , Dorland W. , Garbet X. , Hammett G. W. , Kotschenreuther M. , Rewoldt G. , Synakowski E. J.
Published: June 2003

It is shown here that microturbulence can be stabilized in the presence of steep temperature and density profiles. Indeed in high β plasmas, pressure profile gradients are associated with high |β′|=−∂β/∂ρ, where β=P/(B2/2μ0) and ρ the square root of the toroidal flux normalized to its edge value. It is shown here that high values of |β′| have a stabilizing influence on drift modes. This may form the basis for a positive feedback loop in which high core beta values lead to improved confinement, and to further increase in β. A gyrokinetic electromagnetic flux tube code, GS2 [M. Kotschenreuther, G. Rewoldt, and W. M. Tang, Comput. Phys. Commun. 88, 128 (1995)], is used for analyzing the microstability. In high β spherical tokamak plasmas, high |β′| rather than low aspect ratio is a source of stabilization. Therefore, the effect of high |β′| should be stabilizing in the plasmas of the National Spherical Torus Experiment [Y.-K. Peng, M. G. Bell, R. E. Bell et al., Phys. Plasmas 7, 1681 (2000)].
Atomic-layer-deposited WNxCy thin films as diffusion barrier for copper metallization
Authors: Kim Soo-Hyun , Oh Su Suk , Kim Ki-Bum , Kang Dae-Hwan , Li Wei-Min , Haukka Suvi , Tuominen Marko
Published: June 2003

The properties of WNxCy films deposited by atomic layer deposition (ALD) using WF6, NH3, and triethyl boron as source gases were characterized as a diffusion barrier for copper metallization. It is noted that the as-deposited film shows an extremely low resistivity of about 350 μΩ cm with a film density of 15.37 g/cm3. The film composition measured from Rutherford backscattering spectrometry shows W, C, and N of ∼48, 32, and 20 at. %, respectively. Transmission electron microscopy analyses show that the as-deposited film is composed of face-centered-cubic phase with a lattice parameter similar to both β-WC1−x and β-W2N with an equiaxed microstructure. The barrier property of this ALD–WNxCy film at a nominal thickness of 12 nm deposited between Cu and Si fails only after annealing at 700 °C for 30 min.
Effect of InGaAs capping layer on the properties of InAs/InGaAs quantum dots and lasers
Authors: Chang F. Y. , Wu C. C. , Lin H. H.
Published: June 2003

We report the effects of In0.33Ga0.67As capping layers on the structural and optical properties of InAs self-organized quantum dots grown by gas-source molecular-beam epitaxy. With different deposition methods for the InGaAs capping layer, the quantum-dot density can be adjusted from 2.3×1010 to 1.7×1011 cm−2. As-cleaved 3.98-mm-long diode laser using triple stacks of InAs quantum dots with the capping layer grown by GaAs/InAs sequential binary growth demonstrates an emission wavelength of 1305 nm and a threshold current density of 360 A/cm2. A ground-state saturation gain of 16.6 cm−1 is achieved due to the high dot density.
Interface-reaction-mediated formation of two-dimensional Si islands on CaF2
Authors: Klust Andreas , Grimsehl Michael , Wollschläger Joachim
Published: June 2003

The growth of Si on thin (∼3 nm thick) CaF2/Si(111) films in the temperature range of 500–700 °C was investigated using ultrahigh-vacuum noncontact atomic force microscopy. At 500 °C, the morphology is dominated by small cluster-like shaped Si islands due to weak binding between Si and CaF2; with increasing growth temperatures these islands coexist with an increasing amount of triangular-shaped, flat Si islands. The formation of flattened triangular islands is attributed to an increased binding of Si to the CaF2 film at higher temperatures. The binding changes as a result of an interface reaction leading to the removal of fluorine atoms and the formation of Ca–Si bonds.
Large pure refractive nonlinearity of nanostructure silica aerogel
Authors: Seo J. T. , Yang Q. , Creekmore S. , Tabibi B. , Temple D. , Kim S. Y. , Yoo K. , Mott A. , Namkung M. , Jung S. S.
Published: June 2003

The nonlinear refraction coefficient of silica aerogel was estimated to be ∼−1.5×10−15 m2/W (∼−3.67×10−9 esu) with a signal-beam z-scan spectroscopy. The third-order nonlinear refraction coefficient of nanostructure silica aerogel was almost five orders larger than the nonlinear refraction (χ3) of bulk material. The large nonlinear refraction with high nonlinear figure of merit is an ideal optical property for nonlinear optical applications.
Liquid crystal alignment surface with two easy axes induced by unidirectional rubbing
Authors: Yamaguchi Rumiko , Goto Yusuke , Sato Susumu
Published: June 2003

We report the liquid crystal (LC) alignment surface which has two orthogonal easy axes induced by a unidirectional rubbing treatment. When the photoreactive polymer with chalconyl sidechains is irradiated with nonpolarized UV light and subsequently rubbed with a cloth, LCs align parallel and perpendicular to the rubbing direction on the UV irradiated and nonirradiated surfaces, respectively. The polarized UV absorption of the polymer and the sign of the dichroism are measured to explain the alignment mechanism. The micropatterning of the LC alignment is successfully demonstrated through the simple process using a photomask and nonpolarized UV irradiation technique.
Direct measurements of large near-band edge nonlinear index change from 1.48 to 1.55 μm in InGaAs/InAlGaAs multiquantum wells
Authors: Brzozowski Lukasz , Sargent Edward H. , Thorpe Anthony Spring , Extavour Marcius
Published: June 2003

Direct picosecond measurements of nonlinear refractive index change and nonlinear absorption in In0.530Al0.141Ga0.329As/In0.530Ga0.470As multiquantum wells in the range 1480–1550 nm are reported. Large low-threshold nonlinear index changes are found: Δn of up to 0.14 with figure of merit of 1.38 at a fluence of 116 μJ/cm2. The index-change-over-absorption figure of merit, F, is greater than unity over much of the spectrum, pointing to the prospective applicability of the materials studied to nonlinear switching devices.
Polarization superprism effect in surface polaritonic crystals
Authors: Zayats A. V. , Dickson W. , Smolyaninov I. I. , Davis C. C.
Published: June 2003

Surface plasmon polariton waveguide modes excited in mutually orthogonal line defects of a surface polaritonic crystal (two-dimensional photonic crystal on a metal surface) are observed to be extremely sensitive to the polarization state of excitation light. Full switching between orthogonal waveguides can be achieved with less than 10° variation of the polarization angle. For oblique incidence, the polarization sensitivity of switching is even higher but full switching cannot be realized. This effect can be considered as a polarization-domain analog of the photonic crystal superprism effect. It suggests the possibility of building actively controlled photonic crystal devices for optical signal processing.
Ultraviolet AlGaN multiple-quantum-well laser diodes
Authors: Kneissl Michael , Treat David W. , Teepe Mark , Miyashita Naoko , Johnson Noble M.
Published: June 2003

We demonstrate ultraviolet emission from current-injection AlGaN multiple-quantum-well laser diodes grown on sapphire substrates by metalorganic chemical vapor deposition. Lasing was obtained in gain-guided laser diode test structures with uncoated facets and cavity length ranging from 400 to 1500 μm. Under pulsed bias conditions, threshold current densities as low as 23 kA/cm2 have been achieved for laser diodes with emission wavelengths between 359.7 and 361.6 nm. The maximum output power was 45 mW per facet with differential quantum efficiencies of 1.3%.
Controllable electronic structures and related properties in a double-barrier nanoring
Authors: Yu Xiquan , Zhu Jia-Lin , Hu Xiao
Published: June 2003

The quantum effects of magnetic flux and in-plane electric field on an electron in a two-dimensional double-barrier nanoring have been studied. It has been found that the electronic structures and the related properties in the ring, such as the polarization, persistent currents, and far-infrared spectroscopy, can be tailored and controlled by the double barriers and the electric field. It may be useful for applications in quantum devices design in the future.
Linear, segmented microdischarge array with an active length of ∼1 cm: cw and pulsed operation in the rare gases and evidence of gain on the 460.30 nm transition of Xe+
Authors: von Allmen P. , Sadler D. J. , Jensen C. , Ostrom N. P. , McCain S. T. , Vojak B. A. , Eden J. G.
Published: June 2003

A linear, segmented channel, microdischarge array having an ∼80×380 μm2 aperture and an active length of ∼1 cm, has been fabricated in a ceramic multilayer structure and characterized in Ne, Ar, and Xe. The length of this seven-section device is more than an order of magnitude larger than those for previous arrays (2–3 elements) of coaxial microdischarge devices, and the discharge segments are individually ballasted with integrated thick-film resistors. Pulsed and cw operation of the array for Xe pressures as large as 700 Torr has been demonstrated and experimental indications of gain on the Xe+ 6p 4D3/20→6s 4P3/2 transition at 460.30 nm are presented. This device is well suited for the excitation of microlasers, either by electron impact pumping of gases and vapors, or by photoexcitation of solid state lasers in a slab geometry.
White light source in infrared region from InAs quantum dots grown on (001) InP substrates by droplet heteroepitaxy
Authors: Oga R. , Lee W. S. , Fujiwara Y. , Takeda Y.
Published: June 2003

We have observed extremely wide spectral range electroluminescence (EL) from InAs quantum dots (QDs) on (001) InP substrates at room temperature. The InAs QDs were grown by droplet heteroepitaxy using a low-pressure organometallic vapor phase epitaxial system. Room-temperature EL in a very wide wavelength range from 950 to 2200 nm was observed from InAs QDs embedded in InP matrix. The wide range emission indicates that the QDs have white optical gain in the infrared region at room temperature, which can be applied to efficient optical amplifiers for 1.0–1.6 μm fiber communication.
Tunneling splitting in polyatomic molecules: Application to malonaldehyde
Authors: Mil’nikov Gennady V. , Yagi Kiyoshi , Taketsugu Tetsuya , Nakamura Hiroki , Hirao Kimihiko
Published: June 2003

We report an accurate and efficient full dimensional semiclassical ab initio method for calculation of energy level splitting due to tunneling in polyatomic system. The method is applied to 21-dimensional 9-atomic malonaldehyde molecule. The tunneling splittings obtained are ΔE(H)=21.2 cm−1 for hydrogen atom transfer and ΔE(D)=3.0 cm−1 for deuterium atom transfer, which are in excellent agreement with the experimental values of 21.6 cm−1 and, 2.9 cm−1 respectively. We believe that the present analysis gives the final solution to the longstanding problem.
Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor?
Authors: Arkhipov V. I. , Heremans P. , Bässler H.
Published: June 2003

Although doping of a conjugated polymer by electron acceptors strongly facilitates exciton dissociation into geminate pairs of carriers, the yield of free carrier photogeneration can be high only at high doping levels, that is, in polymer/acceptor blends. We suggest a model that explains how excitons can efficiently dissociate into free carriers at an intrinsic polymer/acceptor interface despite the Coulomb interaction between the charges within precursor geminate pairs.
Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses
Authors: Borowiec A. , Haugen H. K.
Published: June 2003

High-spatial-frequency periodic structures on the surfaces of InP, GaP, and GaAs have been observed after multiple-pulse femtosecond laser irradiation at wavelengths in the transparency regions of the respective solids. The periods of the structures are substantially shorter than the wavelengths of the incident laser fields in the bulk materials. In contrast, high-frequency structures were not observed for laser photon energies above the band gaps of the target materials.
The characteristic carrier–Er interaction distance in Er-doped a-Si/SiO2 superlattices formed by ion sputtering
Authors: Jhe Ji-Hong , Shin Jung H. , Kim Kyung Joong , Moon Dae Won
Published: June 2003

The characteristic interaction distance between Er3+ ions and carriers that excite them in Er-doped a-Si/SiO2 superlattices is investigated. Superlattice thin films consisting of 12 periods of a-Si/SiO2:Er/SiO2/SiO2:Er layers were deposited by ion sputtering and subsequent annealing at 950 °C. The dependence of the Er3+ photoluminescence intensity on the thickness of the Er-doped SiO2 layers is well-described by an exponentially decreasing Er-carrier interaction with a characteristic interaction distance of 0.5±0.1 nm.
Magnetic orderings in Al nanowires suspended between electrodes
Authors: Ono Tomoya , Tsukamoto Shigeru , Hirose Kikuji
Published: June 2003

A theoretical analysis of a relation between atomic and spin-electronic structures for the ground state of single-row aluminum nanowires suspended between Al(001) electrodes is demonstrated using first-principles structural optimizations. We obtain an unusual result that a three-aluminum-atom nanowire sandwiched between the electrodes does not manifest magnetic ordering, although an isolated aluminum trimer molecule in a straight line is spin-polarized. On the other hand, a five-atom nanowire exhibits ferromagnetic ordering, where three central atoms form a spin-polarized trimer. Moreover, in the case of an eight-atom nanowire, the middle atoms in the nanowire form two spin-polarized trimers with antiferromagnetic ordering.
Field-emission properties of molybdenum disulfide nanotubes
Authors: Nemanič Vincenc , Žumer Marko , Zajec Bojan , Pahor Jurij , Remškar Maja , Mrzel Aleš , Panjan Peter , Mihailovič Dragan
Published: June 2003

The field-emission (FE) properties of molybdenum disulfide nanotubes (NTs) are reported for the single-tip geometry. Reproducibly stable FE currents in excess of 10 μA were measured from single NT tips in vacuum of 10−7 mbar. Valuable characteristics of the nanotube material are ease of processing and reproducibility of the FE properties.
Analysis of surface acoustic wave propagation on a cylinder using laser ultrasonics
Authors: Clorennec D. , Royer D.
Published: June 2003

This letter shows an unexpected phenomenon for surface acoustic waves (Rayleigh waves) propagating on a solid cylinder. It was observed that a short Rayleigh wave pulse was reversed during its propagation. In experiments on duraluminum and steel cylinders, transient surface acoustic waves were launched by a pulsed YAG laser focused along a line. The radial component of the displacement was measured by a heterodyne optical interferometer. The wave forms recorded versus the angle θ between the source and the detection point shows that the Rayleigh pulse, which is monopolar (positive) near the source, becomes bipolar at θ=45° and monopolar (negative) at θ=90°. An analytical calculation taking into account the dispersive effect reproduces wave forms whatever the propagation distance. A physical model is proposed to account for the reversal of the acoustic pulse.
Length-scale dependent variation of the first nucleated phase in nickel–silicon multilayers
Authors: Jensen Jacob M. , Kyablue Xavier J. , Ly Sochetra , Johnson David C.
Published: June 2003

We have studied the first phase nucleation behavior of a series of equiatomic Ni–Si multilayers with ultrathin repeating subunits (λ=1.7–5.8 nm) using x-ray reflectivity and x-ray diffraction. The variation in first nucleated phase is understood in terms of the model of Gösele and Tu, who predicted three length-scale dependent reaction routes in thin film diffusion couples: (i) For multilayers with λ>5.0 nm, interfacial nucleation of Ni2Si is observed, consistent with studies of bulk diffusion couples. (ii) For multilayers with 3.8<λ<5.0 nm, interfacial nucleation tends towards phases richer in Si due to a waning supply of Ni within the multilayer. (iii) For multilayers with λ<2.0 nm complete mixing of the Ni–Si multilayers prefigures first phase nucleation of NiSi. In this regime the composition of the first nucleation phase is determined by the amorphous precursor, which in turn is dictated by the relative ratios of Ni and Si in the initial multilayer. A fourth scenario, between (ii) and (iii), in which Ni2Si is the first nucleated phase is also observed. A possible mechanism for this unexpected result is discussed.
Time-delayed indium incorporation in ultrathin (InxGa1−xN/GaN) multiple quantum wells grown by metalorganic vapor phase epitaxy
Authors: Schulze F. , Bläsing J. , Dadgar A. , Krost A.
Published: June 2003

In x Ga 1−x N/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy were investigated by x-ray reflectometry and high-resolution x-ray diffractometry. The combination of both analysis methods allows a very precise determination of the structural parameters of the layer systems. From a systematic variation of the growth temperature from 760 to 840 °C and deposition times from 15 to 200 s, a temperature-dependent time delay of the indium incorporation was observed. Preferentially, at lower growth temperatures, indium seems to be accumulated at the GaN surface before the onset of the InGaN quantum well growth. The growth delay increases with decreasing growth temperature.
Shortest intersubband transition wavelength (1.68 μm) achieved in AlN/GaN multiple quantum wells by metalorganic vapor phase epitaxy
Authors: Waki Ichitaro , Kumtornkittikul Chaiyasit , Shimogaki Yukihiro , Nakano Yoshiaki
Published: June 2003

Two-hundred-period high-quality AlN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy are studied using high-resolution x-ray diffraction, transmission electron microscopy, and optical transmission spectroscopy. Excellent interfaces of the MQWs are confirmed by these measurements. The strong intersubband absorption peak at a wavelength of 1.68 μm is achieved for AlN (1.6 nm)/GaN(1.7 nm) MQW. The full width at half-maximum of the absorption peak is estimated to be 27 meV.
Flexible low-voltage electro-optic polymer modulators
Authors: Song Hyun-Chae , Oh Min-Cheol , Ahn Seh-Won , Steier William H. , Fetterman Harold R. , Zhang Cheng
Published: June 2003

A high-performance electro-optic (EO) polymer modulator on a flexible substrate was fabricated using a polymer substrate layer lift-off method. The SU-8 polymer has widely different adhesion properties on Si and gold substrates that makes selective lift-off possible. The flexible EO polymer modulators on a 100-μm polymer substrate layer have a Vπ of 2.6 V and extinction ratio better than 20 dB at 1550 nm. The bending loss of the flexible waveguide was unchanged at bending radii as small as 1.5 mm, and no effect on Vπ was observed for a 5 mm bending radius.
Near-field scattering of longitudinal fields
Authors: Bouhelier Alexandre , Beversluis Michael R. , Novotny Lukas
Published: June 2003

Longitudinal fields created in strongly focused laser beams are investigated by near-field optical microscopy. Sharp metallic and dielectric tips are raster scanned through the focus of these modes. It is found that regardless of the tip material, the signal scattered by the tip is a measure for the strength of the local longitudinal field. A surprising contrast reversal is observed between the images obtained with a metallic tip and the images obtained with a dielectric tip. The contrast reversal originates from a non-negligible tip–sample interaction.
Exciton and carrier motion in quaternary AlInGaN
Authors: Kazlauskas K. , Tamulaitis G. , Žukauskas A. , Khan M. A. , Yang J. W. , Zhang J. , Kuokstis E. , Simin G. , Shur M. S. , Gaska R.
Published: June 2003

Temperature and excitation power dependences of the photoluminescence Stokes shift and bandwidth were studied in quaternary AlInGaN epilayers as a function of indium content. At low excitation power, gradual incorporation of indium into AlGaN is shown to result in S- and W-shaped temperature dependences of the band peak position and bandwidth, respectively. At high excitation power, the S- and W-behavior disappears; however, increased indium molar fraction boosts the redshift of the luminescence band at high temperatures. Our results indicate that the incorporation of indium into AlGaN has a noticeable impact on the alloy transport properties. At low temperatures and low excitation power, the indium incorporation facilitates hopping of localized excitons, whereas at high temperatures and high excitation power, it sustains free motion of delocalized carriers that results in the band-gap renormalization via screening.
Formation of patterned buried insulating layer in Si substrates by He+ implantation and annealing in oxidation atmosphere
Authors: Ogura Atsushi
Published: June 2003

We fabricated patterned buried insulating layers in a Si substrate by implanting He+ ions through a patterned mask and annealing in an oxidation atmosphere. Compared with patterned O+ implantation and annealing, this technique causes less damage, and can therefore reduce defect density at the edge of the patterned buried insulator. The buried insulator could be SiO2 or a void with thin SiO2 film on the inner surface. The formation of a void could further reduce defect density, probably because the stress induced by SiO2 formation is small. The fabricated surface was extremely smooth within the focal depth limitations of state-of-the-art photolithography techniques.
Electrochemically doped polymeric anode for improving the performance of molecular organic light-emitting diodes
Authors: Zhang F. , Petr A. , Dunsch L.
Published: June 2003

The doping level of spin-coated poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS) films was adjusted quantitatively by an electrochemical doping/dedoping process in toluenesulfonic acid containing solution. The increase in doping level leads to an improvement of hole injection from the PEDT:PSS/ITO (indium–tin oxide) anode to the hole transporting layer, which is attributed to the enhanced work function of PEDT. The performance of vapor-deposited tri(8-hydroxyquinoline) aluminum (Alq3) based organic light-emitting devices with these anodes is shown to depend strongly on the doping level of PEDT:PSS. The device, built on a highly oxidized anode by electrochemical modification (electrochemical equilibrium potential of 0.59 V vs Ag/AgCl), shows a reduction of the operating voltage by 2 V, as well as remarkable enhancement of the luminance compared to the device on a pristine polymer anode.
High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions
Authors: Faure-Vincent J. , Tiusan C. , Jouguelet E. , Canet F. , Sajieddine M. , Bellouard C. , Popova E. , Hehn M. , Montaigne F. , Schuhl A.
Published: June 2003

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (tMgO), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼67% at room temperature) have been observed with tMgO=2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Jullière’s model, can be understood in the framework of ab initio calculations.
High-resolution detection of resonant frequencies of microwave resonators via magnetic measurements
Authors: Amigó R. , Hernandez J. M. , Garcı́a-Santiago A. , Tejada J.
Published: June 2003

An experiment with high spin molecular magnets inside microwave resonators has produced the remarkable observation of sharp peaks in the equilibrium magnetization and the ac susceptibility for the resonant frequencies of these devices. These peaks appear when the magnets are pumped or warmed by microwaves to states of different magnetic moment, and allow one to carry out spectroscopic studies of either the magnetic materials or the resonant device.
Early manifestation of localization effects in diluted Ga(AsN)
Authors: Masia F. , Polimeni A. , Baldassarri Höger von Högersthal G. , Bissiri M. , Capizzi M. , Klar P. J. , Stolz W.
Published: June 2003

The electron effective mass, me, and extent of exciton wave function, rexc, were derived in GaAs1−yNy (y=0.043%–0.5%) from magnetophotoluminescence measurements. With an increase in nitrogen concentration, we find that me and rexc undergo a rapid increase and squeezing, respectively, even for y≈0.1%. This quite early manifestation of nitrogen-induced localization effects imposes important constraints on existing theoretical models.
Fano resonances in open quantum dots and their application as spin filters
Authors: Song J. F. , Ochiai Y. , Bird J. P.
Published: June 2003

We describe how a spin filter may be realized in open quantum-dot systems, by exploiting the Fano resonances that occur in their transmission characteristics. In quantum dots in which the spin degeneracy of carriers is lifted, we show that the Fano resonances may be used as an effective means to generate spin polarization of transmitted carriers, and that electrical detection of the resulting polarization should also be possible.
Strong ultraviolet emission from SiO2/LiNbO3(:Fe)/SiO2 structures
Authors: Yang X. , Wu X. L. , Xue W. W. , Huang G. S. , Siu G. G. , Dong Z. G. , Fang L. , Shen M. R.
Published: June 2003

SiO 2 / LiNbO 3 ( LN)/SiO 2 sandwich structures were fabricated for exploring efficient light emission. After annealing at 1000 °C in O2 for 30 min, this kind of sandwich structure shows a strong ultraviolet photoluminescence (PL) with an asymmetric spectral shape. This PL spectrum may be Gaussian divided into two bands peaked at 310 (α-band) and 346 nm (β-band). If the layer of LN film is replaced by an Fe-doped LN (LN:Fe) one, the β-band vanishes and the α-band redshifts. The α-band is greatly enhanced and simultaneously becomes asymmetrical after this kind of SiO2/LN:Fe/SiO2 structure is annealed for 60 min. Spectral analysis suggests that the α-band arises from an optical transition in positively charged E′ centers at the interfaces between the LN(:Fe) film and the two SiO2 layers, while the β-band arises from intrinsic defects in the LN(:Fe) films. The mechanism for the PL enhancement is discussed in terms of a photorefractive effect in the LN(:Fe) films.
Structural and magnetic properties of triode-sputtered epitaxial γ′-Fe4N films deposited on SrTiO3 (001) substrates
Authors: Nikolaev K. R. , Krivorotov I. N. , Dahlberg E. D. , Vas’ko V. A. , Urazhdin S. , Loloee R. , Pratt W. P.
Published: June 2003

Highly ordered, single-crystal Fe4N films were prepared on single-crystal SrTiO3 (001) substrates by reactive magnetically enhanced dc triode sputtering. Analytical techniques, including x-ray diffraction, reflectivity, ion channeling, and atomic force microscopy, were used to characterize the structure and morphology of the films. The magnetic properties of the films were measured by magnetic force microscopy and superconducting quantum interference device magnetometry. A technique that utilizes anisotropic magnetoresistance was employed for measurement of the magnetocrystalline anisotropy of the Fe4N films.
Note On Darcy’s Law
Authors: Eringen A. Cemal
Published: June 2003

Classical Darcy’s Law expresses pressure gradient in a porous media as a linear function of the velocity difference between solid and fluid constituents. As such, this constitutive equation is frame dependent. The present article is an attempt to express this law in terms of frame independent quantities. The resulting expression shows that classical Darcy’s Law may be justified on an approximate basis when the higher order quantities are neglected.
Model of hole-initiated impact-ionization rate in α-quartz for a full band Monte Carlo simulation
Authors: Kunikiyo Tatsuya , Ishikawa Kiyoshi , Inoue Yasuo , Kamakura Yoshinari , Taniguchi Kenji , Kotani Norihiko
Published: June 2003

The hole-initiated impact-ionization rate in α-quartz was investigated using both an energy band structure and the corresponding pseudowave functions, which are derived from a self-consistent nonlocal pseudopotential method. The ionization probability is calculated formally from Fermi’s golden rule in a straightforward way. The matrix elements for the ionization transition were evaluated from the band structure and the corresponding pseudowave functions. The matrix elements include both direct and the exchange terms with umklapp terms associated with the periodic part of the Bloch function. The hole-initiated impact-ionization rate is discussed with emphasis on both the anisotropy (wave-vector dependence of the primary hole) associated with the band structure and the contribution of the umklapp process to the impact-ionization rate. In contrast to the Keldysh formula, which has a power exponent of 2, the computed impact-ionization rates are fitted to an analytical formula that has a power exponent of 6.7, which originates from the complexity of the α-quartz band structure. We have found that primary holes that exist in the eighth and ninth valence bands labeled from the lowest valence band in ascending order especially contribute to the subthreshold region of ionization. In addition, we show the average energies of the secondary holes and electrons generated at the moment of transition as a function of the initial hole energy.
Highly reflective distributed Bragg reflectors using a deeply etched semiconductor/air grating for InGaN/GaN laser diodes
Authors: Saitoh Tadashi , Kumagai Masami , Wang Hailong , Tawara Takehiko , Nishida Toshio , Akasaka Testuya , Kobayashi Naoki
Published: June 2003

High reflectivity is achieved by deeply etched InGaN/GaN distributed Bragg reflector (DBR) mirrors with tilted sidewalls, which are appropriately designed by using the finite-difference time-domain method. The predicted optimal structure is different from the simple design consisting of a λ/(4n) semiconductor and λ/4 air. If the sidewall of the grating is tilted by 4°, the reflectivity of the DBR mirrors decreases to less than 40%. However, any degradation in the reflectivity of a perfectly vertical sidewall can be suppressed to just a few percent even with a sidewall tilt of 4°, if the DBR structure is properly designed. We fabricated InGaN/GaN multiple-quantum well lasers based on the optimal design. The devices operate as lasers with optical pumping at a lower threshold than devices without DBR mirrors. The DBR mirror reflectivity is characterized by the relation between the threshold pump intensity and the inverse of the cavity length, resulting in a high reflectivity of 62%.
Sound-resonance hydrogen sensor
Authors: Dong Shuxiang , Bai Feiming , Li JieFang , Viehland Dwight
Published: June 2003

A hydrogen sensor is reported in which a small piezoelectric-sound-resonance-cavity (PSRC) is used as the sensing element. Detection utilizes sound resonance and acoustic property differences between H2 and air as a sensing mechanism. Changes in H2 concentration result in a shift of the sound-resonance state of the PSRC. Preliminary experiments have demonstrated a sensitivity limit of 8 ppm, a fast response time ∼1.5 second, and detection capabilities over a broad concentration range 10−5<n<0.2.
Interface energy of Au7Si grown in the interfacial layer of truncated hexagonal dipyramidal Au nanoislands on polycrystalline-silicon
Authors: Wu J. S. , Chen Y. F. , Dhara S. , Wu C. T. , Chen K. H. , Chen L. C.
Published: June 2003

Growth of truncated hexagonal dipyramidal Au nanoislands is reported by annealing Au/amorphous-Si/crystalline-Si at 1030 °C in oxygen and nitrogen ambient. Metastable gold silicide phase of Au7Si is observed to form at the interface of Au and polycrystalline Si, which was transformed from amorphous Si. Oxygen, acting as an impurity, inhibits outdiffusion of Si at the interface and thus controls the growth of Si deficient metastable phase of Au7Si in a diffusion-limited growth process. Interface energies of Au7Si (0.623–0.672 N/m) and Au–Au7Si (1.438–1.471 N/m) are derived from the contact angle measurements of the island structure involving Au and Au7Si.
Strain-induced charge depletion in La2/3Ca1/3MnO3 epitaxial thin films
Authors: Valencia S. , Balcells Ll , Fontcuberta J. , Martı́nez B.
Published: June 2003

Epitaxial thin films of La2/3Ca1/3MnO3 (LCMO), 50 nm thick, have been grown fully strained, on SrTiO3 (001) substrates. Detailed analyses of the structural and magnetotransport properties, after high-temperature annealing processes, have revealed a progressive increase of the out-of-plane cell parameter c, as the annealing temperature rises but no changes in the in-plane cell parameters are observed. Simultaneously, the magnetic transition temperature, TC, and saturation magnetization, MS, substantially increase. We argue that these results can be described assuming that elastic energy in fully strained films could be minimized by introducing oxygen deficiencies in the as-grown films. This observation provides a simple understanding of the commonly observed depressed magnetotransport properties of LCMO grown on nonmatching substrates.
High-fluence Si-implanted diamond: Formation of SiC nanocrystals and sheet resistance
Authors: Weishart H. , Heera V. , Eichhorn F. , Pécz B. , Barna Á. , Skorupa W.
Published: June 2003

The sheet resistance and structural properties of high-fluence Si-implanted diamond were investigated. In order to minimize the radiation damage and to facilitate SiC formation the implantation was performed at 900 °C. All samples were subsequently annealed in a rf-heated furnace at 1500 °C for 10 min in order to remove defects and thermally unstable phases. X-ray diffraction, infrared absorption spectrometry, and high-resolution cross-sectional transmission electron microscopy revealed the formation of a buried layer inside the implanted diamond, which contains SiC nanocrystallites. These SiC nanocrystals have a cubic structure and are nearly perfectly aligned with the diamond lattice. Raman spectroscopy was applied to analyze radiation-damage-induced graphitization in dependence on the implantation conditions. The sheet resistance of the samples was measured as function of temperature by four point probe technique in van-der-Pauw geometry. The decrease of the sheet resistance with increasing ion fluence unambiguously shows the influence of implantation-induced damage. The behavior of the sheet resistance can strongly be modified by additional nitrogen implantation. The resulting higher conductivity is interpreted as partial incorporation of the nitrogen donor into the SiC nanocrystals. However, when the Si fluence exceeds a critical value of 5.3×1017 Si+ cm−2 at 900 °C the diamond is irreversibly damaged and defect related conductivity dominates.
Physical explanation of the barrier height temperature dependence in metal-oxide-semiconductor leakage current models
Authors: Schroeder Dietmar , Avellán Alejandro
Published: June 2003

A temperature dependence of the barrier height between silicon and oxide has been proposed by many authors in order to reflect experimental metal-oxide-semiconductor leakage current results. However, no satisfactory physical explanation of this dependence has yet been given. In this letter, the temperature dependence of the observed macroscopic barrier height is explained by thermal fluctuations of the microscopic local barrier height. Because of the exponential relationship between current and barrier height, the decrease of the barrier during the fluctuation has a dominating effect when compared to the increase, leading on the average to a raised leakage current and correspondingly to a lowered average barrier height.
Residual stress analysis in ferroelectric Pb(Zr0.52Ti0.48)O3 thin films fabricated by a sol-gel process
Authors: Yao Kui , Yu Shuhui , Eng-Hock Tay Francis
Published: June 2003

Ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) films were prepared by a modified sol-gel process, in which polyethylene glycol (PEG) was added to the precursor solutions. An x-ray diffraction (XRD) method that detected the d(123) variation associated with the change in the incline angle ψ of the plane (123) was developed to quantify the residual stress in the PZT films. The stress could not be determined from the wafer curvature due to the effects of both the underlying layers and interdiffusion among layers. Using our XRD method, it was found that the residual stress in the resulting PZT films was significantly reduced when PEG was added to the precursor solutions. Moreover, we observed a further reduction of residual stress with increasing molecular weight of the PEG additive. These results could explain why a thicker, crack-free film can be achieved by adding polymers to the sol-gel precursor solutions.
AlGaN/GaN/AlGaN double heterostructure for high-power III-N field-effect transistors
Authors: Chen C. Q. , Zhang J. P. , Adivarahan V. , Koudymov A. , Fatima H. , Simin G. , Yang J. , Asif Khan M.
Published: June 2003

We propose and demonstrate an AlGaN/GaN/AlGaN double heterostructure (DH) with significantly improved two-dimensional (2D) confinement for high-power III-N heterostructure field-effect transistors (HFETs). The DH was grown directly on an AlN buffer over i-SiC substrate. It enables an excellent confinement of the 2D gas and also does not suffer from the parasitic channel formation as experienced in past designs grown over GaN buffer layers. Elimination of the GaN buffer modifies the strain distribution in the DH, enabling Al contents in the barrier region well over 30%. For the AlGaN/GaN/AlGaN DH design, the 2D electron gas mobility achieved was 1150 cm2/V s at room temperature and 3400 cm2/V s at 77 K, whereas the temperature independent sheet carrier density was NS≈1.1×1013 cm−2. Compared to a regular AlGaN/GaN structure, the channel mobility-concentration profiling shows significant improvement in the carrier confinement. Sample DHFETs with 1-μm long gates demonstrate the threshold voltage of 3.5 V, with a peak saturation current of 0.6–0.8 A/mm.
A Fair Test of the Effect of a Shadow-Incompatible Luminance Gradient on the Simultaneous Lightness Contrast (followed by Discussion)
Authors: Logvinenko Alexander D , Bressan Paola , Logvinenko Alexander D
Published: June 2003

Shadow-compatibility of simultaneous lightness contrast is discussed by Alexander D Logvinenko and Paola Bressan, with examples claiming to provide a test of the hypothesis.
Measuring and Modeling Real-Time Responses to Music: The Dynamics of Tonality Induction
Authors: Toiviainen Petri , Krumhansl Carol L
Published: June 2003

We examined a variety of real-time responses evoked by a single piece of music, the organ Duetto BWV 805 by J S Bach. The primary data came from a concurrent probe-tone method in which the probe-tone is sounded continuously with the music. Listeners judged how well the probe tone fit with the music at each point in time. The process was repeated for all probe tones of the chromatic scale. A self-organizing map (SOM) [Kohonen 1997 Self-organizing Maps (Berlin: Springer)] was used to represent the developing and changing sense of key reflected in these judgments. The SOM was trained on the probe-tone profiles for 24 major and minor keys (Krumhansl and Kessler 1982 Psychological Review89 334–368). Projecting the concurrent probe-tone data onto the map showed changes both in the perceived keys and in their strengths. Two dynamic models of tonality induction were tested. Model 1 is based on pitch class distributions. Model 2 is based on the tone-transition distributions; it tested the idea that the order of tones might provide additional information about tonality. Both models contained dynamic components for characterizing pitch strength and creating pitch memory representations. Both models produced results closely matching those of the concurrent probe-tone data. Finally realtime judgments of tension were measured. Tension correlated with distance away from the predominant key in the direction of keys built on the dominant and supertonic tones, and also correlated with dissonance.
Distance Perception for Points at Equiconvergence and Equidistance Loci
Authors: Ebenholtz Sheldon M , Ebenholtz Jean M
Published: June 2003

Subjects viewed luminescent targets placed at an equidistance (ED) locus or on a comparable equiconvergence locus, the Vieth–Müller circle (VMC). In darkness, subjects moved the index finger to the apparent distance of the visible target. This was repeated at leftward eccentricities of 0° (straight ahead), 15°, 22.5°, 30°, 37.5°, and 45°. Contrary to theoretical expectations, apparent distance to points on the VMC did not describe an equidistance locus, but diminished significantly with increasing eccentricity, whereas ED settings increased slightly but significantly. Binocular and monocular groups performed equivalently. Veridicality of results appears to be the outcome of an accommodation–convergence conflict with a dominant role for accommodation.
Timing Accuracy in Motion Extrapolation: Reverse Effects of Target Size and Visible Extent of Motion at Low and High Speeds
Authors: Sokolov Alexander , Pavlova Marina
Published: June 2003

By varying target size, speed, and extent of visible motion we examined the timing accuracy in motion extrapolation. Small or large targets (0.2 or 0.8 deg) moved at either 2.5, 5, or 10 deg s−1 across a horizontal path (2.5 or 10 deg) and then vanished behind an occluder. Observers responded when they judged that the target had reached a randomly specified position between 0 and 12 deg. With higher speeds, the timing accuracy (the reverse of absolute error) was better for small than for large targets, and for long than for short visible extents. With low speed, these effects were reversed. In addition, while long visible extents yielded a greater accuracy at high than at low speeds, for short extents the accuracy was much better with the low speed. The findings suggest that, when extrapolating motion with targets and visible extents of different sizes, the visual system implements different scaling algorithms depending on target speed. At higher speeds, processing of visible and occluded motion is likely to share a common scaling mechanism based on velocity transposition. Reverse effects for target size and extent of visible motion at low and high speeds converge with the assumption of two distinct speed-tuned motion-processing mechanisms in human vision.
Do Intersections Serve as Basic Features in Visual Search?
Authors: Wolfe Jeremy M , DiMase Jennifer S
Published: June 2003

The status of ‘intersection’ as a basic feature in visual search tasks has been controversial. Under some circumstances, a target possessing this attribute (eg a plus) ‘pops out’ of a display of distractors that lack the attribute (eg Ls). However, those cases may be artifacts of other features such as relative size or number of line terminators. We report two sets of experiments with stimuli intended to control for these factors. Search for the presence or absence of intersections is very inefficient with these stimuli. The results suggest that intersection should not be included among the list of salient features that support efficient search through visual displays.
Visual Context Integration is Not Fully Developed in 4-Year-Old Children
Authors: Káldy Zsuzsa , Kovács Ilona
Published: June 2003

Long-range horizontal interactions supporting contour integration were found to be weaker in children than in adults (Kovács et al, 1999 Proceedings of the National Academy of Sciences of the USA96 12204–12209). In the present study, integration on a larger scale, between a target and its context was investigated. Contextual modulation of the percept of a local target can be directly measured in the case of geometric illusions. We compared the magnitude of a size contrast illusion (Ebbinghaus illusion or Titchener circles) in children and adults. 4-year-old children and adults performed 2AFC size comparisons between two target disks in the classical Ebbinghaus illusion display and in two other modified versions. We found that the magnitude of the illusion effect was significantly smaller in children than in adults. Our interpretation is that context integration is not fully developed in 4-year-old children. Closer-to-veridical-size estimations by children demonstrate that the perception of the local target is less affected by stimulus context in their case. We suggest that immature cortical connectivity is behind the reduced contextual sensitivity in children.
Invalid Cues Impair Auditory Motion Sensitivity
Authors: Stanley Raymond M , Matthews Nestor
Published: June 2003

Compelling lateral motion can be experienced when intensity differences between the two ears change over time. Whether our sensitivity to this dynamic interaural stimulation could be influenced by directional cues was the focus of the present study. On each trial, amplitude-modulated pure tones were presented either diotically (no-motion condition) or dichotically (motion condition), and participants indicated whether lateral motion was present or absent. Randomly across trials, the stimuli were preceded by a valid directional cue, an invalid directional cue, or no cue, while the motion to be detected was identical across these cue conditions. The data indicate that motion sensitivity was comparable in the valid-cue and no-cue conditions. Relative to each of those conditions, however, motion sensitivity was significantly lower in the invalid-cue condition, and motion was reported significantly less often. The results provide evidence that our sensitivity to dynamic interaural intensity differences can be significantly affected by a non-sensory factor, namely cue validity.
Is Color an Intrinsic Property of Object Representation?
Authors: Naor-Raz Galit , Tarr Michael J , Kersten Daniel
Published: June 2003

The role of color in object representation was examined by using a variation of the Stroop paradigm in which observers named the displayed colors of objects or words. In experiment 1, colors of color-diagnostic objects were manipulated to be either typical or atypical of the object (eg a yellow banana versus a purple banana). A Stroop-like effect was obtained, with faster color-naming times for the typical as compared to the atypical condition. In experiment 2, naming colors on words specifying these same color-diagnostic objects reversed this pattern, with the typical condition producing longer response times than the atypical condition. In experiment 3, a blocked condition design that used the same words and colors as experiment 2 produced the standard Stroop-like facilitation for the typical condition. These results indicate that color is an intrinsic property of an object's representation at multiple levels. In experiment 4, we examined the specific level(s) at which color–shape associations arise by following the tasks used in experiments 1 and 2 with a lexical-decision task in which some items were conceptually related to items shown during color naming (eg banana/monkey). Priming for these associates was observed following color naming of words, but not pictures, providing further evidence that the color–shape associations responsible for the differing effects obtained in experiments 1 and 2 are due to the automatic activation of color–shape associations at different levels of representation.