bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanocrystalline silicon electron emitter with a high efficiency enhanced by a planarization technique
Authors: Nishiguchi K. , Zhao X. , Oda S.
Published: September 2002

A cold electron emitter has been fabricated based on nanocrystalline silicon (nc-Si) quantum dots formed in the gas phase by very-high-frequency plasma decomposition of SiH4. A small size of less than 10 nm and the spherical shape of the nc-Si dots facilitated the generation of hot electrons. Electrons with kinetic energies higher than the work function of the top electrode were extracted into vacuum through the electrode. A planarization process of the nc-Si layer by annealing enhanced the electron emission efficiency to 5%. Efficiency was optimized by varying the thicknesses of the nc-Si layer, the SiO2 layer, and the top electrode film.
Intrinsic nanostructural domains: Possible origin of weaklinkless superconductivity in the quenched reaction product of Mg and amorphous B
Authors: Li S. , Prabhakar O. , Tan T. T. , Sun C. Q. , Wang X. L. , Soltanian S. , Horvat J. , Dou S. X.
Published: September 2002

Smooth modulation structure of Mg–B alloy in the quenched reaction product of Mg and amorphous B was studied. It indicates that the MgB2 formed possibly in spinodal decomposition, thus resulting in MgB2 nanodomains. It was found that the nanodomains with small angle boundaries of atomic-scale width were distributed within the subgrains that constitute the clusters in MgB2 grains. This nanostructural characteristic may be intrinsic in the quenched reaction product of Mg and amorphous B. It makes the nanodomain boundaries not act as barriers to the current percolation path, thus exhibiting no weak-link problem in the MgB2.
Europium complex as a highly efficient red emitter in electroluminescent devices
Authors: Sun Pei-Pei , Duan Jiun-Pey , Shih Huai-Ting , Cheng Chien-Hong
Published: September 2002

Several devices using a europium complex Eu(TTA)3(DPPz)(TTA=2-thenoyltrifluoroacetonate, DPPz=dipyrido[3,2-a:2′,3′-c]phenazine) as dopant emitter were fabricated. The performances of these devices are among the best reported for devices incorporating a europium complex as a red emitter. One such device with structure TPD (50 nm)/Eu:CBP (4.5%, 30 nm)/BCP (30 nm)/Alq (25 nm) exhibits an external quantum efficiency 2.1%, current efficiency 4.4 cd/A, power efficiency 2.1 lm/W, and brightness 1670 cd/m2.
An atomic model of the nitrous-oxide-nitrided SiO2/Si interface
Authors: Kushida-Abdelghafar Keiko , Watanabe Kikuo , Kikawa Takeshi , Kamigaki Yoshiaki , Ushio Jiro
Published: September 2002

The interfacial structure of nitrous-oxide- (NO-)nitrided SiO2/Si is determined on the basis of the configuration of the Pb centers and the results of physical analysis. We used electron spin-resonance analysis to observe a decrease in the number of Pb centers after NO annealing, which corresponds to the decrease in the density of interface traps. The nitrogen bonds at the interface were analyzed by x-ray photoelectron spectroscopy. An asymmetric N 1s peak at around 398 eV was detected; the peak may be decomposed into two peaks with a binding-energy difference of 0.6 eV. This core-level shift originates in the difference between the numbers of oxygen atoms that are second-nearest neighbors of the nitrogen which terminates the Pb0 and Pb1 centers.
Short Bragg mirrors with adiabatic modal conversion
Authors: Peyrade D. , Silberstein E. , Lalanne Ph. , Talneau A. , Chen Y.
Published: September 2002

A silicon-on-insulator waveguide-based microcavity with short tapers has been realized by incorporating two identical eight-groove tapers at the two reflector extremities. The microstructure has an overall length of 14 μm and consists of two first-order Bragg mirrors with narrow slits (90 nm) and of four series of slits with progressively varying widths (tapers). A comparison with a nontapered cavity evidences the beneficial effect of the taper, a lowering of the radiation losses.
Ultrahigh room-temperature hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures
Authors: Irisawa T. , Tokumitsu S. , Hattori T. , Nakagawa K. , Koh S. , Shiraki Y.
Published: September 2002

We have obtained ultrahigh room-temperature (RT) hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures with very small parallel conduction. Reducing parallel conduction was achieved by employing Sb doping in Si0.3Ge0.7 buffer layers, which drastically increased RT hole Hall mobility up to 2100 cm2/V s in the strained Ge channel modulation-doped structures and improved device characteristics of the p-type metal–oxide–semiconductor field-effect transistors with the strained Ge channel. The peak effective mobility reached to 2700 cm2/V s at RT, which was much higher than the bulk Ge drift mobility.
A physical explanation of the ground state crossover in the H⊗h Jahn–Teller effect
Authors: Polinger Victor Z. , Huang Ruiwang , Dunn Janette L. , Bates Colin A.
Published: September 2002

By comparison with other systems, vibronic coupling between an h-type vibration and an H-type electronic level in a H⊗h Jahn–Teller system would be expected to result in a vibronic H-type ground state. However, it is already known that an A state can cross over the H state at a given coupling strength and become the ground state if the ground adiabatic potential energy surface contains minima of D3d symmetry. This is an unusual property of the H⊗h Jahn–Teller system. In this article, the physics behind the crossover is analyzed in terms of competition between tunneling paths between D3d wells of C1 and C2 symmetries. The H⊗h Jahn–Teller system is relevant to some fullerenes and other icosahedral complexes.
Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions
Authors: Popova E. , Faure-Vincent J. , Tiusan C. , Bellouard C. , Fischer H. , Hehn M. , Montaigne F. , Alnot M. , Andrieu S. , Schuhl A. , Snoeck E. , da Costa V.
Published: September 2002

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.
Tracer diffusion in salol in the supercooled state
Authors: Wang C. H. , Gong S. S.
Published: September 2002

The tracer diffusion coefficient of camphorquinone in salol has been measured by forced Rayleigh scattering technique at temperatures from 266 to 220 K. Over this temperature range, the shear viscosity of salol changes by nearly nine orders of magnitude. The diffusion coefficient is found to be much faster than the value predicted by the Stokes–Einstein equation. The enhancement of the diffusion coefficient is interpreted as due to the rotation–translation coupling mechanism, a mechanism commonly invoked to account for the intensity of the shear wave doublet in the depolarized Rayleigh scattering spectrum of a rotationally anisotropic molecular liquid in the supercooled state. The rotation–translation coupling parameter extracted from the translation diffusion measurement is compared with that obtained from the depolarized Rayleigh scattering spectrum of salol. Qualitative agreement between the two types of experiments is obtained.
Different role of filled and empty surface states in a polyfunctional molecule adsorption: Geranyl acetone on Si(111)7×7
Authors: Carbone M. , Comtet G. , Dujardin G. , Hellner L. , Mayne A. J.
Published: September 2002

New perspectives in molecular electronics are opening up through controlled surface molecular synthesis. The first step of such a synthesis implies the adsorption of a polyfunctional molecule, which might use one functional group for the surface adsorption, and the other one(s) for further reactions. Here we present an adsorption study of Si(111)7×7 of geranyl-acetone (C13H22O, E-5,9 undecadien-one) characterized by a ketone and two unconjugated double bonds. The study has been performed by temperature and coverage dependent valence band photoemission and room temperature scanning tunneling microscopy. The use of these combined techniques allows us to infer that the interaction between the geranyl acetone and the silicon surface occurs selectively through the rest atom and the carbonyl group, most likely through the oxygen atom. The geranyl acetone does not undergo any fragmentation upon adsorption Si(111)7×7, as has been observed for smaller molecules on the same surface {acetaldehyde [Y. Bu, J. Breslin, M. C. Lin, J. Phys. Chem. B 101, 1872 (1997)] for instance}. The interaction of the chain with the surface is weak and is characterized in the STM images as a darkening of one adatom in positive bias, around the reacted rest atom.
Transformation of BxCyNz nanotubes to pure BN nanotubes
Authors: Han Wei-Qiang , Mickelson W. , Cumings John , Zettl A.
Published: September 2002

We demonstrate that multiwalled BxCyNz nanotubes can be efficiently converted to BN multiwalled nanotubes via an oxidation treatment. The microstructure and composition of the precursors and final products have been characterized by high-resolution transmission electron microscopy, electron energy-loss spectroscopy, and energy dispersive x-ray spectroscopy. The conversion process is monitored by thermogravimetric analysis. Carbon layers of BxCyNz nanotubes start to oxidize at 550 °C, thereby transforming BxCyNz nanotubes into pure BN nanotubes. The remarkable thermal stability of pure BN nanotubes in an oxidizing environment is also established.
Orbital electron densities of ethane: Comparison of electron momentum spectroscopy measurements with near Hartree–Fock limit and density functional theory calculations
Authors: Deng J. K. , Li G. Q. , Wang X. D. , Huang J. D. , Deng H. , Ning C. G. , Wang Y. , Zheng Y.
Published: September 2002

Electron density distributions in momentum space of the valence orbitals of ethane (C2H6) are measured by electron momentum spectroscopy (EMS) in a noncoplanar symmetric geometry. The impact energy was 1200 eV plus binding energy and energy resolution of the EMS spectrometer was 0.95 eV. The measured experimental momentum distributions of the valence orbitals are compared with Hartree–Fock and density functional theory (DFT) calculations. The shapes of the experimental momentum distributions are generally quite well described by both the Hartree–Fock and DFT calculations when large and diffuse basis sets are used. A strong “turn up” of the experimental cross section is observed for the HOMO 1eg orbital in the low momentum region, compared with the theoretical calculations. The pole strengths for the main ionization peaks in the inner-valence region are estimated.
Influence of grain size and structural changes on the electrical properties of nanocrystalline zinc ferrite
Authors: Ponpandian N. , Narayanasamy A.
Published: September 2002

The electrical conductivity and dielectric properties of nanocrystalline zinc ferrite with various grain sizes ranging from 7 to 115 nm have been studied over a wide frequency range 1 Hz–15 MHz by impedance measurements in the temperature range of 300–650 K. The effect of grain size on the conductivity has been investigated by nonlinear least-squares fitting of the impedance data. The conduction mechanism is found to be due to the hopping of both electrons and holes. The activation energies have been obtained from Arrhenius plots of the grain and the grain boundary conductions and their values are characteristic of the hopping of charge carriers. As the grain size decreases, the conductivity also decreases along with a small increase in the activation energy. The real part ε′ of the dielectric constant exhibits anomalous behavior, except for the 115 nm grain size sample, which has been accounted for based on the presence of both types of charge carriers. The dielectric loss factor also decreases as the grain size decreases and thus the relaxation frequency shifts toward lower values. The electrical modulus is fitted to a stretched exponential function and the results clearly indicate the presence of the non-Debye type of dielectric relaxation in these materials.
Synthesized rare-earth doped oxide glasses for nonlinear optics
Authors: Kityk I. V. , Wasylak J. , Benet S. , Dorosz D. , Kucharski J. , Krasowski J. , Sahraoui B.
Published: September 2002

Photostimulated nonlinear optical effects in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Yb3+, Er3+, and Dy3+ rare-earth (RE3+) ions were discovered. Temperature-dependent measurements of optical photoinduced second-harmonic generation (PISHG) and two-photon absorption were performed in the infrared (IR) range. CO pulse laser (λ=5.5 μm, energy power density up to 3.8 GW/cm2 per pulse) was applied as a source of IR-photoinducing and probing (fundamental) light. Absolute values of the PISHG were more than 22% higher than corresponding values obtained for other glasses: As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2] type. The investigated PGBC system possesses a shorter time response (about 18 ps), compared with other IR nonlinear optical glasses. We have also established that all nonlinear optical susceptibilities are dependent on the type of RE3+ ion. A maximal value of the PISHG is achieved for the glasses doped by Yb3+. The PISHG values increase significantly below 25 K. We have carried out ab initio molecular dynamics and quantum chemical simulations in order to evaluate the possible contribution of electron–phonon anharmonic interactions in the observed phenomena. We have modeled the influence of the external CO photoinduced beam through the photoinduced anharmonic electron-phonon interactions. A decrease of the delaying time response is achieved. To obtain independent confirmation of the observed dependencies, we have carried out measurements of the ε2(E) during the external IR pumping. We have also compared the measured and theoretically calculated dependencies of the IR-induced effects.
Predicted electronic properties of polycrystalline silicon from three-dimensional device modeling combined with defect-pool model
Authors: Altermatt Pietro P. , Heiser Gernot
Published: September 2002

We assess a broad range of published experiments to show that the density of states (DOS) at high-energy grain boundaries in silicon is appropriately described by the defect-pool model. This implies that the DOS of such grain boundaries depends strongly on the dopant density and on the position of the Fermi level during device processing. However, since high-energy grain boundaries consist of an amorphous layer that is confined to a width of a few angstroms, the DOS is “frozen in” after material processing and does not suffer the strong degradation effects commonly observed in bulk a-Si:H. By combining three-dimensional device modeling and the defect-pool model, we reproduce various test structures and polycrystalline thin-film Si solar cells considerably more precisely than in the past. Our simulation model potentially provides a link between processing conditions and grain boundary quality.
Double-layer formation in organic light-emitting electrochemical cells
Authors: Ouisse T. , Stéphan O. , Armand M. , Leprêtre J. C.
Published: September 2002

We present a systematic analysis of the current transients accompanying the formation of the electrode–electrolyte double layers in organic light-emitting electrochemical cells. By using various room-temperature molten salts, conducting polymers, and electrodes, we show that the current I always decreases as a power law of time, I∝t−n. The current transients are formed of various time domains, each one being characterized by a power-law exponent n<1. Impedance measurements conducted from 5 Hz to 5 MHz demonstrate that these transients represent the time response of a simple combination of constant phase angle (CPA) impedances, Zn∝(jω)n, and of the electrolyte ionic conductivity. The physical origin of the CPA impedance is attributed to the roughness of the interface between the electrodes and the electrolyte, and to the phase separation within the salt–polymer blend.
High-temperature operation of a third-order mode optically pumped semiconductor laser
Authors: Semaltianos N. G. , De Rossi A. , Vinter B. , Berger V. , Ortiz V.
Published: September 2002

Semiconductor lasers with third-order waveguide mode emission are a promising route toward compact twin-photon sources. In these structures, emission on the third-order mode is required for satisfaction of the phase-matching condition between the pumping frequency and fundamental modes at half frequency and so the production of twin photons. Phase matching depends critically on sample temperature through the dependence of the effective refractive indices of the sample. The dependence of laser mode emission on the temperature of a semiconductor structure specially designed for third-order mode emission at 775 nm is studied. It is shown that the third-order mode emission is preserved up to 40 °C whereas a contribution from other modes becomes significant above that temperature.
Power gain in a quantum-dot cellular automata latch
Authors: Kummamuru Ravi K. , Timler John , Toth Geza , Lent Craig S. , Ramasubramaniam Rajagopal , Orlov Alexei O. , Bernstein Gary H. , Snider Gregory L.
Published: September 2002

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.
Propagation of water waves through finite periodic arrays of vertical cylinders
Authors: Ha Young-Ki , Kim J.-E. , Park H. Y. , Lee In-Won
Published: September 2002

We study the transmission of water waves propagating in finite-size two-dimensional periodic structures which consist of bottom-mounted cylinders using the multiple-scattering method. Complete band gaps exist between the first and the second bands in square and triangular periodic structures, as well as one modeled on a graphite atomic lattice. We investigate the dependence of the band edges on the filling fraction. The graphite-type structure shows the band gap at a lower filling fraction than the others. Therefore, the graphite-type structure may be more suitable for practical coastal protection. We also calculate the first-order force on cylinders located along the symmetry direction.
Synchrotron-radiation-induced wet etching of germanium
Authors: Ma Qing , Moldovan Nicolaie , Mancini Derrick C. , Rosenberg Richard A.
Published: September 2002

A room-temperature photoinduced chemical wet etching process for Ge wafers using x rays from a synchrotron radiation source is described. De-ionized water and H2O:HNO3 solutions were used as etchants. This process produces smoothly etched Ge surfaces with a root-mean-square surface roughness of a few angstroms. Etching chemistry and etching conditions, such as x-ray beam sizes, solution concentration, and doping type, are discussed.
Optical absorption and photoluminescence characteristics of Cd2GeSe4 single crystals
Authors: Lee Jeoung Ju , Park Young Shin , Yang Chang Soo , Kim Kun Ho , Jin Moon Seog , Kim Wha Tek
Published: September 2002

Cd 2 GeSe 4 single crystals were grown by using a modified Bridgmann method. The optical energy band gap obtained from the optical absorption measurement was about 1.922±0.002 eV at 10 K and 1.707±0.002 eV at 298 K. In the photoluminescence (PL) spectra measured at temperatures below 70 K, two peaks were distinguished. One corresponds to the ground state (n=1) and the other corresponds to the excited state (n=2) of free exciton. The binding energy of the exciton was about 5.2 meV. The energy band gap obtained from the PL measurements was 1.9236 eV at 10 K.
Optical switching properties from isotherms of Gd and GdMg hydride mirrors
Authors: Di Vece M. , Zevenhuizen S. J. M. , Kelly J. J.
Published: September 2002

Concentration–pressure isotherms were measured together with the optical transmission in polycrystalline Gd and GdMg thin-film switchable mirrors. Formation plateaus in GdMg alloys, corresponding to gadolinium dihydride and trihydride as well as magnesium dihydride have been found. From these, the formation enthalpies could be calculated. These results show that the GdMg alloys are phase separated. From the onset of the transmission we conclude that the formation of gadolinium trihydride is the final stage in the switching process.
Investigation of hydrogen-induced degradation in Pb(ZrxTi1−x)O3 thin film capacitors for the application of memory devices
Authors: Lee June Key , Park Youngsoo , Chung Ilsub
Published: September 2002

Hydrogen-induced degradation in Pb(ZrxTi1−x)O3 (PZT) thin film capacitors is investigated in terms of three process parameters, such as electrode structures (Pt/PZT/Pt and Ir/IrO2/PZT/Pt/IrO2), Zr/Ti compositional ratios (60/40, 53/47, 40/60, 30/70), and the domain poling states (±5 V). It was found that the hydrogen-induced degradation is enhanced when PZT films have high Ti portion, and can be suppressed by domain poling prior to the hydrogen anneal. From secondary ion mass spectroscopy analysis, it can be concluded that the hydrogen-induced degradation originates mainly from the interface of PZT and Pt electrodes. Hysteresis loop shifts in PZT capacitors indicate that negative charge develops at interfaces by the hydrogen anneal. Chemically, hydrogen-induced degradation can be elucidated as the formation of negatively charged hydroxyl group (OH−).
Correlation between grain size and optical properties in zinc oxide thin films
Authors: Matsumoto Takahiro , Kato Hiroyuki , Miyamoto Kazuhiro , Sano Michihiro , Zhukov Evgeniy A. , Yao Takafumi
Published: September 2002

Photoluminescence (PL) and time-resolved PL spectra of zinc oxide (ZnO) films were investigated as a function of the grain size of the microcrystals. Correlation was found between the grain size and the optical properties—in the bound exciton states, both the PL intensity and PL decay time increased with increasing the grain size. This correlation can be well explained by the existence of nonradiative-surface and/or -interface states in the grain boundaries of ZnO microcrystals.
Spectral measurements of gyrotron oscillator with ferroelectric electron gun
Authors: Einat M. , Jerby E. , Rosenman G.
Published: September 2002

Since the discovery of the ferroelectric electron-emission effect, its implementation in microwave tubes has been impeded by various reasons and in particular by its relatively wide energy spread. Recently, a 1.5 kW microwave output from a gyrotron based on repetitive ferroelectric electron gun has been reported. This letter presents measurements of the spectral variations of the gyrotron output, and relates them to the electron-gun energy spread and to other inherent line-widening causes, such as the pulse length. The result shows that the contribution of the electron energy spread to the spectral content is not significantly larger than the other causes of line broadening.
Structure transformations and superhardness effects in V/Ti nanostructured multilayers
Authors: Xu Junhua , Kamiko Masao , Zhou Yaomin , Lu Guanghong , Yamamoto Ryoichi , Yu Lihua , Kojima Isao
Published: September 2002

An anomalous enhancement of hardness has been found in the V/Ti nanostructured multilayer films, in which the constituent materials have the same elastic modulus. The high-resolution transmission electron microscopy results showed that the V layers take bcc structure at all modulation periods from 2.8 to 150.0 nm. The Ti layers are bcc structured at modulation periods less than 3.0 nm. The Ti layers are hcp structured at modulation periods of more than 5.9 nm. There is a Burgers orientation relation between the bcc V and the hcp Ti. Coherent stress hardening between the interfaces of bcc V and hcp Ti layers and the different crystal structures appear to be one likely reason for the hardness enhancement in V/Ti nanostructured multilayers.
Feasibility of a semiconductor superlattice oscillator based on quenched domains for the generation of submillimeter waves
Authors: Scheuerer R. , Schomburg E. , Renk K. F. , Wacker A. , Schöll E.
Published: September 2002

We discuss the feasibility of a semiconductor superlattice oscillator which exploits the quenching of propagating dipole domains for the generation of submillimeter waves. We studied the dynamics of electrons in a semiconductor superlattice by performing a simulation based on a drift-diffusion model, taking into account feedback from a resonant circuit. The simulation delivers propagating dipole domains which are quenched before they reach the anode. The periodic formation and quenching of domains creates a self-sustained oscillation of the current through the superlattice. The frequency of the oscillation can be more than three times higher than without feedback. We suggest that with already existing superlattices an oscillator working in the quenched domain mode can be realized up to almost 500 GHz.
Inductance-dependent characteristics of high-Tc dc-superconducting quantum interference device amplifiers
Authors: Mitchell E. E. , Tilbrook D. L. , Foley C. P. , MacFarlane J. C.
Published: September 2002

We have experimentally determined the inductance Lsq, transfer function VΦ, and flux noise SΦ of several dc-superconducting quantum interference devices (SQUIDs) of increasing loop size (without an input flux transformer) in open-loop mode. Our primary aim was to provide reliable inductance data to aid in the design of high temperature superconducting magnetometers. Flux was induced by current injection directly into the SQUID loop. The inductances were compared with theoretical values. Correlations between the inductance data and measured values of VΦ and SΦ were obtained. Guidelines for optimization of magnetometer SQUIDs were established; the importance of achieving large VΦ together with a relatively small Lsq was demonstrated, and an order-of-magnitude reduction in noise was achieved by subjecting one of the SQUIDs to an in-house “ion-beam trimming” process.
Thick LiNbO3 layers on diamond-coated silicon for surface acoustic wave filters
Authors: Dogheche E. , Sadaune V. , Lansiaux X. , Remiens D. , Gryba T.
Published: September 2002

Surface acoustic wave (SAW) technology based on lithium niobate/diamond/silicon multilayered structures has been investigated due to the large demand of frequency filters for communication systems, particularly for cellular phones. Thick LiNbO3 layers (∼1 μm) with a good morphology, i.e., surface roughness lower than 10 nm have been grown at 490 °C on diamond-coated silicon substrates by multistep rf magnetron sputtering. A SAW devices operating at 300 MHz was designed and fabricated in order to validate the proposed structure; in particular, our main objective was to verify the high velocity in the layered structure. Preliminary filter responses were very promising: the SAW velocity was estimated to 8200 m/s and the coupling factor was K2∼0.93%. The film growth, the technology, and the electrical measurements are described in this letter.
Mechanism of two-dimensional electron gas formation in AlxGa1−xN/GaN heterostructures
Authors: Jang Ho Won , Jeon Chang Min , Kim Ki Hong , Kim Jong Kyu , Bae Sung-Bum , Lee Jung-Hee , Choi Jae Wu , Lee Jong-Lam
Published: September 2002

Mechanism on the formation of two-dimensional electron gas (2DEG) at the interface of undoped AlxGa1−xN with undoped GaN was interpreted through surface band bending observed using synchrotron radiation photoemission spectroscopy. The surface Fermi level was independent of AlxGa1−xN thickness and Al content in AlxGa1−xN, showing Fermi pinning to surface states at 1.6 eV below conduction band minimum. Oxygen donor impurities in undoped AlxGa1−xN, unintentionally doped during the growth, led to the formation of large density of 2DEG (>∼1013/cm3) at the AlxGa1−xN/GaN interface via electron generation.
Measurement of the thermo-optic coefficients in sol-gel derived inorganic–organic hybrid material films
Authors: Kang Eun-Seok , Lee Tae-Ho , Bae Byeong-Soo
Published: September 2002

Thermo-optic coefficients (dn/dT) of inorganic–organic hybrid material films prepared by sol-gel process of organoalkylsilanes are measured using the prism coupler equipped with autocontrolled hot stage. In order to validate the reliability of this method, dn/dT of polymethylmethacrylate film is measured. dn/dT of inorganic–organic hybrid material films are negative and as high as the order of 10−4, which are comparable to those of optical polymers. Their dn/dT increase with increasing organic content in the film and mainly depend on their thermal expansion.
Lateral indium–indium pair correlations within the wetting layers of buried InAs/GaAs quantum dots
Authors: Shin B. , Lita B. , Goldman R. S. , Phillips J. D. , Bhattacharya P. K.
Published: September 2002

We have investigated lateral In–In pair correlations within the wetting layers of buried InAs/GaAs quantum dots imaged with cross-sectional scanning tunneling microscopy. We quantified the number of In–In pairs along the [110] direction as a function of the spacing between them. Since the number of In–In pairs exceeds that of a randomly generated distribution of In atoms, significant lateral In clustering within the wetting layers is apparent. A comparison of the experimentally determined and randomly generated In–In pair distributions reveals nearest-neighbor In–In pair interaction energies similar to those calculated for InGaAs alloy surfaces [J. -H. Cho, S. B. Zhang, and A. Zunger, Phys. Rev. Lett. 84, 3654 (2000)]. The In–In pair correlations increase with high temperature annealing, indicating that vertical In–Ga interdiffusion occurs simultaneously with lateral In segregation. Together, these results suggest that initial In clustering in the wetting layer may be frozen at the surface during growth, and that annealing increases the effective sizes of these clusters, driving the system toward its segregated equilibrium state.
Metal–insulator–semiconductor electrostatics of carbon nanotubes
Authors: Guo Jing , Goasguen Sebastien , Lundstrom Mark , Datta Supriyo
Published: September 2002

Carbon nanotube metal–insulator–semiconductor capacitors are examined theoretically. For the densely packed array of nanotubes on a planar insulator, the capacitance per tube is reduced due to the screening of the charge on the gate plane by the neighboring nanotubes. In contrast to the silicon metal–oxide–semiconductor capacitors, the calculated C–V curves reflect the local peaks of the one-dimensional density-of-states in the nanotube. This effect provides the possibility to use C–V measurements to diagnose the electronic structures of nanotubes. Results of the electrostatic calculations can also be applied to estimate the upper-limit on-current of carbon nanotube field-effect transistors.
Large ground-to-first-excited-state transition energy separation for InAs quantum dots emitting at 1.3 μm
Authors: Wei Y. Q. , Wang S. M. , Ferdos F. , Vukusic J. , Larsson A. , Zhao Q. X. , Sadeghi M.
Published: September 2002

By capping InAs quantum dots (QDs) with a thin intermediate layer of InAlAs instead of GaAs, the radiative transition wavelengths are redshifted. Surface morphology studies confirm that the redshift is due to a better preserved QD height as compared with capping by GaAs only. In contrast, the energy levels are blueshifted when using AlGaAs instead of GaAs as the barrier material. In both cases, the energy separation between the ground and the first-excited state increases significantly. Combining these approaches, we demonstrate InAs QDs with a record transition energy separation of 108 meV and ground-state emission at 1.3 μm.
Quantum-ballistic transport in an etch-defined Si/SiGe quantum point contact
Authors: Wieser U. , Kunze U. , Ismail K. , Chu J. O.
Published: September 2002

Ballistic constrictions are fabricated on a high-mobility Si/SiGe strained-layer heterostructure which exhibit conductance quantization in units of 4e2/h at T=4.2 K. Under finite drain voltage a half-plateau develops at 2e2/h and a series of oscillations appear which enable us to extract the energy separation ΔEN+1,N between successive one-dimensional subbands. The result is ΔE2,1=2.0 meV and ΔE3,2=1.4 meV.
Suppressing the transit-time instability in large-area electron-beam diodes
Authors: Friedman M. , Myers M. , Swanekamp S. B. , Chan Y. , Sethian J. D. , Obenschain S.
Published: September 2002

Large-area electron-beam diodes are drastically affected by the transit-time instability that introduces a large energy spread to the electrons and generates high-power rf radiation. The instability reduces the energy available for applications such as the excitation of KrF lasers. Simulations and experiments show that replacing the flat cathode with a slotted cathode reduces the growth of the instability. This results in an increase in beam energy transfer efficiency from the Nike diode to the KrF laser from ≈40% to ≈60%. It will be shown that a further increase in efficiency to 76% is possible.
Bioaccumulation of metals in the East Calcutta wetland ecosystem
Authors: Chattopadhyay Buddhadeb , Chatterjee A. , Mukhopadhyay S. K.
Published: September 2002

This study was initiated to examine the extent of toxic metal contamination of the east Calcutta wetland ecosystem. Biological oxygen demand, chemical oxygen demand, total dissolved solids, conductivity, total suspended solids, total hardness, and pH were recorded from six different locations on the sewage carrying canals including an industrial effluent-fed fish pond in the eastern Calcutta (lat. 22°33´–22°40´ N;long. 88°25´–88°35´E). Concentrations of metals such as Pb, Cu, Cr, Zn, Fe, Mg, and Mn in wastewater, and water of adjacent sewage-fed fish tank were also determined. Concentrations of these metals in soil/sludge and accumulation of these metals in plant and fish tissues were also studied. The concentrations of heavy metals in wastewater, namely Pb, Zn, Cu, Mn, and Fe, were reduced by 25 to 45% and total Cr was reduced by 95%, on the course of the 40 km long journey of the composite wastewater from the sources to river mouth via the wastewater carrying canal and the stabilization pond. Reductions of 65% Zn and 99% Cr in soil/sediment were recorded between sources and final discharge sites.
Novel PtdIns(3)P-binding protein Etf1 functions as an effector of the Vps34 PtdIns 3-kinase in autophagy
Authors: Wurmser Andrew E. , Emr Scott D.
Published: September 2002

Autophagy is the process whereby cytoplasmic cargo (e.g., protein and organelles) are sequestered within a double membrane–enclosed transport vesicle and degraded after vesicle fusion with the vacuole/lysosome. Current evidence suggests that the Vps34 phosphatidylinositol 3-kinase is essential for macroautophagy, a starvation-induced autophagy pathway (Kihara et al., 2001). Here, we characterize a requirement for Vps34 in constitutive autophagy by the cytoplasm-to-vacuole targeting (Cvt) pathway. First, we show that transient disruption of phosphatidylinositol (PtdIns) 3-phosphate (PtdIns[3]P) synthesis through inactivation of temperature-sensitive Vps34 or its upstream activator, Vps15, blocks the Cvt and macroautophagy pathways. Yet, PtdIns(3)P-binding FYVE domain-containing proteins, which mediate carboxypeptidase Y (CPY) transport to the vacuole by the CPY pathway, do not account for the requirement of Vps34 in autophagy. Using a genetic selection designed to isolate PtdIns(3)P-binding effectors of Vps34, we identify Etf1, an uncharacterized type II transmembrane protein. Although Etf1 does not contain a known 3-phosphoinositide–binding domain (i.e., FYVE or Phox), we find that Etf1 interacts with PtdIns(3)P and that this interaction requires a basic amino acid motif (KKPAKK) within the cytosolic region of the protein. Moreover, deletion of ETF1 or mutation of the KKPAKK motif results in strong sorting defects in the Cvt pathway but not in macroautophagy or in CPY sorting. We propose that Vps34 regulates the CPY, Cvt, and macroautophagy pathways through distinct sets of PtdIns(3)P-binding effectors and that Vps34 promotes protein trafficking in the Cvt pathway through activation/localization of the effector protein Etf1.
Distinct cell death pathways triggered by the adenovirus early region 4 ORF 4 protein
Authors: Robert Amélie , Miron Marie-Joëlle , Champagne Claudia , Gingras Marie-Claude , Branton Philip E. , Lavoie Josée N.
Published: September 2002

In transformed cells, induction of apoptosis by adenovirus type 2 (Ad2) early region 4 ORF 4 (E4orf4) correlates with accumulation of E4orf4 in the cell membrane–cytoskeleton fraction. However, E4orf4 is largely expressed in nuclear regions before the onset of apoptosis. To determine the relative contribution of nuclear E4orf4 versus membrane-associated E4orf4 to cell death signaling, we engineered green fluorescent fusion proteins to target E4orf4 to specific cell compartments. The targeting of Ad2 E4orf4 to cell membranes through a CAAX-box or a myristylation consensus signal sufficed to mimic the fast Src-dependent apoptotic program induced by wild-type E4orf4. In marked contrast, the nuclear targeting of E4orf4 abolished the early induction of extranuclear apoptosis. However, nuclear E4orf4 still induced a delayed cell death response independent of Src-like activity and of E4orf4 tyrosine phosphorylation. The zVAD.fmk-inhibitable caspases were dispensable for execution of both cell death programs. Nevertheless, both pathways led to caspase activation in some cell types through the mitochondrial pathway. Finally, our data support a critical role for calpains upstream in the death effector pathway triggered by the Src-mediated cytoplasmic death signal. We conclude that Ad2 E4orf4 induces two distinct cell death responses, whose relative contributions to cell killing may be determined by the genetic background.
SNAREs in native plasma membranes are active and readily form core complexes with endogenous and exogenous SNAREs
Authors: Lang Thorsten , Margittai Martin , Hölzler Helmut , Jahn Reinhard
Published: September 2002

During neuronal exocytosis, the vesicle-bound soluble NSF attachment protein (SNAP) receptor (SNARE) synaptobrevin 2 forms complexes with the plasma membrane–bound SNAREs syntaxin 1A and SNAP25 to initiate the fusion reaction. However, it is not known whether in the native membrane SNAREs are constitutively active or whether they are unable to enter SNARE complexes unless activated before membrane fusion. Here we used binding of labeled recombinant SNAREs to inside-out carrier supported plasma membrane sheets of PC12 cells to probe for the activity of endogenous SNAREs. Binding was specific, saturable, and depended on the presence of membrane-resident SNARE partners. Our data show that virtually all of the endogenous syntaxin 1 and SNAP-25 are highly reactive and readily form SNARE complexes with exogenously added SNAREs. Furthermore, complexes between endogenous SNAREs were not detectable when the membranes are freshly prepared, but they slowly form upon prolonged incubation in vitro. We conclude that the activity of membrane-resident SNAREs is not downregulated by control proteins but is constitutively active even if not engaged in fusion events.
Cell cycle– and swelling-induced activation of a Caenorhabditis elegans ClC channel is mediated by CeGLC-7α/β phosphatases
Authors: Rutledge Eric , Denton Jerod , Strange Kevin
Published: September 2002

ClC voltage-gated anion channels have been identified in bacteria, yeast, plants, and animals. The biophysical and structural properties of ClCs have been studied extensively, but relatively little is known about their precise physiological functions. Furthermore, virtually nothing is known about the signaling pathways and molecular mechanisms that regulate channel activity. The nematode Caenorhabditis elegans provides significant experimental advantages for characterizing ion channel function and regulation. We have shown previously that the ClC Cl− channel homologue CLH-3 is expressed in C. elegans oocytes, and that it is activated during meiotic maturation and by cell swelling. We demonstrate here that depletion of intracellular ATP or removal of Mg2+, experimental maneuvers that inhibit kinase function, constitutively activate CLH-3. Maturation- and swelling-induced channel activation are inhibited by type 1 serine/threonine phosphatase inhibitors. RNA interference studies demonstrated that the type 1 protein phosphatases CeGLC-7α and β, both of which play essential regulatory roles in mitotic and meiotic cell cycle events, mediate CLH-3 activation. We have suggested previously that CLH-3 and mammalian ClC-2 are orthologues that play important roles in heterologous cell–cell interactions, intercellular communication, and regulation of cell cycle–dependent physiological processes. Consistent with this hypothesis, we show that heterologously expressed rat ClC-2 is also activated by serine/threonine dephosphorylation, suggesting that the two channels have common regulatory mechanisms.
Hijacking the translation apparatus by RNA viruses
Authors: Bushell Martin , Sarnow Peter
Published: September 2002

As invading viruses do not harbor functional ribosomes in their virions, successful amplification of the viral genomes requires that viral mRNAs compete with cellular mRNAs for the host cell translation apparatus. Several RNA viruses have evolved remarkable strategies to recruit the host translation initiation factors required for the first steps in translation initiation by host cell mRNAs. This review describes the ways that three families of RNA viruses effectively usurp limiting translation initiation factors from the host.
Dangerous Darwinism
Authors: Fleming Chris , Goodall Jane
Published: September 2002

We are concerned with a particular rhetorical narrative that appeared in the early stages of the Darwin debate but that has been fiercely resurgent in the past two decades. Freud wrote of Darwin's theory as one of three major blows to which human vanity had been submitted at the hands of science. Assertions that “Darwin's dangerous idea” came as a horrible shock to the Victorians and that it is still a profound psychological threat, and therefore widely resisted, remain current.

When such assertions are offered as the premise on which Darwin is to be approached by the general reader, they call for some detailed scrutiny, with regard both to the bases on which they are made, and the effect they have on the terms of public debate. This paper offers a critique of the culture-shock myth based on a re-examination of Victorian reactions to Darwin's work, and on an analysis of the ways in which the myth functions as a rhetorical strategy in our own time.
Using reflective reports to improve the case method
Authors: Rosier George
Published: September 2002

The case method has long been accepted as an excellent pedagogical approach and has been used extensively in management education. This paper examines the case method in the light of developments in adult education, and finds that reflection after the class discussion of the case is usually neglected and often inadvertently discouraged. Yet reflection after the event is an important part of the learning process. Requiring students to write brief reflective reports, after the class discussion of a case, has been used successfully to improve student learning and to improve the perceived value and relevance of a case study. The paper concludes that the use of reflective reports does a great deal to overcome concerns about value and relevance in management education.
Knowledge maps for managing Web‐based business
Authors: Wang Shouhong
Published: September 2002

Knowledge management has recently received considerable attention in the Web‐based business community. This paper discusses the key concepts of human‐computer interaction in knowledge development, and identifies new challenges of knowledge management for Web‐based business. Based on theories of knowledge representations and semantic networks, this paper proposes a structure of knowledge maps for knowledge management in the Web‐based business environment. An example of knowledge maps for online auctions is used to illustrate the application of the proposed structure of knowledge maps.
Human Immunodeficiency Virus-Specific CD8+T-Cell Responses Do Not Predict Viral Growth and Clearance Rates during Structured Intermittent Antiretroviral Therapy
Authors: Oxenius Annette , McLean Angela R. , Fischer Marek , Price David A. , Dawson Sarah J. , Hafner Roland , Schneider Christine , Joller Helen , Hirschel Bernard , Phillips Rodney E. , Weber Rainer , Günthard Huldrych F.
Published: September 2002

There is a continuing search for better ways to use existing drugs against human immunodeficiency virus (HIV). One idea is to use short therapy interruptions to “autovaccinate” HIV-infected patients. A group of 13 chronically HIV-infected patients enrolled in a trial of such so-called structured treatment interruptions (STIs) were intensively studied with respect to their viral load (VL) and HIV-specific CD8+T-cell (cytotoxic T-lymphocyte [CTL]) responses. We found that 10 of the 13 patients had plateau VLs after STIs that were lower than their pretreatment VLs. While viral rebound rates became lower over STIs, there were no changes in clearance rates. Although numbers of CTLs did increase over the same time that viral rebounds decreased, there was no correlation between CTL count and either viral rebound rates or clearance rates. Finally, we asked whether absolute numbers of or changes in numbers of CTLs predict plateau VLs after STIs. No measure of CTLs was able to predict plateau VLs. Thus, there was no signature in these data of an important contribution to virological control from HIV-specific CD8+T lymphocytes.
A P22 Scaffold Protein Mutation Increases the Robustness of Head Assembly in the Presence of Excess Portal Protein
Authors: Moore Sean D. , Prevelige, Peter E.
Published: September 2002

Bacteriophage with linear, double-stranded DNA genomes package DNA into preassembled protein shells called procapsids. Located at one vertex in the procapsid is a portal complex composed of a ring of 12 subunits of portal protein. The portal complex serves as a docking site for the DNA packaging enzymes, a conduit for the passage of DNA, and a binding site for the phage tail. An excess of the P22 portal protein alters the assembly pathway of the procapsid, giving rise to defective procapsid-like particles and aberrant heads. In the present study, we report the isolation of escape mutant phage that are able to replicate more efficiently than wild-type phage in the presence of excess portal protein. The escape mutations all mapped to the same phage genome segment spanning the portal, scaffold, coat, and open reading frame 69 genes. The mutations present in five of the escape mutants were determined by DNA sequencing. Interestingly, each mutant contained the same mutation in the scaffold gene, which changes the glycine at position 287 to glutamate. This mutation alone conferred an escape phenotype, and the heads assembled by phage harboring only this mutation had reduced levels of portal protein and exhibited increased head assembly fidelity in the presence of excess portal protein. Because this mutation resides in a region of scaffold protein necessary for coat protein binding, these findings suggest that the P22 scaffold protein may define the portal vertices in an indirect manner, possibly by regulating the fidelity of coat protein polymerization.