bluuebunny/crossref_metadata_2025 huggingface dataset:
Cyclic AMP is sufficient for triggering the exocytic recruitment of aquaporin‐2 in renal epithelial cells
Authors: Lorenz Dorothea , Krylov Andrey , Hahm Daniel , Hagen Volker , Rosenthal Walter , Pohl Peter , Maric Kenan
Published: January 2003

The initial response of renal epithelial cells to the antidiuretic hormone arginine vasopressin (AVP) is an increase in cyclic AMP. By applying immunofluorescence, cell membrane capacitance and transepithelial water flux measurements we show that cAMP alone is sufficient to elicit the antidiuretic cellular response in primary cultured epithelial cells from renal inner medulla, namely the transport of aquaporin‐2 (AQP2)‐bearing vesicles to, and their subsequent fusion with, the plasma membrane (AQP2 shuttle). The AQP2 shuttle is evoked neither by AVP‐independent Ca2+ increases nor by AVP‐induced Ca2+ increases. However, clamping cytosolic Ca2+ concentrations below resting levels at 25 nM inhibited exocytosis. Exocytosis was confined to a slow monophasic response, and readily releasable vesicles were missing. Analysis of endocytic capacitance steps revealed that cAMP does not decelerate the retrieval of AQP2 from the plasma membrane. Our data suggest that cAMP initiates an early step, namely the transport of AQP2‐bearing vesicles towards the plasma membrane, and do not support a regulatory function for Ca2+ in the AQP2 shuttle.
Trithorax interacts with type 1 serine/threonine protein phosphatase in Drosophila
Authors: Rudenko Andrey , Bennett Daimark , Alphey Luke
Published: January 2003

The catalytic subunit of type 1 serine/threonine protein phosphatase (PP1c) was shown to bind trithorax (TRX) in the yeast two‐hybrid system. Interaction between PP1c and TRX was confirmed in vivo by co‐immunoprecipitation from Drosophila extracts. An amino‐terminal fragment of TRX, containing a putative PP1c‐binding motif, was shown to be sufficient for binding to PP1c by in vitro glutathione S‐transferase pull‐down assays using recombinant protein and fly extracts expressing epitope tagged PP1c. Disruption of the PP1c‐binding motif abolished binding, indicating that this motif is necessary for interaction with PP1. On polytene chromosomes, PP1c is found at many discrete bands, which are widely distributed along the chromosomes. Many of the sites that stain strongly for PP1c correspond to sites of TRX, consistent with a physical association of PP1c with chromatin‐bound TRX. Homeotic transformations of haltere to wing in flies mutant for trx are dominantly suppressed by PP1c mutants, indicating that PP1c not only binds TRX, but is a physiologically relevant regulator of TRX function in vivo.
The yeast counterparts of human ‘MELAS’ mutations cause mitochondrial dysfunction that can be rescued by overexpression of the mitochondrial translation factor EF‐Tu
Authors: Feuermann M , Francisci S , Rinaldi T , De Luca C , Rohou H , Frontali L , Bolotin‐Fukuhara M
Published: January 2003

We have taken advantage of the similarity between human and yeast (Saccharomyces cerevisiae) mitochondrial tRNALeu(UUR), and of the possibility of transforming yeast mitochondria, to construct yeast mitochondrial mutations in the gene encoding tRNALeu(UUR) equivalent to the human A3243G, C3256T and T3291C mutations that have been found in patients with the neurodegenerative disease MELAS (for mitochondrial ‘myopathy, e ncephalopathy, lactic acidosis and stroke‐like episodes’). The resulting yeast cells (bearing the equivalent mutations A14G, C26T and T69C) were defective for growth on respiratory substrates, exhibited an abnormal mitochondrial morphology, and accumulated mitochondrial DNA deletions at a very high rate, a trait characteristic of severe mitochondrial defects in protein synthesis. This effect was specific at least in the pathogenic mutation T69C, because when we introduced A or G instead of C, the respiratory defect was absent or very mild. All defective phenotypes returned to normal when the mutant cells were transformed by multicopy plasmids carrying the gene encoding the mitochondrial elongation factor EF‐Tu. The ability to create and analyse such mutated strains and to select correcting genes should make yeast a good model for the study of tRNAs and their interacting partners and a practical tool for the study of pathological mutations and of tRNA sequence polymorphisms.
DmMyD88 controls dorsoventral patterning of the Drosophila embryo
Authors: Kambris Zakaria , Bilak Hana , D'Alessandro Rosalba , Belvin Marcia , Imler Jean‐Luc , Capovilla Maria
Published: January 2003

MyD88 is an adapter protein in the signal transduction pathway mediated by interleukin‐1 (IL‐1) and Toll‐like receptors. A Drosophila homologue of MyD88 (DmMyD88) was recently shown to be required for the Toll‐mediated immune response. In Drosophila, the Toll pathway was originally characterized for its role in the dorsoventral patterning of the embryo. We found that, like Toll, DmMyD88 messenger RNA is maternally supplied to the embryo. Here we report the identification of a new mutant allele of DmMyD88, which generates a protein lacking the carboxy‐terminal extension, normally located downstream of the Toll/IL‐1 receptor domain. Homozygous mutant female flies lay dorsalized embryos that are rescued by expression of a transgenic DmMyD88 complementary DNA. The DmMyD88 mutation blocks the ventralizing activity of a gain‐of‐function Toll mutation. These results show that DmMyD88 encodes an essential component of the Toll pathway in dorsoventral pattern formation.
A carboxy‐terminal domain determines the subunit specificity of KCNQ K+ channel assembly
Authors: Schwake Michael , Jentsch Thomas J , Friedrich Thomas
Published: January 2003

Mutations in KCNQ K+ channel genes underlie several human pathologies. KCNQ α‐subunits form either homotetramers or hetero‐oligomers with a restricted subset of other KCNQ α‐subunits or with KCNE β‐subunits. KCNQ1 assembles with KCNE β‐subunits but not with other KCNQ α‐subunits. By contrast, KCNQ3 interacts with KCNQ2, KCNQ4 and KCNQ5. Using a chimaeric strategy, we show that a cytoplasmic carboxy‐terminal subunit interaction domain (sid) suffices to transfer assembly properties between KCNQ3 and KCNQ1. A chimaera (KCNQ1‐sidQ3) carrying the si domain of KCNQ3 within the KCNQ1 backbone interacted with KCNQ2, KCNQ3 and KCNQ4 but not with KCNQ1. This interaction was shown by enhancement of KCNQ2 currents, testing for dominant‐negative effects of pore mutants, determining its effects on surface expression and co‐immunoprecipitation experiments. Conversely, a KCNQ3‐sidQ1 chimaera no longer affects KCNQ2 but interacts with KCNQ1. We conclude that the si domain suffices to determine the subunit specificity of KCNQ channel assembly.
The two α‐tubulin isotypes in budding yeast have opposing effects on microtubule dynamics in vitro
Authors: Bode Claudia J , Gupta Mohan L , Suprenant Kathy A , Himes Richard H
Published: January 2003

The yeast Saccharomyces cerevisiae has two genes for α‐tubulin, TUB1 and TUB3, and one β‐tubulin gene, TUB2. The gene product of TUB3, Tub3, represents ∼10% of α‐tubulin in the cell. We determined the effects of the two α‐tubulin isotypes on microtubule dynamics in vitro. Tubulin was purified from wild‐type and deletion strains lacking either Tub1 or Tub3, and parameters of microtubule dynamics were examined. Microtubules containing Tub3 as the only α‐tubulin isotype were less dynamic than wild‐type microtubules, as shown by a shrinkage rate and catastrophe frequency that were about one‐third of that for wild‐type microtubules. Conversely, microtubules containing Tub1 as the only α‐tubulin isotype were more dynamic than wild‐type microtubules, as shown by a shrinkage rate that was 50% higher and a catastrophe frequency that was 30% higher than those of wild‐type microtubules. The results suggest that a role of Tub3 in budding yeast is to control microtubule dynamics.
Kerr-resonance-condition-coupled enhancement in magneto-optic media
Authors: De A. , Puri A.
Published: January 2003

We derive an expression for cyclotron frequency ωc, which sets Re[ε+ε−]=1 in a magneto-optic (MO) substrate, at any incident photon energy. Thereby, at any desired part of the optical spectrum, large Kerr effects can be obtained, which are generally known to occur either at the free-charge-carrier-plasma-resonance frequency ωp, or at frequencies where active electronic transitions take place. Under these conditions, ωp is seen to play a very different role; it is seen that for any ω, the magnitude of the Kerr resonance in a single MO (InSb here) substrate increases with decreasing ωp. With the objective of achieving further Kerr enhancement, the effect of coating a thin film of this ωc-coupled InSb layer on a Ag substrate is numerically studied. Further Kerr enhancement, at the plasma-resonance frequency of Ag, is seen, which is dependent on the thickness of the MO layer and its ωp. In this configuration, giant resonances appear in the effective-dielectric-tensor spectra. The spectral locations of these resonances are dependent on the thickness of the MO layer. We interpret these resonance structures to be the effective cyclotron resonance. Our results suggest that there exists a strong correlation between the effective-cyclotron frequency and the plasma-resonance frequency of the noble.
Nanoscale thermal transport
Authors: Cahill David G. , Ford Wayne K. , Goodson Kenneth E. , Mahan Gerald D. , Majumdar Arun , Maris Humphrey J. , Merlin Roberto , Phillpot Simon R.
Published: January 2003

Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.
Ion bombardment effects on microcrystalline silicon growth mechanisms and on the film properties
Authors: Kalache B. , Kosarev A. I. , Vanderhaghen R. , i Cabarrocas P. Roca
Published: January 2003

The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest that ion bombardment, and particularly the implantation of hydrogen ions, favors the formation of a porous layer where the nucleation of crystallites takes place. These results are further supported by in situ spectroscopic ellipsometry measurements of the film morphology as a function of the ion energy.
Energy levels of point defects in SrTiO3 and related oxides
Authors: Robertson J.
Published: January 2003

The energy levels of defects in SrTiO3, PbTiO3 and PbZrO3 have been calculated by the tight-binding Greens function method and compared to experiment. This method is very useful for displaying the chemical trends in defect energy levels. The Ti and O vacancies are found to be shallow. Transition metal impurities at the octahedral ion site are found to give rise to numerous charge states because of the large intra-atomic electron repulsion. The levels are generally in the high spin configuration, except for the high valence transition metals. The levels of Cr4+, Fe4+, and Co4+ lie near midgap. The Ti3+ trapped electron centers become deep at higher Zr contents in Pb(Zr,Ti)O3.
Effect of finite relaxation time on modeling neutral transport in hydrogen plasma
Authors: Miyamoto K. , Ishii Y. , Hatayama A.
Published: January 2003

In low-temperature hydrogen plasma, the effect of neutral transport on the relative population of excited-level neutrals has been investigated. In particular, the validity of the quasi-steady-state approximation of collisional radiative model for a neutral transport simulation of detached divertor plasma is discussed. The relaxation times of the population density are compared with the residence time in detached plasma by solving the time-dependent zero-dimensional rate equations. It is shown that the relaxation times of the population density are comparable with the residence time (10−6–10−5 s) for vibrationally excited hydrogen molecules, which means that the transport of vibrationally excited molecules strongly affects the spatial distributions of their densities and reaction rates for ion conversion and dissociative attachment. The one-dimensional Monte Carlo calculation results support these transport effects. It is important to follow the traces of each vibrationally excited molecule separately.
Surface modification of graphitic foam
Authors: Mukhopadhyay Sharmila M. , Pulikollu Rajasekhar V. , Ripberger Erik , Roy Ajit K.
Published: January 2003

This article discusses surface-related issues and possible modification approaches in high-porosity graphitic foam. The microcellular solid is made of graphitic carbon walls, ligaments, and beams supporting a network of interconnected pores. This makes the exposed surface area very high and its understanding a necessity for most applications. Graphitic planes are seen to be stacked at various orientations with respect to exposed surfaces. Therefore, a simplified analytical model that assumes “random” graphitic planes forming a three-dimensional array of tetrahedral cells may be an appropriate approximation. The influence of oxidizing chemicals such as nitric acid and hydrogen peroxide on surface properties has been studied using electron microscopy, photoelectron spectroscopy, and water absorption tests. Bulk properties such as density measurements and mechanical tests have been performed in parallel. It is seen that exposure to nitric acid results in an increase in oxygen-containing functional groups on the surface, which may lead to increased infiltration of polar matrix fluids such as water and epoxy resins. This possibility is further supported by water absorption studies that show increased water infiltration in foam after nitric acid treatment. Electron microscopy and density studies indicate that some surface etching occurs with concentrated nitric acid exposure, but not significant enough to decrease density. Strength of the stand-alone foam is reduced for concentrated nitric acid treatment, but the elastic modulus is unaffected. The scientific significance of these results in terms of future surface modification of microcellular solids has been discussed.
Sharp photoluminescence of CdSeS nanocrystals embedded in silica glass
Authors: Wang Y. S. , Sun P. , Wang Y. H. , Wang R. Z. , Zheng D. , Li Y. L.
Published: January 2003

CdSeS nanocrystals were grown by annealing silica glass with supersaturated Cd, Se, S at temperatures of 550–800 °C. The linear absorption spectra, photoluminescence (PL) spectra, and photoluminescence excitation spectra were measured. Sharp PL peaks superposed on broadened PL spectra were observed at room temperature in the samples grown at temperatures of 650–800 °C. The sharp PL peak energy was independent of the excited energy and nanocrystal growth conditions. The origin of sharp luminescence is discussed.
Two-dimensional photonic crystal coupled-defect laser diode
Authors: Happ Thomas D. , Kamp Martin , Forchel Alfred , Gentner Jean-Louis , Goldstein L.
Published: January 2003

We demonstrate a two-dimensional photonic crystal defect laser diode based on a coupled cavity waveguide. The laser cavity is formed by 40 coupled hexagonal defect microcavities in a triangular lattice of air cylinders, which are etched into an InGaAsP/InP laser structure. The coupling of the individual cavity modes creates minibands within the photonic band gap. Stable single-mode lasing occurs on the first miniband mode with the lowest group velocity with side mode suppression greater than 40 dB. The lasers emit up to 2.6 mW at a 1.53 μm wavelength under continuous-wave operation at room temperature.
Forbidden transitions and the effective masses of electrons and holes in In1−xGaxAs/InP quantum wells with compressive strain
Authors: Shao Jun , Dörnen Achim , Baars Enno , Härle Volker , Scholz Ferdinand , Guo Shaoling , Chu Junhao
Published: January 2003

Two types of forbidden transitions are identified in In1−xGaxAs/InP undoped quantum wells (QWs) with compressive strain by low-temperature (1.8 K) magneto-optical absorption. One of them is due to the interband transitions with different principal quantum numbers and is observable mainly in a low magnetic field and the other corresponds to P- and D-type exciton states and gets stronger as the magnetic field increases. By analyzing the forbidden transitions the in-plane effective masses of electrons (me,ρ*) and heavy holes (mh,ρ*) are simultaneously determined, together with the z-direction (growth-direction) ones (me,z*,mh,z*). The theoretically predicted relation among the electron effective masses [Sugawara et al., Phys. Rev. B 48, 8102 (1993)], me,Γ6<me,ρ*<me,z*, where me,Γ6 is the band-edge electron effective mass of bulk material, is therefore verified. The difference between the values of me,z* and me,ρ* is found to decrease as the strain in the QW drops.
Analytical approach to integrate the different components of direct tunneling current through ultrathin gate oxides in n-channel metal–oxide–semiconductor field-effect transistors
Authors: Maitra Kingsuk , Bhat Navakanta
Published: January 2003

An analytical scheme to combine the channel component and the edge component of direct tunneling current through ultrathin gate oxides in n-channel metal–oxide–semiconductor field-effect transistors has been developed. The results obtained have been calibrated against the published experimental and numerical simulation data. The inherent simplicity of the proposed analytical model makes it suitable for implementing in circuit simulators. The proposed model is capable of predicting the tunneling current under positive as well as negative gate bias. The impact of gate-source/drain extension overlap length (edge) on total gate leakage is clearly quantified. It is demonstrated that the overlap length should be scaled to decrease the leakage power consumption and also to extend the scalability of gate oxide to lower values.
Ni/Si solid phase reaction studied by temperature-dependent current-voltage technique
Authors: Jiang Yu-Long , Ru Guo-Ping , Lu Fang , Qu Xin-Ping , Li Bing-Zong , Yang Simon
Published: January 2003

The temperature-dependent current–voltage (I–V–T) technique has been used to study the Ni/Si solid phase reaction by measuring the Schottky barrier height (SBH) inhomogeneity of Ni-silicide/Si Schottky diodes. The experimental results show the strong dependence of SBH inhomogeneity on the Ni/Si solid phase reaction. The SBH distribution of the diodes annealed at 500 and 600 °C can be described by a single-Gaussian function and the diode annealed at 500 °C is found to have the best homogeneity and the smallest leakage current. The SBH distribution of the diodes annealed at 400, 700, and 800 °C can be described by a double-Gaussian function in which the mean value of the second Gaussian function is substantially smaller than that of the dominant Gaussian function. The variation of SBH inhomogeneity, an interface property, is related to the phase evolution process in the Ni/Si solid phase reaction, and verified by reverse I–V measurements. Our results indicate that the I–V–T technique may be developed as a wafer-level testing tool to monitor the silicidation process in the complementary metal–oxide–semiconductor device fabrication.
Estimating the band discontinuity at GaInSb/GaSb heterojunction by investigation of single-quantum well photoluminescence
Authors: Donati G. P. , Kaspi R. , Malloy K. J.
Published: January 2003

We have grown single-quantum wells of strained Ga1−xInxSb (x<0.35) embedded in GaSb by molecular-beam epitaxy to investigate the photoluminescence and the band offset of this heterojunction. The photoluminescence shifts to longer wavelengths when the well thickness, or the indium content x, is increased. The band offsets of these heterojunctions are estimated by fitting the photoluminescence data to a single-quantum well model. Our offset estimates support the theoretical prediction by first-principles calculations for these strained heterojunctions.
Fluid demixing in colloid–polymer mixtures: Influence of polymer interactions
Authors: Schmidt Matthias , Denton Alan R. , Brader Joseph M.
Published: January 2003

We consider a binary mixture of hard colloidal spheres and nonadsorbing polymer coils. The polymers are regarded as effective spheres that interact with one another via a repulsive step-function pair potential and with colloids solely via excluded volume. The system is treated with a geometry-based density functional theory based on the exact zero-dimensional limit of the model. For bulk fluid phases, we calculate demixing binodals and find that with increasing strength of polymer–polymer interaction the coexisting colloidal liquid (vapor) phase becomes more concentrated (dilute) in polymer. In contrast to a simple mean-fieldlike perturbative density functional, our approach yields good agreement with an experimental demixing phase diagram.
Infrared and Raman spectroscopy of [Pb(Zn1/3Nb2/3)O3]0.92–[PbTiO3]0.08 and [Pb(Mg1/3Nb2/3)O3]0.71–[PbTiO3]0.29 single crystals
Authors: Kamba S. , Buixaderas E. , Petzelt J. , Fousek J. , Nosek J. , Bridenbaugh P.
Published: January 2003

Far-infrared reflectivity spectra of [Pb(Zn1/3Nb2/3)O3]0.92–[PbTiO3]0.08 and [Pb(Mg1/3Nb2/3)O3]0.71–[PbTiO3]0.29 single crystals were investigated between 10 and 530 K, micro-Raman spectra were recorded between 300 and 800 K. No phonon softening was observed near either of the ferroelectric phase transitions. The low-frequency dielectric anomaly in the paraelectric phase is caused by contribution of dynamic polar nanoclusters with the main dispersion in the microwave range. Infrared and Raman spectra confirm the locally doubled unit cell (Zprim=2) in the paraelectric and ferroelectric phases due to the ordering in the perovskite B sites and occurrence of polar nanoclusters in the paraelectric phase. The lowest-frequency transverse optical (TO1) phonon mode active in the infrared spectra is underdamped in contrast to the recent result of inelastic neutron scattering, where no TO1 mode could be observed for the wave vectors q⩽0.2 Å−1. This discrepancy was explained by different q vectors probed in infrared and neutron experiments. The infrared probe couples with very long-wavelength phonons (q≈10−5 Å−1) which see the homogeneous medium averaged over the nanoclusters, whereas the neutron probe couples with phonons whose wavelength is comparable to the nanocluster size (q⩾10−2 Å−1).
Tunnel current in quantum dot infrared photodetectors
Authors: Duboz J.-Y. , Liu H. C. , Wasilewski Z. R. , Byloss M. , Dudek R.
Published: January 2003

Infrared photodetectors have been fabricated based on InAs/GaAs self-assembled quantum dot (QD) layers, with various QD densities and doping levels. Dark currents have been measured as a function of applied bias and temperature. They show a clear activation energy, which decreases as the QD shell filling increases. Its absolute value and dependence on applied bias indicate that electrons tunnel from QD levels into the wetting layer of the next period. Resonant structures in the current–voltage curve and in its first derivative confirm the tunneling through the GaAs barrier. Negative differential resistances are observed in highly doped samples at low temperature.
Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN
Authors: Motayed Abhishek , Bathe Ravi , Wood Mark C. , Diouf Ousmane S. , Vispute R. D. , Mohammad S. Noor
Published: January 2003

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5×1017 cm−3) were studied. The lowest contact resistivity derived from the annealed contact was ρS=3.0×10−6Ω cm2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti–Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to the formation of Ti–Al alloys with the remaining Al content. Consequently, both the Al and the Ti (from the second Ti layer) contents were almost fully consumed, and none of them were left to appear on the contact surface to form oxides.
Laser-induced surface modes at an air–porous medium interface
Authors: Allard J. F. , Henry M. , Glorieux C. , Petillon S. , Lauriks W.
Published: January 2003

A description of the laser-induced wave excitation of surface modes at a fluid–porous medium interface using the Biot model is presented. For the case of a material much heavier than the fluid, the porous frame behaves like an elastic medium in vacuum with a small frequency-dependent structural damping due to the viscous losses. Predictions concerning the shape of the Rayleigh-like wave are given. Measurements show the effect of scattering for ceramics of different pore sizes on this wave.
Effect of microstructures on nanocrystallite nucleation and growth in hydrogenated amorphous indium–tin–oxide films
Authors: Ando Masahiko , Nishimura Etsuko , Onisawa Ken-ichi , Minemura Tetsuroh
Published: January 2003

Hydrogenated amorphous indium–tin–oxide (ITO) films were prepared by a sputtering method at room temperature with H2O addition. The initial stage of thermal crystallization of the amorphous films was investigated after annealing at 150 °C, just below the crystallization temperature. With increasing H2O addition, the growth of crystallites dispersed in the amorphous matrix was suppressed, while the nucleation of crystallites was sharply enhanced. The amount of bonded hydrogen increased and that of oxygen vacancies decreased at the same time, with introducing inhomogeneites in the amorphous matrix. The effect of these microstructural changes on the nucleation and growth process of crystallites embedded in the amorphous solids was discussed based on these experimental results taking into account a recently proposed theoretical prediction to describe the crystallization of disordered solids.
Evolution of normal stress and surface roughness in buckled thin films
Authors: Palasantzas G. , De Hosson J. Th. M.
Published: January 2003

In this work we investigate buckling of compressed elastic thin films, which are bonded onto a viscous layer of finite thickness. It is found that the normal stress exerted by the viscous layer on the elastic film evolves with time showing a minimum at early buckling stages, while it increases at later stages. The normal stress also shows a minimum as a function of applied compressive stress, which depends strongly on the viscosity of the underlying layer and strain values. Furthermore, with decreasing viscosity the film roughness amplitude also shows a minimum at early buckling stages. The effect of the viscosity becomes more pronounced with increasing strain in the film. Finally, decreasing elastic film thickness and/or increasing viscous layer thickness also enhance buckling roughness.
Self-affine roughness effects on the contact area between elastic bodies
Authors: Palasantzas G. , De Hosson J. Th. M.
Published: January 2003

We have calculated the real contact area between elastic bodies with self-affine rough surfaces, which are described in terms of analytical correlation models in Fourier space. It is found that the roughness has a strong influence on the real contact area A(λ) at lateral length scales λ which are comparable with the in-plane roughness correlation length ξ, and for significant applied loads σo beyond the linear regime (or A∝σo). The effect of the roughness exponent H can be rather complex, depending on the relative magnitude of the roughness correlation length ξ with respect to the lateral length scale λ where the contact area is considered. Finally, we also show that descriptions of the influence of the roughness that is only based on power law approximations of the self-affine roughness spectrum are rather insufficient, especially for large roughness exponents H (>0.5).
Re-evaluation of orientation effects in the friction of a hard ellipsoid sliding on rubber
Authors: Carter J. D.
Published: January 2003

During our effort to quantitatively separate hysteretic friction from adhesional friction, the friction generated along the two principal axes of a solid aluminum ellipsoid, when sliding on a rubber substrate, has been studied. An experimental result running counter to previously published theory was discovered. More friction was generated sliding parallel to the long axis than what was generated sliding parallel to the short axis. Previous theory predicted that more friction should always be generated when sliding parallel to the short axis, since in this direction, more rubber is deformed. It was suspected that this anomaly might be due to changes in sliding contact mechanics. An experimental approach is presented to show that contact area is dependent upon direction of sliding, which negates the assumptions upon which the original theory was based.
Vacancy and interstitial depth profiles in ion-implanted silicon
Authors: Lévêque P. , Nielsen H. Kortegaard , Pellegrino P. , Hallén A. , Svensson B. G. , Kuznetsov A. Yu. , Wong-Leung J. , Jagadish C. , Privitera V.
Published: January 2003

An experimental method of studying shifts between concentration-versus-depth profiles of vacancy- and interstitial-type defects in ion-implanted silicon is demonstrated. The concept is based on deep level transient spectroscopy measurements utilizing the filling pulse variation technique. The vacancy profile, represented by the vacancy–oxygen center, and the interstitial profile, represented by the interstitial carbon–substitutional carbon pair, are obtained at the same sample temperature by varying the duration of the filling pulse. The effect of the capture in the Debye tail has been extensively studied and taken into account. Thus, the two profiles can be recorded with a high relative depth resolution. Using low doses, point defects have been introduced in lightly doped float zone n-type silicon by implantation with 6.8 MeV boron ions and 680 keV and 1.3 MeV protons at room temperature. The effect of the angle of ion incidence has also been investigated. For all implantation conditions the peak of the interstitial profile is displaced towards larger depths compared to that of the vacancy profile. The amplitude of this displacement increases as the width of the initial point defect distribution increases. This behavior is explained by a simple model where the preferential forward momentum of recoiling silicon atoms and the highly efficient direct recombination of primary point defects are taken into account.
Stabilizing wide bandwidth, tuning fork detected force feedback with nonlinear interactions
Authors: Jahncke C. L. , Hallen H. D.
Published: January 2003

Near-field scanning optical microscope force feedback can be destabilized by the anisotropy in response times engendered by nonlinear tip sample interactions. This nonlinear interaction, the tapping of the tip on layers adsorbed on the sample, is important when the intrinsic damping of the system is low. We present strong evidence of tapping on adlayers rather than the sample surface at operational distances, and numerically solve a model to find the dynamics of tip motion. These results illuminate the origins of feedback problems when using tuning fork detection of oscillation amplitude, and show an optimal technique that uses the rapid response of the tip-adlayer nonlinearity to circumvent the slow damping response and enable wide bandwidth, stable distance regulation for these systems.
Parameters of electron–hole scattering in silicon carbide
Authors: Mnatsakanov T. T. , Levinshtein M. E. , Ivanov P. A. , Palmour J. W. , Tandoev A. G. , Yurkov S. N.
Published: January 2003

The parameters of electron–hole scattering (EHS) in silicon carbide (SiC) are estimated by analyzing pulsed isothermal current–voltage characteristics of 6.0 kV 4H–SiC diodes over a wide range of current densities (100–104 A/cm2) and external temperatures (293–553 K). The efficiency of EHS in SiC is approximately two times higher than that in Si and 60 times higher than that in GaAs. The EHS makes a very essential contribution to voltage drop across SiC bipolar devices at high forward current densities.
Electromigration model for the prediction of lifetime based on the failure unit statistics in aluminum metallization
Authors: Park Jong Ho , Ahn Byung Tae
Published: January 2003

A failure model for electromigration based on the “failure unit model” was presented for the prediction of lifetime in metal lines.The failure unit model, which consists of failure units in parallel and series, can predict both the median time to failure (MTTF) and the deviation in the time to failure (DTTF) in Al metal lines. The model can describe them only qualitatively. In our model, both the probability function of the failure unit in single grain segments and polygrain segments are considered instead of in polygrain segments alone. Based on our model, we calculated MTTF, DTTF, and activation energy for different median grain sizes, grain size distributions, linewidths, line lengths, current densities, and temperatures. Comparisons between our results and published experimental data showed good agreements and our model could explain the previously unexplained phenomena. Our advanced failure unit model might be further applied to other electromigration characteristics of metal lines.
Ab initio molecular dynamics with a continuum solvation model
Authors: Senn Hans Martin , Margl Peter M. , Schmid Rochus , Ziegler Tom , Blöchl Peter E.
Published: January 2003

We present an implementation of the conductor-like screening model (COSMO) within the framework of Car–Parrinello ab initio molecular dynamics. In order to obtain the accurate forces needed for energy-conserving dynamics, analytic derivatives with respect to the atomic positions are required for all energy terms. We use a steep, but continuous surface function that effectively switches the surface charges off when they are not exposed on the molecular surface. This allows us to construct the cavity surface in such a way that the required analytic derivatives of the surface charges and surface segments are always available. Furthermore, we treat the surface charges as fictitious dynamic variables within the extended Lagrangian approach, solving the electrostatic problem determining the charges “on the fly” as the system evolves in time. Our implementation makes it possible to perform energy-conserving ab initio molecular dynamics simulations in which continuum solvation is included. It provides solvation energies within the accuracy expected for a COSMO implementation at the density-functional level and allows one to study special features of reactivity that can only be observed at finite temperature in solution.
Self-guided enhanced sampling methods for thermodynamic averages
Authors: Andricioaei Ioan , Dinner Aaron R. , Karplus Martin
Published: January 2003

In the self-guided molecular dynamics (SGMD) simulation method, a continuously updated average force is used to bias the motions of the system. The method appears to sample the configuration space of a number of complex systems more efficiently than ordinary molecular dynamics, and it was argued that it yields canonical averages of observable quantities with only negligible errors. We analyze the dynamic mapping associated with the SGMD algorithm and find that the dynamics lacks reversibility because the effective potential that governs the motion is a functional of the trajectory rather than a function of the coordinates (i.e., the dynamics is not uniquely specified by the initial conditions but depends on past history as well). This irreversibility is shown to result in substantial errors in canonical averages for model systems. Motivated by this analysis, we introduce an alternative self-guided scheme (the momentum-enhanced hybrid Monte Carlo method) that does converge to the canonical distribution in principle. The method differs from the original SGMD algorithm in that momenta, rather than forces, are averaged to bias the initial choice of momenta at each step in a hybrid Monte Carlo procedure. The relation of the method to other enhanced sampling algorithms is discussed.
Experimental studies of peptide bonds: Identification of the C7eq conformation of the alanine dipeptide analog N-acetyl-alanine N′-methylamide from torsion-rotation interactions
Authors: Lavrich R. J. , Plusquellic D. F. , Suenram R. D. , Fraser G. T. , Walker A. R. Hight , Tubergen M. J.
Published: January 2003

Rotational spectra of the biomimetic molecule, alanine dipeptide and the double N15(N215) isotopomer have been observed using a pulsed-molecular-beam Fourier transform microwave spectrometer. The spectra reveal tunneling splittings from the torsional mode structure of two of its three methyl rotors. The torsional states assigned include one AA-state and two AE-states (i.e., AE and EA) for each isotopomer. The AA-states are well-fit to A-reduction asymmetricrotor Hamiltonians. The “infinite-barrier-limit” rotational constants of the N214 isotopomer are A=1710.97(8) MHz, B=991.89(9) MHz, and C=716.12(6) MHz. The AE-states are analyzed independently using “high-barrier” torsion-rotation Hamiltonians, yielding observedminus-calculated standard deviations of <400 kHz. The fits improve substantially (>100-fold for the N215 isotopomer) when analyzed in a ρ-axis frame where ρb=ρc=0. The best-fit torsion-rotation parameters provide accurate V3 barriers and C3 rotor axis angles for both methyl groups. The observed angles are shown to uniquely correlate with those calculated for the acetyl and amide methyl groups in the C7eq conformational form. The V3 barriers of the amide and acetyl methyl groups are 84.0(3) cm−1 and 98.4(2) cm−1 for the N214 and 84.1(1) cm−1 and 98.65(8) cm−1 for the N215 isotopomers, respectively. These results are in good agreement with prior geometry optimizations and with current V3 barrier calculations which predict the C7eq conformation as the lowest energy form in the gas phase. Under certain conditions, the spectrum is dominated by transitions from a thermal decomposition product formed by dehydration of alanine dipeptide. This molecule is tentatively identified as 3,5-dihydro-2,3,5-trimethyl-(9CI) 4H imidazole-4-one (CAS registry #32023-93-1).
Many-body effects in nonadiabatic molecular theory for simultaneous determination of nuclear and electronic wave functions: Ab initio NOMO/MBPT and CC methods
Authors: Nakai Hiromi , Sodeyama Keitaro
Published: January 2003

We have investigated the many-body effects in a molecular theory to determine simultaneously nuclear and electronic wave functions without the Born–Oppenheimer (BO) approximation. We first apply the many-body perturbation theory using the electron–nucleus and nucleus–nucleus interactions to the non-BO theory and show the importance of the electron–nucleus correlation rather than the nucleus–nucleus one. We next combine the non-BO theory with the coupled cluster double and Brueckner double methods using the one-electron plus one-nucleus excitation operators.
Nanoscale analysis on interfacial reactions in Al–Si–Cu alloys and Ti underlayer films
Authors: Yang Jun-Mo , Lee Sukjae , Park Ju-Chul , Lee Deok-Won , Lee Tae-Kwon , Choi Jin-Tae , Lee Soun-Young , Kawasaki Masahiro , Oikawa Tetsuo
Published: January 2003

Solid-phase reactions at the interface between sputtered Al–Si–Cu alloys and Ti films were investigated at the atomic scale by high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy (EDS) coupled with a field-emission (scanning) transmission electron microscope. The analysis results showed that the interface is composed of an amorphous-like Ti–Si layer, an intermediate-crystalline layer, and a Si-dissolved TiAl3 layer containing dissolved Si TiAl3 with a crystallographic relationship with the Al film. The nanometer-scaled interlayers effectively play a role as a barrier suppressing the interdiffusion reaction of Al and Ti during annealing treatment. Further, the quantitative composition of the interlayers was revealed by the analysis of the intensity profiles obtained from EDS elemental maps.
Electric field and responsivity modeling for far-infrared blocked impurity band detectors
Authors: Haegel N. M. , Samperi S. A. , White A. M.
Published: January 2003

One-dimensional numerical modeling is presented for Ge:Ga far-infrared (IR) blocked impurity band (BIB) detectors in the low field, unity gain regime. Spatial variations of space charge, electric field, free carrier, and hopping currents are calculated to illustrate the effects of variations in absorbing layer compensation, blocking layer doping, and blocker/absorber interface gradient. Increased blocking layer doping and broader interface doping gradients lead to significant field variations. These field nonuniformities can increase responsivity by increasing field penetration and associated current collection in the absorbing layer. The ratio of photocurrent to dark current remains constant over a range of blocking layer doping, suggesting that extremely high purity blocking layers may not be required for far-IR BIB fabrication.
Thermal stability and electron irradiation effect on Zr-based amorphous alloys
Authors: Nagase T. , Umakoshi Y.
Published: January 2003

Specimens of Zr66.7Cu33.3, Zr65.0Al7.5Cu27.5, and Zr65.0Al7.5Ni10.0Cu17.5 amorphous alloys with different thermal stability were irradiated by high energy electrons at an accelerated voltage of 2000 and 1000 kV. Electron irradiation was performed at 298 and 103 K. Crystallization of three amorphous alloys was accelerated by this irradiation and nanocrystalline structures were obtained. The critical total dose required for crystallization of the amorphous phase by electron irradiation depends strongly on the irradiation temperature and stability of this amorphous phase. The phase stability and crystallization behavior of the amorphous phase are discussed based on the electron irradiation effect.
Study of deleterious aging effects in GaN/AlGaN heterostructures
Authors: Elhamri S. , Saxler A. , Mitchel W. C. , Berney R. , Elsass C. , Smorchkova Y. , Mishra U. K. , Speck J. S. , Chowdhury U. , Dupuis R. D.
Published: January 2003

A study of aging effects on the electron transport properties of AlGaN/GaN heterostructures grown on sapphire and silicon carbide substrates using temperature dependent Hall effect measurements is presented with the focus on the variations in the mobility, carrier concentration, and resistivity of these structures over time. The transport parameters for several of these structures were measured at various times after the initial measurement, including times exceeding one year. These studies show that these parameters are not stable in most of the samples. The most common effects are an increase in carrier concentration and a decrease in mobility. Changes on the order of several tens of percent were common. Since surface oxidation and other variations in surface states were suspected as possible sources of the observed changes, one sample was treated with HCl and KOH and measured again after each treatment. However, we were unable to recover the initial results although further changes in the electrical properties were observed. The study suggests that surface effects can have profound effects on the properties of the two-dimensional electron gas in these heterostructures.
Ferroelectric thin film properties—Depolarization field and renormalization of a “bulk” free energy coefficients
Authors: Glinchuk M. D. , Eliseev E. A. , Stephanovich V. A. , Farhi R.
Published: January 2003

We suggest an approach to calculate the physical properties of ferroelectric thin films. Our approach takes into account the depolarization field and spatial inhomogeneity of the order parameter inherent to thin films. In or approach, we consider the thin film to be short circuited with the perfectly conducting electrodes and monodomain. The solution of the corresponding Euler–Lagrange equation by the direct variational method shows that the initial free-energy functional of a thin film can be reduced to some effective functional which does not include spatial derivatives of the order parameter. The coefficients of this functional, however, are renormalized as compared to those of the initial free-energy functional. We present the detailed calculation of the spontaneous polarization, pyrocoefficient, and dielectric susceptibility. We analyze in detail the spatial profile of the depolarization field with special attention to its properties near size-driven phase transition in a film.