bluuebunny/crossref_metadata_2025 huggingface dataset:
A role for PPARα in the control of SREBP activity and lipid synthesis in the liver
Authors: Knight Brian L. , Hebbachi Abdel , Hauton David , Brown Anna-Marie , Wiggins David , Patel Dilip D. , Gibbons Geoffrey F.
Published: July 2005

Inclusion of the PPARα (peroxisome-proliferator-activated receptor α) activator WY 14,643 in the diet of normal mice stimulated the hepatic expression of not only genes of the fatty acid oxidation pathway, but also those of the de novo lipid synthetic pathways. Induction of fatty acid synthase mRNA by WY 14,643 was greater during the light phase of the diurnal cycle, when food intake was low and PPARα expression was high. Hepatic fatty acid pathway flux in vivo showed a similar pattern of increases. The abundance of mRNAs for genes involved in hepatic cholesterol synthesis was also increased by WY 14,643, but was associated with a decrease in cholesterogenic carbon flux. None of these changes were apparent in PPARα-null mice. Mice of both genotypes showed the expected decreases in 3-hydroxy-3-methylglutaryl-CoA reductase mRNA levels and cholesterol synthesis in response to an increase in dietary cholesterol. The increase in fatty acid synthesis due to WY 14,643 was not mediated by increased expression of SREBP-1c (sterol regulatory element binding protein-1c) mRNA, but by an increase in cleavage of the protein to the active form. An accompanying rise in stearoyl-CoA desaturase mRNA expression suggested that the increase in lipogenesis could have resulted from an alteration in membrane fatty acid composition that influenced SREBP activation.
The reconstituted P-glycoprotein multidrug transporter is a flippase for glucosylceramide and other simple glycosphingolipids
Authors: Eckford Paul D. W. , Sharom Frances J.
Published: July 2005

The Pgp (P-glycoprotein) multidrug transporter, which is linked to multidrug resistance in human cancers, functions as an efflux pump for non-polar drugs, powered by the hydrolysis of ATP at its nucleotide binding domains. The drug binding sites of Pgp appear to be located within the cytoplasmic leaflet of the membrane bilayer, suggesting that Pgp may function as a ‘flippase’ for hydrophobic compounds. Pgp has been shown to translocate fluorescent phospholipids, and it has been suggested that it may also interact with GlcCer (glucosylceramide). Here we use a dithionite fluorescence quenching technique to show that reconstituted Pgp can flip several NBD (nitrobenzo-2-oxa-1,3-diazole)-labelled simple glycosphingolipids, including NBD–GlcCer, from one leaflet of the bilayer to the other in an ATP-dependent, vanadate-sensitive fashion. The rate of NBD–GlcCer flipping was similar to that observed for NBD-labelled PC (phosphatidylcholine). NBD–GlcCer flipping was inhibited in a concentration-dependent, saturable fashion by various Pgp substrates and modulators, and inhibition correlated well with the Kd for binding to the protein. The addition of a second sugar to the headgroup of the glycolipid to form NBD–lactosylceramide drastically reduced the rate of flipping compared with NBD–PC, probably because of the increased size and polarity contributed by the additional sugar residue. We conclude that Pgp functions as a broad-specificity outwardly-directed flippase for simple glycosphingolipids and membrane phospholipids.
Inhibition of mammalian DNA polymerases by resveratrol: mechanism and structural determinants
Authors: Locatelli Giada A. , Savio Monica , Forti Luca , Shevelev Igor , Ramadan Kristijan , Stivala Lucia A. , Vannini Vanio , Hübscher Ulrich , Spadari Silvio , Maga Giovanni
Published: July 2005

Resveratrol, a natural compound found in many dietary plants and in red wine, plays an important role in the prevention of many human pathological processes, including inflammation, atherosclerosis and carcinogenesis. We have shown that the antiproliferative activity of resveratrol correlated with its ability to inhibit the replicative pols (DNA polymerases) α and δ in vitro [Stivala, Savio, Carafoli, Perucca, Bianchi, Maga, Forti, Pagnoni, Albini, Prosperi and Vannini (2001) J. Biol. Chem. 276, 22586–22594]. In this paper, we present the first detailed biochemical investigation on the mechanism of action of resveratrol towards mammalian pols. Our results suggest that specific structural determinants of the resveratrol molecule are responsible for selective inhibition of different mammalian pols, such as the family B pol α and the family X pol λ. Moreover, the resveratrol derivative trans-3,5-dimethoxy-4-hydroxystilbene, which is endowed with a strong antiproliferative activity (Stivala et al., 2001), can inhibit pols α and λ and also suppress the in vitro SV40 DNA replication. The potency of inhibition is similar to that of aphidicolin, an inhibitor of the three replicative pols α, δ and ε. Our findings establish the necessary background for the synthesis of resveratrol derivatives having more selective and potent antiproliferative activity.
Recruitment of MKLP1 to the spindle midzone/midbody by INCENP is essential for midbody formation and completion of cytokinesis in human cells
Authors: Zhu Changjun , Bossy-Wetzel Ella , Jiang Wei
Published: July 2005

The INCENP (inner centromere protein) is a chromosomal passenger protein that plays multiple roles in regulating mitosis and cytokinesis. The MKLP1 (mitotic kinesin-like protein) is a component of centralspindlin complex that has been implicated in assembly of midzone/midbody during mitosis and is essential for cytokinesis. In the present study, we investigated functions of INCNEP and MKLP1 and their interplay in regulating spindle midzone/midbody formation and cytokinesis in human cells. Immunofluorescence and live-cell imaging analyses have shown that, in addition to multiple chromosome segregation defects, cells that lacked INCENP by RNAi (RNA interference) exhibit abnormal spindle midzone/midbody formation, resulting in formation of binucleated/multinucleated cells. Suppression of MKLP1 expression by siRNA (small interfering RNA) did not cause any abnormality of chromosome segregation and midzone formation, but abrogated midbody formation and completion of cytokinesis. Furthermore, we show that INCENP is required for recruiting MKLP1 to the spindle midzone/midbody. Three-dimensional reconstruction imaging analysis suggests that recruitment of MKLP1 to the midzone/midbody by INCENP is a crucial step for the midbody formation and completion of cytokinesis in mammalian cells.
Architecture of bacterial replication initiation complexes: orisomes from four unrelated bacteria
Authors: Zawilak-PAWLIK Anna , Kois Agnieszka , Majka Jerzy , Jakimowicz Dagmara , Smulczyk-Krawczyszyn Aleksandra , Messer Walter , Zakrzewska-Czerwińska Jolanta
Published: July 2005

Bacterial chromosome replication is mediated by single initiator protein, DnaA, that interacts specifically with multiple DnaA boxes located within the origin (oriC). We compared the architecture of the DnaA–origin complexes of evolutionarily distantly related eubacteria: two Gram-negative organisms, Escherichia coli and Helicobacter pylori, and two Gram-positive organisms, Mycobacterium tuberculosis and Streptomyces coelicolor. Their origins vary in size (from approx. 200 to 1000 bp) and number of DnaA boxes (from 5 to 19). The results indicate that: (i) different DnaA proteins exhibit various affinities toward single DnaA boxes, (ii) spatial arrangement of two DnaA boxes is crucial for the H. pylori and S. coelicolor DnaA proteins, but not for E. coli and M. tuberculosis proteins, and (iii) the oriC regions are optimally adjusted to their cognate DnaA proteins. The primary functions of multiple DnaA boxes are to determine the positioning and order of assembly of the DnaA molecules. Gradual transition from the sequence-specific binding of the DnaA protein to binding through co-operative protein–protein interactions seems to be a common conserved strategy to generate oligomeric initiator complexes bound to multiple sites within the chromosomal, plasmid and virial origins.
Structure, function and molecular adaptations of haemoglobins of the polar cartilaginous fish Bathyraja eatonii and Raja hyperborea
Authors: Verde Cinzia , De Rosa M. Cristina , Giordano Daniela , Mosca Donato , De Pascale Donatella , Raiola Luca , Cocca Ennio , Carratore Vitale , Giardina Bruno , Di Prisco Guido
Published: July 2005

Cartilaginous fish are very ancient organisms. In the Antarctic sea, the modern chondrichthyan genera are poorly represented, with only three species of sharks and eight species of skates; the paucity of chondrichthyans is probably an ecological consequence of unusual trophic or habitat conditions in the Southern Ocean. In the Arctic, there are 26 species belonging to the class Chondrichthyes. Fish in the two polar regions have been subjected to different regional histories that have influenced the development of diversity: Antarctic marine organisms are highly stenothermal, in response to stable water temperatures, whereas the Arctic communities are exposed to seasonal temperature variations. The structure and function of the oxygen-transport haem protein from the Antarctic skate Bathyraja eatonii and from the Arctic skate Raja hyperborea (both of the subclass Elasmobranchii, order Rajiformes, family Rajidae) is reported in the present paper. These species have a single major haemoglobin (Hb 1; over 80% of the total). The Bohr-proton and the organophosphate-binding sites are absent. Thus the haemoglobins of northern and southern polar skates appear functionally similar, whereas differences were observed with several temperate elasmobranchs. Such evidence suggests that, in temperate and polar habitats, physiological adaptations have evolved along distinct pathways, whereas, in this case, the effect of the differences characterizing the two polar environments is negligible.
Air-breakdown charging mechanism of fibrous polytetrafluoroethylene films
Authors: Hu Z. , von Seggern H.
Published: July 2005

Recently air-breakdown phenomena have gained increasing interest with respect to porous electrets for use as piezoelectric polymers. A strong discrepancy between porous (closed-porous) polypropylene and fibrous (open-porous) polytetrafluoroethylene (PTFE) has been recognized concerning the obtainable polarization and the related piezoelectric d33 coefficients. The present study aims to clarify the reasons for a poor charge separation ability of fibrous materials when exposed to electrical breakdown in its air voids. Therefore air-breakdown phenomenon of 91% fibrous PTFE films has been studied in detail, utilizing a corona triode for voltage application. At sufficient surface potential air breakdown sets in at an average field strength of 11.7MV∕m, which corresponds to the value expected by Paschen breakdown in air. For higher applied voltages and therewith higher electric-field strengths in the film, the current generated by such electrical breakdown limits the maximum obtainable surface potential. It will be shown that this limitation is caused by a “breakdown-induced conductivity,” which increases with increasing grid voltage. Such breakdown-induced conductivity hinders the formation of a large internal polarization by mutual charge compensation and therewith accounts for the obtainable low piezoelectric d33 coefficients in fibrous materials.
Edge and scrape-off layer tokamak plasma turbulence simulation using two-field fluid model
Authors: Bisai Nirmal , Das Amita , Deshpande Shishir , Jha Ratneshwar , Kaw Predhiman , Sen Abhijit , Singh Raghvendra
Published: July 2005

A novel two-dimensional (2D) fluid model is proposed for investigating flux-driven plasma turbulence in the tokamak edge and scrape-off layer (SOL). Unlike most previous turbulence simulations of this region, the 2D model treats the two regions in a consolidated manner with a smooth transition region in between. The unified 2D model is simpler and less computer intensive than 3D models, but captures most features of the 3D edge and 2D SOL turbulence. It also illustrates the influence of tokamak edge turbulence on the SOL transport, something not captured by earlier 2D SOL simulations. Existence of an equilibrium radial electric field in the edge and SOL regions has been found. Two different plasma conductivity models have been used for the simulations. Turbulence in the edge is characterized by radially elongated streamers and zonal flows. The streamer structures occasionally break mainly in a region where the radial electric field changes sign. A phenomenological condition for the breaking has been obtained. Effective diffusion co-efficient and density front propagation speed from the simulation have been calculated. Statistical properties of the particle transport obtained from this simulation are compared with earlier flux-driven 2D SOL turbulence simulations and also with Aditya tokamak results.
Evaluation of sliding friction and contact mechanics of elastomers based on dynamic-mechanical analysis
Authors: Le Gal André , Yang Xin , Klüppel Manfred
Published: July 2005

The paper presents a combined experimental and theoretical approach to the understanding of hysteresis and adhesion contributions to rubber friction on dry and lubricated rough surfaces. Based on a proper analysis of the temperature- and frequency-dependent behaviors of nonlinear viscoelastic materials such as filler reinforced elastomer materials, master curves for the viscoelastic moduli are constructed. It is shown that the classical williams–Landel–Ferry equation cannot be applied in its simple form, but needs the introduction of an energy term describing the temperature dependency of glassy polymer bridges, which transmit the forces within flocculated filler clusters. The activation energy for carbon black and silica-filled elastomers is compared based on two different evaluation methods. The obtained dynamic data are shown to be related to a different friction behavior of elastomers regarding the two filler systems. Theoretical predictions of the stationary frictional behavior of the systems are in fair agreement with the experimental friction data at low sliding velocities. It is found that the formulated adhesion plays a dominant role on rough dry surfaces within this range of velocities.
Formation of plasma and ion flux on a target, irradiated by an intense electron beam
Authors: Engelko Vladimir , Mueller Georg
Published: July 2005

Theoretical consideration shows that under the influence of an intense electron beam, first, an ion flux and then plasma are formed on the surface of a target. The target plasma is created when the density of the ion flux achieves a certain limiting value. The time necessary for the plasma formation depends on the beam current density and the efficiency of gas desorption and ionization. This time is few microseconds under typical vacuum conditions of about 5.0×10−5mbar, electron-beam current density in the range of 10A∕cm2, and kinetic energy of electrons in the range of 100keV. When the density of the ion flux reaches a limiting value the beam potential decreases to a level, which is half of the initial one. A transient layer is formed between the plasma boundary and the electron beam. For conditions mentioned above its length are few centimeters. The target plasma expands into the beam drift region, with a velocity increasing in time. The expansion of the target plasma is the main reason for neutralization of the electron-beam space charge.
The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH)
Authors: Wang Suyun , Paul Ankan , DeYonker Nathan J. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2005

Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear Σ+1 and Π1 stationary points, and for the bent ground-state X̃A′1, and excited-states 2A′1 and 1A″1. Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas–Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X̃A′1 and 1A″1 states agree fairly well with available experimental values. However, the 2A′1 structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5kcalmol−1 for the 2A′1 state and 52.6kcalmol−1 for the 1A″1 state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1A″1 state (53.1kcalmol−1) than those for the corresponding 2A′1 state (57.6kcalmol−1), suggesting that the 1A″1 state might be the first singlet excited state while the 2A′1 state might be the second singlet excited state.
Stargenfunctions, generally parametrized systems and a causal formulation of phase space quantum mechanics
Authors: Dias Nuno Costa , Prata João Nuno
Published: July 2005

We address the deformation quantization of generally parametrized systems displaying a natural time variable. The purpose of this exercise is twofold: first, to illustrate through a pedagogical example the potential of quantum phase space methods in the context of constrained systems and particularly of generally covariant systems. Second, to show that a causal representation for quantum phase space quasidistributions can be easily achieved through general parametrization. This result is succinctly discussed.
The planar-to-tubular structural transition in boron clusters from optical absorption
Authors: Marques Miguel A. L. , Botti Silvana
Published: July 2005

The optical response of the lowest-energy isomers of the B20 family is calculated using time-dependent density-functional theory within a real-space, real-time scheme. Significant differences are found among the absorption spectra of the clusters studied. We show that these differences can be easily related to changes in the overall geometry. Optical spectroscopy is thus an efficient tool to characterize the planar-to-tubular structural transition, known to be present in these boron-based systems.
Effect of chain stiffness and entanglements on the elastic behavior of end-linked elastomers
Authors: Bhawe Dhananjay M. , Cohen Claude , Escobedo Fernando A.
Published: July 2005

The effect of chain stiffness and entanglements on the elastic behavior and microscopic structure of cross-linked polymer networks was studied using Monte Carlo simulations. We investigated the behavior of entangled and entanglement-free networks at various degrees of chain stiffness and densities. Based on previous results that indicated that trapped entanglements prevent strain-induced order-disorder transitions in semiflexible chain networks, we prepared the entangled networks by end-linking the chains in very dilute conditions so as to minimize the extent of trapped entanglements. We also considered the entanglement-free case by using a “diamond” structure. We found that the presence of even a very small amount of trapped entanglements is enough to prevent a discontinuous strain-induced transition to an ordered phase. In these mildly entangled networks, a nematiclike order is eventually attained at high extensions but the elastic response remains continuous and the cross-links remain uniformly distributed through the simulation box. The entanglement-free diamond networks on the other hand show discontinuities in their stress-strain data. Networks at higher densities exhibit a more stable ordered phase and show an unusual staircaselike stress-strain curve. This is the result of a stepwise extension mechanism in which the chains form ordered domains that exclude the cross-links. Extension is achieved by increasing the number of these ordered domains in the strain direction. Cross-links aggregate in the spaces between these ordered domains and form periodic bands. Each vertical upturn in the stress-strain data corresponds to the existence of an integer number of ordered domains. This stepwise elastic behavior is found to be similar to that exhibited by some tough natural materials.
A robust design against comb electrode spacing variations for in-plane switching mode thin-film transistor liquid-crystal displays
Authors: Yoneya Makoto , Ohta Masuyuki , Kondo Katsumi
Published: July 2005

A method to reduce the transmittance nonuniformity caused by comb electrode spacing variations of in-plane switching mode thin-film transistor liquid-crystal displays is proposed. The method is based on the compensation of the effect of the comb electrode spacing variations with effective driving voltage variation via feed-through voltage in the thin-film transistor addressing. The conditions for this compensation were analyzed theoretically taking into account gate delay (recharging) effects. It was observed that the common dc driving was superior to the common inversion driving in terms of compensation efficiency. An alternative driving method was proposed to enhance the compensation in common inversion driving.
The combined effect of thermal annealing of MgO substrate and Ca substitution on the surface resistance of YBa2Cu3Oz thin films
Authors: Murugesan M. , Obara H. , Yamasaki H.
Published: July 2005

Single-layer Y1−xCaxBa2Cu3Oz (YCBCO) thin films (x=0.00, 0.02, 0.05, and 0.10) grown on annealed as well as unannealed MgO substrates have been systematically investigated for their carrier concentration, critical current density Jc, and microwave surface resistance Rs. For x⩽0.05, the grain growth follows a three-dimensional-spiral growth mechanism, while for x=0.10 we observed a mainly two-dimensional-like growth of grains. The results of Hall data reveal that the x=0.05 film is overdoped while films with x=0.02 and 0.10 are underdoped with respect to the x=0.00 film. However, the Hall mobility μH is highly enhanced for the x=0.02 film. Thermal annealing of MgO substrates prior to film deposition results to an improvement in the overall superconducting properties of the film such as suppression of normal-state resistivity, enhancement of Jc, and minimization of Rs both for pure as well as Ca-substituted films. Annealing of MgO substrates enhances the Jc value to a magnitude (i) nearly doubled for x=0.00 films and (ii) more than an order for x=0.02 films. Also it leads to a minimization of the Rs value to (i) more than half and (ii) nearly an order of magnitude higher, respectively, for x=0.00 and x=0.02 films. Furthermore, for the x=0.02 film, below 60K, we realized an enhanced Jc value in self- as well as in large-applied fields. For other than the Ca-2% substituted films, a suppression of Jc with a strong field dependency has been noticed. Furthermore, the Rs value of the x=0.02 film (0.1mΩ at 20K, 21.9GHz) was three times lower compared to that of the x=0.00 film (0.35mΩ at 20K, 21.9GHz). At 20K and 21.9GHz, the Rs value for the x=0.05 film is comparable to that of the x=0.00 film, whereas for the x=0.10 film it is twice that of x=0.00. The low normal-state resistivity, enhanced mobility, high Jc, and the minimized Rs observed for x=0.02 films firmly support the possible improvement of superconducting order parameters near the grain boundaries. Thus, the combined effect of thermal annealing of MgO prior to film deposition and substituting Y with 2% Ca may be readily applied to grow high-quality (high Jc and low Rs) YBCO films for microwave device applications.
Dynamics of direct-created trions in coupled double quantum wells: Electron-density effects
Authors: Aceituno P. , Hernández-Cabrera A.
Published: July 2005

We have studied the coherent dynamics of injected electrons when they are either free or bound in excitons and trions (charged excitons). We have considered a remotely doped asymmetric double quantum well where an excess of free electrons together with direct-created excitons generates trions. We have used the matrix density formalism to analyze the electron dynamics for different concentrations of the three species. Calculations show a significant modification of the free-electron intersublevel oscillations caused by electrons bound in excitons and trions. Based on these calculations we propose a method to detect trions through the emitted electromagnetic radiation or the current density.
On the existence and lifetimes of Cu2+ complexes with water, ammonia, and hydrogen cyanide
Authors: Palacios Alicia , Corral Inés , Mó Otilia , Martín Fernando , Yáñez Manuel
Published: July 2005

High-level ab initio calculations have been carried out to evaluate the lifetimes of complexes formed by the association of Cu2+ to water, ammonia, and hydrogen cyanide. The corresponding binding energies were evaluated at the CCSD(T)∕6-311+G(3df,2pd) level of theory. The potential-energy curves corresponding to their dissociation into Cu++L+ (L=H2O, NH3, and HCN) were obtained at the CCSD(T)∕6-311+G(3df,2p) level on BHLYP∕6-311+G(d,p) optimized geometries. Lifetimes were calculated using the exterior complex scaling and the semiclassical WKB methods. Although all the complexes investigated are thermodynamically unstable with respect to their dissociation into Cu++L+ in a typical Coulomb explosion, the activation barrier is high enough to accommodate several vibrational resonances, with very large lifetimes. As a matter of fact, if the three complexes are produced in the lowest vibrational levels, they behave as totally bound (with almost infinite lifetime) species.
Scattering lengths for Li–Cs, Na–Cs, K–Cs, and Rb–Cs ultracold collisions
Authors: Zanelatto A. L. M. , Ribeiro E. M. S. , Napolitano R. d. J.
Published: July 2005

We present triplet and singlet scattering lengths for X–Cs133 collisions (X=Li6,7, Na23, K39,40,41, and Rb85,87). We consider the short-range potential-energy curves based on high-quality ab initio calculations and the long-range potential described by accurate dispersion coefficients available in the literature. Using a Fermi function to smoothly connect the short- and long-range terms we construct the potential for all R and evaluate the scattering length for each mixed species X–Cs. In particular, we predict that the cross section for inelastic processes between Na23 and Cs133 and between K40 and Cs133 will be small as compared with the respective elastic cross sections.
Two new double Rydberg anions plus access to excited states of neutral Rydberg radicals via anion photoelectron spectroscopy
Authors: Radisic Dunja , Stokes Sarah T. , Bowen Kit H.
Published: July 2005

We have observed and characterized two new double Rydberg anions N6H19− and N7H22− through their anion photoelectron spectra. The vertical detachment energies of these anions were found to be 0.443 and 0.438eV, respectively. In addition, for three of the seven double Rydberg anions now known, we measured photodetachment transitions not only to the ground electronic states of their corresponding neutral Rydberg radicals but also to their first electronically excited states. In each spectrum, the energy spacing between the resulting peaks provided the ground-to-first electronically excited-state transition energy for the double Rydberg anion’s corresponding neutral Rydberg radical. For the radicals, N4H13, N5H16, and N6H19, the spacings were found to be 0.83, 0.70, and 0.67eV, respectively. These values are in excellent agreement with ground-to-first excited-state transition energies measured in absorption for the same neutral Rydberg radicals by Fuke and co-workers [Eur. Phys. J. D 9, 309 (1999); J. Phys. Chem. A 106, 5242 (2002).] The duplication of this neutral Rydberg property by photodetachment of double Rydberg anions further confirms that double Rydberg anions are indeed the negative ions of their corresponding neutral Rydberg molecules and cluster-like systems.
Dark current and trailing-edge suppression in ultrafast photoconductive switches and terahertz spiral antennas fabricated on multienergy arsenic-ion-implanted GaAs
Authors: Liu Tze-An , Lin Gong-Ru , Lee Yen-Chi , Wang Shing-Chung , Tani Masahiko , Wu Hsiao-Hua , Pan Ci-Ling
Published: July 2005

We report ultrafast (∼2.7ps, instrument limited) switching responses of a multienergy-implanted GaAs:As+ photoconductive switches (PCSs) with suppressed trailing edge and reduced dark current. This material is highly resistive with dark current as low as 0.94μA at a bias of 40V. The carrier mobility of the former is ∼590cm2∕Vs, resulting in a small-signal optical responsivity of ∼2mA∕W. Pumped at 100mW and biased at 80V, the multienergy-implanted GaAs:As+ PCS exhibits peak response (0.35V) comparable to the best result of single-energy-implanted ones. The improvement on photoconductive response is crucial for the generation of shorter terahertz emission pulses from spiral antennas fabricated on multienergy-implanted GaAs:As+ (0.8ps) than single-energy-implanted GaAs (1.2ps), with the central frequency blueshifted to 0.2THz (from 0.15THz) and the spectral bandwidth broadened to 0.18THz (from 0.11THz).
Highly sensitive micro-Hall devices based on Al0.12In0.88Sb∕InSb heterostructures
Authors: Kunets Vas. P. , Black W. T. , Mazur Yu. I. , Guzun D. , Salamo G. J. , Goel N. , Mishima T. D. , Deen D. A. , Murphy S. Q. , Santos M. B.
Published: July 2005

Micro-Hall devices based on modulation-doped Al0.12In0.88Sb∕InSb heterostructures are fabricated and studied in terms of sensitivity and noise. Extremely high supply-current-related magnetic sensitivities of 1800VA−1T−1 at 77K and 1220VA−1T−1 at 300K are reported and observed to be independent of the bias current. The detection limit of the devices studied at low and room temperature are at nanotesla values throughout the broad frequency range from 20Hzto20kHz. The low detection limit of 28nT at 300K and 18nT at 77K were found at high frequencies where the Johnson noise is dominant. A measured detection limit per unit device width of 630pTmmHz−1∕2 is reported indicating the potential for picotesla detectivity.
Formation of dual-phase HfO2–HfxSi1−xO2 dielectric and its application in memory devices
Authors: Wang Ying Qian , Chen Jing Hao , Yoo Won Jong , Yeo Yee-Chia , Chin Albert , Du An Yan
Published: July 2005

In this paper, we studied the phase-separation phenomenon of Hf0.5Si0.5O2 film deposited on SiO2 or sandwiched by SiO2, by x-ray photoelectron spectroscopy and transmission electron microscopy. The Hf0.5Si0.5O2 film underwent phase separation to form a doublet-phase HfO2–HfxSi1−xO2 (x<0.5) film, and was used as a trapping layer in a metal-blocking oxide-silicon nitride-tunnel oxide-silicon-type memory structure, where the dual-phase HfO2–HfxSi1−xO2 (DPHSO) film replaces the conventional silicon nitride (Si3N4) trapping layer. The charge storage properties of the DPHSO film were investigated and compared with HfO2 and Si3N4. It was found that for a given electric field applied to the tunnel oxide, the programming speed of memory devices using a DPHSO or HfO2 film as a trapping layer is faster than that using Si3N4. This indicates the higher electron-capture efficiency of the DPHSO and HfO2 films. In addition, the double-phase microstructure of the DPHSO film also provided better retention property than pure HfO2.
Study of electromigration in thin tin film using edge displacement method
Authors: Yu H. C. , Liu S. H. , Chen Chih
Published: July 2005

Threshold current density and other electromigration parameters of pure Sn films were measured using edge displacement method. Sn film with a thickness of 5000Å was evaporated on a 1200-Å-thick Ti film on a Si substrate. Electromigration behavior was investigated under the current densities of 2.5×104–1.5×105A∕cm2 at room temperature (RT 27–32°C), 50, 75, and 100°C. Both needle-type and hillock-type whiskers grew in the anode end when the films were stressed at RT and 50°C, but only hillock-type whiskers were observed when they were stressed at 75 and 100°C. The electromigration rate increased linearly with the applied current density for the four stressing temperatures. The threshold current density (Jc) was measured to be 1.93×104, 9.65×103, 9.57×103, and 7.93×103A∕cm2 for RT, 50, 75, and 100°C, respectively. The measured activation energy was 0.32eV. In addition, the measured critical length of the Sn film was 18μm at RT and the products of DZ* were 1.95×10−10, 4.84×10−10, 1.27×10−9, and 1.99×10−9cm2∕s for RT, 50, 75, and 100°C, respectively. These results are fundamental to electromigration in Pb-free solders, since most of their matrices consist of almost pure Sn.
Dynamical clusters of infinite particle dynamics
Authors: Malyshev V. A.
Published: July 2005

For any system {i} of particles with the trajectories xi(t) in Rd on a finite time interval [0,τ] we define the interaction graph G. Vertices of G are the particles, there is an edge between two particles i and j iff for some t∊[0,τ] the distance between particles i and j is not greater than some constant. We undertake a detailed study of this graph for infinite particle dynamics and prove exponential estimates for its finite connected components. This also solves continuous percolation problem for complicated geometrical objects—the tubes around particle trajectories.
Multifractal subgrid-scale modeling for large-eddy simulation. I. Model development and a priori testing
Authors: Burton Gregory C. , Dahm Werner J. A.
Published: July 2005

Results are presented from a new approach to modeling the subgrid-scale stresses in large-eddy simulation of turbulent flows, based on explicit evaluation of the subgrid velocity components from a multifractal representation of the subgrid vorticity field. The approach is motivated by prior studies showing that the enstrophy field exhibits multifractal scale-similarity on inertial-range scales in high Reynolds number turbulence. A scale-invariant multiplicative cascade thus gives the spatial distribution of subgrid vorticity magnitudes within each resolved-scale cell, and an additive cascade gives the progressively isotropic decorrelation of subgrid vorticity orientations from the resolved scale Δ to the viscous scale λν. The subgrid velocities are then obtained from Biot–Savart integrals over this subgrid vorticity field. The resulting subgrid velocity components become simple algebraic expressions in terms of resolved-scale quantities, which then allow explicit evaluation of the subgrid stresses τij*. This new multifractal subgrid-scale model is shown in a priori tests to give good agreement for the filtered subgrid velocities, the subgrid stress components, and the subgrid energy production at both low (ReΔ≈160) and high (ReΔ≈2550) resolved-scale Reynolds numbers. Implementing the model is no more computationally burdensome than traditional eddy-viscosity models. Moreover, evaluation of the subgrid stresses requires no explicit differentiation of the resolved velocity field and is therefore comparatively unaffected by discretization errors.
Multifractal subgrid-scale modeling for large-eddy simulation. II. Backscatter limiting anda posteriorievaluation
Authors: Burton Gregory C. , Dahm Werner J. A.
Published: July 2005

Results are presented from a posteriori evaluations of momentum and energy transfer between the resolved and subgrid scales when the multifractal subgrid-scale model from Part I is implemented in a flow solver for large-eddy simulations of turbulent flows. The multifractal subgrid-stress model is used to evaluate the subgrid part τij* of the stress tensor, with the resolved part u¯iu¯j¯ evaluated by an explicit filter. It is shown that the corresponding subgrid and resolved contributions P* and PR to the resolved-scale energetics produce extremely accurate results for the combined subgrid energy production field P(x,t). A separate backscatter limiter is developed here that removes spurious energy introduced in the resolved scales by including physical backscatter, without sacrificing the high fidelity in the stress and energy production fields produced by the multifractal subgrid-scale model. This limiter makes small reductions only to those components of the stress that contribute to backscatter, and principally in locations where the gradients are large and thus the energy introduced by numerical errors is also largest. Control of the energy introduced by numerical error is thus accomplished in a manner that leaves the modeling of the subgrid-scale turbulence largely unchanged. The multifractal subgrid-scale model and the backscatter limiter are then implemented in a flow solver and shown to provide stable and accurate results in a posteriori tests based on large-eddy simulations of forced homogeneous isotropic turbulence at cell Reynolds numbers ranging from 160⩽ReΔ⩽106, as well as in simulations of decaying turbulence where the model and the limiter must adjust to the changing subgrid conditions.
Dynamo action in a rotating spherical shell at high Rayleigh numbers
Authors: Takahashi Futoshi , Matsushima Masaki
Published: July 2005

We investigate convection-driven dynamos in a rotating spherical shell with the Rayleigh number Ra up to about 53 times the critical value Rac, emphasizing Rayleigh number dependence of the thermal convection and the magnetic field generated by dynamo action. The Rayleigh numbers used in calculations are chosen so as to be in a range which allows us to study the sequence of bifurcation. In the low-Ra-dynamo regime, the flow structure is characterized by columnar convection cells, which mainly generate the magnetic field that is predominantly dipolar. Force balance is essentially in a geostrophic state. Both the magnetic energy and the kinetic energy increase with increase in Ra. In the moderate-Ra-dynamo regime, convective motions appear inside the tangent cylinder (TC), where the azimuthal magnetic field is generated through the so-called ω effect. However, the magnetic energy shows saturation due to relatively inefficient magnetic field generation. In the high-Ra-dynamo regime, dominance of convection inside the TC is remarkable. The advection processes play important roles both in force balance and in magnetic field generation. The magnetic field is generated very inefficiently, leading to the reduced magnetic energy in spite of higher kinetic energy. These three dynamo regimes exhibit distinctive differences in the process of generating magnetic field and characteristic dissipation scales.
Bistability in a complementary metal oxide semiconductor inverter circuit
Authors: Carroll Thomas L.
Published: July 2005

Radiofrequency signals can disrupt the operation of low frequency circuits. A digital inverter circuit would seem to be immune to such disruption, because its output state usually jumps abruptly between 0 and 5 V. Nevertheless, when driven with a high frequency signal, the inverter can have two coexisting stable states (which are not at 0 and 5 V). Slow switching between these states (by changing the rf signal) will produce a low frequency signal. I demonstrate the bistability in a circuit experiment and in a simple model of the circuit.
Non-Hamiltonian dynamics of grains with spatially varying charges
Authors: Zhdanov S. K. , Ivlev A. V. , Morfill G. E.
Published: July 2005

Dynamics of particles with spatially varying charges is studied. It is shown that the motion cannot be described in terms of the Hamiltonian formalism. Strong interparticle interactions give rise to unusual features of the dynamics, e.g., they can cause self-sustaining nonlinear oscillations triggered at finite frictional dissipation. The comprehensive analysis of the non-Hamiltonian dynamics is carried out analytically and numerically by employing a one-dimensional chain of charged particles as a model system. Analysis of available experimental data is performed and implications to strongly coupled complex (dusty) plasmas are discussed.
Nucleation of pentacene on silicon dioxide at hyperthermal energies
Authors: Killampalli Aravind S. , Schroeder Todd W. , Engstrom James R.
Published: July 2005

The nucleation of pentacene on silicon dioxide, incident at hyperthermal energies, has been investigated with atomic force microscopy. The incident kinetic energy of the pentacene molecules strongly influences the process of adsorption—the adsorption probability decreases with increasing incident energy, indicative of trapping-mediated adsorption. In addition, the trapping probability of pentacene decreases with more glancing angles of incidence, a result inconsistent with so-called normal energy scaling. Analysis of the dependence of the island density on the growth rate in the submonolayer regime indicates that growth at all energies is consistent with a critical cluster containing four molecules.
Resonant effect of a background turbulence on the rotating magnetic island
Authors: Marchenko V. S.
Published: July 2005

Resonant interaction between wave packets of the background, short wavelength ion-temperature-gradient mode turbulence and macroscopic, rotating magnetic island is considered. Corresponding contribution to the toroidal torque balance is calculated. This resonant torque is comparable with nonresonant viscous torque produced by turbulent transport of toroidal momentum. Balance of resonant and viscous torques results in the equilibrium island rotation frequency residing in the gap where contribution of the polarization current to the modified Rutherford equation is stabilizing.
Evidence of interface-induced persistent photoconductivity in InP∕In0.53Ga0.47As∕InP double heterostructures grown by molecular-beam epitaxy
Authors: Hudait M. K. , Lin Y. , Goss S. H. , Smith P. , Bradley S. , Brillson L. J. , Johnston S. W. , Ahrenkiel R. K. , Ringel S. A.
Published: July 2005

The impact of interface switching sequences on interface quality and minority carrier recombination in In0.53Ga0.47As∕InP double heterostructure (DH) grown by solid-source molecular-beam epitaxy (MBE) was studied. As2 exposure at the lower In0.53Ga0.47As∕InP interface prior to In0.53Ga0.47As growth was found to cause enhanced As diffusion into the underlying InP that correlates with steadily increased photoconductive decay (PCD) lifetimes beyond the theoretical radiative and Auger limit. Low-temperature PCD measurements reveal that a persistent photoconductivity (PPC) process is responsible for the high “apparent” lifetimes. The PPC effect increases monotonically with As2 exposure on the InP surface, implying the involvement of interfacial defects in the carrier recombination dynamics of In0.53Ga0.47As∕InP DHs grown by MBE.
Quality factors of qubit rotations in single semiconductor quantum dots
Authors: Wang Q. Q. , Muller A. , Bianucci P. , Shih C. K. , Xue Q. K.
Published: July 2005

The quality factors of quantum bit (qubit) rotations in single self-assembled quantum dots were investigated using photoluminescence spectroscopy. Rotations with azimuthal angle ϕ along the equator of the qubit sphere were implemented by varying the time delay between two π∕2 pulses, resulting in a “free rotation” quality factor Q0=9.8×104. Qubit rotations with polar angle θ as large as 10π along the meridian of the qubit sphere (Rabi oscillations with zero detuning) were realized with a quality factor QR∼18, in experiments probing an excited excitonic state.
Tuning of electronic coupling between self-assembled quantum dots
Authors: Rybchenko S. I. , Itskevich I. E. , Skolnick M. S. , Cahill J. , Tartakovskii A. I. , Hill G. , Hopkinson M.
Published: July 2005

Semiconductor self-assembled quantum dots (SAQDs) normally have zero-dimensional properties, but become coupled and acquire higher-dimensional character if the distance between the dots is small. Using photoluminescence spectroscopy under high hydrostatic pressure, we have obtained clear evidence for electronic coupling due to quantum-mechanical tunneling in stacks of InGaAs∕GaAs SAQDs. We demonstrate that application of pressure allows controllable tuning and suppression of the electronic coupling. The effect originates from a pressure-induced increase in the effective mass of Γ-electrons and a related increase in the interdot-barrier height.
Nanoscopic friction as a probe of local phase transitions
Authors: Szoszkiewicz Robert , Riedo Elisa
Published: July 2005

We study nanoscopic friction forces between an atomic force microscope tip and a glass sample. We show how and why it is possible to tune friction forces in a predictable way by changing either the sample temperature, or the humidity in the experimental chamber. We relate the friction behavior to confined water phase transitions. We find that the water gas-liquid phase diagram is the same at the macroscopic scale as at the nanoscopic tip-sample contact.
Crossover from negative to positive magnetoresistance in La0.7Ce0.3MnO3-SrTiO3-Nb heterojunctions
Authors: Sheng Z. G. , Song W. H. , Sun Y. P. , Sun J. R. , Shen B. G.
Published: July 2005

A crossover from negative to positive magnetoresistance (MR) is observed in the heterojunction composed of a La0.7Ce0.3MnO3 film and a 0.5wt.% Nb-doped SrTiO3 substrate. The temperature and bias current dependences of MR sign in the junction are investigated carefully. It is found that the positive MR occurs when temperature or applied bias current is increased to a higher value. The relation between MR and an external magnetic field is also found to be temperature dependent. We attribute the appearance of the positive MR at higher temperatures to the tunneling transport mechanism. These results are helpful in configuring artificial devices using manganite-based heterojunctions.
Zn
x
Cd
1
−
x
Se
alloy nanowires covering the entire compositional range grown by metalorganic chemical vapor deposition
Authors: Shan C. X. , Liu Z. , Ng C. M. , Hark S. K.
Published: July 2005

We show that preferentially oriented, single-crystalline ZnxCd1−xSe alloy nanowires can be grown on GaAs (100) surface using Au as a catalyst over the entire compositional range in a metalorganic chemical vapor deposition system. The composition of the alloy nanowires can be simply adjusted through the ratio of the flow rates of group-II precursors. Electron microscopy shows that the nanowires are smooth and uniform in shape; their diameters range from 20 to 80 nm and lengths exceed a few micrometers. Nanowires containing more than 13% Zn are zinc blende structured and grow along the ⟨110⟩ direction. Those containing less Zn are wurtzite structured and grow along the ⟨210⟩ direction. Compared with the bulk alloy, the change from zinc blende to wurtzite structure in nanowires occurs at far smaller x. The preferred orientation and the persistence of the zinc blende structure both reflect the influence of the substrate on the growth of the nanowires. Photoluminescence measurements identify a strong near-band-edge emission for all samples and show that its peak energy tracks the band gap of ZnxCd1−xSe epilayer for x>0.13. The growth of alloy nanowires at many compositions opens up the possibility of realizing quasi-one-dimensional heterojunctions.
Infrared capacity mapping of semiconductor junctions by lock-in thermography
Authors: Pohl Peter , Brendel Rolf
Published: July 2005

We image the reverse-bias-modulated space charges in a heated p-n junction using infrared (IR) lock-in thermography. The modulation of the space charges leads to a change of the free carrier IR emission signal of the sample, which is detected by an IR camera. This way charge carrier densities are measured, which in combination with the voltage applied yield the capacitance of the junction. Experimentally measured capacitance-voltage curves agree with the theoretical model of an abrupt p-n junction. Using lock-in thermography, we deduce the spatially resolved acceptor doping concentration of a crystalline silicon wafer. A sensitivity analysis shows that our system detects a noise equivalent capacitance of 1nFcm−2 after a measurement period of 28 min at a lateral resolution of 170μm.