bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and transport properties of complementary germanium nanowire field-effect transistors
Authors: Greytak Andrew B. , Lauhon Lincoln J. , Gudiksen Mark S. , Lieber Charles M.
Published: May 2004

n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 μA/μm and 4.9 μA/V, respectively, with device yields of >85%.
Cold electron Josephson transistor
Authors: Savin A. M. , Pekola J. P. , Flyktman J. T. , Anthore A. , Giazotto F.
Published: May 2004

A superconductor-normal metal-superconductor mesoscopic Josephson junction has been realized in which the critical current is tuned through normal current injection using a symmetric electron cooler directly connected to the weak link. Both enhancement of the critical current by more than a factor of two, and supercurrent suppression have been achieved by varying the cooler bias. Furthermore, this transistor-like device demonstrates large current gain (∼20) and low power dissipation.
Coercivity enhancements by high-magnetic-field annealing in sintered Nd–Fe–B magnets
Authors: Kato Hiroaki , Miyazaki Terunobu , Sagawa Masato , Koyama Keiichi
Published: May 2004

An influence of a high-field annealing on the magnetic properties was investigated in sintered Nd–Fe–B magnets. When the sample B, containing 0.32 at. % Al, was annealed at Ta=550 °C under a magnetic field of Ha=140 kOe and then cooled without field, the coercivity Hc increased by 25% with respect to the control sample annealed at zero field. As for the sample C, containing both Cu 0.13 at. % and Al 0.32 at. %, higher Hc values were obtained when it was annealed at Ta=550 °C and the field of Ha=140 kOe was kept applied down to the room temperature. These results were discussed in terms of a field-induced change in interface matching between Nd2Fe14B and intergranular phases.
Influence of O and C co-implantation on the lattice site of Er in GaN
Authors: De Vries B. , Matias V. , Vantomme A. , Wahl U. , Rita E. M. C. , Alves E. , Lopes A. M. L. , Correia J. G. ,   The ISOLDE Collaboration
Published: May 2004

The lattice location of low-dose implanted Er in GaN, GaN:O, and GaN:C was investigated using the emission channeling technique. The conversion electrons emitted by the probe isotope 167mEr give direct evidence that the majority (≈90%) of Er atoms are located on substitutional Ga sites for all samples. Annealing up to 900 °C does not change these fractions, although it reduces the Er root-mean-square (rms) displacements. The only visible effect of oxygen or carbon doping is a small increase in the rms displacements with respect to the undoped sample.
Electronic structure of Zn1−xCoxO using photoemission and x-ray absorption spectroscopy
Authors: Wi S. C. , Kang J.-S. , Kim J. H. , Cho S.-B. , Kim B. J. , Yoon S. , Suh B. J. , Han S. W. , Kim K. H. , Kim K. J. , Kim B. S. , Song H. J. , Shin H. J. , Shim J. H. , Min B. I.
Published: May 2004

Electronic structures of homogeneous bulk samples of Zn0.9Co0.1O which do not exhibit diluted ferromagnetic semiconducting (DMS) behavior have been investigated using photoemission spectroscopy and x-ray absorption spectroscopy. We have found that the Co ions in Zn1−xCoxO are in the divalent Co2+(d7) states under the tetrahedral symmetry. Our finding indicates that the DMS properties will not be produced when Co ions are properly substituted for Zn sites, implying that the DMS properties observed in Zn1−xCoxO thin films are likely to be extrinsic.
Transition of deformation and fracture behaviors in nanostructured face-centered-cubic metals
Authors: Li Hongqi , Ebrahimi Fereshteh
Published: May 2004

Tensile stress–strain curves demonstrate that single-phase nanocrystalline face-centered-cubic (fcc) metals are intrinsically ductile and their failure begins with necking. However, the area reductions and the fracture behaviors were found to be dependent on the grain size. When plastic deformation is governed by dislocation activity, the nanocrystalline samples behave similar to the conventional coarse-grained materials. As the grain size is reduced to the regime where grain boundary sliding dominates, the material shows very high strain-hardening rate and the tensile samples fail by microcracking with no noticeable reduction in area.
Laser trapping and manipulation under focused evanescent wave illumination
Authors: Gu Min , Haumonte Jean-Baptiste , Micheau Yoan , Chon James W. M. , Gan Xiaosong
Published: May 2004

Laser trapping is based on the radiation pressure on a small particle in the focal region of a high numerical-aperture objective. Currently, the focal spot of a trapping beam is elongated along the longitudinal direction and thus the axial size of the trapping volume is approximately three times larger than that in the transverse direction. We report on a laser trapping technique under focused evanescent wave illumination. Therefore laser trapping of micro/nano-objects can be achieved in the near-field region with an axial trapping size of approximately 60 nm, which is reduced by approximately one order of magnitude. Hence, this technique is of significant importance in nanometry including single molecule detection and manipulation.
Brillouin observation of bulk and confined acoustic waves in silica microspheres
Authors: Lim H. S. , Kuok M. H. , Ng S. C. , Wang Z. K.
Published: May 2004

Brillouin scattering from loose silica microspheres has been investigated as a function of their diameter (140 nm to 4 μm). The measured linear dependence of the confined acoustic mode frequencies on the inverse sphere diameters accords well with Lamb’s theory. Bulk acoustic modes are also observed in the larger microspheres, suggesting that the limiting size of a particle for these modes to be observable is about three times their wavelength. Internal consistency in the analyses of the bulk and confined mode data shows the validity of this Brillouin technique in the evaluation of the elastic properties of microspheres. Additionally it also affords a means of sizing the diameters of microspheres.
High-energy x-ray scattering in grazing incidence from nanometer-scale oxide wires
Authors: Sakata Osami , Takata Masaki , Suematsu Hiroyoshi , Matsuda Akifumi , Akiba Shusaku , Sasaki Atsushi , Yoshimoto Mamoru
Published: May 2004

A method has been developed for analyzing the structure of crystalline nanowires deposited on a crystal surface. It combined in-air grazing-incidence surface scattering with high-energy synchrotron x-ray scattering. The technique has allowed a direct reciprocal-lattice space mapping of the x-ray intensities scattered from ultrathin nanowires. Using this method, the sheet-shape diffraction emanating from ultrathin NiO wires was observed. The average nanowire–nanowire distance of 46 nm and a crystallographic domain size of approximately 7.5 nm across the nanowire were evaluated.
High-speed 1.55 μm operation of low-temperature-grown GaAs-based resonant-cavity-enhanced p–i–n photodiodes
Authors: Butun B. , Biyikli N. , Kimukin I. , Aytur O. , Ozbay E. , Postigo P. A. , Silveira J. P. , Alija A. R.
Published: May 2004

We report the design, growth, fabrication, and characterization of GaAs-based high-speed p–i–n photodiodes operating at 1.55 μm. A low-temperature-grown GaAs (LT-GaAs) layer was used as the absorption layer and the photoresponse was selectively enhanced at 1.55 μm using a resonant-cavity-detector structure. The bottom mirror of the resonant cavity was formed by a highly reflecting 15-pair GaAs/AlAs Bragg mirror. Molecular-beam epitaxy was used for wafer growth, where the active LT-GaAs layer was grown at a substrate temperature of 200 °C. The fabricated devices exhibited a resonance around 1548 nm. When compared to the efficiency of a conventional single-pass detector, an enhancement factor of 7.5 was achieved. Temporal pulse-response measurements were carried out at 1.55 μm. Fast pulse responses with 30 ps pulse-width and a corresponding 3 dB bandwidth of 11.2 GHz was measured.
A probe of intrinsic valence band electronic structure: Hard x-ray photoemission
Authors: Takata Y. , Tamasaku K. , Tokushima T. , Miwa D. , Shin S. , Ishikawa T. , Yabashi M. , Kobayashi K. , Kim J. J. , Yao T. , Yamamoto T. , Arita M. , Namatame H. , Taniguchi M.
Published: May 2004

Hard x-ray valence band photoemission spectroscopy (PES) is realized using high-energy and high-brilliance synchrotron radiation. High-energy (∼6 keV) excitation results in larger probing depths of photoelectrons compared to conventional PES, and enables a study of intrinsic electronic property of materials in actual device structures much less influenced by surface condition. With this technique, requirements for surface preparation are greatly reduced, if not eliminated. It is a nondestructive tool to determine electronic structure from surface to genuine bulk as shown by a study on SiO2/Si(100). Electronic structure modification related to the ferromagnetism in the diluted magnetic semiconductor Ga0.96Mn0.04N is also observed.
Determination of the maximum gelation temperature in gelatin gels
Authors: Tosh Susan M. , Marangoni Alejandro G.
Published: May 2004

The maximum gelation temperature, Tgel, is widely used to characterize biopolymers that form thermoreversible gels. The Tgel of gelatin was determined using rheological techniques and compared to other methods in common use. The methods developed in this study defined a Tgel for gelatin, which was independent of gelatin concentration, and which corresponded closely to the theta temperature determined by light scattering.
Generalized Snell’s law and its possible relation to coherent backscattering of ultrasonic waves
Authors: Declercq Nico F. , Degrieck Joris , Leroy Oswald
Published: May 2004

The cause of backscattering in the Rayleigh angle has first been explained by means of finite beam models in which there are inherently present backward traveling components that explain the backscattering effect. Later, the nature of backscattered sound was proved to consist mostly of incoherent sound due to material anomalies. The present work shows that besides the well known real Snell’s law: i.e., continuity of the frequency and continuity of the wave vector component along the interface, there is also the possibility of a complex solution of Snell’s condition of continuity. The latter shows that it is possible that a part of the incident sound gets reflected into nonspecular directions including the backscatter direction. Furthermore, it is shown that this sound must have a different frequency than the incident frequency.
Voltage gain effect in a ring-type magnetoelectric laminate
Authors: Dong Shuxiang , Li J. F. , Viehland D.
Published: May 2004

It has been observed that a ring-type magnetoelectric laminate composite of circumferentially magnetized magnetostrictive Tb1−xDyxFe2 and circumferentially polarized piezoelectric Pb(Zr,Ti)O3 layers has a large magnetoelectric voltage gain effect, offering potential in high-power miniature transformer applications.
Observation of photoluminescence emission in ferromagnetic semiconductor GaCrN
Authors: Hashimoto M. , Tanaka H. , Asano R. , Hasegawa S. , Asahi H.
Published: May 2004

Optical properties of GaN-based diluted magnetic semiconductor GaCrN were studied. The GaCrN layers were grown by electron-cyclotron-resonance plasma-assisted molecular-beam epitaxy. They exhibited ferromagnetic behavior at room temperature and strong photoluminescence (PL) emission at 3.29 eV (10 K). The PL emission peak energy of the GaCrN decreases with increasing temperature in accordance with the Varshini formula similar to the GaN excitonic transition peak. The 3.29 eV PL emission was assigned to be a band-to-band transition in GaCrN from the temperature- and the excitation-power-density dependences of the PL.
Fluidic adaptive lens of transformable lens type
Authors: Zhang De-Ying , Justis Nicole , Lo Yu-Hwa
Published: May 2004

Fluidic adaptive lenses with a transformable lens type were demonstrated. By adjusting the fluidic pressure, not only can the lens properties, such as the focal distance and numerical aperture, be tuned dynamically but also different lens types, such as planoconvex, planoconcave, biconvex, biconcave, positive meniscus, and negative meniscus lenses, can be formed. The shortest focal length for a 20 mm aperture adaptive lens is 14.3 mm when the device is transformed into a positive lens, and −6.3 mm when transformed into a negative lens. The maximum resolution of the fluidic lens is better than 40 line pairs/mm.
Thermal conductivity improvement of silicone elastomer with carbon nanotube loading
Authors: Liu C. H. , Huang H. , Wu Y. , Fan S. S.
Published: May 2004

Unpurified carbon nanotubes were introduced to silicone elastomer to investigate their effect on the thermal conductivity. Microstructure studies by a scanning electron microscope showed that the carbon nanotubes (CNTs) can be well dispersed in the matrix by the grinding method. No notable agglomerates or phase separation between the carbon and silicone matrix were observed, and the CNTs were individually in random orientation. The thermal conductivities of the composites were measured with the ASTM (American Society of Testing Materials) D5470 method. The thermal conductivities κ were found to increase with the carbon amount. There was a 65% enhancement in κ with 3.8 wt % CNT loading. The enhancement by equal loading of carbon black was found to be a little lower than that by the CNT loading. The composites loaded with CNTs displayed an abrupt increase in the electrical conductivity.
Origin and repartition of the oxide fixed charges generated by electrical stress in memory tunnel oxide
Authors: Bernardini Sandrine , Masson Pascal , Houssa Michel , Lalande Frederic
Published: May 2004

A method is proposed to determine the origin and the spatial oxide fixed charge repartition in memory tunnel oxide from Fowler–Nordheim current measurements after electrical stress. The Poisson equation resolution in the dielectric layer is required to account for the nonsymmetric tunneling barrier deformation, resulting from charges generated within the dielectric layer. From current–voltage characteristics measurements and simulations, we have determined the spatial distribution of the oxide fixed charges within the dielectric layer of metal/SiO2/metal structures. In addition, the kinetics of the oxide charge generation can be explained by a dispersive hydrogen transport model.
Blue and near-ultraviolet light-emitting diodes on free-standing GaN substrates
Authors: Cao X. A. , LeBoeuf S. F. , D’Evelyn M. P. , Arthur S. D. , Kretchmer J. , Yan C. H. , Yang Z. H.
Published: May 2004

Blue and near-ultraviolet (UV) InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with peak emission at 465 nm and 405 nm, respectively, were grown on GaN and sapphire substrates. The densities of surface and bulk defects in the homoepitaxially grown LEDs were substantially reduced, leading to a decrease in reverse currents by more than six orders of magnitude. At a typical operating current of 20 mA, the internal quantum efficiency of the UV LED on GaN was twice as high compared to the UV LED on sapphire, whereas the performance of the blue LEDs was found to be comparable. This suggests that the high-density dislocations are of greater influence on the light emission of the UV LEDs due to less In-related localization effects. At high injection currents, both the blue and UV LEDs on GaN exhibited much higher output power than the LEDs on sapphire as a result of improved heat dissipation and current spreading.
Role of Brownian motion in the enhanced thermal conductivity of nanofluids
Authors: Jang Seok Pil , Choi Stephen U. S.
Published: May 2004

We have found that the Brownian motion of nanoparticles at the molecular and nanoscale level is a key mechanism governing the thermal behavior of nanoparticle–fluid suspensions (“nanofluids”). We have devised a theoretical model that accounts for the fundamental role of dynamic nanoparticles in nanofluids. The model not only captures the concentration and temperature-dependent conductivity, but also predicts strongly size-dependent conductivity. Furthermore, we have discovered a fundamental difference between solid/solid composites and solid/liquid suspensions in size-dependent conductivity. This understanding could lead to design of nanoengineered next-generation coolants with industrial and biomedical applications in high-heat-flux cooling.
Quantized charge pumping through a quantum dot by surface acoustic waves
Authors: Ebbecke J. , Fletcher N. E. , Janssen T. J. B. M. , Ahlers F. J. , Pepper M. , Beere H. E. , Ritchie D. A.
Published: May 2004

We present a realization of quantized charge pumping. A lateral quantum dot is defined by metallic split gates in a GaAs/AlGaAs heterostructure. A surface acoustic wave whose wavelength is twice the dot length is used to pump single electrons through the dot at a frequency f=3 GHz. The pumped current shows a regular pattern of quantization at values I=nef over a range of gate voltage and wave amplitude settings. The observed values of n, the number of electrons transported per wave cycle, are determined by the number of electronic states in the quantum dot brought into resonance with the Fermi level of the electron reservoirs during the pumping cycle.
Electric field and temperature-induced removal of moisture in nanoporous organosilicate films
Authors: Biswas N. , Lubguban J. A. , Gangopadhyay S.
Published: May 2004

The effects of bias-temperature-stress (BTS) or simply temperature-stress (TS) on nanoporous low-k methylsilsesquioxane films are studied. Initially, the as-given and O2 ashed/etched films exhibit physical adsorption of moisture as revealed from the electrical behavior of the samples after 15 days. The temperature stressing at 170 °C volatilized the adsorbed water but is unable to remove chemisorb and hydrophillic Si–OH groups. As a result, the TS films remain susceptible to moisture. BTS at 170 °C also removes adsorbed water. More important, the surfaces under the metal-insulator structure were dehydroxylated by breaking the chemisorb Si–OH group facilitating the formation of siloxane bonds that prevents adsorption of moisture even after 60 days.
Indium versus hydrogen-terminated GaN(0001) surfaces: Surfactant effect of indium in a chemical vapor deposition environment
Authors: Northrup John E. , Van de Walle Chris G.
Published: May 2004

First-principles pseudopotential density functional calculations of the relative stability of H- and In-terminated GaN(0001) surfaces are reported. These total energy calculations show that surfaces terminated by one or two monolayers of In are more stable under typical metalorganic vapor deposition conditions than the H-terminated surface structures that have been proposed. Indium may act as a surfactant to improve the growth morphology of GaN films grown by metalorganic vapor deposition via a mechanism similar to that operative in molecular beam epitaxy.
Reduction of spin transfer by synthetic antiferromagnets
Authors: Emley N. C. , Albert F. J. , Ryan E. M. , Krivorotov I. N. , Ralph D. C. , Buhrman R. A. , Daughton J. M. , Jander A.
Published: May 2004

Synthetic antiferromagnetic layers (SAF) are incorporated into spin transfer nanopillars giving a layer composition [Cobottom/Ru/Cofixed]/Cu/Cofree, where square brackets indicate the SAF. The Cobottom and Cofixed layers are aligned antiparallel (AP) by strong indirect exchange coupling through the Ru spacer. All three magnetic layers are patterned, so this AP alignment reduces undesirable dipole fields on the Cofree layer. Adding the Cobottom/Ru layers reduces the spin polarization of the electron current passing through the nanopillar, leading to a decreased spin-torque per unit current incident on the Cofree layer. This may be advantageous for device applications requiring a reduction of the effects of a spin-torque, such as nanoscale current-perpendicular-to-plane magnetoresistive read heads.
Direct evidence of the Fermi-energy-dependent formation of Mn interstitials in modulation-doped Ga1−yAlyAs/Ga1−xMnxAs/Ga1−yAlyAs heterostructures
Authors: Yu K. M. , Walukiewicz W. , Wojtowicz T. , Lim W. L. , Liu X. , Dobrowolska M. , Furdyna J. K.
Published: May 2004

Using ion channeling techniques, we investigate the lattice locations of Mn in Ga1−xMnxAs quantum wells between Be-doped Ga1−yAlyAs barriers. Our earlier results showed that the Curie temperature TC depends on the growth sequence of the epitaxial layers. A lower TC was found in heterostructures in which the Ga1−xMnxAs layer is grown after the modulation-doped barrier. Here, we provide direct evidence that this reduction in TC is directly correlated with an increased formation of magnetically inactive Mn interstitials. The formation of interstitials is induced by a shift of the Fermi energy as a result of the transfer of holes from the barrier to the quantum well during the growth.
Measurement and noise characterization of optically induced index changes using THz interferometry
Authors: Small J. A. , Cheville R. A.
Published: May 2004

A Michelson interferometer designed for broadband single-cycle THz pulses is used to characterize optically induced index changes in semiconductors which result in submicron changes in optical path length. The interferometric measurements are compared both to standard THz time-domain spectroscopy (THz-TDS) and differential THz-TDS based on modulation of the sample. By analyzing noise contributions in THz spectroscopy systems, it is shown that the destructive interference achieved in THz interferometry reduces both some sources of random errors as well as errors due to system drift.
Electrochromism in switchable nematic emulsions
Authors: Nicoletta F. P. , Cupelli D. , De Filpo G. , Chidichimo G.
Published: May 2004

Switchable nematic emulsions are composite systems formed by liquid-crystal droplets dispersed in a fluid, homogeneous, monomer matrix, which can be turned from an opaque to an optically transparent state by application of a suitable ac electric field. An electrochromic device provides a reversible and visible change in its transmittance and/or reflectance as the result of either oxidation or reduction electrochemical processes. Both devices have been proven to be useful for a variety of electro-optical applications as switchable windows, electromagnetic shutters, and displays. This letter reports preliminary results on a bifunctional device based on a switchable nematic emulsion, which hosts electrochemical reactions. The presence of a liquid-crystal dispersion ensures the switching from a scattering and opaque state to a transmissive and transparent state, while the oxidation–reduction reactions allow a contemporary and independent change in color.
Formation of SiGe nanocrystals in HfO2 using in situ chemical vapor deposition for memory applications
Authors: Gupta Rohit , Yoo Won Jong , Wang Yingqian , Tan Zerlinda , Samudra Ganesh , Lee Sungjoo , Chan Daniel S. H. , Loh Kian Ping , Bera L. K. , Balasubramanian N. , Kwong Dim-Lee
Published: May 2004

The in situ deposition process of SiGe nanocrystals on SiO2 and HfO2 substrates was studied using Auger electron spectroscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. The Ge concentration in SiGe nanocrystals increased with deposition time at equal Si/Ge flow rates. Analysis indicated that the nucleation of Ge takes place preferentially on prenucleated Si on the dielectric. Memory transistors were fabricated using SiGe nanocrystals and HfO2 tunneling/control dielectrics as a gate stack. Memory effects from floating SiGe nanocrystals in HfO2 were clearly observed at room temperature from metal–oxide–semiconductor field effect transistor devices with a threshold voltage shift of about 0.5 V with the application of +4 V.
Comparison of Fe/Schottky and Fe/Al2O3 tunnel barrier contacts for electrical spin injection into GaAs
Authors: van ’t Erve O. M. J. , Kioseoglou G. , Hanbicki A. T. , Li C. H. , Jonker B. T. , Mallory R. , Yasar M. , Petrou A.
Published: May 2004

We compare electrical spin injection from Fe films into identical GaAs-based light-emitting diodes (LEDs) using different tunnel barriers—a reverse-biased Fe/AlGaAs Schottky contact and an Fe/Al2O3 barrier. Both types of structures are formed in situ using a multichamber molecular-beam epitaxy system. A detailed analysis of the transport data confirms that tunneling occurs in each case. We find that the spin polarization achieved in the GaAs using the Al2O3 barrier is 40% (best case; 30% typical), but the electrical efficiency is significantly lower than that of the Fe Schottky contact.
Photoinduced birefringence in tellurite glasses
Authors: Tikhomirov V. K. , Asatryan K. E. , Galstian T. V. , Vallee R. , Seddon A. B.
Published: May 2004

We report on photoinduced birefringence in TeO2-based glasses on exposure to linearly polarized subbandgap light. It is shown that the influence of glass-network modifiers, such as ZnO, Na2O, BaO, and of dopant Er2O3, on the photoinduced birefringence is not substantial indicating a dominating role of the TeO2 component. Photoinduced birefringence is optically reversible and its mechanism is proposed to be due to laser-induced alignment of intrinsic anisotropic defects of tellurite glass, such as distorted TeO4 bipyramids.
Frequency locking and wavelength tuning of nanosecond pulsed broad-area semiconductor lasers
Authors: Liu Y. , Kireev V. , Braiman Y.
Published: May 2004

We discuss experimental results of frequency locking and wavelength tuning of a nanosecond pulsed broad-area semiconductor laser. Nanosecond optical pulses with peak power of 25 W and repetition rates of 4–240 kHz are generated from a broad-area laser. An external cavity with a diffractive grating is used to reduce the linewidth of the laser from over 5 nm to less than 0.1 nm. The wavelength of the pulsed laser is tunable over more than 10 nm. The dependence of the laser linewidth on pulse parameters has been investigated.
Ethylene involvement in the regulation of Fe-deficiency stress responses by Strategy I plants
Authors: Romera Francisco J. , Alcántara Esteban
Published: May 2004

Plants have developed different mechanisms for the acquisition of iron (Fe). Depending on the mechanisms, plants are classified into two groups: Strategy I and Strategy II. Strategy I plants include all higher plants except the Gramineae, while Strategy II plants comprise the Gramineae. When plants suffer from Fe-deficiency, they develop several morphological and physiological changes in their roots, known as Fe-deficiency stress responses, which disappear when the plants acquire enough Fe. In Strategy I plants, these changes include subapical swelling with abundant root hairs, transfer cells, acidification of the rhizosphere, enhancement of the capacity to reduce Fe3+ to Fe2+, enhancement of the capacity for Fe2+ uptake, release of flavins, and others. The regulation of these responses is not fully understood but in recent years there has been evidence suggesting the involvement of ethylene in this process. This review summarises different results that support a role for this hormone in the regulation of Fe-deficiency stress responses by Strategy I plants.
Genetic control of lysine metabolism in maize endosperm mutants
Authors: Azevedo Ricardo A. , Damerval Catherine , Lea Peter J. , Landry Jacques , Bellato Cláudia M. , Meinhardt Lyndel W. , Guilloux Martine Le , Delhaye Sonia , Toro Alejandro A. , Gaziola Salete A. , Varisi Vanderlei A. , Gratão Priscila Lupino
Published: May 2004

The capacity of three maize endosperm opaque mutants (o10, o11 and o13) to accumulate soluble lysine in the seed in relation to their wildtype counterpart, W22+, was investigated. The W22o13 and W22o11 mutants exhibited 278% and 186% increases in soluble lysine, respectively, while for W22o10, a 36% decrease was observed, compared with the wildtype. A quantitative and qualitative study of the N constituents of the endosperm has been conducted and data obtained for the total protein, non-protein N, soluble amino acids, albumins / globulins, zeins and glutelins present in the seed of the mutants. Following 2D–PAGE, a total of 38 different forms of zein polypeptides were detected and considerable differences were noted between the three mutant lines. The metabolism of lysine was also studied by analysis of the enzymes aspartate kinase, homoserine dehydrogenase, lysine 2-oxoglutarate reductase and saccharopine dehydrogenase, which exhibited major changes in activity, depending on the genotype, suggesting that the mutant genes may have distinct regulatory activities.
Molecular cloning, characterisation and expression of two catalase genes from peach
Authors: Bagnoli Francesca , Danti Susanna , Magherini Valentina , Cozza Radiana , Innocenti Anna M. , Racchi Milvia L.
Published: May 2004

Two cDNA clones encoding catalase (Cat1 and Cat2) from peach [Prunus persica (L.) Batsch] were identified, that show homologies to other plant catalases. The nucleotide sequences of the two coding regions showed 88% identity to each other. The amino acid sequences predicted from the two full-length clones showed the highest homology to a catalase from cotton and Nicotiana plumbaginifolia L. and included C-terminal tri-peptides typical of those used to target proteins to peroxisomes. Southern hybridisation analysis suggested the existence of two catalase genes in peach. The expression of Cat1 and Cat2 was determined in seeds, vegetative tissue, leaves during the seasonal cycle and in leaves in response to light / dark treatments. Cat1 had high levels of expression only in leaf tissue and was responsive to light and seasonal changes. Cat2 had high levels of expression in in vitro shoots and was also responsive to seasonal changes, but not to light. In situ hybridisations to leaf tissue indicated that the expression of Cat1 was localised mainly in palisade cells, while Cat2 mRNA was present in the vascular tissue. The results of the expression analysis and in situ hybridisation suggest a role for Cat1 in photorespiration and for Cat2 in stress responses.
Plant functional genomics: opportunities in microarray databases and data mining
Authors: Kennedy Gavin C. , Wilson Iain W.
Published: May 2004

High-throughput gene expression profiling using microarrays has given plant biologists a powerful new technology to discover gene function and understand cellular processes. Bioinformatics has rapidly developed to deliver the tools necessary to interpret this gene expression data, but opportunities to further exploit the mass of data from hundreds of experiments are becoming dependent upon the use of sophisticated database repositories. Data mining of these resources will allow plant biologists to compare and link expression profiles and experimental factors to uncover functions and processes that would not normally be visible from analysing a small set of microarray experiments. This in-silico analysis will become critical when designing new experiments and interpreting new results. Consequently microarray databases and their ongoing development are now as important to plant functional genomics as the initial microarray data capture and analysis tools. In order for plant biologists to grasp these new opportunities, an appreciation of microarray database technology and future developments in biological data integration is required. The challenge for plant functional genomics is to embrace these new technologies lest the opportunities for significant discoveries be lost.
Programmed cell death during microgenesis in a Honglian CMS line of rice is correlated with oxidative stress in mitochondria
Authors: Li Shaoqing , Wan Cuixiang , Kong Jin , Zhang Zaijun , Li Yangsheng , Zhu Yingguo
Published: May 2004

The key role of mitochondria in the integration of apoptosis and oxidative stress in mammals has been documented. In plants, mitochondria are implicated in cytoplasmic male sterility (CMS), programmed cell death (PCD), and oxidative stress. However, to date there has been no evidence presented of the interplay among CMS, PCD and oxidative stress. In this study, we demonstrate that the pollen abortion of Honglian CMS line of rice (Oryza sativa L.) displays a PCD phenotype. A premature loss of microspores was accompanied by several biochemical markers of apoptosis. Analysis of mitochondria revealed that during the PCD process there was disruption of the inner mitochondrial membrane potential in microspores. This disruption was correlated with excess production of reactive oxygen species and down-regulation of the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase in mitochondria. The excess accumulation of ROS spanned three stages from pollen mother cell to early-uninucleate stage, and was followed by the occurrence of PCD at meiosis. Our data suggest that the microspores suffered from severe oxidative stress during pollen development. It is likely the chronic oxidative stress triggered the tissue-specific PCD and consequently resulted in the abortion of microspores.
Tissue-specific changes in remobilisation of fructan in the xerophytic tussock species Festuca novae-zelandiae in response to a water deficit
Authors: Clark Greg T. , Zuther Ellen , Outred Heather A. , McManus Michael T. , Heyer Arnd G.
Published: May 2004

Cellular responses of the native New Zealand grass species Festuca novae-zelandiae (Hack.) Cockayne to a water deficit and re-hydration treatment were investigated. Leaf extension ceased after 28 d of withholding water, while mitotic activity within the intercalary meristem still continued. By 35 d, no evidence of mitotic activity could be detected. At the apical meristem, cell division commenced 24 h post re-watering after 49 d of dry-down and was accompanied by synthesis of storage oligosaccharides of the fructan type. Changes in water-soluble carbohydrates over the course of the water deficit were examined in consecutive leaf segments comprising the leaf base (meristem region), elongation zone, the enclosed and the exposed lamina, as well as basal sheath segments from the two next oldest leaves. In fully hydrated leaf tissue, the fructan pool was mainly composed of the low molecular weight fructans of the inulin and neokestose series with higher concentrations towards the leaf base. Fructan concentrations decreased over the course of the water deficit with the leaf base retaining significantly higher concentrations than any other tissue, until tissue water content fell below 45%. Sucrose content increased in each tissue during the course of the dry-down, and was highest at the leaf base, where a concentration of 200 μmol g DW–1 was measured after 49�d of dry-down.In 1.5 h after re-watering, levels of 1-kestotriose increased relative to the level of sucrose at the leaf base, indicating re-synthesis of fructans from accumulated sucrose. By 24 h post re-watering, the ratio of sucrose to 1-kestotriose declined from six in non-watered plants to two and higher molecular weight fructans became detectable. The negative correlation between fructan and sucrose content, which indicates an inter-conversion depending on tissue water content, suggests that, in this species, fructans serve as a carbohydrate pool, while sucrose stabilises the meristem during extreme water deficit.
Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2
Authors: Chen Xiongwen , Gao Kunshan
Published: May 2004

Photosynthetic performance was examined in Skeletonema costatum (Greville) Cleve. under 12 : 12-h light : dark (LD) cycle at ambient CO2 (350 μL L–1) and elevated CO2 (1000 μL L–1). At ambient CO2, the cellular chlorophyll a content, the light-saturated photosynthetic rate (Pm), the initial slope of the light saturation curves (α), the photochemical efficiency of PSII (Fv / Fm), the apparent carboxylating efficiency (ACE) and the photosynthetic affinity for CO2 [1 / Km(CO2)] all showed rhythmical changes with different amplitudes during the light period. The Pm had similar changing pattern in the light period with the ACE and 1 / Km(CO2) rather than with the α and Fv / Fm, indicating that rhythmical changes of photosynthetic capacity may be mainly controlled by the activity of C-reduction associated with CO2 uptake during the light period. The CO2 enrichment reduced the ACE and the affinity to CO2, and increased the α, cellular chlorophyll a content and Pm based on cell number. By contrast, the changing patterns of all photosynthetic parameters examined here during the light period had almost the same for cells grown at ambient CO2 and elevated CO2, suggesting that the photosynthetic rhythms of S. costatum are not affected by CO2 enrichment.
Role of hydrogen peroxide and the redox state of ascorbate in the induction of antioxidant enzymes in pea leaves under excess light stress
Authors: Hernández Jose A. , Escobar Carolina , Creissen Gary , Mullineaux Phil M.
Published: May 2004

In this work we used two different pea cultivars, JI281 is a semidomesticated land race of pea from Ethiopia whereas JI399 is a typical domesticated pea variety. Exposure of pea leaves to excess light (EL) for 1 h caused a reversible photoinhibition of photosynthesis as showed by changes in Fv / Fm. Although little difference existed between the two pea genotypes with respect to photoinhibition, after 60 min of EL the decline in Fv / Fm was higher in JI281 than in JI399 leaves. As a consequence of EL, H2O2 increased in both pea cultivars, whereas lipid peroxidation and protein oxidation slightly increased, although differences between cultivars were minimal. The redox state of ascorbate shifted towards its oxidized form under EL stress in both cultivars. Transcript levels of genes coding antioxidant enzymes varied with EL in both cultivars, but the response was more pronounced in JI399. The induction observed during EL was maintained or increased after the stress period, as occurred for cytGR and chlMDHAR. GR protein accumulation and activity correlated with the transcript accumulation in JI399, but not in JI288. In this work, a possible role for H2O2 and redox status of ascorbate in the photoxidative stress signalling is discussed.
Scattered Hammond plots reveal second level of site-specific information in protein folding: φ′ (β
‡
)
Authors: Hedberg Linda , Oliveberg Mikael
Published: May 2004

Site-specific information about structural heterogeneities of the protein-folding transition-state ensemble is commonly derived from the scatter of the Brønsted plot through the individual values of φ = Δlog
k
f
/Δlog
K
D-N
. Here, we provide a second level of site-specific detail in the transition-state analysis by demonstrating that the scatter of the Hammond plot is related to heterogeneities in the φ-value growth. That is, the extent of transition-state movement (Δβ
‡
) is proportional to the free-energy gradient of the mutational perturbation across the top of the activation barrier, φ′(β
‡
) ∝ Δlog
K
D-N
. The analysis is applied to the two-state protein L23 where the site-specific free-energy gradients are used to identify the interactions that show the highest degree of consolidation after crossing the barrier top. These interactions are distributed as a shell around the high-φ initiation point and denote the side-chain contacts that add criticality to the folding nucleus.
Inducing and modulating anisotropic DNA bends by pseudocomplementary peptide nucleic acids
Authors: Kuhn Heiko , Cherny Dmitry I. , Demidov Vadim V. , Frank-Kamenetskii Maxim D.
Published: May 2004

DNA bending is significant for various DNA functions in the cell. Here, we demonstrate that pseudocomplementary peptide nucleic acids (pcPNAs) represent a class of versatile, sequence-specific DNA-bending agents. The occurrence of anisotropic DNA bends induced by pcPNAs is shown by gel electrophoretic phasing analysis. The magnitude of DNA bending is determined by circular permutation assay and by electron microscopy, with good agreement of calculated mean values between both methods. Binding of a pair of 10-meric pcPNAs to its target DNA sequence results in moderate DNA bending with a mean value of 40–45°, while binding of one self-pc 8-mer PNA to target DNA yields a somewhat larger average value of the induced DNA bend. Both bends are found to be in phase when the pcPNA target sites are separated by distances of half-integer numbers of helical turns of regular duplex DNA, resulting in an enhanced DNA bend with an average value in the range of 80–90°. The occurrence of such a sharp bend within the DNA double helix is confirmed and exploited through efficient formation of 170-bp-long DNA minicircles by means of dimerization of two bent DNA fragments. The pcPNAs offer two main advantages over previously designed classes of nonnatural DNA-bending agents: they have very mild sequence limitations while targeting duplex DNA and they can easily be designed for a chosen target sequence, because their binding obeys the principle of complementarity. We conclude that pcPNAs are promising tools for inducing bends in DNA at virtually any chosen site.
Dissociation of rewarding and dopamine transporter-mediated properties of amphetamine
Authors: Budygin Evgeny A. , Brodie Mark S. , Sotnikova Tatyana D. , Mateo Yolanda , John Carrie E. , Cyr Michel , Gainetdinov Raul R. , Jones Sara R.
Published: May 2004

The interaction of amphetamine (AMPH) with the dopamine (DA) transporter (DAT) is thought to be critically important for the DA-elevating actions of this drug. It is commonly believed that DA elevations are involved in the rewarding/reinforcing properties of AMPH and other drugs of abuse. Here, we found that DAT deletion did not eliminate the rewarding effects of AMPH as measured by conditioned place preference (CPP). In fact, mice in which the DAT gene has been deleted (DAT-KO mice) exhibited AMPH-induced CPP for many weeks after the time when extinction occurred in WT mice. Moreover, systemic AMPH still increased extracellular DA in the nucleus accumbens (NAc) of mice lacking the DAT, although local infusion of AMPH into the NAc did not have this effect. By using voltammetry in NAc slices, we found that AMPH did not decrease the rate of DA clearance. The rate of ventral tegmental area DA neuron firing was dramatically inhibited by AMPH in brain slices from WT mice, but there was no inhibition of firing in DAT-KO mice. AMPH-induced CPP was abolished by pretreatment with WAY-100635, a serotonin 5-HT1Areceptor antagonist, in DAT-KO mice, but the drug did not change AMPH place preference in WT mice. Therefore, despite the absence of the DAT, AMPH displays rewarding effects and causes an increase in extracellular DA in the NAc of DAT-KO mice, acting indirectly in this case. The 5-HT system may be involved in the rewarding effects of AMPH in these mice.
Consistency of a counterexample to Naimark's problem
Authors: Akemann Charles , Weaver Nik
Published: May 2004

We construct a C
*
-algebra that has only one irreducible representation up to unitary equivalence but is not isomorphic to the algebra of compact operators on any Hilbert space. This answers an old question of Naimark. Our construction uses a combinatorial statement called the diamond principle, which is known to be consistent with but not provable from the standard axioms of set theory (assuming that these axioms are consistent). We prove that the statement “there exists a counterexample to Naimark's problem which is generated by

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\aleph}_{1}\end{equation*}\end{document}

elements” is undecidable in standard set theory.
Selective inhibition of calcineurin-NFAT signaling by blocking protein–protein interaction with small organic molecules
Authors: Roehrl Michael H. A. , Kang Sunghyun , Aramburu José , Wagner Gerhard , Rao Anjana , Hogan Patrick G.
Published: May 2004

Transient or reversible protein–protein interactions are commonly used to ensure efficient targeting of signaling enzymes to their cellular substrates. These interactions include direct binding to substrate, interaction with an accessory or scaffold protein, and positioning at subcellular locations in proximity to substrates. The existence of specialized targeting mechanisms raises the possibility of designing inhibitors that do not block enzyme activityper se, but rather interfere with targeting of the enzyme to one or more of its substrates within the cell. Here, we identify small organic molecules that specifically block targeting of the protein phosphatase calcineurin to its substrate nuclear factor of activated T cells (NFAT, also termed NFATc) and show that they are effective inhibitors of calcineurin-NFAT signaling.
An effective kinetic representation of fluctuation-driven neuronal networks with application to simple and complex cells in visual cortex
Authors: Cai David , Tao Louis , Shelley Michael , McLaughlin David W.
Published: May 2004

A coarse-grained representation of neuronal network dynamics is developed in terms of kinetic equations, which are derived by a moment closure, directly from the original large-scale integrate-and-fire (I&F) network. This powerful kinetic theory captures the full dynamic range of neuronal networks, from the mean-driven limit (a limit such as the number of neurons
N
→ ∞, in which the fluctuations vanish) to the fluctuation-dominated limit (such as in small
N
networks). Comparison with full numerical simulations of the original I&F network establishes that the reduced dynamics is very accurate and numerically efficient over all dynamic ranges. Both analytical insights and scale-up of numerical representation can be achieved by this kinetic approach. Here, the theory is illustrated by a study of the dynamical properties of networks of various architectures, including excitatory and inhibitory neurons of both simple and complex type, which exhibit rich dynamic phenomena, such as, transitions to bistability and hysteresis, even in the presence of large fluctuations. The implication for possible connections between the structure of the bifurcations and the behavior of complex cells is discussed. Finally, I&F networks and kinetic theory are used to discuss orientation selectivity of complex cells for “ring-model” architectures that characterize changes in the response of neurons located from near “orientation pinwheel centers” to far from them.
Metabolic engineering with Dof1 transcription factor in plants: Improved nitrogen assimilation and growth under low-nitrogen conditions
Authors: Yanagisawa Shuichi , Akiyama Ai , Kisaka Hiroaki , Uchimiya Hirofumi , Miwa Tetuya
Published: May 2004

Utilization of transcription factors might be a powerful approach to modification of metabolism for a generation of crops having superior characteristics because a single transcription factor frequently regulates coordinated expression of a set of key genes for respective pathways. Here, we apply the plant-specific Dof1 transcription factor to improve nitrogen assimilation, the essential metabolism including the primary assimilation of ammonia to carbon skeletons to biosynthesize amino acids and other organic compounds involving nitrogen in plants. Expressing Dof1 induced the up-regulation of genes encoding enzymes for carbon skeleton production, a marked increase of amino acid contents, and a reduction of the glucose level in transgenic
Arabidopsis
. The results suggest cooperative modification of carbon and nitrogen metabolisms on the basis of their intimate link. Furthermore, elementary analysis revealed that the nitrogen content increased in the Dof1 transgenic plants (≈30%), indicating promotion of net nitrogen assimilation. Most significantly, the Dof1 transgenic plants exhibit improved growth under low-nitrogen conditions, an agronomically important trait. These results highlight the great utility of transcription factors in engineering metabolism in plants.