bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of stochastic resonance in percolative Josephson media
Authors: Glukhov A. M. , Sivakov A. G. , Ustinov A. V.
Published: July 2002

Measurements of the electrical response of granular Sn-Ge thin films below the superconducting transition temperature are reported. The addition of an external noise to the magnetic field applied to the sample is found to increase the sample voltage response to a small externally applied ac signal. The gain coefficient for this signal and the signal-to-noise ratio display clear maxima at particular noise levels. We interpret these observations as a stochastic resonance in the percolative Josephson media which occurs close to the percolation threshold.
Manifestation of Coulomb blockade effects at an arbitrary degeneracy of the levels of a molecular contact
Authors: Klymenko Yu. O.
Published: July 2002

The current-voltage (I–V) characteristics of a metal–molecule–metal structure are investigated under the condition that electron tunneling occurs only via a single molecular level of arbitrary degeneracy. A system of kinetic equations taking into account the accumulation of charge on the molecule is solved, and an exact formula for the steady-state current is obtained for the first time. In the low-temperature limit the steps on the I–V characteristic are analyzed and found to be nonequidistant with respect to current. It is shown that with increasing degeneracy of the level the initial current steps tend toward a completely equidistant spacing. In the case when the coupling parameters between the molecule and external electrodes are substantially different, the behavior of the I–V curves on parts with opposite directions of the current is found to be different: either a single current step is formed, with an amplitude proportional to the degeneracy of the level, or equidistant current steps appear in a number equal to the degeneracy of the level. It is shown that for a given polarity of the applied voltage, the matter of which of the two behaviors of the current is realized is completely conditional on whether the level via which the electron transport occurs is occupied or unoccupied by electrons. The results of the theoretical analysis of the I–V characteristics are confirmed by a numerical simulation.
Kinetics of the low-temperature structural transformation in the In–4.3 at. % Cd solid solution
Authors: Lubenets S. V. , Natsik V. D. , Pal-Val L. N. , Pal-Val P. P. , Fomenko L. S.
Published: July 2002

The reversible structural transformation of the hysteretic type observed previously by the authors in the In–4.3 at. %Cd alloy on low-temperature thermocycling is investigated further. In the hysteresis region (150–290 K) a pronounced instability of the macroscopic characteristics of the alloy is observed, which is manifested as time dependence of the dynamic Young’s modulus and resistivity of the samples at a constant temperature fixed during the cooling or heating run. The kinetic effects are investigated in detail near the boundaries of the hysteresis, where they are most clearly expressed. Less clear signs of the structural transformation are observed on the temperature dependence of the microhardness of the alloy. It is found that the structural transformation is governed by thermally activated processes with several characteristic relaxation times: the analytical form of the kinetic curves and the temperature dependence of the relaxation times are substantially different for the direct (on cooling) and reverse (on heating) transformations. Similarities and differences between this transformation and the known structural rearrangement processes in the physics of alloys are discussed. The structural transformation investigated here is also discussed in relation to the phase diagram of the In–Cd system.
Control of transient enhanced diffusion of boron after laser thermal processing of preamorphized silicon
Authors: Chong Y. F. , Pey K. L. , Wee A. T. S. , Osipowicz T. , See A. , Chan L.
Published: July 2002

In this article we report the role of excess interstitials in the end-of-range region in transient enhanced diffusion of boron during annealing of laser-processed samples. The results show that although the amorphous layer in preamorphized silicon can be completely annealed by laser irradiation, the end-of-range damages were not sufficiently annealed. The end-of-range region contains a supersaturation of interstitial defects that enhance the diffusion of boron during a post-laser processing anneal. It is found that the transient enhanced diffusion is significantly suppressed when the melt depth is extended beyond the amorphous layer such that the interstitial dose in the region adjacent to the laser-melted layer is minimized. In this way, the abruptness of laser-processed ultrashallow junctions can be maintained upon further annealing at moderately high temperatures. Cross-sectional transmission electron microscopy shows that a virtually defect-free regrown layer is obtained by overmelting beyond the amorphous layer into the substrate.
Tuning between negative and positive magnetoresistance in (La0.7Sr0.3MnO3)1−x(La1.85Sr0.15CuO4)x composites
Authors: Li Xiao-Hang , Huang Yun-Hui , Wang Zhe-Ming , Yan Chun-Hua
Published: July 2002

( La 0.7 Sr 0.3 MnO 3 ) 1−x ( La 1.85 Sr 0.15 CuO 4 ) x  [(LSMO)1−x(LSCO)x, x is the weight fraction] composites were prepared by mixing the sol-gel-derived LSMO and LSCO powders. Negative magnetoresistance (MR) is observed for all samples at temperatures above the superconductive Tc of LSCO and for the samples with x<0.5 within the entire measured temperature range. With increasing magnetic field, a positive MR is detected in the samples with x>0.5, which is ascribed to the magnetic suppression of the superconductive coupling within and/or between LSCO grains. This field and composition sensitive competition between positive and negative MR demonstrates a coexistence of superconductive and ferromagnetic orders, and favors a potential application in magnetic-field-sensitive devices.
Low-threshold blue amplified spontaneous emission in a statistical copolymer and its blend
Authors: Lee Tae-Woo , Park O Ok , Choi Dong Hoon , Cho Hyun Nam , Kim Young Chul
Published: July 2002

We report low-threshold, blue amplified spontaneous emission (ASE) in a statistical copolymer, poly(9,9-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene) (CPDHFPV) and its blend with poly(9-vinylcarbazole) (PVK). PVK and CPDHFPV acted as the donor and the acceptor of the Förster-type excitation energy transfer, respectively. A single component waveguide made of CPDHFPV gave the threshold energy of 160 nJ/cm2/pulse. Moreover, that of the PVK/CPDHFPV (95/5 by wt.) blend device was as low as 20 nJ/cm2/pulse. From the relationship between the ASE intensity and the film thickness, we suggest that the optimum thickness for the photopumped ASE should be the transverse electric-mode cutoff thickness.
Two-step As-desorption from (001) InP observed by surface photoabsorption
Authors: Kim T. J. , Ihn Y. S. , Seong G. Y. , Kim Y. D. , Lee T. W. , Hwang H. , Yoon S. , Yoon E.
Published: July 2002

The investigation of As desorption from the (001) InP during metalorganic chemical vapor deposition was performed using surface photoabsorption (SPA). The behavior of the monochromatic SPA signal showed a clear existence of a metastable state after the AsH3 was turned off. SPA spectra at each stable surface were taken to confirm the interpretation. This result indicates that the previous criterion to measure the amount of As/P exchange reaction is not necessarily correct and that the As-desorption process should be understood as a two-step process.
Large blueshift of the H–Kr stretching frequency of HKrCl upon complexation with N2
Authors: Lignell Antti , Khriachtchev Leonid , Pettersson Mika , Räsänen Markku
Published: July 2002

Interaction between HKrCl and N2 is investigated in a low temperature Kr matrix and by using ab initio calculations. Two configurations of the HKrCl⋯N2 complex are found both computationally and experimentally. The complexes show large monomer-to-complex blueshifts (+112.9 and +32.4 cm−1 experimentally) of the H–Kr stretching vibration mode. The large blueshifts are attributed to the enhanced (HKr)+Cl− ion-pair character of complexed HKrCl resulting in stronger H–Kr covalent bonding.
X-ray diffraction and Mössbauer characterization of an Fe/SiO2 catalyst for the synthesis of carbon nanotubes
Authors: Coquay P. , Vandenberghe R. E. , De Grave E. , Fonseca A. , Piedigrosso P. , Nagy J. B.
Published: July 2002

A catalyst powder was prepared by adsorption precipitation of iron acetate on a SiO2 support at a controlled pH. The selective reduction of this powder in a N2/C2H2 atmosphere at 700 °C generates multiwall carbon nanotubes. The study by x-ray diffraction and Mössbauer spectroscopy of the catalyst and reduced powders reveals that hematite particles with a 8–20 nm size range are involved in the formation of these carbon nanotubes and end up as Fe3C after the catalysis process. Moreover, coalescence upon reduction produces large particles responsible for the formation of a minor proportion of large tubes of the order of 100 nm in diameter.
Miscut-angle dependence of perpendicular magnetic anisotropy in thin epitaxial CoPt3 films grown on vicinal MgO
Authors: Maranville B. B. , Shapiro A. L. , Hellman F. , Schaadt D. M. , Yu E. T.
Published: July 2002

The effect of vicinal substrates on the growth-induced perpendicular magnetic anisotropy of epitaxial CoPt3 films has been studied. A small (2°, 4°, or 10°) miscut angle of the vicinal substrate causes the crystallographic axes of the sample to be tilted along the miscut direction. The magnitude of the perpendicular anisotropy is unaffected by the presence of substrate steps produced by the miscut angle, while an additional, in-plane anisotropy develops with a larger miscut angle. Effects of the steps are seen in magnetic force microscopic images of domain wall pinning.
Correlation between the gain profile and the temperature-induced shift in wavelength of quantum-dot lasers
Authors: Klopf F. , Deubert S. , Reithmaier J. P. , Forchel A.
Published: July 2002

The influence of several design parameters on the temperature stability of the emission wavelength of 980 nm GaInAs/(Al)GaAs quantum-dot lasers was studied. The results obtained agree well with a simplified model based on the inhomogeneously broadened transitions of a quantum-dot ensemble. Using this model, the optimum cavity design for a given gain function can be determined. Following this approach, quantum-dot lasers with low wavelength shifts of 0.16 nm/K were realized, which is only half the value of a typical GaInAs/(Al)GaAs quantum well laser.
Embedded anisotropic microreflectors by femtosecond-laser nanomachining
Authors: Mills John D. , Kazansky Peter G. , Bricchi Erica , Baumberg Jeremy J.
Published: July 2002

Directly written embedded structures created within fused silica by a femtosecond Ti:sapphire laser are observed to strongly reflect blue light. Reflection emerges only in a direction parallel to the polarization axis of the writing laser. This anisotropic-effect is caused by a periodic modulation of refractive index of amplitude Δn∼10−2 with a characteristic period Λ∼150 nm over a spot size ∼1.5 μm. We show that the origin of the anisotropic reflection is the primary cause of other anisotropic phenomena reported in recent experiments.
Mechanism of low temperature hydrogen-annealing-induced degradation in Pb(Zr0.4Ti0.6)O3 capacitors
Authors: Seo Sunae , Yoon Jong-Gul , Song T. K. , Kang B. S. , Noh T. W. , Lee Y. K. , Kim Ch. J. , Park Y. S.
Published: July 2002

Changes in the polarization–field hysteresis loop were systematically investigated for Pb(Zr0.4Ti0.6)O3 capacitors after forming gas annealing at 200 °C. Voltage shift in hysteresis was strongly dependent on the polarization states and ascribed to an asymmetric distribution of defect charges and pinned defect dipoles. Field recovery of the imprinted capacitors and increase in coercive field after the recovery were discussed in conjunction with reversible defect dipoles. From the relaxation of the voltage shift with an activation energy of 0.21 eV, it is inferred that charge trapping may be the main cause of the voltage shift and the subsequent degradation of the capacitors by pinning the polarization and defect dipoles.
Integrating micro- and nanoelectrodes into atomic force microscopy cantilevers using focused ion beam techniques
Authors: Lugstein A. , Bertagnolli E. , Kranz C. , Kueng A. , Mizaikoff B.
Published: July 2002

This paper concerns a scanning probe capable of simultaneously measuring topography and local electrochemistry at a sample surface. Our approach ensures the distance regulation of the electrode by maintaining a fixed working distance between the probe and the sample surface independent from the electrochemical response. This is achieved by integrating micro- and nanoelectrodes into atomic force microscopy tips using focused ion beam techniques. The feasibility and functionality of the fully featured tip is demonstrated by a simultaneous topographical and electrochemical measurement of a porous polymer membrane as model surface.
SiGe/Si resonant-cavity-enhanced photodetectors for 1.3 μm operation fabricated using wafer bonding techniques
Authors: Li Cheng , Huang C. J. , Cheng Buwen , Zuo Yuhua , Luo Liping , Yu Jinzhong , Wang Qiming
Published: July 2002

A SiGe/Si multiple-quantum-well resonant-cavity-enhanced (RCE) photodetector for 1.3 μm operation was fabricated using bonding reflector process. A full width at half maximum (FWHM) of 6 nm and a quantum efficiency of 4.2% at 1314 nm were obtained. Compared to our previously reported SiGe RCE photodetectors fabricated on separation-by-implanted-oxygen wafer, the mirrors in the device can be more easily fabricated and the device can be further optimized. The FWHM is expected to be less than 1 nm and the detector is fit for density wavelength division multiplexing applications.
Enhanced figure of merit in thermal to electrical energy conversion using diode structures
Authors: Hagelstein P. L. , Kucherov Y.
Published: July 2002

A characterization of the electrical and thermal properties of thermoelectric diode structures indicates that the figure of merit for thermal to electrical energy conversion is significantly enhanced in our devices over thermoelectric values. Enhancements are due to current injection and blockage of the ohmic return current within the devices. The resulting device takes advantage of both thermoelectric and thermionic effects, and can be considered to be a hybrid. Experiments indicate an enhancement as high as a factor of 8 in the effective figure of merit. The best results are consistent with a conversion efficiency on the order of 35% of the Carnot limit. Enhancements have been observed in InSb and in HgCdTe, and we believe that the approach applies generally to all thermoelectric semiconductors.
Gain dynamics and ultrafast spectral hole burning in In(Ga)As self-organized quantum dots
Authors: Kim K. , Urayama J. , Norris T. B. , Singh J. , Phillips J. , Bhattacharya P.
Published: July 2002

Using a femtosecond three-pulse pump-probe technique, we investigated spectral hole-burning and gain recovery dynamics in self-organized In(Ga)As quantum dots. The spectral hole dynamics are qualitatively different from those observed in quantum wells, and allow us to distinguish unambiguously the gain recovery due to intradot relaxation and that due to carrier capture. The gain recovery due to carrier–carrier scattering-dominated intradot relaxation is very fast (∼130 fs), indicating that this is not the factor limiting the bandwidth of directly modulated quantum dot lasers.
Ultrafast spin dynamics in GaAs/GaSb/InAs heterostructures probed by second harmonic generation
Authors: Glinka Yu. D. , Shahbazyan T. V. , Perakis I. E. , Tolk N. H. , Liu X. , Sasaki Y. , Furdyna J. K.
Published: July 2002

We report the application of pump-probe second harmonic generation (SHG) to monitor spin dynamics in nonmagnetic semiconductor heterostructures. Spin-polarized electrons were selectively excited by a pump beam in the GaAs layer of GaAs/GaSb/InAs structures. However, the induced magnetization manifests itself through the SHG probe signal from the GaSb/InAs interface, thus indicating a spin-polarized electron transport. We find that the magnetization dynamics is governed by an interplay between the spin density evolution at the interfaces and the spin relaxation.
Switch using d-wave symmetry of YBa2Cu3O7−δ Josephson junction
Authors: Enomoto Youichi , Nishihara Tomohiro , Myouren Hiroaki , Takada Susumu
Published: July 2002

Switching functions using a parallel connection of conventional-type and π-type junctions have been investigated. Both junctions consist of Nb/Au/YBa2Cu3O7−δ Josephson junctions on YBa2Cu3O7−δ edges, whose faces are (110) and (100). The modulation patterns of critical currents in low magnetic fields indicate that the d-wave component is dominant: the (100) junctions exhibit the conventional type, but the (110) junctions the π type. Superconducting current path is alternated between the conventional junction and the π junction by applied magnetic fields using these anisotropic magnetic field dependences of the critical current.
Activated cation motions in zeolites
Authors: Maurin G. , Devautour S. , Henn F. , Giuntini J. C. , Senet P.
Published: July 2002

We apply a Monte Carlo technique specialized for the simulation of rare events to study the activated counterions motions in the aluminosilicate Na+-mordenite. Mean activation barriers are obtained from minimum energy paths calculated on realistic potential energy surfaces by using a Metropolis algorithm. Energy barriers for Na+ hops calculated for lattices with various Si/Al ratio are found in good agreement with the Na+ detrapping energies measured by thermally stimulated current spectroscopy. One shows that the dielectric activated motions of Na+ proceed between degenerated many-body ground states with different dipolar moment by either sequential or collective hopping motions. This provides a first microscopic description of dielectric relaxation measured in zeolites.
Influence of leakage current on temperature performance of GaAs/AlGaAs quantum cascade lasers
Authors: Indjin D. , Harrison P. , Kelsall R. W. , Ikonić Z.
Published: July 2002

A detailed analysis of intersubband electron scattering transport in GaAs/AlGaAs quantum cascade lasers (QCLs) is presented, using a full self-consistent rate equation model. Our approach includes all relevant scattering mechanisms between injector/collector, active region and continuumlike states in the cascade structures. In particular, the influence of the Al mole fraction in the quantum barriers on QCLs performance is investigated, by studying GaAs/AlGaAs structures with 33% and 45% Al barrier compositions, respectively. Excellent qualitative and quantitative agreement with recent experimental results at cryogenic and room temperatures is obtained. The model reproduces the gain saturation reported for the 33% Al device, which precludes laser operation at room temperature, and also the much improved room-temperature performance of the 45% Al device, with calculated 300 K threshold current of 17 kA/cm2, and confirms that the superior performance of the 45% Al device is due to suppression of parasitic conduction through continuum states as a consequence of the increased barrier height.
Development and application of a laterally driven electromagnetic microactuator
Authors: Ko Jong Soo , Lee Myung Lae , Lee Dae-Sik , Choi Chang Auck , Tae Kim Youn
Published: July 2002

A laterally driven electromagnetic microactuator (LaDEM) is introduced, and a micro-optical switch is designed and fabricated as an application. LaDEM offers parallel movement of the microactuator to the silicon substrate surface (in-plane mode). Polysilicon-on-insulator wafers and a reactive ion etching process were used to fabricate high-aspect-ratio vertical microstructures, which allowed the equipping of vertical micromirrors. A fabricated single leaf spring had a width of 1.2 μm, thickness of 16 μm, and length of 920 μm. The resistance of the fabricated leaf spring for the optical switch was 5 Ω. The deflection of the leaf spring started to profoundly increase at about 400 mA, and it showed snap-through phenomenon over that current value. Owing to the snap-through phenomenon, a large deflection of 60 μm was detected at 566 mA.
Heteroepitaxial growth of a wide-gap p-type semiconductor, LaCuOS
Authors: Hiramatsu Hidenori , Ueda Kazushige , Ohta Hiromichi , Orita Masahiro , Hirano Masahiro , Hosono Hideo
Published: July 2002

High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 °C results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films.
AlN/AlGaN superlattices as dislocation filter for low-threading-dislocation thick AlGaN layers on sapphire
Authors: Wang Hong-Mei , Zhang Jian-Ping , Chen Chang-Qing , Fareed Q. , Yang Jin-Wei , Khan M. Asif
Published: July 2002

We report on an approach of using AlN/AlGaN superlattices (SLs) for threading-dislocation-density reduction to grow high quality thick AlGaN on sapphire. Using x-ray diffraction (XRD) measurements and etch pits counting by atomic force microscopy, we show that the insertion of AlN/AlGaN SLs suppresses the material mosaicity and decreases the threading dislocation density by two orders of magnitude, and then eliminates cracking. Dislocation densities deduced from the XRD results and those from chemical etching are in a good agreement.
Sensitive imaging of an elastic nonlinear wave-scattering source in a solid
Authors: Kazakov Vyacheslav V. , Sutin Alexander , Johnson Paul A.
Published: July 2002

We have developed an imaging method for locating isolated nonlinear scattering source(s) in solids. It relies on extracting the nonlinear response of a solid by modulation of a high by a low-frequency wave, and employing moving-window, synchronous detection. The resulting image consists of nonlinear wave reflection profiles with remarkable sensitivity to an isolated elastic nonlinear source(s). In creating the image, we can distinguish between a nonlinear scattering source and other wave scatterers in the material. The method should work equally well for imaging the relative nonlinearity of different regions within a volume.
Growth of low-dimensional magnetic nanostructures on an insulator
Authors: Gai Zheng , Farnan G. A. , Pierce J. P. , Shen J.
Published: July 2002

Iron nanometer-scaled wires and ultrathin films have been successfully prepared on an insulating NaCl(001) single-crystal surface by electron beam deposition using different growth conditions. In situ noncontact atomic force microscopy (NC-AFM) shows that the heights and widths of the wires, which are formed by a one-dimensional array of clusters aligned on top of the step edges, are very uniform. The films are atomically flat and are formed due to a high nucleation density achieved through a low temperature growth procedure.
Tailoring the electronic properties of GaAs/AlAs superlattices by InAs layer insertions
Authors: Patanè A. , Sherwood D. , Eaves L. , Fromhold T. M. , Henini M. , Main P. C. , Hill G.
Published: July 2002

We investigate the electrical and optical properties of GaAs/AlAs superlattices (SLs) in which a thin (⩽1.2 monolayers) InAs layer is inserted in the central plane of each GaAs quantum well. The InAs layer modifies the structure of the SL unit cell and provides an additional design parameter for tailoring the energy of the lowest miniband and the size of the minigap. We exploit this effect to enhance electron injection from a doped contact layer into the first miniband and to inhibit interminiband coupling.
The ionization energy of methylene (CH2) from a rotationally resolved photoelectron spectrum and its thermochemical implications
Authors: Willitsch S. , Imbach L. L. , Merkt F.
Published: July 2002

The adiabatic ionization potential of methylene has been determined to be 83772±3 cm−1 from a rotationally resolved photoelectron spectroscopic study of the CH2+ X̃ 2A1 (0,0,0)←CH2 X̃ 3B1(0,0,0) transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and CH2+ D0(CH2=CO+)=33202±7 cm−1, the 0 K dissociation energy of the methyl radical D0(CH2–H)=38179±49 cm−1, the 0 K dissociation threshold of methane in CH2 and H2 D0(CH2–H2)=38232±50 cm−1 and the 0 K enthalpy of formation of CH2 ΔfH⊖(CH2,T=0 K)=390.73±0.66 kJ mol−1.
Pregnancy status and fetal prion genetics determine PrP
Sc
accumulation in placentomes of scrapie-infected sheep
Authors: Tuo Wenbin , O'Rourke Katherine I. , Zhuang Dongyue , Cheevers William P. , Spraker Terry R. , Knowles Donald P.
Published: July 2002

Ovine scrapie is a fatal neurodegenerative disorder that may be transmitted through exposure to infected uterine and placental tissues. Susceptibility to scrapie is primarily controlled by polymorphisms in the prion protein (PrP) gene. Scrapie in the U.S. Suffolk breed and in many breeds in Europe occurs in sheep homozygous for glutamine (171QQ), but rarely in sheep heterozygous for glutamine and arginine (171QR) or homozygous for arginine (171RR) at codon 171 of the PrP gene. This study demonstrated that accumulation of PrP
Sc
in uterine-placental epithelial cells in the placentome was determined by fetal PrP genotype and the pregnancy status of scrapie-infected ewes. PrP
Sc
was detected in 171QQ placentomes of infected ewes, but not in placentomes of infected ewes pregnant with 171QR conceptuses or in the non-pregnant uterus of infected ewes. The distribution of PrP
Sc
plaques in placentomes was temporally associated with stage of gestation. There was a tendency toward increased size and number of placentomal PrP
Sc
plaques from the endometrial stalk (maternal side) to chorionic plate (fetal side). These results indicate that accumulation of PrP
Sc
is eliminated or reduced to undetectable levels in reproductive and placental tissues if infected ewes are not pregnant or conceive conceptuses with a resistant PrP genotype.
Adaptive aberration correction in a confocal microscope
Authors: Booth Martin J. , Neil Mark A. A. , Juškaitis Rimas , Wilson Tony
Published: July 2002

The main advantage of confocal microscopes over their conventional counterparts is their ability to optically “section” thick specimens; the thin image slices thus obtained can be used to reconstruct three-dimensional images, a capability which is particularly useful in biological applications. However, it is well known that the resolution and optical sectioning ability can be severely degraded by system or specimen-induced aberrations. The use of high aperture lenses further exacerbates the problem. Moreover, aberrations can considerably reduce the number of photons that reach the detector, leading to lower contrast. It is rather unfortunate, therefore, that in practical microscopy, aberration-free confocal imaging is rarely achieved. Adaptive optics systems, which have been used widely to correct aberrations in astronomy, offer a solution here but also present new challenges. The optical system and the source of aberrations in a confocal microscope are considerably different and require a novel approach to wavefront sensing. This method, based upon direct measurement of Zernike aberration modes, also exhibits an axial selectivity similar to that of a confocal microscope. We demonstrate an adaptive confocal fluorescence microscope incorporating this modal sensor together with a deformable membrane mirror for aberration correction. Aberration corrected images of biological specimens show considerable improvement in contrast and apparent restoration of axial resolution.
Selective interaction of plant homeodomain proteins mediates high DNA-binding affinity
Authors: Smith Harley M. S. , Boschke Ilja , Hake Sarah
Published: July 2002

Understanding molecular mechanisms that control cell fate in the shoot apical meristem is a fundamental question in plant development. Genetic and molecular studies demonstrate that maize KNOTTED1 (KN1) of the TALE (3-aa acid loop extension) class of homeodomain (HD) proteins is involved in shoot apical meristem function. We show that KN1 interacts with knotted interacting protein (KIP), a BEL1-like TALE HD protein. Interaction between KN1 and KIP is mediated by conserved domains in the N termini of both proteins. The KN1 DNA-binding sequence, TGACAG(G/C)T, was biochemically identified, and
in vitro
DNA-binding assays show that individually KN1 and the HD of KIP bind specifically to this motif with low affinity. The KN1–KIP complex, however, binds specifically to this DNA-binding motif with high affinity, indicating that the association of KN1 and KIP may function in transcriptional regulation.
Neural correlates of instrumental learning in primary auditory cortex
Authors: Blake David T. , Strata Fabrizio , Churchland Anne K. , Merzenich Michael M.
Published: July 2002

In instrumental learning, Thorndike's law of effect states that stimulus–response relations are strengthened if they occur prior to positive reinforcement and weakened if they occur prior to negative reinforcement. In this study, we demonstrate that neural correlates of Thorndike's law may be observed in the primary auditory cortex, A1. Adult owl monkeys learned to discriminate tones higher than a standard frequency. Responses recorded from implanted microelectrodes initially exhibited broad spectral selectivity over a four-to-five octave range. With training, frequency discrimination thresholds changed from close to one octave to about
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octave. Physiological recordings during the week in which the monkey came under behavioral control signaled by a drop in measured threshold had stronger responses to all frequencies. During the same week, A1 neural responses to target stimuli increased relative to standard and nontarget stimuli. This emergent difference in responsiveness persisted throughout the subsequent weeks of behavioral training. These data suggest that behavioral responses to stimuli modulate responsiveness in primary cortical areas.
Evolution of supercolonies: The Argentine ants of southern Europe
Authors: Giraud Tatiana , Pedersen Jes S. , Keller Laurent
Published: July 2002

Some ants have an extraordinary social organization, called unicoloniality, whereby individuals mix freely among physically separated nests. This type of social organization is not only a key attribute responsible for the ecological domination of these ants, but also an evolutionary paradox and a potential problem for kin selection theory because relatedness between nest mates is effectively zero. The introduction of the Argentine ant in Europe was apparently accompanied by a dramatic loss of inter-nest aggression and the formation of two immense supercolonies (which effectively are two unicolonial populations). Introduced populations experienced only limited loss of genetic diversity at neutral markers, indicating that the breakdown of recognition ability is unlikely to be merely due to a genetic bottleneck. Rather, we suggest that a “genetic cleansing” of recognition cues occurred after introduction. Indeed workers of the same supercolony are never aggressive to each other despite the large geographical distance and considerable genetic differentiation between sampling sites. By contrast, aggression is invariably extremely high between the two supercolonies, indicating that they have become fixed for different recognition alleles. The main supercolony, which ranges over 6,000 km from Italy to the Spanish Atlantic coast, effectively forms the largest cooperative unit ever recorded.
Protein–protein interactions among C-4 demethylation enzymes involved in yeast sterol biosynthesis
Authors: Mo C. , Valachovic M. , Randall S. K. , Nickels J. T. , Bard M.
Published: July 2002

A
Saccharomyces cerevisae
microarray expression study indicated that an ORF,
YER044C
, now designated
ERG28
, was strongly coregulated with ergosterol biosynthesis. Disruption of the
ERG28
gene results in slow growth and accumulation of sterol intermediates similar to those observed in
erg26
and
erg27
null strains, suggesting that the Erg28p may interact with Erg26p and/or Erg27p. In this study, a peptide from human hemagglutinin protein (HA) epitope tag was added to
ERG26
and
ERG27
genes, and a Myc tag was added to the
ERG28
gene to detect interactions between Erg28p and Erg26p/Erg27p. Differential centrifugation showed that Erg26p, Erg27p, and Erg28p are all membrane-associated proteins. Green fluorescent protein–fusion protein localization studies showed that Erg26p, Erg27p, and Erg28p are all located in the endoplasmic reticulum. Solubilized membrane protein coimmunoprecipitation studies using rabbit anti-Erg25p indicated that Erg25p coimmunoprecipitates with both Erg27p and Erg28p. Erg28p was also shown to reciprocally coimmunoprecipitate with Erg27p. However, no coimmunoprecipitation was observed with Erg26p, most likely because of the poor solubilization of this protein. Sucrose gradient ultracentrifugation studies suggested that Erg25p/Erg26p/Erg27p/Erg28p, along with other proteins in sterol biosynthesis, might form a complex between 66 and 200 kDa. Using an anti-HA column with Erg27p-HA and Erg26p-HA as target proteins, a complex containing Erg25p/Erg26p/Erg27p/Erg28p was identified. Thus, we suggest that Erg28p works as a transmembrane scaffold to tether Erg27p and possibly other C-4 demethylation proteins (Erg25p, Erg26p), forming a demethylation complex in the endoplasmic reticulum.
All intermediates of the arsenate reductase mechanism, including an intramolecular dynamic disulfide cascade
Authors: Messens Joris , Martins José C. , Van Belle Karolien , Brosens Elke , Desmyter Aline , De Gieter Marjan , Wieruszeski Jean-Michel , Willem Rudolph , Wyns Lode , Zegers Ingrid
Published: July 2002

The mechanism of pI258 arsenate reductase (ArsC) catalyzed arsenate reduction, involving its P-loop structural motif and three redox active cysteines, has been unraveled. All essential intermediates are visualized with x-ray crystallography, and NMR is used to map dynamic regions in a key disulfide intermediate. Steady-state kinetics of ArsC mutants gives a view of the crucial residues for catalysis. ArsC combines a phosphatase-like nucleophilic displacement reaction with a unique intramolecular disulfide bond cascade. Within this cascade, the formation of a disulfide bond triggers a reversible “conformational switch” that transfers the oxidative equivalents to the surface of the protein, while releasing the reduced substrate.
The crystal structure of a tetrameric hemoglobin in a partial hemichrome state
Authors: Riccio Antonio , Vitagliano Luigi , di Prisco Guido , Zagari Adriana , Mazzarella Lelio
Published: July 2002

Tetrameric hemoglobins are the most widely used systems in studying protein cooperativity. Allosteric effects in hemoglobins arise from the switch between a relaxed (R) state and a tense (T) state occurring upon oxygen release. Here we report the 2.0-Å crystal structure of the main hemoglobin component of the Antarctic fish
Trematomus newnesi
, in a partial hemichrome form. The two α-subunit iron atoms are bound to a CO molecule, whereas in the β subunits the distal histidine residue is the sixth ligand of the heme iron. This structure, a tetrameric hemoglobin in the hemichrome state
,
demonstrates that the iron coordination by the distal histidine, usually associated with denaturing states, may be tolerated in a native-like hemoglobin structure. In addition, several features of the tertiary and quaternary organization of this structure are intermediate between the R and T states and agree well with the R → T transition state properties obtained by spectroscopic and kinetic techniques. The analysis of this structure provides a detailed pathway of heme–heme communication and it indicates that the plasticity of the β heme pocket plays a role in the R → T transition of tetrameric hemoglobins.
Dome formation in cell cultures as expression of an early stage of lactogenic differentiation of the mammary gland
Authors: Zucchi I. , Bini L. , Albani D. , Valaperta R. , Liberatori S. , Raggiaschi R. , Montagna C. , Susani L. , Barbieri O. , Pallini V. , Vezzoni P. , Dulbecco R.
Published: July 2002

The study of the development of the mammary gland at the molecular level in the animal is difficult because of the complex tissue organization of the gland. We have previously developed an
in vitro
system for genetic analysis of mammary cell differentiation, based on the cell line LA7 clonally derived from a rat mammary adenocarcinoma. This cell line, after induction with DMSO, differentiates forming structures called domes. This process is under strict gene regulation, and we have previously identified several of the genes involved. In the present paper, we have defined the meaning of dome formation in relation to mammary development, by showing that treatment of LA7 cells with the lactogenic hormones hydrocortisone and prolactin induces dome formation; in the animal, these hormones precede and accompany milk production. Moreover, dome formation is accompanied by expression within the cells of the milk protein genes
WDMN1
and β-casein, which are differentiation markers for the gland during pregnancy and lactation. We also show that two proteins, highly expressed in the mammary gland during lactation,
HSP90-
β and annexin I, are strongly expressed in DMSO-induced LA7 cells. Both proteins are essential in the formation of domes because when their synthesis is blocked by antisense RNA oligonucleotides, dome formation is abolished. Thus our
in vitro
system is a model for lobulo-alveolar development, and the genes identified in the pathway of dome formation are likely to be involved in the early differentiation steps occurring in the rat mammary gland during pregnancy and lactation.
Activation of Cdc2/cyclin B and inhibition of centrosome amplification in cells depleted of Plk1 by siRNA
Authors: Liu Xiaoqi , Erikson Raymond L.
Published: July 2002

The events of the cell cycle, the stages at which the cell proliferates and divides, are facilitated and controlled by multiple signaling pathways. Among the many regulatory enzymes that contribute to these processes is the polo-like kinase (Plk). Plks have been reported to mediate multiple mitotic processes, including bipolar spindle formation, activation of Cdc25C, actin ring formation, centrosome maturation, and activation of the anaphase-promoting complex. To investigate its functions in mammalian cells further, we used the recently developed small interfering RNA technique specifically to deplete Plk1 in cultured cells. We find that Plk1 depletion results in elevated Cdc2 protein kinase activity and thus attenuates cell-cycle progression. About 45% of cells treated with Plk1 small interfering RNA show the formation of a dumbbell-like DNA organization, suggesting that sister chromatids are not completely separated. About 15% of these cells do complete anaphase but do not complete cytokinesis. Finally, Plk1 depletion significantly reduces centrosome amplification in hydroxyurea-treated U2OS cells. These data provide direct evidence that Plk is required for multiple mitotic processes in mammalian cells and their significance is discussed.
Design of protein struts for self-assembling nanoconstructs
Authors: Hyman Paul , Valluzzi Regina , Goldberg Edward
Published: July 2002

Bacteriophage T4 tail fibers have a quaternary structure of bent rigid rods, 3 × 160 nm in size. The four proteins which make up these organelles are able to self-assemble in an essentially irreversible manner. To use the self-assembly domains of these proteins as elements in construction of mesoscale structures, we must be able to rearrange these domains without affecting the self-assembly properties and add internal binding sites for other functional elements. Here we present results on several alterations of the P37 component of the T4 tail fiber that change its length and add novel protein sequences into the protein. One of these sequences is an antibody binding site that is used to inactivate phage carrying the modified gene.
Dynamical model of long-term synaptic plasticity
Authors: Abarbanel Henry D. I. , Huerta R. , Rabinovich M. I.
Published: July 2002

Long-term synaptic plasticity leading to enhancement in synaptic efficacy (long-term potentiation, LTP) or decrease in synaptic efficacy (long-term depression, LTD) is widely regarded as underlying learning and memory in nervous systems. LTP and LTD at excitatory neuronal synapses are observed to be induced by precise timing of pre- and postsynaptic events. Modification of synaptic transmission in long-term plasticity is a complex process involving many pathways; for example, it is also known that both forms of synaptic plasticity can be induced by various time courses of Ca
2+
introduction into the postsynaptic cell. We present a phenomenological description of a two-component process for synaptic plasticity. Our dynamical model reproduces the spike time-dependent plasticity of excitatory synapses as a function of relative timing between pre- and postsynaptic events, as observed in recent experiments. The model accounts for LTP and LTD when the postsynaptic cell is voltage clamped and depolarized (LTP) or hyperpolarized (LTD) and no postsynaptic action potentials are evoked. We are also able to connect our model with the Bienenstock, Cooper, and Munro rule. We give model predictions for changes in synaptic strength when periodic spike trains of varying frequency and Poisson distributed spike trains with varying average frequency are presented pre- and postsynaptically. When the frequency of spike presentation exceeds ≈30–40 Hz, only LTP is induced.
Potent stimulation of transcription-coupled DNA supercoiling by sequence-specific DNA-binding proteins
Authors: Leng Fenfei , McMacken Roger
Published: July 2002

Transcription by RNA polymerase can stimulate localized DNA supercoiling in
Escherichia coli
.
In vivo
, there is extensive experimental support for a “twin-domain” model in which positive DNA supercoils are generated ahead of a translocating RNA polymerase complex and negative supercoils are formed behind it. Negative supercoils accumulate in the template DNA because the positive supercoils are preferentially removed by cellular topoisomerase action. Yet,
in vitro
, clear and convincing support for the twin-domain mechanism has been lacking. In this article, we reconcile this inconsistency by showing that, in a defined
in vitro
system with plasmid DNA templates, a variety of sequence-specific DNA-binding proteins, such as the bacteriophage λ O replication initiator or the
E. coli
lactose or galactose repressors, strikingly stimulate transcription-coupled DNA supercoiling. We demonstrate further that this stimulation requires the presence in the DNA template of a recognition sequence for the relevant DNA-binding protein and depends on the production of long RNA chains by an RNA polymerase. Our data are most consistent with a model in which specific DNA-binding proteins facilitate a twin-domain mechanism to enhance DNA supercoiling during transcription. More precisely, we suggest that some nucleoprotein complexes, perhaps those that contain sharply bent DNA, can form barriers that impede the diffusion and merger of independent chromosomal supercoil domains. Localization of DNA supercoils by nucleoprotein complexes may serve as a general mechanism for modulating DNA transactions that are sensitive to DNA superhelicity.
Pheromone induction promotes Ste11 degradation through a MAPK feedback and ubiquitin-dependent mechanism
Authors: Esch R. K. , Errede B.
Published: July 2002

Ste11 is the mitogen-activated protein kinase (MAPK) kinase kinase in the MAPK cascades that mediate mating, high osmolarity glycerol, and filamentous growth responses in
Saccharomyces cerevisiae
. We show stimulation of the mating pathway by pheromone promotes an accelerated turnover of Ste11 through a MAPK feedback and ubiquitin-dependent mechanism. This degradation is pathway specific, because Ste11 is stable during activation of the high osmolarity glycerol pathway. Because the steady-state amount of Ste11 does not change significantly during pheromone induction, we infer that maintenance of MAPK activation involves repeated cycles in which naïve Ste11 is activated and then targeted for degradation. This model predicts that elimination of active Ste11 would rapidly curtail MAPK activation upon attenuation of the upstream signal. This prediction is confirmed by the finding that blocking ubiquitin-dependent Ste11 degradation during pheromone induction abolishes the characteristic attenuation profile for MAPK activation.
Mutations in the RNase H domain of HIV-1 reverse transcriptase affect the initiation of DNA synthesis and the specificity of RNase H cleavage
in vivo
Authors: Julias John G. , McWilliams Mary Jane , Sarafianos Stefan G. , Arnold Edward , Hughes Stephen H.
Published: July 2002

Retroviral reverse transcriptases contain a DNA polymerase activity that can copy an RNA or DNA template and an RNase H activity that degrades the viral RNA genome during reverse transcription. RNase H makes both specific and nonspecific cleavages; specific cleavages are used to generate and remove the polypurine tract primer used for plus-strand DNA synthesis and to remove the tRNA primer used for minus-strand DNA synthesis. We generated mutations in an HIV-1-based vector to change amino acids in the RNase H domain that contact either the RNA and DNA strands. Some of these mutations affected the initiation of DNA synthesis, demonstrating an interdependence of the polymerase and RNase H activities of HIV-1 reverse transcription during viral DNA synthesis. The ends of the linear DNA form of the HIV-1 genome are defined by the specific RNase H cleavages that remove the plus- and minus-strand primers; these ends can be joined to form two-long-terminal repeat circles. Analysis of two-long-terminal repeat circle junctions showed that mutations in the RNase H domain affect the specificity of RNase H cleavage.