bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of traps on the performance of bulk heterojunction organic solar cells
Authors: Mandoc M. M. , Kooistra F. B. , Hummelen J. C. , de Boer B. , Blom P. W. M.
Published: December 2007

The effect of electron traps on the performance of bulk heterojunction organic solar cells made of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is investigated. By deliberately introducing 7,7,8,8-tetracyanoquinodimethane, which is a stronger electron accepting molecule than PCBM, the electron transport in the solar cells becomes trap limited. We demonstrate that the open circuit voltage and its light-intensity dependence are strongly affected by recombination of holes with trapped electrons. Depending on the amount of traps, their recombination strength, and the light intensity, the losses due to trap-assisted recombination can even dominate over the intrinsic bimolecular recombination.
Tuning the perpendicular magnetic anisotropy in tetragonally distorted FexCo1−x alloy films on Rh (001) by varying the alloy composition
Authors: Luo F. , Fu X. L. , Winkelmann A. , Przybylski M.
Published: December 2007

Tetragonally distorted FexCo1−x alloy films are grown on Rh (001) showing a strong perpendicular magnetic anisotropy in a wide thickness and composition range. This large perpendicular magnetic anisotropy is chemical composition dependent and reaches a maximum at x=0.4. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature with film thickness up to 15 ML. Our experiments show that the proper adjustment of the Fermi level (EF) by the variation of the FexCo1−x alloy composition and the corresponding tetragonal distortion results in a large perpendicular magnetic anisotropy.
Magnetotransport properties of manganite based magnetic tunnel junctions
Authors: Lu Yafeng
Published: December 2007

We have studied spin-dependent tunneling in La2/3Ba1/3MnO3−δ based trilayer magnetic tunnel junctions fabricated by pulsed laser deposition. In the heteroepitaxial trilayer structures, a strained SrTiO3 layer of different thickness was used as a tunneling barrier. For the junctions with a SrTiO3 barrier, the measured tunneling characteristics deviated considerably from the ideal characteristics expected for elastic tunneling through an ideal barrier. The data indicate that inelastic multistep tunneling as described by the Glazman-Matveev model is present. Our analysis indicates that the SrTiO3 barrier layer consists of two different parts: The interface region with the manganite electrodes containing a high density of structural defects, and the central barrier layer with a small density of defects. The observed temperature dependence of the junction resistance can be explained by a series connection of the resistance contributions from the interfacial and central barrier part. The magnetic field dependence of the junction resistance is found to depend strongly on the barrier thickness and can show complicated behavior due to both a complex magnetic state in the junction electrode and magnetic interactions between the ferromagnetic electrodes. A very large tunneling magnetoresistance up to ΔR/Rp≃1200% is observed that, however, depends sensitively on the magnetic history.
Shock reflection and oblique shock waves
Authors: Li Dening
Published: December 2007

The linear stability of steady attached oblique shock wave and pseudosteady regular shock reflection is studied for the nonviscous full Euler system of equations in aerodynamics. A sufficient and necessary condition is obtained for their linear stability under three-dimensional perturbation. The result confirms the sonic point condition in the study of transition point from regular reflection to Mach reflection, in contrast to the von Neumann condition and detachment condition predicted from mathematical constraint.
Magnetization dynamics in planar spin transfer devices and stabilization by repulsion in a spin-flip transistor
Authors: Bazaliy Ya. B.
Published: December 2007

In systems with dominating easy-plane anisotropy, magnetization dynamics is governed by effective one dimensional equation for the in-plane angle. Rederiving this equation in the presence of spin torques, we obtain a convenient and intuitive description of spin transfer devices. In the case of a spin-flip transistor, the method provides a surprising prediction: the device can be stabilized in the (normally unstable) energy saddle point by a spin torque repelling from that point. Stabilization by repulsion happens due to the presence of dissipative environment and requires a Gilbert damping constant that is large enough to ensure overdamped dynamics at zero current.
Tuning the threshold voltage by inserting a thin molybdenum oxide layer into organic field-effect transistors
Authors: Guo Yunlong , Liu Yunqi , Di Chong-an , Yu Gui , Wu Weiping , Ye Shanghui , Wang Ying , Xu Xinjun , Sun Yanming
Published: December 2007

The authors report on the organic field-effect transistors based on copper phthalocyanine with a inserted layer of molybdenum oxide (MoO3). After applying a positive gate voltage of 100V, the threshold voltage had a large shift from −11.8to+66.2V and the device operating model was changed from the enhancement model into the depletion one. A possible mechanism is believed to be originated from the stored charges in the interface between the thin MoO3 layer and the active layer. Largely reversible shift in the threshold voltage and retention time were also obtained in the devices by the programs used.
Phase dynamics of InAs∕GaAs quantum dot semiconductor optical amplifiers
Authors: O‘Driscoll I. , Piwonski T. , Houlihan J. , Huyet G. , Manning R. J. , Corbett B.
Published: December 2007

The gain and phase dynamics of InAs∕GaAs quantum dot amplifiers are studied using single and two-color heterodyne pump probe spectroscopy. The relaxation of the wetting layer carrier density is shown to have a strong effect on the phase dynamics of both ground and excited state transients, while having a much weaker effect on the gain dynamics. In addition, the dynamical alpha factor may also display a constant value after an initial transient. Such behavior is strongly encouraging for reduced pattern effect operation in high speed optical networks.
A (q,γ) analog of the W1+∞ algebra
Authors: Miki Kei
Published: December 2007

A (q,γ) analog of the W1+∞ algebra is introduced. Irreducible quasifinite highest weight modules of this algebra and R matrices acting on a tensor product of these modules are investigated. A connection with the q deformed Virasoro and WN algebras is also discussed.
Wireless drive of piezoelectric plate by focused electric field
Authors: Bhuyan Satyanarayan , Hu Junhui
Published: December 2007

Wireless electric energy transmission to a piezoelectric plate using focused ac electric field is explored in this work. The ac electric field is focused, by using a needle ground electrode and plate-shaped live electrode to the piezoelectric plate placed in between them. The needle ground electrode enables better transmission of electric energy. When frequency of the electric field is close to mechanical resonance frequency of the piezoelectric plate operating in the thickness vibration mode, a maximum output power of 46.10mW has been achieved with an input voltage of 150Vrms and 2cm electrode separation.
Realization of n-Zn1−xMgxO∕i-ZnO∕SiOx∕n+-Si heterostructured n-i-n light-emitting diodes by low-cost ultrasonic spray pyrolysis
Authors: Zhao J. L. , Sun X. W. , Tan S. T. , Lo G. Q. , Kwong D. L. , Cen Z. H.
Published: December 2007

n - Zn 1 − x Mg x O ∕ i - Zn O ∕ Si O x ∕ n + - Si heterostructured light-emitting diodes have been demonstrated by low-cost ultrasonic spray pyrolysis. The current-voltage measurement shows typical characteristics of a back-to-back diode due to the double Schottky barriers induced by the SiOx layer. Blue electroluminescence peaking at 460nm was observed at room temperature when a positive bias of ∼4V was applied on the Si substrate. The electroluminescence is suggested to be dominated by the donor-acceptor pair recombination in the i-ZnO layer, where the holes were injected from the valence band of Si into the acceptor level of i-ZnO.
Plasma-assisted laser ablation of tungsten: Reduction in ablation power threshold due to bursting of holes/bubbles
Authors: Kajita Shin , Ohno Noriyasu , Takamura Shuichi , Sakaguchi Wataru , Nishijima Dai
Published: December 2007

Nanosecond laser ablation of tungsten (W) exposed to helium plasma is investigated using optical emission spectroscopy. Submicrometer-sized holes/bubbles are formed on the surface of W when it was exposed to the helium plasma at a sufficiently high temperature (≳1500–1600K). The emissions from a virgin W (before the helium plasma irradiation) cannot be detected when the fluence is <1J∕cm2; however, the threshold fluence for the detection of neutral W emission after it was exposed to the helium plasma is ∼0.2J∕cm2. The physical mechanism of laser-induced bursting of holes/bubbles is proposed for achieving a significant reduction in ablation power threshold.
Surface waves on a quantum plasma half-space
Authors: Lazar M. , Shukla P. K. , Smolyakov A.
Published: December 2007

Surface modes are coupled electromagnetic/electrostatic excitations of free electrons near the vacuum-plasma interface and can be excited on a sufficiently dense plasma half-space. They propagate along the surface plane and decay in either sides of the boundary. In such dense plasma models, which are of interest in electronic signal transmission or in some astrophysical applications, the dynamics of the electrons is certainly affected by the quantum effects. Thus, the dispersion relation for the surface wave on a quantum electron plasma half-space is derived by employing the quantum hydrodynamical (QHD) and Maxwell–Poisson equations. The QHD include quantum forces involving the Fermi electron temperature and the quantum Bohm potential. It is found that, at room temperature, the quantum effects are mainly relevant for the electrostatic surface plasma waves in a dense gold metallic plasma.
Shot noise of spin-dependent currents in ferromagnetic/semiconductor/ferromagnetic heterojunctions
Authors: An Xing-Tao , Liu Jian-Jun
Published: December 2007

Using the scattering matrix method, we investigate the spin-dependent conductance and the shot noise of the multichannel ferromagnetic/semiconductor/ferromagnetic heterojunctions in the presence of the spin-orbit coupling (SOC). We find that spin-up and spin-down electrons have different contributions to the conductance and the shot noise. The rounded quantum plateaus of the conductance appear when the length of the semiconductor is made shorter. As the number of conducting channels in the system increases, the shot noise power oscillates and the Fano factor is increasingly suppressed. We also find that interband mixing due to the SOC brings significant effects on the spin-dependent conductance and the shot noise.
Stable indium oxide thin-film transistors with fast threshold voltage recovery
Authors: Vygranenko Yuriy , Wang Kai , Nathan Arokia
Published: December 2007

Stable thin-film transistors (TFTs) with semiconducting indium oxide channel and silicon dioxide gate dielectric were fabricated by reactive ion beam assisted evaporation and plasma-enhanced chemical vapor deposition. The field-effect mobility is 3.3cm2∕Vs, along with an on/off current ratio of 106, and subthreshold slope of 0.5V/decade. When subject to long-term gate bias stress, the TFTs show fast recovery of the threshold voltage (VT) when relaxed without annealing, suggesting that charge trapping at the interface and/or in the bulk gate dielectric to be the dominant mechanism underlying VT instability. Device performance and stability make indium oxide TFTs promising for display applications.
Laser Doppler flowmetry signals to quantify effects of isoflurane on the peripheral cardiovascular system of healthy rats
Authors: Humeau Anne , Fizanne Lionel , Roux Jérôme , Asfar Pierre , Cales Paul , Rousseau David , Chapeau-Blondeau François
Published: December 2007

The optical Doppler effect resulting from interactions between laser light photons and red blood cells of the microcirculation is used to characterize the influence of isoflurane, an halogenated volatile anesthetic, on the peripheral cardiovascular system. After having recorded laser Doppler flowmetry blood perfusion signals on isoflurane-induced anesthetized healthy rats, wavelet analyses show a significant decrease of the myogenic and neurogenic activities when isoflurane dose increases from 1.5% to 3%. Moreover, the approximate entropy shows a weak decrease of signal irregularity when dose of isoflurane increases. These findings demonstrate the usefulness of the optical Doppler effect in physiological and pharmacological applications.
Active damping of the e-p instability at the Los Alamos Proton Storage Ring
Authors: Macek R. J. , Assadi S. , Byrd J. M. , Deibele C. E. , Henderson S. D. , Lee S. Y. , McCrady R. C. , Pivi M. F. T. , Plum M. A. , Walbridge S. B. , Zaugg T. J.
Published: December 2007

A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback.
Formation of I2(B3Π) in the presence of O2(a1Δ)
Authors: Azyazov Valeriy N. , Mikheyev Pavel A. , Ufimtsev Nikolay I. , Fomin Evgeny V. , Antonov Ivan O. , Heaven Michael C.
Published: December 2007

The mechanism by which I2(B3Π0) is excited in the chemical oxygen-iodine laser was studied by means of emission spectroscopy. Using the intensity of the O2(b1Σ,υ′=0)→O2(X3Σ,υ″=0) band as a reference, I2(B3Π0) relative number densities were assessed by measuring the I2(B3Π0,υ′)→I2(X1Σ,υ″) emission intensities. Vibrationally excited singlet oxygen molecules O2(a1Δ,υ′=1) were detected using infrared emission spectroscopy. The measured relative density of O2(a1Δ,υ′=1) for the conditions of a typical oxygen-iodine laser medium amounted to ∼15% of the total O2(a1Δ) content. Mechanisms for I2(B3Π0) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A′3Π2u, A3Π1u) as an intermediate. It is proposed that in the dissociation zone the molecular iodine A′3Π2u and A3Π1u states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1Δ,υ′), whereas in the downstream region of the dissociation zone those intermediate states are populated by the atomic iodine recombination process. I2(B3Π0) is subsequently formed in collisions of I2(A′3Π2u, A3Π1u) with singlet oxygen. We also demonstrated that I2(B3Π0) does not participate measurably in the I2 dissociation process, and that energy transfer from O2(b1Σ) does not excite I2(B3Π0) to a significant degree.
Fundamental drift of parameters in chalcogenide phase change memory
Authors: Karpov I. V. , Mitra M. , Kau D. , Spadini G. , Kryukov Y. A. , Karpov V. G.
Published: December 2007

We present the data on temporal (t) drift of parameters in chalcogenide phase change memory that significantly complement the earlier published results. The threshold voltage Vth and the amorphous state resistance R are shown to drift as ΔVth∝v ln t and R∝tα in broad intervals spanning up to nine decades in time; the drift coefficient v depends on glass parameters and temperature, but does not depend on device thickness. We have demonstrated that drift saturates at long enough times that can be shorten with temperature increase. All available data on drift dynamics are fully consistent with the classical double-well-potential model, which gives simple analytical expressions for the observed temporal dependencies including numerical parameters.
Improved operational lifetime of semiconducting polymer lasers by encapsulation
Authors: Richardson S. , Gaudin O. P. M. , Turnbull G. A. , Samuel I. D. W.
Published: December 2007

We report polymer distributed feedback lasers with dramatically extended operational lifetimes by using a simple encapsulation process. The lasers are configured as surface emitting, two-dimensional distributed feedback lasers based on the polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]. The microstructure is transferred to the polymer surface through solvent assisted micromolding. Once encapsulated, a 2500-fold improvement in lifetime is demonstrated under ambient conditions, compared to the unencapsulated device. A blueshift of the emission wavelength observed during operation is characterized by absorption and ellipsometry measurements and attributed to a change in effective index due to a loss of conjugation in the polymer.
Incorporation of Sb in InAs∕GaAs quantum dots
Authors: Molina S. I. , Sánchez A. M. , Beltrán A. M. , Sales D. L. , Ben T. , Chisholm M. F. , Varela M. , Pennycook S. J. , Galindo P. L. , Papworth A. J. , Goodhew P. J. , Ripalda J. M.
Published: December 2007

The formation of a quaternary InGaAsSb alloy is shown to occur in the core of epitaxial GaSb capped InAs∕GaAs quantum dots emitting at 1.3μm. The existence of the four constituent elements is demonstrated by using spatially resolved low-loss electron energy loss spectroscopy and aberration-corrected high angle annular dark field scanning transmission electron microscopy. The intermixing process giving rise to the formation of this quaternary alloy takes place despite the large miscibility gap between InAs and GaSb binary compounds, and is probably driven by the existence of strain in the quantum dots.
Finite-element modeling method for the study of dielectric relaxation at high frequencies of heterostructures made of multilayered particle
Authors: Fourn Cédric , Lasquellec Sophie , Brosseau Christian
Published: December 2007

There has been much recent interest in how morphological descriptors may affect the electromagnetic wave transport in particulate composite mediums. In this work, we present results of finite-element simulations that model the permittivity of two-dimensional (or cross sections of infinite three-dimensional parallel, infinitely long, identical, circular cylinders, where the properties and characteristics are invariant along the perpendicular cross-sectional plane) three-phase heterostructures made of a multilayered discoidal particle. While strictly valid only in a direct current situation, our analysis can be extended to treat electric fields that oscillate with time provided that the wavelengths and attenuation lengths associated with the fields are much larger than the microstructure dimension in order that the homogeneous (effective medium) representation of the composite structure makes sense. From simulations over a range of parameters, our analysis evaluates the effect of the surface fraction of inclusion, the conductivity, and thickness (relative to the particle radius) of the particle conductive coating on the effective complex permittivity of isotropic heterostructures in which the filler particles have a core-shell structure. Four main effects are found. First, the importance of the surface fraction of inclusion on the effective complex permittivity at high frequencies (from microwave to infrared) is illustrated over a broad range of coating thicknesses and conductivities. Second, the encapsulation phase (metallic coating) conductivity is identified as the key property controlling the dielectric relaxation due to interfacial polarization. Third, a simple parametrization of the high-frequency effective permittivity spectrum allowed us to obtain a reliable modelization of the Debye-type relaxation processes. From the least-squares fit of the effective complex permittivity data, we extract information on these relaxation processes, i.e., relaxation frequencies, relaxation strengths, and the limiting high-frequency permittivity. A salient point is that for core-shell structures there is a transition between a single peak and a two-peak relaxation spectrum which is under the dependence of the coating thickness. Fourth, we show how the features of permittivity spectra depend on the local dielectric environment (matrix and inclusion core) and shell conductivity. These results may have experimental consequences in the recent experiments on the dielectric relaxation in nanocoated particles.
Photocurrent of hydrogenated nanocrystalline silicon thin film/crystalline silicon heterostructure
Authors: Zhang R. , Chen X. Y. , Lu J. J. , Shen W. Z.
Published: December 2007

We report on the photocurrent properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin film/crystalline silicon (c-Si) n-p heterostructure. By comparison with the c-Si n-p homojunction, two Gaussian-type photocurrent peaks are observed in the nc-Si:H/c-Si heterostructure and attributed to be transitions from a tail band or discrete levels in quantum dots with localized states, and a miniband with extended states associated with the embedded nanometer crystallites in the amorphous boundaries of the nc-Si:H thin film. The observed strong photocurrent signals and temperature dependency have revealed the unique electronic states of the miniband in the nc-Si:H thin film. Our investigations into the photocurrent properties may help to realize nc-Si:H/c-Si heterostructure-based optoelectronic devices.
Peculiarities of the extraordinary Hall effect of planar arrays Fe nanoparticles embedded in an ultrathin Pt film
Authors: Volkov V. T. , Levashov V. I. , Matveev V. N. , Berezin V. A.
Published: December 2007

The extraordinary Hall effect was investigated for planar arrays Fe nanoparticles embedded in an ultrathin Pt matrix. It was found that the slope of Hall resistance (RH) reversed sign at ±0.15T for Fe particles of mean thickness 0.3nm. The dependence is presumably due to the effect of two spin-orbital induced mechanisms of carriers scattering, skew scattering, and side jump, with the skew component of positive sign and the side jump component of negative sign. For Fe samples, 0.6nm thick RH increases monotonically with the magnetic field value. At low temperatures, the superparamagnetism of the samples disappeared.
Negative characteristic temperature of long wavelength InAs∕AlGaInAs quantum dot lasers grown on InP substrates
Authors: Alghoraibi I. , Rohel T. , Piron R. , Bertru N. , Paranthoen C. , Elias G. , Nakkar A. , Folliot H. , Le Corre A. , Loualiche S.
Published: December 2007

InAs quantum dot lasers grown on (311)B InP substrates with AlGaInAs barriers have been fabricated and studied. A large decrease of the threshold current with temperature was observed from 110to140K. In the same temperature range, electroluminescence spectra showed a shape change, an energy shift with temperature, which cannot be fitted with a Varshni law, and a large decrease of the laser linewidth. These results can be related to a delayed thermalisation of carriers within quantum dot ensemble.
High-efficiency white organic light emitting devices with three separate phosphorescent emission layers
Authors: Sun Yiru , Forrest Stephen R.
Published: December 2007

We demonstrate high-efficiency white organic light emitting devices (WOLEDs) employing three adjacent phosphorescent emission layers (3-EMLs). The metal-organic dopants for red, green, and blue emissions are each doped in separate hosts, allowing for separate optimization of the three dopant-host material combinations. This structure distributes the exciton generation region across the three hosts to form a stepped progression of highest occupied and lowest unoccupied molecular orbitals. The 3-EML WOLED has a color rendering index of 81 and peak forward-viewing external quantum (EQE) and power efficiencies (PE) of (16.6±0.8)% and 32±1lm∕W, respectively, corresponding to a total EQE=(28±1)% and a total PE=54±3lm∕W. When an n-doped electron transporting layer is used, the total PE peaks at 64±3lm∕W, and rolls off to 34±2lm∕W at 1000cd∕m2.
Thermal hysteresis of interface biased ferromagnetic dots
Authors: Dantas Ana L. , Silva A. S. W. T. , Rebouças G. O. G. , Carriço A. S. , Camley R. E.
Published: December 2007

We present a theoretical investigation of the thermal hysteresis of iron dots exchange-coupled to an antiferromagnetic substrate. We consider a temperature interval bounded by the Néel temperature of the substrate, and we calculate the heating and cooling curves in the presence of an external field oriented opposite to the interface exchange field. The thermal hysteresis is due to the temperature variation of the interface field and the influence of the geometrical shapes and sizes of the dots on the magnetic states and switching mechanisms. We show that Fe dots on an uncompensated NiO substrate exhibit large thermal hysteresis at room temperature, and external fields of a few kOe. The width of the hysteresis loops depends on the dimensions of the ferromagnetic dot, and can be significant for dots elongated in the direction of the interface field.
Complicated three-dimensional photonic crystals fabricated by holographic lithography
Authors: Mao Wei-Dong , Liang Guan-Quan , Pu Yi-Ying , Wang He-Zhou , Zeng Zhaohua
Published: December 2007

We have fabricated a kind of three-dimensional photonic crystal in a single exposure by two sets of conventional laser holographic configurations. Specific structural features, such as a kagomelike lattice with p6mm symmetry on the top surface and a chainlike lattice on a cleavage plane, are possessed in this large-area and high-quality complicated microstructure, and many interesting properties such as skyrocketlike diffraction pattern exist. This work demonstrates that holographic lithography is a good method for the fabrication of photonic materials with crystal structure differing from the electronic crystals.
Field-aligned and transverse plasma-potential structures induced by electron cyclotron waves
Authors: Takahashi K. , Kaneko T. , Hatakeyama R.
Published: December 2007

It is demonstrated that potential structures of a magnetized plasma column can be controlled by electron cyclotron waves of azimuthal mode number m=±1. A field-aligned plasma potential structure, i.e., an electric double layer with the potential jump corresponding to the ion flow energy is created around the electron cyclotron resonance point at the center and peripheral areas of the plasma column for m=+1 and −1 modes, respectively. As a result, transversely outward and inward electric fields are locally generated. Our experimental results show that the created potential structures are greatly relevant to the polarization profiles of the injected waves and the resultant heating profiles.
Electron holography study of the temperature variation of the magnetic order parameter within circularly chained nickel nanoparticle rings
Authors: Sugawara Akira , Fukunaga Kei-ichi , Scheinfein M. R. , Kobayashi H. , Kitagawa H. , Tonomura A.
Published: December 2007

The magnetic structure within circular chains composed of 30-nm-diameter, polymer-coated nickel particles was studied by electron holography. The magnetic flux closure due to dipolar coupling was confirmed at zero external magnetic field. Its long-range order parameter was determined from the magnitude of the electron phase shift, and it was found to decrease more rapidly than that of bulk nickel as a function of temperature. The results of Monte Carlo simulations indicated that the temperature variation of the order-parameter was explained in terms of weakened dipolar coupling field ascribed to the decay of the bulk magnetization and enhanced thermal fluctuations.
A comprehensive model for bipolar electrical switching of CuTCNQ memories
Authors: Billen J. , Steudel S. , Müller R. , Genoe J. , Heremans P.
Published: December 2007

The generally observed bipolar electrical switching of Cu\CuTCNQ\metal memories (metal=Al,Yb,Ti) between two stable resistance states is shown to occur at the CuTCNQ\metal interface and not in the bulk of CuTCNQ. The switching is explained by a model involving electrochemical formation and dissolution of Cu filaments at the interface. In this mechanism, CuTCNQ acts as solid ionic conductor and source for the Cu+ cations. The model also explains earlier reported findings of bipolar switching in CuTCNQ devices, including the apparently contradictory observation that neutral TCNQ appears in the low-resistance state.
Self-assembly of silicide quantum dot arrays on stepped silicon surfaces by reactive epitaxy
Authors: Fernández L. , Löffler M. , Cordón J. , Ortega J. E.
Published: December 2007

Reactive epitaxy of Co on vicinal Si(111) surfaces is found to be a flexible and a convenient method for the preparation of dense arrays of Co silicide quantum dots. In the present work, submonolayer amounts of Co were deposited at 800K on vicinal and flat Si surfaces, analyzing the resulting structures by scanning tunneling microscopy. On vicinal Si(111) surfaces with relatively narrow (∼40–100Å) terrace width, such reactive epitaxy leads to self-assembled arrays of CoSi2 quantum dots with a sharp size distribution function. In contrast, the growth of Co on flat Si(111) results in an inhomogeneous array of dots mixed with a variety of silicide phases.
Protein-based biomemory device consisting of the cysteine-modified azurin
Authors: Choi Jeong-Woo , Oh Byung-Keun , Kim Young Jun , Min Junhong
Published: December 2007

We demonstrated a protein-based memory device using recombinant Pseudomonas aeruginosa azurin (azurin), a metalloprotein with a redox property. Azurin was recombined with a cysteine residue to enhance the stability of the self-assembled protein on the gold surface. The memory device characteristics, including the “read,” “write,” and “erase” functions of the self-assembled azurin layer, were well demonstrated with three distinct electrical states of azurin layers by cyclic voltammetry. The robustness of the protein-based biomemory device was validated by the repeated electrochemical performance of 500000cycles.
Full-field imaging of nonclassical acoustic nonlinearity
Authors: Sarens Bart , Kalogiannakis Georgios , Glorieux Christ , Van Hemelrijck Danny
Published: December 2007

The feasibility of full field shearographic detection of nonclassical acoustic nonlinearity is investigated. Traditional frequency analysis of the sinusoidally excited sample, as used in scanning techniques, turns out to be not practical due to the inherent optical detection nonlinearity of the shearography system itself. An alternative method, based on determining the asymmetry between shearographic images stroboscopically obtained for positive and negative displacements, is proposed. This approach allows us to easily and rapidly detect the tension-compression asymmetry which typically arises where nonbounded contact interface defects are present.
Low temperature mobility in hafnium-oxide gated germanium p-channel metal-oxide-semiconductor field-effect transistors
Authors: Beer Chris , Whall Terry , Parker Evan , Leadley David , De Jaeger Brice , Nicholas Gareth , Zimmerman Paul , Meuris Marc , Szostak Slawomir , Gluszko Grzegorz , Lukasiak Lidia
Published: December 2007

Effective mobility measurements have been made at 4.2K on high performance high-k gated germanium p-type metal-oxide-semiconductor field effect transistors with a range of Ge/gate dielectric interface state densities. The mobility is successfully modelled by assuming surface roughness and interface charge scattering at the SiO2 interlayer/Ge interface. The deduced interface charge density is approximately equal to the values obtained from the threshold voltage and subthreshold slope measurements on each device. A hydrogen anneal reduces both the interface state density and the surface root mean square roughness by 20%.
Anomalous phase transition and ionic conductivity of AgI nanowire grown using porous alumina template
Authors: Liang Changhao , Terabe Kazuya , Hasegawa Tsuyoshi , Aono Masakazu , Iyi Nobuo
Published: December 2007

Silver-iodide (AgI)-based superionic conductors are attracting widespread interest for their potential applications in electrochemical devices such as sensors and batteries. A new kind of nanocomposite with highly ordered AgI nanowires embedded in an anodic-aluminum-oxide (AAO) membrane was fabricated by low-temperature step-electrochemical growth. Structural evolution, phase transition, and ionic conductivity were investigated by x-ray diffraction, differential scanning calorimetry, and impedance measurements. The phase transition from β∕γ-AgI phase to α-AgI phase occurred at temperature of 168°C, that is, higher than that of reported bulk AgI (147°C); abnormally, the α to β∕γ phase-transition temperature on cooling was also depressed as large hysteresis formed. The high-temperature phase, namely, α-AgI, remained at temperatures as low as 80°C. The initial highly oriented-growth AgI nanowire disappeared after undergoing heating and cooling processes and a mixture of polycrystalline β∕γ-AgI and amorphouslike interface phases formed. The cooled AgI-AAO composite displayed ionic conductivity in the order of 10−2Scm−1 at room temperature. This array-structured nanocomposite of AgI-AAO may be further developed for usage as a new type of battery, i.e., “nanobatteries” and “nanosensors” with individual AgI nanowires as basic elements.
Hip, an HP1-interacting protein, is a haplo- and triplo-suppressor of position effect variegation
Authors: Schwendemann Alexander , Matkovic Tanja , Linke Christian , Klebes Ansgar , Hofmann Annemarie , Korge Günter
Published: December 2007

The
Drosophila
heterochromatin protein 1 (HP1) regulates epigenetic gene silencing and heterochromatin formation by promoting and maintaining chromatin condensation. Here we report the identification and characterization of an HP1-interacting protein (Hip). Hip interacts with HP1
in vitro
and is associated with HP1
in vivo
. This interaction is mediated by at least three independent but similar HP1-binding modules of the Hip protein. Hip and HP1 completely colocalize in the pericentric heterochromatin, and both haplo- and triplo-dosage mutations act as dominant suppressors of position effect variegation. These findings identify a player in heterochromatinization and suggest that Hip cooperates with HP1 in chromatin remodeling and gene silencing.
Activity of human hippocampal and amygdala neurons during retrieval of declarative memories
Authors: Rutishauser Ueli , Schuman Erin M. , Mamelak Adam N.
Published: December 2007

Episodic memories allow us to remember not only that we have seen an item before but also where and when we have seen it (context). Sometimes, we can confidently report that we have seen something (familiarity) but cannot recollect where or when it was seen. Thus, the two components of episodic recall, familiarity and recollection, can be behaviorally dissociated. It is not clear, however, whether these two components of memory are represented separately by distinct brain structures or different populations of neurons in a single anatomical structure. Here, we report that the spiking activity of single neurons in the human hippocampus and amygdala [the medial temporal lobe (MTL)] contain information about both components of memory. We analyzed a class of neurons that changed its firing rate to the second presentation of a previously novel stimulus. We found that the neuronal activity evoked by the presentation of a familiar stimulus (during retrieval) distinguishes stimuli that will be successfully recollected from stimuli that will not be recollected. Importantly, the ability to predict whether a stimulus is familiar is not influenced by whether the stimulus will later be recollected. We thus conclude that human MTL neurons contain information about both components of memory. These data support a continuous strength of memory model of MTL function: the stronger the neuronal response, the better the memory.
Activity-dependent phosphorylation of Ser187 is required for SNAP-25-negative modulation of neuronal voltage-gated calcium channels
Authors: Pozzi Davide , Condliffe Steven , Bozzi Yuri , Chikhladze Maia , Grumelli Carlotta , Proux-Gillardeaux Véronique , Takahashi Masami , Franceschetti Silvana , Verderio Claudia , Matteoli Michela
Published: December 2007

Synaptosomal-associated protein of 25 kDa (SNAP-25) is a SNARE protein that regulates neurotransmission by the formation of a complex with syntaxin 1 and synaptobrevin/VAMP2. SNAP-25 also reduces neuronal calcium responses to stimuli, but neither the functional relevance nor the molecular mechanisms of this modulation have been clarified. In this study, we demonstrate that hippocampal slices from
Snap25
+/−
mice display a significantly larger facilitation and that higher calcium peaks are reached after depolarization by
Snap25
−/−
and
Snap25
+/−
cultured neurons compared with wild type. We also show that SNAP-25b modulates calcium dynamics by inhibiting voltage-gated calcium channels (VGCCs) and that PKC phosphorylation of SNAP-25 at ser187 is essential for this process, as indicated by the use of phosphomimetic (S187E) or nonphosphorylated (S187A) mutants. Neuronal activity is the trigger that induces the transient phosphorylation of SNAP-25 at ser187. Indeed, enhancement of network activity increases the levels of phosphorylated SNAP-25, whereas network inhibition reduces the extent of protein phosphorylation. A transient peak of SNAP-25 phosphorylation also is detectable in rat hippocampus
in vivo
after i.p. injection with kainate to induce seizures. These findings demonstrate that differences in the expression levels of SNAP-25 impact on calcium dynamics and neuronal plasticity, and that SNAP-25 phosphorylation, by promoting inhibition of VGCCs, may mediate a negative feedback modulation of neuronal activity during intense activation.
Tendons of myostatin-deficient mice are small, brittle, and hypocellular
Authors: Mendias Christopher L. , Bakhurin Konstantin I. , Faulkner John A.
Published: December 2007

Tendons play a significant role in the modulation of forces transmitted between bones and skeletal muscles and consequently protect muscle fibers from contraction-induced, or high-strain, injuries. Myostatin (GDF-8) is a negative regulator of muscle mass. Inhibition of myostatin not only increases the mass and maximum isometric force of muscles, but also increases the susceptibility of muscle fibers to contraction-induced injury. We hypothesized that myostatin would regulate the morphology and mechanical properties of tendons. The expression of myostatin and the myostatin receptors ACVR2B and ACVRB was detectable in tendons. Surprisingly, compared with wild type (
MSTN
+/+
) mice, the tendons of myostatin-null mice (
MSTN
−/−
) were smaller and had a decrease in fibroblast density and a decrease in the expression of type I collagen. Tendons of
MSTN
−/−
mice also had a decrease in the expression of two genes that promote tendon fibroblast proliferation: scleraxis and tenomodulin. Treatment of tendon fibroblasts with myostatin activated the p38 MAPK and Smad2/3 signaling cascades, increased cell proliferation, and increased the expression of type I collagen, scleraxis, and tenomodulin. Compared with the tendons of
MSTN
+/+
mice, the mechanical properties of tibialis anterior tendons from
MSTN
−/−
mice had a greater peak stress, a lower peak strain, and increased stiffness. We conclude that, in addition to the regulation of muscle mass and force, myostatin regulates the structure and function of tendon tissues.
Mechanism of activation of the Formin protein Daam1
Authors: Liu Wei , Sato Akira , Khadka Deepak , Bharti Ritu , Diaz Hector , Runnels Loren W. , Habas Raymond
Published: December 2007

The Formin proteins are central players in mediating cytoskeletal reorganization and are epistatically positioned in a pathway downstream of Rho activation. These proteins exist in the cytoplasm in an autoinhibited state, which is mediated by intramolecular interactions between the amino-terminal GTPase binding domain (GBD) that encompasses the diaphanous inhibitory domain (DID) and the carboxyl-terminal diaphanous autoregulatory domain (DAD). It has been proposed that the binding of Rho within the GBD releases this molecule from autoinhibition by disrupting the DID/DAD interactions. Here we report that Daam1 is not significantly activated by Rho binding but rather by its interaction with Dishevelled (Dvl). Removal of the DAD domain disrupts interactions between Dvl and Daam1, and the binding of Dvl to Daam1 disrupts the interaction between the GBD and DAD that mediates Daam1 autoinhibition. Mutations within or removal of the DAD converts Daam1 into an active protein that can induce Rho activation. We further demonstrate that Dvl synergizes with Daam1 to regulate gastrulation during
Xenopus
embryogenesis and that expression of activated Daam1 can rescue impaired convergent extension movements resulting from deregulated noncanonical Wnt signaling. Our studies together define the importance of a carboxyl-terminal binding partner, Dvl, that leads to the activation of Daam1.
Transgenic system for conditional induction and rescue of chronic myocardial hibernation provides insights into genomic programs of hibernation
Authors: May Dalit , Gilon Dan , Djonov Valentin , Itin Ahuva , Lazarus Alon , Gordon Oren , Rosenberger Christian , Keshet Eli
Published: December 2007

A key energy-saving adaptation to chronic hypoxia that enables cardiomyocytes to withstand severe ischemic insults is hibernation, i.e., a reversible arrest of contractile function. Whereas hibernating cardiomyocytes represent the critical reserve of dysfunctional cells that can be potentially rescued, a lack of a suitable animal model has hampered insights on this medically important condition. We developed a transgenic mouse system for conditional induction of long-term hibernation and a system to rescue hibernating cardiomyocytes at will. Via myocardium-specific induction (and, in turn, deinduction) of a VEGF-sequestering soluble receptor, we show that VEGF is indispensable for adjusting the coronary vasculature to match increased oxygen consumption and exploit this finding to generate a hypoperfused heart. Importantly, ensuing ischemia is tunable to a level at which large cohorts of cardiomyocytes are driven to enter a hibernation mode, without cardiac cell death. Relieving the VEGF blockade even months later resulted in rapid revascularization and full recovery of contractile function. Furthermore, we show that left ventricular remodeling associated with hibernation is also fully reversible. The unique opportunity to uncouple hibernation from other ischemic heart phenotypes (e.g., infarction) was used to determine the genetic program of hibernation; uncovering hypoxia-inducible factor target genes associated with metabolic adjustments and induced expression of several cardioprotective genes. Autophagy, specifically self-digestion of mitochondria, was identified as a key prosurvival mechanism in hibernating cardiomyocytes. This system may lend itself for examining the potential utility of treatments to rescue dysfunctional cardiomyocytes and reverse maladaptive remodeling.
Phase diagram of water in carbon nanotubes
Authors: Takaiwa Daisuke , Hatano Itaru , Koga Kenichiro , Tanaka Hideki
Published: December 2007

A phase diagram of water in single-walled carbon nanotubes at atmospheric pressure is proposed, which summarizes ice structures and their melting points as a function of the tube diameter up to 1.7 nm. The investigation is based on extensive molecular dynamics simulations over numerous thermodynamic states on the temperature–diameter plane. Spontaneous freezing of water in the simulations and the analysis of ice structures at 0 K suggest that there exist at least nine ice phases in the cylindrical space, including those reported by x-ray diffraction studies and those unreported by simulation or experiment. Each ice has a structure that maximizes the number of hydrogen bonds under the cylindrical confinement. The results show that the melting curve has many local maxima, each corresponding to the highest melting point for each ice form. The global maximum in the melting curve is located at ≈11 Å, where water freezes in a square ice nanotube.
Laminopathic mutations interfere with the assembly, localization, and dynamics of nuclear lamins
Authors: Wiesel Naama , Mattout Anna , Melcer Shai , Melamed-Book Naomi , Herrmann Harald , Medalia Ohad , Aebi Ueli , Gruenbaum Yosef
Published: December 2007

Lamins are nuclear intermediate filament proteins and the major building blocks of the nuclear lamina. Besides providing nuclear shape and mechanical stability, lamins are required for chromatin organization, transcription regulation, DNA replication, nuclear assembly, nuclear positioning, and apoptosis. Mutations in human lamins cause many different heritable diseases, affecting various tissues and causing early aging. Although many of these mutations result in nuclear deformation, their effects on lamin filament assembly are unknown.
Caenorhabditis elegans
has a single evolutionarily conserved lamin protein, which can form stable 10-nm-thick filaments
in vitro
. To gain insight into the molecular basis of lamin filament assembly and the effects of laminopathic mutations on this process, we investigated mutations in conserved residues of the rod and tail domains that are known to cause various laminopathies in human. We show that 8 of 14 mutant lamins present WT-like assembly into filaments or paracrystals, whereas 6 mutants show assembly defects. Correspondingly, expressing these mutants in transgenic animals shows abnormal distribution of Ce-lamin, abnormal nuclear shape or change in lamin mobility. These findings help in understanding the role of individual residues and domains in laminopathy pathology and, eventually, promote the development of therapeutic interventions.
Retinal progenitor cells can produce restricted subsets of horizontal cells
Authors: Rompani S. B. , Cepko C. L.
Published: December 2007

Retinal progenitor cells have been shown to be multipotent throughout development. Similarly, many other structures of the developing central nervous system have been found to contain multipotent progenitor cells. Previous lineage studies did not address whether these multipotent progenitor cells were biased in their production of neuronal subtypes. This question is of interest because subtypes are the basis of distinct types of circuits. Here, lentivirus-mediated gene transfer was used to mark single retinal progenitor cells
in vivo
, and the different subtypes of horizontal cells (HCs) in each clone were quantified. Clones with two HCs consistently contained a single HC subtype, a pair of either H1 or H3 cells. This suggests that a multipotent progenitor cell produces a mitotic cell fated to make a terminal division that produces two HCs of only one subtype. This bias in production of one HC subtype suggests a previously undescribed mechanism of cell fate determination in at least a subset of retinal cells that involves decisions made by mitotic cells that are inherited in a symmetric manner by both neuronal daughter cells.