bluuebunny/crossref_metadata_2025 huggingface dataset:
Absence of ferromagnetism in single-phase wurtzite Zn1−xMnxO polycrystalline thin films
Authors: Gilliland S. , Segura A. , Sánchez-Royo J. F. , García L. M. , Bartolomé F. , Sans J. A. , Martínez-Criado G. , Jimenez-Villacorta F.
Published: October 2010

This paper reports on the magnetic properties of Zn1−xMnxO with Mn concentrations from 5% to 25%. The polycrystalline films, deposited by pulsed laser deposition on c-oriented sapphire and mica substrates, were first characterized by x-ray diffraction, absorption, and fluorescence, as well as by photoelectron and optical spectroscopy. Except for the highest Mn content films, all films are single-phase wurtzite, with Mn in tetrahedral coordination. Films with 25% Mn show traces of other crystal phases with spinel or perovskite structure. Their magnetization was then measured in function of applied magnetic field in the range −5 to 5 T and temperature in the range 1.8 to 300 K. All single-phase films exhibit paramagmetic behavior in the whole temperature range. Ferri- or ferromagnetic behavior (involving only a small proportion of the total magnetization) is found only in samples with other phases or in the sapphire substrates. The paramagnetic susceptibility increases with the Mn content. However, the decrease in the mean magnetic moment per Mn atom with increasing Mn concentration and the temperature dependence of the magnetic susceptibility around 1.8 K provide strong evidence of antiferromagnetic interaction between second neighbor Mn atoms.
Shear acoustic waves polarized along the ridged surface of an isotropic solid plate: Mode coupling effects due to the shape profile
Authors: Valier-Brasier Tony , Potel Catherine , Bruneau Michel
Published: October 2010

The aim of the paper is to describe the modes coupling due to scattering on small one-dimensional irregularities (parallel ridges) of the surface of isotropic solid plates, when shear horizontal waves (SH-waves) polarized along the ridges propagate perpendicularly to them. In a previous paper [Valier-Brasier et al., Appl. Phys. Lett. 93, 164101 (2008)], an analytical model was presented for describing the roughness by inertia of “teeth” which bound the ridges, through an impedancelike boundary condition, whatever shape of the roughness is. In the present paper, this shape is accounted for through a more sophisticated model, used previously for describing the effects of the roughness of walls on acoustic pressure fields in fluid-filled waveguides [Valier-Brasier et al., J. Appl. Phys. 106, 034913 (2009)], and adapted here in order to describe the modes coupling due to the scattering of these SH-waves. Moreover, the effect of a spatial periodicity of the ridges on the modes coupling is discussed, emphasizing the role played by the phase-matching (phonon relationship). Finally, comparison between both models (“teeth inertial” and “shape profile” models) is given and discussed.
Ferromagnetic properties of the Mn-doped nanograined ZnO films
Authors: Straumal B. B. , Protasova S. G. , Mazilkin A. A. , Myatiev A. A. , Straumal P. B. , Schütz G. , Goering E. , Baretzky B.
Published: October 2010

Dense nanograined pure and Mn-doped Zn1−xMnxO polycrystals with x ranging between 0.1–34 at. % were synthesized by the wet chemistry method from butanoate precursors. Pure and Mn-doped ZnO possesses ferromagnetic properties only if the ratio of grain boundary (GB) area to grain volume sGB exceeds a certain threshold value sth. The polycrystals in this work satisfy these conditions and, therefore, reveal ferromagnetic properties. The observed dependence of saturation magnetization on the Mn concentration shows an unexpected nonmonotonous behavior. The increase in saturation magnetization at low Mn concentration is explained by the injection of divalent Mn2+ ions and charge carriers into pure ZnO. The decrease in saturation magnetization between 0.1 and 5 at. % Mn can be explained by the increase in the portion of Mn3+ and Mn4+ ions. The second increase in saturation magnetization above 5 at. % Mn is explained by the formation of multilayer Mn segregation layer in ZnO GBs. The shape of the dependence of saturation magnetization on Mn concentration is different for the Mn-doped nanograined ZnO manufactured by different methods. It is most probably controlled by the topology of GB network (ferromagnetic GB foam) in the ZnO polycrystals.
Influence of the transverse dimension on the structure and properties of dc glow discharges
Authors: Bogdanov E. A. , Adams S. F. , Demidov V. I. , Kudryavtsev A. A. , Williamson J. M.
Published: October 2010

Two–dimensional (2D) simulations of a dc glow discharge with a cold cathode in argon have been performed for various radii of the discharge tube. It is shown that the loss of the charged particles to the walls can significantly affect plasma parameters as well as properties of the cathode sheath. The longitude dimensions of the negative glow and Faraday dark space depend on the transverse loss of the charge particles and are not consistently predicted with a 1D model. The common assumption that the cathode sheath can be analyzed independently of the plasma also may not be valid. The transverse inhomogeneity of the plasma leads to a change in the current density distribution over the cathode surface. The thickness of the cathode sheath can vary with radial distance from the discharge axis, even for the case of negligible radial loss of the charge particles. The 2D model results provide an analysis of the conditions of applicability of the 1D model.
Stable coaxial-waveguide gyrotron backward-wave oscillator with distributed losses
Authors: Hung C. L.
Published: October 2010

This study analyzes the performance of a coaxial-waveguide gyrotron backward-wave oscillator (gyro-BWO) operating at the fundamental harmonic by considering mode competition, which may be attributed to higher-order axial modes and competing transverse modes. In the coaxial waveguide with a short length and uniform cross section, the threshold currents of the higher-order axial modes are substantially higher than the operating current. Additionally, when the beam voltage or the magnetic field is adjusted, the oscillation that neighbors the minimum start-oscillation current of a transverse mode has a positive-kz field profile, and is excited near the cutoff frequency. As a result, the distributed wall losses at the downstream end of the interaction structure effectively damp the positive-kz field, and raise significantly the minimum start-oscillation currents of the competing transverse modes. This study also investigates how the parameters, including lossy section length, outer wall resistivity, inner wall resistivity, and ratio of the outer radius to the inner radius, affect the start-oscillation currents of the competing transverse modes in order to obtain stable operation conditions in the frequency tuning range. As is forecasted, when using a 15 A electron beam, the Ka-band coaxial gyro-BWO produces an output power of 137 kW and 3 dB bandwidth of 4.2% by magnetic tuning and an output power of 145 kW and 3dB bandwidth of 2.0% by beam voltage tuning.
Perturbation analysis of trapped-particle dynamics in axisymmetric dipole geometry
Authors: Duthoit F.-X. , Brizard A. J. , Peysson Y. , Decker J.
Published: October 2010

The bounce-action-angle coordinates (J,ζ) for charged particles trapped in an axisymmetric dipole magnetic field are constructed by perturbation analysis. First, the lowest-order bounce-action-angle coordinates (J0,ζ0) are derived for deeply trapped particles in the harmonic-oscillator approximation. Next, the Lie-transform perturbation method is used to derive higher-order anharmonic action-angle corrections (J=J0+ϵtJ1, ζ=ζ0+ϵtζ1), where the dimensionless parameter ϵt≡(sb/re)2⪡1 is defined as the ratio of the turning-point distance |sb| (measured from the equator) along a magnetic field line labeled by the equatorial distance re. Explicit expressions (with anharmonic corrections) for the canonical parallel coordinates s(J,ζ) and p∥(J,ζ) are presented, which satisfy the canonical identity {s,p∥}≡1. Lastly, analytical expressions for the bounce and drift frequencies (which include anharmonic corrections) yield excellent agreement with exact numerical results.
Relaxation dynamics and ionic conductivity in a fragile plastic crystal
Authors: Bauer Th. , Köhler M. , Lunkenheimer P. , Loidl A. , Angell C. A.
Published: October 2010

We report a thorough characterization of the dielectric relaxation behavior and the ionic conductivity in the plastic-crystalline mixture of 60% succinonitrile and 40% glutaronitrile. The plastic phase can be easily supercooled and the relaxational behavior is investigated by broadband dielectric spectroscopy in the liquid, plastic crystalline, and glassy crystal phases. The α-relaxation found in the spectra is characterized in detail. A well-pronounced secondary and faint indications for a third relaxation process were found. The latter most likely is of Johari–Goldstein type. From the temperature dependence of the α-relaxation time, a fragility parameter of 62 was determined. Thus, together with Freon112, this material stands out among all other plastic crystals by being a relatively fragile glass former. This finding provides strong support for an energy-landscape related explanation of the fragility of glass formers. In addition, unusually strong conductivity contributions were detected in the spectra exhibiting the typical features of ionic charge transport making this material a good basis for solid-state electrolytes.
Obliquely propagating generalized lower-hybrid drift instability with nonlocal two-fluid theory in current sheet equilibrium
Authors: Zou Dandan , Yang Weihong , Chen Yinhua , Yoon P. H.
Published: October 2010

By employing nonlocal two-fluid analysis, a class of obliquely propagating current sheet drift instabilities with frequency in the lower-hybrid frequency range is investigated. A series of unstable modes with multiple eigenstates are found by numerical simulation after electrostatic approximation. It is found that the growth rate of the unstable modes, whose eigenfunctions are localized at the current sheet edge, increases as the propagation more oblique. However, as the wave vector attains more and more field-aligned components, the maximum growth rate suffers an acute drop after a certain critical angle, beyond which it finally diminishes. On the other hand, the growth rate associated with modes located near the center of the current sheet is found to be less sensitive to the increase in propagation angle, although it does undergo a gradual decrease until it is stabilized when the mode becomes near-field aligned.
Dissociation of methane under high pressure
Authors: Gao Guoying , Oganov Artem R. , Ma Yanming , Wang Hui , Li Peifang , Li Yinwei , Iitaka Toshiaki , Zou Guangtian
Published: October 2010

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P212121, Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C2H6) at 95 GPa, butane (C4H10) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.
Unified treatment of coherent and incoherent electronic energy transfer dynamics using classical electrodynamics
Authors: Zimanyi Eric N. , Silbey Robert J.
Published: October 2010

Recent experiments on resonance energy transfer (RET) in photosynthetic systems have found evidence of quantum coherence between the donor and the acceptor. Under these conditions, Förster’s theory of RET is no longer applicable and no theory of coherent RET advanced to date rivals the intuitive simplicity of Förster’s theory. Here, we develop a framework for understanding RET that is based on classical electrodynamics but still captures the essence of the quantum coherence between the molecules. Our theory requires only a knowledge of the complex polarizabilities of the two molecules participating in the transfer as well as the distance between them. We compare our results to quantum mechanical calculations and show that the results agree quantitatively.
An efficient method for computing steady state solutions with Gillespie’s direct method
Authors: Mauch S. , Stalzer M.
Published: October 2010

Gillespie’s direct method is a stochastic simulation algorithm that may be used to calculate the steady state solution of a chemically reacting system. Recently the all possible states method was introduced as a way of accelerating the convergence of the simulations. We demonstrate that while the all possible states (APS) method does reduce the number of required trajectories, it is actually much slower than the original algorithm for most problems. We introduce the elapsed time method, which reformulates the process of recording the species populations. The resulting algorithm yields the same results as the original method, but is more efficient, particularly for large models. In implementing the elapsed time method, we present robust methods for recording statistics and empirical probability distributions. We demonstrate how to use the histogram distance to estimate the error in steady state solutions.
Scintillator-based diagnostic for fast ion loss measurements on DIII-D
Authors: Fisher R. K. , Pace D. C. , García-Muñoz M. , Heidbrink W. W. , Muscatello C. M. , Van Zeeland M. A. , Zhu Y. B.
Published: October 2010

A new scintillator-based fast ion loss detector has been installed on DIII-D with the time response (>100 kHz) needed to study energetic ion losses induced by Alfvén eigenmodes and other MHD instabilities. Based on the design used on ASDEX Upgrade, the diagnostic measures the pitch angle and gyroradius of ion losses based on the position of the ions striking the two-dimensional scintillator. For fast time response measurements, a beam splitter and fiberoptics couple a portion of the scintillator light to a photomultiplier. Reverse orbit following techniques trace the lost ions to their possible origin within the plasma. Initial DIII-D results showing prompt losses and energetic ion loss due to MHD instabilities are discussed.
Leakage mechanism of Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films
Authors: Sahoo S. K. , Misra D. , Agrawal D. C. , Mohapatra Y. N. , Majumder S. B. , Katiyar R. S.
Published: October 2010

The temperature and field dependence of the I-V characteristics of Ba0.8Sr0.2TiO3 thin films and Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films on Pt/Ti/SiO2/Si substrates are studied in the temperature range from 310 to 410 K. Leakage current behaviors of the film is analyzed in the light of various models. The bulk limited Poole–Frenkel mechanism is observed to dominate the leakage current in the temperature range of 310–410 K in the high field region. The energy of the trap levels calculated from the Poole–Frenkel model is in the range of 0.2–1.31 eV for different structures. In the low electric field region the conduction is Ohmic where as space charge limited current conduction is the dominant mechanism in the intermediate electric field region for all the temperatures.
Low frequency noise in long channel amorphous In–Ga–Zn–O thin film transistors
Authors: Fung Tze-Ching , Baek Gwanghyeon , Kanicki Jerzy
Published: October 2010

We investigated the low-frequency noise properties in the inverted-staggered amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with the silicon dioxide (SiO2) gate dielectric. The dependence of noise level on gate area indicates that the 1/f noise is the dominate source and the contribution from TFT parasitic resistances can be ignored in long channel devices. The gate voltage dependent noise data closely follow the mobility fluctuation (Δμ) model, and the Hooge’s parameter (αH) was extracted to be ∼1.52×10−3, which is much lower than the reported αH for a-Si:H TFTs. Finally, in the comparative study, the noise level in an unannealed a-IGZO TFT was found to be higher than that in an annealed device. The present results suggest that the 1/f noise in our a-IGZO TFT samples is sensitive to the active layer quality (i.e., concentration of conduction band-tail and/or deep gap states). In addition, the observed low noise in a-IGZO TFT can be associated with the s-orbital conduction in amorphous oxide semiconductor.
A transferable classical potential for the water molecule
Authors: Baranyai András , Kiss Péter T.
Published: October 2010

We developed a new model for the water molecule which contains only three Gaussian charges. Using the gas-phase geometry the dipole moment of the molecule matches, the quadrupole moment closely approximates the experimental values. The negative charge is connected by a harmonic spring to its gas-phase position. The polarized state is identified by the equality of the intermolecular electrostatic force and the spring force acting on the negative charge. In each timestep the instantaneous position of the massless negative charge is determined by iteration. Using the technique of Ewald summation, we derived expressions for the potential energy, the forces, and the pressure for Gaussian charges. The only properties to be fitted are the half-width values of the Gaussian charge distributions and the parameters of the nonelectrostatic repulsion-attraction potential. We determined the properties of gas-phase clusters up to six molecules, the internal energy and density of ambient water and hexagonal ice. We calculated the equilibrium density of ice VII as a function of pressure. As an additional test, we calculated the pair-correlation function, the isotherm compressibility, the heat capacity, and the self-diffusion coefficients for ambient water. As far as we know, this is the first classical model of water which is able to estimate both ends of the phase diagram, the high pressure ice VII, and the gas clusters of water with excellent accuracy.
Note: Collision-induced infrared absorption by gaseous deuterium
Authors: Abel Martin , Frommhold Lothar
Published: October 2010

A recent potential energy surface and an induced dipole surface obtained by advanced quantum chemical methods for collisional H2–H2 complexes [Li et al., Int. J. Spectroscopy.2010 (to be published)] are used to compute the infrared collision-induced absorption spectrum of dense deuterium gas. The computed spectra are in agreement with existing laboratory measurements.
Photodissociation of ozone in the Hartley band: Product state and angular distributions
Authors: McBane George C. , Nguyen Luan T. , Schinke Reinhard
Published: October 2010

Product state properties from the photodissociation of ozone in the ultraviolet Hartley band are investigated by trajectory surface-hopping calculations. The diabatic B and R state potential energy and coupling surfaces of Schinke and McBane [J. Chem. Phys. 132, 044305 (2010)] are employed. The properties computed include rotational and vibrational distributions in both the singlet and triplet channels, the total internal energy distribution in the triplet channel, and the photodissociation anisotropy parameter β in the singlet channel. A method for computing β from trajectories computed in internal Jacobi coordinates is described. In the singlet channel, the vibrational distribution is in good agreement with the experimental results. The observed increase in β with increasing photolysis wavelength is reproduced by the calculations and is attributed to the effects of the bending potential on the B state late in the fragmentation. The computed β values are too high with respect to experiment, and the peaks jmax of the singlet-channel rotational distributions are too low; these discrepancies are attributed to a too steep bending potential at long O–O distances. In the triplet channel, the main part of the internal energy distribution is described well by the calculations, although the detailed structures observed in the experiment are not reproduced. The experimental rotational distributions are well reproduced, although the maxima appear at slightly too high j. The triplet state product energy distributions are shown to depend largely on the distribution of hopping points onto the R state surface. A Landau–Zener model constructed as a function of the O2 bond distance provides a good physical description of the two-state dynamics. The high internal energy O2 products that cannot be attributed to the excitation of the Herzberg states remain unexplained.
DFT
+
U
study of defects in bulk rutile TiO2
Authors: Stausholm-Møller Jess , Kristoffersen Henrik H. , Hinnemann Berit , Madsen Georg K. H. , Hammer Bjørk
Published: October 2010

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5 eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5 eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti3+ ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5 eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.
Excited state dynamics in solid and monomeric tetracene: The roles of superradiance and exciton fission
Authors: Burdett Jonathan J. , Müller Astrid M. , Gosztola David , Bardeen Christopher J.
Published: October 2010

The excited state dynamics in polycrystalline thin films of tetracene are studied using both picosecond fluorescence and femtosecond transient absorption. The solid-state results are compared with those obtained for monomeric tetracene in dilute solution. The room temperature solid-state fluorescence decays are consistent with earlier models that take into account exciton-exciton annihilation and exciton fission but with a reduced delayed fluorescence lifetime, ranging from 20–100 ns as opposed to 2 μs or longer in single crystals. Femtosecond transient absorption measurements on the monomer in solution reveal several excited state absorption features that overlap the ground state bleach and stimulated emission signals. On longer timescales, the initially excited singlet state completely decays due to intersystem crossing, and the triplet state absorption superimposed on the bleach is observed, consistent with earlier flash photolysis experiments. In the solid-state, the transient absorption dynamics are dominated by a negative stimulated emission signal, decaying with a 9.2 ps time constant. The enhanced bleach and stimulated emission signals in the solid are attributed to a superradiant, delocalized S1 state that rapidly fissions into triplets and can also generate a second superradiant state, most likely a crystal defect, that dominates the picosecond luminescence signal. The enhanced absorption strength of the S0→S1 transition, along with the partially oriented nature of our polycrystalline films, obscures the weaker T1→TN absorption features. To confirm that triplets are the major species produced by relaxation of the initially excited state, the delayed fluorescence and ground state bleach recovery are compared. Their identical decays are consistent with triplet diffusion and recombination at trapping or defect sites. The results show that complications like exciton delocalization, the presence of luminescent defect sites, and crystallite orientation must be taken into account to fully describe the photophysical behavior of tetracene thin films. The experimental results are consistent with the traditional picture that tetracene’s photodynamics are dominated by exciton fission and triplet recombination, but suggest that fission occurs within 10 ps, much more rapidly than previously believed.
Growth of seaweed-like TiO2 nanoarrays for dye-sensitized solar cells
Authors: Bala Hari , Jiang Lei , Fu Wuyou , Yuan Guangyu , Wang Xiaodong , Liu Zongrui
Published: October 2010

Seaweed-like titanium dioxide (TiO2) nanoarrays (STNAs) were grown on the surface of a titanium (Ti) sheet by hydrogen peroxide sculpture at low temperature. After calcination, the STNA transformed to highly crystalline anatase phase and exhibited a vertically standing structure, with an average length of 1.35–2.12 μm, leaves breadth of about 30–80 nm, and leaves thickness of about 10–15 nm. STNA-electrode dye-sensitized solar cells (DSCs) fabricated with dye C106 achieved an efficiency of 3.2% under irradiation of 100 mWcm‒2 air mass 1.5 global (AM1.5G) sunlight. Further research showed that the STNA-electrode DSC has much lower recombination rate (Kr) and longer electron life time (τn), thus making this STNA a potential candidate of electrode for fabricating high performance DSC.
Influence of aggregation on thermal conductivity in stable and unstable nanofluids
Authors: Shima P. D. , Philip John , Raj Baldev
Published: October 2010

We study the effect of nanoparticle aggregation on thermal conductivity (k), viscosity (η) and size distribution of a stable and unstable nanofluid of iron oxide (Fe3O4) and copper oxide (CuO), respectively. The k, η, and size are found to be time independent in Fe3O4 while they are time dependant in CuO nanofluid. The microscopic study shows a progressive increase in the size of aggregate from nano to micron in CuO nanofluid. Our results show the prominent role of agglomeration on thermal properties and the importance of surface functionalization for improved stability and performance of nanofluids.
A one-dimensional photonic-crystal nanocavity incorporating a fluorescent molecular dye
Authors: Murshidy Mohamed M. , Adawi Ali M. , Fry Paul W. , Lidzey David G.
Published: October 2010

We model the optical properties of a SiN based one-dimensional waveguide photonic crystal nanocavity with three tapered air holes as a function of the photonic crystal mirror fill factor using finite difference time domain calculations. We show that the Q-factor of this type of cavity has a strong dependence on the mirror fill factor, with a Q-factor as high as 35 000 predicted. Experimental studies on a range of representative structures whose surface is coated with a layer of a fluorescent dye confirm our modeling and reveal that such structures are characterized by a single optical mode having a Q-factor of 2000.
Sign reversal in the dielectric anisotropy as functions of temperature and frequency in the nematic phase of a bent-core mesogen
Authors: Jang Yun , Panov Vitaly P. , Keith C. , Tschierske C. , Vij J. K.
Published: October 2010

The sign reversal in the dielectric anisotropy (ϵ∥′−ϵ⊥′) from positive to negative is observed for 4-cyanoresorcinol bisbenzoate system with temperature and frequency in the nematic phase of a bent-core system as the temperature is reduced. This cross-over in dielectric anisotropy is such that at temperatures much below the cross-over temperature, the negative anisotropy is found to be independent of frequency. The experimental observation is supported by results from optical transmittance spectroscopy. This observation is suggested to arise from the following two sources: the presence of cybotactic clusters found by x-ray scattering in the same compound in its nematic phase and conformers that may change the bent core angle at a certain temperature as observed by nuclear magnetic resonance in a similar compound.
The electronic spectrum and photodissociation of dinitrogen tetroxide (N2O4): Multireference configuration interaction studies
Authors: Grein Friedrich
Published: October 2010

Multireference configuration interaction (MRCI) calculations were performed for vertical excitation energies and potential curves of N2O4 in D2h symmetry using the TZVPP basis set with diffuse functions on the nitrogens. The strong absorption of N2O4 around 185 nm is assigned to the transition from the ground state to 1 B11u (σO→σ∗N–N) rather than 1 B12u (πO→π∗NO2,n→σ∗N–N), as previously assumed. (N2O4 is placed in the yz-plane, with N–N along z.) Transition to 1 B11u is calculated to have an oscillator strength f=0.71 and is z-polarized, in agreement with the experimental observations. Another state, 2 B12u, lies close by, however, at a much lower f-value. The weak absorption around 340 nm is assigned to 1 B13u. Excitation to 1 B12u is calculated at 227 nm. There is no clear assignment of a state for the observed shoulder around 260 nm. TD-DFT (time-dependent density functional theory) vertical excitation energies are close to MRCI values. MRCI singlet and triplet potential curves for the dissociation N2O4→2NO2, combined with a table of NO2 states correlating with those of N2O4, indicate possible products of photodissociation at various wavelengths. The extensive literature on the photodissociation of N2O4 is reviewed. DFT geometry optimizations have been performed on low-lying singlet and triplet states.
Pt-assisted oxidation of (100)-Ge/high-k interfaces and improvement of their electrical quality
Authors: Henkel Christoph , Bethge Ole , Abermann Stephan , Puchner Stefan , Hutter Herbert , Bertagnolli Emmerich
Published: October 2010

We report on the improvement of electrical quality of (100)-Ge/high-k-dielectric interfaces by introducing thin Pt top layers on the dielectric and subsequent oxidative treatments or using a Pt-deposition process with inherent oxidative components. Here, deposition of thin physical vapor deposition-Pt layers, combined with subsequent oxygen treatments, or oxygen assisted atomic layer deposition of Pt on these dielectrics, is applied. Strong reduction of interface trap densities down to mid-1011 eV−1 cm−2 is achieved. The approach is shown for Pt/ZrO2/La2O3/Ge, Pt/ZrO2/GeO2/Ge, and Pt/ZrO2/Ge gate stacks. By x-ray photoelectron spectroscopy evidence is given for oxygen enrichment at Ge/high-k-dielectric interfaces, to be responsible for the improved electrical properties.
Surface segregation of bimetallic alloys in nanoscale confinement
Authors: Hu Ming , Giapis Konstantinos P. , Goicochea Javier V. , Poulikakos Dimos
Published: October 2010

The surface segregation of Pt atoms in liquid bimetallic alloys confined in carbon nanotube cavities was studied using molecular dynamics simulations. Considerable enrichment in the Pt-atom surface density was found to occur in Pt alloys, when the complementary metal has surface energy higher than Pt and simultaneously metal-wall interaction strength lower than that of Pt with the confining wall. The results suggest that solidification of liquid binary alloys in nanochannels could produce core-shell nanorods with the shell enriched in one of the components for catalytic and other applications.
Nonmonotonic crystallite-size dependence of the lattice parameter of nanocrystalline nickel
Authors: Sheng J. , Welzel U. , Mittemeijer E. J.
Published: October 2010

A nonmonotonic crystallite-size dependence of the lattice parameter of ultrathin, nanocrystalline Ni films, i.e., lattice contraction followed by lattice expansion with decreasing crystallite size, was observed using in situ x-ray diffraction stress measurements combined with a dedicated thermal treatment called “loop annealing.” The expected lattice contraction with decreasing crystallite size was found for nanocrystalline Cu and nanocrystalline Pd.
Internal potential distribution in organic light emitting diodes measured by dc bridge
Authors: Yin X. R. , Le Y. K. , Gao X. D. , Sun Z. Y. , Hou X. Y.
Published: October 2010

Internal potential distribution of organic light-emitting diodes (OLEDs) is an essential problem. By using dc bridge to eliminate errors due to high resistance of the devices at low bias, the potential distribution has been accurately measured for both double-layer and single-layer OLEDs. It is found that the electric field inside the device is not uniform, and the potential distribution changes with external bias. This phenomenon could be the effect of space charge originating from the unequal injections of holes and electrons, which is confirmed by the results of the device with modified work function of the anode.
Atomistic simulation of plasticity in silicon nanowires
Authors: Cleri Fabrizio , Ishida Tadashi , Collard Dominique , Fujita Hiroyuki
Published: October 2010

We study the tensile deformation of polycrystalline Si nanowires by means of molecular dynamics simulations. The initial microstructure is composed by a network of nanocrystals glued together by a thin layer of amorphous material. Atomistic simulations could clearly identify liquidlike flow in the constrained amorphous Si as the responsible for the observed elongation. After this first stage of nearly constant-stress flow, a necking instability sets in, eventually leading to fracture, at the point when the nanowire diameter becomes comparable to the size of the nanocrystals.
Regional US carbon sinks from three-dimensional atmospheric CO
2
sampling
Authors: Crevoisier Cyril , Sweeney Colm , Gloor Manuel , Sarmiento Jorge L. , Tans Pieter P.
Published: October 2010

Studies diverge substantially on the actual magnitude of the North American carbon budget. This is due to the lack of appropriate data and also stems from the difficulty to properly model all the details of the flux distribution and transport inside the region of interest. To sidestep these difficulties, we use here a simple budgeting approach to estimate land-atmosphere fluxes across North America by balancing the inflow and outflow of CO
2
from the troposphere. We base our study on the unique sampling strategy of atmospheric CO
2
vertical profiles over North America from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory aircraft network, from which we infer the three-dimensional CO
2
distribution over the continent. We find a moderate sink of 0.5 ± 0.4 PgC y
-1
for the period 2004–2006 for the coterminous United States, in good agreement with the forest-inventory-based estimate of the first North American State of the Carbon Cycle Report, and averaged climate conditions. We find that the highest uptake occurs in the Midwest and in the Southeast. This partitioning agrees with independent estimates of crop uptake in the Midwest, which proves to be a significant part of the US atmospheric sink, and of secondary forest regrowth in the Southeast. Provided that vertical profile measurements are continued, our study offers an independent means to link regional carbon uptake to climate drivers.
ErbB2 receptor controls microtubule capture by recruiting ACF7 to the plasma membrane of migrating cells
Authors: Zaoui Kossay , Benseddik Khedidja , Daou Pascale , Salaün Danièle , Badache Ali
Published: October 2010

Microtubules (MTs) contribute to key processes during cell motility, including the regulation of focal adhesion turnover and the establishment and maintenance of cell orientation. It was previously demonstrated that the ErbB2 receptor tyrosine kinase regulated MT outgrowth to the cell cortex via a complex including Memo, the GTPase RhoA, and the formin mDia1. But the mechanism that linked this signaling module to MTs remained undefined. We report that ErbB2-induced repression of glycogen synthase kinase-3 (GSK3) activity, mediated by Memo and mDia1, is required for MT capture and stabilization. Memo-dependent inhibition of GSK3 allows the relocalization of APC (adenomatous polyposis coli) and cytoplasmic linker-associated protein 2 (CLASP2), known MT-associated proteins, to the plasma membrane and ruffles. Peripheral microtubule extension also requires expression of the plus-end binding protein EB1 and its recently described interactor, the spectraplakin ACF7. In fact, in migrating cells, ACF7 localizes to the plasma membrane and ruffles, in a Memo-, GSK3-, and APC-dependent manner. Finally, we demonstrate that ACF7 targeting to the plasma membrane is both required and sufficient for MT capture downstream of ErbB2. This function of ACF7 does not require its recently described ATPase activity. By defining the signaling pathway by which ErbB2 allows MT capture and stabilization at the cell leading edge, we provide insights into the mechanism underlying cell motility and steering.
Coupling of tandem Smad ubiquitination regulatory factor (Smurf) WW domains modulates target specificity
Authors: Chong P. Andrew , Lin Hong , Wrana Jeffrey L. , Forman-Kay Julie D.
Published: October 2010

Smad ubiquitination regulatory factor 2 (Smurf2) is an E3 ubiquitin ligase that participates in degradation of TGF-β receptors and other targets. Smurf2 WW domains recognize PPXY (PY) motifs on ubiquitin ligase target proteins or on adapters, such as Smad7, that bind to E3 target proteins. We previously demonstrated that the isolated WW3 domain of Smurf2, but not the WW2 domain, can directly bind to a Smad7 PY motif. We show here that the WW2 augments this interaction by binding to the WW3 and making auxiliary contacts with the PY motif and a novel E/D-S/T-P motif, which is N-terminal to all Smad PY motifs. The WW2 likely enhances the selectivity of Smurf2 for the Smad proteins. NMR titrations confirm that Smad1 and Smad2 are bound by Smurf2 with the same coupled WW domain arrangement used to bind Smad7. The analogous WW domains in the short isoform of Smurf1 recognize the Smad7 PY peptide using the same coupled mechanism. However, a longer Smurf1 isoform, which has an additional 26 residues in the inter-WW domain linker, is only partially able to use the coupled WW domain binding mechanism. The longer linker results in a decrease in affinity for the Smad7 peptide. Interdomain coupling of WW domains enhances selectivity and enables the tuning of interactions by isoform switching.
Structural characterization of a β-turn mimic within a protein–protein interface
Authors: Eckhardt Björn , Grosse Wolfgang , Essen Lars-Oliver , Geyer Armin
Published: October 2010

β-turns are secondary structure elements not only exposed on protein surfaces, but also frequently found to be buried in protein–protein interfaces. Protein engineering so far considered mainly the backbone-constraining properties of synthetic β-turn mimics as parts of surface-exposed loops. A β-turn mimic, Hot═Tap, that is available in gram amounts, provides two hydroxyl groups that enhance its turn-inducing properties besides being able to form side-chain-like interactions. NMR studies on cyclic hexapeptides harboring the Hot═Tap dipeptide proved its strong β-turn-inducing capability. Crystallographic analyses of the trimeric fibritin-foldon/Hot═Tap hybrid reveal at atomic resolution how Hot═Tap replaces a βI’-turn by a βII’-type structure. Furthermore, Hot═Tap adapts to the complex protein environment by participating in several direct and water-bridged interactions across the foldon trimer interface. As building blocks, β-turn mimics capable of both backbone and side-chain mimicry may simplify the design of synthetic proteins.
Selectivity of externally facing ion-binding sites in the Na/K pump to alkali metals and organic cations
Authors: Ratheal Ian M. , Virgin Gail K. , Yu Haibo , Roux Benoît , Gatto Craig , Artigas Pablo
Published: October 2010

The Na/K pump is a P-type ATPase that exchanges three intracellular Na
+
ions for two extracellular K
+
ions through the plasmalemma of nearly all animal cells. The mechanisms involved in cation selection by the pump's ion-binding sites (site I and site II bind either Na
+
or K
+
; site III binds only Na
+
) are poorly understood. We studied cation selectivity by outward-facing sites (high K
+
affinity) of Na/K pumps expressed in
Xenopus
oocytes, under voltage clamp. Guanidinium
+
, methylguanidinium
+
, and aminoguanidinium
+
produced two phenomena possibly reflecting actions at site III: (
i
) voltage-dependent inhibition (VDI) of outwardly directed pump current at saturating K
+
, and (
ii
) induction of pump-mediated, guanidinium-derivative–carried inward current at negative potentials without Na
+
and K
+
. In contrast, formamidinium
+
and acetamidinium
+
induced K
+
-like outward currents. Measurement of ouabain-sensitive ATPase activity and radiolabeled cation uptake confirmed that these cations are external K
+
congeners. Molecular dynamics simulations indicate that bound organic cations induce minor distortion of the binding sites. Among tested metals, only Li
+
induced Na
+
-like VDI, whereas all metals tested except Na
+
induced K
+
-like outward currents. Pump-mediated K
+
-like organic cation transport challenges the concept of rigid structural models in which ion specificity at site I and site II arises from a precise and unique arrangement of coordinating ligands. Furthermore, actions by guanidinium
+
derivatives suggest that Na
+
binds to site III in a hydrated form and that the inward current observed without external Na
+
and K
+
represents cation transport when normal occlusion at sites I and II is impaired. These results provide insights on external ion selectivity at the three binding sites.
Plasma membrane-localized transporter for aluminum in rice
Authors: Xia Jixing , Yamaji Naoki , Kasai Tomonari , Ma Jian Feng
Published: October 2010

Aluminum (Al) is the most abundant metal in the Earth's crust, but its trivalent ionic form is highly toxic to all organisms at low concentrations. How Al enters cells has not been elucidated in any organisms. Herein, we report a transporter, Nrat1 (Nramp aluminum transporter 1), specific for trivalent Al ion in rice. Nrat1 belongs to the Nramp (natural resistance-associated macrophage protein) family, but shares a low similarity with other Nramp members. When expressed in yeast, Nrat1 transports trivalent Al ion, but not other divalent ions, such as manganese, iron, and cadmium, or the Al–citrate complex. Nrat1 is localized at the plasma membranes of all cells of root tips except epidermal cells. Knockout of
Nrat1
resulted in decreased Al uptake, increased Al binding to cell wall, and enhanced Al sensitivity, but did not affect the tolerance to other metals. Expression of
Nrat1
is up-regulated by Al in the roots and regulated by a C2H2 zinc finger transcription factor (ART1). We therefore concluded that Nrat1 is a plasma membrane-localized transporter for trivalent Al, which is required for a prior step of final Al detoxification through sequestration of Al into vacuoles.
Large-scale in vivo femtosecond laser neurosurgery screen reveals small-molecule enhancer of regeneration
Authors: Samara Chrysanthi , Rohde Christopher B. , Gilleland Cody L. , Norton Stephanie , Haggarty Stephen J. , Yanik Mehmet Fatih
Published: October 2010

Discovery of molecular mechanisms and chemical compounds that enhance neuronal regeneration can lead to development of therapeutics to combat nervous system injuries and neurodegenerative diseases. By combining high-throughput microfluidics and femtosecond laser microsurgery, we demonstrate for the first time large-scale in vivo screens for identification of compounds that affect neurite regeneration. We performed thousands of microsurgeries at single-axon precision in the nematodeCaenorhabditis elegansat a rate of 20 seconds per animal. Following surgeries, we exposed the animals to a hand-curated library of approximately one hundred small molecules and identified chemicals that significantly alter neurite regeneration. In particular, we found that the PKC kinase inhibitor staurosporine strongly modulates regeneration in a concentration- and neuronal type-specific manner. Two structurally unrelated PKC inhibitors produce similar effects. We further show that regeneration is significantly enhanced by the PKC activator prostratin.
ELOVL1 production of C24 acyl-CoAs is linked to C24 sphingolipid synthesis
Authors: Ohno Yusuke , Suto Shota , Yamanaka Masao , Mizutani Yukiko , Mitsutake Susumu , Igarashi Yasuyuki , Sassa Takayuki , Kihara Akio
Published: October 2010

Very long-chain fatty acids (VLCFAs) exert a variety of cellular functions and are associated with numerous diseases. However, the precise pathway behind their elongation has remained elusive. Moreover, few regulatory mechanisms for VLCFAs synthesis have been identified. Elongases catalyze the first of four steps in the VLCFA elongation cycle; mammals have seven elongases (ELOVL1–7). In the present study, we determined the precise substrate specificities of all the ELOVLs by in vitro analyses. Particularly notable was the high activity exhibited by ELOVL1 toward saturated and monounsaturated C20- and C22-CoAs, and that it was essential for the production of C24 sphingolipids, which are unique in their capacity to interdigitate within the membrane as a result of their long chain length. We further established that ELOVL1 activity is regulated with the ceramide synthase CERS2, an enzyme essential for C24 sphingolipid synthesis. This regulation may ensure that the production of C24-CoA by elongation is coordinated with its utilization. Finally, knockdown of
ELOVL1
caused a reduction in the activity of the Src kinase LYN, confirming that C24-sphingolipids are particularly important in membrane microdomain function.
Critical role of DNAX accessory molecule-1 (DNAM-1) in the development of acute graft-versus-host disease in mice
Authors: Nabekura Tsukasa , Shibuya Kazuko , Takenaka Eri , Kai Hirayasu , Shibata Kai , Yamashita Yumi , Harada Kyoichi , Tahara-Hanaoka Satoko , Honda Shin-ichiro , Shibuya Akira
Published: October 2010

Acute graft-versus-host disease (GVHD) is a life-threatening complication following bone marrow transplantation; however, no effective molecular-targeting therapy has been determined. Here, we show that mice that received allogeneic splenocytes deficient in DNAX accessory molecule-1 (DNAM-1) had significantly milder GVHD and lower mortality than those that received allogeneic WT splenocytes. Donor CD8+T cells deficient in DNAM-1 showed significantly less proliferation and infiltration of the liver and intestines of recipient mice and produced less IFN-γ after coculture with allogeneic splenocytes than WT CD8+T cells. Mice prophylactically treated with an anti–DNAM-1 antibody showed milder GVHD and lower mortality than those treated with a control antibody. Moreover, treatment with a single administration of the antibody after the overt onset of GVHD ameliorated GVHD and prolonged survival. Finally, we show that the anti–DNAM-1 antibody therapy also ameliorated the overt GVHD in lethally irradiated mice after MHC-matched, minor antigen-mismatched bone marrow transplantation. These results indicate that DNAM-1 plays an important role in the development of GVHD and is an ideal molecular target for therapeutic approaches to GVHD.