bluuebunny/crossref_metadata_2025 huggingface dataset:
Microviscoelasticity of soft repulsive sphere dispersions: Tracer particle microrheology of triblock copolymer micellar liquids and soft crystals
Authors: Tanner Shaun A. , Amin Samiul , Kloxin Christopher J. , van Zanten John H.
Published: May 2011

Tracer particle microrheology using diffusing wave spectroscopy-based microrheology is demonstrated to be a useful method to study the dynamics of aqueous Pluronic™ F108 solutions, which are viewed as solutions of repulsive soft spheres. The measured zero-shear microviscosity of noncrystallizing micellar dispersions indicates micelle corona dehydration upon increasing temperature. Colloidal sphere thermal motion is shown to be exquisitely sensitive to the onset of crystallization in these micellar dispersions. High temperature dynamics are dominated by an apparent soft repulsive micelle–micelle interaction potential indicating the important role played by lubrication forces and ultimately micelle corona interpenetration and compression at sufficiently high concentrations. The measured microscopic viscoelastic storage and loss moduli are qualitatively similar to those experimentally observed in mechanical measurements on colloidal dispersions and crystals, and calculated from mode coupling theory of colloidal suspensions. The observation of subdiffusive colloidal sphere thermal motion at short time-scales is strong evidence that the observed microscopic viscoelastic properties reflect the dynamics of individual micelles rather than a dispersion of micellar crystallites.
Exciton dynamics in chromophore aggregates with correlated environment fluctuations
Authors: Abramavicius Darius , Mukamel Shaul
Published: May 2011

We study the effects of correlated molecular transition energy fluctuations in molecular aggregates on the density matrix dynamics, and their signatures in the optical response. Correlated fluctuations do not affect single-exciton dynamics and can be described as a nonlocal contribution to the spectral broadening, which appears as a multiplicative factor in the time-domain response function. Intraband coherences are damped only by uncorrelated transition energy fluctuations. The signal can then be expressed as a spectral convolution of a local contribution of the uncorrelated fluctuations and the nonlocal contribution of the correlated fluctuations.
Design of high-contrast all-optical bistable switches based on coupled nonlinear photonic crystal microcavities
Authors: Li Shaohui , Cai Xuhong , Wang Xiaofan
Published: May 2011

We present a bistable photonic crystal configuration consisting of a waveguide directly coupled to three microcavities with instantaneous Kerr nonlinearity. Coupled mode theory combined with a numerical simulation based on the finite-difference time-domain technique is employed to design and evaluate the optical configuration. We demonstrate that with the proper choice of resonant frequencies of the constitutional photonic crystal microcavities, such a configuration can function as a high contrast switch, and the contrast between the bistable states in its transmission can be as high as 300. We expect this device to be useful in the practical application of all-optical switches.
Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II/III) complex with two crystallographically inequivalent iron sites
Authors: Hazra Susanta , Sasmal Sujit , Fleck Michel , Grandjean Fernande , Sougrati Moulay T. , Ghosh Meenakshi , Harris T. David , Bonville Pierre , Long Gary J. , Mohanta Sasankasekhar
Published: May 2011

The magnetic, electronic, and Mössbauer spectral properties of [Fe2L(μ-OAc)2]ClO4, 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H2L, indicate that 1 is a class III mixed valence iron(II/III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe2]V cationic configuration with a St = 9/2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of −32(2) cm−1 for an electron transfer parameter, B, of 950 cm−1, a zero-field uniaxial D9/2 parameter of −0.9(1) cm−1, and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, Ueff, of 9.8 cm−1. At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10−8 s. A fit of the temperature dependence of the average effective hyperfine field yields |D9/2| = 0.9 cm−1. An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm−1.
Accelerating chemical reactions: Exploring reactive free-energy surfaces using accelerated ab initio molecular dynamics
Authors: Pierce Levi C. T. , Markwick Phineus R. L. , McCammon J. Andrew , Doltsinis Nikos L.
Published: May 2011

A biased potential molecular dynamics simulation approach, accelerated molecular dynamics (AMD), has been implemented in the framework of ab initio molecular dynamics for the study of chemical reactions. Using two examples, the double proton transfer reaction in formic acid dimer and the hypothetical adiabatic ring opening and subsequent rearrangement reactions in methylenecyclopropane, it is demonstrated that ab initio AMD can be readily employed to efficiently explore the reactive potential energy surface, allowing the prediction of chemical reactions and the identification of metastable states. An adaptive variant of the AMD method is developed, which additionally affords an accurate representation of both the free-energy surface and the mechanism associated with the chemical reaction of interest and can also provide an estimate of the reaction rate.
Electric-field-induced dissociation of heavy Rydberg ion-pair states
Authors: Reinhold C. O. , Yoshida S. , Dunning F. B.
Published: May 2011

A classical trajectory Monte Carlo approach is used to simulate the dissociation of H+⋅⋅⋅F− and K+⋅⋅⋅Cl− heavy Rydberg ion pairs induced by a ramped electric field, a technique used experimentally to detect and probe ion-pair states. Simulations that include the effects of the strong short-range repulsive interaction associated with ion-pair scattering are in good agreement with experimental results for Stark wavepackets probed by a ramped field, demonstrating that many of the characteristics of field-induced dissociation can be well described using a quasi-classical model. The data also show that states with a given value of principal quantum number (i.e., binding energy) can dissociate over a broad range of applied fields, the exact field being governed by the initial orbital angular momentum and orientation of the state.
Observation of a time-dependent structural phase transition in potassium sodium bismuth titanate
Authors: Royles A. J. , Bell A. J. , Daniels J. E. , Milne S. J. , Comyn T. P.
Published: May 2011

High-energy synchrotron diffraction has been employed to demonstrate a time-dependent structural phase transition in 0.2K1/2Bi1/2TiO3–0.8Na1/2Bi1/2TiO3. It is demonstrated that the rapid increase in electric field (≥0.25 kV mm−1 s−1) induces a transformation from rhombohedral to mixed phase rhombohedral and tetragonal symmetry. When the electric field is applied slowly (<0.25 kV mm−1 s−1), no transformation occurs, and rhombohedral symmetry is maintained. The extent of transformation increases with the rate of increase in electric field. High-speed capture of diffraction images (7 frames per second) has been used to show the evolution of the phase transformation as a function of time.
Determining partial differential cross sections for low-energy electron photodetachment involving conical intersections using the solution of a Lippmann-Schwinger equation constructed with standard electronic structure techniques
Authors: Han Seungsuk , Yarkony David R.
Published: May 2011

A method for obtaining partial differential cross sections for low energy electron photodetachment in which the electronic states of the residual molecule are strongly coupled by conical intersections is reported. The method is based on the iterative solution to a Lippmann-Schwinger equation, using a zeroth order Hamiltonian consisting of the bound nonadiabatically coupled residual molecule and a free electron. The solution to the Lippmann-Schwinger equation involves only standard electronic structure techniques and a standard three-dimensional free particle Green's function quadrature for which fast techniques exist. The transition dipole moment for electron photodetachment, is a sum of matrix elements each involving one nonorthogonal orbital obtained from the solution to the Lippmann-Schwinger equation. An expression for the electron photodetachment transition dipole matrix element in terms of Dyson orbitals, which does not make the usual orthogonality assumptions, is derived.
Electronic structures and bonding of graphyne sheet and its BN analog
Authors: Zhou Jian , Lv Kun , Wang Qian , Chen X. S. , Sun Qiang , Jena Puru
Published: May 2011

Using density functional theory and generalized gradient approximation for exchange and correlation, we present theoretical analysis of the electronic structure of recently synthesized graphyne and its boron nitride analog (labeled as BN-yne). The former is composed of hexagonal carbon rings joined by C-chains, while the latter is composed of hexagonal BN rings joined by C-chains. We have explored the nature of bonding and energy band structure of these unique systems characterized by sp and sp2 bonding. Both graphyne and BN-yne are found to be direct bandgap semiconductors. The bandgap can be modulated by changing the size of hexagonal ring and the length of carbon chain, providing more flexibilities of energy band engineering for device applications. The present study sheds theoretical insight on better understanding of the properties of the novel carbon-based 2D structures beyond the graphene sheet.
Supersymmetry algebra cohomology. III. Primitive elements in four and five dimensions
Authors: Brandt Friedemann
Published: May 2011

The primitive elements of the supersymmetry algebra cohomology as defined in a previous paper are computed for standard supersymmetry algebras in four and five dimensions, for all signatures of the metric and any number of supersymmetries.
AX centers in II-VI semiconductors: Hybrid functional calculations
Authors: Biswas Koushik , Du Mao-Hua
Published: May 2011

Hybrid functional calculations predict significantly enhanced stability of AX centers against shallow acceptors in selected II-VI semiconductors (ZnO, ZnS, and ZnSe), as compared to the calculations based on local density approximation and generalized gradient approximation. The results agree well with the experimental observations on the p-type doping of ZnS and ZnSe. The improved description of the AX centers by hybrid functional calculations is due to the correction of the valence band maximum of the semiconductor.
Photoabsorption and S 2p photoionization of the SF6 molecule: Resonances in the excitation energy range of 200–280 eV
Authors: Stener M. , Bolognesi P. , Coreno M. , O’Keeffe P. , Feyer V. , Fronzoni G. , Decleva P. , Avaldi L. , Kivimäki A.
Published: May 2011

Photoabsorption and S 2p photoionization of the SF6 molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t2g and 4eg shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s−16t1u excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.
Semiclassical propagator for SU(n) coherent states
Authors: Viscondi Thiago F. , de Aguiar Marcus A. M.
Published: May 2011

We present a detailed derivation of the semiclassical propagator in the SU(n) coherent state representation. In order to provide support for immediate physical applications, we restrict this work to the fully symmetric irreducible representations, which are suitable for the treatment of bosonic dynamics in n modes, considering systems with conservation of total particle number. The derivation described here can be easily extended to other classes of coherent states, thus representing an alternative approach to previously published methods.
Anhydrous crystals of DNA bases are wide gap semiconductors
Authors: Maia F. F. , Freire V. N. , Caetano E. W. S. , Azevedo D. L. , Sales F. A. M. , Albuquerque E. L.
Published: May 2011

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results.
Two-dimensional fringe probing of transient liquid temperatures in a mini space
Authors: Xue Zhenlan , Qiu Huihe
Published: May 2011

A 2D fringe probing transient temperature measurement technique based on photothermal deflection theory was developed. It utilizes material's refractive index dependence on temperature gradient to obtain temperature information from laser deflection. Instead of single beam, this method applies multiple laser beams to obtain 2D temperature information. The laser fringe was generated with a Mach-Zehnder interferometer. A transient heating experiment was conducted using an electric wire to demonstrate this technique. Temperature field around a heating wire and variation with time was obtained utilizing the scattering fringe patterns. This technique provides non-invasive 2D temperature measurements with spatial and temporal resolutions of 3.5 μm and 4 ms, respectively. It is possible to achieve temporal resolution to 500 μs utilizing the existing high speed camera.
Sensitivity analysis of state-specific multireference perturbation theory
Authors: Szabados Ágnes
Published: May 2011

State-specific multireference perturbation theory (SS-MRPT) developed by Mukherjee et al. [Int. J. Mol. Sci. 3, 733 (2002)] is examined focusing on the dependence of the perturbed energy on the initial model space coefficients. It has been observed earlier, that non-physical kinks may appear on the potential energy surface obtained by SS-MRPT while related coupled-cluster methods may face convergence difficulties. Though exclusion or damping of the division by small coefficients may alleviate the problem, it is demonstrated here that the effect does not originate in an ill-defined division. It is shown that non-negligible model space coefficients may also be linked with the problem. Sensitivity analysis is suggested as a tool for detecting the coefficient responsible. By monitoring the singular values of sensitivity matrices, orders of magnitude increase is found in the largest value, in the vicinity of the problematic geometry point on the potential energy surface. The drastic increase of coefficient sensitivities is found to be linked with a degeneracy of the target root of the effective Hamiltonian. The nature of the one-electron orbitals has a profound influence on the picture: a rotation among active orbitals may screen or worsen the effect.
Reactivity indicators for degenerate states in the density-functional theoretic chemical reactivity theory
Authors: Cárdenas Carlos , Ayers Paul W. , Cedillo Andrés
Published: May 2011

Density-functional-theory-based chemical reactivity indicators are formulated for degenerate and near-degenerate ground states. For degenerate states, the functional derivatives of the energy with respect to the external potential do not exist, and must be replaced by the weaker concept of functional variation. The resultant reactivity indicators depend on the specific perturbation. Because it is sometimes impractical to compute reactivity indicators for a specific perturbation, we consider two special cases: point-charge perturbations and Dirac delta function perturbations. The Dirac delta function perturbations provide upper bounds on the chemical reactivity. Reactivity indicators using the common used “average of degenerate states approximation” for degenerate states provide a lower bound on the chemical reactivity. Unfortunately, this lower bound is often extremely weak. Approximate formulas for the reactivity indicators within the frontier-molecular-orbital approximation and special cases (two or three degenerate spatial orbitals) are presented in the supplementary material. One remarkable feature that arises in the frontier molecular orbital approximation, and presumably also in the exact theory, is that removing electrons sometimes causes the electron density to increase at the location of a negative (attractive) Dirac delta function perturbation. That is, the energetic response to a reduction in the external potential can increase even when the number of electrons decreases.
Hyperfine effects on collisional line shape. I. A self-consistent set of equations
Authors: Buffa G. , Tarrini O.
Published: May 2011

We show that the treatment available in the literature for calculating hyperfine effects on collisional line shape is affected by an error making the theory not self-consistent. By correcting such an error we show that the two hyperfine effects on the line shape, modification of the resolved components and collisional coupling between them, cancel each other when hyperfine structure collapses into a single line.
Hyperfine effects on spectral line shape. II. The case DCO+–He
Authors: Buffa G. , Tarrini O.
Published: May 2011

We discuss the hyperfine effect on the shape of rotational spectral lines of DCO+ broadened by collisions with helium. Hyperfine scattering matrix is calculated by the recoupling technique from the spin-free scattering matrix which is obtained by close-coupling calculations and by a previously tested potential. Line shape is calculated for different rotational transitions, perturber density values, and collisional energies. As forecast by a semiclassical treatment and contrary to what may happen for a symmetric top absorber, hyperfine effects are small for a linear absorber. In our case they are of about 2%. We could also verify that the two hyperfine effects on the line shape, modification of resolved components and collisional coupling between them, cancel each other at high values of helium density when hyperfine structure collapses into a single line.
A perturbative density functional theory for square-well fluids
Authors: Jin Zhehui , Tang Yiping , Wu Jianzhong
Published: May 2011

We report a perturbative density functional theory for quantitative description of the structural and thermodynamic properties of square-well fluids in the bulk or at inhomogeneous conditions. The free-energy functional combines a modified fundamental measure theory to account for the short-range repulsion and a quadratic density expansion for the long-range attraction. The long-correlation effects are taken into account by using analytical expressions of the direct correlation functions of bulk fluids recently obtained from the first-order mean-spherical approximation. The density functional theory has been calibrated by extensive comparison with simulation data from this work and from the literature. The theory yields good agreement with simulation results for the radial distribution function of bulk systems and for the density profiles of square-well fluids near the surfaces of spherical cavities or in slit pores over a broad range of the parameter space and thermodynamic conditions.
Hybrid quadrupole mass filter/quadrupole ion trap/time-of-flight-mass spectrometer for infrared multiple photon dissociation spectroscopy of mass-selected ions
Authors: Gulyuz Kerim , Stedwell Corey N. , Wang Da , Polfer Nick C.
Published: May 2011

We present a laboratory-constructed mass spectrometer optimized for recording infrared multiple photon dissociation (IRMPD) spectra of mass-selected ions using a benchtop tunable infrared optical parametric oscillator/amplifier (OPO/A). The instrument is equipped with two ionization sources, an electrospray ionization source, as well as an electron ionization source for troubleshooting. This hybrid mass spectrometer is composed of a quadrupole mass filter for mass selection, a reduced pressure (∼10−5 Torr) quadrupole ion trap (QIT) for OPO irradiation, and a reflectron time-of-flight drift tube for detecting the remaining precursor and photofragment ions. A helium gas pulse is introduced into the QIT to temporarily increase the pressure and hence enhance the trapping efficiency of axially injected ions. After a brief pump-down delay, the compact ion cloud is subjected to the focused output from the continuous wave OPO. In a recent study, we implemented this setup in the study of protonated tryptophan, TrpH+, as well as collision-induced dissociation products of this protonated amino acid [W. K. Mino, Jr., K. Gulyuz, D. Wang, C. N. Stedwell, and N. C. Polfer, J. Phys. Chem. Lett. 2, 299 (2011)]. Here, we give a more detailed account on the figures of merit of such IRMPD experiments. The appreciable photodissociation yields in these measurements demonstrate that IRMPD spectroscopy of covalently bound ions can be routinely carried out using benchtop OPO setups.
Generating transition paths by Langevin bridges
Authors: Orland Henri
Published: May 2011

We propose a novel stochastic method to generate paths conditioned to start in an initial state and end in a given final state during a certain time tf. These paths are weighted with a probability given by the overdamped Langevin dynamics. We show that these paths can be exactly generated by a non-local stochastic differential equation. In the limit of short times, we show that this complicated non-solvable equation can be simplified into an approximate local stochastic differential equation. For longer times, the paths generated by this approximate equation do not satisfy the correct statistics, but this can be corrected by an adequate reweighting of the trajectories. In all cases, the paths are statistically independent and provide a representative sample of transition paths. The method is illustrated on the one-dimensional quartic oscillator.
Temperature dependence of dark current properties of InGaAs/GaAs quantum dot solar cells
Authors: Lu Hao Feng , Fu Lan , Jolley Greg , Tan Hark Hoe , Tatavarti Sudersena Rao , Jagadish Chennupati
Published: May 2011

Self-assembled In0.5Ga0.5As/GaAs quantum dot solar cell (QDSC) was grown by metal organic chemical vapor deposition. Systematic measurements of dark current versus voltage (I-V) characteristics were carried out from 30 to 310 K. Compared with the reference GaAs solar cell, the QDSC exhibits larger dark current however its ideality factor (n) was smaller, which cannot be straightly interpreted by the conventional diode models. These results are important for the fundamental understanding of QDSC properties and further implementation of new solar cell designs for improved efficiency.
Temperature-dependent shifts of ultraviolet multipeak emissions for Mn-doped ZnO nanowires
Authors: Jiang Feihong , Zhang Jun
Published: May 2011

Mn-doped ZnO nanowires have been fabricated through high-temperature vapor-solid deposition process. The temperature-dependent photoluminescence properties of Mn-doped ZnO nanowires under 10 to 300 K have been investigated. The results show that the prepared samples have intensive multipeak emissions in the ultraviolet (UV) region (about 3.4–3.0 eV) at low temperature. The analyses of Gaussian-fitted the UV band from 10 to 300 K reveal that all UV spectra can be well fitted by four Gaussian peaks. With increasing temperature, the four UV bands show different temperature dependences and have obvious redshift. The origins of the intensive UV multipeak emissions can be attributed to the excitonic transition in Mn-doped ZnO nanowires under low temperature. The analysis results indicate that the crystal quality and UV luminescence efficiency of ZnO nanowires are improved with Mn doping.
Gelation in semiflexible polymers
Authors: Padmanabhan Venkat , Kumar Sanat K.
Published: May 2011

Molecular dynamics simulations have been employed to study the formation of a physical gel by semiflexible polymer chains. The formation of a geometrically connected network of these chains is investigated as a function of temperature and rate of cooling. The stiffness of the molecules is controlled via a potential between beads separated by two bonds. As the temperature is lowered, a percolated homogeneous solution phase separates to form a high-density, non-percolated nematic fluid and a low-density gas phase. On further decreasing the temperature, the chains are dynamically arrested preventing the completion of the vapor-liquid (VL) phase separation. As a result, the chains are stuck in a three-dimensional network of nematic bundles forming a percolated gel. Apart from temperature, the rate of cooling also plays an important role in the formation of the gel. Cooling the system at a faster rate yields gel while slower rates result in complete VL phase separation.
Oxidation and disorder in few-layered graphene induced by the electron-beam irradiation
Authors: Xu Zhiwei , Chen Lei , Li Jialu , Wang Rui , Qian Xiaoming , Song Xiaoyan , Liu Liangsen , Chen Guangwei
Published: May 2011

Structural changes caused by an electron beam with the high irradiation energy of 5 MeV were investigated in few-layered graphene. Both the original and the irradiated few-layered graphene were characterized by x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. It was found that a typical diffraction peak of graphene oxide emerged and this may be attributed to a partial oxidation in few-layered graphene which was induced by the irradiation. In addition, the graphitic structure of few-layered graphene was found to be disordered according to the increased intensity ratio of D to G band.
Conversion from unipolar to bipolar resistance switching by inserting Ta2O5 layer in Pt/TaOx/Pt cells
Authors: Yoo H. K. , Lee S. B. , Lee J. S. , Chang S. H. , Yoon M. J. , Kim Y. S. , Kang B. S. , Lee M.-J. , Kim C. J. , Kahng B. , Noh T. W.
Published: May 2011

We observed unipolar resistance switching in Pt/TaOx/Pt cells. We could make the cell have the bipolar resistance switching by inserting a stoichiometric Ta2O5 layer between Pt and TaOx layers. Bipolar resistance switching in Pt/Ta2O5/TaOx/Pt cells occurred reliably without applying an external compliance current. With increase in the Ta2O5 layer thickness, the current value at the low-resistance state became decreased but the forming voltage became increased. We could explain these intriguing phenomena using the interface-modified random circuit breaker network model.
Fast transient charging at the graphene/SiO2 interface causing hysteretic device characteristics
Authors: Lee Young Gon , Kang Chang Goo , Jung Uk Jin , Kim Jin Ju , Hwang Hyeon Jun , Chung Hyun-Jong , Seo Sunae , Choi Rino , Lee Byoung Hun
Published: May 2011

Device instabilities of graphene metal-oxide-semiconductor field effect transistors such as hysteresis and Dirac point shifts have been attributed to charge trapping in the underlying substrate, especially in SiO2. In this letter, trapping time constants around 87 μs and 1.76 ms were identified using a short pulse current-voltage method. The values of two trapping time constants with reversible trapping behavior indicate that the hysteretic behaviors of graphene field effect transistors are due to neither charge trapping in the bulk SiO2 or tunneling into other interfacial materials. Also, it is concluded that the dc measurement method significantly underestimated the performance of graphene devices.
Carbon nanotube semitransparent electrodes for amorphous silicon based photovoltaic devices
Authors: Del Gobbo S. , Castrucci P. , Scarselli M. , Camilli L. , De Crescenzi M. , Mariucci L. , Valletta A. , Minotti A. , Fortunato G.
Published: May 2011

Different amounts of single wall carbon nanotubes (SWCNTs) have been sprayed on amorphous silicon substrates to form Schottky barrier solar cells. The measured external quantum efficiency showed a spectral behavior depending on the SWCNT network optical transparency, presenting a maximum up to 35% at a wavelength of about 460 nm. Ultrathin network of SWCNTs acts as semitransparent electrode and forms Schottky barrier with amorphous silicon, enabling new generation low cost amorphous silicon based solar cells. Numerical simulations show a poor efficiency of SWCNT contacts in collecting holes suggesting that improvement in contact quality is needed to further improve solar cell efficiency.
Imaging sequential dehydrogenation of methanol on Cu(110) with a scanning tunneling microscope
Authors: Kitaguchi Y. , Shiotari A. , Okuyama H. , Hatta S. , Aruga T.
Published: May 2011

Adsorption of methanol and its dehydrogenation on Cu(110) were studied by using a scanning tunneling microscope (STM). Upon adsorption at 12 K, methanol preferentially forms clusters on the surface. The STM could induce dehydrogenation of methanol sequentially to methoxy and formaldehyde. This enabled us to study the binding structures of these products in a single-molecule limit. Methoxy was imaged as a pair of protrusion and depression along the [001] direction. This feature is fully consistent with the previous result that it adsorbs on the short-bridge site with the C-O axis tilted along the [001] direction. The axis was induced to flip back and forth by vibrational excitations with the STM. Two configurations were observed for formaldehyde, whose structures were proposed based on their characteristic images and motions.
Synthesis of few-layered graphene by H2O2 plasma etching of graphite
Authors: Zhao Guixia , Shao Dadong , Chen Changlun , Wang Xiangke
Published: May 2011

Herein, we reported an approach to synthesize few-layered graphene by etching of the graphite using H2O2 plasma technique. The synthesized few-layered graphene was characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). The analysis showed that few-layered graphene was formed in high quality level. The XPS analysis suggested that H2O2 plasma etching of graphite could oxidize graphene and generated COH and >CO groups on the graphene surfaces. The H2O2 plasma technique is an easy and environmental friendly method to synthesize few-layered graphene from the graphite.
CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging
Authors: Kang Zhitao , Zhang Yuelan , Menkara Hisham , Wagner Brent K. , Summers Christopher J. , Lawrence William , Nagarkar Vivek
Published: May 2011

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines/mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.
Communication: New insight into the barrier governing CO2 formation from OH + CO
Authors: Johnson Christopher J. , Poad Berwyck L. J. , Shen Ben B. , Continetti Robert E.
Published: May 2011

Despite its relative simplicity, the role of tunneling in the reaction OH + CO → H + CO2 has eluded the quantitative predictive powers of theoretical reaction dynamics. In this study a one-dimensional effective barrier to the formation of H + CO2 from the HOCO intermediate is directly extracted from dissociative photodetachment experiments on HOCO and DOCO. Comparison of this barrier to a computed minimum-energy barrier shows that tunneling deviates significantly from the calculated minimum-energy pathway, predicting product internal energy distributions that match those found in the experiment and tunneling lifetimes short enough to contribute significantly to the overall reaction. This barrier can be of direct use in kinetic and statistical models and aid in the further refinement of the potential energy surface and reaction dynamics calculations for this system.
Genome-wide remodeling of the epigenetic landscape during myogenic differentiation
Authors: Asp Patrik , Blum Roy , Vethantham Vasupradha , Parisi Fabio , Micsinai Mariann , Cheng Jemmie , Bowman Christopher , Kluger Yuval , Dynlacht Brian David
Published: May 2011

We have examined changes in the chromatin landscape during muscle differentiation by mapping the genome-wide location of ten key histone marks and transcription factors in mouse myoblasts and terminally differentiated myotubes, providing an exceptionally rich dataset that has enabled discovery of key epigenetic changes underlying myogenesis. Using this compendium, we focused on a well-known repressive mark, histone H3 lysine 27 trimethylation, and identified novel regulatory elements flanking the myogenin gene that function as a key differentiation-dependent switch during myogenesis. Next, we examined the role of Polycomb-mediated H3K27 methylation in gene repression by systematically ablating components of both PRC1 and PRC2 complexes. Surprisingly, we found mechanistic differences between transient and permanent repression of muscle differentiation and lineage commitment genes and observed that the loss of PRC1 and PRC2 components produced opposing differentiation defects. These phenotypes illustrate striking differences as compared to embryonic stem cell differentiation and suggest that PRC1 and PRC2 do not operate sequentially in muscle cells. Our studies of PRC1 occupancy also suggested a “fail-safe” mechanism, whereby PRC1/Bmi1 concentrates at genes specifying nonmuscle lineages, helping to retain H3K27me3 in the face of declining Ezh2-mediated methyltransferase activity in differentiated cells.
NifEN-B complex of
Azotobacter vinelandii
is fully functional in nitrogenase FeMo cofactor assembly
Authors: Wiig Jared A. , Hu Yilin , Ribbe Markus W.
Published: May 2011

Assembly of nitrogenase FeMoco is one of the key processes in bioinorganic chemistry. NifB and NifEN are two essential elements immediately adjacent to each other along the biosynthetic pathway of FeMoco. Previously, an 8Fe-precursor of FeMoco was identified on NifEN; however, the identity of the biosynthetic intermediate on NifB has remained elusive to date. Here, we present a combined biochemical and spectroscopic investigation of a His-tagged NifEN-B fusion protein of
Azotobacter vinelandii
. Our data from the EPR and activity analyses confirm the presence of the 8Fe-precursor in the NifEN entity of NifEN-B; whereas those from the metal, EPR, and UV/Vis experiments reveal the presence of additional [Fe
4
S
4
]-type cluster species in the NifB entity of NifEN-B. EPR-, UV/Vis- and metal-based quantitative analyses suggest that the newly identified cluster species in NifEN-B consist of both SAM-motif (CXXXCXXC)- and non-SAM-motif-bound [Fe
4
S
4
]-type clusters. Moreover, EPR and activity experiments indicate that the non-SAM-motif [Fe
4
S
4
] cluster is a NifB-bound intermediate of FeMoco assembly, which could be converted to the 8Fe-precursor in a SAM-dependent mechanism. Combined outcome of this work provides the initial insights into the biosynthetic events of FeMoco on NifB. More importantly, the full capacity of NifEN-B in FeMoco biosynthesis demonstrates the potential of this fusion protein as an excellent platform for further investigations of the role of NifB and its interaction with NifEN during the process of FeMoco assembly.
Concerted heavy-atom bond cleavage and proton and electron transfers illustrated by proton-assisted reductive cleavage of an O–O bond
Authors: Costentin Cyrille , Hajj Viviane , Robert Marc , Savéant Jean-Michel , Tard Cédric
Published: May 2011

Electron transfer may be concerted with proton transfer. It may also be concerted with the cleavage of a bond between heavy atoms. All three events may also be concerted. A model is presented to analyze the kinetics of these all-concerted reactions for homogeneous or electrochemical reduction or oxidation processes. It allows the estimation of the kinetic advantage that derives from the increase of the bond-breaking driving force resulting from the concerted proton transfer. Application of the model to the electrochemical reductive cleavage of the O–O bond of an organic peroxide in the presence of a proximal acid group illustrates the applicability of the model and provides an example demonstrating that electron transfer, heavy-atom bond breaking, and proton transfer may be all concerted. Such analyses are expected to be useful for the invention, analysis, and optimization of reactions involved in contemporary energy challenges as well as for the comprehension of major biochemical processes, a number of which involve electron and proton transfer together with cleavage of bonds between heavy atoms.
Single nucleotide mutation in the barley
acetohydroxy acid synthase
(
AHAS
) gene confers resistance to imidazolinone herbicides
Authors: Lee Hyejin , Rustgi Sachin , Kumar Neeraj , Burke Ian , Yenish Joseph P. , Gill Kulvinder S. , von Wettstein Diter , Ullrich Steven E.
Published: May 2011

Induced mutagenesis can be an effective way to increase variability in self-pollinated crops for a wide variety of agronomically important traits. Crop resistance to a given herbicide can be of practical value to control weeds with efficient chemical use. In some crops (for example, wheat, maize, and canola), resistance to imidazolinone herbicides (IMIs) has been introduced through mutation breeding and is extensively used commercially. However, this production system imposes plant-back restrictions on rotational crops because of herbicide residuals in the soil. In the case of barley, a preferred rotational crop after wheat, a period of 9–18 mo is required. Thus, introduction of barley varieties showing resistance to IMIs will provide greater flexibility as a rotational crop. The objective of the research reported was to identify resistance in barley for IMIs through induced mutagenesis. To achieve this objective, a sodium azide-treated M
2
/M
3
population of barley cultivar Bob was screened for resistance against acetohydroxy acid synthase (AHAS)-inhibiting herbicides. The phenotypic screening allowed identification of a mutant line showing resistance against IMIs. Molecular analysis identified a single-point mutation leading to a serine 653 to asparagine amino acid substitution in the herbicide-binding site of the barley
AHAS
gene. The transcription pattern of the
AHAS
gene in the mutant (Ser653Asn) and WT has been analyzed, and greater than fourfold difference in transcript abundance was observed. Phenotypic characteristics of the mutant line are promising and provide the base for the release of IMI-resistant barley cultivar(s).
Products of squares in finite simple groups
Authors: Liebeck Martin , O’Brien E. , Shalev Aner , Tiep Pham
Published: May 2011

The Ore conjecture, proved by the authors, states that every element of every finite non-abelian simple group is a commutator. In this paper we use similar methods to prove that every element of every finite simple group is a product of two squares. This can be viewed as a non-commutative analogue of Lagrange’s four squares theorem. Results for higher powers are also obtained.
Second eigenvalue of a Jacobi operator of hypersurfaces with constant scalar curvature
Authors: Li Haizhong , Wang Xianfeng
Published: May 2011

Let

x
:
M
→

S

n
+
1

(
1
)

x:M\to \mathbb {S}^{n+1}(1)

be an

n
n

-dimensional compact hypersurface with constant scalar curvature

n
(
n
−
1
)
r
,
 
r
≥
1

n(n-1)r,~r\geq 1

, in a unit sphere

S

n
+
1

(
1
)
,

\mathbb {S}^{n+1}(1),

n
≥
5

n\geq 5

, and let

J
s

J_s

be the Jacobi operator of

M
M

. In 2004, L. J. Alías, A. Brasil and L. A. M. Sousa studied the first eigenvalue of

J
s

J_s

of the hypersurface with constant scalar curvature

n
(
n
−
1
)

n(n-1)

in

S

n
+
1

(
1
)
,
 
n
≥
3

\mathbb {S}^{n+1}(1),~n\geq 3

. In 2008, Q.-M. Cheng studied the first eigenvalue of the Jacobi operator

J
s

J_s

of the hypersurface with constant scalar curvature

n
(
n
−
1
)
r
,
r
>
1

n(n-1)r, r>1

, in

S

n
+
1

(
1
)

\mathbb {S}^{n+1}(1)

. In this paper, we study the second eigenvalue of the Jacobi operator

J
s

J_s

of

M
M

and give an optimal upper bound for the second eigenvalue of

J
s

J_s

.
Beta ensembles, stochastic Airy spectrum, and a diffusion
Authors: Ramírez José , Rider Brian , Virág Bálint
Published: May 2011

We prove that the largest eigenvalues of the beta ensembles of random matrix theory converge in distribution to the low-lying eigenvalues of the random Schrödinger operator

−

d
2

d

x
2

+
x
+

2

β

b
x

′

-\frac {d^2}{dx^2} + x + \frac {2}{\sqrt {\beta }} b_x^{\prime }

restricted to the positive half-line, where

b
x

′

b_x^{\prime }

is white noise. In doing so we extend the definition of the Tracy-Widom(

β
\beta

) distributions to all

β
>
0

\beta >0

and also analyze their tails. Last, in a parallel development, we provide a second characterization of these laws in terms of a one-dimensional diffusion. The proofs rely on the associated tridiagonal matrix models and a universality result showing that the spectrum of such models converges to that of their continuum operator limit. In particular, we show how Tracy-Widom laws arise from a functional central limit theorem.
Glioblastoma motility occurs in the absence of actin polymer
Authors: Panopoulos Andreas , Howell Michael , Fotedar Rati , Margolis Robert L.
Published: May 2011

In fibroblasts and keratocytes, motility is actin dependent, while microtubules play a secondary role, providing directional guidance. We demonstrate here that the motility of glioblastoma cells is exceptional, in that it occurs in cells depleted of assembled actin. Cells display persistent motility in the presence of actin inhibitors at concentrations sufficient to fully disassemble actin. Such actin independent motility is characterized by the extension of cell protrusions containing abundant microtubule polymers. Strikingly, glioblastoma cells exhibit no motility in the presence of microtubule inhibitors, at concentrations that disassemble labile microtubule polymers. In accord with an unconventional mode of motility, glioblastoma cells have some unusual requirements for the Rho GTPases. While Rac1 is required for lamellipodial protrusions in fibroblasts, expression of dominant negative Rac1 does not suppress glioblastoma migration. Other GTPase mutants are largely without unique effect, except dominant positive Rac1-Q61L, and rapidly cycling Rac1-F28L, which substantially suppress glioblastoma motility. We conclude that glioblastoma cells display an unprecedented mode of intrinsic motility that can occur in the absence of actin polymer, and that appears to require polymerized microtubules.
The two actin-interacting protein 1 genes have overlapping and essential function for embryonic development inCaenorhabditis elegans
Authors: Ono Shoichiro , Nomura Kazumi , Hitosugi Sadae , Tu Domena K. , Lee Jocelyn A. , Baillie David L. , Ono Kanako
Published: May 2011

Disassembly of actin filaments by actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1) is a conserved mechanism to promote reorganization of the actin cytoskeleton. We previously reported that unc-78, an AIP1 gene in the nematode Caenorhabditis elegans, is required for organized assembly of sarcomeric actin filaments in the body wall muscle. unc-78 functions in larval and adult muscle, and an unc-78–null mutant is homozygous viable and shows only weak phenotypes in embryos. Here we report that a second AIP1 gene, aipl-1 (AIP1-like gene-1), has overlapping function with unc-78, and that depletion of the two AIP1 isoforms causes embryonic lethality. A single aipl-1–null mutation did not cause a detectable phenotype. However, depletion of both unc-78 and aipl-1 arrested development at late embryonic stages due to severe disorganization of sarcomeric actin filaments in body wall muscle. In vitro, both AIPL-1 and UNC-78 preferentially cooperated with UNC-60B, a muscle-specific ADF/cofilin isoform, in actin filament disassembly but not with UNC-60A, a nonmuscle ADF/cofilin. AIPL-1 is expressed in embryonic muscle, and forced expression of AIPL-1 in adult muscle compensated for the function of UNC-78. Thus our results suggest that enhancement of actin filament disassembly by ADF/cofilin and AIP1 proteins is critical for embryogenesis.
Regulation of dendritic spine growth through activity-dependent recruitment of the brain-enriched Na+/H+ exchanger NHE5
Authors: Diering Graham H. , Mills Fergil , Bamji Shernaz X. , Numata Masayuki
Published: May 2011

Subtle changes in cellular and extracellular pH within the physiological range have profound impacts on synaptic activities. However, the molecular mechanisms underlying local pH regulation at synapses and their influence on synaptic structures have not been elucidated. Dendritic spines undergo dynamic structural changes in response to neuronal activation, which contributes to induction and long-term maintenance of synaptic plasticity. Although previous studies have indicated the importance of cytoskeletal rearrangement, vesicular trafficking, cell signaling, and adhesion in this process, much less is known about the involvement of ion transporters. In this study we demonstrate that N-methyl-d-aspartate (NMDA) receptor activation causes recruitment of the brain-enriched Na+/H+ exchanger NHE5 from endosomes to the plasma membrane. Concomitantly, real-time imaging of green fluorescent protein–tagged NHE5 revealed that NMDA receptor activation triggers redistribution of NHE5 to the spine head. We further show that neuronal activation causes alkalinization of dendritic spines following the initial acidification, and suppression of NHE5 significantly retards the activity-induced alkalinization. Perturbation of NHE5 function induces spontaneous spine growth, which is reversed by inhibition of NMDA receptors. In contrast, overexpression of NHE5 inhibits spine growth in response to neuronal activity. We propose that NHE5 constrains activity-dependent dendritic spine growth via a novel, pH-based negative-feedback mechanism.