bluuebunny/crossref_metadata_2025 huggingface dataset:
A permeation theory for single-file ion channels: One- and two-step models
Authors: Nelson Peter Hugo
Published: April 2011

How many steps are required to model permeation through ion channels? This question is investigated by comparing one- and two-step models of permeation with experiment and MD simulation for the first time. In recent MD simulations, the observed permeation mechanism was identified as resembling a Hodgkin and Keynes knock-on mechanism with one voltage-dependent rate-determining step [Jensen et al., PNAS 107, 5833 (2010)]. These previously published simulation data are fitted to a one-step knock-on model that successfully explains the highly non-Ohmic current–voltage curve observed in the simulation. However, these predictions (and the simulations upon which they are based) are not representative of real channel behavior, which is typically Ohmic at low voltages. A two-step association/dissociation (A/D) model is then compared with experiment for the first time. This two-parameter model is shown to be remarkably consistent with previously published permeation experiments through the MaxiK potassium channel over a wide range of concentrations and positive voltages. The A/D model also provides a first-order explanation of permeation through the Shaker potassium channel, but it does not explain the asymmetry observed experimentally. To address this, a new asymmetric variant of the A/D model is developed using the present theoretical framework. It includes a third parameter that represents the value of the “permeation coordinate” (fractional electric potential energy) corresponding to the triply occupied state n of the channel. This asymmetric A/D model is fitted to published permeation data through the Shaker potassium channel at physiological concentrations, and it successfully predicts qualitative changes in the negative current–voltage data (including a transition to super-Ohmic behavior) based solely on a fit to positive-voltage data (that appear linear). The A/D model appears to be qualitatively consistent with a large group of published MD simulations, but no quantitative comparison has yet been made. The A/D model makes a network of predictions for how the elementary steps and the channel occupancy vary with both concentration and voltage. In addition, the proposed theoretical framework suggests a new way of plotting the energetics of the simulated system using a one-dimensional permeation coordinate that uses electric potential energy as a metric for the net fractional progress through the permeation mechanism. This approach has the potential to provide a quantitative connection between atomistic simulations and permeation experiments for the first time.
Two-dimensional resonance enhanced multiphoton ionization of HiCl; i = 35, 37: State interactions, photofragmentations and energetics of high energy Rydberg states
Authors: Matthíasson Kristján , Long Jingming , Wang Huasheng , Kvaran Ágúst
Published: April 2011

Mass spectra were recorded for (2 + n) resonance enhanced multiphoton ionization (REMPI) of HCl as a function of resonance excitation energy in the 88865-89285 cm−1 region to obtain two-dimensional REMPI data. Band spectra due to two-photon resonance transitions to number of Rydberg states (Ω′ = 0, 1, and 2) and the ion-pair state V(1Σ+(Ω′ = 0)) for H35Cl and H37Cl were identified, assigned, and analyzed with respect to Rydberg to ion-pair interactions. Perturbations show as line-, hence energy level-, shifts, as well as ion signal intensity variations with rotational quantum numbers, J′, which, together, allowed determination of parameters relevant to the nature and strength of the state interactions as well as dissociation and ionization processes. Whereas near-resonance, level-to-level, interactions are found to be dominant in heterogeneous state interactions (ΔΩ ≠ 0) significant off-resonance interactions are observed in homogeneous interactions (ΔΩ = 0). The alterations in Cl+ and HCl+ signal intensities prove to be very useful for spectra assignments. Data relevant to excitations to the j3Σ(0+) Rydberg states and comparison with (3 + n) REMPI spectra allowed reassignment of corresponding spectra peaks. A band previously assigned to an Ω = 0 Rydberg state was reassigned to an Ω = 2 state (ν0 = 88957.6 cm−1).
Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: Bis-(4-hydroxyphenyl)methane
Authors: Rodrigo Chirantha P. , Müller Christian W. , Pillsbury Nathan R. , James William H. , Plusquellic David F. , Zwier Timothy S.
Published: April 2011

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV–UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S0–S1 origins at 35 184 and 35 209 cm−1, respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and $\bar T$T¯), ring-flapping (R and $\bar R$R¯), and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C2 symmetric uu and dd conformers. The S0–S2 origins are assigned to bands located 132 cm−1 above the S0–S1 origins of both conformers. DFL spectra from the S2 origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S2 origin occurs dominantly to S1 $\bar R^1$R¯1 and S1 $\bar R^1 \beta ^1$R¯1β1 levels, which are located –15 and +31 cm−1 from it. Unusual vibronic activity in the ring-breathing (ν1) and ring-deformation (ν6a) modes is also attributed to vibronic coupling involving these Franck–Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S2 origin is strongly mixed (60%/40%) with S1 $\overline {6a} ^1$6a¯1 and $\bar 1^1$1¯1 levels. The direct extension of this model to the T /$\bar T$T¯ and R /$\bar R$R¯ inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S1/S2 mixing compared to the ring modes.
Isotope effects in liquid water by infrared spectroscopy. V. A sea of OH4 of C2v symmetry
Authors: Max Jean-Joseph , Chapados Camille
Published: April 2011

The two water gas OH stretch vibrations that absorb in the infrared (IR) near 3700 cm−1 are redshifted to near 3300 cm−1 upon liquefaction. The bathochromic shift is due to the formation of four H-bonds: two are from the labile hydrogen atoms to neighbors and two are received from neighbors by the oxygen free electron pairs. Therefore, the water oxygen atom is surrounded by four hydrogen atoms, two of these make covalent bonds that make H-bonds and two are oxygen H-bonded. However, these permute at rate in the ps range. When the water molecules are isolated in acetonitrile (MeCN) or acetone (Me2CO), only the labile hydrogen atoms make H-bonds with the solvent. The bathochromic shift of the OH stretch bands is then almost 130 cm−1 with, however, the asymmetric (ν3) and symmetric (ν1) stretch bands maintained. When more water is added to the solutions, the oxygen lone doublets make H-bonds with the available labile hydrogen atoms from neighboring water molecules. With one bond accepted, the bathochromic shift is further displaced by almost 170 cm−1. When the second oxygen doublet is filled, another bathochromic shift by almost 100 cm−1 is observed. The total bathochromic shift is near 400 cm−1 with a full width at half height of near 400 cm1. This is the case of pure liquid water. Notwithstanding the shift and the band broadness, the ν3 and ν1 band individualities are maintained with, however, added satellite companions that come from the far IR (FIR) absorption. These added to the fundamental bands are responsible for the band broadness and almost featureless shape of the massive OH stretch absorption of liquid water. Comparison of light and heavy water mixture spectra indicates that the OH and OD stretch regions show five different configurations: OH4; OH3D; OH2D2; OHD3; and OD4 [J. Chem. Phys. 116, 4626 (2002)]. The comparison of the OH bands of OH4 with that of OHD3 indicates that the main component in OHD3 is νOH, whereas in OH4 two main components are present: ν3 and ν1. Similar results are obtained for the OD bands of OD4 and ODH3. These results indicate that the C2v symmetry of H2O and D2O is preserved in the liquid and aqueous solutions whereas Cs is that of HDO.
Frequency shift keying in vortex-based spin torque oscillators
Authors: Manfrini M. , Devolder T. , Kim Joo-Von , Crozat P. , Chappert C. , Van Roy W. , Lagae L.
Published: April 2011

Vortex-based spin-torque oscillators can be made from extended spin valves connected to an electrical nanocontact. We study the implementation of frequency shift keying modulation in these oscillators. Upon a square modulation of the current in the 10 MHz range, the vortex frequency follows the current command, with easy identification of the two swapping frequencies in the spectral measurements. The frequency distribution of the output power can be accounted for by convolution transformations of the dc current vortex waveform, and the current modulation. Modeling indicates that the frequency transitions are phase coherent and last less than 25 ns. Complementing the multi-octave tunability and first-class agility, the capability of frequency shift keying modulation is an additional milestone for the implementation of vortex-based oscillators in radio frequency circuits.
Laser controlled magnetism in hydrogenated fullerene films
Authors: Makarova Tatiana L. , Kvyatkovskii Oleg E. , Zakharova Irina B. , Buga Sergei G. , Volkov Aleksandr P. , Shelankov Andrei L.
Published: April 2011

Room temperature ferromagnetic-like behavior in fullerene photopolymerized films treated with monatomic hydrogen is reported. The hydrogen treatment controllably varies the paramagnetic spin concentration and laser induced polymerization transforms the paramagnetic phase to a ferromagnetic-like one. Excess laser irradiation destroys magnetic ordering, presumably due to structural changes, which was continuously monitored by Raman spectroscopy. We suggest an interpretation of the data based on first-principles density-functional spin-unrestricted calculations which show that the excess spin from mono-atomic hydrogen is delocalized within the host fullerene and the laser-induced polymerization promotes spin exchange interaction and spin alignment in the polymerized phase.
Voids and Mn-rich inclusions in a (Ga,Mn)As ferromagnetic semiconductor investigated by transmission electron microscopy
Authors: Kovács A. , Sadowski J. , Kasama T. , Domagała J. , Mathieu R. , Dietl T. , Dunin-Borkowski R. E.
Published: April 2011

Voids adjacent to cubic (ZnS-type) and hexagonal (NiAs-type) Mn-rich nanocrystals are characterized using aberration-corrected transmission electron microscopy in an annealed Ga0.995Mn0.005As magnetic semiconductor specimen grown by molecular beam epitaxy. Nanobeam electron diffraction measurements suggest that the nanocrystals exhibit deviations in lattice parameter as compared to bulk MnAs. After annealing at 903 K, the magnetic transition temperature of the specimen is likely to be dominated by the presence of cubic ferromagnetic nanocrystals. In situ annealing inside the electron microscope is used to study the nucleation, coalescence, and grain growth of individual nanocrystals.
Lattice dynamical origin of peak thermoelectric performance in AgPbmSbTe2+m observed by inelastic neutron scattering
Authors: Manley M. E. , Shapiro S. , Li Q. , Llobet A. , Hagen M. E.
Published: April 2011

Phonon densities of states (DOS) for the high performing thermoelectric material, AgPbmSbTe2+m (LAST-m, m = 16, 18, and 20), were extracted from time-of-flight inelastic neutron scattering measurements. The phonon DOS of LAST-18 differs remarkably from LAST-16 and LAST-20 by exhibiting a dramatic broadening of its acoustic modes that increases on heating. This broadening coincides with a minimum in the thermal conductivity, a maximum in the electrical conductivity and Seebeck coefficient, and a related peak in thermoelectric performance. We argue that the anomalous broadening originates with scattering enhanced by modifications to Te-Ag(Sb) bonds caused by their resonant electronic states falling near the Fermi energy for m = 18.
Fiber-coupled laser-driven flyer plates system
Authors: Zhao Xing-hai , Zhao Xiang , Shan Guang-cun , Gao Yang
Published: April 2011

A system for the launch of hypervelocity flyer plates has been developed and characterized. Laser-driven flyers were launched from the substrate backed aluminum–alumina–aluminum sandwiched films. A laser-induced plasma is used to drive flyers with typical thickness of 5.5 μm and diameters of less than 1 mm, to achieve velocities of a few km/s. These flyer plates have many applications, from micrometeorite simulation to laser ignition. The flyer plates considered here have up to three layers: an ablation layer, to form plasma; an insulating layer; and a final, thicker layer that forms the final flyer plates. This technique was developed aiming at improving the energy efficiency of the system. The kinetic energy of flyers launched with the additional layer was found to be enhanced by a factor of near 2 (up to 30%). The optical fiber delivery system governs the output spatial profile of the laser spot and power capacity. Moreover, a technique for coupling high-power laser pulses into an optical fiber has been developed. This fiber optic system has been successfully used to launch flyer plates, and the surface finishing quality of the fiber was found to be an important factor. Importantly, measurements of the flyer performance including the mean velocities and planarity were made by an optical time-of-arrival technique using an optical fiber array probe, demonstrating the good planarity of the flyer and the achievable average velocity of 1.7 km/s with approaching 1 mm diameter. Finally, the relationship between flyer velocities and incident laser pulses energy was also investigated.
Ab initio lattice dynamics of nonconducting crystals by systematic fragmentation
Authors: Collins Michael A.
Published: April 2011

A systematic method for approximating the ab initio electronic energy of crystal lattices has been improved by the incorporation of long range electrostatic and dispersion interactions. The effect of these long range interactions on the optimization of the crystal structure is reported. The harmonic lattice dynamics have been evaluated to give phonon frequencies and neutron scattering intensities. Exemplary results are reported for diamond, silicon, and α-quartz using Hartree–Fock, Möller–Plesset perturbation, and coupled-cluster levels of ab initio theory.
Communication: Comment on the effective temporal and spectral resolution of impulsive stimulated Raman signals
Authors: Mukamel Shaul , Biggs Jason D.
Published: April 2011

A compact correlation-function expression for time-resolved stimulated Raman signals, generated by combining a spectrally narrow (picosecond) with a broad (femtosecond) pulse, is derived using a closed time path loop diagrammatic technique that represents forward and backward time evolution of the vibrational wave function. We show that even though the external spectral and temporal parameters of the pulses may be independently controlled, the effective temporal and spectral resolution of the experiment may not exceed the fundamental bandwidth limitation.
Dielectric relaxation behavior of acceptor (Mg)-doped BaTiO3
Authors: Yoon Seok-Hyun , Kwon Sang-Hoon , Hur Kang-Heon
Published: April 2011

Dielectric relaxation behavior of acceptor (Mg)-doped BaTiO3 ceramics was investigated with the increase of Mg concentration up to 0.6 mol. % in the temperature rang of 120 ∼ 540 °C. In the high temperature range above 320 °C, the activation energies of dielectric relaxation (Eτ) showed nearly similar values of ∼ 1.2 eV irrespective of Mg concentration. However, in the low temperature range below 320 °C, they continuously decreased from ∼ 1.2 eV and then saturated to ∼ 0.4 eV with the increase of Mg concentration. The activation energies of electrical conduction (Eσ) of the bulk grain evaluated by impedance analysis also showed almost the same behavior. Such coincidence demonstrates that the observed dielectric behaviors are caused by the space charge polarization at grain boundaries by conducting charge carriers. The disappearance of the dielectric relaxation in submicrometer fine-grain specimen also supports this mechanism. The variation of Eτ and Eσ with the increase of Mg concentration in the low temperature range was supposed to be caused by the dominant hopping conduction between ionized acceptor (MgTi″) and neutral or hole-trapped acceptor (MgTi×).
ZnO thin film transistor immunosensor with high sensitivity and selectivity
Authors: Reyes Pavel Ivanoff , Ku Chieh-Jen , Duan Ziqing , Lu Yicheng , Solanki Aniruddh , Lee Ki-Bum
Published: April 2011

A zinc oxide thin film transistor-based immunosensor (ZnO-bioTFT) is presented. The back-gate TFT has an on-off ratio of 108 and a threshold voltage of 4.25 V. The ZnO channel surface is biofunctionalized with primary monoclonal antibodies that selectively bind with epidermal growth factor receptor (EGFR). Detection of the antibody-antigen reaction is achieved through channel carrier modulation via pseudo double-gating field effect caused by the biochemical reaction. The sensitivity of 10 fM detection of pure EGFR proteins is achieved. The ZnO-bioTFT immunosensor also enables selectively detecting 10 fM of EGFR in a 5 mg/ml goat serum solution containing various other proteins.
Molecular dynamics scheme for precise estimation of electrostatic interaction via zero-dipole summation principle
Authors: Fukuda Ikuo , Yonezawa Yasushige , Nakamura Haruki
Published: April 2011

We propose a novel idea, zero-dipole summation, for evaluating the electrostatic energy of a classical particle system, and have composed an algorithm for effectively utilizing the idea for molecular dynamics. It conceptually prevents the nonzero-charge and nonzero-dipole states artificially generated by a simple cutoff truncation. The resulting energy formula is nevertheless represented by a simple pairwise function sum, which enables facile application to high-performance computation. By following a heuristic approach to derive the current electrostatic energy formula, we developed an axiomatic approach to construct the method consistently. Explorations of the theoretical details of our method revealed the structure of the generated error, and we analyzed it by comparisons with other methods. A numerical simulation using liquid sodium chloride confirmed that the current method with a small damping factor yielded sufficient accuracy with a practical cutoff distance region. The current energy function also conducts stable numerical integration in a liquid MD simulation. Our method is an extension of the charge neutralized summation developed by Wolf et al. [J. Chem. Phys. 110, 8254 (1999)]. Furthermore, we found that the current method becomes a generalization of the preaveraged potential method proposed by Yakub and Ronchi [J. Chem. Phys. 119, 11556 (2003)], which is based on a viewpoint different from the neutrality. The current study presents these relationships and suggests possibilities for their further applications.
Molecular dynamics simulations of D2O ice photodesorption
Authors: Arasa C. , Andersson S. , Cuppen H. M. , van Dishoeck E. F. , Kroes G. J.
Published: April 2011

Molecular dynamics (MD) calculations have been performed to study the ultraviolet (UV) photodissociation of D2O in an amorphous D2O ice surface at 10, 20, 60, and 90 K, in order to investigate the influence of isotope effects on the photodesorption processes. As for H2O, the main processes after UV photodissociation are trapping and desorption of either fragments or D2O molecules. Trapping mainly takes place in the deeper monolayers of the ice, whereas desorption occurs in the uppermost layers. There are three desorption processes: D atom, OD radical, and D2O molecule photodesorption. D2O desorption takes places either by direct desorption of a recombined D2O molecule, or when an energetic D atom produced by photodissociation kicks a surrounding D2O molecule out of the surface by transferring part of its momentum. Desorption probabilities are calculated for photoexcitation of D2O in the top four monolayers and are compared quantitatively with those for H2O obtained from previous MD simulations of UV photodissociation of amorphous water ice at different ice temperatures [Arasa et al., J. Chem. Phys. 132, 184510 (2010)10.1063/1.3422213]. The main conclusions are the same, but the average D atom photodesorption probability is smaller than that of the H atom (by about a factor of 0.9) because D has lower kinetic energy than H, whereas the average OD radical photodesorption probability is larger than that of OH (by about a factor of 2.5–2.9 depending on ice temperature) because OD has higher translational energy than OH for every ice temperature studied. The average D2O photodesorption probability is larger than that of H2O (by about a factor of 1.4–2.3 depending on ice temperature), and this is entirely due to a larger contribution of the D2O kick-out mechanism. This is an isotope effect: the kick-out mechanism is more efficient for D2O ice, because the D atom formed after D2O photodissociation has a larger momentum than photogenerated H atoms from H2O, and D transfers momentum more easily to D2O than H to H2O. The total (OD + D2O) yield has been compared with experiments and the total (OH + H2O) yield from previous simulations. We find better agreement when we compare experimental yields with calculated yields for D2O ice than when we compare with calculated yields for H2O ice.
First-principles determination of the structure of NaN and ${\rm Na}_N^-$NaN− clusters with up to 80 atoms
Authors: Aguado Andrés , Kostko Oleg
Published: April 2011

We have performed an extensive computational search for the global minimum (GM) structures of both neutral and anionic sodium clusters with up to 80 atoms. The theoretical framework combines basin hopping unbiased optimizations based on a Gupta empirical potential (EP) and subsequent reoptimization of many candidate structures at the density functional theory level. An important technical point is that the candidates are selected based on cluster shape descriptors rather than the relative stabilities of the EP model. An explicit comparison of the electronic density of states of cluster anions to experimental photoemission spectra suggests that the correct GM structures have been identified for all but two sizes (N = 47 and 70). This comparison validates the accuracy of the proposed methodology. Furthermore, our GM structures either match or improve over the results of previous works for all sizes. Sodium clusters are seen to accommodate strain very efficiently because: (a) many structures are based on polyicosahedral packing; (b) others are based on Kasper polyhedra and show polytetrahedral order; (c) finally, some (N + 1)-atom structures are obtained by incorporating one adatom into the outermost atomic shell of a compact N-atom cluster, at the cost of increasing the bond strain. GM structures of neutrals and anions differ for most sizes. Cluster stabilities are analyzed and shown to be dominated by electron shell closing effects for the smaller clusters and by geometrical packing effects for the larger clusters. The critical size separating both regimes is around 55 atoms. Some implications for the melting behavior of sodium clusters are discussed.
Cr(CO)6 photochemistry: Semi-classical study of UV absorption spectral intensities and dynamics of photodissociation
Authors: Crespo-Otero Rachel , Barbatti Mario
Published: April 2011

The UV absorption spectrum of Cr(CO)6 (chromium hexacarbonyl) in gas phase is investigated by theoretical methods with focus on the absorption intensities. It is shown that in spite of good predictions for the excitation energies, the most frequently employed methods for excited-state calculations produce poor predictions for oscillator strengths and absorption cross sections. In particular, time-dependent DFT predicts relative intensities for the two main spectral bands to be up to five times larger than the experimental results depending on the functional. The best results are obtained by a multireference configuration interaction method based on DFT (DFT/MRCI). Spectral shoulders caused by vibronic-coupling absorption are assigned based on symmetry-restricted spectrum simulations. The dynamics of Cr(CO)6 photodissociation was also considered at TDDFT/B3LYP level. The estimated time constants for the Cr(CO)6 relaxation and dissociation are in excellent agreement with experimental values. The time constant for internal conversion, however, is longer than the experimentally observed by factor 2, presumably due to an underestimation of the experimental analysis.
Application of a semiclassical model for the second-quantized many-electron Hamiltonian to nonequilibrium quantum transport: The resonant level model
Authors: Swenson David W. H. , Levy Tal , Cohen Guy , Rabani Eran , Miller William H.
Published: April 2011

A semiclassical approach is developed for nonequilibrium quantum transport in molecular junctions. Following the early work of Miller and White [J. Chem. Phys. 84, 5059 (1986)], the many-electron Hamiltonian in second quantization is mapped onto a classical model that preserves the fermionic character of electrons. The resulting classical electronic Hamiltonian allows for real-time molecular dynamics simulations of the many-body problem from an uncorrelated initial state to the steady state. Comparisons with exact results generated for the resonant level model reveal that a semiclassical treatment of transport provides a quantitative description of the dynamics at all relevant timescales for a wide range of bias and gate potentials, and for different temperatures. The approach opens a door to treating nontrivial quantum transport problems that remain far from the reach of fully quantum methodologies.
Electronic spectra of GdF reanalyzed by decomposing state functions according to f-shell angular momentum
Authors: Yamamoto Shigeyoshi , Tatewaki Hiroshi
Published: April 2011

The electronic structure of the GdF molecule was studied by means of four-component relativistic configuration interaction (CI) calculations [S. Yamamoto, H. Tatewaki, and T. Saue, J. Chem. Phys. 129, 244505 (2008)10.1063/1.3039794]. To analyze the electronic spectra more accurately, the CI wave function is decomposed according to the angular momentum (Ωf) generated from the (4f)7 electrons. The weight of a specified Ωf is referred to as the “f-shell Omega component weight.” This Ωf plays a crucial role in classifying the strong electronic transitions from the upper states (0.7 eV–3.0 eV) to the lower states (∼0.55 eV). For these transitions, the upper and lower states have almost identical Ωf weights. This appears to be a necessary condition for a transition to be strong. The same condition is expected to hold for other lanthanide linear molecules. A point charge model is also studied, acting as a simplified model of GdF; it successfully reproduces the spectra of GdF, justifying studies based on ligand field theory.
Discriminating the structure of exo-2-aminonorbornane using nuclear quadrupole coupling interactions
Authors: Écija Patricia , Cocinero Emilio J. , Lesarri Alberto , Millán Judith , Basterretxea Francisco , Fernández José A. , Castaño Fernando
Published: April 2011

The intrinsic conformational and structural properties of the bicycle exo-2-aminonorbornane have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy and quantum chemical calculations. The rotational spectrum revealed two different conformers arising from the internal rotation of the amino group, exhibiting small (MHz) hyperfine patterns originated by the 14N nuclear quadrupole coupling interaction. Complementary ab initio (MP2) and DFT (B3LYP and M05–2X) calculations provided comparative predictions for the structural properties, rotational and centrifugal distortion data, hyperfine parameters, and isomerization barriers. Due to the similarity of the rotational constants, the structural assignment of the observed rotamers and the calculation of the torsion angles of the amino group were based on the conformational dependence of the 14N nuclear quadrupole coupling hyperfine tensor. In the most stable conformation (ss), the two amino N-H bonds are staggered with respect to the adjacent C-H bond. In the second conformer (st), only one of the N-H bonds is staggered and the other is trans. A third predicted conformer (ts) was not detected, consistent with a predicted conformational relaxation to conformer ss through a low barrier of 5.2 kJ mol−1.
Intermolecular vibrational modes and orientational dynamics of cooperative hydrogen-bonding dimer of 7-azaindole in solution
Authors: Kato Tatsuya , Shirota Hideaki
Published: April 2011

We observed the low-frequency Raman-active intermolecular vibrational modes of 7-azaindole in CCl4 by femtosecond Raman-induced Kerr effect spectroscopy. To understand the dynamical aspects and vibrational modes of 7-azaindole in the solution, the ultrafast dynamics of 1-benzofuran in CCl4 was also examined as a reference and ab initio quantum chemistry calculations were performed for 7-azaindole and 1-benzofuran. The cooperative hydrogen-bonding vibrational bands of 7-azaindole dimer in CCl4 appeared at 89 cm−1 and 105 cm−1 represent the overlap of stagger and wheeling modes and the intermolecular stretching mode, respectively. They are almost independent of the concentration in the solution. We further found from the low-frequency differential Kerr spectra of the solutions with neat CCl4 that the intermolecular motion in the low frequency region below 20 cm−1 was less active in the case of 7-azaindole/CCl4 than in the case of 1-benzofuran/CCl4. The slow orientational relaxation time in 7-azaindole/CCl4 is ∼3.5 times that in 1-benzofuran/CCl4 because of the nature of the dimerization of 7-azaindole.
A fast algorithm for simulating flow-induced nucleation in polymers
Authors: Jolley Kenny , Graham Richard S.
Published: April 2011

We present a fast computer simulation algorithm for high dimensional barrier crossing simulations. The algorithm is described with reference to the Graham and Olmsted (GO) model of flow-induced nucleation in polymers [R. S. Graham and P. D. Olmsted, Phys. Rev Lett. 103, 115702 (2009)]. Inspired by Chandler's barrier crossing algorithm [D. Chandler, J. Chem. Phys 68, 2959 (1978)], our algorithm simulates only the region around the top of the nucleation barrier, where the system deviates most strongly from equilibrium. When applied to the kinetic Monte Carlo (kMC) routine used in the GO model, our algorithm has two advantages: it requires very little additional coding; and it is simple enough to be applied to any barrier crossing problem that can be written in terms of a kMC simulation. Our fast nucleation algorithm is shown to vastly decrease the computer time required to perform the kMC simulations of high barrier crossing.
Plasmonic interferences in two-dimensional stacked double-disk array
Authors: Zhang Zong-Suo , Yang Zhong-Jian , Li Jian-Bo , Hao Zhong-Hua , Wang Qu-Quan
Published: April 2011

We numerically investigate the resonant spectral features of the two-dimensional stacked double-disk array. Narrow plasmon line shapes are observed on resonance spectra which are similar to the case of single-disk array. The electric field shows that the narrow plasmon modes are produced by the dark plasmon mode coupling to the diffraction which may be due to the retardation effect. The narrow mode and the dark mode overlap on resonance spectra, resulting in a Fano-like spectrum. The intensities of the electric fields for these narrow modes arising from the dark mode coupling are much stronger than that of the bright modes.
High resolution study of the strong diamond/silicon nitride interface
Authors: Almeida F. A. , Oliveira F. J. , Silva R. F. , Baptista D. L. , Peripolli S. B. , Achete C. A.
Published: April 2011

Silicon nitride (Si3N4) is known to offer the required adhesion to chemically vapor deposited diamond coatings for demanding mechanical solicitations but the nature of their strong interface is not well know. Focused ion beam preparation preserved such thin layer for high resolution transmission electron microscopy. Contrarily to earlier suppositions, SiC interlinking particles were not found. Instead, the interface shows diamondlike carbon interlayers (approximately 3nm in thickness) intercalated with regions of directly-grown nanomicrometric and submicrometric-diamond crystals. A grain-to-grain epitaxial relationship of the type ⟨111⟩Dia∥⟨010⟩Si3N4 and {111}Dia∥{1¯20}Si3N4 is observed, concomitant with a 7:1 match arrangement, which assists on the interface strength.
Employing exciton transfer molecules to increase the lifetime of phosphorescent red organic light emitting diodes
Authors: Lindla Florian , Boesing Manuel , van Gemmern Philipp , Bertram Dietrich , Keiper Dietmar , Heuken Michael , Kalisch Holger , Jansen Rolf H.
Published: April 2011

The lifetime of phosphorescent red organic light emitting diodes (OLEDs) is investigated employing either N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (NPB), TMM117, or 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as hole-conducting host material (mixed with an electron conductor). All OLED (organic vapor phase deposition-processed) show similar efficiencies around 30 lm/W but strongly different lifetimes. Quickly degrading OLED based on TCTA can be stabilized by doping exciton transfer molecules [tris-(phenyl-pyridyl)-Ir (Ir(ppy)3)] to the emission layer. At a current density of 50 mA/cm2 (12 800 cd/m2), a lifetime of 387 h can be achieved. Employing exciton transfer molecules is suggested to prevent the degradation of the red emission layer in phosphorescent white OLED.
The conducting polymer/polymer interface
Authors: Gadiev R. M. , Lachinov A. N. , Salikhov R. B. , Rakhmeev R. G. , Kornilov V. M. , Yusupov A. R.
Published: April 2011

It is well-known that interfaces between various substances are widely used in modern electronics. Recently it has been found out that the interfaces of two dielectrics can possess unusual electronic properties. This work shows that high-conductivity interface can be obtained not only on the boundary of two crystals but also on the interface of amorphous insulator polymers with the carrier mobility extremely high for organic materials (0.4 cm2/Vs). Conductivity of the structure exceeds the bulk conductivity of the materials used by more than five orders of magnitude. Moreover, the polymer/polymer interface has a metallic conductivity.
Electrical switching behavior from all-polymer-based system of semiconductor/ferroelectrics/semiconductor
Authors: Li Yun , Liu Chuan , Pan Lijia , Pu Lin , Sirringhaus Henning , Shi Yi
Published: April 2011

Ferroelectricity is attractive in data storage application as the polarizations can be used as binary levels. However, conductivity and ferroelectricity cannot be tuned independently in inorganic materials, thus two-terminal resistive memories with ferroelectrics are yet to be achieved. Here, we present an all-polymer-based system of semiconductor/ferroelectrics/semiconductor. Electrical switching behavior, which is critical to resistive memories, is observed. The mechanism of the conduction transition is attributed to the transmission probability of charge carriers tuned by electrical polarization. The importance of polarization in controlling the charge transport deserves further investigation for the realization and optimization of the two-terminal resistive memories with ferroelectrics.
Polarization analysis of luminescence for the characterization of silicon wafer solar cells
Authors: Peloso Matthew P. , Hoex Bram , Aberle Armin G.
Published: April 2011

Luminescence imaging is routinely used to extract important information from photovoltaic materials and devices. We extend the existing luminescence imaging technique to yield the partial polarization of luminescence. It is observed that certain material structures of silicon wafer solar cells generate strongly polarized luminescence. The luminescence polarization effect is related to internal charge anisotropy of certain defects in the silicon wafer solar cells. These observations may be used, for example, to advance the characterization of solar cells, to understand the electrical properties of defects in silicon wafer solar cells, or to study the formation of defects during crystal growth.
Field-dependent carrier trapping induced kink effect in AlGaN/GaN high electron mobility transistors
Authors: Fu Lihua , Lu Hai , Chen Dunjun , Zhang Rong , Zheng Youdou , Chen Tangsheng , Wei Ke , Liu Xinyu
Published: April 2011

An anomalous kink effect featuring an abrupt recovery of drain current following current collapse is observed in the room-temperature output characteristics of AlGaN/GaN high electron mobility transistors. The kink is largely caused by trapping electrons from the gate leakage current by deep levels within the AlGaN barrier at high drain bias, resulting in a positive shift in threshold voltage and a reduction in reverse gate leakage current. The release of the trapped electrons is likely due to impact ionization of traps by hot electrons, which starts to play a role at relatively lower drain bias. Both sub-bandgap illumination and temperature rise could reduce the kink.
Schottky barrier height reduction for metal/n-GaSb contact by inserting TiO2 interfacial layer with low tunneling resistance
Authors: Yuan Ze , Nainani Aneesh , Sun Yun , Lin J.-Y. Jason , Pianetta Piero , Saraswat Krishna C.
Published: April 2011

Fermi level pinning near GaSb valence band edge leads to high Schottky barrier height for metal/n-type GaSb contacts. However, this effect can be alleviated by depinning of the Fermi level with the introduction of thin interfacial dielectric. In this paper, the use of TiO2 allows depinning of the Fermi level without introducing excessive tunneling resistance due to the low conduction band offset, estimated by synchrotron radiation photoemission spectroscopy. It is shown the insertion of TiO2 results in reduction in Schottky barrier height and greater than four orders of magnitude increase in current density for metal contacts on n-type GaSb.
Note: A simple technique for reduction of the fall time and enhancement of the peak power of diode side-pumped intracavity frequency doubled repetitively Q-switched green laser pulse
Authors: Mukhopadhyay Pranab K. , Sharma Sunil K. , Singh Amarjeet , Oak Shrikant M.
Published: April 2011

A simple method to reduce the fall time and enhancement of peak power of intracavity generated Q-switched green pulses is demonstrated. In this method, a mirror with partial transmission at the fundamental lasing wavelength is inserted in front of the nonlinear crystal inside a conventional linear intracavity frequency doubled configuration to form two coupled sub-cavities. The coupled cavity configuration exhibits considerable improvement in the pulse quality. The fall time and timing jitter of the green pulses are reduced by a factor of 2.5 and 4.5, respectively, in comparison to an intracavity frequency doubled system with nearly identical cavity parameters and the peak power of the green pulses is considerably increased. The pulse shape from the Q-switched coupled cavity green laser system is analyzed using the rate equation model. The simulation results are in good agreement with the experiment.
A precise method for visualizing dispersive features in image plots
Authors: Zhang P. , Richard P. , Qian T. , Xu Y.-M. , Dai X. , Ding H.
Published: April 2011

In order to improve the advantages and the reliability of the second derivative method in tracking the position of extrema from experimental curves, we develop a novel analysis method based on the mathematical concept of curvature. We derive the formulas for the curvature in one and two dimensions and demonstrate their applicability to simulated and experimental angle-resolved photoemission spectroscopy data. As compared to the second derivative, our new method improves the localization of the extrema and reduces the peak broadness for a better visualization on intensity image plots.
Measurement of the electric quadrupole moment of CO
Authors: Chetty Naven , Couling Vincent W.
Published: April 2011

Measurements of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous carbon monoxide are presented. The measurements span the temperature range 301.2–473.9 K, which allows for separation of the temperature-independent hyperpolarizability contribution from the temperature-dependent quadrupole contribution. It is demonstrated that in the case of carbon monoxide, quantization of the rotational motion of the molecules needs to be considered, the analysis yielding a quadrupole moment of Θ = (−8.77 ± 0.31) × 10−40 C m2 and a hyperpolarizability term of b′ = (−0.1243 ± 0.0078)  × 10−60 C3 m4 J−2. For dipolar molecules, the quadrupole moment is origin dependent, and the value reported here is referred to an origin called the effective quadrupole center. Comparison of this value with the center-of-mass quadrupole moment obtained from other experiments yields information about the dynamic dipole-quadrupole and dipole-magnetic dipole polarizabilities. The temperature-independent term, which contributes (7.0 ± 0.6)% to the EFGIB at room temperature, is by no means insignificant, and must necessarily be accounted for if the quadrupole moment is to be definitively established. The measured Θ and b′ are compared with the best available ab initio calculated values.
On the properties of X⋅⋅⋅N noncovalent interactions for first-, second-, and third-row X atoms
Authors: Scheiner Steve
Published: April 2011

In addition to a structure with a PH⋅⋅⋅N H-bond, a second complex of greater stability is formed when the PH3 is rotated such that its P-H bond is pointing away from the approaching N lone pair of NH3. Quantum calculations are applied to examine whether such a complex is characteristic only of P, or may occur as well for other atoms of the first, second, or third rows of the periodic table. The molecules PH3, H2S, HCl, AsH3, and NH3 are all paired with NH3 as electron donor. While NH3 will not engage in an N⋅⋅⋅N attraction, all the others do form a X⋅⋅⋅N complex. The energetics, geometries, and other properties of these complexes are relatively insensitive to the nature of the X atom. This uniformity contrasts sharply with the H-bonded XH⋅⋅⋅N complexes where a strong sensitivity to X is observed. The three-dimensional nature of the electrostatic potential, in conjunction with the striving for a linear H-X⋅⋅⋅N orientation that maximizes charge transfer, serves as an excellent tool in understanding both the shape of the potential energy surface and the proclivity to engage in a X⋅⋅⋅N interaction.
Rectifying behavior in La2/3Sr1/3MnO3/MgO/SrRuO3 magnetic tunnel junctions
Authors: Cheng Hongguang , Liu Zuli , Yao Kailun
Published: April 2011

We report first principles calculations of transport properties of the all-oxide La2/3Sr1/3MnO3/MgO/SrRuO3 magnetic tunnel junctions. A sizeable rectifying behavior which can be enhanced by increasing the MgO barrier thickness is predicted theoretically. For the device with 13 layers of MgO barrier, the positive current is about two orders of magnitude larger than the reverse leakage current. The rectifying behavior arises from the symmetry-filtering properties of the MgO barrier. This rectifying effect which is totally dominated by quantum tunneling could be used to design faster quantum devices such as tunnel diode and tunnel transistor.
Plastic and unpredictable responses of stream invertebrates to leaf pack patches across sandy-bottomed streams
Authors: Downes Barbara J. , Lancaster Jill , Hale Robin , Glaister Alena , Bovill William D.
Published: April 2011

Detrital inputs to ecosystems provide potential food sources and can produce trophic cascades, but this effect is influenced by whether species specialise in consuming or inhabiting accumulations of detritus. To test whether species are differentially associated with leaves or sand, we compared densities of stream invertebrate species in patches of leaves and bare sand in two sandy-bed creeks in south-eastern Australia, in summer and spring. We also assessed the quality of information on diet and substrate association in the literature. Most species showed no density differences between leaf and sand patches (‘microhabitat generalists’), but categorisation as generalists, leaf or sand species differed between datasets. We developed a method for identifying important effect sizes; power analyses showed that many species were true generalists, but many non-significant results were potentially Type II errors. The literature provided information that was broadly consistent with our data, but few studies publish reliable information about either diet or patch use. Our results support a contention that few Australian stream invertebrates are obligate shredders, and this may also be true for streams elsewhere. Predicting and detecting the responses of such generalist taxa to detrital inputs will be very challenging.
Experimental evolution of selfish policing in social bacteria
Authors: Manhes Pauline , Velicer Gregory J.
Published: April 2011

Cooperative organisms evolve within socially diverse populations. In populations harboring both cooperators and cheaters, cooperators might adapt by evolving novel interactions with either social type or both. Diverse animal traits suppress selfish behaviors when cooperation is important for fitness, but the potential for prokaryotes to evolve such traits is unclear. We allowed a strain of the bacterium
Myxococcus xanthus
that is proficient at cooperative fruiting body development to evolve while repeatedly encountering a non-evolving developmental cheater. Evolving populations greatly increased their fitness in the presence of the cheater, both relative to their ancestor and in terms of absolute spore productivity. However, the same evolved lineages exhibited a net disadvantage to the ancestor in the cheater's absence. Evolving populations reversed a large ancestral disadvantage to the cheater into competitive superiority and also evolved to strongly suppress cheater productivity. Moreover, in three-party mixes with the cheater, evolved populations enhanced their ancestor's productivity relative to mixes of only the ancestor and cheater. Thus, our evolved populations function as selfish police that inhibit cheaters, both to their own advantage and to the benefit of others as well. Cheater suppression was general across multiple unfamiliar cheaters but was more pronounced against the evolutionarily familiar cheater. Also, evolution generated three new mutually beneficial relationships, including complementary defect rescue between evolved cells and the selection-regime cheater. The rapid evolution of cheater suppression documented here suggests that coevolving social strategies within natural populations of prokaryotes are more diverse and complex than previously appreciated.