bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular dynamics of excited state intramolecular proton transfer: 3-hydroxyflavone in solution
Authors: Bellucci Michael A. , Coker David F.
Published: May 2012

The ultrafast enol-keto photoisomerization in the lowest singlet excited state of 3-hydroxyflavone is investigated using classical molecular dynamics in conjunction with empirical valence bond (EVB) potentials for the description of intramolecular interactions, and a molecular mechanics and variable partial charge model, dependent on transferring proton position, for the description of solute-solvent interactions. A parallel multi-level genetic program was used to accurately fit the EVB potential energy surfaces to high level ab initio data. We have studied the excited state intramolecular proton transfer (ESIPT) reaction in three different solvent environments: methylcyclohexane, acetonitrile, and methanol. The effects of the environment on the proton transfer time and the underlying mechanisms responsible for the varied time scales of the ESIPT reaction rates are analyzed. We find that simulations with our EVB potential energy surfaces accurately reproduce experimentally determined reaction rates, fluorescence spectra, and vibrational frequency spectra in all three solvents. Furthermore, we find that the ultrafast ESIPT process results from a combination of ballistic transfer, and intramolecular vibrational redistribution, which leads to the excitation of a set of low frequency promoting vibrational modes. From this set of promoting modes, we find that an O−O in plane bend and a C−H out of plane bend are present in all three solvents, indicating that they are fundamental to the ultrafast proton transfer. Analysis of the slow proton transfer trajectories reveals a solvent mediated proton transfer mechanism, which is diffusion limited.
Unstable kinetic roughening during the island coalescence stage of sputtered tantalum films
Authors: Yang J. J. , Tang J. , Liu N. , Ma F. , Tang W. , Xu K. W.
Published: May 2012

Kinetic roughening of tantalum films during the initial growth stages has been studied by atomic force microscopy, scanning electron microscopy, and dynamic scaling theory. Different from the time-independent scaling behavior for continuous film growth, an intriguing unstable kinetic roughening occurs during island coalescence. In such case, roughness exponent α increases with growth time, accompanied by lower growth exponent β and higher coarsening exponent η. Detailed analysis of film surface morphology and simple phenomenological models suggests that this unstable behavior is related to the pronounced lateral growth of surface islands, which arises from the combined effect of the formation of grain boundary and the covering of heterogeneous substrate surface.
Elastic network normal modes provide a basis for protein structure refinement
Authors: Gniewek Pawel , Kolinski Andrzej , Jernigan Robert L. , Kloczkowski Andrzej
Published: May 2012

It is well recognized that thermal motions of atoms in the protein native state, the fluctuations about the minimum of the global free energy, are well reproduced by the simple elastic network models (ENMs) such as the anisotropic network model (ANM). Elastic network models represent protein dynamics as vibrations of a network of nodes (usually represented by positions of the heavy atoms or by the Cα atoms only for coarse-grained representations) in which the spatially close nodes are connected by harmonic springs. These models provide a reliable representation of the fluctuational dynamics of proteins and RNA, and explain various conformational changes in protein structures including those important for ligand binding. In the present paper, we study the problem of protein structure refinement by analyzing thermal motions of proteins in non-native states. We represent the conformational space close to the native state by a set of decoys generated by the I-TASSER protein structure prediction server utilizing template-free modeling. The protein substates are selected by hierarchical structure clustering. The main finding is that thermal motions for some substates, overlap significantly with the deformations necessary to reach the native state. Additionally, more mobile residues yield higher overlaps with the required deformations than do the less mobile ones. These findings suggest that structural refinement of poorly resolved protein models can be significantly enhanced by reduction of the conformational space to the motions imposed by the dominant normal modes.
Localized mid-gap-states limited reverse current of diamond Schottky diodes
Authors: Teraji T. , Liao M. Y. , Koide Y.
Published: May 2012

Electric properties of the lateral-type tungsten carbide/p-diamond (100) Schottky diodes were investigated. At the lower voltage below 10 V, the reverse current density JR increased steeply and then became saturate at a certain value. This saturated JR value was kept constant in the wider voltage range of 10–500 V. A large variation of ∼5 orders of magnitude in the constant JR value was observed among the electrodes examined, whereas there was a clear correlation between the constant JR values and the current level of the shoulder component in the forward characteristics. The generation current induced by the spatially localized mid-gap states was presumed as the transport mode of this constant JR. The fitting procedure indicates the mid-gap states localized ∼2 μm far from the Schottky electrode edge. The maximum density of mid-gap states divided by the effective carrier lifetime was estimated to be ∼1017 cm−2s−1. The mid-gap states locating energetically at 0.12 eV and 0.62 eV above the valence-band top were detected in the temperature range of 300–700 K.
Lorentz group and oriented McIntosh-Cisneros-Zwanziger-Kepler orbits
Authors: Meng Guowu
Published: May 2012

The McIntosh-Cisneros-Zwanziger (MICZ)-Kepler orbits are the non-colliding orbits of the MICZ-Kepler problems (the magnetized versions of the Kepler problem). The oriented MICZ-Kepler orbits can be parametrized by the canonical angular momentum L and the Lenz vector A, with the parameter space consisting of the pairs of 3D vectors \documentclass[12pt]{minimal}\begin{document}$(\mathbf A, \mathbf L)$\end{document}(A,L) with \documentclass[12pt]{minimal}\begin{document}${\mathbf L}\cdot {\mathbf L} > (\mathbf L\cdot \mathbf A)^2$\end{document}L·L>(L·A)2. The recent 4D perspective of the Kepler problem yields a new parametrization, with the parameter space consisting of the pairs of Minkowski vectors (a, l) with l · l = −1, a · l = 0, a0 > 0. Here, a0 is the temporal component of a. This new parametrization of orbits implies that the MICZ-Kepler orbits of different magnetic charges are related to each other by symmetries: \documentclass[12pt]{minimal}\begin{document}${\mathrm{SO}}^+(1,3)\times {\mathbb R}_+$\end{document} SO +(1,3)×R+ acts transitively on both the set of oriented elliptic MICZ-Kepler orbits and the set of oriented parabolic MICZ-Kepler orbits. This action extends to \documentclass[12pt]{minimal}\begin{document}${\mathrm{O}}^+(1,3)\times {\mathbb R}_+$\end{document}O+(1,3)×R+, the structure group for the rank-two Euclidean Jordan algebra whose underlying Lorentz space is the Minkowski space.
Rovibrational dynamics of the strontium molecule in the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}$AΣu+1, c3Πu, and ${\rm a}\sideset{^3}{_{u}^{+}}{\Sigma}$aΣu+3 manifold from state-of-the-art ab initio calculations
Authors: Skomorowski Wojciech , Pawłowski Filip , Koch Christiane P. , Moszynski Robert
Published: May 2012

State-of-the-art ab initio techniques have been applied to compute the potential energy curves for the electronic states in the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}$AΣu+1, c3Πu, and ${\rm a}\sideset{^3}{_{u}^{+}}{\Sigma}$aΣu+3 manifold of the strontium dimer, the spin-orbit and nonadiabatic coupling matrix elements between the states in the manifold, and the electric transition dipole moment from the ground ${\rm X}\sideset{^1}{_{g}^{+}}{\Sigma}$XΣg+1 to the nonrelativistic and relativistic states in the A+c+a manifold. The potential energy curves and transition moments were obtained with the linear response (equation of motion) coupled cluster method limited to single, double, and linear triple excitations for the potentials and limited to single and double excitations for the transition moments. The spin-orbit and nonadiabatic coupling matrix elements were computed with the multireference configuration interaction method limited to single and double excitations. Our results for the nonrelativistic and relativistic (spin-orbit coupled) potentials deviate substantially from recent ab initio calculations. The potential energy curve for the spectroscopically active (1)$0_u^+$0u+ state is in quantitative agreement with the empirical potential fitted to high-resolution Fourier transform spectra [A. Stein, H. Knöckel, and E. Tiemann, Eur. Phys. J. D 64, 227 (2011)]10.1140/epjd/e2011-20229-6. The computed ab initio points were fitted to physically sound analytical expressions, and used in converged coupled channel calculations of the rovibrational energy levels in the A+c+a manifold and line strengths for the ${\rm A}\sideset{^1}{_{u}^{+}}{\Sigma}\leftarrow {\rm X}\sideset{^1}{_{g}^{+}}{\Sigma}$AΣu+1←XΣg+1 transitions. Positions and lifetimes of quasi-bound Feshbach resonances lying above the 1S0 + 3P1 dissociation limit were also obtained. Our results reproduce (semi)quantitatively the experimental data observed thus far. Predictions for on-going and future experiments are also reported.
Drift wave dispersion relation for arbitrarily collisional plasma
Authors: Angus Justin R. , Krasheninnikov Sergei I.
Published: May 2012

The standard local linear analysis of drift waves in a plasma slab is generalized to be valid for arbitrarily collisional electrons by considering the electrons to be governed by the drift-kinetic equation with a BGK-like (Bhatnagar-Gross-Krook) collision operator. The obtained dispersion relation reduces to that found from collisionless kinetic theory when the collision frequency is zero. Electron temperature fluctuations must be retained in the standard fluid analysis in order to obtain good quantitative agreement with our general solution in the highly collisional limit. Any discrepancies between the fluid solution and our general solution in this limit are attributed to the limitations of the BGK collision operator. The maximum growth rates in both the collisional and collisionless limits are comparable and are both on the order of the fundamental drift wave frequency. The main role of the destabilizing mechanism is found to be in determining the parallel wave number at which the maximum growth rate will occur. The parallel wave number corresponding to the maximum growth rate is set by the wave-particle resonance condition in the collisionless limit and transitions to being set by the real frequency being on the order of the rate for electrons to diffuse a parallel wavelength in the collisional limit.
Dielectric properties and electrical conduction of high-k LaGdO3 ceramics
Authors: Pavunny S. P. , Thomas R. , Kumar A. , Murari N. M. , Katiyar R. S.
Published: May 2012

The temperature and frequency dependent dielectric properties and leakage conduction mechanism in LaGdO3 (LGO) ceramics have been studied, and this material has been identified as a potential high-k candidate for the future complementary metal-oxide-semiconductor (CMOS) and dynamic random access memory (DRAM) technology nodes. The dielectric constant and the loss tangent at 100 kHz were ∼21.5 and ∼0.003, respectively, at ambient conditions without any significant temperature and voltage dependence. The ac conductivity shows the typical features of universal dynamic response (UDR) and obey the double power law σac=σdc+Aωn1+Bωn2 with three types of temperature dependent conduction processes involved; (i) a dc plateau (< 3 kHz) due to long range translational hopping, (ii) a mid frequency region due to the short range hopping (3–100 kHz), and (iii) a high frequency region due to localized or reorientational hopping (100–1000 kHz). The temperature dependent dc conductivity follows the Arrhenius relation with activation energies of 0.05 eV in the 200–400 K range and 0.92 eV in the 400–600 K range. The leakage current behavior reveals bulk limited Poole-Frenkel (PF) conduction mechanism and the estimated optical dielectric constant (ɛ∞) is 3.6.
On the small-signal theory of stimulated Cherenkov emission in dielectric-lined waveguides
Authors: Fares Hesham
Published: May 2012

The growth characteristics of a two-dimensional Cherenkov free-electron laser (CFEL) in the small-signal low gain regime are studied for both single-electron and collective models. In the single-electron regime, for a thick enough beam, we prove that the gain of the CFEL can be increased by a factor of 2 than that obtained for an infinitely thin electron beam. Thus, the Cherenkov laser should be considered as a hybrid longitudinal/transverse free-electron laser device in general. In the collective-regime, we demonstrate the damping effects of Coulomb collisions among electrons on the amplification characteristics of a Cherenkov laser.
Photoelectron spectroscopy and theoretical studies of UF5− and UF6−
Authors: Dau Phuong Diem , Su Jing , Liu Hong-Tao , Huang Dao-Ling , Wei Fan , Li Jun , Wang Lai-Sheng
Published: May 2012

The UF5− and UF6− anions are produced using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemistry. An extensive vibrational progression is observed in the spectra of UF5−, indicating significant geometry changes between the anion and neutral ground state. Franck-Condon factor simulations of the observed vibrational progression yield an adiabatic electron detachment energy of 3.82 ± 0.05 eV for UF5−. Relativistic quantum calculations using density functional and ab initio theories are performed on UF5− and UF6− and their neutrals. The ground states of UF5− and UF5 are found to have C4v symmetry, but with a large U−F bond length change. The ground state of UF5− is a triplet state (3B2) with the two 5f electrons occupying a 5fz3-based 8a1 highest occupied molecular orbital (HOMO) and the 5fxyz-based 2b2 HOMO-1 orbital. The detachment cross section from the 5fxyz orbital is observed to be extremely small and the detachment transition from the 2b2 orbital is more than ten times weaker than that from the 8a1 orbital at the photon energies available. The UF6− anion is found to be octahedral, similar to neutral UF6 with the extra electron occupying the 5fxyz-based a2u orbital. Surprisingly, no photoelectron spectrum could be observed for UF6− due to the extremely low detachment cross section from the 5fxyz-based HOMO of UF6−.
The role of 4f-electron on spin reorientation transition of NdFeO3: A first principle study
Authors: Chen La , Li Tongwei , Cao Shixun , Yuan Shujuan , Hong Feng , Zhang Jincang
Published: May 2012

Magnetic properties and electronic structures of NdFeO3 have been studied by performing accurate first principle calculation based on density functional theory. The 4f-electrons of Nd are explicitly treated as valence electrons. The simulation results of crystal structure and magnetic structure of this compound agree well with the experimental observations. Importantly, our study indicates that the spin reorientation transition of Fe3+ spin sublattice can be ascribed to the exchange interaction between Nd-4f and Fe-3d electrons, which are mediated by O2-2p state in Fe-O plane. As the temperature decreases, the Fe-O and Nd-O bonds become more covalent, and the exchange interactions become stronger.
Phase transition behavior in microcantilevers coated with M1-phase VO2 and M2-phase VO2:Cr thin films
Authors: Rúa Armando , Cabrera Rafmag , Coy Horacio , Merced Emmanuelle , Sepúlveda Nelson , Fernández Félix E.
Published: May 2012

Silicon microcantilevers were coated by pulsed laser deposition with vanadium dioxide (VO2) (monoclinic M1 phase) and V1−xCrxO2 with x near 0.024 (monoclinic M2 phase), and their mechanical characteristics were studied as a function of temperature through the films’ insulator-to-metal transition (IMT). The undoped VO2 films grew with (011)M1 planes parallel to the substrate, while Cr-doped VO2 films grew oriented with (201)M2 and (2¯01)M2 planes parallel to the substrate. In both cases, the films transformed reversibly through the IMT to the tetragonal (rutile, R) phase, with film (110)R planes oriented parallel to the substrate. The fundamental resonant frequencies of the cantilevers were measured as the temperature was cycled from ambient temperature, through the IMT, and up to 100  °C. Very high resonant frequency changes were observed through the transition for both types of samples, with increases during heating of over 11% and over 15% for the cantilevers coated with pure and Cr-doped VO2, respectively. From the resonant frequencies measured at room temperature for the bare and coated cantilevers in each case, the effective Young’s moduli of the films were determined. The values obtained, assuming bulk densities for the films, are 156 ± 7.5 GPa for VO2 (M1 phase) and 102 ± 3 GPa for Cr-doped VO2 (M2 phase). Strong curvature changes during the transition to the R phase were also observed for cantilevers coated with both types of films, but these were significantly higher in the case for the Cr-doped film. Curvature changes for temperature ranges outside the IMT region were small and attributed to differential thermal expansion between film and silicon substrate. From measured cantilever tip displacements in this post-transition range—for the undoped VO2-coated microcantilevers—a rough estimate of 110 GPa was obtained for the effective Young’s modulus for R-phase VO2. The substantially higher changes in resonant frequency and curvature for V1−xCrxO2-coated cantilevers suggest that this material may be even more useful than M1-phase VO2 for prospective microelectromechanical or optomechanical device applications in which ample frequency tunability—in oscillators or filters—or large displacements—in actuators—within a small temperature range is desirable. Since M2-phase V1−xCrxO2 with Cr composition of a few atomic percent retains other desirable properties of VO2, such as very high resistivity changes through the IMT and a transition temperature fairly close to ambient temperature, multifunctionality is not impaired and in fact may be enhanced for some applications.
Z-scan study of nonlinear absorption in reduced LiNbO3 crystals
Authors: Kostritskii S. M. , Aillerie M.
Published: May 2012

The nonlinear absorption (NLA) was studied by open-aperture Z-scan experiments in the chemically reduced nominally pure LiNbO3 crystals at cw-illumination with the red (644 nm) and green (514.5 nm) laser beams. The magnitude of the measured NLA is considerably different from the reported Z-scan results obtained in as-grown LiNbO3. The positive sign of NLA obtained with the red light has been related to the generation of the small bound polarons absorbing in red and near-IR ranges. Application of green light results in the light-induced transparency, i.e., the Z-scan traces show negative sign of NLA. Intensity dependence of Z-scan traces allows for conclusion that the photo-induced dissociation of small NbLi4+:NbNb4+ bipolarons and sequent generation of small polarons gives the dominating contribution to the nonlinear optical absorption in reduced crystals with a large bipolarons concentration.
Spatially localized measurement of thermal conductivity using a hybrid photothermal technique
Authors: Hua Zilong , Ban Heng , Khafizov Marat , Schley Robert , Kennedy Rory , Hurley David H.
Published: May 2012

A photothermal technique capable of measuring thermal conductivity with micrometer lateral resolution is presented. This technique involves measuring separately the thermal diffusivity, D, and thermal effusivity, e, to extract the thermal conductivity, k = (e2/D)1/2. To generalize this approach, sensitivity analysis is conducted for materials having a range of thermal conductivities. Application to nuclear fuel is consider by performing experimental validation using two materials (CaF2 and SiO2) having thermal properties representative of fresh and high burnup nuclear fuel. The measured conductivities compare favorably with literature values.
Azimuthal dependence of switching field strength for nematic liquid crystal bistability on patterned alignment layers
Authors: Niitsuma Jun-ichi , Yoneya Makoto , Yokoyama Hiroshi
Published: May 2012

We investigate the azimuthal dependence of the switching field strength for nematic liquid crystal bistability on patterned alignment layers to determine the validity of a switching theory proposed by Kim et al. [Appl. Phys. Lett. 78, 3055 (2001)] and to evaluate higher-order azimuthal anchoring coefficients. Director behavior during switching is described in detail for general azimuthal directions of an applied electric field and the experimental results are explained on the basis of the theory. We obtain azimuthal anchoring coefficients up to the second higher-order term in an expansion of the anchoring energy function. Detailed analysis of the director behavior reveals a close analogy between the proposed model of nematic liquid crystal bistability and a coherent rotation (Stoner-Wohlfarth) model of ferromagnetism. This suggests a reversal-asymmetric property of the substrate-nematic interface director, differing from the reversal symmetry of the bulk nematic director, as far as the nematic bistability switching is concerned.
Coexistence of unipolar and bipolar resistive switching in BiFeO3 and Bi0.8Ca0.2FeO3 films
Authors: Liu Lu , Zhang Shantao , Luo Ying , Yuan Guoliang , Liu Junming , Yin Jiang , Liu Zhiguo
Published: May 2012

Ferroelectric BiFeO3 and paraelectric Bi0.8Ca0.2FeO3 polycrystalline films were prepared to study the dependence of resistive switch on defect density. With defect density and the corresponding leakage current increasing, current-voltage loops allow four different types, i.e., overlapping, hysteresis without memory effect, bipolar resistive switch, and unipolar resistive switch. The first three types can transform to the last one, once electroforming introduces enough charged defects to films and the resistance monotonically increases to a certain value. Unipolar resistive switch is due to conductive filamentary type and can be treated as an especial electroforming process. Furthermore, its high resistance status allows the second or third type at low voltage region.
Single molecules probe the polarization dynamics of poly (methyl methacrylate) in external electric field
Authors: Chen Ruiyun , Zhang Guofeng , Gao Yan , Xiao Liantuan , Jia Suotang
Published: May 2012

We demonstrate the electric field (EF) induced polarization dynamics of poly (methyl methacrylate) (PMMA) by observing the fluorescence modulation of embedded non-polar single squaraine-derived rotaxane molecules. It is established that interaction between the molecular energy level and the potential valley formed by surrounding PMMA matrix can be detuned by the EF, which induces the changing of electron transfer rates between them effectively. The EF-induced response time of the fluorescence quenching or enhancement and the fluorescence recovery time reflect the diverse polarization and relaxation dynamics of PMMA.
Modeling of carrier lifetimes in uniaxially strained GaAs
Authors: Grivickas P.
Published: May 2012

A carrier dynamics model based on the continuity equation is constructed for doped GaAs in order to explain lifetime changes under uniaxial strain. The model includes detailed expressions of carrier generation, diffusion, recombination, and trapping processes and is solved numerically for fitting carrier decays obtained from time- and spectral-resolved photoluminescence measurements. First, a set of baseline model parameters is established at ambient conditions by fitting experimental data from a GaAs wafer at different excitations. Then, the parameters are adjusted to model carrier lifetimes detected in the samples cut from the same wafer, under the conditions of uniaxial strain applied along the [100] crystallographic orientation. It is shown that the observed linear reduction of effective lifetimes is dominated by the changes in recombination processes. Increase in the strength of the non-radiative Shockley-Read-Hall recombination mechanism versus the radiative band-to-band recombination mechanism is consistent with the quantum efficiency loss in GaAs at increasing uniaxial strain.
Synchronization of a network coupled with complex-variable chaotic systems
Authors: Wu Zhaoyan , Chen Guanrong , Fu Xinchu
Published: May 2012

In this paper, synchronization of a network coupled with complex-variable chaotic systems is investigated. Adaptive feedback control and intermittent control schemes are adopted for achieving adaptive synchronization and exponential synchronization, respectively. Several synchronization criteria are established. In these schemes, the outer coupling matrix is not necessarily assumed to be symmetric or irreducible. Further, for a class of networks with an irreducible and balanced outer coupling matrix, a pinning control scheme is adopted for achieving synchronization. Numerical simulations are demonstrated to verify the effectiveness of the theoretical results.
A new concept in underwater high fidelity low frequency sound generation
Authors: Fonseca Paulo J. , Alves J. Maia
Published: May 2012

This article reports on a new type of system for high fidelity underwater sound generation (patent pending PT105474). The system includes an underwater sound actuator and the corresponding electronic driver. The sound is generated by a rigid plate that is actuated (both for positioning/dumping and excitation) using purely electromagnetic forces, thus, avoiding the use of any elastic membrane. Since there is no compressible air inside the device, which is flooded by water, the operation of this device is independent from depth, broadening its applications to any water pressure. Characterization of the frequency response, the radiation characteristics, and the dynamic range of this new device for underwater sound generation is presented.
Excess protons in water-acetone mixtures
Authors: Semino Rocío , Laria Daniel
Published: May 2012

Using molecular dynamics experiments, we analyze equilibrium and dynamical characteristics related to the solvation of excess protons in water-acetone mixtures. Our approach is based on the implementation of an extended valence-bond Hamiltonian, which incorporates translocation of the excess charge between neighboring water molecules. Different mixtures have been analyzed, starting from the pure water case down to solutions with a water molar fraction xw = 0.25. In all cases, we have verified that the structure of the first solvation shell of the H3O+ moiety remains practically unchanged, compared to the one observed in pure water. This shell is composed by three water molecules acting as hydrogen bond acceptors, with no evidence of hydrogen bond donor-like connectivity. Moreover, the increment in the acetone concentration leads to a gradual stabilization of Eigen-like [H3O·(H2O)3]+ configurations, in detriment of Zundel-like [H·(H2O)2]+ ones. Rates of proton transfer and proton diffusion coefficients have been recorded at various water-acetone relative concentrations. In both cases, we have found a transition region, in the vicinity of xw ∼ 0.8, where the concentration dependences of the two magnitudes change at a quantitative level. A crude estimate shows that, at this tagged concentration, the volumes “occupied” by the two solvents become comparable. The origins of this transition separating water-rich from acetone-rich realms is rationalized in terms of modifications operated in the nearby, second solvation shell, which in the latter solutions, normally includes at least, one acetone molecule. Our results would suggest that one possible mechanism controlling the proton transfer in acetone-rich solutions is the exchange of one of these tagged acetone molecules, by nearby water ones. This exchange would give rise to Zundel-like structures, exhibiting a symmetric, first solvation shell composed exclusively by water molecules, and would facilitate the transfer between neighboring water molecules along the resonant complex.
A surface enhanced Raman spectroscopy study of aminothiophenol and aminothiophenol-C60 self-assembled monolayers: Evolution of Raman modes with experimental parameters
Authors: Delafosse G. , Merlen A. , Clair S. , Patrone L.
Published: May 2012

P-aminothiophenol (PATP) is a well-known molecule for the preparation of self-assembled monolayers on gold via its thiol functional group. After adsorption, it has been demonstrated that this molecule is anchored to gold through its thiol group, and standing nearly upright at the surface with the amino functional group on top. This molecule has been extensively studied by surface enhanced Raman spectroscopy but its exact SERS spectrum remains unclear. Here, we demonstrate that it can be strongly affected by at least two experimental parameters: laser power and layer density. Those features are discussed in terms of a dimerization of the PATP molecules. The free amino group affords the adsorption of other molecules such as C60. In this case, a complex multilayer system is formed and the question of its precise characterisation remains a key point. In this article, we demonstrate that surface enhanced Raman spectroscopy combined with x ray photoelectron spectroscopy can bring very important information about the organization of such a self-assembled multilayer on gold. In our study, the strong evolution of Raman modes after C60 adsorption suggests a change in the organization of aminothiophenol molecules during C60 adsorption. These changes, also observed when the aminothiophenol layer is annealed in toluene, do not prevent the adsorption of C60 molecules.
Surface-induced truly half-metallicity in VTe with rocksalt structure from first-principles calculations
Authors: Gao G. Y. , Xu Bin , Yao Kai-Lun
Published: May 2012

Motivated by the experimental synthesis of bulk MnTe with rocksalt structure, we use the first-principles calculations to explore the structural, electronic, and magnetic properties of VTe in four different phases: rocksalt, zinc-blende, wurtzite, and ground-state NiAs. It is found that the ground-state NiAs phase is antiferromagnetic, but both the zinc-blende and wurtzite phases exhibit truly half-metallic ferromagnetism, and the rocksalt phase is a nearly half-metal. Importantly, the rocksalt phase is energetically more stable than the zinc-blende and wurtzite ones. We further reveal that there is an interesting transition from the nearly half-metallicity in bulk rocksalt VTe to the truly half-metallicity at the Te-terminated (111) surface, and the Te-terminated (111) surface is energetically more stable than the V-terminated (111) surface, which makes VTe with rocksalt structure promising candidate for spintronic applications.
Inhomogeneous nanostructured honeycomb optical media for enhanced cathodo- and under-x-ray luminescence
Authors: Gaponenko N. V. , Kortov V. S. , Rudenko M. V. , Pustovarov V. A. , Zvonarev S. V. , Slesarev A. I. , Molchan I. S. , Thompson G. E. , Khoroshko L. S. , Prislopskii S. Ya.
Published: May 2012

Photo-, radio-, and pulse cathodoluminescence spectra from sol-gel derived titania, doped with strontium and terbium, deposited on porous anodic alumina (PAA) films are reported. The morphology and qualitative elemental depth distributions have been examined by transmission electron microscopy, scanning electron microscopy, and radio-frequency glow discharge optical emission spectroscopy. PAA films with pore and cell sizes ranging from 170 to 190 and 240 to 270 nm, respectively, have been generated on aluminum and monocrystalline silicon substrates followed by spin-on sol-gel derived coating with the subsequent thermal treatment. The resultant PAA surface is not coated with a continuous xerogel film; the xerogel is mainly distributed near the pore bases, leaving much of the pore volume unfilled. The xerogel/PAA structures reveal terbium-related luminescence under x-ray excitation and cathodoluminescence. The same xerogels generated on monocrystalline silicon revealed no cathode- or under-x-ray luminescence. Thus, PAA enhances strongly the cathode- and under x-ray luminescence from terbium and strontium-doped titania xerogels confined in the porous matrix. The fabricated structures are considered as a type of low-cost, thin-film convertor of x-rays, and cathode ray irradiation into visible light, with an average cell size of the convertor of about 250 nm.
Coupling between whistler waves and slow-mode solitary waves
Authors: Tenerani A. , Califano F. , Pegoraro F. , Le Contel O.
Published: May 2012

The interplay between electron- and ion-scale phenomena is of general interest for both laboratory and space plasma physics. In this paper, we investigate the linear coupling between whistler waves and slow magnetosonic solitons through two-fluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ion-scales. This raises the interesting question of how ion-scale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons.
Observation of band alignment transition in InAs/GaAsSb quantum dots by photoluminescence
Authors: Ban Keun-Yong , Kuciauskas Darius , Bremner Stephen P. , Honsberg Christiana B.
Published: May 2012

The band alignment of InAs quantum dots (QDs) embedded in GaAsSb barriers with various Sb compositions is investigated by photoluminescence (PL) measurements. InAs/GaAsSb samples with 13% and 15% Sb compositions show distinct differences in emission spectra as the PL excitation power increases. Whilst no discernible shift is seen for the 13% sample, a blue-shift of PL spectra following a 1/3 exponent of the excitation power is observed for the 15% sample suggesting a transition from a type I to type II band alignment. Time-resolved PL data show a significant increase in carrier lifetime as the Sb composition increases between 13% and 15% implying that the transformation from a type I to type II band alignment occurs between 13% and 15% Sb compositions. These results taken together lead to the conclusion that a zero valence band offset (VBO) can be achieved for the InAs/GaAsSb system in the vicinity of 14% Sb composition.
Solid-phase crystallization of ultra high growth rate amorphous silicon films
Authors: Sharma K. , Ponomarev M. V. , Verheijen M. A. , Kunz O. , Tichelaar F. D. , van de Sanden M. C. M. , Creatore M.
Published: May 2012

In this paper, we report on the deposition of amorphous silicon (a-Si:H) films at ultra-high growth rate (11–60 nm/s) by means of the expanding thermal plasma technique, followed by solid-phase crystallization (SPC). Large-grain (∼1.5 μm) polycrystalline silicon was obtained after SPC of high growth rate (∼25 nm/s) deposited a-Si:H films. The obtained results are discussed by taking into account the impact of the a-Si:H microstructure parameter R* as well as of its morphology, on the final grain size development.
Ion impact energy distribution and sputtering of Si and Ge
Authors: Hossain M. Z. , Freund J. B. , Johnson H. T.
Published: May 2012

The spatial distribution of ion deposited energy is often assumed to linearly relate to the local ion-induced sputtering of atoms from a solid surface. This—along with the assumption of an ellipsoidal region of energy deposition—is the central mechanism used in the Bradley and Harper [J. Vac. Sci. Technol. A 6, 2390 (1988)] explanation of ion-induced surface instabilities, but it has never been assessed directly. To do this, we use molecular dynamics to compute the actual distribution of deposited energy and relate this to the source of sputtered atoms for a range of ion energies (250 eV and 1500 eV), ion species (Ar, Kr, Xe, and Rn), targets (Si and Ge), and incidence angles (0°, 10°, 20°, 30°, 40°, 50°, 60°, 70°, and 80°). It is found that the energy deposition profile is remarkably ellipsoidal but that the relation between local deposited energy and local sputtering is not simple. It depends significantly upon the incidence angle, and the relation between energy and local sputter yield is nonlinear, though with a nearly uniform power-law relation. These results will affect, in particular, surface instability models based upon simpler approximations.
Annealing dependence of diamond-metal Schottky barrier heights probed by hard x-ray photoelectron spectroscopy
Authors: Gaowei M. , Muller E. M. , Rumaiz A. K. , Weiland C. , Cockayne E. , Jordan-Sweet J. , Smedley J. , Woicik J. C.
Published: May 2012

Hard x-ray photoelectron spectroscopy was applied to investigate the diamond-metal Schottky barrier heights for several metals and diamond surface terminations. The position of the diamond valence-band maximum was determined by theoretically calculating the diamond density of states and applying cross section corrections. The diamond-platinum Schottky barrier height was lowered by 0.2 eV after thermal annealing, indicating annealing may increase carrier injection in diamond devices leading to photoconductive gain. The platinum contacts on oxygen-terminated diamond was found to provide a higher Schottky barrier and therefore a better blocking contact than that of the silver contact in diamond-based electronic devices.
Theoretical study of Cl-related defect complexes in cubic SiC
Authors: Alfieri G. , Kimoto T.
Published: May 2012

First-principles calculations were employed to investigate the electronic properties and formation energy of Cl-related complex defects, with either carbon (silicon) vacancies, VC (VSi) or dopants (nitrogen/aluminum). Our investigation revealed that Cl at a Si-site related complexes, such as the ClSiVC and ClSiNC, are either structurally unstable or possess a rather high formation energy, in agreement with the fact that Cl energetically favors a C-site. By employing the defect-molecule model, we found that ClCVSi and ClCAlSi complexes have C3v symmetry and are both donors. However, contrarily to the case of the ClCVSi complex, for the ClCAlSi complex the singlet (A1) orbitals can lie higher in energy position than the doubly degenerate orbital (E). The formation energies of Cl-vacancy/dopant complex defects are discussed in the light of recent experimental results, reported for 4 H-SiC grown by Cl-based chemical vapor deposition.
D-T gamma-to-neutron branching ratio determined from inertial confinement fusion plasmas
Authors: Kim Y. , Mack J. M. , Herrmann H. W. , Young C. S. , Hale G. M. , Caldwell S. , Hoffman N. M. , Evans S. C. , Sedillo T. J. , McEvoy A. , Langenbrunner J. , Hsu H. H. , Huff M. A. , Batha S. , Horsfield C. J. , Rubery M. S. , Garbett W. J. , Stoeffl W. , Grafil E. , Bernstein L. , Church J. A. , Sayre D. B. , Rosenberg M. J. , Waugh C. , Rinderknecht H. G. , Gatu Johnson M. , Zylstra A. B. , Frenje J. A. , Casey D. T. , Petrasso R. D. , Kirk Miller E. , Yu Glebov V. , Stoeckl C. , Sangster T. C.
Published: May 2012

A new deuterium-tritium (D-T) fusion gamma-to-neutron branching ratio [3H(d,γ)5He/3H(d,n)4He] value of (4.2 ± 2.0) × 10−5 was recently reported by this group [Y. Kim et al. Phys. Rev. C (submitted)]. This measurement, conducted at the OMEGA laser facility located at the University of Rochester, was made for the first time using inertial confinement fusion (ICF) plasmas. Neutron-induced backgrounds are significantly reduced in these experiments as compared to traditional beam-target accelerator-based experiments due to the short pulse nature of ICF implosions and the use of gas Cherenkov γ-ray detectors with fast temporal responses and inherent energy thresholds. It is expected that this ICF-based measurement will help resolve the large and long-standing inconsistencies in previously reported accelerator-based values, which vary by a factor of approximately 30. The reported value at ICF conditions was determined by averaging the results of two methods: (1) a direct measurement of ICF D-T γ-ray and neutron emissions using absolutely calibrated detectors and (2) a separate cross-calibration against the better known D-3He gamma-to-proton branching ratio [3He(d, γ)5Li/3He(d,p)4He]. Here we include a detailed explanation of these results, and introduce as a corroborative method an in-situ γ-ray detector calibration using neutron-induced γ-rays. Also, by extending the established techniques to two additional series of implosions with significantly different ion temperatures, we test the branching ratio dependence on ion temperature. The data show a D-T branching ratio is nearly constant over the temperature range 2–9 keV. These studies motivate further investigation into the 5He and 5Li systems resulting from D-T and D-3He fusion, respectively, and result in improved ICF γ-ray reaction history diagnosis at the National Ignition Facility.
Classical confinement and outward convection of impurity ions in the MST RFP
Authors: Kumar S. T. A. , Den Hartog D. J. , Mirnov V. V. , Caspary K. J. , Magee R. M. , Brower D. L. , Chapman B. E. , Craig D. , Ding W. X. , Eilerman S. , Fiksel G. , Lin L. , Nornberg M. , Parke E. , Reusch J. A. , Sarff J. S.
Published: May 2012

Impurity ion dynamics measured with simultaneously high spatial and temporal resolution reveal classical ion transport in the reversed-field pinch. The boron, carbon, oxygen, and aluminum impurity ion density profiles are obtained in the Madison Symmetric Torus [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)] using a fast, active charge-exchange-recombination-spectroscopy diagnostic. Measurements are made during improved-confinement plasmas obtained using inductive control of tearing instability to mitigate stochastic transport. At the onset of the transition to improved confinement, the impurity ion density profile becomes hollow, with a slow decay in the core region concurrent with an increase in the outer region, implying an outward convection of impurities. Impurity transport from Coulomb collisions in the reversed-field pinch is classical for all collisionality regimes, and analysis shows that the observed hollow profile and outward convection can be explained by the classical temperature screening mechanism. The profile agrees well with classical expectations. Experiments performed with impurity pellet injection provide further evidence for classical impurity ion confinement.
Humidity-controlled preparation of frozen-hydrated biological samples for cryogenic coherent x-ray diffraction microscopy
Authors: Takayama Yuki , Nakasako Masayoshi
Published: May 2012

Coherent x-ray diffraction microscopy (CXDM) has the potential to visualize the structures of micro- to sub-micrometer-sized biological particles, such as cells and organelles, at high resolution. Toward advancing structural studies on the functional states of such particles, here, we developed a system for the preparation of frozen-hydrated biological samples for cryogenic CXDM experiments. The system, which comprised a moist air generator, microscope, micro-injector mounted on a micromanipulator, custom-made sample preparation chamber, and flash-cooling device, allowed for the manipulation of sample particles in the relative humidity range of 20%–94%rh at 293 K to maintain their hydrated and functional states. Here, we report the details of the system and the operation procedure, including its application to the preparation of a frozen-hydrated chloroplast sample. Sample quality was evaluated through a cryogenic CXDM experiment conducted at BL29XUL of SPring-8. Taking the performance of the system and the quality of the sample, the system was suitable to prepare frozen-hydrated biological samples for cryogenic CXDM experiments.
Molecular orientation and anisotropic carrier mobility in poorly soluble polythiophene thin films
Authors: Hosokawa Yuki , Misaki Masahiro , Yamamoto Satoshi , Torii Masafumi , Ishida Kenji , Ueda Yasukiyo
Published: May 2012

Oriented thin films of a poorly soluble polythiophene, poly[benzo[1,2-d:4,5-d′]bisthiazole-2,6-diyl(3′,4,4″,4″′-tetradodecyl[2,2′:5′,2″:5″,2″′-quaterthiophene]-5,5″′-diyl)], were fabricated using the friction-transfer technique. The detail of the film structure was investigated using polarized UV-vis spectroscopy, transmission electron microscopy, and grazing-incidence x-ray diffraction observations. The friction-transfer technique enables control of the anisotropic carrier transport according to the face-on and/or edge-on molecular orientations in films with one-directionally aligned polymer backbones.
Ab initio quantum dynamics using coupled-cluster
Authors: Kvaal Simen
Published: May 2012

The curse of dimensionality (COD) limits the current state-of-the-art ab initio propagation methods for non-relativistic quantum mechanics to relatively few particles. For stationary structure calculations, the coupled-cluster (CC) method overcomes the COD in the sense that the method scales polynomially with the number of particles while still being size-consistent and extensive. We generalize the CC method to the time domain while allowing the single-particle functions to vary in an adaptive fashion as well, thereby creating a highly flexible, polynomially scaling approximation to the time-dependent Schrödinger equation. The method inherits size-consistency and extensivity from the CC method. The method is dubbed orbital-adaptive time-dependent coupled-cluster, and is a hierarchy of approximations to the now standard multi-configurational time-dependent Hartree method for fermions. A numerical experiment is also given.
Molecular dynamics simulation studies of the influence of imidazolium structure on the properties of imidazolium/azide ionic liquids
Authors: Hooper Justin B. , Starovoytov Oleg N. , Borodin Oleg , Bedrov Dmitry , Smith Grant D.
Published: May 2012

Atomistic molecular dynamics simulations were performed on 1-butyl-3-methyl-imidazolium azide [bmim][N3], 1-butyl-2,3-dimethylimidazolium azide [bmmim][N3], and 1-butynyl-3-methyl-imidazolium azide [bumim][N3] ionic liquids. The many-body polarizable APPLE&P force field was augmented with parameters for the azide anion and the bumim cation. Good agreement between the experimentally determined and simulated crystal structure of [bumim][N3] as well as the liquid-state density and ionic conductivity of [bmmim][N3] were found. Methylation of bmim (yielding bmmim) resulted in dramatic changes in ion structuring in the liquid and slowing of ion motion. Conversely, replacing the butyl group of bmim with the smaller 2-butynyl group resulted in an increase of ion dynamics.
Dramatic enhancement of structure-borne wave energy harvesting using an elliptical acoustic mirror
Authors: Carrara M. , Cacan M. R. , Leamy M. J. , Ruzzene M. , Erturk A.
Published: May 2012

Broadband structure-borne wave energy harvesting is reported by wave focusing using an elliptical acoustic mirror (EAM). The EAM is formed by an array of cylindrical stubs mounted along a semi-elliptical path on the surface of a plate. The array back-scatters incoming guided waves and focuses them at the focal location where a piezoelectric energy harvester is located. Multiple scattering simulations and experiments illustrate the broadband focusing characteristics of the EAM. More than an order of magnitude improvement in piezoelectric power generation is documented for an EAM-based energy harvester with respect to a free harvester over the 30–70 kHz frequency range.