bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature effects on quasi-isolated conjugated polymers as revealed by temperature-dependent optical spectra of 16-mer oligothiophene diluted in a sold matrix
Authors: Kanemoto Katsuichi , Akai Ichiro , Sugisaki Mitsuru , Hashimoto Hideki , Karasawa Tsutomu , Negishi Nobukazu , Aso Yoshio
Published: June 2009

Temperature dependences (4–300 K) of photoluminescence (PL) and absorption spectra of 16-mer oligothiophene (16 T) extremely diluted in polypropylene (PP) have been investigated in order to clarify temperature effects on quasi-isolated conjugated polymers. The PL and absorption spectra are found to blueshift with increasing temperature. The reason for the blueshift is discussed by comparing models based on the refractive index of the solvent (PP) and on the thermal conformational change of 16 T. The blueshift is concluded to result from the thermal conformational change. Time-resolved PL spectra show a redshift of PL band following photoexcitation (spectral migration). The amount of the migration is shown to increase with increasing temperature. The increased migration is concluded to be due to the thermal conformational change. The temperature dependence of the effective conjugation length (ECL) of 16 T is calculated for the absorption and PL transitions. The calculation suggests that ECL is reduced at room temperature to two-thirds of the intrinsic chain length. The activation energy of the conformational change is estimated to be 22.4 meV from the temperature dependence of ECL. We demonstrate that the steady-state PL spectra are well reproduced by simple Franck–Condon analyses using a single Huang–Ryes factor over a wide temperature range. The analyses reveal features of temperature dependence in important spectral parameters such as the Stokes shift, linewidth, and Huang–Ryes factor.
Endohedral nitrogen storage in carbon fullerene structures: Physisorption to chemisorption transition with increasing gas pressure
Authors: Barajas-Barraza R. E. , Guirado-López R. A.
Published: June 2009

We present extensive pseudopotential density functional theory (DFT) calculations in order to analyze the structural properties and chemical reactivity of nitrogen molecules confined in spheroidal (C82) and tubelike (C110) carbon fullerene structures. For a small number of encapsulated nitrogens, the N2 species exist in a nonbonded state within the cavities and form well defined molecular conformations such as linear chains, zigzag arrays, as well as both spheroidal and tubular configurations. However, with increasing the number of stored molecules, the interaction among the confined nitrogens as well as between the N2 species and the fullerene wall is not always mainly repulsive. Actually, at high densities of the encapsulated gas, we found both adsorption of N2 to the inner carbon surface together with the formation of (N2)m molecular clusters. Total energy DFT calculations reveal that the shape of the interaction potential of a test molecule moving within the carbon cavities strongly varies with the number and proximity of the coadsorbed N2 from being purely repulsive to having short-range attractive contributions close to the inner wall. In particular, the latter are always found when a group of closely spaced nitrogens is located near the carbon cage (a fact that will naturally occur at high densities of the encapsulated gas), inducing the formation of covalent bonds between the N2 and the fullerene network. Interestingly, in some cases, the previous nitrogen adsorption to the inner surface is reversible by reducing the gas pressure. The calculated average density of states of our considered carbon compounds reveals the appearance of well defined features that clearly reflect the occurring structural changes and modifications in the adsorption properties in the systems. Our results clearly underline the crucial role played by confinement effects on the reactivity of our endohedral compounds, define this kind of materials as nonideal nanocontainers for high density nitrogen storage applications, and must be taken into account when analyzing the diffusion properties of the encapsulated species.
Study of the interaction between water and hydrogen sulfide with polycyclic aromatic hydrocarbons
Authors: Cabaleiro-Lago Enrique M. , Carrazana-García Jorge A. , Rodríguez-Otero Jesús
Published: June 2009

A computational study has been carried out for determining the characteristics of the interaction between one water and hydrogen sulfide molecule with a series of polycyclic aromatic hydrocarbons of increasing size, namely, benzene, anthracene, triphenylene, coronene, circumcoronene, and dicircumcoronene. Potential energy curves were calculated for structures where H2X (X=O,S) molecule is located over the central six-membered ring with its hydrogen atoms pointing toward to (mode A) or away from (mode B) the hydrocarbon. The accuracy of different methods has been tested against the results of coupled cluster calculations extrapolated to basis set limit for the smaller hydrocarbons. The spin component scaled MP2 (SCS-MP2) method and a density functional theory method empirically corrected for dispersion (DFT-D) reproduce fairly well the results of high level calculations and therefore were employed for studying the larger systems, though DFT-D seems to underestimate the interaction in hydrogen sulfide clusters. Water complexes in mode A have interaction energies that hardly change with the size of the hydrocarbon due to compensation between the increase in the correlation contribution to the interaction energy and the increase in the repulsive character of the Hartree–Fock energy. For all the other clusters studied, there is a continuous increase in the intensity of the interaction as the size of the hydrocarbon increases, suggesting already converged values for circumcoronene. The interaction energy for water clusters extrapolated to an infinite number of carbon atoms amounts to −13.0 and −15.8 kJ/mol with SCS-MP2 and DFT-D, respectively. Hydrogen sulfide interacts more strongly than water with the hydrocarbons studied, leading to a limiting value of −21.7 kJ/mol with the SCS-MP2 method. Also, complexes in mode B are less stable than the corresponding A structures, with interaction energies amounting to −8.2 and −18.2 kJ/mol for water and hydrogen sulfide, respectively. The DFT-D calculations give values of −16.2 and −9.3 kJ/mol for hydrogen sulfide complexes in modes A and B, less negative than those predicted by the SCS-MP2 method, probably indicating problems with sulfur dispersion parameters.
A crossed beams and ab initio investigation on the formation of cyanodiacetylene in the reaction of cyano radicals with diacetylene
Authors: Zhang Fangtong , Kim Seol , Kaiser Ralf I. , Jamal Adeel , Mebel Alexander M.
Published: June 2009

The crossed molecular beams reaction of ground state cyano radicals (CN) with diacetylene (HCCCCH) was studied in the laboratory under single collision conditions. Combining the derived center-of-mass translational energy and angular distributions with novel electronic structure calculations, we show that the linear cyanodiacetylene molecule (HCCCCCN) is the sole reaction product. Our study provided no substantiation of two alternative products which have been suggested previously: cyanoacetylene (HCCCN), speculated to be synthesized via the exchange of the ethynyl by the cyano group, and the 1,3-butadiynyl radical (HCCCC), thought to be formed via hydrogen abstraction. The unambiguous identification of cyanodiacetylene formed in an exoergic, barrierless bimolecular reaction of the cyano radical with diacetylene strongly suggests that cyanodiacetylene can be also synthesized via this process in the interstellar medium (cold molecular clouds) and in hydrocarbon-rich atmospheres of planets and their moons such as Titan.
Generic coarse-grained model for protein folding and aggregation
Authors: Bereau Tristan , Deserno Markus
Published: June 2009

A generic coarse-grained (CG) protein model is presented. The intermediate level of resolution (four beads per amino acid, implicit solvent) allows for accurate sampling of local conformations. It relies on simple interactions that emphasize structure, such as hydrogen bonds and hydrophobicity. Realistic α/β content is achieved by including an effective nearest-neighbor dipolar interaction. Parameters are tuned to reproduce both local conformations and tertiary structures. The thermodynamics and kinetics of a three-helix bundle are studied. We check that the CG model is able to fold proteins with tertiary structures and amino acid sequences different from the one used for parameter tuning. By studying both helical and extended conformations we make sure the force field is not biased toward any particular secondary structure. The accuracy involved in folding not only the test protein but also other ones show strong evidence for amino acid cooperativity embedded in the model. Without any further adjustments or bias a realistic oligopeptide aggregation scenario is observed.
Effect of the static magnetic field strength on parahydrogen induced polarization NMR spectra
Authors: Bouguet-Bonnet Sabine , Reineri Francesca , Canet Daniel
Published: June 2009

Spin polarization transfer from parahydrogen (p-H2) to another molecular entity is generally thought to be mediated by longitudinal spin order (represented by the operator product IzAIzB, A and B being the two hydrogen nuclei which originate from p-H2 after a hydrogenation reaction). The longitudinal spin order leads to antiphase patterns in the proton NMR spectrum. In addition to these antiphase patterns, in-phase patterns, arising from polarization differences (represented by (IzA−IzB)), have been experimentally observed. A complete theory, based on a density operator treatment, has been worked out and applied to the two types of parahydrogen induced polarization experiments: PASADENA (PArahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment; hydrogenation reaction inside the NMR magnet) and (ALTADENA) (Adiabatic Longitudinal Transport After Dissociation Engenders Nuclear Alignment; hydrogenation reaction outside the NMR magnet). It is shown that polarization differences are always created in the case of a PASADENA experiment but that their amplitude depends critically on the ratio of the J coupling over the frequency difference between A and B. In the case of an ALTADENA experiment, if the sample is slowly transferred toward the NMR magnet, polarization differences are definitely created and their amplitude can be larger than the amplitude of the longitudinal spin order. Some test experiments demonstrate the validity of the proposed theory.
Standard electrode potential, Tafel equation, and the solvation thermodynamics
Authors: Matyushov Dmitry V.
Published: June 2009

Equilibrium in the electronic subsystem across the solution-metal interface is considered to connect the standard electrode potential to the statistics of localized electronic states in solution. We argue that a correct derivation of the Nernst equation for the electrode potential requires a careful separation of the relevant time scales. An equation for the standard metal potential is derived linking it to the thermodynamics of solvation. The Anderson–Newns model for electronic delocalization between the solution and the electrode is combined with a bilinear model of solute-solvent coupling introducing nonlinear solvation into the theory of heterogeneous electron transfer. We therefore are capable of addressing the question of how nonlinear solvation affects electrochemical observables. The transfer coefficient of electrode kinetics is shown to be equal to the derivative of the free energy, or generalized force, required to shift the unoccupied electronic level in the bulk. The transfer coefficient thus directly quantifies the extent of nonlinear solvation of the redox couple. The current model allows the transfer coefficient to deviate from the value of 0.5 of the linear solvation models at zero electrode overpotential. The electrode current curves become asymmetric in respect to the change in the sign of the electrode overpotential.
Photoabsorption cross section of C70 thin films from visible to vacuum ultraviolet
Authors: Yagi H. , Nakajima K. , Koswattage K. R. , Nakagawa K. , Katayanagi H. , Mitsuke K.
Published: June 2009

Absolute photoabsorption cross sections of C70 thin films were determined for hv values from 1.3 to 42 eV using photon attenuation. The spectrum showed a prominent peak of 1320 Mb at 21.4 eV with several fine structures mostly due to σ→σ∗ single-electron excitation. The complex refractive index and complex dielectric function were calculated up to 42 eV with Kramers–Kronig analyses. From the present data of C70 thin films, the cross section curve of molecular C70 was calculated using the standard Clausius–Mossotti relation dealing with correction of the local electromagnetic field, with a plausible assumption that the anisotropy in molecular structure of C70 was smeared out by molecular rotation at room temperature.
Glassy dynamics of liquid crystalline 4′-n-pentyl-4-cyanobiphenyl in the isotropic and supercooled nematic phases
Authors: Drozd-Rzoska Aleksandra
Published: June 2009

The temperature evolution of dielectric relaxation times (τ(T)) in the isotropic (I) and supercooled nematic (N) phases of 4′-n-pentyl-4-cyanobiphenyl is discussed. This compound may be considered as the classical model-material for the physics of liquid crystals and recently also for the glass transition physics. Experimental results extends from ∼TI-N+100 K down to Tg≈TI-N−100 K, where TI-N denotes the I-N phase transition temperature. The supporting derivative-based analysis enabled the distortion-sensitive tests of the validity of τ(T) parametrization using the Vogel–Fulcher–Tammann, Avramov model, and “critical-like” equations. Only the latter yielded the possibility of portraying τ(T) data using a single equation for the isotropic phase within the mode coupling theory and for the supercooled nematic phase within the dynamical scaling model. Issues related to the fragility of the system are also discussed. The existence of the hidden N-SmA transition, damped due to the earlier vitrification at the glass temperature (Tg), is speculatively suggested. For the isotropic phase the supplementary discussion of dynamics in 4′-n-butyl-4-cyanobiphenyl is presented.
Effective atomic orbitals for fuzzy atoms
Authors: Mayer I. , Salvador P.
Published: June 2009

The method of extracting effective atomic orbitals and effective minimal basis sets from molecular wave function characterizing the state of an atom in a molecule is developed in the framework of the “fuzzy” atoms. In all cases studied, there were as many effective orbitals that have considerable occupation numbers as orbitals in the classical minimal basis. That is considered to be of high conceptual importance.
Discharge parameters and dominant electron conductivity mechanism in a low-pressure planar magnetron discharge
Authors: Baranov O. , Romanov M. , Ostrikov Kostya (Ken)
Published: June 2009

Parameters of a discharge sustained in a planar magnetron configuration with crossed electric and magnetic fields are studied experimentally and numerically. By comparing the data obtained in the experiment with the results of calculations made using the proposed theoretical model, conclusion was made about the leading role of the turbulence-driven Bohm electron conductivity in the low-pressure operation mode (up to 1 Pa) of the discharge in crossed electric and magnetic fields. A strong dependence of the width of the cathode sputter trench, associated with the ionization region of the magnetron discharge, on the discharge parameters was observed in the experiments. The experimental data were used as input parameters in the discharge model that describes the motion of secondary electrons across the magnetic field in the ionization region and takes into account the classical, near-wall, and Bohm mechanisms of electron conductivity.
Simultaneous escaping of explicit and hidden free energy barriers: Application of the orthogonal space random walk strategy in generalized ensemble based conformational sampling
Authors: Zheng Lianqing , Chen Mengen , Yang Wei
Published: June 2009

To overcome the pseudoergodicity problem, conformational sampling can be accelerated via generalized ensemble methods, e.g., through the realization of random walks along prechosen collective variables, such as spatial order parameters, energy scaling parameters, or even system temperatures or pressures, etc. As usually observed, in generalized ensemble simulations, hidden barriers are likely to exist in the space perpendicular to the collective variable direction and these residual free energy barriers could greatly abolish the sampling efficiency. This sampling issue is particularly severe when the collective variable is defined in a low-dimension subset of the target system; then the “Hamiltonian lagging” problem, which reveals the fact that necessary structural relaxation falls behind the move of the collective variable, may be likely to occur. To overcome this problem in equilibrium conformational sampling, we adopted the orthogonal space random walk (OSRW) strategy, which was originally developed in the context of free energy simulation [L. Zheng, M. Chen, and W. Yang, Proc. Natl. Acad. Sci. U.S.A. 105, 20227 (2008)]. Thereby, generalized ensemble simulations can simultaneously escape both the explicit barriers along the collective variable direction and the hidden barriers that are strongly coupled with the collective variable move. As demonstrated in our model studies, the present OSRW based generalized ensemble treatments show improved sampling capability over the corresponding classical generalized ensemble treatments.
Surface enhanced Raman scattering of pyridine adsorbed on Au@Pd core/shell nanoparticles
Authors: Yang Zhilin , Li Yan , Li Zhipeng , Wu Deyin , Kang Junyong , Xu Hongxing , Sun Mengtao
Published: June 2009

Surface enhanced Raman scattering (SERS) of pyridine adsorbed on Au@Pd core/shell nanoparticles has been investigated theoretically with quantum chemical method, generalized Mie theory and three-dimensional finite-difference time domain (3D-FDTD) method. We first studied the influence of the coated Pd on the electronic structure of Au nanoparticle, and compared the electronic structure of Au20 cluster with that of Au10Pd10 (core/shell) cluster. Second, we studied SERS spectroscopy of pyridine on Au@Pd core/shell nanoparticles, which revealed the rate of static chemical enhancement and electromagnetic enhancement in the experimental reports. Third, the influence of the Pd shell thickness to the optical absorption of Au@Pd core/shell nanoparticles was investigated with generalized Mie theory. Fourth, we studied the influence of the shell thickness to the local electric field enhancement with 3D-FDTD method. The theoretical results reveal that the static chemical enhancement and electromagnetic enhancement are in the order of 10 and 103, respectively. These theoretical studies promote the deeper understanding of the electronic structure and optical absorption properties of Au@Pd, and the mechanisms for SERS of molecule adsorbed on Au@Pd.
Microphase separation induced by differential interactions in diblock copolymer/homopolymer blends
Authors: Zhou Jiajia , Shi An-Chang
Published: June 2009

Phase behavior of diblock copolymer/homopolymer blends (AB/C) is investigated theoretically. The study focuses on a special case where all three binary pairs, A/B, B/C, and C/A, are miscible. Despite the miscibility of the binary pairs, a closed-loop immiscible region exists in the AB/C blends when the A/C and B/C pair interactions are sufficiently different. Inside the closed-loop, the system undergoes microphase separation, exhibiting different ordered structures. This phenomenon is enhanced when the homopolymer (C) interacts more strongly to one of the blocks (A or B).
The preserved aromaticity of aniline molecules adsorbed on a Si(5 5 12)−2×1 surface
Authors: Jang Sang Hoon , Jeong Sukmin , Hahn Jae Ryang
Published: June 2009

We present a scanning tunneling microscopy and first-principles calculations study of the adsorption structures of aniline on a Si(5 5 12)−2×1 surface. Dissociation from the aniline molecules of one or two H atom(s) bonded to N is favored, and then adsorption onto adatom, tetramer, and dimer rows of Si(5 5 12)−2×1 occurs in several distinct configurations. On the adatom row, aniline binds to an adatom in a tilted configuration, which is formed via a σ bond between the adatom and N, with one dissociated H atom adsorbed on a nearby adatom. No further hydrogen dissociation occurs. On the tetramer and dimer rows, the structures with two dissociated hydrogens and upright configurations are the most stable. Aniline does not adsorb onto the honeycomb chains; this adsorption configuration has a low adsorption energy. In all the adsorption configurations of aniline on this surface, the molecule’s aromaticity is preserved. Thus Si–N bonding of aromatic amine molecules provides a strategy for the homogeneous aromatic functionalization of high index Si surfaces.
Shear and extensional deformation of droplets containing polymers and nanoparticles
Authors: Usta O. Berk , Perchak Dennis , Clarke Andrew , Yeomans Julia M. , Balazs Anna C.
Published: June 2009

We investigate the effects of polymer chains and nanoparticles on the deformation of a droplet in shear and extensional flow using computational modeling that accounts for both the solid and fluid phases explicitly. We show that under shear flow, both the nanoparticles and the encapsulated polymers reduce the shear-induced deformation of the droplet at intermediate capillary numbers. At high capillary numbers, however, long polymer chains can induce the breakup of the droplet. We find that the latter behavior is dependent on the nature of the imposed flow. Specifically, under extensional flow, long polymers inhibit the droplet breakup and reduce deformation. Overall, the findings provide guidelines for tailoring the stability of filled droplets under an imposed flow, and thus, the results can provide useful design rules in a range of technological applications.
The influence of dissipation on the quantum-classical correspondence: Stability of stochastic trajectories
Authors: Kryvohuz Maksym , Cao Jianshu
Published: June 2009

The quantum-classical correspondence in the presence of dissipation is studied. The semiclassical expression for the linear response function of an anharmonic system is expressed in a series containing classical stability matrix elements, which can diverge due to the chaotic behavior of stochastic trajectories. The presence of dissipation in most cases removes the divergence of higher-order correction terms, thus suppressing quantum effects and making the system more classical. The regime of system-bath coupling, which makes quantum dynamics completely classical, is obtained in terms of friction, temperature, and anharmonicity. Special cases when bath coupling may lead to enhancement of quantum effects are discussed.
Electronic states in Si nanocrystal thin films
Authors: Zhang Rong , Wu Hua , Chen Xinyi , Shen Wenzhong
Published: June 2009

We report on the investigation of electronic states in hydrogenated nanocrystalline silicon (nc-Si:H) thin films through the electronic transitions by photocurrent measurements. Higher photocurrent response has been observed above the bulk Si band gap of 1.05 eV in the nc-Si:H films with larger crystalline faction. We attribute the high photocurrent response to the enhancement of the photocarrier transport due to the formation of the extended electronic states and the direct electronic transition caused by the discrete states. The interaction of the extended states and discrete states has been further demonstrated by the Fano resonance observed in the Raman scattering spectra.
Orientation dependence of the Schottky barrier height for La0.6Sr0.4MnO3/SrTiO3 heterojunctions
Authors: Minohara M. , Furukawa Y. , Yasuhara R. , Kumigashira H. , Oshima M.
Published: June 2009

The authors report on the crystallographic orientation dependence of the Schottky properties for heterojunctions between a half-metallic ferromagnet La0.6Sr0.4MnO3 (LSMO) and Nb-doped SrTiO3 semiconductor. The Schottky barrier height determined by in situ photoemission measurements is independent for the substrate orientations (001) and (110), while the magnetic properties of LSMO (110) films are more enhanced than for (001) films. These results suggest that the performance of magnetic devices based on ferromagnetic manganite is improved by using (110)-oriented substrates.
Blinking suppression of colloidal CdSe/ZnS quantum dots by coupling to silver nanoprisms
Authors: Yuan C. T. , Yu Pyng , Tang Jau
Published: June 2009

Fluorescence blinking of quantum dots (QDs) is a common but annoying behavior for practical applications as fluorescent markers or single photon sources. We demonstrate here, by coupling colloidal CdSe/ZnS QDs to silver nanoprisms, fluorescence blinking behavior can be suppressed. For most of coupled QDs, two emissive states with different fluorescence intensities and lifetime were observed. We suggest that energy transfer could play an important role in suppressing blinking behavior.
Concentration fluctuations in fluid mixtures. II.
Authors: Mazo Robert M. , Matteoli Enrico , Smith Paul E.
Published: June 2009

The method developed in a previous paper [R. Mazo, J. Chem. Phys. 129, 154101 (2008)] for extracting information on the size of relative fluctuations in multicomponent systems is tested on ten binary systems and one ternary system. For the binary systems, it is found that the approximation works well for mole fractions in the range of 0.15–0.85 in most cases. For the ternary case, the method inherently yields less information and is valid only over a more restricted range for the case studied (chloroform-methanol-acetone). It is found that the predicted ratio of number fluctuations is approximately equal to the ratio of molar volumes of the components.
Adaptive control and synchronization in a class of partially unknown chaotic systems
Authors: Noroozi N. , Roopaei M. , Karimaghaee P.
Published: June 2009

This paper first addresses a novel control scheme to control a class of chaotic systems. In this method, no knowledge on the bounds of perturbations and disturbances is required in advance and parameters of the proposed controller are updated through an adaptive algorithm. Using the Lyapunov theory is employed to guarantee the stability of the closed loop system. Then synchronization of two nonidentical uncertain chaotic systems is investigated. To demonstrate the feasibility of the proposed scheme, numerical simulations on the application of control/synchronization of some famous chaotic systems are provided.
A mathematical model of the Leidenfrost effect on an axisymmetric droplet
Authors: Myers T. G. , Charpin J. P. F.
Published: June 2009

A simple mathematical model is developed for the Leidenfrost effect on an axisymmetric droplet. We first examine the practice of describing large droplets as cylindrical and small droplets as almost spherical, by solving the Young–Laplace equation. Best fit formulas are presented relating the droplet surface area to volume. The full model for Leidenfrost is then discussed. This shows that the calculation may be reduced to the solution of three first order ordinary differential equations involving the volume, evaporation rate, and vapor film thickness. Comparison with experimental results shows good agreement provided the model incorporates two components neglected in previous theoretical studies. First, a finite heat transfer coefficient between the vapor and substrate is required. Second evaporation from the upper surface of the droplet must be included, even for large droplets.
Modifications of direct transition energies in β-FeSi2 epitaxial films grown by molecular beam epitaxy
Authors: Noda K. , Terai Y. , Hashimoto S. , Yoneda K. , Fujiwara Y.
Published: June 2009

Modifications of direct transition energies by crystal lattice deformations were confirmed in β-FeSi2 epitaxial films on Si(111) substrates. With an increasing of annealing temperature (Ta), lattice constants of a-axis expanded, and those of b- and c-axis shrank, resulting in the volume reduction in −0.2%. In photoreflectance measurements, the direct transition energy at Y point in the Brillouin zone of β-FeSi2 (∼0.92 eV) was shifted to lower photon energy with the increase in Ta. These results revealed that the band-gap energy was modulated systematically by the lattice deformation, which suggests a possibility of band-gap engineering by the lattice deformation in β-FeSi2 epitaxial films on Si(111) substrates.
Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers
Authors: Zhao Hui , Chen Yu-Guang , Zhang Xu-Ming , An Zhong , Wu Chang-Qin
Published: June 2009

By employing an adaptive time-dependent density-matrix-renormalization-group method, we investigate the dynamics of a charged bipolaron in the presence of both electron-phonon and electron-electron interactions. We use a Su–Schrieffer–Heeger model modified to include electron-electron interactions via a Hubbard Hamiltonian, a Brazovskii–Kirova symmetry-breaking term, and an external electric field. Our results show that the velocity of the bipolaron increases first and then decreases with the increasing of the on-site Coulomb interaction, U. Furthermore, the dependence of the bipolaron velocity, bipolaron effective mass, and bipolaron stability on the lattice structures is discussed.
80 GHz field-effect transistors produced using high purity semiconducting single-walled carbon nanotubes
Authors: Nougaret L. , Happy H. , Dambrine G. , Derycke V. , Bourgoin J. -P. , Green A. A. , Hersam M. C.
Published: June 2009

This paper presents the high frequency performance of single-walled carbon nanotube (SWNT) field-effect transistors, with channel consisting of dense networks of high purity semiconducting SWNTs. Using SWNT samples containing 99% pure semiconducting SWNTs, we achieved operating frequencies above 80 GHz. This record frequency does not require aligned SWNTs, thus demonstrating the remarkable potential of random networks of sorted SWNTs for high frequency electronics.
On the adequacy of the Redfield equation and related approaches to the study of quantum dynamics in electronic energy transfer
Authors: Ishizaki Akihito , Fleming Graham R.
Published: June 2009

The observation of long-lived electronic coherence in photosynthetic excitation energy transfer (EET) by Engel et al. [Nature (London) 446, 782 (2007)] raises questions about the role of the protein environment in protecting this coherence and the significance of the quantum coherence in light harvesting efficiency. In this paper we explore the applicability of the Redfield equation in its full form, in the secular approximation and with neglect of the imaginary part of the relaxation terms for the study of these phenomena. We find that none of the methods can give a reliable picture of the role of the environment in photosynthetic EET. In particular the popular secular approximation (or the corresponding Lindblad equation) produces anomalous behavior in the incoherent transfer region leading to overestimation of the contribution of environment-assisted transfer. The full Redfield expression on the other hand produces environment-independent dynamics in the large reorganization energy region. A companion paper presents an improved approach, which corrects these deficiencies [A. Ishizaki and G. R. Fleming, J. Chem. Phys. 130, 234111 (2009)].
Unified treatment of quantum coherent and incoherent hopping dynamics in electronic energy transfer: Reduced hierarchy equation approach
Authors: Ishizaki Akihito , Fleming Graham R.
Published: June 2009

A new quantum dynamic equation for excitation energy transfer is developed which can describe quantum coherent wavelike motion and incoherent hopping in a unified manner. The developed equation reduces to the conventional Redfield theory and Förster theory in their respective limits of validity. In the regime of coherent wavelike motion, the equation predicts several times longer lifetime of electronic coherence between chromophores than does the conventional Redfield equation. Furthermore, we show quantum coherent motion can be observed even when reorganization energy is large in comparison to intersite electronic coupling (the Förster incoherent regime). In the region of small reorganization energy, slow fluctuation sustains longer-lived coherent oscillation, whereas the Markov approximation in the Redfield framework causes infinitely fast fluctuation and then collapses the quantum coherence. In the region of large reorganization energy, sluggish dissipation of reorganization energy increases the time electronic excitation stays above an energy barrier separating chromophores and thus prolongs delocalization over the chromophores.
On the determination of optimized, fully quadratic, coupled state quasidiabatic Hamiltonians for determining bound state vibronic spectra
Authors: Zhu Xiaolei , Yarkony David R.
Published: June 2009

The quasidiabatic, coupled electronic state, fully quadratic Hamiltonian (Hd), suitable for the simulation of spectra exhibiting strong vibronic couplings and constructed using a recently introduced pseudonormal equations approach, is studied. The flexibility inherent in the normal equations approach is shown to provide a robust means for (i) improving the accuracy of Hd, (ii) extending its domain of utility, and (iii) determining the limits of the fully quadratic model. The two lowest electronic states of pyrrolyl which are coupled by conical intersections are used as a test case. The requisite ab initio data are obtained from large multireference configuration interaction expansions comprised of 108.5×106 configuration state functions and based on polarized triple zeta quality atomic orbital bases.
Leakage current improvement of nitride-based light emitting diodes using CrN buffer layer and its vertical type application by chemical lift-off process
Authors: Fujii Katsushi , Lee Seogwoo , Ha Jun-Seok , Lee Hyun-Jae , Lee Hyo-Jong , Lee Sang-Hyun , Kato Takashi , Cho Meoung-Whan , Yao Takafumi
Published: June 2009

We report the electrical characteristics of vertical and lateral type light emitting diodes (LEDs) grown with CrN buffer layer. The LED with CrN buffer showed lower reverse leakage current than the reference sample grown with conventional low-temperature GaN buffer. It was also observed that the density of open core screw dislocation was smaller by one order of magnitude, which was thought to relate to the leakage current of devices. The vertical type LED fabricated by chemical etching of CrN buffer showed lower series resistance, lower turn-on voltage, and larger light output power than those of the conventional LEDs.
Spin dependent recombination in magnetic semiconductor
Authors: Tashpour Hojjat , Vesaghi M. A.
Published: June 2009

The effect of spin dependent recombination on the transport properties of magnetic semiconductors is investigated theoretically. In particular, for p-type direct band gap semiconductors, a theory based on classic Shockley equations is formulated. In this theory the density of spin and charge has been evaluated analytically by solving the diffusive transport equation and it is shown that the difference between recombination rates affect the lifetimes of spin and charge significantly. It is also demonstrated that a considerable spin charge coupling occur. Application of this theory to pure band to band recombination process is discussed.
Controlling the outcoupled power in a dual wavelength optically pumped semiconductor laser
Authors: Ongstad Andrew P. , Kaspi Ron , Tauke-Pedretti Anna , Chavez Joseph C. , Tilton Michael L. , Dente Gregory C.
Published: June 2009

We present a study of a dual wavelength optically pumped midinfrared semiconductor laser. In order to control the outcoupled power of each wavelength, the modal overlap between the adjacent gain regions was minimized while the pump absorbance for each gain region was balanced. The nominal power expectation, based solely on the absorbed power per gain region, was observed to be generally in good agreement with measurement. Improved power accounting can be accomplished if the waveguide loss and internal efficiency at each operational wavelength is known.
Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature
Authors: Ma Hong , Jin Zuanming , Ma Guohong , Liu Weiming , Hai Tang Sing
Published: June 2009

Excitation photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal was studied by time resolved pump-probe reflectivity technique at room temperature. The results show that spin relaxation time decreases monotonously. While with increasing excitation carrier density, the time constants increases initially then decreases after reaching a maximum value. Our experimental results reveal that both D’yakonov–Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)] and Elliot–Yafet [R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, Solid State Phys. 14, 1 (1963)] mechanisms dominate the spin relaxation process in CdTe crystal.
Calculation of resonant short-crested waves in deep water
Authors: Debiane Mohammed , Kharif Christian
Published: June 2009

Solving the problem of resonant short-crested waves is very challenging because the appearance of small divisors causes the classical perturbation methods to fail. From a numerical point of view, the case of resonant gravity short-crested waves has been studied, but as far as we know, there are very few results in the case of resonant capillary-gravity short-crested waves. In fact, to the best of our knowledge, the most related study which has been made is the one of Craig and Nicholls [SIAM J. Math. Anal. 32, 323 (2000)] who gave existence theorems for the case where the surface tension is supposed not to be too small. There is a need for such an investigation, and the work considered herein therefore provides a calculation technique and presents new results on resonant short-crested gravity-capillary waves. We overcome the technical problems associated with small divisors by using a method derived from Whitham’s variational formulation of the classical problem of short-crested waves. Whitham’s method is not modified in essence, but computations are carried out and organized to obtain a method that has been applied to series of cases demonstrating the robustness and flexibility of the approach. In particular, numerical solutions corresponding to three-dimensional Wilton ripples have been obtained. Moreover, these waves are also obtained for long wave configurations. This method is able to handle the case of small or zero surface tension, including the resonant cases, and works well very near the limiting two-dimensional cases.
Instability in gravity-driven flow over uneven surfaces
Authors: D’Alessio S. J. D. , Pascal J. P. , Jasmine H. A.
Published: June 2009

We consider the gravity-driven laminar flow of a shallow fluid layer down an uneven incline with the principal objective of investigating the effect of bottom topography and surface tension on the stability of the flow. The equations of motion are approximations to the Navier–Stokes equations which exploit the assumed relative shallowness of the fluid layer. Included in these equations are diffusive terms that are second order relative to the shallowness parameter. These terms, while small in magnitude, represent an important dependence of the flow dynamics on the variation in bottom topography and play a significant role in theoretically capturing important aspects of the flow. Some of the second-order terms include normal shear contributions, while others lead to a nonhydrostatic pressure distribution. The explicit dependence on the cross-stream coordinate is eliminated from the equations of motion by means of a weighted residual approach. The resulting mathematical formulation constitutes an extension of the modified integral-boundary-layer equations proposed by Ruyer-Quil and Manneville [Eur. Phys. J. B 15, 357 (2000)] for flows over even surfaces to flows over variable topography. A linear stability analysis of the steady flow is carried out by taking advantage of Floquet–Bloch theory. A numerical scheme is devised for solving the nonlinear governing equations and is used to calculate the evolution of the perturbed equilibrium flow. The simulations are used to confirm the analytical predictions and to investigate the interfacial wave structure. The bottom profile considered in this investigation corresponds to periodic undulations characterized by measures of wavelength and amplitude. Conclusions are drawn on the combined effect of bottom topography and surface tension.
Harmonic oscillator in presence of nonequilibrium environment
Authors: Chaudhuri Jyotipratim Ray , Chaudhury Pinaki , Chattopadhyay Sudip
Published: June 2009

Based on a microscopic Hamiltonian picture where the system is coupled with the nonequilibrium environment, comprising of a set of harmonic oscillators, the Langevin equation with proper microscopic specification of Langevin force is formulated analytically. In our case, the reservoir is perturbed by an external force, either executing rapid or showing periodic fluctuations, hence the reservoir is not in thermal equilibrium. In the presence of external fluctuating force, using Shapiro–Loginov procedure, we arrive at the linear coupled first order differential equations for the two-time correlations and examine the time evolution of the same considering the system as a simple harmonic oscillator. We study the stochastic resonance phenomena of a Kubo-type oscillator (assumed to be the system) when the bath is modulated by a periodic force. The result(s) obtained here is of general significance and can be used to analyze the signature of stochastic resonance.
Enhanced radiation resistance of nanocrystalline pyrochlore Gd2(Ti0.65Zr0.35)2O7
Authors: Zhang Jiaming , Lian Jie , Fuentes Antonio F. , Zhang Fuxiang , Lang Maik , Lu Fengyuan , Ewing Rodney C.
Published: June 2009

The radiation response of nanostructured materials is of great interest because of the potential of nanoscale materials design for mitigating radiation damage. We report a greatly enhanced resistance to radiation-induced amorphization in nanocrystalline Gd2(Ti0.65Zr0.35)2O7 at a particle size less than 20 nm while larger crystals with the size >100 nm are radiation sensitive. The grain size of pyrochlore, Gd2(Ti0.65Zr0.35)2O7, can be controlled by mechanical milling and subsequent thermal treatment (from 800 to 1500 °C), offering the possibility of designing pyrochlore materials at the nanoscale with enhanced performance for specific radiation environments.
Softer region at boundary of supercooled liquid–crystal in glassy fresnoite
Authors: Takahashi Yoshihiro , Osada Minoru , Masai Hirokazu , Fujiwara Takumi
Published: June 2009

The transition between the supercooled liquid (SCL) and crystal phases was investigated by in situ observation of boson peak in a glassy fresnoite (Ba2TiSi2O8) with an extremely high nucleation ability. The glassy fresnoite revealed anomalous boson behavior, in that the boson peak's full width at half maximum diverged at T/Tg∼1.2. This behavior suggests that an elastically softer region exists at the interface of the fresnoite nucleus/crystallite in the SCL state.
Anomalous capacitance-voltage characteristics of Al/Al-rich Al2O3/p-Si capacitors and their reconstruction
Authors: Liu Z. , Chen T. P. , Liu Y. , Yang M. , Wong J. I. , Cen Z. H. , Zhang S. , Li Y. B.
Published: June 2009

Anomalous capacitance-voltage (C-V) characteristics of Al/Al-rich Al2O3/p-Si capacitors have been observed. The measured C-V curves exhibit rolloffs and frequency dispersion in the accumulation region and voltage stresses cause both horizontal and vertical shifts of the C-V curves. These anomalous behaviors are mainly due to the large current conduction and the charge trapping in the Al-rich Al2O3 layer. The anomalous C-V characteristics have been reconstructed based on a four-element circuit model. With the reconstructed C-V curves, the capacitance of the Al-rich Al2O3 layer and the charging-induced flatband voltage shift can be determined.
Orientation dependent emission properties of columnar quantum dash laser structures
Authors: Hein S. , Podemski P. , Sęk G. , Misiewicz J. , Ridha P. , Fiore A. , Patriarche G. , Höfling S. , Forchel A.
Published: June 2009

InAs columnar quantum dash (CQDash) structures on (100) InP have been realized by gas source molecular beam epitaxy for stacking numbers of up to 24. Laser devices show low threshold current densities between 0.73 and 3.5 kA/cm2, dependent on the CQDash orientation within the cavity. Photoluminescence and electroluminescence measurements confirm a strong relationship between the polarization degree of the emission and the orientation of the CQDashes. Eventually, the polarization of the CQDash emission could be changed from predominantly transverse electric to transverse magnetic by simply altering the dash alignment relative to the light propagation axis.
Enhanced dielectric constants and shielding effectiveness of, uniformly dispersed, functionalized carbon nanotube composites
Authors: Park Sung Hoon , Thielemann Paul , Asbeck Peter , Bandaru Prabhakar R.
Published: June 2009

It was seen that composites constituted of functionalized single-walled nanotubes (SWNTs) dispersed in a reactive ethylene terpolymer (RET) matrix possess a complex dielectric permittivity an order of magnitude larger than composites composed of pristine SWNTs and two orders of magnitude larger than functionalized multiwalled nanotube-RET composites. We seek to understand such an enhancement, both in terms of uniform nanotube dispersion and through a parallel resistor-capacitor model. We subsequently show that the ac electrical conductivity is a good predictor of the electromagnetic interference shielding effectiveness of nanocomposites.
Monte Carlo simulations of air shielding effect on lubricant transfer at the head disk interface
Authors: Kubotera H. , Imamura T.
Published: June 2009

The lubricant transfer at the head disk interface of hard disk drives was investigated by the direct simulation Monte Carlo method. It was found that the air molecules between the disk and the slider have an effect of shielding the slider surface, preventing the evaporated lubricant molecules from reaching it. We propose a model that well describes the shielding effect and show that the shielding effect has distinct characteristics depending on the Knudsen number.
Single mode coupled optofluidic ring resonator dye lasers
Authors: Wu Xiang , Sun Yuze , Suter Jonathan D. , Fan Xudong
Published: June 2009

The authors demonstrate the single mode dye laser from a coupled optofluidic ring resonator that consists of a cylindrical ring resonator fused onto the inner surface of a thin-walled capillary ring resonator. The whispering gallery modes in each ring resonator interact strongly and utilize the Vernier effect to generate single mode laser emission. The lasing threshold is 3.0 μJ/mm2. The light can be coupled out through an optical taper in contact with the capillary.
Unipolar resistive switching characteristics of room temperature grown SnO2 thin films
Authors: Nagashima Kazuki , Yanagida Takeshi , Oka Keisuke , Kawai Tomoji
Published: June 2009

The resistive switching characteristics of room temperature grown SnO2 films were investigated by fabricating the metal-oxide-metal sandwich structures. The unipolar operation was found in all devices. Experiments, including the size and material dependencies of the top electrodes and the three terminal device structures, demonstrated the rupture and formation of conducting filaments near the anode. The Ohmic behavior was observed in both on- and off-states when using Au and Ti top electrodes, whereas the Schottky behavior was only found in the off-state for Pt. The analysis on the transport properties indicates the presence of insulative crystalline SnO2 near the anode in the off-state.