bluuebunny/crossref_metadata_2025 huggingface dataset:
The mechanism of paramagnetic NMR relaxation produced by Mn(II): Role of orthorhombic and fourth-order zero field splitting terms
Authors: Sharp Robert
Published: October 2008

Mn(II) is a spin-5/2 paramagnetic ion that mediates a characteristically large NMR paramagnetic relaxation enhancement (NMR-PRE) of nuclear spins in solution. In the range of high magnetic field strengths (above about 0.3 T), where the electronic Zeeman interaction provides the largest term of the electron spin Hamiltonian, NMR relaxation mechanism is well understood. In the lower field range, the physical picture is more complex because of the presence in the spin Hamiltonian of zero field splitting (ZFS) terms that are comparable to or greater than the Zeeman term. This work describes a systematic study of the relaxation mechanism in the low field range, particularly aspects involving the dependence of NMR-PRE on the orthorhombic (E) and fourth-order (aq(4), q=0,2,4) ZFS tensor components. It is shown that the fourfold (a4(4)) and twofold (a2(4)) fourth-order components exert large orientation-dependent influences on the NMR-PRE. Thus, fourth-order terms with magnitudes equal to only a few percent of the quadratic ZFS terms (D,E) produce large changes in the shape of the magnetic field profile of the PRE. Effects arising from the orthorhombic quadratic ZFS term (E) are much smaller than those of the fourth-order terms and can in most cases be neglected. However, effects due to a4(4) and a2(4) need to be included in simulations of low field data.
Density measurement of shock compressed foam using two-dimensional x-ray radiography
Authors: Le Pape Sebastien , Macphee Andrew , Hey Daniel , Patel Pravesh , Mackinnon Andrew , Key Mike , Pasley John , Wei Mingsheng , Chen Sophia , Ma Tammy , Beg Farhat , Alexander N. , Stephens Rich , Offerman Dustin , Link A. , Van-Woerkom Lynn , Freeman R.
Published: October 2008

We have used spherically bent quartz crystal to image a laser-generated shock in a foam medium. The foam targets had a density of 0.16 g/cm3 and thickness of 150 μm, an aluminum/copper pusher drove the shock. The experiment was performed at the Titan facility at Lawrence Livermore National Laboratory using a 2 ns, 250 J laser pulse to compress the foam target, and a short pulse (10 ps, 350 J) to generate a bright Ti Kα x-ray source at 4.5 keV to radiograph the shocked target. The crystal used gives a high resolution (∼20 μm) monochromatic image of the shock compressed foam.
On the origin of the large electron correlation contribution to the hyperpolarizabilities of some diacetylene rare gas compounds
Authors: Pluta Tadeusz , Avramopoulos Aggelos , Papadopoulos Manthos G. , Leszczynski Jerzy
Published: October 2008

A comprehensive study of the linear and nonlinear molecular optical properties of HRgC4H, where Rg=Ar,Kr,Xe, has been performed. Dynamical electron correlation effects were computed by employing the coupled cluster methodology. A large electron correlation contribution to the nonlinear properties of HArC4H has been revealed. This contribution decreases by increasing the atomic number of the inserted rare gas atom. In order to interpret the origin of this noteworthy property, the complete active space self-consistent field method was employed. We have performed a systematic study of the linear and nonlinear electric properties by modifying the active space. The calculations have shown the significant contribution of the doubly excited σ*2 configuration and a negligible contribution of π*2. A quite remarkable discrepancy between numerically and analytically evaluated hyperpolarizabilities has also been observed for HArC4H. This was attributed to the contribution of near degenerate states.
Forced chemical mixing in model immiscible systems under plastic deformation
Authors: Delogu Francesco
Published: October 2008

Molecular dynamics has been employed to investigate forced chemical mixing in binary immiscible systems induced by plastic deformation. Four X matrix-Y precipitate model systems with thermodynamic and structural features almost identical to the ones of Ag–Cu solid solutions but different mechanical properties were generated. With the positive enthalpy of mixing roughly constant, mixing is shown to depend on the precipitate size as well as on the difference between X and Y tetragonal shear moduli.
Diagnostics and growth control of single-walled carbon nanotube forests using a telecentric optical system for in situ height monitoring
Authors: Yasuda Satoshi , Futaba Don. N. , Yumura Motoo , Iijima Sumio , Hata Kenji
Published: October 2008

An in situ telecentric height monitoring system was employed as a standard characterization tool for in situ height monitoring of nanotube forests. We demonstrated that the system possesses a wide dynamic range beyond the centimeter scale, high resolution of 1 μm, free from routine maintenance, simple installation, and use in contrast to previous methods. These features were highlighted by the monitoring of a 5 mm tall forest, the examination of the effect of gas species on growth rates, and the automatic control forest height.
Lower spectral branches of a spin-boson model
Authors: Angelescu Nicolae , Minlos Robert A. , Ruiz Jean , Zagrebnov Valentin A.
Published: October 2008

We study the structure of the spectrum of a two-level quantum system weakly coupled to a boson field (spin-boson model). Our analysis allows to avoid the cutoff in the number of bosons, if their spectrum is bounded below by a positive constant. We show that, for small coupling constant, the lower part of the spectrum of the spin-boson Hamiltonian contains (one or two) isolated eigenvalues and (one or two) manifolds of atom +1-boson states indexed by the boson momentum q. The dispersion laws and generalized eigenfunctions of the latter are calculated.
Nonlinear characteristics of the hysteretic magnetoresistance of a hybrid nanomagnetic field-effect transistor
Authors: Bae J.-U. , Lin T.-Y. , Reno J. L. , Bird J. P.
Published: October 2008

We analyze the influence of applied source-drain bias (Vsd) on the magnetoresistance (MR) of a field-effect transistor (FET) whose gate is formed by a nanoscale magnet. Using an external magnetic field to modulate the fringing magnetic fields that emanate into the channel of the hybrid FET from its gate, we observe a strongly hysteretic MR that is suppressed by the application of Vsd. Our analysis suggests that the effect of Vsd is to reduce the effective barrier in the channel and that the tunneling/activated MR is quenched due to the associated increase in carrier transmission.
Real space method for the electronic structure of one-dimensional periodic systems
Authors: Han Jiaxin , Tiago Murilo L. , Chan T.-L. , Chelikowsky James R.
Published: October 2008

We present a real space pseudopotential method for calculating the electronic structure of one-dimensional periodic systems such as nanowires. As an application of this method, we examine H-passivated Si nanowires. The band structure and heat of formation of the Si nanowires are presented and compared to plane wave methods. Our method is able to offer the same accuracy as the traditional plane wave methods but offers a number of computational advantages such as faster convergence for heteropolar nanowires.
Electron-induced surface reactivity modification in Zinc oxide-based thin films
Authors: Sabayev V. , Aronov D. , Oster L. , Rosenman G.
Published: October 2008

Chemical surface reactivity is a key parameter in modern microelectronic and display technology that is defined by basic physical interactions at the liquid etcher/material surface interface. We apply recently developed low-energy electron irradiation method for surface modification of zinc oxide-based thin films affording to vary physical processes at the liquid agent/material surface interface/and widely tune its chemical reactivity. Electron irradiation leads to the formation of ultrathin layer on irradiated surface, without generation of volumetric defects, and preserves original optical and conductive properties. The method allows fabrication of high-resolution patterned templates with modified chemical etching resistance for the fabrication of three-dimensional patterned arrays.
Multibody scattering, correlation, molecular conduction, and the 0.7 anomaly
Authors: Subotnik Joseph E. , Nitzan Abraham
Published: October 2008

We describe a new grid-based (or localized orbital-based) method for treating the effects of exchange and correlation on electronic transmission through a molecular target where there are initially other bound electrons. Our algorithm combines the approaches of (i) solid-state grid-based algorithms using self-energies and (ii) the complex Kohn method from electron-molecule scattering. For the general problem of a molecular target with n-electrons, our algorithm should ideally solve for electronic transmission with a computational cost scaling as n2, although the present implementation is limited to one-dimensional problems. In this paper, we implement our algorithm to solve three one-dimensional model problems involving two electrons: (i) Single-channel resonant transmission through a double-barrier well (DBW), where the target already contains one bound-state electron [Rejec et al., Phys. Rev. B 67, 075311 (2003)]; (ii) multichannel resonant transmission through a DBW, where the incoming electron can exchange energy with the bound electron; (iii) transmission through a triple-barrier well (TBW), where the incoming electron can knock forward the bound electron, yielding a physical model of electron-assisted electron transfer. This article offers some insight about the role and size of exchange and correlation effects in molecular conduction, where few such rigorous calculations have yet been made. Such multibody effects have already been experimentally identified in mesoscopic electron transport, giving rise to the “0.7 anomaly,” whereby electrons traveling through a narrow channel pair up as singlets and triplets. We expect the effect of electronic correlation to be even more visible for conduction through molecules, where electrons should partially localize into bonding and antibonding orbitals.
On the divine clockwork: The spectral gap for the correspondence limit of the Nelson diffusion generator for the atomic elliptic state
Authors: Durran Richard , Neate Andrew , Truman Aubrey , Wang Feng-Yu
Published: October 2008

The correspondence limit of the atomic elliptic state in three dimensions is discussed in terms of Nelson’s stochastic mechanics. In previous work we have shown that this approach leads to a limiting Nelson diffusion, and here we discuss in detail the invariant measure for this process and show that it is concentrated on the Kepler ellipse in the plane z=0. We then show that the limiting Nelson diffusion generator has a spectral gap; thereby proving that in the infinite time limit the density for the limiting Nelson diffusion will converge to its invariant measure. We also include a summary of the Cheeger and Poincaré inequalities, both of which are used in our proof of the existence of the spectral gap.
High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap
Authors: Neufeld Carl J. , Toledo Nikholas G. , Cruz Samantha C. , Iza Michael , DenBaars Steven P. , Mishra Umesh K.
Published: October 2008

We report on III-nitride photovoltaic cells with external quantum efficiency as high as 63%. InxGa1−xN/GaN p-i-n double heterojunction solar cells are grown by metal-organic chemical vapor deposition on (0001) sapphire substrates with xIn=12%. A reciprocal space map of the epitaxial structure showed that the InGaN was coherently strained to the GaN buffer. The solar cells have a fill factor of 75%, short circuit current density of 4.2 mA/cm2, and open circuit voltage of 1.81 V under concentrated AM0 illumination. It was observed that the external quantum efficiency can be improved by optimizing the top contact grid.
Terahertz heterodyne receiver with quantum cascade laser and hot electron bolometer mixer in a pulse tube cooler
Authors: Richter H. , Semenov A. D. , Pavlov S. G. , Mahler L. , Tredicucci A. , Beere H. E. , Ritchie D. A. , Il’in K. S. , Siegel M. , Hübers H.-W.
Published: October 2008

A liquid cryogen-free terahertz heterodyne receiver in a pulse tube cooler has been realized. The receiver operates at 2.5 THz. It is based on a quantum cascade laser (QCL) as local oscillator and a hot electron bolometric mixer. A detailed study of the QCL beam quality yielded a beam propagation factor of 1.1–1.2. The double sideband noise temperature of the system is 2000 K and when corrected for optical losses in the signal path it is ∼800 K.
Terahertz surface plasmon polaritons on a helically grooved wire
Authors: Fernández-Domínguez A. I. , Williams C. R. , García-Vidal F. J. , Martín-Moreno L. , Andrews S. R. , Maier S. A.
Published: October 2008

The propagation of tightly confined terahertz surface plasmon polaritons on a helically grooved metal wire has been studied both experimentally and theoretically. The periodic structure of such metawires allows the formation of surface electromagnetic modes with different azimuthal symmetry at wavelengths longer than the pitch. There is good agreement between the experimentally determined band edge frequencies of these modes and numerical calculations for the structure investigated.
Interface as the origin of ferromagnetism in cobalt doped ZnO film grown on silicon substrate
Authors: Yin Z. G. , Chen N. F. , Li Y. , Zhang X. W. , Bai Y. M. , Chai C. L. , Xie Y. N. , Zhang J.
Published: October 2008

We have investigated the magnetic properties of Co-doped zinc oxide (ZnO) film deposited on silicon substrate by magnetron sputtering. Co ions have a valence of 2+ and substitute for Zn sites in the lattice. By using a chemical etching method, an extrinsic ferromagnetism was demonstrated. The observed ferromagnetism is neither associated with magnetic precipitates nor with contamination, but originates from the silicon/silicon oxide interface. This interface ferromagnetism is characterized by being temperature independent and by having a parallel magnetic anisotropy.
Thermodynamical and structural properties of binary mixtures of imidazolium chloride ionic liquids and alcohols from molecular simulation
Authors: Raabe Gabriele , Köhler Jürgen
Published: October 2008

We have performed molecular dynamics simulations to determine the densities, excess energies of mixing, and structural properties of binary mixtures of the 1-alkyl-3-methylimidazolium chloride ionic liquids (ILs) [amim][Cl] and ethanol and 1-propanol in the temperature range from 298.15to363.15K. As in our previous work [J. Chem. Phys. 128, 154509 (2008)], our simulation studies are based on a united atom model from Liu et al. [Phys. Chem. Chem. Phys. 8, 1096 (2006)] for the 1-ethyl- and 1-butyl-3-methylimidazolium cations [emim+] and [bmim+], which we have extended to the 1-hexyl-3-methylimidazolium [hmim+] cation and combined with parameters of Canongia Lopes et al. [J. Phys. Chem. B 108, 2038 (2004)] for the chloride anion [Cl−] and the force field by Khare et al. for the alcohols [J. Phys. Chem. B 108, 10071 (2004)]. With this, we provide both prediction for the densities of the mixtures that have mostly not been investigated experimentally yet and a molecular picture of the interactions between the alcohol molecules and the ions. The negative excess energies of all mixtures indicate an energetically favorable mixing of [amim][Cl] ILs and alcohols. To gain insight into the nonideality of the mixtures on the molecular level, we analyzed their local structures by radial and spatial distribution functions. These analyses show that the local ordering in these mixtures is determined by strong hydrogen-bond interactions between the chloride anion and the hydroxyls of the alcohols, enhanced interactions between the anion and the charged domain of the cation, and an increasing aggregation of the nonpolar alkyl tails of the alcohols and the cations with increasing cation size, which results in a segregation of polar and nonpolar domains.
Experimental interrogation of the multidimensional He+ICl(E,v†) and He+ICl(β,v†) intermolecular potential energy surfaces
Authors: Darr Joshua P. , Loomis Richard A.
Published: October 2008

Resonant two-photon excitation of the T-shaped and linear He⋯IC35l(X,v″=0) complexes is used to access the intermolecular vibrational levels bound within the He+ICl(β,v†=0–2) and He+ICl(E,v†=11,12) intermolecular potentials. The excitation utilizes different metastable intermolecular vibrational levels within the He+ICl(A,v′=15) and He+ICl(B,v′=2,3) potentials to access levels with varying intermolecular vibrational excitation in the ion-pair states. In addition to providing data revealing properties of the He+ICl(E,v†) and He+ICl(β,v†) potentials, the transition energies of the observed features permit the relative binding energies of the T-shaped and linear ground-state He⋯ICl(X,v″=0) conformers to be accurately measured. The binding energies of the T-shaped and linear He⋯IC35l(X,v″=0) conformers are 16.6(3) and 22.0(2) cm−1, respectively. These values and the observed transition energies are then used to set the binding energies of the T-shaped He⋯IC35l complexes in the He+ICl(A,v′=15), He+ICl(B,v′=3), He+ICl(β,v†=1), and He+ICl(E,v†=12) potentials as 13.4(3), 13.3(3), 41(1), and 39.2(4) cm−1, respectively. Nonadiabatic coupling between specific intermolecular vibrational levels within the He+ICl(β,v†) state and the ICl(D′,v†) molecular state is observed.
Determination of rain age via γ rays from accreted radon progeny
Authors: Greenfield M. B. , Ito N. , Iwata A. , Kubo K. , Ishigaki M. , Komura K.
Published: October 2008

The relative γ ray activities from P214b and B214i condensed from precipitation are used to determine its “age,” the average time the accreted activity has been removed from secular equilibrium. A verifiable assumption that radon progeny on/in the surface/volume of droplets mostly remains in secular equilibrium until they begin their descent, enables estimates of their transit times to ground of typically a few tens of minutes. This agrees well with the time expected for the activity on the surface of droplets to reach the ground from heights of a few kilometers. The half lives of γ activities from B214i and P214b, 19.7 and 26.9 min, respectively, are on the same scale as transit time to ground and close enough to each other to measure ratios of activities from secular equilibrium (1.00) to transient equilibrium (3.88) within a few hundreds of minutes. The ratio of γ count rates is independent of knowledge of either initial activity or any systematic errors and thus limited only by the uncertainty from counting statistics, which from condensates of 5–30 l of rain viewed with 2π solid angle by a 50% efficient, high-resolution Ge detector is only a few percent. These ratios fit extremely well to known theoretical curves, which cannot only be used to date rain but can also be extrapolated backward to determine radon progeny activities in rain prior to its descent, knowledge of which may facilitate further studies using radon progeny as tracers.
Estimating the rate constant of cyclic GMP hydrolysis by activated phosphodiesterase in photoreceptors
Authors: Reingruber Jürgen , Holcman David
Published: October 2008

The early steps of light response occur in the outer segment of rod and cone photoreceptor. They involve the hydrolysis of cGMP, a soluble cyclic nucleotide, that gates ionic channels located in the outer segment membrane. We shall study here the rate by which cGMP is hydrolyzed by activated phosphodiesterase (PDE). This process has been characterized experimentally by two different rate constants βd and βsub: βd accounts for the effect of all spontaneously active PDE in the outer segment, and βsub characterizes cGMP hydrolysis induced by a single light-activated PDE. So far, no attempt has been made to derive the experimental values of βd and βsub from a theoretical model, which is the goal of this work. Using a model of diffusion in the confined rod geometry, we derive analytical expressions for βd and βsub by calculating the flux of cGMP molecules to an activated PDE site. We obtain the dependency of these rate constants as a function of the outer segment geometry, the PDE activation and deactivation rates and the aqueous cGMP diffusion constant. Our formulas show good agreement with experimental measurements. Finally, we use our derivation to model the time course of the cGMP concentration in a transversally well-stirred outer segment.
Molecular mechanism of pressure denaturation of proteins
Authors: Harano Yuichi , Yoshidome Takashi , Kinoshita Masahiro
Published: October 2008

We investigate the molecular mechanism of pressure denaturation of proteins using the angle-dependent integral equation theory combined with the multipole water model and the morphometric approach. We argue that the hydration entropy of a protein is the key quantity. It is verified that at an elevated pressure, a swelling structure—which has only moderately less compact than the native structure but has a much larger water-accessible surface area—turns more stable than the native structure in terms of the water entropy. The swelling structure is characterized by the penetration of water into the interior. The hydration entropy is decomposed into contributions from the translational and rotational restrictions for the molecular motions of water. Each contribution is further decomposed into the water-protein pair correlation component and the water-water-protein triplet and higher-order correlation components. The pair correlation component in the translational contribution is divided into two terms arising from the excluded volume and the water structure near the protein, respectively. It is found that pressure denaturation accompanies a loss of the translational and rotational entropies at the pair correlation level but a much larger gain of the translational entropy at the triplet and higher-order correlation levels. Although the translational and rotational motions of water molecules penetrating the protein interior and contacting the protein surface are constrained, the translational restriction for the water molecules well outside the protein is greatly reduced. The latter entropic gain dominates, leading to the denaturation.
A Langevin equation approach to electron transfer reactions in the diabatic basis
Authors: Song XiaoGeng , Wang Haobin , Van Voorhis Troy
Published: October 2008

A linear Langevin equation that governs the population dynamics of electron transfer reactions is derived. The noise in the Langevin equation is eliminated by treating the diabatic population fluctuations as the relevant variables, leaving only the memory kernel responsible for the population relaxation. Within the memory kernel, the diabatic coupling is treated perturbatively and a second order expansion is found to give a simple closed form expression for the kernel. The accuracy of the second order truncation is maximized by performing a fixed rotation of the diabatic electronic states that minimizes the first order free energy of the system and thus minimizes the effect of the perturbation on the thermodynamics. The resulting two-hop Langevin equation (THLE) is then validated by applying it to a simple spin-boson model, where exact results exist. Excellent agreement is found in a wide parameter range, even where the perturbation is moderately strong. Results obtained in the rotated electronic basis are found to be consistently more accurate than those from the unrotated basis. These benchmark calculations also allow us to demonstrate the advantage of treating the population fluctuations instead of the populations as the relevant variables, as only the former lead to reliable results at long time. Thus, the THLE appears to provide a viable alternative to established methods—such as Ehrenfest dynamics or surface hopping—for the treatment of nonadiabatic effects in electron transfer simulations.
An ab initio calculation of the vibronic energy levels of the X Π2 and 1 Δ2 electronic states of C2P
Authors: Jerosimić Stanka , Perić Miljenko
Published: October 2008

In the present study, the results of an ab initio calculation of the vibronic energy levels in the X Π2 and 1 Δ2 electronic states of C2P are reported. This work is motivated by recent measurements carried out by [Sunahori et al. J. Chem. Phys. 128, 244311 (2008)]. The vertical electronic spectrum, excitation energies, bending potential curves, and spin-orbit constants for the title molecule are computed by means of the state-average complete active space self-consistent field and multireference configuration interaction approach. Vibronic energy levels of the X Π2 and 1 Δ2 states are calculated with the help of a simple, effectively one-dimensional model. The results of the present study strongly support the analysis of experimental data by Sunahori et al. and offer reliable predictions for experimental searches for heretofore unobserved electronic states.
Rayleigh–Taylor instability in cylindrical geometry with compressible fluids
Authors: Yu Huidan , Livescu Daniel
Published: October 2008

A linear stability analysis of the Rayleigh–Taylor instability (RTI) between two ideal inviscid immiscible compressible fluids in cylindrical geometry is performed. Three-dimensional (3D) cylindrical as well as two-dimensional (2D) axisymmetric and circular unperturbed interfaces are considered and compared to the Cartesian cases with planar interface. Focuses are on the effects of compressibility, geometry, and differences between the convergent (gravity acting inward) and divergent (gravity acting outward) cases on the early instability growth. Compressibility can be characterized by two independent parameters—a static Mach number based on the isothermal sound speed and the ratio of specific heats. For a steady initial unperturbed state, these have opposite influence, stabilization and destabilization, on the instability growth, similar to the Cartesian case [D. Livescu, Phys. Fluids 16, 118 (2004)]. The instability is found to grow faster in the 3D cylindrical than in the Cartesian case in the convergent configuration but slower in the divergent configuration. In general, the direction of gravity has a profound influence in the cylindrical cases but marginal for planar interface. For the 3D cylindrical case, instability grows faster in the convergent than in the divergent arrangement. Similar results are obtained for the 2D axisymmetric case. However, as the flow transitions from the 3D cylindrical to the 2D circular case, the results above can be qualitatively different depending on the Atwood number, interface radius, and compressibility parameters. Thus, 2D circular calculations of RTI growth do not seem to be a good model for the fully 3D cylindrical case.
Solvent effects on optical properties of molecules: A combined time-dependent density functional theory/effective fragment potential approach
Authors: Yoo Soohaeng , Zahariev Federico , Sok Sarom , Gordon Mark S.
Published: October 2008

A quantum mechanics/molecular mechanics (QM/MM) type of scheme is employed to calculate the solvent-induced shifts of molecular electronic excitations. The effective fragment potential (EFP) method was used for the classical potential. Since EFP has a density dependent functional form, in contrast with most other MM potentials, time-dependent density functional theory (TDDFT) has been modified to combine TDDFT with EFP. This new method is then used to perform a hybrid QM/MM molecular dynamics simulation to generate a simulated spectrum of the n→π∗ vertical excitation energy of acetone in vacuum and with 100 water molecules. The calculated water solvent effect on the vertical excitation energy exhibits a blueshift of the n→π∗ vertical excitation energy in acetone (Δω1=0.211 eV), which is in good agreement with the experimental blueshift.
The relative entropy is fundamental to multiscale and inverse thermodynamic problems
Authors: Shell M. Scott
Published: October 2008

We show that the relative entropy, Srel≡∑pT ln(pT/pM), provides a fundamental and unifying framework for multiscale analysis and for inverse molecular-thermodynamic problems involving optimization of a model system (M) to reproduce the properties of a target one (T). We demonstrate that the relative entropy serves as a generating function for principles in variational mean-field theory and uniqueness and gives intuitive results for simple case scenarios in model development. Moreover, we suggest that the relative entropy provides a rigorous framework for multiscale simulations and offers new numerical techniques for linking models at different scales. Finally, we show that Srel carries physical significance by using it to quantify the deviations of a three-site model of water from simple liquids, finding that the relative entropy, a thermodynamic concept, even predicts water’s kinetic anomalies.
Critical mass and a new periodic four-ring vortex wake mode for freely rising and falling spheres
Authors: Horowitz M. , Williamson C. H. K.
Published: October 2008

In this letter, we study the dynamics and vortex wakes of spheres rising or falling freely through a fluid. Since this problem was first considered by Newton in 1726, the conditions under which a sphere will vibrate are still not understood clearly. In our experiments, all falling spheres (where the relative density, m∗>1) descend rectilinearly. Although previous studies conclude that all rising spheres (m∗<1) vibrate, we find instead that there exists a critical value of the relative density (for example, mcrit∗=0.36, for Reynolds numbers 400–500) above which there is a significant regime where rising spheres do not vibrate. Lighter spheres undergo large-amplitude periodic oscillations, confined to a single vertical plane. We discover a new mode of vortex formation comprising four vortex rings formed in each cycle, distinct from previous vortex modes for fixed and tethered bodies.
Influence of swirl on the initial merging zone of a turbulent annular jet
Authors: Vanierschot M. , Van den Bulck E.
Published: October 2008

This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S=0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of the CRZ significantly, increasing its complexity. The CRZ becomes toroidal and the jet fluid is entrained near the apex. At intermediate swirl numbers a vortex breakdown bubble appears downstream which moves upstream with increasing swirl. At high swirl, the CRZ and breakdown bubble merge which creates a complex and highly anisotropic flow field.
Fabrication and electrical characterization of Si-based rolled-up microtubes
Authors: Cavallo F. , Songmuang R. , Schmidt O. G.
Published: October 2008

Freestanding SiGe:B∕Si:B tubes are fabricated by rolled-up technology. Linear I-V curves are measured both for unreleased and rolled-up films. The bilayer resistance increases after release from the substrate. The electrical resistance of tubes with diameters varying in the range of 0.8−2.2μm, scales from 110to9kΩ as a function of bilayer thickness. Rapid thermal annealing is used to investigate the effect of B activation and Si–Ge interdiffusion on structural and electrical properties of unreleased and rolled-up films.
Ice-water and liquid-vapor phase transitions by a Ginzburg–Landau model
Authors: Fabrizio Mauro
Published: October 2008

A model for the first order phase transitions as ice-water and liquid-vapor is proposed using the Ginzburg–Landau equation for the order parameter φ. In this model the density ρ is composed of two quantities ρ0 and ρ1 such that 1/ρ=1/ρ0+1/ρ1, where ρ1 is strictly connected to the order parameter φ. By means of this decomposition, we are able to represent the Andrew diagram without the use of the heuristic van der Waals equation.
Reduction of nonhomogeneous quasilinear 2×2 systems to homogeneous and autonomous form
Authors: Currò Carmela , Oliveri Francesco
Published: October 2008

By using the invariance with respect to suitable Lie groups of point transformations, necessary and sufficient conditions are determined allowing one to map through an invertible point transformation nonhomogeneous and nonautonomous quasilinear 2×2 systems to homogeneous and autonomous form. The procedure is constructive and the new independent and dependent variables are obtained by determining the canonical variables associated with the Lie groups of point symmetries admitted by the source system. Various examples of physical interest arising from different contexts are considered.
Ray-based calculations of backscatter in laser fusion targets
Authors: Strozzi D. J. , Williams E. A. , Hinkel D. E. , Froula D. H. , London R. A. , Callahan D. A.
Published: October 2008

A one-dimensional, steady-state model for Brillouin and Raman backscatter from an inhomogeneous plasma is presented. The daughter plasma waves are treated in the strong damping limit, and have amplitudes given by the (linear) kinetic response to the ponderomotive drive. Pump depletion, inverse-bremsstrahlung damping, bremsstrahlung emission, Thomson scattering off density fluctuations, and whole-beam focusing are included. The numerical code DEPLETE, which implements this model, is described. The model is compared with traditional linear gain calculations, as well as “plane-wave” simulations with the paraxial propagation code PF3D. Comparisons with Brillouin-scattering experiments at the OMEGA Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] show that laser speckles greatly enhance the reflectivity over the DEPLETE results. An approximate upper bound on this enhancement, motivated by phase conjugation, is given by doubling the DEPLETE coupling coefficient. Analysis with DEPLETE of an ignition design for the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], with a peak radiation temperature of 285eV, shows encouragingly low reflectivity. Re-absorption of Raman light is seen to be significant in this design.
Correlation between the glass-rubber transition and ionic conductivity in poly(3-hexylthiophene)
Authors: Lada M.
Published: October 2008

Using impedance spectroscopy and metal-insulator-semiconductor structures, the small-signal bulk conductivity of annealed poly(3-hexylthiophene) (P3HT) has been extracted in the temperature range of 150–420K. A faster-than-Arrhenius increase in the conductivity of P3HT observed near and above room temperature is shown to be a result of the glass-rubber transition and is closely related to the amorphous phase of the polymer. The super-Arrhenius conductivity can be modeled and interpreted as ionic, arising from the thermal motion of the polymer segments. In addition, a percolative conduction mechanism induced by the glass transition can phenomenologically model the conductivity increase.
Freezing of fluids in disordered mesopores
Authors: Dvoyashkin Muslim , Khokhlov Alexey , Valiullin Rustem , Kärger Jörg
Published: October 2008

Freezing and melting behaviors of a fluid confined to pores of mesoporous silicon with a modulated structure have been studied using NMR techniques. The molecular self-diffusivities, measured along the freezing and melting transitions, unveiled essential differences in the configuration of the frozen domains. This suggests that freezing is dominated by a pore-blocking mechanism. Freezing kinetics is found to exhibit very slow long-time dynamics, following a ln2(t) dependence. This type of time dependence may result if the front of the frozen phase is assumed to propagate in the random potential field created by the disorder of the porous silicon channels, similar to the mechanism of Sinai diffusion. The free energy barriers calculated from the kinetic measurements and estimated using a thermodynamical model yield a consistent picture of the freezing process in the presence of quenched disorder.
Self-assembled molecular monolayers as ultrathin gate dielectric in carbon nanotube transistors
Authors: Robert Gaël , Derycke Vincent , Goffman Marcelo F. , Lenfant Stéphane , Vuillaume Dominique , Bourgoin Jean-Philippe
Published: October 2008

We demonstrate the use of a self-assembled monolayer of octadecanethiol on gold as thin gate dielectric for a single-walled carbon nanotube field-effect transistor. P-type transistors display very steep subthreshold slopes, greatly reduced hysteresis, and band-to-band tunneling. The suppression of the gate efficiency for n-type transistors emphasizes the key role of the electrical dipole of the molecular layer in controlling the switching. Combining the versatility of organic dielectrics with the exceptional electronic and mechanical properties of carbon nanotubes opens interesting ways toward the realization of fully organic nanoscale transistors.
Recognition of biomolecular interactions by plasmon resonance shifts in single- and multicomponent magnetic nanoparticles
Authors: Schotter Joerg , Bethge Ole , Maier Thomas , Brueckl Hubert
Published: October 2008

Composite biomarkers open prospects to combine the targeting advantages of magnetic nanoparticles with direct plasmon-based optical detection of biomolecular interactions. Although nanoparticles from ferromagnetic 3d-transition metals could be suitable for such a task, they are shown to be rather large, thus tending to agglomerate in aqueous suspensions. A superior alternative uses composite nanoparticles consisting of a superparamagnetic core and a noble metal shell. Systematic Mie-theory based calculations of their plasmon peak shifts and sensitivity against biomolecular binding events on their surfaces are presented for this hybrid particle class.
Large magnetic circular dichroism of Co clusters in Co-doped ZnO
Authors: Fukuma Y. , Asada H. , Yamamoto J. , Odawara F. , Koyanagi T.
Published: October 2008

Large magnetic circular dichroism (MCD) in Co-doped ZnO films is reported. The Co-(∼20%) doped ZnO films with oxygen deficiencies show ferromagnetic behavior at room temperature. While paramagnetic films exhibit a small MCD signal, broad MCD spectra with large amplitude of ∼20kdeg∕cm are observed for the ferromagnetic films. In the ferromagnetic films, Co cluster formation occurs, especially near the interface between the substrate and the film. The amount of the Co clusters is consistent with the spontaneous magnetization and the amplitude of the MCD.
Influence of reaction with XeF2 on surface adhesion of Al and Al2O3 surfaces
Authors: Zhang Tianfu , Park Jeong Y. , Huang Wenyu , Somorjai Gabor A.
Published: October 2008

The change in surface adhesion after fluorination of Al and Al2O3 surfaces using XeF2 was investigated with atomic force microscopy. The chemical interaction between XeF2 and Al and Al2O3 surfaces was studied by in situ x-ray photoelectron spectroscopy. Fresh Al and Al2O3 surfaces were obtained by etching top silicon layers of Si∕Al and Si∕Al2O3 with XeF2. The surface adhesion and chemical composition were measured after the exposure to air or annealing (at 200°C under vacuum). The correlation between the adhesion force increase and presence of AlF3 on the surface was revealed.
Adiabatically switched-on electrical bias and the Landauer–Büttiker formula
Authors: Cornean H. D. , Duclos P. , Nenciu G. , Purice R.
Published: October 2008

Consider a three dimensional system which looks like a cross connected pipe system, i.e., a small sample coupled to a finite number of leads. We investigate the current running through this system, in the linear response regime, when we adiabatically turn on an electrical bias between leads. The main technical tool is the use of a finite volume regularization, which allows us to define the current coming out of a lead as the time derivative of its charge. We finally prove that in virtually all physically interesting situations, the conductivity tensor is given by a Landauer–Büttiker type formula.
Blob dynamics in an inhomogeneous plasma
Authors: Bodi K. , Krasheninnikov S. I. , Smolyakov A. I.
Published: October 2008

Convective blob propagation in the scrape-off-layer and/or limiter shadow region is analyzed analytically and numerically with emphasis on effects of a gradient in the equilibrium plasma density. The gradient of the equilibrium plasma density is taken into account beyond the Boussineque approximation. It is shown that the vorticity modification due to the plasma density gradient leads to the acceleration for the blobs propagating into the region of lower density and de-acceleration for the blobs propagating toward the regions of higher density. Analytical estimates are corroborated by direct numerical simulations.
Current perpendicular to the plane spin-valves with CoFeGe magnetic layers
Authors: Maat S. , Carey M. J. , Childress J. R.
Published: October 2008

The magnetotransport properties of current perpendicular to the plane giant magnetoresistive spin-valves utilizing (CoFe)100−xGex alloys are investigated. The composition range of 21≤x≤32 at. % is determined to be the alloy composition that maximizes magnetoresistance. ΔRA values of 2.6 mΩ–μm2 are measured for spin-valves with CoFeGe in the free and reference layer resulting in magnetoresistance values of greater than 6%, which is almost twice the value of similar spin-valves with CoFeAl alloys. An analysis of the physical properties of the CoFeGe thin films in the range of interest is also presented.
Determination of helicities in unidirectional assemblies of graphitic or graphiticlike tubular structures
Authors: Jiang H. , Brown D. P. , Nikolaev P. , Nasibulin A. G. , Kauppinen E. I.
Published: October 2008

Here we propose a universal method for the determination of all helicities present in unidirectional assemblies of hexagon-based graphitic or graphiticlike tubular structures, e.g., multiwalled or bundled carbon nanotubes (CNTs) or boron-nitride nanotubes and their structural analogs. A critical dimension characteristic of a fundamental structural property, i.e., the atomic bond length, is discerned from electron diffraction patterns by which all helicities present in the assemblies are identified. Using this method, we determine the helicity population in a single-walled CNT sample produced by laser ablation technique.
Two insulating phases in compressed Pr1−xCaxMnO3 thin films
Authors: Filippi M. , Prellier W. , Auban-Senzier P. , Pasquier C. R.
Published: October 2008

The temperature-dependent resistivity of two Pr1−xCaxMnO3 (x=0.5 and 0.6) thin films grown on LaAlO3 has been studied as a function of hydrostatic pressure (up to 2.5 GPa) and magnetic field (up to 9 T). Both samples show a monotonic decrease in the resistivity with an increase in pressure, corresponding to a change of −35% at 2.5 GPa. No pressure induced metal-to-insulator transition was observed in the temperature-dependent resistivity. The nontrivial interaction between high pressure and magnetic field reveals that the effect of pressure cannot be simply rescaled to that of a specific field, as has been reported for the corresponding bulk material. We propose an interpretation of the data based on phase separation, where two different insulating phases coexist: the charge ordered phase, which is sensitive to both magnetic field and pressure, and a second insulating phase that can be tuned by magnetic field. Such a result demonstrates that phase separation can be manipulated in thin films by independent application of magnetic field and/or external pressure.
Enhancement of light trapping in thin-film hydrogenated microcrystalline Si solar cells using back reflectors with self-ordered dimple pattern
Authors: Sai Hitoshi , Fujiwara Hiroyuki , Kondo Michio , Kanamori Yoshiaki
Published: October 2008

In this article, the light-trapping effect of textured back surface reflectors in thin-film Si solar cells is investigated. A unique substrate with a periodic dimple pattern has been developed by utilizing anodic oxidation of Al as a self-ordering process. n-i-p hydrogenated microcrystalline Si (μc-Si:H) cells fabricated on the Al substrate with a period of 0.9 μm show an improved infrared response compared to those fabricated on randomly textured substrates. A high short circuit current density of 24.3 mA/cm2 has been achieved in a 1-μm-thick μc-Si:H cell by adopting the patterned Al substrate.
Charging effect of Al2O3 thin films containing Al nanocrystals
Authors: Liu Y. , Chen T. P. , Zhu W. , Yang M. , Cen Z. H. , Wong J. I. , Li Y. B. , Zhang S. , Chen X. B. , Fung S.
Published: October 2008

In this work, Al2O3 thin film containing Al nanocrystals (nc-Al) is deposited on Si substrate by radio frequency sputtering to form a metal-insulator-semiconductor structure. Both electron and hole trapping in nc-Al are observed. The charge storage ability of the nc-Al/Al2O3 thin films provides the possibility of memory applications. Charging in the nc-Al also leads to a change in the dc resistance of the thin films, namely, the electron trapping in the nc-Al leads to an increase in the resistance, whereas the resistance is reduced if there is hole trapping in the nc-Al.
Single-dot optical emission from ultralow density well-isolated InP quantum dots
Authors: Ugur A. , Hatami F. , Masselink W. T. , Vamivakas A. N. , Lombez L. , Atatüre M.
Published: October 2008

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.
Response spectrum of coupled nanomechanical resonators
Authors: Dorignac J. , Gaidarzhy A. , Mohanty P.
Published: October 2008

We develop a simple continuum model to analyze the vibrational modes of a nanomechanical multielement structure. In this model, arrays of submicron cantilevers located symmetrically on both sides of the central clamped-clamped nanobeam are replaced by a continuum. In this approach, the equations of motion of the structure become exactly solvable. Our analytical results capture the main features of the vibrational modes observed both numerically and experimentally and can be applied to a general class of scale-independent elasticaly coupled resonator structures.