bluuebunny/crossref_metadata_2025 huggingface dataset:
Molar volume, excess enthalpy, and Prigogine-Defay ratio of some silicate glasses with different (P,T) histories
Authors: Wondraczek Lothar , Behrens Harald
Published: October 2007

Structural relaxation in silicate glasses with different (p,T) histories was experimentally examined by differential scanning calorimetry and measurements of molar volume under ambient pressure. Temperature and pressure-dependent rates of changes in molar volume and generation of excess enthalpy were determined for sodium trisilicate, soda lime silicate, and sodium borosilicate (NBS) compositions. From the derived data, Prigogine-Defay ratios are calculated and discussed. Changes of excess enthalpy are governed mainly by changes in short-range structure, as is shown for NBS where boron coordination is highly sensitive to pressure. For all three glasses, it is shown how the relaxation functions that underlie volume, enthalpy, and structural relaxation decouple for changes in cooling rates and pressure of freezing, respectively. The magnitude of the divergence between enthalpy and volume may be related to differences in structural sensitivity to changes in the (p,V,T,t) space on different length scales. The findings suggest that the Prigogine-Defay ratio is related to the magnitude of the discussed decoupling effect.
Accelerated entropy estimates with accelerated dynamics
Authors: Minh David D. L. , Hamelberg Donald , McCammon J. Andrew
Published: October 2007

Accelerated dynamics is applied to entropy calculations on a set of toy and molecular systems and is found to enhance the rate of convergence.
An eight-degree-of-freedom, time-dependent quantum dynamics study for the H2+C2H reaction on a new modified potential energy surface
Authors: Wang Dunyou , Huo Winifred M.
Published: October 2007

An eight-dimensional time-dependent quantum dynamics wave packet approach is performed for the study of the H2+C2H→H+C2H2 reaction system on a new modified potential energy surface (PES) [L.-P. Ju et al., Chem. Phys. Lett. 409, 249 (2005)]. This new potential energy surface is obtained by modifying Wang and Bowman’s old PES [J. Chem. Phys. 101, 8646 (1994)] based on the new ab initio calculation. This new modified PES has a much lower transition state barrier height at 2.29kcal∕mol than Wang and Bowman’s old PES at 4.3kcal∕mol. This study shows that the reactivity for this diatom-triatom reaction system is enhanced by vibrational excitations of H2, whereas the vibrational excitations of C2H only have a small effect on the reactivity. Furthermore, the bending excitations of C2H, compared to the ground state reaction probability, hinder the reactivity. The comparison of the rate constant between this calculation and experimental results agrees with each other very well. This comparison indicates that the new modified PES corrects the large barrier height problem in Wang and Bowman’s old PES.
Accounting for the dependence of P(E′,E) on the maximum impact parameter in classical trajectory calculations: Application to the H2O–H2O collisional relaxation
Authors: Bustos-Marún Raúl A. , Coronado Eduardo A. , Ferrero Juan C.
Published: October 2007

In this work we report a novel methodology that is able to predict how energy transfer transition probability density functions [P(E′,E)] change with the maximum impact parameter (bmax) used in trajectory calculations (TC’s). The method assumes that P(E′,E) can be described by a sum of exponential functions and that all the trajectories with an initial impact parameter beyond a certain critical value will contribute only to the elastic peak [P(E′,E) for E′=E]. This approach is applied to H2O–H2O collisions at different initial vibrational energies of the excited molecules and temperatures of bath gas. The results show that it is possible to reproduce with high accuracy the whole P(E′,E) obtained from a given bmax, using the results of TC’s performed at another bmax. The new methodology also leads us to propose a new criterion to choose the value of bmax.
Conditional Lie Bäcklund symmetries and solutions to (n+1)-dimensional nonlinear diffusion equations
Authors: Ji Lina , Qu Changzheng
Published: October 2007

This paper discusses a class of (n+1)-dimensional nonlinear diffusion equations with source term which arises in nonlinear shear flows of non-Newtonian fluids. It is shown that some radially symmetric equations admit certain types of conditional Lie Bäcklund symmetries. As a result, exact solutions to the resulting equations are obtained. Those solutions extend the known ones such as instantaneous source solutions of the porous medium equation with absorption term. The phenomena of extinction and blow up and behavior to many of the solutions are described.
Ultrasonic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys
Authors: Fukuhara Mikio , Wada Takeshi , Inoue Akihisa
Published: October 2007

Using ultrasonics, the acoustic characteristics of porous Pd42.5Cu30Ni7.5P20 glassy alloys were examined in terms of complex elasticity. Bulk modulus (K) and Lamè parameter (λ) decrease in a parabolic manner as porosity increases, while Young (E) and shear (G) moduli decrease linearly. Consequently, the order of K>λ>E>G, which is monopolistically characterized by nonporous glassy alloys, changes to the order of E>K>λ⩾G at 58% porosity. The decrease in Poisson’s ratio and the increase in G∕K ratio up to 58% porosity indicate crystallinelike characteristics that resist three-dimensional volume-nonpreserving deformation. An increase in porosity induces frequency convergence for longitudinal waves and an increment of the imaginary parts in the complex waves. The viscoelasticity of the porous glassy alloys is predominated by shear motion. In contrast to the continuous increment in dilational damping, the porosity-dependent increment in shear damping could be elucidated based on the accumulation of strains induced by the formation of pores.
Hyperspherical coupled channel calculations for the spectra and structure parameters of rare gas trimers NeAr2 and Ne2Ar
Authors: Han Hui-li , Li Yong , Zhang Xian-zhou , Shi Ting-yun
Published: October 2007

We calculate the L=0 vibration energies and rotational constants for the van der Waals trimers Ne20Ar2, Ne220Ar, and their corresponding isotopologues within the framework of hyperspherical coordinates. The Schrödinger equation in hyperangular coordinates is solved at a series of fixed hyper-radii using B-splines and the resulting coupled hyper-radial equation is solved using the slow variable discretization method developed by Tolstikhin et al. [J. Phys. B 29, L389 (1996)]. Using the special properties of B-splines, we make the knot distributions more precisely, characterizing the behavior of channel functions. Our method improves the convergence greatly. It turns out that our numerical tool works quite well in study of rare gas trimers. Calculations are performed on two kinds of pair potentials, the HFD-B and Tang-Toennies (TT) potentials, and the resultant rotational constants and their isotope shifts are compared with the experimental results obtained from high-resolution spectroscopy. The TT pair potentials give much better agreement with the experimental values for Ne220Ar and Ne222Ar trimers, while the HFD-B pair potentials give much better agreement with the experimental values for Ne20Ar2 and Ne22Ar2 trimers.
Presmectic wetting and supercritical-like phase behavior of octylcyanobiphenyl liquid crystal confined to controlled-pore glass matrices
Authors: Kralj Samo , Cordoyiannis George , Zidanšek Aleksander , Lahajnar Gojmir , Amenitsch Heinz , Žumer Slobodan , Kutnjak Zdravko
Published: October 2007

The influence of controlled-pore glass (CPG) confinement on the phase behavior of octylcyanobiphenyl liquid crystal (LC) is studied by means of x-ray scattering and high precision calorimetry. For CPG samples with pore diameter 2R>24nm, the smectic order parameter temperature dependence η(T) reveals apparent presmectic ordering far above the bulk smectic A–nematic (SmA-N) phase transition for both nontreated and silane-treated CPG matrices. The behavior of η(T) is qualitatively similar in all samples, well obeying the mean field approach (MFA) in which the surface wetting tendency plays the dominant role. In contrast, the critical fluctuations remain important in the specific heat data, which cannot be described within the MFA. We show experimentally that randomness and surface wetting become dominant over finite-size effects for 2R≲10nm, in agreement with theoretical analysis. In nontreated samples, the noncritical character of the static disorder and the interfacial LC-CPG coupling almost completely suppress the quasi-SmA-N and nematic-isotropic phase transitions at 2R∼15.1 and ∼7.5nm, respectively.
Concurrence revisited
Authors: Hildebrand Roland
Published: October 2007

Concurrence is a widely used entanglement measure of bipartite mixed states. We propose to consider the concurrence as being defined by a quadratic form on the space of self-adjoint operators. The square root of this form determines the values of the concurrence on the pure states, while the values on the mixed states are obtained as the largest convex extension, the convex roof. This viewpoint admits a generalization. Namely, the space of self-adjoint operators and the convex cone of positive semidefinite operators contained therein can be replaced by an arbitrary real vector space containing a convex cone. Then the concurrence is determined on the extreme rays of the cone by the square root of a quadratic form, and on the rest of the cone by the convex roof. We compute this generalized concurrence in the case when the cone is a second order cone. This enables us to compute the concurrence of arbitrary bipartite mixed states of rank 2. As an application, we compute the concurrences of the density matrices of all graphs with two edges or with three edges forming a triangle. We also consider the problem of maximizing the concurrence on the set of mixed states having a fixed spectrum.
An accurate, repeatable, and well characterized measurement of laser damage density of optical materials
Authors: Lamaignère L. , Bouillet S. , Courchinoux R. , Donval T. , Josse M. , Poncetta J.-C. , Bercegol H.
Published: October 2007

Known for more than 40years, laser damage phenomena have not been measured reproducibly up to now. Laser resistance of optical components is decreased by the presence of material defects, the distribution of which can initiate a distribution of damage sites. A raster scan test procedure has been used for several years in order to determine laser damage density of large aperture UV fused silica optics. This procedure was improved in terms of accuracy and repeatability. We describe the equipment, test procedure, and data analysis to perform this damage test of large aperture optics with small beams. The originality of the refined procedure is that a shot to shot correlation is performed between the damage occurrence and the corresponding fluence by recording beam parameters of hundreds of thousands of shots during the test at 10Hz. We characterize the distribution of damaging defects by the fluence at which they cause damage. Because tests are realized with small Gaussian beams (about 1mm at 1∕e), beam overlap and beam shape are two key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, we reached a repeatable metrology of laser damage performance. The measurement is destructive for the sample. However, the consideration of error bars on defect distributions in a series of parts allows us to compare data with other installations. This will permit to look for reproducibility, a necessary condition in order to test theoretical predictions.
Curve crossing and negative refraction in simulations of near-field coupled metallic nanoparticle arrays
Authors: Lopata Kenneth , Neuhauser Daniel , Baer Roi
Published: October 2007

We extend our previous results [R. Baer et al., J. Chem. Phys. 126, 014705 (2007).] to develop a simple theory of localized surface plasmon-polariton (LSPP) dispersion on regular arrays of metal nanoparticles in the weak-field and weak-damping limits. This theory describes the energy-momentum as well as the polarization-momentum properties of LSPP waves, both of which are crucial to plasmonic device design. We then explicitly compute the dispersion relation for isotropic and anisotropic two-dimensional square lattices, and show curve crossings between all three levels as well as negative refraction where the phase and group velocities (refractive indices), or at least their projection along the main axis, have different signs. The curve crossing implies that scattering between the different polarizations, and therefore different velocities, is easy at the curve crossing momenta, so that a quick change in wave packet direction can be achieved. Time-resolved wave packet dynamics simulations demonstrate negative refraction and the easy scattering over nanometer length scales. This paper also gives some computational schemes for future applications, such as a way to include source terms and how to efficiently treat dissipative effects.
Pressure induced structural phase transition in triglycine sulfate and triglycine selenate
Authors: Choudhury Rajul Ranjan , Poswal H. K. , Chitra R. , Sharma Surinder M.
Published: October 2007

To elucidate the cause of destruction of ferroelectricity with pressure in triglycine sulfate and triglycine selenate, we have investigated these compounds with the help of Raman measurements as well as first principles total energy and structural optimization calculations. Our results show that, beyond the critical pressures, the loss of ferroelectricity in these compounds is due to the conformational change in one of the three glycine ions of these crystals. Our studies suggest that pressure induced phase transition might be of displacive nature unlike the temperature induced ferroelectric phase transition in these crystals which is known to be of order-disorder type.
Reconfigurable magnetic logic for all basic logic functions produced by ion bombardment induced magnetic patterning
Authors: Höink V. , Meyners D. , Schmalhorst J. , Reiss G. , Junk D. , Engel D. , Ehresmann A.
Published: October 2007

In most common logic gates based on transistors, different logic functions have to be realized by applying a large number of logic gates, which are capable of performing only one function. A promising approach to overcome this obstacle with the additional advantage of a nonvolatile output is a reconfigurable logic based on small arrays of magnetic tunnel junctions. Here, an approach utilizing an ion bombardment induced patterning of the reference layer is proposed where the same logic unit consisting of only two magnetic tunnel junctions can be used for the AND, OR, NAND, NOR, and X(N)OR, functions.
Efficient surface-conducted field emission from ZnO nanotetrapods
Authors: Li Chi , Hou Kai , Lei Wei , Zhang Xiaobing , Wang Baoping , Sun X. W.
Published: October 2007

We report a surface-conducted field emission cathode using ZnO nanotetrapods as the surface conduction emitter. The dependences of electron emission efficiency on ZnO nanotetrapod film thickness and the spacing between adjacent electrodes of surface emitter were studied. A maximum electron emission efficiency of 60% was obtained for an optimal device with film thickness of 8μm and electrode spacing of 0.1mm. A low turn-on voltage of about 100V (corresponding to a field of 1V∕μm) at an emission current density of 0.6mA∕cm2 was obtained which is good enough for field emission displays. Good stability and uniformity were also demonstrated.
Light output improvement of InGaN ultraviolet light-emitting diodes by using wet-etched stripe-patterned sapphire substrates
Authors: Pan Chang-Chi , Hsieh Chi-Hsun , Lin Chih-Wei , Chyi Jen-Inn
Published: October 2007

GaN-based epilayers are grown on wet-etched stripe-patterned sapphire substrates, with stripes along the ⟨11−20⟩sapphire and ⟨1−100⟩sapphire directions, for 400nm ultraviolet light-emitting diodes (LEDs). The effects of the etching depth and stripe orientation on the structural and optical properties of the GaN layer as well as on the LEDs are investigated. Much better material quality and light output power are obtained when the GaN and the LEDs are grown on a 0.9μm deep patterned sapphire substrate with stripes along the ⟨1−100⟩sapphire direction. Stripe-orientation dependent growth modes accounting for the observed experimental results are proposed.
Lateral heat flow method for thickness independent determination of thermal diffusivity
Authors: Tralshawala Nilesh , Howard Donald R. , Knight Bryon , Plotnikov Yuri , Ringermacher Harry I.
Published: October 2007

A pulsed transient thermography method is described where a high-intensity light pulse is used to heat a long, uniform stripe on the surface of a plate. A high spatial resolution, high frame rate focal plane array infrared camera is used to monitor surface temperature. We explain the theoretical model and data analysis framework used to experimentally determine all three thermal diffusivity components from the temperature measurements. The analysis does not require any fitting to the temperature profile and is based on the creation of thermal time-of-flight (tof) images from the temperature data and the relationship between tof and the distance from the stripe edge. The in-plane components of thermal diffusivity are obtained without the need for thickness information. Experimental validation of this procedure was carried out using anisotropic carbon fiber reinforced polymer composites.
ZnO nanorods as an intracellular sensor for pH measurements
Authors: Al-Hilli Safaa M. , Willander M. , Öst Anita , Strålfors Peter
Published: October 2007

ZnO nanorods with 80nm diameter and 700nm length and grown on the tip of a borosilicate glass capillary (0.7μm in diameter) were used to create a highly sensitive pH sensor for monitoring in vivo biological process within single cells. The ZnO nanorods, functionalized by proton H3O+ and hydroxyl OH− groups, exhibit a pH-dependent electrochemical potential difference versus a Ag∕AgCl microelectrode. The potential difference was linear over a large dynamic range (4–11), which could be understood in terms of the change in surface charge during protonation and deprotonation. These nanoelectrode devices have the ability to enable analytical measurements in single living cells and have the capability to sense individual chemical species in specific locations within a cell.
Deformation potentials of Si-doped GaAs from microscopic residual stress fields
Authors: Porporati Alessandro Alan , Furukawa Naohide , Zhu Wenliang , Pezzotti Giuseppe
Published: October 2007

Underlying physics has been put forward and an experimental verification given for in situ determination of deformation potentials in Si-doped GaAs through a quantitative assessment of micro-/nanoscopic surface stress fields. Highly localized spectroscopic stress assessments could be achieved using a field emission scanning electron microscope as an energy source for stimulating cathodoluminescence emission from a Si-doped GaAs wafer. The deformation potentials were obtained from the local (elastic) residual strain fields stored in the neighborhood of an indentation print. The three independent GaAs deformation potentials could be obtained from a single measurement set and from mixed strain fields including tensile and compressive strains, while all the previously published characterizations were made in compression and on different samples. For these reasons, the deformation potentials determined in this study may prove more reliable and valid in a wider strain range as compared to those from previously published study. The proposed experimental method is suitable for in situ assessments of epitaxially grown thin-film materials and other zinc-blende-like III-V semiconductor heterostructures and alloys.
Growth-temperature optimization for low-carrier-density In0.75Ga0.25As-based high electron mobility transistors on InP
Authors: Simmonds P. J. , Beere H. E. , Ritchie D. A. , Holmes S. N.
Published: October 2007

Two-dimensional electron gases (2DEGs) were formed in undoped In0.75Al0.25As∕In0.75Ga0.25As∕In0.75Al0.25As quantum wells. The optimal growth temperature for this structure is 410°C, with peak 2DEG electron mobility and density values of μ=221000cm2∕Vs and n=1.36×1011cm−2 at 1.5K. This electron mobility is equal to the highest previously published for these undoped structures but with a factor of 2 reduction in n. This has been achieved through the use of a significantly thinner InAlAs graded buffer, which supports the theory that this is the source of the 2DEG electrons. For n>1.6×1011cm−2, μ is shown to be 10% higher for transport parallel to the [011] crystal axis. This is a direct result of anisotropic surface morphology.
Carbon, dopant, and vacancy interactions in germanium
Authors: Chroneos A. , Uberuaga B. P. , Grimes R. W.
Published: October 2007

Electronic structure calculations have been used to study the interaction of carbon with isolated substitutional dopants (boron, phosphorus, or arsenic), vacancies, and dopant-vacancy pairs in germanium. For comparison, equivalent defects were examined in silicon. The calculations employed a plane-wave basis set and pseudopotentials within the generalized gradient approximation of density functional theory. The results predict a range of different association preferences, with carbon being strongly bound in some cases and unbound in others. For example, in germanium, the carbon-vacancy cluster is weakly bound whereas in silicon it is more strongly bound. Conversely, dopant-carbon pairs are not stable in either germanium or silicon compared to their isolated components. If, however, they are formed during implantation, they will act as strong vacancy traps. Details of clusters comprised of a dopant, carbon, and vacancy are also discussed with respect to their formation by the association of a vacancy or cluster pair.
Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites
Authors: Lioudakis Emmanouil , Othonos Andreas , Alexandrou Ioannis , Hayashi Yasuhiko
Published: October 2007

In this work, we present the evolution of optical constants as a function of [6,6]-phenylC61-butyric acid methyl ester (PCBM) concentration for conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites. The PCBM concentration of the utilized samples varies from 1to50wt%. The dielectric functions for all these composites reveal electronic structural changes as a result of the addition of PCBM. We have deconvoluted the contribution of the substrate using a two-layer Fabry-Pérot structural model. The extracted optical properties contain crucial absorption peaks of singlet exciton states and vibronic sidebands for poly(3-hexylthiophene) (P3HT) conjugated polymer as well as two PCBM-related states at higher energies. With the addition of PCBM, we have observed a limit of 20wt% PCBM beyond which two discrete energy levels (3.64 and 4.67eV) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic contribution in the absorption spectrum at energies where the conjugated polymer absorbs (1.85–2.7eV) and a strong rise of PCBM states (3.64 and 4.67eV) which are responsible for the subsequent exciton dissociation. In addition, the energy gap between the higher occupied molecular orbitals and the lower unoccupied molecular orbitals of the highest concentration composite (50wt%) is 1.85eV. The tuning of the optical properties of P3HT with the addition of PCBM shows that ellipsometry can be used to monitor layer concentration toward optimization of plastic solar cells.
Pairwise cobalt doping of boron carbides with cobaltocene
Authors: Ignatov A. Yu. , Losovyj Ya. B. , Carlson L. , LaGraffe D. , Brand J. I. , Dowben P. A.
Published: October 2007

We have performed Co K-edge x-ray absorption fine structure and x-ray absorption near edge structure measurements of Co-doped plasma enhanced chemical vapor phase deposition (PECVD) grown “C2B10Hx” semiconducting boron carbides, using cobaltocene. Cobalt does not dope PECVD grown boron carbides as a random fragment of the cobaltocene source gas. The Co atoms are fivefold boron coordinated (R=2.10±0.02Å) and are chemically bonded to the icosahedral cages of B10CHx or B9C2Hy. Pairwise Co doping occurs, with the cobalt atoms favoring sites some 5.28±0.02Å apart.
Optical characterization and ligand-field splitting of Er3+(4f11) energy levels in a fluorine containing tellurite glass
Authors: Sardar Dhiraj K. , Dee Douglas M. , Nash Kelly L. , Yow Raylon M. , Gruber John B. , Debnath Radhaballav
Published: October 2007

An in-depth spectroscopic study is performed on Er3+(4f11) ions doped into a fluorine containing (lead, lanthanum)-tellurite glass host, containing nominal 1.93at.% of Er3+. The standard Judd-Ofelt (JO) model is applied to the room temperature absorption intensities of Er3+(4f11) transitions in the tellurite glass host to obtain three phenomenological intensity parameters, Ω2, Ω4, and Ω6. These parameters are subsequently used to determine the radiative decay rates, radiative lifetimes, and branching ratios of the Er3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds LJ2S+1 of Er3+(4f11) in the tellurite glass host. The emission cross sections of the intermanifold Er3+ I13∕24→I15∕24 (1.5μm) and S3∕24→I15∕24 (540nm) transitions have been determined. The room temperature fluorescence lifetimes of the I13∕24→I15∕24 and S3∕24→I15∕24 transitions in this tellurite glass host were also measured. From the radiative lifetime determined from the JO model and measured fluorescence lifetime, the quantum efficiency of this material was also determined. The spectroscopic properties were compared with those of Er3+(4f11) in other halotellurite glass hosts. We also analyzed the band structure observed in the absorption spectra of Er3+ in the tellurite glass at 8K. The structure can be interpreted in terms of the ligand-field splitting of the energy levels of the Er3+ ion in the local environment established by the glass matrix into which Er2O3 has been introduced. The spectroscopic analysis of Er3+(4f11) suggests that the tellurite glass is an excellent candidate for various photonic applications.
Intermediate-band solar cells based on quantum dot supracrystals
Authors: Shao Q. , Balandin A. A. , Fedoseyev A. I. , Turowski M.
Published: October 2007

The authors show that the ordered three-dimensional arrays of quantum dots, i.e., quantum dot supracrystals, can be used to implement the intermediate-band solar cell with the efficiency exceeding the Shockley-Queisser limit for a single junction cell. The strong electron wave function overlap resulting in minibands formation allows one to tune the band structure and enhance the light absorption and carrier transport. A first-principles semianalytical approach was used to determine the optimum dimensions of the quantum dots and the interdot spacing to achieve a maximum efficiency in the InAs0.9N0.1∕GaAs0.98Sb0.02 quantum dot supracrystal photovoltaic cells.
Formation and optical characterization of single InAs quantum dots grown on GaAs nanoholes
Authors: Alonso-González P. , Alén B. , Fuster D. , González Y. , González L. , Martínez-Pastor J.
Published: October 2007

We present a study of the structural and optical properties of InAs quantum dots formed in a low density template of nanoholes fabricated by droplet epitaxy on GaAs (001). The growth conditions used here promote the formation of isolated quantum dots only inside the templated nanoholes. Due to the good optical quality and low density of these nanostructures, their ensemble and individual emission properties could be investigated and related to the particular growth method employed and the quantum dot morphology.
Coherence in Y-coupled quantum cascade lasers
Authors: Hoffmann L. K. , Hurni C. A. , Schartner S. , Austerer M. , Mujagić E. , Nobile M. , Benz A. , Schrenk W. , Andrews A. M. , Klang P. , Strasser G.
Published: October 2007

A monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for a GaAs∕AlGaAs quantum cascade laser. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field pattern of a double slit. The mode distribution is comprehensively derived by matching the far field profiles to simulated values and shows a weak current dependence. The device demonstrates the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays.
Forming single molecular junctions between indium tin oxide electrodes
Authors: Chen Fang , Huang Zhifeng , Tao Nongjian
Published: October 2007

We have demonstrated that indium tin oxide (ITO) can be used as contact electrodes to form molecular junctions via ITO-carboxylic acid bonds. We measured the conductance of n-alkane terminated with dicarboxylic acids using conducting atomic force microscope break junction. The conductance decreases exponentially with molecular length with a decay constant of ∼0.80Å−1, which is similar to that of Au-carboxylic acid junctions. However, the contact conductance of ITO-carboxylic acid is smaller than that of Au-carboxylic acid, reflecting different electronic couplings between the linker group and electrodes. From the average breakdown force and stretching length, we estimated the binding energy of ITO-carboxylic acid.
Digital resonance tuning of high-Q∕Vm silicon photonic crystal nanocavities by atomic layer deposition
Authors: Yang Xiaodong , Chen Charlton J. , Husko Chad A. , Wong Chee Wei
Published: October 2007

We propose and demonstrate the digital resonance tuning of high-Q∕Vm silicon photonic crystal nanocavities using a self-limiting atomic layer deposition technique. Control of resonances in discrete steps of 122±18pm/hafnium oxide atomic layer is achieved through this postfabrication process, nearly linear over a full 17nm tuning range. The cavity Q is maintained in this perturbative process, and can reach up to its initial values of 49 000 or more. Our results are highly controllable, applicable to many material systems, and particularly critical to matching resonances and transitions involving mesoscopic optical cavities.
Investigation on the diffusion barrier properties of sputtered Mo∕W–N thin films in Cu interconnects
Authors: Majumder Prodyut , Takoudis Christos G.
Published: October 2007

Mo ∕ W – N bilayer thin film structures deposited on Si using sputtering have been studied as a copper diffusion barrier. The thermal stability of the barrier structure after annealing Cu∕Mo∕W–N∕⟨Si⟩ samples in N2 for 5min is studied using x-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy, and four point probe measurements. The failure of the barrier structure is indicated by the abrupt increase in sheet resistance value and the formation of Cu3Si phase as probed by XRD. Our results suggest that the Mo (5nm)∕W–N (5nm) barrier is stable and can prevent the formation of Cu3Si at least up to 775°C.
Nonequilibrium electron charging in carbon-nanotube-based molecular bridges
Authors: Deretzis I. , La Magna A.
Published: October 2007

We evidence the importance of electron charging under nonequilibrium conditions for carbon-nanotube-based molecular bridges, using a self-consistent Green’s function method with an extended Hückel Hamiltonian and a three-dimensional Poisson solver. Our analysis demonstrates that such feature is highly dependent on the chirality of the carbon nanotube as well as on the type of the contact metal, conditioning in a nongeneralized way the system’s conduction mechanism. Based on its impact on transport, we argue that self-consistency is essential for the current-voltage calculations of semiconducting nanotubes, whereas less significant in the case of metallic ones.
Bilayer model of polarization offset of compositionally graded ferroelectric thin films
Authors: Zhang J. , Tang M. H. , Tang J. X. , Yang F. , Xu H. Y. , Zhao W. F. , Zheng X. J. , Zhou Y. C. , He J.
Published: October 2007

The anomalous vertical shift behavior of hysteresis loops observed in compositionally graded ferroelectric films was investigated using an analytical bilayer model. By introducing a parameter A which is defined by grad(c)∕c(0), the model demonstrated that the compositional graded structure is sufficient to allow for the hysteresis loop shift along the polarization axis, and the shift is affected significantly by the concentration distribution in the films. Two reversed electric field buildups were also observed in the layers as indicated by the E-t profile.
Memory effect of nonvolatile bistable devices based on CdSe∕ZnS nanoparticles sandwiched between C60 layers
Authors: Li Fushan , Son Dong-Ick , Ham Jung-Hun , Kim Bong-Jun , Jung Jae Hun , Kim Tae Whan
Published: October 2007

Current-voltage and conductance-voltage (G-V) measurements on three-layer Al∕C60∕CdSe nanoparticles∕C60∕indium tin oxide (ITO) structures fabricated by using a spin-coating method showed a nonvolatile electrical bistable behavior. Capacitance-voltage (C-V) measurements on Al∕C60∕CdSe nanoparticles∕C60∕ITO structures showed a clockwise hysteresis with a flatband voltage shift due to the existence of the CdSe nanoparticles, indicative of memory effects in the devices. Current-time measurements showed that the devices exhibited excellent memory retention ability at ambient conditions. Possible operating mechanisms for the memory effects in the Al∕C60∕CdSe nanoparticles∕C60∕ITO devices are described on the basis of the G-V and the C-V results.
Theory for toggle magnetic random access memory: The asymmetric case
Authors: Worledge D. C.
Published: October 2007

A single domain model is used to analyze the magnetic switching of two coupled magnetic layers, relevant for toggle magnetic random access memory, for the nonideal case where the layers are not identical. In general, we show that discontinuous transitions occur across the critical switching curve and continuous transitions occur around cusps in the critical switching curve. We illustrate this with examples of thickness imbalance, anisotropy mismatch, and Néel coupling and derive the corresponding switching behavior. Features in the direct-write switching can be used to experimentally distinguish between these various sources of asymmetry.
Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation
Authors: Hinkle C. L. , Sonnet A. M. , Vogel E. M. , McDonnell S. , Hughes G. J. , Milojevic M. , Lee B. , Aguirre-Tostado F. S. , Choi K. J. , Kim J. , Wallace R. M.
Published: October 2007

The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined.
Grassrooting Network Imaginaries: Relationality, Power, and Mutual Solidarity in Global Justice Networks
Authors: Routledge Paul , Cumbers Andrew , Nativel Corinne
Published: October 2007

In this paper we draw critically upon actor network theory (ANT) in order to analyse the contours of relationality, communication, and operational logic within a global justice network—People's Global Action Asia. Drawing upon the concept of translation, we consider how connections are fostered and sustained within the network, focusing upon the work of key organisers (those we term the ‘imagineers’) and key events in producing the network. In so doing, we ground ANT in direct political engagement and introduce the concept of ‘grassrooting vectors’ to highlight the power relations at work within global justice networks, a consideration which is crucial to the formation of mutual solidarity between social movements.
Beyond Trenches and Grassroots? Reflections on Urban Mobilization, Fragmentation, and the Anti-Wal-Mart Campaign in Chicago
Authors: Sites William
Published: October 2007

Studies of the relationship between urban-based contention and neoliberal capitalism have recognized the relatively fluid and potentially empowering ways in which networked and multiscaled mobilizations strive to overcome fragmentation. Yet this work has not focused sufficiently on national urban politics for understanding the legacies of division and circumscription that may shape the city as a terrain of conflict. Reconsidering two classic treatments of fragmentation in the work of Katznelson and Castells, I contend that the historically racialized and politically decentralized institutional patterns characteristic of US urban governance continue to hamper urban social-justice mobilizations that seek to grapple with sectoral and scalar cleavages. Drawing on recent work in urban history, labor sociology, and urban politics, my discussion acknowledges the emerging potential of certain kinds of cross-sectoral and multiscalar efforts—such as labor–community coalitions and translocally supported multilocal campaigns—while also emphasizing that these mobilizations take shape within urban political arenas that, in the United States, are notoriously divisive and ‘sticky’. The paper illustrates these points through a brief case example involving the anti-Wal-Mart movement in Chicago.
Between Relationality and Territoriality: Investigating the Geographies of Justice Movements in the Netherlands and the United States
Authors: Beaumont Justin , Nicholls Walter
Published: October 2007

This paper examines the geographies of justice movements in Rotterdam in The Netherlands and Los Angeles in the United States. In their wider national and international frameworks movements in both countries continue to contest unjust forms of urbanization characterized by neoliberal initiatives that undermine the socioeconomic status of low-income residents. These movements are constituted by relations that stretch across several geographical levels. There remain, however, significant differences in their spatial organizational form: Rotterdam is characterized by loose networks of local associations which relate to constellations of nationally based Christian churches, unions, and humanist organizations, whereas networks between associations, unions, and university activists in Los Angeles have undergone institutionalization at the urban level. We show that movement territorialization is particularly evident at the urban level in Los Angeles while embedded at the national level in the shadow of state–corporatist institutional legacies and power relations in Rotterdam. By drawing upon important insights from several economic geographers, we develop a conceptual framework for explaining the spatialities of contention and contribute to contemporary controversies over relationality, territoriality, and political action at a variety of scales. A normative implication of the paper concerns the learning capacities of contesting actors to forge alliances and achieve their ambitions within path-dependent institutional frameworks.
Mutinous Eruptions: Autonomous Spaces of Radical Queer Activism
Authors: Brown Gavin
Published: October 2007

This paper offers a reflexive ethnography of a set of queer autonomous spaces created in London over the last five years. It traces the political genealogies of a recent strand of radical queer activism that is broadly aligned with the anarchist and anticapitalist wings of the global justice movement. In line with the usage of the term ‘queer’ by these activists themselves, to refer to a variety of states of being that challenge both homonormativity and heteronormativity, this paper utilises a definition of ‘queer’ that moves beyond the ways in which it has been mobilised by many sexual geographers. The ethnography poses questions about the ‘queer’ in ‘queer geography’ and what it means to be an ‘activist’. This work considers the importance (as well as the limits) of these autonomous queer spaces. It suggests that the process of collective experimentation to build autonomous queer spaces is ultimately more transformative and empowering than the resulting structures.
Spaces of Opposition: Activism and Deliberation in Post-Apartheid Environmental Politics
Authors: Barnett Clive , Scott Dianne
Published: October 2007

Drawing on recent political theory that examines the relationship between inclusive deliberation and oppositional activism in processes of democratisation, we develop a case study of environmental justice mobilisation in post-apartheid South Africa. We focus on the emergence of a network of social movement organisations embedded in particular localities in the city of Durban, connected into national and transnational campaigns, and centred on grievances around industrial air pollution. We analyse how the geographies of uneven industrial and urban development in Durban combine with sedimented place-based histories of activism to make particular locations spaces of democratic contention, in which the scope and operation of formal democratic procedures are challenged and transformed. We examine the range of strategic engagements adopted by social movement organisations in pursuing their objectives, looking in particular at the dynamic interaction between inclusion in deliberative forums and more adversarial, activist strategies of legal challenge and dramaturgical protest. We identify the key organisational features of groups involved in this environmental justice network, which both enable and constrain particular patterns of democratic engagement with the state and capital. We also identify a disjuncture between the interpretative frames of different actors involved in participatory policy making. These factors help to explain the difficulties faced by social movement organisations in opening up the space for legitimate nonparliamentary opposition in a political culture shaped by norms of conciliation and consensus.
Making New Political Spaces: Mobilizing Spatial Imaginaries, Instrumentalizing Spatial Practices, and Strategically Using Spatial Tools
Authors: Boudreau Julie-Anne
Published: October 2007

This paper explores space as the object of mobilization (rather than focusing on space as resource or constraint, or on the spatial configuration of actors within the organizational structure of a movement). In the context of state-restructuring processes, it is argued that new political spaces result not only from social movement activities (as in the drive for ‘free spaces’), but also in a dynamic interaction between state and civil society actors. The author asks what it takes to create a new, effective, and significant political space. Three elements are explored empirically and theoretically: the production of allegiance and legitimacy through spatial imaginaries, the instrumentalization of spatial practices and of the political culture, and the strategic use of spatial tools. In light of the case of Toronto, where a new regional political space eased the normalization of neoliberalism, it is concluded that new political spaces create the conditions for political exchange, but do not guarantee emancipation, democracy, and justice. Overall, the author's aim is to discuss the concept of political space and the analytical advantages provided by its openness to fluidity, uncertainties, uninstitutionalized interactions, and various forms of rationalities (imaginaries, everyday practices, as well as strategic calculation) in the state-restructuring and rescaling debate.
Using Eye-Tracking to Study Audio — Visual Perceptual Integration
Authors: Xiao Mei , Wong May , Umali Michelle , Pomplun Marc
Published: October 2007

Perceptual integration of audio—visual stimuli is fundamental to our everyday conscious experience. Eye-movement analysis may be a suitable tool for studying such integration, since eye movements respond to auditory as well as visual input. Previous studies have shown that additional auditory cues in visual-search tasks can guide eye movements more efficiently and reduce their latency. However, these auditory cues were task-relevant since they indicated the target position and onset time. Therefore, the observed effects may have been due to subjects using the cues as additional information to maximize their performance, without perceptually integrating them with the visual displays. Here, we combine a visual-tracking task with a continuous, task-irrelevant sound from a stationary source to demonstrate that audio—visual perceptual integration affects low-level oculomotor mechanisms. Auditory stimuli of constant, increasing, or decreasing pitch were presented. All sound categories induced more smooth-pursuit eye movement than silence, with the greatest effect occurring with stimuli of increasing pitch. A possible explanation is that integration of the visual scene with continuous sound creates the perception of continuous visual motion. Increasing pitch may amplify this effect through its common association with accelerating motion.
Linking double-stranded DNA breaks to the recombination activating gene complex directs repair to the nonhomologous end-joining pathway
Authors: Cui Xiaoping , Meek Katheryn
Published: October 2007

Two major DNA repair pathways, nonhomologous end-joining (NHEJ) and homologous recombination (HR), repair double-stranded DNA breaks (DSBs) in all eukaryotes. Additionally, several alternative end-joining pathways (or subpathways) have been reported that characteristically use short-sequence homologies at the DNA ends to facilitate joining. How a cell chooses which DNA repair pathway to use (at any particular DSB) is a central and largely unanswered question. For one type of DSB, there is apparently no choice. DSBs mediated by the lymphocyte-specific recombination activating gene (RAG) endonuclease are repaired virtually exclusively by NHEJ. Here we demonstrate that non-RAG-mediated DSBs can be similarly forced into the NHEJ pathway by physical association with the RAG endonuclease.
Gossip as an alternative for direct observation in games of indirect reciprocity
Authors: Sommerfeld Ralf D. , Krambeck Hans-Jürgen , Semmann Dirk , Milinski Manfred
Published: October 2007

Communication about social topics is abundant in human societies, and many functions have been attributed to such gossiping. One of these proposed functions is the management of reputations. Reputation by itself has been shown to have a strong influence on cooperation dynamics in games of indirect reciprocity, and this notion helps to explain the observed high level of cooperation in humans. Here we designed a game to test a widespread assumption that gossip functions as a vector for the transmission of social information. This empirical study (with 14 groups of nine students each) focuses on the composition of gossip, information transfer by gossip, and the behavior based on gossip information. We show that gossip has a strong influence on the resulting behavior even when participants have access to the original information (i.e., direct observation) as well as gossip about the same information. Thus, it is evident that gossip has a strong manipulative potential. Furthermore, gossip about cooperative individuals is more positive than gossip about uncooperative individuals, gossip comments transmit social information successfully, and cooperation levels are higher when people encounter positive compared with negative gossip.