bluuebunny/crossref_metadata_2025 huggingface dataset:
Recombination at laser-processed local base contacts by dynamic infrared lifetime mapping
Authors: Müller Jens , Bothe Karsten , Gatz Sebastian , Haase Felix , Mader Christoph , Brendel Rolf
Published: December 2010

Laser-processed local metal contacts to Si solar cells are a promising approach, to combine high efficiency and low production cost. Understanding carrier transport and recombination in locally contacted solar cells requires numerical simulations with experimentally verified input parameters. One of these input parameters is the reverse saturation current density J0,cont at the local base contact. We determine J0,cont by means of area averaged charge carrier lifetime measurements and an analytical model, which distinguishes between recombination at the metal contacts and at the passivated interface in between the contacts. The calibration-free dynamic infrared lifetime mapping technique is used. We measure local reverse saturation current densities J0,cont=2×103 to 2×107 fA/cm2 at metal contacts to p-type float-zone material with resistivities ρ=0.5 to 200 Ω cm. Laser contact openings (LCOs) formed by laser ablation of an amorphous Si/SiNx passivation stack and subsequent physical vapor deposition of aluminum are used as contact formation technique. As well laser fired contacts (LFCs) are applied to the same passivation stack and metallization. We observe no difference in J0,cont between LCO and LFC. Our results indicate degradation of the passivation stack by the laser treatment in the vicinity of the LCO and LFC.
Understanding cardiac alternans: A piecewise linear modeling framework
Authors: Thul R. , Coombes S.
Published: December 2010

Cardiac alternans is a beat-to-beat alternation in action potential duration (APD) and intracellular calcium (Ca2+) cycling seen in cardiac myocytes under rapid pacing that is believed to be a precursor to fibrillation. The cellular mechanisms of these rhythms and the coupling between cellular Ca2+ and voltage dynamics have been extensively studied leading to the development of a class of physiologically detailed models. These have been shown numerically to reproduce many of the features of myocyte response to pacing, including alternans, and have been analyzed mathematically using various approximation techniques that allow for the formulation of a low dimensional map to describe the evolution of APDs. The seminal work by Shiferaw and Karma is of particular interest in this regard [Shiferaw, Y. and Karma, A., “Turing instability mediated by voltage and calcium diffusion in paced cardiac cells,” Proc. Natl. Acad. Sci. U.S.A. 103, 5670–5675 (2006)]. Here, we establish that the key dynamical behaviors of the Shiferaw–Karma model are arranged around a set of switches. These are shown to be the main elements for organizing the nonlinear behavior of the model. Exploiting this observation, we show that a piecewise linear caricature of the Shiferaw–Karma model, with a set of appropriate switching manifolds, can be constructed that preserves the physiological interpretation of the original model while being amenable to a systematic mathematical analysis. In illustration of this point, we formulate the dynamics of Ca2+ cycling (in response to pacing) and compute the properties of periodic orbits in terms of a stroboscopic map that can be constructed without approximation. Using this, we show that alternans emerge via a period-doubling instability and track this bifurcation in terms of physiologically important parameters. We also show that when coupled to a spatially extended model for Ca2+ transport, the model supports spatially varying patterns of alternans. We analyze the onset of this instability with a generalization of the master stability approach to accommodate the nonsmooth nature of our system.
Photodissociation and photoionization of 2,5-dihydroxybenzoic acid at 193 and 355 nm
Authors: Bagchi Arnab , Dyakov Yuri A. , Ni Chi-Kung
Published: December 2010

Photodissociation and photoionization of 2,5-dihydroxybenzoic acid (25DHBA), at 193 and 355 nm were investigated separately in a molecular beam using multimass ion imaging techniques. Two channels competed after excitation by one 193 nm photon. One channel is dissociation from the repulsive excited state along O-H bond distance, resulting in H atom elimination from meta-OH functional group. The other channel is internal conversion to the ground state, followed by H2O elimination. Some of the fragments further proceeded to secondary dissociation. On the other hand, absorption of one 355 nm photon gave rise to H2O elimination channel on the ground state. Absorption of more than one 355 nm photon resulted in the three-body dissociation which also occurs on the ground state. Dissociation on the excited state does not play a role at 355 nm. The large concentration ratio (2×105), between neutral fragments and cations produced from 355 nm multiphoton excitation indicates that internal conversion followed by dissociation, is the major channel for 355 nm multiphoton excitation. Multiphoton ionization is a minor channel. Multiphoton ionization of 25DHBA clusters only produces 25DHBA cations. Neither anion nor protonated 25DHBA cation were observed. It is very different from the ions produced from solid matrix-assisted laser desorption/ionization (MALDI), experiments. This suggests that protonated 25DHBA and negatively charged 25DHBA generated in MALDI experiments does not simply result from the ionization following proton transfer reactions or charge transfer reactions of the clusters in the gas phase.
Modulated 3D cross-correlation light scattering: Improving turbid sample characterization
Authors: Block Ian D. , Scheffold Frank
Published: December 2010

Accurate characterization using static light scattering (SLS) and dynamic light scattering (DLS) methods mandates the measurement and analysis of singly scattered light. In turbid samples, the suppression of multiple scattering is therefore required to obtain meaningful results. One powerful technique for achieving this, known as 3D cross-correlation, uses two simultaneous light scattering experiments performed at the same scattering vector on the same sample volume in order to extract only the single scattering information common to both. Here we present a significant improvement to this method in which the two scattering experiments are temporally separated by modulating the incident laser beams and gating the detector outputs at frequencies exceeding the timescale of the system dynamics. This robust modulation scheme eliminates cross-talk between the two beam-detector pairs and leads to a fourfold improvement in the cross-correlation intercept. We measure the dynamic and angular-dependent scattering intensity of turbid colloidal suspensions and exploit the improved signal quality of the modulated 3D cross-correlation DLS and SLS techniques.
Synthesis of wirelike silicon nanostructures by dispersion of silicon on insulator using electroless etching
Authors: Mantey Kevin , Shams Somayeh , Nayfeh Munir H. , Nayfeh Osama , Alhoshan Mansour , Alrokayan Salman
Published: December 2010

We employ electroless etching to disperse silicon on insulator (SOI) wafers in ionic silver HF into wirelike silicon nanostructures. The procedure allows detachment of the nanowires at the oxide interface and enables easy recovery of dispersions for subsequent controlled delivery. Nanowires 10 μm long and 50–100 nm thick are demonstrated using an SOI substrate of 10 μm device thickness. Direct material analysis shows no silver contamination after a wet-etch silver clean. Anodization treatment of the wires was conducted. Our measurements and analysis show that the wires are not amenable to anodization resulting from the fact that the nanowire radius is less than the thickness of depletion layer. The procedure has the potential of providing dispersions of arbitrarily long wirelike nanostructures which are useful for composite and energy applications.
Non-Abelian Berry phase, instantons, and ${\mathcal N} = (0,4)$N=(,4) supersymmetry
Authors: Laia João N.
Published: December 2010

In supersymmetric quantum mechanics, the non-Abelian Berry phase is known to obey certain differential equations. Here we study \documentclass[12pt]{minimal}\begin{document}${\cal N}=(0,4)$\end{document}N=(0,4) systems and show that the non-Abelian Berry connection over \documentclass[12pt]{minimal}\begin{document}${\bf R}^{4n}$\end{document}R4n satisfies a generalization of the self-dual Yang–Mills equations. Upon dimensional reduction, these become the tt* equations. We further study the Berry connection in \documentclass[12pt]{minimal}\begin{document}${\cal N}=(4,4)$\end{document}N=(4,4) theories and show that the curvature is covariantly constant.
Homogenization of the linearized ionic transport equations in rigid periodic porous media
Authors: Allaire Grégoire , Mikelić Andro , Piatnitski Andrey
Published: December 2010

In this paper we undertake the rigorous homogenization of a system of partial differential equations describing the transport of a N-component electrolyte in a dilute Newtonian solvent through a rigid porous medium. The motion is governed by a small static electric field and a small hydrodynamic force, which allows us to use O’Brien's linearized equations as the starting model. We establish convergence of the homogenization procedure and discuss the homogenized equations. Even if the symmetry of the effective tensor is known from the literature [J. R. Looker and S. L. Carnie, Transp. Porous Media, 65, 107 (2006)], its positive definiteness does not seem to be known. Based on the rigorous study of the underlying equations, we prove that the effective tensor satisfies Onsager properties, namely is symmetric positive definite. This result justifies the approach of many authors who use Onsager theory as starting point.
Understanding anomalous delays in a model of intracellular calcium dynamics
Authors: Harvey Emily , Kirk Vivien , Osinga Hinke M. , Sneyd James , Wechselberger Martin
Published: December 2010

In many cell types, oscillations in the concentration of free intracellular calcium ions are used to control a variety of cellular functions. It has been suggested [J. Sneyd et al., “A method for determining the dependence of calcium oscillations on inositol trisphosphate oscillations,” Proc. Natl. Acad. Sci. U.S.A. 103, 1675–1680 (2006)] that the mechanisms underlying the generation and control of such oscillations can be determined by means of a simple experiment, whereby a single exogenous pulse of inositol trisphosphate (IP3) is applied to the cell. However, more detailed mathematical investigations [M. Domijan et al., “Dynamical probing of the mechanisms underlying calcium oscillations,” J. Nonlinear Sci. 16, 483–506 (2006)] have shown that this is not necessarily always true, and that the experimental data are more difficult to interpret than first thought. Here, we use geometric singular perturbation techniques to study the dynamics of models that make different assumptions about the mechanisms underlying the calcium oscillations. In particular, we show how recently developed canard theory for singularly perturbed systems with three or more slow variables [M. Wechselberger, “A propos de canards (Apropos canards),” Preprint, 2010] applies to these calcium models and how the presence of a curve of folded singularities and corresponding canards can result in anomalous delays in the response of these models to a pulse of IP3.
Spontaneous sarcomere dynamics
Authors: Günther Stefan , Kruse Karsten
Published: December 2010

Sarcomeres are the basic force generating units of striated muscles and consist of an interdigitating arrangement of actin and myosin filaments. While muscle contraction is usually triggered by neural signals, which eventually set myosin motors into motion, isolated sarcomeres can oscillate spontaneously between a contracted and a relaxed state. We analyze a model for sarcomere dynamics, which is based on a force-dependent detachment rate of myosin from actin. Our numerical bifurcation analysis of the spontaneous sarcomere dynamics reveals notably Hopf bifurcations, canard explosions, and gluing bifurcations. We discuss possible implications for experiments.
Evolution of functional subnetworks in complex systems
Authors: Li Menghui , Wang Xingang , Lai Choy-Heng
Published: December 2010

Links in a realistic network may have different functions, which makes the network virtually a combination of some small-size functional subnetworks. Here, by a model of coupled phase oscillators, we investigate how such functional subnetworks are evolved and developed according to the network structure and dynamics. In particular, we study the case of evolutionary clustered networks in which the function type of each link (attractive or repulsive coupling) is adaptively updated according to the local network dynamics. It is found that during the process of system evolution, the network is gradually stabilized into a particular form in which the attractive (repulsive) subnetwork consists only of the intralinks (interlinks). Based on the observed properties of subnetwork evolution, we also propose a new algorithm for network partition which, compared with the conventional algorithms, is distinguished by its convenient operation and fast computing speed.
The extension problem for partial Boolean structures in quantum mechanics
Authors: Budroni Costantino , Morchio Giovanni
Published: December 2010

Alternative partial Boolean structures, implicit in the discussion of classical representability of sets of quantum mechanical predictions, are characterized, with definite general conclusions on the equivalence of the approaches going back to Bell and Kochen–Specker. An algebraic approach is presented, allowing for a discussion of partial classical extension, amounting to reduction of the “number of contexts,” classical representability arising as a special case. As a result, known techniques are generalized and some of the associated computational difficulties overcome. The implications on the discussion of Boole–Bell inequalities are indicated.
A collective dynamics description of dipolar interactions and the coercive field of magnetic nanoparticles
Authors: Das R. K. , Rawal S. , Norton D. , Hebard A. F.
Published: December 2010

The effect of dipolar interactions on the coercive field of Ni nanoparticles embedded as layers in a Al2O3 host matrix is discussed. The results for two sets of 5 layer samples with different interlayer spacings and a set of single layer samples are compared for samples with particle size varying from 3 nm (single domain) to 60 nm (multiple domain). The dipolar interactions are strongest in the samples with shorter interlayer distances and weakest for the single layer samples. Our observation that dipolar interactions increase the coercive field and decrease the critical diameter separating single domain from multiple domain behavior reinforces a description based on collective dynamics.
Stochastic characterization of small-scale algorithms for human sensory processing
Authors: Neri Peter
Published: December 2010

Human sensory processing can be viewed as a functional H mapping a stimulus vector s into a decisional variable r. We currently have no direct access to r; rather, the human makes a decision based on r in order to drive subsequent behavior. It is this (typically binary) decision that we can measure. For example, there may be two external stimuli s[0] and s[1], mapped onto r[0] and r[1] by the sensory apparatus H; the human chooses the stimulus associated with largest r. This kind of decisional transduction poses a major challenge for an accurate characterization of H. In this article, we explore a specific approach based on a behavioral variant of reverse correlation techniques, where the input s contains a target signal corrupted by a controlled noisy perturbation. The presence of the target signal poses an additional challenge because it distorts the otherwise unbiased nature of the noise source. We consider issues arising from both the decisional transducer and the target signal, their impact on system identification, and ways to handle them effectively for system characterizations that extend to second-order functional approximations with associated small-scale cascade models.
Magnetic-field-induced recovery strain in polycrystalline Ni–Mn–Ga foam
Authors: Chmielus Markus , Witherspoon Cassie , Wimpory Robert C. , Paulke Andreas , Hilger André , Zhang Xuexi , Dunand David C. , Müllner Peter
Published: December 2010

Recently, we have shown that a polycrystalline Ni–Mn–Ga magnetic shape-memory alloy, when containing two populations of pore sizes, shows very high magnetic-field-induced strain of up to 8.7%. Here, this double-porosity sample is imaged by x-ray microtomography, showing a homogenous distribution of both pore populations. The orientation of six large grains—four with 10M and two with 14M structure—is identified with neutron diffraction. In situ magnetomechanical experiments with a rotating magnetic field demonstrate that strain incompatibilities between misoriented grains are effectively screened by the pores which also stop the propagation of microcracks. During uniaxial compression performed with an orthogonal magnetic bias field, a strain as high as 1% is recovered on unloading by twinning, which is much larger than the elastic value of <0.1% measured without field. At the same time, repeated loading and unloading results in a reduction in the yield stress, which is a training effect similar to that in single crystals.
Relaxation dynamics and frequency response of a noisy cell signaling network
Authors: Rué P. , Pons A. J. , Domedel-Puig N. , García-Ojalvo J.
Published: December 2010

We investigate the dynamics of cell signaling using an experimentally based Boolean model of the human fibroblast signal transduction network. We determine via systematic numerical simulations the relaxation dynamics of the network in response to a constant set of inputs, both in the absence and in the presence of environmental fluctuations. We then study the network’s response to periodically modulated signals, uncovering different types of behaviors for different pairs of driven input and output nodes. The phenomena observed include low-pass, high-pass, and band-pass filtering of the input modulations, among other nontrivial responses, at frequencies around the relaxation frequency of the network. The results reveal that the dynamic response to the external modulation of biologically realistic signaling networks is versatile and robust to noise.
Evaluating the effect of magnetocaloric properties on magnetic refrigeration performance
Authors: Engelbrecht K. , Bahl C. R. H.
Published: December 2010

Active magnetic regenerator (AMR) refrigerators represent an alternative to vapor compression technology that relies on the magnetocaloric effect in a solid refrigerant. Magnetocaloric materials are in development and properties are reported regularly. Recently, there has been an emphasis on developing materials with a high entropy change with magnetization while placing lower emphasis on the adiabatic temperature change. This work uses model magnetocaloric materials and a numerical AMR model to predict how the temperature change and entropy change with magnetization interact and how they affect the performance of a practical system. The distribution of the magnetocaloric effect as a function of temperature was also studied. It was found that the adiabatic temperature change in a magnetocaloric material can be more important than the isothermal entropy change for certain conditions. A material that exhibits a sharp peak in isothermal entropy change was shown to produce a significantly lower cooling power than a material with a wide peak in a practical AMR system.
Curie point, susceptibility, and temperature measurements of rapidly heated ferromagnetic wires
Authors: Anwar Muhammad Sabieh , Zia Wasif
Published: December 2010

This article describes a technique to measure the temperature of a resistively heated ferromagnetic wire. The wire's temperature rapidly increases, a scenario in which a thermocouple or thermistor's thermal inertia prevents it from keeping up with the rapid temperature variation. The temperature is derived from electrical measurands (voltage and current) and time, as well as thermophysical data such as heat losses and emissivity, and is based on a dynamical thermal–electrical energy conservation principle. We go on to use our technique for the quantitative determination of the Curie point as well as the magnetic susceptibility at elevated temperatures. The results are in good agreement with accepted values.
Tunable Josephson current in a lateral triple quantum dot molecule
Authors: Bai Long , Jiang Liang , Zhang Zheng-Zhong , Duan Chen-Long
Published: December 2010

With the help of the nonequilibrium Green’s function method, we have investigated the Josephson current properties of a triple quantum dot molecule. It is found that the sign of the Josephson current can be changed from positive to negative with increasing the spin-flip scattering, which leads to the π-junction transition. By tuning the system parameters, such as the gate voltage, the interdot coupling, and spin-flip strength, it is manifested that the π-junction not only can be controlled, but also the oscillation of the Josephson current versus magnetic flux can be changed. The π-junction is explained in terms of the picture of the current carrying density of states. These results provide the ways of manipulating the Josephson current.
Three-dimensional multiexcitation magnetoacoustic tomography with magnetic induction
Authors: Li Xu , Mariappan Leo , He Bin
Published: December 2010

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a hybrid imaging modality proposed to image electrical conductivity contrast of biological tissue with high spatial resolution. This modality combines magnetic excitations with ultrasound detection through the Lorentz force based coupling mechanism. However, previous studies have shown that MAT-MI method with single type of magnetic excitation can only reconstruct the conductivity boundaries of a sample. In order to achieve more complete conductivity contrast reconstruction, we proposed a multiexcitation MAT-MI approach. In this approach, multiple magnetic excitations using different coil configurations are applied to the object sequentially and ultrasonic signals corresponding to each excitation are collected for conductivity image reconstruction. In this study, we validate the new multiexcitation MAT-MI method for three-dimensional (3D) conductivity imaging through both computer simulations and phantom experiments. 3D volume data are obtained by utilizing acoustic focusing and cylindrical scanning under each magnetic excitation. It is shown in our simulation and experiment results that with a common ultrasound probe that has limited bandwidth we are able to correctly reconstruct the 3D relative conductivity contrast of the imaging object. As compared to those conductivity boundary images generated by previous single-excitation MAT-MI, the new multiexcitation MAT-MI method provides more complete conductivity contrast reconstruction, and therefore, more valuable information in possible clinical and research applications.
Different effects of redundant feedback loops on a bistable switch
Authors: Domingo-Sananes Maria Rosa , Novak Bela
Published: December 2010

Bistable switches have important roles in cellular decision-making processes. Bistability can be the consequence of positive or double-negative feedback loops. Although necessary, such feedback is not sufficient for bistability, which also requires nonlinearity. Nonlinearity can be provided by synergy of multiple feedback loops or by an ultrasensitive response within a single feedback loop. However, these two possibilities are not mutually exclusive; a combination of them is also possible. Here we analyze a biochemical regulatory network that controls a crucial cell cycle transition in all eukaryotic cells and contains multiple redundant feedback loops and nonlinearity. We show in this realistic biological example that two redundant feedback loops have different effects on the position of one of the saddle-node bifurcations of the system, which determines where the system switches. This illustrates that even though the roles of positive and double-negative feedbacks have been regarded as equivalent, the difference in their architectures can lead to differences in their effects on the system. We speculate that this conclusion could be general for other bistable systems with redundant feedback loops.
Hybrid optimal scheduling for intermittent androgen suppression of prostate cancer
Authors: Hirata Yoshito , di Bernardo Mario , Bruchovsky Nicholas , Aihara Kazuyuki
Published: December 2010

We propose a method for achieving an optimal protocol of intermittent androgen suppression for the treatment of prostate cancer. Since the model that reproduces the dynamical behavior of the surrogate tumor marker, prostate specific antigen, is piecewise linear, we can obtain an analytical solution for the model. Based on this, we derive conditions for either stopping or delaying recurrent disease. The solution also provides a design principle for the most favorable schedule of treatment that minimizes the rate of expansion of the malignant cell population.
Counting SO(9) × SU(2) representations in coordinate independent state space of SU(2) matrix theory
Authors: Michishita Yoji
Published: December 2010

We consider decomposition of coordinate independent states into SO(9) × SU(2) representations in SU(2) matrix theory. To see what and how many representations appear in the decomposition, we compute the character, which is given by a trace over the coordinate independent states, and decompose it into the sum of products of SO(9) and SU(2) characters.
Peculiarities of impact ionization of Impurity Al in SiC polytypes
Authors: Sankin V. I. , Monakhov A. M. , Shkrebiy P. P. , Abramov P. L. , Sablina N. I. , Averkiev N. S.
Published: December 2010

The specific character of impact ionization of a small radius impurities was investigated in polytypes 6H-, 4H-, and 15R-SiC. The I-V characteristics of Al breakdown in 6H-, 4H-, and 15R-SiC in electrical field were studied. It was shown that at 77 K the impurity breakdown takes place only under the large impurity concentrations (three orders larger than that in Ge). The following data were obtained: the decreasing dependence of breakdown field due to the concentration increase, the absence of the low temperature breakdown at low impurity concentration, and gigantic enhancement of breakdown field when the field is parallel to the crystal axis.
On the role of subthreshold currents in the Huber–Braun cold receptor model
Authors: Finke Christian , Freund Jan A. , Rosa Epaminondas , Braun Hans A. , Feudel Ulrike
Published: December 2010

We study the role of the strength of subthreshold currents in a four-dimensional Hodgkin–Huxley-type model of mammalian cold receptors. Since a total diminution of subthreshold activity corresponds to a decomposition of the model into a slow, subthreshold, and a fast, spiking subsystem, we first elucidate their respective dynamics separately and draw conclusions about their role for the generation of different spiking patterns. These results motivate a numerical bifurcation analysis of the effect of varying the strength of subthreshold currents, which is done by varying a suitable control parameter. We work out the key mechanisms which can be attributed to subthreshold activity and furthermore elucidate the dynamical backbone of different activity patterns generated by this model.
Mass synchronization: Occurrence and its control with possible applications to brain dynamics
Authors: Chandrasekar V. K. , Sheeba Jane H. , Lakshmanan M.
Published: December 2010

Occurrence of strong or mass synchronization of a large number of neuronal populations in the brain characterizes its pathological states. In order to establish an understanding of the mechanism underlying such pathological synchronization, we present a model of coupled populations of phase oscillators representing the interacting neuronal populations. Through numerical analysis, we discuss the occurrence of mass synchronization in the model, where a source population which gets strongly synchronized drives the target populations onto mass synchronization. We hypothesize and identify a possible cause for the occurrence of such a synchronization, which is so far unknown: Pathological synchronization is caused not just because of the increase in the strength of coupling between the populations but also because of the strength of the strong synchronization of the drive population. We propose a demand controlled method to control this pathological synchronization by providing a delayed feedback where the strength and frequency of the synchronization determine the strength and the time delay of the feedback. We provide an analytical explanation for the occurrence of pathological synchronization and its control in the thermodynamic limit.
Fibroblasts alter spiral wave stability
Authors: Petrov Valentin S. , Osipov Grigory V. , Kurths Jürgen
Published: December 2010

We consider a three-domain model of cardiac tissue consisting of fibroblasts, myocytes, and extracellular space. We show in the one dimensional case that the fibroblasts with different resting potentials may alter restitution properties of tissue. On this basis we demonstrated that in two dimensional slice of cardiac tissue, a spiral wave break up can be caused purely by the influence of fibroblasts and, vice-versa, initially unstable spiral can be stabilized by fibroblasts depending on the value of their resting potential.
Communication: Evidence of hydrated electrons injected by a metallic electrode in a high voltage system
Authors: Perles Carlos Eduardo , Volpe Pedro Luiz Onófrio
Published: December 2010

In this work it a strong evidence of the hydrated electrons production was shown in a film of condensed water, by directing the injection of electrons in localized and/or delocalized water electronic states using a system of high voltage made in laboratory. The results show that the water layers on the silica particles are electrically charged by injection of electrons from a metal electrode when silica is placed in high electric field. This charging process also appears to depend on the thickness of these water layers and of the spatial arrangement required by the silica surface.
Impact of edge-barrier pinning in superconducting thin films
Authors: Jones W. A. , Barnes P. N. , Mullins M. J. , Baca F. J. , Emergo R. L. S. , Wu J. , Haugan T. J. , Clem J. R.
Published: December 2010

It has been suggested that edge-barrier pinning might cause the critical current density (Jc) in bridged superconducting films to increase. Subsequent work indicated that this edge-barrier effect does not impact bridges larger than 1 μm. However, we provide a theoretical assessment with supporting experimental data suggesting edge-barrier pinning can significantly enhance Jc for bridges of a few microns or even tens of microns thus skewing any comparisons among institutions. As such, when reporting flux pinning and superconductor processing improvements for Jc comparisons, the width of the sample has to be taken into consideration as is currently done with film thickness.
Dynamic synchronization of a time-evolving optical network of chaotic oscillators
Authors: Cohen Adam B. , Ravoori Bhargava , Sorrentino Francesco , Murphy Thomas E. , Ott Edward , Roy Rajarshi
Published: December 2010

We present and experimentally demonstrate a technique for achieving and maintaining a global state of identical synchrony of an arbitrary network of chaotic oscillators even when the coupling strengths are unknown and time-varying. At each node an adaptive synchronization algorithm dynamically estimates the current strength of the net coupling signal to that node. We experimentally demonstrate this scheme in a network of three bidirectionally coupled chaotic optoelectronic feedback loops and we present numerical simulations showing its application in larger networks. The stability of the synchronous state for arbitrary coupling topologies is analyzed via a master stability function approach.
Shear Alfvén wave continuous spectrum within magnetic islands
Authors: Biancalani Alessandro , Chen Liu , Pegoraro Francesco , Zonca Fulvio
Published: December 2010

The radial structure of the continuous spectrum of shear Alfvén waves is calculated in this paper within the separatrix of a magnetic island. Geometrical effects due to the noncircularity of the flux surface’s cross section are retained to all orders. On the other hand, only curvature effects responsible for the beta-induced gap in the low-frequency part of the continuous spectrum are kept. Modes with different helicity from that of the magnetic island are considered. The main result is that, inside a magnetic island, there is a continuous spectrum very similar to that of tokamak plasmas, where a generalized safety factor q can be defined and where a wide frequency gap is formed, analogous to the ellipticity induced Alfvén eigenmode gap in tokamaks. The presence of this gap is due to the strong eccentricity of the island cross section. The importance of the existence of such a gap is recognized in potentially hosting magnetic island induced Alfvén eigenmodes (MiAE). Due to the frequency dependence of the shear Alfvén wave continuum on the magnetic island size, the possibility of utilizing MiAE frequency scalings as a novel magnetic island diagnostic is also discussed.
Axial dispersion in segmented gas-liquid flow: Effects of alternating channel curvature
Authors: Muradoglu Metin
Published: December 2010

The effects of channel curvature on the axial dispersion in segmented gas-liquid flows are studied computationally in a two-dimensional setting using a finite-volume/front-tracking method. Passive tracer particles are used to visualize and quantify the axial dispersion. The molecular diffusion is modeled by random walk of tracer particles. It is found that there is significant axial dispersion in serpentine channels even in the absence of molecular diffusion. The lubricating thin liquid layer that persists on the wall of a straight channel is periodically broken in the serpentine channel leading to enhanced axial dispersion. It is also found that the axial dispersion is always larger in the serpentine channel than that in the straight channel but the effects of channel curvature are more pronounced at high Peclet numbers, i.e., Pe>104. A model is proposed based on the difference between the liquid film thicknesses on the inner and outer side of the bend in the limit as Pe→∞. Good agreement is found between the computational results and the model when the liquid slug is well mixed by the chaotic advection.
Room temperature negative differential resistance based on a single ZnO nanowire/CuPc nanofilm hybrid heterojunction
Authors: Guo Wen , Yang Ya , Qi Junjie , Zhang Yue
Published: December 2010

We report a room temperature negative differential resistance (RT-NDR) phenomenon in the ZnO nanowire/CuPc nanofilm hybrid heterojunction by using conductive atomic force microscope (C-AFM). The NDR behavior is observed only in the positive sample bias range with the peak-to-valley current ratio as high as 4.7 in the dark. Under light illumination (centered at 716 nm), the NDR peak current increases from 2.1 to 7.53 nA and the peak voltage decreases from 4.2 to 3.1 V with increasing the light intensity. The origin and the modulation of the NDR are discussed in terms of energy band diagram of the heterojunction.
Spin-transfer pulse switching: From the dynamic to the thermally activated regime
Authors: Bedau D. , Liu H. , Sun J. Z. , Katine J. A. , Fullerton E. E. , Mangin S. , Kent A. D.
Published: December 2010

The effect of thermal fluctuations on spin-transfer switching has been studied for a broad range of time scales (subnanoseconds to seconds) in a model system, a uniaxial thin film nanomagnet. The nanomagnet is incorporated into a spin-valve nanopillar, which is subject to spin-polarized current pulses of variable amplitude and duration. Two physical regimes are clearly distinguished: a long pulse duration regime, in which reversal occurs by spin-transfer assisted thermal activation over an energy barrier, and a short-time large pulse amplitude regime, in which the switching probability is determined by the spin-angular momentum in the current pulse.
Insight into the contact resistance problem by direct probing of the potential drop in organic field-effect transistors
Authors: Weis Martin , Lin Jack , Taguchi Dai , Manaka Takaaki , Iwamoto Mitsumasa
Published: December 2010

The microscopic electric field induced second-harmonic generation technique is used for direct observation of electric field profiles and evaluation of the potential drop on the injection electrode in the organic field-effect transistors with various channel lengths L. It is found that the potential drop on injection electrode is not a function of L. We show that the analysis of the transmission line model (TLM) cannot distinguish channel length independent contact resistance and potential drop. Tracing back to the conceptional idea of contact resistance proposed by Shockley in 1964, the TLM approach is discussed to explain L-dependent contact resistance.
Observation of anomalous phonons in orthorhombic rare-earth manganites
Authors: Gao P. , Chen H. Y. , Tyson T. A. , Liu Z. X. , Bai J. M. , Wang L. P. , Choi Y. J. , Cheong S.-W.
Published: December 2010

We observe the appearance of a phonon near the lock-in temperature in orthorhombic REMnO3 (RE denotes rare earth) (RE: Lu and Ho) and anomalous phonon hardening in orthorhombic LuMnO3. The anomalous phonon occurs at the onset of spontaneous polarization. No such changes were found in incommensurate orthorhombic DyMnO3. These observations directly reveal different electric polarization mechanisms in the E-type and incommensurate-type orthorhombic REMnO3.
Separation of ultrafast photoexcited electron and hole dynamics in CdSe quantum dots adsorbed onto nanostructured TiO2 films
Authors: Shen Qing , Ayuzawa Yasumasa , Katayama Kenji , Sawada Tsuguo , Toyoda Taro
Published: December 2010

The ultrafast photoexcited electron and hole dynamics of CdSe quantum dots (QDs) deposited onto nanostructured TiO2 films were studied using an improved transient grating (TG) technique. By comparing the TG responses measured in air and in a Na2S solution (hole acceptor), we succeeded in separating the dynamic characteristics of photoexcited electrons and holes in the CdSe QDs. We found that charge separation in the CdSe QDs occurred over a very fast time scale from a few hundreds of femtoseconds in the Na2S solution via hole transfer to S2− ions to a few picoseconds in air via hole trapping.
Isolation blocking voltage of nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structure
Authors: Lo C. F. , Kang T. S. , Liu L. , Chang C. Y. , Pearton S. J. , Kravchenko I. I. , Laboutin O. , Johnson J. W. , Ren F.
Published: December 2010

Nitrogen ion-implanted AlGaN/GaN high electron mobility transistor structures showed an isolation blocking voltage of 900 V with a leakage current at 1 μA/mm across an implanted isolation-gap of 10 μm between two Ohmic pads. The effect of implanted gap distance (1.7, 5, or 10 μm) between two Ohmic contact pads was evaluated. The isolation current density was determined to be solely dependent on the applied field between the contact pads. A model using a combination of resistive current and Poole–Frenkel current is consistent with the experimental data. The resistance of the isolation implantation region significantly decreased after the sample was annealed at temperatures above 600 °C.
Electronic conduction in amorphous and polycrystalline zinc-indium oxide films
Authors: Kuznetsov Vladimir L. , O’Neil David H. , Pepper Michael , Edwards Peter P.
Published: December 2010

We report on the electronic properties of both amorphous and polycrystalline zinc-indium oxide thin films with similar degenerate electron concentrations just above the insulator-to-metal transition. The highest electron mobilities occur in amorphous oxide films deposited at 100 °C; for these, structural disorder is on a spatial scale much smaller than the characteristic electron wavelength (∼3 nm) of the conduction electron gas. For polycrystalline films fabricated at 200–300 °C enhanced electron scattering occurs at evolving grain boundaries when the grain size is comparable to the electron wavelength. Larger, highly crystalline grains form for deposition at 500 °C with concomitant higher carrier mobilities.
Longitudinal spin-Seebeck effect in sintered polycrystalline (Mn,Zn)Fe2O4
Authors: Uchida Ken-ichi , Nonaka Tatsumi , Ota Takeru , Saitoh Eiji
Published: December 2010

This paper reports that the longitudinal spin-Seebeck effect appears even in a sintered polycrystalline Mn-Zn ferrite [(Mn,Zn)Fe2O4] slab. The effect drives a spin current flowing across an interface between the (Mn,Zn)Fe2O4 slab and an attached Pt film along a temperature gradient and it generates electric voltage via the inverse spin-Hall effect in the Pt film. Since the observed phenomenon enables thermal generation of electric voltage from commonly used polycrystalline insulators, it can be used as a versatile and low-cost thermoelectric generator.
Spin-polarized energy-gap opening in asymmetric bilayer graphene nanoribbons
Authors: Kim Gyubong , Jhi Seung-Hoon
Published: December 2010

Electronic and magnetic properties of bilayer zigzag graphene nanoribbon (bZGNR) are studied using pseudopotential density functional method. The edge atoms in the top and bottom layers of bZGNR make a weak hybridization, which leads to electronic structures different from monolayer ZGNR. For asymmetric bZGNR, where the top and bottom layers have different widths, one edge is pinched by the interlayer bonding and the other sustains antiferromagnetic ordering. A small amount of charge transfer occurs from narrower to wider layer, producing spin-polarized electron and hole pockets. External electric field produces asymmetric energy-gap opening for each spin component, inducing half-metallicity in bZGNR.
Optimum areal coverage for perfect transmission in a periodic metal hole array
Authors: Lee J. W. , Park T. H. , Nordlander Peter , Mittleman Daniel M.
Published: December 2010

We investigate the conditions for perfect transmission of terahertz radiation in periodic two-dimensional plasmonic system of square holes. Changing the period in the direction perpendicular to the incident light polarization reveals the optimum areal coverage for perfect transmission at a specific period. The simulated near-zone energy flow distributions show that all incident light is accumulated on the apertures, under the condition that the period is shorter than the resonant wavelength, acting as static local capacitors restricted to a wavelength-confined region. Our finding can be exploited for designing cost-effective terahertz filters and may be applicable to terahertz spectroscopy requiring strong local field enhancement.
Ab initio studies of the effect of nanoclusters on magnetostriction of Fe1−xGax alloys
Authors: Wang Hui , Zhang Y. N. , Yang Teng , Zhang Z. D. , Sun L. Z. , Wu R. Q.
Published: December 2010

Using the density functional calculations, we investigated the effect of nanoprecipitation on the magnetostriction of Fe1−xGax alloys. While the B2-like FeGa clusters undergo slight tetragonal distortion, D03-like FeGa clusters remain cubic in the Fe matrix. Moreover, we found that B2-like nanostructures produce negative magnetostriction, whereas D03-like nanostructures give small positive magnetostriction in the hypothetical inhomogeneous structures. Therefore, the formation of nanoscale precipitates cannot be the reason for the extraordinary enhancement of magnetostriction of Fe1−xGax alloys.
Simultaneous changes of nuclear and magnetic structures across the morphotropic phase boundary in (1−x)BiFeO3−xPbTiO3
Authors: Bhattacharjee Shuvrajyoti , Senyshyn Anatoliy , Krishna P. S. R. , Fuess H. , Pandey Dhananjai
Published: December 2010

We present here the results of a powder neutron diffraction study on the (1−x)BiFeO3–xPbTiO3 (BF-xPT) system, which reveal an abrupt change in magnetic structure from a noncollinear antiferromagnetic one to a collinear G-type antiferromagnetic as the nuclear structure of the ferroelectric phase changes from monoclinic in Cc space group for x≤0.27 to tetragonal in P4mm space group for x≥0.31 across the morphotropic phase boundary (MPB). This reflects the multiferroic character of the MPB in the BF-xPT system.
Preface to Special Topic: Optofluidics
Authors: Liu Ai-Qun
Published: December 2010

This Special Topic section of Biomicrofluidics is on optofluidics or micro-optofluidic systems (MOFS), a burgeoning technology that aims to manipulate light and fluid at microscale and exploits their interaction to create highly versatile devices and integrated systems. This special issue puts together various contributed articles focusing on optofluidics or MOFS, which help inspire new research ideas and innovation in the microfluidics and nanofluidics community.
Carbon-based nanomaterials as contacts to graphene nanoribbons
Authors: Ouyang Yijian , Guo Jing
Published: December 2010

Making good electrical contacts to nanodevice channels is crucial for device performance. Carbon-based nanomaterials are studied and compared to each other as the contact materials for graphene nanoribbons (GNRs) by atomistic quantum transport simulations. A monolayer graphene that has an AB stacking structure on the GNR offers the best electrical contact. Rotation of the graphene layer with regard to the GNR leads to a considerable increase in the contact resistance and contact transfer length. Importance of wave vector matching and interfacial defects is examined for improving electrical contacts to GNRs. This paper could be useful for designing all-carbon-based circuits.
Microwave readout scheme for a Josephson phase qubit
Authors: Wirth T. , Lisenfeld J. , Lukashenko A. , Ustinov A. V.
Published: December 2010

We present experimental results on a microwave scheme for reading out a Josephson phase qubit. A capacitively shunted superconducting quantum interference device (SQUID) is used as a nonlinear resonator which is inductively coupled to the qubit. The flux state of the qubit is detected by measuring the amplitude and phase of a microwave pulse reflected from the SQUID resonator. By this low-dissipative method, the qubit state measurement time is reduced to 25 μs, which is much faster than the conventional readout performed by switching the SQUID to its nonzero dc voltage state. The readout scheme presented here allows for reading out multiple qubits using a single microwave line by employing frequency-division multiplexing.
Formation of nickel germanide contacts to Ge nanowires
Authors: Dellas N. S. , Minassian S. , Redwing J. M. , Mohney S. E.
Published: December 2010

Nickel germanide contacts are expected to play an important role in Ge-based electronics similar to that of their nickel silicide counterparts in Si devices. Here we have studied the solid state reaction between Ni contact pads and Ge nanowires. We observe the formation of axial nickel germanide segments after annealing at temperatures as low as 300 °C for 2 min. The nickel germanide segments are polycrystalline, without an epitaxial relationship to the Ge nanowire, in contrast to observations of epitaxial nickel silicide formation from Si nanowires. The crystal structure of the nickel germanide phase is consistent with the Ni2In prototype structure. Annealing above 400 °C results in fracture in the nickel germanide segment; however, nickel germanide segments as long as 1.7 μm can be formed by annealing at 400 °C for 5 min.