Showing papers in "EPL in 2005"
TL;DR: In this paper, a spin transfer torque term in the Landau-Lifchitz-Gilbert equation was proposed to explain the motion of magnetic domain walls (DW) in nanowires carrying a current.
Abstract: In order to explain recent experiments reporting a motion of magnetic domain walls (DW) in nanowires carrying a current, we propose a modification of the spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that it explains, with reasonable parameters, the measured DW velocities as well as the variation of DW propagation field under current. We also introduce coercivity by considering rough wires. This leads to a finite DW propagation field and finite threshold current for DW propagation, hence we conclude that threshold currents are extrinsic. Some possible models that support this new term are discussed.
992 citations
TL;DR: In this paper, the effects of confinement on active polar gels such as the actin network of eukaryotic cells were studied theoretically and it was shown that spontaneous flow transition from a homogeneously polarized immobile state for small thicknesses to a perturbed flowing state for larger thicknesses is not driven by an external field but by the activity of the system.
Abstract: We study theoretically the effects of confinement on active polar gels such as the actin network of eukaryotic cells. Using generalized hydrodynamics equations derived for active gels, we predict, in the case of quasi–one-dimensional geometry, a spontaneous flow transition from a homogeneously polarized immobile state for small thicknesses, to a perturbed flowing state for larger thicknesses. The transition is not driven by an external field but by the activity of the system. We suggest several possible experimental realizations.
339 citations
TL;DR: In this article, the authors uncover a condition for enhanced synchronization in weighted networks with asymmetric coupling and show that synchronizability is solely determined by the average degree and does not depend on the system size and the details of the degree distribution.
Abstract: Heterogeneity in the degree (connectivity) distribution has been shown to suppress synchronization in networks of symmetrically coupled oscillators with uniform coupling strength (unweighted coupling). Here we uncover a condition for enhanced synchronization in weighted networks with asymmetric coupling. We show that, in the optimum regime, synchronizability is solely determined by the average degree and does not depend on the system size and the details of the degree distribution. In scale-free networks, where the average degree may increase with heterogeneity, synchronizability is drastically enhanced and may become positively correlated with heterogeneity, while the overall cost involved in the network coupling is significantly reduced as compared to the case of unweighted coupling.
338 citations
TL;DR: O'Hern et al. as discussed by the authors analyzed the density D(ω) of normal-mode frequencies ω and the nature of the low-frequency normal modes of a recently simulated system, comprised of weakly compressed spheres at zero temperature.
Abstract: Glasses have an excess number of low-frequency vibrational modes in comparison with most crystalline solids. We show that such a feature necessarily occurs in solids with low coordination. In particular, we analyze the density D(ω) of normal-mode frequencies ω and the nature of the low-frequency normal modes of a recently simulated system (O'Hern C., Silbert L. E., Liu A. J. and Nagel S. R., Phys. Rev. E, 68 (2003) 011306) comprised of weakly compressed spheres at zero temperature. We account for the observed a) convergence of D(ω) toward a non-zero constant as the frequency goes to zero, b) appearance of a low-frequency cutoff ω*, and c) power law increase of ω* with compression. We introduce a length scale l* which characterizes the vibrational modes that appear at ω*.
298 citations
TL;DR: A theoretical model for ionic transport through synthetic conical nanopores obtained using the track-etching technique is presented in this paper, based on the Poisson and Nernst-Planck equations.
Abstract: A theoretical model for ionic transport through synthetic conical nanopores obtained using the track-etching technique is presented. The model is based on the Poisson and Nernst-Planck equations. The results provided by the theory are compared with recent experimental current-voltage curves obtained for polymeric membranes containing single, gold-coated conical nanopores. The calculated profiles of average concentration and electric potential along the pore symmetry axis allow for an intuitive explanation of the rectification properties observed in these systems.
253 citations
TL;DR: A conceptually simple method for hierarchical clustering of data called mutual information clustering (MIC) algorithm that uses mutual information (MI) as a similarity measure and exploits its grouping property.
Abstract: We present a conceptually simple method for hierarchical clustering of data called mutual information clustering (MIC) algorithm. It uses mutual information (MI) as a similarity measure and exploits its grouping property: The MI between three objects X, Y, and Z is equal to the sum of the MI between X and Y, plus the MI between Z and the combined object (XY). We use this both in the Shannon (probabilistic) version of information theory and in the Kolmogorov (algorithmic) version. We apply our method to the construction of phylogenetic trees from mitochondrial DNA sequences and to the output of independent components analysis (ICA) as illustrated with the ECG of a pregnant woman.
244 citations
TL;DR: This model of community network is presented and it is found that, compared to the random network, the community network has a broader degree distribution, a smaller threshold of epidemic outbreak, and more prevalence to keep the outbreak endemic.
Abstract: Social networks have the structure of communities To understand how the community structure affects epidemic spreading, we present a simplified model of community network, and investigate the epidemic propagation in this model We find that, compared to the random network, the community network has a broader degree distribution, a smaller threshold of epidemic outbreak, and more prevalence to keep the outbreak endemic The formulae of epidemic threshold are given and confirmed by numerical simulations
235 citations
TL;DR: In this paper, a model of magnetic interactions in the ordered ferromagnetic FePt is proposed on the basis of first-principles calculations of non-collinear magnetic configurations and shown to be capable of explaining recent measurements of magnetic-anisotropy energy (MAE).
Abstract: A model of magnetic interactions in the ordered ferromagnetic FePt is proposed on the basis of first-principles calculations of non-collinear magnetic configurations and shown to be capable of explaining recent measurements of magnetic-anisotropy energy (MAE). The site (Fe,Pt) resolved contributions to the MAE have been distinguished with small Fe easy-plane and large Pt easy-axis terms. This model has been tested against available experimental data on the temperature dependence of MAE showing scaling of uniaxial MAE (K1(T)) with magnetization (M(T)) K1(T) ~ M(T)γ characterized by the unusual exponent of γ = 2.1. It is shown that this unusual behavior of the FePt can be quantitatively explained within the proposed model and originates from an effective anisotropic exchange mediated by the induced Pt moment. The latter is expected to be a common feature of 3d-5d(4d) alloys having 5d/4d elements with large spin-orbit coupling and exchange-enhanced Stoner susceptibility.
233 citations
TL;DR: A general model that incorporates local traffic information through a tunable parameter is studied, showing that whether the transition from a low-traffic regime to a congested phase is of first- or second-order type is determined by the protocol at work.
Abstract: We numerically investigate jamming transitions in complex heterogeneous networks. Inspired by Internet routing protocols, we study a general model that incorporates local traffic information through a tunable parameter. The results show that whether the transition from a low-traffic regime to a congested phase is of first- or second-order type is determined by the protocol at work. The microscopic dynamics reveals that these two radically different behaviors are due to the way in which traffic jams propagate through the network. Our results are discussed in the context of Internet dynamics and other transport processes that take place on complex networks and provide insights for the design of routing policies based on traffic awareness in communication systems.
227 citations
TL;DR: In this paper, the electronic structure of the monoclinic structure of Fe3O4 was studied using both the local density approximation (LDA) and the LDA+U.
Abstract: The electronic structure of the monoclinic structure of Fe3O4 is studied using both the local density approximation (LDA) and the LDA+U. The LDA gives only a small charge disproportionation, thus excluding that the structural distortion should be sufficient to give a charge order. The LDA+U results in a charge disproportion along the c-axis in good agreement with the experiment. We also show how the effective U can be calculated within the augmented plane-wave methods.
212 citations
TL;DR: In this article, the free energy difference ΔF between two equilibrium states is estimated using the Jarzynski equality and the Crooks relation, independently of the time scale and amplitude of the driving force.
Abstract: We have experimentally checked the Jarzynski equality and the Crooks relation (see Jarzynski C., Phys. Rev. Lett., 78 (1997) 2690; Crooks G. E., Phys. Rev. E, 60 (1999) 2721; Jarzynski C., J. Stat. Phys., 98 (2000) 77) on the thermal fluctuations of a macroscopic mechanical oscillator in contact with a heat reservoir. We found that, independently of the time scale and amplitude of the driving force, both relations are satisfied. These results give credit, at least in the case of Gaussian fluctuations, to the use of these relations in biological and chemical systems to estimate the free energy difference ΔF between two equilibrium states (see Ritort F., Seminaire Poincare, 2 (2003) 63; Liphardt J., Dumont S., Smith S. B., Ticono I. jr and Bustamante C., Science, 296 (2002) 1832). An alternative method to estimate ΔF in an isothermal process is proposed too.
TL;DR: In this article, a double-cavity set-up capable of generating a stationary entangled state of two movable mirrors at cryogenic temperatures is proposed, based on the optimal transfer of squeezing of input optical fields to mechanical vibrational modes of the mirrors.
Abstract: We propose a double-cavity set-up capable of generating a stationary entangled state of two movable mirrors at cryogenic temperatures. The scheme is based on the optimal transfer of squeezing of input optical fields to mechanical vibrational modes of the mirrors, realized by the radiation pressure of the intracavity light. We show that the presence of macroscopic entanglement can be demonstrated by an appropriate readout of the output light of the two cavities.
TL;DR: In this paper, the concept of equilibrium inequality in a society, based on the principle of maximal entropy, and quantitatively show that it applies to the majority of population is discussed.
Abstract: Personal income distribution in the USA has a well-defined two-class structure. The majority of population (97–99%) belongs to the lower class characterized by the exponential Boltzmann-Gibbs ("thermal") distribution, whereas the upper class (1–3% of the population) has a Pareto power law ("superthermal") distribution. By analyzing income data for 1983–2001, we show that the "thermal" part is stationary in time, save for a gradual increase of the effective temperature, whereas the "superthermal" tail swells and shrinks following the stock market. We discuss the concept of equilibrium inequality in a society, based on the principle of maximal entropy, and quantitatively show that it applies to the majority of population.
TL;DR: In this article, the authors developed a theory determining the quantities J, λ 1, √ λ 2 from the statistical properties of the gas flow, in the limit of short correlation times.
Abstract: Networks of caustics can occur in the distribution of particles suspended in a randomly moving gas. These can facilitate coagulation of particles by bringing them into close proximity, even in cases where the trajectories do not coalesce. The evolution of these caustic patterns depends upon the Lyapunov exponents λ1, λ2 of the suspended particles, as well as the rate J at which particles encounter caustics. We develop a theory determining the quantities J, λ1, λ2 from the statistical properties of the gas flow, in the limit of short correlation times.
TL;DR: Inelastic neutron scattering, susceptibility, and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic, next-nearest-nighbour frustrated, quasi-one-dimensional helimagnet as discussed by the authors.
Abstract: Inelastic neutron scattering,susceptibility,and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic,next-nearest-neighbour frustrated,quasi-one- dimensional helimagnet,which is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals.
TL;DR: In this paper, the effect of pressure on the contact angle of a water drop on superhydrophobic carbon nanotube (CNT) forests is studied, and it is shown that these surfaces are able to withstand high excess pressures (larger than 10 kPa) without transiting toward a roughness-invaded state.
Abstract: In this paper the effect of pressure on the contact angle of a water drop on superhydrophobic carbon nanotube (CNT) forests is studied. Superhydrophobic CNT forests are obtained from a new and simple functionalization strategy, based on the gold-thiol affinity. Using a specifically devised experimental setup, we then show that these surfaces are able to withstand high excess pressures (larger than 10 kPa) without transiting toward a roughness-invaded state, therefore preserving their low adhesion properties. Together with the relatively low technical cost of the process, this robustness vs. pressure makes such surfaces very appealing for practical integration into microfluidic systems.
TL;DR: In this paper, the lattice Boltzmann algorithm is adapted to include noise, satisfying a fluctuation-dissipation theorem (FDT) directly at lattice level: this gives correct fluctuations for mass and momentum densities, and for stresses, at all wave vectors k. Unlike previous work which recovers FDT only as k → 0, our algorithm offers full statistical mechanical consistency in mesoscale simulations of, e.g., fluctuating colloidal hydrodynamics.
Abstract: The lattice Boltzmann algorithm efficiently simulates the Navier-Stokes equation of isothermal fluid flow, but ignores thermal fluctuations of the fluid, important in mesoscopic flows. We show how to adapt the algorithm to include noise, satisfying a fluctuation-dissipation theorem (FDT) directly at lattice level: this gives correct fluctuations for mass and momentum densities, and for stresses, at all wave vectors k. Unlike previous work, which recovers FDT only as k → 0, our algorithm offers full statistical mechanical consistency in mesoscale simulations of, e.g., fluctuating colloidal hydrodynamics.
TL;DR: In this article, the voter model dynamics in random networks with an arbitrary distribution of the degree of the nodes were considered and it was shown that for the usual node-update dynamics the average magnetization is not conserved.
Abstract: We consider the voter model dynamics in random networks with an arbitrary distribution of the degree of the nodes. We find that for the usual node-update dynamics the average magnetization is not conserved, while an average magnetization weighted by the degree of the node is conserved. However, for a link-update dynamics the average magnetization is still conserved. For the particular case of a Barabasi-Albert scale-free network, the voter model dynamics leads to a partially ordered metastable state with a finite-size survival time. This characteristic time scales linearly with system size only when the updating rule respects the conservation law of the average magnetization. This scaling identifies a universal or generic property of the voter model dynamics associated with the conservation law of the magnetization.
TL;DR: A new method to detect phase as well as generalized synchronization in a wide class of complex systems based on the recurrences of the system's trajectory to the neighborhood of a former state in phase space is presented.
Abstract: We present a new method to detect phase as well as generalized synchronization in a wide class of complex systems. It is based on the recurrences of the system's trajectory to the neighborhood of a former state in phase space. We illustrate the applicability of the algorithm for the paradigmatic chaotic Rossler system in the funnel regime and for noisy data, where other methods to detect phase synchronization fail. Furthermore, we demonstrate for electrochemical experiments that the method can easily detect phase and generalized synchronization in non-phase-coherent and even non-stationary time series.
TL;DR: In this article, an exact quantization rule for the Schrodinger equation is presented, in which in addition to Nπ, there is an integral term, called the quantum correction, which is invariant, independent of the number of nodes in the wave function.
Abstract: An exact quantization rule for the Schrodinger equation is presented. In the exact quantization rule, in addition to Nπ, there is an integral term, called the quantum correction. For the exactly solvable systems we find that the quantum correction is an invariant, independent of the number of nodes in the wave function. In those systems, the energy levels of all the bound states can be easily calculated from the exact quantization rule and the solution for the ground state, which can be obtained by solving the Riccati equation. With this new method, we re-calculate the energy levels for the one-dimensional systems with a finite square well, with the Morse potential, with the symmetric and asymmetric Rosen-Morse potentials, and with the first and the second Poschl-Teller potentials, for the harmonic oscillators both in one dimension and in three dimensions, and for the hydrogen atom.
TL;DR: In this paper, the authors established a dynamic length scale below which Fickian diffusion breaks down, as is observed in experiments and simulations, and provided analytical estimates for the van Hove and self-intermediate scattering functions.
Abstract: The interplay between self-diffusion and excitation lines in space-time was recently studied in kinetically constrained models to explain the breakdown of the Stokes-Einstein law in supercooled liquids. Here, we further examine this interplay and its manifestation in incoherent scattering functions. In particular, we establish a dynamic length scale below which Fickian diffusion breaks down, as is observed in experiments and simulations. We describe the temperature dependence of this length scale in liquids of various fragilities, and provide analytical estimates for the van Hove and self-intermediate scattering functions.
TL;DR: In this paper, a superconducting electromagnetic resonant cavity with a wall covered by a semiconductor layer whose reflectivity is driven by a laser at gigahertz frequencies is used as a moving mirror.
Abstract: In order to observe the Casimir radiation we propose a new experimental scheme with no mechanically moving mirror. In fact we estimate that the power required for a sustained mechanical vibration would be beyond present experimental possibilities. Our apparatus consists of a superconducting electromagnetic resonant cavity with a wall covered by a semiconductor layer whose reflectivity is driven by a laser at gigahertz frequencies. The semiconductor thus acts as a moving mirror. Preliminary laboratory tests showed that a semiconductor can indeed reflect microwaves as efficiently as a conductor. In this paper we present the complete scheme that we intend to set up for the detection of the Casimir radiation.
TL;DR: In this article, a general sequential probabilistic frame is presented, which extends a series of earlier opinion dynamics models, and orders and classifies all of the existing two-state spin systems.
Abstract: We present a general sequential probabilistic frame, which extends a series of earlier opinion dynamics models. In addition, it orders and classifies all of the existing two-state spin systems. The scheme operates via local updates where a majority rule is applied differently in each possible configuration of a local group. It is weighted by a local probability which is a function of the local value of the order parameter, i.e., the majority-to-minority ratio. The system is thus driven from one equilibrium state into another equilibrium state till no collective change occurs. A phase diagram can thus be constructed. It has two phases, one where the collective opinion ends up broken along one opinion, and another with an even coexistence of both opinions. Two different regimes, monotonic and dampened oscillatory, are found for the coexistence phase. At the phase transition local probabilities conserve the density of opinions and reproduce the collective dynamics of the Voter model. The essential behavior of all existing discrete two-state models (Galam, Sznajd, Ochrombel, Stauffer, Krapivsky-Redner, Mobilia-Redner, Behera-Schweitzer, Slanina-Lavicka, Sanchez ...) is recovered and found to depart from each other only in the value of their local probabilities. Corresponding simulations are discussed. It is concluded that one should not judge from the above model results the validity of their respective psycho-social assumptions.
TL;DR: In this paper, a simple surface potential favoring the planar degenerate anchoring of nematic liquid crystals was proposed, i.e., the tendency of the molecules to align parallel to one another along any direction parallel to the surface.
Abstract: We propose a simple surface potential favoring the planar degenerate anchoring of nematic liquid crystals, i.e., the tendency of the molecules to align parallel to one another along any direction parallel to the surface. We show that, at lowest order in the tensorial Landau-de Gennes order-parameter, fourth-order terms must be included. We analyze the anchoring and wetting properties of this surface potential. In the nematic phase, we find the desired degenerate planar anchoring, with positive scalar order-parameter and some surface biaxiality. In the isotropic phase, we find, in agreement with experiments, that the wetting layer may exhibit a uniaxial ordering with negative scalar order-parameter. For large enough anchoring strength, this negative ordering transits towards the planar degenerate state.
TL;DR: In this article, the authors analyze the evolution of political organizations using a model in which agents change their opinions via two competing mechanisms: two agents may interact and reach consensus, and additionally, individual agents may spontaneously change their opinion by a random, diffusive process.
Abstract: We analyze the evolution of political organizations using a model in which agents change their opinions via two competing mechanisms. Two agents may interact and reach consensus, and additionally, individual agents may spontaneously change their opinions by a random, diffusive process. We find three distinct possibilities. For strong diffusion, the distribution of opinions is uniform and no political organizations (parties) are formed. For weak diffusion, parties do form and furthermore, the political landscape continually evolves as small parties merge into larger ones. Without diffusion, a pattern develops: parties have the same size and they possess equal niches. These phenomena are analyzed using pattern formation and scaling techniques.
TL;DR: In this article, a modified lattice Boltzmann model with a stochastic relaxation mechanism mimicking virtual collisions between free-streaming particles and solid walls is introduced, which permits to compute plane channel flows in satisfactory agreement with analytical results.
Abstract: A modified lattice Boltzmann model with a stochastic relaxation mechanism mimicking "virtual" collisions between free-streaming particles and solid walls is introduced. This modified scheme permits to compute plane channel flows in satisfactory agreement with analytical results over a broad spectrum of Knudsen numbers, ranging from the hydrodynamic regime, all the way to quasi-free flow regimes up to Kn ~ 30.
TL;DR: It is shown that, contrary to the model's initial prediction, a stable bilingual situation is possible if the languages in competition are similar enough and a simple parameter is described, whose value can be estimated from fits of the data.
Abstract: We analyze the time evolution of a system of two coexisting languages (Castillian Spanish and Galician, both spoken in northwest Spain) in the framework of a model given by Abrams and Strogatz (Nature 424 (2003) 900). It is shown that, contrary to the model's initial prediction, a stable bilingual situation is possible if the languages in competition are similar enough. Similarity is described with a simple parameter, whose value can be estimated from fits of the data.
TL;DR: In this paper, the rotational and translational dynamics of interfacial, non-crystalline, water from 77 to 280 K were measured and combined with calorimetric and diffraction data to show that after exhibiting a glass transition at 165 K, interfacial water experiences a first-order liquid-liquid transition at 240 K from a low density to a high-density liquid.
Abstract: At ambient pressure, bulk liquid water shows an anomalous increase of thermodynamic quantities and apparent divergences of dynamic properties on approaching a temperature Ts of 228 K. At normal pressure, supercooled water spontaneously freezes below the homogeneous nucleation temperature, TH = 235 K. Upon heating, the two forms of Amorphous Solid Water (ASW), LDA (Low Density Amorphous Ice) and HDA (High Density Amorphous Ice), crystallise above TX = 150 K. As a consequence, up to now no experiment has been able to explore the properties of liquid water in this very interesting temperature range between 150 and 235 K. We present nanosecond-time-scale measurements of local rotational and translational dynamics of interfacial, non-crystalline, water from 77 to 280 K. These experimental dynamic results are combined with calorimetric and diffraction data to show that after exhibiting a glass transition at 165 K, interfacial water experiences a first-order liquid-liquid transition at 240 K from a low-density to a high-density liquid. This is the first direct evidence of the existence of a liquid-liquid transition involving water.
TL;DR: In this article, the authors developed a theory for the full counting statistics for a class of nanoelectromechanical systems (NEMS), describable by a Markovian generalized master equation.
Abstract: We develop a theory for the full counting statistics (FCS) for a class of nanoelectromechanical systems (NEMS), describable by a Markovian generalized master equation The theory is applied to two specific examples of current interest: vibrating C60-molecules and quantum shuttles We report a numerical evaluation of the first three cumulants for the C60 setup; for the quantum shuttle we use the third cumulant to substantiate that the giant enhancement in noise observed at the shuttling transition is due to a slow switching between two competing conduction channels Especially the last example illustrates the power of the FCS
TL;DR: In this article, the authors use minimum spanning trees (MSTs) to explore the effect of correlations between link weights and network topology on scale-free networks and show that the structure of the MSTs can change from scale free to exponential with increasing network size.
Abstract: A complete characterization of real networks requires us to understand the consequences of the uneven interaction strengths between a system's components. Here we use minimum spanning trees (MSTs) to explore the effect of correlations between link weights and network topology on scale-free networks. Solely by changing the nature of the correlations between weights and network topology, the structure of the MSTs can change from scale-free to exponential. Additionally, for some choices of weight correlations, the efficiency of the MSTs increases with increasing network size, a result with potential implications for the design and scalability of communication networks.