Showing papers in "EPL in 2002"

Scale-free networks from optimal design

Abstract: A large number of complex networks, both natural and artificial, share the presence of highly heterogeneous, scale-free degree distributions. A few mechanisms for the emergence of such patterns have been suggested, optimization not being one of them. In this letter we present the first evidence for the emergence of scaling (and the presence of small-world behavior) in software architecture graphs from a well-defined local optimization process. Although the rules that define the strategies involved in software engineering should lead to a tree-like structure, the final net is scale-free, perhaps reflecting the presence of conflicting constraints unavoidable in a multidimensional optimization process. The consequences for other complex networks are outlined.

Non?diffraction-limited light transport by gold nanowires

Abstract: We show that metal nanowires sustaining collective electron oscillations (surface plasmon polaritons) can be used as optical waveguides. Thereby, the use of a metal allows to overcome the limitations of miniaturization imposed on conventional dielectric waveguides due to diffraction. To demonstrate this effect we investigate a 200 nm wide and 50 nm high gold nanowire locally excited at a light wavelength of 800 nm. By direct imaging the optical near-field with subwavelength-resolution photon scanning tunneling microscopy we observe light transport along the nanowire over a distance of a few μm. Besides the realization of unprecedented integration densities of photonic devices, metal nanowires could be effectively used to optically address individual nanostructures or molecules.

Rectification and voltage gating of ion currents in a nanofabricated pore

Abstract: We have fabricated a voltage sensor in the form of a conically shaped nanopore in a polyethylene terephthalate (PET) foil. The pore is produced by irradiation of the foil with a single heavy ion and subsequent etching in alkaline solution. The resulting pore functions as a voltage gate and rectifies ion current due to changes of its diameter in an electrical field. Ion currents through the pore show voltage-dependent fluctuations, whose kinetics are similar as in voltage-gated biological ion channels and pores.

Cooperative motion and growing length scales in supercooled confined liquids

Abstract: Using molecular-dynamics simulations we investigate the relaxation dynamics of a supercooled liquid close to a rough as well as to a smooth wall. For the former situation the relaxation times increase strongly with decreasing distance from the wall, whereas in the second case they strongly decrease. We use this dependence to extract various dynamical length scales and show that they grow with decreasing temperature. Finally, we calculate the frequency-dependent average susceptibility of such confined systems and show that they compare very well with experimental data.

Instability of scale-free networks under node-breaking avalanches

Abstract: The instability introduced in a large scale-free network by the triggering of node-breaking avalanches is analyzed using the fiber-bundle model as conceptual framework. We found, by measuring the size of the giant component, the avalanche size distribution and other quantities, the existence of an abrupt transition. This test of strength for complex networks like Internet is more stringent than others recently considered like the random removal of nodes, analyzed within the framework of percolation theory. Finally, we discuss the possible implications of our results and their relevance in forecasting cascading failures in scale-free networks.

The non-Abelian density matrix renormalization group algorithm

Abstract: We describe here the extension of the density matrix renormalization group algorithm to the case where the Hamiltonian has a non-Abelian global symmetry group. The block states transform as irreducible representations of the non-Abelian group. Since the representations are multi-dimensional, a single block state in the new representation corresponds to multiple states of the original density matrix renormalization group basis. We demonstrate the usefulness of the construction via the one-dimensional Hubbard model as the symmetry group is enlarged from U(1)×U(1), up to SU(2)×SU(2).

Compaction dynamics of a granular medium under vertical tapping

Abstract: We report new experimental results on granular compaction under consecutive vertical taps. The evolution of the mean volume fraction and of the mean potential energy of a granular packing presents a slow densification until a final steady state, and is reminiscent of usual relaxation in glasses via a stretched exponential law. The intensity of the taps seems to rule the characteristic time of the relaxation according to an Arrhenius's type relation. Finally, the analysis of the vertical volume fraction profile reveals an almost homogeneous densification in the packing.

Dynamics of sleep-wake transitions during sleep

Abstract: We study the dynamics of the awakening during the night for healthy subjects and find that the wake and the sleep periods exhibit completely different behavior: the durations of wake periods are characterized by a scale-free power law distribution, while the durations of sleep periods have an exponential distribution with a characteristic time scale. We find that the characteristic time scale of sleep periods changes throughout the night. In contrast, there is no measurable variation in the power law behavior for the durations of wake periods. We develop a stochastic model which agrees with the data and suggests that the difference in the dynamics of sleep and wake states arises from the constraints on the number of microstates in the sleep-wake system.

Dendritic magnetic instability in superconducting MgB2 films

Abstract: Magneto-optical imaging reveal that below 10 K the penetration of magnetic flux in MgB2 films is dominated by dendritic structures abruptly formed in response to an applied field. The dendrites show a temperature-dependent morphology ranging from quasi-1D at 4 K to large tree-like structures near 10 K. This behaviour is responsible for the anomalous noise found in magnetization curves, and strongly suppresses the apparent critical current. The instability is of thermo-magnetic origin, as supported by our simulations of vortex dynamics reproducing the variety of dendritic flux patterns.

Secure exchange of information by synchronization of neural networks

Abstract: A connection between the theory of neural networks and cryptography is presented. A new phenomenon, namely synchronization of neural networks is leading to a new method of exchange of secret messages. Numerical simulations show that two artificial networks being trained by Hebbian learning rule on their mutual outputs develop an antiparallel state of their synaptic weights. The synchronized weights are used to construct an ephemeral key exchange protocol for a secure transmission of secret data. It is shown that an opponent who knows the protocol and all details of any transmission of the data has no chance to decrypt the secret message, since tracking the weights is a hard problem compared to synchronization. The complexity of the generation of the secure channel is linear with the size of the network.

The Stokes efficiency for molecular motors and its applications

Abstract: In many experiments it is only possible to load a molecular motor by manipulating the viscous drag from the fluid medium In these situations, it is not clear how to measure the efficiency of energy transduction Here we define the Stokes efficiency as the average viscous drag times the average velocity divided by the rate of free-energy consumption We show that this efficiency is bounded by 100% and thus is well defined We compare the Stokes efficiency with the usual thermodynamic efficiency, and discuss what information it provides about the motor mechanism

Keyword detection in natural languages and DNA

Abstract: We show that words in a text present long-range frequency fluctuations due to a strong self-attraction, that is directly related to the relevance of the term to the text considered. The standard deviation of the distance between successive occurrences of a word is an excellent parameter to quantify this self-attraction, and provides us with an effective tool for automatic keyword extraction. DNA sequences also present the same features: words, for example codons in the coding part of the sequences, attract between themselves.

Water structure and solute association in dilute aqueous methanol

Abstract: The molecular structure of a dilute aqueous methanol solution (1:19 molar ratio) is studied using neutron diffraction with hydrogen/deuterium isotope substitution. The Empirical Potential Structure Refinement (EPSR) method is used to create a structural model of the solution which is consistent with the measured diffraction data. Direct evidence of methanol association is found with more than 80% of the methanol molecules in clusters of 3 to 8 molecules. Contrary to several previous studies, and conventional views on the hydrophobic interaction, no enhancement of the water structure surrounding the non-polar groups is observed. Consistent with results from other related systems, a compression of the second-neighbour water-water contact distance is observed that may be a structural feature of the hydrophobic driving force.

Elastic properties of a-SiO2 up to 2300 K from Brillouin scattering measurements

Abstract: The velocities of the transverse and longitudinal acoustic modes of silica glass and liquid have been measured by Brillouin spectroscopy from 300 to 2300 K. Elastic properties of liquid SiO2 could thus be studied over an interval of 800 K above the standard glass transition temperature. The velocity of the transverse mode, and thus the shear modulus, first increases with temperature and then decreases after reaching a maximum near the glass transition temperature. The velocity of the longitudinal mode and the bulk modulus, in contrast, continuously increase up to the maximum temperatures investigated. This unusual increase of the bulk modulus with temperature becomes less strong above 900 K. The vibrational (high frequency) contribution to the compressibility of the liquid, which is about three times smaller than the configurational (static) part, is thus insensitive to the configurational state of the liquid.

Semicausal operations are semilocalizable

Abstract: We prove a conjecture by DiVincenzo, which in the terminology of Preskill et al. states that semicausal operations are semilocalizable. That is, we show that any operation on the combined system of Alice and Bob, which does not allow Bob to send messages to Alice, can be represented as an operation by Alice, transmitting a quantum particle to Bob, and a local operation by Bob. The proof is based on the uniqueness of the Stinespring representation for a completely positive map. We sketch some of the problems in transferring these concepts to the context of relativistic quantum field theory.

Density-dependent pair interactions in 2D

Abstract: We experimentally determine effective interparticle potentials in a two-dimensional (2D) colloidal system of charge-stabilized polystyrene particles at different particle densities ρ. Density variation is achieved by means of a scanned optical laser tweezer which serves to create a boundary box for the system. By changing the size of this boundary box, ρ can be systematically varied without having to prepare a new system. From the measured radial distribution functions we can then obtain the effective pair potentials of the particles. While for low particle densities perfect agreement with Yukawa-like potentials is observed, considerable deviations from this form are found at higher densities. We interpret this result as a many-body effect produced by macroion screening which is expected to become more pronounced as the density is increased.

Mean-field threshold systems and phase dynamics: An application to earthquake fault systems

Abstract: Driven mean-field threshold systems demonstrate complex observable space-time patterns of behavior that are difficult to understand or predict without knowledge of the underlying dynamics, which are typically unobservable. Here we describe a new method based on phase dynamics techniques to analyze and forecast the space-time patterns of activity in these systems. Application to earthquake data from a typical, seismically active region shows that the method holds considerable promise for forecasting the temporal occurrence of the largest future events. We demonstrate the power of our technique via an application to the difficult problem of earthquake forecasting in southern California.

Crystallization of PDMS: The effect of physical and chemical crosslinks

Abstract: Calorimetric studies of the crystallization behavior of physically and chemically crosslinked semicrystalline polymer, polydimethylsiloxane (PDMS) are presented. Physical crosslinks are introduced either via entanglements in high-molecular-weight PDMS, or by anchoring chain ends to rigid polyethylene oxide (PEO) endblocks in a PEO-b-PDMS-b-PEO triblock copolymer. Chemical end-linking of di-vinyl PDMS chains results in the formation of a crosslinked network. Comparison of the thermograms obtained for each of these systems at constant cooling/heating rates with their noncrosslinked analogues indicates that, contrary to conventional wisdom, the different types of crosslinks result in an increased crystallization tendency. We suggest that this effect is a manifestation of the enhancement of local ordering together with reduced dynamics as compared to the non-crosslinked melt.

Overcharging: The crucial role of excluded volume

Abstract: In this letter we investigate the mechanism for the overcharging of a single spherical colloid in the presence of aqueous salts within the framework of the primitive model by molecular dynamics (MD) simulations as well as integral-equation theory. We find that the occurrence and strength of overcharging strongly depends on the salt-ion size, and the available volume in the fluid. To understand the role of the excluded volume of the microions, we first consider an uncharged system. For a fixed bulk concentration we find that upon increasing the fluid particle size one strongly increases the local concentration nearby the colloidal surface and that the particles become laterally ordered. For a charged system the first surface layer is built up predominantly by strongly correlated counterions. We argue that this is a key mechanism to produce overcharging with a low electrostatic coupling, and as a more practical consequence, to account for charge inversion with monovalent aqueous salt ions.

Influence of correlations on the velocity statistics of scalar granular gases

Abstract: The free evolution of inelastic particles in one dimension is studied by means of Molecular Dynamics (MD), of an inelastic pseudo-Maxwell model and of a lattice model, with emphasis on the role of spatial correlations. We present a new exact solution of the 1d granular pseudo-Maxwell model for the scaling distribution of velocities and discuss how this model fails to describe correctly the homogeneous cooling stage of the 1d granular gas. Embedding the pseudo-Maxwell gas on a lattice (hence allowing for the onset of spatial correlations), we find a much better agreement with the MD simulations even in the inhomogeneous regime. This is seen by comparing the velocity distributions, the velocity profiles and the structure factors of the velocity field.

Superluminal tunneling through two successive barriers

Abstract: We study the phenomenon of one-dimensional non-resonant tunneling through two successive (opaque) potential barriers, separated by an intermediate free region R ,b y analyzing the relevant solutions to the Schroedinger equation. We find that the total traversal time does not depend not only on the barrier widths (the so-called "Hartman effect"), but also on the R width: so that the effective velocity in the region R, between the two barriers, can be regarded as practically infinite. This agrees with the results known from the corresponding waveguide experiments, which simulated the tunneling experiment herein considered due to the known formal identity between the Schroedinger and the Helmholtz equation.

Vibrations of amorphous, nanometric structures: When does continuum theory apply?

Abstract: We investigate the low-frequency end of the vibrational spectrum in small (nanometric) disordered systems. Using numerical simulation and exact diagonalization for simple two-dimensional models, we show that continuum elasticity, applied to these systems, actually breaks down below a length scale of typically 30 to 50 molecular sizes. This length scale is likely related to the one which is generally invoked to explain the peculiar vibrational properties of glassy systems.

On the formation/dissolution of equilibrium droplets

Abstract: We consider liquid-vapor systems in finite-volume V ⊂ R d at parameter values corresponding to phase coexistence and study droplet formation due to a fixed excess δN of par- ticles above the ambient gas density. We identify a dimensionless parameter ∆ ∼ (δN) (d+1)/d /V and a universal value ∆c =∆ c(d), and show that a droplet of the dense phase occurs when- ever ∆ > ∆c, while, for ∆ < ∆c, the excess is entirely absorbed into the gaseous background. When the droplet first forms, it comprises a non-trivial, universal fraction of excess particles. Similar reasoning applies to generic two-phase systems at phase coexistence including solid/gas —where the "droplet" is crystalline— and polymorphic systems. A sketch of a rigorous proof for the 2D Ising lattice gas is presented; generalizations are discussed heuristically.

Uniaxial relaxor ferroelectrics: The ferroic random-field Ising model materialized at last

Abstract: Owing to their intrinsic charge disorder ferroelectric crystals of strontium-barium-niobate doped with Ce3+ materialize the three-dimensional ferroic random-field Ising model (RFIM) as evidenced by order paramenter and susceptibility criticalities with 93Nb NMR and dielectric spectroscopy, respectively. Upon cooling towards Tc, extreme critical slowing-down due to activated dynamic scaling gives rise to relaxor-like dispersion of the susceptibility and to a metastable ferroelectric nanodomain state with fractal size distribution as imaged by piezoelectric force microscopy.

The measurement by SAXS of the nematic order parameter of laponite gels

Abstract: We performed small-angle X-ray scattering (SAXS) experiments on oriented samples of laponite clay gels obtained by slow evaporation. The SAXS patterns are clearly anisotropic, which demonstrates the existence of nematic-like orientational correlations of the laponite disc-like particles. The iso-intensity lines of the SAXS patterns are elliptical and roughly homothetic over the whole scattering vector range examined in this experiment. The value of the nematic order parameter, , derived from the SAXS patterns is comparable to that of usual liquid crystals. This large value proves the importance of orientational correlations in these gels at concentrations higher than 0.02 g·cm−3, even in the absence of shear.

Sensitivity to initial conditions at bifurcations in one-dimensional nonlinear maps: Rigorous nonextensive solutions

Abstract: Using the Feigenbaum renormalization group (RG) transformation we work out exactly the dynamics and the sensitivity to initial conditions for unimodal maps of nonlinearity ζ > 1 at both their pitchfork and tangent bifurcations. These functions have the form of q-exponentials as proposed in Tsallis' generalization of statistical mechanics. We determine the q-indices that characterize these universality classes and perform for the first time the calculation of the q-generalized Lyapunov coefficient λq. The pitchfork and the left-hand side of the tangent bifurcations display weak insensitivity to initial conditions, while the right-hand side of the tangent bifurcations presents a super-strong (faster than exponential) sensitivity to initial conditions. We corroborate our analytical results with a priori numerical calculations.

High-energy tails for inelastic Maxwell models

Abstract: Monte Carlo simulations of the spatially homogeneous Boltzmann equation for inelastic Maxwell molecules, performed by Baldassarri et al. (cond-mat/0111066), have shown that general classes of initial distributions evolve for large times into a singular nonlinear scaling solution with a power law tail. By applying an asymptotic analysis we derive these results from the nonlinear Boltzmann equation, and obtain a transcendental equation from which the exponents, appearing in the power law tails, can be calculated. The dynamics of this model describes a dissipative flow in v-space, which drives the system to an attractor, the nonlinear scaling solution, with a constant negative rate of irreversible entropy production, given by − ¼(1 − α2), where α is the coefficient of restitution.

Broken-bond rule for the surface energies of noble metals

Abstract: Using two different full-potential ab initio techniques we introduce a simple rule based on the number of broken first-neighbour bonds to determine the surface energies of the three noble metals Cu, Ag and Au When one bond is broken, the rearrangement of the electronic charge for these metals does not practically lead to a change of the remaining bonds Thus the energy needed to break a bond is independent of the surface orientation, so that the surface energy is in good approximation proportional to the number of broken nearest-neighbour bonds

Two-dimensional long-range ordered growth of uniform cobalt nanostructures on a Au(111) vicinal template

Abstract: A novel approach based on strain-relief vicinal patterned substrates is demonstrated for two-dimensional (2D) self-ordered growth. Long-range–ordered cobalt nanodots growth is achieved using a Au(788) vicinal surface which is spontaneously patterned in two dimensions at a nanometer scale with a macroscopic coherence length. Performing the cobalt growth on a substrate cooled down to 130 K allows a high degree of nanostructure uniformity. The atomic processes responsible for such a behavior are discussed. Such a high quality of both long-range–and local–ordered cobalt growth opens up the possibility of making measurements of physical properties of such nanostructures by macroscopic integration techniques.

Rheology of soft glassy materials

Abstract: We compare the linear and non-linear rheological properties of a typical soft glassy material, the colloidal glass of Laponite, with the predictions of recently developed rheological models based on glassy dynamics, in which particles are trapped in a free-energy landscape with many minima. We find qualitative agreement between theory and experiment for the linear visco-elastic properties. However, for the non-linear response, notably the viscosity under continuous shear flow, important qualitative differences are found. This shows that a developed flow affects the dynamics of relaxation of the material in a more complicated way than what is assumed in the models: not only does the flow alter the energy landscape, but it also modifies the effective temperature controlling the escape rate for a given energy barrier.