Table of contents

The jump process introduced by J. S. Bell in 1986, for defining a quantum field theory without observers, presupposes that space is discrete whereas time is continuous. In this letter, our interest is to find an analogous process in discrete time. We argue that a genuine analog does not exist, but provide examples of processes in discrete time that could be used as a replacement.

We report an analytic and numerical study of a phase transition in a P problem (the assignment problem) that separates two phases whose representatives are the simple matching problem (an easy P problem) and the traveling-salesman problem (a NP-complete problem). Like other phase transitions found in combinatoric problems (K-satisfiability, number partitioning) this can help to understand the nature of the difficulties in solving NP problems an to find more accurate algorithms for them.

We predict new generic types of vorticity-carrying soliton complexes in a class of physical systems including an attractive Bose-Einstein condensate in a square optical lattice (OL) and photonic lattices in photorefractive media. The patterns include ring-shaped higher-order vortex solitons and supervortices. Stability diagrams for these patterns, based on direct simulations, are presented. The vortex ring solitons are stable if the phase difference Δϕ between adjacent solitons in the ring is larger than π/2, while the supervortices are stable in the opposite case, Δϕ < π/2. A qualitative explanation to the stability is given.

We present a new approximation scheme to solve the Non-Perturbative Renormalization Group equations and obtain the full momentum dependence of n-point functions. The core of the method consists in deriving approximate flow equations that are then solved exactly. The results thus obtained can next be improved through an iteration procedure. The approximations proposed in this paper lead to approximate equations that are accurate both in the perturbative and the scaling regimes. The scheme is tested with the calculation of the shift ΔT_c in the transition temperature of the weakly repulsive Bose gas, a quantity which is very sensitive to all momentum scales. The leading-order result is in agreement with lattice calculations, albeit with a theoretical uncertainty of about 25%. The next-to-leading order differs by about 10% from the best accepted result.

We study effects of spatiotemporal additive noise in conjunction with subthreshold travelling waves on the spatial dynamics of excitable media. We show that solely additive noise is able to extract an inherent spatial periodicity of the media in a resonant manner, thus marking the existence of spatial coherence resonance in the studied system. Next, in addition to noise, we introduce to the media excitatory waves to investigate the possibility of spatial stochastic resonance. We find that the solely noise-induced inherent spatial periodicity of the media cannot be altered by the spatial frequency of the waves. This so-called persistency of inherent spatial periodicity is attributed to the noise-robust excursion time that is characteristic for the local excitable dynamics.

We investigate global persistence properties for the non-equilibrium critical dynamics of the randomly diluted Ising model. The disorder-averaged persistence probability _c(t) of the global magnetization is found to decay algebraically with an exponent θ_c that we compute analytically in a dimensional expansion in d = 4 − . Corrections to Markov process are found to occur already at one loop order and θ_c is thus a novel exponent characterizing this disordered critical point. Our result is thoroughly compared with Monte Carlo simulations in d = 3, which also include a measurement of the initial slip exponent. Taking carefully into account corrections to scaling, θ_c is found to be universal, independent of the dilution factor p along the critical line at T_c(p), and in good agreement with our one-loop calculation.

The fluctuation theorem (FT) and the work relation (WR) are two relations that extend our understanding of thermodynamics to non-equilibrium systems. While often treated as distinct relations, they are in fact closely related. In this letter we generalise these relations, showing that they are fundamental relations of statistical systems, and use these generalised forms to connect the FT and WR through a new set of relations, the conjugate work relations. We then take these general forms of the FT and WR, and show that they reduce to original forms under deterministic dynamics, before finally exploring their application to an experimental system.

A model based on the local gauge group SU(3)_c⊗SU(3)_L⊗U(1)_X without particles with exotic electric charges is shown to be able to provide the quark mass spectrum and their mixing, by means of universal see-saw mechanisms, avoiding a hierarchy in the Yukawa coupling constants.

We show that coherent-control techniques are uniquely suited for high-resolution hyperfine spectroscopy. We apply this method to a V-type system in the D₂ line of ¹³³Cs The spectrum of a weak probe beam shows Doppler-free transparency peaks when the strong control beam is resonant with a neighboring hyperfine transition. The lineshape and linewidth are consistent with theoretical calculations. Using an acousto-optic modulator to calibrate the frequency of the control beam, we measure hyperfine intervals in the 6P_3/2 state with an accuracy of 6 kHz. The measurement is important because there is a large discrepancy between two earlier measurements on this state.

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.

It is proposed, by way of studies made using tris(8-hydroxyquinoline) aluminium (Alq3) in an ITO/Alq3/Al structure, that dielectric spectroscopy (represented by log ε'' = f[log ω]) and a determination of I(V) characteristics permit a good estimation of charge mobility (μ). The value of μ determined by this new method was found to be ca. 2 × 10⁻⁶ cm² V⁻¹ s⁻¹, without bias voltage. This method also allows a facile determination of field effects, and it was possible to demonstrate that μ followed a Poole-Frenkel law. In addition, mobilities determined via field effects alone generally led to overestimation of around one order of magnitude for an electric field of around 1 MV cm^-1.

Molecular-dynamics simulations are used to sample the single-chain form factor of labelled sub-chains in model polymer networks under elongational strain. We observe very similar results for randomly cross-linked and for randomly end-linked networks with the same average strand length and see no indication of lozenge-like scattering patterns reported for some experimental systems. Our data analysis shows that a recent variant of the tube model quantitatively describes scattering in the Guinier regime as well as the macroscopic elastic properties. The observed failure of the theory outside the Guinier regime is shown to be due to non-Gaussian pair-distance distributions.

The free energy of a crystalline domain coexisting with a liquid phase on a spherical vesicle may be approximated by an elastic or stretching energy and a line tension term. The stretching energy generally grows as the area of the domain, while the line tension term grows with its perimeter. We show that if the crystalline domain contains defect arrays consisting of finite-length grain boundaries of dislocations (scars), the stretching energy grows linearly with a characteristic length of the crystalline domain. We show that this result is critical to understand the existence of solid domains in lipid-bilayers in the strongly segregated two-phase region even for small relative area coverages. The domains evolve from caps to stripes that become thinner as the line tension is decreased. We also discuss the implications of the results for other experimental systems and for the general problem that consists in finding the ground state of a very large number of particles constrained to move on a fixed geometry and interacting with an isotropic potential.

The starting formula of Bosvieux and Friedel (J. Phys. Chem. Solids, 23 (1962) 123) for the force on an ion in a metal due to an applied voltage is shown to lead to the same description as the linear-response approach used in the field since its introduction by Kumar and Sorbello (Thin Solid Films, 25 (1975) 25). By this electromigration theory has become a unified theory. This follows after accounting for a treacherous trap term, which at first sight seems to be zero. Up to now, Bosvieux and Friedel claimed to predict a completely screened direct force, which means that only a wind force would be operative. In addition, the amount of screening has been calculated up to second order in the potential of the migrating impurity, using a finite temperature version of the screening term derived by Sham (Phys. Rev. B, 12 (1975) 3142). For a proton in a metal modeled as a jellium the screening appears to be about 15%, which is neither negligible nor reconcilable with the old full-screening point of view.

Dewetting of thin polystyrene films deposited onto silicon wafers exhibits unusual dynamics and rim morphologies. Here, we present a new theoretical approach of these phenomena taking into account both the viscoelastic properties of the film and the non-zero velocity of the film at the interface with the substrate (i.e. slippage). We show how these two ingredients lead to: a) a very asymmetric shape of the rim as the film dewets; b) a decrease of the dewetting velocity with time like t^−1/2 for times shorter than the reptation time (for larger times, the dewetting velocity reaches a constant value). Recent experiments by Damman, Baudelet and Reiter (Phys. Rev. Lett., 91 (2003) 216101) present, however, a much faster decrease of the dewetting velocity. We then show how this striking result may be explained by the presence of residual stresses in the film.

We study fermions in a Mach-Zehnder interferometer, subject to a quantum-mechanical environment leading to inelastic scattering, decoherence, renormalization effects, and time-dependent conductance fluctuations. We present a method to derive both the loss of interference contrast as well as the shot noise, using equations of motion and leading-order perturbation theory. The dependence of the shot noise on the Aharonov-Bohm phase acquires an unexpected average phase shift, due to correlations between the fluctuating renormalized phase shift and the output current. We discuss the limiting behaviours at low and high voltages, compare with simpler models of dephasing, and present implications for experiments.

The normal-state transport properties of La_{1.6 − x}Nd_0.4Sr_xCuO₄ films (x = 0.08, 0.1, 0.12, 0.14, 0.16) have been discussed with a one-dimensional transport model proposed by Moshchalkov et al. The linear-T dependent resistivity ρ_ab in the high-temperature regime is dominated by the charge diffusion in two dimensions, while the superlinear behavior of ρ_ab upon cooling results from one-dimensional charge transport. With further decreasing temperature, a low-temperature logarithmic behavior is observed from underdoped to optimally doped samples. The mechanism of the localization is ascribed to the freezing of charges into the stripes. We emphasize that the pseudogap can be well understood within the charge stripe scenario.

We present a model for the dynamics in energy space of multicanonical simulation methods that lends itself to a rather complete analytic characterization. The dynamics is completely determined by the density of states. In the ±J 2D spin glass the transitions between the ground state level and the first excited one control the long time dynamics. We are able to calculate the distribution of tunneling times and relate it to the equilibration time of a starting probability distribution. In this model, and possibly in any model in which entering and exiting regions with low density of states are the slowest processes in the simulations, tunneling time can be much larger (by a factor of O(N)) than the equilibration time of the probability distribution. We find that these features also hold for the energy projection of single spin flip dynamics.

The approach to magnetic saturation has been used to estimate the magnetic anisotropy in single-crystal La_0.73Ba_0.27MnO₃ and polycrystalline La_0.5Sr_0.5CoO₃, and these estimates are compared with those found from the measured coercive field. While this analysis confirms the order-of-magnitude discrepancy between these two estimates reported previously and linked, via the enhanced anisotropic magnetostrictive behaviour, to a field-induced low- to high-spin state transition of the Co ion, it also indicates that such discrepancies may be a ubiquitous feature of these transition metal oxides, occurring in systems where orbital instabilities appear unlikely.

The magnetic anisotropy of thin Fe films on (001)-oriented GaAs has been investigated using the fully relativistic TB-KKR method of band structure calculation. In line with experimental findings an in-plane anisotropy has been found with the easy axis pointing along the [110]-direction. The theoretical results for the anisotropy energy ΔE are reproduced in a semi-quantitative way by the models of Bruno and van der Laan, respectively, which relate ΔE to the anisotropy of the spin-orbit–induced orbital magnetic moment.

By taking into account the four-spin interaction in the transverse Ising model (TIM), the transition feature of the ferroelectric thin film is investigated within the framework of the Green's function technique. Various transition regions in the parameter space with respect to the ratios of the tunneling frequency and the four-spin interaction to the two-spin interaction are calculated by a higher-order decoupling approximation beyond the usual mean-field approximation. The results show that the transition regions are greatly different from the ones obtained within the framework of the mean-field approximation. It is suggested that the usual mean-field approximation exaggerates not only the second-order but also the first-order ferroelectric transition regions for a ferroelectric thin film.

This paper deals with a theoretical analysis of the reflection and refraction of light at the interface of a bicrystal by use of Maxwell's equations. For a general case, the formulas of Snell's Law and the four Fresnel coefficients for the reflection and refraction of extraordinary light at the interface of a uniaxial bicrystal are derived for the first time, as well as the Brewster angle value. The condition for total reflection is presented and the electromagnetic fields distributions at both sides of a bicrystal are presented when total reflection occurs.

The Helmholtz-Smoluchowski (HS) relation between the local electrostatic potential and lateral solvent velocity in a continuum fluid at a planar charged surface under the action of a tangential electric field is exact for smooth surfaces, even in the presence of ion correlations. At rough surfaces, the interplay of hydrodynamics and surface friction due to electrostatic and excluded-volume interactions between surface groups and counterions gives rise to pronounced deviations from HS-theory. This is demonstrated by hydrodynamic simulations of ions at charged corrugated surfaces. Derived effective electrokinetic surface charges are highly reduced, in agreement with the experimental trend.

We peel off cationic and anionic polyelectrolytes from different substrates (cationic, hydrophobic and metallic) with the atomic force microscope (AFM). By varying the salt concentration, the measured plateau forces can be separated into electrostatic and non-electrostatic contributions to the adsorption energy. Surprisingly, i) the non-electrostatic contribution dominates even for highly charged substrates, and ii) hydrophobic substrates act like negatively charged surfaces. In the theoretical modeling the surface-induced charge regulation of the polymers has to be included.

We introduce stochastic driving in the Sznajd model of opinion spreading. This stochastic effect is meant to mimic a social temperature, so that agents can take random decisions with a varying probability. We show that a stochastic driving has a tremendous impact on the system dynamics as a whole by inducing an order-disorder nonequilibrium phase transition. Interestingly, under certain conditions, this stochastic dynamics can spontaneously lead to agents in the system who are analogous to Galam's contarians.

In a recent paper, Krapivsky and Redner (Phys. Rev. E, 71 (2005) 036118) proposed a new growing network model with new nodes being attached to a randomly selected node, as well to all ancestors of the target node. The model leads to a sparse graph with an average degree growing logarithmically with the system size. Here we present compeling evidence for software networks being the result of a similar class of growing dynamics. The predicted pattern of network growth, as well as the stationary in- and out-degree distributions are consistent with the model. Our results confirm the view of large-scale software topology being generated through duplication-rewiring mechanisms. Implications of these findings are outlined.

We analyze data sets containing the circulation of magazines and newspapers. We show that the cumulative distribution follows, in the range of large circulation, a power law behavior whose exponent is μ ≃ 1.5; and deviations from the asymptotic power law behavior can be well described by a q-exponential distribution (Zipf-Mandelbrot law) from Tsallis statistics. We also show that, in the range of large circulation, the distribution of logarithmic growth rates is consistent with an exponential; and the standard deviation of the growth rates is practically independent of the circulation (size). Moreover, we employ a model, inspired in one of the simplest model for firm growth, in order to reproduce some of our findings.

Erratum of Europhys. Lett.72 p. 823