Complex oscillation modes in the Belousov-Zhabotinsky reaction by weak diffusive coupling.

We study the diffusive coupling of oscillating or excitable Belousov-Zhabotinsky reaction units arranged in a square lattice array and show that for certain sizes of the units and for certain distances between the units, complex oscillation modes of individual spots occur, which manifest themselves in multi-periodic, amplitude-modulated, and multi-mode oscillations. This experimental finding can be reproduced in simulations of the FitzHugh-Nagumo model mimicking the experimental setup, suggesting that it is a generic phenomenon in systems of coupled excitable units such as excitable cell tissues or coupled oscillators such as neurons. Further analysis let us conclude that the complex oscillation modes occur close to the transition from quiescent to coupling-induced oscillations states if this transition is taking place at weak coupling strength.

Growth kinetics of racemic heptahelicene-2-carboxylic acid nanowires on calcite (104).

Molecular self-assembly of racemic heptahelicene-2-carboxylic acid on a dielectric substrate at room temperature can be used to generate wire-like organic nanostructures consisting of single and double molecular rows. By means of non-contact atomic force microscopy, we investigate the growth of the wire-like pattern after deposition by experimental and theoretical means. From analyzing the time dependence of the mean row length, two distinct regimes were found.

Maximum efficiency of state-space models of nanoscale energy conversion devices.

The performance of nano-scale energy conversion devices is studied in the framework of state-space models where a device is described by a graph comprising states and transitions between them represented by nodes and links, respectively. Particular segments of this network represent input (driving) and output processes whose properly chosen flux ratio provides the energy conversion efficiency. Simple cyclical graphs yield Carnot efficiency for the maximum conversion yield.

Initiation of atrial fibrillation by interaction of pacemakers with geometrical constraints.

Atrial fibrillation (AF) is the most common arrhythmia of the heart in industrialized countries. Its generation and the transitory behavior of paroxysmal AF are still not well understood. In this work we examine the interaction of two activation sources via an isthmus as possible cause for the initiation of fibrillation episodes.

Phase transitions in Brownian pumps.

We study stochastic particle transport between two reservoirs along a channel, where the particles are pumped against a bias by a traveling wave potential. It is shown that phase transitions of period-averaged densities or currents occur inside the channel when exclusion interactions between the particles are taken into account. These transitions reflect those known for the asymmetric simple exclusion process.

One-dimensional transport of interacting particles: currents, density profiles, phase diagrams, and symmetries.

Driven lattice gases serve as canonical models for investigating collective transport phenomena and properties of nonequilibrium steady states. Here we study one-dimensional transport with nearest-neighbor interactions both in closed bulk systems and in open channels coupled to two particle reservoirs at the ends of the channel. For the widely employed Glauber rates we derive an exact current-density relation in the bulk for unidirectional hopping.

Irregular excitation patterns in reaction-diffusion systems due to perturbation by secondary pacemakers.

Spatiotemporal excitation patterns in the FitzHugh-Nagumo model are studied, which result from the disturbance of a primary pacemaker by a secondary pacemaker. The primary and secondary pacemakers generate regular waves with frequencies f(pace) and f(pert), respectively. The pacemakers are spatially separated, but waves emanating from them encounter each other via a small bridge.

Classical driven transport in open systems with particle interactions and general couplings to reservoirs.

We study nonequilibrium steady states of lattice gases with nearest-neighbor interactions that are driven between two reservoirs. Density profiles in these systems exhibit oscillations close to the reservoirs. We demonstrate that an approach based on time-dependent density functional theory copes with these oscillations and predicts phase diagrams of bulk densities to a good approximation under arbitrary boundary-reservoir couplings.

Second-layer induced island morphologies in thin-film growth of fullerenes.

Deposition of fullerenes on the CaF(2)(111) surface yields peculiar island morphologies with close similarities to previous findings for (100) surfaces of other ionic crystals. By means of noncontact atomic force microscopy we find a smooth transition from compact, triangular islands to branched hexagonal islands upon lowering the temperature. While triangular islands are two monolayers high, hexagonal islands have a base of one monolayer and exhibit a complicated structure with a second-layer outer rim and trenches oriented towards the interior.

Unidirectional hopping transport of interacting particles on a finite chain.

Particle transport through an open, discrete one-dimensional channel against a mechanical or chemical bias is analyzed within a master equation approach. The channel, externally driven by time-dependent site energies, allows multiple occupation due to the coupling to reservoirs. Performance criteria and optimization of active transport in a two-site channel are discussed as a function of reservoir chemical potentials, the load potential, interparticle interaction strength, driving mode, and driving period.

Nonlinear hopping transport in ring systems and open channels.

We study the nonlinear hopping transport in one-dimensional rings and open channels. Analytical results are derived for the stationary current response to a constant bias without assuming any specific coupling of the rates to the external fields. It is shown that anomalous large effective jump lengths, as observed in recent experiments by taking the ratio of the third-order nonlinear and the linear conductivity, can occur already in ordered systems.

Work distributions for Ising chains in a time-dependent magnetic field.

Master equations can be conveniently used to investigate many particle systems driven out of equilibrium by time-dependent external fields. This topic is of vital interest in connection with fluctuation theorems and the associated microscopic work and heat distributions. We present an exact Monte Carlo simulation algorithm, which allows us to study interaction effects on these distributions.

Scaling of island densities in submonolayer growth of binary alloys.

We study the nucleation kinetics of binary alloys formed by codeposition of A and B atoms onto a planar substrate with fluxes F(alpha) (alpha=A, B). Based on a generalization of mean-field rate equations, we derive scaling relations for the dependence of the number density of stable islands on the ratios D(alpha)/F(alpha), where D(alpha) are the diffusion coefficients of two types of adatoms. Novel scaling laws are predicted for different situations of cluster stabilities with respect to their size and composition.

Friction and second-order phase transitions.

A microscopic model is studied numerically to describe wearless dry friction without thermal fluctuations between atomically flat contact interfaces. The analysis is based on a double-chain model with a Lennard-Jones interaction between the chains which are the respective upper flexible monolayers of the rigid bulk systems. Whereas below a critical interaction strength epsilon(c) the system exhibits a frictionless state, it offers static friction above epsilon(c) .