Study of the key biotic and abiotic parameters influencing ammonium removal from wastewaters by Fe-mediated anaerobic ammonium oxidation (Feammox).

The release of ammonia (as NH) into water bodies causes serious environmental problems. Therefore, the removal of ammonia from wastewater effluents has become a worldwide concern. New autotrophic biological alternatives for ammonia removal could reduce the limitations of conventional organic carbon-dependent nitrification-denitrification methods.

Algorithmic decoding of dense OAM signal constellations for optical communications in turbulence.

We demonstrate an optical detection and decoding strategy to increase the information rate and spectral efficiency of free-space laser communication links affected by turbulence by means of dense orbital angular momentum (OAM) modulation. Using three candidate receiver architectures-based on a Shack-Hartmann sensor, a Mode Sorter, and a complex conjugate projection scheme as a base case-we demonstrate an algorithmic classification system based on the received OAM spectra produced by these architectures. This classification scheme allows low-error-rate data transmission in turbulence using 16-OAM, 32-OAM, and 64-OAM symbol constellations, with OAM states between -20 and 20.

Reaction-diffusion fronts and the butterfly set.

A single-species reaction-diffusion model is used for studying the coexistence of multiple stable steady states. In these systems, one can define a potential-like functional that contains the stability properties of the states, and the essentials of the motion of wave fronts in one- and two-dimensional space. Using a quintic polynomial for the reaction term and taking advantage of the well-known butterfly bifurcation, we analyze the different scenarios involving the competition of two and three stable steady states, based on equipotential curves and points in parameter space.

Spatial-diversity detection of optical vortices for OAM signal modulation.

We propose a method for identifying orbital angular momentum (OAM) states within a vortex superposition using a Shack-Hartmann (SH) sensor as a spatial-diversity detector. We define a local OAM at every pixel of the SH image, from which we construct an OAM spectrum. The topological charges are determined from the OAM spectrum using a low-complexity algorithm, resulting in estimates that are robust to beam wandering.

Random walks of trains of dissipative solitons.

The propagation of light pulses in dual-core nonlinear optical fibers is studied using a model proposed by Sakaguchi and Malomed. The system consists of a supercritical complex Ginzburg-Landau equation coupled to a linear equation. Our analysis includes single standing and walking solitons as well as walking trains of 3, 5, 6, and 12 solitons.

Synchronization and fluctuations: Coupling a finite number of stochastic units.

It is well established that ensembles of globally coupled stochastic oscillators may exhibit a nonequilibrium phase transition to synchronization in the thermodynamic limit (infinite number of elements). In fact, since the early work of Kuramoto, mean-field theory has been used to analyze this transition. In contrast, work that directly deals with finite arrays is relatively scarce in the context of synchronization.

Competing ternary surface reaction CO + O + H on Ir(111).

The CO oxidation on platinum-group metals under ultra-high-vacuum conditions is one of the most studied surface reactions. However, the presence of disturbing species and competing reactions are often neglected. One of the most interesting additional gases to be treated is hydrogen, due to its importance in technical applications and its inevitability under vacuum conditions.

Mechanism of dissipative soliton stabilization by nonlinear gradient terms.

We present a feedback mechanism for dissipative solitons in the cubic complex Ginzburg-Landau (CGL) equation with a nonlinear gradient term. We are making use of a mechanical analog containing contributions from a potential and from a nonlinear viscous term. The feedback mechanism relies on the continuous supply of energy as well as on dissipation of the stable pulse.

Normal and anomalous random walks of 2-d solitons.

Solitons, which describe the propagation of concentrated beams of light through nonlinear media, can exhibit a variety of behaviors as a result of the intrinsic dissipation, diffraction, and the nonlinear effects. One of these phenomena, modeled by the complex Ginzburg-Landau equation, is chaotic explosions, transient enlargements of the soliton that may induce random transversal displacements, which in the long run lead to a random walk of the soliton center. As we show in this work, the transition from nonmoving to moving solitons is not a simple bifurcation but includes a sequence of normal and anomalous random walks.

Detailed analysis of transitions in the CO oxidation on palladium(111) under noisy conditions.

It has been shown that CO oxidation on Pd(111) under ultrahigh vacuum conditions can suffer rare transitions between two stable states triggered by weak intrinsic perturbations. Here we study the effects of adding controlled noise by varying the concentrations of O and CO that feed the vacuum chamber, while the total flux stays constant. In addition to the regime of rare transitions between states of different CO reaction rates induced by intrinsic fluctuations, we found three distinct effects of external noise depending on its strength: small noise suppresses transitions and stabilizes the upper rate state; medium noise induces bursting; and large noise gives rise to reversible transitions in both directions.

Study of Resting-State Functional Connectivity Networks Using EEG Electrodes Position As Seed.

Electroencephalography (EEG) is the standard diagnosis method for a wide variety of diseases such as epilepsy, sleep disorders, encephalopathies, and coma, among others. Resting-state functional magnetic resonance (rs-fMRI) is currently a technique used in research in both healthy individuals as well as patients. EEG and fMRI are procedures used to obtain direct and indirect measurements of brain neural activity: EEG measures the electrical activity of the brain using electrodes placed on the scalp, and fMRI detects the changes in blood oxygenation that occur in response to neural activity.

Anomalous Diffusion of Dissipative Solitons in the Cubic-Quintic Complex Ginzburg-Landau Equation in Two Spatial Dimensions.

We demonstrate the occurrence of anomalous diffusion of dissipative solitons in a "simple" and deterministic prototype model: the cubic-quintic complex Ginzburg-Landau equation in two spatial dimensions. The main features of their dynamics, induced by symmetric-asymmetric explosions, can be modeled by a subdiffusive continuous-time random walk, while in the case dominated by only asymmetric explosions, it becomes characterized by normal diffusion.

Coarse-Grained Clustering Dynamics of Heterogeneously Coupled Neurons.

The formation of oscillating phase clusters in a network of identical Hodgkin-Huxley neurons is studied, along with their dynamic behavior. The neurons are synaptically coupled in an all-to-all manner, yet the synaptic coupling characteristic time is heterogeneous across the connections. In a network of N neurons where this heterogeneity is characterized by a prescribed random variable, the oscillatory single-cluster state can transition-through [Formula: see text] (possibly perturbed) period-doubling and subsequent bifurcations-to a variety of multiple-cluster states.

Travelling fronts of the CO oxidation on Pd(111) with coverage-dependent diffusion.

In this work, we study a surface reaction on Pd(111) crystals under ultra-high-vacuum conditions that can be modeled by two coupled reaction-diffusion equations. In the bistable regime, the reaction exhibits travelling fronts that can be observed experimentally using photo electron emission microscopy. The spatial profile of the fronts reveals a coverage-dependent diffusivity for one of the species.

Intermittent explosions of dissipative solitons and noise-induced crisis.

Dissipative solitons show a variety of behaviors not exhibited by their conservative counterparts. For instance, a dissipative soliton can remain localized for a long period of time without major profile changes, then grow and become broader for a short time-explode-and return to the original spatial profile afterward. Here we consider the dynamics of dissipative solitons and the onset of explosions in detail.

Model of a two-dimensional extended chaotic system: evidence of diffusing dissipative solitons.

We investigate a two-dimensional extended system showing chaotic and localized structures. We demonstrate the robust and stable existence of two types of exploding dissipative solitons. We show that the center of mass of asymmetric dissipative solitons undergoes a random walk despite the deterministic character of the underlying model.

CO oxidation on Ir(111) surfaces under large non-gaussian noise.

In this article we consider the CO oxidation on Ir(111) surfaces under large external noise with large autocorrelation imposed on the composition of the feed gas, both in experiments and in theory. We report new experimental results that show how the fluctuations force the reaction rate to jump between two well defined states. The statistics of the reaction rate depend on those of the external noise, and neither of them have a gaussian distribution, and thus they cannot be modeled by white or colored noise.

Exploding dissipative solitons: the analog of the Ruelle-Takens route for spatially localized solutions.

We investigate the route to exploding dissipative solitons in the complex cubic-quintic Ginzburg-Landau equation, as the bifurcation parameter, the distance from linear onset, is increased. We find for a large class of initial conditions the sequence: stationary localized solutions, oscillatory localized solutions with one frequency, oscillatory localized solutions with two frequencies, and exploding localized solutions. The transition between localized solutions with one and with two frequencies, respectively, is analyzed in detail.

Influence of turbulence strength on temporal correlation of scintillation.

Through extensive laboratory experimentation we demonstrate that the temporal frequency content of turbulence-induced scintillation strongly depends on the temperature gradient exerted at the propagation path of a collimated laser beam. We find a power law relating the turbulence strength induced by convection with the vertical temperature gradient and we show that the cutoff frequency of scintillation shows an approximately linear growth with turbulence strength, measured by angle-of-arrival fluctuations. The impact of these findings are discussed in the context of free-space optical communications.

Bifurcations of lurching waves in a thalamic neuronal network.

We consider a two-layer, one-dimensional lattice of neurons; one layer consists of excitatory thalamocortical neurons, while the other is comprised of inhibitory reticular thalamic neurons. Such networks are known to support "lurching" waves, for which propagation does not appear smooth, but rather progresses in a saltatory fashion; these waves can be characterized by different spatial widths (different numbers of neurons active at the same time). We show that these lurching waves are fixed points of appropriately defined Poincaré maps, and follow these fixed points as parameters are varied.

Noise induces partial annihilation of colliding dissipative solitons.

Partial annihilation of two counterpropagating dissipative solitons, with only one pulse surviving the collision, has been widely observed in different experimental contexts, over a large range of parameters, from hydrodynamics to chemical reactions. However, a generic picture accounting for partial annihilation is missing. Based on our results for coupled complex cubic-quintic Ginzburg-Landau equations as well as for the FitzHugh-Nagumo equation we conjecture that noise induces partial annihilation of colliding dissipative solitons in many systems.

Collisions of pulses can lead to holes via front interaction in the cubic-quintic complex Ginzburg-Landau equation in an annular geometry.

We study the interaction of counterpropagating pulse solutions for two coupled complex cubic-quintic Ginzburg-Landau equations in an annular geometry. For small approach velocity we find as an outcome of such collisions several results including zigzag bound pulses, stationary bound states of 2pi holes, zigzag 2pi holes, stationary bound states of pi holes, zigzag bound states of pi holes, propagating 2pi holes, and propagating pi holes as the real part of the cubic cross coupling between the counterpropagating waves is increased. We characterize in detail the collisions giving rise to the three states involving pi holes as an outcome.

Oscillatory thermomechanical instability of an ultrathin catalyst.

Because of the small thermal capacity of ultrathin ( approximately 200 nanometers) metal single crystals, it is possible to explore the coupling of catalytic and thermal action at low pressures. We analyzed a chemothermomechanical instability in this regime, in which catalytic reaction kinetics interact with heat transfer and mechanical buckling to create oscillations. These interacting components are separated and explored through experimentation, mathematical modeling, and scientific computation, and an explanation of the phenomenon emerges from their synthesis.