Hybrid model for chaotic front dynamics: from semiconductors to water tanks

Hyperchaos, Intermittency, Noise and Disorder in Modified Semiconductor Superlattices

Weakly coupled semiconductor superlattices under DC voltage bias are nonlinear systems with many degrees of freedom whose nonlinearity is due to sequential tunneling of electrons. They may exhibit spontaneous chaos at room temperature and act as fast physical random number generator devices. Here we present a general sequential transport model with different voltage drops at quantum wells and barriers that includes noise and fluctuations due to the superlattice epitaxial growth. Excitability and oscillations of the current in superlattices with identical periods are due to nucleation and motion of charge dipole waves that form at the emitter contact when the current drops below a critical value. Insertion of wider wells increases superlattice excitability by allowing wave nucleation at the modified wells and more complex dynamics. Then hyperchaos and different types of intermittent chaos are possible on extended DC voltage ranges. Intrinsic shot and thermal noises and external noises produce minor effects on chaotic attractors. However, random disorder due to growth fluctuations may suppress any regular or chaotic current oscillations. Numerical simulations show that more than 70% of samples remain chaotic when the standard deviation of their fluctuations due to epitaxial growth is below 0.024 nm (10% of a single monolayer) whereas for 0.015 nm disorder suppresses chaos.

Designing Hyperchaos and Intermittency in Semiconductor Superlattices

Weakly coupled semiconductor superlattices under dc voltage bias are excitable systems with many degrees of freedom that may exhibit spontaneous chaos at room temperature and act as fast physical random number generator devices. Superlattices with identical periods exhibit current self-oscillations due to the dynamics of charge dipole waves but chaotic oscillations exist on narrow voltage intervals. They disappear easily due to variation in structural growth parameters. Based on numerical simulations, we predict that inserting two identical sufficiently separated wider wells increases superlattice excitability by allowing wave nucleation at the modified wells and more complex dynamics. This system exhibits hyperchaos and varieties of intermittent chaos in extended dc voltage ranges. Unlike in ideal superlattices, our chaotic attractors are robust and resilient against noises and against controlled random disorder due to growth fluctuations.

Organic Light-Emitting Transistors: Materials, Device Configurations, and Operations

Organic light-emitting transistors (OLETs) represent an emerging class of organic optoelectronic devices, wherein the electrical switching capability of organic field-effect transistors (OFETs) and the light-generation capability of organic light-emitting diodes (OLEDs) are inherently incorporated in a single device. In contrast to conventional OFETs and OLEDs, the planar device geometry and the versatile multifunctional nature of OLETs not only endow them with numerous technological opportunities in the frontier fields of highly integrated organic electronics, but also render them ideal scientific scaffolds to address the fundamental physical events of organic semiconductors and devices. This review article summarizes the recent advancements on OLETs in light of materials, device configurations, operation conditions, etc. Diverse state-of-the-art protocols, including bulk heterojunction, layered heterojunction and laterally arranged heterojunction structures, as well as asymmetric source-drain electrodes, and innovative dielectric layers, which have been developed for the construction of qualified OLETs and for shedding new and deep light on the working principles of OLETs, are highlighted by addressing representative paradigms. This review intends to provide readers with a deeper understanding of the design of future OLETs.

Adaptation controls synchrony and cluster states of coupled threshold-model neurons

We analyze zero-lag and cluster synchrony of delay-coupled nonsmooth dynamical systems by extending the master stability approach, and apply this to networks of adaptive threshold-model neurons. For a homogeneous population of excitatory and inhibitory neurons we find (i) that subthreshold adaptation stabilizes or destabilizes synchrony depending on whether the recurrent synaptic excitatory or inhibitory couplings dominate, and (ii) that synchrony is always unstable for networks with balanced recurrent synaptic inputs. If couplings are not too strong, synchronization properties are similar for very different coupling topologies, i.e., random connections or spatial networks with localized connectivity. We generalize our approach for two subpopulations of neurons with nonidentical local dynamics, including bursting, for which activity-based adaptation controls the stability of cluster states, independent of a specific coupling topology.

On a bifurcation structure mimicking period adding

In this work, we investigate a piecewise-linear discontinuous scalar map defined on three partitions. This map is specifically constructed in such a way that it shows a recently discovered bifurcation scenario in its pure form. Owing to its structure on the one hand and the similarities to the nested period-adding scenario on the other hand, we denoted the new bifurcation scenario as nested period-incrementing bifurcation scenario. The new bifurcation scenario occurs in several physical and electronical systems but usually not isolated, which makes the description complicated. By isolating the scenario and using a suitable symbolic description for the asymptotically stable periodic orbits, we derive a set of rules in the space of symbolic sequences that explain the structure of the stable periodic domain in the parameter space entirely. Hence, the presented work is a necessary step for the understanding of the more complicated bifurcation scenarios mentioned above.

Control of coherence resonance in semiconductor superlattices

We study the effect of time-delayed feedback control and Gaussian white noise on the spatiotemporal charge dynamics in a semiconductor superlattice. The system is prepared in a regime where the deterministic dynamics is close to a global bifurcation, namely, a saddle-node bifurcation on a limit cycle. In the absence of control, noise can induce electron charge front motion through the entire device, and coherence resonance is observed. We show that with appropriate selection of the time-delayed feedback parameters the effect of coherence resonance can be either enhanced or destroyed, and the coherence of stochastic domain motion at low noise intensity is dramatically increased. Additionally, the purely delay-induced dynamics in the system is investigated, and a homoclinic bifurcation of a limit cycle is found.

Noise-induced front motion: signature of a global bifurcation

We show that front motion can be induced by noise in a spatially extended excitable system with a global constraint. Our model system is a semiconductor superlattice exhibiting complex dynamics of electron accumulation and depletion fronts. The presence of noise induces a global change in the dynamics of the system forcing stationary fronts to move through the entire device. We demonstrate the effect of coherence resonance in our model; i.e., there is an optimal level of noise at which the regularity of front motion is enhanced. Physical insight is provided by relating the space-time dynamics of the fronts with a phase-space analysis.

Long-lived transient behavior in an n+-n-n+ semiconductor device with optical stochasticity

We numerically study the response time of an ultrafast microwave switch, i.e., a GaAs-based Gunn device, under stochastic stimuli. The conducting states in this semiconductor device can be controlled in the presence of laser illumination near the doping notch. With the consideration of the additional randomness in laser intensity, the switching time from the initial unstable state to the final stable state will increase. Therefore, the so-called noise delayed decay of unstable states in an n+-n-n+ semiconductor device is demonstrated.