Amplitude chimeras and chimera death in dynamical networks

We find chimera states with respect to amplitude dynamics in a network of Stuart-Landau oscillators. These partially coherent and partially incoherent spatio-temporal patterns appear due to the interplay of nonlocal network topology and symmetry-breaking coupling. As the coupling range is increased, the oscillations are quenched, amplitude chimeras disappear and the network enters a symmetry-breaking stationary state. This particular regime is a novel pattern which we call chimera death. It is characterized by the coexistence of spatially coherent and incoherent inhomogeneous steady states and therefore combines the features of chimera state and oscillation death. Additionally, we show two different transition scenarios from amplitude chimera to chimera death. Moreover, for amplitude chimeras we uncover the mechanism of transition towards in-phase synchronized regime and discuss the role of initial conditions

Magnetic and electronic properties of oxides heterostructures probed with x-ray spectroscopy.

Fundamental investigation of interfacial properties in complex oxide heterostructures is crucial to boost the development of modern electronics. Coupled degrees of freedom in transition metal oxides (TMOs) provide a rich playground to tailor precisely properties of artificial materials. Yet, entangling complex correlated behaviour is a very challenging task, although it is necessary for heterostructures application. A variety of x-rays techniques allowed us to build a connection between magnetic prop- erties and the electronic and structural behaviour of TMO as a function of the proximity layer. We applied x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and x-ray linear dichroism (XLD) to probe the magnetic and orbital properties of Mn in ultra-thin La0.7Sr0.3MnO3 in proximity to SrRuO3 in comparison to the interface with SrTiO3. We have revealed the large impact of proximity and dimensionality, resulting in the magnetic dead layer in La0.7Sr0.3MnO3/SrRuO3 bilayers being drastically diminished. This restoration of magnetism in the ultra-thin La0.7Sr0.3MnO3 layer could be evoked by strong hybridization between La0.7Sr0.3MnO3 and SrRuO3, hindering the quantum confinement effect. The orbital rearrangement at the La0.7Sr0.3MnO3/SrRuO3 interface, its origin and consequences are discussed. In addition to the strong magnetic and electronic coupling to SrRuO3, the preservation of the mixed-valence and the remaining dx2−y2 occupation still allows for a strong double exchange coupling in the plane. Based on quantitative analysis and theoretical simulation of the x-ray spectra, the magnetic stability mechanisms in La0.7Sr0.3MnO3/SrRuO3 are discussed. Further, we explored phenomena of emerged ferromagnetism in NdNiO3 using x- ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). When interfaced with La0.7Sr0.3MnO3 we found a net ferromagnetic moment on NdNiO3. Reduction of the layer averaged magnetic moment between 5uc and 10uc NdNiO3 on La0.7Sr0.3MnO3 demonstrated the interfacial nature of the effect. We have found a larger charge transfer between manganite and nickelate layers in compressively strained bilayers. In addition, compressive strain promotes ferromagnetism in the NdNiO3 layer due to superexchange between Mn4+-Ni2+ at the interface. On the other hand, films under small tensile strain were demonstrated to have almost no sizeable Mn4+-Ni2+ charge transfer. Resonant x-ray reflectivity reveals a complex coupling with antiferromagnetic components on NdNiO3. It gradually decreases size of the magnetic moment causing still measurable net ferromagnetic moment on NdNiO3. It may originate from antiferromagnetic superexchange between Ni2+/3+ and Mn3+/4+ planes. When the manganite layer is insulating (La0.88 Sr0.12 MnO3 ), NdNiO3 ferromagnetic ordering is almost quenched. Indeed, the exchange between two insulating layers is blocked suppressing the appearance of ferromagnetism in the NNO layer. At the interface with La0.88Sr0.12MnO3, Ni is in a weakly ferromagnetic or frustrated magnetic phase, in contrast to the Ni in NdNiO3/La0.7Sr0.3MnO3. By employing resonant x-ray reflectivity we have found different magnetic reconstructions at the NdNiO3/La1−xSrxMnO3 interface. Summarizing our investigation discovers the role of the proximity layer in TMO heterostructure. By presenting novel experimental data we found complex magnetic behaviour when uniting two dissimilar TMO utilizing x-ray resonant absorption spectroscopy and resonant x-ray reflectivity. We have explored switching between non-magnetic, ferromagnetic and antiferromagnetic phases revealing fine control over the magnetism in bilayers

Time-delayed feedback control of coherence resonance chimeras

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Chaos 27, 114320 (2017) and may be found at https://doi.org/10.1063/1.5008385.Using the model of a FitzHugh-Nagumo system in the excitable regime, we investigate the influence of time-delayed feedback on noise-induced chimera states in a network with nonlocal coupling, i.e., coherence resonance chimeras. It is shown that time-delayed feedback allows for the control of the range of parameter values where these chimera states occur. Moreover, for the feedback delay close to the intrinsic period of the system, we find a novel regime which we call period-two coherence resonance chimera. Coherence resonance chimeras in nonlocally coupled networks of excitable elements represent partial synchronization patterns composed of spatially separated domains of coherent and incoherent spiking behavior, which are induced by noise. These patterns are different from classical chimera states occurring in deterministic oscillatory systems and combine properties of the counter-intuitive phenomenon of coherence resonance, i.e., a constructive role of noise, and chimera states, i.e., the coexistence of spatially synchronized and desynchronized domains in a network of identical elements. Another distinctive feature of the particular type of chimera we study here is its alternating behavior, i.e., periodic switching of the location of coherent and incoherent domains. Applying time-delayed feedback, we demonstrate how to control coherence resonance chimeras by adjusting delay time and feedback strength. In particular, we show that feedback increases the parameter intervals of existence of chimera states and has a significant impact on their alternating dynamics leading to the appearance of novel patterns, which we call period-two coherence resonance chimera. Since the dynamics of every individual network element in our study is given by the FitzHugh-Nagumo (FHN) system, which is a paradigmatic model for neurons in the excitable regime, we expect wide-range applications of our results to neural networks.DFG, 163436311, SFB 910: Kontrolle selbstorganisierender nichtlinearer Systeme: Theoretische Methoden und Anwendungskonzept

Birth and Death of Chimera: Interplay of Delay and Multiplexing

The chimera state with co-existing coherent-incoherent dynamics has recently attracted a lot of attention due to its wide applicability. We investigate non-locally coupled identical chaotic maps with delayed interactions in the multiplex network framework and find that an interplay of delay and multiplexing brings about an enhanced or suppressed appearance of chimera state depending on the distribution as well as the parity of delay values in the layers. Additionally, we report a layer chimera state with an existence of one layer displaying coherent and another layer demonstrating incoherent dynamical evolution. The rich variety of dynamical behavior demonstrated here can be used to gain further insight into the real-world networks which inherently possess such multi-layer architecture with delayed interactions

Chimera states in complex networks: interplay of fractal topology and delay

Chimera states are an example of intriguing partial synchronization patterns emerging in networks of identical oscillators. They consist of spatially coexisting domains of coherent (synchronized) and incoherent (desynchronized) dynamics. We analyze chimera states in networks of Van der Pol oscillators with hierarchical connectivities, and elaborate the role of time delay introduced in the coupling term. In the parameter plane of coupling strength and delay time we find tongue-like regions of existence of chimera states alternating with regions of existence of coherent travelling waves. We demonstrate that by varying the time delay one can deliberately stabilize desired spatio-temporal patterns in the system.Comment: arXiv admin note: text overlap with arXiv:1603.0017

Chimera states and the interplay between initial conditions and non-local coupling

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Chaos 27, 033110 (2017) and may be found at https://doi.org/10.1063/1.4977866.Chimera states are complex spatio-temporal patterns that consist of coexisting domains of coherent and incoherent dynamics. We study chimera states in a network of non-locally coupled Stuart-Landau oscillators. We investigate the impact of initial conditions in combination with non-local coupling. Based on an analytical argument, we show how the coupling phase and the coupling strength are linked to the occurrence of chimera states, flipped profiles of the mean phase velocity, and the transition from a phase- to an amplitude-mediated chimera state. Chimera states are an example of intriguing partial synchronization patterns appearing in networks of identical oscillators with symmetric coupling scheme. They exhibit a hybrid structure combining coexisting spatial domains of coherent (synchronized) and incoherent (desynchronized) dynamics and were first reported for the model of phase oscillators. Recent studies have demonstrated the emergence of chimera states in a variety of topologies and for different types of individual dynamics. In this paper, the interplay between initial conditions and non-local coupling is studied. We show that, based on an analytical argument incorporating the initial conditions and the range of non-local coupling, the occurrence of phase chimeras can be seen as caused by a phase lag in the coupling. Considering the dynamics of chimera states, our argument shows how “flipped” profiles of the mean phase velocities can be explained by a change of sign of the coupling phase. By this, one can either choose a concave (“upside”) profile of the mean phase velocities or a “flipped” one. Extending our reasoning, we show that this argument intuitively explains the transition from a phase- to an amplitude-mediated chimera state as a result of increasing coupling strength.DFG, 163436311, SFB 910: Kontrolle selbstorganisierender nichtlinearer Systeme: Theoretische Methoden und Anwendungskonzept