External Perturbations Research Articles

Photorefraction‐Assisted Self‐Emergence of Dissipative Kerr Solitons

AbstractGenerated in high‐Q optical microresonators, dissipative Kerr soliton microcombs constitute broadband optical frequency combs with chip sizes and repetition rates in the microwave to millimeter‐wave range. For frequency metrology applications such as spectroscopy, optical atomic clocks, and frequency synthesizers, octave‐spanning soliton microcombs generated in dispersion‐optimized microresonators are required, which allow self‐referencing for full frequency stabilization. In addition, field‐deployable applications require the generation of such soliton microcombs to be simple, deterministic, and reproducible. Here, a novel scheme to generate self‐emerging solitons in integrated lithium‐niobate microresonators is demonstrated. The single soliton features a broadband spectral bandwidth with dual dispersive waves, allowing 2f–3f self‐referencing. Via harnessing the photorefractive effect of lithium niobate to significantly extend the soliton existence range, a spontaneous yet deterministic single‐soliton formation is observed. The soliton is immune to external perturbation and can operate continuously for over 13 h without active feedback control. Finally, via integration with a pre‐programmed distributed feedback (DFB) laser, turnkey soliton generation is demonstrated. With further improvement of microresonator Q and hybrid integration with chip‐scale laser chips, compact soliton microcomb devices with electronic actuation can be created, which can become central elements for future LiDAR, microwave photonics, and optical telecommunications.

Practical Adaptive Fast Terminal Sliding Mode Control for Servo Motors

Position control of servo motor systems is a challenging task because of inevitable factors such as uncertainties, nonlinearities, parametric variations, and external perturbations. In this article, to alleviate the above issues, a practical adaptive fast terminal sliding mode control (PAFTSMC) is proposed for better tracking performance of the servo motor system by using a state observer and bidirectional adaptive law. First, a smooth-tangent-hyperbolic-function-based practical fast terminal sliding mode control (PFTSM) surface is designed to ensure not only fast finite time tracking error convergence but also chattering reduction. Second, the PAFTSMC is proposed for the servo motor, in which a two-way adaptive law is designed to further suppress the chattering and overestimation problems. More importantly, the proposed adaptive technique can update the switching gain according to the system uncertainties, which can provide high gain in the reaching phase and then decrease to the smallest value in the sliding phase to avoid the monotonically increasing gain that exists in most adaptation methods. Third, the finite-time stability of the closed-loop system is proved based on the Lyapunov theorem. Finally, the simulation studies and experimental tests verify the effectiveness of the proposed control in terms of better tracking, strong robustness, and reduced chattering, compared to existing algorithms.

Biological thermodynamics: Ervin Bauer and the unification of life sciences and physics

Biological systems operate toward the maximization of their self-maintenance and adaptability. This is achieved through the establishment of robust self-maintaining configurations acting as attractors resistant to external and internal perturbations. Ervin Bauer (1890–1938) was the first who formulated this essential thermodynamic constraint in the operation of biological systems, which he defined as the stable non-equilibrium state. The latter appears as the basic attractor relative to which biological organization is established. The stable non-equilibrium state represents a generalized cell energy status corresponding to efficient spatiotemporal organization of the fluxes of matter and energy and constantly reproducing the conditions of self-maintenance of metabolism and controlling the rates of major metabolic fluxes that follow thermodynamically and kinetically defined computational principles. This state is realized in the autopoietic structures having closed loops of causation based on the operation of biological codes. The principle of thermodynamic buffering determines the conditions for optimization of the fluxes of load and consumption in metabolism establishing the conditions of metabolic stable non-equilibrium. In developing and evolving biological systems, the principle of stable non-equilibrium is transformed into the principle of increasing external work, which is grounded in the hyper-restorative non-equilibrium dynamics. Bauer's concept of the stable non-equilibrium state puts thermodynamics into the frames of the internal biological causality governing self-maintenance and development of living systems. It can be defined as a relational theory of biological thermodynamics since the standard to which it refers represents the actual biological function rather than the abstract state of thermodynamic equilibrium.

Self-excited converging shock structure in a complex plasma medium.

We report the study of a self-excited converging shock structure observed in a complex plasma medium. A high-density dust cloud of melamine formaldehyde particles is created and horizontally confined by a circular ring in a dc glow discharge plasma at a particular discharge voltage and pressure. Later, as the discharge voltage is increased, a circular density crest is spontaneously generated around the outer boundary of the dust cloud. This nonlinear density structure is seen to propagate inward towards the center of the dust cloud. The properties of the excited structure are analyzed and found to follow the characteristics of a converging shock structure. A three-dimensional molecular dynamics simulation has also been performed in which a stable dust cloud is formed and levitated by the balance of forces due to gravity and an external electric field mimicking the cathode sheath electric field in the experiment. Particles are also horizontally confined by an external electric field, representing the sheath electric field of the circular ring present in the experiment. A circular shock structure has been excited by applying an external perturbation in the horizontal electric field around the outer boundary of the dust cloud. The characteristic properties of the shock are analyzed in the simulation and qualitatively compared with the experimental findings. This paper is not only of fundamental interest but has many implications concerning the study of converging shock waves excited in other media for various potential applications.

Robust proportional control for trajectory tracking using parameter and perturbation adaptive estimators

The principal contribution of this paper is the feedback design of an output trajectory tracking controller applied to a certain class of mechanical systems. The controller does not need velocity measurements for its implementation and achieves the trajectory tracking control aim. We propose adaptive estimators to determine the plant parameters and constant external perturbations. When the plant is affected by time-variant external perturbations, an observer filter that can be implemented to recover an estimation of the time-variant perturbations can replace the adaptive estimator. The closed-loop stability is analysed throughout Lyapunov tools. We illustrate the performance of the proposed control structure via numerical simulations and experiments, the numerical simulations are carried out for a pendulum coupled to a DC motor and the experiments are accomplished for a mass-spring-damper system.

Controlled caging, flat band and thermoelectric response in a quasi-one dimensional kagomé ribbon

A quasi-one dimensional kagomé ribbon geometry is studied in respect of its band spectrum, electronic transport as well as the thermopower within the tight-binding formalism. Analytic proposition of Bloch bands along with the dispersionless flat band states is seen to be consistent with the numerical evaluation of density of states, transmission and band dispersion. A uniform magnetic flux can act as an external agency to tune the spectral landscape and the transport at will. Flux sensitive band engineering leads to an interesting proposal of tunable thermoelectric performance of an identical model device. The tunable behavior can be explored by the calculation of Seebeck co-efficient. The external magnetic perturbation plays the key role in this demonstration and it is an easily adjustable parameter. We have critically investigated all the characteristic features using the Sommerfeld expansion within a tight-binding framework based on Green’s function formalism. The present analysis may provide useful directive for constructing efficient thermoelectric devices at the nanoscale level.

Robust Super‐Resolution Image Transmission Based on a Ring Core Fiber with Orbital Angular Momentum

AbstractSingle‐fiber imaging technology offers unique insights for research and inspection in difficult‐to‐reach and narrow spaces. In particular, ultra‐compact multimode fiber (MMF) imaging, has received increasing interest over the past decade. However, MMF imaging will be seriously distorted when subjected to dynamic perturbations due to time‐varying mode coupling, and the imaging of space objects via Gaussian beam will be relatively degraded at the edge due to insufficient contrast. Here, a robust super‐resolution image transmission method based on a ring core fiber (RCF) with orbital angular momentum (OAM) is proposed and experimentally demonstrated. The OAM modes propagating in the RCF form a series of weakly coupled mode groups, making the imaging system robust to external perturbations. In addition, a spiral phase plate is used as a vortex filter to produce OAM for edge enhancement, thus improving the image resolution. Furthermore, a few‐shot U‐Transformer neural network is proposed to enhance the resilience of the developed RCF‐OAM imaging system against environmental perturbations. Finally, the developed RCF‐OAM imaging system achieves biological image transmission, demonstrating the practicality of the scheme. This pioneering RCF OAM image transmission system may have broad applications, potentially revolutionizing fields such as biological imaging and industrial non‐destructive testing.

A high performance piezoelectric hetero-junction based on the configuration reform on interfacial potential barrier

In this paper, the barrier region of a mixed hetero-junction consisting of a p-type Si and an n-type ZnO was fully opened by abandoning the depletion layer approximation in advance. The barrier configuration was reconstructed by the artificial potential barrier based on a new fully coupled model in which the interaction between physical fields and charge carriers was taken into account. Then, we put forward an interesting design idea of elevating the mechanical regulation performance of a PN junction by manufacturing it with a third-generation piezoelectric semiconductor and a narrow bandgap semiconductor. It is through the deformation of the third-generation piezoelectric semiconductor on one side under mechanical loadings to excite polarization charges at the PN interface such that the movement of charge carriers in the narrow bandgap semiconductor on the other side is able to be tuned by the interface polarization charges. Besides, it is found a specific current-stress characteristic for such a mixed hetero-junction, which will present three stages, i.e., rising stage, platform stage and falling stage. The rising stage is caused by the improving recombination rate, which can be used as a high sensitivity stress sensor (output current improved by more than 10 times in a small stress). Oppositely, the falling stage is caused by the weakened recombination rate due to carrier type-inversion, which can be used to determine overload (output current reduced dramatically when the stress increased to a specific value). Finally, the platform stage is caused by the competition of the above two cases. At this stage, the device operates very stable and can resist the external perturbation. Obviously, the study possesses referential significance to the design and mechanical tuning on performance of piezotronic devices.

Activity of deep trunk muscles for postural control to predictable and unpredictable sagittal plane perturbations: Electromyographic analysis.

The activity of deep trunk muscles (psoas major; PM, quadratus lumborum; QL, transverse abdominis; TrA, and lumbar multifidus; MF) in response to external perturbation is not clearly known. This study aimed to record the onset and amount of activity of the deep trunk muscles during sagittal plane perturbations. Fourteen healthy males participated in this study. The activity of the right deep trunk muscles was recorded using wire electrodes. In standing, the participants performed three tasks: a pendulum impacted from anterior with predictable and unpredictable and posterior with unpredictable. In predictable anterior perturbation, the TrA and PM demonstrated feedforward activation, while all deep trunk muscles demonstrated feedback activation in unpredictable anterior and posterior perturbations. In the anticipatory postural adjustment phase, the activity of the TrA was large in predictable anterior perturbation, while that of all deep trunk muscles was slight in other perturbations. In the compensatory postural adjustment phase, the activity of the PM, QL, and TrA in unpredictable anterior perturbation and those of the PM, QL, and MF in unpredictable posterior perturbation were large. These results showed that the onset and magnitude of deep trunk muscle activity changed depending on both predictable or unpredictable perturbation and the direction of perturbation.

Influence of structural variables and external perturbations on the nonlinear optical rectification, second, and third-harmonic generation in the InP/InGaAs triple quantum well structure

Influence of structural variables and external perturbations on the nonlinear optical rectification, second, and third-harmonic generation in the InP/InGaAs triple quantum well structure

Analysis of bifurcation and chaos in the traveling wave solution in optical fibers using the Radhakrishnan–Kundu–Lakshmanan equation

The study examines the propagation of solitary waves in optical fibers controlled by the Radhakrishnan–Kundu–Lakshmanan model (RKL) equation, which includes a cubic–quintic non-Kerr refractive index. The first step in this study involves utilizing the extended algebraic method to derive solitonic solutions for the given equation. These solutions are subsequently graphically represented for a variety of parameter values. The proposed strategy allows for the discovery of a range of solutions.The controlling model is incorporated into a planar dynamical system via the Galilean transformation, resulting in the transformation of the governing model. The effects of both the quintic nonlinearity and self-phase modulation on the perturbed and unperturbed systems are explored using the effective potential and related phase portrait. The governing model is thus transformed into a planar dynamical system. Furthermore, this study harnessed a variety of numerical techniques. These methods encompassed time series analysis, phase portraits examination, and meticulous sensitivity inspections. Additionally, we employed a Poincaré section to assess the equation’s responsiveness to external perturbations. The outcomes reveal a fascinating observation: the perturbed system’s behavior shifts from a quasi-periodic state to a chaotic one, contingent upon the amplitude and frequency of the external perturbation. Moreover, we conducted multistability analysis for specific sets of physical parameters, uncovering the equation’s tendency to exhibit multistability in such scenarios.

Quantifying Individual Variability in Neural Control Circuit Regulation Using Single-Subject fMRI

As a field, control systems engineering has developed quantitative methods to characterize the regulation of systems or processes, whose functioning is ubiquitous within synthetic systems. In this context, a control circuit is objectively “well regulated” when discrepancy between desired and achieved output trajectories is minimized and “robust” to the degree that it can regulate well in response to a wide range of stimuli. Most psychiatric disorders are assumed to reflect dysregulation of brain circuits. Yet, probing circuit regulation requires fundamentally different analytic strategies than the correlations relied upon for analyses of connectivity and their resultant networks. Here, we demonstrate how well-established methods for system identification in control systems engineering may be applied to functional magnetic resonance imaging (fMRI) data to extract generative computational models of human brain circuits. As required for clinical neurodiagnostics, we show these models to be extractable even at the level of the single subject. Control parameters provide two quantitative measures of direct relevance for psychiatric disorders: a circuit’s sensitivity to external perturbation and its dysregulation.

Tuning the electronic heat capacity and thermal Schottky anomaly of monolayer [formula omitted]-borophene via adsorbed gas molecules

Owing to the inherent inversion symmetry between five atoms {a,b,c,d,e} and metallic phase, β12-borophene possess fantastic physical properties, such as various electronic phase transitions in the presence of external perturbations. Here, the effect of adsorbed gas molecules (NH3, NO, NO2, and CO) on the electronic heat capacity (EHC) and thermal Schottky anomaly of β12-borophene is calculated employing the tight-binding model and the Green’s function approach. The results show that the gas-adsorbed EHC is enhanced with the dominant contribution of NO on individual {a,b,d,e} atoms, while it is NO2 on c atoms. However, EHC is suppressed when the molecule is adsorbed on the pair of boron atoms. Moreover, the enhancement of EHC is dominated by the simultaneous adsorption of NH3 on all atoms. On average, the pristine Schottky anomaly moves to the higher thermal energies upon the adsorption of gas molecules. These distinct differences originate from the generation of various new energy levels. This study provides a detailed understanding of β12-borophene response to the gas molecules for tuning field effect transistor-based gas sensors.

No effects of different perturbations on the performance in a mental body-rotation task (MBRT) with egocentric perspective transformations and object-based transformations

The present study investigates participants' performance in two different mental body-rotation tasks (MBRTs) under conditions in which dynamic stability is challenged in two different balancing conditions: active balance control (Experiment 1), where participants actively maneuver, and re-active balance control (Experiment 2), where participants react to an external perturbation. The two MBRTs induced either an object-based spatial transformation (based on a same-different judgment) or an egocentric transformation (based on a left-right judgment). In Experiment 1, 48 participants were tested while standing on an even ground (low balancing requirements) or on a balance board (high balancing requirements). In Experiment 2, 32 participants performed while either standing still on a vibration plate or with the vibration plate moving in a low (20 Hz) or high (180 Hz) frequency. In both experiments, the results for response time and response error revealed effects of rotation angle and type of task. An effect of balancing condition was only observed for response error in Experiment 1. More precisely, response times and response errors increased for higher rotation angles. Also, performance was better for egocentric than for object-based spatial transformations. However, the different challenges to dynamic stability in Experiments 1 and 2 did not influence performance in the two MBRTs (except for response errors in Experiment 1) nor in a control condition (Experiment 1) without mental rotation.

A robust finite–time model reference adaptive controller for arbitrary order disturbed LTI systems

This manuscript deals with the trajectory-tracking problem for linear time-invariant systems with parameter uncertainties and time-dependent external perturbations. A robust finite-time model reference adaptive controller is proposed. In the absence of external perturbations, the proposed controller ensures finite-time convergence to zero of the tracking and parameter identification errors. In presence of time-dependent external perturbations, the tracking and parameter identification errors converge to a region around the origin in a finite time. The convergence proofs are developed based on Lyapunov and input-to-state stability theory. Finally, simulation results in an academic example and a flexible-joint robot manipulator show the feasibility of the proposed approach.

Reactive postural adjustment in response to predictable and unpredictable perturbations in healthy adults: A comparison between swayback, hyperlordotic and erect postures

BackgroundPromoting an erect posture in standing has been advocated to offer superior protection to the spine when compared to hyperlordotic and swayback postures. Research questionDo postural adjustments towards external perturbation differ between erect, hyperlordotic and swayback postures? If so, which posture offers better protection to lumbar spine? MethodsForty-four healthy adults received top-down perturbations under unpredictable (without visual-and-auditory input) and predictable (with visual-and-auditory input) conditions in three simulated postures: erect, hyperlordotic, and swayback. Postural adjustments namely the centre of pressure parameters, joint angle onsets, and neuromuscular responses measured by muscle onsets and co-contraction between muscle pairs upon the perturbation were compared using the two-way repeated measures ANOVA. Post-hoc analysis with Bonferroni correction was conducted to identify the between-posture differences for the respective postural adjustment parameters. ResultsPath length, ellipse area and average velocity of centre of pressure were significantly greater under unpredictable condition as compared to predictable condition (p < 0.001). Significant between-posture difference was detected in centre of pressure path length (p < 0.035), pelvic tilt onset (p < 0.038) and all muscle co-contraction indexes (p < 0.001). Post-hoc analysis revealed significantly smaller centre of pressure path length in erect posture as compared to hyperlordotic and swayback postures (p < 0.01) under unpredictable conditions. Significantly greater co-contraction indexes of lumbar multifidus and erector spinae, and internal oblique and lumbar multifidus were found in hyperlordotic as compared to erect and swayback postures (p < 0.05). SignificanceCompared to erect posture, adoption of hyperlordotic and swayback postures altered the contributions of the active and passive subsystems of the spine that regulates postural control upon external perturbations. Such differences in neuromuscular control may lessen the capacity of the human spine to withstand loading and shear forces. Prospective studies are required to validate if habitually adopted hyperlordotic and swayback postures contribute to an earlier/ higher prevalence of spinal dysfunctions.

Sensitivity and bifurcation analysis of an analytical model of a trapped object in an externally excited acoustic radiation force field

Acoustic radiation force (ARF) is a nonlinear acoustic phenomenon for which the acoustic field properties and, to an even greater extent, the explicit dynamics of the object, have received limited attention in the published literature to date. Any oscillations due to the flow field or external perturbations are thereby negligible while the particle is trapped in a stable position. By changing the viewpoint from the acoustic field to the dynamics of a levitated particle, the amplitude and frequency of external oscillation is non-negligible, we ask the question of how external excitation changes the dynamics of the object. We explicitly derive an analytical formulation of a trapped object in the form of a Duffing-like equation with its constants being defined by the object itself, the fluid, the acoustic wave, and the external vibration properties. In this case, the bifurcation behaviour is studied, and we show this together with a sensitivity analysis to represent correct dynamic behaviour in certain regimes of the bifurcation diagram.

Unravelling diverse spatiotemporal orders in chlorine dioxide-iodine-malonic acid reaction-diffusion system through circularly polarized electric field and photo-illumination.

Designing and predicting self-organized pattern formation in out-of-equilibrium chemical and biochemical reactions holds fundamental significance. External perturbations like light and electric fields exert a crucial influence on reaction-diffusion systems involving ionic species. While the separate impacts of light and electric fields have been extensively studied, comprehending their combined effects on spatiotemporal dynamics is paramount for designing versatile spatial orders. Here, we theoretically investigate the spatiotemporal dynamics of chlorine dioxide-iodine-malonic acid reaction-diffusion system under photo-illumination and circularly polarized electric field (CPEF). By applying CPEF at varying intensities and frequencies, we observe the predominant emergence of oscillating hexagonal spot-like patterns from homogeneous stable steady states. Furthermore, our study unveils a spectrum of intriguing spatiotemporal instabilities, encompassing stripe-like patterns, oscillating dumbbell-shaped patterns, spot-like instabilities with square-based symmetry, and irregular chaotic patterns. However, when we introduce periodic photo-illumination to the hexagonal spot-like instabilities induced by CPEF in homogeneous steady states, we observe periodic size fluctuations. Additionally, the stripe-like instabilities undergo alternating transitions between hexagonal spots and stripes. Notably, within the Turing region, the interplay between these two external influences leads to the emergence of distinct superlattice patterns characterized by hexagonal-and square-based symmetry. These patterns include parallel lines of spots, target-like formations, black-eye patterns, and other captivating structures. Remarkably, the simple perturbation of the system through the application of these two external fields offers a versatile tool for generating a wide range of pattern-forming instabilities, thereby opening up exciting possibilities for future experimental validation.

Novel perturbation mechanism underlying the network fragility evolution

Studies have shown that fragility is an effective marker for seizures and seizure onset zone (SOZ). Through analysis and simulation of a probabilistic neural network under different inputs, the regularization mechanism of external input perturbations on the fragility is explored. It is theoretically found that the fragility of a perturbed node within seizure network is inversely associated with the received perturbation input, while the fragility of the other unperturbed nodes always oppositely changes with this perturbed node. By terming the node with high fragility as the fragile node (FN), it is interestingly shown that the FN would evolve to the node with the smallest input. Then, the network fragility is further investigated. Results show that the non-uniform perturbation inputs can more easily impact the network fragility. In addition, noise-induced variations of network connection can degrade the network fragility to some extent. Finally, the real data from a patient with epilepsy have verified the universality of the above obtained findings. These results may provide possible insights into stimulation strategies for seizure control in clinic.

Generalisation of fluctuation-dissipation theorem to systems with absorbing states

Systems that evolve towards a state from which they cannot depart are common in nature. But the fluctuation-dissipation theorem (FDT), a fundamental result in statistical mechanics, is mainly restricted to systems near-stationarity. In processes with absorbing states, the total probability decays with time, eventually reaching zero and rendering the predictions from the standard response theory invalid. In this article, we investigate how such processes respond to external perturbations and develop a new theory that extends the framework of the FDT. We apply our theory to two paradigmatic examples that span vastly different fields—a birth–death process in forest ecosystems and a targeted search on DNA by proteins. These systems can be affected by perturbations which increase their rate of extinction/absorption, even though the average or the variance of population sizes are left unmodified. These effects, which are not captured by the standard response theory, are exactly predicted by our framework. Our theoretical approach is general and applicable to any system with absorbing states. It can unveil important features of the path to extinction masked by standard approaches.