Slow Dynamics Research Articles

The Na+/K+-ATPase generically enables deterministic bursting in class I neurons by shearing the spike-onset bifurcation structure.

Slow brain rhythms, for example during slow-wave sleep or pathological conditions like seizures and spreading depolarization, can be accompanied by oscillations in extracellular potassium concentration. Such slow brain rhythms typically have a lower frequency than tonic action-potential firing. They are assumed to arise from network-level mechanisms, involving synaptic interactions and delays, or from intrinsically bursting neurons. Neuronal burst generation is commonly attributed to ion channels with slow kinetics. Here, we explore an alternative mechanism generically available to all neurons with class I excitability. It is based on the interplay of fast-spiking voltage dynamics with a one-dimensional slow dynamics of the extracellular potassium concentration, mediated by the activity of the Na+/K+-ATPase. We use bifurcation analysis of the complete system as well as the slow-fast method to reveal that this coupling suffices to generate a hysteresis loop organized around a bistable region that emerges from a saddle-node loop bifurcation-a common feature of class I excitable neurons. Depending on the strength of the Na+/K+-ATPase, bursts are generated from pump-induced shearing the bifurcation structure, spiking is tonic, or cells are silenced via depolarization block. We suggest that transitions between these dynamics can result from disturbances in extracellular potassium regulation, such as glial malfunction or hypoxia affecting the Na+/K+-ATPase activity. The identified minimal mechanistic model outlining the sodium-potassium pump's generic contribution to burst dynamics can, therefore, contribute to a better mechanistic understanding of pathologies such as epilepsy syndromes and, potentially, inform therapeutic strategies.

Firing patterns and fast–slow dynamics in an N-type LAM-based FitzHugh–Nagumo circuit

The diversity of firing patterns and their bifurcation mechanisms of a neuron circuit are crucial for exploiting bionic applications. This paper constructs an N-type locally active memristor (LAM)-based FitzHugh–Nagumo (FHN) circuit by replacing the tunnel-diode in the original FHN circuit. Numerical simulations and hardware measurements reveal that the N-type LAM-based FHN circuit can reproduce rich neuromorphic firing patterns of quasi-periodic/periodic bursting behaviors and chaotic/periodic spiking behaviors. The bursting and spiking behaviors are triggered by a low-frequency stimulus and a high-frequency one, respectively. Besides, the fold and Hopf bifurcation sets are depicted. Then, the bifurcation mechanisms for the quasi-periodic and periodic bursting behaviors via Hopf/Hopf and Hopf/fold bifurcations are theoretically deduced by time-domain waveform and equilibrium trajectory. The numerical results and hardware experiments exhibit the effectiveness of the proposed memristive FHN circuit in reproducing abundant firing patterns of bursting and spiking behaviors.

(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy have been used to elucidate the electron and hole dynamics in high quality InP/ZnSe/ZnS quantum dots (QDs) at room temperature. These dynamics depend critically on the overall In:P ratio. Stoichiometric QDs (those having an In:P ratio close to unity), have PL quantum yields of > 90% and exhibit dynamics that are very simple: both electron and hole relaxation to the bandedge is fast compared to the ~ 40 ps time-correlated photon-counting instrument response and the PL follows a single exponential decay having a time constant of about 26 ns. However, QDs having an In:P ratio greater than about 1.1 maintain PL quantum yields > 90%, but show much more complicated PL kinetics. In the non-stoichiometric QDs, a significant fraction of the PL exhibits a slower risetime. The PL rises over a range of timescales varying from close to the instrument response to approximately 2.0 ns. The time constant of the slowest component increases with increasing InP core size and the fraction of the PL that has the slow risetime increases with excess indium in the particle. The slow risetime is absent in the TA results, indicating that the slow dynamics may be assigned to relaxation in the valence band. The risetime increases with the thickness of the ZnSe shell, varying from < 40 ps for the thinnest (1.1 nm) to about 2.0 ns for the thickest (2.7 nm) ZnSe shells. In addition to the slow risetime, the QDs having excess indium show a significant long-lived (>100 ns) decay component. The magnitude and time constant of this slow decay also increases with ZnSe shell thickness.These results suggest that holes are transiently trapped at sites associated with indium dopants in the ZnSe shell. The trapped hole has negligible overlap with the conduction band electron, making this an optically dark state. The holes are in an equilibrium between the shell traps and the valence band, giving rise to the delayed emission. Several possible assignments of the trapping species can be considered, and a substitutional indium adjacent to a zinc vacancy, In3+/VZn 2-, is the most likely. This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. Further experiments show that addition of alkyl carboxylic acids causes increased trapping, presumably by the formation of zinc oleate, thereby creating additional zinc vacancies. Chloride ion is a common impurity in the synthesis of InP/ZnSe/ZnS QDs and the possibility of In3+/Cl- impurities acting as the hole trap can be also considered. However, additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride show the same PL risetimes and delayed emission, eliminating this possibility. Density functional theory calculations further corroborate assignment of the trapping species to In3+/VZn 2-. These calculations show that either a single In2+ ion or an In2+-In3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In3+ is far too difficult to oxidize. However, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band.

Different bifurcations and slow dynamics underlying different stochastic dynamics of slow, medium, and fast bursting of β-cell

Different bifurcations and slow dynamics underlying different stochastic dynamics of slow, medium, and fast bursting of β-cell

Slow dynamics observed on highly shifted spectral component of $${^{23}}$$Na NMR in low silica X zeolite loaded with potassium for saturating level

Slow dynamics observed on highly shifted spectral component of $${^{23}}$$Na NMR in low silica X zeolite loaded with potassium for saturating level

Molten Salt Synthesis of Ti3C2/Cu Cocatalyst for Enhanced TiO2 Photocatalytic CO2 Reduction

AbstractPhotocatalytic reduction of CO2 is considered as a crucial pathway towards achieving sustainable energy and environmental goals. Nonetheless, attaining efficient CO2 conversion poses significant challenges, primarily due to the slow dynamics of charge carriers and the high activation energy required to break C=O bonds. In this study, a novel strategy involving Lewis acid molten salt etching is investigated to engineer a titanium oxide (TiO2)‐based photocatalysts with dual electron transfer channels (i. e., Ti3C2/Cu), which targets the photoreduction of CO2 to CO and CH4. Thanks to the dual electron transfer channels presented in the cocatalysts (Ti3C2/Cu), in conjunction with the numerous heterogeneous interfaces between TiO2 and the Ti3C2/Cu cocatalysts, this hybrid catalyst not only reduces charge transfer resistance but also accelerates the dynamics of photogenerated charge carriers. Consequently, the TiO2/Ti3C2/Cu hybrid catalyst demonstrates an exceptional photocatalytic CO2 reduction rate of 13.67 μmol g−1 h−1, with a total utilized photoelectron number (UPN) of 102.34 μmol g−1 h−1. which is 3.99‐fold higher than that of unmodified TiO2. This research provides a new approach for the preparation of dual cocatalysts through a one‐step molten salt synthesis process.

Synchronization under saturable nonlinearity.

In this theoretical work, we introduce a nonlinear gain saturation law representative of the experimentally observed properties manifested by phenomena ranging from aeroacoustic shear layers in self-sustained cavity oscillations to flame heat release rate in thermoacoustic instabilities. Furthermore, this type of saturable gain may be relevant for a wider class of physical systems, such as active laser media in photonics. The nonlinearity discussed herein governs the fullscale behavior of a self-oscillator exhibiting linear loss under large amplitude perturbations, in contrast to the cubic damping and linear gain of the Van der Pol model. A distinctive characteristic of the proposed equation is the simple, well behaved gain term in the slow timescale dynamics.

PepBinding: A Workflow for Predicting Peptide Binding Structures by Combining Peptide Docking and Peptide Gaussian Accelerated Molecular Dynamics Simulations.

Predicting protein-peptide interactions is crucial for understanding peptide binding processes and designing peptide drugs. However, traditional computational modeling approaches face challenges in accurately predicting peptide-protein binding structures due to the slow dynamics and high flexibility of the peptides. Here, we introduce a new workflow termed "PepBinding" for predicting peptide binding structures, which combines peptide docking, all-atom enhanced sampling simulations using the Peptide Gaussian accelerated Molecular Dynamics (Pep-GaMD) method, and structural clustering. PepBinding has been demonstrated on seven distinct model peptides. In peptide docking using HPEPDOCK, the peptide backbone root-mean-square deviations (RMSDs) of their bound conformations relative to X-ray structures ranged from 3.8 to 16.0 Å, corresponding to the medium to inaccurate quality models according to the Critical Assessment of PRediction of Interactions (CAPRI) criteria. The Pep-GaMD simulations performed for only 200 ns significantly improved the docking models, resulting in five medium and two acceptable quality models. Therefore, PepBinding is an efficient workflow for predicting peptide binding structures and is publicly available at https://github.com/MiaoLab20/PepBinding.

Quantum logarithmic multifractality

Through a combination of rigorous analytical derivations and extensive numerical simulations, this work reports an exotic multifractal behavior, dubbed “logarithmic multifractality,” in effectively infinite-dimensional systems undergoing the Anderson transition. In contrast to conventional multifractality observed in finite dimensions, logarithmic multifractality at infinite dimension introduces an algebraic behavior with respect to the logarithm of system size or time. We demonstrate this phenomenon across eigenstate statistics, spatial correlations, and wave packet dynamics. Our findings offer crucial insights into strong finite-size effects and slow dynamics in complex systems undergoing the Anderson transition, such as the many-body localization transition. Published by the American Physical Society 2024

One-step synthesis of g-C3N4/TiVCTx MXene electrodes for lithium-ion batteries and supercapacitors

The wide-ranging application of lithium-ion batteries (LIBs) is currently restricted by their slow ion dynamics and low capacity, which hinders the development of innovative electrode materials. Herein, the graphitic carbon nitride (g-C3N4)/TiVCTx composite was fabricated through in situ pyrolysis of urea to generate g-C3N4 nanosheets on the surface of TiVCTx nanosheets. The as-prepared g-C3N4 protective layer can reduce the degree of oxidation of TiVCTx MXene, restrain the re-stacking of TiVCTx nanosheets and minimize the risk of undesirable side reactions. As expected, the g-C3N4/TiVCTx electrode exhibits remarkable specific capacity of 1025.3 mAh/g after 150 cycles at 0.1 A/g and 558.3 mAh/g after 1000 cycles at 1.0 A/g for LIBs and a high specific capacitance of 508.9F g−1 in 0.5 M Na2SO4 electrolyte at 1.0 A/g after 7000 cycles for supercapacitors (SCs). Furthermore, the assembled g-C3N4/TiVCTx-30//LiFePO4 full cell presents stable specific capacity of 505.5 mAh/g at 0.1 A/g after 100 cycles and 255.2 mAh/g at 0.1 A/g along with a Coulombic efficiency of 97.9 % after 50 cycles at − 20 °C, demonstrating excellent low temperature tolerance. Density functional theory (DFT) calculation results indicated that the g-C3N4/TiVCTx-30 exhibits strongest adsorption capacity for Li+ with an adsorption energy of −3.85 eV. The introduction of g-C3N4 improves the insertion and extraction kinetics of Li+, effectively reducing the diffusion barrier. Consequently, the g-C3N4/TiVCTx-30 electrode exhibits potential application prospects in high-performance electrochemical energy storage devices. Our work provides valuable insights and significant research implications for improving the specific capacity and stability of TiVCTx MXene in the field of energy storage.

Designofhighperformancealuminum-airbatterybasedoncheapnon-preciousmetalcathodes

With the rapid development of areas such as mobile electronic devices and electric vehicles, environmental problems and energy problems have become increasingly prominent. Therefore, it is particularly important to develop green, safe, efficient, cheap, sustainable energy storage and large-scale application of energy storage devices. Aluminum-air battery is a device that converts the chemical energy of anode aluminum directly into electric energy through electrochemical reaction. The theoretical voltage of aluminum-air battery is high (2.75 V), large specific capacity (2.98 Ah/g), high specific energy (8.1 Wh/g), and metal aluminum is a high strength energy carrier, has the advantages of rich resources, low price, environment friendly, perfectly matching the current situation of power supply requirements. However, the existing aluminum-air batteries face the economic problem of high cost, due to the slow oxygen reduction reaction dynamics of the cathode and the use of the expensive precious metal Pt/C as a catalyst. In order to reduce the cost of aluminum-air batteries, this paper plans to use the cheap non-precious metal nano carbon materials as the cathode catalyst, and flexibly use the structural design to construct the structure of the aluminum-air battery with double cathodes, which effectively increases the cathode reaction area. In this paper, the discharge performance was studied by experimental method, and its peak power is 1.42 times that of the traditional single cathode battery, which has good catalytic performance and stability, and shows that the non-precious metal double-cathode structure battery is both practical and economical.

Unified understanding of the impact of semiflexibility, concentration, and molecular weight on macromolecular-scale ring diffusion

Conformationally fluctuating, globally compact macromolecules such as polymeric rings, single-chain nanoparticles, microgels, and many-arm stars display complex dynamic behaviors due to their rich topological structure and intermolecular organization. Synthetic rings are hybrid objects with conformations that display both ideal random walk and compact globular features, which can serve as models of genomic DNA. To date, emphasis has been placed on the effect of ring molecular weight on their unusual behaviors. Here, we combine simulations and a microscopic force-level theory to build a unified understanding for how key aspects of ring dynamics depend on different tunable molecular properties including backbone rigidity, monomer concentration, degree of traditional entanglement, and molecular weight. Our large-scale molecular dynamics simulations of ring melts with very different backbone stiffnesses reveal unanticipated behaviors which agree well with our generalized theory. This includes a universal master curve for center-of-mass diffusion constants as a function of molecular weight scaled by a chemistry and thermodynamic state-dependent critical molecular weight that generalizes the concept of an entanglement cross-over for linear chains. The key physics is how backbone rigidity and monomer concentration induced changes of the entanglement length, interring packing, degree of interpenetration, and liquid compressibility slow down space-time dynamic-force correlations on macromolecular scales. A power law decay of the center-of-mass diffusion constant with inverse molecular weight squared is the first consequence, followed by an ultraslow activated hopping transport regime. Our results set the stage to address slow dynamics and kinetic arrest in different families of compact synthetic and biological polymeric systems.

Spin-Glass States Generated in a van der Waals Magnet by Alkali-Ion Intercalation.

Tuning magnetic properties in layered van der Waals (vdW) materials has captured significant attention due to the efficient control of ground states by heterostructuring and external stimuli. Electron doping by electrostatic gating, interfacial charge transfer, and intercalation is particularly effective in manipulating the exchange and spin-orbit properties, resulting in a control of Curie temperature (TC) and magnetic anisotropy. Here, an uncharted role of intercalation is discovered to generate magnetic frustration. As a model study, Na atoms are intercalated into the vdW gaps of pristine Cr2Ge2Te6 (CGT) where generated magnetic frustration leads to emerging spin-glass states coexisting with a ferromagnetic order. A series of dynamic magnetic susceptibility measurements/analysis confirms the formation of magnetic clusters representing slow dynamics with a distribution of relaxation times. The intercalation also modifies other macroscopic physical parameters including the significant enhancement of TC from 66 to 240K and the switching of magnetic easy-hard axis direction. This study identifies intercalation as a unique route to generate emerging frustrated spin states in simple vdW crystals.

Efficient spatial separation for chiral molecules via optically induced forces.

We investigate an efficient spatial enantioseparation method of chiral molecules in cyclic three-level systems coupled with three optical fields using optically induced forces. When the overall phase differs by π between two enantiomers, significant variations in the magnitude and direction of the optically induced forces are observed. The manipulation of the center of mass of chiral molecules in optical fields can be achieved through the induced gauge force, primarily generated from the variations in the chirality-dependent scalar potentials created by the three inhomogeneous laser fields. By appropriately configuring the system, we can completely separate the slow spatial and fast inner dynamics, making instantaneous eigenstates of the inner Hamiltonian independent of the transverse profiles of the laser beams. Compared to previous methods, which required adiabatic conditions to be satisfied, the proposed method overcomes the limitations of the adiabatic approximation by utilizing a specific system configuration. This allows for increased flexibility in the transverse profiles of the laser beams and relaxes the constraints on the velocity of chiral molecules, leading to significantly greater spatial separations achievable across a broader range of parameters.

Slow polymer dynamics in poly(3-hexylthiophene) probed by muon spin relaxation

Slow polymer dynamics in poly(3-hexylthiophene) probed by muon spin relaxation

Similarities and Differences Between Natural and Simulated Slow Earthquakes

AbstractWe investigate similarities and differences between natural and simulated slow earthquakes using nonlinear dynamical system tools. We use spatio‐temporal slip potency rate data derived from Global Navigation Satellite System (GNSS) position time series in the Cascadia subduction zone and numerical simulations intended to reproduce their pulse‐like behavior and scaling laws. We provide metrics to evaluate the accuracy of simulations in mimicking slow earthquake dynamics. We investigate the influence of spatio‐temporal coarsening as well as observational noise. Despite the use of many degrees of freedom, numerical simulations display a surprisingly low average dimension, akin to natural slow earthquakes. Instantaneous dynamical indices can reach large values (&gt;10) instead, and differences persist between numerical simulations and natural observations. We propose to use the suggested metrics as an additional tool to narrow the divergence between slow earthquake observations and dynamical simulations.

Massively parallel free energy calculations for in silico affinity maturation of designed miniproteins.

Computational protein design efforts continue to make remarkable advances, yet the discovery of high-affinity binders typically requires large-scale experimental screening of site-saturated mutant (SSM) libraries. Here, we explore how massively parallel free energy methods can be used for in silico affinity maturation of de novo designed binding proteins. Using an expanded ensemble (EE) approach, we perform exhaustive relative binding free energy calculations for SSM variants of three miniproteins designed to bind influenza A H1 hemagglutinin by Chevalier et al. (2017). We compare our predictions to experimental ΔΔ G values inferred from a Bayesian analysis of the high-throughput sequencing data, and to state-of-the-art predictions made using the Flex ddG Rosetta protocol. A systematic comparison reveals prediction accuracies around 2 kcal/mol, and identifies net charge changes, large numbers of alchemical atoms, and slow side chain conformational dynamics as key contributors to the uncertainty of the EE predictions. Flex ddG predictions are more accurate on average, but highly conservative. In contrast, EE predictions can better classify stabilizing and destabilizing mutations. We also explored the ability of SSM scans to rationalize known affinity-matured variants containing multiple mutations, which are non-additive due to epistatic effects. Simple electrostatic models fail to explain non-additivity, but observed mutations are found at positions with higher Shannon entropies. Overall, this work suggests that simulation-based free energy methods can provide predictive information for in silico affinity maturation of designed miniproteins, with many feasible improvements to the efficiency and accuracy within reach.

Two glass transitions in substrate-supported polymer ultrathin films: A molecular dynamics simulation

Broadening of glass transition and even two glass transitions in polymer ultrathin films were repeatedly reported in experiments. The glass transition and dynamic gradient in substrate-supported polymer films are studied by performing molecular dynamics simulations in this work. Two glass transitions are observed in ultrathin films with sufficiently strong attraction of the substrate. Due to the strong attraction of the substrate and spatial confinement of the ultrathin film, the adsorbed chains adopt a flattened conformation with slow dynamics, resulting in a pronounced dynamic gradient across the ultrathin film. The dynamics of chains near the free surface are suppressed by the adsorbed chain as the tail of a few of adsorbed chains is longer than the film thickness, therefore two glass transitions occur at higher temperatures than the bulk glass transition temperature. However, only a single glass transition is observed when the substrate attraction weakens or the film thickness increases.

Collective photon emission in solid state environments: Concatenating non-Markovian and Markovian dynamics

Collective light emission and multiqubit dynamics of solid-state quantum emitters are affected both by their coupling to the light field and to lattice vibrations. The effect of phonons on quantum emitters is twofold: polaron formation is described by ultrafast non-Markovian dynamics, while slower dephasing is well described by exponential decay. Of the two temperature-dependent processes, the effect of the former on the collective emission and the entanglement decay of emitters is usually not modeled, and also the latter is sometimes neglected. Here we propose and compare two methods that are efficient also for several emitters: the first method concatenates the fast and slow phonon dynamics, and the second is the polaron method. For a single quantum emitter, we show that the dynamical equations are identical in both methods, while predictions for two or more emitters also agree very well. Both of our methods incorporate non-Markovian dynamics due to phonons demonstrating the temperature sensitivity of the collective photon emission. Utilizing a simplified Markovian model instead may not be accurate enough especially for quantum information applications: for example, we show how the Markovian model may considerably overestimate the two-emitter concurrence, except at very low temperatures. Our concatenation and polaron methods can be applied to an arbitrary number and type of quantum emitters, and beyond the bulk GaAs environment that we consider here. Especially the concatenation method can take phonon effects into account at the same computational cost as modeling the emitter-photon interaction alone. Finally, we present approximate analytical expressions for the collective emission spectrum for N emitters on a one-dimensional chain. Published by the American Physical Society 2024

Frequency switching leads to distinctive fast–slow behaviors in Duffing system

The slowly forced Duffing system has been found to exhibit unique fast–slow behaviors in descending frequency switching scheme, the typical of which is the sliding bursting, which is instructive for understanding the dynamics of ubiquitous frequency switching systems in scientific research and practical engineering. This paper is devoted to further refining the fast–slow dynamics of the slowly forced Duffing system with another commonly encountered frequency switching scheme, i.e., the ascending frequency switching, characterized by switching the frequency synchronously according to the increase or decrease of state variable values. Taking the forcing amplitude as an example, this paper provides a theoretical way to fully summarize the fast–slow behaviors in frequency switching systems with the variation of parameter values based on the proposed superposition analysis of one- and two-parameter bifurcations of subsystems. As a result, two typical bifurcation structures and eight threshold windows with distinct switching vector fields therein are identified, inducing up to ten different bursting patterns. Among them, several novel fast–slow dynamics, such as multiple jumps hysteresis loop formed by boundary equilibrium bifurcations and the switching failure phenomena, are presented and investigated. In particular, the underlying mechanism of threshold modulation, i.e., the evolution of unconventional bifurcations, is also found. These findings contribute to complementing and contrasting the existing studies on the fast–slow dynamics of frequency switching systems.