Stochastic thermodynamics is an extension of classical nonequilibrium thermodynamics to small systems, where fluctuations are expected to play an important role. A central difficulty met when developing such an extension is how to define a nonequilibrium fluctuating entropy. Typically, the expression used is based on Gibbs' formula for entropy at equilibrium. In this work, we show that one can construct an alternative framework for stochastic thermodynamics based on an extension of Einstein's formula connecting the probability of fluctuations and entropy around equilibrium states. We compare the two approaches and discuss, in particular, how they lead to different interpretations of what a stochastic entropy and entropy production represent.

We present a classical analog of the quantum metric tensor, which is defined for classical integrable systems that undergo an adiabatic evolution governed by slowly varying parameters. This classical metric measures the distance, on the parameter space, between two infinitesimally different points in phase space, whereas the quantum metric tensor measures the distance between two infinitesimally different quantum states. We discuss the properties of this metric and calculate its components, exactly in the cases of the generalized harmonic oscillator, the generalized harmonic oscillator with a linear term, and perturbatively for the quartic anharmonic oscillator. Finally, we propose alternative expressions for the quantum metric tensor and Berry's connection in terms of quantum operators.

Collective behavior of an ensemble of directly or indirectly coupled oscillators can be a function of population density. Experiments using autonomous mercury beating heart (MBH) oscillators coupled through their surroundings are employed, to study the existence of quorum-like (population dependent) phenomena. Two coupling mechanisms are used, namely, static and dynamic coupling. For the static coupling scheme, the transitions of a subset of the coupled oscillators occur from active (oscillatory) to inactive (quiescent) state and vice versa. A continuous variation of collective dynamics was observed as the population of the oscillators increased. For the dynamic coupling scheme, the time for which the coupled oscillators are active changes sharply as the population increases beyond a certain threshold.

The dynamics of a weakly dissipative Hamiltonian system submitted to stochastic perturbations has been investigated by means of asymptotic methods. The probability of noise-induced separatrix crossing, which drastically changes the fate of the system, is derived analytically in the case where noise is an additive Kubo-Anderson process. This theory shows how the geometry of the separatrix, as well as the noise intensity and correlation time, affect the statistics of crossing. Results can be applied to a wide variety of systems, and are valid in the limit where the noise correlation time scale is much smaller than the time scale of the undisturbed Hamiltonian dynamics.

For a reliable prediction of an epidemic or information spreading pattern in complex systems, well-defined measures are essential. In the susceptible-infected model on heterogeneous networks, the cluster of infected nodes in the intermediate-time regime exhibits too large fluctuation in size to use its mean size as a representative value. The cluster size follows quite a broad distribution, which is shown to be derived from the variation of the cluster size with the time when a hub node was first infected. On the contrary, the distribution of the time taken to infect a given number of nodes is well concentrated at its mean, suggesting the mean infection time is a better measure. We show that the mean infection time can be evaluated by using the scaling behaviors of the boundary area of the infected cluster and use it to find a nonexponential but algebraic spreading phase in the intermediate stage on strongly heterogeneous networks. Such slow spreading originates in only small-degree nodes left susceptible, while most hub nodes are already infected in the early exponential-spreading stage. Our results offer a way to detour around large statistical fluctuations and quantify reliably the temporal pattern of spread under structural heterogeneity.

To measure, predict, and prevent social segregation, it is necessary to understand the factors that cause it. While in most available descriptions space plays an essential role, one outstanding question is whether and how this phenomenon is possible in a well-mixed social network. We define and solve a simple model of segregation on networks based on discrete convictions. In our model, space does not play a role, and individuals never change their conviction, but they may choose to connect socially to other individuals based on two criteria: sharing the same conviction and individual popularity (regardless of conviction). The tradeoff between these two moves defines a parameter, analogous to the “tolerance” parameter in classical models of spatial segregation. We show numerically and analytically that this parameter determines a true phase transition (somewhat reminiscent of phase separation in a binary mixture) between a well-mixed and a segregated state. Additionally, minority convictions segregate faster and inter-specific aversion alone may lead to a segregation threshold with similar properties. Together, our results highlight the general principle that a segregation transition is possible in absence of spatial degrees of freedom, provided that conviction-based rewiring occurs on the same time scale of popularity rewirings.

In a recent letter [S. Bouzat et al., Phys. Rev. Lett. 120, 178101 (2018)], a mathematical model for eyeball and pupil motion was developed allowing for the understanding of the postsaccadic oscillations (PSO) as inertial effects. The model assumes that the inner part of the iris, which defines the pupil, moves driven by inertial forces induced by the eyeball rotation, in addition to viscous and elastic forces. Among other achievements, the model correctly reproduces eye-tracking experiments concerning PSO profiles and their dependence on the saccade size. In this paper we propose various extensions of the mentioned model, we provide analytical solutions, and we perform an exhaustive analysis of the dynamics. In particular, we consider a more general time dependence for the eyeball velocity enabling the description of saccades with vanishing initial acceleration. Moreover, we give the analytical solution in terms of hypergeometric functions for the constant parameter version of the model and we provide particular expressions for some cases of interest. We also introduce a new version of the model with inhomogeneous viscosity that can improve the fitting of the experimental results. Our analysis of the solutions explores the dependence of the PSO profiles on the system parameters for varying saccade sizes. We show that the PSO emerge in critical-like ways when parameters such as the elasticity of the iris, the global eyeball velocity, or the saccade size vary. Moreover, we find that the PSO profiles with the first overshoot smaller than the second one, which are usually observed in experiments, can be associated to parameter regions close to criticality.

The empirical Kohlrausch-Williams-Watts (KWW) relaxation function has been widely used to describe the nonexponential relaxation behavior of glassy polymers during their physical aging. The characteristic relaxation time (${\tau }_{\text{KWW}}$) and the stretching parameter (${\beta }_{\text{KWW}}$) of the KWW equation are obtained from experimental relaxation data by a curve-fitting procedure. However, knowing the values of KWW parameters is not sufficient to completely characterize the physical aging kinetics of glassy materials in order to compare, for example, the aging rate of different systems. To be able to overcome such problems, at least, an exact interpretation of these parameters is absolutely vital and then it would be quite desirable to convert them into a single relaxation-time constant. This approach, indeed, provides the possibility to directly evaluate and compare the kinetics and aging rate of different systems as well as different methods used for monitoring the physical aging of the same materials. To do these, a method based on the notion of the relaxation spectrum is employed which is modeled via the well-known statistical distribution of log-normal. By detailed numerical simulations, some explicit expressions between KWW parameters with descriptor parameters of the distribution are established. From this point of view, it is tried to interpret ${\tau }_{\text{KWW}}$ and ${\beta }_{\text{KWW}}$ and convert them into a single relaxation-time constant. Finally, the results of suggested simulations are compared with those obtained in the literature.

We investigated the low speed impact on a granular bed by a hydrogel sphere and especially focused on the resultant crater diameter. Though the crater diameter depends only on the impact kinetic energy for the solid sphere impact cratering, it also depends on the elastic energy for the hydrogel sphere. The hydrogel sphere impact cratering is classified into two dynamics: small and large indentation of the sphere. For the small indentation, the crater diameter is proportional to the 1/4 power of the energy for ejecting granular materials, which is calculated by substituting the elastic energy stored in the sphere from the impact kinetic energy. Considering the force balance between the inertial force and the indentation, we derived the relation between the impact kinetic energy, Young's modulus, and the crater diameter. In the large limit of Young's modulus, the relation leads to the 1/4 power law observed in solid sphere impact. The dependency of Young's modulus on the crater diameter for the large indentation of the sphere is larger than the impact with small indentation.

We propose a mechanism for solving the “negative sign problem”—the inability to assign non-negative weights to quantum Monte Carlo configurations—for a toy model consisting of a frustrated triplet of spin-1/2 particles interacting antiferromagnetically. The introduced technique is based on the systematic grouping of the weights of the recently developed off-diagonal series expansion of the canonical partition function [Phys. Rev. E 96, 063309 (2017)]. We show that, although the examined model is easily diagonalizable, the sign problem it encounters can nonetheless be very pronounced, and we offer a systematic mechanism to resolve it. We discuss the prospects of generalizing the suggested scheme and the steps required to extend it to more general and larger spin models.

In an electromagnetic code, a wave can be injected in the simulation domain by prescribing an oscillating field profile at the domain boundary. The process is straightforward when the field profile has a known analytical expression (typically, paraxial Gaussian beams). However, if the field profile is known at some other plane, but not at the boundary (typically, nonparaxial beams), some preprocessing is needed to calculate the field profile after propagation back to the boundary. We present a parallel numerical technique for this propagation between an arbitrary tilted plane and a given boundary of the simulation domain, implemented in the Maxwell-Vlasov particle-in-cell code Smilei.

Defining an entropy function out of equilibrium is an outstanding challenge. For stochastic lattice models in spatially uniform nonequilibrium steady states, definitions of temperature $T$ and chemical potential $\mu$ have been verified using coexistence with heat and particle reservoirs. For an appropriate choice of exchange rates, $T$ and $\mu$ satisfy the zeroth law, marking an important step in the development of steady-state thermodynamics. These results suggest that an associated steady-state entropy $S_{th}$ be constructed via thermodynamic integration, using relations such as ${(\partial S/\partial E)}_{V,N}=1/T$, ensuring that derivatives of $S_{th}$ with respect to energy and particle number yield the expected intensive parameters. We determine via direct calculation the stationary nonequilibrium probability distribution of the driven lattice gas with nearest-neighbor exclusion, the Katz-Lebowitz-Spohn driven lattice gas, and a two-temperature Ising model so that we may evaluate the Shannon entropy $S_S$ as well as $S_{th}$ defined above. Although the two entropies are identical in equilibrium (as expected), they differ out of equilibrium; for small values of the drive, $D$, we find $|S_S-S_{th}|\propto D^2$, as expected on the basis of symmetry. We verify that $S_{th}$ is not a state function: changes $\mathrm{\Delta }{S}_{th}$ depend not only on the initial and final points, but also on the path in parameter space. The inequivalence of $S_S$ and $S_{th}$ implies that derivatives of $S_S$ are not predictive of coexistence. In other words, a nonequilibrium steady state is not determined by maximizing the Shannon entropy. Our results cast doubt on the possibility of defining a state function that plays the role of a thermodynamic entropy for nonequilibrium steady states.

We study negative differential thermal resistance (NDTR) of one-dimensional hard-point gas models. The models are non-integrable unless all particles have the same mass. We show that NDTR can exist in both the integrable case and the non-integrable case. In the integrable case, the existence of NDTR is analytically predicted and numerically confirmed and a mechanism for NDTR different from that observed in lattice models is unveiled. In the non-integrable case, we show that the mechanism also works under certain conditions and the properties of NDTR are found to depend on the particle masses and the system size by the molecular dynamics simulations. These results shed new light on the mechanisms for NDTR and may help design new functional heat devices.

Circular KPZ interfaces spreading radially in the plane have Gaussian unitary ensemble (GUE) Tracy-Widom (TW) height distribution (HD) and ${\mathrm{Airy}}_2$ spatial covariance, but what are their statistics if they evolve on the surface of a different background space, such as a bowl, a mountain, or any surface of revolution? To give an answer to this, we report here extensive numerical analyses of several one-dimensional KPZ models on substrates whose size enlarges as $\langle L\left(t\right)\rangle =L_0+\omega t^{\gamma }$, while their mean height $\langle h\rangle$ increases as usual $[\langle h\rangle \sim t]$. We show that the competition between the $L$ enlargement and the correlation length ($\xi \simeq c{t}^{1/z}$) plays a key role in the asymptotic statistics of the interfaces. While systems with $\gamma >1/z$ have HDs given by GUE and the interface width increasing as $w\sim t^{\beta }$, for $\gamma <1/z$ the HDs are Gaussian, in a correlated regime where $w\sim t^{\alpha \gamma }$. For the special case $\gamma =1/z$, a continuous class of distributions exists, which interpolate between Gaussian (for small $\omega /c$) and GUE (for $\omega /c\gg 1$). Interestingly, the HD seems to agree with the Gaussian symplectic ensemble (GSE) TW distribution for $\omega /c\approx 10$. Despite the GUE HDs for $\gamma >1/z$, the spatial covariances present a strong dependence on the parameters $\omega$ and $\gamma$, agreeing with ${\mathrm{Airy}}_2$ only for $\omega \gg 1$, for a given $\gamma$, or when $\gamma =1$, for a fixed $\omega$. These results considerably generalize our knowledge on 1D KPZ systems, unveiling the importance of the background space on their statistics.

An adapted correlation dimension algorithm is used to numerically determine the number of integrals of motion in a variety of conservative classical systems. The method is demonstrated on three sample systems that display various degrees of integrable and chaotic behavior: the Hénon-Heiles Hamiltonian, an asymmetric top molecule in an electric field, and the planetary system HD128311. Two additional applications of the method emerge: using the adapted correlation dimension algorithm (a) to study partial barriers and turnstiles in phase space and (b) to predict the long-time stability of planetary systems using short-time data.

Large cascades are a common occurrence in many natural and engineered complex systems. In this paper we explore the propagation of cascades across networks using realistic network topologies, such as heterogeneous degree distributions, as well as intra- and interlayer degree correlations. We find that three properties, scale-free degree distribution, internal network assortativity, and cross-network hub-to-hub connections, are all necessary components to significantly reduce the size of large cascades in the Bak-Tang-Wiesenfeld sandpile model. We demonstrate that correlations present in the structure of the multilayer network influence the dynamical cascading process and can prevent failures from spreading across connected layers. These findings highlight the importance of internal and cross-network topology in optimizing robustness of interconnected systems.

Inspired by recent experiments on chromosomal dynamics, we introduce an exactly solvable model for the interaction between a flexible polymer and a set of motorlike enzymes. The enzymes can bind and unbind to specific sites of the polymer and produce a dipolar force on two neighboring monomers when bound. We study the resulting nonequilibrium dynamics of the polymer and find that the motion of the monomers has several properties that were observed experimentally for chromosomal loci: a subdiffusive mean-square displacement and the appearance of regions of correlated motion. We also determine the velocity autocorrelation of the monomers and find that the underlying stochastic process is not fractional Brownian motion. Finally, we show that the active forces swell the polymer by an amount that becomes constant for large polymers.

The propagation of mechanical energy in granular materials has been intensively studied in recent years given the wide range of fields that have processes related to this phenomena, from geology to impact mitigation and protection of buildings and structures. In this paper, we experimentally explore the effect of an interstitial fluid on the dynamics of the propagation of a mechanical pulse in a granular packing under controlled confinement pressure. The experimental results reveal the occurrence of an elastohydrodynamic mechanism at the scale of the contacts between wet particles. We describe our results in terms of an effective medium theory, including the presence of the viscous fluid. Finally, we study the nonlinear weakening of the granular packing as a function of the amplitude of the pulses. Our observations demonstrate that the softening of the material can be impeded by adjusting the viscosity of the interstitial fluid above a threshold at which the elastohydrodynamic interaction overcomes the elastic repulsion due to the confinement.

Large-eddy simulations of thermal convection are presented and discussed for a cube with rough horizontal surfaces. Two types of roughness are considered: uniformly placed pyramids, and grooves aligned parallel to one set of sidewalls. The Rayleigh number is ${10}^8$, the Prandtl number 0.7, and the aspect ratio 1, as in a previous study [N. Foroozani, J. J. Niemela, V. Armenio, and K. R. Sreenivasan, Phys. Rev. E 95, 033107 (2017)], except that the meshes here are finer. When the thermal boundary layers are sufficiently large relative to the characteristic roughness height, i.e., for hydrodynamically smooth conditions, the mean properties of the large scale circulation (LSC) are qualitatively similar to the case of smooth surfaces. In particular, the LSC is always aligned along one of the diagonals of the cube. When the boundaries are hydrodynamically rough, the same result holds true only for the case of pyramidal structures; for grooved surfaces, the LSC is forced to be parallel to the sidewalls on average, alternating rapidly between the two diagonals of the cube with a mean period of the order 10 turnover times. Our analysis suggests that the difference from the pyramidal case is due to the breaking of the horizontal $x-z$ symmetry under conditions of hydrodynamical roughness, and the corresponding directional concentration of plume emission along the grooves, from which the LSC is generated, providing a strong restoring force. Furthermore, in this study we observed a small reduction in heat transport for both roughness configurations which is in good agreement with past studies.