We study the effect of population variability on the collective dynamics of interacting cyclic active elements, modeled as oscillators whose frequencies are a sum of a deterministic part, identical for all elements, and a random part. The random part can vary both in amplitude and over time yielding quenched or annealed temporal disorder, as well as an intermediate regime where the noise varies over a timescale comparable to the period. In the absence of randomness, the elements synchronize in phase and spontaneous coherent motion emerges. However, if the population variability is large enough, then synchronization is lost. This transition to a nonsynchronized state is governed by properties of the tail of the population distribution that can be understood by an analysis of the properties of a random matrix. We find that collections of elements with annealed temporal disorder can tolerate, remaining functional (i.e., synchronized), higher levels of variability than the quenched cases. This places design constraints on functional synthetic biological systems in the presence of unavoidable disorder.

The critical behavior at the transition from a uniform nematic to a twist-bend modulated nematic phase is revealed and shown to be well explained by the mean field approximation. The study was performed on a group of materials that exhibit an unusually broad temperature range of the nematic phase above the twist-bend modulated nematic phase, so the critical range in which the order parameter fluctuations are strong and can be experimentally observed is wide. The formation of instantaneous helices is observed already several degrees above the transition temperature, strongly influencing the birefringence of the system. The analysis of a critical part of the birefringence changes provides a set of critical exponents that are consistent with the mean field approximation, indicating a large correlation length of helical fluctuations in the nematic phase.

We report a systematic study of the role of Marangoni convection in the evaporation kinetics of pure water drops, considering the influence of the heating regime and surface wettability. Marangoni flows were induced via heating under constant wall temperature (uniform heating) and constant heat flux (local heating) regimes below the drops. To visualize the thermal patterns emerging during the evaporation, we employed infrared thermography and we captured the evolution of the drop profile with a CCD camera to follow the evaporation kinetics of each drop. We observed a strong correlation between the temperature difference within the drop and the evolution of the drop shape during different modes of evaporation (i.e., constant radius, angle, or stick-slip) resulting in different Marangoni flow patterns. Under uniform heating, stable recirculatory vortices due to Marangoni convection emerged at high temperature, but they faded at later stages of the evaporation process. On the other hand, in the localized heating case, the constant heat flux resulted in a rapid increase in the temperature difference within the drop capable of sustaining Marangoni flows throughout the evaporation. Surface wettability was found also to play a role in both the emergence of the Marangoni flows and the evaporation kinetics. In particular, recirculatory flows in drops on hydrophobic surfaces were stronger when compared to flows on hydrophilic surfaces for both uniform and local heating. To quantify the effect of the heating mode and the importance of Marangoni flows, we calculated the evaporative flux for each case and found it to be much higher in the localized heating case. Evaporative flux depends on both diffusion and natural convection of the vapor phase to the ambient. Hence, we estimated the Grashof number for each case and found a strong relation between natural convection in the vapor phase and heating regime or Marangoni convection in the liquid phase. Subsequently, we demonstrate the limitation of the previously reported diffusion-only model in describing the evaporation of heated drops.

We perform simulations of obstructed diffusion in periodic lattice configurations of solid, nonoverlapping, impermeable superballs to study effective diffusivity as a function of shape and solid volume fraction for simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) lattices. In the simulations, point particles diffuse in the intermediate space between the solid superballs. For sc lattices, we find that for a constant solid volume fraction, the optimal effective diffusivity is obtained for a particular superball between a sphere and a cube, whereas for bcc and fcc lattices, no such optimum exists. Further, we investigate the prediction of the effective diffusivity in these systems using an approximate analytical expression based on two- and three-point correlation functions and the microstructural parameter ${\zeta }_2$.

We propose an alternative strategy to modulate the acoustic radiation force (ARF) acting on an elastic sphere (ES) using laser irradiation. A mathematical model of the interaction of the acoustic plane wave with the laser-irradiated ES is developed to calculate the ARF acting on the ES. It is demonstrated that the action of this laser-induced photoacoustic effect on a polystyrene (PS) sphere could strongly affect the ARFs of acoustic plane waves that are incident on the PS sphere and negative ARFs could thus be realized. The variational ARF behavior observed is ascribed to competition between the photoacoustic effect and the acoustic scattering. Furthermore, we find that both the laser intensity and the phase difference between the incident laser beam and the acoustic plane wave could be used to modulate the ARF acting on the irradiated PS sphere, with the phase difference playing the dominant role. One effective method for control of the negative ARF involves changing the laser intensity with a suitable phase difference. The work presented here may lead to an alternative way to perform single-beam acoustic manipulation.

A three-dimensional (3D) transition route for zero-pressure-gradient boundary layer over a flat plate is computationally investigated, for impulsive and nonimpulsive startup of harmonic wall excitation. A monochromatic frequency of excitation is chosen to perturb the boundary layer. The exciter placed near the leading edge of the flat plate is stapled in the spanwise direction, as in the classical experiment by Klebanoff, Tidstrom, and Sargent [J. Fluid Mech. 12, 1 (1962)]. The computational domain includes the leading edge and the 3D Navier-Stokes equation is solved first for the equilibrium flow without any excitation. This is followed by computing the disturbed flow with the exciter started in either an impulsive or in a nonimpulsive manner. Detailed description of the transition process is provided for the startup cases, which shows that the transition is essentially caused by the spatiotemporal wave front for the moderate frequency of excitation. The present investigation follows the formulation and methodology given by Bhaumik and Sengupta [Phys. Rev. E 89, 043018 (2014)], which specifically reported only the impulsive startup of harmonic wall Gaussian circular patch excitation for a moderate frequency that led to the aligned pattern of $\mathrm{\Lambda }$ vortices.

By using multidimensional particle-in-cell simulations, we demonstrate that the coherent interaction of two relativistic laser pulses in plasmas offers a degree of freedom for manipulating the electron acceleration process. It is shown that by adjusting the relative phase between two intense laser pulses, their interaction features in plasmas are well controlled, which subsequently induce different electron acceleration processes. In this case, the properties of the accelerated electron beam, including the directionality, the maximum energy, and the conversion efficiency, can be controlled though the manipulation of the energy redistribution of these intense laser pulses. In particular, compared to the cases with a single laser pulse or two out-of-phase laser pulses, two in-phase laser pulses fuse into a much stronger laser pulse, which significantly prolongs the electron acceleration length in plasmas and leads to the generation of energetic electron beam with much higher conversion efficiency.

The precision of reaction-diffusion models for mesoscopic physical systems is limited by fluctuations. To account for this uncertainty, Van Kampen derived a stochastic Langevin-like reaction-diffusion equation that incorporates spatiotemporal white noise. The resulting solutions, however, have infinite standard deviation. Ad hoc modifications that address this issue by introducing microscopic correlations are inconvenient in many physical contexts of wide interest. We instead estimate the magnitude of fluctuations by coarse-graining solutions of the Van Kampen equation at a relevant mesoscopic scale. The ensuing theory yields fluctuations of finite magnitude. Our approach is demonstrated for a specific biophysical model—the encoding of positional information. A numerical example is given for bicoid signaling in Drosophila. We discuss the properties of the fluctuations and the role played by the macroscopic parameters of the underlying reaction-diffusion model. The analysis and numerical methods developed here can be applied in physical problems to predict the magnitude of fluctuations. This general approach can also be extended to other classes of dynamical systems that are described by partial differential equations.

We consider the construction of a generalized Gibbs ensemble composed of complete bases of conserved charges in the repulsive Lieb-Liniger model. We will show that it is possible to construct these bases with varying locality as well as demonstrating that such constructions are always possible provided one has in hand at least one complete basis set of charges. This procedure enables the construction of bases of charges that possess well-defined, finite expectation values given an arbitrary initial state. We demonstrate the use of these charges in the context of two different quantum quenches: a quench where the strength of the interactions in a one-dimensional gas is switched suddenly from zero to some finite value and the release of a one-dimensional cold atomic gas from a confining parabolic trap. While we focus on the Lieb-Liniger model in this paper, the principle of the construction of these charges applies to all integrable models, both in continuum and lattice form.

We study Lévy flights confined in a parabolic potential. This has to do with a fractional generalization of an ordinary quantum-mechanical oscillator problem. To solve the spectral problem for the fractional quantum oscillator, we pass to the momentum space, where we apply the variational method. This permits one to obtain approximate analytical expressions for eigenvalues and eigenfunctions with very good accuracy. The latter fact has been checked by a numerical solution to the problem. We point to the realistic physical systems ranging from multiferroics and oxide heterostructures to quantum chaotic excitons, where obtained results can be used.

The helical conformation of electric dipoles in some chiral molecules, such as DNA and bacteriorhodopsin, induces a spin-orbit coupling that results in a sizable spin selectivity of electrons. The local deformation of the molecule about the moving electron may affect the spin dynamics due to the appearance of bright solitons with well-defined spin projection onto the molecule axis. In this work, we introduce an effective model for electron transport in a deformable helical molecular lattice that resembles the nonlinear Kronig-Penney model in the adiabatic approximation. In addition, the continuum limit of our model is achieved when the dipole-dipole distance is smaller than the spatial extent of the bright soliton, as discussed by E. Díaz et al. [N. J. Phys. 20, 043055 (2018)]. In this limit, our model reduces to an extended Davydov model. Finally, we also focus on perturbations to the bright soliton that arise naturally in the context of real helical molecules. We conclude that the continuum approximation provides excellent results in more complex scenarios.

Simplicial complexes are increasingly used to understand the topology of complex systems as different as brain networks and social interactions. It is therefore of special interest to extend the study of percolation to simplicial complexes. Here we propose a topological theory of percolation for discrete hyperbolic simplicial complexes. Specifically, we consider hyperbolic manifolds in dimension $d=2$ and $d=3$ formed by simplicial complexes, and we investigate their percolation properties in the presence of topological damage, i.e., when nodes, links, triangles or tetrahedra are randomly removed. We show that in $d=2$ simplicial complexes there are four topological percolation problems and in $d=3$ there are six. We demonstrate the presence of two percolation phase transitions characteristic of hyperbolic spaces for the different variants of topological percolation. While most of the known results on percolation in hyperbolic manifolds are in $d=2$, here we uncover the rich critical behavior of $d=3$ hyperbolic manifolds, and show that triangle percolation displays a Berezinskii-Kosterlitz-Thouless (BKT) transition. Finally, we provide evidence that topological percolation can display a critical behavior that is unexpected if only node and link percolation are considered.

In the present work, using the generalized Boltzmann equation of the first author [Phys. Rev. E 94, 023316 (2016)], a quantum kinetic model in the complex phase space is proposed. Employing the Chapman-Enskog analysis and applying the Wick rotation (using complex-valued relaxation time), it is shown that the present model recovers the time-dependent Schröedinger equation, while preserving the main features of the conventional lattice Boltzmann models, e.g., simplicity of implementation, second-order accuracy (in space), and convenience in parallel programming. The present results are numerically verified by simulating three benchmark problems.

Population annealing Monte Carlo is an efficient sequential algorithm for simulating $k$-local Boolean Hamiltonians. Because of its structure, the algorithm is inherently parallel and therefore well suited for large-scale simulations of computationally hard problems. Here we present various ways of optimizing population annealing Monte Carlo using 2-local spin-glass Hamiltonians as a case study. We demonstrate how the algorithm can be optimized from an implementation, algorithmic accelerator, as well as scalable parallelization points of view. This makes population annealing Monte Carlo perfectly suited to study other frustrated problems such as pyrochlore lattices, constraint-satisfaction problems, as well as higher-order Hamiltonians commonly found in, e.g., topological color codes.

The dynamic magnetic response of a ferrofluid to a weak ac magnetic field is studied using statistical mechanical theory and Brownian dynamics simulations, taking account of dipole-dipole interactions between the constituent ferromagnetic colloidal particles, and the presence of a range of particle sizes. The effects of interactions and polydispersity on the frequency dispersion are shown to be substantial: the amplitude of the response can be about twice that of a noninteracting system; the frequency for peak power loss can be reduced by about one half; and polydispersity effects can even change the qualitative appearance of the susceptibility spectrum.

We consider a minimalist model of overtaking dynamics in one dimension. On each site of a one-dimensional infinite lattice sits an agent carrying a random number specifying the agent's preferred velocity, which is drawn initially for each agent independently from a common distribution. The time evolution is Markovian, where a pair of agents at adjacent sites exchange their positions with a specified rate, while retaining their respective preferred velocities, only if the preferred velocity of the agent on the “left” site is higher. We discuss two different cases: one in which a pair of agents at sites $i$ and $i+1$ exchange their positions with rate 1, independent of their velocity difference, and another in which a pair exchange their positions with a rate equal to the modulus of the velocity difference. In both cases, we find that the net number of overtake events by a tagged agent in a given duration $t$, denoted by $m\left(t\right)$, increases linearly with time $t$, for large $t$. In the first case, for a randomly picked agent, $m/t$, in the limit $t\to \infty$, is distributed uniformly on $[-1,1]$, independent of the distributions of preferred velocities. In the second case, the distribution is given by the distribution of the preferred velocities itself, with a Galilean shift by the mean velocity. We also find the large time approach to the limiting forms and compare the results with numerical simulations.

This is a continuation of previous work [J. Stat. Phys., 172, 880 (2018)] that introduces the presumably simplest model of kinetic theory for phase transition. Here the main concern is to clarify the stability of uniform equilibrium states in the kinetic regime rather than that in the continuum limit. It is found by linear stability analysis that the linear neutral curve is invariant with respect to the Knudsen number, although the transition process is dependent on the Knudsen number. In addition, numerical computations of the (nonlinear) kinetic model are performed to investigate the transition processes in detail. Numerical results show that (unexpected) incomplete transitions may happen as well as clear phase transitions.

We explore the signatures of a topological phase transition (TPT) in the work and efficiency of a quantum heat engine, which uses a single-layer topological insulator, stanene, in an external electric field as a working substance. The magnitude of the electric field controls the trivial and topological insulator phases of the stanene. The effect of the TPT is investigated in two types of thermodynamic cycles, with and without adiabatic stages. We examine a quantum Otto cycle for the adiabatic case and an idealized Stirling cycle for the nonadiabatic case. In both cycles, investigations are done for high and low temperatures. It is found that the Otto cycle can distinguish the critical point of the TPT as an extremum point in the work output with respect to applied fields at all temperatures. The Stirling cycle can identify the critical point of the TPT as the maximum work point with respect to the applied fields only at relatively lower temperatures. As temperatures increase toward room temperature, the maximum work point of the Stirling cycle shifts away from the critical point of the TPT. In both cycles, increasing the temperature causes considerable enhancement in work and efficiency from the order of meV to eV.

The dynamical behavior resulting from an initial discontinuity in focusing media is investigated using a combination of numerical simulations and Whitham modulation theory for the focusing nonlinear Schrödinger equation. Initial conditions with a jump in either or both the amplitude and the local wave number are considered. It is shown analytically and numerically that the space-time plane divides into expanding domains in which the solution is described by a slow modulation of genus-0, genus-1 or genus-2 solutions, their precise arrangement depending on the specifics of the initial datum.

We consider random walks on dynamical networks where edges appear and disappear during finite time intervals. The process is grounded on three independent stochastic processes determining the walker's waiting time, the up time, and the down time of the edges. We first propose a comprehensive analytical and numerical treatment on directed acyclic graphs. Once cycles are allowed in the network, non-Markovian trajectories may emerge, remarkably even if the walker and the evolution of the network edges are governed by memoryless Poisson processes. We then introduce a general analytical framework to characterize such non-Markovian walks and validate our findings with numerical simulations.