The goal of this article is to identify and understand the fundamental role of spatial symmetries in the emergence of undulatory swimming using an anguilliform robot. Here, the local torque at the joints of the robot is controlled by a chain of oscillators forming a central pattern generator (CPG). By implementing a symmetric CPG with respect to the transverse plane, motor activation waves are inhibited, preventing the emergence of undulatory swimming and resulting in an oscillatory gait. We show experimentally that the swimmer can recover from the traveling wave inhibition by using distributed fluid force feedback to modulate the phase dynamics of each oscillator. This transition from oscillatory to undulating swimming is characterized by a symmetry breaking in the CPG and the body dynamics. By studying the stability of the oscillator chain, we show that the sensory feedback produces a frequency detuning gradient along the CPG chain while preserving its stability. To explain the origin of the instability, we introduce a toy model where the couplings between the dynamics of the oscillators and the body deformation reinforce the symmetry breaking.

Collisionless shocks are ubiquitous in space and astrophysical plasmas, and they are essential dynamical features of these systems. Lacking Coulomb collisions, these shocks are mediated by the anomalous dissipation provided by nonlinear plasma instabilities. By numerically resolving the structure of a steady-state, ion gyroviscous shock, we show that ion gyroviscosity, alone, can produce weak ($M\lesssim 1.1$, where $M$ is the sonic Mach number) shocks in a collisionless, magnetized plasma. We emphasize that this effect does not require an appeal to plasma microturbulence. Moreover, while most collisionless systems may be unsuitable to support purely gyroviscous shocks, we argue that gyro-viscous heating may be an overlooked mechanism, generally; and it may be a key driver within magnetohydrodynamic shocks at large. Representative examples include the plasma environments produced on the plasma liner experiment and the magnetized liner inertial fusion platforms.

We present two methods for computing the dynamic structure factor for warm dense hydrogen without invoking either the Born-Oppenheimer approximation or the Chihara decomposition, by employing a wave-packet description that resolves the electron dynamics during ion evolution. First, a semiclassical method is discussed, which is corrected based on known quantum constraints, and second, a direct computation of the density response function within the molecular dynamics. The wave-packet models are compared to PIMC and DFT-MD for the static and low-frequency behavior. For the high-frequency behavior the models recover the expected behavior in the limits of small and large momentum transfers and show the characteristic flattening of the plasmon dispersion for intermediate momentum transfers due to interactions, in agreement with commonly used models for x-ray Thomson scattering. By modeling the electrons and ions on an equal footing, both the ion and free electron part of the spectrum can now be treated within a single framework where we simultaneously resolve the ion-acoustic and plasmon mode, with a self-consistent description of collisions and screening.

Dynamics of non-Markovian systems is a classic problem yet it attracts everlasting activity in physics and beyond. A powerful tool for modeling such setups is the generalized Langevin equation, however, its analysis typically poses a major challenge even for numerical means. For this reason, various approximations have been proposed over the years that simplify the original model. In this paper, we compare two methods allowing us to tackle this great challenge: (i) the well-known and successful Markovian embedding technique and (ii) the recently developed effective mass approach. We discuss their scope of applicability, numerical accuracy, and computational efficiency. In doing so, we consider a paradigmatic model of a free Brownian particle subjected to power-law correlated thermal noise. We show that when the memory time is short, the effective mass approach offers satisfying precision and typically is much faster than the Markovian embedding. Moreover, the concept of effective mass can be used to find optimal parameters allowing us to reach supreme accuracy and minimal computational cost within the embedding. Our paper therefore provides a blueprint for investigating the dynamics of non-Markovian systems.

A new scaling regime characterized by a $z=1$ dynamical critical exponent has been reported in several numerical simulations of the one-dimensional Kardar-Parisi-Zhang and noisy Burgers equations. In these works, this scaling, differing from the well-known KPZ one $z=3/2$, was found to emerge in the tensionless limit for the interface and in the inviscid limit for the fluid. Based on functional renormalization group, the origin of this scaling has been elucidated. It was shown to be controlled by a yet unpredicted fixed point of the one-dimensional Burgers-KPZ equation, termed inviscid Burgers (IB) fixed point. The associated universal properties, including the scaling function, were calculated. All these findings were restricted to $d=1$, and it raises the intriguing question of the fate of this new IB fixed point in higher dimensions. In this work, we address this issue and analyze the multidimensional Burgers-KPZ equation using functional renormalization group. We show that the IB fixed point exists in all dimensions $d\ge 0$, and that it controls the large momentum behavior of the correlation functions in the inviscid limit. It turns out that it yields in all $d$ the same super-universal value $z=1$ for the dynamical exponent.

On applying a small bias force, nonequilibrium systems may respond in paradoxical ways such as with giant negative mobility (GNM)—a large net drift opposite to the applied bias, or giant positive mobility (GPM)—an anomalously large drift in the same direction as the applied bias. Such behaviors have been extensively studied in idealized models of externally driven passive inertial particles. Here, we consider a minimal model of a memory-driven active particle inspired from experiments with walking and superwalking droplets, whose equation of motion maps to the celebrated Lorenz system. By adding a small bias force to this Lorenz model for the active particle, we uncover a dynamical mechanism for simultaneous emergence of GNM and GPM in the parameter space. Within the chaotic sea of the parameter space, a symmetric pair of coexisting asymmetric limit cycles separate and migrate under applied bias force, resulting in anomalous transport behaviors that are sensitive to the active particle's memory. Our work highlights a general dynamical mechanism for the emergence of anomalous transport behaviors for active particles described by low-dimensional nonlinear models.

It has long been known that, fundamentally different from a large body of rarefied gas, when a Knudsen gas is immersed in a thermal bath, it may never reach thermal equilibrium. The root cause is nonchaoticity: as the particle-particle collisions are sparse, the particle trajectories tend to be independent of each other. Usually, this counterintuitive phenomenon is studied through kinetic theory and is not considered a thermodynamic problem. In current research, we show that if incorporated in a compound setup, such an intrinsically nonequilibrium behavior has nontrivial consequences and cannot circumvent thermodynamics: cold-to-hot heat transfer may happen spontaneously, either continuously (with an energy barrier), or cyclically (with time-dependent entropy barriers). It allows for production of useful work by absorbing heat from a single thermal reservoir without any other effect. As the system obeys the first law of thermodynamics, it breaks the boundaries of the second law of thermodynamics.

We study the fluctuational behavior of overdamped elastic filaments (e.g., strings or rods) driven by active matter which induces irreversibility. The statistics of discrete normal modes are translated into the continuum of the position representation which allows discernment of the spatial structure of dissipation and fluctuational work done by the active forces. The mapping of force statistics onto filament statistics leads to a generalized fluctuation-dissipation relation which predicts the components of the stochastic area tensor and its spatial proxy, the irreversibility field. We illustrate the general theory with explicit results for a tensioned string between two fixed endpoints. Plots of the stochastic area tensor components in the discrete plane of mode pairs reveal how the active forces induce spatial correlations of displacement along the filament. The irreversibility field provides additional quantitative insight into the relative spatial distributions of fluctuational work and dissipative response.

Thermal resonance, that is, the heat flux obtained by means of a periodic external driving, offers the possibility of controlling heat flux in nanoscale devices suitable for power generation, cooling, and thermoelectrics, among others. In this work we study the effect of the onsite potential period on the thermal resonance phenomenon present in a one-dimensional system composed of two dissimilar Frenkel-Kontorova lattices connected by a time-modulated coupling and in contact with two heat reservoirs operating at different temperature by means of molecular dynamics simulations. When the periods of the onsite potential on both sides of the system are equal, the maximum resonance is obtained for the lowest considered value of the period. For highly structurally asymmetric lattices, the heat flux toward the cold reservoir is maximized, and asymmetric periods of the onsite potential afford an extra way to control the magnitude of the heat fluxes in each side of the system. Our results highlight the importance of the substrate structure on thermal resonance and could inspire further developments in designing thermal devices.

The ultimate goal of this paper is to develop a systematic method for deriving quantum master equations that satisfy the requirements of a completely positive and trace-preserving (CPTP) map, further describing thermal relaxation processes. In this paper, we assume that the quantum master equation is obtained through the canonical quantization of the generalized Brownian motion proposed in our recent paper [T. Koide and F. Nicacio, Phys. Lett. A 494, 129277 (2024)]. At least classically, this dynamics describes the thermal relaxation process regardless of the choice of the system Hamiltonian. The remaining task is to identify the parameters ensuring that the quantum master equation meets complete positivity. We limit our discussion to many-body quadratic Hamiltonians and establish a CPTP criterion for our quantum master equation. This criterion is useful for applying our quantum master equation to models with interaction such as a network model, which has been used to investigate how quantum effects modify heat conduction.

We investigate synchronization behaviors of a Kuramoto oscillator network with a two-dimensional square-lattice configuration. We show that the oscillator network can reach a phase-locking vortex synchronized state in the long time limit starting from random initial oscillator phases sampled according to the von Mises distribution characterized by a zero mean and a finite concentration parameter. We further reveal that the stability of the vortex synchronized state is sensitive to the presence of local node defects, in contrast to the usual knowledge that oscillator networks should exhibit robustness against local perturbations. Moreover, we explore the behaviors of the vortex synchronized state in networks with an additional temporal white noise on the oscillator phases or a spatial noise due to randomly distributed oscillator frequencies. Interestingly, we find that the vortex synchronized state can become immune to local node defects when the variance of spatial noise is above a certain threshold, suggesting a beneficial role of usually unwanted spatial noise in protecting vortex-synchronized networks.

The need for effective multibody interactions is asserted in understanding the entropically driven phase separation of diblock copolymers, arising from disparity in self cohesion and association between dissimilar components. Through Landau analysis combined with a molecular equation of state to describe associability, it is demonstrated that diblock copolymers can exhibit dual critical points. We highlight the significance of multibody effects in correctly locating these critical points. Additionally, a region of first-order transition beyond the ${\varphi }^4$ Landau framework is identified, which is further examined using a companion self-consistent field theory.

It has recently been shown that large collections of self-propelled entities (a.k.a. “flocks” or, more technically, polar-ordered active fluids), all spontaneously moving in the same direction, can undergo phase separation in the presence of sufficiently strong attractive interactions (which can be caused by, e.g., autochemotaxis). At this transition, the system spontaneously separates into a high-density band and a low-density band, moving parallel to each other, and to the direction of mean flock motion, at different speeds. We show here that phase separation in ordered polar active fluids belongs to a new universality class, completely different from that of phase separation in equilibrium fluids. The upper critical dimension for this transition is $d_c=5$, in contrast to the well-known $d_c=4$ of equilibrium phase separation. With a dynamical renormalization group analysis, we obtain the large-distance, long-time scaling laws of the velocity and density fluctuations, which are characterized by universal critical correlation length and order parameter exponents ${\nu }_{\perp }, {\nu }_{\parallel }$, and $\beta$, respectively. We calculate these to $O\left(\epsilon \right)$ in a $d=5-\epsilon$ expansion.

Composite materials made of polymers and liquid crystals have been widely employed in smart windows, optical filters, and bistable displays. However, it is often difficult to decipher the role of the polymer network architecture on the alignment and the texture of liquid crystals. In this study, we use a simple model system where a small amount of polymerizable liquid crystalline monomer is mixed in a liquid crystal that exhibits both a smectic phase and a cholesteric phase with a large helical pitch. By cross-linking the monomer in the smectic phase, the liquid crystal textures become significantly altered. The existence of a polymer network not only creates resistance to form a helix and therefore hinders the formation of cholesteric fingerprints but also modifies the structure of the cholesteric texture. We investigate the effects of changing the concentration of chiral dopant and monomer on the formation and growth of the cholesteric fingerprint texture during the phase transition and on the width of the fingerprints. We find that, within a certain parameter range, the cholesteric fingerprints do not depend on the concentration of the chiral dopant. We explain this phenomenon by hypothesizing that the polymer network acts as a bulk alignment field.

Characterizing current fluctuations in a steady state is of fundamental interest and has attracted considerable attention in the recent past. However, the bulk of the studies are limited to systems that either do not exhibit a phase transition or are far from criticality. Here we consider a symmetric zero-range process on a ring that is known to show a phase transition in the steady state. We analytically calculate two density-dependent transport coefficients, namely, the bulk-diffusion coefficient and the particle mobility, that characterize the first two cumulants of the time-integrated current. We show that on the hydrodynamic scale, away from the critical point, the variance of the time-integrated current in the steady state grows with time $t$ as $\sqrt{t}$ and $t$ at short and long times, respectively. Moreover, we find an expression of the full scaling function for the variance of the time-integrated current and thereby the amplitude of the temporal growth of the current fluctuations. At the critical point, using a scaling theory, we find that, while the above-mentioned long-time scaling of the variance of the cumulative current continues to hold, the short-time behavior is anomalous in that the growth exponent is larger than one-half and varies continuously with the model parameters.

Droplet rebound on nonwetting surfaces is a common phenomenon. However, the underlying physics regulating the contact time remains unclear. In this work, we investigate droplet impacts on superamphiphobic surfaces through experiments and theoretical analyses. By analyzing the spreading and retraction of droplet impinging processes over a wide range of Weber numbers ($We$), it is revealed that droplet impacts experience three regimes as $We$ is varied, which are denoted as the Hertz collision ($We<1$), transition ($1<We<10$), and oscillation ($We>10$) regimes. In the Hertz collision regime, the droplet impinging process is temporally symmetric, i.e., the spreading time, $t_S$, and the retraction time, $t_R$, are almost the same. Furthermore, $t_S$ and $t_R$ decrease with increasing $We$ and follow a power-law dependence, which is different from previous theories. In the transition regime, $t_S$ remains dominated by the Hertz collision, while $t_R$ is governed by droplet oscillation. In the oscillation regime, $t_S$, $t_R$, and the total contact time, $t_C$, become independent of $We$. These three regimes are valid for both monophase and compound droplets. The findings in this work advance the understanding and offer a clear picture of droplet impact dynamics.

The Griffiths phase in systems with quenched disorder occurs below the ordering transition of the pure system down to the ordering transition of the actual disordered system. While it does not exhibit long-range order, large fluctuations in the disorder degrees of freedom result in exponentially rare, long-range ordered states and hence the occurrence of broad distributions in response functions. Inside the Griffiths phase of the two-dimensional bond-diluted Ising model the distribution of the magnetic susceptibility is expected to have such a broad, exponential tail. A large-deviation Monte Carlo algorithm is used to sample this distribution and the exponential tail is extracted over a wide range of the support down to very small probabilities of the order of ${10}^{-300}$. We study the behavior of the susceptibility distribution across the full phase diagram, from the paramagnetic state through the Griffiths phase to the ferromagnetically ordered system and down to the zero-temperature point. We extract the rate function of large-deviation theory as well as its finite-size scaling behavior and we reveal interesting differences and similarities between the cases. A connection between the fraction of ferromagnetic bonds in a given disorder sample and the size of the magnetic susceptibility is demonstrated numerically.

We investigate the pattern dynamics of the one-dimensional nonreciprocal Swift-Hohenberg model. Characteristic spatiotemporal patterns such as disordered, aligned, swap, chiral-swap, and chiral phases emerge depending on the parameters. We classify the characteristic spatiotemporal patterns obtained in numerical simulation by focusing on the spatiotemporal Fourier spectrum of the order parameters. We derive a reduced dynamical system by using the spatial Fourier series expansion. We analyze the bifurcation structure around the fixed points corresponding to the aligned and chiral phases, and explain the transitions between them. The disordered phase is destabilized either to the aligned phase by the Turing bifurcation or to the chiral phase by the wave bifurcation, while the aligned phase and the chiral phase are connected by the pitchfork bifurcation.

We demonstrate that in situ coherent diffractive imaging (CDI), which leverages the coherent interference between strong and weak beams to illuminate static and dynamic structures, can serve as a highly dose-efficient imaging method. At low doses, in situ CDI can achieve higher resolution than perfect lenses with the point spread function as a delta function. Both our numerical simulations and experimental results demonstrate that combining in situ CDI with ptychography can reduce the required dose by up to two orders of magnitude compared with ptychography alone. We anticipate that computational microscopy based on in situ CDI can be applied across various imaging modalities using photons and electrons for low-dose imaging of radiation-sensitive materials and biological samples.

Active curling of epithelial tissues, as an indispensable component of developmental morphogenesis, occurs frequently both in vivo and in vitro microenvironments. Deciphering the mechanisms underlying the active curling of epithelial monolayers is crucial for understanding many physiological and pathological processes. Here, a multiscale structure-based cell monolayer model and an active constitutive relation are established to characterize this spontaneous curling of the epithelial tissue. It is shown that the asymmetric distribution of Myosin II along the apicobasal axis generates an active moment that drives spontaneous curling of the suspended epithelial tissue. The time-dependent deflection and rotation angle of the active curling are analytically solved, proving that the curling is driven by the active bending moment directly associated with the apicobasal asymmetric contractile force. Moreover, we demonstrate that the rotation angle is proportional to the apicobasal force ratio and inversely proportional to the thickness of epithelial tissues. In addition, we derive an approximate analytical relation between the out-of-plane curling behavior and in-plane stress, in good agreement with the experimental data and our simulation results. This study provides a pathway to elucidate the mechanical mechanism underlying complex morphological development as well as the physiological and pathological phenomena of epithelial tissues.