Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R08: NoiseDriven Dynamics in FarFromEquilibrium Systems IIFocus Live

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Sponsoring Units: GSNP DBIO Chair: Luis Bonilla, Univ Carlos III De Madrid 
Thursday, March 18, 2021 8:00AM  8:36AM Live 
R08.00001: Climate variability: a manifestation of fluctuations in a nonequilibrium steadystate Invited Speaker: Jeffrey Weiss The climate system is forced by incoming solar radiation, is damped by outgoing longwave radiation and is, apart from timedependent natural and anthropogenic forcing, approximately in a nonequilibrium steadystate. The natural variability about the timemean climate state takes the form of coherent, preferred, spatiotemporal patterns with names such as the ElNiño Southern Oscillation (ENSO), the MaddenJulien Oscillation (MJO), the Atlantic Multidecadal Oscillation, and the Pacific Decadal Oscillation. Climate oscillations have large human impacts and their response to anthropogenic climate change is difficult to predict. Nonequilibrium steadystates can be characterized by persistent phase space currents and we interpret climate oscillations as the physical space manifestation of those currents. We describe a diagnostic for phase space currents, the phase space angular momentum, closely related to the Lévy stochastic area. The phase space angular momentum describes the rotational flow of trajectories in phase space by analogy to the mass angular momentum of a fluid rotating in physical space. An advantage of the phase space angular momentum and stochastic area over other measures of nonequilibrium currents, such as entropy production, is that they can be readily calculated from an observed time series with no assumptions about an underlying model. We find that the phase space angular momentum in simple stochastic models of ENSO and the MJO agree surprisingly well with that seen in observations of the climate system, providing additional evidence for its utility in quantifying nonequilibrium steadystates. We propose that the phase space angular momentum and the Lévy stochastic area are useful diagnostics to intercompare climate models and to compare climate models with observations of the climate system. 
Thursday, March 18, 2021 8:36AM  8:48AM Live 
R08.00002: GreenKubo relations for nonequilibrium hydrodynamics transport coefficients HyunMyung Chun, Qi Gao, Jordan Horowitz For nearequilibrium macroscopic transport, the fluctuationdissipation theorem (FDT) has been used to link transport coefficients to correlation functions of local observables, a connection known as GreenKubo relations. Although there are several known equalities that generalize the FDT around nonequilibrium steady states, none have led to the prediction of analogous nonequilibrium GreenKubo relations. Here, we demonstrate that there is a class of perturbations whose response maintains the equilibriumform of the FDT, yet remains valid arbitrarily far from equilibrium. As a consequence of this novel FDT, we derive GreenKubo relations for nonequilibrium hydrodynamic transport coefficients without invoking Onsager's regression hypothesis. We illustrate the theoretical results with molecular dynamics simulations of interacting active Brownian particles. Our simulations show that the diffusion coefficient of active Brownian particles is determined by the fluctuations of the local particle density and its current. 
Thursday, March 18, 2021 8:48AM  9:00AM Live 
R08.00003: Parallel temperature interfaces in the KatzLebowitzSpohn driven lattice gas Ruslan Mukhamadiarov, Priyanka Priyanka, Uwe Tauber We explore a variant of the KatzLebowitzSpohn (KLS) driven lattice gas in two dimensions, where the lattice is split into two regions that are coupled to heat baths with distinct temperatures. The temperature boundaries are oriented parallel to the external particle drive. If the hopping rates at the interfaces satisfy particlehole symmetry, the current difference across them generates a vector flow diagram akin to a flat vortex sheet. We have studied the finitesize scaling of the particle density fluctuations in both temperature regions, and observed that it is controlled by the respective temperature values. If the colder subsystem is maintained at the KLS critical temperature, while the hotter subsystem's temperature is set much higher, the interface current greatly suppresses particle exchange between the two regions, whence strong fluctuations persist in the critical region and the particle density fluctuations scale with the KLS critical exponents. However, if both temperatures are set well above the critical temperature, the particle density fluctuations scale according to the totally asymmetric exclusion process (TASEP). 
Thursday, March 18, 2021 9:00AM  9:12AM Live 
R08.00004: Steadystate Entropy of Dynamical Systems Described by Intermittent Iterated Maps Yunxiang Song, Thomas Witten A stochastic perturbation applied to a set of randomlyphased, noninteracting oscillators is known to induce synchronization provided the noise is very weak [1]. We use iterated phase maps to describe the dynamics of nonlinear oscillators in response to intermittent impulsive forcing. That is, we repeatedly subject all the oscillators to a common phase map imposed at random times. This synchronization can be exploited to create orientational order in a dispersion of forced, rotating, asymmetric colloidal particles [2]. Here we study the case where the forcing creates strong but incomplete synchronization. The strength of the forcing can be characterized by the average Lyapunov exponent Λ of its phase map. We demonstrate a regime of small, positive Λ in which the distribution of oscillator phases and its corresponding entropy H fluctuates wildly. We characterize this novel state numerically for a class of phase maps. The distribution of entropies achieves an apparent steadystate consistent with the form e^{HΛ/C}, where C is a constant. For small Λ, H shows apparent 1/Λ behavior. 
Thursday, March 18, 2021 9:12AM  9:24AM Live 
R08.00005: Critical Dynamics of Anisotropic Antiferromagnets in an External Field Riya Nandi, Uwe Tauber We numerically investigate the nonequilibrium critical dynamics in threedimensional anisotropic antiferromagnets in the presence of an external magnetic field. The phase diagram of this system exhibits two critical lines that meet at a bicritical point. The nonconserved components of the staggered magnetization order parameter couple dynamically to the conserved component of the magnetization density along the direction of the external field. Employing a hybrid computational algorithm that combines reversible spin precession with relaxational Monte Carlo updates, we study the aging scaling dynamics for the model C critical line, identifying the critical initial slip, autocorrelation, and aging exponents for both the order parameter and conserved field, thus also verifying the dynamic critical exponent. We further probe the model F critical line by investigating the system size dependence of the characteristic spinwave frequencies near criticality and measure the dynamic critical exponents for the order parameter including its aging scaling at the bicritical point. 
Thursday, March 18, 2021 9:24AM  9:36AM Live 
R08.00006: Mechanism of noiseinduced wavenumber selection in the stabilized KuramotoSivashisky equation Saloni Saxena, John Michael Kosterlitz We revisit the question of wavenumber selection in patternforming systems by studying the one dimensional stabilized KuramotoSivashinsky equation with additive Gaussian noise. It was found in previous work that the presence of noise leads the system to prefer one of many possible periodic steady states, establishing the critical role of noise in the selection process. However, the detailed mechanism by which the noise picked out the selected wave number was not understood. Here, we look at the ensemble averaged growth of each unstable Fourier mode from the spatially homogeneous state, with and without noise. We find drastic differences between the two cases. In particular, we find that noise opposes the growth of perturbations with wave numbers in a small band around the critical wave number and boosts the growth of perturbations with wave numbers much smaller than the critical wave number. This process determines the selected wave number. We further propose a partial explanation for this effect, which is confirmed by numerical simulations. 
Thursday, March 18, 2021 9:36AM  9:48AM Live 
R08.00007: Recovering Classical Langevin Dynamics by Coupling the System to Quantum Noise Hong Yao, Uwe Tauber We consider a general Hamiltonian system and couple it to a large heat bath consisting of an infinite number of harmonic oscillators at fixed temperature. By linearly coupling the canonical momentum of the system to the heat bath and taking the classical limit as well as a properly defined Markov limit, we recover the purely relaxational Langevin dynamics of a nonconserved order parameter. Using the same formalism and additionally fixing the symmetry of the Hamiltonian system, one may also derive the ensuingLangevin dynamics of conserved modes, as originally constructed from phenomenological hydrodynamics arguments. The formalism can be extendedto systems with additional conserved modes to obtain reversible mode coupling terms in the classical limit. We exemplify this procedure forthe critical dynamics of isotropic ferro and antiferromagnets. 
Thursday, March 18, 2021 9:48AM  10:00AM Live 
R08.00008: Stochastic charge fluctuations analyzed by factorial cumulants Philipp Stegmann, Annika Kurzmann, Pia Lochner, Jens Kerski, Rüdiger Schott, Arne Ludwig, Andreas D. Wieck, Axel H Lorke, Martin Paul Geller, Jurgen Konig Transport of electrons through nanoscale systems is a stochastic process determined, for instance, by the probabilistic nature of tunneling or a randomly changing environment. Nowadays, the resulting charge fluctuations can be detected by sensitive electrometers in real time. However, it is quite challenging to analyze the information contained in the obtained statistical data. In my talk, I will introduce an evaluation scheme based on generalized factorial cumulants [1,2] which can reveal correlations between tunneling electrons [1,2,3], a violation of detailed balance [4], hidden states and relaxation rates [5], or coherent dynamics [6]. 
Thursday, March 18, 2021 10:00AM  10:12AM Live 
R08.00009: Modelling Spontaneous Thermal Fluctuations of Ripples in Suspended Graphene Antonio Lasanta Becerra, Luis Bonilla, Paul M Thibado, Pradeep Kumar, Surendra Pal Singh, Miguel Ruiz Garcia At room temperature, micronsize sheets of freestanding graphene are in constant motion, even in the presence of an applied 
Thursday, March 18, 2021 10:12AM  10:24AM Live 
R08.00010: Stochastic Line Integrals as Metrics of Irreversibility and Heat Transfer Stephen Teitsworth, John Neu Stochastic line integrals allow quantitative characterization of irreversibility and detailed balance violation in noisedriven dynamical systems. One example of such integrals, the stochastic area, was introduced for linear systems and tested experimentally in coupled linear electrical circuits [1,2]. Here we establish the general properties of stochastic line integrals and clarify their implementation for experiments and simulations as well as their utility for quantifying nonequilibrium behavior. Theoretical results are supported by numerical studies of an overdamped, twodimensional massspring system driven out of equilibrium. In this case, the stochastic area can be concisely expressed in terms of a streamfunction the sign of which determines the orientation of probability current loops. The streamfunction provides analytical insight to the dependence of stochastic area on parameters such as the noise strength for both nonlinear and linear springs; in particular, we find distinct scaling regimes for stochastic area versus noise amplitude depending on the character of nonlinearity. 
Thursday, March 18, 2021 10:24AM  10:36AM Live 
R08.00011: Response and flux of information in extended nonequilibrium dynamics Angelo Vulpiani, Marco Baldovin, Camilla Sarra It is well known that entropy production is a proxy to the detection of nonequilibrium, i.e. of the absence of detailed balance, in stochastic processes and physical models; however, due to the global character of this quantity, its knowledge does not allow to identify spatial currents or fluxes of information among specific elements of the system under study. In this respect, much more insight can be gained, for instance, by studying transfer entropy and response, which allow to quantify the relative influence of parts of the system and the asymmetry of the fluxes. 
Thursday, March 18, 2021 10:36AM  10:48AM Live 
R08.00012: A coupled twospecies model for the pair contact process with diffusion Shengfeng Deng, Wei Li, Uwe Tauber The contact process with diffusion (PCPD) defined by the binary reactions B+B→B+B+B, B+B→0 and diffusive particle spreading exhibits an unusual active to absorbing phase transition. Multiple studies indicated that further coarsegraining of the model may be needed for its effective longtime, largescale description. We introduce a twospecies representation for the PCPD in which single particles B and particle pairs A are dynamically coupled according to the stochastic reaction processes B+B→A, A→A+B, A→0, and A→B+B, with each particle type diffusing independently. Meanfield analysis reveals that the phase transition of this model is driven by competition and balance between the two species. We employ Monte Carlo simulations in one, two, and three dimensions to demonstrate that this model consistently captures the pertinent features of the PCPD and the upper critical dimension is dc=2. The extracted moment ratios from our simulations indicate that our model displays the same temporal crossover behavior as the PCPD, which further corroborates its full dynamical equivalence with our coupled model. 
Thursday, March 18, 2021 10:48AM  11:00AM Live 
R08.00013: Thermodynamic Signalling in Underdamped Networks John Neu

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