Statistical Mechanics
Autonomous engines driven by active matter: Energetics and design principles (1905.00373v1)
Patrick Pietzonka, Étienne Fodor, Christoph Lohrmann, Michael E. Cates, Udo Seifert
2019-05-01
Due to its non-equilibrium character, active matter in a steady state can drive engines that autonomously deliver work against a constant mechanical force or torque. As a generic model for such an engine, we consider systems that contain one or several active components and a single passive one that is asymmetric in its geometrical shape or its interactions. Generally, one expects that such an asymmetry leads to a persistent, directed current in the passive component, which can be used for the extraction of work. We validate this expectation for a minimal model consisting of an active and a passive particle on a one-dimensional lattice. It leads us to identify thermodynamically consistent measures for the efficiency of the conversion of isotropic activity to directed work. For systems with continuous degrees of freedom, work cannot be extracted using a one-dimensional geometry under quite general conditions. In contrast, we put forward two-dimensional shapes of a movable passive obstacle that are best suited for the extraction of work, which we compare with analytical results for an idealised work-extraction mechanism. For a setting with many non-interacting active particles, we use a mean-field approach to calculate the power and the efficiency, which we validate by simulations. Surprisingly, this approach reveals that the interaction with the passive obstacle can mediate cooperativity between otherwise non-interacting active particles, which enhances the extracted power per active particle significantly.
Emergence and spontaneous breaking of approximate O(4) symmetry at a weakly first-order deconfined phase transition (1805.03759v2)
Pablo Serna, Adam Nahum
2018-05-10
We investigate approximate emergent nonabelian symmetry in a class of weakly first order
deconfined' phase transitions using Monte Carlo simulations and a renormalization group analysis. We study a transition in a 3D classical loop model that is analogous to a deconfined 2+1D quantum phase transition in a magnet with reduced lattice symmetry. The transition is between the N\'eel phase and a twofold degenerate valence bond solid (lattice-symmetry-breaking) phase. The combined order parameter at the transition is effectively a four-component superspin. It has been argued that in some weakly first orderpseudocritical' deconfined phase transitions, the renormalization group flow can take the system very close to the ordered fixed point of the symmetricsigma model, where
is the total number of
soft' order parameter components, despite the fact that  is not a microscopic symmetry. This yields a first order transition with unconventional phenomenology. We argue that this occurs in the present model, with . This means that there is a regime of lengthscales in which the transition resembles aspin-flop' transition in the orderedsigma model. We give numerical evidence for (i) the first order nature of the transition, (ii) the emergence of
spontaneously broken', with distinctive features in the order parameter probability distribution. These results may be relevant for other models studied in the literature, including 2+1D QED with two flavours, theeasy-plane' deconfined critical point, and the N'eel--VBS transition on the rectangular lattice.
Density fluctuations and random walks in overdamped and supercooled simple liquid (1901.05265v2)
Eugene B. Postnikov
2019-01-16
In this work, the short-time dynamics of simple liquid mimicked by particles interacting via the Lennard-Jones potential, which corresponds to liquid argon, is explored both analytically and numerically with the focus on interplay of the density fluctuations in a volume surrounding a chosen particle and its random walk motion. For large times, analytical calculations based on the fluctuation theory provides an explicit expression reproducing isothermal change of the self-diffusion coefficient in liquid argon corresponding to the experimental data. These results lead to the conclusion that such behavior is based on the reduced mobility of particles reflected in their density fluctuations that can be equivalently achieved in the cases either low temperatures and pressures (supercooling) or moderate temperatures and high pressures (overdamping)
Many-body effects on the thermodynamics of closed quantum systems (1905.00318v1)
A. H. Skelt, K. Zawadzki, I. D'Amico
2019-05-01
Thermodynamics of quantum systems out-of-equilibrium is very important for the progress of quantum technologies, however, the effects of many body interactions and their interplay with temperature, different drives and dynamical regimes is still largely unknown. Here we present a systematic study of these interplays: we consider a variety of interaction (from non-interacting to strongly correlated) and dynamical (from sudden quench to quasi-adiabatic) regimes, and draw some general conclusions in relation to work extraction and entropy production. As treatment of many-body interacting systems is highly challenging, we introduce a simple approximation which includes, for the average quantum work, many-body interactions only via the initial state, while the dynamics is fully non-interacting. We demonstrate that this simple approximation is surprisingly good for estimating both the average quantum work and the related entropy variation, even when many-body correlations are significant.
Bridging of liquid drops at chemically structured walls (1905.00308v1)
Alexandr Malijevský, A. O. Parry, Martin Pospíšil
2019-05-01
Using mesoscopic interfacial models and microscopic density functional theory we study fluid adsorption at a dry wall decorated with three completely wet stripes of width
separated by distances
and
. The stripes interact with the fluid with long-range forces inducing a large finite-size contribution to the surface free-energy. We show that this non-extensive free-energy contribution scales with
and drives different types of bridging transition corresponding to the merging of liquid drops adsorbed at neighbouring wetting stripes when the separation between them is molecularly small. We determine the surface phase diagram and show that this exhibits two triple points, where isolated drops, double drops and triple drops coexist. For the symmetric case,
, our results also confirm that the equilbrium droplet configuration always has the symmetry of the substrate corresponding to either three isolated drops when
is large or a single triple drop when
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