1,721,034 research outputs found
Time asymmetry of the Kramers equation with nonlinear friction: Fluctuation-dissipation relation and ratchet effect
No full textWe show by numerical simulations that the presence of nonlinear velocity-dependent friction forces can induce a finite net drift in the stochastic motion of a particle in contact with an equilibrium thermal bath and in an asymmetric periodic spatial potential. In particular, we study the Kramers equation for a particle subjected to Coulomb friction, namely a constant force acting in the direction opposite to the particle's velocity. We characterize the nonequilibrium irreversible dynamics by studying the generalized fluctuation-dissipation relation for this ratchet model driven by Coulomb friction. © 2013 American Physical Society
Nonlinear Response of Inertial Tracers in Steady Laminar Flows: Differential and Absolute Negative Mobility
Full text linkWe study the mobility and the diffusion coefficient of an inertial tracer advected by a two-dimensional incompressible laminar flow, in the presence ofthermal noise and under the actionof an external force. We show, with extensive numerical simulations, that the force-velocity rela-tion for the tracer, in the nonlinear regime, displays complex and rich behaviors, including negativedifferential and absolute mobility. These effects rely upon asubtle coupling between inertia andapplied force which induce the tracer to persist in particular regions of phase space with a velocityopposite to the force. The relevance of this coupling is revisited in the framework of non-equilibriumresponse theory, applying a generalized Einstein relationto our system. The possibility of experi-mental observation of these results is also discussed
Ratchet effect driven by Coulomb friction: The asymmetric Rayleigh piston
No full textThe effect of Coulomb friction is studied in the framework of collisional ratchets. It turns out that the average drift of these devices can be expressed as the combination of a term related to the lack of equipartition between the probe and the surrounding bath, and a term featuring the average frictional force. We illustrate this general result in the asymmetric Rayleigh piston, showing how Coulomb friction can induce a ratchet effect in a Brownian particle in contact with an equilibrium bath. An explicit analytical expression for the average velocity of the piston is obtained in the rare collision limit. Numerical simulations support the analytical findings. © 2013 American Physical Society
Entropy production in non-equilibrium fluctuating hydrodynamics
No full textFluctuating entropy production is studied for a set of linearly coupled complex fields. The general result is applied to non-equilibrium fluctuating hydrodynamic equations for coarse-grained fields (density, temperature, and velocity), in the framework of model granular fluids. We find that the average entropy production, obtained from the microscopic stochastic description, can be expressed in terms of macroscopic quantities, in analogy with linear non-equilibrium thermodynamics. We consider the specific cases of driven granular fluids with two different kinds of thermostat and the homogeneous cooling regime. In all cases, the average entropy production turns out to be the product of a thermodynamic force and a current: the former depends on the specific energy injection mechanism, the latter takes always the form of a static correlation between fluctuations of density and temperature time-derivative. Both vanish in the elastic limit. The behavior of the entropy production is studied at different length scales and the qualitative differences arising for the different granular models are discussed
Role of inhibitory neurons in temporal correlations of critical and supercritical spontaneous activity
No full textExperimental and numerical results suggest that the brain can be viewed as a systemacting close to a critical point, as confirmed by scale–free distributions of relevantquantities in a variety of different systems and models. Less attention has received theinvestigation of the temporal correlation functions in brain activity in different, healthyand pathological, conditions. Here we perform this analysis by means of a model withshort and long-term plasticity which implements the novel feature of different recoveryrates for excitatory and inhibitory neurons, found experimentally. We evidence theimportant role played by inhibitory neurons in the supercritical state: We detect anunexpected oscillatory behaviour of the correlation decay, whose frequency depends onthe fraction of inhibitory neurons and their connectivity degree. This behaviour can berationalized by the observation that bursts in activity become more frequent and witha smaller amplitude as inhibition becomes more relevant
Identification of the critical temperature from non-equilibrium time-dependent quantities
No full textWe present a new procedure that can identify and measure the critical temperature. This method is based on the divergence of the relaxation time approaching the critical point in quenches from infinite temperature. We introduce a dimensionless quantity that turns out to be time independent at the critical temperature. The procedure does not need equilibration and allows for a relatively fast identification of the critical temperature. The method is first tested in the ferromagnetic Ising model and in the two-dimensional EA model and then applied to the one-dimensional Ising spin glass with power law interactions. Here we always find a finite critical temperature also in the presence of a uniform external field, in agreement with the mean-field picture for the low-temperature phase of spin glasses
About thermometers and temperature
No full textWe discuss a class of mechanical models of thermometers and their minimal requirements to determine the temperature for systems out of the common scope of thermometry. In particular we consider: (1) anharmonic chains with long time of thermalization, such as the Fermi-Pasta-Ulam (FPU) model; (2) systems with long-range interactions where the equivalence of ensembles does not always hold; (3) systems featuring absolute negative temperatures. We show that for all the three classes of systems a mechanical thermometer
model can be designed: a temporal average of a suitable mechanical observable of the thermometer is sufficient to get an estimate of the system’s temperature.
Several interesting lessons are learnt from our numerical study: (1) the long thermalization times in FPU-like systems do not affect the thermometer, which is not coupled to normal modes but to a group of microscopic degrees of freedom; (2) a thermometer coupled to a long-range system measures its microcanonical temperature, even at values of the total energy where its canonical temperature
would be very different; (3) a thermometer to read absolute negative temperatures must have a bounded total energy (as the system), otherwise it heavily perturbs the system changing the sign of its temperature. Our study shows that in order to also work in a correct way in ‘non standard’ cases, the proper model of thermometer must have a special functional form, e.g. the kinetic part cannot be quadratic
"La trappola delle disgiunzioni. Emozioni/conoscenza; cervello/corpo/mente"
No full textIl contributo indaga, dal punto di vista teorico e scientifico, la storia delle disgiunzioni che "parassitano" la nostra cultura separando-gerarchizzando le conoscenze: in particolare, si occupa delle disgiunzioni tra emozioni e processi cognitivi, tra cervello e corpo. Obiettivo del saggio è mostrare le plurali connessione tra questi ambiti; nonché proporre una particolare concezione di mente come essere-nel-mondo del soggetto. La mente in questa accezione diventa l'ultima “congiunzione” da operare, al fine di proporre un modello educativo teso, razionalmente, all’integrazione di tutti i piani dell’esperienza esistenziale dei soggetti e per connettere, secondo il paradigma della complessità, ciò che, per motivi diversi, è stato separato e contrapposto
On anomalous diffusion and the out of equilibrium response function in one-dimensional models
No full textWe study how the Einstein relation between spontaneous fluctuations and the response to an external perturbation holds in the absence of currents, for the comb model and the elastic single-file, which are examples of systems with subdiffusive transport properties. The relevance of non-equilibrium conditions is investigated: when a stationary current (in the form of a drift or an energy flux) is present, the Einstein relation breaks down, as is known to happen in systems with standard diffusion. In the case of the comb model, a general relation, which has appeared in the recent literature, between the response function and an unperturbed suitable correlation function, allows us to explain the observed results. This suggests that a relevant ingredient in breaking the Einstein formula, for stationary regimes, is not the anomalous diffusion but the presence of currents driving the system out of equilibriu
Dynamics of a massive intruder in a homogeneously driven granular fluid
No full textA massive intruder in a homogeneously driven granular fluid, in dilute configurations, performs a memory-less Brownian motion with drag and temperature simply related to the average density and temperature of the fluid. At volume fraction∼10–50% the intruder’s velocity correlates with the local fluid velocity field: such situation is approximately described by a system of coupled linear Langevin equations equivalent to a generalized Brownian motion with memory. Here one may verify the breakdown of the Fluctuation-Dissipation relation and the presence of a net entropy flux -from the fluid to the intruder- whose fluctuations satisfy the Fluctuation Relation
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