String ratchets: ac driven asymmetric kinks.

We simulated numerically the time evolution of a one-kink bearing, damped elastic string sitting on noiseless periodic substrates of two types: (I) asymmetric, time independent, (II) symmetric, periodically deformable. An asymmetric kink subjected to an ac drive is shown to drift steadily with finite average speed independent of its initial kinetic conditions. In the overdamped regime the resulting net kink transport can be attributed to the rectification of the Brownian motion of a pointlike particle with oscillating mass. For intermediate to low damping completely different features show up, due to the finite size of the objects being transported; in particular, the kink current hits a maximum for an optimal value of the damping constant, resonates at the kink internal-mode frequency and, finally, reverses sign within a certain range of the drive parameters

Nonequilibrium steady state of the kinetic Glauber-Ising model under an alternating magnetic field

When periodically driven by an external magnetic field, a spin system can enter a phase of steady entrained oscillations with nonequilibrium probability distribution function. We consider an arbitrary magnetic field switching its direction with frequency comparable with the spin-flip rate and show that the resulting nonequilibrium probability distribution can be related to the system equilibrium distribution in the presence of a constant magnetic field of the same magnitude. We derive convenient approximate expressions for this exact relation and discuss their implications.Comment: 6 pages, 4 figure

Order out of noise

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Taxis of Artificial Swimmers in a Spatio-Temporally Modulated Activation Medium

Contrary to microbial taxis, where a tactic response to external stimuli is controlled by complex chemical pathways acting like sensor-actuator loops, taxis of artificial microswimmers is a purely stochastic effect associated with a non-uniform activation of the particles' self-propulsion. We study the tactic response of such swimmers in a spatio-temporally modulated activating medium by means of both numerical and analytical techniques. In the opposite limits of very fast and very slow rotational particle dynamics, we obtain analytic approximations that closely reproduce the numerical description. A swimmer drifts on average either parallel or anti-parallel to the propagation direction of the activating pulses, depending on their speed and width. The drift in line with the pulses is solely determined by the finite persistence length of the active Brownian motion performed by the swimmer, whereas the drift in the opposite direction results from the combination of ballistic and diffusive properties of the swimmer's dynamics.Comment: 19 pages, 6 figures; Entropy (in press

Rectification of spatial disorder

We demonstrate that a large ensemble of noiseless globally coupled-pinned oscillators is capable of rectifying spatial disorder with spontaneous current activated through a dynamical phase transition mechanism, either of first or second order, depending on the profile of the pinning potential. In the presence of an external weak drive, the same collective mechanism can result in an absolute negative mobility, which, though not immediately related to symmetry breaking, is most prominent at the phase transition

Low-frequency internal friction in clamped-free thin wires

We present a series of internal friction measurements for the normal modes of circular fibres made of different materials, that can suspend the test masses of an interferometric gravity wave detector. For metallic wires, the frequency independent loss angle ranges between 10 y3 and 10 y4 . The losses in fused silica are two orders of magnitude lower than those in metals

Anomalous cross-field diffusion in a magnetic trap

We numerically simulated the diffusion of a charged Brownian particle confined to a plane under the action of an orthogonal magnetic field with intensity depending on the distance from a center. Despite its apparent simplicity, this system exhibits anomalous diffusion. For positive field gradients, radial and angular dynamics are asymptotically subdiffusive, with exponents given by simple analytical expressions. In contrast, when driven by a weakly decaying field, the particle attains normal diffusion only after exceedingly long superdiffusive transients. These mechanisms can be related to Bohm diffusion in magnetized plasmas