Efficiency of the SQUID Ratchet Driven by External Current

We study theoretically the efficiency of an asymmetric superconducting quantum interference device (SQUID) which is constructed as a loop with three capacitively and resistively shunted Josephson junctions. Two junctions are placed in series in one arm and the remaining one is located in the other arm. The SQUID is threaded by an external magnetic flux and driven by an external current of both constant (dc) and time periodic (ac) components. This system acts as a nonequilibrium ratchet for the dc voltage across the SQUID with the external current as a source of energy. We analyze the power delivered by the external current and find that it strongly depends on thermal noise and the external magnetic flux. We explore a space of the system parameters to reveal a set for which the SQUID efficiency is globally maximal. We detect the intriguing feature of the thermal noise enhanced efficiency and show how the efficiency of the device can be tuned by tailoring the external magnetic flux.Comment: accepted for publication in New Journal of Physic

Subdiffusion via dynamical localization induced by thermal equilibrium fluctuations

We reveal the mechanism of subdiffusion which emerges in a straightforward, one dimensional classical nonequilibrium dynamics of a Brownian ratchet driven by both a time-periodic force and Gaussian white noise. In a tailored parameter set for which the deterministic counterpart is in a non-chaotic regime, subdiffusion is a long-living transient whose lifetime can be many, many orders of magnitude larger than characteristic time scales of the setup thus being amenable to experimental observations. As a reason for this subdiffusive behaviour in the coordinate space we identify thermal noise induced dynamical localization in the velocity (momentum) space. This novel idea is distinct from existing knowledge and has never been reported for any classical or quantum systems. It suggests reconsideration of generally accepted opinion that subdiffusion is due to road distributions or strong correlations which reflect disorder, trapping, viscoelasticity of the medium or geometrical constraints.Comment: in press in Scientific Reports (2017

Flux-biased mesoscopic rings

Kinetics of magnetic flux in a thin mesoscopic ring biased by a strong external magnetic field is described equivalently by dynamics of a Brownian particle in a tilted washboard potential. The 'flux velocity', i.e. the averaged time derivative of the total magnetic flux in the ring, is a candidate for a novel characteristics of mesoscopic rings. Its global properties reflect the possibility of accommodating persistent currents in the ring.Comment: 7 pages, 4 figures, Presented at the XXII International Conference of Theoretical Physics - Electron Correlations in Nano- and Macrosystems, 9 - 14 September 2006, Ustron, Poland; phys. stat. sol. (b) (in press) (2007

Josephson junction ratchet: effects of finite capacitances

We study transport in an asymmetric SQUID which is composed of a loop with three capacitively and resistively shunted Josephson junctions: two in series in one arm and the remaining one in the other arm. The loop is threaded by an external magnetic flux and the system is subjected to both a time-periodic and a constant current. We formulate the deterministic and, as well, the stochastic dynamics of the SQUID in terms of the Stewart-McCumber model and derive an equation for the phase difference across one arm, in which an effective periodic potential is of the ratchet type, i.e. its reflection symmetry is broken. In doing so, we extend and generalize earlier study by Zapata et al. [Phys. Rev. Lett. 77, 2292 (1996)] and analyze directed transport in wide parameter regimes: covering the over-damped to moderate damping regime up to its fully under-damped regime. As a result we detect the intriguing features of a negative (differential) conductance, repeated voltage reversals, noise induced voltage reversals and solely thermal noise-induced ratchet currents. We identify a set of parameters for which the ratchet effect is most pronounced and show how the direction of transport can be controlled by tailoring the external magnetic flux.Comment: accepted for publication in Phys. Rev.