We consider overdamped Brownian particles in anisotropic, periodic structures (ratchets) that are rocked periodically. Together with the periodic forcing, white thermal noise can generate a non-zero, macroscopic velocity. By tuning the parameters, the direction of the current can be reversed. Additionally, the current as a function of the driving amplitude exhibits several local maxima at finite driving frequencies. For zero thermal noise, the deterministic current assumes an intriguing structure, reflecting the complex dynamics of particle excursions along the ratchet.

We argue that the phase across an asymmetric dc SQUID threaded by a magnetic flux can experience an effective ratchet (periodic and asymmetric) potential. Under an external ac current, a rocking ratchet mechanism operates whereby one sign of the time derivative of the phase is favored. We show that there exists a range of parameters in which a fixed sign (and, in a narrower range, even a fixed value) of the average voltage across the ring occurs, regardless of the sign of the external current dc component.PACS numbers: 05.40.+j, 74.40.+k, 74.50.+r, 85.25.Cp, 85.25.Dq Although the nonequibrium dynamics of a particle in a ratchet potential (i.e., a periodic potential that lacks reflection symmetry) has for long been considered a fundamental problem in statistical physics [1], it has become the object of more intense attention in recent years, because of its newly found relevance in diverse areas of physics, chemistry, and biology. A characteristic effect is that, when the ratchet is subject to a stationary nonequilibrium perturbation, particle motion in one direction is favored. Within this context, an important class of dynamical systems is formed by the so-called rocking ratchets, for which the external perturbation is a time periodic, uniform force [2][3][4]. The effect of dynamically induced unidirectional motion can overcome the drift effect of a small bias that would push the particle into the non favored direction. Thus, for not very strong tilts, uphill movement is possible provided the ratchet structure is conveniently rocked.In this letter, we propose a realization of the rocking ratchet mechanism in a new type of superconducting quantum interference device (SQUID) containing a characteristic asymmetry. The system we propose, depicted in Fig. 1, is formed by a ring with two Josephson junctions in series in one of the arms and only one junction in the other arm. We will show that, when the ring is threaded by a flux Φ ext that is not an integer multiple of Φ 0 /2 (Φ 0 ≡ h/2e being the flux quantum), the effective potential experienced by the total phase ϕ across the ring displays a ratchet structure. As a consequence, when the asymmetric SQUID is 'rocked' by an external ac current I(t), one sign of the phase velocityφ is favored. From the Josephson voltage-phase relation, we conclude that there must be a range of parameters for which a fixed sign of the average voltage V 0 ≡h φ /2e occurs regardless of the sign of the external current dc component I 0 .We focus on SQUID structures formed by conventional Josephson junctions whose phase is a classical variable and which can be adequately described by the 'resistively shunted junction' model [5,6]. Thus, the phase ϕ i across Josephson junction i on the left arm (see Fig. 1a) obeys the equation (i = 1, 2):where I l (t) is the current through the left arm, and R i , C i , and J i are the resistance, capacitance, and critical current of junction i. For simplicity, we assume here that the two junctions in series are identical, and will comment later on the cas...

No abstract

No abstract