A single particle confined in an asymmetric potential demonstrates an anticipated ratchet effect by drifting along the 'easy' ratchet direction when subjected to non-equilibrium fluctuations. This well-known effect can, however, be dramatically changed if the potential captures several interacting particles. Here we demonstrate that the inter-particle interactions in a chain of repelling particles captured by a ratchet potential can, in a controllable way, lead to multiple drift reversals, with the drift sign alternating from positive to negative as the number of particles per ratchet period changes from odd to even. To demonstrate experimentally the validity of this very general prediction, we performed transport measurements on a.c.-driven vortices trapped in a superconductor by an array of nanometre-scale asymmetric traps. We found that the direction of the vortex drift does undergo multiple reversals as the vortex density is increased, in excellent agreement with the model predictions. This drastic change in the drift behaviour between single- and multi-particle systems can shed some light on the different behaviour of ratchets and biomembranes in two drift regimes: diluted (single particles) and concentrated (interacting particles).

We study the transport of vortices excited by an ac current in an Al film with an array of nanoengineered asymmetric antidots. The vortex response to the ac current is investigated by detailed measurements of the voltage output as a function of ac current amplitude, magnetic field and temperature. The measurements revealed pronounced voltage rectification effects which are mainly characterized by the two critical depinning forces of the asymmetric potential. The shape of the net dc voltage as a function of the excitation amplitude indicates that our vortex ratchet behaves in a way very different from standard overdamped models. Rather, as demonstrated by the observed output signal, the repinning force, necessary to stop vortex motion, is considerably smaller than the depinning force, resembling the behavior of the so-called inertia ratchets. Calculations based on an underdamped ratchet model provide a very good fit to the experimental data.PACS numbers: 05.40. 74.78.Na., 74.40.+k, From the point of view of classical thermodynamics, it is not possible to induce directed motion of particles by using equilibrium fluctuations only, otherwise it would constitute a perpetuum mobile of the second kind [1]. Nevertheless, non-equilibrium fluctuations, such as periodic excitations or a "colored" noise, are allowed to take advantage of the asymmetry of a periodic ratchet potential to promote motion of particles in a preferential direction [2]. New solid-state-based ratchet systems are currently being developed for controlling the motion of electrons [3] and fluxons, as well as for particle separation [4] and electrophoresis [5]. In particular, ratchet potentials in superconducting devices may be very useful to control the dissipative motion of fluxons, which causes undesired internal noise.Modern lithographic technics make it possible to fabricate periodic arrays of vortex pinning sites with size and shape that can be easily tuned, thus giving an interesting perspective for making different asymmetric pinning potentials. In this context, several ideas to control flux motion by applying an ac excitation have been proposed [6,7,8,9], but up to now only a few experiments have been realized [10,11]. One realization has been recently implemented on a Nb film with a square array of nanoscopic triangular magnetic dots [10]. The authors reported rectification of the ac driven vortices due to the asymmetric shape of the dots. Nevertheless, the detailed dynamics of vortices in such structures is not yet completely understood.In this Letter we investigate a composite square array of pinning sites, with its unit cell consisting of a small and a big square antidot separated by a narrow superconducting wall, as a vortex rectifier. As demonstrated by our dc and ac transport measurements at several fields and temperatures, this configuration is able to break the reflection symmetry of the total effective pinning potential and promote flux quanta rectification. Moreover, our data reveals a remarkable hysteresis in the currentinduced pinni...

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by magnetic force microscopy imaging, the magnetic state of both systems can be adjusted to produce arrays of almost pointlike magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role in the transport properties of this hybrid system. DOI: 10.1103/PhysRevLett.98.117005 PACS numbers: 74.78.Na, 05.40.ÿa, 74.25.Fy, 85.25.ÿj Nanoengineered arrays of vortex pinning sites have recently spawned numerous novel phenomena and potential applications based on vortex manipulation in superconductors. These structures are very suitable for tailoring the critical current and equilibrium properties [1-3] as well as for manipulating the distribution and direction of motion of flux quanta in superconducting devices [4 -7]. Such a control of vortex motion can be achieved when the periodic pinning potential lacks the inversion symmetry in a given direction, in which case any correlated fluctuating force induces a net vortex motion based on the phenomenon known as ratchet effect [8].As recently proposed by Carneiro [9], a different way to create vortex ratchets can be realized by using in-plane magnetized dots. Here the spatial inversion symmetry is broken not by the shape of the pinning sites but rather by the vortex-magnetic-dipole interaction. This dipoleinduced ratchet motion depends on the orientation and strength of the local magnetic moments thus allowing one to control the direction of the vortex drift. It is particularly this flexibility to manipulate the vortex motion which makes this kind of pinning potentials attractive for practical applications, although still a clear experimental corroboration is pending. In the present work, we demonstrate in a series of transport experiments that in-plane magnetized dipoles can indeed rectify vortex motion. Moreover, the rectified voltage induced by the ratchet motion depends strongly on temperature and field intensity and is nonzero even at zero field. Our analysis suggests that this behavior results from the interaction between the external field-induced vortices and vortex-antivortex pairs generated by magnetic dipoles.Our measurements were performed on two samples with different magnetic templates: (i) a 50 nm thick Al film with an on-top square array (period a p 3 m) of Si=Co=Au bars (with thicknesses 5 nm=47 nm=5 nm and lateral dimensions 2:6 0:5 m 2 ), labeled Bar-Al, and (ii) a Ge=Pb=Ge trilayer (20 nm=25 nm=5 nm thick) evaporated on top of a close-packed square array of equilateral triangular Co rings (250 nm wide, 23 nm thick, with lateral siz...