Vortex-Rectification Effects in Films with Periodic Asymmetric Pinning

Abstract

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...

Tunable ratchet effects for vortices pinned by periodic magnetic dipole arrays

“…Applications to removal of trapped flux from superconductors, and to voltage rectification have been proposed [2,3,4,5,6,7]. Experimental realizations of the ratchet effect for vortices have been reported by some groups [8,9,10]. The work so far has concentrated on vortex pinning potentials resulting from periodic modulations of the film thickness or from periodic arrays of anti-dots or blind holes with asymmetric shape.…”

Magnetic field tunable vortex diode made of YBa2Cu3O7−δ Josephson junction asymmetrical arrays

“…7-12) and realized. [13][14][15][16][17][18][19] It is important to stress that a significant feature of the fluxtronic devices is their great tunability as compared to their electronic counterparts, for instance, the vortex diode fabricated and tested in Ref. 13 allows the direction of rectification to be reversed simply by tuning the applied B.…”

Reversible transport of interacting Brownian ratchets

“…In this case, the net current is given by J(F ) = J + (F ) + J − (F ), with J +(−) being the mean particle current for the force applied to the right (left). Such dc measurements of the ratchet effect is equivalent to an ac measurement where the oscillating force is an adiabatic square wave [10]. The advantage of such a strategy is that the dynamical states can be characterized for a constant force during an arbitrary long time interval, thus allowing one to compute statistical quantities for that specific force value.…”

“…These systems are able to promote the directed transport of particles on a periodic substrate lacking reflection symmetry by means of symmetrical, unbiased, non-equilibrium fluctuations. Their applicability can range from understanding the molecular transport of proteins [6,7] to the controlled transport of flux quanta in superconductors [8,9,10,11,12] and separation of mixtures in nanometric devices [13]. Albeit equilibrium (thermal) fluctuations alone cannot induce such a motion, it plays a key role within transport efficiency when the system is driven out of equilibrium.…”