Experimental ratchet effect in superconducting films with periodic arrays of asymmetric potentials

Abstract

A vortex lattice ratchet effect has been investigated in Nb films grown on arrays of nanometric Ni triangles, which induce periodic asymmetric pinning potentials. The vortex lattice motion yields a net dc voltage when an ac driving current is applied to the sample and the vortex lattice moves through the field of asymmetric potentials. This ratchet effect is studied taking into account the array geometry, the temperature, the number of vortices per unit cell of the array, and the applied ac currents. DOI: 10.1103/PhysRevB.71.024519 PACS number͑s͒: 74.78.Ϫw, 05.60.Ϫk, 74.40.ϩk Feynman used, in his Lectures on Physics, 1 a ratchet to show how anisotropy never could lead to net motion in an equilibrium system. Since then, asymmetric sawtooth potentials are called ratchet potentials and, in general, a device with broken inversion symmetry is called a ratchet device. The ratchet effect occurs when asymmetric potentials induce outward particle flow under external fluctuations in the lack of any driving direct outward forces. The ratchet effect changes an ac source in a dc one. Ratchet effect spans from Nature phenomena to laboratory fabricated devices. In a ratchet, the energy necessary for net motion is provided by raising and lowering the barriers and wells, either via an external time-dependent modulation, for example an ac current injected in a superconducting film with asymmetric pinning centers, 2 or by energy input from a nonequilibrium source, such as a chemical reaction, as for instance in biological motors. 3 During the past years, ratchet effect has called the attention of many researchers. A state of the art on the related topics Brownian motion and ratchet potential could be found in Ref. 4.The use of ratchetlike pinning potentials in superconductors has been the subject of theoretical approaches which deal with very different topics, for instance, to remove flux trapped in superconducting devices, 5 fluxon optic, 6 logic devices, 7 etc. From the experimental point of view, some progress has been reported related to superconducting circuits, [8][9][10] and very recently vortex motion ratchet effect has been reported in superconducting films with artificially fabricated arrays of asymmetric pinning centers. 2 In the present paper, we will address some of the properties of this superconducting ratchet effect. We will explore the dependence of the ratchet with the applied alternating current, the array shape, the temperature, and the number of vortices per array unit cell. We will show that periodic asymmetric potentials are crucial to produce the ratchet behavior, that the effect is enhanced decreasing the temperature, and finally, that the effect decreases when the applied magnetic field ͑number of vortices per unit cell of the array͒ increases. The paper is organized as follows: First, we will summarize some results on the behavior of vortex lattice on artificially induced pinning potentials. After this, we will present the fabrication method and main characteristics of the films. Finally, the experim...

Transverse rectification in superconducting thin films with arrays of asymmetric defects

“…The same behavior is found in both cases ͑ac and dc driving forces͒. 6 The experimental data shown in Figs. 2 and 3 are taken in the adiabatic limit.…”

“…2,5 Moreover, this vortex ratchet is an adiabatic ratchet; i.e., the effect shows a nonfrequency dependence that allows us to study and mimic the vortex ratchet effect by I͑V͒ curves. 6,7 The possible transverse rectification of particles has been theoretically studied by several authors. 8,9 The transverse ratchet effect for vortices moving in arrays of triangular pinning sites was specifically studied by Reichhardt-Olson and Reichhardt.…”

“…The suitable temperatures and driving forces for exploring ratchet and matching effects have been discussed elsewhere. 6,13 In summary, the temperature should be very close to the superconducting critical temperature and the driving forces ͑currents͒ should be between the two current values. Below a threshold current value the vortex lattice velocity is too low and the intrinsic and random potentials overcome the periodic pinning, therefore the periodic pinning effect vanishes; above a second threshold current value ͑high driving force͒ the vortex lattice velocity is enhanced and the vortex lattice does not undergo the periodic potentials.…”

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.…”

Experimental Vortex Ratchet Effect in Nanostructured Superconductors

“…Worth a while to note that the ratchet effect is not frequency dependent in the range attainable in our experiment up to 10 kHz. Villegas et al (3) have explored this effect, recently van Vondel et al (7) has found the same behavior in superconducting Al films with array of asymmetric antidotes (holes). Both found that the vortex ratchet effect is in the adiabatic limit, but in both cases the analysis is done for n=2, (2) and n=1 We are going to study the possible adiabatic behavior when both types of vortices develop, that is vortices pinned in the Ni triangles and interstitial vortices.…”