Vortex Ratchets Research Articles

Ratchet Effect in Triangle-Shaped Vertical Nb/TiO/Nb Josephson Junctions

We fabricate vertical Nb/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junctions with the shape of right-angled isosceles triangles of different sizes. The TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer is primarily metallic, indicating that our junctions belong to the superconductor-normal metal-superconductor (S-N-S) junction. When applying bipolar dc currents across the junctions in parallel magnetic fields, there are many obvious differences between the positive and negative parts of the current-voltage characteristics. We attributed this asymmetry behavior (or ratchet behavior) mainly to the asymmetric pinnings for Josephson vortex penetration at the asymmetric edges (the right-angled corner and the hypotenuse) of the triangled junction, in addition to the effect of current-induced magnetic self-fields. For comparison with superconducting thin-film ratchets, the rectified dc voltages of our junctions with applied ac currents are investigated at different temperatures, magnetic fields, and driving frequencies. The waveforms of the time-resolved voltage with sinusoid currents are also measured. The ratchet effect in the junctions decreases monotonically with increasing temperature, increases to the peak first, and then decreases with the increasing magnetic field. It remains almost unchanged in the driving frequency range of 0.1 to 100 kHz. Compared with Abrikosov vortex ratchets, the ratchet effect in the junctions is much more sensitive to temperatures far from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , demonstrating the larger temperature-regulating window. This article shows the distinct influence of the asymmetric junction boundaries on the characteristics of the junctions under certain conditions, which can be useful for further understanding and inspiring new applications of Josephson junctions.

Vortex ratchets based on asymmetric arrays of Josephson junctions

Magnetic vortex-ratchet effects in various designs of asymmetric arrays of Josephson junctions (JJs) ratchets have been simulated numerically and measured experimentally. Asymmetry in the designs is achieved by spatial asymmetric variations of either the critical current (j c) of the JJ or the area (A) of the superconducting loops in between the JJ or both. It was found that larger arrays result in larger ratchet efficiency and are less susceptible to noise fluctuations. The simulation results were compared with measurements of various designs of JJ arrays ratchets made of YBa2Cu3O7−δ thin film superconductor and having either asymmetric or symmetric loop areas. We demonstrate that the ratchet efficiency can be very efficiently tuned by an applied magnetic field. Such vortex ratchets could be used as vortex diodes to pump out Josephson vortices from sensitive superconducting electronic or as sensitive magnetic sensors.

Switchable reversal of vortex ratchet with dynamic pinning landscape

Vortex rectifications are well known as drifting vortices along an “easy” direction when asymmetric potential is introduced to break the inversion symmetry for the vortex motion. Using the time-dependent Ginzburg–Landau formalism, we show an approach to switch the reversal of vortex ratchets by a dynamic pinning landscape, which is highly tuned by varying the sliding velocity of the dynamic pinning potential and its characteristic sizes and densities. Besides the anticipated positive rectified voltage with an easy vortex motion along the sliding direction of dynamic pinning sites, contrary to intuition, we also observe a negative dc voltage with vortices moving preferentially uphill, i.e., against the sliding direction. The mechanism of such reversal of vortex ratchets is different from previous work, which is revealed based on the dragging effect on the vortex motion by the sliding pinning sites.

Tuning the structure of the Josephson vortex lattice in Bi2Sr2CaCu2O8+\u03b4 single crystals with pancake vortices

In extremely anisotropic cuprate superconductors a lattice of stacks of pancake vortices nucleates when a magnetic field is applied perpendicular to the copper oxide layers, while an orthogonal lattice of highly elliptical Josephson vortices forms when the applied field is parallel to the layers. Under tilted magnetic fields these sublattices can interact in complex ways to form systems of vortex chains and composite vortex lattices. Here we have used high-resolution scanning Hall microscopy (SHM) to map the rich tilted-field vortex phase diagram in an underdoped Bi2Sr2CaCu2O8+δ single crystal. We find that the Josephson vortex lattice spacing has an unexpected non-monotonic dependence on the pancake vortex density reflecting the delicate balance between attractive and repulsive vortex interactions, and actually undergoes a field-driven structural transformation with increasing out-of-plane fields. We also identify particularly stable composite structures composed of vortex chains separated by an integer number of rows of interstitial pancake vortex stacks and are able to establish the precise evolution of vortex-chain phases as the out-of-plane field is increased at small in-plane fields. Our results are in good semi-quantitative agreement with theoretical models and could enable the development of vortex ratchets and lenses based on the interactions between Josephson and pancake vortices.

Nanostructured Superconductors With Asymmetric Pinning Potentials: Vortex Ratchets

Ratchets formed from spatially asymmetric confining potentials can rectify an oscillatory driving force and generate directed motion. Such devices can probe the fundamental nature of particle transport in nanoscale systems, both solid state and biological. Vortices in superconductors form an ideal system for exploring ratchet phenomena. Various techniques are available for producing nanostructured pinning landscapes that can provide tailored asymmetries for vortex ratchets. Progress in the theory and experimental implementations of vortex ratchets will be reviewed. In many cases, intervortex interactions in ratchet structures result in intriguing collective effects in the vortex transport, such as reversals in the sense of the rectification. Future vortex ratchet investigations may probe possible quantum mechanical ratchet effects, explore the dynamics of single vortices in ratchets, and test vortex devices based on ratchet phenomena.

Mapping the dynamic interactions between vortex species in highly anisotropicsuperconductors

Here we use highly sensitive magnetization measurements performed using a Hall probesensor on single crystals of highly anisotropic high temperature superconductorsBi2Sr2CaCu2O8 to study the dynamic interactions between the two species of vortices that exist in suchsuperconductors. We observe a remarkable and clearly delineated high temperature regimethat mirrors the underlying vortex phase diagram. Our results map out the parameterspace over which these dynamic interaction processes can be used to create vortex ratchets,pumps and other fluxonic devices.

Current reversal in collective ratchets induced by lattice instability

A collective mechanism for current reversal in superconducting vortex ratchets is proposed. The mechanism is based on a two-dimensional instability of the ground state (T=0) of the system. We illustrate our results with numerical simulations and experiments in Nb superconducting films fabricated on top of Si substrates with artificially induced asymmetric pinning centers.

Asymmetric weak-pinning superconducting channels: Vortex ratchets

The controlled motion of objects through narrow channels is important in many fields. We have fabricated asymmetric weak-pinning channels in a superconducting thin-film strip for controlling the dynamics of vortices. The lack of pinning allows the vortices to move through the channels with the dominant interaction determined by the shape of the channel walls. We present measurements of vortex dynamics in the channels and compare these with similar measurements on a set of uniform-width channels. While the uniform-width channels exhibit a symmetric response for both directions through the channel, the vortex motion through the asymmetric channels is quite different, with substantial asymmetries in both the static depinning and dynamic flux flow. This vortex ratchet effect has a rich dependence on magnetic field and driving force amplitude.

Dipole-Induced Vortex Ratchets in Superconducting Films with Arrays of Micromagnets

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.

Reversible vortex ratchet effects and ordering in superconductors with simple asymmetric potential arrays

Using computer simulations, we demonstrate that the simplest vortex ratchet system for type-II superconductors with artificial pinning arrays, a simple asymmetric potential array, exhibits the same features as those of more complicated two-dimensional (2D) vortex ratchets that have been studied in recent experiments. We show that the simple geometry, proposed by Lee et al. [Nature (London) 400, 337 (1999)], undergoes multiple reversals in the sign of the ratchet effect as a function of vortex density, substrate strength, and ac drive amplitude, and that the sign of the ratchet effect is related to the type of vortex lattice structure present. Thus, although the ratchet geometry has the appearance of being effectively one dimensional, the behavior of the ratchet is affected by the 2D structure of the vortex configuration. When the vortex lattice is highly ordered, an ordinary vortex ratchet effect occurs, which is similar to the response of an isolated particle in the same ratchet geometry. In regimes where the vortices form a smectic or a disordered phase, the vortex-vortex interactions are relevant, and we show with force balance arguments that the ratchet effect can reverse in sign. The dc response of this system features a reversible diode effect and a variety of vortex states including triangular, smectic, disordered, and square.

Josephson vortex in a ratchet potential: theory.

We propose a type of Josephson vortex ratchet. In this system a Josephson vortex moves in a periodic asymmetric potential under the action of a deterministic or random force with zero time average. For some implementations the amplitude of the potential can be controlled during the experiment, thus allowing us to tune the performance of the system and build rocking as well as flashing ratchets. We discuss the differences between conventional and Josephson vortex ratchets and present a model describing the dynamics of the fluxon in such a system. We show numerical simulation results that predict rectification of a monochromatic, deterministic signal with zero time average. The investigation of this system may lead to the development of the fluxon rectifier-a device that produces a dc voltage from nonequilibrium fluctuations.