Vortex Ratchet Effect Research Articles

Electric Tuning of Vortex Ratchet Effect in NbSe2.

Inversion symmetry breaking has played an important role in recent discoveries of nonreciprocal charge transport. Niobium diselenide, for example, lacks an inversion center in the monolayer form and can host prominent nonreciprocal transport property. Here, however, we observe a nonreciprocal transport signal in the second-harmonic channel of bulk-like NbSe2, in which inversion symmetry of the lattice seems preserved. The second-harmonic signal occurs along different in-plane current orientations and appears not only in the vortex-liquid regime but also even in the superconducting fluctuation regime without an applied magnetic field. By adding a direct current (DC) bias, we quantify the symmetry breaking effect in the vortex-liquid regime. The DC bias also suggests that the rectification effect at the contacts may account for the seemingly nonreciprocal transport at zero magnetic field. Our results demonstrate that DC biasing is a useful knob for addressing nonreciprocal charge transport in a wide range of materials.

Nonreciprocal magnon fluxonics upon ferromagnet/superconductor hybrids

Ferromagnet/superconductor heterostructures allow for the combination of unique physical phenomena offered by the both fields of magnetism and superconductivity. It was shown recently that spin waves can be efficiently scattered in such structures by a lattice of static or moving magnetic flux quanta (Abrikosov vortices), resulting in bandgaps in the spin-wave spectra. Here, we realize a nonreciprocal motion of a vortex lattice in nanoengineered symmetric and asymmetric pinning landscapes and investigate the non-reciprocal scattering of magnons on fluxons. We demonstrate that the magnon bandgap frequencies can be tuned by the application of a low-dissipative transport current and by its polarity reversal. Furthermore, we exploit the rectifying (vortex diode or ratchet) effect by the application of a 100MHz-frequency ac current to deliberately realize bandgap up- or downshifts during one ac halfwave while keeping the bandgap frequency constant during the other ac halfwave. The investigated phenomena allow for the realization of energy-efficient hybrid magnonic devices, such as microwave filters with an ultra-high bandgap tunability of 10GHz/mA and a fast modulation of the transmission characteristics on the 10ns time scale.

Multiple reversals of vortex ratchet effects in a superconducting strip with inclined dynamic pinning landscape* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11702034, 11972298, and 11702218), the China Postdoctoral Science Foundation (Grant No. 2019M663812), the Fundamental Research Funds for the Central Universities, China (Grant Nos. 300102129104, 3102018zy013, and 3102017jc01003), and the Young Talent Fund of

Using time-dependent Ginzburg–Landau formalism, we investigate the multiple reversals of ratchet effects in an unpatterned superconducting strip by the tilted dynamic pinning potential. In the case of collinear sliding potential and Lorentz force, vortices are always confined in the channels induced by sliding potential. However, due to the inclination angle of sliding pinning potential with respect to the Lorentz force, vortices could be driven out of the channels, and unexpected results with multiple reversals of vortex rectifications are observed. The mechanism of multiple reversals of vortex rectifications is explored by analyzing different vortex motion scenarios with increasing ac current amplitudes. The multiple reversals of transverse and longitudinal ratchet effects can be highly controlled by ac amplitude and dynamic pinning velocity. What’s more, at certain large current the ratchet effect reaches strongest within a wide range of pinning sliding velocity.

Upper Frequency Limits for Vortex Guiding and Ratchet Effects

The guided and rectified motion of magnetic flux quanta are important effects governing the magnetoresistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestations at higher ac frequencies remain poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial $\mathrm{Nb}$ films decorated with ferromagnetic nanostripes. By combining broadband electrical spectroscopy with resistance measurements, we reveal that the rectified voltage vanishes at a geometrically defined frequency of about 700 MHz. By contrast, a vortex-guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency ${f}_{d}^{s}$ associated with the washboard pinning potential induced by the nanostripes, which exhibits peaks for commensurate vortex-lattice configurations. By applying a sum of dc and microwave ac currents at an angle $\ensuremath{\alpha}$ with respect to the nanostripes, the angle dependence of ${f}_{d}^{s}(\ensuremath{\alpha})$ is found to be correlated with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher ${f}_{d}^{s}$ should be favorable for efficient vortex manipulation in the gigahertz ac frequency range.

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.

Ratchet effects in superconducting ring-shaped devices

The vortex ratchet effects in superconducting ring-shaped NbN devices are experimentally investigated. We have studied three types of samples: a narrowing eccentric circular loop, standard circular loop, and widening eccentric circular loop. Using the time-dependent Ginzburg–Landau simulations, we demonstrate that the sample edges significantly influence the dynamics of the vortices entering the samples. Through transport measurements with dc and ac currents, strong rectified voltage responses were observed for all the samples over a wide range of temperatures, critical currents, and magnetic fields. Moreover, there are some differences in the detailed ratchet behaviors among them. Furthermore, we also observed the ratchet effects in our samples caused by the current-induced field, which deserves further investigations.

Vortex ratchet effects in a superconducting asymmetric ring-shaped device

We investigate the vortex ratchet effects in a superconducting asymmetric ring-shaped NbN device. Through transport measurements, we find that the rectified dc voltages are significantly enhanced, and we observe time-dependent asymmetric voltage waveforms over a single cycle. Our vortex ratchet device operates over a wide range of temperatures, critical currents, and magnetic fields. We demonstrate that in this asymmetric structure giant ratchet effects are mainly caused by the collective behavior of vortices, which differs clearly from one-particle vortex effects studied in conventional vortex ratchet systems.

Pinning, flux diodes and ratchets for vortices interacting with conformal pinning arrays

A conformal pinning array can be created by conformally transforming a uniform triangular pinning lattice to produce a new structure in which the six-fold ordering of the original lattice is conserved but where there is a spatial gradient in the density of pinning sites. Here we examine several aspects of vortices interacting with conformal pinning arrays and how they can be used to create a flux flow diode effect for driving vortices in different directions across the arrays. Under the application of an ac drive, a pronounced vortex ratchet effect occurs where the vortices flow in the easy direction of the array asymmetry. When the ac drive is applied perpendicular to the asymmetry direction of the array, it is possible to realize a transverse vortex ratchet effect where there is a generation of a dc flow of vortices perpendicular to the ac drive due to the creation of a noise correlation ratchet by the plastic motion of the vortices. We also examine vortex transport in experiments and compare the pinning effectiveness of conformal arrays to uniform triangular pinning arrays. We find that a triangular array generally pins the vortices more effectively at the first matching field and below, while the conformal array is more effective at higher fields where interstitial vortex flow occurs.

Superconducting/magnetic Three-state Nanodevice for Memory and Reading Applications.

We present a simple nanodevice that can operate in two modes: i) non-volatile three-state memory and ii) reading device. The nanodevice can retain three well defined states −1, 0 and +1 and can operate in a second mode as a sensor for external magnetic fields. The nanodevice is fabricated with an array of ordered triangular-shaped nanomagnets embedded in a superconducting thin film gown on Si substrates. The device runs based on the combination of superconducting vortex ratchet effect (superconducting film) with the out of plane magnetization (nanomagnets). The input signals are ac currents and the output signal are dc voltages. The memory mode is realized without applying a magnetic field and the nanomagnet stray magnetic fields govern the effect. In the sensor mode an external magnetic field is applied. The main characteristic of this mode is that the output signal is null for a precise value of the external magnetic field that only depends on the fabrication characteristics of the nanodevice.

Onset, evolution, and magnetic braking of vortex lattice instabilities in nanostructured superconducting films

In 1976 Larkin and Ovchinnikov [Sov. Phys. JETP 41, 960 (1976)] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow regime to a more dissipative state at sufficiently high vortex velocities. Typically this transition manifests itself as a large voltage jump at a particular current density, so-called instability current density $J^*$, which is smaller than the depairing current. By tuning the effective pinning strength in Al films, using an artificial periodic pinning array of triangular holes, we show that a unique and well defined instability current density exists if the pinning is strong, whereas a series of multiple voltage transitions appear in the relatively weaker pinning regime. This behavior is consistent with time-dependent Ginzburg-Landau simulations, where the multiple-step transition can be unambiguously attributed to the progressive development of vortex chains and subsequently phase-slip lines. In addition, we explore experimentally the magnetic braking effects, caused by a thick Cu layer deposited on top of the superconductor, on the instabilities and the vortex ratchet effect

Guided vortex motion and ratchet effect in an anisotropic superconductor with a periodic pinning potential

Two-dimensional adiabatic ratchet dynamics of Abrikosov vortices in a symmetric periodic pinning potential is considered in the presence of dc and ac transport currents and anisotropy of the viscous vortex motion at zero temperature. Exact analytical expressions for two anisotropic nonlinear current-voltage responses along and across the transport current direction are derived and analyzed within the framework of the washboard model of pinning potential. The physical origin of these voltages, which are odd with respect to the current direction reversal, is the interplay between the vortex guiding effect, which is even with respect to the magnetic field reversal, and the asymmetry of the pinning potential due to its tilt induced by the dc component of the transport current.

Vortex ratchet induced by controlled edge roughness

We demonstrate theoretically and experimentally the generation of rectified mean vortex displacement resulting from a controlled difference between the surface barriers at the opposite borders of a superconducting strip. Our investigation focuses on Al superconducting strips where, in one of the two sample borders, a saw tooth-like array of micro-indentations has been imprinted. The origin of the vortex ratchet effect is based on the fact that (i) the onset of vortex motion is mainly governed by the entrance/nucleation of vortices and (ii) the current lines bunching produced by the indentations facilitates the entrance/nucleation of vortices. Only for one current direction the indentations are positioned at the side of vortex entry and the onset of the resistive regime is lowered compared to the opposite current direction. This investigation points to the relevance of ubiquitous border effects typically neglected when interpreting vortex ratchet measurements on samples with arrays of local asymmetric pinning sites.

Ratchet Effect of Single Vortex Motion in Superconducting Asymmetrical Nanobridges

We have successfully obtained asymmetrical current–voltage (I–V) characteristics in YBa2Cu3O7-x (YBCO) nanobridges patterned with asymmetrical geometry. These asymmetrical nanobridges (ANBs) are shown to control vortices one at a time. The critical current Ic, of 200-nm-wide and 100-nm-thick ANB changes by a maximum of 9% when a magnetic field of 2.4 mT is applied. The Ic variation ΔIc increases monotonically with increased magnetic field from -2.2 to 2.4 mT. The asymmetrical I–V characteristics obtained in our experiment are due to the restriction of vortex motion by the asymmetrical surface barrier (the so-called vortex ratchet effect). The width and thickness dependencies of the I–V asymmetry are also studied, and indicate that the bias current and the Meissner screening current, determined by the relative width compared to the effective penetration depth of the YBCO film, dominates the I–V asymmetry characteristics.

Vortex Core Deformation and Stepper-Motor Ratchet Behavior in a Superconducting Aluminum Film Containing an Array of Holes

We investigated experimentally the frequency dependence of a superconducting vortex ratchet effect by means of electrical transport measurements and modeled it theoretically using the time-dependent Ginzburg-Landau formalism. We demonstrate that the high frequency vortex behavior can be described as a discrete motion of a particle in a periodic potential, i.e., the so-called stepper-motor behavior. Strikingly, in the more conventional low frequency response a transition takes place from an Abrikosov vortex rectifier to a phase slip line rectifier. This transition is characterized by a strong increase in the rectified voltage and the appearance of a pronounced hysteretic behavior.

High-frequency vortex ratchet effect in a superconducting film with a nanoengineered array of asymmetric pinning sites

Vortex ratchet effect is investigated experimentally in the frequency range between 0.5 MHz and 2 GHz. The ratchet potential is provided by an array of about a quarter of a million nanoengineered asymmetric antidots in a Pb film. A square vortex lattice is stabilized at the first matching field, when each asymmetric antidot is occupied by a single vortex. We have found that (1) the transition from adiabatic to nonadiabatic cases occurring at about 1 MHz, above which the ratchet windows shift upwards with the applied frequency due to the fact that the time for a vortex to escape from the pinning potential is comparable to the period of the applied rf driving current ${I}_{\text{rf}}$; (2) a sudden ${V}_{\text{dc}}$ reversal at large ${I}_{\text{rf}}$, which can be attributed to inertia effect; (3) the collective step-motor behavior in the MHz region, i.e., the vortex lattice moves forward by an integer number of the period of pinning array at each cycle of ${I}_{\text{rf}}$; and (4) very weak ratchet effect at several GHz, indicating the possibility of stronger inertia effects in the vortex motion at such high frequencies. These results reveal rich physics information in the nonadiabatic ratchet system and are of particular importance for particle separation and molecular motor in biology.

Vortex ratchet effect in single-crystal films of Bi 2Sr 2CaCu 2O 8+ y

To explore the vortex ratchet effect in underlying spatially asymmetric potentials in high- T c superconductors, we have measured vortex-flow and rectification resistance in Bi 2Sr 2CaCu 2O 8 + y (Bi2212) single-crystal films with an array of triangular holes fabricated using focused ion beam. The matching effect was observed in vortex-flow resistance in the sample with the array of triangular holes as well as that with the symmetric antidots arrays. In addition to that, we found a rectification effect in this sample. Multiple reversals of the rectification direction reflecting the net motion of vortices were observed as the magnetic field changes. In contrast to the vortex ratchet effects observed in conventional superconductors, the sign of the rectification sharply flipped in the matching fields.

Magneto-optical observations of vortices in anisotropic and nanostructured superconductors

Magneto-optical imaging technique is applied to visualize vortex distribution in superconducting networks and in highly anisotropic superconductor Bi2Sr2CaCu2O8+y under tilted magnetic fields. We have succeeded in visualizing the vortex distribution in a finite-size superconducting network with the unit cell size as small as 6×6μm2. We have also visualized the ratchet effect of pancake vortices induced by the time-asymmetric motion of Josephson vortices. The ratchet effect appears most clearly at the similar condition as the vortex lensing effect. The ratchet efficiency diminishes strongly as a function of frequency and it is almost completely suppressed above 1kHz. Such a strong frequency dependence of the vortex ratchet effect is discussed based on the skin effect controlled by c-axis resistivity.

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.

Asymmetry reversal of thermomagnetic avalanches in Pb films with a ratchet pinning potential

We combine transport measurements and magneto-optical imaging in order to study vortex motion in Pb samples with a square lattice of asymmetric pinning potentials formed by two antidots of different sizes. Near the boundary with the normal phase, a vortex ratchet effect is detected when an ac-current is applied. As previously observed in similar systems, inversion of vortex rectification direction occurs as the magnetic field is changed. Further inside the superconducting phase, anisotropic flux penetration patterns are formed. An inversion of anisotropy is observed as the temperature is lowered. This effect can be understood in terms of a thermomagnetic avalanche model. © 2007 The American Physical Society.

Vortex ratchet effect in a niobium film with spacing-graded density of pinning sites

The vortex propagation exhibits rectification effect in Nb superconductors with spacing-graded density of holes. A rectified dc voltage is obtained when the vortex lattice is driven by ac current. The asymmetric geometry of the pinning array produces a significant influence on the vortex motion. The rectified voltage depends considerably on the amplitude of the applied ac current and the magnetic field. The experimental results reveal a drastic change of the vortex rectification for magnetic field above∕below the first matching field. The reason may be that the interstitial vortices are formed in the film above the first matching field. A reversible vortex motion is induced by the interstitial vortices for the field above the first matching field.