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Register a new userUpper frequency limits for vortex guiding and ratchet effects
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Added by
Oleksandr Dobrovolskiy V.
Publication date
2020
fields
Physics
and research's language is
English
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Guided and rectified motion of magnetic flux quanta are important effects governing the magneto-resistive response of nanostructured superconductors. While at low ac frequencies these effects are rather well understood, their manifestation at higher ac frequencies remains poorly investigated. Here, we explore the upper frequency limits for guided and rectified net motion of superconducting vortices in epitaxial 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, vortex guiding-related low-ac-loss response persists up to about 2 GHz. This value corresponds to the depinning frequency $f_mathrm{d}^mathrm{s}$ associated with the washboard pinning potential induced by the nanostripes and exhibiting peaks for the commensurate vortex lattice configurations. Applying a sum of dc and microwave ac currents at an angle $alpha$ with respect to the nanostripes, the angle dependence of $f_mathrm{d}^mathrm{s}(alpha)$ has been found to correlate with the angle dependence of the depinning current. In all, our findings suggest that superconductors with higher $f_mathrm{d}^mathrm{s}$ should be favored for an efficient vortex manipulation in the GHz ac frequency range.
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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.
We study the dynamics of vortices in an asymmetric ring channel driven by an external current I in a Corbino setup. The asymmetric potential can rectify the motion of vortices and cause a net flow without any unbiased external drive, which is called ratchet effect. With an applied ac current, the potential can rectify the motion of vortices in the channel and induce a dc net flow. We show that the net flow of vortices strongly depends on vortex density and frequency of the driving current. Depending on the density, we distinguish a single-vortex rectification regime (low density) determined by the potential-energy landscape inside each cell of the channel (i.e., hard and easy directions of motion) and multi-vortex, or collective, rectification (high density) when the interaction between vortices becomes important. The frequency of the driving ac current determines a possible distance that a vortex could move during one period. For high frequency current, vortices only oscillate in the triangular cell. For low frequency, the vortex angular velocity $omega$ increases nearly linearly until the driving force reaches the maximum friction force in the hard direction. Furthermore, the commensurability between the number of vortices and the number of cells results in a stepwise $omega-I$ curve. Besides the integer steps, i.e., the large steps found in the single vortex case, we also found fractional steps corresponding to fractional ratio between the numbers of vortices and triangular cells. The principal and fractional frequencies for different currents are found, when the net flow of vortices reaches the maximum that is proportional to the frequency when the density of vortices is low. We have performed preliminary measurements on a device containing a single weak-pinning circular ratchet channel in a Corbino geometry and observed a substantial asymmetric vortex response.
We experimentally demonstrate that the origin of multiply reversed rectified vortex motion in an asymmetric pinning landscape is a consequence not only of the vortex-vortex interactions but also essentially depends on the ratio between the characteristic interaction distance and the period of the asymmetric pinning potential. Our system consists of an Al film deposited on top of a square array of size-graded magnetic dots with a constant lattice period a=2mu m. Four samples with different periods of the size gradient d were investigated. For large d the dc voltage Vdc recorded under a sinusoidal ac excitation indicates that the average vortex drift is from bigger to smaller dots for all explored positive fields. As d is reduced a series of sign reversals in the dc response are observed as a function of field. We show that the number of sign reversals increases as d decreases. These findings are in agreement with recent computer simulations and illustrate the relevance of the different characteristic lengths for the vortex rectification effects.
We study the ratchet effect in a narrow pinning-free superconductive ring based on time-dependent Ginzburg-Landau (TDGL) equations. Voltage responses to external dc an ac currents at various magnetic fields are studied. Due to asymmetric barriers for flux penetration and flux exit in the ring-shaped superconductor, the critical current above which the flux-flow state is reached, as well as the critical current for the transition to the normal state, are different for the two directions of applied current. These effects cooperatively cause ratchet signal reversal at high magnetic fields, which has not been reported to date in a pinning-free system. The ratchet signal found here is larger than those induced by asymmetric pinning potentials. Our results also demonstrate the feasibility of using mesoscopic superconductors to employ superconducting diode effect in versatile superconducting devices.
Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe2 single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.
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