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Register a new userReversible ratchet effects in a narrow superconducting ring
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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.
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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 have studied Ni-substitution effect in LaFe$_{1-x}$Ni$_{x}$AsO ($0leq x leq0.1$) by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03leq x leq0.06$ with the maximum $T_c$ of 6.5 K at $x$=0.04, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered $T_c$ in comparison with LaFeAsO$_{1-x}$F$_{x}$ system is discussed.
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.
We have designed, fabricated and tested a robust superconducting ratchet device based on topologically frustrated spin-ice nanomagnets. The device is made of a magnetic Co honeycomb array embedded in a superconducting Nb film. This device is based on three simple mechanisms: i) the topology of the Co honeycomb array frustrates in-plane magnetic configurations in the array yielding a distribution of magnetic charges which can be ordered or disordered with in-plane magnetic fields, following spin-ice rules, ii) the local vertex magnetization, which consists of a magnetic half vortex with two charged magnetic Neel walls, iii) the interaction between superconducting vortices and the asymmetric potentials provided by the Neel walls. The combination of these elements leads to a superconducting ratchet effect. Thus, superconducting vortices driven by alternating forces and moving on magnetic half vortices generate a unidirectional net vortex flow. This ratchet effect is independent of the distribution of magnetic charges in the array.
The theory of current transport in a narrow superconducting channel accounting for thermal fluctuations is revisited. The value of voltage appearing in the sample is found as the function of temperature (close to transition temperature $T-T_{mathrm{c}}$ $ll T_{mathrm{c}}$) and bias current $J<J_{mathrm{c}}$ ( $J_{mathrm{c}}$ is a value of critical current calculated in the framework of the BCS approximation, neglecting thermal fluctuations). It is shown that the careful analysis of vortex crossing of the stripe results in considerable increase of the activation energy.
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