Ring-shaped Superconductor Research Articles

Mixed superconducting state without applied magnetic field

A superconducting (SC) mixed state occurs in type-II superconductors where the upper critical field Hc2 is higher than the thermodynamic critical field Hc. When an applied field is in between these fields, the free energy depends weakly on the order parameter which therefore can be small (SC state) or zero (normal state) at different parts of the sample. In this paper we demonstrate how a normal state along a line traversing a superconductor can be turned on and off externally in zero field. The concept is based on a long, current-carrying excitation coil, piercing a ringshaped superconductor. The ring experiences zero field, but the vector potential produced by the coil generates a circular current that destroys superconductivity along a radial line starting at preexisting nucleation points in the sample. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil, and different cooldowns yield the same normal lines. We suggest potential applications of this magnetic-field-free mixed state.

Reversible ratchet effects in a narrow superconducting ring

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.

Superconducting levitation analysis of a flywheel system using H-formulation

In this research study, two-dimensional axisymmetric finite element model based on the H-formulation was studied to analyze the levitation characteristics of a flywheel system by COMSOL Multiphysics 5.3. This model provides the designer with some reliable guidelines for electromagnetic behaviours of the flywheel system. Levitation forces are generated through separate interaction for axial and radial bearing in such an application. However, the use of more than one permanent magnet (PM) with a single ring-shaped superconductor can provide higher stiffness characteristic. To ensure this condition, the proper polarization of the permanent magnets is a key factor. The effect of polarity is investigated through the distributions of the induced current density and the force characteristic. Finally, the effectiveness of the simulation method has been validated through comparisons with experimentally obtained values.

Inverse Flux Creep in Ring-Shaped Superconductors

Anomalous flux creep is depicted in ring-shaped superconductors, where the vortex movement goes to the inner edge of the sample. The current dissipation phenomenon is inverted and an induction of the current is achieved. Thus, an increase in the current circulating by the superconductor is obtained. The experimental technique consists in the utilization of a closed magnetic circuit for the current induction and a magnet ring with the field opposite to the vortex trapped in the superconducting wall.

Inducting technique and trapped field in ring-shaped superconductors

A comparative study of trapped flux depending on the inducting technique was made in superconducting rings. The inducting procedures used where the traditional field cooling with electromagnet (in this work FC1 or FC2) and the field cooling with a closed magnetic circuit placed through the ring’s hole (FC3). The mentioned study was made by means of flux creep experiences in conventional samples and maps of trapped field in cut samples (current circulation disabled). The current induced in the samples was the critical value and the field trapped in the ring’s wall depended on the magnetic field of FC1, FC2 or FC3. Data obtained from flux creep measurements exhibited the lowest relaxation rate for FC3. On the other hand, maps of trapped field show that the flux trapped was also the lowest when the induction was made by using FC3. The data depicted that samples with similar trapped flux density exhibited similarities in their relaxation rates with very different critical current.

Improving the Dynamics of a Flywheel Energy Storage System With Superconducting Magnetic Bearings

This paper describes a new method for improving the dynamics of a flywheel energy storage system using superconducting magnetic bearings (SMBs). The SMBs are composed of a ring-shaped superconductor and some permanent magnets (PM). Changes in stiffness and damping coefficient of the SMBs were investigated. Forced displacements are given to the SMBs to improve the stiffness and the damping coefficient. The stiffness and the damping coefficient are also evaluated by using the energy storage flywheel system.

Quantum transport of vortices in a weakly dissipative ring threaded by an Aharonov-Casher flux: A tight-binding model

Based on the tight-binding model and taking into account the effect of dissipation as well as the disorder, we study quantum interference via the Aharonov-Casher effect for fluxons in a ring-shaped superconductor. The electrical resistance in terms of the transmission probability for a quantum vortex to traverse the ring is calculated. It is shown that a quantum interference effect is exhibited in the resistance in the presence of weak dissipation. Our analysis may also be applied to a Josephson-junction array system. In particular, by including the disorder effect, we are able to explain the experimental measurements in the dissipative regime done by Elion et al. [Phys. Rev. Lett. 71, 2311 (1993)].

The thin-film disk and ring shaped type II superconductors in perpendicular magnetic fields

Properties of small, thin-film, disk-shaped and ring-shaped superconductors in the presence of perpendicular magnetic fields were considered within the framework of a model that takes into account the difference between magnetic induction and vortex density, vortex movement, and generation of vortices if current density exceeded its critical value. The linear complex magnetic susceptibility of a ring was calculated. It was shown that the distribution of the dissipative current differed considerably from the reactive one. The influence of the edge barrier on the form of the hysteresis loop, and current and field patterns for the ring in the critical state were shown.

Ginzburg–Landau Equations and Stable Solutions in a Rotational Domain

The Ginzburg–Landau (GL) equations, with or without magnetic effect, are studied in the case of a rotational domain in $\mathbb{R}^3 $. It can be shown that there exist rotational solutions which describe the physical state of permanent current of electrons in a ring-shaped superconductor. Moreover, if a physical parameter—called the GL parameter—is sufficiently large, then these solutions are stable, that is, they are local minimizers of an energy functional (GL energy). This is proved by the spectral analysis on the linearized equation.

Aharonov-Cacher effect for fluxons in a superconducting ring with weak dissipation

Based on the path integral formalism and taking into account the effect of dissipation, we study quantum interference via the Aharonov-Casher effect for fluxons in a ring-shaped superconductor. Both the transmission probability and the electric resistance are derived. It is shown that a quantum interference effect is exhibited in the resistance in the presence of weak dissipation. Our analysis may also be applied to a Josephson-junction array system.

QUANTIZATION AND METASTABILITY OF SUPERCURRENTS

The idea that the metastability of currents in ringshaped superconductors may have the same origin as the phenomenon of magnetic flux quantization was examined, with emphasis on the effects of thermal excitation. A relation with Bardeen's two-fluid model was considered. Model caicuiations are presented. (C.H.)