Controlling magnetic flux penetration in low-T _C superconducting films and hybrids

Abstract

In realistic situations, a typical scenario for applications of type II superconductors necessarily includes vortices, usually in the presence of currents. Under these circumstances, a Lorentz force acts on each individual vortex, propelling its dissipative movement across the superconducting medium, what increases the local temperature and, eventually, endangers the preservation of the superconducting state. Thus, preventing vortices from moving or, at least, designing strategies to minimize undesirable effects from their inevitable presence, is on the list of the most important demands. Vortex dynamics in superconducting films is a fundamental and complex subject in which efficiency is mainly represented by the ability of pinning vortices and thus increasing the current-carrying capability of the specimen. This Topical Review, focused on the strategies developed to control magnetic flux entry and restrict vortex mobility in low-T C superconducting films, compiles the most relevant literature on this subject, starting from a revision of the physical response of a plain film to externally applied magnetic fields and, subsequently, discussing specific changes caused to such a response when obstacles devoted to refrain vortices from moving around are cast into the scene, like arrays of antidots, conducting or magnetically active cap layers, and in-plane magnetic fields. An additional method of control discussed here is provided by the so-called Superconducting Flux Injector, through which one can decide where and when to feed the film with magnetic flux. It should also be mentioned that superconducting films are likely to exhibit avalanches at certain ranges of the applied magnetic field and the temperature, a feature that can be sharpened or mitigated by each of the above approaches, which is also discussed in this paper.

Metamorphosis of discontinuity lines and rectification of magnetic flux avalanches in the presence of noncentrosymmetric pinning forces

“…At low fields, the avalanches are seen to penetrate exclusively from the bottom border and develop along an easy-axis direction with a well defined needle-like shape and very short perpendicular branching. At high fields, avalanches are also triggered at the top edge and show 45-degree ramifications, as those observed in thin films decorated with a square array of square holes [29,30]. Moreover, 90-degree branches also develop and follow the main directions of the square lattice of holes.…”

Superconducting Properties and Electron Scattering Mechanisms in a Nb Film with a Single Weak-Link Excavated by Focused Ion Beam

“…Concerning the terminology “grain” to define the polycrystalline unpatterned Nb film regions, one should be careful to inspect if, indeed, both areas behave uniformly as unique superconducting entities. A powerful technique to do so is MOI, which allows one to map the flux distribution [ 54 , 55 ]. Figure 1 a shows a magneto-optical (MO) image for the grooved Nb specimen captured using MOI at 8 K for an applied field of 50 Oe after a zero field cooling (ZFC) procedure.…”

Vortex jets generated by edge defects in current-carrying superconductor thin strips

“…II B and II D we have analyzed the shape of vortex jets in narrow and wide strips and phenomena that arise due to the difference between the vortex jet and the vortex chain. It should be noted that the case of a superconducting strip in the Meissner state under consideration is different from the often studied case of vortex penetration into a strip that is in the critical state, where vortex avalanches (dendrites) are observed [62][63][64][65][66][67][68][69]. The considered regime is also different from the case of strips with rather strong intrinsic pinning at low magnetic fields, where the flux does not penetrate with a smooth advancing front, but instead as a series of irregularly shaped protrusions, resulting in v * 0 at B 0 [70].…”