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

Abstract

Granularity is one of the main features restricting the maximum current which a superconductor can carry without losses, persisting as an important research topic when applications are concerned. To directly observe its effects on a typical thin superconducting specimen, we have modeled the simplest possible granular system by fabricating a single artificial weak-link in the center of a high-quality Nb film using the focused ion beam technique. Then, its microstructural, magnetic, and electric properties in both normal and superconducting states were studied. AC susceptibility, DC magnetization, and magneto-transport measurements reveal well-known granularity signatures and how they negatively affect superconductivity. Moreover, we also investigate the normal state electron scattering mechanisms in the Boltzmann theory framework. The results clearly demonstrate the effect of the milling technique, giving rise to an additional quadratic-in-temperature contribution to the usual cubic-in-temperature sd band scattering for the Nb film. Finally, by analyzing samples with varying density of incorporated defects, the emergence of the additional contribution is correlated to a decrease in their critical temperature, in agreement with recent theoretical results.

Magnetic field induced weak-to-strong-link transformation in patterned superconducting films

“…From 92.6 to 48.0 Oe, the black profile decreases less and becomes increasingly similar to the red one, indicating that the flux pushed inward by the d-line movement is partially retained by the WL. This is related to an increased vortex pinning potential in the WL due to its reduced thickness and FIB-induced defects, which act as pinning centers and locally suppress T c [3,30]-see Appendix A. The observed relative increase in B corroborates the increase in m l across the WL seen in Fig.…”

“…Sample preparation details are given in Ref. [30]. Global dc magnetic responses are obtained in a Quantum Design MPMS-5S in the perpendicular geometry.…”

“…However, FIB unavoidably introduces defects along the patterned regions, arising from ionic implantation [26][27][28], locally increasing vortex pinning in superconductors [3,8,29]. Recently, we investigated the effects of a single FIB-milled WL on the properties of prototypical low-critical temperature Nb films, revealing the existence of local maxima in their magnetization loops [30].…”

Magnetic flux penetration in nanoscale wedge-shaped superconducting thin films

“…Figures 2(a)-2(d) were selected to highlight the difference among the thicker and the thinner regions of the specimen. The fact that we do not observe additional symmetrical d lines anywhere in the film [53,68] is a strong indication that there exists a smooth thickness gradient, as represented in Fig. 1(c).…”

Soft Superconductors: Materials and Applications

“…In 1911, Heike Kamerlingh Onnes determined that when liquid helium was heated to -269ºC, the electrical resistance of the material was disappeared. Thus, superconductivity is a phenomenon first understood, approaching close to 4 degrees Celsius of absolute temperature in Kelvin [16]. Twenty-two years after this work, German researchers Walther Meissner and Robert Ochsenfeld, while studying the behavior of matter at extremely cold temperatures, found that a superconducting material affects the magnetic field [17][18][19].…”