DOE OSTI2014
Superconductors are materials that conduct electricity with no resistance, i.e. no energy dissipation. In future electrically activated nanodevices, the use of superconducting interconnects is highly desirable, since it circumvents the damaging heat produced by energy dissipation in normal nano-conductors whose high resistance is inversely proportional to its cross-section area. Furthermore, the wave functions of the charge carriers - Cooper pairs will be strongly modified in a superconductor when they are squeezed into a small volume comparable to the coherence length or penetration depth. Thus, superconducting nanostructures provide unique platforms to investigate and discover novel superconducting phenomena in confined geometries. The objective of this DOE funded project was the development of novel approaches to synthesizing a new class of superconducting nanostructures such as nanowires and nanoribbons that are stable in atmosphere, enabling the exploration of superconducting properties and potential applications of individual nanostructures. We also pursued the modification of properties of the fabricated nanostructures by introducing artificial defects through nanoengineering. We developed a conversion approach comprised of two-steps to synthesize NbSe 2 and NbN nanowires/ribbons that were not achievable using other routes: first, we synthesized one-dimensional (1D) nanostructures of NbSe 3 and second, we converted these into desired 1D superconducting nanostructures of NbSe 2 and NbN by adjusting the composition of NbSe 3 nanostructure precursors while maintaining their shapes. Novel phenomena such as resistance oscillations with increasing magnetic fields were observed in both NbSe 2 and NbN nanowires/ribbons. By replacing the catalytic microparticles used in conventional whisker growth with the same kind of nanoparticles that are available now commercially, we improved whisker technologies for nanofabrication of high-T c superconductor nanowires/ribbons: we succeeded in the growth of Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO-2212) and YBa 2 Cu 3 O 7-x (YBCO) nanowires and nanoribbons by annealing glassy precursors with alumina nanoparticles as growth sites. The high-quality whiskers enabled us to observe the ‘matching effect’ that represents itself as dips in the field dependence of the magnetoresistance and the influence of the periodic defects on the melting behavior of vortex matter. We also observed N-shaped V-I characteristics similar to those observed in semiconductor and plasma devices for the first time. Each nanofabrication technique has its advantages and drawbacks. For example, electron-beam lithography (EBL), in combination thin film deposition, has been widely used to fabricate nanowires (nanobridge) with well-defined cross-sections. However, electron-beam resist usually contaminates the sample up to a few hundred nanometers and hence degrades the quality of the entire nanowires if their widths are only a few hundred nanometers. We found that e-beam resist can degrade the superconducting critical temperature of a Nb nanowire from 9 K down to below 4 K if its width is less than 200 nm. We investigated the suitability of the various nanofabrication methods for achieving high-quality samples with desired properties. For example, we developed a novel way to achieve high quality Nb nanostrips with transverse dimensions of 150 nm or smaller. In the new process a superconducting bridge a few micrometers wide is fabricated with EBL. Subsequently, focused-ion-beam (FIB) is used to mill nano- 2 grooves to isolate an uncontaminated area of the microbridge to form a nanostrip. The high-quality superconducting Nb nanostrips with well-defined rectangular cross-sections fabricated using this new technique have enabled experimental confirmation of the theoretically predicted confinement-induced anisotropy effect on the magnetoresistance and critical field. We also found that FIB milling is more suitable than EBL in producing a pinning potential array with desired barrier between them into a MoGe film, which enabled us to realize the predicted vortex ices for the first time. By utilizing a filtration membrane as the substrate, we developed a new method to achieve networks of superconducting MoGe nanowires with transverse dimensions smaller than 10 nm. We observed superconducting confinement effect such as thermally activated phase slips and Little-Parks effect in these new samples. This research significantly advanced our knowledge on confined superconductors, in both their synthesis and physical properties. Though the new fabrication methods developed in this project aimed to achieve samples for studying new phenomena in confined superconductors, they can be utilized to obtain nanostructures of other functional materials. For example, we utilized the method developed to produce superconducting nanowire networks for the fabrication of networks of ultrasmall palladium (Pd) nanowires, which take advantage of single Pd nanowires while eliminating their nanofabrication obstacles, resulting in high-performance hydrogen sensors that are much more cost effective than those based on single Pd nanowires fabricated with lithography methods.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗