Fluxon manipulation by nanoscale artificial pinning lattices

Superconductivity is an exciting phenomenon that raises many unexplored fundamental questions and has enormous relevance for emerging and disruptive technologies. Apart from the well-known applications of almost loss-less energy transport in superconducting cables and the creation of intense magnetic fields by superconducting coils, recent advances in the nanostructuring of superconductors make them promising candidates for novel quantum technologies Inside a superconductor, a magnetic field can exist only in small quantized units, called fluxons or vortices. In a homogeneous and pure superconductor, the fluxons are arranged in a hexagonal lattice. It is by the introduction of controlled defects that the properties of superconductors can be tailored to allow for a control of these fluxons, for instance by forcing them into a predefined formation by traps. While the brittle nature and the complicated crystal structure of the copper-oxide high- temperature superconductors pose a challenge to their technical usability, this project will turn this susceptibility into an asset to fabricate superconducting nanostructures with unprecedented resolution. Point defects are created by helium ion irradiation with moderate energy that readily suppress superconductivity locally. These individual small regions of non-superconducting material act as the traps for the fluxons and they all together form a pinning landscape in the superconductor. Using shadow projection of a wide-field ion beam through a stencil mask or the focused beam of a helium-ion microscope, it is possible to pattern nanoscale pinning landscapes into copper-oxide superconductors with unprecedented small distances. This leads to a much stronger interaction of the fluxons than it could be achieved in previous experiments and opens the door to investigate many novel phenomena. The scientific results of this project may have significant implications on the understanding of vortex physics, on the development of new functional materials, on the development of ultrafast low-dissipation superconducting circuits, and may lead to improvements of very small magnetic field sensors (SQUIDs) based on copper-oxide superconductors. This research program will be performed in tight collaboration of three expert groups at the University of Vienna, the Johannes Kepler University of Linz, and the Eberhard Karls University in Tübingen. Theoretical support with molecular-dynamic simulations will be provided by collaborators at the University of Antwerp and the visualization of the fluxon patterns will be performed at the Universidad Autnoma de Madrid.

Since October 2020, our research project has been exploring new ways to enhance the performance of high-temperature superconductors, materials capable of conducting electricity without resistance at relatively high temperatures. One primary focus has been understanding how nanoscopic defects, introduced via helium-ion irradiation, interact with "Abrikosov vortices." These are tiny current whirlpools that carry one magnetic flux quantum and emerge within superconductors under a strong electric current or in an external magnetic field. Unless these vortices are "pinned" in place, they move and cause energy losses, undermining the superconductor's zero-resistance advantage. Understanding and controlling vortices is a key challenge for real-world applications of superconductors. Our approach uses helium-ion irradiation to introduce nanoscopic defects in the cuprate superconductor YBaCuO (YBCO). By computer simulations of how helium ions collide with atoms in thin YBCO films, we identified the optimal conditions to create tightly packed defect columns. These defects act as effective anchors, or pinning centers, for the vortices. Building on these insights, our project partners at the University of Tübingen applied a focused 30 keV helium-ion beam to produce ultra-dense patterns of pinning sites. Measurements in Vienna revealed an unprecedented high commensurability field of 6 Tesla for a triangular defect lattice with only 20 nanometers between sites. This cutting-edge fabrication technology unlocked experimental access to new areas of vortex physics. We proposed an "ordered Bose glass" phase in nanopatterned YBCO films, suggesting that under specific conditions, the vortices arrange themselves into a unique glass-like structure with remarkable properties. Intriguingly, the nanostructured samples exhibited a reentrant zero resistance at high magnetic fields and revealed the theoretically predicted "transverse Meissner effect," in which the vortex arrangement strongly resists any tilt of the magnetic field. Complementary studies into vortex motion in metallic superconductors were helping us understand how edge features and tiny slits can control the flow of vortices. In addition to YBCO, the project has spurred the synthesis and analysis of other high-temperature superconductors by our national coworkers at the Johannes Kepler University in Linz, such as the highly anisotropic BiSrCaCuOx (Bi-2212). Detailed measurements revealed its excellent superconducting qualities. Preliminary experiments indicate that nanostructuring by focused helium ion-beam irradiation also applies to Bi-2212 thin films. The outcomes of this research project have direct relevance for applications requiring zero resistance under extreme conditions, such as powerful electromagnets for medical imaging. The advanced techniques for nanostructuring these complex materials are indispensable for manufacturing electronic devices and sensors, possibly extending into quantum computing. We expect the follow-up research embedded in our network of collaborators will unravel more of the fascinating quantum properties of these remarkable materials.