Superconductivity is a physical phenomenon that occurs at ultra-low temperatures in many materials, manifesting itself through a vanishing electrical resistance and the expulsion of magnetic fields from the materials' interiors.
The fact that electrical resistance ceases to exist in superconductors means that they are particularly attractive for electronics where substantial amounts of energy can be wasted through heat. Today, they are already used for applications in medical imaging and hold lots of potential for consumer-grade applications.
However, the most technologically relevant superconductors' conductivity, so-called type-II superconductors, is not so "super" after all. In these superconductors, an external magnetic field penetrates the material in the form of quantized lines of magnetic flux. These flux lines are known as Abrikosov vortices and when a moderately strong electrical current is applied to the materials, these vortices move, and the superconductor can no longer carry the current with zero resistance.
A Rare Combination of Properties
In most superconductors, a low state of resistance is limited by vortex velocities of 1 km/s. This limits the practical use of superconductors in various applications. Simultaneously, such velocities are not sufficiently high to address the rich physics generic to nonequilibrium collective systems.
This is what the international research team, led by scientists from the University of Vienna, sought to address. According to the research team, their work has resulted in a new superconducting system in which magnetic flux quanta can move at velocities of 10-15 km/s.
The new superconductor also exhibits a rare combination of properties: high structural uniformity, large critical current, and fast relaxation of heated electrons. This combination ensures that flux-flow instability—the abrupt transition of a superconductor from the low-resistant to the normal conducting state—occurs at sufficiently large transport currents.
Abrikosov lattice at moderate vortex velocities (left); ultra-fast moving Abrikosov-Josephson "vortex rivers" (right). Image Credit: University of Vienna.
"In recent years, there have appeared experimental and theoretical works pointing to a remarkable issue; it has been argued that current-driven vortices can move even faster than the superconducting charge carriers.", says Oleksandr Dobrovolskiy, lead author of the team's research.
However, in these works, locally non-uniform structures were used. Initially, Dobrovolskiy and colleagues worked with high-quality clean films, but it later transpired that dirty superconductors better support ultra-fast vortex dynamics. "Though the intrinsic pinning in these is not necessarily as weak as in other amorphous superconductors, the fast relaxation of heated electrons becomes the dominating factor allowing for ultrafast vortex motion," he added.
Potential Applications in Quantum Information Processing
For their study, the researchers fabricated an Nb-C superconductor via focused ion beam induced deposition. In addition to ultra-fast vortex velocities in Nb-C, the direct-write nanofabrication technology enables the fabrication of complex-shaped nano-architectures and 3D fluxonic circuits with complex interconnectivity. This may point to applications in quantum information processing.
This approach opens perspectives for building large-area single-photon detectors which could be used in e.g. confocal microscopy, free-space quantum cryptography, deep-space optical communication," says Denis Vodolazov, Senior Researcher at the Institute for Microstructures of RAS, Russia.