Towards super fast vortex movement in superconductors

An international team of scientists from Austria, Germany and Ukraine has found a new superconducting system in which magnetic flux quanta can move at speeds of 10 to 15 km / s. This opens access to research on the rich physics of unbalanced collective systems and generates an Nb-C direct write superconductor as candidate material for individual photon detectors. The results are published in Nature’s Communications.

Superconductivity is a physical phenomenon that occurs at low temperatures in many materials that manifests itself through a disappearing electrical resistance and the expulsion of magnetic fields from within the material. Superconductors are already used for medical imaging, fast digital circuits, or sensitive magnetometers and have great potential for other applications. However, the conductivity of most technologically important superconductors is not “super”. In these so-called Type II superconductors, an external magnetic field penetrates the material in the form of quantized lines of magnetic flux. These flow lines are known as Abrikosov vortices, named after Alexei Abrikosov, whose prediction brought him the Nobel Prize in Physics in 2003. Already at moderately strong electric currents, the vortices begin to move and the superconductor can no longer transport the current without resistance.

In most superconductors, a low resistance state is limited by vortex speeds of the order of 1 km / s that set practical limits for the use of superconductors in various applications. At the same time, such speeds are not high enough to address the rich generic physics of unbalanced collective systems. Now, an international team of scientists from the University of Vienna, the Goethe University in Frankfurt, the RAS Institute for Microstructures, the National V. Karazin University in Kharkiv, the B. Verkin Institute for Physics and Engineering of Low Temperature NAS has found a new superconducting system in which magnetic flux quanta can move at speeds of 10 to 15 km / s. The new superconductor exhibits a rare combination of properties: high structural uniformity, high critical current, and rapid relaxation of heated electrons. The combination of these properties ensures that the phenomenon of flux flow instability (abrupt transition of a superconductor from the low resistance state to the normal conduction state) occurs at sufficiently large transport currents.

“In recent years, experimental and theoretical work has emerged that points to a remarkable topic; it has been argued that current vortices can move even faster than superconducting charge carriers,” says Oleksandr Dobrovolskiy, lead author of the recent publication in Nature’s Communications and head of the Laboratory of Superconductivity and Spintronics of the University of Vienna. “However, these studies used locally non-uniform structures. Initially, we worked with high-quality clean films, but later it turned out that dirty superconductors are better candidate materials to support ultrafast vortex dynamics. Although the intrinsic fixation in these is not necessarily as Weak as in other amorphous superconductors, rapid relaxation of heated electrons becomes the dominant factor allowing for ultra-fast vortex motion. “

For their research, the researchers manufactured an ion beam-induced deposition Nb-C superconductor focused on Professor Michael Huth’s group at Goethe University in Frankfurt am Main, Germany. Surprisingly, in addition to the ultra-fast vortex speeds in Nb-C, direct-write nanofabrication technology enables one to fabricate complexly shaped nano-architectures and 3-D fluxonic circuits with intricate interconnectivity that can find application in processing quantum information.

Challenges for ultrafast vortex matter investigations

“To achieve the maximum current that a superconductor can carry, the so-called separation current, fairly uniform samples are needed on a macroscopic length scale that is due in part to small defects in a material. Achieving the separation current is not just a Fundamental problem, but it is also important for applications; a one micrometer wide superconducting strip can be switched to a resistive state by a single near infrared or optical photon if the strip is polarized by a current close to the value of the separation current, as predicted and confirmed in recent experiments. This approach opens perspectives for the construction of large area single photon detectors that could be used, for example, in confocal microscopy, free-space quantum cryptography, deep-space optical communication, “says Denis Vodolazov, Principal Investigator, RAS Institute of Microstructures, Russia.

The researchers successfully studied how fast vortices can move in dirty Nb-C superconducting strips that have a critical current in a zero magnetic field near the decay current. Their results indicate that the instability of the flow flow begins near the edge where the vortices enter the sample due to the locally improved current density. This provides information on the applicability of widely used flux flux instability models and suggests that Nb-C is a good candidate material for rapid single photon detectors.