A new way towards super-fast motion of vortices in superconductors
discovered

Date:
July 6, 2020
Source:
University of Vienna
Summary:
An international team of scientists has found a new superconducting
system in which magnetic flux quanta can move at velocities of 10-15
km/ s. This opens access to investigations of the rich physics of
non- equilibrium collective systems and renders a direct-write Nb- C
superconductor as a candidate material for single-photon detectors.



FULL STORY ========================================================================== Superconductivity is a physical phenomenon occurring at low temperatures
in many materials which manifests itself through a vanishing electrical resistance and the expulsion of magnetic fields from the material's
interior.

Superconductors are already used for medical imaging, fast digital
circuits or sensitive magnetometers and hold a great potential for
further applications.

However, the conductivity of the majority of technologically important superconductors is in fact 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 flux 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 carry the current without resistance.


==========================================================================
In most superconductors, a low-resistive state is limited by vortex
velocities of the order of 1 km/s setting the practical limits of use of superconductors in various applications. At the same time, such velocities
are not high enough to address the rich physics generic to nonequilibrium collective systems. Now, an international team of scientists from the University of Vienna, the Goethe University Frankfurt, the Institute for Microstructures of RAS, the V. Karazin National University of Kharkiv,
the B. Verkin Institute for Low Temperature Physics and Engineering of NAS
has found a new superconducting system in which magnetic flux quanta can
move at velocities of 10-15 km/s. The new superconductor exhibits a rare combination of properties -- high structural uniformity, large critical
current and fast 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-resistive to the normal conducting state -- takes place at sufficiently large transport currents.

"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 recent publication in
Nature Communications and head of the Superconductivity and Spintronics Laboratory at 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 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." For their investigations the researchers fabricated a Nb-C superconductor by focused ion beam induced deposition in the group of Prof. Michael Huth at the Goethe University
in Frankfurt am Main, Germany. Remarkably, in addition to ultra-fast
vortex velocities in Nb-C, the direct-write nanofabrication technology
allows one to fabricate complex-shaped nano-architectures and 3D fluxonic circuits with intricate interconnectivity which may find application in
quantum information processing.

Challenges for investigations of ultra-fast vortex matter "In order
to reach the maximum current that a superconductor can carry, the so-
called depairing current, one needs rather uniform samples over a
macroscopic length scale which is partially owed to small defects in
a material. Reaching the depairing current is not only a fundamental
problem, but it is also important for applications; a micrometer-wide superconducting strip can be switched to a resistive state by a single
near infrared or optical photon if the strip is biased by a current
close to the depairing current value, as was predicted and confirmed
in recent experiments. 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.

The researchers successfully studied how fast vortices can move in dirty
Nb- C superconducting strips having a critical current at zero magnetic
field close to the depairing current. Their results indicate that the
flux flow instability starts near the edge where vortices enter the sample because of the locally enhanced current density. This offers insights into
the applicability of widely used flux flow instability models and suggest
Nb-C to be a good candidate material for fast single-photon detectors.


========================================================================== Story Source: Materials provided by University_of_Vienna. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. O. V. Dobrovolskiy, D. Yu Vodolazov, F. Porrati, R. Sachser,
V. M. Bevz,
M. Yu Mikhailov, A. V. Chumak, M. Huth. Ultra-fast vortex motion
in a direct-write Nb-C superconductor. Nature Communications,
2020; 11 (1) DOI: 10.1038/s41467-020-16987-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200706100822.htm

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