A research team has made the first direct visual observation and measurement of ultrafast vortex dynamics in superconductors. The technique used, detailed in the journal Nature Communications, could contribute to the development of novel practical applications by optimizing superconductor properties for use in electronics, according to the scientists.
Superconductivity is, generally speaking, suppressed in the presence of magnetic fields, limiting the ability to use the materials in real life applications. A certain family of superconductors, called type 2, can withstand much higher values of magnetic fields thanks to an ability to allow the magnetic field to thread through the material in a quantized manner, in a local tubular-shaped form called a vortex. Unfortunately, in the presence of electric currents these vortices experience a force and may begin to move. Motion of vortices allows for electrical resistance, again posing an obstacle for applications.
Understanding when and how vortices will move or remain localized is the focus of much scientific research, but addressing the physics of fast moving vortices experimentally has proven extremely challenging, mainly because of the lack of adequate tools. Now an international team of researchers, led by professor Eli Zeldov from the Weizmann Institute of Science and Dr. Yonathan Anahory, senior lecturer at the Hebrew University of Jerusalem’s Racah Institute of Physics, has shown for the first time how these vortices move in superconducting materials and how fast they may travel.
The team used a novel microscopy technique called scanning SQUID-on-tip, that allows magnetic imaging at unprecedented high resolution (about 50 nm) and magnetic sensitivity. Using the microscope, the scientists observed vortices flowing through a thin superconducting film at rates of tens of GHz, and traveling at velocities much faster than previously thought possible, up to about 72,000 km/hr (45,000 mph). The velocitiy is not only much faster than the speed of sound, but also exceeds the pair-breaking speed limit of superconducting condensate, meaning that a vortex can travel 50 times faster than the speed limit of the supercurrent that drives it.
The vortex trajectories appear in photos and video as smeared lines crossing from one side of the film to another, similar to the blurring of images in photographs of fast-moving objects. The images show a tree-like structure with a single stem that undergoes a series of bifurcations into branches. This channel flow is quite surprising since vortices normally repel each other and try to spread out as much as possible. Here vortices tend to follow each other, which generates the tree-like structure.
A team of theoretical physicists from the USA and Belgium, led by Professors Alexander Gurevich and Milorad Miloševic, partially explained this finding by the fact that when a vortex moves, the appearance of resistance locally heats the material, which makes it easier for following vortices to travel the same route.
“This work offers an insight into the fundamental physics of vortex dynamics in superconductors, crucial for many applications,” says Dr. Lior Embon, who was, at the time, the student in charge of this study. “These findings can be essential for further development of superconducting electronics, opening new challenges for theories and experiments in the yet unexplored range of very high electromagnetic fields and currents.”
“The research shows that the SQUID-on-tip technique can address some outstanding problems of non-equilibrium superconductivity, ultrafast vortices and many other magnetic phenomena at the nanometer scale,” says Dr. Yonathan Anahory, senior lecturer at the Hebrew University’s Racah Institute of Physics.