Recent research from the U.S. Department of Energy’s Argonne National Laboratory has revealed that a novel form of superconductivity the researchers call “critical superconductivity” may be accessed in a special régime lying at the boundary between type I and type II superconductivity. Inside this régime, small superconducting objects called Abrikosov vortices obey the same laws that participate in shaping the deep structure of the universe, which means that scientists could probe for novel aspects of this structure by simply manipulating the behavior of these vortices in the laboratory.
“Vortices control the current carrying ability of all superconductors,” said Argonne Distinguished Fellow and materials scientist Valerii Vinokur, a member of the international team of researchers who made the discovery. “At the same time, these objects can mimic cosmic strings, elucidating concepts that address fundamental features of the evolution of our universe. Our experiment on the intriguing behavior of vortices in mesoscopic lead crystals opens new horizons for probing cosmological models in bench-top laboratory experiments.”
Scientists have long known that something special happens when a type I superconductor changes to type II in the presence of an external magnetic field, but they conceptualized this as being limited to a single point called the Bogomolny critical point. This misconception arose because the superconducting materials under study were very large in size. Recent advances in vortex measurement allowed the researchers to study lead crystals that were mesoscopic (between microscopic and macroscopic in size), so the crystals were about the same size as the vortices the researchers were studying.
All superconducting materials offer no resistance to electrical currents passing through them at sufficiently low temperatures. Differences arise when a superconducting material is placed in an external magnetic field. A large-scale type I superconductor will prevent the magnetic field from penetrating it by creating a counteracting magnetic field of its own through the agency of a supercurrent near its boundary. A type II superconductor will react similarly except that it will additionally develop Abrikosov vortices inside, each consisting of a supercurrent encircling a tiny normal phase core; the normal phase filaments will permit penetration by the external magnetic field.
In contrast, an external magnetic field will cause a somewhat small sample of a type I superconductor to exhibit an intermediate state consisting of alternating domains of normal and superconducting phases. As the type I superconductor transitions to type II, the intermediate state becomes unstable, with the domains changing into a lattice of Abrikosov vortices. At the mesoscopic scale, only a single domain forms, which then becomes a single unstable giant vortex. The researchers studied the behavior of these giant vortices using triangular lead crystals that were about 2.2 µm on a side and about 0.7 µm thick. They caused the type I crystals to transition to type II by inducing changes in temperature and studied the transient behavior of the vortices by varying the strength of the external magnetic field as well. The researchers found that in a decreasing magnetic field, individual vortices sequentially detached from the giant vortex and eventually escaped from the lead samples. Most importantly, the behavior of the vortices strictly obeyed the Abelian Higgs model of quantum field theory.
The research has possible practical consequences as well. “Our discovery of unique vortex behavior in critical superconductors can lead to new avenues for controlling and enhancing the performance of emergent mesoscopic superconducting devices,” Vinokur said.
Also involved in the research were scientists from the University of Picardie, Stockholm University, the University of Antwerpen, and the University of Bath. Funding for the research came from the U.S. Department of Energy’s Office of Science.
An article describing the research, “Rayleigh instability of confined vortex droplets in critical superconductors,” appeared online in Nature Physics on November 10.
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