February 13, 2013

Nanostructured nanowires appears to solve loss of superconductivity in very strong magnetic fields

Researchers have discovered a way to efficiently stabilize tiny magnetic vortices that interfere with superconductivity—a problem that has plagued scientists trying to engineer real-world applications for decades. The discovery could remove one of the most significant roadblocks to advances in superconductor technology.

When magnetic fields reach a certain strength, they cause a superconductor to lose its superconductivity. But there is a type of superconductor—known as “Type II”—which is better at surviving in relatively high magnetic fields. In these superconductors, magnetic fields create tiny whirlpools or “vortices.” Superconducting current continues to travel around these vortices to a point, but eventually, as the magnetic field strengthens, the vortices begin to move about and interfere with the material’s superconductivity, introducing resistance.

“These vortices dissipate the energy when moving under applied currents and bury all hopes for a technological revolution—unless we find ways to efficiently pin them,” said Argonne Distinguished Fellow Valerii Vinokur, who co-authored the study.

Scientists have spent a lot of time and effort over the past few decades trying to immobilize these vortices, but until now, the results have been mixed. They found ways to pin down the vortices, but these only worked in a restricted range of low temperatures and magnetic fields.

Vinokur and his colleagues, however, discovered a surprise. They began with very thin superconducting wires—just 50 nanometers in diameter. (A stack of 2,000 of these wires would equal the height of a sheet of paper.) These thin wires can accommodate only one row of vortices. When they applied a high magnetic field, the vortices crowded together in long clusters and stopped moving. Increasing the magnetic field restored the material’s superconductivity, instead of destroying it.

Next, the team carved superconducting film into an array of holes so that only a few vortices could squeeze between the holes, where they stayed, unable to interfere with current.

The resistance of the superconductor dropped dramatically—at temperatures and magnetic fields where no one has been able to pin vortices before. “The results were quite striking,” Vinokur said.

The team has only experimented with low-temperature superconductors so far, Vinokur said, “but there is no reason why the approach we used should be restricted to just low-temperature superconductors.”

This mosaic represents the distribution of superconductivity around holes (white) in a thin sheet of superconducting film. Green indicates strong superconductivity. Further away from the holes, the superconductivity decreases (yellow, red and finally black, where the material is densely populated with vortices that interfere with superconductivity.

Nature Communications- Magnetic field-induced dissipation-free state in superconducting nanostructures

A superconductor in a magnetic field acquires a finite electrical resistance caused by vortex motion. A quest to immobilize vortices and recover zero resistance at high fields made intense studies of vortex pinning one of the mainstreams of superconducting research. Yet, the decades of efforts resulted in a realization that even promising nanostructures, utilizing vortex matching, cannot withstand high vortex density at large magnetic fields. Here, we report a giant reentrance of vortex pinning induced by increasing magnetic field in a W-based nanowire and a TiN-perforated film densely populated with vortices. We find an extended range of zero resistance with vortex motion arrested by self-induced collective traps. The latter emerge due to order parameter suppression by vortices confined in narrow constrictions by surface superconductivity. Our findings show that geometric restrictions can radically change magnetic properties of superconductors and reverse detrimental effects of magnetic field.

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