Superconductivity roadblock breakthrough

14 February 2013
By Edward Gent
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This mosaic represents the distribution of superconductivity around holes, marked in white, in a thin sheet of superconducting film. Green indicates strong superconductivity. Further away from the holes, the superconductivity decreases, from yellow to red and finally to black, where the material is densely populated with vortices that interfere with superconductivity

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

Researchers believe they have cracked one of the major roadblocks to applying superconductor technology to the real world.

A team of researchers from Russia, Spain, Belgium, the UK and the US Department of Energy's (DOE) Argonne National Laboratory have announced they have discovered a way to efficiently stabilise tiny magnetic vortices that interfere with superconductivity.

The problem has plagued scientists trying to engineer real-world applications for decades and the discovery could remove one of the most significant obstacles to advances in superconductor technology.

When magnetic fields reach a certain strength they cause a superconductor to lose its superconductivity. There is a type of superconductor, known as "Type II", which are better at surviving in relatively high magnetic fields.

In these materials, magnetic fields create tiny whirlpools or "vortices" and while superconducting current continues to travel around these vortices to a point, 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 tried to immobilise these vortices for decades, but until now, they had only found ways to pin down the vortices in a restricted range of low temperatures and magnetic fields.

However, Vinokur and his colleagues discovered a surprise when using very thin superconducting wires just 50 nanometers in diameter which 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, though the team has only experimented with low-temperature superconductors so far.

"The results were quite striking," Vinokur said. “There is no reason why the approach we used should be restricted to just low-temperature superconductors."

The paper, 'Magnetic field-induced dissipation-free state in superconducting nanostructures', is published this week in Nature Communications.

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