Scientists Solve Mystery of Superinsulators

The opposite of superconductivity might lead to strange new circuits

Photo: Argonne National Laboratory

Well Chilled: Superinsulators like titanium nitride show their stuff only when cooled to nearly absolute zero.

In 2008 a team of physicists from Argonne National Laboratory, in Illinois, and other institutions stumbled upon an odd phenomenon. They called it superinsulation, because in many ways it was the opposite of superconductivity. Now they’ve worked out the theory behind it, potentially opening the doors to better batteries, supersensitive sensors, and strange new circuits.

Superconductors lose all resistance once they fall below a certain temperature. In superinsulators, on the other hand, the resistance to the flow of electricity becomes infinite at very low temperatures, preventing any flow of electric current.

Valerii Vinokur of Argonne and Tatyana Baturina from the Institute of Semiconductor Physics, in Novosibirsk, Russia, discovered superinsulators when the pair chilled a thin film of titanium nitride to nearly absolute zero and tried to send a current through it. They found that the resistance shot up to 100 000 times its original level. The effect vanished at higher temperatures. The researchers also noticed that the effect was sensitive to the strength of a magnetic field; as they increased the strength of an external magnetic field, the resistance disappeared.

Vinokur and his colleagues say the effect could make new kinds of batteries possible. In most batteries, there is a certain amount of leakage when the battery is left exposed to air, because air is not a perfect insulator. Thus, an unused battery eventually drains. ”If you pass a current through a superconductor, then it will carry the current forever; conversely, if you have a superinsulator, then it will hold a charge forever,” says Vinokur. In fact, he points out, a device made from superconductors and superinsulators might lose no heat at all during operation.

Vinokur, Baturina, and Nikolai Chtchelkatchev, a theoretical physicist from the Moscow Institute of Physics and Technology, who also has an affiliation with Argonne, recently worked out a theory of how superinsulation works at microscopic scales, which they reported this past December in Physical Review Letters. They say that superinsulation, like superconductivity, is caused at low temperatures by electrons that form what are known as Cooper pairs. In a superconductor the pairs move together collectively, which means there is no resistance to impede the flow of current. In a superinsulator, on the other hand, the Cooper pairs repel one another, and thus prevent any current from flowing.

Vinokur says that while the qualitative picture of the phenomenon emerged in 2008, it was only recently that the team succeeded in working out the first set of detailed calculations.

Eugene Chudnovsky, an expert on superconductivity and a physics professor at Lehman College of the City University of New York, says that the nature of superinsulators has been hotly debated by physicists and that the theory by Vinokur and the Russian physicists is promising. Gergely Zimanyi, a physics professor at the University of California, Davis, says the theoretical physics community has ”extremely high respect” for Vinokur’s work.

So far the theory and experiments have been confined to thin films of titanium nitride, says Vinokur, who intends to investigate other compounds at higher temperatures next. ”There are still plenty of unresolved questions,” he says. ”We can only remind [ourselves] that the microscopic theory of superconductivity appeared almost 50 years after the discovery. In this respect, we are proceeding incredibly fast.”

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