Superinsulators, the Reverse of Superconductors

Well, we know how to turn materials into superconductors; it is about the time to learn how to create superinsulators. We cannot have one without the other, can we? In fact, superinsulators are just the opposite of superconductors. While superconductors experience zero or close to zero electrical resistance, superinsulators should have infinite or a very large electrical resistance, basically retaining electrical charge for an indefinite amount of time.

In order to create such a material, a team of researchers from Regensburg University, along with colleagues from Argonne National Laboratory, started experimenting with titanium nitride films, which were cooled at close to absolute zero temperatures while being placed in a magnetic field. Most materials, thus titanium nitride as well, experience superconductivity while being cooled at close to absolute zero temperatures; however, in the conditions recreated by the team, the film's electrical resistance raised to infinity.

"In the 1990s it became apparent in a number of measurements that a quantum phase transition – that is, a transition between two ordered states at zero Kelvin – is a great place to look for new kinds of ordered states. This research seems to be quite an unexpected and beautiful example of this: a superinsulator on the boundary between the ordinary insulator and the superconducting ground state," said low-temperature physicist at the University of Toronto, Stephen Julian.

Puddles of charge

Superconductivity is available for a certain material when the electrons inside it bind into pairs, called ‘Cooper pairs’ by physicists, and behave as a single entity. This is available for bulk material, however if the superconductor is shaped into a granular film, the Cooper pairs will turn to puddles isolated from each other by separation areas called Josephson junctions. The electron pair can only pass from one superconducting puddle to the other through quantum tunneling effects.

If the temperature is dropped even more, the Josephson junctions may block the free flow of charge, thus that of electrical current. However, by applying a magnetic field measuring 0.9 Tesla, the titanium nitride film may retain this property even when heated at temperatures of 70 mK.

Researchers believe that, in order to retain the superinsulating properties of the material at even higher temperatures, they would have to somehow reverse the roles of charge and magnetic flux. Magnetic fields create vortices inside the superconducting material, which rotate in opposite directions, thus enabling the free circulation of Cooper pairs between puddles by tunneling through the vortices.

On the other hand, in superinsulators vortices Cooper pairs of opposite charge circulate, thus disabling electrical current traffic. "A superinsulator cannot appear at all without the existence of superconductivity in the same film. That is why we refer to the superinsulator as the reverse of superconductivity," said Valerii Vinokur from the Argonne National Laboratory.

Ideal batteries

Vinokur believes that, with the help of such superinsulators, ideal batteries may be built – devices that store charge for indefinite amounts of time. "It is still a long way to commercial devices. However, as usual, the speed of technological development is hard to predict," added Vinokur.

"Their theoretical interpretation is still under heavy dispute. It seems to me that the community needs a little time to digest this stuff," pointed Paul Mueller from Erlangen-Nuernberg University.


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