An international team of researchers has discovered what it describes as the reverse side of a superconductor — a “superinsulator” that indefinitely retains electrical charge.

Christoph Strunk of Regensburg University in Germany, whose team includes Valerii Vinokur of Argonne National Laboratory in the US and other colleagues from Germany, the US and Belgium, found the state in thin films of titanium nitride cooled towards absolute zero in a magnetic field. Although the material is usually a superconductor, in which electrical current can propagate without resistance, the team have found that in these conditions the material’s resistance rises to infinity (Nature 452 613).

“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,” says Stephen Julian, a low-temperature physicist at the University of Toronto, Canada. “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.”

Superconducting ‘puddles’

In a superconductor, the lack of resistance arises because electrons bind together into pairs called Cooper pairs. These pairs act collectively, moving as single quantum entity. When a superconducting material is flattened into a granular film, however, the entity becomes partitioned. Strong disorder forces the Cooper pairs into isolated “puddles” separated by insulating regions known as Josephson junctions, and individual Cooper pairs can only pass between puddles by quantum tunnelling.

Physicists have previously found that, very near to absolute zero, the insulating regions can become clogged with charge, blocking the flow of current. But Strunk, Vinokur and colleagues have found that, given a magnetic field of 0.9 T, their films of titanium nitride persist with this zero-conduction state as warm as 70 mK.

To explain this superinsulation, in which current is blocked even at finite temperature, the team has suggested the roles of charge and magnetic flux become mirrored. In the superconducting phase, a magnetic field penetrates the material in quanta called vortices, which rotate in alternate directions. The Cooper pairs are free to circulate the vortices by tunnelling between puddles.

But in the superinsulating phase, the roles of charge and vortices are swapped. Vortices circulate bound pairs of opposite charge, which prevents a current from flowing. “A superinsulator cannot appear at all without the existence of superconductivity in the same film,” explains Vinokour. “That is why we refer to the superinsulator as the reverse side of superconductivity.”

Storing charge

Vinokour told that the phenomenon could be exploited to make “ideal” batteries, because a superinsulating material — as well as blocking the transit of charge — would never let charge escape. “It is still a long way to commercial devices,” he says. “However, as usual, the speed of technological development is hard to predict.”

Nevertheless, the team might face some more immediate problems in getting their work accepted by other condensed-matter physicists. “[Their] theoretical interpretation is still under heavy dispute,” says Paul Mueller of Erlangen–Nuernberg University in Germany. “It seems to me that the community needs a little time to digest this stuff.”