For materials that carry electricity without resistance, a little nanotechnology turns a major turnoff into a turn-on, says a team of researchers. Ordinarily, a magnetic field quashes the currents flowing freely through a superconductor. But when decked out in tiny magnetic dots, a superconductor may behave just the opposite way and carry electricity freely only when exposed to a magnetic field, the team reports in the 16 May PRL. Their technique might someday boost the current-carrying capacity of superconducting wires, or set the bits in quantum computers.
Usually, superconductivity and magnetic fields get along about as well as a pair of squabbling three-year-olds. When made sufficiently strong, a magnetic field disrupts the resistance-free flow of electricity and turns a superconductor into an ordinary conductor. This effect limits the amount of free-flowing current a superconductor can carry, as the current itself produces a magnetic field, which can grow large enough to scramble the superconductivity. For years, physicists have known, however, that a few exotic compounds have a paradoxical penchant for magnetic fields and will conduct electricity freely only when subjected to one–a phenomenon known as field-induced superconductivity.
But any superconductor can be made to pull off that trick if it is studded with little magnetic dots, say Martin Lange, Victor Moshchalkov, and their colleagues from the Catholic University of Leuven in Belgium. To prove it, the researchers studied microchips covered with films of lead, which becomes a superconductor when cooled within a few degrees of absolute zero. On top of the lead, they spread a grid of tiny magnetic dots, each measuring 800 nanometers across and separated from its neighbors by 1.5 micrometers. The magnetization of the dots could point either up or down. If it pointed up, the dots created a field pattern that pointed up through the lead directly beneath each dot and then curled around to point down through the lead in between the dots. These fields killed the superconductivity in the film, just as expected.
Then the researchers applied an external magnetic field that also pointed up through the film. Between the dots this external field cancelled the field from the dots themselves, allowing the superconductivity to switch on and the film to carry electricity without resistance. Of course, directly under the dots, the external field reinforced the one that was already there and further suppressed the superconductivity, leaving little islands where current was inhibited. “It’s like a Swiss cheese,” Moshchalkov says. “You have holes. But luckily for us, a lot of [superconducting] material remains.”
The magnetic dots might someday be incorporated into superconducting wires to counter the fields the wires themselves produce and thus increase the amount of current they can carry, Moshchalkov says. Or they might be used to set up currents flowing both ways at once around tiny superconducting rings. Such rings might someday server as the bits in ultra-fast quantum computers.
Those applications may be years away, but the technique already demonstrates the tremendous potential of combining disparate materials, such as magnets and superconductors, with nanotechnology, says Francesco Tafuri of the Italian National Institute for the Physics of Matter and the Second University of Naples. “It’s the best combination of two things that probably hold the keys to the future,” Tafuri says, “hybrid systems and nanostructures.”