Nanodots. Atomic force microscope picture of the palladium-cobalt dots.

Superconductors and Magnets Learn to Get Along

Magnetic fields normally ruin superconductivity, the highly sought-after ability to conduct electricity without resistance. Because any electrical current generates a magnetic field, this roadblock greatly limits the practicality of harnessing the electron superhighway effect. Now a study finds that miniature magnetic dots--each just billionths of a meter wide--may help superconductors resist even the strongest synthetic magnetic fields. The technique could expand the range of substances used in supercomputing applications, and it might allow scientists to create magnetic scanners more powerful than before.

Superconductivity relies on delicate couples of electrons known as Cooper pairs, which are disappointingly easy to break apart by magnetic fields. Even if a device with superconducting wires were shielded from magnets, any electricity flowing through the system would generate an unwanted magnetic field. In very rare instances, however, magnetic fields can trigger superconductivity rather than being its downfall. Until now this unusual effect has been seen in only two compounds among hundreds of superconductors. Researchers at the University of Leuven in Belgium now show that lattices of tiny magnets can, counterintuitively, make any superconductor magnet-activated.

The researchers implanted arrays of magnetic palladium-cobalt dots on a superconducting film of lead, lining up the dots' magnetic poles so they all pointed the same way, explains team leader and physicist Victor Moshchalkov. When a magnetic field is applied perpendicular to the magnetic dots, superconductivity is triggered wherever the two fields cancel each other out, although superconductivity remains thwarted wherever the fields align. Moshchalkov compares the effect to Swiss cheese: "You have holes, of course, but you have a lot of cheese in between." Moshchalkov and his colleagues have published their findings on the online preprint server arXiv and submitted them to Physical Review Letters.

The results are "simple and beautiful," says experimental solid-state physicist Luis Balicas of Florida State University in Tallahassee. He says the study helps confirm theories that such magnet-induced superconductivity in previously discovered materials was linked to magnetized elements within them that aid the superconductors much as the nanodots do. And the finding may also help researchers circumvent superconductivity interference from magnetic fields induced by electric currents. Positioning the dots even closer together would enable engineers to make superconductors more magnet-tolerant. These could be used to build superconducting power lines, or even extremely strong magnets, which Balicas says could "open frontiers" in magnetic scanning of everything from gene structure to brain activity.

Related sites
A preprint of the paper
Superconductor history

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