Magnetism and superconductivity are often thought to be incompatible with each other. However, Martin Lange and colleagues from the Catholic University of Leuven in Belgium have now shown that a nanoengineered field, in the form of tiny magnetic dots, can actually enhance superconductivity instead of destroying it (M Lange et al. 2003 Phys. Rev. Lett. 90 197006).
According to the Bardeen-Cooper-Schreiffer theory of superconductivity, electrons with opposite spins form pairs that can move through a material without resistance. A magnetic field can destroy superconductivity in two ways: by breaking up the electron pair, or by trying to make both of the electron spins point in the same direction. These effects also limit how much current can flow through the superconductor because of the disruptive effect of the magnetic field produced by the current itself.
Until now, only a few compounds remained superconducting under the influence of an applied magnetic field. Moreover, the number of materials in which an applied field could actually induce superconductivity – by the so-called magnetic field induced superconductivity effect – were very few.
Lange and co-workers placed a layer of cobalt-palladium ferromagnetic dots, each 800 nanometres in diameter and separated by 1.5 micrometres, on top of a superconducting thin film made of lead. Each dot produces a stray magnetic field that destroys the superconductivity in the thin film. The researchers then applied an external magnetic field, which enhanced the destructive effect of the dots’ magnetic field in the area directly beneath the dots and, to compensate, reduced it everywhere else in the film. The overall effect was an increase in the current carried by the superconductor.
“In fact, the technique can be compared with an array of small sponges, which are already a bit wet, put on a wet floor: the sponges will absorb the water,” said team leader Victor Moshchalkov. “The floor between the sponges will become dry at the expense of having more water in and under the sponges.”
This new ‘field compensation effect’ is not restricted to specific superconductors, the researchers say, so magnetic field induced superconductivity could be achieved in any superconducting thin film. The team believes that using nanodots and nanopillars, which have larger stray fields, could allow superconducting materials to remain in higher magnetic fields. The nano-dot array could also be used to design logical devices for use in quantum computers.
