Instead of the usual triangle pattern,
vortices in a tiny superconducting disk
can form a more complicated pattern:
or merge into a giant vortex.
IN SEBASTIAN JUNGER’S 1997 BESTSELLER, THE PERFECT STORM, two storms merge to form a gargantuan cyclone. Now, physicists have spotted the quantum-mechanical equivalent: the merging of several tiny whirlpools of current in a superconductor into a single “giant vortex.” Fulfilling a decades-old prediction, the observation may foreshadow stranger things to come and help lay the groundwork for the budding field of”fluxonics.”
Researchers have had indirect evidence of the giant vortices (actually less than a micrometer across) and have been striving to image them with sophisticated scanning techniques. But Akinobu Kanda of the University of Tsukuba, Japan, Ben Baelus of the University of Antwerp, Belgium, and colleagues have taken a shortcut to the first direct evidence for the jumbo swirls, as they report in the 17 December Physical Review Letters. “It’s clever,” says Simon Bending, a physicist at the University of Bath U.K. “In hindsight, I don’t know why we didn’t do this.”
Whirlpools of current arise when a magnetic field penetrates a superconductor, in which current flows without loss of energy. The mdgnetic field threads the eyes of the vortices and thanks to quantum mechanics each vortex contains precisely one fundamental quantum of magnetic flux. The vortices repel one another, so they arrange themselves in a triangular pattern. If the superconductor is tiny, however, the cramped vortices should form more exotic patterns and even merge into one jumbo vortex containing several flux quanta, according to the prevailing Ginzburg- Landau theory of superconductivity.
Since the 1990s, physicists had found indirect evidence of the giant vortices by studying the magnetization of a tiny superconducting disk in a varying magnetic field, among other techniques. But they inferred the current distribution from computer simulations. Kanda and colleagues probed the currents directly, by placing two tiny electrodes called “tunnel junctions” on the edge of a 1.5-micro-meter-wide aluminum disk 120 degrees apart. They measured the voltage from each junction to a third electrode 120 degrees from each of the other two. The voltages depended on the currents flowing under the tunnel junctions. So if the disk contained a single, symmetrical giant vortex, the two voltages should go up and down together as the magnetic field through the disk changed slightly. If the disk contained a less symmetrical pattern of several vortices, the two voltages should change independently.
The researchers ramped up the magnetic field so that the disk contained several flux quanta and then varied the field to change number. Each time the number changed, the two voltages jumped, which allowed the experimenters to keep the tally as they looked for the subtler signals. In the relatively quiescent times between some jumps, the two voltages went their own ways, indicating several vortices. In between others, the voltages varied in parallel indicating a single vortex. Thus, the researchers demonstrated the merging of individual vortices into one big vortex.
“This evidence is probably 10 times stronger than before,” says Andre Geim of the University of Manchester, U.K., who performed the magnetization measurement. Victor N4oshchalkov of Catholic University of Leuven in Belgium says the experiment is a step toward observing even stranger vortices, including ones containing fractional flux: “There’s a lot of new physics coming up.”
In the meantime, Kanda hopes to use the technique to monitor and control the positions of vortices in so-called fluxonic devices. Whereas electronic microchips shuttle electrons, fluxonic chips would shuttle vortices, so that information would literally swirl through them.
17 December 2004, Vol 306 (pg.2021
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