As described in the Jan. 14 issue of the journal Physical Review Letters, researchers at the University of Illinois at Urbana-Champaign have created high-quality superconducting wires with molecular dimensions, and measured their behavior in magnetic fields of various strengths. The observational results have confirmed that theories developed for bulk superconductors also apply to molecular-scale superconductors.

"Our experimental results show an excellent agreement with the theory of pair-breaking perturbations, even at high magnetic fields," said Alexey Bezryadin, a professor of physics at Illinois. "The theory takes into account both spin and orbital contributions."

To study this phenomenon, the researchers began by placing a single-wall carbon nanotube across a narrow trench (about 100 nanometers wide) etched in the surface of a silicon wafer. The nanotube was then coated with a thin film of superconducting material (molybdenum-germanium), chilled below its critical temperature, and its properties measured in the presence of a magnetic field.

"Usually, when you apply a magnetic field to a superconductor, the field suppresses or even destroys the superconductivity," Bezryadin said. "The magnetic field pulls apart the two electrons forming Cooper pairs and also rotates their spins. As the superconductor becomes smaller, however, the destructive effects of the magnetic field become weaker."

The magnetic field showed a remarkably weak effect on nanowires, the researchers report. Both the orbital and the spin pair-breaking effects were strongly suppressed in the nanowires. The orbital effect was weak because of the small dimensions of the wire (ab
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Contact: James E. Kloeppel, Physical Sciences Editor
kloeppel@uiuc.edu
217-244-1073
University of Illinois at Urbana-Champaign
4-Feb-2005