Superconducting nanowires help in study of phase transistors

By creating superconducting nanowires using carbon nanotube molecules, researchers at the University of Illinois are investigating just how small a wire can become and remain a superconductor.

The answer could prove useful in applications such as supercomputing, where short superconducting wires can connect circuit elements.

"The phenomenon of superconductivity depends on the phase coherence of the condensate," said Alexey Bezryadin, a UI professor of physics.

"But for small systems, such as ultrathin wires, the phase is a quantum variable which may or may not have a definite value, corresponding to both superconducting and insulating states."

To study these quantum effects, Bezryadin and colleagues at Harvard University developed a technique that creates tiny superconducting wires from carbon nanotubes. The nanotubes serve as scaffolds, on which uniform superconducting films of molybdenum-germanium alloy are deposited.

When the nanowires are placed across a narrow slit etched in a silicon chip, the researchers can apply a voltage and measure the resulting current.

"The molybdenum-germanium films have a sharp superconducting transition, and show no signs of granularity down to a thickness of about one nanometer," Bezryadin said. "By changing how much material is deposited, we can make wires of different diameters and study important phase transitions between superconducting and insulating states."

Because the proximity of normal metals also can alter a superconducting system's properties, it's nearly impossible to distinguish between current that flows through the nanotube and current that flows through the film deposited on the tube, Bezryadin said.

"One concern in our previous studies was that the metallic carbon nanotube itself was contributing to the current that we measured."

Recent measurements by Bezryadin and UI graduate research assistant Anthony Bollinger are shedding new light on superconducting phase transitions while also clarifying earlier findings.

The researchers fabricated superconducting nanowires from specially prepared, fluorinated carbon nanotubes prepared by chemist John Margrave at Rice University.

Unlike typical carbon nanotubes, Margrave's fluorinated nanotubes are insulators. "With these non-conducting nanotubes, we have no doubt that the current we measure is flowing through the molybdenum-germanium film and not through the carbon scaffold," Bezryadin said.

"The fluorotubes also appear to allow the fabrication of even thinner nanowires."