Nanowires Demonstrate Quantum Interference Phenomenon


6/1/2005

Strands of DNA have been used as tiny scaffolds to create superconducting nanodevices that demonstrate a new type of quantum interference phenomenon. These nanowire quantum interference devices (NQIDs), which comprise a pair of suspended superconducting wires as thin as three to four molecular diameters (5 nm to 15 nm), could be used to measure magnetic fields and to map out the phase of the order parameter in regions of superconductivity.

The image at the left shows an artist's conception of the NQID. Two strands of DNA have been suspended over a trench cut in a substrate and then sputtered with a superconducting alloy to create the nanowires and leads. The cutaway wire on the right shows the DNA scaffold.

The team of researchers, led by Professors of Physics Alexey Bezryadin and Paul Goldbart and including their graduate students David Hopkins and David Pekker, created the NQIDs and provided the theoretical explanations for the unexpected quantum effects that they observed. Their work is described in the June 17 issue of the journal Science.

“Initially, this project started as a search for the well-known Little–Parks oscillations in superconducting nanodevices. Unexpectedly, our measurements on these two-nanowire devices revealed a strange class of periodic oscillations in resistance with applied magnetic field that were qualitatively different from the expected Little–Parks effect,” Bezryadin said. “Through experimentation and theory, we found both an explanation for this odd behavior and a way to put it to work.”

To make their nanodevices, Bezryadin and Hopkins began by placing molecules of DNA across a narrow trench (about 100-nm wide) etched in a silicon wafer. The molecules and trench banks were then coated with a thin film of superconducting MoGe. The result was a device containing a pair of homogeneous, superconducting nanowires having molecular-scale dimensions.

“In the absence of a magnetic field, these ultra-narrow wires exhibited a nonzero resistance over a broad temperature range,” Bezryadin said. “At temperatures where thicker wires would already be superconducting, these DNA-templated wires remained resistive.” Tuning the strength of a magnetic field applied to the device, however, caused highly pronounced and periodic oscillations in resistance, at any temperature in the transition region.

Goldbart and Pekker provided the theoretical explanation for this surprising behavior (see also, the team's recent preprint). "The key point is this," said Goldbart. "The resistance originates in the narrow wires. It's caused by random Brownian fluctuations of the superconducting condensate in them, which occasionally disrupt the superconductivity enough to dissipate some supercurrent." But why does the resistance oscillate with the magnetic field?  "The magnetic field causes a supercurrent to flow in the leads and," said Goldbart, "which alters the pattern of superconductivity in the leads and modulates the superconductivity in the wires. At certain regularly spaced values of the magnetic field, the superconductivity in the wires turns out to be as stable as it can be, making dissipative events rare and the resistance low. But at other values of the field, the superconductivty is partially frustrated so that dissipation occurs more readily and the resistance is higher. As the leads are so narrow, the period of the oscillations is set by nothing more than the width of the leads and the separation of the wires, together with the so-called fundamental constantsh, c, and e." The resulting resistance oscillations are a reflection of the wave nature of matter dictated by quantum mechanics, Goldbart said. “One of the marvels of superconductivity is that it liberates quantum effects from their usual atomic and subatomic habitats."

Metallic nanodevices based on DNA scaffolds could be used in applications such as local magnetometry and the imaging of phase profiles created by supercurrents—in essence a superconducting phase gradiometer, the researchers report. “By taking advantage of DNA self-assembly processes, complex scaffolds could be created for molecular-scale electronic devices,” Bezryadin said.

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