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Superconducting nanowires show ability to
measure magnetic fields
June 16 -- By using DNA molecules as
scaffolds, scientists have created superconducting nanodevices that demonstrate
a new type of quantum interference and could be used to measure magnetic fields
and map regions of superconductivity.
Researchers at the University of Illinois
at Urbana-Champaign have fabricated and studied nanostructures consisting of
pairs of suspended superconducting wires as tiny as 3 to 4 molecular diameters
(typically 5 to 15 nanometers) in width. The team consisted of physics
professors Alexey Bezryadin and Paul Goldbart, and graduate students David
Hopkins and David Pekker. Their work is described in the June 17 issue of the
journal Science.
"Our measurements on these two-nanowire
devices revealed a strange class of periodic oscillations in resistance with
applied magnetic field," 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, the researchers
began by placing molecules of DNA across a narrow trench (about 100 nanometers
wide) etched in a silicon wafer. The molecules and trench banks were then coated
with a thin film of superconducting material (molybdenum-germanium). The result
was a device containing a pair of homogeneous, superconducting nanowires with
extremely fine features.
"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.
"The applied magnetic field causes a small
current to flow along the trench banks, and this current then causes a large
change in resistance," Goldbart said. "The strength of the current is controlled
only by the magnetic field and the width of the banks supporting the wires."
The resulting periodic oscillation is a
reflection of the wave nature of matter that goes to the very heart of quantum
mechanics, Goldbart said. "Unlike ordinary matter, the electrons in these wires
are behaving as though they are one quantum mechanical object in one great
quantum mechanical wave function."
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 electronic devices with
features having molecular-scale dimensions," Bezryadin said.
In related work, to appear in the August
issue of the journal Nanotechnology (published online in May), Bezryadin and
undergraduate student Mikas Remeika improved the nanofabrication process by
using a focused electron beam to locally alter the shape and structure of
metallized nanowires.
Performed in a transmission electron
microscope, electron-beam sculpting and crystallization can modify small
segments of the nanowires, with a spatial resolution of approximately 3
nanometers, Bezryadin said. The technique could be used to fabricate novel
electronic nanodevices, such as single-electron transistors, with dimensions
less than 10 nanometers.
Funding came from the National Science
Foundation, the Alfred P. Sloan Foundation and the U.S. Department of Energy.
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