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University of Illinois explains
superconductivity in nanowires
Champaign, IL, October 18: Superconducting wires are used in magnetic
resonance imaging machines, high-speed magnetic-levitation trains, and in
sensitive devices that detect variations in the magnetic field of a brain.
Eventually, ultra-narrow superconducting wires might be used in power lines
designed to carry electrical energy long distances with little loss.
Now, researchers at the
University of Illinois at
Urbana-Champaign not only have discovered an unusual phenomenon in which
ultra-narrow wires show enhanced superconductivity when exposed to strong
magnetic fields, they also have developed a theory to explain it.
Magnetic fields are generally observed to
suppress a material's ability to exhibit superconductivity – the ability of
materials to carry electrical current without any resistance at low enough
temperatures. Deviations from this convention have been observed, but there
is no commonly accepted explanation for these exceptions, although several
ideas have been proposed.
As reported in the Sept. 29 issue of Physical
Review Letters, U. of I. physics professor Alexey Bezryadin (pronounced
BEZ-ree-ah-dun) and his research group have studied the effect of applying a
magnetic field to ultra-narrow superconducting wires only a few hundred
atoms across, and have used a microscopic theory proposed by physics
professor Paul Goldbart and his team to explain the results.
"My group discovered that magnetic fields can
enhance the critical current in superconducting wires with very small
diameters," Bezryadin said. "We spoke with many colleagues and reached the
consensus that this phenomenon is indeed curious."
Magnetic fields have long been known to
suppress superconductivity by raising the kinetic energy of the electrons
and by influencing the electron spins. Magnetic atoms, if present in the
wires, also inhibit superconductivity.
Nevertheless, as reported in the Sept. 15
issue of Europhysics Letters, Goldbart, postdoctoral researcher Tzu-Chieh
Wei and graduate student David Pekker proposed that the enhancement observed
by Bezyradin's group was due to magnetic moments in the wires.
"Even though the two effects – magnetic
fields and magnetic moments – work separately to diminish superconductivity,
together one effect weakens the other, leading to an enhancement of the
superconducting properties, at least until very large fields are applied,"
Goldbart said.
As for the origin of these magnetic moments,
the collaborating groups proposed that exposure of the wires to oxygen in
the atmosphere causes magnetic moments to form on the wire surfaces. On
their own, the moments weaken the superconductivity, but the magnetic field
inhibits their ability to do this. This effect shows up in ultra-narrow
wires because so many of their atoms lie near the surface, where the
magnetic moments form.
With postdoctoral research associate Andrey
Rogachev (now a physics professor at the University of Utah) and graduate
student Anthony Bollinger, Bezryadin deposited either niobium or an alloy of
molybdenum and germanium onto carbon nanotubes to fabricate wires that were
less than 10 nanometers wide. The superconductivity of these wires under a
range of applied magnetic fields was examined, and the experimental results
were compared with the proposed theory, revealing an excellent correlation
between the two.
"The results of this work may provide a key
to explaining our previous findings that nanowires undergo an abrupt
transition from superconductor to insulator as they get smaller," said
Bezryadin, referring to work published in the Sept. 27 issue of Europhysics
Letters.
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