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New theory explains enhanced superconductivity in nanowires
Kristen
Aramthanapon 217-244-8780
aramthan@uiuc.edu
10/18/2006
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CHAMPAIGN,
Ill. —
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.
The work was funded by the U.S. Department of Energy and the National
Science Foundation.
Editor’s note: To reach Alexey Bezryadin,
call 217-333-9580; e-mail: bezryadi@uiuc.edu.
To reach Paul Goldbart, call 217-333-1195; e-mail: goldbart@uiuc.edu.
