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news release:First vortex
'chains' observed in engineered superconductor
Argonne, IL, Dec. 9: They look like tiny swirling
dust devils on the surface of the superconductor: "vortices" that appear where
magnetic fields interact with the material. Unlike harmless dust devils,
however, vortices can sap a superconductor's ability to transmit current without
resistance.
Knowing how the vortices move and arrange
themselves under various temperatures and magnetic fields, as well as how they
are influenced by the physical properties of the material, is critical in
maintaining supercurrent flow.
As part of Argonne's intense focus on
superconductors, a team of scientists in the laboratory's
Materials Science Division
(MSD) has obtained, for the first time, detailed images of the interaction of
magnetic vortices with artificial, nanoscale engineered defects in a
superconductor. Understanding this interaction could help scientists reduce the
vortices' current-sapping effects or lead to fundamentally new superconductor
designs for transmitting DC and AC electric power, and quantum logic devices
based on vortex manipulation.
High-temperature superconductors, discovered in
1986, have attracted intense interest due to their ability to conduct
electricity without resistance when cooled with liquid nitrogen. Previously,
superconductivity was only known in metals cooled with liquid helium, which is
much more difficult and expensive to produce and handle. High-temperature
superconductors are now used in many applications, including RF filters for
mobile telephone networks, magnetic resonance imaging (MRI) machines and
particle accelerators.
A critical factor limiting applications for these
superconductors is their response to magnetic fields, such as in electric
motors. Magnetic fields reduce the amount of current a superconducting material
can carry. The fields create swirling tubes of electrical current vortices
in the superconducting material. Superconductivity is completely suppressed
within these structures. (The individual structure of vortices and their
arrangement were predicted by Alexei Abrikosov of Argonne's Materials Science
Division, who won the 2003 Nobel Prize in physics for his work on
superconductors.) And as current flows through the superconductor, the vortices
are pushed at right angles to the current flow by the Lorentz force. The vortex
movement inside the material dissipates energy and produces resistance.
Scientists have discovered that vortices can be
locked into position by "pinning" them to defects tiny grains of
non-superconducting substances embedded in the superconductor.
"Today, vortex pinning is the main thrust of
superconductor research," said Goran Karapetrov (MSD), a lead researcher of the
Argonne team that includes Maria Iavarone, Jan Fedor, Dan Rosenmann and Wai Kwok
(all MSD). "We are concentrating on the microscopic physics behind defects that
hold the vortices in place and increase the current-carrying capability of the
materials."
To learn more about vortices and their effects,
the Argonne team uses low temperature scanning tunneling microscopy, or STM.
This sophisticated technique is used in fundamental research to obtain
atomic-scale images of surfaces as well as essential information on the
electronic states at and just beneath the surface. The atomic-scale images of
the surface and its electronic structure allow the Argonne team to pin-point the
positions of both the vortices that control the superconductivity and the
defects that pin the vortices. Applying this powerful imaging technique to
engineered defect structures is a major advance.
STM requires an extremely sharp conducting probe
held close to the sample at a distance of only a few atom-diameters. Electrons
can jump the gap or tunnel between the sample material and the stylus,
producing an electrical signal. The stylus slowly scans across the surface,
raising and lowering to keep the gap between the surface and the tip constant.
Recording the vertical movement of the stylus reveals the structure of the
surface atom by atom.
"If the scanning tip touches the surface, the
experiment is over," Karapetrov said. "These experiments are very precise. To
visualize the surface, the tip comes within three to four Angstroms of the
material. It has to be precisely that distance within one hundredth of an
Angstrom in order to observe these effects."
Beyond developing sophisticated STM techniques,
the team devised a method of preparing a sample with an atomically flat surface
containing a periodic array of defects to pin the vortices.
"The size of each defect allows it to hold up to
six vortices," Karapetrov said. "As the magnetic field increases beyond the
saturation number of the defect, vortices appear outside the defect."
The vortices induced by a weak magnetic field
attached themselves to the defects, as expected. As the scientists increased the
magnetic field, STM images revealed additional vortices; those that couldn't
find a home in a defect appeared alongside in orderly lines a "chain." As the
magnetic field was increased further, the vortex chains became denser, up to a
specific, critical intensity; at this critical field the vortex chain split into
two parallel chains. The transition was accompanied by a peak in the
superconductor's critical current density the measure of how well the
superconductor carries large electric currents. The scientists were able to
create additional parallel chains by further increasing the magnetic field.
"It's basically a phase transition," Karapetrov
said. "This behavior was predicted theoretically more than 10 years ago, but it
hadn't been possible to see it until this scanning technique was perfected."
The experiments marked the first time this phase
transition from single to multiple chains had been directly observed. It was
also the first time vortices have been studied in engineered samples with STM.
Previously, creating superconductors with varying defect properties was done
using randomly distributed defects created with heavy-particle accelerators like
Argonne's ATLAS.
"The STM experiments using samples irradiated at
ATLAS helped us a lot. But the ability to create engineered samples means we are
free to make whatever geometry of defects fits best for the application,"
Karapetrov said. "We can design the material for vortex-pinning abilities and
the best critical current by changing the fabrication parameters. Since the
defects are created by lithography, we have full control over the geometry and
internal structure." The lithographic process also allows researchers to vary
the material in the defects, opening up a new avenue for research.
The research resulted in two published papers in
Applied Physics Letters
and Physics Review Letters:
the first discussing the sample preparation, the second focusing on the research
results. (APL 87, 162515 (2005), Phys. Rev. Lett. 95 (2005) 167002).
The research was funded by the U.S. Department of
Energy's Office of
Basic Energy
Sciences Materials Science under contract No.W-31-109-ENG-38. The research
is a result of an on-going collaboration with the
Center
for Microanalysis of Materials at the
University of Illinois,
Urbana-Champaign, and the Center for
Nanoscale Materials at Argonne.
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