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Magneto-optical image of magnetic
fields within a YBCO superconductor showing electrically connected
grains (yellow) and grain boundaries (green) that form barriers to
superconducting currents. The large reversible effect of strain
observed by NIST might be due to associated changes in grain
boundaries, which raise the barriers to current flow and lowering
the material's current-carrying capability.
Image by D.C. van der Laan/NIST |
The measurements, reported in Applied Physics
Letters,* revealed a 40 percent reduction in critical current, the
point at which superconductivity breaks down, at just 1 percent
compressive strain. This effect can be readily accommodated in the
engineering design of practical applications, NIST project leader Jack
Ekin says, but knowing about it ahead of time will be important to the
success of many large-scale devices. The effect was measured in three
types of yttrium-barium-copper-oxide (YBCO), a brittle ceramic
considered the best prospect for making low-cost, high-current,
superconducting wires. The researchers developed a “four point" bend
technique that enables studies of superconducting properties over a
wide range of uniform strain at high current levels. The
superconductor is soldered on top of a flexible metal beam, which is
then bent up or down at both ends while the critical current is
measured.
The discovery is the first major reversible strain
effect found in practical high-temperature superconductors, which
generally have been tested under smaller tensile strains only, or at
strains so high they caused the material to break down permanently.
The newly discovered effect is totally reversible and symmetric for
both compressive and tensile (pushing and pulling) strains, suggesting
it is intrinsic to the fundamental mechanism of superconductivity in
YBCO.
The NIST team is now pursuing the possibility of
using the effect as a new tool for probing the elusive mechanism
underlying high-temperature superconductivity. The next step is to
investigate how magnetic fields affect the strain effect, and several
collaborations are under way with universities and other research
organizations to study the interplay of the effect with other factors
affecting high-temperature superconductivity. The research described
in the new paper was supported in part by the U.S. Department of
Energy. |
