Data supports role of magnetism in iron-based superconductors: The height of each dot in this image represents the superconducting energy gap—a measure of the strength of electron pairing—for electrons moving at a particular momentum (speed in a given direction) on each electronic band (red and yellow rings) of a particular iron superconductor. The data show that the magnitude of the gap (height of the dots) varies by its momentum (position along the base plane) and the band it is on—which is exactly what was predicted by theories in which magnetism plays a primary role in the emergence of superconductivity. The results therefore strengthen confidence that those theories may help scientists discover or design new superconductors. Image: Brookhaven National Laboratory |
By measuring how strongly electrons are bound together to form
Cooper pairs in an iron-based superconductor, scientists at the U.S. Department
of Energy’s (DOE) Brookhaven National Laboratory, Cornell University, St.
Andrews University, and collaborators provide direct evidence supporting
theories in which magnetism holds the key to this material’s ability to carry
current with no resistance. Because the measurements take into account the
electronic bands and directions in which the electrons are traveling, which was
central to testing the theoretical predictions, this research strengthens
confidence that this type of theory may one day be used to identify or design
new materials with improved properties—namely, superconductors operating at
temperatures far higher than today’s. The findings are published in Science.
“In the best possible world you would be able to take this
theory and plug in different chemical elements until you find a combination
that should work as a superconductor at higher temperatures,” said team leader
Séamus Davis, Director of the Center for Emergent Superconductivity at Brookhaven
and the J.G. White Distinguished Professor of Physical Sciences at Cornell
University. Such materials could be used for real world, energy-saving
technologies, such as zero-loss power transmission lines, without the need for
expensive coolants.
Scientists have been trying to understand the mechanism
underlying so-called “high-temperature” superconductivity ever since
discovering materials that could carry current with no resistance at
temperatures somewhat above the operating realm of conventional
superconductors, which must be chilled to near absolute zero (0 K, or -273 C).
Though still mighty chilly, these high-Tc materials’ operating
temperatures—some as high as 145 K (-130 C)—offer hope that such materials
could one day be designed to operate at room temperature.
One key to superconductivity is the formation of electron pairs.
Scientists hypothesized that if these negatively charged particles have their
magnetic moments pointing in opposite directions, they could overcome their mutual
repulsion to join forces in so-called Cooper pairs—thus carrying current with
no loss.
“Many people suspected you could take materials that naturally
have alternating magnetic moments on adjacent electrons—antiferromagnetic
materials—and convert them into superconductors,” Davis said. But to prove this conjecture
hasn’t been possible with copper-based, or cuprate, superconductors—the first
high-Tc superconductors discovered starting some 25 years ago. “You
can make a robust antiferromagnetic cuprate insulator, but in that state it’s
hard to get the magnetic electrons to pair and then move around and make a
superconductor,” Davis
said.
Then, in 2008, when iron-based superconductors were discovered,
the idea that magnetism plays a role in high-Tc superconductivity
was revived. But determining that role was a very complex problem.
“In each iron atom there are five magnetic electrons, not just
one,” Davis
said. “And each, as it moves around the crystal, does so in a separate
electronic band. In order to find out if the magnetic interactions between
electrons are generating the superconductivity, you have to measure what’s called
the anisotropic energy gap—how strongly bound together the electrons are in a
pair—depending on the electrons’ directions on the different electronic bands.”
Theorists Dung-Hai Lee of the University of California at
Berkeley, Peter Hirschfeld of the University of Florida, and Andrey Chubukov of
the University of Wisconsin among others had developed different versions of a
theory that predicts what those measurements should be if magnetism were the
mechanism for superconductivity.
“It was our job to test those predictions,” Davis said. But at first, the techniques
didn’t exist to make the measurements. “We had to invent them,” Davis said.
Two scientists working with Davis, Milan P. Allan of Brookhaven,
Cornell, and the University of Saint Andrews (where Davis also teaches) and
Andreas W. Rost of Cornell and St. Andrews—the lead authors on the paper—figured
out how to do the experiments and identified an iron-based material (lithium
iron arsenide) in which to test the predictions.
Their method, multi-band Bogoliubov quasiparticle scattering
interference, found the “signature” predicted by the theorists:
“The strength of the ‘glue’ holding the pairs together is
different on the different bands, and on each band it depends on the direction
that the electrons are traveling—with the pairing usually being stronger in a
given direction than at 45 degrees to that direction,” Davis said.
“This is the first experimental evidence direct from the electronic structure in support of the theories that
the mechanism for superconductivity in iron-based superconductors is due
primarily to magnetic interactions,” he said.
The next step is to use the same technique to determine whether
the theory holds true for other iron superconductors. “We and others are
working on that now,” Davis
said.
If those experiments show that the theory is indeed correct, the
model could then be used to predict the properties of other elements and
combinations—and ideally point the way toward engineering new materials and
higher-temperature superconductors.
