Top: Visualization of nanoscale disruptions in electron interactions in a Kondo-hole doped heavy-fermion compound. The black-and-white inset shows directly how oscillations in electron behavior are centered on the Thorium impurities, “rippling” outward like disturbances caused by drops of water on a still pond. The rippling oscillations in electron energy are shown in more detail in the close-up view (bottom), where the bands of different shades of blue represent the distance between the ripples. Images: Brookhaven National Laboratory |
It’s a basic technique learned early, maybe even before
kindergarten: Pulling things apart—from toy cars to complicated electronic
materials—can reveal a lot about how they work. “That’s one way physicists
study the things that they love; they do it by destroying them,” says Séamus
Davis, a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National
Laboratory and the J.G. White Distinguished Professor of Physical Sciences at
Cornell University.
Davis and colleagues recently turned this destructive
approach—and a sophisticated tool for “seeing” the effects—on a material
they’ve been studying for its own intrinsic beauty, and for the clues it may
offer about superconductivity, the ability of some materials to carry electric
current with no resistance. The findings, published in the Proceedings of the National Academy of Sciences, reveal how
substituting just a few atoms can cause widespread disruption of the delicate
interactions that give the material its unique properties, including
superconductivity.
The material, a compound of uranium, ruthenium, and silicon,
is known as a “heavy-fermion” system. “It’s a system where the electrons
zooming through the material stop periodically to interact with electrons
localized on the uranium atoms that make up the lattice, or framework of the
crystal,” Davis says. These stop-and-go magnetic interactions slow down the
electrons, making them appear as if they’ve taken on extra mass, but also
contribute to the material’s superconductivity.
In 2010, Davis and a group of collaborators visualized
these heavy fermions for the first time using a technique developed by Davis, known as
spectroscopic imaging scanning tunneling microscopy (SI-STM), which measures
the wavelength of electrons of the material in relation to their energy.
The idea of the present study was to “destroy” the heavy
fermion system by substituting thorium for some of the uranium atoms. Thorium,
unlike uranium, is non-magnetic, so in theory, the electrons should be able to
move freely around the thorium atoms, instead of stopping for the brief
magnetic encounters they have at each uranium atom. These areas where the
electrons should flow freely are known as “Kondo holes,” named for the
physicist who first described the scattering of conductive electrons due to
magnetic impurities.
Free-flowing electrons might sound like a good thing if
you want a material that can carry current with no resistance. But Kondo holes
turn out to be quite destructive to superconductivity. By visualizing the behavior
of electrons around Kondo holes for the first time, Davis’ current research helps to explain why.
“There have been beautiful theories that predict the
effects of Kondo holes, but no one knew how to look at the behavior of the
electrons, until now,” Davis
says.
Working with thorium-doped samples made by physicist
Graeme Luke at McMaster University in Ontario,
Davis’ team
used SI-STM to visualize the electron behavior.
“First we identified the sites of the thorium atoms in the
lattice, then we looked at the quantum mechanical wave functions of the
electrons surrounding those sites,” Davis
says.
The SI-STM measurements bore out many of the theoretical
predictions, including the idea proposed just last year by physicist Dirk Morr
of the University
of Illinois that the
electron waves would oscillate wildly around the Kondo holes, like ocean waves
hitting a lighthouse.
“Our measurements revealed waves of disturbance in the ‘quantum glue’ holding the heavy fermions together,” Davis says.
So, by destroying the heavy fermions—which must pair up
for the material to act as a superconductor—the Kondo holes disrupt the
material’s superconductivity.
Davis’
visualization technique also reveals how just a few Kondo holes can cause such
widespread destruction: “The waves of disturbance surrounding each thorium atom
are like the ripples that emanate from raindrops suddenly hitting a still pond
on a calm day,” he says. “And like those ripples, the electronic disturbances
travel out quite a distance, interacting with one another. So it takes a tiny
number of these impurities to make a lot of disorder.”
What the scientists learn by studying the exotic heavy
fermion system may also pertain to the mechanism of other superconductors that
can operate at warmer temperatures.
“The interactions in high-temperature superconductors are
horribly complicated,” Davis
says. “But understanding the magnetic mechanism that leads to pairing in heavy
fermion superconductors—and how it can so easily be disrupted—may offer clues
to how similar magnetic interactions might contribute to superconductivity in
other materials.”