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deepens mystery of heavy-fermion superconductors
Kloeppel, Physical Sciences Editor
photo to enlarge
by Kwame Ross
Greene, a Swanlund Endowed Chair in physics at Illinois,
and colleagues at Los Alamos National Laboratory recently
used a sensitive technique called point-contact spectroscopy
to explore Andreev reflection between a normal metal
and a heavy-fermion superconductor.
CHAMPAIGN, Ill. —
Theoretical understanding of heavy-fermion superconductors has just
slipped a notch or two, says a team of experimentalists.
Researchers from the University of Illinois at Urbana-Champaign and
Los Alamos National Laboratory recently used a sensitive technique called
point-contact spectroscopy to explore Andreev reflection between a normal
metal and a heavy-fermion superconductor. Conventional theories cannot
account for their data, the scientists report.
“According to conventional theories, the Fermi velocity mismatch
between a normal metal and a heavy-fermion superconductor is too large
for Andreev reflection to occur,” said Laura Greene, a Swanlund
Endowed Chair in physics at Illinois. “But we can clearly and reproducibly measure it as
a matter of course.”
Andreev reflection is a particle-hole conversion process that occurs
at the interface of a normal metal and a superconductor. Using point-contact
spectroscopy, Greene and postdoctoral research associate Wan Kyu Park
obtained measurements of Andreev reflection at the interface of a normal
metal (gold) and the heavy-fermion superconductor CeCoIn5 (cerium-cobalt-indium-five).
Andreev reflection can be better understood by drawing an analogy to
light, Greene said. When light is incident on glass, some of the light
is reflected due to the difference in index of refraction between air
and glass, and some is transmitted into the glass. The index of refraction
is essentially the velocity difference: light travels slower in glass
than in air.
Shining light on a diamond, which has an index of refraction larger
than glass, causes more light to be reflected (one of the reasons diamonds
appear to glow). The larger the velocity mismatch, the more light will
Similarly, when a metal and a superconductor are in good electrical
contact and have different Fermi velocities (the speed of electrons
at the Fermi energy), some of the electrons will be reflected in a normal
fashion. The larger the mismatch, the more electrons will be reflected
and the less transmitted.
The Andreev reflection process requires at least some penetration of
the electrons into the superconductor, Greene said. If the Fermi velocities
are quite disparate, then there is a large fraction of normal reflection
and less of a chance for Andreev reflection.
Conventional theory dictated that all of the reflection between a normal
metal and a heavy-fermion superconductor would be normal and there would
be no Andreev reflection.
“Our measurements prove that existing theories can’t account
for Andreev reflection between normal metals and heavy-fermion superconductors,”
said Greene, who will present the team’s findings at the spring
meeting of the American Physical Society, to be held in Los Angeles,
“The bottom line is, we can understand Andreev reflection occurring
between a normal metal and a superconductor, but we can’t understand
it occurring between a normal metal and a heavy-fermion superconductor,”
Greene said. “We need a whole new theoretical formulation to explain
In addition to Greene and Park, the research team included physicists
John Sarrao and Joe Thompson at Los Alamos. Tony Leggett (Macarthur
Professor of Physics at Illinois), his graduate student, Vladimir Lukic,
and an undergraduate student in Greene’s group, Justin Elenewski,
are collaborating on this experimental effort in investigating new theoretical
frameworks to account for these results. The U.S. Department of Energy
funded the work.
Editor’s note: To reach Laura Greene, call 217-333-7315; e-mail: firstname.lastname@example.org.