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New particle explains odd behavior in cuprate superconductors
Kloeppel, Physical Sciences Editor
photo to enlarge
by L. Brian Stauffer
|Scientists at the University of Illinois have discovered a new particle hidden in copper-oxide ceramics (cuprates). From left are physics graduate student Ting-Pong Choy, and physics professors Robert G. Leigh and Philip Phillips.
CHAMPAIGN, Ill. —
New fundamental particles aren’t found only at Fermilab and at
other particle accelerators. They also can be found hiding in plain
pieces of ceramic, scientists at the University of Illinois report.
The newly formulated particle is a boson and has a charge of 2e, but
does not consist of two electrons, the scientists say. The particle
arises from the strong, repulsive interactions between electrons, and
provides another piece of the high-temperature superconductivity puzzle.
Twenty-one years ago, superconductivity at high temperatures was discovered
in copper-oxide ceramics (cuprates). Existing explanations of superconductivity
proved inadequate because, unlike low-temperature superconductors, which
are metals, the parent materials from which all high-temperature superconductors
arise are insulators.
Now, a new theory suggests something has been overlooked. “Hidden
in the copper-oxide materials is a new particle, a boson with a charge
of 2e,” said Philip Phillips, a professor of physics at Illinois.
Surprisingly, this boson is not formed from the elementary excitations
– that is, electrons and ions. Instead, the particle emerges as
a remnant of the strong interactions between electrons in the normal
“High- and low-energy scales are inextricably coupled in the cuprates,”
Phillips said. “Normally, when you remove a single electron from
most systems, one empty state is created. In the cuprates, however,
when you remove an electron, you create two empty states – both
of which occur at low energy, but paradoxically, one of the states comes
from the high-energy scale.”
Experimental evidence of this “one to two” phenomenon was
first reported in 1990 and explained phenomenologically by University
of Groningen physicist George A. Sawatzky (now at the University of
British Columbia) and colleagues. What was missing was a low-energy
theory that explained how a high-energy state could live at low energy.
Phillips, with physics professor Robert G. Leigh and graduate student
Ting-Pong Choy, have constructed such a theory, and have shown that
a charged 2e boson makes this all possible.
“When this 2e boson binds with a hole, the result is a new electronic
state that has a charge of e,” Phillips said. “In this case,
the electron is a combination of this new state and the standard, low-energy
state. Electrons are not as simple as we thought.”
The new boson is an example of an emergent phenomenon – something
that can’t be seen in any of the constituents, but is present
as the constituents interact with one another.
By constructing a low-energy theory of the cuprates, the researchers
have moved a step closer to unraveling the mystery of high-temperature
“Until we understand how these materials behave in their normal
state, we cannot understand the mechanism behind their high-temperature
superconductivity,” Phillips said.
Phillips, Leigh and Choy present their mathematical proof for the new
boson in a paper accepted for publication in the journal Physical Review
Letters. The National Science Foundation provided partial funding for
Editor’s note: To reach Philip Phillips, call 217-244-2003; e-mail: email@example.com.
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