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New wrinkle in the mystery of
high-Tc superconductors
Upton, NY, Mar. 13: In the twenty years since the
discovery of high-temperature (Tc) superconductors, scientists have been trying
to understand the mechanism by which electrons pair up and move coherently to
carry electrical current with no resistance. "We are still at the beginning,"
says Tonica
Valla, a physicist at the U.S. Department of Energy's
Brookhaven National
Laboratory, who will give a talk on his group's latest results at the
American Physical Society meeting in Baltimore, Maryland on Thursday, March
16, 2006. "If anything," he adds, "it looks like the story is getting more
complicated."
In 1999, Valla's group was the first to observe a
"kink" in the energy level of electrons in high-Tc superconductors just as they
went through the transition temperature from their normal to superconducting
state. The kink was the first clue to explaining what the mechanism of electron
pairing might be.
| "The kink gave us the hope that we could
identify the interaction that was responsible for the electron pairing,"
said Valla. Some groups hold that the mechanism is the same as in
conventional superconductors -- that is, that phonons, or vibrations in
the crystal lattice, are responsible for electron pairing. Other
scientists believe that changes in the spin alignment, or magnetic
polarity, of adjacent electrons -- known as magnons -- are responsible.
"The problem is that there are both phonons and magnons in the crystal
with the energy where we see the kink, so it is still not clear," Valla
says.
|
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The latest wrinkle uncovered by Valla's group is
the observation of similar energy scales and gaps in a material that is not a
superconductor. The material is a special form of a compound made of lanthanum,
barium, copper, and oxygen, where there is exactly one barium atom for every
eight copper atoms. With less or more barium, the material acts as a high-Tc
superconductor (in fact, this was the very first high-Tc superconductor
discovered). But at the 1:8 ratio, the material momentarily loses its
superconductivity.
"The fact that this system, which is not a
superconductor, has similar properties to the superconducting system is not
helping to solve the mystery," Valla says. But then he notes that 20 years since
the discovery of high-Tc superconductors is still not that long. "For
conventional superconductors," he says, "it took about 50 years to come up with
a good explanation for the behavior."
Valla's talk is part of a session on the use of
angle-resolved photoemission spectroscopy in the study of high-Tc
superconductors. It will include a discussion of advances in this technique. His
group uses bright beams of ultraviolet light at the National Synchrotron Light
Source, one of Brookhaven Lab's premiere research facilities, to emit electrons
from the samples they are studying. Using high-resolution spectrometers, the
scientists measure the energy and the angle at which the electrons exit the
crystal, allowing them to reconstruct the electrons' state while in the crystal
-- their energy level and whether they had any interactions with phonons/magnons.
The talk will take place on Thursday, March 16,
2006 at 3:06 p.m. in Ballroom IV of the Baltimore Convention Center.
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