Scientists Discover New Wrinkle in Mystery of High-Temperature Superconductors
AEN News
Baltimore, MD - 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.
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."
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