One Mystery of High-Tc Superconductivity Resolved
Posted on Thursday, November 16, 2006 @ 21:22:14 UTC by vlad
|
|
 Research published online in the journal Science
this week by Tonica Valla, a physicist at the U.S. Department of
Energy’s Brookhaven National Laboratory, appears to resolve one mystery
in the 20-year study of high-temperature (high Tc) superconductors — materials that lose their resistance to the flow of electricity at relatively high temperatures.
The research shows that a “pseudogap” in the energy level of the
material’s electronic spectrum is the result of the electrons being
bound into pairs above the so-called transition temperature to the
superconducting state, but unable to superconduct because the pairs
move incoherently.
In conventional superconductors, which operate at much lower
temperatures (near absolute zero), superconductivity occurs as soon as
electron pairs are formed. But in the case of the high-Tc
materials, the electrons, though paired, “do not ‘see’ each other,”
Valla says, “so they cannot establish ‘phase coherence,’ with all the
pairs behaving as a ‘collective.’”
The origin of this pseudogap, along with the mechanism for forming
the pairs necessary for superconductivity, has been one of the biggest
mysteries scientists have been trying to understand about high-Tc
superconductors since their discovery some 20 years ago. Because of
their higher operating temperatures (up to 134 kelvins at ambient
pressure and up to 164 K under high pressure), high-Tc
superconductors have much greater potential for real world
applications, such as zero-loss power transmission lines, than do
conventional superconductors.
The material studied by Valla’s group — 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 — is actually not
a superconductor. 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.
Yet despite the fact that this material, at this ratio, is not a
superconductor, it has a very similar energy signature — including the
energy gap in the electronic spectrum (pseudogap) — as other high-Tc superconductors in their superconducting states.
Valla’s group interprets the finding as evidence that the electron
pairs are formed first (as “preformed pairs”) and phase coherence
occurs later, at some lower temperature (the transition temperature, or
Tc), when thermal fluctuations of the phase are suppressed enough to cause superconductivity.
“Our research shows that the pseudogap is caused by the same
interactions that are responsible for superconductivity — interactions
that bind two electrons into a pair,” Valla says.
“In high-Tc superconductors, however, this pairing is only the
first step,” he continues. “The superconducting transition is delayed,
possibly — and ironically — because the pairing might be too strong.
Figuratively speaking, a strong pairing produces “small” pairs with
strongly fluctuating phases. Only by cooling the material to much lower
temperatures do the phase fluctuations become suppressed. At that
point, the phase becomes locked so the electron pairs can act
coherently — and the system becomes a superconductor.”
Source: Brookhaven National Laboratory
News via: http://www.physorg.com/news82915060.html
|
| |
|
Don't have an account yet? You can create one. As a registered user you have some advantages like theme manager, comments configuration and post comments with your name.
| |
Average Score: 4
Votes: 1
| |
|
