New
Release -- Superconductor Week does not edit or endorse the following
news release:One Mystery of
High-Tc Superconductivity Resolved
Upton, NY, November 16: 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.”
This research was funded by the Office of Basic
Energy Sciences within the U.S. Department of Energy’s Office of Science. The
Department of Energy has a keen interest in understanding the mechanisms of
superconducting materials — particularly those that can carry current with zero
resistance at higher temperatures — because these materials have many potential
applications in improving the efficiency of energy generation and transmission.
Co-authors on the study are: Alexei Fedorov of
the Advanced Light Source at Lawrence Berkeley National Laboratory, Jinho Lee
and Seamus Davis of Cornell University, and Genda Gu of Brookhaven Lab.
Return
to industry news releases
|
