One Mystery of High-Tc Superconductivity Resolved

Nov 16, 2006
Tonica Vall
Tonica Vall

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

Explore further: New insights found in black hole collisions

add to favorites email to friend print save as pdf

Related Stories

Recommended for you

New insights found in black hole collisions

Mar 27, 2015

New research provides revelations about the most energetic event in the universe—the merging of two spinning, orbiting black holes into a much larger black hole.

X-rays probe LHC for cause of short circuit

Mar 27, 2015

The LHC has now transitioned from powering tests to the machine checkout phase. This phase involves the full-scale tests of all systems in preparation for beam. Early last Saturday morning, during the ramp-down, ...

Swimming algae offer insights into living fluid dynamics

Mar 27, 2015

None of us would be alive if sperm cells didn't know how to swim, or if the cilia in our lungs couldn't prevent fluid buildup. But we know very little about the dynamics of so-called "living fluids," those ...

Fluctuation X-ray scattering

Mar 26, 2015

In biology, materials science and the energy sciences, structural information provides important insights into the understanding of matter. The link between a structure and its properties can suggest new ...

Hydrodynamics approaches to granular matter

Mar 26, 2015

Sand, rocks, grains, salt or sugar are what physicists call granular media. A better understanding of granular media is important - particularly when mixed with water and air, as it forms the foundations of houses and off-shore ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Mar 18, 2009
"But at the 1:8 ratio, the material momentarily loses its superconductivity. "

It is obvious Peierls instability doubling period in three dimension: 2*2*2=8

There is pinning of superconducting current at 1:8

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.