We have been able to compare
three separate materials working at the same approximate temperatures and
suggest structure that helps explain it.
Perhaps someday we will have a
fully satisfying solution to the problem.
This is another incremental improvement.
Great work as we are now making
several materials that work at relatively high temperatures. Liquid nitrogen temperatures are more that
good enough for what we want to do. Room
temperature was always dreaming in Technicolor and actually unnecessary.
Study helps explain behavior of latest high-temp superconductors
by Staff Writers
A Rice University-led team of physicists this week offered up one of the first theoretical explanations of how two dissimilar types of high-temperature superconductors behave in similar ways.
The research appears online this week in the journalPhysical Review Letters.
It describes how the magnetic properties of electrons in
two dissimilar families of iron-based materials called "pnictides"
(pronounced: NICK-tides) could give rise to superconductivity. One of the
parent families of pnictides is a metal and was discovered in 2008; the other
is an insulator and was discovered in late 2010.
Experiments have shown that each material, if prepared in a particular
way, can become a superconductor at roughly the same temperature. This has left
theoretical physicists scrambling to determine what might account for the
similar behavior between such different compounds.
Rice physicist Qimiao Si, the lead researcher on the new paper, said
the explanation is tied to subtle differences in the way iron atoms are
arranged in each material. The pnictides are laminates that contain layers of
iron separated by layers of other compounds.
In the newest family of insulating materials, Chinese scientists found
a way to selectively remove iron atoms and leave an orderly pattern of
"vacancies" in the iron layers.
Si, who learned about the discovery of the new insulating compounds
during a visit to China in late December, suspected that the explanation for
the similar behavior between the new and old compounds could lie in the
collective way that electrons behave in each as they are cooled to the point of
superconductivity.
His prior work had shown that the arrangement of the iron atoms in the
older materials could give rise to collective behavior of the magnetic moments,
or "spins," of electrons.
These collective behaviors, or "quasi-localizations," have
been linked to high-temperature superconductivity in both pnictides and other
high-temperature superconductors.
"The reason we got there first is we were in a position to really
quickly incorporate the effect of vacancies in our model," Si said.
"Intuitively, on my flight back (from China
Si conducted the calculations and analyses with co-authors Rong Yu,
postdoctoral research associate at Rice, and Jian-Xin Zhu, staff scientist at Los Alamos National Laboratory.
"We found that ordered vacancies enhance the tendency of the
electrons to lock themselves some distance away from their neighbors in a
pattern that physicists call 'Mott localization,' which gives rise to an
insulating state," Yu said. "This is an entirely new route toward
Mott localization."
By showing that merely creating ordered vacancies can prevent the
material from being electrical conductors like their relatives, the researchers
concluded that even the metallic parents of the iron pnictides are close to
Mott localization.
"What we are learning by comparing the new materials with the
older ones is that these quasi-localized spins and the interactions among them
are crucial for superconductivity, and that's a lesson that can be potentially
applied to tell experimentalists what is good for raising the transition
temperature in new families of compounds," Zhu said.
Superconductivity occurs when electrons pair up and flow freely through
a material without any loss of energy due to resistance. This most often occurs
at extremely low temperatures, but compounds like the pnictides and others
become superconductors at higher temperatures - close to or above the
temperature of liquid nitrogen - which creates the possibility that they could
be used on an industrial scale.
One impediment to their broader use has been the struggle to precisely
explain what causes them to become superconductors in the first place. The race
to find that has been called the biggest mystery in modern physics.
"The new superconductors are arguably the most important
iron-based materials that have been discovered since the initial discovery of
iron pnictide high-temperature superconductors in 2008," Si said.
"Our theoretical results provide a natural link between the new and old
iron-based superconductors, thereby suggesting a universal origin of the
superconductivity in these materials."
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