A Rice Univ.-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 in Physical Review Letters. It describes how the magnetic properties
of electrons in two dissimilar families of iron-based materials called
“pnictides” 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 last Christmas), I was
thinking through the calculations we should begin doing.”
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.”
The research was funded by the National Science Foundation,
the Robert A. Welch Foundation and the Department of Energy. It was facilitated
by the International Collaborative Center
on Quantum Matter, a collaborative entity Rice formed with partner institutions
from China, Germany, and United Kingdom.
