(Press-News.org) HOUSTON -- (April 5, 2013) -- Rice University physicists on the hunt for the
origins of high-temperature superconductivity have published new findings
this week about a seemingly contradictory state in which a material
simultaneously exhibits the conflicting characteristics of both a metallic
conductor and an insulator.
In a theoretical analysis this week in Physical Review Letters (PRL), Rice
physicists Qimiao Si and Rong Yu offer an explanation for a strange series
of observations described earlier this year by researchers at the Stanford
Linear Accelerator Center in Menlo Park, Calif. In those experiments,
physicists used X-rays to probe the behavior of electrons in superconducting
materials made of potassium, iron and selenium. The material becomes
superconducting at extremely cold temperatures, and the experiments revealed
that at a slightly higher temperature, the material exhibited a seemingly
contradictory electronic state in which some electrons in the iron atoms
became frozen in place while electrons in neighboring orbitals continued to
move.
"We have proposed a unified phase diagram for the alkaline iron selenides in
which this phase connects between the lower-temperature, superconducting
phase at one extreme and a higher-temperature insulating phase at the
other," said Si, Rice's Harry C. and Olga K. Wiess Professor of Physics and
Astronomy.
Flowing electrons power all the world's energy grids, and a significant
amount of power in those grids is lost to electrical resistance -- a kind of
electronic friction that occurs when electrons move through metallic wires.
Superconducting materials, which were discovered more than a century ago,
conduct electricity without any loss of power, but they only operate at
extremely cold temperatures. Since the 1980s, scientists have discovered a
number of new materials that become superconducting at temperatures that,
while still cold, are above or close to the temperature of liquid nitrogen
-- an important threshold for engineering applications. The hope is that
these "high-temperature" superconductors may one day be used to
revolutionize power transmission and other technologies, but physicists have
yet to develop a clear-cut understanding of how high-temperature
superconductors work.
In classical superconductors, frictionless conduction is achieved when
electrons pair up in a way that allows them to flow effortlessly, without
the bumping and jostling that normally leads to electrical resistance.
Electron pairing is uncommon because the rules of quantum mechanics
typically make electrons loners. Under normal circumstances, electrons repel
one another, and the mechanism that causes them to pair up in classical
superconductors doesn't account for their behavior in high-temperature
superconductors.
Iron-based high-temperature superconductors were discovered in 2008. Si and
collaborators, including UCLA physicist Elihu Abrahams, were among the first
to propose a way in which superconductivity might arise in the iron-based
materials due to a phenomenon known as "correlated electron" behavior. In
correlated-electron systems, the behavior of electrons in a material can
only be understood by viewing the electrons as a collective system rather
than many individual objects.
Si and Yu's new paper focuses on experiments with an alkaline iron selenide,
one family of materials that is included in the larger class of iron-based
superconductors. Prior experiments had found that alkaline iron selenides
exhibited odd electronic behaviors at temperatures above the critical
temperature in which they transition to the superconducting state.
In the PRL paper, Si and Yu describe a new electronic state, or phase,
marked by electronic traffic congestion. They show that electrons in
different quantum states, or orbitals, react differently to the bad traffic
situation. In particular, the new phase is marked by electrons in selected
orbitals becoming locked in a place -- a phenomenon known as a Mott
insulating state.
"In a theoretical model containing several orbitals, we identified an
'orbital-selective Mott phase,'" said Yu, a postdoctoral research associate
at Rice. "In this phase, electrons in some orbitals behave like an
insulator, while those in the other orbitals act as a metal."
Si and Yu said they saw the first hints of the new phase in a 2011 model
they designed to study a different family of iron-based superconductors. In
that model, the orbital-selective Mott phase ultimately proved to be
unstable, so they were somewhat surprised when the phase appeared and proved
stable in the model for the alkaline iron selenides.
"This is the first time anybody has identified an orbital-selective Mott
phase in any model for the iron-based superconductors," Yu said.
Si said characterizing the orbital-selective Mott phase in the alkaline iron
selenides provides more clues about the fundamental origins of
superconductivity.
"Ultimately, our goal is to understand superconductivity and the conditions
to optimize superconductivity," Si said.
The premise is that this kind of bad traffic situation -- the contradictory
phase where electrons are in conflict as to whether they should freeze or
move -- is good for superconductivity.
"Our results provide evidence that electron correlations play a vital role
in the superconductivity of the iron-based superconductors," he said.
INFORMATION:
The research was funded by the National Science Foundation and the Robert A.
Welch Foundation.
High-resolution IMAGES are available for download at:
http://news.rice.edu/wp-content/uploads/2013/04/0408-QIMIAO-3-lg.jpg
CAPTION: Rice University physicists Qimiao Si (left) and Rong Yu.
CREDIT: Jeff Fitlow/Rice University
A copy of the PRL paper is available at:
http://prl.aps.org/abstract/PRL/v110/i14/e146402
This release can be found online at news.rice.edu.
Electron conflict leads to 'bad traffic' on way to superconductivity
Strange state: Some electrons remain mobile while their neighbors are locked down
2013-04-06
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[Press-News.org] Electron conflict leads to 'bad traffic' on way to superconductivityStrange state: Some electrons remain mobile while their neighbors are locked down