|
An
international team of researchers has used SLAC’s Linac Coherent Light
Source (LCLS) to discover never-before-seen behavior by electrons in
complex materials with extraordinary properties.
The
result is an important step forward in the investigation of so-called
strongly correlated materials, whose unusual qualities and futuristic
applications stem from the collective behavior of their electrons. By
understanding how these materials work, scientists hope to ultimately
design novel materials that, for instance, conduct electricity with
absolutely no resistance at room temperature, dramatically improving the
performance and efficiency of energy transmission and electronic
devices.
In a report published yesterday in Nature Communications,
researchers led by SLAC Chief Scientist Zhi-Xun Shen and Lawrence
Berkeley National Laboratory Scientist Zahid Hussain describe
experiments at the LCLS with a material called striped nickelate.
It
gets its name from the pattern of alternating stripes of enhanced
charge and spin that its electrons collectively assume under certain
conditions. This pattern constitutes a new quantum state, and it
provides a model system that scientists can use to learn about electron
correlations and their impact on the properties of materials.
The
researchers hit the material with a pulse from an infrared laser, and
then used an exceedingly intense, brief flash of X-ray laser light from
LCLS—just a few millionths of a billionth of a second long—to record
what happened.
The
initial pulse jarred the nickelate out of its striped state. By varying
the interval between the two pulses, the researchers created images
that showed how the charge stripes reemerged. They were surprised to
find that variations in the locations of minimum and maximum charge,
controlled by a quantity called phase, persisted long after the stripes’
charge distribution returned to its original magnitude.
“These
phase fluctuations are very important for understanding how these
materials behave,” said Wei-Sheng Lee, a SLAC physicist and lead author
on the research. “But until now, they have been impossible to discern
directly. Being able to see this electron behavior represents a new era
in materials science research.”
Other
members of the research team come from the University of
California-Berkeley, Lawrence Berkeley National Laboratory, Swiss Light
Source, European X-ray Free-Electron Laser, Max-Planck Research Group
for Structural Dynamics in Germany and the Tokyo Institute of
Technology.
Yesterday’s
report is the fourth in the past six months describing experiments at
LCLS that use pairs of optical and X-ray pulses to excite and probe
materials that combine oxygen with so-called transition metals, such as
nickel, copper, titantium, or manganese.
While
these transition-metal oxides can have many fantastic properties, the
one that has caused the most scientific excitement is the prospect of
being able to conduct electricity without resistance at much higher
temperatures than is possible today.
Until
1986, all the known superconductors worked only at extremely low
temperatures, limiting their usefulness. But that year, two Swiss
scientists discovered that a copper-based oxide lost all its electrical
resistance at 32 Kelvin—about minus 400 F. While this is still close to
absolute zero, it was 12 degrees Kelvin higher than had ever been seen
before.
The Swiss researchers received the Nobel Prize in Physics the very next year.
In a frantic worldwide scramble, scientists found dozens of
transition-metal oxide combinations that became superconducting at even
higher temperatures—up to 135 Kelvin at atmospheric pressure.
However,
the dream of developing room-temperature superconductors for a new
generation of magnetically levitated trains, superfast computers and
super-efficient electrical power lines has not been realized, because no
one knows why these complex materials behave as they do or how to
predict their properties.
“When
scientists first looked at these materials 26 years ago, they had no
clue that such spectacular properties would appear,” Shen said. “With
LCLS, we now we have a new tool to help us learn how these properties
arise.”
Source: SLAC
