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| A new material has exhibited colossal magnetoresistance, switching its electrical conductivity by a billion percent in response to a magnetic field: Depositphotos |
A group of physicists, including two Georgia Tech researchers, have
discovered a new quantum state. The study, published in the journal Nature,
uncovered novel looping currents flowing along the edges of octahedral cells
in a crystal of Mn3Si2Te6, which allowed for a billion percent increase in
the material’s electric conductivity. The findings could lead to a new
paradigm for quantum devices and superconductors.
The team consisted of Georgia Tech theoretical physicists Sami Hakani and
Itamar Kimchi, along with experimental physicists Feng Ye (Oak Ridge
National Lab), Lance DeLong (University of Kentucky), and, from the
University of Colorado at Boulder: Gang Cao, Yifei Ni, Yu Zhang, and Hengdi
Zhao. The group was drawn to the research after their
previous study
investigated the same material.
“Because this material did not fit any preexisting models, we had to develop
new ideas to understand it,” said Georgia Tech graduate student Hakani, who
played a key role in developing the theory. “These new ideas will help us
study related materials that could be used for next-generation magnetic
field devices.”
An Exception to the Rule
The physicists first became interested in the Mn3Si2Te6 material due to its
unique electrical properties — in particular, a property called colossal
magnetoresistance, an extreme enhancement in a material’s electrical
conductivity when a magnetic field is applied.
In most materials, applying a magnetic field does not change that material’s
conductivity. However, in another class of materials, applying a magnetic
field does change conductivity; this is called magnetoresistance, and it can
scale to “giant” and “colossal” changes in conductivity. In instances of
colossal magnetoresistance, a material can change from behaving like an
insulator (like Styrofoam) to being as conductive as a metal wire.
This change is not altogether unusual. Materials displaying giant
magnetoresistance are not uncommon and are often used in computers; however,
in all of these known materials, the material does not change its behavior
in a way that significantly depends on the direction of the applied magnetic
field. This new trimer-honeycomb material does.
“The phenomenon defies all existing theoretical models and experimental
precedents,” said Kimchi, theoretical physicist and assistant professor in
the School of Physics at Georgia Tech. And that’s where he and Hakani come
in.
Uncovering Looping Currents
“As theoretical physicists, we develop new kinds of mathematical models,”
said Kimchi. “When it’s qualitatively difficult to understand how anything
can make sense in experimental data — when there’s something qualitatively
shocking — we try to come up with that basic picture.”
Using the information uncovered by the experimental physicists, Hakani and
Kimchi set out to understand why the extreme change in conductivity only
happens when the magnetic field is applied perpendicularly to the
honeycomb-like surface of the material.
“Our idea smelled promising, but, unfortunately, we quickly realized that
currents between the magnetic manganese ions would be forbidden by symmetry,
which was discouraging,” said Kimchi. “However, Sami then did the symmetry
analysis for the octahedrally arranged tellurium ions, and, for them,
currents were symmetry-allowed and could work out!”
Viewed from above, the material looks like a series of two-dimensional
honeycombs. From the side, however, the material is composed of “sheets,”
like a layer cake. Within each “sheet” of honeycomb, electrons can move in
circular paths around each octahedral cell. These looping, circular-moving
currents within the material are responsible for the material’s unique
behavior.
On its own, without a magnetic field present, electrons move both
counterclockwise and clockwise around the honeycomb “cells,” like cars going
in both directions around a roundabout. Just like in uncontrolled traffic,
“traffic jams” make it difficult for electrons to move quickly throughout
the material. Without a way to streamline traffic, the material acts more
like an insulator.
However, if a magnetic field is applied perpendicular to the honeycomb-like
surface, a “flow of traffic” is established, and electrons navigate the
loops more quickly. The material then acts as a conductor, showing a
seven-magnitude increase in conductivity — equivalent to an increase of a
billion percent.
A New Paradigm
The transformation from insulator to conductor can also be driven by
applying electrical currents in the material, but in that case, it doesn’t
happen instantaneously. It can take seconds or even minutes for the material
to switch from insulator to conductor.
The team believes that this tunability and slower type of switching, coupled
with the material’s sensitivity to currents, could lead to new applications
and discoveries in current-controlled quantum devices, a field of devices
that range from sensors to computers to secure communication.
The next step? Working to better understand the newly discovered quantum
state, and finding other materials where the quantum state might exist.
“Looking forward, we hope to understand not only what makes this material
special, but also which microscopic ingredients are needed for related
materials to become useful quantum technologies in our future,” said Hakani.
Reference:
Zhang, Y., Ni, Y., Zhao, H. et al. Control of chiral orbital currents in a
colossal magnetoresistance material. Nature 611, 467–472 (2022).
DOI: 10.1038/s41586-022-05262-3