Scientists at the U.S. Department of Energy's Brookhaven National Laboratory
have discovered a long-predicted magnetic state of matter called an
"antiferromagnetic excitonic insulator."
"Broadly speaking, this is a novel type of magnet," said Brookhaven Lab
physicist Mark Dean, senior author on a paper describing the research just
published in Nature Communications. "Since magnetic materials lie at the
heart of much of the technology around us, new types of magnets are both
fundamentally fascinating and promising for future applications."
The new magnetic state involves strong magnetic attraction between electrons
in a layered material that make the electrons want to arrange their magnetic
moments, or "spins," into a regular up-down "antiferromagnetic" pattern. The
idea that such antiferromagnetism could be driven by quirky electron
coupling in an insulating material was first predicted in the 1960s as
physicists explored the differing properties of metals, semiconductors, and
insulators.
"Sixty years ago, physicists were just starting to consider how the rules of
quantum mechanics apply to the electronic properties of materials," said
Daniel Mazzone, a former Brookhaven Lab physicist who led the study and is
now at the Paul Scherrer Institut in Switzerland. "They were trying to work
out what happens as you make the electronic 'energy gap' between an
insulator and a conductor smaller and smaller. Do you just change a simple
insulator into a simple metal where the electrons can move freely, or does
something more interesting happen?"
The prediction was that, under certain conditions, you could get something
more interesting: namely, the "antiferromagnetic excitonic insulator" just
discovered by the Brookhaven team.
Why is this material so exotic and interesting? To understand, let's dive
into those terms and explore how this new state of matter forms.
In an antiferromagnet, the electrons on adjacent atoms have their axes of
magnetic polarization (spins) aligned in alternating directions: up, down,
up, down and so on. On the scale of the entire material, those alternating
internal magnetic orientations cancel one another out, resulting in no net
magnetism of the overall material. Such materials can be switched quickly
between different states. They're also resistant to information being lost
due to interference from external magnetic fields. These properties make
antiferromagnetic materials attractive for modern communication
technologies.
Next, we have the excitonic. Excitons arise when certain conditions allow
electrons to move around and interact strongly with one another to form
bound states. Electrons can also form bound states with "holes," the
vacancies left behind when electrons jump to a different position or energy
level in a material. In the case of electron-electron interactions, the
binding is driven by magnetic attractions that are strong enough to overcome
the repulsive force between the two like-charged particles. In the case of
electron-hole interactions, the attraction must be strong enough to overcome
the material's "energy gap," a characteristic of an insulator.
"An insulator is the opposite of a metal; it's a material that doesn't
conduct electricity," said Dean. Electrons in the material generally stay in
a low, or "ground," energy state. "The electrons are all jammed in place,
like people in a filled amphitheater; they can't move around," he said. To
get the electrons to move, you have to give them a boost in energy that's
big enough to overcome a characteristic gap between the ground state and a
higher energy level.
In very special circumstances, the energy gain from magnetic electron-hole
interactions can outweigh the energy cost of electrons jumping across the
energy gap.
Now, thanks to advanced techniques, physicists can explore those special
circumstances to learn how the antiferromagnetic excitonic insulator state
emerges.
A collaborative team worked with a material called strontium iridium oxide
(Sr3Ir2O7), which is only barely insulating at high temperature. Daniel
Mazzone, Yao Shen (Brookhaven Lab), Gilberto Fabbris (Argonne National
Laboratory), and Jennifer Sears (Brookhaven Lab) used X-rays at the Advanced
Photon Source—a DOE Office of Science user facility at Argonne National
Laboratory—to measure the magnetic interactions and associated energy cost
of moving electrons. Jian Liu and Junyi Yang from the University of
Tennessee and Argonne scientists Mary Upton and Diego Casa also made
important contributions.
The team started their investigation at high temperature and gradually
cooled the material. With cooling, the energy gap gradually narrowed. At 285
Kelvin (about 53 degrees Fahrenheit), electrons started jumping between the
magnetic layers of the material but immediately formed bound pairs with the
holes they'd left behind, simultaneously triggering the antiferromagnetic
alignment of adjacent electron spins. Hidemaro Suwa and Christian Batista of
the University of Tennessee performed calculations to develop a model using
the concept of the predicted antiferromagnetic excitonic insulator, and
showed that this model comprehensively explains the experimental results.
"Using X-rays we observed that the binding triggered by the attraction
between electrons and holes actually gives back more energy than when the
electron jumped over the band gap," explained Yao Shen. "Because energy is
saved by this process, all the electrons want to do this. Then, after all
electrons have accomplished the transition, the material looks different
from the high-temperature state in terms of the overall arrangement of
electrons and spins. The new configuration involves the electron spins being
ordered in an antiferromagnetic pattern while the bound pairs create a
'locked-in' insulating state."
The identification of the antiferromagnetic excitonic insulator completes a
long journey exploring the fascinating ways electrons choose to arrange
themselves in materials. In the future, understanding the connections
between spin and charge in such materials could have potential for realizing
new technologies.
Reference:
D. G. Mazzone et al, Antiferromagnetic excitonic insulator state in
Sr3Ir2O7, Nature Communications (2022).
DOI: 10.1038/s41467-022-28207-w
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Physics