Evidence for exotic magnetic phase of matter
Scientists identify a long-sought magnetic state predicted nearly 60
years ago

Date:
February 22, 2022
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists have discovered a long-predicted magnetic state of
matter called an antiferromagnetic excitonic insulator -- broadly
speaking, a novel type of magnet. Understanding the connections
between electrons' 'spin' and charge in such materials could have
potential for realizing new technologies.



FULL STORY ========================================================================== 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 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.

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. D. G. Mazzone, Y. Shen, H. Suwa, G. Fabbris, J. Yang, S.-S. Zhang,
H.

Miao, J. Sears, Ke Jia, Y. G. Shi, M. H. Upton, D. M. Casa, X. Liu,
Jian Liu, C. D. Batista, M. P. M. Dean. Antiferromagnetic excitonic
insulator state in Sr3Ir2O7. Nature Communications, 2022; 13 (1)
DOI: 10.1038/ s41467-022-28207-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220222135435.htm

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