Lasers trigger magnetism in atomically thin quantum materials

Date:
April 20, 2022
Source:
University of Washington
Summary:
Researchers have discovered that light -- from a laser -- can
trigger a form of magnetism in a normally nonmagnetic material. This
magnetism centers on the behavior of electrons 'spins,' which
have a potential applications in quantum computing. Scientists
discovered that electrons within the material became oriented in
the same direction when illuminated by photons from a laser. By
controlling and aligning electron spins at this level of detail
and accuracy, this platform could have applications in the field
of quantum simulation.



FULL STORY ========================================================================== Researchers have discovered that light -- in the form of a laser --
can trigger a form of magnetism in a normally nonmagnetic material. This magnetism centers on the behavior of electrons. These subatomic particles
have an electronic property called "spin," which has a potential
application in quantum computing.

The researchers found that electrons within the material became oriented
in the same direction when illuminated by photons from a laser.


==========================================================================
The experiment, led by scientists at the University of Washington and
the University of Hong Kong, was published April 20 in Nature.

By controlling and aligning electron spins at this level of detail
and accuracy, this platform could have applications in the field of
quantum simulation, according to co-senior author Xiaodong Xu, a Boeing Distinguished Professor at the UW in the Department of Physics and the Department of Materials Science and Engineering.

"In this system, we can use photons essentially to control the 'ground
state' properties -- such as magnetism -- of charges trapped within the semiconductor material," said Xu, who is also a faculty researcher with
the UW's Clean Energy Institute and the Molecular Engineering & Sciences Institute. "This is a necessary level of control for developing certain
types of qubits -- or 'quantum bits' -- for quantum computing and other applications." Xu, whose research team spearheaded the experiments, led
the study with co- senior author Wang Yao, professor of physics at the University of Hong Kong, whose team worked on the theory underpinning the results. Other UW faculty members involved in this study are co-authors Di Xiao, a UW professor of physics and of materials science and engineering
who also holds a joint appointment at the Pacific Northwest National Laboratory, and Daniel Gamelin, a UW professor of chemistry and director
of the Molecular Engineering Materials Center.

The team worked with ultrathin sheets -- each just three layers of
atoms thick -- of tungsten diselenide and tungsten disulfide. Both are semiconductor materials, so named because electrons move through them
at a rate between that of a fully conducting metal and an insulator,
with potential uses in photonics and solar cells. Researchers stacked
the two sheets to form a "moire' superlattice," a stacked structure made
up of repeating units.



========================================================================== Stacked sheets like these are powerful platforms for quantum physics
and materials research because the superlattice structure can hold
excitons in place. Excitons are bound pairs of "excited" electrons and
their associated positive charges, and scientists can measure how their properties and behavior change in different superlattice configurations.

The researchers were studying the exciton properties within the material
when they made the surprising discovery that light triggers a key
magnetic property within the normally nonmagnetic material. Photons
provided by the laser "excited" excitons within the laser beam's path,
and these excitons induced a type of long-range correlation among other electrons, with their spins all orienting in the same direction.

"It's as if the excitons within the superlattice had started to 'talk'
to spatially separated electrons," said Xu. "Then, via excitons, the
electrons established exchange interactions, forming what's known as
an 'ordered state' with aligned spins." The spin alignment that the researchers witnessed within the superlattice is a characteristic of ferromagnetism, the form of magnetism intrinsic to materials like iron. It
is normally absent from tungsten diselenide and tungsten disulfide. Each repeating unit within the moire' superlattice is essentially acting like
a quantum dot to "trap" an electron spin, said Xu. Trapped electron spins
that can "talk" to each other, as these can, have been suggested as the
basis for a type of qubit, the basic unit for quantum computers that
could harness the unique properties of quantum mechanics for computation.

In a separate paper published Nov. 25 in Science, Xu and his collaborators found new magnetic properties in moire' superlattices formed by
ultrathin sheets of chromium triiodide. Unlike the tungsten diselenide
and tungsten disulfide, chromium triiodide harbors intrinsic magnetic properties, even as a single atomic sheet. Stacked chromium triiodide
layers formed alternating magnetic domains: one that is ferromagnetic
-- with spins all aligned in the same direction -- and another that is "antiferromagnetic," where spins point in opposite directions between
adjacent layers of the superlattice and essentially "cancel each other
out," according to Xu. That discovery also illuminates relationships
between a material's structure and its magnetism that could propel future advances in computing, data storage and other fields.

"It shows you the magnetic 'surprises' that can be hiding within moire' superlattices formed by 2D quantum materials," said Xu. "You can never be
sure what you'll find unless you look." First author of the Nature paper
is Xi Wang, a UW postdoctoral researcher in physics and chemistry. Other co-authors are Chengxin Xiao at the University of Hong Kong; UW physics doctoral students Heonjoon Park and Jiayi Zhu; Chong Wang, a UW researcher
in materials science and engineering; Takashi Taniguchi and Kenji Watanabe
at the National Institute for Materials Science in Japan; and Jiaqiang
Yan at the Oak Ridge National Laboratory. The research was funded by
the U.S. Department of Energy; the U.S. Army Research Office; the U.S.

National Science Foundation; the Croucher Foundation; the University Grant Committee/Research Grants Council of Hong Kong Special Administrative
Region; the Japanese Ministry of Education, Culture, Sports, Science and Technology; the Japan Society for the Promotion of Science; the Japan
Science and Technology Agency; the state of Washington; and the UW.


========================================================================== Story Source: Materials provided by University_of_Washington. Original
written by James Urton. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Illustration_of_light-induced_ferromagnetism ========================================================================== Journal Reference:
1. Xi Wang, Chengxin Xiao, Heonjoon Park, Jiayi Zhu, Chong Wang,
Takashi
Taniguchi, Kenji Watanabe, Jiaqiang Yan, Di Xiao, Daniel R. Gamelin,
Wang Yao, Xiaodong Xu. Light-induced ferromagnetism in moire'
superlattices.

Nature, 2022; 604 (7906): 468 DOI: 10.1038/s41586-022-04472-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220420133602.htm

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