Scientists capture the fleeting dance of moire' excitons

Date:
March 9, 2022
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
Scientists have visualized the two parts of an elusive particle --
the moire' exciton -- to reveal its structure and unique properties.



FULL STORY ========================================================================== Published in leading journalNature, scientists from the Okinawa Institute
of Science and Technology Graduate University (OIST), SLAC National
Accelerator Laboratory (SLAC), and Stanford University have, in a
world-first, imaged and measured the two parts of a unique particle,
called amoire' exciton. Excitons are hailed as having the potential to revolutionize technological and quantum devices, but they are usually
fleeting in nature, often lasting no longer than a few thousandths
of a billionth of a second, which makes them incredibly difficult to
study. Moire' excitons tend to be longer-lived and are thus an attractive
way to study the particle and its potential applications. Yet information
about their size, shape, and behavior remains unclear.


========================================================================== Excitons are created when light is shone on a semiconducting material. The particles of light, called photons, interact with the material's
electrons.

This causes the electrons to jump to a higher energy level. Each one
leaves behind a hole at the lower energy level. The two are oppositely
charged so they mutually attract and revolve around each other, forming
the short-lived exciton.

"We've previously visualized the electron part of the exciton before it disappeared," stated Mr. Vivek Pareek, one of the six first authors in
this paper and PhD candidate in OIST's Femtosecond Spectroscopy Unit. "In
this new study, we measured both parts of the exciton. This was the
first time we've seen the 'hole', or lack of an electron." "This is
really exciting research and could have vast implications," added Dr.

Michael Man, a staff scientist in the same unit and also one of the
first authors. "Excitons adopt the properties of the material they're
in. By controlling the materials and the environment of the excitons,
we can control the exciton itself." In the previous research, the
scientists visualized excitons in a two- dimensional, single layer semiconductor. Now, the researchers have stacked two semiconducting
layers on top of each other. When the exciton forms, the electron jumps
from one layer to the other. This forces the electron and hole to remain
apart for longer, extending the lifespan of the exciton.

The two-dimensional, two-layered samples were created in the
state-of-the-art laboratories at SLAC and Stanford University. The two
layers had to be aligned in a very specific way to create a pattern,
called a moire' pattern.



========================================================================== "Imagine that the atomic structure of the two different materials are like
two nets. The structures resemble each other but they are not exactly the same," explained Dr. Ouri Karni, also a first author and a postdoctoral researcher working in the group of Prof. Tony Heinz at SLAC and Stanford University. "When the nets are placed on top of each other, there are
certain places where the gaps in the nets overlap, and others where
they do not. This results in a moire' pattern. We aligned these atomic
'nets' at very specific angles so they would clearly exhibit the moire' pattern, which in turn expresses what is called moire' potential -- a
periodic 'landscape' of electronic energy levels across the material."
The samples were then sent to OIST where scientists utilized a powerful
and unique technique. They shone a beam of light within the extreme
ultraviolet range at the material. The energy was so high that the
excitons were broken apart and their electrons were sent flying out of
the material. By measuring the speed and angles of the electrons as they
left the material, the scientists were able to backtrack this information
and construct an image of the exciton.

Perhaps the most important and exciting part of this research, both conceptually and experimentally, was that the scientists were also able
to see the hole. Since the hole is actually the absence of an electron,
it doesn't emit any of its own signals and its presence can only be
detected by what is around it, similar to how black holes are detected.

"This is an incredibly powerful tool, which allowed us to obtain a
full picture of the exciton-how far apart the electron and hole were
from each other, and how much the two moved together in the material,"
said Mr. Jonathan Georgaras, another first author and PhD candidate in
Prof. Felipe Jornada's theory group at Stanford University.

Furthermore, they were also able to estimate how many excitons were
present, something they were unable to do with just the electron signals.

The researchers found that, due to the moire' potential, the excitons were
very localized and formed in places where the energy was minimal. This
meant that the excitons were effectively pinned in tiny pockets of
around 1.8 nanometers, despite their relatively large diameter, at around
5.2 nanometers.

"After nearly a century of knowing about the existence of excitons, we
are now able to get a near-holistic view of this important particle by
peering into it and imaging both its constituent particles," concluded
Prof. Keshav Dani, who leads OIST's Femtosecond Spectroscopy Unit and
is a senior author of the paper.

"This research opens the doors to studying more sophisticated phenomena
with excitons for quantum technology. Our current demonstration of
the pinning of the large moire' exciton in tiny pockets is just the
beginning."

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Lucy Dickie. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Ouri Karni, Elyse Barre', Vivek Pareek, Johnathan D. Georgaras,
Michael
K. L. Man, Chakradhar Sahoo, David R. Bacon, Xing Zhu, Henrique B.

Ribeiro, Aidan L. O'Beirne, Jenny Hu, Abdullah Al-Mahboob, Mohamed
M. M.

Abdelrasoul, Nicholas S. Chan, Arka Karmakar, Andrew J. Winchester,
Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Katayun Barmak,
Julien Made'o, Felipe H. da Jornada, Tony F. Heinz, Keshav
M. Dani. Structure of the moire' exciton captured by imaging
its electron and hole. Nature, 2022; 603 (7900): 247 DOI:
10.1038/s41586-021-04360-y ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220309131907.htm

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