A highly unusual movement of light emitting particles in atomically-thin
semiconductors was experimentally confirmed by scientists from the
Würzburg–Dresden Cluster of Excellence ct.qmat–Complexity and Topology in
Quantum Matter. Electronic quasiparticles, known as excitons, seemed to move
in opposite directions at the same time. Professor Alexey Chernikov–newly
appointed physicist at the Technische Universität Dresden–and his team were
able to reveal the consequences of this quantum phenomenon by monitoring
light emission from mobile excitons using ultrafast microscopy at extremely
low temperatures. These findings move the topic of quantum transport of
excitonic many-body states into the focus of modern research. The results of
this work have been published in the Physical Review Letters journal.
Light emitters in atomically-thin matter
Quantum materials studied by Alexey Chernikov and his team are only a few
atoms thin. Due to extremely strong interactions in these systems, electrons
come together to form new states known as excitons. Excitons behave like
independent particles and are able to absorb and emit light with high
efficiency. In atomically-thin layers they are stable from lowest
temperatures such as minus 268 degree Celsius up to room temperature.
Regarding the current research project that focuses on the movement of
excitons in ultra-thin matter, the physicist Chernikov explains: "Excitons
can be understood as a kind of moving light sources. Like other quantum
mechanical objects, they combine both wave and particle properties,
propagating through atomically-thin crystals. It means that they can store
and transport both energy and information, but also convert them again to
light. That makes them particularly interesting for us."
On the trail of "crazy" quasiparticles
Rapid movement of excitons in atomically-thin semiconductors was visualized
using highly sensitive optical microscopy: "First we applied a short laser
pulse to the material that generated the excitons. Then we used an ultrafast
detector to observe when and where the light was reemitted. When we repeated
these experiments at very low temperatures, however, the movement of
quasiparticles appeared rather astonishing," says Chernikov.
Moving in two directions at the same time
So far, two general types of exciton movement were broadly known to the
scientific community: either the excitons "jump" from one molecule to
another (process known as hopping)–or they move rather "classically" like
billiard balls that change their direction after random scattering events.
"In the ultra-thin semiconductors, however, the excitons behaved in a way
that we have never seen before. In the end, the only possible explanation
was that the excitons would occasionally move through closed loops in
opposite directions at the same time. Such behavior was in fact known from
individual electrons. However, to observe this experimentally for
luminescent excitons–that was quite unusual," notes Chernikov.
After all control experiments confirmed the result, the scientists looked
for the cause of their unusual observation. A recently published theoretical
work by the Russian researcher Mikhail M. Glazov from the Ioffe Institute in
Saint Petersburg provided the key insight: Glazov describes how excitons in
atomically-thin semiconductors can indeed move through closed, ring-like
paths and enter superimposed states. This means that the excitons seem to
essentially move both clockwise and counterclockwise at the same time. This
effect is a purely quantum mechanical phenomenon, which does not occur for
classical particles. Together with the team of Ermin Malic from the Philipps
University of Marburg, who provided additional insights into the exciton
dynamics, the scientists were finally able to track down this unusual
behavior.
Outlook
In a collaboration with international colleagues Alexey Chernikov's team has
shown a way to experimentally monitor quantum mechanical effects in the
movement of interacting many-particle complexes. Research into the quantum
transport of excitonic quasiparticles, however, is still at the very
beginning. In the future, materials such as the ultra-thin layers examined
by Chernikov could also serve as a basis for new types of laser sources,
light sensors, solar cells or even building blocks for quantum computers.
Reference:
Koloman Wagner et al, Nonclassical Exciton Diffusion in Monolayer WSe2,
Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.076801
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