Light derails electrons through graphene

Date:
March 24, 2022
Source:
ICFO-The Institute of Photonic Sciences
Summary:
Researchers have experimentally caused electrons to bend in
bilayer graphene with the use of light. The way electrons flow in
materials determine its electronic properties. For example, when a
voltage is sustained across a conducting material, electrons start
flowing, generating an electrical current. These electrons are often
thought to flow in straight paths, moving along the electric field,
much like a ball rolling down a hill. Yet these are not the only
trajectories electrons can take: when a magnetic field is applied,
the electrons no longer travel in straight paths along the electric
field, but in fact, they bend. The bent electronic flows lead to
transverse signals called 'Hall' responses.



FULL STORY ========================================================================== Researchers have experimentally caused electrons to bend in bilayer
graphene with the use of light. The way electrons flow in materials
determine its electronic properties. For example, when a voltage
is sustained across a conducting material, electrons start flowing,
generating an electrical current.

These electrons are often thought to flow in straight paths, moving
along the electric field, much like a ball rolling down a hill. Yet these
are not the only trajectories electrons can take: when a magnetic field
is applied, the electrons no longer travel in straight paths along the
electric field, but in fact, they bend. The bent electronic flows lead
to transverse signals called "Hall" responses.


==========================================================================
Now, is it possible to bend electrons without applying a magnetic
field? In a study recently published in Science, an international team
of researchers report that circular polarized light can induce bent
electronic flows in bilayer graphene. The study has been carried out by
a team including ICFO scientists Jianbo Yin (currently researcher from
the Beijing Graphene Institute, China), David Barcons, Iacopo Torre,
led by ICREA Prof. at ICFO Frank Koppens, in collaboration with Cheng
Tan and James Hone from Columbia University, Kenji Watanabe and Takashi Taniguchi from NIMS Japan and Prof.

Justin Song from Nanyang Technological University (NTU) in Singapore.

Jianbo Yin, first author of the study, remembers how it all started. "This collaborative study began in 2016 with a conversation between Justin
Song and Frank Koppens at a scientific conference." As Justin Song
explains, "Electrons are not just particles, but can have a quantum
wave-like nature." In quantum materials, such as bilayer graphene, the
wave pattern of electrons can exhibit a complex winding often referred
to as quantum geometry. "Frank and I talked about the possibility of
harnessing quantum geometry in bilayer graphene to bend the flow of
electrons with light instead of using magnetic fields." With this in
mind, Jianbo Yin, a researcher in Frank Koppens' team, decided to take on
the challenge of experimentally realizing this unusual phenomenon. "Our
device was very complicated to build. It took building many devices and
flying to Columbia University to work with Cheng Tan and James Hone to
improve the device quality." Quantum geometry and Valley selectivity In bilayer graphene, there are two pockets of electron valleys (K and K'):
when a perpendicular electric field is applied, the quantum geometrical properties of electrons in these two valleys can cause them to bend in
opposite directions. As a result, their Hall effects are cancelled out.

In their study, the team of scientists found that by applying circular polarized infrared light onto the bilayer graphene device, they were
able to selectively excite one specific valley population of electrons in
the material, which generated a photovoltage perpendicular to the usual electron flow. As Koppens highlights," we now engineered the device and
setup in such a way that current only flows with light illumination. With
this, we were able to avoid the background noise that hampers measurements
and achieve a sensitivity in the detection several orders of magnitude
better than any other 2D material." This development is significant
because conventional photodetectors often require large voltage biases
that can lead to "dark currents" that flow even when there is no light.

Yin remarks that "we can control the bending of the electrons with the
out-of- plane electric field we apply. We can change the bending angle
of these electrons, which can be quantified by the Hall conductivity. By controlling the voltage 'knob', the Berry curvature [one characteristic
of quantum geometry], can be tuned, which can lead to a giant Hall conductivity." The results of the study open a new realm of many
detection and imaging applications, as Koppens finally concludes. "Such discovery could have major implications in applications for infrared
and terahertz sensing since bilayer graphene can be transformed from
semimetal to semiconductor with a very small bandgap, so it can detect
photons of very small energies. It may be also useful, for example, for
imaging in space, medical imaging, e.g. for tissue skin cancer, or even
for security applications such as the quality inspection of materials."
The possibilities are manifold and the next steps of research focused on
new 2D materials, such as the moire' material twisted bilayer graphene,
may find new ways of controlling electron flows and unconventional opto-electronic properties.


========================================================================== Story Source: Materials provided by
ICFO-The_Institute_of_Photonic_Sciences. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Jianbo Yin, Cheng Tan, David Barcons-Ruiz, Iacopo Torre, Kenji
Watanabe,
Takashi Taniguchi, Justin C. W. Song, James Hone, Frank
H. L. Koppens.

Tunable and giant valley selective Hall effect in gapped bilayer
graphene. Science, 2022 DOI: 10.1126/science.abl4266 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220324143737.htm

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