Physicists find misaligned carbon sheets yield unparalleled properties


Date:
July 31, 2020
Source:
University of Texas at Dallas
Summary:
A material composed of two one-atom-thick layers of carbon has
grabbed the attention of physicists worldwide for its intriguing --
and potentially exploitable -- conductive properties. University of
Texas at Dallas physicists are studying how the ability of twisted
bilayer graphene to conduct electrical current changes in response
to mid- infrared light.



FULL STORY ==========================================================================
A material composed of two one-atom-thick layers of carbon has grabbed the attention of physicists worldwide for its intriguing -- and potentially exploitable -- conductive properties.


==========================================================================
Dr. Fan Zhang, assistant professor of physics in the School of Natural
Sciences and Mathematics at The University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with Dr. Fengnian Xia's group at Yale University in Nature Photonics that describes how
the ability of twisted bilayer graphene to conduct electrical current
changes in response to mid-infrared light.

From One to Two Layers Graphene is a single layer of carbon atoms arranged
in a flat honeycomb pattern, where each hexagon is formed by six carbon
atoms at its vertices.

Since graphene's first isolation in 2004, its unique properties have been intensely studied by scientists for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of one another, and one
layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graphene, yields electronic properties that differ significantly from those exhibited by
a single layer alone or by two aligned layers.

"Graphene has been of interest for about 15 years," Zhang said. "A
single layer is interesting to study, but if we have two layers, their interaction should render much richer and more interesting physics. This
is why we want to study bilayer graphene systems." A New Field Emerges


==========================================================================
When the graphene layers are misaligned, a new periodic design in the mesh emerges, called a moire' pattern. The moire' pattern is also a hexagon,
but it can be made up of more than 10,000 carbon atoms.

"The angle at which the two layers of graphene are misaligned -- the
twist angle -- is critically important to the material's electronic properties," Wang said. "The smaller the twist angle, the larger
the moire' periodicity." The unusual effects of specific twist
angles on electron behavior were first proposed in a 2011 article by
Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this field as a doctoral student
in MacDonald's group.

"At that time, others really paid no attention to the theory, but now
it has become arguably the hottest topic in physics," Zhang said.

In that 2011 research MacDonald and Bistritzer predicted that electrons' kinetic energy can vanish in a graphene bilayer misaligned by the
so-called "magic angle" of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory, finding that offsetting two graphene layers by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current with no
resistance and no energy loss.



==========================================================================
In a 2019 article in Science Advances, Zhang and Wang, together with
Dr. Jeanie Lau's group at The Ohio State University, showed that
when offset by 0.93 degrees, twisted bilayer graphene exhibits both superconducting and insulating states, thereby widening the magic angle significantly.

"In our previous work, we saw superconductivity as well as
insulation. That's what's making the study of twisted bilayer graphene
such a hot field - - superconductivity. The fact that you can manipulate
pure carbon to superconduct is amazing and unprecedented," Wang said.

New UT Dallas Findings In his most recent research in Nature Photonics,
Zhang and his collaborators at Yale investigated whether and how
twisted bilayer graphene interacts with mid- infrared light, which
humans can't see but can detect as heat. "Interactions between light and
matter are useful in many devices -- for example, converting sunlight
into electrical power," Wang said. "Almost every object emits infrared
light, including people, and this light can be detected with devices."
Zhang is a theoretical physicist, so he and Wang set out to determine how
mid- infrared light might affect the conductance of electrons in twisted bilayer graphene. Their work involved calculating the light absorption
based on the moire' pattern's band structure, a concept that determines
how electrons move in a material quantum mechanically.

"There are standard ways to calculate the band structure and light
absorption in a regular crystal, but this is an artificial crystal,
so we had to come up with a new method," Wang said. Using resources of
the Texas Advanced Computing Center, a supercomputer facility on the
UT Austin campus, Wang calculated the band structure and showed how the material absorbs light.

The Yale group fabricated devices and ran experiments showing that the
mid- infrared photoresponse -- the increase in conductance due to the
light shining -- was unusually strong and largest at the twist angle of
1.8 degrees. The strong photoresponse vanished for a twist angle less
than 0.5 degrees.

"Our theoretical results not only matched well with the experimental
findings, but also pointed to a mechanism that is fundamentally connected
to the period of moire' pattern, which itself is connected to the twist
angle between the two graphene layers," Zhang said.

Next Step "The twist angle is clearly very important in determining
the properties of twisted bilayer graphene," Zhang added. "The question
arises: Can we apply this to tune other two-dimensional materials to get unprecedented features? Also, can we combine the photoresponse and the superconductivity in twisted bilayer graphene? For example, can shining
a light induce or somehow modulate superconductivity? That will be very interesting to study." "This new breakthrough will potentially enable a
new class of infrared detectors based on graphene with high sensitivity,"
said Dr. Joe Qiu, program manager for solid-state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of
the U.S. Army Combat Capabilities Development Command's Army Research Laboratory. "These new detectors will potentially impact applications
such as night vision, which is of critical importance for the U.S. Army."
In addition to the Yale researchers, other authors included scientists
from the National Institute for Materials Science in Japan. The ARO, the National Science Foundation and the Office of Naval Research supported
the study.


========================================================================== Story Source: Materials provided
by University_of_Texas_at_Dallas. Original written by Amanda
Siegfried. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Bingchen Deng, Chao Ma, Qiyue Wang, Shaofan Yuan, Kenji Watanabe,
Takashi
Taniguchi, Fan Zhang, Fengnian Xia. Strong mid-infrared
photoresponse in small-twist-angle bilayer graphene. Nature
Photonics, 2020; DOI: 10.1038/ s41566-020-0644-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200731145134.htm

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