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Physicists have found that misaligned carbon sheets provide unique properties



Physicists have found that misaligned carbon sheets provide unique properties

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, each hexagon being formed by six carbon atoms at its vertices. UT Dallas physicists study the electrical properties that appear when two layers of graphene are stacked. Credit: University of Texas at Dallas

The material composed of two carbon layers with one atom has attracted the attention of physicists around the world for its interesting and potentially useful conductive properties.

Fan Zhang, an assistant professor of physics at the School of Science and Mathematics at the University of Texas at Dallas, and Qiyue Wang, a doctoral student of physics, published an article in June with Dr. Fengnian Xia at Yale University v Natural photonics , which describes the ability of twisted bilayer graphene to make changes in electric current 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, each hexagon being formed by six carbon atoms at its vertices. Since the first isolation of graphene in 2004, scientists have intensively studied its unique properties for potential use in advanced computers, materials and devices.

If two sheets of graphene are stacked on top of each other and one layer rotates so that the layers are slightly out of alignment, the resulting physical configuration called twisted two-layer graphene provides electronic properties that differ significantly from those of a single layer or two aligned layers. .

“Graphene has been of interest for about 1

5 years,” Zhang said. “A single layer is interesting to study, but if we have two layers, their interaction should result in a much richer and more interesting physics. That’s why we want to study two-layer graphene systems. “

A new field is created

When the graphene layers are misaligned, a new periodic design called moiré appears in the network. The moire pattern is also a hexagon, but may consist of more than 10,000 carbon atoms.

“The angle at which both graphene layers are misaligned – the twist angle – is critical to the electronic properties of the material,” Wang said. “The smaller the twist angle, the greater the moiré periodicity.”

The unusual effects of specific bending angles on the behavior of electrons 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 department as a doctoral student in the MacDonald’s group.

“At the time, another theory didn’t really address that theory, but now it’s becoming the hottest topic in physics,” Zhang said.

In this 2011 survey, MacDonald and Bistritzer predicted that the kinetic energy of electrons could disappear in a two-layer graphene, which is unbalanced in the so-called “Magic angle” of 1.1 degrees. In 2018, scientists at the Massachusetts Institute of Technology demonstrated this theory when they discovered that an offset of two graphene layers of 1.1 degrees created a two-dimensional superconductor, a material that conducts electricity without resistance and without energy loss.





This animation shows what happens when two stacked layers of graphene are aligned by a small amount called the angle of curvature. A new periodic design called moire will appear. Physicists at UT Dallas are investigating how the twist angle affects the electronic properties of such a twisted two-layer graphene. Credit: University of Texas at Dallas

In a 2019 article in Science Advances, Zhang and Wang, along with Dr. Jeanie Lau at Ohio State University have shown that when shifted by 0.93 degrees, twisted two-layer graphene exhibits both superconducting and insulating states, greatly expanding the magic angle.

“In our previous work, we saw superconductivity as well as insulation. This makes the study of twisted two-layer graphene such a hot field – superconductivity. The fact that you can handle pure carbon with superconductivity is amazing and unprecedented, ”said Wang.

New findings from UT Dallas

In their latest research in the field of Nature Photonics, Zhang and his colleagues at Yale examined whether and how twisted two-layer graphene interacts with mid-infrared light, which humans cannot see but can detect as heat.

“Interactions between light and matter are useful in many devices – for example, in converting sunlight into electricity,” Wang said. “Almost every object emits infrared light, including people, and this light can be detected by devices.”

Zhang is a theoretical physicist, so he and Wang decided to determine how mid-infrared light can affect the conductivity of electrons in a twisted-layer graphene. Their work was to calculate the absorption of light based on the structure of the moire band, a concept that determines how electrons move mechanically in a quantum material.

“Graphene has been of interest for about 15 years. A single layer is interesting to study, but if we have two layers, their interaction should cause a much richer and more interesting physics. That’s why we want to study two-layer graphene systems,” he says.

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

The Yale Group manufactured devices and performed experiments that showed that the mid-infrared photoreaction – an increase in conductivity due to light radiation – was unusually strong and largest at a twist angle of 1.8 degrees. The strong photoreaction disappeared at a twist angle of less than 0.5 degrees.

“Our theoretical results not only corresponded well with the experimental findings, but also pointed to a mechanism that is fundamentally associated with the moire period, which in itself is related 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 two-layer graphene,” adds Zhang. “The question arises: Can we use it to tune other two-dimensional materials to get unprecedented properties? Can we also combine photoreaction and superconductivity in twisted two-layer graphene? For example, can inductive light shine or somehow modulate superconductivity? will be very interesting to study. “

“This new breakthrough will potentially enable a new class of high-sensitivity graphene-based infrared detectors,” said Dr. Joe Qiu, program manager for solid state electronics and electromagnetics at the US Army Research Office (ARO), an element of the US Army Research Laboratory for Combat Development Capabilities. “These new detectors will have a potential impact on applications such as night vision, which are critical to the U.S. military.”


Physicists are discovering graphene, which could help in the development of superconductors


More information:
Bingchen Deng et al. Strong medium infrared photoreaction in two-layer graphene with a small angle of rotation, Natural photonics (2020). DOI: 10,1038 / s41566-020-0644-7

Provided by the University of Texas at Dallas



Citations: Physicists find misaligned carbon sheets provide unique properties (2020, July 31) obtained on July 31, 2020 from https://phys.org/news/2020-07-physicists-misaligned-carbon-sheets-yield.html

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