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Physicists find that uneven carbon sheets give incomparable properties



Physicists find that uneven carbon sheets give incomparable properties

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. UT Dallas physicists study the electrical properties that occur when two layers of graphene are laid. Credit: University of Texas at Dallas

The material, made up of two layers of carbon one atom thick, has attracted the attention of physicists around the world for its intriguing ̵

1; and potentially operational – conductive properties.

Dr. Fang Zhang, an associate professor of physics at the School of Natural and Mathematical Sciences at the University of Texas at Dallas, and Ph.D. Photography of nature describing the ability of twisted bilayer graphene to make changes in electric current in response to light of 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 the first isolation of graphene in 2004, its unique properties have been intensively studied by scientists 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 not slightly aligned, the resulting physical configuration, called twisted two-layer graphene, gives electronic properties that are significantly different from those developed by one layer alone or two aligned layers.

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

A new field appears

When the layers of graphene are uneven, a new periodic grid design is obtained, which is called a moire pattern. The moire pattern is also a hexagon, but it can consist of more than 10,000 carbon atoms.

“The angle at which the two layers of graphene are unequal, the angle of rotation, is critical to the electronic properties of the material,” Wang said. “The smaller the angle of rotation, the greater the frequency of the moire.”

Unusual effects of specific angles of rotation on the behavior of electrons were first proposed in an article in 2011 by Dr. Allan MacDonald, Professor of Physics at UT Austin, and Dr. Rafi Bistritzer. Zhang witnessed the birth of this industry as a doctoral student of the McDonald’s.

“At the time, others didn’t really pay attention to the theory, but now it’s probably the hottest topic in physics,” Zhang said.

In that 2011 study, MacDonald and Bistritzer predicted that the kinetic energy of electrons could disappear in two-layer graphene, interspersed with a so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory by finding that shifting two layers of graphene by 1.1 degrees yielded a two-dimensional superconductor, a material that conducts electricity without resistance and without energy loss.





This animation shows what happens when two folded layers of graphene are unequal, aligned by a small number called the angle of rotation. There is a new periodic design in the grid, which is called the moire pattern. UT Dallas physicists study how the angle of rotation affects the electronic properties of such twisted two-layer graphene. Credit: University of Texas at Dallas

In an article on Science in 2019, Zhang and Wang, along with Dr. Jenny Lau’s group at Ohio State University, showed that when shifted by 0.93 degrees, twisted bilayer graphene detects both superconducting and insulating states, thereby greatly expanding the magic angle.

“In previous work, we’ve seen both superconductivity and insulation. That’s what makes the study of twisted bilayer graphene such a hot field – superconductivity. The fact that you can manipulate pure carbon for superconductivity is amazing and unprecedented,” Wang said.

New findings of UT Dallas

In their latest study, Nature Photonics, Zhang and his colleagues at Yel investigated whether twisted bilayer graphene interacts with medium infrared light, which humans cannot see but can detect heat.

“The interaction between light and matter is useful in many devices – for example, to convert sunlight into electricity,” Wang said. “Virtually every object emits infrared light, including humans, and that light can be detected by devices.”

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

“Graphene has been interesting for about 15 years. One layer is interesting to study, but if we have two layers, their interaction should make the physics much richer and more interesting. That’s why we want to study two-layer graphene systems,” he said.

“There are standard ways to calculate the band structure and light absorption in an ordinary crystal, but it’s 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 equipment on the UT Austin campus, Wang calculated the structure of the strip and showed how the material absorbs light.

The Yale group made devices and conducted experiments, showing that the average infrared photoreaction – an increase in electrical conductivity through radiant light – was unusually strong and the largest at a rotation angle of 1.8 degrees. The strong photo response disappeared at a rotation angle of less than 0.5 degrees.

“Our theoretical results not only agreed well with the experimental conclusions, but also indicated a mechanism that is fundamentally related to the moire model period, which itself is related to the angle of rotation between the two layers of graphene,” Zhang said.

The next step

“The angle of rotation is obviously very important in determining the properties of twisted two-layer graphene,” Zhang added. “The question is: can we use this to tune other two-dimensional materials to get unprecedented features? Also, can we combine photo response and superconductivity in twisted two-layer graphene? For example, can bright light induce or somehow modulate superconductivity? It will be very interesting. to study “.

“This new breakthrough has the potential to provide a new class of high-sensitivity graphene-based infrared detectors,” said Dr. Joe Qiu, head of the solid-state electronics and electromagnetism program at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Research Laboratory. . “These new detectors will potentially affect programs such as night vision, which is critical for the U.S. military.”


Physicists are discovering graphene, which can help develop superconductors


More information:
Bingchen Deng et al. Strong medium-infrared photo response in two-layer two-layer graphene, Photography of nature (2020). DOI: 10.1038 / s41566-020-0644-7

Provided by the University of Texas at Dallas



Citation: Physicists have found that uneven carbon sheets provide incomparable properties (2020, July 31), obtained July 31, 2020 from https://phys.org/news/2020-07-physicists-misaligned-carbon-sheets-yield.html

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