Physicists find that misaligned carbon sheets produce incomparable properties

A material consisting of two carbon layers of an atom thick has caught the attention of physicists around the world for its intriguing and potentially exploitable conductive properties.

Dr. Fan Zhang, an assistant professor of physics at the University of Texas School of Natural Sciences and Mathematics at Dallas, and Ph.D. student in physics Qiyue Wang published an article in June with Dr. Fengnian Xia’s group at the 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.

One to two layers

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, where each hexagon is made up of six carbon atoms at their vertices. Since graphene’s first isolation in 2004, its unique properties have been intensively studied by scientists for its potential use in computers, materials, and advanced devices.

If two sheets of graphene are stacked on top of each other, and one layer is rotated so that the layers are slightly misaligned, the resulting physical configuration, called twisted bilayer graphene, produces 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,” said Zhang. “A single layer is interesting to study, but if we have two layers, their interaction should make physics much richer and more interesting. That is why we want to study double-layer graphene systems.”

A new field emerges

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

“The angle at which the two graphene layers are misaligned, the angle of gyration, is of vital importance to the electronic properties of the material,” said Wang. “The smaller the angle of gyration, the greater the periodicity of the moire. “

The unusual effects of specific gyration 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 the time, others weren’t really paying attention to the theory, but now it has become possibly the hottest topic in physics,” said Zhang.

In that 2011 investigation, MacDonald and Bistritzer predicted that the kinetic energy of electrons can disappear in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology tested this theory and found that compensating two layers of graphene by 1.1 degrees produced a two-dimensional superconductor, a material that conducts electrical current without resistance and without loss of energy.

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

“In our previous work, we looked at superconductivity as well as insulation. That’s what makes studying twisted double-layer graphene such a hot field: superconductivity. The fact that pure carbon can be manipulated to superconductivity is surprising and unprecedented, “said Wang.

New UT Dallas Findings

In their most recent research at Nature Photonics, Zhang and his colleagues at Yale investigated whether twisted bilayer graphene and how it interacts with mid-infrared light, which humans cannot see but can detect as heat.

“The interactions between light and matter are useful in many devices, for example, converting sunlight into electrical energy,” said Wang. “Almost all objects emit 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 could affect the conductance of electrons in twisted bilayer graphene. His work consisted of calculating light absorption based on the band structure of the moire pattern, a concept that determines how electrons in a quantum material move mechanically.

“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 make physics much richer and more interesting. That is why we want to study graphene systems. double layer. ” He says.

“There are standard ways of calculating band structure and light absorption in normal glass, but this is an artificial glass, so we had to come up with a new method,” said Wang. Using resources from the Advanced Computing Center at Texas, a supercomputer facility on the UT Austin campus, Wang calculated the band’s structure and showed how the material absorbs light.

The Yale group made devices and conducted experiments showing that the mid-infrared photoresponse (the increase in conductance due to the brightness of light) was unusually strong and greatest at the 1.8-degree turn angle. The strong photoresponse faded at a turning angle of less than 0.5 degrees.

“Our theoretical results not only agreed well with the experimental findings, but also pointed to a mechanism that is fundamentally connected to the moire pattern period, which in turn is connected to the angle of gyration between the two graphene layers,” he said. Zhang.

Next step

“The angle of gyration is clearly very important in determining the properties of twisted bilayer graphene,” added Zhang. “The question that arises is: Can we apply this to fit other two-dimensional materials for unprecedented characteristics? Also, can we combine photoresponse and superconductivity in twisted bilayer graphene? It will be very interesting to study.”

“This new advancement will potentially enable a new class of graphene-based infrared detectors with high sensitivity,” said Dr. Joe Qiu, program manager for solid state and electromagnetic electronics in the US Army Research Office. . (ARO), an element of the US Army Combat Capability Development Command. “These new detectors will have a potential impact on applications such as night vision, which is of critical importance to the United States Army. ”