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MIT Physicists Transform Pencil Lead Into Electronic “Gold”


Turning Pencil Lead Into Gold

MIT researchers have uncovered unique properties in graphite by stacking five graphene layers in a precise order. This pentalayer rhombohedral stacked graphene can manifest insulating, magnetic, or topological characteristics, marking a significant discovery in material physics using innovative nanoscale microscopy techniques.

Isolate thin flakes that can be tuned to exhibit three important properties.

“It is kind of like one-stop shopping,” says Long Ju, an assistant professor in the MIT Department of Physics and leader of the work, which is reported in the October 5 issue of Nature Nanotechnology. “Nature has plenty of surprises. In this case, we never realized that all of these interesting things are embedded in graphite.”

Further, he says, “It is very rare material to find materials that can host this many properties.”

The Rise of “Twistronics”

Graphite is composed of

In the current work, “we discovered interesting properties with no twisting at all,” says Ju, who is also affiliated with the Materials Research Laboratory.

Electron Correlation Artist’s Rendition

Artist’s rendition of the electron correlation, or ability of electrons to talk with each other, that can occur in a special kind of graphite (pencil lead). Credit: Sampson Wilcox, MIT Research Laboratory of Electronics

He and colleagues discovered that five layers of graphene arranged in a certain order allow the electrons moving around inside the material to talk with each other. That phenomenon, known as electron correlation, “is the magic that makes all of these new properties possible,” Ju says.

Bulk graphite–and even single sheets of graphene–are good electrical conductors, but that’s it. The material Ju and colleagues isolated, which they call pentalayer rhombohedral stacked graphene, becomes much more than the sum of its parts.

A Novel Microscope and Its Discoveries

Key to isolating the material was a novel microscope Ju built at MIT in 2021 that can quickly and relatively inexpensively determine a variety of important characteristics of a material at the Zhengguang Lu, Long Ju, and Tonghang Han

MIT Postdoctoral Associate Zhengguang Lu, Assistant Professor Long Ju, and Graduate Student Tonghang Han in the lab. The three are authors, with seven others, of a paper in Nature Nanotechnology about a special kind of graphite (pencil lead). Credit: Ju Lab

“We found that the material could be insulating, magnetic, or topological,” Ju says. The latter is somewhat related to both conductors and insulators. Essentially, Ju explains, a topological material allows the unimpeded movement of electrons around the edges of a material, but not through the middle. The electrons are traveling in one direction along a “highway” at the edge of the material separated by a median that makes up the center of the material. So the edge of a topological material is a perfect conductor, while the center is an insulator.

“Our work establishes rhombohedral stacked multilayer graphene as a highly tunable platform to study these new possibilities of strongly correlated and topological physics,” Ju and his coauthors conclude in Nature Nanotechnology.

Reference: “Correlated insulator and Chern insulators in pentalayer rhombohedral-stacked graphene” by Tonghang Han, Zhengguang Lu, Giovanni Scuri, Jiho Sung, Jue Wang, Tianyi Han, Kenji Watanabe, Takashi Taniguchi, Hongkun Park and Long Ju, 5 October 2023, Nature Nanotechnology.
DOI: 10.1038/s41565-023-01520-1

In addition to Ju, authors of the paper are Tonghang Han and Zhengguang Lu. Han is a graduate student in the Department of Physics; Lu is a postdoctoral associate in the Materials Research Laboratory. The two are co-first authors of the paper.

Other authors are Giovanni Scuri, Jiho Sung, Jue Wang and Hongkun Park of Harvard University; Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan, and Tianyi Han of MIT Physics.

This work was supported by a Sloan Fellowship; the U.S. National Science Foundation; the U.S. Office of the Under Secretary of Defense for Research and Engineering; the Japan Society for the Promotion of Science KAKENHI; the World Premier International Research Initiative of Japan; and the U.S. Air Force Office of Scientific Research.

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