MIT Researchers Uncover Unique Properties in Stacked Graphene Layers
MIT researchers have made a significant breakthrough in material physics by precisely stacking five layers of graphene, resulting in a distinctive pentalayer rhombohedral stacked graphene. This groundbreaking discovery, achieved through innovative nanoscale microscopy techniques, reveals the manifestation of insulating, magnetic, and topological characteristics in this material.
The team of physicists, led by Assistant Professor Long Ju from the MIT Department of Physics and affiliated with the Materials Research Laboratory, has transformed graphite into a remarkable material by isolating and arranging ultrathin flakes in a specific order. This newly developed material can be effectively tuned to exhibit three important properties that were previously unknown to exist in natural graphite.
“It is kind of like one-stop shopping,” says Professor Ju. “Nature has plenty of surprises, and we never realized that all of these interesting things are embedded in graphite. We are fortunate to have discovered a material that can host this many properties.”
The Rise of “Twistronics”
Graphite consists of graphene, which is a single layer of carbon atoms arranged in hexagons, resembling a honeycomb structure. Graphene has been the focus of extensive research since its isolation two decades ago. In recent years, scientists, including a team at MIT, uncovered that stacking individual graphene sheets and twisting them at slight angles can bestow new properties upon the material, such as superconductivity and magnetism. This field of research, known as “twistronics,” emerged.
In their latest work, the MIT researchers made an astonishing discovery without employing any twisting techniques. They found that arranging five layers of graphene in a particular order enables the electrons inside the material to communicate with each other, resulting in a phenomenon known as electron correlation. This electron correlation is the key factor that enables the emergence of various new properties in the pentalayer rhombohedral stacked graphene.
Artist’s rendition of electron correlation in pentalayer rhombohedral stacked graphene (graphite)
A Novel Microscope and Its Revelations
Professor Ju built a novel microscope at MIT in 2021, called Scattering-type Scanning Nearfield Optical Microscopy (s-SNOM), which played a crucial role in isolating the pentalayer rhombohedral stacked graphene. This innovative microscope can quickly and inexpensively determine essential characteristics of materials at the nanoscale. The isolated material is only a few billionths of a meter thick.
The researchers utilized s-SNOM to identify and isolate specific pentalayers in the desired rhombohedral stacking order out of more than 10 possible stacking orders. By attaching electrodes to a boron nitride sandwich, which acts as a protective layer for the pentalayer rhombohedral stacked graphene, the team could manipulate the system with different voltages and study the emergent properties.
Multifaceted Material Phenomena
Through their experiments, the researchers observed the emergence of three distinct phenomena depending on the number of electrons within the system. The material could exhibit insulating, magnetic, or topological characteristics.
Professor Ju explains that a topological material allows electrons to move freely along the edges but not through the center. It acts as a perfect conductor at the edge while functioning as an insulator in the middle.
The researchers state in their conclusion published in Nature Nanotechnology, “Our work establishes rhombohedral stacked multilayer graphene as a highly tunable platform to study these new possibilities of strongly correlated and topological physics.”
This research, titled “Correlated insulator and Chern insulators in pentalayer rhombohedral-stacked graphene,” was conducted by Tonghang Han, Zhengguang Lu, Giovanni Scuri, Jiho Sung, Jue Wang, Tianyi Han, Kenji Watanabe, Takashi Taniguchi, Hongkun Park, and Long Ju. It was published in Nature Nanotechnology on October 5, 2023.
The study received support from various sources, including 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.
