Blog

MIT physicists have developed a new form of graphenecreating a five-lane electron superhighway that allows ultra-efficient electron movement without energy loss.

This breakthrough in rhombohedral five-layer graphene could transform low-power electronic devices and works through the quantum anomalous Hall effect at zero magnetic field.

MIT physicists and their collaborators have created a five-lane electron superhighway that could enable ultra-efficient electronics and much more.

The work, recently reported in the journal Scienceis one of several important discoveries by the same team in the past year involving a material that is a unique form of graphene.

“This finding has direct implications for low-power electronic devices because no energy is lost during electron propagation, which is not the case in ordinary materials where electrons are scattered,” said Long Zhu, an assistant professor in the Department of Physics and corresponding author of Science paper.

The phenomenon is similar to cars driving on an open highway as opposed to those driving through neighborhoods. Cars in the neighborhood can be stopped or slowed by other drivers making sharp stops or U-turns that disrupt an otherwise smooth commute.

New material: rhombohedral graphene

The material behind this work, known as rhombohedral pentaloene graphene, was discovered two years ago by physicists led by Zhu. “We found a gold mine, and every scoop reveals something new,” said Zhu, who is also affiliated with MIT’s Materials Research Laboratory.

IN Natural nanotechnologies paper last October, Zhu and colleagues reported the discovery of three important properties arising from rhombohedral graphene. For example, they showed that it can be topological, or allow the unimpeded movement of electrons around the edge of the material but not through the middle. This led to the superhighway, but required the application of a large magnetic field, about tens of thousands of times stronger than Earth’s magnetic field.

Six of the MIT physicists who created the five-lane electron superhighway are (left to right) undergraduates Jixiang Yang, Junseok Seo and Tonghang Han; visiting student Yuxuan Yao; Assistant Professor Long Ju; and postdoc Zhengguang Lu. Credit: Shenyong Ye

Enhancing the electron channels of graphene

In ongoing work, the team reports the creation of a superhighway without any magnetic field.

Tonghang Han, a physics graduate student at MIT, co-authored the paper. “We are not the first to discover this general phenomenon, but we did so in a very different system. And compared to previous systems, ours is simpler and also supports more electronic channels. Zhu explains, “other materials can only support one lane of motion at the edge of the material. Suddenly we raised it to five.

Additional co-authors of the paper who contributed equally to the work are Zhengguang Lu and Yuxuan Yao. Lu is a postdoctoral fellow in the Materials Research Laboratory. Yao led the work as a visiting student from Tsinghua University. Other authors include MIT physics professor Liang Fu; Jixiang Yang and Junseok Seo, both graduate students in physics at MIT; Chiho Yoon and Fan Zhang of the University of Texas at Dallas; and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

How it works

Graphite, the main component of pencil lead, consists of many layers of graphene, a single layer of carbon atoms arranged in hexagons resembling a honeycomb structure. Rhombohedral graphene consists of five layers of graphene arranged in a specific overlapping order.

Ju and colleagues isolated rhombohedral graphene thanks to a new microscope Ju built at MIT in 2021, which can quickly and relatively cheaply determine various important characteristics of a material in nanoscale. The five-layer rhombohedral stacked graphene is only a few billionths of a meter thick.

In the current work, the team tinkered with the original system, adding a layer of tungsten disulfide (WS₂). “The interaction between WS₂ and five-layer rhombohedral graphene led to this five-band superhighway that works at zero magnetic field,” Zhu says.

Comparison with superconductivity

The phenomenon that Ju’s group discovered in rhombohedral graphene, which allows electrons to travel without resistance in a zero magnetic field, is known as the quantum anomalous Hall effect. Most people are more familiar with superconductivity, a completely different phenomenon that does the same thing but occurs in very different materials.

Zhu notes that although superconductors were discovered in 1910, it took about 100 years of research to make the system work at the higher temperatures needed for applications. “And the world record is still well below room temperature,” he notes.

Similarly, the rhombohedral graphene superhighway currently operates at about 2 Kelvin, or -456 degrees Fahrenheit. “It will take a lot of effort to raise the temperature, but as physicists, our job is to provide insight; different way to implement this [phenomenon]” says Zhu.

Implications and future perspectives

Discoveries involving rhombohedral graphene came as a result of painstaking research that was not guaranteed to work. “We tried many recipes over many months,” says Hahn, “so it was very exciting when we cooled the system down to a very low temperature and [a five-lane superhighway operating at zero magnetic field] it just popped out.”

Zhu says, “It’s very exciting to be the first to discover a phenomenon in a new system, especially in a material we’ve discovered.”

Reference: “Large quantum anomalous Hall effect in spin-orbit proximate rhombohedral graphene” by Tonghang Han, Zhengguang Lu, Yuxuan Yao, Jixiang Yang, Junseok Seo, Chiho Yoon, Kenji Watanabe, Takashi Taniguchi, Liang Fu, Fan Zhang, and Long Ju May 9, 2024 Science.
DOI: 10.1126/science.adk9749

This work was supported by a Sloan fellowship; US National Science Foundation; Office of the Under Secretary of Defense for Research and Engineering; Japan Society for the Promotion of Science KAKENHI; and Japan’s World Leading International Research Initiative.