MIT physicists forge a five-lane quantum highway for electrons

MIT Physicists have developed a new shape graphenecreating a five-lane electron highway that allows ultra-efficient electron movement without energy loss.

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

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

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

“This discovery 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 Ju, assistant professor in the Department of Physics and corresponding author of the Science paper.

The phenomenon is similar to cars driving on an open highway as opposed to cars driving through neighborhoods. The cars in the neighborhood can be stopped or slowed down by other drivers who make abrupt stops or U-turns that disrupt an otherwise smooth ride.

A new material: Rhombohedral graphene

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

In a Nature Nanotechnology In a paper last October, Ju and colleagues reported the discovery of three key properties that emerge from rhombohedral graphene. For example, they showed that it could be topological, or allow the unhindered movement of electrons around the edge of the material but not through the middle. That resulted in a superhighway, but required the application of a large magnetic field that was several tens of thousands of times stronger than the Earth’s magnetic field.

Improvement of the electron channels of graphene

In the current work, the team reports that the superhighway was realized without any magnetic field.

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

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

How it works

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

Ju and colleagues isolated rhombohedral graphene thanks to a new microscope Ju built at MIT in 2021 that can quickly and relatively cheaply determine a variety of important features of a material on its surface. nanoscalePentalayer rhombohedral stacked graphene is only a few billionths of a meter thick.

In the current work, the team experimented with the original system by adding a layer of tungsten disulfide (WS)₂). “The interaction between the WS₂ and the pentalayer rhombohedral graphene resulted in this five-lane highway that operates at zero magnetic field,” Ju says.

Comparison with superconductivity

The phenomenon that the Ju 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 very different phenomenon that does the same thing but occurs in very different materials.

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

Similarly, the rhombohedral graphene highway currently operates at about 2 Kelvin, or -456 degrees Fahrenheit“It will take a lot of effort to increase the temperature, but as physicists it is our job to provide insight; another way to achieve this [phenomenon]”, says Ju.

Implications and future prospects

The discoveries regarding rhombohedral graphene are the result of painstaking research that was not guaranteed to work. “We tried many recipes for months,” says Han, “so it was very exciting when we allowed the system to cool down to a very low temperature and [a five-lane superhighway operating at zero magnetic field] “It just came out.”

Ju says: “It is very exciting to be the first to discover a phenomenon in a new system, especially in a material that we have discovered.”

Reference: “Large quantum anomalous Hall effect in spin-orbit proximitized 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; the US National Science Foundation; the US Office of the Under Secretary of Defense for Research and Engineering; the Japan Society for the Promotion of Science KAKENHI; and Japan’s World Premier International Research Initiative.