Researchers report quantum anomalous Hall effect in rhombohedral graphene

Researchers at the Massachusetts Institute of Technology (MIT), University of Texas at Dallas and Japan's National Institute for Materials Science have reported the quantum anomalous Hall effect (QAHE), a topological phenomenon that features quantized Hall resistance at zero magnetic field, in a rhombohedral pentalayer graphene-monolayer tungsten disulfide (WS2) heterostructure. 

This achievement can also be described as a 'five-lane superhighway' for electrons, that could allow ultra-efficient electronics and more. The team explained that its discovery could have direct implications for low-power electronic devices because no energy is lost during the propagation of electrons, which is not the case in regular materials where the electrons are scattered.


As opposed to other experimentally confirmed QAHE systems, this system has neither magnetic element nor moiré superlattice effect. 

The material behind this work, known as rhombohedral pentalayer graphene, was discovered two years ago by physicists led by Long Ju, an assistant professor in the MIT Department of Physics and corresponding author of the paper. 

In a research paper last October, Ju and colleagues reported the discovery of three important properties arising from rhombohedral graphene. For example, they showed that it could be topological, or allow the unimpeded 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 some tens of thousands times stronger than the Earth's magnetic field.

In the current work, the team reports creating the superhighway without any magnetic field.

The scientists said they are not the first to discover this general phenomenon, but they did so in a very different system. And compared to previous systems, this one is simpler and also supports more electron channels. Other materials can only support one lane of traffic on the edge of the material - which this work managed to raise to five.

Rhombohedral graphene is composed of five layers of graphene stacked in a specific overlapping order. Ju and colleagues isolated rhombohedral graphene thanks to 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 nanoscale. 

In the current work, the team adjusted the original system, adding a layer of tungsten disulfide (WS2). The interaction between the WS2 and the pentalayer rhombohedral graphene reportedly resulted in this five-lane superhighway that operates at zero magnetic field.

The phenomenon that the Ju group discovered in rhombohedral graphene that allows electrons to travel with no resistance at 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 happens in very different materials.

Ju notes that although superconductors were discovered in the 1910s, it took some 100 years of research to coax the system to work at the higher temperatures necessary for applications - and the world record is still well below room temperature. Similarly, the rhombohedral graphene superhighway currently operates at about 2 Kelvin, or -456 Fahrenheit. 

The discoveries involving rhombohedral graphene came as a result of much research that wasn't guaranteed to work. The team tried many recipes over many months, and finally managed to cool the system to a very low temperature.

Posted: May 14,2024 by Roni Peleg