University of Texas at Dallas scientists are investigating how structures made from several layers of graphene stack up in terms of their fundamental physics and their potential as reconfigurable semiconductors for advanced electronics.
Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern whereby each hexagon is formed by six carbon atoms at its vertices. Since graphene’s first isolation in 2004 — which later led to a Nobel Prize in physics — scientists and engineers have intensely studied its unique physical properties as well as its potential applications.
Dr. Fan Zhang, professor of physics in the School of Natural Sciences and Mathematics at UT Dallas, is a theorist who for more than a decade has been examining the electronic properties that emerge when layers of graphene are stacked in a chiral manner to form a rhombohedral structure.
Tianyi Xu is a physics doctoral student who has worked with Zhang on the material. “There are two types of stacking orders that occur in graphite, which is a crystal found in pencil lead,” Xu said. “A typical pencil lead consists of about 10 million layers of graphene.”
Hexagonal stacking, also known as AB stacking, occurs when even-numbered graphene layers are aligned, with the odd-numbered layers rotated 60 degrees relative to the even layers. In contrast, rhombohedral stacking, or ABC stacking, features a unidirectional, or chiral, 60-degree rotation for each successive layer.
“Hexagonal stacking is more structurally stable, but rhombohedral stacking is much more remarkable because the chirality in stacking can make the electrons strongly correlated for extraordinary macroscopic quantum phenomena,” Zhang said.
In recent articles published in Science, Nature Physics and other journals, Zhang’s group and experimental collaborators at the University of Göttingen in Germany and the Massachusetts Institute of Technology reported new findings about rhombohedral graphene.
The researchers found that the family of materials behaves as semiconductors whose band gaps and electron densities can be continuously tuned by gate electric fields. They also discovered that, at extremely low temperatures, rhombohedral graphene exhibits novel magnetism and superconductivity, as well as the quantum anomalous Hall effect, depending on the gate electric fields that are applied to the device.
“You would not usually observe all these effects in the same material, but they can occur in the same rhombohedral graphene device, and we can switch and even tune between the semiconducting, magnetic and superconducting properties in a single device,” said Praveen Pai, a physics doctoral student in Zhang’s group. “There is a great deal of interest in the quantum anomalous Hall effect, not to mention the coexistence of magnetism and superconductivity.”
“You would not usually observe all these effects in the same material, but they can occur in the same rhombohedral graphene device, and we can switch and even tune between the semiconducting, magnetic and superconducting properties in a single device.”
Praveen Pai, physics doctoral student
Ninad Dongre, another UT Dallas physics doctoral student who works with Zhang, said that in order to change a conventional semiconductor’s electron density or band gap, researchers have to adjust the chemical composition and make new samples.
“This is time consuming and sometimes challenging,” Dongre said. “With rhombohedral graphene, we can change a single sample with electric gates.”
Zhang will share his expertise and recent advancements in the physics of rhombohedral graphene with fellow scientists and students at an invited tutorial and an invited symposium during the week of March 16 at the American Physical Society’s (APS) Global Physics Summit in Anaheim, California. His graduate students also will present research at the six-day annual conference, which is expected to draw more than 14,000 physicists and 6,400 students from around the world.
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One of the challenges of studying rhombohedral graphene is that it is difficult to fabricate an isolated sample in the lab, said Dr. Chiho Yoon, a research scientist in Zhang’s group and a presenter at the APS meeting. A graphene few-layer sample typically consists of a combination of hexagonal graphene and rhombohedral graphene.
“One side of the sample may exhibit ABC stacking, while the other side has AB stacking, with a defect in between,” Yoon said. “It’s crucial to identify the line defects and isolate the ABC region; otherwise, the AB region will overtake the ABC region during device fabrication. This process is exceptionally challenging, and we are fortunate to collaborate with experimentalists who possess cutting-edge nanotechnologies.”
In October 2024, the Alexander von Humboldt Foundation in Germany selected Zhang for a Humboldt Research Award, which recognizes academics whose fundamental discoveries, new theories or insights have made a significant impact on their own field and beyond, and who are expected to continue making groundbreaking contributions.
“I have been very fortunate. Not only have I seen our predictions confirmed, but the topic we initiated 15 years ago has evolved into a rapidly expanding field,” Zhang said. “What makes me most proud are my students, postdocs and experimental collaborators — together we are writing new chapters of discovery.”
Zhang’s research is primarily funded by the National Science Foundation, including a Faculty Early Career Development Program (CAREER) award, a Designing Materials to Revolutionize and Engineer our Future (DMREF) award and a recent grant focusing on 2D semiconductors.
