Researchers at Harvard University and their collaborators have used a specially designed microscope to probe the properties of supermoiré patterns in trilayer graphene to an extent that was never possible before.
Using the unique microscope, they detected many new states of matter in which electrons would get stuck or form unusual groups, leading to changes in the entire system’s electronic behavior and opening doors to studying layered materials with precisely controllable properties.
The ultra-long supermoiré patterns visible in twisted trilayer materials had been considered by some to be imperfections of little consequence amidst the simpler moiré structures that emerge when only two layers are present. The new paper challenges that assumption and introduces the concept of supermoiré engineering – how that additional pattern-on-pattern could be used as a probe to uncover the overall properties of these special materials. The supermoiré pattern is relatively large and can be easily controlled, introducing potential for designing exotic new materials for thin electronics and other applications.
“Going into this study, if you asked me if I thought the supermoiré was good for anything, I probably would’ve said it’ll just be a nuisance,” said co-author Andrew Pierce, now a postdoctoral researcher at Cornell. “But it turned out to give us new information about the system – information that would’ve been hard to get with other techniques besides ours.”
Understanding of supermoiré patterns had been limited by the fact that the patterns can vary significantly across different regions in a sample. To solve this problem, the researchers used their single-electron transistor microscope, developed in Amir Yacoby’s lab at SEAS, that’s capable of examining materials with spatial resolution of about 100 nanometers and is sensitive to perturbations in individual electrons. A sharp needle with a sensor at its tip scans the sample and captures these details.
The microscope allowed the team to detect very slight changes in moiré and supermoiré patterns in two- and -three-layer graphene, and the resulting electronic properties per pixel. By analyzing the correlations between these quantities, they gleaned new insights into how the supermoiré patterns in particular influence the entire system.
“This additional long-range pattern that until now was largely overlooked could be used as a probe to understand the material properties of the parent material,” said co-author Yonglong Xie, now an assistant professor at Rice University.
The results could enhance understanding of quantum phenomena, including the lossless conduction of electrons known as superconductivity, and lead to next-generation materials that contain multiple tunable properties.
The paper was co-authored by Jeong Min Park, Daniel E. Parker, Jie Wang, Patrick Ledwith, Zhuozhen Cai, Kenji Watanabe, Takashi Taniguchi, Eslam Khalaf, Ashvin Vishwanath, and Pablo Jarillo-Herrero. The research received federal support from the Army Research Office, the National Science Foundation, and the Department of Defense.