New Tool Simplifies Twistronics Research

A recent paper published in Nature describes a fingernail-sized device developed by a Harvard University team. This device can twist thin materials at will, eliminating the need to create twisted devices individually.

Six years ago, a discovery that completely changed the field of condensed-matter physics was made: ultra-thin carbon stacked in two slightly asymmetrical layers became a superconductor, and the electrical properties of the layers could be switched by varying the twist angle between them.

Yuan Cao, a recent Harvard Junior Fellow and MIT graduate student, was the first author of the seminal 2018 paper describing "magic-angle graphene superlattices," which introduced the field of "twistronics. " 

Building on this foundational work, Cao and colleagues—including Harvard physicists Amir Yacoby, Eric Mazur, and others—have developed a method to more easily twist and study a variety of materials, opening up further research in twistronics.

These highly manipulable, thin, two-dimensional materials hold significant potential for advancements in quantum computing, solar cells, and higher-performance transistors.

This development makes twisting as easy as controlling the electron density of 2D materials. Controlling density has been the primary knob for discovering new phases of matter in low-dimensional matter, and now, we can control both density and twist angle, opening endless possibilities for discovery.

Amir Yacoby, Professor, Department of Physics and Applied Physics, Harvard University

In Pablo Jarillo-Herrero's lab at MIT, Yuan Cao first created twisted bilayer graphene as a graduate student. While the achievement was groundbreaking, it was muted by the challenge of replicating the precise twist.

At the time, each twisted device had to be made by hand, making them unique and labor-intensive. According to Cao, the team needed tens or even hundreds of devices to conduct their experiments, leading them to contemplate the idea of creating "one device to twist them all"—a micromachine capable of arbitrarily twisting two layers of material, eliminating the need for numerous samples.

The researchers developed the MEGA2D, or micro-electromechanical system-based generic actuation platform for 2D materials. This novel apparatus, designed in collaboration between the labs of Amir Yacoby and Eric Mazur, can be applied to graphene and other materials.

By having this new ‘knob’ via our MEGA2D technology, we envision that many underlying puzzles in twisted graphene and other materials could be resolved in a breeze. It will certainly also bring other new discoveries along the way.

Yuan Cao, Assistant Professor, University of California Berkeley

The scientists demonstrated the utility of their apparatus by using two pieces of hexagonal boron nitride, a material closely related to graphene. They were able to examine the optical properties of the bilayer device and found evidence of quasiparticles with desirable topological characteristics.

The simplicity of the new system opens up numerous scientific possibilities. For example, it can be used to create light sources for low-loss optical communication by leveraging hexagonal boron nitride twistronics.

“We hope that our approach will be adopted by many other researchers in this prosperous field, and all can benefit from these new capabilities,” Cao said.

Haoning Tang, a Postdoctoral Researcher in Mazur's lab and a Harvard Quantum Initiative fellow, is the paper's first author. Tang, who specializes in Nanoscience and Optics, noted that the development of MEGA2D involved a long process of trial and error.

We didn’t know much about how to control the interfaces of 2D materials in real-time, and the existing methods just weren’t cutting it. After spending countless hours in the cleanroom and refining the MEMS design — despite many failed attempts — we finally found the working solution after about a year of experiments.

Haoning Tang, Postdoctoral Researcher, Quantum Initiative Fellow, and Study First Author, Harvard University

Tang added that all nanofabrication took place at Harvard’s Center for Nanoscale Systems, where staff provided invaluable technical support.

“The nanofabrication of a device combining MEMS technology with a bilayer structure is a veritable tour de force. Being able to tune the nonlinear response of the resulting device opens the door to a whole new class of devices in optics and photonics,” said Mazur, the Balkanski Professor of Physics and Applied Physics.

Federal funding for the research came from the Defense Advanced Research Projects Agency, the Army Research Office, the US Air Force Office of Scientific Research, and the National Science Foundation.

Journal Reference:

Tang, H., et al. (2024) On-chip multi-degree-of-freedom control of two-dimensional materials. Nature. doi.org/10.1038/s41586-024-07826-x