A smoother way to study ‘twistronics’

A discovery six years ago took the condensed-matter physics world by storm: Ultra-thin carbon stacked in two slightly askew layers became a superconductor, and changing the twist angle between layers could toggle their electrical properties. The landmark 2018 paper describing “magic-angle graphene superlattices” launched a new field called “twistronics,” and the first author was then-MIT graduate student and recent Harvard Junior Fellow Yuan Cao.  

Together with Harvard physicists Amir Yacoby, Eric Mazur, and others, Cao and colleagues have built on that foundational work, smoothing a path for more twistronics science by inventing an easier way to twist and study many types of materials.   

A new paper in Nature describes the team’s fingernail-sized machine that can twist thin materials at will, replacing the need to fabricate twisted devices one by one. Thin, 2D materials with properties that can be studied and manipulated easily have immense implications for higher-performance transistors, optical devices such as solar cells, and quantum computers, among other things.

“This development makes twisting as easy as controlling the electron density of 2D materials,” said Yacoby, Harvard professor of physics and applied physics. “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.”

Cao first made twisted bilayer graphene as a graduate student in the lab of MIT’s Pablo Jarillo-Herrero. Exciting as it was, the achievement was tempered by challenges with replicating the actual twisting.

At the time, each twisted device was hard to produce, and as a result, unique and time-consuming, Cao explained. To do science with these devices, they needed tens or even hundreds of them. They wondered if they could make “one device to twist them all,” Cao said — a micromachine that could twist two layers of material at will, eliminating the need for hundreds of unique samples. They call their new device a MEMS (micro-electromechanical system)-based generic actuation platform for 2D materials, or MEGA2D for short.

The Yacoby and Mazur labs collaborated on the design of this new tool kit, which is generalizable 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,” said Cao, now an assistant professor at University of California Berkeley. “It will certainly also bring other new discoveries along the way.”

In the paper, the researchers demonstrated the utility of their device with two pieces of hexagonal boron nitride, a close relative of graphene. They were able to study the bilayer device’s optical properties, finding evidence of quasiparticles with coveted topological properties.

The ease of their new system opens several scientific roadways, for example, employing hexagonal boron nitride twistronics to produce light sources that can be used for low-loss optical communication.

“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.

The paper’s first author is nanoscience and optics expert Haoning Tang, a postdoctoral researcher in Mazur’s lab and a Harvard Quantum Initiative fellow, who noted that developing the MEGA2D technology was 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,” she said. “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.” All nanofabrication took place at Harvard’s Center for Nanoscale Systems, where staff provided invaluable technical support, Tang added.

“The nanofabrication of a device combining MEMS technology with a bilayer structure is a veritable tour de force,” said Mazur, the Balkanski Professor of Physics and Applied Physics. “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.”

Federal funding for the research was provided by the Defense Advanced Research Projects Agency, the Army Research Office, the U.S. Air Force Office of Scientific Research, and the National Science Foundation.