In materials science labs around the world, researchers regularly reproduce a peculiar experiment. Under a microscope, they use tiny robotic manipulators to lay two sheets of single carbon atoms against one another. They then rotate the top sheet against the bottom. The two identical lattices of atoms, when twisted relative to one another, bloom into a complex moiré pattern, a shifting complex of honeycomb shapes that grow and shrink as the angle between the layers changes.
Try it yourself: In the box below, move the rotation slider back and forth with your mouse or arrow keys to see how the two sheets of atoms create different patterns:
A landmark study, published by MIT researchers in 2018, showed that when the top layer is rotated to the so-called “magic angle” of 1.1 degrees and cooled to near absolute zero, the material can become superconducting, a state in which electrons flow freely through the material with no resistance. The discovery ignited hopes of future breakthroughs in superconducting materials, which could help create extremely efficient computers, power grids and new quantum devices.
In the years following that key experiment, scientists have teased new properties out of the “moiré material” by introducing temperature changes, voltages and magnetic fields. Now, researchers at the University of Washington have added a new variable by stretching and compressing the material. In a paper published Nov. 12 in Science, the team showed that adding strain to the experiment produced big swings in the material’s electrical resistance.
The results give researchers an additional tool for probing and controlling these mysterious materials.
“Atomically thin materials can host superconductivity, magnetism and other exotic quantum states,” said senior author Matthew Yankowitz, a UW associate professor of both physics and materials science and engineering. “We’ve now added a fundamentally new way to manipulate them, by directly deforming the atomic lattice they live on. That enables experiments we simply couldn’t do before.”
Since the MIT discovery, researchers have branched out from carbon and now manipulate a variety of atomically thin materials. They found that tweaking variables like voltage, magnetic fields and temperature within these materials can abruptly change them from superconductors to insulators, which resist the flow of electrons entirely.
“By turning these knobs, we can cycle through different states of matter in very predictable and repeatable ways,” Yankowitz said. “You can go from a metal, to an insulator, then back to a metal, then to an insulator, then to a superconductor. This allows us to use one device to stand in for a whole family of materials, so we can see how superconductivity relates to the states around it in a single sample.”
In the new paper, Yankowitz’s team, in close collaboration with the group of UW physics professor Jiun-Haw Chu, added a new knob to turn: strain. The discovery grew out of a common frustration in moiré material research: It’s incredibly difficult to place two microscopic flecks of atoms directly on top of one another without distorting one or both of them.
“It’s like saying, ‘I’m going to give you two massive sheets of shrink wrap and demand that you place them perfectly flat without any wrinkles,’” Yankowitz said.
In practice, the sheets of atoms often stretch and warp relative to each other, creating a messier, more complex moiré pattern.
Yankowitz and Chu wondered whether new properties might emerge if they could control the stretch of the material. By attaching the tiny flakes of matter to a device that expands and contracts as voltage is applied, the team found that they could continuously control the degree of strain on the lattices. The result was striking: Adjusting the strain changed the material’s resistance more than 100 times more than it would in an ordinary metal, an effect that was further amplified when the sample was cooled to very low temperatures.
Next, Yankowitz and Chu want to simplify the experiment so that other researchers can reproduce it more easily and bring it to other atomically thin materials. The broader goal is to squeeze as much knowledge as possible out of this enigmatic material — to inch toward another computing revolution, perhaps, but also to better understand the hidden behaviors that govern matter.
“What is remarkable about this giant response to strain is that it also appears in other exotic materials, including high-temperature superconductors, one of the great unsolved problems in physics,” Chu said. “This common link brings us a step closer to understanding what underlies these fascinating and complex phenomena.”
Co-authors include Xuetao Ma, a former UW doctoral student of materials science and engineering who is now a postdoctoral researcher at the University of Basel; Zhaoyu Liu, a former UW postdoctoral researcher of physics who is now the Director of Quantum Matter Facility at Clemson University; Jiaqi Cai, a former UW doctoral student of physics who is now a research fellow at MIT; Xiaodong Xu, a former UW professor of physics now at Stanford University; and Kenji Watanabe and Takashi Taniguchi at the National Institute for Materials Science.
This research was funded by the National Science Foundation through the UW Molecular Engineering Materials Center, the U.S. Army Research Office, the UW Clean Energy Institute, the David and Lucile Packard Foundation and the Gordon and Betty Moore Foundation.
For more information, contact Yankowitz at myank@uw.edu.