For decades, scientists have been probing the potential of two-dimensional materials to transform our world. 2D materials are only a single layer of atoms thick. Within them, subatomic particles like electrons can only move in two dimensions. This simple restriction can trigger unusual electron behavior, imbuing the materials with “exotic” properties like bizarre forms of magnetism, superconductivity and other collective behaviors among electrons — all of which could be useful in computing, communication, energy and other fields.

But researchers have generally assumed that these exotic 2D properties exist only in single-layer sheets, or short stacks. The so-called “bulk” versions of these materials — with their more complex 3D atomic structures — should behave differently.

Or so they thought.

In a published July 19 in Nature, a team led by researchers at the 91̽�� reports that it is possible to imbue graphite — the bulk, 3D material found in No. 2 pencils — with physical properties similar to graphite’s 2D counterpart, graphene. Not only was this breakthrough unexpected, the team also believes its approach could be used to test whether similar types of bulk materials can also take on 2D-like properties. If so, 2D sheets won’t be the only source for scientists to fuel technological revolutions. Bulk, 3D materials could be just as useful.

“Stacking single layer on single layer — or two layers on two layers — has been the focus for unlocking new physics in 2D materials for several years now. In these experimental approaches, that’s where many interesting properties emerge,” said senior author , a 91̽��assistant professor of physics and of materials science and engineering. “But what happens if you keep adding layers? Eventually it has to stop, right? That’s what intuition suggests. But in this case, intuition is wrong. It’s possible to mix 2D properties into 3D materials.”

The team, which also includes researchers at Osaka University and the National Institute for Materials Science in Japan, adapted an approach commonly used to probe and manipulate the properties of 2D materials: stacking 2D sheets together at a small twist angle. Yankowitz and his colleagues placed a single layer of graphene on top of a thin, bulk graphite crystal, and then introduced a twist angle of around 1 degree between graphite and graphene. They detected novel and unexpected electrical properties not just at the twisted interface, but deep in the bulk graphite as well.

The twist angle is critical to generating these properties, said Yankowitz, who is also a faculty member in the 91̽��Clean Energy Institute and the 91̽��Institute for Nano-Engineered Systems. A twist angle between 2D sheets, like two sheets of graphene, creates what’s called a moiré pattern, which alters the flow of charged particles like electrons and induces exotic properties in the material.

In the UW-led experiments with graphite and graphene, the twist angle also induced a moiré pattern, with surprising results. Even though only a single sheet of graphene atop the bulk crystal was twisted, researchers found that the electrical properties of the whole material differed markedly from typical graphite. And when they turned on a magnetic field, electrons deep in the graphite crystal adopted unusual properties similar to those of electrons at the twisted interface. Essentially, the single twisted graphene-graphite interface became inextricably mixed with the rest of the bulk graphite.

“Though we were generating the moiré pattern only at the surface of the graphite, the resulting properties were bleeding across the whole crystal,” said co-lead author , a 91̽��postdoctoral researcher in physics.

For 2D sheets, moiré patterns generate properties that could be useful for quantum computing and other applications. Inducing similar phenomena in 3D materials unlocks new approaches for studying unusual and exotic states of matter and how to bring them out of the laboratory and into our everyday lives.

“The entire crystal takes on this 2D state,” said co-lead author Ellis Thompson, a 91̽��doctoral student in physics. “This is a fundamentally new way to affect electron behavior in a bulk material.”

Yankowitz and his team believe their approach of generating a twist angle between graphene and a bulk graphite crystal could be used to create 2D-3D hybrids of its sister materials, including tungsten ditelluride and zirconium pentatelluride. This could unlock a new approach to re-engineering the properties of conventional bulk materials using a single 2D interface.

“This method could become a really rich playground for studying exciting new physical phenomena in materials with mixed 2D and 3D properties,” said Yankowitz.

Co-authors on paper are 91̽��graduate student Esmeralda Arreguin-Martinez and 91̽��postdoctoral researcher Yafei Ren, both in the Department of Materials Science and Engineering; , a 91̽��assistant professor of materials science and engineering; , a 91̽��professor of physics and chair of materials science and engineering; Manato Fujimoto of Osaka University; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan. The research was funded by the National Science Foundation; the U.S. Department of Energy; the 91̽��Clean Energy Institute; the Office of the Director of National Intelligence; the Japan Science and Technology Agency; the Japan Society for the Promotion of Science; the Japanese Ministry of Education, Culture, Sports, Science and Technology; and the M.J. Murdock Charitable Trust.

For more information, contact Yankowitz at myank@uw.edu.

Scientists can have ambitious goals: curing disease, exploring distant worlds, clean-energy revolutions. In physics and materials research, some of these ambitious goals are to make ordinary-sounding objects with extraordinary properties: wires that can transport power without any energy loss, or quantum computers that can perform complex calculations that today’s computers cannot achieve. And the emerging workbenches for the experiments that gradually move us toward these goals are 2D materials — sheets of material that are a single layer of atoms thick.

In a published Sept. 14 in the journal Nature Physics, a team led by the 91̽�� reports that carefully constructed stacks of graphene — a 2D form of carbon — can exhibit highly correlated electron properties. The team also found evidence that this type of collective behavior likely relates to the emergence of exotic magnetic states.

“We’ve created an experimental setup that allows us to manipulate electrons in the graphene layers in a number of exciting new ways,” said co-senior author , a 91̽��assistant professor of physics and of materials science and engineering, as well as a faculty researcher at the UW .

Yankowitz led the team with co-senior author , a 91̽��professor of physics and of materials science and engineering. Xu is also a faculty researcher with the 91̽��, the 91̽�� and the Clean Energy Institute.

Since 2D materials are one layer of atoms thick, bonds between atoms only form in two dimensions and particles like electrons can only move like pieces on a board game: side-to-side, front-to-back or diagonally, but not up or down. These restrictions can imbue 2D materials with properties that their 3D counterparts lack, and scientists have been probing 2D sheets of different materials to characterize and understand these potentially useful qualities.

But over the past decade, scientists like Yankowitz have also started layering 2D materials — like a stack of pancakes — and have discovered that, if stacked and rotated in a particular configuration and exposed to extremely low temperatures, these layers can exhibit exotic and unexpected properties.

The 91̽��team worked with building blocks of bilayer graphene: two sheets of graphene naturally layered together. They stacked one bilayer on top of another — for a total of four graphene layers — and twisted them so that the layout of carbon atoms between the two bilayers were slightly out of alignment. Past research has shown that introducing these small twist angles between single layers or bilayers of graphene can have big consequences for the behavior of their electrons. With specific configurations of the electric field and charge distribution across the stacked bilayers, electrons display highly correlated behaviors. In other words, they all start doing the same thing — or displaying the same properties — at the same time.

“In these instances, it no longer makes sense to describe what an individual electron is doing, but what all electrons are doing at once,” said Yankowitz.

“It’s like having a room full of people in which a change in any one person’s behavior will cause everyone else to react similarly,” said lead author , a 91̽��doctoral student in physics and a former Clean Energy Institute fellow.

Quantum mechanics underlies these correlated properties, and since the stacked graphene bilayers have a density of more than 10¹², or one trillion, electrons per square centimeter, a lot of electrons are behaving collectively.

The team sought to unravel some of the mysteries of the correlated states in their experimental setup. At temperatures of just a few degrees above absolute zero, the team discovered that they could “tune” the system into a type of correlated insulating state — where it would conduct no electrical charge. Near these insulating states, the team found pockets of highly conducting states with features resembling superconductivity.

Though other teams have recently reported these states, the origins of these features remained a mystery. But the 91̽��team’s work has found evidence for a possible explanation. They found that these states appeared to be driven by a quantum mechanical property of electrons called “spin” — a type of angular momentum. In regions near the correlated insulating states, they found evidence that all the electron spins spontaneously align. This may indicate that, near the regions showing correlated insulating states, a form of is emerging — not superconductivity. But additional experiments would need to verify this.

These discoveries are the latest example of the many surprises that are in store when conducting experiments with 2D materials.

“Much of what we’re doing in this line of research is to try to create, understand and control emerging electronic states, which can be either correlated or topological, or possess both properties,” said Xu. “There could be a lot we can do with these states down the road — a form of quantum computing, a new energy-harvesting device, or some new types of sensors, for example — and frankly we won’t know until we try.”

In the meantime, expect stacks, bilayers and twist angles to keep making waves.

Co-authors are 91̽��researchers Yuhao Li and Yang Liu; 91̽��physics doctoral student and Clean Energy Institute fellow Jiaqi Cai; and K. Watanabe and T. Taniguchi with the National Institute for Materials Science in Japan. The research was funded by the 91̽��Molecular Engineering Materials Center, a National Science Foundation Materials Research Science and Engineering Center; the China Scholarship Council; the Ministry of Education, Culture, Sports, Science and Technology of Japan; and the Japan Science and Technology Agency.

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For more information, contact Xu at xuxd@uw.edu and Yankowitz at myank@uw.edu.