Scientists can have ambitious goals (Experiments With Twisted 2D Materials) to cure diseases, to study distant worlds, to clean energy revolutions. In physics and materials science, some of these ambitious goals are to create ordinary sound objects with unique properties: wires that can transmit energy without losing energy, or quantum computers that can perform complex calculations that are not available to today’s computers. And evolving workbenches that are slowly moving us toward these goals are 2D material sheets of material that are a thick layer of atoms.
In an article published in the journal Nature Physics in September 2020, a team led by the University of Washington said that careful design of stacks of graphene – a 2D form of carbon – could demonstrate excellent electronic properties. The team also found evidence that this type of collective behavior is likely related to the emergence of exotic magnetic states. “We’ve created an experimental setup (Experiments With Twisted 2D Materials) that allows us to manipulate electrons in graphene layers in many exciting new ways,” said co-author Matthew Yankowitz, an assistant professor of physics at UW and in materials science and engineering as well as faculty. researcher at the UW Clean Energy Institute.
Yankowitz led the team with co-author Xiaodong Xu, professor of physics and materials science and engineering at UW. Xu is also a researcher at the UW Molecular Engineering and Sciences Institute, the UW Institute for Nano-Engineered Systems and the Clean Energy Institute.
Because 2D materials (Experiments With Twisted 2D Materials) are a layer of strong atoms, connections between atoms can only be made in two dimensions, and particles such as electrons can only function as game elements on a board: side to side, front to back, or diagonally, but not up or down. . These limitations can fill 2D materials with properties that their 3D counterparts lack, and researchers examine 2D sheets of various materials to identify and understand these potentially useful properties. But in recent decades, scientists like Yankowitz have also begun laying 2D materials – such as a stack of pancakes – and have found that when stacked and rotated in a configuration and exposed to extremely low temperatures, these layers can be exotic and unexpected. exhibit properties. .
The UW team worked on the building blocks of two-layer graphene: two graphene sheets naturally joined together. They laid one double layer on top of each other – a total of four graphene layers – and rotated them so that the arrangement of the carbon atoms between the two layers was a little off. Previous research has shown that the introduction of these small twist angles between a layer or bilayers of graphene can have major implications for the behavior of their electrons. With specific configurations in the field of energy distribution and charge in layered bilayers, electrons show more relevant behavior. This means that everyone starts doing the same thing – or showing the same qualities – at the same time.
“In these cases, it is no longer reasonable to describe what a single electron does, but what all electrons do at the same time,” Yankowitz said.
“It’s like having a room full of people, where a change in everyone’s behavior can make everyone react like that,” says lead author Minhao. Hi, PhD student in physics UW and former colleague from the Institute of Clean Energy.
Quantum mechanics is based on these related properties, and because stacked graphene particles have a density of more than 1012, or one trillion electrons per square centimeter, many electrons move together.
The team tried to uncover some of the mysteries of the correlated states in their experimental setup. At temperatures just a few degrees above absolute zero, the team found that they could “set” the system to some correlated insulation state – where it had no electric charge. Near these insulation states, the team found pockets of highly conductive states with superconductivity-like properties.
Courtesy of Matthew Yankowitz / University of Washington.
Although other teams have recently reported these conditions, the origins of these elements remain a mystery. But the work of the UW team found evidence for a possible explanation. They found that these states seem to be controlled by a quantum mechanical property of electrons called “spin” – a kind of momentum. In areas near adjacent isolation states, they found evidence that all electron spins are strongly interconnected. It can be shown that a form of ferromagnetism – not superconductivity – has developed in the vicinity of areas showing correlated isolation states. However, further experiments are needed to prove this.
These findings are the latest example of the many surprises that exist when performing experiments on 2D materials. “Most of what we do in this line of research is to try to create, understand and manage evolving electronic states that may be related or topological or have similar characteristics,” Xu said. . “There’s a lot we can do in these situations – such as quantum computing, a new energy recovery device or some new class of sensors – and we honestly won’t know until we try.”
In the meantime, expect stacks, bilayers, and twisted corners to continue to make waves.
