Electrons Caught Behaving Collectively in Experiments With Twisted 2D Materials

Scientists can pursue ambitious goals: cure diseases, explore distant worlds, revolutions with clean energy. In physics and materials research, some of these ambitious goals are to make normal-sounding objects with extraordinary properties: wires that can carry energy without losing energy, or quantum computers that can perform complex calculations that today’s computers cannot. And the new workbenches for the experiments that are gradually moving us towards these goals are 2D materials – sheets of material that are a single layer of atoms thick.

In an article published in the magazine in September 2020 Natural physics, a team from the University of Washington reported that carefully stacked stacks of Graph – a 2D form of carbon – can have strongly correlated electron properties. The team also found evidence that this type of collective behavior is likely related to the creation of exotic magnetic states.

“We 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 Matthew Yankowitz, a UW assistant professor of physics and materials science and engineering as a faculty researcher at UW Clean Energy Institute.

Yankowitz led the team with co-senior author Xiaodong Xu, a UW professor of physics and materials science and technology. Xu is also a faculty researcher at the UW Institute for Molecular Technology and Sciences, the UW Institute for Nanotechnical Systems and the Institute for Clean Energy.

Because 2D materials are a layer of atoms thick, bonds between atoms are only formed in two dimensions and particles like electrons can only move like parts of a board game: side to side, front to back or diagonally, but not up or down down below. These limitations can give 2D materials properties that their 3D counterparts lack, and scientists have studied 2D sheets made from various materials to characterize and understand these potentially useful properties.

Over the past decade, scientists like Yankowitz have also begun layering 2D materials – like a stack of pancakes – and found that when these layers are stacked and rotated in a particular configuration and exposed to extremely low temperatures, they become exotic and may have unexpected properties.

The UW team worked with building blocks made of two-layer graphene: two naturally layered layers of graphene. They stacked a bilayer on top of each other – for a total of four graphene layers – and twisted them so that the arrangement of the carbon atoms between the two bilayers was slightly out of alignment. Previous research has shown that introducing these small twist angles between individual layers or bilayers of graphene can have major consequences for the behavior of your electrons. With specific configurations of the electric field and the charge distribution over the stacked bilayers, electrons show a strongly correlated behavior. In other words, they all start doing the same thing – or displaying the same properties – at the same time.

“In these cases it no longer makes sense to describe what a single electron does, but what all electrons do at the same time,” said Yankowitz.

“It’s like having a room full of people where changing one person’s behavior causes everyone else to react similarly,” said lead author Minhao He, a UW PhD student in physics and a former associate of the Clean Energy Institute.

Quantum mechanics underlies these correlated properties, and because the stacked graphene bilayers have a density greater than 10¹²or a trillion electrons per square centimeter, many electrons behave together.

The team tried to solve some of the puzzles of the correlated states in their experimental setup. At temperatures just a few degrees above that Absolute zeroThe team discovered that they could “tune” the system to a correlated state of isolation – in which it would not conduct an electrical charge. In the vicinity of these insulating states, the team found pockets with highly conductive states with characteristics that resemble superconductivity.

Although other teams have recently reported these conditions, the origin of these traits has remained a mystery. However, the work of the UW team has found evidence of a possible explanation. They found that these states appear to be driven by a quantum mechanical property of electrons called “spin” – a type of angular momentum. In regions near the correlated isolation states, they found evidence that all electron spins align spontaneously. This may indicate that near the areas with correlated isolation states, some form of ferromagnetism is occurring – not superconductivity. However, additional experiments would have to check this.

These discoveries are the latest example of the many surprises that await you when experimenting with 2D materials.

“Much of what we’re doing in this line of research is trying to create, understand, and control emerging electronic states that can be either correlated, topological, or both,” Xu said. “We could do a lot with these states later – a form of Quantum computingFor example a new energy harvesting device or some new types of sensors – and frankly, we won’t know until we try. ”

In the meantime, expect stacks, double layers, and twist angles to continue to ripple.

Reference: “Symmetry Break in Twisted Double-Layer Graph” by Minhao He, Yuhao Li, Jiaqi Cai, Yang Liu, K. Watanabe, T. Taniguchi, Xiaodong Xu and Matthew Yankowitz, September 14, 2020, Natural physics.
DOI: 10.1038 / s41567-020-1030-6

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