Researchers Probe Spin Structure in 2D Materials for First Time

By observing spin structure in “magic-angle” graphene, a team of scientists led by Brown University researchers have found a workaround for a long-standing roadblock in the field of two-dimensional electronics.

In the study, researchers describe what they believe to be the first measurement showing direct interaction between electrons spinning in a 2D material and photons coming from microwave radiation. Graphic by Jia Li, an assistant professor of physics at Brown.

Researchers Probe Spin Structure in 2D Materials for First Time: For two decades, physicists have tried to directly manipulate the spin of electrons in 2D materials like graphene. Doing so could spark key advances in the burgeoning field of 2D electronics, where super-fast, small and flexible electronic devices carry out computations based on quantum mechanics.

Standing in the way is the fact that the typical method by which scientists measure the spin of electrons – an essential behavior that gives everything in the physical universe its structure – usually doesn’t work in 2D materials. This makes it incredibly difficult to fully understand such 2D materials and propel forward technological advances based on them. But now a team of scientists, led by researchers at Brown University, believe they have a way around this longstanding challenge. They report their solution in a paper in Nature Physics.

In the paper, the team — which also includes scientists from the Center for Integrated Nanotechnologies at Sandia National Laboratories, and the University of Innsbruck in Austria — describe what they believe to be the first measurement showing direct interaction between electrons spinning in a 2D material and the photons making up microwave radiation. Called a coupling, the absorption of microwave photons by electrons establishes a novel experimental technique for directly studying the properties of how electrons spin in these 2D quantum materials. According to the researchers, this coupling could serve as a foundation for developing computational and communicational technologies based on 2D materials.

“Spin structure is the most important part of a quantum phenomenon, but we’ve never really had a direct probe for it in these 2D materials,” said Jia Li, an assistant professor of physics at Brown and senior author of the paper. “That challenge has prevented us from theoretically studying spin in these fascinating materials for the last two decades. We can now use this method to study a lot of different systems that we could not study before.”

Researchers Probe Spin Structure in 2D Materials for First Time: The researchers made the measurements on a relatively new 2D material called ‘magic-angle’ twisted bilayer graphene. This graphene-based material is created when two sheets of ultrathin layers of carbon are stacked and twisted at just the right angle, converting the new double-layered structure into a superconductor that allows electricity to flow without resistance or energy waste. Only discovered in 2018, the researchers focused on this material because of the potential and mystery surrounding it.

“A lot of the major questions that were posed in 2018 have still yet to be answered,” said Erin Morissette, a graduate student in Li’s lab at Brown, who led the work.

Physicists usually use nuclear magnetic resonance (NMR) to measure the spin of electrons. They do this by exciting the nuclear magnetic properties in a sample material using microwave radiation and then reading the different signatures this radiation causes.

The challenge is that the magnetic signature of the electrons in 2D materials, as generated by microwave excitation, is too small to detect. So the research team decided to improvise. Instead of directly detecting the magnetization of the electrons, they measured subtle changes in electronic resistance caused by the changes in magnetization from the radiation. They did this using a device fabricated at Brown’s Institute for Molecular and Nanoscale Innovation. These small variations in the flow of the electronic currents allowed the researchers to determine that the electrons were absorbing the microwave photons.

The researchers were able to observe novel information from these experiments. They noticed, for instance, that interactions between the photons and electrons made the electrons in certain sections of the system behave as they would in an anti-ferromagnetic system. In such a system, the magnetism of some atoms is canceled out by a set of magnetic atoms that are aligned in a reverse direction.

This new method for studying spin in 2D materials and the current findings won’t be applicable to technologies today, but the research team sees potential for future applications. They plan to continue to apply their method to twisted bilayer graphene but also to expand it to other 2D materials.

“It’s a really diverse toolset that we can use to access an important part of the electronic order in these strongly correlated systems and in general to understand how electrons can behave in 2D materials,” Morissette said.


Researchers Probe Spin Structure in 2D Materials for First Time: Original Article


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