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For two decades, physicists have attempted to directly manipulate the spin of electrons in two-dimensional materials such as graphene. Doing so could herald major advances in the burgeoning world of two-dimensional electronics, a field in which ultrafast, small, and flexible electronic devices perform computations based on quantum mechanics.
What stands in the way is that the typical way scientists measure the spin of electrons — a fundamental behavior that gives everything in the physical universe its structure — doesn’t usually work in two-dimensional materials. This makes it very difficult to fully understand materials and drive technological advances based on them. But a team of scientists led by researchers from Brown University believe they now have a way around this long-standing challenge. They describe their solution in a new study published in the journal nature physics.
In the study, the team — which also includes scientists from the Center for Integrated Nanotechnologies at Sandia National Laboratories and the University of Innsbruck — describe what they believe is the first measurement showing the direct interaction between electrons orbiting in a two-dimensional material and photons coming from microwave radiation. Called conjugation, electron absorption of microwave photons establishes a new experimental technique for directly studying the properties of how electrons spin in these two-dimensional quantum materials — which could serve as a basis for developing computational and communication technologies based on those materials, according to the researchers.
“Spin structure is the most important part of a quantum phenomenon, but we’ve never had a direct probe of it in these two-dimensional materials,” said Jia Li, associate professor of physics at Brown and senior author on the paper. “This challenge has prevented us from theoretically studying spin in these fascinating materials for the past two decades. We can now use this method to study a lot of different systems that we couldn’t study before.”
The researchers performed the measurements on a relatively new 2D material called “magic angle” twisted bilayer graphene. This graphene-based material is created when two extremely thin layers of carbon are stacked and twisted at just the right angle, turning the new double-layered structure into a superconductor that allows electricity to flow without resistance or wasted energy. Just discovered in 2018, researchers have focused on the substance because of the potential and mystery surrounding it.
“A lot of the key questions that were asked in 2018 remain unanswered,” said Erin Morissette, a graduate student in Lee’s lab at Brown who led the work.
Physicists commonly use nuclear magnetic resonance, or NMR, to measure the spin of electrons. They do this by exciting the nuclear magnetic properties in a sample of material using microwave radiation and then reading the various signatures caused by that radiation to measure the rotation.
The challenge with 2D materials is that the magnetic signature of electrons in response to microwave excitation is too small to detect. The research team decided to improvise. Instead of directly detecting the electrons’ magnetization, they measured subtle changes in electronic resistance, which were caused by changes in radiation magnetization, using a device made at the Institute for Molecular and Nanotechnology Innovation at Brown. These small differences in the flow of electronic currents allowed the researchers to use the device to discover that electrons absorb images from microwave radiation.
The researchers were able to note new information from the experiments. The team observed, for example, that the interactions between photons and electrons caused the electrons in certain sections of the system to behave as if they were in an antiferromagnetic system — meaning that some atoms are demagnetized by a group of magnetic atoms that are aligned in an opposite direction.
The new method for studying spin in 2D materials and the current findings will not be applicable to the technology today, but the research team sees potential applications the method might lead to in the future. They plan to continue applying their method to twisted bilayer graphene but also extend it to other 2D materials.
“It’s a really diverse set of tools that we can use to get into an important part of the electronic system in these tightly coupled systems and more generally to understand how electrons behave in two-dimensional materials,” Morissette said.
The experiment was conducted remotely in 2021 at the Center for Integrated Nanotechnologies in New Mexico. Presented by Matthias S. Schürer from the University of Innsbruck Theoretical support for modeling and result understanding. The work included funding from the National Science Foundation, the US Department of Defense and the US Department of Energy’s Office of Science.
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