New technique probes spin structure in 2D materials

By observing the spin structure in ‘magic-angle’ graphene, a team of scientists led by Brown University researchers have found a workaround for a longstanding roadblock in the field of 2D electronics. Physicists have previously tried to directly manipulate the spin of electrons in 2D materials like graphene. Standing in the way is that the typical way in which scientists measure the spin of electronics usually doesn’t work in 2D materials. This makes it difficult to fully understand the materials and propel technological advances based on them. However, a team of researchers have found a way around this challenge and have described their solution in a study published in Nature Physics.

In the study, the team described what they believe to be the first measurement showing direct interaction between electrons spinning in a 2D material and photons coming from 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 — one that could serve as a foundation for developing computational and communicational technologies based on those materials, according to the researchers.

Jia Li, an assistant professor of physics at Brown University, said that although spin structure is the most important part of a quantum phenomenon, researchers have been unable to probe it directly in these 2D materials. “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,” Li said.

The researchers made the measurements on a 2D material called ‘magic angle’ twisted bilayer graphene. This material is created when two sheets of ultrathin layers of carbon are stacked and twisted to just the right angle, converting the new double-layered structure into a superconductor that allows electricity to flow without resistance or energy waste.

Physicists usually use nuclear magnetic resonance (NMR) to measure the spin of electrons. This is done by exciting the nuclear magnetic properties in a sample material using microwave radiation and then reading the different signatures this radiation causes to measure spin. The challenge with 2D materials is that the magnetic signature of electrons in response to the microwave excitation is too small to detect.

Instead of directly detecting the magnetisation of the electrons, the researchers measured the subtle changes in electronic resistance, which were caused by the changes in magnetisation from the radiation using a device fabricated at Brown University. These small variations in the flow of the electronic currents allowed the researchers to use the device to detect that the electrons were absorbing the photons from the microwave radiation.

The researchers also observed that the interactions between the photons and electrons made electrons in certain sections of the system behave as they would in an anti-ferromagnetic system — meaning the magnetism of some atoms was cancelled out by a set of magnetic atoms that are aligned in a reverse direction. The new method for studying spin in 2D materials and the current findings won’t be applicable to technology today, but the researchers see potential applications for the method in the future. They plan to continue to apply their method to twisted bilayer graphene but also expand it to other 2D material.

“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,” said Erin Morissette, a graduate student in Li’s lab.

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