Shubhayu Chatterjee Explores Spin Properties in Materials

Shubhayu Chatterjee is putting a new spin on magnetism research and physics teaching.

In February 2023 Chatterjee, assistant professor in Carnegie Mellon University's Department of Physics, was part of a team that conducted theoretical analysis and simulations for research that was published in Nature. The paper, "Continuous symmetry breaking in a two-dimensional Rydberg array," explores magnetism in synthetic platforms that closely simulate real materials.

"Two-dimensional magnets are very rare. If you have a two-dimensional magnet and all the spins are trying to align with each other, thermal fluctuations at any finite temperature usually destroy that kind of ordering," Chatterjee said. "It will only be a magnet at 0 temperature if the alignment of spins breaks a continuous symmetry."

This idea is known as the Mermin-Wagner-Hohenberg Theorem. But, Chatterjee said, there is a way around this.

If the atoms of a two-dimensional material have degrees of freedom, or spins, which have long-range interaction with other spins, then the thermal fluctuations become suppressed, and all of the spins try to point in the same direction.

"It turns out these spins can align to form a ferromagnet at non-zero temperature," Chatterjee said. "This hasn't been observed in any material before, because these types of long-range interactions are very hard to engineer."

The phenomenon was observed in an artificial platform built with the use of optical tools by collaborators in Paris. At the time, Chatterjee was a postdoctoral fellow prior at the University of California, Berkeley, before joining Carnegie Mellon in January 2023.

"With this new design, we were able to show that at finite temperatures this long-range order persists, and that's what is new and exciting," he said. "Observing that this actually exists opens the door to many different things."

Chatterjee said the researchers are moving forward with the research in different directions.

"For example, having determined that there is magnetism due to alignment of spins — one natural question is: how do the spins fluctuate due to thermal effects, or due to interaction with other spins?" Chatterjee said. "Is there something special about long-range interacting platforms? We are trying to answer these questions, both theoretically and experimentally, as we speak."

This summer, Andrew Kim, a senior in physics, worked with Chatterjee and postdoctoral researcher Ahmed Khalifa on these problems.

As a theoretical condensed physicist, Chatterjee is motivated by questions about the fundamental principles behind how nature works. His work intersects with not only the condensed matter group in Carnegie Mellon's Department of Physics but also with the high energy physics group.

"It's exciting to be part of a department that has been growing on both the theoretical and experimental sides," Chatterjee said. "CMU has a pretty big group working on what is called effective field theory. While part of that focus is on cosmological aspects like black holes, the universe, etc., there are other parts that are related to materials science."

As a teacher, Chatterjee aims to get all of his students — inside and outside the Department of Physics — thinking like physicists.

"When you are doing physics, you are learning to look at data that may come from very different sources and then trying to compile it in terms of a simple model that is consistent with the laws of physics," Chatterjee said. "In doing this type of work, you gain a lot of skills that may not be immediately apparent, but they are very valuable later on."