Sufei Shi Brings Department of Physics New Expertise

Sufei Shi is electrified to be working with Carnegie Mellon University physicists.

"Collaboration is one of the reasons I came here," said Shi, associate professor of physics. "The Department of Physics is really strong in condensed matter physics, so I know quite a few people already here."

Shi joined the Mellon College of Science in September 2023. Previously, he was an associate professor in the Department of Chemical and Biological Engineering at Rensselaer Polytechnic Institute.

Shi's research focuses on synthesizing and analyzing low-dimensional quantum materials. In the past year, his group has published two papers in Nature Communications on moiré superlattices: "Quadrupolar Excitons and Hybridized Interlayer Mott Insulator in a Trilayer Moiré Superlattice" and "Exciton Superposition Across Moiré States in a Semiconducting Moiré Superlattice."

Moiré superlattices are formed by overlapping patterns of crystals, typically graphene. The orientation of the layers gives the materials different properties. For example, when the patterns are turned at a specific angle, 1.1 degrees, which is called the "magic angle," electron motion will be significantly slowed, and unconventional insulating states or even superconducting states emerge.

Shi's work with moiré lattices focuses on excitons in two-dimensional (2D) semiconductors, pairings of an electron and an electron hole. Typically, excitons are created when electrons interact with light, and they are able to transport energy without conveying a charge. As a result, excitons play a role in determining the optical properties of semiconductors and capturing solar energy. Additionally, excitons can be manipulated by moiré superlattices, which Shi constructs.

In the paper on quadrupolar excitons, Shi's group constructed atomically thin superconductors known as transitional metal dichalcogenides (TMDCs) with three different layers to form a trilayer moiré superlattice. This setup allowed for two dipolar excitons to create a superposition, a quantum state where they exist half in one space and half in another, and form quadripolar excitons.

"The quadrupolar excitons in the trilayer moiré superlattices use a new platform to manipulate excitons and electrons for new quantum states, which are critical for realizing quantum computing and quantum simulations," Shi said.

After Shi's group found that they could manipulate excitons in moiré superlattices, they investigated other potential functions. Typically, excitons in moiré superlattices are locked in one place, but they wanted to see if they could make excitons interact through superposition.

"One route is to realize the so-called superposition of moiré excitons, which is a new quantum state, saying that the exciton is half at one site but half at the other," Shi said. "You cannot determine where they are, only the probability, which is the essence of quantum mechanics. If we can do that, that is also a new way for quantum computing with the moiré exciton as the qubit, as well as a way to construct a new quantum state."

In the paper on exciton superposition, Shi's group created a trilayer WSe₂ and a monolayer WS₂ moiré superlattice where the excitons could interact through the third layer WSe₂, away from the interface. Now that Shi knows how to make excitons interact through this kind of superlattice, he can investigate what other possibilities the materials have.

Shi plans to collaborate with other researchers at Carnegie Mellon to expand the materials he works with and fabricates. He is particularly excited to work with theorists and experimentalists investigating condensed matter physics.

"On my first day, Professor Ben Hunt stopped me, and then we talked for over an hour," Shi said. "He showed me his lab, and some of the things we can start working on together."

Shi will begin teaching experimental physics in the spring of 2024 and hopes to develop further courses on quantum physics and optical spectroscopy.

Scott Dodelson, professor and department head of physics, said that Shi and his work will have a significant impact for MCS.

"Sufei is both a superb scientist and superb team player," Dodelson said. "Students who take his courses or who work in his lab will benefit from the experience. Colleagues who interact with him will become better than the sum of their parts."