By combining approaches from two rapidly growing fields of quantum physics, researchers at Penn State and Saint Louis University have demonstrated a novel specialized material can naturally enable a new way to study unusual physical phenomena known as non-Hermitian dynamics.
The work lays the foundation to build a new platform to explore phenomena that could power devices capable of transporting and grouping electrical signals and quantum states in ways not traditionally achievable without relying on optic or engineered systems. The team detailed their findings in a paper published in Science Advances.
Non-Hermitian physics refers to systems that exhibit behaviors not found in conventional physical models, explained Morteza Kayyalha, assistant professor of electrical engineering at Penn State and corresponding author on the paper. These systems can display unusual behaviors, such as enhanced responses to perturbations and external stimulus. They can also demonstrate the non-Hermitian skin effect, where quantum states - which researchers can use to predict the physical properties of a material - become concentrated near a specific boundary or point in the material, rather than spreading uniformly throughout.
Kayyalha and his collaborators' work focuses on the development of a magnetic topological insulator, otherwise known as a quantum anomalous Hall (QAH) insulator, which can achieve this behavior. The interior of this material is insulating, stopping the flow of electricity, with electrical current instead passing along the material's edge in a single direction. These one-way edge paths, called chiral edge channels, offer a natural way to build an electronic network whose effective connections are direction dependent, Kayyalha said.
Ordinary electronic networks showcase reciprocal responses between two points, meaning the connection from one point in the system to another is balanced by the connection in the reverse direction, like a two-way highway into a city where cars can enter, but only if the same number of cars leaves. Non-reciprocal systems relax this symmetry - their effective connections can depend on direction, allowing states or electrical signals to accumulate in ways that would not occur in a conventional reciprocal system.
"We wanted to show that these phenomena can emerge naturally in a quantum material," Kayyalha said. "Our work lays the groundwork for achieving scalable, non-Hermitian behavior with a quantum material platform rather than relying only on optical or circuit-based designs."
The QAH devices used in the study were made from thin films of the topological insulator bismuth antimony telluride, synthesized in the two-dimensional crystal consortium (2DCC), a facility at Penn State funded by the U.S. National Science Foundation (NSF). The insulator is magnetically doped, a process that introduces magnetic atoms to a non-magnetic base material, creating a quantum state in which current travels along the device boundary through a chiral edge channel. According to Kayyalha, the QAH insulators do not require external magnetic fields, which are usually necessary to achieve non-Hermitian behavior in quantum Hall devices during operation.
"A key advantage of this QAH platform is that, after the material is magnetized, the chiral edge state can be studied at zero applied magnetic field," Kayyalha said. "That makes it a promising platform for exploring non-Hermitian physics in electronic quantum materials."
The team built ring-shaped devices from the QAH insulator, connecting multiple electrical contacts around the perimeter of each ring. By carefully measuring how electrical signals traveled between the contacts in one of the rings, the team reconstructed the system's conductance network, a collection of measurements that visualize how electricity moves through a material. They then compared these measurements to theoretical models, specifically the Hatano-Nelson model, which is a standard model used to identify non-Hermitian behavior in systems.
"We can compare the measured conductance matrix directly with theoretical models of non-Hermitian physics," Kayyalha said. "From there, we can identify signatures of non-Hermitian dynamics in the quantum material."
