Restricting the flow of various entities, such as sound, electricity, or heat, to a single direction is often desirable. However, natural systems typically do not allow for unidirectional flow. Nonetheless, scientists have been able to engineer unidirectional flow under specific conditions, and such systems are said to exhibit chiral behavior.
Traditionally, the concept of chirality has been limited to one-dimensional flows in a single direction. However, in 2021, researchers, including Taylor Hughes, a physics professor at the University of Illinois Urbana-Champaign, introduced a theoretical extension that accounts for more complex chiral flows in two dimensions.
Now, Hughes and Gaurav Bahl, a professor of mechanical science and engineering at UIUC, have successfully demonstrated this extended chirality experimentally. In their study published in Nature Communications, they constructed a topological circuit network—a system of electronics that simulates the microscopic behavior of quantum materials—to explore the novel behaviors predicted by this higher-rank chirality.
“In essence, we have expanded the concept of a one-way street into two dimensions,” explained Hughes. “In two dimensions, there is no inherent notion of something going strictly one way or the other. However, by carrying a fixed arrow or vector quantity, it becomes possible to describe chiral motion relative to that reference.”
Higher-rank chirality manifests as a correlation between the direction of particle flow and the direction of an accompanying arrow or vector. In this study, the researchers focused on rank-2 chirality, where the flow is transverse to the momentum vector carried by the particles.
Lead author Penghao Zhu, a physics graduate student at UIUC, elucidated, “In conventional chirality, flows can only go in one direction, let’s say to the right. However, in a rank-2 system, if a particle’s momentum is upward, it flows to the right; if the momentum points downward, it flows to the left.”
By exploring these higher-rank chirality phenomena, the researchers open up possibilities for new advancements and applications in the field of engineered directional flows in complex systems.
The research conducted by Hughes’ team in 2021 proposed a quantum material system to explore rank-2 chirality. However, they realized that they could investigate the behaviors of this system using a topological circuit network instead. In this network, chirality arises as a consequence of engineered microscopic dissipation or friction, known as non-Hermiticity, which selectively affects flows in specific directions, causing undesired flows to decay rapidly and leaving only the desired flow.
Zhu and Sun, a postdoctoral fellow, designed a circuit network that exhibits the required non-Hermiticity. They collaborated with Bahl to construct this “meta” material and conduct experimental measurements. The material exhibited a crucial characteristic of chiral systems known as the non-Hermitian skin effect, where flow accumulates on the boundary of the system due to the imposed unidirectionality.
Zhu explained that their experiment also revealed novel phenomena, such as corner localization, where flows accumulate at the corners of the material. This is a unique feature specific to rank-2 chirality and has not been observed in previously demonstrated skin effects.
The generalizations offered by higher-rank chirality suggest the possibility of developing new types of devices for flow filtering and engineering optical beams. Sun envisioned a device capable of separating photons based on their travel direction, where a rank-2 chiral material could eliminate photons propagating in the undesired direction by redirecting them elsewhere for disposal.
Bahl further suggested that these concepts could be applied to semiconductor electronic devices, potentially enabling novel filtering operations with electrons. Since the control of electron flow is fundamental to electronic computation and communication devices, replicating higher-rank chiral behavior in microelectronics could lead to transformative applications.
Sun emphasized that the true value of studying higher-rank systems lies in gaining a deeper understanding of the possibilities that exist.
“By designing and constructing systems that expand our knowledge, we are taking the first step towards a much more comprehensive understanding of the universe,” he said.