Researchers Extend the Concept of Directionality and Propose a New Class of Materials
It is often desirable to restrict flow—whether sound, electricity, or heat—in one direction, but naturally occurring systems almost never allow this. However, unidirectional flow can indeed be engineered under certain conditions, and the resulting system is said to exhibit chiral behavior.
The concept of chirality has traditionally been limited to one-way flow in one dimension. However, in 2021, researchers working with Taylor Hughes, a professor of physics at the University of Illinois Urbana-Champaign, introduced a theoretical extension that could explain more complicated chiral flows in two dimensions. Now, a team led by Hughes and Gaurav Bahl, a UIUC mechanical science & engineering professor, have experimentally realized this extension. As researchers report in Nature Communications, they built a network of topological circuits, electronic systems that simulate the microscopic behavior of quantum materials, to explore entirely new behaviors predicted by these extended chiralities or higher ranks.
“As a result, we have generalized the idea of a one-way street into two dimensions,” said Hughes. “In two dimensions there is no absolute sense of something going one way or the other, but if you hold a fixed arrow, then you can still describe chiral movement relative to that arrow.”
Indeed, high levels of chirality manifest as locking between the direction of flow of the particles and the direction of the arrow, or vector quantity, they carry. For this study, the team focused on rank-2 chirality in which flow is locked transversely to the momentum vector carried by the particles. Penghao Zhu, lead author of the study and UIUC physics graduate student, explained, “In standard chirality, flow can only go one way—to the right, say. However, a rank-2 system is designed so that if the momentum of the particles is up, the particles will flow to the right, and if the momentum is downward, then the particles will flow to the left.
In a 2021 study, Hughes’ group proposed a quantum material system for rank-2 chirality, but their interdisciplinary team realized they could explore the behavior of this system with a network of topological circuits. On these platforms, chirality is a consequence of microscopic dissipation or friction, called non-hermiticity, which has been engineered to only affect flow in a certain direction so that unwanted flow dies quickly, leaving only flow in the desired direction.
Zhu and postdoctoral fellow Xiao-Qi Sun designed a network of circuits that demonstrated the required non-hermiticity, and they collaborated with Bahl to build this “meta” material and carry out experimental measurements. According to Zhu, the material displays an important hallmark of the chiral system: the non-Hermitian skin effect, in which forced unidirectionality makes flows accumulate at the system boundary.
“In addition, our experiments show new phenomena that have not been explored before, such as corner localization, in which flows accumulate in the corners of the material,” he says. “This is something very special for rank-2 chirality and cannot be seen in any of the skin effects that have been shown before.”
The generalization offered by the high degree of chirality suggests a new class of devices that can be used to filter streams and engineer optical beams. Sun envisioned a device that separated photons, or particles of light, based on the direction they traveled: if only photons moving to the right were desired, then a rank-2 chiral material could eliminate left-diffusing photons by forcing them in different directions. direction to be removed.
Another useful mapping of this idea can be made for semiconductor electronic devices, where new and unique filtering operations can be performed with electrons. Bahl said. “Nearly all of the electronic calculation and communication devices we use today rely on controlling the flow of electrons. If we are able to replicate this high-level chiral behavior in microelectronics, behavior we had not had access to before, it could lead to some transformative new applications.
Sun added that the real benefit of learning a higher ranking system was a deeper understanding of what was possible.
“By designing and building systems that expand our understanding, we are taking the first step toward a much more general universe.” he says.