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(上) ランク 2 のキラル物質の分散面。 (下) ランク 2 回路メタマテリアルからの実験測定。 クレジット: イリノイ大学アーバナ・シャンペーン校グレンジャー工科大学

イリノイ大学アーバナ・シャンペーン校のチームは、トポロジカル回路ネットワークを使用して、ランク 2 キラリティーと呼ばれる新しい形式の 2 次元カイラル フローを実証しました。 この画期的な進歩により、フローのフィルタリング、光ビーム工学、マイクロエレクトロニクスにおける革新的なデバイスへの道が開かれる可能性があります。

音、電気、熱などの流れを一方向に制限することが望ましいことがよくありますが、自然に存在するシステムではこれが許可されることはほとんどありません。 しかし、一方向の流れは特定の条件下で実際に設計することができ、その結果得られるシステムはキラルな挙動を示すと言われています。

キラリティーの概念は伝統的に、1 次元における単一方向の流れに限定されています。 しかし、2021年に、イリノイ大学アーバナシャンペーン校の物理学教授であるテイラー・ヒューズと共同研究している研究者らは、二次元におけるより複雑なカイラル流を説明できる理論的拡張を導入した。 今回、ヒューズ氏と UIUC の機械科学および工学教授であるガウラフ・バール氏が率いるチームが、この拡張を実験的に実現しました。 研究者らが雑誌で報告したように、

Indeed, higher-rank chirality manifests as locking between a particle’s flow direction and the direction of an arrow, or vector quantity, that it carries along with it. For this study, the team focused on rank-2 chirality where the flow is locked to be transverse to the momentum vector carried by the particles. Penghao Zhu, the study’s lead author and a UIUC physics graduate student, explained, “In standard chirality, flows can only go one way—to the right, let’s say. However, a rank-2 system is designed so that if a particle’s momentum is up, then it flows to the right, and if the momentum points down, then it flows to the left.”

In the 2021 study, Hughes’ group proposed a quantum material system for rank-2 chirality, but their interdisciplinary team realized they could explore the behaviors of this system with a topological circuit network instead. On this platform, chirality is a consequence of microscopic dissipation or friction, called non-Hermiticity, that has been engineered to only impact flows in particular directions such that unwanted flows die off quickly, leaving only flow in the desired direction.

Zhu and postdoctoral fellow Xiao-Qi Sun designed a circuit network that exhibits the needed non-Hermiticity, and they collaborated with Bahl to construct this “meta” material and perform experimental measurements. According to Zhu, the material displayed an important signature of chiral systems: the non-Hermitian skin effect, where the imposed unidirectionality makes the flow accumulate on the system’s boundary.

“Moreover, our experiment displays new phenomena that have not been previously explored, like corner localization, where the flows accumulate at the material corners,” he said. “This is something very special to rank-2 chirality and cannot be seen in any skin effect that has been previously demonstrated.”

The generalizations offered by higher-rank chirality suggest a new class of devices that could be used to filter flows and engineer optical beams. Sun imagines a device that separates photons, or particles of light, based on the direction they travel: if only the photons traveling to the right are desired, then a rank-2 chiral material could remove the leftward propagating photons by forcing them into a different direction to be discarded.

“Another useful mapping of this idea could be made to semiconductor electronic devices, where new and unique filtering operations may be accomplished with electrons,” Bahl said. “Pretty much every electronic computation and communication device that we use today relies on controlling the flow of electrons. If we are able to replicate this higher-rank chiral behavior in microelectronics, a behavior that we’ve never had access to before, it could lead to some transformative new applications.”

Sun added that the true reward of studying higher-rank systems is a deeper understanding of what is possible.

“By designing and constructing systems that extend our understanding, we are taking the first step towards a much more generalized universe,” he said.

Reference: “Higher rank chirality and non-Hermitian skin effect in a topolectrical circuit” by Penghao Zhu, Xiao-Qi Sun, Taylor L. Hughes and Gaurav Bahl, 9 February 2023, Nature Communications.
DOI: 10.1038/s41467-023-36130-x

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