Significance
Non-Hermitian Skin Effect (NHSE) is observed in non-Hermitian systems where eigenstates accumulate at the boundaries of a system instead of being uniformly distributed throughout its bulk. The phenomenon has been extensively studied in one-dimensional (1D) systems where its origins are linked to nontrivial spectral winding topologies. However, one of the limitations in advancing the field of non-Hermitian physics is the difficulty in constructing and manipulating high-dimensional NHSEs. The topological mechanisms driving these effects in two or three dimensions are not as well understood as in 1D systems. Additionally, experimental demonstrations of high-dimensional NHSEs have been rare and previous studies were focused on isolated case studies rather than providing a systematic approach for constructing and controlling these effects across different physical platforms. To address these gaps, recent study published in Advanced Materials Journal and conducted by Associate Professor Qicheng Zhang, graduate student Yufei Leng, Liwei Xiong, Yuzeng Li, Kun Zhang, Liangjun Qi, and led by Professor Chunyin Qiu from the Department of Physics at Wuhan University developed a new method for systematically constructing higher-dimensional NHSEs using a combination of well-understood 1D systems. Their goal was to provide a flexible and intuitive framework that allows for precise control over the behavior of NHSEs in higher dimensions. This innovative approach could unlock new possibilities for applications in sensing, filtering and amplification where the ability to localize energy at specific boundaries or corners of a system is essential. To validate their theoretical framework for constructing high-dimensional NHSEs and to demonstrate the flexibility and control afforded by their approach, the researchers designed two experimental setups using 2D nonreciprocal acoustic metamaterials which consisted of air-filled cavities connected by narrow tubes which emulated the desired 2D non-Hermitian lattice structures. The authors were able to control the directionality of sound transmission by using active unidirectional couplers which enabled the experimental observation of both multi-polar NHSEs and hybrid skin-topological effects. To dig deep in more detail in the first experiment, the team focused on demonstrating multi-polar NHSEs with a setup consisted of a 5×5 array of air-filled cavities with a point sound source placed at the center of the system. By measuring the sound pressure at various cavities in the array, the researchers observed that the sound fields localized at different corners depending on the frequency of the input. For example, at a specific frequency, the sound field accumulated at the lower-left corner while at another frequency it shifted to the upper-right corner and such behavior was consistent with the theoretical predictions of frequency-selective localization which confirmed that the multi-polar NHSE can be controlled by varying the input frequency. The authors findings showed a strong agreement with the simulated data with a relative deviation of about 22%, indicating the robustness of their approach in constructing 2D NHSEs. Moreover, the researchers constructed a hybrid system to investigate hybrid skin-topological effects, combining a 1D Su-Schrieffer–Heeger lattice along one axis and a long-range non-Hermitian lattice along the other axis which allowed them to investigate the interplay between topological edge states and non-Hermitian skin effects. The experimental setup consisted of a 6×5 array of cavities and the researchers placed sound sources at strategic positions along the edges of the system and they observed that the sound fields localized at two adjacent corners which was predicted by their hybrid model. The hybrid skin-topological states had combination of properties from both topological and skin effects and demonstrated that the interplay of different physical mechanisms could yield novel localized states. The frequency selectivity of these states further highlighted the researchers’ ability to control not only the localization but also the specific regions where the energy accumulated within the system.
In conclusion Professor Chunyin Qiu and colleagues successfully developed a systematic and flexible method for synthesizing higher-dimensional NHSEs from well-understood 1D systems. According to the authors, the precise control of the localization of energy in high-dimensional systems that they achieved in their study could revolutionize a variety of fields such as signal processing, sensing, and amplification technologies and also devices such as selective filters, directional amplifiers, and energy concentrators could be designed with improved efficiency and precision thanks to the advantage of frequency-selective nature of NHSEs. Moreover, the authors’ findings on hybrid skin-topological effects suggest the possibility of combining traditional topological states with non-Hermitian phenomena which can open the door for innovative solutions in topological insulators, quantum computing, and advanced materials. Finally, the methods proposed by Wuhan University scientists can be an important toolkit for theoretical and applied physicists to explore complex non-Hermitian systems.
Reference
Q. Zhang, Y. Leng, L. Xiong, Y. Li, K. Zhang, L. Qi, C. Qiu, Construction and Observation of Flexibly Controllable High-Dimensional Non-Hermitian Skin Effects. Adv. Mater. 2024, 36, 2403108. https://doi.org/10.1002/adma.202403108