Sone Kazuki, Ashida Yuto, Sagawa Takahiro
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan.
Nat Commun. 2020 Nov 12;11(1):5745. doi: 10.1038/s41467-020-19488-0.
Topological materials exhibit edge-localized scattering-free modes protected by their nontrivial bulk topology through the bulk-edge correspondence in Hermitian systems. While topological phenomena have recently been much investigated in non-Hermitian systems with dissipations and injections, the fundamental principle of their edge modes has not fully been established. Here, we reveal that, in non-Hermitian systems, robust gapless edge modes can ubiquitously appear owing to a mechanism that is distinct from bulk topology, thus indicating the breakdown of the bulk-edge correspondence. The robustness of these edge modes originates from yet another topological structure accompanying the branchpoint singularity around an exceptional point, at which eigenvectors coalesce and the Hamiltonian becomes nondiagonalizable. Their characteristic complex eigenenergy spectra are applicable to realize lasing wave packets that propagate along the edge of the sample. We numerically confirm the emergence and the robustness of the proposed edge modes in the prototypical lattice models. Furthermore, we show that these edge modes appear in a model of chiral active matter based on the hydrodynamic description, demonstrating that active matter can exhibit an inherently non-Hermitian topological feature. The proposed general mechanism would serve as an alternative designing principle to realize scattering-free edge current in non-Hermitian devices, going beyond the existing frameworks of non-Hermitian topological phases.
拓扑材料在厄米系统中通过体边对应关系展现出受其非平凡体拓扑保护的边缘局域无散射模式。虽然最近在具有耗散和注入的非厄米系统中对拓扑现象进行了大量研究，但其边缘模式的基本原理尚未完全确立。在此，我们揭示，在非厄米系统中，由于一种不同于体拓扑的机制，稳健的无隙边缘模式可以普遍出现，从而表明体边对应关系的失效。这些边缘模式的稳健性源于围绕一个例外点的分支点奇点所伴随的另一种拓扑结构，在该点本征向量合并且哈密顿量变得不可对角化。它们独特的复本征能量谱适用于实现沿样品边缘传播的激光波包。我们通过数值方法证实了在典型晶格模型中所提出的边缘模式的出现及其稳健性。此外，我们表明这些边缘模式出现在基于流体动力学描述的手性活性物质模型中，这表明活性物质可以展现出一种内在的非厄米拓扑特征。所提出的一般机制将作为一种替代设计原理，用于在非厄米器件中实现无散射边缘电流，超越了现有的非厄米拓扑相框架。
