论文标题

辫子受保护的拓扑结构,具有未配对的特殊点

Braid Protected Topological Band Structures with Unpaired Exceptional Points

论文作者

König, J. Lukas K., Yang, Kang, Budich, Jan Carl, Bergholtz, Emil J.

论文摘要

我们证明存在拓扑稳定的未配合特殊点(EPS)的存在,并构建了简单的非热(NH)紧密结合模型,以示例这种显着的淋巴结相。尽管费米恩加倍,即通过反毒剂补偿稳定节点点的拓扑费的必要性,但在Hermitian半学领域中排除了我们发现的直接对应物,在这里,我们在这里得出如何稳定复杂能量水平的非强制性辫子可以稳定不成熟的EPS。利用这种见解,我们揭示了一个单一的,未配对的EP的发生,该EP在最小的三波段模型的Brillouin区域中被表现为非亚洲单子。这种三阶退化性代表了在较大的拓扑阶段中的最佳位置,任何局部扰动都无法完全掩盖。取而代之的是,它只能分成更简单的(二阶)变性,这些变性只能在布里鲁因区域的不相等大圆圈中移动后,才能通过成对an灭。我们的结果暗示了基于绕组数字的拓扑分类的不完整性,这是由于编织组的非 - 阿贝尔表示,使三个或更多复杂的能量水平相互交织,并提供了对非亚美特式系统及其非阿布尔相的拓扑鲁棒性的见解。

We demonstrate the existence of topologically stable unpaired exceptional points (EPs), and construct simple non-Hermitian (NH) tight-binding models exemplifying such remarkable nodal phases. While fermion doubling, i.e. the necessity of compensating the topological charge of a stable nodal point by an anti-dote, rules out a direct counterpart of our findings in the realm of Hermitian semimetals, here we derive how noncommuting braids of complex energy levels may stabilize unpaired EPs. Drawing on this insight, we reveal the occurrence of a single, unpaired EP, manifested as a non-Abelian monopole in the Brillouin zone of a minimal three-band model. This third-order degeneracy represents a sweet spot within a larger topological phase that cannot be fully gapped by any local perturbation. Instead, it may only split into simpler (second-order) degeneracies that can only gap out by pairwise annihilation after having moved around inequivalent large circles of the Brillouin zone. Our results imply the incompleteness of a topological classification based on winding numbers, due to non-Abelian representations of the braid group intertwining three or more complex energy levels, and provide insights into the topological robustness of non-Hermitian systems and their non-Abelian phase transitions.

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