Observation of flat-band skin effect

• URL: http://arxiv.org/abs/2512.19745v1
• Date: Thu, 18 Dec 2025 05:11:48 GMT
• Title: Observation of flat-band skin effect
• Authors: Xulong Wang, Dongyi Wang, Congwei Lu, Ruo-Yang Zhang, Ching Hua Lee, Kun Ding, Guancong Ma,
• Abstract summary: The non-Hermitian skin effect (NHSE) can appear in a flat band.<n>NHSE is associated with the non-trivial spectral topology of the dispersive bands enclosing the flat band.<n>Our work reveals flat-band phenomena unique to non-Hermitian systems.
• Score: 9.721404989450905
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Symmetry-protected ideal flat bands in one-dimensional (1D) Hermitian lattices are populated by compact localized states (CLS) - a special class of localization with wavefunctions confined within a small region. In this work, we discover that the non-Hermitian skin effect (NHSE) can appear in a flat band. Unlike conventional NHSEs for dispersive bands that are protected by nontrivial point-gap topology, the flat band remains a point on the complex-energy plane and is therefore always topologically trivial. We found that, intriguingly, the flat-band skin effect (FBSE) is associated with the non-trivial spectral topology of the dispersive bands enclosing the flat band on the complex-energy plane, so it only emerges within a finite range of non-Hermitian parameters and can counterintuitively disappear at large non-Hermiticity. Moreover, the gaps between the flat and the dispersive bands can close at higher-order exceptional points under both periodic and open boundary conditions. The flat-band wavefunctions are discontinuous in quantum distance across these exceptional points, signifying that the gap-closing is singular. The FBSE was experimentally observed in a non-Hermitian mechanical lattice. Our work reveals flat-band phenomena unique to non-Hermitian systems and highlights new possibilities in quantum geometry and localization control.

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