Observation of flat-band skin effect

TL;DR

This paper reports the discovery and experimental observation of a flat-band skin effect in non-Hermitian systems, revealing new flat-band phenomena linked to spectral topology and exceptional points.

Contribution

It introduces the flat-band skin effect, showing its dependence on spectral topology and exceptional points, and demonstrates experimental observation in a mechanical lattice.

Findings

Flat-band skin effect (FBSE) exists in non-Hermitian systems.

FBSE is linked to the spectral topology of dispersive bands.

Experimental observation of FBSE in a mechanical lattice.

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.