Experimental probe of point gap topology from non-Hermitian Fermi-arcs

The gap in spectra of a physical system is fundamental in physics, while gap topology further restricts possible occurrent gaps of topological boundary states. The emergence of non-Hermiticity unveils a unique gap type known as the point gap, which forecasts the wavefunction localization, known as the non-Hermitian skin effect. Therefore, experimentally identifying the point gap in the complex frequency plane through a real operating frequency can become a tool for the systematic investigation of skin effects. Here, we utilize a Weyl phononic crystal to demonstrate that the point gap constituted by bulk and Fermi-arc surface states can be observed experimentally by a real-space field mapping technique. The identified point gaps forecast various skin effects and their evolutions. We further experimentally demonstrate the hinge skin effect in a parallelogram structure. Our work provides a feasible recipe to explore point gap topology experimentally in a variety of systems and certainly stimulates the research on skin effects in three-dimensional systems. Point gap is signature of non-Hermitian systems, but the experimental identification of nontrivial point gaps is elusive. Here, the authors use a Weyl phononic crystal to demonstrate that the point gap constituted by bulk and Fermi-arc surface states can be observed experimentally by a real-space field mapping technique and discover various skin effects and their evolutions.