Observation of an acoustic topological Euler insulator with meronic waves

Abstract

Topological band theory has conventionally been concerned with the topology of bands around a single gap. Only recently non-Abelian {topologies that thrive on involving multiple gaps} were studied, unveiling a new horizon {in topological physics} beyond the conventional paradigm. Here, we report on the first experimental realization of a topological Euler insulator phase with unique meronic characterization in an acoustic metamaterial. We demonstrate that this topological phase has several nontrivial features: First, the system cannot be {described} by conventional topological band theory, but has a nontrivial Euler class that captures the unconventional geometry {of the Bloch} bands {in the Brillouin zone}. Second, we uncover in theory and probe in experiments a meronic configuration of the bulk Bloch states for the first time. Third, using a detailed symmetry {analysis}, we show that the topological Euler insulator evolves from {a non-Abelian topological semimetal phase via the annihilation of Dirac points in pairs in one of the band gaps}. With these nontrivial properties, we establish concretely an unconventional bulk-edge correspondence which is confirmed by directly measuring the edge states via {pump-probe techniques}. Our work thus unveils a nontrivial topological Euler insulator phase with {a unique} meronic {pattern} and paves the way as a platform for {non-Abelian topological} phenomena.