Stable self-connecting boundary arcs of triply degenerate point in a two-dimensional acoustic crystal
Phys. Rev. B 111, 214117 – Published 24 June, 2025
DOI: https://doi.org/10.1103/lqkm-44dr
Abstract
Weyl semimetals have attracted significant attention due to their special topological properties, especially the Fermi arcs connecting the projection of two oppositely charged Weyl points. However, these boundary arcs are typically confined to local regions in momentum space, which is influenced by the boundary formation and restricts their universality and stability. Here, we report our realization of a triply degenerate point hosting self-connecting boundary arcs that are stable to the boundary formation in a two-dimensional acoustic crystal. By symmetry enhancement, we merge two Weyl points into a quadratic Weyl point that supports a global boundary arc. Further adjustment of geometric parameters leads to the formation of a triply degenerate point, serving as a phase transition between distinct quadratic Weyl states. Despite exhibiting a zero Berry phase, the triply degenerate point supports experimentally observed boundary arcs that are stable and immune to boundary formation. Our findings reveal the variation in topological properties during the evolution of different degenerate points and present a different approach for engineering boundary arcs, with potential applications in topological acoustics and beyond.
