Symmetry-protected, zero-energy disclination modes and their observation in an acoustic lattice

Different from the bulk-boundary correspondence, bulk-defect correspondence gives rise to new wave behavior in topological systems. Such crystalline topological defects can induce a curvature singularity in a lattice, thus trapping modes within the bulk of the synthetic lattice. These trapped modes are useful in photonic and sonic crystals because the bulk surrounding a defect generates a bandgap that isolates the mode from the gapless free space.

However, in current realizations, these modes are not protected from delocalization because so far, experimentally realized mode-trapping disclinations have broken chiral symmetry. As such, their modes do not lie at mid-gap and can hybridize with bulk modes to form resonances, losing their confinement. Here, we devise a fundamentally new paradigm that allows the protection of modes bound to disclinations that preserve chiral symmetry so that the disclination modes are consequently pinned at the mid-gap. In this way, they are protected at maximal confinement. By presenting a protection mechanism that rests on the interplay between the topology of the lattice and the point group symmetry of defects and by experimentally probing these modes in judiciously designed chiral-symmetric acoustic lattices, our work demonstrates the existence of protected modes within the bulk of synthetic lattices.