Experimental observation of the acoustic Z2 Weyl semimetallic phase in synthetic dimensions

Scalar waves such as airborne sound lack an intrinsic spin degree of freedom, making the realization of sonic Z2 topological phases based on spin degeneracy challenging. Here, we demonstrate the relevance of synthetic dimensions and higher-dimensional topological physics for exploring topological phases based on acoustic pseudo-spin with exact Kramers degeneracy. Interestingly, we find that a carefully designed two-band one-dimensional Hamiltonian with two additional phason degrees of freedom can enter a Z2 semimetallic phase with nonzero topological invariants carried by pairs of Weyl points in a three-dimensional synthetic momentum space. Taking advantage of the high localization of sonic quasibound states, embedded in the modal continuum of a one-dimensional acoustic waveguide, we implement a Z2 topological Weyl system and experimentally observe its signature in far-field sound scattering experiments. Our findings establish sonic quasibound states in continuum as a fertile ground for exploring higher dimensional Weyl physics in scattering media, and provide a viable experimental path to study spin-related topological effects in acoustics.