Deterministic double-zero media and robust wave manipulation using a phononic Weyl semimetal

Double-zero media exhibit unparalleled impedance-independent wave manipulation properties. However, their reliance on accidental degeneracy makes them fragile to disturbances in structural geometry and material parameters. Here, we propose a topological approach to address this limitation. By constructing a Weyl semimetal with uniaxial screw symmetry, which exhibits a charge-2 Weyl point at the Brillouin zone center, we propose a deterministic directional double-zero medium characterized by linear dispersion along one direction and quadratic dispersion along the perpendicular directions, leading to impedance-independent total transmission and divergent impedance mismatch along different directions. By experimentally constructing this Weyl semimetal, we have observed the existence of a charge-2 Weyl point and its unique wave manipulation capabilities as directional double-zero media, including frequency-tunable wave collimation and a configurable transition between positive and negative refraction. These findings open new pathways for realizing topology-protected metamaterials with unprecedented capabilities for bulk-wave manipulation.