High energy in plastic

A magnetic field is a tunable knob that can drastically change the electrons' behavior in a material. Such a magnetic field couples to the electronic charge and cannot be used to control neutral excitations such as sound waves or mechanical vibrations. In this work, we employ ideas drawn from high-energy theories to experimentally observe Landau levels in an acoustic metamaterial.

Weyl fermions are massless particles that carry a particular topological charge. Albeit originally predicted as elementary particles, they have been observed only as low-energy excitations around linearly dispersing touching points in the band structure of electronic materials and photonic or phononic crystals. Instead of coupling to the electric charge of these particles, one could envision an exotic gauge field that couples to their topological charge. This field, dubbed axial gauge field, has been introduce in the high-energy literature but has never been observed in nature.

By carefully designing spatial inhomogeneities in our 3D acoustic metamaterial, we successfully couple to the topological charge associated with the nodal points in the band structure of the sample. In this way, we can probe the phenomenology associated to an axial gauge field. In particular, we observe the distinctive chiral Landau levels dispersing along the field with a direction that depends on the associated topological charges. This unidirectional chiral channels in the bulk of a 3D material hold the promise for interesting applications for acoustic lensing and sensing. Moreover, this work provides an accessible platform to simulate and explore magneto-transport phenomena associated to an axial gauge field that might be difficult to probe in a more conventional electronic system.