No abstract

Acoustic media with Willis coupling possess intrinsic directionality owing to the vector nature of coupling coefficients in constitutive relations. Here, we report a type of sonic topological insulator that exhibits the valley Hall effect by using an acoustic fluid of the Willis type. We find that the valley Hall phase transition can be triggered by tuning the coupling vector. In addition, the Dirac cones or valley position are displaced away from high symmetry points in the Brillouin zone. The tunability of the valley offset offered by Willis coupling helps to realize equally robust one-way transport for both zigzag and armchair domain walls and for more tortuous wave channels.

Acoustics Willis media, known as bianisotropic acoustic media, incorporate additional coupling between pressure and velocity and between momentum and volumetric strain in their constitutive equation. The extra coupling terms have a significant influence on acoustic wave behavior. In this paper, the unusual wave phenomena relevant to interfaces between homogeneous acoustic Willis media are theoretically studied. We show that Willis media offer more flexible control in wave front and energy flow when waves are transmitted through an interface. Different from traditional acoustic fluid, Willis acoustic media support edge and interface waves, for which the existence conditions and corresponding wave features are systematically investigated. The study unveils more possibilities for manipulating acoustic waves and may inspire new functional designs with acoustic Willis metamaterials.

No abstract