Geometric phases, synthetic gauge flux and Weyl points and in acoustic systems

Sound waves do not respond to external magnetic fields and do not have an intrinsic spin degree of freedom. For these reasons, phenomena that are analogues of quantum Hall effect and quantum spin Hall effect are difficult to realize in acoustic systems. By designing acoustic three dimensional meta-crystals with special structures, we show that one can create synthetic staggered flux and hence one-way edge modes in the x−y plane in acoustic meta-crystals which have simple “static” structures with no moving fluid and no dynamic modulation [1]. The three dimensional band structure of acoustic meta-crystals have topological Weyl points in the Brillouin zone and for fixed values of k
z
, the systems are acoustic analogues of the topological Haldane model. We also constructed a simple acoustic crystal with a topological transition point in the acoustic band structure where the band inverts and the Zak phase in the bulk band changes upon a shift in system parameters [2]. As a consequence, the topological characteristics of the band gap changes and interface states form at the boundary separating two phononic crystals. Experiments were performed to measure the Zak phase of the bulk bands of these acoustic crystals using two different methods.