Experimental Observation of Acoustic Weyl Points and Topological Surface States

Abstract

Weyl points emerge as topological monopoles of Berry flux in the three-dimensional (3D) momentum space and have been extensively studied in topological semimetals. As the underlying topological principles apply to any type of waves under periodic boundary conditions, Weyl points can also be realized in classical wave systems, which are easier to engineer compared to condensed matter materials. Here, we made an acoustic Weyl phononic crystal by breaking space inversion (P) symmetry using a combination of slanted acoustic waveguides. We conducted angle-resolved transmission measurements to characterize the acoustic Weyl points. We also experimentally confirmed the existence of acoustic "Fermi arcs" and demonstrated robust one-way acoustic transport, where the surface waves can overcome a step barrier without reflection. This work lays a solid foundation for the basic research in 3D topological acoustic effects.

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

“…parametric phason degrees of freedom). We demonstrate the construction of pairs of Weyl points [25][26][27][28][29][30][31][32] carrying Z 2 topological charges in the synthetic momentum space. In order to observe the associated topological phase transitions in simple sound scattering tests, we synthesize our system in a standard acoustic pipe supporting evanescently coupled quasibound states [33][34][35][36][37][38][39][40] embedded in the continuum of the single propagating mode.…”

Chirality-assisted three-dimensional acoustic Floquet lattices

“…Since then, tremendous attention has been drawn to the field of classical analogues of electronic topological insulators in photonics, phononics, and mechanics systems. Recently, various schemes for realizing topological acoustic transport were proposed to construct pesudospin/valley degrees of freedom in two-dimensional (2D) static systems, which are passive and not time-varying, to mimicking the quantum spin/valley Hall effects [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. However, these reported pesudospin/valley-based acoustic topological insulators are still not strictly robust in oneway topological transport, since the bosonic-like time-reversal symmetry in the linear and static acoustic system does not allow any Kramers doublet [22].…”

“…In the past years, investigations on topological transport in 3D lattices are rising, mostly based on the delicate tuning of band structures of the meta-atom-based 3D phoxonic crystals [6][7][8][29][30][31][32]. In this paper, we choose a different strategy and propose a unique type of 3D acoustic topological lattice, i.e., acoustic Floquet insulator, where a mapping relation is established between the time dimension and the space dimension.…”

Turing-complete mechanical processor via automated nonlinear system design

“…This limitation exists despite the significant progress in systematic design of mechanical systems with thousands of degrees of freedom, driven primarily by the metamaterials community. In these recent works, the desired performance is first expressed as a set of symmetries [12,13], a stiffness or deformation map [14][15][16] or a discrete mass-spring model [17,18]. Then, it is translated by a systematic algorithm into a device geometry that can be fabricated.…”