Experimental Realization of Type-II Weyl Points and Fermi Arcs in Phononic Crystal

Abstract

Weyl points (WPs), as the doubly degenerate points in three-dimensional momentum band structures, carry quantized topological charges and give rise to a variety of extraordinary properties, such as robust surface wave and chiral anomaly. Type-II Weyl semimetals, which have conical dispersions in Fermi surfaces and a strongly tilted dispersion with respect to type I, have recently been proposed in condensedmatter systems and photonics. Although the type-II WPs have been theoretically predicted in acoustics, the experimental realization in phononic crystals has not been reported so far. Here, we experimentally realize a type-II Weyl phononic crystal. We demonstrate the topological transitions observed at the WP frequencies and the topological surface acoustic waves between the Weyl frequencies. The experiment results are in good accordance with our theoretical analyses. Due to the violation of the Lorentz symmetry, the type-II WPs only exist in low energy systems. As the analog counterpart in classical waves, the phononic crystal brings a platform for the research of type-II WPs in macroscopic systems.

Discovering Topological Surface States of Dirac Points

“…The surface states are measured through the Fouriertransformed field scan (FTFS). Similar approaches have been used to study other topological acoustic crystals [42][43][44][45][46]. A pressure-field microphone (diameter of 3.5mm, B&K-4138-A-015) is used as the scanning probe (receiver).…”

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.…”

Symmetry-controlled edge states in the type-II phase of Dirac photonic lattices

“…In certain classes of systems displaying a broken Lorentz invariance, the Fermi surfaces become conical (type-II) instead of point-like as in type-I, owing to the underlying strong tilt of the associated Weyl cones. Such type-II Weyl fermion excitations have been lately observed in semimetals, e.g., WTe 2 , MoTe 2 , PdTe 2 , along with Fermi arcs and unusual chiral photogalvanic effects [23][24][25][26][27][28][29][30] . An issue of interest is how the rich topological properties of type-II Weyl and Dirac systems can alter the properties and dynamics of localized edge states.…”