Chunyin Qiu

The emergent higher-order topological insulators significantly deepen our understanding of topological physics. Recently, the study has been extended to topological semimetals featuring gapless bulk band nodes. To date, higher-order nodal point and line semimetals have been successfully realized in different physical platforms. However, for the conceptually expected higher-order nodal surface semimetals, a concrete model has yet to be proposed, let alone experimentally observed.

Higher-order topological insulators, which go beyond the conventional bulk-boundary correspondence, have been attracting extensive interest in past years. Very recently, it was pointed out that chiral-symmetric higher-order topological insulators can be characterized by a Z topological invariant (dubbed the multipole chiral number), which dictates the number of degenerate zero-energy states per corner.

Abstract Novel quasiparticles beyond those mimicking the elementary high-energy particles such as Dirac and Weyl fermions have attracted great interest in condensed-matter physics and materials science. Here we report an experimental observation of the long-desired quadratic Weyl points by using a three-dimensional chiral metacrystal of sound waves.