  Very recently, 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 science1-9. Here we report
the first experimental observation of the long-desired quadratic Weyl points10
by using a three-dimensional chiral metacrystal of sound waves. Markedly
different from the newly observed unconventional quasiparticles5-9, such as the
spin-1 Weyl points and the charge-2 Dirac points that are featured respectively
with threefold and fourfold band crossings, the charge-2 Weyl points identified
here are simply twofold degenerate, and the dispersions around them are
quadratic in two directions and linear in the third one10. Besides the
essential nonlinear bulk dispersions, we further unveil the exotic
double-helicoid surface arcs that emanate from a projected quadratic Weyl point
and terminate at two projected conventional Weyl points through Fourier
transformation of the scanned surface fields. This unique global surface
connectivity provides conclusive evidence for the double topological charges of
such unconventional topological nodes.
