Topological triply degenerate point with double Fermi arcs

Unconventional chiral particles have recently been predicted to appear in certain three-dimensional crystal structures containing three-or more-fold linear band degeneracy points (BDPs)(1-4). These BDPs carry topological charges, but are distinct from the standard twofold Weyl points or fourfold Dirac points, and cannot be described in terms of an emergent relativistic field theory(1). Here we report on the experimental observation of a topological threefold BDP in a three-dimensional phononic crystal. Using direct acoustic field mapping, we demonstrate the existence of the threefold BDP in the bulk band structure, as well as doubled Fermi arcs of surface states consistent with a topological charge of 2. Another novel BDP, similar to a Dirac point but carrying non-zero topological charge, is connected to the threefold BDP via the doubled Fermi arcs. The Fermi arcs form double helicoids spanning a broad frequency range (relative bandwidth >25%). We show that the non-contractibility of these arcs gives rise to the phenomenon of topologically protected negative refraction of surface states on all surfaces of the sample. Our work paves the way to using these unconventional particles for exploring new emergent physical phenomena, and may find applications in symmetry-stabilized three-dimensional zero-index metamaterials.