Observation of nodal links in momentum space

In crystals, two bands may cross each other and form degeneracies along a closed loop in the three-dimensional momentum space, which is called nodal line. Nodal line degeneracies can be designed to exhibit nodal rings, chains, links and knots. With rich global topological structure and other peculiar properties, such as diverse surface state variations, nodal links have attracted extensive theoretical attentions. However, they impose stringent requirements on the Hamiltonian form and hence have not yet been observed in realistic materials thus far. Here, we propose a simple realization of nodal links in a biaxial hyperbolic metamaterial. The two linked nodal lines threading through each other are formed by the crossings between three adjacent bands. On the interface between the metamaterial and air, surface bound states in the continuum (BICs) are observed, which serve as the symmetry-enforced derivative of drumhead surface states from the linked nodal lines. Our discovery not only experimentally demonstrates the global topological structure of nodal links, but also opens the gate towards manipulating topological surface states.