Observation of Non-Abelian Charged Nodes Linking Nonadjacent Gaps

Abstract

<p>Nodal links are special configurations of band degeneracies in the momentum space, where nodal line<br>branches encircle each other. In PT symmetric systems, nodal lines can be topologically characterized<br>using the eigenvector frame rotations along an encircling loop and the linking structure can be described<br>with non-Abelian frame charges involving adjacent bands. While the commutation rules between the<br>frame charges are well established, the underlying relationship between distant band gap closing nodes<br>remains to be explored. In this Letter, we present a photonic multiple nodal links system, where the<br>nodal lines of nonadjacent bands are investigated with symmetry constraints on frame charges. Through<br>an orthogonal nodal chain, the nodal line from the lower two bands predicts the existence of nodal lines<br>formed between the higher bands. We designed and fabricated a metamaterial, with which the multiple<br>nodal links and the topological connection between nonadjacent nodal lines are experimentally<br>demonstrated.<br></p>