Nodal lines in a honeycomb plasmonic crystal with synthetic spin

Nodal line degeneracies can occur either at accidental or symmetry-enforced band crossings. In the case of symmetry-enforced nodal lines, nonsymmorphic symmetries have been shown to play a critical role. Here we propose a honeycomb plasmonic crystal which hosts nodal lines in the absence of nonsymmorphic symmetries. Using both continuum and tight-binding models, we show that a combination of synthetic time-reversal symmetry, inversion symmetry, and particle-hole symmetry guarantee that a nodal line enclosing the K and K' points exists. The nodal lines are not directly gapped even when the symmetries are weakly broken. The existence of the nodal lines are verified using full-wave electromagnetic simulations. The nodal line can be gapped by introducing a Kekule distortion that mixes the nodal lines at the K, K' points. Our work presents an alternative route to investigating the properties of symmetry-enforced nodal lines using plasmonic crystals.