A hidden-essential nodal line (HENL) is a symmetry-enforced band degeneracy in a two-dimensional crystal that, unlike a conventional essential nodal line lying on a high-symmetry line of the Brillouin zone, is buried inside a high-symmetry plane and is therefore less observable. Its existence is guaranteed by a horizontal glide-mirror symmetry, so it arises in both non-magnetic and magnetic systems; in magnetic layer groups it can also be enforced by horizontal mirror plus time-reversal symmetry under spin-orbit coupling. The HENL is required at electronic fillings 4N+2 (spinless) or 2N+1 (spinful) independent of material detail, and its spinful form carries a persistent, momentum-independent spin texture, defining a new topological semimetal state.
Li and coworkers propose a new type of band degeneracy in two-dimensional crystals: the hidden-essential nodal line (HENL). Unlike ordinary essential nodal lines, which sit on high-symmetry lines of …