Sci. Adv. 3, e1602415 (2017)

Topological insulators have bulk valence and conduction bands that are connected by conducting surface states characterized by a linear dispersion, hosting Dirac-like fermions. These surface states are topologically non-trivial and are protected by a discrete symmetry. Junzhang Ma and colleagues have found evidence that a certain type of crystal symmetry can protect a new type of surface state with an hourglass-shaped dispersion.

All topological insulators discovered so far have relied on time-reversal and/or mirror crystal symmorphic symmetry. But a class of material has been predicted whose band topology relies on non-symmorphic symmetries, which involve translations that are a fraction of a lattice period. Crystalline KHgSb is one of the materials predicted to belong to such a class, with a glide mirror symmetry protecting the surface states.

Using photoemission experiments, Ma et al. looked at the electronic structure in crystals of KHgSb. They found support for the idea that these crystals host surface fermions whose dispersion relation is shaped like an hourglass — making KHgSb potentially the first example of a non-symmorphic topological insulator.