Experimental Evidence of the First Nonsymmorphic Topological Insulator KHgSb.

Topological insulators (TIs) host novel states of quantum matter characterized by nontrivial conducting boundary states connecting valence and conduction bulk bands. All TIs discovered experimentally so far rely either on time reversal or mirror crystal symmorphic symmetries to protect massless Dirac-like boundary states. Several materials were recently proposed to be TIs with nonsymmorphic symmetry, where a glide-mirror protects exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of such new fermion is missing. Using angle- resolved photoemission spectroscopy, we provide experimental evidence of hourglass fermions on the (010) surface of crystalline KHgSb while the (001) surface has no boundary state, in agreement with first-principles calculations. Our study will stimulate further research activities of topological properties of nonsymmorphic materials.