Experimental Discovery of the First Nonsymmorphic Topological Insulator KHgSb

TL;DR

This paper reports the experimental discovery of KHgSb as the first nonsymmorphic topological insulator, confirming theoretical predictions and revealing hourglass fermions protected by nonsymmorphic symmetry.

Contribution

It provides the first experimental evidence of a nonsymmorphic topological insulator, expanding the classification of topological phases and demonstrating hourglass fermions in KHgSb.

Findings

KHgSb hosts hourglass fermions on the (010) surface.

The (001) surface of KHgSb has no boundary states.

Experimental results agree with first-principles calculations.

Abstract

Topological insulators (TIs) host novel states of quantum matter, distinguished from trivial insulators by the presence of nontrivial conducting boundary states connecting the valence and conduction bulk bands. Up to date, all the TIs discovered experimentally rely on the presence of either time reversal or symmorphic mirror symmetry to protect massless Dirac-like boundary states. Very recently, it has been theoretically proposed that several materials are a new type of TIs protected by nonsymmorphic symmetry, where glide-mirror can protect novel exotic surface fermions with hourglass-shaped dispersion. However, an experimental confirmation of such new nonsymmorphic TI (NSTI) is still missing. Using angle-resolved photoemission spectroscopy, we reveal that such hourglass topology exists on the (010) surface of crystalline KHgSb while the (001) surface has no boundary state, which is fully consistent with first-principles calculations. We thus experimentally demonstrate that KHgSb is a NSTI hosting hourglass fermions. By expanding the classification of topological insulators, this discovery opens a new direction in the research of nonsymmorphic topological properties of materials.