Custodial glide symmetry of quantum spin Hall edge modes in monolayer WTe 2

A monolayer of ${\mathrm{WTe}}_{2}$ has been shown to display quantum spin Hall (QSH) edge modes persisting up to 100 K in transport experiments. Based on density-functional theory calculations and symmetry-based model building including the role of correlations and substrate support, we develop an effective electronic model for ${\mathrm{WTe}}_{2}$ that fundamentally differs from other prototypical QSH settings: we find a remarkably strong transverse localization of QSH edge modes in ${\mathrm{WTe}}_{2}$ related to the glide symmetry due to which the topological gap opens away from high-symmetry points in momentum space. While the indirect bulk gap is much smaller, a large direct gap of up to 1 eV in the Brillouin zone region of the dispersing edge modes determines their properties.