Spinful hinge states in the higher-order topological insulators WTe2

Higher-order topological insulators are recently discovered quantum materials exhibiting distinct topological phases with the generalized bulk-boundary correspondence. T d -WTe 2 is a promising candidate to reveal topological hinge excitation in an atomically thin regime. However, with initial theories and experiments focusing on localized one-dimensional conductance only, no experimental reports exist on how the spin orientations are distributed over the helical hinges—this is critical, yet one missing puzzle. Here, we employ the magneto-optic Kerr effect to visualize the spinful characteristics of the hinge states in a few-layer T d -WTe 2 . By examining the spin polarization of electrons injected from WTe 2 to graphene under external electric and magnetic fields, we conclude that WTe 2 hosts a spinful and helical topological hinge state protected by the time-reversal symmetry. Our experiment provides a fertile diagnosis to investigate the topologically protected gapless hinge states, and may call for new theoretical studies to extend the previous spinless model. The standard topological insulator is characterized by an insulating bulk and a conducting boundary, so a three dimensional insulating bulk of a topological insulator has a conducting surface. Recently, this idea was extended to the edges of the surfaces of the three dimensional material as a new topological phase, referred to as a higher-order topological insulator. Here, the authors find evidence of such higher order topological insulator states in tungsten ditelluride using heterostructures composed of tungsten ditelluride and graphene.