Inherited Weak Topological Insulator Signatures in the Topological Hourglass Semimetal Nb3XTe6 (X = Si, Ge)
Using spin-resolved and angle-resolved photoemission spectroscopy and first-principles calculations, we have identified bulk band inversion and the spin-polarized surface state evolved from a weak topological insulator (TI) phase in van der Waals materials Nb3XTe6 (X=Si, Ge). The fingerprints of weak TIs homologically emerge with hourglass fermions as multinodal chains composed by the same pair of valence and conduction bands gapped by spin-orbit coupling. The unique topological state, with a pair of valence and conduction bands encoding both weak TI and hourglass semimetal nature, is essential and guaranteed by nonsymmorphic symmetry. It is distinct from TIs studied previously based on band inversions without symmetry protections.