  Using spin-resolved and angle-resolved photoemission spectroscopy and
first-principles calculations, we have identified bulk band inversion and 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 TI
homologically emerge with hourglass fermions, as multi nodal chains composed by
the same pair of valence and conduction bands gapped by spin orbit coupling.
The novel 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.
