  Owing to the natural compatibility with current semiconductor industry,
silicon allotropes with diverse structural and electronic properties provide
promising platforms for the next-generation Si-based devices. After screening
230 all-silicon crystals in the zeolite frameworks by first-principles
calculations, we disclose two structurally stable Si allotropes (AHT-Si24 and
VFI-Si36) containing open channels as topological node-line semimetals with
Dirac nodal points forming a nodal loop in the kz=0 plane of Brillouin zone.
Interestingly, their nodal loops protected by inversion and time-reversal
symmetries are robust against SU(2) symmetry breaking due to very weak
spin-orbit coupling of Si. When the nodal lines are projected onto the (001)
surface, flat surface bands can be observed because of the nontrivial topology
of the bulk band structures. Our discoveries extend the topological physics to
the three-dimensional Si materials, highlighting the possibility to realize
low-cost, nontoxic and semiconductor-compatible Si-based electronics with
topological quantum states.
